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Randi Sue Price - The Schanze Research Group

1. A solution can be deoxygenated by substitution with an inert gas like N2 or Ar achieved by a gentle flow of the inert gas into the solution for about 15 minutes for every 3 mL while stirring This purging process works rather well for solutions where the triplet excited lifetime of interest is on the order of a few microseconds For solutions containing chromophores with substantially long lifetimes gt 100 us a more complicated deoxygenation procedure is needed One method of removing oxygen from a solution is by freeze pump thaw Freeze pump thaw cycling is when a solution in a borosilicate tube is flash frozen with liquid nitrogen and the gas present in the headspace is removed by applying vacuum for 10 30 minutes The tube is then sealed off from the vacuum and the solution is allowed to thaw During thawing gas trapped in solution will bubble out and escape This freeze pump thaw process is repeated a minimum of three times For compounds with relatively low phosphorescence quantum yields the emission spectrum can be collected with the sample in a glass forming solvent such as 2 methyltetrahydrofuran MeTHF at very low temperatures At low temperatures phosphorescence is promoted by slowing the rate of radiationless decay Typically a 29 cryostat or cold finger containing liquid nitrogen is used to cool a sample to 77 K Sometimes this involves modification of the sample holder to mount the cryostat securely in the optical path
2. 0 000 830 nm O Purged 05 10 15 Time microseconds 2 0 Figure 4 8 Transient absorption spectra A and transient decays B of pure MEH PPV solution 1 in benzene under deoxygenated and oxygen saturated conditions Aexc 532 nm 2 6 mJ pulse 112 The transient absorption spectra of solution 2 MEH PPV with 40 uM Hg are shown in Figure 4 9 In deoxygenated solution the triplet triplet absorption peak at 830 nm is observed and another peak tentatively assigned to MEH PPV appears at 950 nm There is a slight red shift of the triplet triplet absorption peak from 810 nm in the pure MEH PPV solution to 830 nm When the solution is saturated with oxygen the transient absorption at 830 nm is very quickly quenched disappearing within 88 ns while the absorption at 950 nm remains Looking at the transient decays the very fast component appears to maintain the same temporal profile although it changes in magnitude between wavelength and purging conditions 0 016 Ar Purged O Purged 0 014 Si 2 9 Inc 88 ns inc 88 ns O C S 2 O Yn Q lt oO Q Ki Peas SE S 0 000 Gees jam oe 850 900 950 1000 1050 1100 850 900 950 1000 1050 1100 B Wavelength nm Wavelength nm 0 018 F 0 015L Raw Data 1 Raw Data 0 0121 Exponential Fit Exponential Fit Q O bal 2 i 5 0
3. 127 128 129 130 131 132 133 134 135 Yang K Arif M Foerster M Scherf U Guha S Synth Met 2009 159 2338 2341 Haneline M R Tsunoda M Gabbai F P J Am Chem Soc 2002 124 3737 3742 Gardinier J R Gabbai F P J Chem Soc Dalton Trans 2000 2861 2865 Tsunoda M Gabbai F P J Am Chem Soc 2000 122 8335 8336 King J B Haneline M R Tsunoda M Gabbai F P J Am Chem Soc 2002 124 9350 9351 Taylor T J Elbjeirami O Burress C N Tsunoda M Bodine ML Omary M A Gabbai F P J Inorg Organomet Polym 2008 18 175 179 Omary M A Kassab R M Haneline M R Elbjeirami O Gabbai F P Inorg Chem 2003 42 2176 2178 Elbjeirami O Burress C N Gabbai F P Omary M A J Phys Chem C 2007 177 9522 9529 Burini A Fackler J P Galassi R Grant T A Omary M A Rawashdeh Omary M A Pietroni B R Staples R J J Am Chem Soc 2000 122 11264 11265 Haneline M R Gabbai F P Inorg Chem 2005 44 6248 6255 Forero S Nguyen P H Brutting W Schwoerer M PCCP 1999 1 1769 1776 Dai L M Winkler B Dong L M Tong L Mau A W H Adv Mater 2001 13 915 925 Hoppe H Arnold N Sariciftci N S Meissner D Sol Energy Mater Sol Cells 2003 80 105 113 Sariciftci N S Braun D Zhang C Srdanov V I Heeger A J Stucky G Wudl F Ap
4. and their derivatives are the most promising of all organic polymers with reported 123 efficiencies of up to 5 5 5 P3HT can be prepared in two flavors regioregular and regiorandom depending on the alignment of the polymer chains Regioregular P3HT is aligned in a head to tail stereoregular order and forms high mobility films due to the formation of nanocrystalline lamellae 5 5 Regiorandom P3HT does not display supramolecular structure in films and generally has very small hole mobility in contrast to the high mobility found in regioregular P3HT 154158 CH2 CH2 4CH3 S n Figure 4 19 Structure of P3HT Poly 3 hexylthiophene 2 5 diyl The interaction between P3HT regioregular and Hg3 was investigated There was found to be some solvent dependence as shown in Figure 4 20 Unlike MEH PPV P3HT tends to aggregate in benzene so the addition of Hg3 only darkens the purplish red color slightly In chloroform initially the P3HT is unaggregated and the solution is orange but with addition of Hg3 the solution quickly darkens Benzene Chloroform Figure 4 20 P3HT solutions with increasing amount of Hg3 in benzene and chloroform 124 P3HT in chloroform has an absorption peak at 450 nm When Hg is added the shoulder around 595 nm rises and a peak appears at 551 nm Ordered P3HT aggregates in solution such as is induced by poor solvent is reported to absorb at 551 nm where the intensity is dependent on the deg
5. A AAbsorbance AAbsorbance TPV2 T2 a crossTPV3 avelength nm 0 2 y TPVO l crossTPV1 e TPV1 Ph crossTPV3 TPV1 A crossTPV1 DPAF TPV2 TPV2 T2 Figure 3 3 Transient triplet triplet absorption spectra of the linear A and cross conjugated B p phenylene vinylene platinum Il acetylide chromophores in argon purged THF using nanosecond pulsed excitation at 355 nm energy 5 mJ pulse The solutions were prepared to the same optical density at 355 nm OD 0 4 and the spectra shown here were collected after a camera delay of 50 ns after the laser pulse 70 To examine the effect of the platinum on the nonlinear absorption the two photon absorption properties of the ligands TPVO L TPV1 L TPV2 L were investigated and compared to the properties of the linear and cross conjugated platinum acetylides The ligands consist of oligomeric OPVs with terminal triphenylamine groups that are not attached to a platinum acetylide Generally the peak values of the ligands correspond to approximately one half of the peak values in the platinum acetylide complex This is reasonable since there are two OPV ligands present in each linear and cross conjugated platinum acetylide complex The discrepancy in the NLT peak values for the TPVO L and TPVO compounds are likely due to photobleaching of the high concentration samples used in the NLT experiment The pe
6. Lan 2 E ay 60 S Lan 5 D 3 50004 34 40 3 4 Gs a e 3 10 3 sill SN 20 S a lt 0 oln e 0 a a H i N TPV2 T2 d V 100004 F 80 g 20 gt t 60 20907 49 5000 40 e ae Ca E Ry Ki HG r r T H Kb r crossTPV1 D crossTPV3 L 40 2000 J 60004 20 d E 30 sooo kx 1000 aa p 20 IS fe 10 coco gore 2000 10 ey NE wf SS ol ess D o ged H 600 700 800 900 1000 600 700 800 900 1000 Laser wavelength nm Laser wavelength nm Figure 3 4 Two photon absorption 2PA spectra and values of the linear and cross conjugated OPV Pt acetylides and ligands measured by 2PEF open red symbols and NLT filled blue symbols methods by Dr Rebane and his research group at Montana State University The one photon absorption spectra are shown in black for comparison The nonlinear absorption of each complex and ligand to nanosecond pulses 680 nm 800 pJ pulse was measured by open aperture z scan Well known nonlinear absorbing platinum acetylide Pt DPAF was used as a reference compound in each scan as a means of comparing Z scan measurements performed on separate occasions 72 4 CH S oo Bet N Eat D TPV2 T2 crossTPV1 J crossTPV3 crossTPV1 DPAF C Relative Transmittance CH CO 10 5 0 5 Z Position mm E Pt DPAF TPVO L TPV1 L W TPV2 L 10 10 5 0 5 Z Position mm E Pt DPAF TPV1 Ph TPVO 10 10 5 0 5 Z Position mm E Pt
7. The fluorescence or phosphorescence quantum yield is the ratio of photons emitted by fluorescence or phosphorescence to photons absorbed A comparative method by Williams et al is the most commonly used method for determining quantum yields The fluorescence intensity of a standard sample with a known quantum yield Or is compared to that of a test sample prepared to the same optical density at the same excitation wavelength A range of concentrations of the standard and sample solutions are prepared in a clean optical quality cuvette such that the optical density at the excitation wavelength is lt 0 1 Higher absorbances can introduce reabsorption effects The integrated fluorescence intensity of the solutions are acquired and plotted against the absorbance at the excitation wavelength The absolute quantum yield of the sample can be calculated with the following equation by Gan SP iy 2 slopesta NSta where the subscripts x and std denote standard and unknown sample respectively is the quantum yield slope is the slope of emission intensity vs absorbance and n is the refractive indices of the solvents used Some considerations are important in order to get the most accurate results First two standard samples should be cross calibrated to confirm good technique and that the standard r value can be used confidently Second the same instrumental parameters should be used for both sample and standard includ
8. s Internal conversion IC is nonradiative decay between states with the same multiplicity S gt So or T2 gt T1 seen as energy loss due to collisions or less of heat IC occurs when there is significant orbital overlap between the excited vibrational levels between electronic states Due to the loss of energy during vibrational relaxation of the initially excited S1 state to a relaxed S state from which fluorescence occurs the emitted photons will be of slightly lower energy than those absorbed This red shift of the fluorescence spectrum compared to the absorption spectrum is called the Stokes shift Intersystem crossing ISC can also occur from the S1 state Intersystem crossing is a radiationless transition between two states of different multiplicity such as a singlet to a triplet excited state or vice versa Formally this process is spin forbidden by the law of conservation of angular momentum For organic molecules the yield of ISC is typically very low However when heavy atoms are present spin orbit coupling occurs between the orbital and spin angular momenta and the singlet triplet transition becomes more allowed The total angular momentum in the system is conserved even though the individual electron s spin momentum and the orbital momentum are changed The triplet excited state T1 produced by intersystem crossing from the Si state can be deactivated by phosphorescence or nonradiative decay The lowest triplet
9. B Gresser G Havaux M Van Breusegem F Mueller M J Plant Physiol 2008 148 960 968 Brezova V Valko M Breza M Morris H Telser J Dvoranova D Kaiserova K Varecka L Mazur M Leibfritz D J Phys Chem B 2003 107 2415 2425 Kuimova M K Yahioglu G Ogilby P R J Am Chem Soc 2009 131 332 340 Walsh S E Maillard J Y Russell A D Catrenich C E Charbonneau DL Bartolo R G Journal of Applied Microbiology 2003 94 240 247 Cen L Neoh K G Kang E T Langmuir 2003 19 10295 10303 Andresen M Stenstad P Moretro T Langsrud S Syverud K Johansson L S Stenius P Biomacromolecules 2007 8 2149 2155 Tiller J C Liao C J Lewis K Klibanov A M Proc Natl Acad Sci U S A 2001 98 5981 5985 McCluskey D M Smith T N Madasu P K Coumbe C E Mackey M A Fulmer P A Wynne J H Stevenson S Phillips J P Acs Applied Materials amp Interfaces 2009 1 882 887 Schulz Ekloff G Wohrle D van Duffel B Schoonheydt R A Microporous Mesoporous Mater 2002 57 91 138 Ogawa M Kuroda K Chem Rev 1995 95 399 438 Intercalation Chemistry Whittingham M S Jacobson A J Ed Academic Press New York 1982 Jones W In Photochemistry in Organized amp Constrained Media Ramamurthy V Ed VCH Publishers Inc New York 1991 Brinker C J Current Opinion in Colloid amp Interface Sc
10. excited state T1 is always lower in energy than S because of decreased Coulombic repulsion between electrons due to Hund s Rule Phosphorescence is a radiative decay 19 process between states of different spin For this reason the observation of phosphorescence from T always occurs at longer wavelengths than the fluorescence observed from S4 Although the gaps between electronic energy levels have discrete values the absorption and emission spectra always appear as broad bands Light absorption is a very fast process fs and occurs much faster than nuclear motion as described by the Franck Condon Principle Immediately upon absorption of light an electronic excited state is produced that initially is also vibrationally excited Vibrational relaxation occurs quickly after excitation typically involving molecular collisions which dissipate small amounts of energy to the environment in each collision Bimolecular Excited State Dynamics It is possible that an excited molecule may encounter another molecule during its excited state lifetime There are several processes that can occur including the enhancement of intersystem crossing electronic energy transfer and complex formation An important feature of bimolecular excited state dynamics is that these processes are dependent on the concentration of the excited molecules and quenchers and is also a diffusion controlled process The enhancement of nonradiative decay by intersy
11. occurs in the presence of the heavy metal atom Hg however the decreased emission intensity is more likely due to the aggregated polymer Additionally self absorption of the emission may also be present due to the red shifted absorption when Hg is added Hg3 in benzene exhibited no absorption or photoluminescence in the detection range The changes in the absorption and emission spectra upon addition of Hg3 in benzene are strikingly similar with reported aggregation of pure MEH PPV under various conditions reported in the literature 28127 In fact reported results by Collison et al in 2001 mentioned an instantaneous visible color change when the poor solvent hexane was added to a toluene solution of MEH PPV accompanied with a red shifted structured absorption and emission along with a lower fluorescence quantum yield 8 In this report the authors concluded that the formation of locally ordered MEH PPV chains in poor solvent that were torsionally constrained effectively increased the conjugation length of the polymer Using a two species model they were able to describe the changes seen in the absorption and emission spectra Time resolved emission measured with excitation at 375 nm showed decreased fluorescence lifetimes upon addition of Hg3 The measured fluorescence lifetime of pure MEH PPV in benzene was tave 270 ps monitored at A 520 nm which agrees with fluorescence lifetimes previously reported for MEH PPV 9 9 Upon add
12. 1971 Vol Volume 1 Part A Marini A Munoz Losa A Biancardi A Mennucci B J Phys Chem B 2010 114 17128 17135 Williams A T R Winfield S A Miller J N Analyst 1983 108 1067 1071 Dhami S Demello A J Rumbles G Bishop S M Phillips D Beeby A Photochem Photobiol 1995 61 341 346 Palsson L Monkman A Beeby A Ltd J Y Ed Norrish R G W Porter G Nature 1949 164 658 658 Lindqvist L Hebd Seances Acad Sci Ser C 1966 263 852 854 Small R D Scaiano J C J Am Chem Soc 1978 100 296 298 Goppert Mayer M Annalen Der Physik 1931 9 273 294 Kaiser W Garrett C G B Phys Rev Lett 1961 7 229 231 Franken P A Weinreich G Peters C W Hill A E Phys Rev Lett 1961 7 118 120 167 40 41 42 43 44 45 46 47 48 49 50 52 53 54 55 He G S Tan L S Zheng Q Prasad P N Chem Rev 2008 108 1245 1330 Ehrlich J E Wu X L Lee Y S Hu Z Y Rockel H Marder S R Perry J W Opt Lett 1997 22 1843 1845 Sutherland R Brant M Heinrichs J Rogers J Slagle J McLean D Fleitz P J Opt Soc Am B Opt Phys 2005 22 1939 1948 Liao C Shelton A H Kim K Y Schanze K S ACS Appl Mater Inter 2011 3 3225 3238 Rao S V Rao D N Akkara J A DeCristofano B S Rao D Chem Phys Lett 1998 297 491 498 Pa
13. 2 4 will appear upside down 52 Shutter Laser O pens Fires Shutter Closes Figure 2 4 Schematic of TA data acquisition and processing for a PMT signal The software averages a user defined smoothing factor number of sequential points to create a smoothed waveform Smoothing serves to reduce random noise increasing the signal noise ratio of the signal but most importantly it reduces the number of data points which provides more manageable file sizes A smoothing factor of 2 reduces the 10 000 data points to 5 000 data points and a smoothing factor of 10 reduces the 10 000 data points to 1 000 data points The described system acquires transient absorption decays with low signal to noise with intensities on the order of 10 AA and can even resolve decays on the order of 104 AA Analysis between 400 and 1000 nm is possible on timescales of 4 us or longer with the available instrumentation and software As triplets in solid state typically have longer lifetimes than those found in solution shorter timescales are rare A sample decay profile is shown in Figure 2 5 53 0 003 0 002 AA 0 001 4 0 001 0 Time us Figure 2 5 Transient decay signal of a platinum acetylide polymer film pPtPh from the described film TA Further processing can be performed to produce AA vs wavelength transient absorption spectra from the AA vs time raw data using an in house written Matlab program TA
14. 28 cm GW This effective nonlinear absorption coefficient Ber allows an easy comparison between the nonlinear absorption of different compounds at the same excitation wavelength Aexc In Chapter 3 these calculations will be applied to assign quantitative values to the overall nonlinear absorption of a series of platinum acetylide complexes for the purpose of evaluating their use in optical power limiting applications This Study The goal of this research is to study the excited state dynamics of conjugated molecules oligomers and polymers and compare the properties in solution and in the 42 solid state Instrumentation was developed to study the triplet excited state dynamics of solid state films and materials by transient absorption This new transient absorption system was used to study photoactive materials ranging in applications from nonlinear absorbing PMMA monoliths trimeric mercury containing polymer films and conjugated cationic oligomers absorbed onto clay scaffolds Chapter 2 introduces the development of new transient absorption instrumentation specifically optimized for use with thin films and solids is described The instrumental components and optical layout is discussed A custom Labview program was made to make the transient absorption experiment a user friendly routine experiment Further the instrument was validated and optimized using platinum acetylide polymer films to examine the capabilities of the instrument Sensit
15. 400 450 500 550 250 300 350 400 450 500 550 Wavelength nm Wavelength nm n TPV1 Ph p Te TPV1 ee L I ogsstpv Fg 1 TPVO cross 1 04 Na Wem 1 04 Nf we 5 E B 084 2 E 1 E W 0 64 Ww CH CH N na N E 0 44 z E e 0 24 fe Z Z 0 0 0 0 350 400 450 500 550 600 650 700 750 800 400 450 500 550 600 650 700 Wavelength nm Wavelength nm y TPVO f crossTPV1 j e TPV1 Ph crossTPV3 TPV1 crossTPV1 DPAF e TPV2 TPV2 T2 Figure 3 2 Normalized A absorption and B emission spectra of the linear and cross conjugated p phenylene vinylene platinum Il acetylide chromophores in THF 67 The fluorescence quantum yields and lifetimes shown in Table 3 1 demonstrate several interesting trends For the TPVn series the fluorescence quantum yield and fluorescence lifetimes are increased with longer OPV chain which suggests that the competing nonradiative decay ISC is significantly less efficient when the conjugated OPV chromophore is lengthened The mixing of the platinum centered orbitals with the frontier orbitals decreases with increasing OPV length resulting in decreasing spin orbit coupling and intersystem crossing which explains the decreased dr and t TPV2 T2 had significantly lower and faster Tr indicating that ISC is particularly efficient likely due to the presence of thiophene units which are known to increase the rate of ISC in conjugated organic molecules The crossTPVn
16. 61 2 8 47 3 TPV1 0 99 2 0 59 8 11972 1186 TPV2 1 04 2 8 54 0 TPV2 T2 2 40 1 2 56 0 crossTPV1 1 02 1 8 61 4 3423 285 crossTPV3 1 02 1 9 60 7 13343 1107 crossTPV1 2 09 1 7 71 9 DPAF a2PA cross section measured in THF by the NLT method gt 2PA cross section measured in PMMA monolith by the NLT method C2PA was not measured for these monoliths Femtosecond 2PA of PMMA monoliths was measured by Dr Rebane s group at Montana State University using the NLT method with high accuracy The 2PA cross sections and values for TPV1 crossTPV1 and crossTPV3 monoliths were measured and compared to the 2PA properties in THF solution Figure 3 8 The absolute 2PA cross section values were calculated using the monolith concentrations listed in Table 3 2 The solid state measurements are very consistent with the 2PA previously 76 measured in THF solution In the blue wavelength region the 2PA is higher for crossTPV1 and crossTPV3 monoliths compared to the 2PA in THF This indicates that in the PMMA monoliths more excited state absorption is present probably due to slower vibrational relaxation in the solid state 6000 Be 4500 a o GM 3000 4 og 1500 a a crossTPV1 4000 3000 o GM 2000 1000 tS en KE ee FT ge crossTPV3 10000 8000 6000 53 4000 Ki H Di 20001 see bh Zu Sec c GM Figure 3 8 Femtosecond 2P
17. 66 PMMA Monoliths for OPL Applications sissies iannnscanaicosiveneiererventinnmdnieianieinasieaiuius 74 Quantitative Analysis of Nonlinear Absorption in Solution and Monolith 79 Triplet Triplet Annihilation Studies of Platinum Acetylide Polvmers 81 Summa y o RoS a i a pha a et a da Sie Sa ae ee a 87 ee hc eicnne een annn min nonin 88 Triplet Molar Extinction Determination by Relative Actinometry 93 4 TRIPLET SENSITIZATION OF CONJUGATED POLYMERS BY Hg3 nana J9 Introducti eieiei TAR ET LAA ET ENEE AEAT Triple n by Hg3 EN tM Conjugated Polymers and H93 EEN O ia fiber PSHT enter Section 5 PHOTOPHYSICAL STUDY OF CATIONIC OLIGOMERS IN A CLAY HOST 134 En Triplet Sens Photophysical S APPENDIX USER MANUAL FOR THE TRANSIENT ABSORPTION INSTRUMENT FOR THIN FILMS E ENS LIST OF REFERENCES RE BIOGRAPHICAL SKETCH EK LIST OF TABLES Table page Table 3 1 One Photon Photophysical Properties in THF Solution ccccccees 66 Table 3 2 Monolith concentrations and thicknesses and their nonlinear photophysical Properes eege neem 76 Table 3 3 Effective nonlinear absorption coefficients Ber and effective nonlinear absorption cross sections oa for the linear and cross conjugated platinum acetylides in THF solution and in PMMA monolith ccccccsesseesseeeeeeeeeeeeees 80 Table 3 4 Calculated values of the tr
18. 900 950 1000 1050 1100 Wavelength nm Wavelength nm 0 016 8 Raw Data i Raw Data E 0 012 Exponential Fit d Exponential Fit 5 0 008 es a AC ugen 950 nm Ar Purged S 0 000 P J D 0 016F g 0 012 Q 5 0 008 8 lt s 3 Q Figure 4 10 Transient absorption spectra A and transient decays B of MEH PPV with 100 uM Hg in benzene solution 3 under deoxygenated and oxygen saturated conditions Aexc 532 nm 2 6 mJ pulse 114 The transient decays reveal interesting information as shown in Table 4 2 These decays are fitted without including the initial very fast feature in the transients which will be discussed later First upon addition of Hg3 to the MEH PPYV the single exponential decay becomes obviously bi exponential with a short 200 400 ns and a longer 1 2 us lifetime indicating a transition from a single component MEH PPV to a two component MEH PPV and MEH PPV system Second the lifetimes after oxygen purge are very consistent between both 40 uM and 100 uM Hg3 solutions with lifetimes around 200 ns while the average lifetime decreases upon increased Hg3 addition Table 4 2 Exponential fits to the transient decays at 830 nm and 950 nm for MEH PPV and Hg solutions in argon purged and oxygen saturated benzene All fits were to single or bi exponential decays with R values of gt 0 997 except for pure MEH PPV O2 purged which had an R value of 0 4
19. The platinum II acetylide chromophores initial concentration 1 mM and 6 mg of azoisobutyronitrile AIBN were dissolved in 1 1 mL of inhibitor free methyl methacrylate MMA The solution was poured into a 0 5 circular Teflon mold and placed in an oven to polymerize at 50 C for 3 days then at 80 C for 3 days Some of the initial PMMA monoliths showed a darkening of the top surface of the monolith indicative of decomposition of the chromophore so the procedure was modified with a gentle nitrogen flow into the oven to reduce oxygen sensitized degradation All monoliths prepared under N2 appeared homogeneous with no evidence of degradation and were highly transparent Figure 1 Some gas bubbles were trapped near the edges of the monoliths however they did not obstruct the photophysical measurements Trapped bubbles did not occur in the monoliths prepared without N2 flow The monoliths were cut to about 1 mm thickness using a thin rotary blade Dremel Tools and polished by hand using fine grit sandpaper placed on a flat surface for photophysical measurement Different concentrations of chromophore were 88 attempted but monoliths containing higher initial concentrations gt 2 mM of chromophore were not transparent due to phase separation Monolith solutions were prepared to 1 mM initial concentration before polymerization Final calculated concentrations of the polymerized monoliths were approximated by using the density of PMMA 1 2 g
20. These materials are used in applications for the protection of sensitive optical sensors such as the human eye from being harmed by exposure to laser light 59 6 After the input fluence reaches an initial threshold these materials strongly attenuate the transmitted light to a safe clamping energy The key property in OPL activity of these materials is their strong nonlinear absorption 6 Platinum Il acetylide complexes exhibit nonlinear absorption via a dual mechanism pathway and their nonlinear absorption and optical power limiting properties have been studied extensively in the past decade or so 3 646 When the two photon absorption 2PA spectrum and the triplet triplet excited state absorption ESA spectrum have significant overlap particularly efficient nonlinear absorption is achieved 4267 69 Initially instantaneous two photon absorption occurs which populates the singlet excited state S Spin orbit coupling due to the heavy platinum atom promotes intersystem crossing from the first singlet excited state with efficient yield to form the triplet state T The triplet state T1 which in platinum acetylides typically has a particularly large molar extinction coefficient readily absorbs additional photons after a significant triplet population is achieved 7 Nonlinear absorption by two photon absorption allows for the production of transparent materials because no ground state 62 absorption is required at the active wavelen
21. absorption spectrum is rising and 3 high Hg3 concentration 100 uM where the complex is near the saturation point of the polymer Solution 1 was bright orange in color solution 2 was a bright pink grapefruit color and solution 3 was a deep red Transient absorption spectra and decays were obtained with excitation using nanosecond pulses Aexc 532 nm at approximately 2 5 mJ pulse laser energies for all measurements under both deoxygenated and oxygen saturated conditions It is 108 important to note that the solutions were matched in polymer concentration and not optical density at the laser excitation wavelength Solution 3 with the highest Hg3 concentration was free from visible particulates during TA measurement although after Absorbance OD ie 1 Pure MEH PPV 0 8 2 40 uM Hg3 0 6 _ 3 100 M Hg3 0 4 0 2 D 300 400 500 600 700 800 Wavelength nm Figure 4 6 Ground state absorption spectra of the MEH PPV solutions in benzene used for the transient absorption spectroscopy experiments the solution sat overnight some particulates had settled on the bottom of the solution vial Hg3 in benzene shows no transient absorption The transient absorption spectra are shown in Figure 4 7 Initially the transient absorption measurements were taken in argon purged solutions Figure 4 7A In all solutions there is a transient absorption peak centered around 830 nm which matches reported t
22. and measure the interlayer spacing The XRD spectra are shown in Figure 5 8 142 Pure SSA shows a peak around 6 7 that shifts to smaller angles upon addition of EO OPE C2 and EO OPE Th 5 7 and 5 5 respectively From these results it is evident that the layer spacing of the SSA is increased upon addition of both oligomers and that intercalation of the oligomers into the interlayer spaces of the SSA has occurred The width of a single layer of SSA is 0 960 nm with an interlayer distance of 0 360 nm The layer spacing of EO OPE C2 SSA hybrid is 1 530 nm giving an interlayer distance of 0 570 nm The layer spacing of the more nonplanar oligomer EO OPE Th SSA hybrid is 1 571 nm giving an interlayer distance of 0 611 nm B 10 5 4 10 SSA 5 3 10 SSA EO OPE C2 SSA fp 52 be EO OPE Th SSA fp 5 1 10 5 10 1 2 10 1 107 Intensity 8000 6000 6000 4000 4000 2000 2000 10 20 3 oe 6 70 80 10 20 30 40 50 60 70 ap 290 deg Cu Ka 290 deg Cu Ka Figure 5 8 X ray diffraction patterns of the oligomer SSA hybrids acquired by Akino Uchikoshi at Tokyo Metropolitan Univesity A EO OPE C2 and B EO OPE Th The unmodified SSA powder XRD is shown in red while the oligomer SSA hybrid XRD is shown in brown With the assistance of Dr Galyna Dubinina Chimera 1 8 software after MM2 geometry optimization was used to estimate the length and width of the oligomers in order to get
23. and patience has kept me sane and who continues to laugh with me throughout this crazy adventure called life TABLE OF CONTENTS page E EE T Sssosmaine ienne enie eelere tt 4 TABLE EE Ree 6 KETTEN 8 EES Fae ee een ise 9 ng EEN 13 CHAPTER t INTROD TION EE 15 The Interaction of Light with Matter sc serxenrioasvinanestncesanssenetrenrnanvineisnadaneerceneaassinnreene 15 Excted state TE 17 Unimolecular Excited State Dynamics aso ssc seen stre rations ENEE AE AEedEe E 18 Bimolecular Excited State Dynamics vous dacceccsas ve ececcssesgctcdervassna cceiccocansareseces 20 Photophysics of Conjugated Polymers gp isunnscnrasdanchenaieoncnnnciazendedeianemimacuaiians 24 Photophysical Characterization Mettet egsestkabreretark s Eegk degen 25 Absorpiion and Elek See 25 Transient Absorption Spectroscopy so sstacatasitiawaiseodyuciandeinanienaiatasadiianeeaneaaedorn 32 Nonlinear RY CA E 34 TRS AUY EE 42 2 TRANSIENT ABSORPTION INSTRUMENTATION FOR THIN EILMGS 45 len E 45 Instrumentation of a Transient Absorption System for Thin Films and Solids 46 SEENEN 47 lui Kee e TE 48 Instrument Validation and Optimization with pPtPh Polymer Films 54 3 SOLID STATE MATERIALS FOR OPTICAL POWER LIMITING sssssssseseseeeeeeneen 62 Optical Power Limiting by Nonlinear Absorption s sssssesssseesnneenrrrrnrernrrrrnesnreeee 62 Linear and Cross Conjugated PI OPV Series AEN 64 Solution Photophysical StUdIES sishaciciiisaseeccrreraeternennGamceente eerie
24. benzene d6 green top NMR experiment performed by Russ Winkel X ray diffraction XRD experiments at room temperature shown in Figure 4 17 were carried out on drop coated films containing MEH PPV and 0 5 15 30 and 50 wt Hg3 The pure MEH PPV 0 wt Hg3 film had a featureless diffraction pattern between 26 0 25 indicating that MEH PPV has amorphous structure Relatively weak diffraction peaks in the scattering angle range of 20 4 5 to 7 5 for the MEH PPV 5 wt Hg3 film and increasingly strong diffraction peaks for the higher wt Hg3 films up to the scattering angle range of 20 3 5 to 8 5 for the MEH 121 PPV 50 wt Hg3 film were observed This indicates efficient packing of the side chains of MEH PPV polymer in the presence of the Hg3 Studies of MEH PPV after thermal annealing treatment by other groups report a peak around 5 in XRD owing to the regular packing of the polymer side chains 48 This XRD data supports the photophysical results that Hg3 templates ordering of the MEH PPV Powder XRD on the Hg3 crystals show a strong diffraction peak at 20 9 6 Because the Hg3 peak at 9 6 is not seen in the MEH PPV Hg3 films it s likely that all the Hg3 is associated with the polymer creating the strong diffraction pattern at 5 A B Intensity a u Intensity a u 5 10 15 20 25 5 10 15 20 25 2 6 deg 2 0 deg Figure 4 17 X ray diffraction profiles at room temperature
25. e 161 File Edit View Project Operate Tools Window Help gt Wa ele Start at Temp File R i Wavelength End UPDATE SEAN Smoothing Factor Sue 10 ei Wavelength Step IZERO Wavelength ED RUN 0 0 Delta A o 5 Choose a smoothing parameter Smoothing results in a less noisy baseline and more manageable file size See attached plots a A smoothing parameter of 1 is no smoothing b A smoothing parameter of 2 will average every 2 data points reducing the 10 000 point data record to 5 000 points c A smoothing parameter of 10 will average every 10 data points reducing the 10 000 point data record to 1 000 points 6 The white graph at the bottom of the screen can be manually rescaled by double clicking on the y axis numbers and typing in the desired values A good place to start is 0 001 and 0 001 The signal will appear for review after each iteration 7 Click Run a The program will first compute Izero and then perform the TA acquisition for each wavelength using the settings specified on the TA vi page 5 b The indicator box on the top right of the screen will update with the wavelength being acquired the Izero and the x of y progress of the acquisition 162 c After acquisition at each specified wavelength the TA signal will be displayed on the white graph in the middle of the screen and a pop up box will appea
26. for a variety of organic electrooptical devices such as photovoltaic cells light emitting diodes lasers and thin film transistors due to their exceptional charge carrier transport electroluminescent and photoluminescent properties In addition to desirable photoluminescent properties conjugated polymers can act as semiconductors due to the highly tr conjugated backbone along the polymer chain with delocalized rr and rr orbitals which can act as valence and conduction bands which support charge carrier transport along the polymer chain Compared to inorganic semiconductors organic polymers can offer mechanical flexibility optical transparency and easy processibility Indeed the field of organic plastic electronics has such vastly impacted current technology that the study of T conjugated polymers and their semiconductor properties earned Alan J Heeger Alan G MacDiarmid and Hideki Shirakawa the 2000 Nobel Prize in Chemistry Furthering the technological applications of these polymer materials there has been much attention on understanding the phenomena that control the photophysics and carrier transport such as conformation of the polymer chains interchain aggregation 100 102 and intramolecular dynamics 1 1 103 104 Recently some focus has shifted to the study of triplet states of small molecules and conjugated polymers for photoactive applications Triplet states due to the spin forbidden nature of intersystem crossing
27. have substantially longer lifetimes than singlet 99 states Thus triplet excitons may have a greater probability of travelling throughout the medium to the desired interface in a device where charge separation for photovoltaic cells or photoluminescence for LEDs can occur The trade off to this longer lived excited state is that singlet excitons can travel long distance by F rster energy transfer whereas triplet exciton diffusion is governed by Dexter exchange and is dependent on orbital overlap between neighboring chromopbores 17106 The triplet excitons move along a polymer chain or efficiently t stacked adjacent chains in a hopping motion according to a random walk pattern 107 109 Organometallic small molecules and polymers are actively being studied as materials for triplet solar cells 11 111 Shao and Yang in 2005 reported a heterojunction solar cell device using PtOEP platinum Il octaethylporphyrin as the donor with PCBM phenyl C61 butyric acid methyl ester as the acceptor and found relatively high performance of the device with a triplet exciton diffusion length of 30 nm 117 The Reynolds and Schanze groups in 2006 studied a phosphorescent platinum acetylide polymer and acceptor PCBM in a bulk heterojunction solar cell and demonstrating for the first time that the involvement of the triplet excited state in photoinduced charge separation leads to efficient charge generation Numerous organometallic polymers have been studied s
28. in d THF Ga ee to ae eteten men gehier E EE EEN CH Figure 4 16 H NMR of MEH PPV MEH PPV with Ho 121 dee eo 7 See GEN lte yl Nee S pe room sie aed ll We of MEH PPV tions of oe Figure 4 19 Structure of P3HT DN E Figure 4 20 P3HT solutions with pha amount of it in benzene and chloroform E Figure 4 21 eierlech and emission of P3HT upon addition of increasing i nee See eee ee EE EE EE EE EE EE EE EE EE EE e 126 with increasing concentrations Of Ho 127 and chlorof el FERIR a 1 aiea oo a the intercalation of cationic SE into the Figure 5 2 Structure of Sumectin SA SSA EN O7 Figure 5 3 Structures of the cationic oligomers EO OPE C2 and EO OPE Th 137 Figure 5 6 Emission spectra of oligomer SSA hybrid ms 141 Figure 5 7 Transient absorption spectra of oligomer clay hybrid films 142 Figure 5 8 X ray diffraction Geet of the isan ene EO OPE C2 and EO OPE Th heen weenie s 143 12 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy EXCITED STATE PROPERTIES OF ORGANIC AND ORGANOMETALLIC OLIGOMERS POLYMERS AND MATERIALS AND THEIR SOLID STATE APPLICATIONS By Randi Sue Price August 2013 Chair Kirk Schanze Major Chemistry The goal of this research is to study the excited state dynamics of conjugated
29. is not frozen exit the TA vu program and check the GPIB connection If the GPIB cable seems to be tight open the TA vi program again 4 TA vu program will not do anything after the Acquire vi window has been opened This is normal The Acquire button should only be pressed when all the settings are correct to acquire data Click the red stop sign button and then the white arrow on the TA vi window 5 Laser PCU starts beeping Stop the program turn the laser key to off and restart the laser following the startup procedure 165 LIST OF REFERENCES Huygens C Trait de la Lumi re Macmillan London 1912 Fresnel A Ann Chem Phys 1816 2 239 281 Young T In A Course of Lectures on Natural Philosophy and the Mechanical Arts Printed for J Johnson London 1807 Vol Volume 1 p 457 471 Marion J B Heald M A In Classical Electromagnetic Radiation Second Edition Jerry M Ed Academic Press 1980 p 1 33 Maxwell J C A Treatise on Electricity and Magnetism Cambridge University Press Cambridge 2010 Vol 1 Planck M Annalen Der Physik 1901 4 553 561 Einstein A Annalen der Physik 1905 322 132 148 de Broglie L Annalen Der Physik 1925 Paris 3 22 Davisson C Germer L H Physical Review 1927 30 705 740 Thomson G P C W Theory and practice of electron diffraction Macmillan and co limited London 1939 Shankar R Principles of quantum mechanics 2nd e
30. of oligomer incorporated onto the clay substrate which is supported by the absorption fluorescence titration data Transient absorption of the oligomer clay thin films shows strong triplet triplet absorption of both oligomers which indicates that these materials may have a promising future for triplet state applications such as singlet oxygen sensitized light activated biocides Solution Studies The cationic oligomers EO OPE C2 and EO OPE Th in water have absorption in the near UV region with maxima of 327 and 354 nm respectively The absorption of the thienylene containing oligomer EO OPE Th is slightly red shifted Upon addition of SSA to water solutions of the oligomers there is an increasing red shift in the absorption maxima accompanied with a growth of a structured red shifted absorption peak which indicates the presence of aggregation of the oligomers onto the clay substrate as shown in Fig 5 4 A B 0 5 0 6 05 0 4 o 0 4 o 0 3 0 3 f 4 lt 0 2 0 2 0 1 4 0 1 0 0 0 0 250 300 350 400 450 500 250 300 350 400 450 500 550 Wavelength nm Wavelength nm Figure 5 4 The absorption spectra acquired by Eduardo Acosta of 4 uM of oligomers EO OPE C2 A and EO OPE Th B with increasing concentration of SSA in water from 0 to 16 yeq L 138 The fluorescence of the oligomers is shown in Fig 5 5 The emission is quenched with increasing additions of SSA and a new peak appea
31. of MEH PPV changes when Hg3 is added Figure 4 15 Color change of MEH PPV in methyl THF upon freezing to liquid nitrogen temperatures Vial right MEH PPV in methyl THF at room temperature Tube left MEH PPV in frozen methyl THF The Hg3 complex does not contain any protons so H NMR experiments were performed in deuterated benzene to examine at the structural changes of MEH PPV when increasing amounts of the Hg3 complex were added Figure 4 16 The H NMR spectrum of pure MEH PPV in benzene d6 matches the spectra of MEH PPV previously reported 46 When Hg3 is added there is a dramatic change in the NMR spectrum Due to the insolubility of Hg3 in benzene as Hg3 is added and associates with MEH PPV the polymer also becomes less soluble The NMR signal becomes noticeably weaker The protons around the aromatic ring in the polymer labeled as peaks A and B in the spectra almost completely disappear indicating significant interaction between Hg3 and the aromatic protons The shift of A and the side chain proton signals C and D downfield indicates significant shielding of the aromatic region by large planar molecule Hg3 120 1100 C 1000 m 900 r800 M700 M600 M500 m 400 C 200 H D Fy m C E M100 100 aS 10 5 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 05 00 0 5 f1 ppm Figure 4 16 1H NMR of pure MEH PPV in benzene d6 red bottom and H NMR of MEH PPV with Hg3 in
32. the transition Using TA spectroscopy alongside the other spectroscopic techniques described above it is possible to complete a photophysical profile of the most important electronic transitions of a molecule Norrish and Porter first developed flash photolysis to study reactive intermediates by excitation with a flashlamp in 1949 34 which later was recognized when they shared the Nobel Prize in 1967 for their development of the flash photolysis method The time 32 resolution for transient absorption spectroscopy was significantly improved with the introduction of laser technology As shown in Figure 1 4 modern instrumentation typically uses laser light for quickly creating large populations of reactive chemical species in a variation of the laser flash photolysis system reported by Lindqvist in 1966 35 Transient absorption instrumentation works by using a pump source laser to create a detectable population of the transient species A probe source lamp with a detector such as a photomultiplier tube PMT is used to detect differences in the transmitted light through the sample as the population of the transient species changes through time laser light oscilloscope sample Figure 1 4 Laser flash photolysis system first reported by Lindqvist Laser light is used as the pump source and a broad spectrum white light source is used as the probe source Single wavelength detection is achieved with
33. thin film the laser must hit either the front or the back face of the sample film In the instrumentation described here a nearly antiparallel laser path was chosen For protection against thermal degradation of the films low laser energies of 300 800 uJ pulse are used for excitation and longpass filters in a manual filter wheel are incorporated to block 46 unwanted wavelength regions of the probe lamp This decreases the overall intensity of the light without any information loss in detection Maybe the most significant feature of the newly developed TA system is that sensitive detection for thin films is acquired by using a very intense stable light source and a low light sensitive detector Polymer thin films typically give TA intensities of 103 104 AA whereas a solution sample under optimal conditions will have a transient absorption intensity on the order of 10 AA Instrument Description A transient absorption system for thin film analysis was constructed The optical schematic is shown in Figure 2 1 A Surelite l 10 Nd YAG laser with second and third harmonic generators delivers nanosecond pulses for excitation at 532 or 355 nm respectively A 250 watt quartz tungsten halogen lamp and power supply Newport catalog 6334NS lamp and 69931 radiometric power supply is used as the probe source Light exposure of the sample is controlled by a series of mechanical shutters and a manual filter wheel is placed between lens 6 and the sample
34. 0 The optimal slit width was determined to be 0 70 mm This gives a bandpass of about 5 nm which is an acceptable value for the broad spectral features detected with transient absorption spectroscopy 58 SR 0 30 mm slit width 0 40 mm slit width 0 50 mm slit width 0 0005 AA 0 0000 KE oder 0 0005 0 0010 0 60 mm slit width 0 70 mm slit width 0 80 mm slit width 0 0005 AA 0 0000 0 0005 0 0010 RE 2 4 6 8 10 0 90 mm slit width 1 mm slit width Time us 0 0005 AA 0 0000 0 0005 0 2 4 6 8 0 2 4 6 8 10 Time us Time us Figure 2 10 Transient decays collected of a pPtPh film prepared from a 20 mg mL solution in toluene spun at 2000 RPM with increasing slit width on the monochromator Detection was with a PMT at Amax 650 nm Aexc 355 nm 600 uJ pulse 1 KQ termination 128 avgs smooth 10 The number of waveforms to average was also optimized Figure 2 11 With the weak TA signals produced by thin films whose thicknesses aka pathlengths are on the order of nanometers a significant amount of averaging in the signal is necessary to get meaningful data The more a signal is averaged the less noisy it becomes Films however are very sensitive to the heat produced by intense light sources and cannot dissipate heat as easily as solutions can Thus it is necessary to find a balance between the number of times the sample film is shot with the relati
35. 0 700 750 Wavelength nm Wavelength nm Wavelength nm Figure 3 7 Nanosecond triplet triplet absorption of the linear and cross conjugated complexes in PMMA monolith black compared to the triplet triplet absorption in THF solution reproduced from Figure 3 3 red Pulse energies were approximately 5 mJ pulse for solution measurements and 800 udJ pulse for monolith measurements with Aexc 355 nm A blue shift in the transient absorption spectrum was also observed for two other well known nonlinear absorbing platinum acetylides Pt BTF benzothiazolfluorene and Pt DPAF after incorporation into PMMA glass monoliths This effect is probably due to the donor tr donor or acceptor tr acceptor structure of the chromophore in the polar PMMA matrix 75 Monolith nonlinear absorption properties were measured using the non linear transmission method for 2PA and open aperture z scan method for nonlinear absorption A summary of their nonlinear photophysical properties monolith thickness and calculated concentration can be found in Table 3 2 Monolith thickness was measured with high precision calipers and the concentration was calculated as described in the experimental section Table 3 2 Monolith concentrations and thicknesses and their nonlinear photophysical properties Monolith Thickness Calculated Chromophore Transmission 2PA Cross mm Concentration at z 0 from Section at 680 nm mM Z scan GM TPV1 Ph 1 01 2 0 75 9 TPVO 2
36. 00 400 500 600 700 800 Wavelength nm 1 2 1 0 10 mg mL pPtPh 0 8 Q Q 0 6 500 RPM 0 4 1000 RPM 0 2 Ve 0 0 SS 300 400 500 600 700 800 Wavelength nm 2 0 15 20 mg mL pPtPh S 1 0 500 RPM EN 1000 RPM 05 2000 RPM 0 0 ge 300 400 500 600 700 800 Wavelength nm Figure 2 7 Absorption spectra of pPtPh spin coated films prepared from various solution concentrations in toluene and spin speeds 55 The solutions were stirred for 4 hours before films were spin coated at varying spin speeds at each concentration level 500 RPM 1000 RPM and 2000 RPM producing films of different thicknesses The spin coating process has been described in the literature 5 Although the film thicknesses were never directly measured films of pPtPh made under similar conditions had thicknesses on the order of 50 nm Di The ground state absorption spectra of the films are shown in Figure 2 7 Thin film transient absorption spectra were taken with the thin film transient absorption system described previously and the transient absorption of a pPtPh film is shown in Figure 2 8 Transient absorption decays were collected from 550 800 nm and processed in Matlab to produce the transient absorption spectra 0 0030 TA Spectrum pPtPh film 0 0025 44 ns increment 0 0020 0 0015 AA 0 0010 0 0005 0 0000 550 600 650 700 750 800 Wavelength Figure 2 8 Transient absorption spectrum of p
37. 000 data points 0 001 E i 0 000 0 001 0 002 Se Smooth Factor209 2 4 6 8 10 lt 0 001 i 500 data points Time us 0 000 0 001 0 2 4 6 8 0 2 4 6 8 10 Time us Time us Figure 2 12 Transient decays of a pPtPh film prepared from a 20 mg mL solution in toluene spun at 2000 RPM with varying smooth factors Detection was with a PMT at Ans 650 nm Aexc 355 nm 600 uJ pulse 1 KQ termination slits 0 5 mm 128 averages This new instrumentation was developed to measure the transient absorption of thin films and solids The experimental parameters that were found to be optimal for routine use of this instrument are as follows 1 KQ termination at the oscilloscope entrance and exit slits of 0 70 mm on the monochromator 128 waveforms to average and a smoothing factor of 10 Altering these parameters may be necessary depending on unique experimental factors however these settings should ensure optimal transient absorption data for most analyses 61 CHAPTER 3 SOLID STATE MATERIALS FOR OPTICAL POWER LIMITING Optical Power Limiting by Nonlinear Absorption Due to the rapid development of lasers there has been significant research focused on materials that can provide defense to some of the threats accidental and hostile involved with laser technology 55 Optical power limiting OPL materials are materials that allow the transmission of low intensity light while strongly attenuating high intensity light
38. 009 1 T 950 nm Ar Purged ZS 0 006 830 nm Ar Purged T a 0 003 T 0 000 H l i e eg i l i D 0 018 i t 8 oos f Z g 0012 r 5 ooog t 950 nm O Purged 2 0 006 830 nm O Purged w 0 003 T 0 000 f s EES i 0 1 2 3 4 0 1 2 3 4 Time microseconds Time microseconds Figure 4 9 Transient absorption spectra A and transient decays B of MEH PPV with 40 uM Hg3 in benzene solution 2 under deoxygenated and oxygen saturated conditions Aexc 532 nm 2 6 mJ pulse 113 Figure 4 10 shows the transient absorption spectra of solution 3 MEH PPV with 100 uM Hg3 in deoxygenated and oxygenated solution In deoxygenated solution the triplet triplet absorption peak at 830 nm is almost completely overwhelmed by the absorption band at 950 nm tentatively assigned to MEH PPV After purging the solution with oxygen little change in the absorption spectrum is apparent The small peak visible at 830 nm is quickly quenched after the first 88 ns and the 950 nm peak decays more slowly less affected by the presence of oxygen 0 004 e io am O Purged 0 000 w D EEEE 0 1 2 3 40 1 2 3 4 Time microseconds Time microseconds A 0 020 0 018 Ar Purged O Purged 0 016 inc 88 ns inc 88 ns o 0 014 oO t E 0 012 S 0 010 Le lt 0 008 oe 7 S ool e Sa H ooo GE 0 002 oe le a 0 000 SSE SS SE E 850 900 950 1000 1050 1100 850
39. 5 cm f l plano convex 2 lens 10 5 cm f l concave mirror Figure 2 1 Transient absorption system for films Computational Considerations A custom executable for this system was written in Labview 8 0 and saved as executable file TA vi Figure 2 2 The TA vi program enables live control of all instrumental components in the TA system via GPIB communications so that the optimum settings for transient absorption measurement such as the oscilloscope time div number of waveforms to average etc can be obtained before the acquisition begins The Izero vi subvi performs a routine to calculate Izero at a given wavelength as follows The slow and fast shutters are activated so that the probe beam passes through the sample and the oscilloscope collects an average of 16 waveforms The 48 average value in mV of the collected waveform is Izero The laser Q switch is not enabled so Izero essentially measures the baseline transmittance of the probe beam through the sample This Izero is then saved on the scope as the vertical offset This allows the oscilloscope to zoom in on the TA signal when viewed live on the TA vi window or on the oscilloscope screen itself when the appropriate instrument controls are enabled E TA vi File Edit View Project agin Operate Tools Window Help Set Wavelength Wavelength Grating Edit gt Reinitialize Values to Default J 400 i Then click Initialize to set computer defaults S
40. 500 600 700 Wavelength nm Wavelength nm Wavelength nm Figure 3 6 Photoluminescence spectra of the linear and cross conjugated complexes in THF solution red and in PMMA monolith black 74 Nanosecond transient absorption was employed to probe the triplet state of the complexes in PMMA monolith and is shown in Figure 3 7 All monoliths displayed strong broad triplet triplet absorption similar to the measurements in THF Transient absorption lifetimes could not be measured in the monoliths because they were longer than could be measured with the current instrumentation gt 1 ms The linear and cross conjugated complexes in PMMA monolith exhibit a very slight red shift except for crossTPV1 DPAF which has a large blue shift in the monolith transient absorption This indicates a similar triplet state in both THF solution and monolith for all complexes but crossTPV1 DPAF 0 25 Solution A 0 20 Monolith Ra A EI 4 4 1 i wod jo lt x2 lt e a wer ay wi Be 10 e SS f 7 im l x4N lt q 0 10 v pre Be Ki Lemma Zeie RN ie e E a NA 0 05 AE es I z ee K Ra TR Speed TPM ell U E 0 4 500 550 600 650 700 750 500 550 600 650 700 750 0 31 a 4 Wavelength nm Wavelength nm S g 0 2 dee 0 1L E AC 1 SE Z d J an erossTPVI sj crossTPV3 H grossTPV1 DPAF 500 550 600 650 700 750 500 550 600 650 700 750 500 550 600 65
41. 55 Pure MEH PPV MEH PPV 40u M Hg3 MEH PPV 100u M Hg3 A 830 nm 830 nm 950 nm 830 nm 950 nm Ar 262 ns 418 ns 82 400 ns 40 329 ns 82 199 ns 75 purged 2 63 us 18 1 62 us 60 1 18 us 18 1 03 us 25 O2 30 ns 218 ns 190 ns 224 ns 222 ns Purged The origin of the initial very fast feature in the transient decays shown in Figure 4 8 Figure 4 9 and Figure 4 10 is unknown Preliminary experiments find that the proportion of this fast component to the longer triplet decay is independent of the laser excitation energy from 200 uJ pulse to 2 mJ pulse so concentration based effects such as triplet triplet annihilation can be ruled out Interestingly regardless of whether the transient absorption spectrum is extrapolated from the peak of the fast component or from the start of the longer lived component at each wavelength the resulting spectra 115 are equivalent This holds true even for solution 2 and solution 3 where Hg3 has been added and the spectrum includes a rising peak at 950 nm Experiments performed on a benzene blank solution and other compounds in solution under similar conditions do not show this fast lived component so it does not seem to be an instrumental effect Cyclic voltammetry CV measurements were performed on the Hg3 complex to support the observation of the polymer cation by transient absorption as shown in Figure 4 11 Reduction of the Hg3 complex dichloromethane 3 mM concentratio
42. 998 39 3793 3799 Closs G L Piotrowiak P Macinnis J M Fleming G R J Am Chem Soc 1988 110 2652 2653 Burrows H D de Melo J S Serpa C Arnaut L G Miguel M D Monkman A P Hamblett Navaratnam S Chem Phys 2002 285 3 11 Monkman A D Burrows H D Hamblett Navaratnam S Chem Phys Lett 2001 340 467 472 Samiullah M Moghe D Scherf U Guha S Phys Rev B Condens Matter 2010 82 6 Archer S Weinstein J A Coord Chem Rev 2012 256 2530 2561 Liu S J Chen Y Xu W J Zhao Q Huang W Macromol Rapid Commun 2012 33 461 480 Shao Y Yang Y Adv Mater 2005 17 2841 Guo F Q Kim Y G Reynolds J R Schanze K S Chem Commun 2006 1887 1889 Wong W Y Wang X Z He Z Chan K K Djurisic A B Cheung K Y Yip C T Ng A M C Xi Y Y Mak C S K Chan W K J Am Chem Soc 2007 129 14372 14380 Wong W Y Chow W C Cheung K Y Fung M K Djurisic A B Chan W K J Organomet Chem 2009 694 2717 2726 Schulz G L Holdcroft S Chem Mater 2008 20 5351 5355 Cheng K W Mak C S C Chan W K Ng A M C Djurisic A B J Polym Sci Part A Polym Chem 2008 46 1305 1317 Padhy H Sahu D Chiang H Patra D Kekuda D Chu C W Lin H C J Mater Chem 2011 27 1196 1205 172 119 120 121 122 123 124 125 126
43. A of TPV1 crossTPV1 and crossTPV3 in THF solution red and in PMMA monolith black acquired by Dr Rebane s research group at Montana State University by the NLT method The dual mode property of nonlinear absorption is retained in the solid state The nanosecond open aperture Z scan method was used to measure the nonlinear 77 absorption response to 680 nm laser pulses Figure 3 9 All PMMA monoliths show strong nonlinear absorption The monolith thicknesses and concentrations were not matched so only qualitative information can be obtained from the Z scan results In general similar to the solution z scan results the nonlinear absorption increased with increasing length of the conjugated chain and the linear complexes seemed to have a stronger nonlinear response than the cross conjugated complexes in PMMA monoliths g 1 0 1 0 g is is 08 10 8 E Tp Ka crossTPV1 m 0 6 crossTPV3 06 o crossTPV1 DPAF D A 0 4 L 0 4 X un es o 5 1010 5 O 5 10 Z Position mm Z Position mm S 5 erregt E PMMA Blank crossTPV1 he crossTPV3 AE TPV2 T2 a crossT PV1 DPAF Figure 3 9 Open aperture Z scan response to 680 nm nanosecond pulses for the linear A and cross conjugated B complexes in PMMA monoliths A blank PMMA monolith was included as a reference black Pulse energies were approximately 700 pJ pulse When the z scan results for TPV1 Ph TPV1 and TPV2 are compared the NLA increases w
44. About 2 5 3 mg of solid was placed into the sample pan and the temperature program used was as follows a ramp of 10 C min from room temperature to 110 C a 5 minute hold a 10 C min ramp to 700 C and a final 30 min hold XRD analysis was carried out at Tokyo Metropolitan University by Akino Uchikoshi on a Rigaku RINT TTR II instrument with monochromatic Cu Ka radiation A 1 540562 A 50 kV 30 mA Scan rate was 1 min in 20 9 mode The oligomer clay films studied by XRD were prepared by the filter paper transfer method and compared against the XRD profile of pure SSA powder 150 CHAPTER 6 CONCLUSION The work reported here investigated the excited state dynamics of conjugated molecules oligomers and polymers in solution and in the solid state with a focus on the triplet state properties for applications such as optical power limiting photovoltaic devices and biocidal action Demonstrated were three methods of designing solid state materials with desired triplet state properties The first was by covalent bonding of a two highly conjugated ligand with a heavy metal such as platinum and incorporating the complex into an inert polymer matrix which was discussed in Chapter 3 Another approach was to prepare blended films containing a conjugated polymer and a heavy metal containing compound such as Hg as described in Chapter 4 Lastly intercalation of a cationic oligomer into a solid matrix like clay that contains heavy met
45. B Nature 1990 347 539 541 Friend R H Gymer R W Holmes A B Burroughes J H Marks R N Taliani C Bradley D D C Dos Santos D A Bredas J L Logdlund M Salaneck W R Nature 1999 397 121 128 Schwartz B J Annu Rev Phys Chem 2003 54 141 172 Sumpter B G Kumar P Mehta A Barnes M D Shelton W A Harrison R J J Phys Chem B 2005 109 7671 7685 Traiphol R Srikhirin T Kerdcharoen T Osotchan T Scharnagl N Willumeit R Eur Polym J 2007 43 478 487 Collison C J Rothberg L J Treemaneekarn V Li Y Macromolecules 2001 34 2346 2352 Hennebicq E Deleener C Bredas J L Scholes G D Beljonne D J Chem Phys 2006 125 Wang P Collison C J Rothberg L J J Photochem Photobiol A 2001 144 63 68 Yan M Rothberg L J Kwock E W Miller T M Phys Rev Lett 1995 75 1992 1995 Sherwood G A Cheng R Smith T M Werner J H Shreve A P Peteanu L A Wildeman J J Phys Chem C 2009 113 18851 18862 171 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 Lim S H Bjorklund T G Bardeen C J Chem Phys Lett 2001 342 555 562 Rolczynski B S Szarko J M Son H J Liang Y Y Yu L P Chen L X J Am Chem Soc 2012 134 4142 4152 Kodaira T Watanabe A Fujitsuka M Ito O Polymer 1
46. CQuire Vi program cccceceeteeeceeeeeeeeeeeeeeeeteeeeeeeeenes 51 Figure 2 4 Schematic of TA data acquisition and processing for a PMT signal 53 Figure 2 5 Transient decay signal of a pPtPh film from the described film TA 54 Figure 2 6 Structure of the platinum acetylide polymer ph 54 Figure 2 7 Absorption spectra of pPtPh spin coated films prepared from various solution concentrations in toluene and spin Speeds cceeeee serrer neren 55 Figure 2 8 Transient absorption spectrum of PPtPh Tim 56 Figure 2 9 Transient absorption spectrum of pPtPh in THF solution 000oaesesseeeeane 58 Figure 2 10 Transient decays collected of a pPtPh film with increasing slit width on ihe a Eeer 59 Figure 2 11 Transient decays of a pPtPh film with an increasing number of e Wa ON Sossen ianen nni E Rie ieee 60 Figure 2 12 Transient decays of a pPtPh film with varying smooth factors 61 84 10 Figure 4 1 Structure of trimeric perfluoro ortho phenylmercury Ho 102 Figure 4 2 Crystal structure of Hg3 and tolane reported by Gabbai e 103 Figure 4 3 Structure of MEH PPV wsisincsinseioriieaseusnmnoriraccaumnssonasicnsninencesnanbsinnereaannan 104 1 05 ii 409 concen 1s of Hg3 D CERN PE DEE pl Figure 4 8 Transient were and transient we of as MEH PPV in benzene EE Daat t A TAmax for increasing wt opreegen ES lor change of MEH PPV
47. D G Rogers J E Slagle J E Fleitz P A J Phys Chem A 2006 110 4369 4375 Barkana Y Belkin M Surv Ophthalmol 2000 44 459 478 Hudson S J Aviat Space Env Med 1998 69 519 524 Hollins R C Curr Opin Solid State Mater Sci 1999 4 189 196 Gvishi R Narang U Ruland G Kumar D N Prasad P N Appl Organomet Chem 1997 11 107 127 He G S Xu G C Prasad P N Reinhardt B A Bhatt J C Dillard A G Opt Lett 1995 20 435 437 Zhou G J Wong W Y Chem Soc Rev 2011 40 2541 2566 McKay T J Bolger J A Staromlynska J Davy J R J Chem Phys 1998 108 5537 5541 Rogers J E Cooper T M Fleitz P A Glass D J McLean D G J Phys Chem A 2002 106 10108 10115 Rogers J E Slagle J E Krein D M Burke A R Hall B C Fratini A McLean D G Fleitz P A Cooper T M Drobizhev M Makarov N S Rebane A Kim K Y Farley R Schanze K S Inorg Chem 2007 46 6483 6494 Kim K Y Shelton A H Drobizhev M Makarov N Rebane A Schanze K S J Phys Chem A 2010 114 7003 Perry J W Nonlinear Optics of Organic Molecules and Polymers CRC Press Boca Raton FL 1997 Staromlynska J McKay T J Bolger J A Davy J R J Opt Soc Am B Opt Phys 1998 15 1731 1736 Chateau D Chaput F Lopes C Lindgren M Brannlund C Ohgren J Djourelov N Nedelec P Desroc
48. DPAF z crossTPV1 crossTPV3 CH Relative Transmittance o oo 0 8 10 TPV1 TPV2 T2 7v crossTPV1 DPAF Figure 3 5 Open aperture Z scan measurement showing nonlinear absorption to nanosecond laser pulses at 680 nm 800 uJ pulse for the A ligands B linear series and C cross conjugated series in THF Pt DPAF a well known nonlinear absorbing platinum acetylide was used as a reference compound for purposes of comparison between different scans As shown by the z scans in Figure 3 5 all complexes and ligands exhibit nonlinear absorption under these conditions The nonlinear absorption of the ligands in plot A is only due to simultaneous two photon absorption because the ligands have near zero ISC yields whereas the nonlinear absorption of the complexes is due to the dual mode mechanism of 2PA and ESA By comparing the nonlinear absorption of the ligands Figure 3 5 A to that of the platinum acetylide complexes Figure 3 5 B and C it is clear that the nonlinear absorption to nanosecond pulses is considerably enhanced when both 2PA and ESA are present Sustaining the trends in 2PA cross section values and triplet triplet absorption molar extinction coefficients the longer conjugated OPVs display stronger attenuation of the transmittance at the focus 73 PMMA Monoliths for OPL Applications PMMA monoliths were prepared by a procedure adapted from Westlund and coworkers The monoliths ar
49. EXCITED STATE PROPERTIES OF ORGANIC AND ORGANOMETALLIC OLIGOMERS POLYMERS AND MATERIALS AND THEIR SOLID STATE APPLICATIONS By RANDI SUE PRICE A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2013 2013 Randi Sue Price To Zach ACKNOWLEDGMENTS Much gratitude goes to my advisor Prof Kirk Schanze who has given me an amazing amount of support and guidance He challenged me with designing and programming new instrumentation which gave me an intimate knowledge of the processes involved in chemical measurement and data acquisition Most importantly he encouraged me to become an independent scientist with the freedom to make my own discoveries Also would like to thank my committee members Dr Brucat Dr Hagen Dr Kleiman and Dr Omenetto for their time and support Everyone in the Schanze group former and current have been great colleagues and friends For the better part of my two years of research Dr Abigail Shelton let me follow her around asking many hundreds of questions and she was so patient to teach answer them all The past couple of years it has been great fun working together with Dr Galyna Dubinina on the nonlinear absorption project We shared many laughs Dr Gyu Leem introduced me to the materials side of my research and helped me to better understand the Hg3 project by pe
50. J Am Chem Soc 2001 123 9412 9417 Dubinina G Price R Abboud K Wicks G Wnuk P Stepanenko Y Drobizhev M Rebane A Schanze K J Am Chem Soc 2012 134 19346 19349 Dubinina G G Price R S Wicks G Drobizhev M Wnuk P Stepanenko Y Rebane A Schanze K S Proc SPIE 8622 2013 862218 Makarov N S Drobizhev M Rebane A Opt Express 2008 16 4029 Bonneau R Carmichael Hug G L Pure Appl Chem 1991 63 290 299 Porter G Windsor M W J Chem Phys 1953 21 2088 2088 Porter G Windsor M W Proc R Soc London Ser A 1958 245 238 258 170 88 89 90 92 93 94 95 96 97 98 99 100 101 102 Porter G Windsor M W Discuss Faraday Soc 1954 178 amp Candeias L P Wildeman J Hadziioannou G Warman J M J Phys Chem B 2000 104 8366 8371 Inigo A R Chiu H C Fann W Huang Y S Jeng U S Lin T L Hsu C H Peng K Y Chen S A Phys Rev B Condens Matter 2004 69 Jeng U Hsu C H Sheu H S Lee H Y Inigo A R Chiu H C Fann W S Chen S H Su A C Lin T L Peng K Y Chen S A Macromolecules 2005 38 6566 6574 Tan C H Inigo A R Fann W Wei P K Perng G Y Chen S A Org Electron 2002 3 81 88 Burroughes J H Bradley D D C Brown A R Marks R N Mackay K Friend R H Burns P L Holmes A
51. M and the units of do are mol L giving units to B A of cm GW This equation is valid only when the population of the ground state is much larger than the population of the excited state promoted by 2PA The 2PA cross section o zis also expressed as a factor of the energy of a photon in the incident light beam which has units of cm s For simplicity an informal unit GM for Gdppert Mayer is sometimes used which is equal to 10 cm s o 0 2 A xX hv 1 16 Converting Equation 1 14 to describe the nonlinear transmission through a medium with optical path length the equation becomes as written below IAL 1 TU A a 14 BA o ADL SS 37 Excited state absorption ESA has an enhancing effect on the observed two photon absorption of a medium The population of an excited state can be significantly increased by 2PA resulting in further ESA via a two step process involving the absorption of three photons In fact at higher intensities of incoming light it has been shown that ESA becomes the dominant source of reduced transmission In part because of this magnifying effect the combination of 2PA and ESA have been studied extensively in recent years for nonlinear absorption applications such as optical power limiting 7 It is difficult to separate the contribution of 2PA from that of ESA so an effective 2PA cross section value Ber has been used in the literature to describe the nonlinear absorption of chromophores under sp
52. PAF have triplet molar extinction coefficients of 64 300 and 30 300 M1 cm respectively The OPV ligands on the platinum significantly enhances the triplet molar extinction coefficient Table 3 4 Calculated values of the triplet molar extinction coefficients eT for the platinum Il acetylides from the slope of the relative actinometry experiment and isc 1 F according to Equation 2 7 above The linear and cross conjugated series were analyzed on different days in THF solutions prepared to ODss55nm of 0 77 and 0 79 respectively Slope Slope er Compound 0 d Compound p P aos Mtcm aos Mie Benzophenone 1 6 78707879 Benzophenone 7 0 78707879 TPVO 13 7 0 10 74 000 crossTPV1 52 3 0 01 77 000 TPV1 Ph 20 2 0 46 180 000 cross TPV3 87 8 0 04 81 000 crossTPV1 TPV1 15 9 0 30 110 000 DPAF 105 0 005 118 000 TPV2 9 9 0 52 100 000 TPV2 T2 58 3 0 28 390 000 increasing with the length of the OPV chain 2 to 8 oligomeric units Benchmark platinum acetylide complexes which do not contain OPV units Pt BTF and Pt DPAF have triplet molar extinction coefficients of 64 300 and 30 300 MI cm respectively The OPV ligands on the platinum significantly enhances the triplet molar extinction coefficient 98 CHAPTER 4 TRIPLET SENSITIZATION OF CONJUGATED POLYMERS BY Hg3 Introduction Conjugated polymers such as MEH PPV poly 2 methoxy 5 2 ethylhexyloxy 1 4 phenylene vinylene are studied
53. PtPh film 20 mg mL 500 RPM with nanosecond excitation at Aexc 355 nm 500 uJ pulse 1 KQ termination 128 averages slits 0 7 mm smoothing factor of 10 56 The transient absorption decay of a pPtPh film prepared from a 10 mg mL solution and spin coated at 1000 RPM at TAmax 650 nm was fit to a single exponential decay of the form y ae with an R 0 9247 a 0 001298 b 0 446 units in us This corresponds to a triplet lifetime of pPtPh in thin films of 2 2 us which is on the same order as the 0 996 us reported by Jarrett Vella by a time correlated single photon counting technique TCSPC 4 The transient absorption of pPtPh in tetrahydrofuran THF was performed for comparison to the film TA measurements A solution of pPtPh in THF was prepared to an optical density of 0 7 at 355 nm and purged with argon The transient absorption excitation source was the 355 nm third harmonic of a Continuum Surelite Il 10 Nd YAG laser The probe source was a Xe arc flashlamp and detection was with a PI Max intensified CCD camera mounted onto a spectrograph Data processing was done with an in house Matlab program The excitation energy was about 5 mJ The TA spectrum was collected from 350 nm to 850 nm with a 20 ns initial camera delay and 750 ns increments One hundred averages were taken at each time The solution showed a broad transient absorption peak over the wavelength range 410 nm to acquisition end with AAmax at approximately 650 nm Figur
54. S Ramsdale C M Sirringhaus H Friend R H Phys Rev B Condens Matter 2003 67 Gurau M C Delongchamp D M Vogel B M Lin E K Fischer DA Sambasivan S Richter L J Langmuir 2007 23 834 842 Guo J Ohkita H Benten H Ito S J Am Chem Soc 2009 137 16869 16880 Jiang X M Osterbacka R An C P Vardeny Z V Synth Met 2003 137 1465 1468 Osterbacka H An C P Jiang X M Vardeny Z V Science 2000 287 839 842 Sontag S K Marshall N Locklin J Chem Commun 2009 3354 3356 Corbitt T S Sommer J R Chemburu S Ogawa K Ista L K Lopez G P Whitten D G Schanze K S ACS Appl Mater Inter 2009 1 48 52 Zhou Z Corbitt T S Parthasarathy A Tang Y Ista L F Schanze K S Whitten D G J Phys Chem Lett 2010 7 3207 3212 Chemburu S Corbitt T S Ista L K Ji E Fulghum J Lopez G P Ogawa K Schanze K S Whitten D G Langmuir 2008 24 11053 11062 Lu L D Rininsland F H Wittenburg S K Achyuthan K E McBranch D W Whitten D G Langmuir 2005 27 10154 10159 175 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 Epe B Chem Biol Interact 1991 80 239 260 Bachowski G J Pintar T J Girotti A W Photochem Photobiol 1991 53 481 491 Triantaphylides C Krischke M Hoeberichts F A Ksas
55. S m The TAS m program allows the user to graphically select time zero to Time zero is taken to be end of the laser pulse indicated on the raw decay by the peak maximum of the transient absorption Using to a transient absorption spectrum AA vs wavelength is generated for the wavelength range of the TA vi acquisition run Several spectra at regular time intervals can also be generated and overlaid onto the to spectrum to visualize the spectral decay Instrument Validation and Optimization with pPtPh Polymer Films The platinum acetylide polymer pPtPh shown in Figure 2 6 was used in the validation of the transient absorption system The properties of this polymer have been well studied in the literature 4 It is a model platinum acetylide and has a maximum ground state absorption at 370 nm with fluorescence at 390 nm and phosphorescence at 515 nm 14 EE ee Figure 2 6 Structure of the platinum acetylide polymer pPtPh 54 To prepare thin films of pPtPh glass slides were cut into 1 cm wide pieces with a glass scribe and were thoroughly cleaned with sonication in a SDS water solution isopropanol and acetone followed by plasma cleaning Three solutions of varying concentration of pPtPh were prepared in toluene 5 mg mL 10 mg mL and 20 mg mL 1 0 0 8 5 mg mL pPhPt Q 0 6 O 500 RPM lt 0 4 1000RPM D r 0 9 3
56. TET E a a e g 0 03 5 wt Ha3 10 wt Hg3 ae N 0 02 wE ng 0 0051 L E En y 5 on 235 ns 32 1 09 us 68 W a lt oo 0 wt Hg3 i 0 01 0 000 i KR S 8 750 800 850 900 950 1000 Time microseconds Wavelength nm Figure 4 14 Nanosecond transient absorption spectra A and decays at A TAmax B for dropcoated films of MEH PPV prepared in benzene containing increasing wt Hg3 Aexc 532 nm 800 uJ pulse 128 averages 1 KQ termination smooth 10 Transient decays were fit to biexponential decays shown in red Structure Morphology Studies An interesting effect was noticed while attempting to perform low temperature emission experiments on MEH PPV When MEH PPV was frozen in the glass forming solvent methyl THF the color of the solution changed from orange to pink as shown in Figure 4 15 This is analogous to the color change upon addition of Hg3 to MEH PPV in benzene solution at room temperature The increased conjugation length intermolecular interactions and exciton migration of MEH PPV and other PPVSs at decreasing temperature in organic glasses leads to red shifted and narrower emission bands as has been well documented with many detailed studies by other groups 103 142 145 For the purposes of this work the observed color change offered further evidence that Hg3 simply acts as an instigator for MEH PPV to self aggregate and motivated 119 further experiments to examine how the structure and morphology
57. The Rayleigh length zr gives an approximate value for the length over which the beam stays focused The Rayleigh length can be calculated by equation 1 26 Zp 1 26 The wavelength A is the vacuum wavelength divided by the refractive index n of the material For a beam waist of 30 um the Rayleigh length is approximately 6 mm Interestingly the dip in the transmittance of the z scans obtained using the described 41 instrumental setup as shown in Figure 1 4 and Figure 1 5 begins about 6 mm away from z 0 This supports that a 30 um beam waist is a good approximation Typical open aperture Z scan experimental paramaters as are used in Chapter 3 are as follows Excitation is with a Aexc 600 nm laser beam through an OPO with a pulse energy of 600 uJ pulse 10 Hz repetition rate 5 ns fwhm Samples are prepared at 1 mM concentration in THF in a 1 mm pathlength quartz cuvette To calculate the effective nonlinear absorption coefficient Ber for a sample with a transmittance at the focus z 0 of 0 9 the following calculations would be used 1 The peak power of the laser pulse is 1 13 x 10 W If a 30 um beam waist is assumed the intensity of the beam is L 113x105 W G 1 30x10 4 cm 2 4 00 x 109 W cm 2 The pathlength is 1 mm The refractive index of THF is 1 40 at 25 C l 1 0 mm 0 10 cm 3 Equation 1 22 can be rearranged to solve for Bett 1 T _ 1 0 90 Txloxl 0 90 x 4 002 5 x 0 10 cm cm Ber f 0
58. a monochromator and photomultiplier tube The signal is amplified and collected on an oscilloscope One of the difficulties in acquiring transient absorption is that the excited state population of interest is relatively small leading to only tiny changes in the measured absorbance of the sample To achieve the highest signal possible an intense laser pulse and high power lamp are used as the pump and probe beams respectively 33 Computer control and data processing has certainly streamlined this process since starting in the 1970s making transient absorption a more routine experiment for even less experienced users Depending on the instrumentation that is used different transient species can be detected The development of femtosecond picosecond lasers has pushed the detection of transient absorption to shorter lived species allowing the investigation of the dynamics of singlet excited states whose lifetimes are typically on the order of 10 seconds Nanosecond instrumentation typically with the use of Nd YAG Q Switched lasers allows the routine study of triplet excited states which have gt ns lifetimes Further discussion about nanosecond transient absorption instrumentation is given in Chapter 2 Nonlinear Absorption In the early development of the interaction between matter and light only one photon processes were considered Two photon absorption 2PA by which a transition between a lower and higher energy level occu
59. a better understanding of the orientation of the oligomers in the clay 143 interlayer space The distances are measured from atom center to atom center Distances between the van der Waals radii will be slightly larger The 3D structures and EO OPE C2 0 50 nm N N distance 2 32 nm H H distance 2 75 nm EO OPE Th 0 62 nm N N distance 2 24 nm H H distance 2 64 nm Figure 5 9 3D structures of the cationic oligomers made by Chimera 1 8 software after MM2 geometry optimization Distances are measured from the atom centers approximate distances are shown in Figure 5 9 The widths of the oligomers 0 50 nm for EO OPE C2 and 0 62 nm for EO OPE Th calculated by Chimera are similar to the interlayer spacing of the SSA layers after insertion of the oligomer measured by XRD in Figure 5 8 0 570 nm for EO OPE C2 SSA the film and 0 611 nm for the EO OPE Th SSA film This suggests that the oligomers are intercalated into the clay in a single layer The lengths between the nitrogen atoms of the cationic endgroups are 2 32 nm 144 for the EO OPE C2 oligomer and 2 24 nm for the EO OPE Th oligomer Since the distance between anionic sites in the clay is about half the length of the oligomers it s likely that at least one unused anionic site will lie between the two anionic sites in the clay layer which are accessed by the cationic endgroups of the oligomer Thus this A es the limit to adsorption at less than 2 3 of t
60. a points iii Smooth 5 2 000 data points iv Smooth 10 1 000 data points v Smooth 20 500 data points 11 The output file will contain all the requested time slices in the current directory with the filename TA This file can be imported into Excel Data Processing TA Lifetimes 1 Raw data can be imported into Excel worksheets The units are AA vs Time ms 2 Delete AA values up to the peak of the TA decay when the laser fires but do not delete any time data points Now Time 0 should be at the peak of the TA decay 3 Fit this decay to the exponential decay function of your choice Troubleshooting 1 Laser does not fire Make sure the BNC cables are connected to the Pulse Generator via a T connector 2 Program Freezes Wait a few minutes and if nothing happens click the red stop sign button at the top of the program Wait a few seconds and click the white arrow to run the program again and pick up where you left off 3 Program is not frozen but TA vi toggles do not do anything Look at the BNC 575 Pulse Generator Display a If the circle on the pulse generator display screen is not flashing press the RUN STOP button on the front of the pulse generator If the circle flashes the problem should be solved b If the pulse generator display is frozen turn off the laser see Shutdown procedure above and then turn off the BNC 575 Pulse Generator for 10 seconds then turn it back on c If the pulse generator display
61. ak in the 2PA absorption spectra of the complexes is different than the So S transition in the ground state 1PA absorption spectrum This is explained by the selection rules for 1PA and 2PA transitions which require alternative parity in the 1PA and 2PA transitions for centrosymmetric molecules Analogous to the ground state absorption spectra shown in Figure 3 2 the 2PA peaks red shift with increasing conjugation length Both the 2PA spectra and T T absorption spectra red shift with lengthened OPV unit so the spectral overlap of these two processes remain well maintained The large discrepancy in the 2PA cross sections at shorter wavelengths seen in the NLT compared to the 2PEF measurement for TPV2 T2 and crossTPV3 may be due to a large contribution in the NLT from excited state absorption in the singlet state S1 Sn during the laser pulse duration Singlet excited state absorption is not measured by the 2PEF method 71 Transition wavelength nm Transition wavelength nm 300 350 400 450 500 300 350 400 450 500 800 4 z d h S 2000 gt h d d TPVO L I TPVO Lag 1500 1 20 les K 15 1000 J E Sei Pgs i 10 500 Me 5 WRF o Et 0 TPV1 H 4000 gt 30 ote 19 A a 20 BE ON gt E A n A N T 1000 fF BS R 2000 eS S oF EN S 4 fF S 5 2 z 10 2 I 4 P Ki S owe 2 o n a o4 H a TPV2 L e gt IPV2 T gt 10000 80
62. als was investigated in Chapter 5 Transient Absorption Instrumentation for Thin Films In all the work herein the triplet state properties were investigated by use of a newly constructed nanosecond transient absorption spectroscopy system specifically designed for solid state analysis A Surelite l 10 Q switched Nd YAG laser was used as the pump source and a Newport 250 W quartz tungsten halogen lamp is used as the probe Detection is with either a Hamamatsu R928 photomultiplier tube using 5 of the 9 stages for amplification or a Thorlabs A110 avalanche photodiode for detection in the UV Vis and nIR regions respectively The transient absorption system for thin films was used and validated using thin films made by spin coating solutions of a model platinum acetylide polymer pPtPh in toluene under various conditions The optimal settings for routine analysis were deduced by systematically varying the experimental parameters 151 and observing the quality of the transient decay collected of pPtPh films Transient absorption of lower than AA 10 was achieved which makes this system a valuable tool for analyzing the triplet state properties of thin films Solid State Materials for Optical Power Limiting A series of p phenylene vinylene platinum acetylide complexes were incorporated into PMMA monoliths with high transparency stability and optical quality for optical power limiting application The mechanism for optical power limiting in thes
63. amp was used as the probe source The 89 transient absorption signal was detected with a gated intensified CCD mounted on a 0 18 M spectrograph Princeton Pl Max Acton Pro 180 after an initial camera delay of 50 ns Samples were prepared to an optical density of 0 4 at the excitation wavelength Triplet lifetimes were calculated with a single exponential global fitting of the transient absorption decay data using SpecFit analysis software Triplet molar extinction coefficients were determined by relative actinometry using benzophenone t 1 00 et 7870 L mol cm 9 as the actinometer Monolith photophysics were measured freestanding open to atmosphere without a cuvette Absorption spectra were not obtained due to saturated OD Emission spectra were obtained by a front face alignment on a Spex Fluorolog 3 spectrophotometer Transient absorption measurements on monoliths cut to a thickness of approximately 1 mm were made using an in house designed TA instrument dedicated to solid state measurements The third harmonic of a Continuum Surelite l 10 Nd YAG laser at 355 nm 600 uJ pulse was used as the pump beam in an antiparallel alignment to the probe beam provided by a 250 W QTH lamp Newport Single wavelength transient absorption decays were acquired as an average of 128 shots every 25 nm using a Hamamatsu R928 PMT and an in house modified base using 5 of the 9 stages for amplification The transient absorption spectrum was acquire
64. amsplitter is split into a sample beam and a reference beam which are focused onto two matched detectors A neutral density filter can be used to attenuate the energy pulse The sample beam is brought into a very tight focus by a plano convex focusing lens The sample in a 1 mm pathlength cuvette is mounted on a translational stage and moved along the z axis through the focus of the laser The transmitted beam is collected by a detector and the transmittance relative to that of the reference beam is recorded by computer software Parle Energy Meter PC Plano SE ug M 15cm Sample Laser source Variable OPO 3 ND 8 Plano Translation Plano Hs 0 3 100 T SS a Convex Stage Detector A lens lens f 50 8 mm f 50mm Figure 1 5 Schematic of an open aperture Z scan system 35 Nonlinear absorption is observed as a dip in the relative transmittance of the laser beam through the sample with the lowest transmittance at the focus of the beam as shown in Figure 1 6 At the focus of the laser beam the intensity of the light is the highest 1 05 1 00 h 0 95 0 90 Relative Transmittance 0 85 Increasing 0 80 t NLA 0 75 0 70 0 65 L 1 1 1 1 1 1 L 1 10 8 6 4 2 0 2 4 6 8 10 Z Position mm Figure 1 6 Representative data demonstrating the nonlinear absorption of samples with increasing NLA measured by the open aperture Z scan apparatus If no nonlinear absorption is pre
65. ar momentum of the electron thus transitioning between the singlet ground state So and the first singlet excited state S4 Deactivation from S4 involves either fluorescence or nonradiative decay back to the ground state So or if there is sufficient spin orbit coupling intersystem crossing to the triplet state T1 can occur followed by phosphorescence or nonradiative decay It is known that upon excitation of a polymer the excited state tends to be delocalized over a few repeat units This delocalized quantum of excited energy in a polymer is called a Frenkel exciton There are several fundamental differences in the photophysical processes of polymers compared to discrete molecules 2779 First the large size and weight of a polymer may slow down diffusional motions of the polymer such as translation or rotational movement in a solvent or speed up restricted relative motions such as vibration of side groups or segmental motion along the polymer chain Second polymers tend towards heterogeneity For example a sample of polymer will typically contain a distribution of molecular weights stereoregularity etc Third in polymers 24 where the chromophore is a repeating unit the local concentration of chromophores is always high even when the number of polymer chains in a solution or media are very low This means that bimolecular processes such as energy transfer concentration quenching and excimer formation will be more prevalent in po
66. are enabled A user chosen number of waveforms are averaged on the scope and the resulting waveform is transmitted to the computer The waveform data off the scope is in mV vs data record The time parameter is computed externally and added to the waveform data by taking the ms div user defined setting and multiplying by the total number of divisions 10 and dividing by the number of data points 10 000 to get the ms data point The TA vi software at this time is not able to perform computations with scale settings of 200 ns div and lower In post processing Matlab software time zero to is selected manually by the user The raw mV data is converted to AA as demonstrated by Figure 2 4 and Equations 2 9 and 2 10 where A t is the change in transmittance intensity observed due to transient absorption AA log 2 9 A Absorbance log 1 A Transmittance 2 10 The mean of the first 50 data points of the acquisition for a 4 us div timescale 50 data points is 20 ns is taken and this is designated as lo lo is computed similarly to the Izero previously computed but with more precision due to the expanded y axis scale Each raw data point is subtracted from lo to get Al t The signal Al t lo I t divided by the background lo is the change in transmittance The output from a PMT is negative current so the Izero appears as a negative mV value Other detectors such as the APD output positive so the schematic shown in Figure
67. atter The nature of light and its interaction with matter has an interesting history ranging from the discovery of fire which provided portable lamps in the form of a bundle of sticks set ablaze to aid early man in sight to the evolution of laser technology for controlling chemical reactions and even the harvesting of light for conversion to energy Light is categorized into three regions of the electromagnetic spectrum the ultra violet 200 400 nm the visible 400 700 nm and the near infrared 700 1000 nm In the wave model light is treated as a wave propagating from a source characterized by Equation 1 1 EE 1 1 where A is the wavelength of light v the frequency and c the velocity of light 2 998 x 108 m s in vacuum The wave theory of light was supported by the observation of refraction in different media by Huygens and Fresnel and also by the wave like interference pattern observed by Thomas Young during his double slit experiment 1 3 The most convincing evidence was when Maxwell combined four simple equations previously developed by Coulomb Faraday Ampere and Gauss to describe the propagation of oscillating electric and magnetic fields and found that light waves propagate at 3 x 108 m s71 45 In the particle model of light light is quantized Planck developed the particle model of light during his blackbody experiment in the early 20th century where he found that blackbody radiation is limited to discrete value
68. be single beam or double beam A single beam spectrophotometer compares the light intensity of a single beam before and after a sample is inserted into the path A double beam spectrophotometer compares the light intensity between two light paths one which passes through the sample and one that does not The modern spectrophotometer typically consists of a broad spectrum white light source a monochromator containing a diffraction grating and a single wavelength detector such as a photomultiplier tube or photodiode The broad spectrum white light source must have a continuous emission spectrum and be temporally stable in regards to emission intensity The monochromator allows discrete wavelengths of the light source to pass through the sample and the transmission is measured by the detector The grating can be scanned step wise so that transmission of light through the sample can be measured at each wavelength On the other hand detectors such as charge coupled devices CCD or photodiode arrays PDA can be coupled with a fixed grating to capture the transmittance of every wavelength of light at the same time The molar absorptivity or molar extinction coefficient is a measurement of how strongly a chemical species absorbs light at a given wavelength and is an intrinsic property of the molecule that relates the absorbance to the concentration The units of molar absorptivity are MI cm The molar absorptivity can be calculated by taking the absorpti
69. ble 3 3 Effective nonlinear absorption coefficients Ber and effective nonlinear absorption cross sections 02 for the linear and cross conjugated platinum acetylides in THF solution and in PMMA monolith Compound Ber cm GW Ber CmM GW ge 10 cm4 GW os 101 cem4 GW Solution PMMA Monolith Solution PMMA Monolith TPV1 Ph 0 21 0 64 3 5 5 3 TPVO 0 20 0 92 3 4 5 9 TPV1 0 65 1 5 11 12 TPV2 N A 1 8 N A 10 TPV2 T2 0 89 0 7 15 4 9 crossTPV1 0 36 1 3 6 0 12 crossTPV3 0 65 1 4 11 12 crossTPV1 0 35 0 40 5 8 3 4 DPAF Qpen aperture Z scan for TPV2 was not measured in solution 80 Using these effective nonlinear absorption cross section values the trend seen in the solution z scan measurements of increasing nonlinear absorption with longer OPV conjugation length is now observed for the monolith measurements of the linear series however there is no difference in the oz values of the cross conjugated containing monoliths between crossTPV1 and crossTPV3 Inconsistencies are likely due to error in determining the concentration of chromophore in the monoliths Overall this method of calculating a quantitative measure of the nonlinear absorption of chromophores may be a promising way to compare nonlinear absorption properties in solution and solid state materials Through future work on OPL materials more accurate methods of controlling and determining the chromophore concentration in polymer glasses will be devised Also this effect
70. by a red shifted absorption spectrum and the growth of a shoulder at 578 nm as shown in Figure 4 5 Visibly the color changes from a bright orange to a deep red Large aggregated particles can be seen floating in solution at higher concentrations of Hg3 where the absorption spectrum nears the saturation point of Hg3 addition at concentrations greater than about 200 uM These particulates settle to the bottom if the solution is left to sit overnight although the solution still remains a deep red color indicating that not all Hg3 aggregated polymer precipitates from solution A em H 3e 6 2 3e46 0 8 S Ss 2e 6 5 oe T 2e 6 lt E DA ZG 1e 6 D 0 2 LO E 5e 5 0 0 r r A 0 300 400 500 600 700 800 500 550 600 650 700 750 800 Wavelength nm Wavelength nm Figure 4 5 Absorption A and emission B spectra of MEH PPV in benzene with a concentration of 10 ug mL 15 uM repeat unit with increasing additions of Hg3 from 0 uM to 240 uM Excitation for the emission measurements was at 490 nm The fluorescence spectrum also shows interesting changes upon addition of Hg3 Figure 4 5 The fluorescence of MEH PPV in benzene is seen as a structured 106 emission with maxima at 558 nm and 602 nm with a shoulder around 660 nm When Hg3 is added the fluorescence intensity was effectively quenched and the structured emission was red shifted This quenching may indicate that intersystem crossing
71. ced due to the insolubility of the Hg3 complex in benzene at room temperature The Hg8 results in solvation into the polymer chains ordering the polymer chains as seen in the absorption and emission 104 spectra Furthermore others have reported properties of a solid state complex of benzene and Hg3 produced by dissolving Hg3 in boiling benzene and slow evaporation of the solvent 177 so the interaction between the Hg3 and the polymer may also be mediated by pi stacking of the solvent between polymer chains benzene j Wgd 0 uM 19 uM 39 uM Hg3 0 uM 16 uM 19 uM Hg3 1 0 A 0 uM Ho B 0uMHg3 0 8 39 uMHg3 16uMHg3 19 uM Ho o 0 6 O lt 04 0 2 0 0 S 300 400 500 600 700 800 400 500 600 700 800 Wavelength nm Wavelength nm Figure 4 4 Visible color change of the solution and the absorption spectra of MEH PPV upon addition of Hg3 in o dichlorobenzene A and benzene B The changes in the absorption and emission spectra were monitored carefully with addition of Hg3 MEH PPV in good solvents like benzene or toluene exists as 105 isolated unaggregated polymer chains with an absorbance maximum at 497 nm due to relatively short conjugation lengths from twisting along the polymer backbone 98 Upon addition of increasing amounts of the Hg3 complex to the polymer solution aggregation of the MEH PPV polymer occurs shown
72. cking of the long branched alkyl side chains increased conjugation length along the polymer chain and overall increased crystallinity of the polymer in untreated films These properties are important to the design of efficient optoelectronic devices Future studies will be focused on experiments to measure the charge and exciton transport of the polymer in the presence of Ho Experimental Section Preparation of Polymer Solutions and Films The mercury complex Hg3 was synthesized by a former postdoc Kye Young Kim and the purity was checked by 9F NMR The molecular weight of Hg3 is 1046 g mol MEH PPV was obtained from a commercial supplier American Dye Source Inc and stored in an oxygen and moisture free environment although portions were taken out and kept in tightly closed vials covered with foil for up to several weeks before use The molecular weight and purity of the polymer were checked by GPC Polymer solutions for photophysical measurement and the preparation of films were prepared by first dissolving the polymer into high purity spectroscopy grade solvent stirring for at least 2 3 hours to obtain a homogenous solution A polymer solution was prepared by dilution of a concentrated MEH PPV stock solution 3 mg mL which was left to stir overnight to a 3 mL quartz cuvette toa final concentration of 15 uM repeat unit for soectrophotometric analysis The Ho complex is not completely soluble in benzene so a dispersion was prepared at 0 6
73. complexes had low fluorescence quantum yields lt 5 and short fluorescence lifetimes lt instrument response 100 ps indicative of especially efficient ISC Nanosecond transient absorption was used to investigate the triplet state properties of these complexes The triplet triplet absorption spectra are shown in Figure 3 3 Each of the chromophores exhibits strong broad triplet triplet absorption throughout the visible and near IR regions with bleaching lt 500 nm corresponding to ground state absorption The chromophores are prepared to matched optical density at Ae 355 nm in THF The Pt complexes containing the longer conjugated ligands TPV2 T2 and crossTPV3 have the largest T T absorption which is supported by the triplet molar extinction coefficients shown in Table 3 1 The T T absorption bands for the cross conjugated series are distinctly sharper and more intense than for the linear series because the triplet chromophore is more influenced by the Pt centers Comparing the crossTPV1 crossTPV3 and crossTPV1 DPAF complexes there is a 68 red shift in the TA with increasing conjugation length of the OPV axis indicating that the triplet excited state is primarily localized along the OPV axis and not the phenylethynl axis This is further supported by the fact that in crossTPV1 DPAF the triplet triplet absorption spectrum aligns with that of crossTPV1 Amax of T1 Tn 660 nm so the axis containing the two DPAF units does not con
74. d ed Plenum Press New York 1994 Rohatgi Mukherjee K K Fundamentals of Photochemistry New Age International Ltd Publishers Sanjeev Offset Delhi 1986 Smith K C Stanford University School of Medicine Vol 2013 Shriver D F Atkins P W Inorganic Chemistry 3rd ed ed W H Freeman and Company New York 1999 Turro N J Modern Molecular Photochemistry University Science Books Sausalito California 1991 Forster T Annalen Der Physik 1948 2 55 75 Dexter D L J Chem Phys 1953 27 836 850 Forster T Angewandte Chemie International Edition 1969 8 333 amp Birks J B Photophysics of Aromatic Molecules 1970 704 704 166 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Birks J B Rep Prog Phys 1975 38 903 974 Epple R Forster T Z Elektrochem 1954 58 783 787 Cohen M D Mol Cryst Lig Cryst 1979 50 1 10 Ferguson J J Chem Phys 1965 43 306 amp Avis P Porter G J Chem Soc Faraday Trans 1974 8 1057 1065 Winnik F M Macromolecules 1990 23 233 242 Winnik F M Chem Rev 1993 93 587 614 Polymer Photophysics Phillips D Ed Chapman and Hall New York 1985 Somersall A C Guillet J E J Macromol Sci Rev Macromol Chem Phys 1975 C 13 135 187 Creation and Detection of the Excited State Lamola A A Ed Marcel Dekker Inc New York
75. d by using a custom Matlab program to convert the transient absorption decays at each wavelength to a transient absorption spectrum at t 0 The transient absorption of the monoliths did not fully decay to the baseline in the timescale of the instrument 2 ms so transient absorption lifetime data could not be obtained 90 Fluorescence lifetimes for both THF solutions and monoliths were obtained by time correlated single photon counting technique TCSPC with a PicoQuant FluoTime 100 compact fluorescence lifetime spectrophotometer A UV pulsed diode laser provided excitation at 375 nm power lt 10 mW The laser was pulsed by a PDL800 B pulsed diode laser driver Fluorescence decays were obtained for at least five wavelengths across the emission spectrum using 10 nm bandpass interference filters for solution measurements in air saturated THF and analyzed by a global fit as average amplitude weighted from the fluorescence decays using biexponential fitting parameters FluoFit software For monolith fluorescence lifetime measurements the monoliths were placed at 45 in the sample holder with respect to the incoming laser beam Fluorescence lifetimes were acquired and fit at one wavelength for comparison to solution measurements Nonlinear transmission measurements were performed via an open aperture z scan apparatus The excitation wavelength 680 nm was generated by a Continuum Surelite OPO Plus pumped with the third harmonic 355 nm of a Conti
76. d pulses The transient absorption intensity of the EO OPE C2 SSA hybrid compared to the EO OPE Th SSA hybrid is very weak A higher triplet yield in the EO OPE Th oligomer is expected due to the enhanced intersystem crossing by the presence of the thiophene unit 141 0 014 FO OPE C2 SSA DER ee EO OPE Th SSA 0 010 0 008 AA 0 006 0 004 0 002 0 000 500 550 600 650 700 750 800 Wavelength nm Figure 5 7 Transient absorption spectra of oligomer clay hybrid films Excitation was with nanosecond pulses at 355 nm 800 uJ pulse 128 averages 1 kO termination smooth 10 The transient decays at Amax appeared biexponential with a very long lived component and a shorter component The long component of the EO OPE C2 was out of the instrumental range gt 10 ms but the shorter component was measured to be 18 5 us Previous measurements of the free EO OPE C2 oligomer in water give a lifetime of 18 7 us IP This again illustrates the similarity between the excited state properties in solution and in the clay substrate The average lifetime of the EO OPE Th oligomer was measured to be Tave 144 us T1 67 3 24 4 us and T2 32 7 391 us The solution transient absorption lifetime for EO OPE Th in water was previously reported to be 7 2 ys Oligomer SSA hybrid solid was subjected to XRD analysis compared to unmodified SSA powder to verify intercalation of the oligomer into the interlayer of the clay
77. d to the solution measurements after addition of SSA indicating similar excited state energetics The EO OPE C2 SSA film exhibited a broad featureless emission whereas the EO OPE Th SSA film emission displayed a red shifted band which may correspond uneven intercalation of the oligomer The nanosecond transient absorption spectra of the films show relatively strong broad and long lived transient absorption for both EO OPE C2 SSA and EO OPE Th SSA In agreement with the fluorescence lifetime results for EO OPE Th in solution the transient absorption for EO OPE Th SSA was significantly more intense compared to EO OPE C2 due to the triplet yield of the EO OPE Th oligomer The interlayer spacing of the clay increased upon addition of the oligomer as was measured by XRD confirming that intercalation has occurred For EO OPE C2 SSA the interlayer space increased from 0 360 nm in pure SSA to 0 570 nm upon intercalation of the oligomer The EO OPE Th SSA film had an interlayer space of 0 611 nm The interlayer spacing is larger for EO OPE Th SSA than that for EO OPE C2 SSA due to the rigid non linear backbone introduced by the thiophene unit present in the middle of the oligomer 3D models of the oligomers support the interlayer spacings calculated from the XRD data suggesting that the oligomers are unagreggated and in a single layer in the interlayer space The amount of oligomer absorbed onto the 147 clay was measured by TGA It was found that the a
78. e Hg3 powder was weighed and directly added to the solution stirring moderately overnight For the lower wt solutions a stock solution in benzene 0 6 mg Hg3 dispersed per mL was prepared and while stirring the stock solution an aliquot of Hg3 was transferred into the TEM solution For TEM sample preparation involved the deposition of the polymer solution onto a 300 mesh Holey carbon coated copper grid and allowing the sample to dry before analysis Vapor phase staining with OsOs is used for imaging the polymer samples by TEM X ray powder diffraction measurements XPert Powder with monochromatic Cu Ka radiation A 1 540562 A were performed to determine the structure of the powder Diffraction data were collected at room temperature by step scanning in the range 3 lt 20 lt 25 with a step size of 0 0167 and a counting time of 30 s at each step Films were prepared from sample solutions in benzene with 5 mg mL MEH PPV concentration stirred overnight Hg3 powder was weighed and added directly to the solution stirring 132 vigorously for 2 days Films were made by doctor blading the solutions onto clean glass slides for XRD analysis The doctor blading procedure is as follows Three layers of Scotch Magic Tape were used to section off a portion of the glass slide about 3 cm x 2 5 cm and 150 uL of solution was pipetted to one edge of the slide A flat edge was used to sweep along the tape evenly covering the glass slide betwe
79. e complexes is nonlinear absorption by combined simultaneous two photon absorption and triplet triplet excited state absorption The photophysical properties of the PMMA monoliths were studied and compared to the properties of the p phenylene vinylene platinum acetylides in solution The important photophysical properties to nonlinear absorption such as strong two photon absorption intense triplet triplet excited state absorption and good spectral overlap between the two were retained in the PMMA monolith The PMMA monoliths containing the platinum acetylide complexes exhibited nonlinear absorption to nanosecond pulses An effective nonlinear absorption coefficient Ber was used to compare the nonlinear absorption of the chromophores in solution and in the PMMA monoliths Similar values for Ber were found between the solution z scan results and those of the monolith measurement especially for the linear p phenyene vinylene platinum acetylide series indicating that these platinum acetylide chromophores are as effective optical power limiters in PMMA monolith as in solution Triplet triplet annihilation was present in conjugated co polymers containing phenylene and p phenylene vinylene platinum acetylide units which was observed as very fast initial decay with increasing amplitude at increasing laser fluence in the 152 transient absorption Representative monomer complexes did not show evidence of triplet triplet annihilation Tripet trip
80. e 2 9 The spectra between the solution and thin film are similar with little shift in the absorption maximum Instrument optimization was performed using pPtPh films The optimal settings for slit width number of waveforms to average and smoothing of the acquired data were determined These are the default settings that will be used for routine sample analysis although optimization should be performed for each individual sample 57 0 08 0 06 0 04 AA 0 02 550 600 650 700 750 800 Wavelength nm Figure 2 9 Nanosecond transient absorption spectrum of pPtPh in THF solution Aexc 355 nm 5 mJ pulse initial camera delay 20 ns camera delay increment 750 ns 25 transients 100 averages The wavelength resolution at the detector is dependent on the slit width of the monochromator and the bandpass of the grating used The grating used in for the film TA is a 500 nm blaze grating with 1200 lines mm which has a bandpass of about 7 nm mm Generally a smaller slit width gives higher wavelength resolution Using larger slits more light hits the detector which normally decreases the noise in the acquired data however a slit that is too large will also allow stray and scattered light to affect the signal The optimal slit width for this instrument was determined by observing the signal intensity vs the signal to noise ratio of TA decays acquired under the same conditions varying only the slit width as shown in Figure 2 1
81. e at each point in space The evolution of the wave can be described by a differential equation known generally as the Schrodinger equation developed in 1928 The propagation of this wave causes the observed effects such as interference and diffraction Excited State Dynamics The Grotthuss Draper law proposed in the early 1800s asserts that only light that is absorbed can cause photochemical changes Only the photons with energy equal to the energy gap of that transition will be absorbed promoting valence to a higher energy level The result is an electronically excited state The Beer Lambert law shown in Equation 1 3 gives a quantitative relation describing the absorption of light through a material A elc 1 3 where A is the absorbance is the molar absorptivity MT cm is the pathlength in cm and c is the molar concentration of the absorbing molecule An excited molecule can relax radiatively or nonradiatively Radiative decay is accompanied by emission of a photon either by fluorescence from the singlet excited state or by phosphorescence from the triplet excited state Nonradiative decay is a relaxation process that does not involve emitting photons Generally photophysical characterization involves the measurement of the quantum yield and lifetime of the produced excited states The quantum yield describes the efficiency of a photophysical process For example the quantum yield of emission is the ratio of the number of photon
82. e highly transparent and could potentially be made to any size and shape with the appropriate mold Figure 1 For measurement of their photophysical properties the monoliths were cut and polished to approximately 1 mm to high optical quality Varying concentrations of chromophore were attempted but monoliths containing higher initial concentrations gt 2 mM of chromophore were not transparent due to phase separation Table 3 2 lists the final thickness and calculated concentration of each monolith The absorption spectra could not be measured for the monoliths due to saturated optical density The photoluminescence spectra of the monoliths were measured in front face alignment and compared to the spectra in THF solution in Figure 3 6 The linear TPV complexes display a slight blue shift in the PMMA monolith however the peak shape and broadness is similar to solution measurements The cross conjugated complexes do not exhibit a shifted spectrum in the monoliths 1 0 Solution os IN Ween f E A Di BN a A VI N I 0 4 Ii I i S CK Uh LEX PE Normalized Emission Normalized Emission Ei TPV1 Ph TPVO__ _ TPV1 TPV2 TPV2 T2 aaa d i 400 500 600 700 400 500 600 700 0 8 L I d i Wavelength nm Wavelength nm of M CG A Cla ost Lf Hl Oe H A J s 0 01 crossTPV 1 ccfossTPV3 T_ rossTPV1 DPAF 400 500 600 700 400 500 600 700 400
83. e promoting a molecule to a higher energy state After some time the molecule 31 relaxes back to the ground state and a photon is emitted This photon is detected by the PMT and the clock stops marking the time from excitation to emission The probability that a photon is emitted decreases over time After many repeated acquisitions the exponential decay of the emission process is generated and the fluorescence lifetime can be obtained Phosphorescence lifetimes can be obtained by less sensitive methods due to the long lifetime natural to the triplet excited state Because of the longer lifetime a larger population of the emitting state T1 can be produced over a sustained laser pulse and single photon detection becomes unnecessary As such emission from a large ensemble of molecules can be detected in a single shot using a PMT Transient Absorption Spectroscopy Transient absorption TA spectroscopy also Known as pump probe spectroscopy and flash photolysis is a powerful technique which allows the study of short lived transient species These transient species can involve photoreaction intermediates as well as higher excited states of a molecule accessed by the absorption of photons Spectral and dynamic information are acquired which provide information about the evolution of excited states that are populated upon excitation with light The absorption spectra acquired by TA depends on the energy gap of the excited states involved in
84. ear and cross conjugated platinum Il acetylide chromophores are listed in Table 3 3 For the calculation the following values were used The solution z scan measurements were performed with 1 mM solutions in 1 mm pathlength cuvette For the monoliths the concentrations and pathlengths monolith thickness used in the calculations are those listed in Table 3 2 Intensities were calculated based on the laser pulse energies of each experiment 800 uJ pulse for solution measurements and 700 uJ pulse for monoliths 79 Overall there is good agreement between the effective cross section values o2 for each chromophore calculated from the solution and monolith open aperture z scan measurements Discrepancies which are largest in the case of TPV2 T2 crossTPV1 and crossTPV1 DPAF may be due to inaccuracy in the measurement of the concentration of the monoliths The monolith concentrations were estimated based on weight loss when they are polymerized and cured in an oven for 6 days The assumption is that the weight loss is only due to evaporation of the monomer methyl methacrylate There could be some leaking or splashing of the solution out of the Teflon mold which would result in a lower actual concentration than was estimated Additionally if the monolith is not truly homogeneous and it is cut and polished the final calculated concentration will not be very accurate leading to error in the effective nonlinear absorption cross section values Ta
85. ecific conditions Several models for describing experimental NLT to nanosecond laser pulses have been proposed 42 44 Sutherland et al successfully modeled the nonlinear absorption of alkyl fluorenyl AFX chromophores accounting for 2PA and ESA from both the first singlet excited state and triplet excited state with good agreement They used a five level system and a simple three photon absorption model where two of the photons are absorbed simultaneously through a virtual intermediate state as shown in Figure 1 7 The attenuation of the light intensity through a sample along the z axis can be described by a modified equation of Equation 1 12 al Oz 0 NgoI gehen o7 N71 1 18 where Nso Ns1 and Nr are the number densities in 1 cm of molecules in So S1 and T states respectively and o2 Os and o7 are the absorption cross sections The absorption cross sections can be experimentally determined by picosecond and nanosecond photophysical techniques The difficulty of this calculation is in determining 38 the number densities Equation 1 18 must be solved simultaneously with kinetic equations which describe the population of each state Nso Ns7 and Nr based on the excited state lifetimes and quantum yields ON Sic Oa ay Ns t 2hw Ts 1 19 NT _ PT age Nra ae rs ale 1 20 Nso N Ns Mou 1 21 Using several approximations Sutherland et al also derived an analytical express
86. ed on a relative scale for comparison The triplet triplet transient absorption for regiorandom P3HT films occurs around 800 nm while regioregular P3HT is not reported to show triplet triplet absorption 180 182 Nanosecond transient absorption measurements were attempted on P3HT in chloroform and P3HT films 0 50 wt Hg3 with addition of Hg3 with no transient absorption detected Summary of Results In solution the interaction between MEH PPV and Hg3 is strongly solvent dependent In solvents such as o dichlorobenzene no interaction between the Hg3 and the polymer is observed however in benzene which does not dissolve Hg3 the Hg3 complex solvates into the polymer chains leading to aggregation of the polymer In 127 films prepared by either solvent aggregation of the polymer occurs as the solvent is evaporated The photophysical data suggests increased conjugation length of the polymer due to self organization of the polymer chains promoted by the interaction with Hg3 The fluorescence emission intensity is quenched indicating that the presence of the heavy mercury atoms may be promoting intersystem crossing in the polymer to the triplet state however the transient absorption results in solution do not show evidence of increased triplet concentration as Hg3 concentration increases Instead the polymer cation is detected in increasing magnitude due to electron transfer to Hg3 which was shown to have irreversible reduction by cycl
87. elkrug D J Chem Phys 2002 176 8596 8609 Rahman MH Chen H L Chen S A Chu P P J J Chin Chem Soc 2010 57 490 495 Ou Yang W C Wu T Y Lin Y C ran Polym J 2009 18 453 464 Chen S H Su A C Huang Y F Su C H Peng G Y Chen S A Macromolecules 2002 35 4229 4232 Chen S H Su A C Chang C S Chen H L Ho D L Tsao C S Peng K Y Chen S A Langmuir 2004 20 8909 8915 Lobez J M Andrew T L Bulovic V Swager T M ACS Nano 2012 6 3044 3056 174 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 Brabec C J Gowrisanker S Halls J J M Laird D Jia S Williams S P Aav Mater 2010 22 3839 3856 Chen TA Wu X M Rieke R D J Am Chem Soc 1995 117 233 244 Prosa T J Winokur M J McCullough R D Macromolecules 1996 29 3654 3656 Bao Z Dodabalapur A Lovinger A J Appl Phys Lett 1996 69 4108 4110 Sirringhaus H Tessler N Friend R H Science 1998 280 1741 1744 Sirringhaus H Brown P J Friend R H Nielsen M M Bechgaard K Langeveld Voss B M W Spiering A J H Janssen R A J Meijer E W Herwig P de Leeuw D M Nature 1999 407 685 688 Li L G Lu G H Yang X N J Mater Chem 2008 78 1984 1990 Brown P J Thomas D S Kohler A Wilson J S Kim J
88. en and wiping off excess solution The films were allowed to dry for several hours before analysis The cyclic voltammetry measurement was performed in dichloromethane with a conventional three electrode cell configuration on a BAS CV 50W voltammetric analyzer equipped with a platinum disk working electrode 2 mm and platinum and silver wires were used as the counter and reference electrodes respectively All electrodes were polished and rinsed with water before use Tetrabutylammonium hexafluorophosphate TBAH 0 1 M was used as the supporting electrolyte The concentration of Hg3 was approximately 3 mM The cyclic voltammogram was calibrated using ferrocene as a standard taking its potential as 0 41 vs Ag AgCl The scan rate was 100 mV s and the sensitivity was 10 UA 133 CHAPTER 5 PHOTOPHYSICAL STUDY OF CATIONIC OLIGOMERS IN A CLAY HOST Introduction Poly phenylene ethynylene PPE and oligo phenylene ethynylene OPE derivatives have been studied extensively in recent years due to their photophysical and photochemical properties One application of PPEs with cationic endgroups are aseffective light activated biocides in water solution due to their ability to coordinate with the anionic bacterial membrane which can itself cause cell death It has also been shown that singlet oxygen plays a vital role in the light inactivation of bacteria 6 7 Upon irradiation with light singlet oxygen is readily produced by energy tran
89. ers and exciplexes is that they are associative when one of the molecules is in the excited state but are dissociative in the ground state gt 7 In both excimers and exciplexes the new emission will be highly concentration dependent and typically has a broad featureless red shifted emission compared to the component molecules This loss in vibrational structure of the emission is due to the repulsive nature of the component molecules in the ground state that leads to rapid separation of the complex after loss of the excited state Excimers can be used as fluorescent labels yielding information about macromolecular conformation due to the contact required for excimer formation between label pairs A classic example of an excimer is 23 that formed by pyrene which has been studied in detail in systems ranging from pyrene labelled polymers to pyrenes in crystals and other matrices 1 26 Photophysics of Conjugated Polymers Conjugated polymers are molecules that consist of many repeating monomer units having contiguous conjugation along the polymer backbone These polymers are known as conductive polymers due to their semi conductive properties Polymers interact with light in much the same way as their small molecule analogues After the absorption of light an electron from the highest occupied molecular orbital HOMO gets promoted to the lowest unoccupied molecular orbital LUMO This absorption takes place without a change in the spin angul
90. es indicating that the S1 state is dominated by the OPV chromophore No phosphorescence from the polymers or model complexes was observed crossTPV1 crossTPV3 co crossTPV1 P co crossTPV3 P Normalized Absorption a u Normalized Emission a u 300 400 500 600 700 Wavelength nm Figure 3 11 Normalized absorption and emission spectra for the model complexes top and the copolymers bottom in THF solution Nanosecond transient absorption was carried out to examine the triplet excited state properties of the complexes and copolymers Transient absorption spectra are 83 shown in Figure 3 12 Samples were prepared at matched optical densities at 355 nm in THF The triplet triplet absorption spectra of the copolymers is very similar to the spectra of the model complexes indicating that the triplet state is localized on the OPV chromophore axis with little involvement of the platinum acetylide polymer backbone Interestingly the transient absorption intensity of the copolymers is less than half that of the model complexes crossTPV1 and crossTPV3 At matched optical densities and with similar fluorescence quantum yields 1 for crossTPV1 and co crossTPV1 P 4 for crossTPV3 P and co crossTPV3 P it is expected the triplet yield should be the same between each copolymer and model complex pair One possibility for marked decrease of triplet state concentration in a conjugated polyme
91. et Shutter Shutter Fiter p Pren jo Laser Flashlamp Shutters Q Switch Oscilloscope QSwitch Delay us Channel A and E Channel B Channel C a Channel D i SEN D Run D Run A Run i Stop J Stop Stop AJ UPDATE PULSE SETTINGS Volts corre Set IZERO in mV GR 2 TZERO in mi DH I DH 1 D 15E5 2 amp 5 2565 365 3565 465 Jo Time in seconds Seen Vertical Scale m Div Horizontal Scale per Division e 1000 mV Joo 500 mV 200 mV F Kan 100 mV di SO my 20 mv Ue Horizontal Scale 5mY 2m im INNMNMNNKRMNNMNMNNNNNKRMNNNNNNNNI 2ns 1 7 SS mi Ave for Acquisition ACQUIRE Vertical Scale Figure 2 2 Screenshot of the TA vi program 49 After the transient absorption settings have been optimized for acquisition the acquisition window is opened by clicking the Acquire button in the TA vi window Figure 2 2 to open the Acquire vi subvi In the Acquisition window shown in Figure 2 3 parameters for a full acquisition scanning many wavelengths can be set On the top left hand part of the screen the start stop and interval wavelengths are input by the user and the progress indicator on the right is updated with the correct number of iterations A smoothing parameter is selected to reduce the number of data points collected for each run from the 10 000 sent by the oscilloscope to a more manageable number for manipulation in post acquisiti
92. gth Dual mode nonlinear absorption utilizing 2PA and ESA has broad temporal response offering protection from both short pulses fs ps and longer pulses ns us 41 65 69 70 The majority of research on platinum acetylides for OPL properties has been reported in solution experiments but recently attention has shifted to the incorporation of these complexes into solid state materials that are more useful to real world applications Inorganic sol gel glasses offer high stability and excellent optical quality 8 Platinum acetylides incorporated into sol gels have demonstrated good optical power limiting abilities 717 however the sol gel method is time consuming requires careful attention to pH and temperature and typically requires additional functionalization of the active chromophores in order to reach adequately high concentrations for effective optical power limiting On the other hand polymer glasses such as those made with poly methyl methacrylate PMMA make good solid state hosts due to their simple preparation high optical transparency and ability to be cut and polished with good optical quality In 2008 Westlund and coworkers prepared platinum Il acetylides containing phenylene ethynylenes units into PMMA polymer glasses with high doping concentrations up to 100 mM as both blended and covalently bonded materials They found that these materials exhibited nonlinear absorption as shown by strong attenuation of transmit
93. he sites or 66 vs CEC 2 60 2 55 2 50 2 45 2 40 2 35 2 30 2 25 2 20 Weight mg Temperature degrees Celsius Time min U N Get Ge Ve N Weight mg N E 2 7 O 100 200 300 400 500 600 700 0 20 40 60 80 100 Temperature degrees Celsius Time min Figure 5 10 TGA analysis of the A EO OPE C2 SSA and B EO OPE Th SSA solid Correcting for water loss the weight change due to loss of EO OPE C2 is 8 32 and the weight change due to loss of EO OPE Th is 7 55 To determine the amount of oligomer adsorbed into the clay substrate solids of each oligomer SSA hybrid were subjected to TGA analysis as shown in Figure 5 10 with 145 a temperature ranging from room temperature up to 700 C The inorganic clay SSA is stable at 700 C so the mass loss of the hybrid can be attributed to water and oligomer The oligomer mass was calculated as the difference in the ending mass at 700 C 100 minutes and the mass after water evaporation during a hold at 110 C The adsorption calculated from the TGA data is 37 7 vs CEC for EO OPE C2 and 33 5 vs CEC for EO OPE Th Summary of Results The intercalation of cationic oligomers EO OPE C2 and EO OPE Th into Sumectin SA clay and the effect on the photophysics of the oligomers was studied In solution the interaction with the clay led to a slight red shift in both the absorption and emission spectra In the absorption spectra with increasing conce
94. her one or three OPV units between terminal triphenylamine groups in the OPV axis The copolymers co crossTPV1 P and co crossTPV3 P contain alternating platinum acetylides of an OPV unit and a phenyl ring with long alkyl side chains to increase solubility The structures of crossTPV1 crossTPV3 co crossTPV1 P and co crossTPV3 P are shown in Figure 3 10 BuP ra Pre O SHO ene fax W d ty Ze Ri crossTPV3 d A Su G Ae d S CH O Bu P PtPBu ch crossTPVL nu IS d 1 oun Vv Q ML p IN Lf d KE E yo e J N o 4 ma G J Le ZA Vase Kant P lt P el BE ie ZA vig PBu3 Bop PBus e n x CY l d d LN wy _ co crossTPV3 P Or A YY co crossTPV1 P Figure 3 10 Structures and acronyms for the cross conjugated platinum Il acetylide p phenylene vinylene model complexes and copolymers synthesized by Galyna Dubinina 82 The photophysical properties of the copolymers in comparison to the model complexes were investigated in THF solution The absorption and emission spectra are shown in Figure 3 11 CrossTPV3 and co crossTPV3 P containing the longer OPV unit are both red shifted compared to crossTPV1 and co crossTPV1 P Both polymers have very pronounced peak around 360 nm due to the conjugation of the polymer along the organometallic platinum acetylide axis The emission spectra of the polymers are identical to the spectra of the model complex
95. hes C Eliasson B Kindahl T Lerouge F Andraud C Parola S ACS Appl Mater Inter 2012 4 2369 2377 169 72 73 74 75 76 es 78 79 80 81 82 83 84 85 86 87 Zieba H Desroches C Chaput F Carlsson M Eliasson B Lopes C Lindgren M Parola S Adv Funct Mater 2009 19 235 241 Westlund R Malmstrom E Lopes C Ohgren J Rodgers T Saito Y Kawata S Glimsdal E Lindgren M Adv Funct Mater 2008 18 1939 1948 Drobizhev M Rebane A Suo Z Spangler C W J Lumin 2005 177 291 305 Albota M Beljonne D Bredas J L Ehrlich J E Fu J Y Heikal A A Hess S E Kogej T Levin M D Marder S R McCord Maughon D Perry J W Rockel H Rumi M Subramaniam C Webb W W Wu X L Xu C Science 1998 281 1653 1656 Mcllroy S P Clo E Nikolajsen L Frederiksen P K Nielsen C B Mikkelsen K V Gothelf K V Ogilby P R J Org Chem 2005 70 1134 1146 Mardelli M Olmsted J J Photochem 1977 7 277 285 Carmichael Helman W Hug G J Phys Chem Ref Data 1987 16 239 260 Amand B Bensasson R Chem Phys Lett 1975 34 44 48 Scaiano J C Redmond R W Mehta B Arnason J T Photochem Photobiol 1990 52 655 659 Wilson J S Chawdhury N Al Mandhary M R A Younus M Khan M S Raithby P R Kohler A Friend R H
96. highly two photon absorbing oligomeric p phenylene vinylene tr conjugated chromophores OPV are connected to platinum Il centers and the one and two photon photophysical properties are examined in solution and in solid state PMMA monoliths with a focus on optical power limiting Two series of oligo j phenylene vinylene platinum Il acetylides were studied to elucidate the structure property relationship for optical power limiting Fig 3 1 A linear series TPVn has two OPV chromophores linked to a platinum center through a 64 triphenylamino group with an increasing number of OPV units n 0 to 2 where n is the number of phenylenes between the terminal triphenylamine groups The complex TPV2 T2 has two OPV chromophores each connected to a platinum atom that is connected through a dithiophene center The cross conjugated series crossTPVn has two OPV chromophores attached to a center phenyl unit that is conjugated through platinum phenyl ethynylenes n 1 and 3 where n is the number of phenylenes between the terminal triphenylamine groups and the center phenyl ring The complex crossTPV1 DPAF has diphenylaminofluorene DPAF groups instead of the terminal triphenylamines J E Bu P NE N PBu Wee Q ett ens S ee Mi en 7 j A A Z A Bu P V A N Pt Zi O J PBug MORA gt TPV1 Ph C IANN n 0 TPVO n Ch nei TPV1 n 2 TPV2 ia Ne MAN A rna QO Ka A mN ANA BusR As s _ PBu
97. ic programming that controlled the old electronic components that were replaced in the latest upgrade of this system Instrumentation of a Transient Absorption System for Thin Films and Solids Here we describe the instrumentation and optimization of a nanosecond transient absorption system for thin films There are some considerations for optical instrumentation for films compared to solution samples Solutions placed into a standard cuvette can easily be prepared to a set concentration and the sample will be the same size 1 cm pathlength and shape making placement of the sample into the instrument very straightforward Thin films may require more preparation time than solutions and for many analyses are more sensitive to decomposition under pulsed laser irradiation due to the fact that molecules in solid films cannot dissipate heat as effectively as molecules in solutions Typically films also produce weaker signals with an increased signal to noise ratio due to the shorter pathlength nm um thickness Nonetheless many real world applications involve the incorporation of chromophores into solid state materials so measuring the properties of these compounds in thin films or polymer monoliths is particularly important A typical TA instrumentation setup for solution measurements involves an excitation path perpendicular to that of the probe beam and the detector in order to minimize laser reflection and scattered light into the detector For a
98. ic voltammetry The aggregation of the polymer by interaction with Hg3 was confirmed by XRD and TEM analysis With increasing Hg3 concentration XRD analysis shows efficient packing of the polymer side chains and the TEM analysis demonstrates the templated ordering of the polymer backbone by the appearance of worm rod shapes Taken together with the NMR results which show shielding of the aromatic protons with Hg3 this ordering of the polymer is likely due to an intercalation of Hg3 between the aryl regions of adjacent polymer chains Another conjugated polymer P3HT was examined for its interaction with Hg3 Aggregation of P3HT occurred in chloroform with increasing amounts of Hg3 added observed by changes in the absorption and emission spectra Fluorescence quenching suggested enhanced intersystem crossing however no transient absorption in solution or in film was observed P3HT naturally forms well ordered films so it s not surprising that Hg3 had little observed effect on the photophysical properties 128 From these results it can be concluded that Hg3 may enhance ordering of amorphous conjugated polymer chains without the need for complicated heat treatments While the P3HT polymer showed little benefit from addition of Hg3 the MEH PPV Hg3 system showed interesting properties While there is no evidence to support that Hg3 sensitizes the triplet excited state of the polymer it is proven that the addition of Hg3 leads to efficient pa
99. ience 1998 3 166 173 176 183 184 185 186 187 188 189 190 191 192 Decher G Eckle M Schmitt J Struth B Current Opinion in Colloid amp Interface Science 1998 3 32 39 Winkler B Dai L Mau A W H J Mater Sci Lett 1999 18 1539 1541 Cione A P P Gessner F Neumann M G J Colloid Interface Sci 2001 240 539 543 Conjugated Polymers Bredas J L Silbey R Ed Kluwer Dordrecht 1991 Ray S S Okamoto M Prog Polym Sci 2003 28 1539 1641 Cornil J Beljonne D Calbert J P Bredas J L Adv Mater 2001 13 1053 1067 Tang Y Zhou Z Ogawa K Lopez G P Schanze K S Whitten D G Langmuir 2009 25 21 25 Tozoni J R Guimaraes F E G Atvars T D Z Nowacki B Marlleta A Akcelrud L Bonagamba T J Eur Polym J 2011 47 2259 2265 Eguchi M Takagi S Tachibana H Inoue H J Phys Chem Solids 2004 65 403 407 Takagi K Shichi T In Solid State and Surface Photochemistry Ramamurthy V Schanze K S Ed Marcel Dekker New York 2000 Vol 5 p 31 177 BIOGRAPHICAL SKETCH Randi Sue Price grew up in LaPorte Indiana with her parents Pamela and Dennis Price In 2006 she graduated with her B S in chemistry from Purdue University in West Lafayette Indiana After graduation she worked as an analytical chemist at the Office of Indiana State Chemist in West Lafayette Indiana for two yea
100. ight Oligomer SSA film emission spectra are reported in Fig 5 6 The emission spectra of the oligomer SSA films are very similar to the solution measurements with the highest concentration of clay There is only a slight red shift of the emission maxima compared to the solution measurements revealing that the excited state of the oligomer after incorporation into the SSA is very similar to that present in water solution 140 EO OPE C2 1 0 es EO OPE Th 2 08 bd I V E 0 6 d 7 E P 0 4 KM W i I SN z S 02 K 400 450 500 550 600 650 Wavelength nm Figure 5 6 Emission spectra of oligomer SSA hybrid films Emission scans were performed using a front face configuration with excitation at 350 nm There is a short wavelength shoulder on the EO OPE Th fluorescence spectrum may indicate the presence of unbound oligomer on the film Since the films were rinsed copiously with water this is unlikely Most likely due to the non linear structure of EO OPE Th there is less uniform packing of the oligomer into the interlayer Possibly some oligomers are absorbed to the clay at only one of the two cationic side groups causing a free oligomer like band in the emission spectrum of EO OPE Th The transient absorption spectra of the oligomer SSA films are shown in Figure 5 7 Both films display triplet triplet transient absorption between 500 800 nm with excitation at 355 nm nanosecon
101. ilar to 139 previously reported fluorescence lifetimes in methanol At 470 nm in the 25 CEC oligomer SSA solutions the fluorescence lifetimes become more complex reflecting an increasingly heterogeneous environment Fitting the lifetimes to a triexponential the average lifetime at 470 nm increases to Tave 1 4 ns for EO OPE C2 but decreases for EO OPE Th to Tave 0 034 ns The increased lifetime in the EO OPE C2 oligomer indicates a reduction of the nonradiative decay processes such as vibrational relaxation due to restricted movement in the rigid clay substrate The EO OPE Th oligomer has a shorter fluorescence lifetime than EO OPE C2 because of spin orbit coupling induced by the heavy sulfur atom When the EO OPE Th oligomer is incorporated into the clay interspace molecular movement is restricted A decreased fluorescence lifetime is observed because this enhances the conjugation along the backbone leading to increased intersystem crossing by spin orbit coupling Solid State Measurements Films of the oligomer clay hybrids were prepared by the filter paper transfer method Oligomer SSA solutions were prepared in water with an excess of oligomer to ensure saturation of the clay The solutions were filtered and rinsed with copious amounts of water to remove any free oligomer The wet filter paper was then pressed firmly onto clean glass slides and peeled gently to leave behind the oligomer clay hybrid film Films were dried in an oven overn
102. ince then for triplet photovoltaic devices incorporating heavy metals such as PI 114115 Ir Ill 17 Ru tl 117 and others In 2009 a group from University of Missouri reported a ladder type poly para phenylene polymer LPPP containing trace amounts of Pd impurity less than 100 ppm PhLPPP that was used to study the effect of triplet excitons on power conversion efficiency in photovoltaic cells They compared LPPP PCBM photovoltaic cells 100 which contain the Pd impurity to a MeLPPP which does not contain any heavy metal impurity Remarkably the cells made with PhLPPP PCBM containing the heavy metal impurity had an external quantum efficiency EQE higher by a factor of 8 than that for the metal free MeLPPP PCBM in the blue region of the spectrum Their work introduced the possibility of enhancing efficiency using the triplet state without complicated synthetic routes to introduce heavy metals into the backbone of the polymer itself These studies demonstrate that the triplet states of conjugated polymers have interesting implications for the performance of photovoltaic devices The next step is to devise the best methods of introducing triplet excited states in conjugated materials The results presented here involve the mixing of conjugated polymers with a planar mercury containing complex in solution and films in order to investigate the effects on the photophysics by the presence of a heavy metal mercury It is postulated tha
103. ing slit width excitation wavelength number of averages etc They should absorb in the same region such that 30 the same excitation wavelength can be used and it is best if they share a similar range of emission wavelengths vi p Be _ amp Gradient B 319 2 RK 1 00 Q N Integrated Fluorescence Intensity arbitrary units 0 000 0 002 0 004 0 006 0 008 0 010 0 012 0 014 0 016 Absorbance Figure 1 3 Absorbance vs integrated fluorescence intensities for two samples measured at 5 different concentrations for quantum yield determinatton 27 The gradients or slopes from this plot for each sample can be used with Equation 1 11 to calculate the fluorescence quantum yield Time resolved fluorescence allows measurement of the emitting singlet excited state lifetime Si and can be acquired using the Time Correlated Single Photon Counting TCSPC method The fluorescence of a sample is monitored at a wavelength as a function of time after photoexcitation TCSPC works by detecting emission from individual molecules in a sample and totaling the outcomes resulting in an intensity vs time plot that represents the exponential decay of the process The excitation pulse is split into a reference beam and a sample beam The reference beam travels directly to the detector PMT while the sample beam excites the sample When the reference beam is detected by the PMT it starts an internal clock The sample beam excites the sampl
104. ion which gives good agreement with experimental results for AFX chromophores 27 However both methods involve challenging calculations and require accurate determination of experimental values such as lifetimes and quantum yields Assumptions about the laser beam profile both spatially and temporally are also necessary to calculate the intensity S Os Aw S ag O A2hw So Figure 1 7 Five level energy diagram a and the three photon absorption model b used by Sutherland et al to describe the nonlinear absorption of AFX chromophores 2 39 An easier way to quantify the nonlinear absorption experiment is to define an effective nonlinear absorption coefficient which describes the overall nonlinear absorption without separating 2PA and ESA Equations 1 13 to 1 17 describe a purely 2PA process A parallel equation can be realized to relate the transmittance to an effective nonlinear absorption coefficient Bac IAL 1 TU A Ia 1 beff AAL 1 22 To calculate lo it is necessary to know the parameters of the excitation laser beam The peak power of the laser pulse can be calculated from the energy per pulse and the pulse duration For Gaussian shaped pulses the constant 0 94 is used 7 P 0 94 2 1 23 p SCH In the case of a Gaussian beam as is depicted in Figure 1 8 the beam is radially symmetrical The beam waist or radius wo is the radius at which the intensity decreases to 1 e of its peak
105. ion when working with the laser 156 Components A Continuum Surelite I 10 Nd YAG laser B Second and Third Harmonic Generator C Newport 250 W QTH Lamp D Slow Shutter E Fast Shutter F Cornerstone 130 Monochromator G Hamamatsu R928 PMT with in house modified base H Thorlabs APD 110A Optics 1 2 1 prisms 3 4 10 1 mirrors 5 10 cm focal length plano convex 2 lens 6 10 cm focal length plano convex 2 lens 7 10 cm focal length plano convex 2 lens 8 10 cm focal length concave mirror 9 5 cm focal length plano convex 2 lens 10 5 cm focal length concave mirror System Startup Routine _ Turn on the BNC 575 Pulse Generator 2 Make sure the laser Q Switch and Flashlamp BNC cables from the pulse generator are connected via a T connector to the Film TA Q Switch and Film TA Flashlamp BNC cables from the laser control box 3 Turn on the laser a On the laser control box turn the key a quarter turn counterclockwise and allow the display to cycle through several groups of numbers b After the Laser On indicator lights up and the cooling pump is starts running push the Start Stop button and then the Shutter button You will hear the shutter click open and the indicator lights on both buttons will be lit 4 Turn on the Tektronix 3032B Oscilloscope The oscilloscope will run a self check When it is finished press any key to clear the screen 5 Turn on the high voltage sou
106. iplet molar extinction coefficients eT for the ue DEER 98 Table 4 1 Average fluorescence lifetimes tave for MEH PPV in benzene upon addition of increasing amounts of AB oissscsecvciiccsscserscerssssasccvcesadernccextiedanniense 108 Table 4 2 Exponential fits to the transient decays at 830 nm and 950 nm for MEH PPV and Hg solutions in argon purged and oxygen saturated benzene 115 LIST OF FIGURES Figure page Figure 1 1 Jablonski diagram of intramolecular photophysical pathways 18 Figure 1 2 Energy diagrams of a donor and acceptor demonstrating FRET 22 Figure 1 3 Absorbance vs integrated fluorescence intensities for two samples for guamum EIERE sessirnir E mammenennniaied 31 Figure 1 4 Laser flash photolysis system first reported by Lindqvist 0 33 Figure 1 5 Schematic of an open aperture Z scan System ccccceceeeeeceeeeeeeeteeeeeeees 35 Figure 1 6 Nonlinear absorption of samples with increasing NLA measured by the open aperture Z scan apparatus ageet wrianeracerenlasdpdalateidenea etree 36 Figure 1 7 Five level energy diagram and the three photon absorption model used by Sutherland et al to describe nonlinear abeortton 39 Figure 1 8 Properties of a Gaussian E 40 Figure 2 1 Transient absorption system for TINS cseceesesrnnscccsicerereccreres cetereesvsanennenss 48 Figure 2 2 Screenshot of the TA vi program een 49 Figure 2 3 Screenshot of the A
107. ith increasing conjugation of the OPV unit which supports the trend seen in the solution measurements The TPVO monolith has more than double the thickness of the other monoliths so it displays a surprisingly strong z scan response However the 78 effect of monolith thickness vs concentration on the nonlinear absorption property measured by the open aperture z scan method cannot be elucidated from these results Quantitative Analysis of Nonlinear Absorption in Solution and Monolith In Chapter 1 the calculation of an effective nonlinear absorption coefficient Ber was introduced A Ber can be calculated for each solution and monolith sample by using equation 1 22 and using the transmittance of the sample at z 0 in the open aperture z scan data shown in Figures 3 5 and 3 9 The value beris a macroscopic quantity that depends on the concentration of chromophore in the material Because the chromophores in solution and in PMMA monolith do not have the same concentration their Ber values cannot be directly compared Instead an effective molecular nonlinear absorption cross section oz can also be defined to compare the nonlinear absorption between materials with different chromophore concentrations 1 Bere S 2 N 4d x1073 E The constant Na is Avogadro s number and do is the molar concentration of absorbing molecules in M The units of o2 are cm4 GW The effective nonlinear absorption coefficients and cross sections for the lin
108. ition of Hg3 the fluorescence lifetime decreased as shown in Table 4 1 however because the lifetime of pure MEH PPV is close to the instrument response function IRF accurate 107 lifetime quenching data could not be obtained Low temperature emission 80 Kelvin measurements were attempted to observe phosphorescence from MEH PPV upon addition of the Hg3 but the experiments were unsuccessful due to instrumental constraints Table 4 1 Average fluorescence lifetimes tave for MEH PPV in benzene upon addition of increasing amounts of Hg3 using narrow band filters 10 nm Concentration Fluorescence Fluorescence Fluorescence Hg3 uM Lifetime Tave Lifetime Tave Lifetime Tave 520 nm 600 nm 670 nm 0 uM added 270 ps 280 ps 266 ps 15 uM added 246 ps 172 ps 171 ps 30 uM added lt IRF lt IRF lt IRF Nanosecond transient absorption spectroscopy was used to observe the excited state dynamics of the polymer in the presence of Hg3 Solutions of MEH PPV in benzene were prepared to an optical density at Aexc 532 nm of 0 61 from a stock solution 3 mg mL in benzene Hg3 crystals were added directly to the solutions Solutions were stirred for 2 3 hours until the color change was complete and all Hg3 was dissolved Three concentration regimes were examined for TA analysis as shown in the absorption spectra in Figure 4 6 1 pure MEH PPV with no Hg3 added 2 low Hg3 concentration 40 uM such that the aggregation peak in the
109. ive detection of as low as 10 AA was achieved which is over 10x the sensitivity of previously available instrumentation for solution measurements In Chapter 3 the photophysical properties and nonlinear absorption of series of p phenylene vinylene platinum acetylide complexes are investigated The photophysical properties of the series are investigated in THF The triplet molar extinction coefficients of a series of p phenylene vinylene platinum acetylide complexes are measured with transient absorption by relative actinometry against benzophenone as the actinometer The complexes are incorporated into PMMA monoliths with high transparency stability and optical quality for optical power limiting application Properties of the PMMA monoliths are studied and the photostability of the complexes in the polymer glasses is discussed Triplet triplet annihilation in conjugated co polymers containing the phenylene and p phenylene vinylene platinum acetylide units is 43 observed and compared against representative monomer complexes using transient absorption spectroscopy as a function of increasing laser fluence of the excitation pulse The nonlinear absorption of the complexes in solution and in monolith is measured with open aperture z scan and the effective nonlinear absorption coefficient Ber is calculated Chapter 4 details the interaction between conjugated polymer MEH PPV and a trimeric perfluoro p phenylene mercury complex Hg3 in s
110. ive nonlinear absorption coefficient and cross section will be used for other studies and its usefulness in comparing the nonlinear absorption of chromophores under various conditions will be realized Triplet Triplet Annihilation Studies of Platinum Acetylide Polymers When a high concentration of triplet excitons is produced and the diffusion of triplet excitons along a polymer chain is very efficient triplet triplet annihilation may occur Triplet triplet annihilation TTA is the deactivation of two triplet excitons into an excited singlet state and a ground state as shown in Equation 3 2 The observation of delayed fluorescence can often be observed from Gi 3T EE 3 2 The triplet state plays an important role in platinum acetylide photophysics due to the high yield of intersystem crossing caused by the presence of the heavy platinum atom Thus the study of triplet dynamics in these systems is very important Triplet triplet 81 annihilation effectively places a limit on the concentration of triplets that can be present in a conjugated system In the study presented here the effect of triplet triplet annihilation in conjugated platinum acetylide polymers versus their model molecular complexes is examined by nanosecond transient absorption spectroscopy The model complexes crossTPV1 and crossTPV3 contain two cross conjugated axes where two platinum acetylides are attached to a center phenyl unit of oligo phenylene vinylene OPV with eit
111. length nm Figure 4 7 Nanosecond transient absorption spectra for MEH PPV with three concentrations of Hg3 added 0 uM 40 uM and 100 uM with Aexc 532 nm 2 6 mJ pulse A Transient absorption spectra at time 0 ns immediately after laser pulse under deoxygenated benzene solutions B Transient absorption spectra at time 0 ns in oxygen saturated benzene C Transient absorption spectra at time 88 ns in oxygen saturated benzene 111 triplet triplet absorption of MEH PPV An exponential fit including the initial fast component could not be found with good results so only the longer lived decay was fit to an single exponential function with a lifetime of tra 299 ns R 0 989 This lifetime is surprisingly short compared to lifetimes reported in benzene for MEH PPV triplets created by triplet energy transfer from a donor like biphenyl which are typically on the order of 100 us 4 The best exponential fit to the weak noisy decay in the oxygen saturated solution at 830 nm shows a decreased lifetime of 30 ns R 0 455 Delta Absorbance LI Delta Absorbance Delta Absorbance Ar Purged inc 88 ns O Purged inc 88 ns 900 950 1000 Wavelength nm 950 1000 Wavelength nm 1050 1100 850 1050 1100 0 025 0 020 0 015 0 010 0 005 0 000 Lal Raw Data Exponential Fit 830 nm Ar Purged 0 025 0 020 0 015 0 010 0 005
112. let annihilation reduces the concentration of triplets after a certain threshold population is present which can affect nonlinear absorption by reducing the amount of photons absorbed by the triplet excited state These results demonstrate that discrete molecules capable of two photon absorption and strong triplet triplet excited state absorption which are incorporated into polymer monoliths are a promising route to producing solid state optical power limiting materials Triplet Sensitization of Conjugated Polymers by Hg3 The addition of trimeric perfluoro p phenylene mercury Hg3 to conjugated polymers was investigated to determine whether Hg3 could sensitize the triplet state of the polymers by spin orbit coupling which could be useful in photovoltaic applications where long lived excitons may increase efficiency The addition of Hg3 to MEH PPV had interesting effects on the photophysical properties Aggregation peaks in the absorption and emission of MEH PPV characteristic of self organization of the polymer were observed with increasing concentration of Hg3 added Quenching of the fluorescence suggested enhanced intersystem crossing to the triplet state however nanosecond transient absorption in solution did not show evidence of increased triplet state population upon addition of the Hg3 The transient absorption did show an absorption corresponding to the MEH PPV radical cation which suggested that Hg3 can act as an electron acceptor which was confir
113. linear and no nonlinear effects are observed The laser fluence is increased while keeping the same beam quality by use of a variable beamsplitter The slopes of AA vs laser fluence for the sample and the actinometer are used to calculate the triplet molar extinction coefficient by Equation 3 9 95 slopesample Iscref Eref EE 3 9 sloperefPiscsample The actinometer used was benzophenone which has a known triplet molar extinction coefficient cr san nm 7870 1200 MI cm and triplet quantum yield Disc 1 0 7879 Under the conditions used in this experiment benzophenone in benzene is linear for laser energies up to about 1 mJ pulse Figure 3 15 0 10 e e Oo oO AAbsorbance CH CH IK 00 0 0 0 5 1 0 1 5 2 0 2 5 3 0 3 5 Laser Energy mJ pulse Figure 3 15 AAbsorbance vs Laser Energy for the nanosecond transient absorption monitored at 525 nm for benzophenone in benzene ODsss5nm 0 78 To obtain iscsample the approximation that Discsample is equal to one minus the fluorescence quantum yield Br can be used The singlet oxygen quantum yield Pa was used but the singlet oxygen measurement typically has a higher degree of uncertainty due to the reactivity of the singlet oxygen produced and the sensitivity of some of the platinum acetylide complexes to singlet oxygen sensitized photodegradation At higher laser energies the AA response of the platinum acetylides 96 becomes nonlinear either due to satu
114. lues measured by NLT method are similar for the complexes in solution and in the monoliths Open aperture Z scan measurement also shows that strong nonlinear absorption is preserved Alongside the design of increasingly efficient NLA chromophores future endeavors are focused on improved methods of achieving solid state OPL materials with a focus on greater stability higher concentrations of incorporated chromophores and more effective optical power limiting The photophysical properties of copolymers consisting of alternating phenylene and cross conjugated platinum acetylide units were investigated In the transient decays of copolymers a significant decrease in TA intensity was observed which indicated that triplet triplet annihilation may be present The transient decays of the 87 copolymer had a fast component in the decay which exhibited increasing amplitude with higher laser fluence which confirmed the presence of triplet triplet annihilation The monomer complexes did not show evidence for triplet triplet annihilation Further investigations using femtosecond transient absorption are in progress with a collaboration with Dr Papanikolas and his group at the University of North Carolina at Chapel Hill Experimental Section Preparation of Monoliths The synthesis of the linear and cross conjugated complexes in Scheme 1 have been reported previously 835 PMMA monoliths were prepared by a procedure adapted from Westlund and coworkers
115. ly high 3 357 eV 28 so orbital mixing between Hg3 and the organic aromatics is highly unlikely Most importantly in these studies the Hg3 arene complexes exhibit phosphorescence from the T state of the arene due to heavy atom effect from the proximity of the mercury R F F F F Hg Hg F F Hg F F F F F Figure 4 1 Structure of trimeric perfluoro ortho phenylmercury Hg3 Gabbai and his group at Texas A amp M University have done extensive research on the supramolecular interactions between conjugated molecules and Hg3 along with 102 similar fluorinated organomercurials 2 21123 126 In 2008 they reported the supramolecular structure between Hg3 and tolane diphenylacetylene which is comprised of an alternating stacked configuration where mercury atoms interact with both the acetylenic and aromatic carbons on the tolane Figure 4 2 The tolane adopts a non planar geometry with a dihedral angle between phenyl groups of 5 9 It was mentioned that the interaction of Hg3 and tolane was heavily dependent on the presence of the solvent CH2Cl2 as upon complete evaporation of the solvent to form crystals decomposition of the supramolecular complex occurred Thus no phosphorescence of the crystalline supramolecular structure could be observed Figure 4 2 Crystal structure of Hg3 and tolane reported by Gabbai a ORTEP view 50 ellipsoids of compound 3 tolane CH2Cl2 b Space filling model of the binary stack prese
116. lymers compared to free molecules Energy transfer in polymers can be divided into three main types 1 Intramolecular energy transfer along a single polymer chain usually by hopping of the exciton between overlapped orbitals in a conjugated polymer backbone 2 Intramolecular energy transfer between two regions of the polymer when the polymer is coiled or aggregated and 3 Intermolecular energy transfer between two separate polymer chains that are in proximity due to collision or interchain aggregation In dilute solutions it is rare that a polymer chain will diffuse and collide with another polymer chain before the excited state decays so 1 and 2 are the main mechanism of energy transfer In thin films or solids where the polymer is aggregated or organized into a small volume with other polymer chains all three types of energy transfer occur Singlet excitons can diffuse in a polymer by Forster transfer whereas triplet excitons diffuse by Dexter exchange Photophysical Characterization Methods Absorption and Emission An absorption spectrum measures the light absorbed by a substance as a function of wavelength The absorption spectrum can be measured using a spectrophotometer Spectrophotometers quantitatively measure the transmission of light through a sample comparing it to a reference usually a sample of solvent The transmission is then converted to absorbance by Beer s Law in Equation 1 4 25 Spectrophotometers can either
117. m with the maximum pulse energy 10 200 uJ The maximum average pulse duration was 80 120 fs fwhm The OPA output beam was slightly expanded and then collimated with long focal length lenses f 500 1000 mm to give an average beam diameter on the sample d 0 3 1 0 mm Minimum beam diameter was chosen to eliminate spurious nonlinear effects such as self focusing which may occur in the sample and in the optical elements at high photon flux density values The pulse energy incident at the sample was changed by means of a continuously variable reflective circular attenuator Thorlabs maximum OD 2 0 Nonlinear transmittance of the sample as a function of the incident pulse energy was determined by measuring the relative pulse energy before and after the sample with two silicon photodiodes Thorlabs PDA10A The measured nonlinear transmittance at each wavelength was fitted with a linear function The 2PA cross sections at each wavelength were determined by comparing the CB NLT slope values in the sample with those obtained in 1 cm path length solution of 2PA 92 reference standards under same experimental conditions Rhodamine B solution in methanol fluorescein solution in water and bis diphenylanimostilbene solution in dichloromethane were used as reference standards Triplet Molar Extinction Determination by Relative Actinometry Triplet absorption properties are generally difficult to characterize due to the transient nature of the
118. m on the desktop In the Current Directory window open the directory where your data files are stored Note The data files must have sequentially numeric filenames In the Command window type TAS to begin the processing program be a file in the middle of the run where the signal is strong A plot of the chosen data file will pop up in a separate window Use the mouse cursor to mark the TO point after the laser pulse This is the maximum of the transient absorption signal Right click on the data point just created and select Export Cursor Data to Workspace Press OK on the box that pops up to keep the variable name the default cursor_info Close the Plot window and return to the Command window Type in y and press Enter Enter another filename and make sure the data point created still marks a TO that is correct for this data file Next to the Plot window a popup box will appear with three options Choose the appropriate option It is recommended to check at least one data file near the beginning middle and end of the run to make sure the TO point is valid After it has been verified that TO is good for all data files choose the Process Data option 10 Enter in the requested parameters a It is assumed that the signal fit nicely in the scope screen during acquisition The signal decays all the way back to the baseline 164 i Smooth 1 10 000 data points ii Smooth 2 5 000 dat
119. mL and the final weights after polymerization Weight loss due to the evaporation of MMA in the monoliths was inconsistent due to uncontrolled flow of N2 into the oven It is assumed that the monoliths are homogenous and that cutting and polishing the monoliths do not change their concentration Table 3 2 lists the final thickness and calculated concentration of each monolith Photophysical Measurements One photon photophysical properties for solutions were measured using a 1 cm pathlength quartz or borosilicate cuvette in spectroscopic grade dry THF Solutions were deoxygenated by purging with argon for phosphorescence and transient absorption measurements Ground state absorption spectra were collected on a Varian Cary 100 dual beam spectrophotometer with THF as the instrument baseline blank Corrected steady state emission measurements were collected by a Photon Technology International PTI photon counting fluorescence spectrophotometer with optically dilute solutions ODmax lt 0 10 Fluorescence quantum yields were measured at room temperature in air saturated THF using 9 10 diphenylanthracene in cyclohexane as a standard du 0 75 Nanosecond triplet triplet transient absorption was measured in a continuously circulating 1 cm pathlength borosilicate cuvette in argon purged THF solutions The third harmonic of a Continuum Surelite Il 10 Nd YAG laser at 355 nm 10 mJ pulse was used as the pump anda Perkin Elmer LS1130 3 pulsed xenon l
120. mM 129 concentration and was continuously stirred during the experiment to ensure as consistent additions as possible to the polymer solution After each addition the solution in the cuvette was stirred for at least 1 minute before data acquisition For very concentrated polymer solutions such as for films the solutions were stirred for at least 24 hours before addition of the Hg3 complex Due to the insolubility of Hg3 in most solvents Hg3 powder was weighed and directly added into the polymer solution for all TA measurements The Hg3 particles are initially colorless but when they were added to bright orange MEH PPV polymer solution the particles became reddish from aggregation with the polymer and fell to the bottom of the orange MEH PPV solution After stirring for 3 4 hours or overnight the Hg3 particles dissolved and the MEH PPV Hg38 solution changed in color from orange to red depending on the amount of Hg3 added At saturated concentrations of Hg3 some visible chunks of aggregated polymer became stick to the side of the vial Sometimes the aggregated polymer could be redissolved by sonication but the highest Hg3 concentrations always had aggregated polymer along the sides of the sample vial Solutions were wrapped with foil to protect from light when not in use Films were prepared by dropcoating from concentrated solutions and allowing the solvent to evaporate at room temperature in the dark overnight before measurement A 50
121. med by cyclic voltammetry experiments In MEH PPV films containing increasing amounts of Hg3 there was a small increase in the transient absorption intensity of the triplet exciton peak of MEH PPV and the MEH PPV radical cation absorption at 950 nm was also observed at high concentrations of Ho Additionally the structure and morphology of MEH PPV in films 153 upon addition of Hg3 was observed using H NMR powder XRD and TEM It was found that the side chains of MEH PPV become efficiently packed when Hg3 is added and worm rod shapes observed in the TEM point to templated aggregation of the polymer backbone by Hg3 similar to the ordering that is seen when MEH PPYV films are annealed after casting This may be due to intercalation of the Hg3 between the aryl groups of adjacent polymer chains since the H NMR shows significant shielding of the aromatic protons of the polymer once Hg3 is added The interaction of another conjugated polymer P3HT with Hg3 was also investigated The absorption and emission spectra in solution displayed an increasing aggregation peak with increasing concentration of Hg3 however since P3HT inherently produces crystalline films there was little effect on the photophysical properties of PSHT films when Hg3 was added From these results it is concluded that Hg3 may benefit photovoltaic applications of amorphous conjugated polymers such as MEH PPV by improving the morphology of films without the need for additional t
122. molecules oligomers and polymers in the solid state and compare these photophysical properties to those found in solution To study the triplet excited state dynamics of films a new nanosecond transient absorption instrumentation was developed This instrument can detect transient absorption with AA as low as of 10 Using this newly available instrumentation it is possible to measure the triplet excited state dynamics so as to gain insight into several solid state applications of photoactive molecules oligomers and polymers A series of p phenylene vinylene platinum acetylide complexes are studied for optical power limiting applications and they are incorporated into poly methy methacrylate PMMA monoliths The photophysical properties of the monoliths and Triplet triplet annihilation in conjugated co polymers containing phenylene and p phenylene vinylene platinum acetylide units is observed and compared against representative monomer complexes using transient absorption spectroscopy as a function of increasing laser fluence of the excitation pulse The interaction between conjugated polymer MEH PPV and a trimeric perfluoro ortho phenylene mercury complex Hg is investigated in solution and in films Finally cationic oligomers are intercalated into a clay host and the photophysical properties of the clay oligomer hybrid are investigated in disperse solutions and in films CHAPTER 1 INTRODUCTION The Interaction of Light with M
123. mount of the oligomer adsorbed was at 37 7 vs CEC for EO OPE C2 SSA and 33 5 vs CEC for EO OPE Th SSA These findings are consistent with the observed precipitation during the fluorescence titration in water Experimental Section Materials Sumectin SA was a gift from Dr Haruo Inoue from Tokyo Metropolitan University and was purchased from Kunimine Industries Co Ltd The oligomers EO OPE C2 and EO OPE Th were synthesized in the Schanze group by Anand Parthasarathy and Subhadip Goswami Water was purified using a Millipore system to a resistivity of 18 0 megaohms Preparation of Films Films were prepared by the filter paper transfer method A solution of clay saturated with oligomer was prepared in ultrapure water and filtered through filter paper washing at least 3 times with copious amounts of water to eliminate any excess oligomer The wet filter paper was pressed firmly onto a clean glass slide and slowly peeled away leaving behind a clay oligomer hybrid film Photophysical Measurements Photophysical properties for solutions were measured in ultrapure water using a 1 cm pathlength quartz or borosilicate cuvette Ground state absorption and emission experiments were performed by undergraduate research student Eduardo Acosta Ground state absorption spectra were collected on a Varian Cary 100 dual beam spectrophotometer with water as the instrument baseline blank Corrected steady state emission measurements were collected by a Photo
124. n Technology International PTI photon counting fluorescence spectrophotometer Fluorescence lifetimes were obtained by time correlated single photon counting technique TCSPC with a PicoQuant FluoTime 100 compact fluorescence lifetime 148 spectrophotometer A UV pulsed diode laser provided excitation at 375 nm power lt 10 mW The laser was pulsed by a PDL800 B pulsed diode laser driver Fluorescence decays were obtained using 10 nm bandpass interference and analyzed from the fluorescence decays using exponential fitting parameters FluoFit software The emission lifetimes of the pure oligomers in water were fit to biexponential decays 0 84 ns 20 5 and 0 46 ns 79 5 for EO OPE C2 and 1 1 ns 3 2 and 0 19 ns 96 8 for EO OPE Th at 410 nm Upon addition of SSA to an approximate CEC of 25 at 410 nm the EO OPE C2 SSA solution lifetime was fit to a triexponential decay 7 9 ns 2 1 2 4 ns 12 8 and 0 48 ns 85 1 and for EO OPE Th SSA solution triexponential fit is 3 3 ns 0 8 0 96 ns 8 9 and 0 20 ns 90 3 The lifetime was also measured for both oligomers at 470 nm where the new emission peak arises after addition of the SSA In the 25 CEC EO OPE C2 SSA solution the lifetime components were the same but the long lifetime components have a larger contribution to the overall lifetime than in the emission at 410 nm 7 6 ns 7 0 2 3 ns 30 7 0 34 ns 62 3 However in the 25 CEC EO OPE Th SSA solution measured a
125. n in argon purged was observed with a non reversible reduction peak around 1 4 V vs Ag AgCl The CV shown in Figure 4 11 is the first scan of a freshly prepared solution The reduced Hg3 complex was not stable and a shift of the peak to higher potential and eventual disappearance of the peak with precipitation occurred after subsequent CV measurements Ferrocene Standard 0 41 0 01 1 40 V K _ 0 00 lt x E z 0 01 5 O 0 02 0 03 1 97 V7 2 1 0 1 2 Potential V vs Ag AgCl Figure 4 11 Cyclic voltammetric response from a 3 mM solution of Hg3 at a scan rate of 100 mV s in dichloromethane at platinum working electrode supporting electrolyte 0 1 M TBAH Ferrocene was used as a standard 116 Film Photophysical Studies Films were prepared with varying amounts of Hg3 added to examine the photophysics in the solid state Solutions were prepared in o dichlorobenzene or benzene and drop coated onto clean glass slides with similar optical properties between films made with different solvents As shown in Figure 4 12 pure MEH PPV films prepared in o dichlorobenzene solutions are a bright orange color but the color changes from bright orange to deep red with increasing amounts of Hg3 as the solvent is evaporated to match the visible color change in benzene solution This illustrates that the aggregation between Ho and MEH PPV forced by the evaporation of solvent does not require the presence of benzene to form agg
126. n Horizontal Scale 14 2mv J Vertical Scale ACQUIRE Program Start 1 On the computer double click the TA vi program on the desktop 2 Click the white arrow to run the VI program continuously The arrow will turn black when the program is running 3 Click on the Initialize button at the top right hand corner This opens communication to all the components and inputs the correct settings for the TA experiment The indicator box below will indicate when the initialization is complete The laser flashlamps will begin to fire 158 Acquisition Setup Optical Alignment 1 Make sure that Prism 1 marked P1 on the optical table is directed toward the Film TA system If necessary move the prism to the post closest to the laser output port and angled towards the Film TA 2 Insert sample into the sample holder using a slitted business card 3 Toggle on the Shutters switch and click Update Settings a Make sure the sample slit is in the center of the lamp light beam If necessary adjust the vertical position of the sample holder or the horizontal position of the sample b Do not adjust the sample holder mount position or any of the surrounding optics 4 Toggle off the Shutters switch and toggle on the Laser Flashlamp and Q Switch switches Make sure the Q Switch delay is 380 us or higher and click Update Settings a Follow the laser beam path from the laser outpu
127. nt in 3 tolane CH2Cl2 H atoms and solvate CH2Cle molecules omitted Conjugated Polymers and Hg3 MEH PPV MEH PPV poly 2 methoxy 5 2 ethylhexyloxy 1 4 phenylene vinylene is one of the most well studied electroluminescent polymers It and other poly phenylene vinylene PPV analogs are commonly used in LED applications due to their excellent 103 luminescent properties 94 129 130 These polymers have also been studied for use in photovoltaic devices with an acceptor like fullerene for example due to its charge transport properties and high absorption coefficient in the visible region of the spectrum 131 133 MEH PPV is an amorphous polymer with a glass transition temperature of Tg 65 C 134135 The structure of MEH PPV is shown in Figure 4 3 below O Figure 4 3 Structure of MEH PPV Solution Photophysical Studies The interaction between MEH PPV and the trimeric mercury complex Hg3 was found to be strongly solvent dependent as is shown in Figure 4 4 In a nonpolar aromatic solvent such as benzene the addition of Hg3 to MEH PPV causes a visible color change of the solution after stirring for several minutes however in a polar chlorinated solvent such as o dichlorobenzene or chloroform the addition of Hg3 complex to the polymer MEH PPV has no effect on the absorption spectrum indicating no interaction between the Hg3 and the polymer The solvent dependency of this interaction indicates that the interaction may be for
128. ntrations of clay added there was a more structured emission indicating ordered aggregation of the oligomers onto the clay The fluorescence of the oligomers with increasing amounts of clay is quenched and a very broad structureless band appears Precipitation occurred at concentrations of clay corresponding to 25 vs CEC for EO OPE C2 and 22 vs CEC for EO OPE Th placing a lower limit on the amount of oligomer adsorbed onto the clay The fluorescence lifetimes in solution of the oligomers in concentrated clay solutions became more complex The fluorescence lifetime of the EO OPE C2 SSA solution increased compared to free EO OPE C2 in water from Tave 0 54 ns to 1 4 ns Conversely for the EO OPE Th SSA solution the fluorescence lifetime increased from Tave 0 29 ns measured of the pure EO OPE Th solution to Tave 0 034 ns Both changes in lifetime can be explained by the more restrictive environment of the rigid clay substrate The molecular geometry of the oligomers in the clay interlayer is more confined and reduces the rate of internal conversion by vibrational relaxation thus 146 increasing the lifetime of the radiative process In EO OPE Th a heavy sulfur atom is present A more restrictive geometry can improve orbital overlap and enhance intersystem crossing by increased spin orbital coupling leading to a decreased fluorescence lifetime In oligomer SSA film measurements the emission spectra is only slightly red shifted compare
129. number is inputted the scope will default to the closest option b Consider how sensitive the sample is to the laser pulse and how weak the signal might be If sample damage is a concern a lower number of averages potentially allows more acquisitions to be taken before a sample is damaged but the baseline will be more noisy c 128 averages is a good choice for most samples 2 Click Acquire and the Acquire vi window will pop up 3 On the top left of the screen enter in the Wavelength Start Wavelength End and the Wavelength Step Click Update Scan Settings a If only one acquisition is needed type the same wavelength into the starting and ending wavelength boxes and a 1 in the step wavelength box b On the right hand top of the screen the x of y indicator should update with the correct number of acquisitions 4 In the center of the screen type in a starting number filename for saving the data in the Start at Temp File box a Data files will be saved to a Temp Data Files folder on the desktop b If the number in the Start at Temp File box is 0 the first wavelength acquisition will be saved to file 0 the second to file 1 the third to file 2 etc Data files saved to the same Temp File will be overwritten d If for any reason a run needs to be restarted make sure the data temp file numbers remain continuous This is important for the data processing program
130. nuum Surelite Il 10 Nd YAG laser The laser beam was split with a 50 50 beam splitter to two pyroelectric detectors which measured the transmitted pulse energy through the sample as a function of the input pulse energy using an Ophir Laserstar dual channel optical laser energy meter The beam was focused with a 25 4 mm diameter 50 8 mm focal length concave lens A ThorLabs motorized translation stage Z825B and TDC001 allowed millimeter movement along the z axis Solution samples were prepared with dry THF to a concentration of 1 mM and put into a 1 mm pathlength quartz cuvette for analysis A sample of Pt DPAF was used as a reference standard for 91 comparison between different runs Monolith samples cut to approximately 1 mm thickness were placed between two glass slides with a few drops of refractive index matching fluid on each side of the monolith Newport F IMF 105 1 52 at 589 nm The femtosecond 2PA measurements were performed using collimated beam nonlinear transmission CB NLT technique by Dr Aleks Rebane and his group at the Montana State University A detailed description of the laser system is described elsewhere Briefly the laser system comprised of a Ti Sapphire femtosecond oscillator Lighthouse Inc femtosecond regenerative amplifier Legend H Coherent Inc and an optical parametric amplifier OPA TOPAS C Light Conversion The second harmonic of the signal output of the OPA was continuously tunable from 540 to 810 n
131. o crossTPV1 2 4 6 8 10 12 14 Laser Fluence mJ cm AA 0 25 gial 0 15 0 10 0 05 0 00 0 WM crossTPV3 co crossTPV3 4 6 8 10 12 14 Laser Fluence mJ cm Figure 3 13 Nanosecond transient absorption intensity AA at TAmax vs laser fluence for the copolymers and model complexes in THF prepared to an OD at 355 nm of 0 78 Aexc 355 nm 128 averages detection at AtAmax Additional experiments were performed on crossTPV1 and co crossTPV1 P to monitor the triplet decay kinetics Transient decays collected at short timescales 0 200 85 ns at A TAmax at increasing laser fluence are shown in Figure 3 14 CrossTPV1 shows no fast decay in the timescale monitored while a very fast initial decay in co crossTPV1 P appears at higher laser fluences At low laser fluence lt 0 8 mJ cm no fast decay is observed in the copolymer sample The amplitude of the fast decay increases with increasing laser fluence which is proportional to the number of triplets present This supports the presence of triplet triplet annihilation A B A 17 mJ cm 53 mJ cm E 12 mJ cm 33 mem 8 0 mem ee F 3 H 2 N 2 Ki 16 mJ cm T 5 0 mJ cm E E O a Z Z E 8 0 mJ cm a 0 50 100 150 200 Time ns Time ns Figure 3 14 Transient decays of crossTPV1 A and co crossTPV1 P B in THF at TAmax with increasing laser fluence Aexc 355 nm 128 averages solution
132. obtained by Gyu Leem of A films of MEH PPV with increasing wt of Hg3 added and B Hg3 powder To examine the morphology of MEH PPV films containing Hg3 at various concentration conventional bright field transmission electron microscopy TEM was used The images shown in Figure 4 18 demonstrates the remarkable effect on the morphology of MEH PPV in films by the presence of Hg3 The pure MEH PPV film at room temperature gives a featureless TEM image due to the amorphous nature of the polymer From the 5 wt Hg film some aggregation of the polymer is visible as worm rod shapes on the TEM image As the concentration of Hg3 increases to 50 122 wt the worm rod shapes become interconnected and the TEM image becomes strikingly similar to the image of MEH PPV after heat treatment reported by Chen in 2004 149 Taken together with the XRD results these TEM studies show that the increasing the amount of Hg3 from 0 to 50 wt led to increased aggregation and higher ordered structures of MEH PPV containing Hg3 Figure 4 18 TEM images of MEH PPV films acquired by Gyu Leem scale bar is 1 um Various concentrations of Hg3 were added A 0 wt B 5 wt and C 50 wt D For comparison a TEM image of MEH PPV after annealing treatment reported by Chen in Langmuir 2004 149 P3HT The interaction between P3HT and Hg3 was also investigated P3HT Poly 3 hexylthiophene 2 5 diyl shown in Figure 4 19 Organic solar cells comprising of PSHT
133. olution and in films The addition of Hg3 to MEH PPV templates aggregation of the polymer leading to interesting effects on the photophysical properties Nanosecond transient absorption is employed to study the excited state dynamics of the polymer Additionally the changes in structure and morphology of MEH PPYV in films upon addition of Hg3 are observed using 1H NMR powder XRD and TEM The interaction of Hg3 with another conjugated polymer P3HT is briefly discussed Chapter 5 reports the intercalation of cationic oligomers into a synthetic saponite clay matrix and the study of the photophysical properties of clay oligomer hybrid in dispersed solutions and in films is reported The absorption and emission measurements reveal ordering of the oligomers once they are incorporated into the clay An increase in interlayer spacing of the clay observed by XRD confirms the intercalation of the oligomer Thermogravimetric analysis and fluorescence titration studies are performed to determine the quantity of oligomer integrated into the clay 44 CHAPTER 2 TRANSIENT ABSORPTION INSTRUMENTATION FOR THIN FILMS Background Transient absorption can be measured using two modes of detection The first acquires a whole transient absorption spectrum at a given time t with very precise wavelength detection To get decay information many spectra need to be collected at varied t and then imported into a spectral kinetics program such as SpecFit An alternative
134. on of a species at varied concentrations Rearranging the Beer Lambert law shown in Equation 1 3 the slope of absorbance vs concentration will be the molar absorptivity value _A 2 Saal 1 9 26 where A is the absorbance measured by the spectrophotometer C is the concentration of the solution in M and is the pathlength of the cuvette in cm usually 1 The experimental absorption intensity provides a measure of the theoretical quantity of oscillator strength These values are related by the following equation f 432x107 fre dp 1 10 where Vi and Vz are the limits of the transition measurement in wave numbers cm and the molar extinction values are in MI em Thus the integrated absorption band is the transition probability Values of fare normalized so that a highly probable transition has an fvalue of close to 1 whereas a forbidden transition will have a value approaching zero There are some experimental considerations to be aware of when measuring absorption spectra 2 The environment around the absorbing species can play a large role in the observed absorption spectrum The absorption spectrum of a gas at low pressure will have sharp lines with rotational and vibrational fine structure If absorbing molecules are allowed to interact with each other such as in high pressure gas or with a solvent the band width of the absorption bands will become more diffuse as intermolecular interactions cause small changes in the elec
135. on processing in Matlab or other plotting programs Typically a smooth factor of 10 is used which decreases the number of data points to 1 000 The acquisitions for the wavelength series indicated by the user are saved to temporary files with consecutive numeric filenames When the acquisition program is run the monochromator is set to the starting wavelength At that wavelength Izero is calculated The oscilloscope is set to the horizontal scale the user defined in the TA vi program for transient absorption data acquisition and the vertical scale defined by the user for the Izero computation The shutters and oscilloscope trigger are activated and sixteen waveforms are averaged on the scope The Q switch is not enabled for Izero calculation The resulting waveform is transmitted to the computer where the software takes the mean value of the 10 000 data points The waveform data sent from the scope is in mV vs data point 1 to 10 000 The time parameter is not important for the Izero computation 50 Figure 2 3 Screenshot of the Acquire vi program For the TA data acquisition at each defined wavelength the scope offset is set to the Izero computed by the software The oscilloscope is set to the horizontal and vertical scales the user defined in the TA vi program for data acquisition The shutters 51 oscilloscope laser flashlamps and laser Q Switch
136. ount of oligomer absorbed onto the clay measured by TGA was 37 7 vs CEC for EO OPE C2 SSA and 33 5 vs CEC for EO OPE Th SSA 155 APPENDIX A USER MANUAL FOR THE TRANSIENT ABSORPTION INSTRUMENT FOR THIN FILMS TA System for Films Randi Price Installed June 2010 Latest revision July 2013 This is the instruction manual for a transient absorption system for films installed 2010 These instructions are intended for users already aware of the safety precautions and operating limitations of all the components Please check with Dr Schanze or the TA Lab Manager if you are unsure how to operate any part of this system For more detailed explanations of each component please refer to the manufacturer s original booklet For a detailed explanation of experimental settings and calculations refer to the additional information included at the end of this manual Common troubleshooting problems are also included at the end of the acquisition instructions gt u A few points on laser safety 1 Never look directly into a laser beam Be aware of reflections and scattered laser light If you are unsure about where the laser path do not turn on the laser 2 Keep the beam at waist level and contained within the blackouts around the laser table Do not lean over the table putting your eye level to the laser beam 3 For alignment keep the laser power as low as possible generally with a Q Switch gt 400 us Wear proper eye protect
137. pl Phys Lett 1993 62 585 587 Yu G Gao J Hummelen J C Wudl F Heeger A J Science 1995 270 1789 1791 Liu Y Q Liu M S Li X C Jen A K Y Chem Mater 1998 10 3301 3304 Lee T W Park O O Adv Mater 2000 12 801 804 173 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 Nguyen T Q Martini LB Liu J Schwartz B J J Phys Chem B 2000 104 237 255 Koehler A Hoffmann S T Baessler H J Am Chem Soc 2012 134 11594 11601 Smilowitz L Hays A Heeger A J Wang G Bowers J E J Chem Phys 1993 98 6504 6509 Samuel D W Crystall B Rumbles G Burn P L Holmes A B Friend R H Chem Phys Lett 1993 213 472 478 Monkman A P Burrows H D Miguel M D Hamblett I Navaratnam S Chem Phys Lett 1999 307 303 309 Candeias L P Grozema F C Padmanaban G Ramakrishnan S Siebbeles L D A Warman J M J Phys Chem B 2003 107 1554 1558 Wachsmann Hogiu S Peteanu L A Liu L A Yaron D J Wildeman J J Phys Chem B 2003 107 5133 5148 Bjorklund T G Lim S H Bardeen C J J Phys Chem B 2001 105 11970 11977 Cornil J Beljonne D Heller C M Campbell LH Launch B K Smith D L Bradley D D C Mullen K Bredas J L Chem Phys Lett 1997 278 139 145 Gierschner J Mack H G Luer L O
138. r Keep the same settings for the next iteration i Yes The program will keep the horizontal and vertical scope settings the same for the next wavelength ii Select No to change settings If you select No a new screen will pop up where settings can be changed for the next wavelength 1 IMPORTANT There are default settings that appear on this screen make sure to change ALL settings to your settings 2 After all settings are correct click on Save settings and return to acquisition and the screen will close d Select End VI and this will abort the VI Any data already acquired will be in the Temp File folder on the desktop To restart the acquisition return to the TA window and click on the white arrow Click on the Acquire button to return to the Acquisition window 8 Repeat for all iterations 9 After all data has been acquired minimize the Acquisition and TA windows and transfer the temporary data files to your user folder on the server a Data is output in AA vs Time ms b NO PARAMETERS are saved in the data files li Parameters that are important wavelength start stop step Izero mV div time div of averages smoothing factor scope termination iii Make note of these acquisition parameters in notebook or text file 1 System Shutdown Routine 1 Turn off the laser a Toggle off the Flashlamp and Q Switch switches and click Update Settings b Lis
139. r is triplet triplet annihilation 0 4 crossTPV1 co crossTPV1 P 0 3 crossTPV3 co crossTPV3 P 0 2 lt 0 1 lt 0 0 600 700 0 1 Wavelength nm 0 2 Figure 3 12 Nanosecond transient absorption spectra for the model complexes red and the copolymers blue in THF solution prepared to OD at 355 nm of 0 57 with exc 355 nm 5 mJ pulse initial camera delay 50 ns 100 averages 84 To confirm the presence of triplet triplet annihilation in the copolymer solutions the intensity of the transient absorption at Amax was monitored with increasing laser fluence and is shown in Figure 3 13 The transient absorption of the copolymers saturated with lower laser fluence compared to the monomeric complexes which supports the presence of triplet triplet annihilation The model complex and copolymer should share similar triplet molar extinction values since the triplet exciton is localized on the OPV axis as seen by the matching triplet triplet absorption spectra So in the absence of other effects like triplet triplet annihilation the slope of AA versus laser fluence should be the same Most noticeably for crossTPV3 and co crossTPV3 it appears that the slopes are indeed similar at very low range 0 1 mJ cm This effect is not as obvious in crossTPV1 and the corresponding copolymer due to weaker TA intensity AA 0 25 0 20 0 157 0 10 0 05 0 00 0 RW crossTPV1 c
140. rameters of the experiment should be well characterized with a linear response curve of detected transmittance and a temporal response that is substantially shorter than the decay time of the triplet excited state Most importantly care must be taken to ensure the analyzed volume of the sample has a homogenous distribution of the triplet excited state and perfectly overlaps the volume of the excitation used to produce the excited state Ideal homogeneity of the sample and overlap of the excitation and analysis beams is difficult to manage due to many reasons however most methods for calculating the triplet extinction coefficient allow correction factors for these experimental imperfections One method commonly used is the ground state depletion method where a two species system is assumed first used by Porter and Windsor in the 1950s during their study of triplet states of aromatic complexes by flash photolysis 8888 This method assumes that the ground state is converted into the triplet state T by intersystem crossing after excitation to S1 with a 100 yield So there are no other ground state depletion pathways The ground state is repopulated by decay of the triplet state So the concentration of the triplet state can be calculated by Equation 3 5 T A G G o G 3 5 At a wavelength A1 where the ground state depletion can be monitored the transient has no absorption and within the transient s absorption band at another waveleng
141. ration or self quenching effects and so lower laser energies were used for the platinum complexes Figure 3 16 0 065 0 10 2 e 0 054 ans 3 E A lt 0 04 g 4 g 0 064 a Van 4 S 0 034 4 E S S a 8 lt a 4 B 0 04 lt JS a lt 8 2 0 02 lt ja see vA 0 01 S 0 02 e D 4 wy i E ep 8 8 sn PR Di a 0 00 0 00 200 400 600 800 0 50 100 150 200 250 300 Laser Energy uJ pulse MM Benzophenone M Benzophenone TPVO crossTPV1 A TPV1 Ph A crossTPV3 wv TPVI WY _ crossTPV1 DPAF PV a TPVv2 T2 Laser Energy uJ pulse Figure 3 16 Relative actinometry plots of AAbsorbance vs Laser Energy uJ pulse for the linear TPVn and cross conjugated crossTPVn series against benzophenone measured at A AtAmax Solutions were prepared to an OD at 355 nm of 0 78 in benzene The linear and cross conjugated series relative actinometry experiments were performed on different days The triplet molar extinction coefficients calculated for both series in Table 3 4 demonstrate the strong triplet triplet absorption present in these complexes Pulse radiolysis studies on oligomeric p phenylene vinylenes OPVs to measure the triplet state properties reported triplet molar extinction coefficients of 140 000 M cm to 190 000 MT cm increasing with the length of the OPV chain 2 to 8 oligomeric units 89 Benchmark platinum acetylide complexes which do not contain OPV units Pt BTF and 97 Pt D
142. rce for the PMT by flipping the orange switch Set the dials to 730 V 0 on the first dial 700 on the second dial 30 on the third dial and 0 on the last dial 6 Turn on the lamp by flipping the ON OFF switch on the lamp power supply and pressing the Lamp Start button The indicator should show 250 W It 157 may fluctuate for a few minutes before it stabilizes No warm up time is required 7 The default for this TA instrument is excitation with 355 nm laser and detection with the PMT detector Please contact a lab manager to make the necessary changes to the system if necessary Excitation at 532 nm and detection into the nIR is available File Edit Yiew Project Operate Tools Window Help 2 oln D aveleng Edit gt Reinitialize Values to Default f Set Wavelength KR 2 EE Then click Initialize to set computer defaults am Set Shutter Shutter rte Fon Ze mj h Shutters Laser Flashlamp Q Switch Oscilloscope Switch Delay us Channel A and E Channel B Channel C Ha Run Run Run Run Stop Stop Stop _ Stop Volts ser orrser Set IZERO in mV d D y g i y i i k 1 5E 5 Ep 2 5E 5 3 5 3565 ER ESAT Time in seconds 1o Vertical Scale mW Div 1000 mY 500 mY DUDU 2ns 10 SET IZERO My DIV Horizontal Scale per Division om 100m proms 20 mv ba mi Ave for Acquisitio
143. reatment such as annealing Photophysical Study of Cationic Oligomers in a Clay Host The intercalation of cationic oligomers into a synthetic saponite clay SSA was reported Absorption and emission measurements revealed ordered aggregation of the oligomers upon addition of the SSA A red shift in both the absorption and emission spectra occurred and a quenching of the fluorescence was observed when SSA was added to solutions of the oligomers in water Fluorescence quenching may be due to the presence of heavy atoms in the clay promoting intersystem crossing to the triplet state The nanosecond transient absorption spectra of the films show relatively strong broad and long lived transient absorption for both EO OPE C2 SSA and EO OPE 154 Th SSA Greater transient absorption was present in the EO OPE Th SSA film than in the EO OPE C2 SSA film which is as expected due to the presence of the sulfur atom in the thiophene unit of EO OPE Th Analysis by XRD confirmed that the interlayer spacing of the clay increased upon addition of the oligomer The interlayer space measured for the EO OPE C2 SSA film was 0 570 nm and for the EO OPE Th SSA film it was 0 611 nm The interlayer space of pure SSA was 0 360 nm A larger interlayer space for EO OPE Th SSA is expected due to the more rigid non linear backbone The XRD results and 3D models of the oligomers suggest that the oligomers are unagreggated and in a single layer in the interlayer space The am
144. ree of interchain ordered aggregation of the polymer possibly due to rn stacking 5 5 The fluorescence of P3HT occurs with maxima at 580 and 620 nm with a shoulder at 690 nm This fluorescence was effectively quenched upon addition of Hg3 with little red shift but the peak ratios at high concentrations of Hg3 compared to pure P3HT in chloroform are changed Unlike MEH PP V large aggregates were not visible in solution at the highest concentrations of Hg3 This may be due to the fact that Hg3 is soluble in chloroform whereas in benzene it is mostly insoluble A AOD Emission Intensity Arb Units 500 600 700 300 Wavelength nm Wavelength nm Figure 4 21 Absorption A and emission B spectra of P3HT 14 g mL 9 uM repeat unit upon addition of increasing concentration of Hg3 from 0 uM 105 uM 300 400 500 600 700 ie Preparing films from P3HT in chloroform with varying concentrations of Hg3 it is interesting to note that although the solution color varies from orange to purple depending on the amount of Hg3 present upon evaporation of the solvent all films 125 become a similar dark purple in color Figure 4 22 This is seen most clearly in the 0 film The pure P3HT film starts out a bright orange color but as the solvent evaporates the P3HT undergoes self organization and the purple color that appears is that of the aggregated polymer Figure 4 22 P3HT films dropcoated from chloroform
145. regates Even in a solvent where Hg3 is fairly soluble like o dichlorobenzene as the solvent evaporates and the polymer and mercury complex are forced closer together by the shrinking volume aggregation of the MEH PPV occurs similarly as in a poor solvent 0 2 5 5 11 5 Figure 4 12 Visible color change of films made with o dichlorobenzene with increasing weight of Hg3 added where 50 wt means equal weights of polymer and Hg3 The absorption and emission spectra for drop coated films prepared from MEH PPV solutions in benzene with increasing amounts of Hg3 added are shown in Figure 4 13 The films varied in thickness due to aggregation so the intensity of the absorption 117 or emission cannot be directly compared Interestingly the structure of the absorption and emission spectra of the films evolve similarly upon increased concentration of Hg3 compared to the spectra measured in benzene solution shown in Figure 4 5 of MEH PPV 25 uM repeat unit with 0 240 uM Hg3 added In general upon addition of Hg3 there is a red shift in the absorption and emission with more structured bands indicative of the aggregation of MEH PPV A 12 0 50 wt He B 20 o 50 wt Hg3 2 1 84 1 0 5 a S 08 lt I S 0 6 E N E E 0 44 S N 2 a 0 0 T T y i s 0 0 T M T T bi T ii T 1 300 400 500 600 700 550 600 650 700 750 800 Wavelength nm Wavelength nm Figure 4 13 Absorption A and emis
146. region The cationic exchange capacity CEC of SSA is 99 7 meq 100 g and the layer thickness is 0 96 nm This CEC means that a solution of SSA prepared to 20 mg L will be approximately 20 yeq L 2 0 x 10 anionic sites per L Between anionic sites the average distance is approximately 1 2 nm 191192 Tetrahedral Layer 0 96 nm Octahedral Layer Tetrahedral Layer Figure 5 2 Structure of Sumectin SA SSA clay that was used in this analysis 19 Two cationic oligomers shown in Figure 5 3 EO OPE C2 and EO OPE Th which were synthesized by Anand Parthasarathy and Subhadip Goswami were used in this study These oligomers have been previously studied for their light activated killing of bacteria and their photophysical properties in methanol and in water have been fully characterized Both oligomers have two quaternary nitrogen endgroups capable of cation exchange with the ions present in the clay The goal of this research is to intercalate the oligomers into the clay interlayer spacings and examine the effect on the photophysical properties of the oligomers SZ EO OPE C2 EO OPE Th Eege E eg E Figure 5 3 Structures of the cationic oligomers EO OPE C2 and EO OPE Th N Absorption and emission measurements show aggregation of the oligomers onto the clay surface and XRD analysis is used to verify insertion of the oligomers into the 137 interlayer spaces of the SSA TGA allows a quantitative measurement of the amount
147. rforming XRD and TEM experiments on my films Thanks to Russ Winkel and his skills in NMR was able to get NMR data on MEH PPV Hg8 solutions even though they were difficult samples to run The undergraduate research student who worked on the clay oligomer project with me Eduardo Acosta did great work and helped me to become a better teacher of difficult concepts A big thanks also to the guys in Sisler 420 who always laughed at my jokes and made being in lab a joy every day was also given the amazing opportunity to spend ten weeks at Tokyo Metropolitan University in Japan with Dr Haruo Inoue and his research group which was supported by Dr Schanze Dr Inoue and the graduate student research abroad award from the International Center at UF Dr Inoue was a wonderful host and he Masae Asano and the group members took great care of me It was a fun experience and have great memories of everyone met there Many thanks particularly to Dr Yu Nabetani and Satomi Onuki for guiding me through the Japanese lab and for helping to teach me new research experiments Finally have had nothing but the most heartfelt support from my friends and family even though they thought that maybe was a little weird for being so in love with science owe a lot of my success to my Mom and Dad They have always supported my education so that could have the best chance to achieve my own success So much gratitude goes to my husband Zach whose love
148. rge transfer between donor and acceptor chromophores give the largest shifts This is because charge transfer states typically have a large change in dipole moment upon excitation which is very sensitive to small changes in the environment such as solvent polarity or hydrogen bonding capability Emission spectra can be measured with a spectrofluorimeter which consists of an excitation light source a grating monochromator and a detector The excitation light source is typically a white light source such as a xenon arc lamp which passes through a monochromator to single out the excitation wavelengths of interest before hitting the sample The luminescence from the sample is collected at 90 to the excitation beam with condensing optics and passed through a second monochromator with a grating which can be scanned stepwise through the detection region of interest Detection is 28 usually with a photomultiplier tube which produces an amplified signal upon detection of photons Spectrofluorimeters can also be double beam to correct for fluctuations in lamp intensity during the experiment Both fluorescence and phosphorescence can be detected by spectrofluorimeters assuming that the solution has been sufficiently deoxygenated Molecular oxygen is a ground state triplet and efficiently quenches other triplet excited states with a second order quenching constant in the range of the diffusional quenching constant typically on the order of 109 L molt s
149. riplet triplet absorption for MEH PPV 4 The intensity of the triplet triplet absorption peak decreases with increasing Hg3 but that may be due to lower optical density at the excitation wavelength as the aggregation 109 peak in the ground state absorption spectrum around 600 nm rises It can be concluded however that the addition of Hg3 to the polymer does not greatly increase the concentration of triplet excitons produced upon excitation as would be expected by the increased presence of heavy metal atoms in the polymer As the concentration of added Hg3 increases another peak arises around 950 nm which indicate the formation of MEH PPV radical cations that are reported to absorb at 955 nm Triplet states are readily quenched by molecular oxygen which is a ground state triplet to form singlet oxygen while the radical cation should be unaffected by the presence of oxygen in solution After purging the solutions with oxygen Figures 4 7 B and C the peak around 830 nm is efficiently quenched after 88 ns while the peak at 950 nm remains Therefore the peak at 830 nm must be the triplet state The rise of 950 nm upon addition of Hg3 indicates that Hg3 may be acting as an electron acceptor in benzene solution Little if any photodegradation of the polymer occurred from the production of singlet oxygen during this experiment as repurging the oxygen saturated benzene solutions with argon recovers the original long lived transient absorp
150. rs analyzing commercially available pesticide formulations for label claim requirements set forth by state and local governments In 2008 she attended the University of Florida in Gainesville FL to pursue her graduate degree under the advisement of Dr Kirk Schanze in the study of the photophysics of organic and organometallic molecules oligomers and polymers including the development and instrumentation of a nanosecond transient absorption instrumentation system for analysis of solid state materials and thin films She received her Ph D from the University of Florida in the summer of 2013 178
151. rs around 470 nm which can be assigned to the oligomer SSA hybrid In both cases the fluorescence emission after addition of SSA becomes broadened and in the fluorescence spectrum of EO OPE Th the fluorescence loses its vibronic structure in the clay substrate An isosbestic point is present in both oligomer SSA titrations indicating a two component system whose components are the free oligomer and the oligomer clay hybrid After a certain amount of SSA has been added the original oligomer fluorescence is nearly entirely quenched and settling of the clay oligomer hybrid occurs as indicated by a loss of the isosbestic point in the fluorescence spectrum This settling occurred ata concentration of 16 yeq L SSA for EO OPE C2 and 18 peq L for EO OPE Th This corresponds to 25 vs CEC for EO OPE C2 and 22 vs CEC for EO OPE Th This provides a minimum approximation of the saturation point of the clay surface A e 25 E gs5 Z D D S 2 0 g 4 lt T gt 1 5 gt 3 c f D ZS 1 0 2 5 5 2 KA 350 400 450 500 550 600 350 400 450 500 550 600 Wavelength nm Wavelength nm Figure 5 5 The emission spectra acquired by Eduardo Acosta of 4 uM of the oligomers A EO OPE C2 and B EO OPE Th upon addition of SSA in water from 0 to to 16 yeq L The fluorescence lifetimes of the oligomers at 410 nm in water were measured to be Tave 0 54 ns for EO OPE C2 and Tave 0 29 ns for EO OPE Th which is sim
152. rs by absorption of two identical photons through an intermediate virtual state was first proposed in 1931 in a doctoral dissertation by M G ppert Mayer under the supervision of Max Born 27 The probability of this transition was too small to be measured however without the use of laser technology developed in the 1960s The first reports of nonlinear optical phenomena that were observed to prove her theory were in 1961 Kaiser and Garrett irradiated a CaF2 Eu crystal with a ruby laser 694 3 nm and observed the characteristic bright blue fluorescence of the crystal at 425 nm 28 Also reported around the same time was the observation of the second harmonic generation of a quartz crystal irradiated with a 34 ruby laser 9 Since then the field of nonlinear optics and nonlinear absorption has been growing steadily Nonlinear absorption can be experimentally determined by the nonlinear transmission method NLT where the transmittance of light through a sample is measured as the intensity of the light increases An open aperture z scan instrument can be used to measure the nonlinear absorption of a sample to nanosecond laser pulses A schematic of the open aperture z scan system used in the studies reported in Chapter 3 is shown in Figure 1 5 A laser beam produced by an optical parametric oscillator OPO pumped by the third harmonic of a Nd YAG laser 355 nm Continuum Surelite Il 10 5 ns fwhm passes through an iris and using a 50 50 be
153. s Shortly after Einstein further developed the idea of quantized photons in 1905 when he discovered the photoelectric effect for which he was recognized with the Nobel Prize in 1921 Photons are massless particles which are localized energy in a small volume of space and radiate from the source with a velocity c The energy of a given photon is given by he E w 7 1 2 where his Planck s constant 6 62 x 104 J s vis the frequency of the wave associated with the photon cis the speed of light and 4 is the wavelength Originally the wave particle duality of light was troubling to many scientists In many circumstances such as when observing interference and diffraction phenomena it seemed that light behaved as a wave that could generally be described by Maxwell s wave equations In others such as the quantized energies detected by blackbody radiation or the photoelectric effect light acts like discrete particles with discrete energies This duality was extended to other particles such as electrons when de Broglie theorized that all particles can also act as waves This theory was confirmed with the observation of electron diffraction patterns by independent research done by Thomson and by Germer Owing to the development of quantum mechanics the wave particle duality of light and other particles can be described in a single theory A particle can be described by its wavefunction which describes the amplitude of a wav
154. s prepared to an OD at 355 nm of 0 78 While the observation of triplet triplet annihilation has interesting implications for applications involving the triplet state of platinum acetylide polymers due to the limited time resolution of our nanosecond instrumentation at such short timescales the triplet kinetic analysis described here is preliminary Further in depth studies will be performed with collaboration with Dr John Papanikolas and his group at the University 86 of North Carolina at Chapel Hill They have femtosecond transient absorption capabilities to more closely study early time scale dynamics of the triplet state after intersystem crossing It is hoped that this collaboration with provide a better understanding of the triplet dynamics in polymers with high intersystem crossing yield and efficient exciton diffusion when high concentrations of triplet excited states are produced Summary of Results A series of linear and cross conjugated phenylene vinylene platinum acetylide complexes with terminal triphenylamine groups were incorporated into PMMA glass monoliths for solid state study of their one photon and two photon photophysical properties with a focus on their application to optical power limiting materials by dual mechanism nonlinear absorption All the properties important to nonlinear absorption are retained in the solid state with increased photostability by the polymer host Absolute two photon absorption cross section va
155. s emitted to the number of photons absorbed A quantum yield of 1 indicates that 100 of the photons absorbed by a molecule produce emitted photons Quantum yield measurements can be used to quantify both photophysical processes fluorescence phosphorescence and intersystem crossing and photochemical reactions where the absorption of a photon converts a reactant R to a product P 17 Unimolecular Excited State Dynamics The excited state dynamics of an optically excited molecule can be described with a Jablonski diagram as shown in Figure 1 1 Most molecules exist as ground state singlets where all electrons are paired with another of opposite spin The most notable exception is molecular oxygen O2 which exists as a ground state triplet Upon absorption of a photon of appropriate energy the molecule becomes excited and an electron is promoted to a higher energy orbital and produces a singlet excited state S4 A t hv gt A 1 4 Internal Conversion Vibrational Relaxation Internal Absorption onversion Internal H Conversion Intersystem Absorption Fluorescence Intersyste Crossing Phosphorescence Figure 1 1 Jablonski diagram demonstrating intramolecular photophysical pathways 1 18 From the Si state the system can lose energy by fluorescence or nonradiative decay IC returning back to the ground state Fluorescence is a spin allowed radiative transition so it is a very fast process 10
156. s of the excited donor D and the acceptor A D L Dexter worked out the rate constant of energy transfer by electron exchange 17 2R kgr Exchange KJe 1 1 8 where K is related to the specific orbital interactions J is a spectral overlap integral Roa is the donor acceptor separation and L is the sum of their van der Waals radii J is 22 normalized such that the absorption and emission spectra are on the same scale So ket of Dexter energy transfer is independent of the absorption characteristics of the acceptor in contrast to the ket of FRET which is dependent on ea The rate of electron exchange decreases exponentially as the distance between D and A increases Complex formation between an excited molecule and a molecule in the ground state can precede or enhance the bimolecular decay processes discussed earlier In the case where a complex forms between an excited molecule and another of the same molecule in the ground state it is called an excimer whereas if the complex forms between two different molecules in the ground and excited state it is termed an exciplex 51819 Excimers can be formed with either locally excited or charge transfer character In an excimer resonance interactions involving either intermolecular exchange of the excitation energy and or charge causes weak intermolecular forces that hold the two molecules together Most commonly excimers occur between aromatic molecules A key feature of excim
157. s or LS KAS ry KR SAE LAA on PBuz Bu3P NA ES LAN Aye ei TPV2 T2 gt ON ZA Q WO aS N Bas Ch e 2 PB B P Q 5 e DU u NL ous im e PBus E Q GE le pa FRA j Zen e ee yoy ee a Pe PBu PBug3 ey 7 But Dis pa y SC ZA E ON crossTPV1 DPAF n 0 crossTPV1 E O n 1 crossTPV3 WO O Figure 3 1 Structures and acronyms for p phenylene vinylene platinum ll acetylide chromophores synthesized by Galyna Dubinina 65 Solution Photophysical Studies The one photon photophysical properties in solution were measured for each complex to investigate the structure property relationship of increasing OPV chain length and the linear or cross conjugated geometry The results in THF are summarized in Table 3 1 Table 3 1 One Photon Photophysical Properties in THF Solution Es b lt T gt ET TTA 0 Name Nabs x104 Me Or Ae Ar x104 us 1029 TPVO 395 13 5 459 0 10 0 18 585 7 4 0 3 0 13 TPV1 415 20 9 502 0 30 0 78 678 11 0 1 19 0 73 TPV2 424 29 5 525 0 52 1 15 687 100 67 9 0 51 TPV2 T2 423 245 522 0 28 0 66 683 39 3 0 79 0 79 TPV1 Ph 415 9 33 502 0 46 1 44 671 18 1 0 59 0 35 crossTPV1 355 11 2 476 0 01 lt 0 1 662 7 7 0 85 0 83 405 7 6 507 crossTPV3 343 95 501 004 lt 0 1 719 8 1 0 43 0 98 415 10 7 537 crossTPV1 301 8 7 478 0 005 lt 0 1 660 11 8 0 96 NA DPAF 381 21 4 511 aWavelengths of ground state absorption Aabs triplet triplet absorption Ar and fluorescence A maxima are in nm oMeasured a
158. s were assembled to anionic nanoparticulate scaffolds such as Laponite The cationic OPEs 135 participated in strong host guest interactions with these anionic scaffolds and the formation of J dimers upon intercalation into the clay suggested an increase in effective conjugation length More recently a polyfluorene derivative was incorporated into a clay nanocomposite by Tozoni resulting in a blue shifted photoluminescence with a significant increase in intensity In this case it was observed that the strong tendency for polyfluorenes to aggregate by tr stacking was overcome simply by stirring a solution of the polymer with clay particles This current study involves the solution intercalation of oligo phenylene ethynylenes OPEs into a clay matrix as represented in Figure 5 1 and the properties of the oligomer clay hybrid are investigated Figure 5 1 Representative drawing of the intercalation of a cationic oligomer into the clay interlayer spaces in a film Sumectin SA SSA is an artificially synthesized cation exchangeable clay synthetic saponite obtained from Kuminine Ind Co with a stoichiometric formula of Siz 20Alo 80 Mgs 97Alo 03 O20 OH a 77 Nao 49 Mgo 14 as shown in Figure 5 2 9 It is a layered silicate which consists of a layer of Al or Mg octahedra between two layers of Si tetrahedra SSA a highly pure clay that is easily dispersed in water and 136 transparent in the UV visible
159. schotta R In Field Guide to Laser Pulse Generation SPIE Press Bellingham WA 2008 Gaussian Beam Wikipedia The Free Encyclopedia Online Early Access http en wikipedia org w index php title Gaussian_beam amp oldid 560465837 accessed July 4 2013 Wong W Y Harvey P D Macromol Rapid Commun 2010 31 671 713 Chawdhury N Kohler A Friend R H Younus M Long N J Raithby P R Lewis J Macromolecules 1998 31 722 727 Dray A E Wittmann F Friend R H Donald A M Khan M S Lewis J Johnson B F G Synth Met 1991 47 871 874 Wittmann H F Fuhrmann K Friend R H Khan M S Lewis J Synth Met 1993 55 56 61 Daughton W J G F L J Electrochem Soc 1982 129 173 179 Lawrence C J Phys Fluids 1988 31 2786 2796 Jaczewska J B A Bernasik A Moons E Rysz J Macromolecules 2008 41 4802 4810 Vella J Ph D Dissertation University of Florida 2009 Glimsdal E Carlsson M Eliasson B Minaev B Lindgren M J Phys Chem A 2007 111 244 250 168 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 Gubler U Bosshard C In Polymers for Photonics Applications I Lee K S Ed Springer Verlag Berlin Heidelberg Germany 2002 Vol 158 p 123 191 Chaumel F Jiang H W Kakkar A Chem Mater 2001 13 3389 3395 Cooper T M Krein D M Burke A R McLean
160. sent the relative transmittance will be the same at all sample intensities whereas a sample with strong nonlinear absorption will show a large dip in the transmittance at z 0 The attenuation of light through a medium can be expressed by the following general equations ZO al z PP yP 1 12 The function z is the intensity of the incident beam propagating along the z axis and a B and y are the one two and three photon absorption coefficients of the transmitting medium under the assumption that the light has a uniform transverse intensity distribution and that the initial intensity is independent of time If only 2PA is 36 present the change in intensity of the propagating light beam through the absorbing medium is dependent on the square of the intensity of the light and the two photon absorption coefficient of the medium E E R72 S az PI 1 13 Assuming a top hat pulse shape a solution for Equation 1 13 can be calculated for a given position z where o A is the incident light intensity z is the propagation length in the medium and GOU is the two photon absorption coefficient of the medium Ig A te ESCHETTE 1 14 The value A can be expressed as a macroscopic constant for a medium at a given wavelength where o is the 2PA cross section Na is Avogadro s number and do is the molar concentration of the absorbing species DO 0 2 A Ngdyx10 3 1 15 The units of o 2are cm G
161. sfer from the triplet state There is a positive correlation between the triplet quantum yield and lifetime of oligo phenylene ethynylenes and their biocidal activity 185 Immobilizing light induced biocidal complexes onto a material surface has been a topic of study in recent years as more efficient and effective antimicrobial agents are needed to combat increasing antibiotic resistent bacterial strains 7 In an article by Tiller and coworkers in 2001 methods were explored for attaching N alkylated poly 4 vinylpyridine to a glass surface and evaluated the cell killing on the dry glass Surface 178 The biocidal activity of polymer films containing singlet oxygen sensitizers such as fullerene have also been reported Incorporating photoactive chromophores into clay scaffolds can produce interesting effects relevant to photocatalysis photoluminescence photochromism and nonlinear optics 178 179 Layered materials such as clay offer an expandable interlayer space where guest compounds can be intercalated This intercalation constrains and organizes the geometry of the guest compound resulting in nanostructured materials with unique properties Further 134 nanofilms of clays containing cationic dyes or polymers can be produced with two levels of organization an association an organization of molecules onto the clay surface and a secondary organization on the individual clay sheets 182 183 In 1999 Winkler reported the
162. sion B spectra of drop coated films of MEH PPV with increasing wt of Hg3 prepared in benzene 0 wt 10 wt 20 wt and 50 wt Hg3 Nanosecond transient absorption measurements shown in Figure 4 14 on dropcoated films of MEH PPV and Hg3 are a remarkable parallel to the solution experiments Initially upon addition of Hg3 a rising peak centered around 825 nm appears and when the concentration is increased to 20 wt Hg3 and finally 35 wt a prominent peak around 925 nm takes over The biexponential fits to the transient decays at Amax of the 10 20 and 35 wt films give lifetimes similar to those measured in benzene solution with a longer component that ranges from 877 ns to 1 09 us and a shorter component that ranges from 162 ns to 235 ns as the amount of Ho increases The longer component around 1 us has a larger contribution to the overall 118 decay from 48 to 68 as the amount of Hg3 increases This trend resembles the trend seen in solution TA measurements The increasing lifetime is probably due to the restrictive organization of the polymer when it is aggregated by the Hg3 0 025 g 0 006 E 0 004 0 wt Hg3 ee Data 35 wt Hg3 3 0 002 SE 163 ns 52 Sportal Ft Fit 0 020 ans Z 0 000 es IRn aanas 7 KL g 0 002 e g 0 0154 e 8 0 008 20 wt Hg3 PE S 0 004 989 ns 40 218 ns 60 Pees lt 0 000 e GEES S 0 0104 e 20 wt Hg3 lt 0 004 lt ARAT
163. solution as the solvent evaporates Left 0 Hg3 Right 50 Hg3 The absorption and emission spectra of the P3HT films prepared in benzene and chloroform containing increasing weight of Hg3 are shown in Figure 4 23 The P3HT films have similar absorption and emission properties regardless of the solvent used for casting The structured absorption of the pure P3HT films is lost when Hg3 is added and the absorption maxima slightly red shifts However a dramatic change in the absorption spectrum seen in the presence of Hg3 such as with MEH PPV does not occur with P3HT It appears as though the P3HT polymer naturally self organizes in a film and the Hg3 only has a small effect The emission of P3HT seems to be quenched by the addition of Hg3 and the peak ratios in the emission change similarly to the effect seen in chloroform solution shown in Figure 4 20 126 A 03 B 0 4 0 wt 50 wt Hg3 0 wt 50 wt Hg3 8 8 oa S 0 2 S 2 2 2 2 oz lt lt o 0 1 2 gt E E 0 1 d d LZ z 0 0 0 0 gt 400 500 600 700 800 400 500 600 700 800 1 0 1 0 Wavelength nm Wavelength nm 2 0 wt A 0 wt BEE 2 0 8 0 8 oO E c 0 6 2 2 G 0 4 LU L 2 0 2150 wt oe a 550 600 650 700 750 800 Figure 4 23 Absorption and emission spectra of P3HT films prepared in benzene A and chloroform B with increasing concentrations of Hg3 0 wt black 20 wt red and 50 wt blue Spectra are scal
164. solution intercalation of poly p phenylene vinylene into a smectite clay host and the resulting photoluminescent and electroluminescent properties including those of a single layer LED device containing the polymer clay hybrid 84 An interesting report by Neumann Cione and Gessner in 2001 shows that the emission properties of aromatic hydrocarbons in a Laponite clay can be modified by the exchange of the Na or Li cations in the clay with heavier metals Rb Cer and TI 8 In this study decreased singlet lifetimes and increased phosphorescence were observed as an effect from enhanced spin orbit coupling provided by the presence of the heavier metal atoms Additionally the hydrocarbons on the clay surface exhibited aggregation dependence on the concentration of heavier cations due to a decrease in surface area of the heavy cation exchanged clay Their results illustrate the ability to tune the photophysical properties of chromophores by incorporation into a clay framework Conjugated oligomers and polymers in clay hosts have received considerable attention for their novel electronic and optical properties in the confined interlayer space 187 One of the major difficulties in working with conjugated oligomers and polymers is controlling aggregation because of the presence of strong interchain interactions 16 188 An investigation of the effect of supramolecular organization on oligo phenylene ethynylenes by Whitten and coworkers in 2009 when the OPE
165. stem crossing occurs if a molecule M1 in the excited state comes across a heavy metal containing complex Me which is capable of inducing spin orbit coupling This can be seen as a quenching of the fluorescence of Mi Phosphorescence may also be observed Intermolecular energy transfer can occur between colliding molecules which is a form of nonradiative decay An excited state is quenched if it is lost by energy transfer to another molecule or an excited state can be sensitized if it is created by energy 20 transfer from another molecule This quenching process involves transfer from an energy donor D to suitable acceptor A D A gt A D 1 5 The kinetics of energy transfer are more complicated than for unimolecular decay First there is a dependence on the concentration of the species involved Second in order for Dexter energy transfer or photochemical reaction to occur the excited donor must collide with the acceptor This collision may involve actual overlap between the orbitals of the two species or the two molecules can come together within a specified reaction radius After collision either energy can be transferred between donor and acceptor or they can separate without further interaction Energy transfer between donor and acceptor can occur by either F rster energy transfer or Dexter exchange Forster resonance energy transfer FRET is a Coulombic interaction by dipole dipole coupling FRET requires spec
166. t 470 nm all the lifetime components at 470 nm are significantly shorter than the lifetime components at 410 nm 1 4 ns lt 1 0 19 ns 7 5 0 007 ns 92 4 Film photophysics were measured freestanding open to atmosphere without a cuvette Absorption spectra were not obtained due to saturated OD Emission spectra were obtained by a front face alignment on a Fluorolog 3 spectrophotometer Transient absorption measurements on films were performed using an in house designed TA instrument dedicated to film measurements The third harmonic of a Continuum Surelite I 10 Nd YAG laser at 355 nm 600 uJ pulse was used as the pump beam in an 149 antiparallel alignment to the probe beam provided by a 250 W QTH lamp Newport Single wavelength transient absorption decays were acquired as an average of 128 shots every 25 nm using a Hamamatsu R928 PMT and an in house modified base using 5 of the 9 stages for amplification The transient absorption spectrum was acquired by using a custom Matlab program to convert the transient absorption decays at each wavelength to a transient absorption spectrum at t 0 The transient decay at Amax for the EO OPE C2 SSA film shows a biexponential decay with a shorter lifetime of 18 5 us 54 8 and a very long lived component of 431 ms 45 2 The lifetime fitting of the EO OPE Th SSA to a biexponential decay yields lifetimes of 24 4 us 67 3 and 391 us 32 7 TGA was measured on a TA instruments model Q5000
167. t a planar heavy metal complex may be able to coordinate with a highly conjugated aromatic polymer backbone by Tr Tr interaction to efficiently enhance intersystem crossing to the triplet state by spin orbit coupling while simultaneously stabilizing the planarity of the polymer backbone to improve the charge transport ability Triplet Sensitization by Hg3 Trimeric perfluoro ortho phenylene mercury Hg3 shown in Figure 4 1 is a colorless solid with no photoluminescence in room temperature solutions of CH2Cl2 and with intense orange photoluminescence in its crystalline form when irradiated by UV light At low temperatures 77 K a very broad emission band with Amax 440 nm exc 355 nm appears with a broad shoulder centered at 530 nm and extending past the 101 visible region of the spectrum possibly due to the formation of tight dimers in its crystalline state 177 Hg3 and similar organomercurial fluoroarenes have been shown to complex with planar aromatic compounds such as benzene 1 1 122 biphenyl 2 naphthalene triphenylene 177 and acetone 2 among others 124126 forming compact alternating stacks in sandwich type configurations due to the Hg tr interaction in the solid state These interactions are likely electrostatic as DFT calculations suggest the center of the trinuclear macrocycle of Hg3 has a positive electrostatic potential while the perimeter is negative Also the HOMO LUMO gap for Hg3 is particular
168. t port to the sample slit Make sure that the laser hits in the center of all optics with no clipping of the beam b If necessary adjust the horizontal or vertical knobs on the preceding optic to fix an offcenter or clipping beam on a prism or mirror c At this point do not further adjust the sample holder Optimize Izero 1 Toggle off the Laser Flashlamp and Q Switch switches and toggle on the Shutters and Oscilloscope switches and click Update Settings 2 At the top of the screen is the monochromator control Choose a wavelength within your data acquisition and press Enter The movement of the gratings should be audible and the wavelength indicator next to the control box should reflect the change in wavelength A wavelength around 500 600 is a good wavelength at which to optimize IZero Move the filter wheel to an appropriate position for the wavelength At the lower right hand corner of the screen are the horizontal and vertical adjusters for the scope Start with a 100 mV div vertical setting and a 4 us div horizontal setting Observe the Izero level the position of the baseline directly on the scope screen The PMT gives a negative signal Increasing the magnitude of this negative signal will give a better signal to noise ratio A good Izero should be over 2V Maximize Izero using the following steps 159 a Adjust the large curved mirror before the monochromator using the horizontal and ver
169. t rt using 9 10 diphenylanthracene in cyclohexane as a std M1 0 75 Determined by relative actinometry with benzophenone as the actinometer 1T 1 00 T 7870 mol cm 879 dMeasured in CDCl using terthiophene as a std o2 0 75 Lifetime is less than the instrument time resolution which is 100 ps The normalized absorption and emission spectra of the complexes are shown in Figure 3 2 For the linear TPVn series the peak molar absorptivity values increase with OPV length and the ground state absorption and fluorescence maxima red shift The cross conjugated chromophores crossTPVn have very broad absorption peaks with two maxima As the length of the OPV unit increases from crossTPV1 to crossTPV3 the fluorescence maxima red shift and the longer wavelength absorption 66 band becomes more pronounced This suggests that the shorter wavelength transition is due to the platinum phenylene ethynylene unit while the longer wavelength band can be assigned to the OPV chromophore axis Although the crossTPVn series absorbs more broadly in the blue all complexes are optically transparent in the visible region A gt 525 nm which is important for OPL applications A TPV1 1 27 TPV1 Ph TPV2 1 2 crossTPV1 DPAF TPV2 T2 crossTPV1 g 1 0 g 10 SC l ao COSSTPV3 P TA 2 0 84 2 08 O n 1 wn 2 0 64 2 0 6 ai Baal N 0 4 A 0 44 T T E 0 2 E 0 24 fe 3 Z 0 04 IV B 300 350
170. ted laser energy to nanosecond pulses Their results demonstrated that platinum Il acetylides could be incorporated into PMMA glasses for optical power limiting applications and found that the blended glasses actually 63 performed better than those prepared by covalent attachment of the chromophore to the polymer backbone The work discussed here involves the incorporation of highly two photon absorbing platinum acetylide chromophores into PMMA polymer monoliths and the evaluation of the photophysical properties in the monoliths compared to the properties measured in solution One goal is to establish whether the photophysical properties important to optical power limiting are retained in the solid state Also proposed here is a quantitative analysis of nonlinear absorption to nanosecond pulses and this quantitative method is applied to the open aperture z scan results of these platinum acetylide complexes in solution and in PMMA monolith Linear and Cross Conjugated PI OPV Series Previous research on tr conjugated chromophores containing triphenylamine groups at the ends of p phenylene vinylene units have found that these types of complexes have very large 2PA cross section values in the visible to near infrared region 4 7476 Linking these types of chromophores to a heavy metal atom such as platinum the triplet triplet absorption can be enhanced and optical power limiting by dual mode nonlinear absorption can be achieved In this study
171. ten for the flashlamps to stop firing c On the Laser Control Box i push the Shutter button and then the Start Stop button ii Turn the key to the off position The LED on the laser PCU should read OFF 2 Turn off the Tektronix 3032B Oscilloscope 2 On the high voltage source for the PMT dial the first knob to HV Off and the other knobs to 0 Turn off the source by flipping the orange switch 163 Turn off the BNC 575 Pulse Generator Turn off the lamp by pressing the Lamp Off button on the lamp power supply Allow the lamp fans to run for a few minutes before turning off the power supply as the power supply also powers the cooling fan Flip the orange ON OFF switch once the fan stops If termination other than 1 KQ was used replace the 1 kQ BNC terminator to the T on the channel 1 input of the scope If slits other than 0 70 mm were used return the monochromator entrance and exit slits to 0 70 mm Cover all optics with bags Exit out of TA vi and Acquisition vi and double check that the acquired data has been transferred from the temporary file folder on the desktop to the appropriate non temporary storage location Data Processing TA Spectrum L 2 3 Type in a filename to select TO and press Enter Ex 5 Ideally this should EN To process the raw data and create a TA spectrum at various time values double click to open the Matlab progra
172. th A2 where there is no ground state absorption the change in absorbance can be described by Equations 3 6 and 3 7 where l is the pathlength of the sample AA geil 3 6 94 AA A2 Er A2 IT II 3 7 Combining these two equations the triplet molar extinction coefficient can be calculated by the following relation AA A2 Er Azg Eg Ax AACA 3 8 The uncertainty in this method is determining a wavelength A where one can be certain there is no triplet triplet absorption Depending on the sample this can be difficult because there is often at least some overlap between the ground state absorption spectrum and the triplet triplet absorption spectrum Using this method other reasons for ground state depletion such as photodecomposition can contribute significant error to the calculation of er To calculate the molar triplet extinction coefficients of the series of linear and cross conjugated p phenylene vinylene platinum Il acetylides Figure 3 1 a different method was used called the relative actinometry method Use of an actinometer whose erand Disc are Known is an accurate way of determining the triplet molar extinction coefficients for unknown samples The sample and actinometer solutions are prepared in benzene to matched optical density ODs55nm 0 78 at the excitation wavelength At Amax of their transient absorption the AA intensity is measured at increasing laser fluence in a region where the response is
173. the top of the choose a QSwitch setting suitable for the acquisition 380 us and click Update Settings signal a b Click the View Live button next to the waveform graph to view the TA on the computer or view the signal on the scope Adjust the horizontal and vertical scale using the slide bars at the lower right hand corner of the screen The vertical scale should be such that the signal is fully on the screen and fills up as much of the screen as possible The horizontal scale should be such that the signal decays all the way back to the baseline before the acquisition end IMPORTANT A vertical scale of less than 100 mV div cannot be used Note these settings 160 4 Choose a few more wavelengths in the region of data acquisition and repeat steps 2 5 a Make note of the Izero signal and whether a larger mV div setting is needed to compute Izero b Always use the largest needed Izero mV div setting 5 For data acquisition the last inputted Izero mV div setting will be used for EVERY wavelength During data acquisition this setting CANNOT be adjusted Make sure to use an Izero mV div setting large enough for all wavelengths of interest Usually Izeromax will be around 500 nm Acquiring TA Data 1 At the bottom of the TA vi screen type in the number of waveforms to average for each acquisition and press Enter a The only available options are 4 8 16 32 64 128 512 If an unacceptable
174. tical adjuster knobs watching the scope to maximize the negative signal If necessary this mirror can be pivoted in its mount but the mount should not be moved Change the vertical setting on the scope using the software as needed to see the signal b If Izero is still unsatisfactory lt 1000 mV after optimizing the position of the mirror the monochromator slits can be adjusted Note The bandpass for this monochromator is 6 7 nm mm So a slit width of 0 5 mm gives a bandpass of 3 nm Adjust the entrance and exit slits as desired For help in reading the micrometer refer to the Cornerstone 130 motorized 1 8m Monochromator user manual i il ill Apply a 10X multiplier to the micrometer markings to readout slit opening Each mark on the micrometer barrel corresponds to 10 um of slit width The 0 position is closed One full turn of the barrel is equal to 250 um of slit width The default slit width is 500 um Make sure both the entrance and exit slits are the same width c If necessary the PMT voltage can be adjusted up to 770 V to increase Izero This will also slightly increase the noise level The optimal PMT operating voltage is 730 V Acquisition Parameters In the software click Get Offset The program will compute Izero and L return offset screen the value in the Izero indicator box The oscilloscope will set the equal to Izero After a suitable Izero is computed toggle on all 4 switches at
175. tion at 830 nm The transient absorption of solution 1 pure MEH PPV is shown in Figure 4 8 In deoxygenated solution a relatively strong transient absorption peak is present centered at 810 nm and tails off with no transient absorption after 925 nm When the solution is saturated with oxygen the transient absorption is very quickly quenched disappearing within 88 ns Looking at the transient decays at 830 nm there is a very fast component in both the deoxygenated and O2 saturated measurements The 110 longer lived component of the transient decay at 830 nm is nearly completely quenched in the presence of molecular oxygen confirming the assignment of this peak to the A 0 018 A 0 016 Deoxygenated time 0 ns 0 014 alt 0 012 OR 100 uM Hg3 S 0 010 yos P S onge 40 pM H93 f ee 0 006 SS a M 0 004 EA 0 002 L 0 uMHg3 gt g 0 000 L De E D 850 900 950 1000 1050 1100 Wavelength nm 0 030 O Purged time 0 ns 0 025 0 020 8 8 3 0 015 100 uM Ho G i ZZ lt 0 010 l pos X a 7 40 uM Hg3 goe 0 uM Hg3 gt 0 000 1 R b Te 850 900 950 1000 1050 1100 C Wavelength nm 0 008 r r 0 007 L O Purged time 88ns a 100 uM Hg3 g 0 005 ZZ Za a S 0 0044 D J E F SE D ZS 0 003 e 40 uM Hg3 Ka D D lt i 0 002 F Mia sy L ke Yv 0 001 ba 0 uM Hg3 d 0 000 et eet tail 850 900 950 1000 1050 1100 Wave
176. to protect from heat damage caused by the intense light A Cornerstone 130 motorized monochromator is used to isolate a single wavelength using one of two available gratings one for UV Vis analysis 1200 I mm blaze 500 and one for nIR analysis 800 I mm blaze 1000 and detection is by either a photomultiplier tube Hamamatsu R928 PMT 400 800 nm or an avalanche photodiode APD 750 1000 nm to observe the TA signal decay versus time For UV VIS detection a Hamamatsu R928 PMT powered by 730 volts is mounted to the monochromator in a custom base modified in house using 5 of the 9 PMT stages for signal amplification For nlR detection the PMT is removed lens 9 is slid into place to collect the output light from the monochromator and concave mirror 10 focuses the light onto a Si avalanche photodiode Thorlabs APD110A The signal from the 47 detector is collected with a Tektronics 3032B Oscilloscope and sent to a computer Laser flashlamp Q switch slow shutter fast shutter and oscilloscope timings are controlled by a BNC Model 575 8 channel pulse generator Components A Surelite l 10 Nd YAG laser B Second and Third Harmonic Generator C 250 W QTH Lamp D Slow Shutter E Fast Shutter F Cornerstone 130 Monochromator G PMT H APD Optics 1 2 1 prisms 3 4 10 1 mirrors 5 10 cm f l plano convex 2 lens 6 10 cm f l plano convex 2 lens 7 10 cm f l plano convex 2 lens 8 10 cm f l concave mirror 9
177. tral overlap between the emission of the donor and the absorption of the acceptor as shown in Figure 1 2 Energy transfer by FRET is related to the transition dipoles of the acceptor and the donor and the distance between them yy ke Coulombic eg 1 6 FRET is most favored if 1 the emission D D and the absorption A A spectrally coincide 2 the radiative rate constant kr of D is very large 3 A has a high molar extinction coefficient and 4 A and D are within close proximity 21 S Spectral S 7 Overlap Donor Absorption Fluorescence Acceptor I I I I FRET i 1 Fluorescence Intensity Acceptor Absorption Fluorescence l So So Donor Acceptor Figure 1 2 Energy diagrams of a donor and acceptor demonstrating FRET when the emission of the donor overlaps with the absorption of the acceptor The FRET efficiency E describes the fraction of energy transfer occurring per donor excitation SE ker _ 1 krikeri k ER 1 7 where ker is the rate of energy transfer kr is the radiative decay rate of the donor ki are other decay pathways of D and Ro is the Forster distance which is defined as the energy transfer between donor and acceptor has 50 efficiency When FRET occurs fluorescence may occur from the acceptor Dexter exchange involves energy transfer by an electron exchange mechanism Electron exchange requires an overlap of the wavefunction
178. tribute significantly to the triplet excited state In order to be effective optical power limiters two photon absorption is a necessary process To quantify the two photon absorption properties of these complexes the femtosecond 2PA cross sections across the wavelength range of 570 1070 nm was measured by two methods two photon excited fluorescence 2PEF and a newly developed intensity dependent nonlinear transmission NLT method by collaboration with Dr Aleks Rebane at Montana State University Similar results were obtained using the wavelength tunable output of an amplified Ti sapphire source 80 120 fs by both the NLT and 2PEF methods as shown in Figure 3 4 pulse repetition rates 100 Hz and 1 kHz respectively Several features are significant with respect to the 2PA spectra of each complex shown in Figure 3 4 Specifically each system exhibits strong 2PA with o2 greater than 500 GM over a large range of the visible to near IR region 600 nm 900 nm Also the longest linear OPV containing complexes have peak o2 values larger than 10 000 GM which is one of the largest 2PA values reported The cross conjugated chromophores have particularly broad and intense spectra over a 200 300 nm window In all cases there is exceptional spectral overlap between the 2PA spectra and the 69 triplet triplet absorption spectra which is the most important property in dual mode nonlinear absorption for optical power limiting applications
179. triplet excited state The triplet absorption spectrum is relatively easy to measure in regions where little to no ground state absorption exists however if strong ground state absorption is present the triplet absorption can usually not be detected Particularly difficult to measure is the quantity of the molar triplet extinction coefficient The molar triplet extinction coefficient et MT cmt is a quantitative measure of the amount of light absorbed by the triplet excited state T1 of a molecule and depends on the concentration of T1 c and the absorbance of T A along a given optical pathlength I as shown in Equation 3 3 er 3 3 The absorbance is calculated experimentally from the transmittance of light through a sample as in Equation 3 4 where To and T are the transmitted light through the sample in the absence and presence respectively of the transient species A logio 3 4 Some considerations of any experiment to calculate the triplet extinction coefficient should be taken into account P First the beam used for analyzing the absorbance of the sample should not suffer from reflection or scattering such that the beam is only transmitted or absorbed Also the measured transmittance should only be due to the transmitted beam and not to other light sources such as emission from the 93 sample This is relatively easy to control with good optical design of the beam path and sample conditions Second the instrumental pa
180. tronic energy states eventually becoming a nearly infinite smearing of energy levels At very low temperatures in rigid media collisions with other molecules are minimized and the spectrum becomes more structured Also low temperatures will eliminate the absorption transitions from the higher energy states as the Boltzmann distribution mandates that only the lowest vibrational level in the ground state will be populated Conversely at higher temperatures there exists more population of 27 molecules in higher vibrational states in the ground state which again introduces a different absorption spectrum Solvent polarity also has a consequence because molecules in the ground state versus an excited state may exhibit a change in polarity A change in polarity leads to different stabilization of the ground and excited states causing a difference in the energy gap This introduces significant shifts in the absorption spectra and is termed solvatochromism Solvatochromism is a complex phenomenon dependent on many factors such as polarity solubility and hydrogen bonding capability of the solvent Generally if the excited state is more polarizable than the ground state then a red shift bathochromic shift will occur in more polar solvent If the excited state is less polarizable then the spectrum will exhibit a blue shift hypsochromic shift in more polar solvent Charge transfer states either MLCT or LMCT which involve intramolecular cha
181. value The Rayleigh length Zr is defined as the distance from wo where the cross sectional area of the beam doubles Figure 1 8 Properties of a Gaussian beam wo is the beam waist Zr is the Rayleigh range bis the depth of focus and is the beam divergence 40 The diameter of the beam waist 2wo can be calculated by Equation 1 24 2Wy 1 24 Using the small angle approximation tan 8 8 can be approximated as the inverse of the f number of the focusing lens The f number is the focal length F of the lens divided by the diameter D illuminated onto the lens Thus the diameter of the beam waist is approximately equal to Equation 1 25 zm 8E vn D In the open aperture Z scan apparatus shown in Figure 1 4 the focal length of the focusing lens is F 50 8 mm If the excitation wavelength is A 600 nm and the diameter of the beam illuminated onto the lens is D 2 cm then the beam diameter is 3 2 um This is an unreasonably small value for our beam diameter A necessary assumption in Equation 1 24 is that the beam is collimated however the beam output from the OPO is slightly divergent There are ways of accurately measuring the beam waist but they are expensive upwards of several thousand dollars A reasonable estimate for the beam waist would be 30 50 um As 2PA only occurs in the focused region of the beam it is necessary to check the assumption that the beam stays focused throughout the entire cuvette
182. velength to spectra at specified time intervals MEH PPV samples were prepared to an optical density of 0 61 at 532 nm before Hg3 addition Transient absorption lifetimes were calculated using exponential fitting in Origin 8 5 software Polymer film photophysics were measured freestanding open to the atmosphere Absorption spectra were measured on a Varian Cary 100 dual beam spectrophotometer with a clean glass slide as the instrument blank Emission spectra were obtained by a front face alignment on a Fluorolog 3 spectrophotometer Transient absorption measurements were made using an in house designed TA instrument dedicated to 131 solid state measurements The second harmonic of a Continuum Surelite l 10 Nd YAG laser at 532 nm 600 uJ pulse was used as the pump beam in an antiparallel alignment to the probe beam provided by a 250 W QTH lamp Newport Single wavelength transient absorption decays were acquired as an average of 128 shots every 25 nm using a Thorlabs Avalanche Photodetector APD 110A The transient absorption spectrum was acquired by using a custom Matlab program to convert the transient absorption decays at each wavelength to a transient absorption spectrum at t 0 Other Measurements To characterize the morphology of the MEH PPV with Hg analysis by transmission electron microscope TEM JEM 2000 FX electron microscope JEOL was performed at a bias voltage of 100 kV Polymer solutions were prepared at 1 mg mL MEH PPV in benzen
183. vely intense lamp and laser beams and the quality of the data acquired It was found that 128 averages was sufficient to produce a quality transient decay 59 0 002 16 averages 64 averages D g 32 averages g AA 0 000 0 001 0 002 0 001 AA 0 000 0 001 Time us Time us Time us Figure 2 11 Transient decays of a pPtPh film prepared from a 20 mg mL solution in toluene spun at 2000 RPM with an increasing number of averaged waveforms Detection was with a PMT at Amax 650 nm Aexc 355 nm 600 uJ pulse 1 KQ termination slits 0 5 mm smooth 10 The oscilloscope outputs 10 000 data points which is a burden for plotting programs such as Excel or Origin to handle Smoothing was introduced to reduce the number of data points output by the scope and it also decreases the signal to noise ratio of the acquired transient Smoothing does affect the time resolution of the decay but after smoothing the transients still display a sharp rise indicating that the time resolution is still acceptable Based on the decays collected with varying smooth factors as shown in Figure 2 12 a smooth factor of 10 was shown to satisfactorily decrease the noise of the signal while keeping a workable amount of data points 1 000 The smooth factor of 10 takes an average of each 10 data points 60 0 002 Smooth Factor 1 Smooth Factor 2 Smooth Factor 5 10 000 data pts 5 000 data points 2
184. way to collect transient absorption data is by single wavelength detection such as is described in the introduction Single wavelength detection gives more precise transient decay lifetime information at a single wavelength but to acquire a spectrum many transient decays at different wavelengths are collected and then collated using a custom executable Matlab program Depending on which system is used there is a trade off between time and wavelength resolution The pump beam used in these transient absorption experiments comes from a Nd YAG Q Switched laser with excitation possible at 355 nm or 532 nm by use of harmonic generation of the 1064 nm fundamental For single wavelength detection the probe source is a xenon arclamp Detection is with a PMT which provides detection up to 800 nm or with a Si Photodiode for detection into the near IR A Triax 180 Monochromator is used for single wavelength detection This transient absorption system has been used for many years however the electronic components and programming was beginning to become incompatible with upgraded computer hardware necessitating a considerable upgrade of components The system was updated with new electronic components and Labview programming including a Berkely Nucleonics Corporation Model 575 and a Tektronix 4032B Phosphor oscilloscope Labview 45 programming offers complete automation and data collection for the transient absorption system and replaced the old Visual Bas
185. weight film means equal mass of polymer and Hg3 P3HT solutions and films were prepared and analyzed similarly Hg3 is soluble in chloroform so a stock solution of Hg3 in chloroform was easily prepared P3HT regioregular was obtained from Sigma Aldrich and PFO was obtained from American Dye Source Inc 130 Photophysical Measurements Photophysical properties for solutions were measured using a 1 cm pathlength quartz or borosilicate cuvette in spectroscopic solvents Solutions were deoxygenated by purging with argon or oxygen saturated by purging with oxygen for transient absorption measurements Ground state absorption spectra were collected on a Varian Cary 100 dual beam spectrophotometer with the appropriate solvent as the instrument baseline blank Corrected steady state emission measurements were collected by a Photon Technology International PTI photon counting fluorescence spectrophotometer Nanosecond triplet triplet transient absorption was measured in a continuously circulating 1 cm pathlength borosilicate cuvette The second harmonic of a Continuum Surelite Il 10 Nd YAG laser at 532 nm 2 4 mJ pulse was used as the pump and a Newport xenon arclamp was used as the probe source The transient absorption signal was detected in the nl between 810 1100 nm every 10 nm with a Thorlabs Si Photodiode PDA8A The transient absorption spectra were obtained by using a custom Matlab program to convert the transient absorption decays at each wa

Randi Sue Price - The Schanze Research Group

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