The use of functionalized polymers as photosensitizers in an energy storage reaction

Solar Energy, Vol. 19, pp. 503-508. Pergamon Press 1977. Printed in Great Britain

THE USE OF FUNCTIONALIZED POLYMERS AS PHOTOSENSITIZERS IN AN ENERGY STORAGE REACTIONt RICHARD R. HAUTALA,JAMES LITTLE and EDWARD SWEET Department of Chemistry, University of Georgia, Athens, GA 30602, U.S.A. (Received 20 October 1976; in revised form 3 November 1976)

Abstract--Insoluble polymer bound photosensitizers, useful for the conversion of norbornadiene (1) to quadricyclene (2), have been synthesized. An acetophenone analogue was produced by Friedel-Crafts acylation of polystyrene resin while treatment of chloromethylated resin with salicyladehyde and triethylamine produced an aniogue of benzyloxybenzaldehyde.Reaction of lithio-polystyreneresin with methyl 4-(N,N-dimethylamino)benzoate gave a ketone equivalent to 4-(N,N-dimethylamino)benzophenone(3). Quantum yields for the conversion of 1 and 2 using the polymer bound sensitizers were generally comparable to, but slightly lower than, the analogous compound in homogeneous solution. The quantum yield of polymer bound 3 was less solvent dependent than that of the homogeneous counterpart. The advantages of isolating the photosensitizer to the photochemical reactor stage of a photochemical solar energy storage device are discussed. Efficient sensitization by polymer bound photosensitizers demonstrates the feasibility of this approach. 1. l l f f l l O I ) l J C T I O N ---.

The interconversion between norbornadiene 1 and quadricyclene 2 is a promising system for potential use in

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solar energy storage. Briefly the attractive features of this system include: 1. Norbornadiene is readily available and comparatively inexpensive. 2. Quadricyclene has a high volumetric storage capacity of approximately 1050J cc -1 (250 ca] cc -1) [1]. 3. Conditions exist for which both steps in the cyclical process can be made virtually quantitative. 4. Both norbornadiene and quadricyclene are liquids; quadricyclene is stable indefinitely at ambient temperatures but with an appropriate catalyst can be rapidly converted to norbornadiene under ambient conditions [2]. There are, of course, a number of difficulties which must be overcome before the system could seriously be considered as the basis for a practical device. Most prominent among these is the total lack of overlap between the electronic absorption spectrum of norbornadiene and the solar radiance spectrum (Fig. 1). Thus the direct conversion of 1 ~ 2 by sunlight is precluded. However, it is well known that sensitizers exist which are capable of effecting the conversion with quantum efficiencies near 100 per cent [3]. The absorption spectra of many of these sensitizers overlap at least in part with the solar radiance spectrum. A significant effort in these and other laboratories is currently devoted to the

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7OO 7 WAVELENGTH, nm Fig. 1. Overlap of the electronic absorption spectra (in non-polar solvents) norbornadiene (a), Acetophenone (b), o-benzyloxybenzaldehyde (c), and 4-(N,N-dimethylamino)benzophenone (d) with the solar radiance spectrum (e): Midday, Midsummer at Lat. 4&N (Ref. [15]). 200

tPresented in part at the First International Conference on the Photochemical Conversion and Storage of Solar Energy, London, Ontario, Canada (24-28 August 1976).

300

400

500

600

development of efficient photosensitizers which absorb a greater fraction of available sunlight. A second serious problem involves side reactions arising during either the photochemical or catalytic processes. For example, certain of the known photosensitizers produce undesirable photoadducts with either norbomadiene or quadricyclene [4]. Fortunately this does not appear to be a totally general phenomenon. Clearly sensitizers and/or conditions must be developed which scrupulously avoid this problem. Long term recyclability requires that the extent of side reactions be absolutely minimal in order to avoid frequent replacement of the photochemical fluid. Equally important are the long-term stabilities of the sensitizer and catalytic systems. Again, intensive efforts are underway in the investigation of each of these aspects. It is likely that the problems and features of the norbornadiene-quadricyclene interconversion are common to many potential chemical-based solar energy storage systems [5]. For this reason we have begun to look ahead to other aspects of the development which

503

504

RICHARD R. HAUTALA et al.

are necessary for an actual working device. The obvious need to confine the catalyst for the heat releasing reaction (2~1) to the "catalytic" chamber (Fig. 2) can be met by covalent attachment of the catalyst to.an insoluble polymer matrix. Efforts in this regard are in progress and some success has been achieved [6]. Similar confinement of the photosensitizer to the "irradiation" chamber is desirable for a number of simple reasons. The absolute quantity of sensitizer needed can thus be reduced by several orders of magnitude. Periodic replacement of the photochemical fluid would not necessitate replacement of the sensitizer and vice versa. Undesirable interactions with the catalyst (in the catalytic chamber) would also be avoided. For example these might involve destruction of the sensitizer by the catalyst or poisoning of the catalyst by the sensitizer.

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SOLAR ENERGY COLLECTOR

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Fig. 2. Schematic diagram of a potential system. We have begun investigations in the preparation and study of sensitizers covalently bound to polymeric matrices. At the outset of this work little information was available regarding the complications and problems that might be encountered in heterogeneous photosensitization[7]. We anticipated low quantum efficiencies relative to homogeneous systems. Among the effects which could decrease the efficiency are excessive light scattering and inefficient liquid diffusion to sensitizer sites within the polymer. The importance of these and other factors were virtually unknown. We undertook this study with the intent of assessing these factors and attempting to solve any difficulties encountered. Furthermore, we had the optimistic expectation that certain photosensitizers might exhibit a more favorable performance when polymer bound due to perturbations in the photophysical or photochemical behavior of the chromophore. Finally, if there were any selective adsorptive affinity of the polymer toward norbornadiene

(relative to that toward quadricyclene or any co-solvent) high local concentrations of norbornadiene could be maintained. Since the majority of undesired side reactions results from interaction of the sensitizer with quadricyclene[4] the impact of this annoying difficulty could be significantly reduced. We report here the synthesis and results concerning three functionalized polymers. These are compared to the corresponding sensitization efficiencies for the monomeric chromophores. In addition, two interesting effects have been observed that offer encouragement for the polymeric sensitizer concept. 2, EXPERIMENTAL

Reagents. Norbornadiene (Aldrich) was retluxed over potassium and distilled under N2. Authentic samples of quadricyclene (97 per cent) were prepared by the method of Smith [8]. Acetophenone (Aldrich) was distilled before use. 4-(N,N-dimethylamino)benzophenone (Aldrich) was used as received, ortho-Benzyloxybenzaldehyde was prepared by treating salicylaldehyde with benzyi chloride and triethylamine in acetonitrile. The resulting product was purified by distillation. Methyl-4-(N,N-dimethylamino)benzoate was prepared by treatment of 4-(N,Ndimethylamino)benzoic acid with diazomethane. The crude yellow ester was recrystallized from methanol at -78°C until white. Solvents were spectral quality and were distilled prior to use. Preparation of polymer A. A mixture of anhydrous AiC13 (11 g), acetic anhydride (3 ml) and nitrobenzene (50 ml) was added dropwise to a flask containing 2 per cent linked microporous polystyrene (Dowex, 6.8 g) in 25 ml nitrobenzene. The flask was fitted with a calcium chloride drying tube, and the mixture was stirred at room temperature for 3 days. After the nitrobenzene was removed by steam distillation, the beads were washed with tetrahydrofuran (THF), stirred for 2 days in a solution of THF containing triethylamine (10 per cent) extracted with THF for 1 day in a Soxhelet extractor and dried under vacuum. The resulting polymer was light brown. Preparation of polymer S. A solution of stannic chloride (2.7 ml) in chloromethyl ethyl ether (270 ml) was stirred for 5 min at 5°C and then added to a flask containing 30 g of 20 per cent crosslinked macroporous polystyrene (Dowex XFS 4022)[9]. The flask was fitted with a Dfierite drying tube, and the mixture was allowed to stir at room temperature for 5 hr. The beads were then filtered and washed with a 1:1 solution of 10 per cent aqueous HCI and dioxane followed by exhaustive washing with dioxane and drying under vacuum overnight. Analysis indicated 1.36 meq CI per g. A solution containing freshly distilled salicyladehyde (20ml) and dry triethylamine (1.12ml) was added to 4.94g of the chloromethylated polymer suspended in 20 ml of THF (dried over potassium metal and benzophenone). The mixture was heated to 78°C and stirred under nitrogen for 1 week. The beads were exhaustively washed with benzene and dried under vacuum. The resulting polymer was light tan in color. Preparation o[ Polymer N. A solution containing 3 ml

The use of functionalizedpolymers as photosensitizersin an energy storage reaction n-butyl lithium and 2.16 g tetramethylethylenediamine in 15 ml of dry, deoxygenated cyclohexane was prepared and stirred under nitrogen at room temperature for 30min[10]. A portion (8.1ml) of this solution was transferred by syringe to a flask containing 5 g of 20 per cent crosslinked macroporous polystyrene (Dowex XFS 4022, previously washed with THF and vacuum dried) suspended in 20ml deoxygenated cyclohexane (previously dried over potassium). The mixture was allowed to stir for 2 hr at 50°C in a flask designed to rigorously maintain an oxygen-free atmosphere. The light red beads were then washed 4 times under a dry, oxygen-free atmosphere with cyclohexane and dried under vacuum for I hr. The solution containing 2.0 g methyl 4-(N,N-dimethylamino)benzoate in dry, oxygen-free THF was transferred by syringe into a flask containing the polymer suspended in 25 ml THF. The beads immediately turned green and were stirred at room temperature overnight. After the addition of 15 ml deoxygenated methanol the beads turned bright yellow and were allowed to stir for 1 hr. The system was opened to air, washed exhaustively with THF (12hr in a Soxhelet extraction), and dried under a vacuum. Analytical procedure. Analysis for norbornadiene and quadricyclene was made by gas chromatography (Varian Model 2740 dual flame ionization instrument) using a 6 ft OV-101 column at 65°C. Under these conditions the retention times for norbornadiene and quadricyclene were 1.68 and 2.58 min respectively. Response factors were routinely derived from injections of standard solutions of authentic samples. Photolysis conditions. Samples were irradiated simultaneously in a custom-built merry-go-round apparatus similar to that described in the literature[l 1]. A watercooled 450W medium pressure Hg arc (Hanovia) provided irradiation through the windows of a centrally located cylindrical enclosure. The 366 nm Hg line was isolated with four Corning No. 7-37 filters in the windows of the cylinder. The 313 nm Hg line was isolated by circulating a 10-3 M K2CrO4-1% K2CO3solution through the water cooling jacket in combination with Corning No. 7-54 filters in the windows. The irradiation vessels were custom made from optically flat Pyrex 5.5 cm in height and 0.7 cm wide with an inside pathlength of 0.3 cm. The top of the cell consisted to a T 10/18 joint. When mounted in the slots of the merry-go-round apparatus the effective surface area irradiated was 4 cm high by 0.8 cm wide. Polymer samples were prepared for irradiation by filling the cell with polymer to a height of 4 cm and then pipetting 1.30 ml of the appropriate solution into the cell. Approximately 20 rain was allowed for equilibration prior to photolysis. Polymer samples were prepared for irradiation by filling the cell with polymer to a height of 4 cm and then pipetting 1.30 ml of the appropriate solution into the cell. Approximately 20 min was allowed for equilibration prior to photolysis. Actinometry. Incident light intensities were determined by using optically dense actinometer solutions which

505

were irradiated simultaneously in similar vessels. The conversion of valerophenone to acetophenone in benzene[12] (q~ = 0.33) and the decomposition of potassium ferrioxalate[13] (4,= 1.24) were used as primary actinometers. 3. RESULTS Acetophenone [3], ortho-benzyloxybenzaldehyde [14] and 4-(N,N-dimethylamino)-benzophenone[14] have been shown to be effective sensitizers for the conversion of norbornadiene to quadricyclene. The overall quantum efficienty is dependent on the concentration of norbornadiene (NBD). Our observations are consistent with the following abbreviated scheme. hv

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The quantum yield, ~b, for quadricyclene formation is given by / k:[NBD] 'l = d~v~bp~k,.[NBD] + k, + k~iO.qJ" A plot of ~)-1 VS [NBD] -1 is linear with a slope given by kl + k3[O2][c~r~bek2and an intercept given by l/q~rd~e. Thus an important index for the effectiveness of a sensitizer is the value of the intercept which optimally would be 1. A small value for the slope is also desirable. Acetophenone meets each of these requirements admirably. In air saturated hexane the values for the intercept and slope are 1.02 and 0.014 M respectively. In deoxygenated hexane the values are 1.00 and 0.0063 M[14]. It should be noted that the values for the slopes are dependent on the purity and nature of the solvent (due to quenching by adventitious impurities) and are not to be regarded as intrinsic values. It can be concluded, however, that the demands for a high concentration of NBD and for the absence of oxygen in order to achieve efficient photosensitization are low for acetophenone. It is further deduced that, based on the value of the intercept, there is no "inherent" inefficiency in the sensitization. The inherent sensitization efficiency of 4-(N,N-dimethylamino)benzophenone is also 100 per cent when the solvent used is non-polar (e.g. hexane or cyclohexane). The slopes in air saturated and deoxygenated solutions are 0.16 and 0.06 respectively[15]. Interestingly, however, when the polarity of the solvent is increased the sensitization efficiency drops markedly. For example, in aerated acetonitrile the intercept is -15-20 and the slope is -40. At a norbornadiene concentration of 0.5 M the quantum efficiency drops from 50 per cent in hexane to - 1 per cent in acetonitrile.

RICHARDR. HAUTALAet al.

506

The mechanistic studies concerning this effect will be described elsewhere. Detailed studies using o-benzyloxybenzaldehyde have not been completed but the compound appears to be a moderately efficient sensitizer. At 0.5 M norbornadiene the quantum yield for quadricyclene formation in air saturated hexane is 0.25 [14]. The absorption spectrum of each of these sensitizers overlaps at least in part with the solar radiance spectrum. The extent of this overlap is least favorable for acetophenone and most favorable for 4-(N,N-dimethylamino)benzophenone (Fig. 1). Even in the least favorable case, a solution (3.5 ml) consisting of 0.7 M acetophenone and 0.5 M norbornadiene in hexane can be quantitatively (>98 per cent) converted solely by sunlight to quadricyclene in less than one day. In order to evaluate the potential of using insoluble polymeric sensitizers, polymers were modified by covalently incorporating each of these chromophores. The synthetic schemes employed are presented in Fig. 3. Each of these preparations began with spherical Dowex polystyrene beads. The properties and appearance of the beads remained unchanged except for color. The size, porosity and extent of cross-linking are among the variables which could potentially affect the sensitization

efficiency. For our initial studies two types of beads were randomly selected. The properties of each are listed in Table 1. We have not yet completed detailed studies regarding the comparative merits of these two types but preliminary results indicate that the sensitization efficiency is comparable. The advantage of functionalizing performed polystyrene rather than polymerizing a monomeric sensitizer derivative seemed obvious. In the former, the polymeric sites which are functionalized are those most accessible to chemical reagent employed. These sites should then also be accessible to norbornadiene. In contrast, copolymerization of a sensitizer monomer would likely result in chromophores being randomly distributed and buried, at least in part, in the interior of the polymer matrix where contact with the liquid solution would be minimal. The probability of light absorption is roughly independent of the location since the polymer matrix itself is transparent to the useful wavelengths of light. Thus excitation of these internal chromophores would not result in sensitization and a large source of inefficiency would be encountered. The degree of functionalization is another variable which could potentially influence the sentitization efficiency. Our objective was to achieve sufficient functionalization so that a single layer of beads would absorb at least 99 per cent of the Polymer A light incident on the surface of the layer at wavelengths employed in the study (313 or 366 nm). Due to the high Et3N Ac20 • IP extinction coefficients of the chromophores, this nit robenzene THF requirement can be met by functionalization so low that C=O I the composition of the beads cannot be distinguished CH 3 from the unfunctionalized polymer by any analytical Polymer S technique other than UV spectroscopy. Thus the chemical composition of the beads remains only partially SnCI4 aq. HCI salicylaldeh~e " ~ n characterized. It should be noted that the extent of CICH2OEt • ~ Et3N THF CH2 functionalization is highly dependent on the success of I O O the synthetic pathway. It is clear that the degree of functionalization achieved fell short of the theoretical yield but was still sufficient to meet the light absorbing Polymer N requirements. The influence of solvents in "opening up" or swelling butyl lithium MDAB methanol the polymers is an intriguing factor for consideration. It TMDE I~ THF b I~ seemed clear that if the chemical functionalization took CyCIohexane place under solvent conditions which swelled the polymer and if the sensitization were carried out under H~(CH3)2 conditions where the polymer contracted, then light Fig. 3. Synthetic schemes for preparation of the polymeric absorbing chromophores might be buried and inaccessible to the liquid solution containing norbornadiene. sensitizers. Table 1. Properties of the polystyrene beads used in this study Type Dow XFS 4022

Appearance

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Mesh size§

Porosity type

Percent crosslinkingi

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20%

~ 50

Microreticular

2%

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The use of functionalizedpolymers as photosensitizersin an energy storage reaction

507

noted that some of our earlier preparations produced polymers which were far less effÉcient. It is clear that the polymeric sensitizers are minimally competitive with the homogeneous systems, and, in certain cases, may be even more efficient. The recyclability of the polymeric sensitizers was tested in the following manner. After using the polymer for several hours it was washed in a Soxhelet extractor, dried under vacuum, and then used with a fresh solution. No variation in quantum efficiency was observed through several such cycles. The effect of solvent was examined using polymer N. It had been shown that the monomeric analog of this sensitizer is very sensitive to solvent with respect to the efficiency of sensitization. For example, the maximum (extrapolated) efficiency for sensitization is 0.06 in acetonitrile (1.0 in hexane), and the value of 0.5 M norbornadiene is approximately 0.01. The effect is thought to be due to three concurrent factors[14]: (1) a decrease in triplet yield (to 0.06); (2) a decrease in the rate constant for sensitization; (3) an increase in rate constant for radiationless deactivation of the triplet excited state. In contrast, the use of acetonitrile as a solvent with polymer N results in no significant decrease in efficiency relative to the use of cyclohexane. The measured quantum efficiency using 0.5 M norbornadiene is 0.32 which is well above the maximum (extrapolated) value in homogeneous solution. It should also be mentioned that the quantum efficiency for polymer N using neat norbornadiene is high (0.47). Finally we have observed that polystyrene appears to exhibit some sort of adsorptive affinity for norborTable 2. Comparison of insoluble polymeric vs soluble nadiene. As shown in Table 3, the apparent concenmonomeric sensitizers for the photoisomerization of nor- tration of certain solutions containing norbornadiene in bornadiene to quadricyclene the present of polystyrene beads gradually decreases over a period of 5-10m i n until a limiting value is Quantum efficiency reached. It is possible that a locally high concentration of for production SensRizert quadricyclene~; norbornadiene is maintained within the polymer even though the norbornadiene concentration in bulk solution acetophenone 0.9 would, of course, decrease during a photolysis experiPolymer A 0.26 ment. We, in fact, have observed no significant decrease o-Benzyloxybenzaldehyde 0.3 in measured quantum yield for polymeric solutions Polymer S 0.24 4-(N,N-dimethylamino)benzophenone 0.5 photolyzed to high conversions. This observation is Polymer N 0.55 consistent with our speculation. While we have also observed a similar effect for quadricyclene using polyt0.5 M norbornadiene in cyclohexanein aerated solutions. ~t313nm irradiationemployed;sensitizersall absorbed >99 per styrene beads, it might be possible to prepare polymers cent of incident light. with a selective affinity for norbornadiene.

The opposite sets of conditions might well be optimal. Unfortunately, there is frequently little latitude in the choice of reaction conditions and those required vary markedly depending on the type of chemical functionalization desired. Furthermore an actual solar device based on this system would operate most efficiently (in terms of liquid storage volume) using neat norbornadiene. Polystyrene does not swell to any significant degree in norbornadiene. Another factor imposed on the system by choice of the solvent is the degree of reflection of light by the polymer. This is very effectively minimized by choosing a solvent with a refractive index closely matching that of the polymer. Benzene and carbon tetrachioride are excellent in this regard. Indeed, a cell filled with polymer and benzene is quite transparent, and acceptable ultraviolet-visible spectra can be taken under such conditions. The primary purpose of the initial phase of our studies was to examine the feasibility of attaining any photosensitization of the norbornadiene-quadricyclene conversion by heterogeneous polymeric sensitizers and, if success were achieved, to compare the quantum efliciencies with analogous homogeneous systems. In Table 2, the results from three parallel systems are presented. We have attempted to devise experimental conditions which make this comparison as meaningful as possible. However, reflective light losses by the polymer cannot be totally avoided. Consequently, the values obtained for polymer systems are likely to be low for that reason. In any event the efficiencies observed for the polymeric sensitizers were surprisingly high. It should be

Table 3. Adsorptiveaffinityof polystyrene for norbornadiene

Solvent

Norbornadieneconc. in originalsolution§ (M)

Hexane Benzene Ethanol

0.50 0.25 0.25

Measurednorbornadiene conc. after equiliPer cent bration¶with polymer adsorbed (M) (%)

tMacroporous polystyrene. ~Microporous polystyrene. §0.6g polymer and 1.7 ml solution. ¶Approximately20 rain.

0.45 0.25 0.18

10 0 29

RaCHARD R. HAUTALAet al.

508 4. CONCLUSIONS

Our preliminary studies concerning the use of polymeric sensitizers in the photoisomerization of norbornadiene to quadricyclene have demonstrated that high quantum efficiencies are obtainable. A multitude of experimental parameters exist for refining and optimizing the efficiencies. Most promising among our initial findings are the observations that under certain conditions (for example, Polymer N in acetonitrile) the polymeric sensitizer is dramatically superior to the homogeneous counterpart and that an adsorptive affinity of the polymer for norbornadiene is possible. Hopefully these and other characteristics of polymers can be exploited to make the polymeric sensitizers much more than simply a heterogeneous substitute. Acknowledgements--We gratefully acknowledge generous financial support from the Energy Research and Development Administration (Contract No. E(38-1)-893) and the National Science Foundation (Grant No. CHE75-13752). REFERENCES

1. D. S. Kabakoff et al., Enthalpy and kinetics of isomerization of quadricyclene to norbornadiene. Strain energy of quadricyclene. J. Am. Chem. Soc. 97, 1510 (1975). 2. H. Hogeveen and H. C. Voger, Valence isomerization of quadricyclene to norbornadiene catalysed by transition metal complexes. J. Am. Chem. Soc. 89, 2486 (1%7). 3. S. Murov and G. S. Hammond, Mechanism of photochemical reactions in solution, LVI. J. Phys. Chem. 72, 3797 (1%8); G. S. Hammond et al. Photosensitized isomerization involving saturated centers. J. Am. Chem. Soc. 86, 2532 (1%4).

4. A. A. Gorman et al, Concerning the mechanism of interaction of triplet benzophenone with norbornadienes and quadricyclesnes. Tetrahedron Letters 5085 (1973). 5. G. Jones and B. R. Ramachandran, Catalytic activity in the reversion of an energy storing valence photoisomerization. J. Org. Chem. 41, 798 (1976); M. D. Archer in Photochemistry, Vol. 5, Specialist Periodical Reports, Chemical Society Publications (1975); W. Sasse, Plenary Lecture, First international conference on the photochemical conversion and storage of solar energy. London, Ontario, Canada (1976). 6. R. B. King and E. M. Sweet, The University of Georgia. Unpublished results. 7. J. R. Williams et al. Preparation of singlet oxygen by heterogeneous photosensitization. Tetrahedron Letters, 4603 (1973); E. C. Blossey et al. Polymer-based photosensitizers for photooxidations. J. Am. Chem. Soc. 95, 5820 (1973). 8. C. D. Smith, Quadricyclene (Tetracyclo[3.2.0.02'7.04'6] heptane). Org. Syn. 51, 133 (1971). 9. K. W. Pepper et al. Properties of ion exchange resins in relation to their structure. Part VI. Anion-exchange resins derived from styrene-divinylbenzene copolymers. J. Chem. Soc. 4097 (1953). 10. D. C. Evans, et al., Reaction of polystyrene with nButyllithium-N,N,N',N'-Tetramethylethylenediamine. J. Polym. Sci., Polym Chem. Educ. 12, 247 (1974). 11. F. G. Moses et al., Merry-go-round quantum yield apparatus. Mol. Photochem. 1,245 (1%9). 12. P. J. Wagner and A. E. Kemppainen, Type II photoprocesses of phenyl ketones. Triplet state reactivity as a function of 3, and 8 substitution. J. Am. Chem. Soc. 94, 7495 (1972). 13. A. J. Gordon and R. A. Ford, The Chemist's Companion, p. 362. Wiley, New York (1972). 14. R. R. Hautala, K. Kim, J. Little and S. Harris, The University of Georgia, Unpublished Results. 15. R. G. Zepp and D. M. Cline, Rates of direct photolysis in the aquatic environment. Env. Sci. Technol., in press.

Resumen--Ha sido sintetizada una uni6n polimera fotosensibilizadora, fttil para la conversitn de norbornadiene (1) a quadricyclene (2). Una acetofenona amiloga fu6 producida por acilacitn Friedel-Crafts de resina de poliestireno mientras que el tratamiento de resina clorometilada con salicilaldehido y trietilamina produjo un anfilogo de benciloxibenzaldehido. La reacci6n de resina de litio-polistireno con metil 4-(N,N-dimetilamino) benzoate di6 una acetona equivalente a 4-(N,N-dimetilamino) benzofenona (3). El consumo cmintico para la conversi6n de 1 a 2 usando la unitn polfmera sensibilizadora fueron generalmente comparables, pero levemente menores al compuesto amilogo en soluci6n homogtnea. El consumo cmintico de la unitn polfmera 3 fu6 menos dependiente del solvente que su contraparte homogtnea. Se discuten las ventajas de aislar el fotosensibilizaror de la etapa reactora fotoqufmica de un dispositivo fotoqufmico de acumulacitn de la energfa solar. La eficiente sensibilizacitn por la unitn polfmera fotosensibilizadora demuestra la factibilidad de este enfoque. Rtsumt--Des polymtres insolubles photosensibilisateurs de liaison, utilisables pour la conversion du norbornadi~ne (1) en quadricycl/me (2), on 6tg synthttist~s. Une acgtophtnone analogue a 6tg produite par la rtaction de Friedel et Crafts d'acylation de rtsine de polystyrene, tandis que le traitement de la rtsine chlorom~thylte avec I'aldthyde salicylique et la tritthylamine produit un analogue de la benzyloxybenzaldthyde. La rtaction de la rtsine tithio-polystyr~ne avec la 4 (N,N-dimgthylamino) benzoate de mtthyl a donn6 une cttone gquivalente ~ la 4 (N,N-dimtthylamino) benzophtnone (3). Les rendements quantiques pour la conversion de 1 en 2 utilisant les sensibilisateurs de liaison polymgris~s ont &6 en gtn6ral comparables, mais 16g~rement inftrieurs, au compost analogue dans une solution homog~ne. Le rendement quantique du polym&re 3 dtpendait moins du solvant que celui de son correspondant homog~ne. On a 6tudi6 les avantages d'isoler le photosensibilisateur de l'ttage du r6acteur photochimique d'un syst~me de stockage photochimique de rgnergie solaire. Une sensibilisation eflicace par polym~res sensibilisateurs de liaison d6montre la faisabilit6 de cette approche.