J. Phys. Chem. 1992, 96, 10365-10371 (7) Nakamoto, N. Infrared and Roman Spectra of Inorganic and Coordination Compounds, 3rd 4.Wiley: ; New York, 1978. (8) Stampfl, S.R.;Chcn, Y.; Dumesic, J. A.; Niu, C.; Hill, C. G. J. Carol. 1987, 105, 445. (9) Oganowski, W.; Hanuza, J.; Jezowska-Trzebiatowsk, B.; Wryszcz, J. J. Coral. 1975, 39, 161. (10) Hanuza, J.; Jezowska-Trzebiatowska, B.; Oganowski, W. J. Mol. Carol. 1978, 4, 271. (1 1) Hanuza, J.; Jezowska-Trzebiatowska,B.; Oganowski, W. Bull. Acad. Pol. Sci.. Ser. Sci. Chim. 1977, 25, 569. (12) Vuurman, M. A.; Wachs, I. E. J. Phys. Chem. 1992, 96, 5008. (13) Liegeois-Duyckaerts. M.; Tarte, P. Spectrochim. Acra 1974, 3OA, 1771. (14) Cheng, C. P.; Ludowise, J. D.; Schrader. G. L. Appl. Specrrosc. 1980, 34, 146. (15) Went, G. T.; Oyama, S.T.; Bell, A. T. J. Phys. Chem. 1990,94,4240. (16) Buckley. R. I.; Clark, R. J. H. Coord. Chem. Rev. 1985, 65, 167. (17) Horsley, J. A.; Wachs, I. E.; Brown, J. M.; Via, G. H.; Hardcastle, F. D. J. Phys. Chem. 1987, 91, 4014. (18) Payen, E.; Grimblot, J.; Kasztelan, S.J. Phys. Chem. 1987, 91,6642. (19) Lcciejewicz, J. Z . Krisrallogr. 1965, 121, 158. (20) Chiu, N.-S.; Bauer, S.H. A d o Crysrallogr. 1984, C40, 1646. (21) Hardcastle, F. D.; Wachs, I. E. J . Raman. Spectrosc. 1990,21,683. (22) Wdliams, C. C.; Ekcrdt, J. G.; Jehng, J.-M.; Hardcastle, F.D.; Turek, A. M.; Wachs, 1. E. J. Phys. Chem. 1991, 95, 8781. (23) Dawson, P.; Hadfield, C. D.; Wilkinson, G. R.Phys. Chem. Solids 1973,34, 1217. (24) Rao, C. N. R. In Solid Srare Chemistry;Marcel Dekker: New York, 1974; p 107. (25) Bare, S.R.;Mitchell, G. E.; Gland, J. L. To be submitted. (26) Jezowska-Trzebiatowska,9. Pure Appl. Chem. 1971, 27, 89. (27) Griffith, W. P. Coord. Chem. Rev. 1970, 5,459.
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(28) Lindquist, I. Nova Acra Regiae Soc. Sci. Ups. 1950, 5. 15. (29) Lindquist, I. Acra Chem. Scand. 1951, 5, 568. (30)Daniele, G. G u n . Chim. Ral. 1960, 90, 1371. (31) Sasaki, Y.; Lindqvist, I.; Sillen, L. G. J . Inorg. Nucl. Chem. 1959, 9, 93. (32) Glemr, 0.;Holznagel, W.; ALi, S.I. 2.Narwforsch. 1965,2W, 192. (33) Aveston, J.; Anacker, E. W.; Johnson, J. S.Inorg. Chem. 1%4,3,735. (34) Carpeni, G . Bull. SOC.Chim. Fr. 1947,14,484. (35) Sasaki, Y. Acra Chem. Scand. 1%1,59,710. (36) Linquist, I. Acra Chem. Scand. 1956, 4, 324. (37) Tanabe, K. Solid Acids and Bases; Academic: New York, 1970; p 50. (38) Jeziorowski, H.; Knozinger, H. J . Phys. Chem. 1979,83, 1166. (39) Kihlborg, L. Ark. Kemi 1963, 21, 357. (40) Malinowski, St.; Szczepanska, S.;Bielanski, A,; Sloczynski, J. J. Coral. 1965, 4, 324. (41) Russo, S.;Noguera, C. Surf. Sci. 1992, 262, 245. (42) de Vleesschauwer, W. F. N. M. In Physical and Chemical Aspects of Adsorbenrs and Caralysrs; Linsen, B. G., Ed.; Academic: New York, 1970; Chapter 6. (43) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 4th ed.; Wiley: New York, 1980. (44) Volpe, L.; Boudart, M. Coral. Rev. Sci. Eng. 1985, 27, 515. (45) Chan, S.S.;Wachs, I. E.; Murrell, L. L.; Dispenziere, N. C. J. Caral. 1985, 92, 1. (46) Jenhng, J.-W.; Wachs, I. E. J . Phys. Chem. 1991, 95, 7373. (47) Stencel, J. M.; Makovsky, L. E.; Sarkus, T. A.; De Vrits, J.; Thomas, R.; Moulijn, J. A. J . Catal. 1984, 90, 314. (48) Lagers, M. A. Unpublished data. (49) Liegeois-Duyckaerts, M.; Tarte, P. Spectrochim. Acra 1972, 28A. 2037. (50) Parks, G. A. Chem. Rev. 1965,65, 177.
Infrared Reflection Absorption Spectroscopic Study on the Photodimerization Process of a Stilbazollum Cation Embedded in Langmuir-Biodgett Films Masato Yamamoto, Takuo Wajima, Akiko Kameyama, and Koichi Itoh* of Chemistry, School of Science and Engineering, Waseda University, Shinjuku- ku, Tokyo 169, Japan (Received: May 4, 1992; In Final Form: July 21, 1992) Department
Infrared reflection absorption (IRA) spectroscopy was applied to photodimerization process induced by irradiation (at 340 nm) of N-(l-octadecyl)-4-stilbazoliumcations (C18S)incorporated in the Langmuir-Blodgett films of perdeuterated arachidic acid (CD3(CD2)18COOH,A). The IRA spectra proved that the main process proceeds through cyclobutane ring formation of the stilbazolium cations in a syn head-to-head arrangement. The extent and rate of photodimerization were followed by measuring the intensity of an olefinic 0-c stretching vibration of ClsS,changing the incorporation ratio (C18S:A)and the state of the matrix molecule, A (acid form or dissociated form). The extent and rate of photodimerization in the matrix with the acid form does not depend on the incorporation ratio (( 1:4) and (1:9)); the result indicates that the ClSS molecules assemble with each other, forming a phase separated from the matrix. The rate and extent of photodiierization in the matrix with the dissociated form are much lower than those observed in the matrix with the acid form, suggesting more dispersed distribution of CI8Sin the former matrix. The photodimerization consists of two steps of second-order processes; the first process (in the irradiation period of 0 to ca. 45 s) proceeds much faster than the second one (in the period of 45 to ca. 180 s). These kinetic data correspond to the time course of orientation changes of the alkyl chains of C18S,which was elucidated by the IRA spectra in CH3 and CH2 stretching vibration regions. In the first process the stilbazolium cations, which are favorably stacked for dimerization,readily form cyclobutanerings, causing a rapid change in the orientation. This orientation change results in a repulsive interaction between the alkyl chains and the matrix molecules. A slow change in the orientation observed for the second step was interpreted as a relaxation process of the repulsive interaction.
Introduction
It has been known that, on irradiation by UV light, olefins such as trans-cinnamic in a crystalline state undergo dimerization, resulting in the formation of cyclobutane rings; the geometrical structures of the products are lattice controlled and topochemical principles have been proposed for the relationship between the configurations of the side chains around the cyclobutane ring and the arrangements of the olefins in the ~ r y s t a l . ~ 7 ~ Photodimerization of stilbazoliumcations in monolayer assemblies such as Langmuir-Blodgett films and micelles has also been studied.s.6 Quina and Whittens studied the photodimerization process of N-(l-octadecyl)-4-stilbazoliucation m (CIsS,see Figure 1) incorporated in the LangmubBlodgett (LB) films of arachidic
acid. They pointed out the importance of preferential packing into dimeric sita in the monolayer assemblies for the photodimerization. The photodimerization of tram-stilbazolium cations leads to the formation of one of four possible structures, as depicted in Figure 1. Takagi et al.6 analyzed structures of photodimerization products from trans-4-stilbazolium and trans-Nmethyl-4-stilbazolium (MS,see Figure 1) cations in revemd micelles. They reported that product distributions can be controlled by changing the stilbazolium/surfactant and surfactantlwater ( 0 )ratios; especially in the case where u is less than 20, dimer 3 (Figure 1) is selectively formed. All these studies have proved that topological control due to the presence of stilbazolium cations in organized molecular assemblies plays an
0022-3654/92/2096-10365S03.00/0Cb 1992 American Chemical Society
10366 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
DIMER 2
DIMER 1
Ph
Ph
DIMER 4
DIMER 3
TYPE
1
TYPE
11
Figure 1. Structures of ClsS and MS and those of possible photodimerization products. (A trans conformation is assumed for the stilbazole derivatives.)
important role in photodimerization processes. In order to get more insight into the photodimerization of stilbazole derivatives in LB films, we should monitor the rate and extent of the reaction as well as the dormation and orientation change of the reactants during irradiation of UV light. In the present paper, Fouriertransform infrared reflection absorption (IRA) spectroscopy was applied to study the photodimerization of C18Sembedded in the LB films of perdeuterated arachidic acid (CD3(CD2)18COOH), which is abbreviated to A. The IRA spectroscopy has been established as a powerful method to determine structures and orientations of monomolecular layers prepared by various ways such as LB7-12and self-assembling methods.13J4 As mentioned below, the IRA spectra present detailed information about structural and orientation changes of C18S during photodimerization processes. Further, comparison of the IRA spectra of photodimerization products with the infrared (IR) spectra of dimers 1, 2, and 3, which were prepared from N-methyl-l-stilbazolium salts (MS), provides information about configurations around the cyclobutane rings of the products, revealing new aspects of topologically controlled photodimerization processes of C18S in the LB films.
Experimental Section Matcrtla CIBSwas synthesized as both bromide and pchlotobenzmesulfonte salts by the method of Quina and Whittens and plrifiedby ncrystallization from 2 - p r o ~ l The . preparation method reported by Takagi et aL6 was applied to obtain transand cis-MS and dimers 1 and 2 synthesized from MS. The comapondingdimer 3 was prepared by the procedure of Takagi.I5 Purity and configuration of dimers 1 , 2 and 3 were checked by measuring proton 400-MHz NMR spectra in chloroform-d. Pnpurtion of LB Films. The method of preparation of LB films was essentially the same as that employed in the previous paper.16 Water used for the preparation was purified with a Millipore water purification system (Milli-Q, 4-bowl). A silver film with a thickness of about 100 nm, which was deposited on a slide glass by an evaporation method, was used as a substrate. A Kyowa Kaimen Kagaku, HBM-AP2, Langmuir t r o u a with a Wilhelmy balance was used for LB film fabrication. A monolayer was spread on trough water by dropwise addition of one of the sample solutions (1:4 and 1:9 (C18S:A) mixtures and A) in chloroform (total concentration of 5 X lV3mol/L). The pH of the troughwater was kept at 4.5 for the preparationof LB films with the acid form of the matrix molecule (A). A CdC12solution was added to the trough water to the level of 2 X 10-4 mol/L at pH 6.8 for preparing LB f h with the dissociated form of the matrix molecule. The pH values were adjusted by using NaHC03 and HCl. The monolayers were transferred to the substrate by a vertical dipping method at the surface pressure of 30.0 mN/m, at the dipping s p e d of 2.5 mm/min and at 23.5 OC. The first transfer was always perfarmed in an upatrob mode. The transfer ratio was near 1.0 for the LB films with the matrix molecule of the dissociated form. In the case of the LB films with the acid form of the matrix molecule, the ratios of odd and even number
Yamamoto et al. TABLE I: Comprriroll betweem IB Band Freqmeack (em-’) Cbmcteristie of Dimers 1,2, and 3 ud Tkrc obrarrd for the Photodimedutloa Products in the 15-Mowbyer LB Fila of the 1:4 Mixture (A)and Tboee in tk Alternatb# LB Fua (B) (See Text) dimer 1 dimer 2 dimer 3 sample A sample B 985 972 912 914 971 945 958 930 922 935 931 930 920 908 906 893 885 889 889 889 878 876 862 866 860 860 835 823 822 791 793 791 169 769 768 768 764 754 756 154 756 758
layers were 0.97 and 0.92, respectively. An alternating LB film consisting of monolayers of the (1:4) mixture and those of A was fabricated by successive transfer of the component monolayers. All the LB films were of a Y-type structure. Pbotodimerizrtioa. Photodimerization was performed by irradiating LB films with a 15O.W xenon lamp. A Hoya U340 filter was used to get a light source around 340 nm. MeasuremenL IRA and IR spectra were recorded by using a JEOL JIR-5500 Fourier-transfm infrared spectrometer equippad with TGS (for IR measurement) and liquid nitrogen cooled MCT (for IRA measurement) detectors with a resolution of 4 cm-’. The angle of incidence was kept at 80° for IRA measurements. ‘H NMR (400 MHz) spectra were recorded by using a JEOL GSX-400 NMR spectrometer. ReSdtS I n i n r e d ~ o f D i m e r s 1 , 2 , m d 3 ~ t r o m M SThe . infrared spectra of dimers 1,2, and 3 prepared from MS were measured to elucidate correlation between IR bands and configurations of the cyclobutane ring (or the arrangement of the N-methylpyridiniumyl and phenyl group around the cyclobutane ring). The spectra in the 1050-700-cm-1 region are summarized in Figure 2 together with the spectrum of trans-MS (see Figure 1). The 976-cm-’ band of rrans-MS (Figure 2A) can be asigned to a CH out-of-plane bending vibration of the olefinic group,” The IR spectrum o k e d for cis-MS (not shown in this paper18) indicates that the IR bands at 891 and 781 cm-’characterize the cis conformation. The IR bands of the dimers in Figure 2, parts B-D, contain coupled vibrations of CH bending and C-C stretching vibrations, and the spectral features of each dimer are expected to characterize its configuration. Comparing the IR spectra of trans- and cis-MS with those of the dimers, we can conclude that the IR f r c q u d a listed in Table I are characteristic of each configuration of the dimers. From the table it is clear that the frequencies listed for dimer 1 correspond well to those for dimer 3. As can be seen from Figure 1,if we look upon the phenyl and N-methylpyridiniumyl groups as identical units, the codigurations of dimers 1 and 3 are virtually identical with each other, which explains the similarity of the characteristicf r c q d e s of both dimers. (The slight frequency differences between the corresponding IR bands of dimers 1 and 3 are ascribable to a crystal packing effect.) This meam that, resorting only to the IR h”tI‘c frcqwncies, we can dimMnate neither between dimers 1 and 3 nor between dimers 2 and 4. Then, the confguration of dimers 1 and 3 is called type I as a whole and those of dimers 2 and 4 type 11. IRA spcctnl amoge during pbtodircriptk.ofC,ls in tbe 15-Mollohyer LB FLlm of tbe 1:4 Mixture with the Add Form of the Matrix Mdccpk. Figure 3A illustrates the IRA spectral change in the 1800- 1550-cm-’ region observed for the 15-monolayer LB film of the 1:4 Cl8S:A mixture during irradiation at
The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10367
Spectra of Stilbazolium Cation Embedded in LB Films
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340 nm. With the onset of irradiation, the 1626-cm-' band due to an olefinic C = C stretching vibration of c18s shows a precipitous intensity decrease, indicating that a photochemical reaction is taking place. Quina and Whitten5observed a similar decrease in absorption intensity near 350 nm induced by irradiating the LB films containing c18s and arachidic acid at 366 nm. They ascribed the result to photodimerization of the stilbazole derivative. The IR bands at 1707 cm-l are assigned to a C = O stretching vibration of the carboxyl group of A, and its intensity remains unchanged during the photoreaction. On the other hand, the 1645-cm-' band, which is assigned to a ring stretching vibration of the N-methylpyridiniumyl group of c18s, shows a slight intensity increase, and the band near 1600 cm-l, which is due to the phenyl group of c& decreases its intensity. These results suggest an orientation change of the N-methylpyridiniumyl and phenyl groups during the photoreaction (vide infra). Figure 3B exhibits the IRA spectral changes in the 104Cb 72O-m-' region during irradiation (at 340 nm) of the same sample
as that of Figure 3A. The 969-m-l band, which is the counterpart of the 976-cm-' band of trans-MS (Figure 2A) associated with a CH out-of-plane bending vibration of the olefinic group, reduces its intensity with the onset of irradiation, indicating that the photoreaction accompanies disappearance of the olefinic group. Intensity decrease observed for the bands at 999, 887, and 762 cm-l is ascribable to the photoreaction. The irradiation does not cause any feature near 891 and 78 1 an-', which are characteristic of cis-MS (vide supra). Thus, as far as the IR data are concerned, trans-cis isomerization does not take place in the LB film. Quina and Whitten5 also reported that photoirradiation does not lead to detectable isomerization reaction in the monomolecular assemblies containing c18s. As Figure 3B shows, there appear IR bands at 974, 930, 866, 860, and 754 cm-l at the beginning of irradiation and, after about 30 s of irradiation, another set of IR bands appear at 958 and 769 cm-l. As listed in Table I, the former set of IR bands have a counterpart in the characteristic IR frequencies of both dimers 1 and 3. Thus, a cyclobutane ring with the configuration of type I (see Figure 1) is formed with the onset of irradiation. The latter set of IR bands (958 and 769 cm-l) do not correspond well to the characteristic bands of type I or type I1 configuration. We tentatively considered, however, that the 958- and 769-cm-l bands correspond to the IR bands at 945 and 769 cm-l observed for dimer 2 (Figure 2). The discrepancy in relative intensity between the two sets of bands and that in frequency between the 958- and 945-cm-l bands can be explained
Yamamoto et al.
18368 The Journal of Physical Chemistry, Vol. 96, No. 25, 1992
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by considering that the configuration of the cyclobutane ring formed in the LB film is largely distorted from that of dimer 2 prepared from MS. As Figure 2 shows, dimer 2 gives rise to a characteristic band at 920 cm-'.The band near 930 cm-'in Figure 3B may contain a contribution from the 920-cm-' band. Thus, the IR spectra in Figure 3B suggest that formation of a cyclobutane ring with type I1 conf@ration starts after a certain period of irradiation (about 30 s). Although it is not clear compared to the IR bands listed above, the bands at 935,889, and 791 cm-' in Figure 3B may also be assigned to photodimerization products. The 791-m-I band is a counterpart of the 793-cm-' band of dimer 2 (Figure 2C). As listed in Table I, the other bands correspond to one of the IR bands characteristic of dimers 1 and 3. IRA spectral changes during wotodimerizationof C,& in tbe Alternating LB Fihn Consisting of a Monolayer of the 1:4 Mixture and a Monolayer of A. Figure 4, parts A and B, summarizes the IRA spectral changes in the 1800-1600 and 1040-720-~m-~regions, respectively, during irradiation (at 340 nm) of the alternating LB film consisting of 15 monolayers of the 1:4 mixture and 14 monolayers of A. (At first the 1:4 mixture monolayer was transferred to the substrate. The matrix molecule in both layers assumes the acid form.) Upon irradiation at 340 nm the LB fdm shows an intensity decrease of the olefinic C=C stretching vibration at 1626 cm-', which is virtually completed after several minutes. Spectral changes in the 1050-700-~m-~ region (Figure 4B) are not clear compared to those observed in Figure 3B because of the presence of intense bands near 960 and 840 cm-'ascribable
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to the matrix molecule. Figure 4B indicates, however, that with the onset of irradiation there appear IR bands at 97 1,931, 893, 889, and 756 cm-'. The frequencies are compared with those of dimers in Table I, which indicates that each of the bands has a counterpart among the characteristic bands of dimers 1 and 3. The IR bands at 791 and 768 cm-' in Figure 4B arise after a certain period of irradiation (30-40 s). The 791- and 768-cm-' bands correspond to the 793- and 769-cm-' bands of dimer 2 (Figure 2C), respectively. Then, as in the case of the photodimerization in the LB film of the 1:4 mixture, the dimerization of ClsS in the alternating LB film initiates with the formation of a cyclobutane ring of type I configuration and, after about 35 s of irradiation, starts to give a dimerization product of type I1 configuration. comparisonof the Rate and Extent of Photodherimh of C,& in Various LB Filnrs.In addition to the LB films discussed above, we extended the IRA study to the 15-monolayer LB film of the 1:9 CIIS:Amixture in the matrix with the acid form and to those of the 1:4 and 1:9 mixtures in the matrix with the dissociated form. On irradiation at 340 nm, these LB films also show a similar intensity decrease in the olefinic C = C stretching vibration. The IRA spectra in the 1040-720-~m-~ region of the LB films give rise to the IR bands ascribable to the dimers, proving that photodimerization takes place in these LB films. In Figure 5 the intensity ratio of the c=C stretching vibration relative to the intensity observed without irradiation is plotted against irradiation time for the LB films. The plots, which express the rate and extent of photodimerization in the LB films, reveal the following points. 1. The rate and extent of photodimerization of the LB film of the 1:4 mixture in the matrix molecule with the acid form is roughly identical with those of dimerization in the corresponding LB film of the 1:9 mixture. 2. The rate and extent of photodimerization in the LB films of the 1:4 and 1:9 mixtum in the matrix with the dissociated form are much smaller than those of the photodimerization observed for the corresponding LB films in the matrix with the acid form. 3. The extent of photodimerization in the alternating LB film containing the 1:4 mixture monolayers and the A monolayers is appreciably smaller than that of the 15-monolayer LB film of the 1:4 mixture in the matrix molecule with the acid form; thus, the intervening monolayer of A reduces the extent of dimerization. 4. The decrease in the intensity ratio is virtually completed within several minutes of irradiation for all the LB f h . Gradual decrease in the ratio, however, is still observed after the period especially for the LB films in the matrix with the dissociated form; presumably, this is due to a bleaching process other than the dimerization in the LB films.
Spectra of Stilbazolium Cation Embedded in LB Films
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The Journal of Physical Chemistry, Vol. 96, No. 25, 1992 10369
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More insight into photodimerization in the irradiation period of &180 s can be obtained by plotting the reciprocal of the intensity ratio of the C-C stretching vibration against irradiation time. As Figure 6 shows, the plots for each LB film clearly conform to two sets of straight lines, proving that the photodimerization in all the LB f h follows two steps of secondsrder reactions. The dimerization procesds at a faster rate in the initial step (0 to ca. 45 s) than in the second step (45-180 s). orientation auqp ofthe side croups ofthe C& in LB Films during Wotodimerizrtion. Figure 7A represents IRA spectral changes induced by irradiation at 340 nm in the CH, and CH, stretchingvibrations of the alkyl group of C& in the 15-monolayer LB film of the 1:4 mixture in the matrix molecule with the acid form. The bands at 2956,2921,2873,and 2850 cm-'are assigned to CH3 asymmetric (v,(CH3)), CH2 antisymmetric (v,(CH,)), CH3 symmetric (v,(CH,)), and CH2 symmetric (v,(CH,)) stretching vibrations, respecti~e1y.l~Figure 7B shows the IRA spectral features in the CD3and CD2stretching vibration regions ascribable to the matrix molecule, A, during photodimerization. In contrast to the case of the CD3and CD2stretching vibrations, the intensities of which remain unchanged, the CH, and CH, stretching vibrations show appreciable intensity change during the photodimerization. This can be recognized from Figure 8, which summarizes the relative intensity change of the u,(CH,), v,(CH3), and v,(CHJ vibrations in Figure 7A. (The intensities are normalized by those of the correspondingvibrations observed without irradiation.) In Figure 9 the relative intensity changes of the u,(CH3), u,(CH3), and v,(CHJ vibrations are also plotted for the photodimerization of the alternating LB film consisting of the 1:4 mixture monolayers and the A monolayers. Plots at the bottom of Figures 8 and 9 exhibit the relative intensity change of the IR band near 1645 cm-' due to a ring stretching vibration
Fgrre 8. Relative intensity changes in the v,(CH3) (2956 cm-l), v,(CH3) (2873 cm-I), v,(CH2) (2850 cm-I), and pyridiniumyl ring stretching (1645 an-')vibrations during the photodimerization of the 15-monolayer LB films of the 1:4 mixture with the acid form of the matrix molecule.
of the N-methylpyridiniumyl moiety (see Figures 3A and 4A). From Figures 8 and 9 it is clear that the intensities of all the CH3 and CH2stretching vibrations change following two steps. That is, the relative intensities change at much faster rate during an initial irradiation period (0 to ca. 45 s) than the intensities in the following period (45-180 s), and the features of the intensity changes correspond well to the kinetic features of photodimerization depicted in Figure 6. The relative intensity of v,(CH,) in Figures 8 and 9 shows a rapid decrease in the fmt step and, then, a slight increase in the second step. On the other hand, as can be sem from Figures 8 and 9, the v,(CH,) vibration exhibits an opposite trend of intensity change; that is, at fmt the relative intensities rapidly increase and, then, slightly decrease. The direction of the transition moment of the v,(CH3) vibration is perpendicular to the symmetry axis of the methyl group and that of the v,(CH3) vibration parallel to the axis. According to the surface selection rule of IRA spectnwcopy,' the abovementioned intensity changes can be explained by considering that, in the initial step of photodimerization, the symmetry axis of the methyl group changes its orientation from a tilted state to a l e s tilted one, and, in the second step, the methyl group tilts its orientation in the direction of the initial orientation. The intensity change of the v,(CH2) vibration, which has the transition moment perpendicular to the axis of the alkyl chain, shows a trend similar to that of the u,(CH3) vibration in Figures 8 and 9. The result suggests that the alkyl group also orients its axis from a tilted state to a less tilted one during the first step of photodimerization. This orientation change is consistent with that of the methyl group. The intensity change of the 1645-cm-' band (depicted at the bottom of Figures 8 and 9) reveals the following points. (1) The relative intensity observed for both samples increases in an initial step of dimerization, indicating an orientation change of the N-methylpyridiniumyl moiety. (2) The relative intensity umvergm to a constant level after a certain period of irradiation (ca.45 s
10370 The Journal of Physical Chemistry, Vol '6, No. 25, 1992
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Figure 9. The same as Figure 8 except for the sample (the alternating LB film consisting of 15 monolayers of the 1:4 mixture and 14 monolayers of A, the matrix molccule assume8 the acid form).
for the 15-monolayer LB film of the 1:4 mixture and ca. 100 s for the alternating LB film). DiSCIdOIl
~ o f p b o t o d b n e r i p t k a p r o d u c g .AlthougllQuina and Whittens postulated that the photodimerization product of clgs incorporated in the LB films of arachidic acid assumes a codigwation similar to that of dimer 3, there has been no direct evidence for this. The IRA spectra in Figures 3B and 4B prove the formation of the cyclobutane ring with type I configuration. Although the IR data cannot alone identify the exact configuration (the configuration corresponding to dimer 1 or to dimer 3), the latter configuration is more plausible, since a syn head-to-head dimerization h the most probable processin the LB films.5*6After about 35 s of irradiation, the type I1 configwation is also formed both in the LB film of the 1:4 mixture and in the alternating LB film oonsieting of the 1:4 mixture monolayers intervened by the monolayer of A. This fact indicates that the formation of the cyclobutane rings with both type I and I1 configurations takes place within the same monolayer of the 1:4 mixture. The IRA spectra in Figurea 3B and 4B suggest that the formation of the type I1 configuration takes place in the second step of photodimhation, which is proved by the kinetic plots in Figure 6. As d d in the following d o n , the second step is explained as a relaxation process of repulsive interaction between the Photodimcrization products (especially the alkyl side chains) and the matrix molccule. Presumably, during the relaxation process some of the Cl$ mlcculesarc convcrtsd to a new stacking state, which causa the formation of the cyclobutane ring with the type I1 configuration. TbR8bdErtgtOfpbodoQaaiptbaiaVuioplLB~ The kinetic plots for the photodimerization in the 15-monolayer LB f h of the 1:4 and 1:9 mixtures in the matrix with the acid form ( F i i 5 ) indicate that the f d extent of photodimerization is turned out to be more than 90% the value being almost independent of the incorporation ratios. This is explained by con-
Yamamoto et al. sidering that most of the CIgS molecules in the matrix assemble with each other, forming a stacking state favorable for dimerization. Pnsumably, under the conditions of the LB film formation (at pH 4423.5 OC and the surface pressure of 30.0 mN/m; see Eixperimental Section) the stilbamlium cations in the monolayers of the 1:4 and 1:9 mixtures on the LB trough water surface exist in a phasaseparated state. This kind of phase separation has been reported also for a stilbene derivative in the bilayer matrix of dialkylammonium bromide.20 The extent and rate of photodimerization in the matrix with the dissociated form are much smaller than those of the dimerization in the matrix with the acid form (see Figure 5 ) . Presumably, the positively charged stilbazolium cation and the negatively charged arachidate ion associate with each other in the monolayers on the LB trough water, resulting in a more dispersed distribution of the cations in the matrix with the dissociated form. This may explain the reduced rate and extent of photodimerization. The phase equilibrium (or monomer-cluster equilibrium) in the monolayers on the trough water should be dependent on pH, temperature and compositions (C,,S:A). Then, an analysis of the IRA spectra of photodimerization products of the LB films prepared under various conditions will give a more detailed information about the phase equilibrium. The reduced extent and rate of photodimerization in the alternating LB film compared to those of the photodimerization in the 15-monolayer LB film of the 1:4 mixture in the matrix with the acid form can be explained by considering that the intervening monolayer of A interacts with the neighboring monolayers of the 1:4 mixture, rendering a stacking state of the stilbazolium cations less favorable for dimerization. (Meatrtioa Cbnpofthe Alkyl Side cbsirred Its Rehtioasbip to Wotodimeriution Mecbaai" Figure 6 proves that the intensity decrease in the C = C stretching vibration due to the photodimerization of CIgS in the LB films conforms to secondorder plots, which consists of the initial faster reaction and the second slower one. The kinetic nature of the photodimerization corresponds well to the time course of the orientation changes of the alkyl side chains of clgs (Figures 8 and 9). As already explained, most of the stilbazoliumcations especially in the matrix molecule with the acid form assume a stacking state favorable for dimerization (a syn head-to-head arrangement). With the onset of irradiation, these molecules readily undergo dimerization, resulting in both the fast step of the reaction and the rapid orientation change of the alkyl group.This rapid orientation change should accumulate repulsive interaction between the alkyl chains and the matrix molecules. Figum 8 and 9 suggest that the gradual orientation change of the alkyl groups in the second step takes place in the direction of the original orientation. This change can be interpreted as a relaxation pmcegp of the repulsive interaction. During the relaxation, a part of the CI8Smolecules, which are originally in an unfavorable stacking state for dimerization, are rearranged to dimerize, cawing the m n d reaction step with the slower rate constant. The plots at the bottom of Figures 8 and 9 indicate that the intensity of the 1645-cm-l band of the Nmethylpyridiniumyl moiety increases during the initial step of photodimerization and remains constant during the second step. Thus, the relaxation process does not accompany orientation change of the N-methylpyridiniumyl moiety. From Figures 3A and 4A, it is recognized that the intensity of the band near 1600 cm-* due to the phenyl ring rapidly decreasa during an initial step of photoirradiation and seems to converge to a certain level after several minutes of the irradiation. Probably these results suggest that the orientational change of the phenyl group also takes place during the photodimerization. In order to get more detailed structural aspects of the photodimerization, however, we need to analyze the spectral changes in Figures 3,4, and 7 based on a quantitative manner such as that proposed by Umemura et a1.I0 This is in progress in our laboratory. Acknowledgment. We are grateful to Professor Takagi of Nagoya University for giving us detailed information about the
J. Pkys. Ckem. 1992, 96, 10371-10379 method of preparation of dimer 3.
Referenem and Notes (1) Liebermann, C. Chem. Eer. 1889, 22, 124, 782. (2) Bentein, H. I.; Quimby, W. C. J. Am. Chem. Soc. 1943, 65, 1845. (3) Cohcn. M. D.:Schmidt, G. M. J. Pure A d . Chem. 1971, 27, 647. (4) Hasegawa. Mi Chem. Rev. 1983,83,507: (5) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 99, 877. (6) Takagi, K.; Suddaby, B. R.; Vadas, S. L.; Backer, C. A.; Whitten, D. G. J. Am. Chem. Soc. 1986,108,7865. (7) Allara, D. L.; Swalen, J. D. J. Phys. Chem. 1982, 86, 2700. (8) Kimura, F.; Umcmura, J.; Takcnaka, T. Langmuir 1986, 2, 96. (9) Naselli, C.; Swalcn, J. D.; Rabolt, J. F. J. Chem. Phys. 1989,90, 3855.
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(10) Umemura, J.; Kamata, T.; Kawai, T.; Takenaka, T. J. Phys. Chem. 1990, 94,62. (11) Kawai, T.; Umcmura, J.; Takenaka, T. Langmuir 1990,6,672. (12) Stroeve, P.; Sapentein, D. D.; Rabolt, J. F. J. Chem. Phys. 1990,92, 6958. (13) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (14) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 52. (15) Takagi, K., private communication. (16) Itoh, K.; Hayashi, K.; Hamanaka, Y.; Yamamoto, M.; Araki, T.; Iriyama, K. Lungmuir 1992,8, 140. 117) Meic. Z.: Gusten. H. Soectrochim. Acra 1978. 34A. 101. (18j Wajima,'T.; Yankmot;, M.; Itoh, K., unpublkhcd work. (19) Snyder, R. G. J. Chem. Phys. 1%7,47, 1316. (20) Shimomura, M.; Hashimoto, H.; Kunitake, T. Langmuir 1989,5, 174.
Carrier Relaxation at Semiconductor Interfaces and Essential Features of a Quantltative Model M. L. Shumaker, W. J. Dohrd, and D. H. Waldeck* Department of Ckemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: May 18, 1992; In Final Form: July 22, 1992)
Experimental studies of the time-resolved bandgap emission of CdSe are used to probe the attributes required in a theoretical model for the carrier dynamics of semiconductor electrodes. By variation of excitation conditions and material properties, the validity of approximations in a commonly used model are tested. The experimental results show that the details of bulk recombination, the effects of self-absorption, and the influence of the space charge field are quite important factors for the carrier dynamics. Simulation results show that the detailed kinetics of the interfacial recombination can also have important consequences for the carrier relaxation. Appropriate modeling of these processes is needed before a surface recombination velocity can be extracted from such studies in a quantitative manner.
htroduction
Heterogeneous charge-transfer processes are ubiquitous in nature and crucially important in many technologies. Recombination of minority carriers at the semiconductor/electrolyte interface is an important physical proteas which nceds to be clearly understood before an adequate mechanistic description of heterogeneous charge transfer is possible. Such recombination processes may be either desirable or undesirable, depending upon one's goal. For example, such recombination processes may be the first step in corrosion of electrodes. Also they may limit the operating efficiencies of photovoltaic and photocatalytic devices. In contrast, these processes are desirable when corrosion is used in the fabrication of devices. Also these processes may act as mediators that enhance heterogeneous charge-transfer rates. Although recombination of carriers in the bulk of semiconductor materials has bem studied for quite some time, less effort has been expended on interfacial recombination.lS2 Yet in many cases, chemical reactions are dominated by interfacial properties and processes. The experimental methods used to quantify the minority carrier recombination at interfaces can be divided into two major c a t e gories, steady-state measurements and transient (or AC) mea~urements.~.'Clearly the steady-state measurements do not directly probe the relaxation kinetics, and they only provide 'effective" values (or time-integrated values) of the kinetic parameters. For this reason these methods are less useful as a guide to understanding the dynamics of carrier relaxation; however, it should be reaiized that these steady-state methods are generally quite useful for the characterization of devices which operate on long time males. A variety of direct methods have been used to probe the carrier evolution in electrodes. Transient electrical methods, such as photopotmtial and photocurrent, arc limited by the RC of the electrochemical cell.43 All optical methods provide the opportunity to probe the carrier dynamics on the picosecond 0022-3654/92/2096-1037 1$03.00/0
and subpicosecond time scales. Both timeresolved fluorescence61o and transient grating".'* spectroscopic methods have been used to study carrier recombination. This study experimentally probes some of the limits of a diffusion mode11J0J3-16which has been widely usedb12to analyze the recombination kinetics of photogenerated minority carriers from the observed photoluminescence decay. The emphasis of this work is on the relaxation of minority carriers under low injection conditions (Le. the photogenerated minority carrier density is significantly smaller than the thermal majority carrier population density) and the transition from high injection to low injection. If possible, one d e s k to monitor the carrier dynamics under low injection conditions for two reasons. First the system's response is expected to be linear and analytical models can be evaluated. Second this regime is the same as that obtained under solar irradiation, making such studies directly relevant to solar powered devices and processes. The relaxation kinetics of photogenerated minority carriers in n-CdSe single crystals and single crystal electrodes are used as a test system in order to probe the available models. In these studies an ultrashort light pulse (approximately 10 ps) with suprabandgap energy is used to excite the material, and the bandgap emission is monitored from the front surface as a function of time (see Figure 1 for a sketch of the experimental geometry). The geometry and the use of intense laser excitation is consistent with the requirements of the model13used to describe the carrier dynamia. The limitations of these models are probed by variation of the excitation conditions and of the material properties. A variety of approximations are inherent in the modeling which is typically used, and four of these are studied in this work. First the influence of the space charge field on the carrier dynamics is not treated in the time dependent models, although it has been included in steady-state analyses1' and in numerical solutions of the diffusion e q ~ a t i o n . ~ *Usually J~ this neglect is justified by 0 1992 American Chemical Society
