Excited-State Singlet Energy Transport in Polystyrene - American

Chapter 22

Excited-State Singlet Energy Transport in Polystyrene 1

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Daniel R. Coulter , Amitava Gupta , Vincent M . Miskowski , and Gary W. Scott

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on September 20, 2016 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch022

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Jet Propulsion Laboratory, Chemical and Mechanical Systems Division, California Institute of Technology, Pasadena, CA 91109 Department of Chemistry, University of California, Riverside, CA 92521 2

Photophysical properties of polystyrene and copolymers of styrene and a chemically attachable quencher have been determined in dilute solution and in s o l i d f i l m s . In dilute s o l u t i o n , it i s found that energy migration i s not extensive. In room temperature s o l i d f i l m s , quenching studies shown e f f e c t i v e long range energy transport. Rough estimates of the excimer forming s i t e concentration and hopping rate have been reported. Temperature dependent studies on films down to ~20K have shown that both monomer and excimer emission come from i n t r i n s i c traps. Steady state and time resolved fluorescence measurements as a function of temperature have allowed determination of the activation energy for monomer detrapping and have placed an upper l i m i t on the activation energy of migration. Singlet excited state energy migration in aromatic polymers has been proposed to be important for many years (1). There remains, however, considerable disagreement (1,2) as to the" extent of excitation mobility, and little information (3) i s available as to the time-scale of energy migration. Our current work (1,4) in this f i e l d attempts to d i r e c t l y address these questions. We here report the results and interpretation of photophysical experiments involving two different types of perturbation of the phototypical aromatic polymer, polystyrene. F i r s t , we have prepared a series of co-polymers of styrene with an excitation quencher, 2-(2'-hydroxy-5'-vinylphenyl)-2H-benzotriazole, abbreviated 2H5V. Secondly, we have looked at the e f f e c t of temperature on the photophysical properties of these polymers. Experimental Preparation and characterization of the polymers has been described in detail (4). Results are summarized in Table I. Fluorescence measurements employed a Perkin-Elmer MPF-66 spectrofluorimeter with an excitation wavelength of 260 nm and e x c i t a 0097-6156/87/0358-0286S06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


22.

COULTER ET AL.

Singlet Energy Transport

287

tion and emission spectral s l i t widths of 2 run. A band pass f i l t e r was used to further isolate 260 nm excitation l i g h t . The excitation was chopped at a frequency of 34Hz and phasesensitive detection was used. In this way, the long-lived (seconds at 20K) phosphorescence and delayed fluorescence were s e l e c t i v e l y ex cluded from these data, and only the "prompt" fluorescence was anal yzed. Data including the long-lived signals were measured in separ ate experiments, and w i l l be reported elsewhere. Time-resolved studies employed previously-described (4) equip ment but were improved upon as follows. Part of the excitation NdYAG laser beam was s p l i t o f f and delivered to the phototube, serving as a marker pulse to trigger our Biomation 6500 for data a c q u i s i t i o n ; the sample emission signal was timed by a laser pulse delay l i n e to arrive ^15 ns a f t e r the marker pulse. Laser j i t t e r was thereby mini mized, and computer signal averaging more precise. Variable-temperature experiments employed a Cryogenic Technol ogy Model 21 closed-cycle helium r e f r i g e r a t o r . Samples were deposi ted as films upon quartz disks from CH2CI2 s o l u t i o n , with minimal ex posure to l i g h t . They were mounted on a copper sample holder bolted to the cold stage of the r e f r i g e r a t o r , with copper grease used to give good thermal contact of a l l surfaces. A Chromel-Au (0.07%Fe) thermocouple was mounted on the opposite side of the sample holder, at the same height as the sample. A copper heat shield was mounted over the cold stage/sample, a shroud with quartz windows was emplaced and the sample space was pumped on for ^24 hours in order to remove O2 and CH2CI2 dissolved in the polymer. Samples prepared in this way were extremely photostable. Background:

Aromatic Excited States and Excimers

Excimers, that i s complexes of excited state molecules with corres ponding ground state molecules, are [5) c h a r a c t e r i s t i c of aromatic excited states. For our present purposes we wish to consider the lowest excited state of benzene and substituted derivatives together with the corresponding excimer. The lowest singlet excited state (imr ) o f , for example, ethyl benzene, shows absorption and emission maxima a t , respectively ( 1 ) , ^260 and ^280 nm. The temperature dependence of the emission in d i lute polar organic solution has been investigated ( l b ) , and i t was found that there i s a temperature dependent non-radiative rate compo nent that follows an Arrhenius law A e " / with an activation energy ΔΕ^2300 cm" and A ^ I O ^ " . The ratio of the emission quantum y i e l d (ΦΕ) and lifetime(-r) is temperature independent, consistent with a temperature independent radiative rate constant k , and l i m i t i n g low temperature values of Φ£ and τ , achieved by ^250K, are 0.12 and 21 ns, respectively, in dichloroethane s o l u t i o n . Excimer formation of ethylbenzene in excited state has been studied in concentrated (.1-1M) methylcyclohexane solution at -78°C by Hirayama, et al (6). They found, in addition to the "monomer" emission, a broad concentration dependent excimer emission with ^ = 322 nm. From the concentration dependence they were able to ex t r a c t " i n t r i n s i c " monomer and excimer y i e l d s , that i s , the quantum y i e l d s i f a l l excited states were present as either monomer or e x c i mer; these amounted t o , respectively, 0.30 and 0.021 under the given conditions. A E

1

1

k T

1

v

m a x


PHOTOPHYSICS OF POLYMERS

288

The i n t r i n s i c weakness of benzene excimer emission derives from i t s extreme dipole forbiddeness (7_). Its weakness r e l a t i v e to mono mer emission under dilute solution conditions additionally results from the diffusion l i m i t upon excimer formation r a t e s , along with thermally induced excimer d i s s o c i a t i o n . These l a t t e r considerations are largely irrelevant to polymers, vide i n f r a .


Polystyrene in Dilute Solution In Figure 1 we compare emission spectra f o r polystyrene in dilute solution and as a s o l i d f i l m , and for a model monomer, ethyl benzene, in dilute s o l u t i o n . Polystyrene in solution e x h i b i t s , in addition to a monomer-like emission, a broad excimer emission maximizing at ^330 nm. This emission spectrum i s not unique to high molecular weight polymer. Indeed, 1,3-diphenylpropane exhibits (_la,8) very s i m i l a r total emission spectra. The excimer emission lifetime is (4) 12.5 ns in CH2C12 at room temperature, while monomer-like emission decay and excimer emission r i s e times are reported (3) to be of the order of a nanosecond in cyclohexane s o l u t i o n . Importantly, emission spectra of polystyrene and of model com pounds in r i g i d dilute solution ( 3 j , e . g . , 1:1 diethylether-tetrahydrofuran at 77K, are a l l e s s e n t i a l l y i d e n t i c a l , with no trace of ex cimer emission. The model which has been developed (1,3) to f i t these results involves formation of excimers v i a thermally activated phenyl group motion. Such motion i s r e s t r i c t e d in a r i g i d matrix, and "pre-formed excimer s i t e s , where l i t t l e phenyl motion would be required f o r collapse to the excimer, are evidently rare. However, Itagaki, et al (_3) propose that energy migration along polymer chains i s s t i l l important, as a result of a detailed analysis of molecular weight effects on photophysical parameters, with the average number of phenyl rings covered by s i n g l e t energy mi gration estimated to be ^7-8 in the high molecular weight polymers; the c h a r a c t e r i s t i c timescales of energy hopping and of polymer i n t e r nal rotation to an excimer-forming conformation were proposed to be, respectively, ^30 ps and ^7 ns. The r e l a t i v e l y short 1 ns monomer decay lifetime and excimer r i s e time thus was inferred to r e s u l t from excitation sampling along the chain to find favorable conforma t i o n s , with the additional r e s t r i c t i o n that excimer dissociation to monomer was negligibly slow on the timescale of the excimer l i f e t i m e . The l a t t e r r e s t r i c t i o n i s c h a r a c t e r i s t i c of aromatic polymers in gen eral . We determined photophysical properties for our copolymers in dilute s o l u t i o n , and found effects (9) upon either total emission spectra (Figure 2) or emission l i f e t i m e s . However, because the mole % quencher in our copolymers i s quite small, this r e s u l t is complet ely consistent with the interpretation of Itagaki, et al (3). Α Π we can say from our results i s that energy migration i s not extensive in dilute s o l u t i o n . n

Solid Polystyrene The emission spectrum of s o l i d polystyrene at room temperature has already been shown in Figure 1. It shows a broad excimer emission at 325 nm and only a trace of monomer-like emission. The excimer and



22. C O U L T E R ET A L .

Figure 1.


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Emission spectra of ethylbenzene and polystyrene.

CURVE mole % 2H5V ο 0.0 0.00616 Δ 0.0285 0.154 •

Qpft Δ Δ •

•

Ό •

• Δ

• Δθ Ρ Δ

α

ο Δ • Ο L

280

Figure 2.

300

JL 320

340 360 λ, nm

380

400 420

Emission spectra for CH2CI2 solutions of poly(styrene-Co-2H5V) copolymers. The phenyl group concen tration is constant at 5 X 10" M. 3


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Table I. COMPOSITION AND MOLECULAR WEIGHT DATA FOR P0LY(STYRENE-C0-2H5V) COPOLYMER SAMPLES


2H5V, mol %

2H5Y, mol/L

0.0 0.00155 0.00273 0.00616 0.0114 0.0285 0.527 0.154 0.268 0.483 0.570 2.55 4.78

0.0 1.55X10"•4 2.73X10-•4 6.16X10"•4 1.15X10-•3 2.88X10-•3 5.32X10-•3 1.55X10"•2 2.70X10-•2 4.88X10-•2 5.76X10-•2 2.58X10"•1 4.83X10-•1

328000 402000 463000 364000 410000 511000 511000 480000 240000 242600 306000 241700

M "η

V n

initiator

87000 202000 218000 192000 200000 302000 302000 280000 133300 132500 167200 116400

3.78 2.06 2.12 1.89 2.05 1.69 1.69 1.71 1.80 1.83 1.83 2.08

Polysciences peroxide peroxide peroxide peroxide AIBN AIBN AIBN AIBN AIBN AIBN peroxide peroxi de

monomer-like emission decay lifetimes are respectively 21.5 ns and 1 ns. We have not been able to observe an excimer rise-time to date and conclude that i t must be 150K excimer emission as one l i m i t . The resulting mono mer emissions are shown in Figure 7; the spectral p r o f i l e compares well with that f o r d i l u t e ethyl benzene Figure 1 and ( l b ) . The tem perature dependence of the integrated i n t e n s i t i e s is sTTown in Figure 8, and an Arrhenius plot of the monomer emission data i s shown in Figure 9. We also determined the temperature dependence of the emission l i f e t i m e s , as shown in Figure 10. It i s remarkable that the excimer decay lifetime i s almost completely temperature independent, showing no detectable r i s e time, while the monomer lifetime decreases rapid ly with temperature. An Arrhenius plot of the monomer lifetime data i s shown in Figure 11. It gives, within experimental e r r o r , the same activation energy as that f o r the monomer i n t e n s i t y , Figure 9. F i n a l l y , we performed s i m i l a r experiments f o r the various copo lymers. As shown in Figure 12 f o r one of the copolymers, despite a large (70%) decrease in excimer i n t e n s i t y , the thermal behavior i s q u a l i t a t i v e l y i d e n t i c a l , and an Arrhenius plot of the monomer inten s i t y data, Figure 13, gives the same activation energy, ν ^ Ο - Ι δ Ο ς ^ . As indicated in Figure 8, the l i m i t i n g (at presently accessible temperatures) low-temperature excimer emission intensity i s only about 20% lower than the room-temperature value and no rise-time con s i s t e n t with the monomer decay can be discerned. The l i m i t i n g low


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C O U L T E R E T AL.



Ί

1

293

Γ

mole% HPB

Figure 5.

Excimer emission intensity as a function of composi tion for copolymers at room temperature.

nm Figure 6 .

Temperature dependent fluorescence for a f i l m of pure polystyrene.




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Figure 7.

Data of Figure 6 after subtraction of excimer emis sion. See text.

τ

ι

ι

ι

ι

I

I

I

Γ

TEMPERATURE (K) Figure 8.

Temperature dependence of integrated resolved emis sion for a polystyrene f i l m .


COULTER ET AL.



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Figure 1 0 . Temperature dependence of fluorescence decay l i f e times of a polystyrene f i l m .


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Figure 12.

Temperature dependent fluorescence for a poly(styrene-Co-2H5V) copolymer, 0.0527 mole % 2H5V.



COULTER ET AL.

+


ι

"\ ·

I

1

1 „ .-AE/kT - — = α + Ae em

1.00

+

1

ΔΕ = 159 cm" 0.80 \

0.60

\

-

0.40 -

0.20

1

I

0

ι

0.02

!i 0.04

ι

0.03

ι, 0.05

+J X 0.06

1/T Figure 13.

Arrhenius f i t to the monomer intensity data of Fig ure 12.



298

temperature monomer lifetime (^22 ns) i s very s i m i l a r to that of ethyl benzene in dilute solution (16.). We therefore conclude that the emissive monomer i s predominantly not_ the source of low-temperature excimers, but that at higher temperatures i t can be detrapped in a thermally activated process, activation energy VL40cm , to y i e l d excimer additional to the low-T l i m i t . Note that the l i m i t i n g lowtemperature r e l a t i v e emission i n t e n s i t i e s are consistent with this hypothesis. Hirayama's (6) " i n t r i n s i c " monomer and excimer quantum y i e l d s o f , respectively, 0.30 and 0.021, f o r ethylbenzene predict that i f the emissive monomer comprises 20% of the total i n i t i a l e x c i tation that monomer emission should be ^3 times as intense as excimer emission, which i s consistent with Figure 6. Moreover, the hypothes i s that the emissive monomer i s a r e l a t i v e l y small fraction of the total i n i t i a l excitation i s consistent with our i n a b i l i t y to measure excimer r i s e times corresponding to monomer decay as simulations of the biphasic curves resulting from a ^20% r i s e component convoluted with the normal excimer decay proved to be e f f e c t i v e l y i n d i s t i n g u i s h able from single component decays (15). This would not be true i f the emissive monomer were the exclusive excimer precursor. We think that this shallow trap monomer i s predominantly "preformed" and achieved by the i n i t i a l absorption of a photon. Thus, comparison of Figures 6 and 12 shows that monomer emission i s decreasing considerably less rapidly than excimer emission at low temperature. The activation energy i s consistent with expectation (_3, lcU16) f o r a phenyl rotation and the activated process is l i k e l y then sucTTa rotation into a conformation where energy-transfer to a neighboring phenyl group can occur. We have not attempted to correlate the population of the shallow trap with chain dyads because we suspect that interactions with phenyl groups of adjacent chains are much more important in the s o l i d polymer. On the other hand, the excimer emission because i t is 80% noncorrelated with monomer trap emission and because i t is e f f e c t i v e l y quenched in the copolymers even at low temperatures, must largely arise from a mobile precursor. The activation energy for hopping of this precursor is implied to be

Excited-State Singlet Energy Transport in Polystyrene - American

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