Langmuir 1989,5, 174-180
174 Or
DMMP and of the probable dissociation product, methyl methylphosphonate, are identical in the spectral region inve~tigated.~~ The small change observed in the peak height is consistent with the low reversibility of adsorption of the organophosphorus compounds on oxides."
a2
30
a4
1000x(T/°K)-l
Figure 6. Arrhenius graphs for the powders studied; the scatter
was too large for tungsten oxide.
Under the experimental conditions used in the present work, we have been unable to observe any effect on the spectra of heat or illumination with light with photon energy larger than the band gap of the oxide (with the exception of alumina and magnesia, for which no appropriate light is available). Also, the niobium- containing titanium dioxide, reported to decompose p-nitrophenyl diethylphosphonate ~atalytically,~ showed no catalytic activity for DMMP decomposition. The fact that no new peaks were observed does not necessarily mean that no dissociation occurred, because the vibrational spectra of
Conclusions The experimental results indicate that DMMP is labilized by adsorption on oxide surfaces. The change in the bond strength with the electronegatively of the cations supports the hypothesis of bonding of DMMP on the surface through the P=O bond. The adsorption of DMMP on oxidic surfaces is an activated process. In the experimental conditions studied, no significant catalysis could be observed; i.e., the adsorption was virtually irreversible, in agreement with previous studies.8a An immediate consequence is that if metal oxides are to be used as decontaminants,their surface must be suitably modified by decomposition catalysts; in other words, the metal oxides are good for the concentration stage of the decontamination process. Acknowledgment. This work was supported by the Office of Naval Research. We are indebted to Drs. T. J. Lewis and T. L. Rose for illuminating discussions. Special thanks to Drs. R. D. Rauh and L.S. Robblee for advice and critical reading of the manuscript. Registry No. DMMP, 868-85-9; TiOz, 13463-67-7; W03, 1314-35-8;MgO, 1309-48-4;ZnO, 1314-13-2; AZO3, 1344-28-1; Nb, 7440-03-1.
Controlled Stilbene Photochemistry in Ammonium Bilayer Membranes Masatsugu Shimomura,l Hiromasa Hashimoto, and Toyoki Kunitake* Department of Organic Synthesis,? Faculty of Engineering, Kyushu University, Fukuoka 812, Japan Received September 21, 1988 Long-chain ammonium amphiphiles containing the stilbene chromophore (C12StbC,N+)formed bilayer assemblies in water, as confirmed by electron microscopy and other means. Their absorption spectra showed blue shifts characteristic of parallel chromophore stacking in the bilayer. The extent of the blue shift was greater for the C12StbCld+(Clo spacer methylene) bilayer than for the C12StbC4N+(C1 spacer methylene) bilayer. The emission spectra were red-shifted accordingly. The monomer-cluster equilibrium (phase separation) of the stilbene chromophore in the bilayer matrix of dialkylammonium bromides was readily detected by absorption spectral changes, and it was promoted in the crystalline matrix. The photoreactivity of the C12StbCld+bilayer was much smaller than that of the C1.$tbC4N+ bilayer, which underwent ready dimerization. This difference was apparently produced by typical parallel stacking of the stilbene unit in the former. The course of stilbene photochemistry was affected by phase separation. Bimolecular cycloaddition,cyclobutane formation, was dominant for the stilbene component in a phase-separated cluster, whereas unimolecular trans-cis isomerization and the subsequentphenanthrene generation were observed when the stilbene was molecularly dispersed either in a matrix membrane or in ethanol. The photochemical processes can be controlled by the membrane physical state.
Introduction Chemical reactions in organized assemblies such as bilayer membranes and molecular multilayers are expected to show peculiar behaviors different from those in homogeneous solutions. This should be particularly true in the photochemical reaction^.^?^ Many attempts have been conducted to delineate photoreaction mechanisms and/or 'Contribution No. 885.
to control photochemical reactivities by taking advantage of controlled distribution and orientation of molecules in these assemblies. The interest in the photochemical and photophysical study in bilayer membranes also arises in (1) Present address: Department of Industrial Chemistry, Tokyo University of Agriculture and Technology, Tokyo 184, Japan. (2) Kuhn, h.; Mobius, D. Angew. Chern., Int. Ed. Engl. 1971, 10, 620-637. (3) Whitten, D. G. Angew. Chern., Int. Ed. Engl. 1979, 18,440-450.
0743-7463/89/2405-Ol74$01.50/0 0 1989 American Chemical Society
Langmuir, Vol. 5, No. 1, 1989 175
Controlled Stilbene Photochemistry in Bilayers
relation to attempts to develop artificial photosynthetic ~ystems.~JPhotoinduced electron transfer across bilayer membranes has been extensively studied in this respect, as exemplified by Matsuo and his co-workers, who succeeded-in photochemical charge separation within the two-dimensional molecular array of certain bilayem6 We have shown that stable bilayer membranes are produced from a large number of synthetic amphiphiles.' Various chromophores such as anthracene, naphthalene, azobenzene, and carbazole are covalently incorporated into the synthetic bilayer, and unique photochemical behavior observed can be discussed in terms of the two-dimensional stacking of chromophores."" The stilbene chromophore occupies a central position in mechanistic photochemistry. The photochemical trans-cis isomerization of stilbenes was scrutinized,12and the experimental and theoretical elucidation of the isomerization mechanism had an important bearing on the excited state of stilbene^.'^ Stilbene was also a molecule of choice in the photophysical study in the picosecond dynamic spectroscopy, thanks to its strong fluorescence. Sumitani and Yoshihara14determined directly the rates for cis-trans and trans-cis photoisomerizations and concluded that the isomerization proceed through the potential surface of the singlet excited state. A strong dependence of the fluorescence characteristics on the microenvironment makes stilbene a useful probe for the nature of the solubilization site.16 Whitten and co-workers found that the fluorescence quantum yield of a long-chain stilbene derivative changed significantly by the fluidity change (phase transition) of the matrix bilayer.18 The quantum yield for the trans-cis photoisomerization is similarly affected by the solubilzation site. Photodimerization in which cyclobutane derivatives are formed by [2 + 21 cycloaddition is the other important reaction of stilbene. This is a bimolecular reaction, and its rate in homogeneous media is slower than that of the photoisomerization." However, the reaction is much facilitated in the solid state, where stilbene molecules are oriented in such a way as to favor dimerization.'* Selection between unimolecular isomerization and bimolecular dimerization may be accomplished through the use of molecular assemblies such as bilayer membranes and molecular multilayers (Langmuir-Blodgett films). Quina and Whitten conducted this selection for stilbene derivatives by the control of molecular distribution in the (4) Fendler, J. Membrane Mimetic Chemistry; Wiley-Interscience: New York, 1982; pp 492-505. (5) Monserrat, K.; Griitzel, M. J. Chem. SOC., Chem. Commun. 1981, 183-184.
(6) Takeyama, N.; Sakaguchi, H.; Hashiguchi, Y.; Shimomura, M.; Nakamura, H.; Kunitake, T.; Matsuo, T. Chem. Lett. 1985,1735-1738. (7) For example: Kunitake, T.; Okahata, Y.; Shimomura, M.; Yasunami, S.; Takarabe, K. J. Am. Chem. SOC.1981,103,5401-5413. (8) Shimomura, M.; Haehimoto, H.; Kunitake, T. Chem. Lett. 1982, 1285-1288. (9) Nakashima, N.; Kimizuka, N.; Kunitake, T. Chem. Lett. 1986, 1817-1820. (10) Shimomura, M.; Ando, R.; Kunitake, T. Ber. Bunsen-Ges. Phys. Chem. 1983,87, 1134-1143. (11) Kunitake, T.; Shimomura, M.; Hashiguchi, Y.; Kawanaka, T. J. Chem. SOC.,Chem. Commun. 1985, 833-835. (12) Brown, P. E.; Whitten, D. G. J.Phys. Chem. 1985,89,1217-1220
and references cited therein. (13) Olbrich, G. Ber. Bunsen-Ges. Phys. Chern. 1982,86,209-214 and references cited therein. (14) Sumitani, M.; Yoshihara, K. Bull. Chem. SOC. Jpn. 1982. 55,
85-89. (15) Saltiel, J.; DAgostino, J. J. Am. Chem. SOC. 1972,94,6445-6456. (16) Suddaby, B.; Brown, P.; Russell, J.; Whitten, D. J.Am. Chem. SOC. 1985,107, 5609-5617. (17) Shechter, H.; Links, W. J.; Tiers, G. V. D. J. Am. C h m . SOC. 1963, 85, 1601-1605. (18) Lewis, F. Acc. Chem. Res. 1979,12, 152-158.
Langmuir-Blodgett film.lg The selectivity in this case appears to be determined predominantly by the deposition conditions. Unfortunately, the photoreactivity of stilbene derivatives is drastically suppressed in multilayers, as demonstrated by Whitten and co-worker@ and by Fukuda and NakaharaS2l This made the control of the course of the photoreaction rather difficult. Our current interest lies in elucidation of the relation between the mode of molecular orientation in bilayers and their photochemical behavior. We have reported in the case of azobenzene bilayers that (1) absorption spectra show large ,A, shifts (both bathochromic and hypsochromic) due to different modes of the chromophore stacking,1° (2) photoisomerization is influenced by the mode of the chromophore stacking,22and (3) azobenzene bilayer components undergo monomer-cluster equilibria, depending on the physical state of the matrix membrane and other experimental conditions.23-26Also, we reported that the photochemical behavior changes accordingly.22 Efficient energy migration was also observed for naphthalene- and carbazole-containing b i l a y e r ~ . ~These J ~ results suggest that stilbene photochemistry, too, is affected by the mode of chromophore interaction. We selected two stilbene derivatives, Cl2StbCnN+( n = 4 and lo), as candidates for the photochemical probe by taking into account the influence of the tail and spacer methylenes on the stability of and the molecular orientation in the azobenzene bilayer. According to our previous the dodecyl tail is required to enhance stabilization of the bilayer structure, and the spacer methylene length (C,,, n = 4 or 10) strongly affects the chromophore orientation and the consequent photophysical property.
C12 Stb C,N'
2Cn N'2C1
Experimental Section Amphiphiles. T h e stilbene amphiphiles, Cl2StbC4N+and C12StbCloN+, were prepared by stepwise alkylation and the subsequent quaternization of 4,4'-dihydroxystilbene prepared from phenol and chloral.27*284,4'-Dihydroxystilbene, m p 285-293 OC (2.0 g, 9.4 mmol) and 2.0 g (8.0 mmol) of dodecyl bromide were dissolved in 100 mL of ethanol t h a t contained 0.53 g (8.0 mmol) of 85% KOH and refluxed for 4 h. The precipitates were washed with water and ethanol. Recrystallization from ethanol gave 1.4 g (46%) of p-(dodecy1oxy)-p'-hydroxystilbene: colorless flakes, m p 165 177 "C (the arrow indicates the liquid crystalline region). T h e didodecylated byproduct was insoluble in ethanol. p-(Dodecy1oxy)-p '-hydroxystilbene was allowed t o react with 1,4-dibromobutane or 1,lO-dibromodecane (2-3 times excess) in
-
(19) Quina, F.; Whitten, D. G. J.Am. Chem. SOC. 1976,97,1602-1603. (20) Mooney, W.; Brown, P.; Russell, J.; Costa, S.; Pedenen, L.; Whitten, D. G. J. Am. Chem. SOC. 1984,106, 5659-5667. (21) Fukuda, K. Proc. International Symposium on Future Electron . . Devices, 1985, Tokyo, p 15. (22) Shimomura, M.; Kunitake, T. J. Am. Chem. SOC. 1987, 109, 5175-5183. (23) Shimomura, M.; Kunitake, T. Chem. Lett. 1981, 1001-1004. (24) Shimomura, M.; Kunitake, T. J. Am. Chem. SOC. 1982, 104, 1757-1759. (25) Kunitake, T.; Ihara, H.; Okahata, Y. J. Am. Chem. SOC. 1983,105, 6070-6078. (26) Kunitake, T.; Okahata, Y. J.Am. Chem. SOC. 1980,102,549-553. (27) Hubacker, M. H. J. Org. Chem. 1959,24, 1949-1951. (28) Zincke, Th.; Fries, K. Liebigs. Ann. Chem. 1902, 325, 19-92.
Shimomura et al.
176 Langmuir, Vol. 5, No. 1, 1989
Figure 2. Absorption spectra of the stilbene amphiphileaat 20 'C: (a) C, tbC,N' and CI&thC,$Il* in ethanol. 0.4 mM; (b) C,StbC.N m water, 0.35 mM; (e)CIStbCi$rl'in water, 0.4mht.
P
(Taahiba) that is transparent at 300-400 nm or the cell compartment of the spwtmfluorimeter(light so~vce,1W-WXe lamp). The monochromatic light was obtained in the latter case. Experimental p d m of electron micmxmpyand difFerential scanning ealorimetry were described else~here.~.~'
Figure I. Electron mierogrsphsof stilbene amphiphilea Stained by uranyl acetate. Sample C,&tbC,N? (A) m = 4; (B)m = 10. Original magnification, X300000. the presence of amell exeeases of KOH in refluxing ethanol for 3-6 h. The precipitates were washed with water, ethanol, and hexane and then recrystallized from benzene: p(dodecy1oxy)p'-((4-hromobutyl)ory)stilbena. yield 60%. colorless flakes, mp 124
-
-
154 O C ; p-(dodecyloxy)-p'-((l0-bromodefyl)oxy)stilbene,
yield 6370,colorleas flakes, mp 120 127 O C . The subsequent quaternization with large excesses (ea. 30 mol/mol) of N,N-dimethylaminoethanol was conducted for 1-2 days in refluxing benzene. The precipitates were collected, after ice eooling if necessary. washed with hexane, and recrystallized either from CHCl,/hexane or from CHCI,: p-[(4-(dimethyl(hydroxyethyl)ammonio)butyl)oxy]-p'-(dodecyloxy)atilhene bromide (C,&tbC,N+). yield 6870, colorless flakes, mp 180 240 "C. Anal. Calcd for C,HuOaNBr: C. 67.53; H, 9.00: N. 2.32. Found C, 67.01; H, 8.99; N, 2.30. p-[(IO-(Dimethyl(hydroxyethyl)ammonio)decyl)oxy]-p~(d~ decy1oxy)stilbenebromide, yield 18%. colorlesspowder, mp 110 225 OC. Anal. Calcd for C&O3NBr4.SH20 C, 68.W. H. 9.68, N, 2.01. Found C,68.71;H,9.59,N,201. LHNh4RandIRspectra were consistent with the exwcted structures. Preparation of dialkylammonium salts, 2CIfl*2Ci and 2CI.N'2C,. wan described elsewhere.?g Photoreaction and Measurement. Abeorption and fluorescence spectra were obtained hy using 1-mm quartz ells with a Hitachi 200 UV-vis spectrometer and a Hitachi 650-10s spectrofluorimeter, respectively, which were equipped with thermoatated cell holders. Aqueous stmk solutions (10mM) of the stilbene amphiphiles were prepared by sonication and diluted, as n y by water or by ethanol. Two kinds of aqueous stock solutions were mixed, sonicated, and diluted as necessary when mixed bilayer samples were required. Photoreaction was conducted by using either a 500-Whighpressure Hg lamp (Ushio Den& Co.)together with a W - D S fdtu
-
-
(28) Kunituke, T.; Otahata, Y.;Tamaki. K.; Kumamaru. F.: Takay-
.nSgi, M. Chom. Lett. 1977.381-390.
Results and Discussion Aggregation Behavior of Stilbene Amphiphiles. The aggregation behavior of the stilbene amphiphiles was studied by electron microscopy and other physical means. Figure 1 shows electron micrographs of negatively stained samples of the stilbene bilayers. Stacked disks (side view) are found abundantly. The spherical objects appear to be top views of the stacked disks. The disk possesses a thickness of ca. 60 A and a diameter of 200 A. The extended molecular length of C1$tbC,N+ as estimated from the CPK molecular model is ca. 33 A, and the disk thickness corresponds approximately to the bilayer arrangement. In contrast, lamellar bundles with a layer thickness of ca.60 A are found for C1$tbCIJV*. The longer spacer methylene apparently promotes development of the twodimensional bilayer structure, in agreement with our previous observation on the azobenzene bilayer.l0 The critical micelle (aggregate) concentration of CI2StbCloN+ was estimated by using (2,g-dichloropheny1)indophenol (DCPI) as a spectroscopic Neutral DCPI (A- 510 nm) in an acetate buffer is converted to the anionic form ,A( 610 nm) in the presence of cationic molecular aggregates.= In the present case, a break in the absorbance at 610 nm was found at 3 X lod M of C12StbClJV+ (20 "C, pH 4.1, p = 0.02). This cmc value is typical of ammonium bilayer membranes.' Typical bilayer membranes undergo the gel-to-liquid crystal-phase transition. The phase transition behavior of the bilayer membrane of single-chain amphiphiles is extensively documented." DSC thermograms of aqueous dispersions (20 mM) of the stilbene amphiphiles did not display any endothermic peak at 5-80 OC. A small endothermic peak (AH, ca. 10 kJ/mol) was found a t 18 "C in the case of CI2StbC6N+." DSC peaks might as well be observed for CI2StbC,N+ and C12StbCloN+. That this is not the case indicates that enthalpy changes are too small to be detected or that the phase transition occurs outside (30)Kunitake, T.; O b h a y Y.J.Am. Chem. Soc. In?,%, 3860-3BSI. (31) Okahata, Y.; Ando, R.;Kunitake. T. Ber. Bunsen-Goa. Phys. Chem. 1981.85.189-198. (32) Collin. M. L.: Harkins, W. D. J. Am. Chem Soc. 1941, 69, 61It683. (33) KunitaLs.T.;Olrahata,Y.;Ando.R.;ShinLai,S.;HiraLaakS.J. Am. Chem. Soe. l9SO.102,1811-1881. (34) Kunitaks, T.; Ando, R; Ishikawa, Y.Mom. Foe. Eng., Kywhu Uniu. 1986.46,245-263. (35) Hashimob, H. Mastsn W,Faculty %.Kyushu Univ., 1981.
Controlled Stilbene Photochemistry in Bilayers I
200
Langmuir, Vol. 5, No. 1, 1989 177 I
30 0 400 WdVQlength (nrn)
500
I
200
300 wavelength ( n r n )
Figure 3. Fluorescence spectra of C12StbC,N+at 20 "C: (a) excitation spectrum in ethanol, monitored at A,, 380 nm; (b) emission spectrum in ethanol, X,330 nm; (c)excitation spectrum in water, monitored at X,390 nm; (d) emission spectrum in water, A,, 290 nm.
the temperature range (5-80 "C) of the measurement: probably below 5 "C. Absorption and Emission Spectra of Stilbene Bilayers. UV-vis absorption spectra of the stilbene amphiphiles in ethanol possess maxima at 305 and 325 nm with shoulders at 285 and 342 nm, as given in Figure 2. This spectrum is essentially identical with that of 4,4'dimetho~ystilbene~~~' and indicates that the amphiphiles are molecularly dispersed in this solvent. On the other hand, C12StbC4N+in water gives, A at 295 nm (e 24 500) with shoulders at 330 and 350 nm. A still larger blue shift is found for aqueous CI2StbCloN+with, , A at 282 nm ( E 26300) and shoulders at 330 and 350 nm. The blue shifts in the bilayer assemby can be attributed to the electronic interaction of the chromophore. We have reported that absorption spectra of azobenzene-containing bilayer membranes show large blue and red shifts depending on the molecular structure (especially the spacer methylene length) of the component.1° The blue shift was ascribed to the card-packed orientation (the H aggregation) of the neighboring azobenzene units on the basis of the molecular exciton theory of K a ~ h a . The ~ ~ red shift, in contrast, was ascribed to the head-to-tail orientation (the J aggregation) of azobenzenes. This spectroscopic inference was later confirmed by X-ray structure determinat i ~ n If. a~similar ~ ~ ~argument is applied to the stilbene bilayer, the observed blue shifts are indicative of the card-packed structure. Blue shifts due to the assembled stilbene chromophore have been observed in molecular multilayers. Heesemann41 prepared a series of monolayer compounds with stilbene chromophores of which the molecular long axis would be oriented perpendicular to the plane of the molecular layer. His combined observation of absorption spectra and surface pressurearea isotherms suggested that the flat-lying species is converted to the H aggregate with increasing surface pressures. Whitten and co-workers20 prepared stilbene-containing fatty acids and conducted a spectral and photochemical study of the corresponding supported multilayers. Their photochemical reactivities were very low, and absorption and fluorescence spectra indicated the (36) Bernsteh, J. Spectrochim. Acta 1973,29A, 147-149. (37) Suzuki, H. Bull. Chem. SOC.Jpn. 1960,33, 406-410. (38) Kaeha, M. Radiat. Res. 1963,20, 55-71. (39) Kunitake, T.; Shimomura, M.; Hrada, A.; Okuyama, K.; Takayanagi, M. Thin Solid F i l m . 1984,121, L89-L91. (40) Okuyama, K.; Watanabe, H.; Shimomura, M.; Hirabayashi, K.; Kunitake, T.; Kajiyama, T.; Yasuoka, N. BuZ1. Chem. SOC.Jpn. 1986,59, 3351-3356. (41) Heesemann, J. J. Am. Chem. SOC.1980,102, 2167-2176.
4 00
Figure 4. AbsorDtion mectra of mixed bilaver membranes of C1&tbCloN+and*2C18Ni2C1in water: C12S&CloN+, 5.0 X lo4 M; 2Cl8N+2C1, 1.0 X lo-' M.
- 0 10
30
50
70
Temperature ('C)
Figure 5. Temperature dependence of phme separation as given by A325/A282.The conditions are identical with those of Figure 4: 0,cooling cycle; 0 , heating cycle.
presence of the card-packed stilbene units (the H aggregate) in the multilayer. Fluorescence spectra of the stilbene bilayer of C12StbC4N+similarly display the influence of the chromophore interaction. As shown in Figure 3, the excitation spectrum in water is blue shifted relative to that in ethanol, in agreement with the absorption spectral data of Figure 2. Accordingly, the emission spectrum in water shows a red shift with sharper splitting compared with that in ethanol. The emission intensity in water is ca. 1/15 of that in ethanol. The C12StbCloN+bilayer gives very similar fluorescence spectra. The spacer length does not appear to affect emission characteristics. Phase Separation. Since azobenzene components show large spectral shifts depepending on their physical states in the bilayer assembly, the cluster-monomer equilibrium (phase separation) of the component molecule is readily estimated from the spectral shift.= This phenomenon was later used to evaluate reactivity signal transd u ~ t i o nand , ~ ~spectral control.42 The spectral data of Figure 2 suggest that the isolated stilbene component is distinguishable from the clustered component. Figure 4 demonstrates spectral changes of C1$tbCloN+ embedded in the bilayer matrix of 2ClSN+2C1 (0.05 mol/mol). The two components were sonicated for 3-4 min in water and incubated at 65 "C in a UV cell for 1 h. The temperature was then lowered to 60 "C and maintained until equilibration is attained. A twin peak at 305 and 325 nm that is very similar to that in ethanol was observed at this temperature. The spectrum was (42) Nakashima, N.; Morimitsu, K.; Kunitake, T. Bull. Chem. SOC. Jpn. 1984,57,3253-3257.
178 Langmuir, Vol. 5, No. 1, 1989
Shimomura et al.
10W
4
- log
K?Cia N'2CiI Figure 6. Effect of component ratio and temperature on the monomer-cluster equilibrium in water: 0,20 "C; 0 , 60 "C. measured at lower temperatures by the same procedure. With lowering temperature, the intensity of the twin peak was lessened and a sharp peak characteristic of the stilbene bilayer appeared at 282 nm. This spectral change was reversible, and the original spectrum was regained upon raising the temperature. The presence of a clear isosbestic point at 295 nm suggests that only the two species are in equilibrium and that the monomeric (isolated) stilbene present at high temperatures is converted to the stilbene cluster at low temperatures. The molar ratio of these two stilbene species can be estimated by the relative absorbance at 325 and 282 nm (A3a/A=2). Figure 5 gives A3%/AB2values plotted against temperature. The absorbance ratio decreases rapidly between 50 and 40 "C in the cooling process. The change is much smaller outside this temperature range. A similar change is observed in the heating process, though small hysteresis remains. The most important factor to affect the stilbene equilibrium is apparently the physical state of the matrix membrane, since the phase transition temperature (DSCpeak top) of the 2C18N+2C1bilayer is 45 "C. The stilbene component is much more miscible with the liquid crystalline matrix than with the crystalline matrix. The same conclusion has been obtained for azobenzene-containing bilayer component^.^^-^^^^^ The miscibility, of course, depends on the molar ratio of the probe and the matrix. Figure 6 displays the influence of the molar ratio on A3,/Am at 20 and 60 "C where the matrix bilayer is in the crystalline state and in the liquid crystalline state, respectively. The fraction of the monomeric stilbene component gradually increases at 60 "C with lowering stilbene contents (50-1%) in the liquid crystalline matrix. In contrast, the stilbene component in the crystalline matrix is mostly in the cluster form until its content becomes less than a few percent. In the bilayer matrix of 2C16N+2C1of which T,is 28 "C, the miscibility is not very different between 41 and 18 "C. The phase separation is affected less drastically by temperature in this case. Fluorescence spectra also reflect the cluster/monomer equilibrium. The stilbene cluster and monomer show fluorescence characteristics typical of the bilayer species and the isolated species (in ethanol), respectively (see Figure Kano3). and others43examined the phase separation behavior of stilbene amphiphiles in a lipid bilayer matrix of dipalmitoylphosphatidylcholine (DPPC) liposomes. The existence of the stilbene cluster was, however, not detected (43) Tanaka,Y. Masters Thesis, Graduate School of Eng.Sci., Kyushu Univ., 1981.
300
200
[CizStbCioN'I
400
wavelength (nm)
Figure 7. Photoreactionof Cl#tbClg+ in ethanol: Cl#tbCl,,N+, 3.0 X lo4 M; 15 "C,irradiation at 330 nm. The dotted-line spectrum was obtained after prolonged irradiation.
in a similar temperature range. Photochemistry of the Stilbene Bilayer. The stilbene photochemistry includes monomolecular trans-cis isomerization and bimolecular dimerization.18 In homogeneous solution, the efficiency of photodimerization is usually small, and the major course of reaction becomes the trans-cis isomerization. Shechter and co-workers" obtained the photodimer of stilbene in 27% yield upon irradiation of ultraviolet light for 2 months. The photodimerization is facilitated by concentration of the reactant, as is evidenced by the photochemical results of stilbene multilayers.ls The molecular orientation is an additional factor to affect this reaction. For instance, photodimerization does not proceed in the stilbene crystal, whereas it proceeds readily in the crystal of 2,4-dichlorotrans-stilbene." The observed reactivity difference was attributed to a change in the crystal structure. As in the stilbene multilayer, the long axis of the stilbene chromophore is aligned in parallel fashion in the bilayer assembly, as inferred from the absorption spectrum. The photochemistry in the bilayer would naturally reflect this structural characteristics. Figure 7 shows spectral changes of an ethanol solution of Cl2StbCl0N+(in a quartz cell) due to photoirradiation at 330 nm. The absorption of the monomeric trans-stilbene decreases with time with an isosbestic point a t 270 nm. The spectral change ceases to be observed after 150 s with peaks at 290 and 230 nm. Therefore, the spectra after 150-300 s correspond apparently to that of the photostationary state between the trans- and cis-stilbene moieties. When the irradiation is extended much longer (e.g., 15 min), the spectrum deviates from the isosbestic point, and a new peak appears at 260 nm. This peak has been assigned to the phenanthrene unit.45 Thus, there is a slow conversion of the cis ClzStbCloN+unit formed by photoisomerization to the corresponding dihydrophenanthrene derivative (intramolecular cyclization) and then to the phenanthrene unit (oxidation by air), as given by eq 1. A very similar spectral change was observed for C12StbC4N+in ethanol. R
J?=fie&2& R
R
R
R
(1) R
R
(44) Cohen, M. D.; Green, B. S.; Ludwer, Z.; Schmidt, G. M. J. Chem. Phys. Lett. 1972, 7,486-490. (45) Mallory, F . B.; Wood, C. S.; Gordon, J. T.; Lindquist, L. C.; Savitz, M. L. J. Am. Chem. SOC.1962,84, 4361-4362.
Controlled Stilbene Photochemistry in Bilayers
Langmuir, Vol. 5, No. 1, 1989 179
k
.. 8
ppm
300 wavelength (nm)
200
400
Figure 8. Photoreaction of the CI2StbC4N+bilayer in water: ClzStbC4N+,4.0 X lo4 M; 15 OC, irradiation at 330 nm.
7 ppm
6
8
7 w m
6
'""A! 8
7
6
5
4
3
ppm
Figure 10. Partial Nh4R spectra of C1$tbC4N+ before and after photoirradiation (CDC1, TMS standard): (a) before irradiation; (b) 1-h irradiation in ethanol; (c) 2-h irradiation in ethanol; (d) 2-h irradiation in D,O.
We recently carried out a spectroscopic and photoisomerization study of aqueous bilayer aggregates of azobenzene-containing ammonium amphiphiles C,AZOC,N+.~
8
U \ c
0
40 80 Time ( s e c )
120
Figure 9. Second-order kinetics observed for the absorbance decrease at 300 nm in Figure 8.
No spectral changes were observed upon photoirradiation of the C12StbCloN+bilayer in water under identical conditions. Therefore, the C12StbCloN+bilayer shows a much reduced photoreactivity compared with that of unassembled C12StbCloN+.In contrast, the C12StbC4N+ bilayer underwent photoreactions under the identical conditions, as shown by Figure 8. The peak intensity at 295 nm decreases monotonously with irradiation time, and a peak at 230 nm becomes stronger. This spectral change is ascribed to the formation of the photodimer, as shown in the following section. The time course of the absorbance decrease follows the second-orderplots as shown in Figure 9. These kinetics are consistent with the dimerization process. The same photoreaction is made to proceed in the case of the aqueous C1$tbCl&+ bilayer by using a stronger W source. When a 500-W ultra-high-pressure Hg lamp was used in combination with a UV-D35 filter (313-, 334-, and 365-nm light), a spectral change similar to that of Figure 8 is observed. However, the change is still quite slow, and the spectral reduction after the 45-min irradiation is roughly equivalent to that after 200-9 irradiation in Figure 8. The exceptionally sluggish reaction, both isomerization and dimerization, of the C12StbCloN+bilayer relative to the bilayer of CI2StbC,N+ is conceivably related to the mode of the chromophore orientation. The absorption spectrum of the C12StbCloN+bilayer indicates that the stilbene chromophores are in the head-to-head arrangement. Mooney et aL20 employed an exciton treatment in order to explain the behavior of the stilbene chromophore in the multilayer assembly and concluded that there exist small patches of the head-to-head (pincushion) chromophore cluster which give rise to blue-shifted absorption and efficient energy transfer to doped surfactant pyrene.
Their behaviors are very sensitive to the chromophore orientation that is predominantly determined by the length of spacer methylenes. The rate of the trans-to-cis photoisomerization in the crystalline bilayer was much smaller for the head-to-head (parallel) azobenzene orientation (for the C8AzoCloN+bilayer) than for the head-to-tail orientation (for the C12AzoC6N+bilayer). The difference in the excited-state structure was assumed to be responsible for these behaviors. It is interesting that the parallel chromophore stacking leads to lower reactivities in both azobenzene and stilbene bilayers. A greater reactivity of the C12StbC4N+bilayer is consistent with its absorption spectrum (Figure 2), which indicates less typical parallel stacking. Identification of Photoproducts. The course of the photoreaction inferred from the absorption spectral results can be confirmed by identification of the photoproduct by NMR spectroscopy. C12StbC4N+in D20 and in ethanol was irradiated with UV light from a 500-W Hg lamp (UV-D35 filter used). Solvents were removed in vacuo after the irradiation, and the residues were dissolved in CDC1, for NMR measurement. Figure 10 displays partial NMR spectra before and after photoirradiation. Before irradiation (Figure loa), the 4,4'-disubstituted trans-stilbene unit gives an olefinic singlet and aromatic AzBz quartet centered at 6.7-7.5 ppm. When irradiated in ethanol, the trans-stilbene peaks collapse to a complex multiplet, and upon further irradiation, a sharp aromatic peak appears at 8.0 ppm. These results are qualitatively consistent with the spectral data of Figure 6, which indicate the occurrence of the trans-to-cis photoisomerization followed by cyclization-dehydrogenation to the phenanthrene unit. An NMR spectrum of the irradiated aqueous bilayer (Figure 10d) gives an aromatic quartet (6.8 ppm, J = 0.13 Hz) which is located upfield relative to the original one (0.3 pprn). This upfield shift is probably caused by the paramagnetic deshielding. A t the same time, a new singlet appears at 4.3 ppm which is reminiscent of the cyclobutane proton at 4.4 ppml' of l-trans-2-trans-3-cis-4-tetra-
180 Langmuir, Vol. 5, No. 1, 1989
wavelength (nm)
Shimomura et al.
wavelength ( n m )
Figure 11. Photoreaction of the isolated stilbene amphiphile in the liquid crystalline bilayer matrix: Cl&bCl&P, 5.0 X 10"' M; 2C18N+2C1,1.0 X lo-'- M; 60 O C . (a) Irradiation at 330 nm in the cell compartment of a spectrometer. (b) Prolonged irradiation with a 500-W Hg lamp. phenylcyclobutane isomer A. Isomer B gives the cyclobutane proton peak at 3.63 ppm.
isomer A
isomer B
These NMR data are again consistent with the absorption spectral data, indicating the predominance of photodimerization in the aqueous bilayer. The syn-type dimerization (isomer A) is apparently preferred on the basis of NMR evidence. Photoreaction in Inert Bilayer Matrices. The stilbene bilayer, typically that of C12StbCloN+,exists in the bilayer matrix of 2C,N+2C1, either in the monomeric state or in the phase-separated state (Figures 4-6). These changes in the membrane physical state naturally affect the course of the photoreaction. Figure 11demonstrates the spectral change due to photoreaction of C12StbCl,J+ embedded in the bilayer matrix of 2Cl$r1+2C1at 60 "C. At this temperature, the matrix bilayer is in the liquid crystalline state (T,= 45 "C) and the absorption spectrum of the stilbene component (A, 305 and 325 nm) is typical of that of the monomeric dispersion. The absorbance decreases with irradiation a t 330 nm, and the photostationary state is attained in ca. 30 s. The subsequent use of a much stronger UV source (500-W Hg lamp with a DV-D35 filter) causes a further decrease in absorbance at 300-320 nm and an increase at 260 nm. These changes are essentially the same as that shown in Figure 7. Thus, the photoreaction of the monomeric C12StbCl$J+includes, just like that in ethanol, trans-to-cis photoisomerization accompanied by oxidative cyclization to phenanthrene. The photoreaction of the stilbene cluster in the crystalline bilayer matrix of 2C18N+2C1is quite different, as shown in Figure 12. Blue-shifted cluster absorption is observed for C12StbC1,J+under these conditions (20 "C). The absorption gradually decreases with a concomitant increase at 230 nm with irradiation by the 500-W Hg lamp. This spectral change which obeys the second-order kinetics is virtually identical with that in Figure 8, indicating occurrence of photodimerization. When shorter C1&bC4N+ is mixed with 2C18N+2C1,a blue-shifted spectrum characteristic of phase separation could not be observed. Irradiation of this bilayer mixture showed spectral changes
wavelength ( n m )
Figure 12. Photoreaction of the clustered stilbene amphiphile in bilayer: C12StbC4N+, 4.0 X 10"' M; 2C18N+2C1,8 X M; 20 "C. Irradiation at 330 nm in the cell compartment of a spectrometer. that indicate trans-to-cis photoisomerization and the subsequent formation of the phenanthrene unit, similar to those of Figure 11. Kano and others43 studied the photoreaction of C8StbC4N+in a lipid bilayer matrix of DPPC. The formation of the stilbene cluster was not confirmed by absorption spectroscopy, although small A, shifts were found. When the bilayer mixture (C8StbC4N+,5 mol 5%) was photoirradiated at 25 "C, the formation of the photodimer from the trans-stilbene was indicated by absorption spectra. When the experiment was conducted at 50 "C, the trans-to-cis photoisomerization was the major pathway of reaction. The formation of the phenanthrene ring was promoted by irradiation of the cis isomer. The phase transition of the DPPC bilayer occurs at 42 "C. Though not confirmed spectroscopically, the cluster of C8StbC4N+appears to be present in the crystalline bilayer matrix of DPPC, leading to the photodimer. In the liquid crystalline DPPC matrix, unimolecular photoisomerization is preferred probably due to better miscibility.
Conclusion The photochemical course of the stilbene-containing component was dependent on the physical state of the bilayer assembly. The present result, together with our previous data on the photochemistry of the azobenzene bilayer, establishes that the bilayer assembly is convenient media for controlling photochemical processes. The use of bilayer membranes for controlling photophysical processes such as electron transfer and energy migration has been studied extensively in relation to modeling the photosynthetic system. Coordinated control of the photophysical and photochemical processes should be essential in order to construct complex photosystems. The synthetic bilayer membrane was shown to be useful for that purpose, apart from the fact that the biological photoprocess often proceeds in the lipid bilayer. Acknowledgment. We are grateful to Prof. K. Kano of Doshisha University for his help and stimulating discussion. Registry No. C12StbCloN+Br-, 107004-31-9;ClzStbC4N+Br-, 107004-32-0; 2C18N+2ClBr-,3700-67-2; 2Cl&J+2ClBr-, 70755-47-4; 4,4'-dihydroxystilbene, 659-22-3; p-(dodecy1oxy)-p-hydroxystilbene, 117755-22-3; p-(dodecyloxy)-p'-((4-bromobutyl)oxy)stilbene, 117755-23-4;p-(dodecy1oxy)-p'-((10-bromodecy1)oxy)stilbene, 117755-24-5.
