Photochemistry in solid matrices: Laser flash photolysis of benzophenone in polymethyl-methacrylate and polystyrene matrices

Polymer Photochemistry 5 (1984) 215-230

Photochemistry in Solid Matrices: Laser Flash Photolysis of Benzophenone in Polymethylmethacrylate and Polystyrene Matrices

Ali Salmassi and Wolfram Schnabel Hahn-Meitner-Institut fiir Kernforschung, Berlin, Bereich Strahlenchemie GmbH, D-1000 Berlin 39, Federal Republic of Germany

ABSTRACT Benzophenone triplets were produced in matrices of polymethylmethacrylate ( P M M A ) and polystyrene (PSt) at concentrations up to ca. 6 × 10-4M upon irradiation with single flashes of 3 4 7 n m light. With P M M A matrices a single triplet decay mode following first-order kinetics was observed at T<~150 K and at T>~410 K. Two distinct modes of triplet decay were observed in the temperature range in between: a fast first-order process, the lifetime (ca. 4 t~s at 295 K) being independent of the initial triplet concentration [T]o, and a slow second-order process, the first half-life being proportional to [T]o 1. Similar results were obtained with PSt matrices. A t temperatures between 180 and 350 K two modes of triplet decay were also observed, a fast first-order and a slow second-order process. From these results it is concluded that T - T annihilation is an important deactivation route between the glass temperature range and T~, where the rotation of a-methyl groups (PMMA ) and of phenyl groups (PSt), respectively, commences. Above Tg, deactivation processes involving matrix molecules become dominant. Hydrogen abstraction, evidenced by the absorption spectrum of ketyl radicals, occurs to some extent here. Moreover energy transfer from vibronically excited triplet levels to chromophores of the matrix is feasible at elevated temperatures. On the basis that T - T annihilation is a diffusion-controlled process, the application of Smoluchowski' s equation yields, for the temperature range between T~ and T~, diffusion constants much higher than 215 Polymer Photochemistry 0144-2880/84/$3-00 © Elsevier Applied Science Publishers Ltd, England, 1984. Printed in Northern Ireland

216

Ali Salmassi and Wolfram Schnabel expected. This indicates a strong reduction of the micro-viscosity of the matrix if benzophenone is admixed.

INTRODUCTION During recent years polymeric matrices have attracted attention for various reasons: they were considered, for example, as convenient media for spectroscopic investigations of excited triplet states over wide temperature ranges and, vice versa, the usage of photophysically easily detectable probes allowed the investigation of the structure of polymeric matrices and of physical transitions connected with changes in the mobility of certain structural units such as the t e m p e r a t u r e - d e p e n d e n t rotation of methyl groups or ester sidegroups in polymethylmethacrylate. A n o t h e r interesting aspect pertains to reactions of low molecular weight compounds e m b e d d e d in polymeric matrices. This point is of relevance for practical applications with respect to behavior and reactivity of additives which are admixed to polymers as stabilizers against photodegradation or thermal degradation, etc. Despite the great body of published papers devoted to photochemistry and photophysics of glassy polymers and to polymer/additive systems, only relatively few papers have been concerned with the kinetics of formation and decay of transient species in solid polymeric matrices. Moreover, almost exclusively luminescence studies were carried out in these cases. The present work was aimed at studying the behavior of triplet excited low molecular weight compounds in polymethylmethacrylate (PMMA) and polystyrene (PSt) matrices over a large temperature range with the aid of both absorption and emission measurements. B e n z o p h e n o n e (BP) was used as probe material because its triplets can be conveniently detected both by optical absorption and by phosphorescence measurements. Our interest concentrated in part on the mobility of low molecular weight compounds in polymeric matrices. In this respect, work of E1-Sayed et al. 1 is noteworthy. These authors interpreted the non-exponential luminescence decay observed with some aromatic hydrocarbons in P M M A matrices at

Photochemistry in solid matrices

217

temperatures above 280 K in terms of diffusion-controlled triplettriplet annihilation

T1 + T1 ,

So + $1

(la)

So + So + energy

(lb)

~ Singlet excimer

(lc)

The occurrence of T - T annihilation, which was confirmed by the detection of P-type delayed fluorescence at 298 K following reaction (la), implies an appreciable mobility of the hydrocarbons in the P M M A matrix. It appeared to be justifiable to carry out similar experiments employing experimental conditions improved by using a laser flash photolysis set-up, operating in the nanosecond region, which allowed the generation of initial triplet concentrations up to 6 x 10 -4 mol litre -1. This way, evidence for T - T annihilation could be obtained from the linear dependence of the first second-order halflife of the triplet decay on the reciprocal absorbed dose per flash, Do 1, according to eqn (2) 1 r~/2- kT-T[T]0

(2)

where proportionality between IT]0, the initial triplet concentration, and Do is assumed. The importance of high initial triplet concentrations derives from the fact that reaction (1) always competes with other routes of triplet deactivation, i.e. first-order processes such as phosphorescence T1 ~ So + hv (3) and radiationless transitions T~ ~ So + energy

(4)

_ d[T] = ~ k~[T] + 2kT_T[T]2 dt

(5)

With respect to eqn (5)

where Y. k~ denotes the sum of all first-order or pseudo first-order rate constants of triplet decay, T - T annihilation becomes dominating when Y. kl << kT-T[T].

218

Ali Salmassi and Wolfram Schnabel

It is n o t e w o r t h y that non-exponential decay behavior was d e t e c t e d with electronically excited probes in P M M A matrices by o t h e r researchers, 2-6 but it was attributed to a variety of causes o t h e r than diffusion-controlled T - T annihilation. Details w e r e discussed by E1Sayed et al. 1 A n o t h e r aspect of this w o r k concerns the influence of temperature, i.e. the physical state of the matrix, on m o d e s and kinetics of triplet decay. Papers dealing with this subject 7-H w e r e generally based on emission m e a s u r e m e n t s obtained during continuous irradiations. Plots of the emission intensity versus the reciprocal absolute t e m p e r ature exhibited abrupt changes at t e m p e r a t u r e s corresponding to the onset of local relaxation processes in the matrix. It a p p e a r e d interesting, therefore, to find out w h e t h e r flash photolysis experiments yield similar results with respect to emission intensities m e a s u r e d immediately at the end of the flash. Additional information concerning physical transitions were expected, in the present study, from the t e m p e r a t u r e d e p e n d e n c e of the kinetics of triplet decay. Finally, the reaction of triplet molecules with matrix molecules was studied and the kinetics w e r e c o m p a r e d with those of the reaction of triplet molecules with low molecular weight model compounds. B e n z o p h e n o n e appeared to be an appropriate p r o b e because it forms triplets with a high yield upon U V irradiation. ~2 Moreover, triplettriplet annihilation has b e e n detected, in this c a s e , 13 and occurs, according to a recent report, ~4 with almost diffusion-controlled rate constants at r o o m t e m p e r a t u r e in solutions of benzene. It was also shown that a hopping m e c h a n i s m (self-energy transfer) according to reaction (6) T1 + So ---> So + T1

(6)

is not responsible for the high rate constants of about 6 - 8 × 1091itresmol-~s-X. ~4 The reaction of b e n z o p h e n o n e triplets with P M M A was investigated by Kucenova and Karpuchin, 15 who determ i n e d the rate constant for h y d r o g e n abstraction from P M M A in b e n z e n e solution at r o o m t e m p e r a t u r e as kR --- 1 0 6 litres tool -1 s -1 and in the polymeric matrix at r o o m t e m p e r a t u r e as kR----180 litres mol -~ s -~. ka

3BP* + P H

> K" + P-

w h e r e P H = polymer, P- = macroradical, and K. = ketyl radical.

(7)

Photochemistry in solid matrices

219

EXPERIMENTAL PART Materials Polymethylmethacrylate (R6hm /~rw=4.2× 10 4) w a s reprecipitated four times from acetone solution with petroleum benzene (b.p. 5070°C). Prior to the last reprecipitation the polymer solution was filtrated through a membrane filter (Sartorius, 0.8 tzm pore size). Polystyrene, Mw -- 1.0 x 105, was polymerized by thermal initiation. It was reprecipitated three times using a 4 : 2 mixture of acetone and cyclohexane as solvent, and methanol as precipitant. Prior to the last reprecipitation the polymer solution was filtered through a membrane filter (Sartorius, 0.8 ~ m pore size). For film preparation and irradiation the following solvents were used. 2-Ethoxyethylacetate ( C H 3 C O O C H z C H 2 O C 2 H s ) , obtained from Merck-Schuchardt (99%), was passed through a silica gel/charcoal column prior to distillation via a 1-m splitting tube column (Fischer, Bonn) (b.p. 60-70°C, 30 Torr). Ethylbenzene was washed several times with concentrated H2SO4, an aqueous solution of NaHCO3 (10%) and water. After drying with CaSO4 it was distilled from Call2 (b.p. 50°C/70 Torr). Ethylbenzene and methyl isobutyrate (b.p. 92°C/760 Torr) were both obtained from E. Merck (98% purity). Benzophenone (E. Merck, 98%) was recrystallized three times from ethanol solution (m.p. 47.1°C).

Preparation of polymer films The polymers were dissolved in 2-ethoxyethylacetate containing appropriate amounts of BP at concentrations of ca. 500 g per litre P M M A and ca. 150 g per litre PSt. The solutions were poured on plane quartz plates (7.5 × 0.6 cm), which were kept for 30 min at 80°C and 600 Torr. They were subsequently annealed for 2 h at 125°C and tempered for 55 h at 65°C and 5 × 10-1Torr. The film thickness (measured with a microdigit meter from Sviluppo Tecnologie Avanzate) varied between 80 and 170 ~m.

Irradiation of films All irradiations were carried out in a cryostat (Oxford Instruments, model DN 1704), which was evacuated to 10-5-10-6Torr for 3 h

220

Ali Salmassi and Wolfram Schnabel

after insertion of a sample plate. With the aid of a vacuum passage device (Balzers rotary linear motion feed-through DN 40 KF, Cat. No. 2258IT) the position of the plate could be varied, thus allowing the irradiation of six different sections of the film. Both the photolyzing and the analyzing light beams passed the polymer film at angles of 45 °. Prior to irradiation, the optical density (O.D.) at 2, = 347 nm in all positions of the film to be irradiated was measured. Irradiations were performed at temperatures between 77 and 430 K. A ruby laser (Korad, model K1QS2) in conjunction with a frequency doubler was used as photolyzing light source (h = 347 nm, flash duration 25 ns). Optical absorption measurements were carried out with the aid of a xenon lamp (Osram, XBO 450 W) and a Bausch and Lomb monochromator (33-86-76). The output of the photomultiplier (RCA, 1P28) was fed into a 7A13 or 7A22 plug-in (Tektronix) and displayed on a storage oscilloscope (Tektronix). Alternatively, a computer ( P D P l l ) in conjunction with a transient recorder 8100 (Gould/Biomation) was used for storage and evaluation of data. For emission measurements the photomultiplier was operated with a gating system, having only two of the nine dynodes in action at the time of the flash, the others becoming operative after a variable gating time (minimum 0-4 txs). For actinometry the photochromic compound (E)-a-(2,5-dimethyl3-furyl-ethylidene) (isopropylidene) succinic anhydride (Aberchrome 540), obtained from Aberchromics Ltd, w a s u s e d . 16 P M M A films containing 3-8 wt% Aberchrome 540 were prepared via spin-coating of a solution of P M M A in 2-ethoxy-ethylacetate, containing about 10 wt% polymer at 3000 rpm for 60 s. Typically, the film thickness was about 0.5 ~ m and the optical absorption at 347 nm about 15%. The absorbed dose per flash was calculated from the increase of the O.D. at 494 nm using ~b = 0-20 as quantum yield for photocoloration and 494 = 7.8 x 103 litres mol -~ cm -1 as extinction coefficient. The film thickness was measured with the aid of a Talystep-1 instrument (Taylor/Hobson).

RESULTS Tgiplet-triplet annilu'lation Upon irradiation of P M M A and PSt films, containing benzophenone (BP) with 25 ns flashes of 347 nm light at temperatures between 77

Photochemistry in solid matrices

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Fig. 1. The optical density at )t =530nm (normalized for a film thickness of 140 ~m), measured at the end of the flash at 295 K versus the absorbed dose per flash. SoLutions of benzophenone in (a) PMMA (2-3x10 2mollitre 1), and (b) PSt (5-6 x 10 3 mol litre '). and 430 K, the well-known triplet-triplet absorption spectrum as well as the characteristic phosphorescence spectrum of BP was observed. T h e triplet concentration increased linearly with the absorbed dose, as can be seen from Fig. 1 where the optical density m e a s u r e d at 530 nm at the end of the flash is plotted against the absorbed dose. A n estimate of the q u a n t u m yield for triplet formation at r o o m temperature, using the extinction coefficient for BP triplets in benzene e s30 = 7- 5 x 103 litres mol-1 cm- I reported by Land, 26 yields ~ (T) ~0-5 (PMMA) and &(T)--~0-3 (PSt). A b o u t the same ratio of dffT) values was f o u n d with the low molecular weight model compounds methylisobutyrate and ethylbenzene. It appears, therefore, that the q u a n t u m yield for intersystem crossing in the aromatic matrix is lower than in the matrix consisting of aliphatic esters, probably due to singlet quenching by aromatics competing with intersystem crossing. The decay of the triplets was studied by both absorption and emission measurements. It d e p e n d e d strongly on temperature. With P M M A matrices a single decay m o d e following first-order kinetics was observed at low temperatures (T~<150 K) and at temperatures

All Salmassi and Wolfram Schnabel

222

ABSORPTION

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Fig. 2. D e c a y of b e n z o p h e n o n e triplets in a P M M A matrix at 295 K. Oscilloscope traces illustrating the decay of the absorption (a) at 530 nm and the decay of the phosphorescence (b) at 450 nm, and respective second-order plots (a') and (b'). [BP] = 2.3 × 10 -2 mol litre -1. Film thickness = 80 ~ m . A b s o r b e d dose per flash = 4.5 x 10 -4 (a) and 4-8 × 10 -s (b) einstein litre -1.

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Photochemistry in solid matrices

223

above 410 K. In the temperature range in between, the decay comprised two distinct modes. Typical results obtained at 295 K with a solution of BP (2-3 x 10 -2 mol litre -1) in P M M A are shown in Fig. 2. The decrease of the rapid mode followed first-order kinetics, the lifetime (ca. 4 Us) being independent of the absorbed dose per flash. The fraction of triplets decaying according to the rapid mode increased with increasing absorbed dose as is depicted in Fig. 3. An analogous behavior was observed with P M M A films in the whole temperature range between 150 and 410 K. It is noteworthy that the slow mode of triplet decay followed second-order kinetics. This result is best exemplified by the data presented in Fig. 4, where it is shown that the reciprocal first second-order half-life increases linearly with the absorbed dose per flash, as is expected if triplet-triplet annihilation according to reaction (1) is the dominating deactivation route for the decay of benzophenone triplets (see eqn (2)). The absorbed dose was, in this case, corrected for the fraction which produced triplets decaying according to the fast mode. Similar results were obtained with polystyrene matrices containing benzophenone. In the temperature range between 180 and 350 K the 30 "7 U~

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Fig. 4. R e c i p r o c a l first s e c o n d - o r d e r half-lives of b e n z o p h e n o n e triplet decay in P M M A a n d PSt matrices at 295 K. [ B P ] = 2 . 3 x l 0 - 2 m o l l i t r e -~ ( P M M A ) a n d 5.6 x 10 3 mol litre 1 (PSt). O , • = a b s o r p t i o n , a n d /% • = e m i s s i o n m e a s u r e m e n t s . Cor (Dab s = (1 -- f)Dabs).

224

Ali Salmassi and Wolfram Schnabel

triplets d e c a y e d according to two modes: a rapid first-order process (-r ~ 3/~s at 295 K, i n d e p e n d e n t of the a b s o r b e d dose per flash) and a slow s e c o n d - o r d e r process. Also, in this case, a linear relationship b e t w e e n the reciprocal first-order half-life and the a b s o r b e d dose per flash was established, as can b e seen from Fig. 4, and the fraction of triplets decaying according to the rapid m o d e increased with increasing a b s o r b e d dose as is shown in Fig. 3, straight line (b). These results are strong evidence for the occurrence of diffusioncontrolled reactions b e t w e e n low molecular weight c o m p o u n d s in polymeric matrices below the glass transition range.

Influence of the physical state of the matrix on formation and decay of triplets For b o t h P M M A and PSt various transition t e m p e r a t u r e s are known from mechanical and dielectric relaxation m e a s u r e m e n t s as well as from luminescence studies. 17-19 In the case of P M M A , transitions o b s e r v e d at 1 5 5 - 1 7 0 K, 2 4 0 - 2 7 0 K and 3 7 5 - 3 9 0 K were attributed to the onset of the rotation of a - m e t h y l groups, ester side-groups and segmental motions (glass transition), respectively. A n additional transition at a b o u t 330 K, detectable, for example, by dilatometric measurements, 2° was assigned to the presence of heterotactic segments. In the case of PSt, transitions d e t e c t e d at a b o u t 1 5 0 - 1 8 0 K and at 3 6 0 - 3 9 0 K, respectively, correspond to the onset of the rotation of phenyl groups and of segmental motions (glass transition). 21-2s In this work, the m e t h o d e m p l o y e d to study physical changes in the matrix as a function of t e m p e r a t u r e consisted of the m e a s u r e m e n t of the rate of triplet decay. B e c a u s e the kinetics varied r e m a r k a b l y with the t e m p e r a t u r e (clear exponential decay b e l o w 150 K and a b o v e 410 K, and a combination of first- and s e c o n d - o r d e r processes bet w e e n these temperatures, in the case of P M M A , for example), the first half-life was taken as a m e a s u r e for the detection of physical changes. The results o b t a i n e d with P M M A and PSt are p r e s e n t e d in Figs 5(a) and 5(b), respectively, w h e r e the reciprocal first-order half-lives are plotted against the reciprocal absolute temperature. T h e curves exhibit significant discontinuities at the transition t e m p e r atures r e p o r t e d in the literature.

Photochemistry in solid matrices 10'

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225

I J I , I , I , I , I , I 2 ~, 6 8 10 12 14

(K -1)

103/T

(K -1)

Fig. 5. Reciprocal first half-lives of the decrease of the optical density at 530 nm or of the phosphorescence at 450 nm, measured at constant absorbed dose, versus the reciprocal temperature. (a) Benzophenone in PMMA (2-3 × l0 2 mol litre-1). O a t s = 6"8 × 10 -4 einstein litre 1. (b) Benzophenone in PSt (5"6× 10 3 mol litre 1). D,ts = 8.9 × 10 -.4 einstein litre ~. In the temperature range between Tg and T, half-lives corresponding to the 'slow' second-order decrease were plotted.

Reactions of triplets with nmtHx molecules F i g u r e 6 shows o s c i l l o s c o p e t r a c e s r e c o r d e d with a P M M A m a t r i x c o n t a i n i n g b e n z o p h e n o n e at 4 3 0 K, i.e. at a t e m p e r a t u r e a b o v e Tg. F r o m Fig. 6(c), w h i c h s h o w s t h e d e c a y o f t h e p h o s p h o r e s c e n c e at 4 5 0 nm, it b e c o m e s o b v i o u s t h a t t h e triplets d e c a y r a t h e r r a p i d l y with k t - - ( 5 . 5 ± 0 - 5 ) × 105s 1. It is n o t e w o r t h y that t h e d e c r e a s e of the triplets follows strictly f i r s t - o r d e r kinetics in c o n t r a s t t o t h e kinetic b e h a v i o r o b s e r v e d at t e m p e r a t u r e s b e l o w Tg (vide ante). P a r a l l e l to t h e triplet d e c a y , a l o n g - l a s t i n g a b s o r p t i o n is f o r m e d w h i c h is attrib u t e d t o k e t y l radicals. T h i s can b e s e e n f r o m Figs 6(a) a n d 6(b) w h i c h d e p i c t t h e c h a n g e o f t h e a b s o r p t i o n at 3 3 0 n m w h e r e ketyl radicals a b s o r b s t r o n g l y . Similar results w e r e o b t a i n e d at h i g h e r w a v e l e n g t h s , e.g. at 5 3 0 n m , w h e r e t h e a b s o r p t i o n o f B P ketyl

226

Ali Salmassi and Wolfram Schnabel

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Emission: x- 450nm Fig. 6. Oscilloscope traces depicting formation and decrease of the optical density at 330 nm (a) and (b) and of the phosphorescence at 450 nm (c). Benzophenone in PMMA (2-3 x 1 0 - 2 mol litre-1). T = 430 K. Absorbed dose per flash = 8.2 × 10 -4 einstein litre -1 (a) and (b), and 2-9 × 10 -4 einstein litre -z (c). radicals has frequently been measured by other authors. These results document the importance of hydrogen abstraction reactions according t o r e a c t i o n (7) at r e l a t i v e l y h i g h t e m p e r a t u r e s . B e l o w T~ o t h e r t r i p l e t d e c a y r o u t e s p r e d o m i n a t e d , as w a s i n f e r r e d f r o m s i m i l a r experiments which showed that only weak absorptions corresponding t o k e t y l r a d i c a l s w e r e f o r m e d • T h i s is c o n s i s t e n t w i t h t h e c o n c l u s i o n

Photochemistry in solid matrices

227

arrived at above, that triplet-triplet annihilation reactions prevail at temperatures below Tg. Analogous results were obtained with polystyrene matrices containing benzophenone. Also here, at high temperatures above Tg, a rather strong long-lasting absorption, ascribed to BP ketyl radicals, was observed. The decay of triplets followed first-order kinetics. At temperatures below Tg the extent of BP ketyl radical formation was rather small although noticeable. The results obtained with both matrix systems clearly demonstrate that BP triplets are capable of reacting with the macromolecules of the matrix thus forming ketyl radicals. This activation-controlled reaction only becomes important at higher temperatures where it can compete with other triplet deactivation routes. It might be recalled, in this connection, that the formation of ketyl radicals during the reaction of BP triplets with polystyrene at room temperature was evidenced by ESR measurements carried out by David et a l . 27 Evidence for the analogous reaction with P M M A was obtained by Kucenova and Karpuchin 15 and Melhuish 3 from optical absorption measurements.

DISCUSSION There are two aspects concerning the results reported in this paper which deserve further comment: (1) reactions between low molecular weight molecules in polymeric glasses at temperatures below the glass transition range, and (2) the reaction of benzophenone triplets with matrix molecules. With respect to (1), attention has to be drawn to the fact that triplet decay according to two modes was observed. The results obtained upon varying the absorbed dose per pulse are in accord with the assumption that the two modes correspond to two kinds of BP triplets which only differ by their location; the 'rapidly' decaying triplets were formed in close proximity. Therefore interaction was possible without or only with very little displacement from the original position. This conclusion was arrived at by taking into account the fact that the lifetime of the fast mode was independent of the absorbed dose, i.e. independent of the initial triplet concentration and that the fraction of triplets corresponding to the fast mode

228

Ali Salmassi and Wolfram Schnabel

increased with increasing absorbed dose. The higher the absorbed dose, the greater the probability for two triplets being generated in close proximity. The 'slowly' decaying triplets were formed at loci separated by relatively large distances. In order to interact they must diffuse through the matrix. That interaction between these triplets is possible is inferred from the results of this work. The fact that the slow mode of triplet decay follows second-order kinetics is taken as evidence for the occurrence of T - T annihilation at temperatures below the glass transition range in matrices of both P M M A and PSt. It is noteworthy that our results corroborate the conclusions of E1-Sayed et al. t arrived at from luminescence studies with aromatic hydrocarbons in PMMA. Application of the Smoluchowski equation, which relates the diffusion-controlled rate constant kD to the diffusion constant 8, the reaction distance r, and Avogadro's number NA kD = 167r 10-t°NAr 8

(litres mol -~ s -t)

(8)

yields ~ values in the order of magnitude of values obtained for the diffusion of 02 in PMMA. For example, 8 ~ 1 × 10 -8 cm 2 s-1 was estimated with r = 1.5 nm from kT_~r~-5× 1 0 7 litres mo1-1 s-t at T = 295 K (in PMMA). For benzophenone which is more bulky than O2 a much lower 8 value would be expected. It appears, therefore, that, in our case, the structure of the matrix differed significantly from that of a closely packed molecular arrangement. As a consequence of this, the decreased micro-viscosity of the matrix affords opportunity for small molecules to diffuse more easily. At present it cannot be decided whether structural changes of the matrix are caused by the addition of benzophenone or whether the film preparation technique employed in this work results in a rather loosely packed molecular arrangement favouring a rather high mobility of solutes. In this connection it is noteworthy that benzene has been reported to achieve an approximately exponentially (with its own concentration) increasing mobility in P S T . 3° The other important result of this work concerns the reaction of benzophenone triplets with matrix molecules. In the case of P M M A matrices, ketyl radical formation at room temperature according to reaction (7) was rather unimportant. At higher temperatures, however, hydrogen abstraction became more pronounced. The following estimate demonstrates that only a small fraction (about 10%) of BP

Photochemistry in solid matrices

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triplets was c o n v e r t e d to ketyl radicals at 430 K, the highest t e m p e r a ture e m p l o y e d in the present work; typically, the O . D . at 530 nm, corresponding to ketyl radicals, was 1-8× 1 0 - 3 . This gives, with d = 1 . 4 × 10 -2 cm and ~-~ketyl 530 = 5 )< 1 0 3 litres mol 1 cm 1,29 [K.] = 2.57 × 10 5 mol litre -1. With Dabs = 4.3 × 10 4 einsteins litre -1, one obtains & ( K . ) = 0 - 0 6 . Taking into account that ~b(T)=0-5, it is concluded that only a b o u t 12% of the B P triplets were converted to ketyl radicals. This result is in accordance with an assumption of H o r i e and Mita, z8 according to which the decay of B P triplets in a P M M A matrix, at t e m p e r a t u r e s a b o v e Tg, is d u e to a quenching reaction involving carbonyl groups of P M M A . O u r results substantiate the assumption of these authors, 28 w h o were not able to discriminate b e t w e e n hydrogen abstraction and energy transfer. Similar conclusions were drawn from the experiments with PSt matrices. The formation of ketyl radicals at t e m p e r a t u r e s a b o v e T~ 30 was clearly evidenced by O.D. measurements. B a s e d on E 5ke,yj = 5 × 1 0 3 litres mo1-1 cm 129 the fraction of triplets being converted to ketyl radicals was estimated as ca. 2 0 % .

AC~O~E~E~NTS The authors are grateful to D r G. B e c k (HMI) for maintaining the ruby laser, to D r J. Lilie (HMI) for assisting in data processing, and to D r W. Wunderlich ( R 6 h m G m b H ) for making p o l y m e t h y l m e t h acrylate of appropriate average molecular weight available. T h e assistance and the advice of D r R. B. Frings are highly appreciated.

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