Photoisomerization and Photodimerization in Self-Assembled

Feb 1, 1996 - Michael O. Wolf† and Marye Anne Fox*. Department of Chemistry and ... Thin films of cis-4-cyano-4′-(10-mercaptodecoxy)stilbene (1) a...
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Langmuir 1996, 12, 955-962

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Photoisomerization and Photodimerization in Self-Assembled Monolayers of cis- and trans-4-Cyano-4′-(10-mercaptodecoxy)stilbene on Gold Michael O. Wolf† and Marye Anne Fox* Department of Chemistry and Biochemistry, University of Texas at Austin, Austin, Texas 78712 Received October 23, 1995. In Final Form: October 30, 1995X Thin films of cis-4-cyano-4′-(10-mercaptodecoxy)stilbene (1) and trans-4-cyano-4′-(10-mercaptodecoxy)stilbene (2) on quartz or self-assembled monolayers of these substrates on optically transparent gold undergo photoisomerization and photodimerization reactions when irradiated with >350 nm visible light. The quantum yield for cyclodimerization in a monolayer of 1 is Φ ) 1.0 ( 0.5 × 10-6, whereas that observed in the solid-state thin film was Φcisftrans ) 4 ( 1 × 10-3 and Φcisfdimer ) 6 ( 2 × 10-4. Two photodimers are isolated by irradiation of melts of the derivatives cis-4-cyano-4′-(10-(acetylthio)decoxy)stilbene (3) and trans-4-cyano-4′-(10-(acetylthio)decoxy)stilbene (4). Irradiation of 3 resulted in photoisomerization to 4 as well as dimerization, whereas irradiation of 4 resulted only in photodimerization. A monolayer of 1 on gold blocks the electrochemical oxidation of decamethylferrocene (DMFc) dissolved in CH3CN in the potential region from -0.5 to +0.5 V vs Ag wire. After scanning between -0.5 and +1.0 V vs Ag, the monolayer no longer blocks the oxidation of DMFc present in the contacting electrolyte solution. Cycling of the potential of monolayers of 1 or 2 on Au in 0.1 M KOH between 0 and -1.2 V vs Ag results in the reductive stripping of the monolayer from the electrode surface. The cyclic voltammogram of a monolayer of 1 (Γ ) 1 × 10-9 mol/cm2) has a broad reduction peak, which disappears after the first scan, whereas that of 2 (Γ ) 3 × 10-10 mol/cm2) has a single, sharp reduction peak, which persists upon repeated cycling.

Introduction Self-assembled monolayers (SAMs) of organic thiols on metal surfaces have proven to be fascinating and useful structures.1 Many studies have focused on examining monolayers of simple n-alkyl thiols, and a detailed understanding of the factors involved in the formation,2-4 stability,5 structure,6-9 properties,10,11 and applications12,13 of these systems has emerged. Recently much work has focused on the preparation of thiols containing more complex and interesting functionalities and the study of the behavior of monolayers of these molecules on Au surfaces. Electron transfer,14-17 wettability,18-22 adhesion,18 and optical signal transduction23 have all been studied in this fashion. In addition, it has been possible † Current address: Department of Chemistry, University of British Columbia, Vancouver BC, V6T 1Z1. X Abstract published in Advance ACS Abstracts, February 1, 1996.

(1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Bain, C. D.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1989, 5, 723-727. (3) Ha¨hner, G.; Wo¨ll, C.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955-1958. (4) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860-5862. (5) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 65616561. (6) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506-511. (7) Li, Y.; Huang, Y. L.; McIver, R. T.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428-2432. (8) Strong, L.; Whitesides, G. M. Langmuir 1988, 4, 546-558. (9) Camillone, N. I.; Chidsey, C.; Eisenberger, P.; Fenter, P.; Li, J.; Liang, K.; Liu, G.; Scoles, G. J. Chem. Phys. 1993, 99, 744-747. (10) Lo´pez, G. P.; Biebuyck, H. A.; Frisbie, C. D.; Whitesides, G. M. Science 1993, 260, 647-649. (11) Lo´pez, G. P.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1513-1516. (12) Xia, Y.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 32743275. (13) Kumar, A.; Biebuyck, H. A.; Abbott, N. L.; Whitesides, G. M. J. Am. Chem. Soc. 1992, 114, 9188-9189. (14) Chidsey, C. E. D. Science 1991, 251, 919-922. (15) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657-2668. (16) Bard, A. J.; Abrun˜a, H. D.; Chidsey, C. E. D.; Faulkner, L. R.; Feldberg, S.; Itaya, K.; Majda, M. M.; Melroy, O.; Murray, R. W.; Porter, M.; Soriaga, M.; White, H. S. J. Phys. Chem. 1993, 97, 7147-7173.

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to pattern modified surfaces by generating excited states when bound directly to the surface.24-28 There is significant interest in the preparation of such patterned surfaces for applications in information storage, lithography, solar energy conversion, and microsensors. A significant advantage of using a molecular photoreaction to effect surface patterning is that solution photochemistry can be used to understand and control the surface reactivity. There are, however, important differences between a solution and surface photoreaction which are important. In order to evaluate these differences, we became interested in examining the photochemistry of a stilbene-containing SAM. The photochemistry and photophysics of stilbene and its derivatives have been extensively studied.29,30 Stilbene undergoes photochemical cis-trans isomerization, dimerization, and cyclization in solution. The photoisomerization of stilbene involves a twisting about the central carbon-carbon bond, (17) Finklea, H. O.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223-227. (18) DiMilla, P. A.; Folkers, J. P.; Biebuyck, H. A.; Ha¨rter, R.; Lo´pez, G. P.; Whitesides, G. M. J. Am. Chem. Soc. 1994, 116, 2225. (19) Abbott, N. L.; Folkers, J. P.; Whitesides, G. M. Science 1992, 257, 1380-1382. (20) Abbott, N. L.; Whitesides, G. M. Langmuir 1994, 10, 14931497. (21) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16-18. (22) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897-5898. (23) Katz, E.; Lion-Dagan, M.; Willner, I. J. Electroanal. Chem. 1995, 382, 25-31. (24) Calvert, J. M.; Georger, J. H.; Peckerar, M. C.; Pehrsson, P. E.; Schnur, J. M.; Schoen, P. E. Thin Solid Films 1992, 210, 359-363. (25) Kang, D.; Wrighton, M. S. Langmuir 1991, 7, 2169-2174. (26) Rozsnyai, L. F.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 5993-5994 and references therein. (27) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395-4404. (28) Tarlov, M. J.; Burgess, D. R. F. J.; Gillen, G. J. Am. Chem. Soc. 1993, 115, 5305-5306. (29) Go¨rner, H.; Kunh, H. J. In Advances in Photochemistry; D. C. Neckers, D. H. Volman and G. von Bu¨nau, Eds.; John Wiley & Sons, Inc.: 1995; Vol. 19; pp 1-117. (30) Saltiel, J.; D’Agostino, J.; Megarity, E. D.; Metts, L.; Neuberger, K. R.; Wrighton, M.; Zafirou, O. C. Org. Photochem. 1973, 3, 1-113.

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and we were interested in determining the effect of a constrained monolayer environment on such an isomerization. Other studies have revealed that the photochemistry of stilbene in the solid state and in micelles31-34 is quite different from that observed in solution. Photochemical reactions in which the chromophore is close to a metal surface, as in SAMs, are of interest because the metal may quench or otherwise perturb the excited state. Other studies of the photochemistry of monolayers on metal surfaces have demonstrated that the excited state is not completely quenched by the metal,24-28 although the observed reaction quantum yields may be lower than either in solution or the solid state. By studying the photochemistry of substituted stilbenes within a monolayer, we hoped to probe this effect directly. Finally, by incorporating a polar functional group in the stilbene monolayer, we could simultaneously examine the effect of isomerization on various surface properties such as the wetting angle with water. Control of hydrophobicity is important for the preparation of switching devices in which the state of the device is controlled by the degree of wetting exhibiting by the control surface.35-37 In this paper, we use absorption spectroscopy to study the photochemical reactions occurring within monolayers of cis-4-cyano-4′-(10-mercaptodecoxy)stilbene (1) and trans-

4-cyano-4′-(10-mercaptodecoxy)stilbene (2) on gold surfaces. We compare the photoreactivity of these wellordered monolayers with that of thin films of 1 and 2 and of closely related thioesters cis-4-cyano-4′-(10-(acetylthio)decoxy)stilbene (3) and trans-4-cyano-4′-(10-(acetylthio)decoxy)stilbene (4). In a previous communication we described changes in the wetting properties that result from the irradiation of monolayers of 1.38 Experimental Section General. Compounds 1-4 were prepared as described previously.38 Solvents were reagent grade and were used as received. 1H and 13C NMR spectra were acquired on either Bruker AC-250 or General Electric GN-500 instruments using CDCl3 as solvent. Absorption spectra were acquired on a HP 8451A diode array spectrophotometer. Mass spectra were obtained on either a Finnigan TSQ70 or a Fisons TS250 instrument. 1,2-(4′-Cyanophenyl)-3,4-(4′-(10-(acetylthio)decoxy)phenyl)cyclobutane (5a) and 1,3-(4′-Cyanophenyl)-2,4-(4′-(10(acetylthio)decoxy)phenyl)cyclobutane (5b). cis-4-Cyano4′-(10-(acetylthio)decoxy)stilbene (3) (0.3 g, 7 mmol) was heated on a hot plate in a 20 mL beaker to its melting point. The stirred mixture was then irradiated with a 450 W Hanovia mediumpressure mercury lamp filtered through a cooled water filter and a Schott BG40 filter for 16 h. During the course of the (31) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1975, 97, 16021603. (32) Quina, F. H.; Whitten, D. G. J. Am. Chem. Soc. 1977, 1977, 877-883. (33) Whitten, D. G. J. Am. Chem. Soc. 1974, 96, 594-596. (34) Whitten, D. G. Angew. Chem. Int. Ed. Engl. 1979, 18, 440-450. (35) Jackel, J. L.; Hackwood, S.; Veselka, J. J.; Beni, G. Appl. Opt. 1983, 22, 1765-1770. (36) Beni, G.; Tenan, M. A. J. Appl. Phys. 1981, 52, 6011-6015. (37) Beni, G.; Hackwood, S. Appl. Phys. Lett. 1981, 38, 207-209. (38) Wolf, M. O.; Fox, M. A. J. Am. Chem. Soc. 1995, 1845-1846. (39) Oesch, U.; Janata, J. Electrochem. Acta 1983, 28, 1237-1246. (40) Ulrich, H.; Rao, D. V.; Stuber, F. A.; Sayigh, A. A. R. J. Org. Chem. 1970, 35, 1121-1125. (41) Cohen, M. D.; Green, B. S.; Ludmer, Z.; Schmidt, G. M. J. Chem. Phys. Lett. 1979, 7, 486-489.

Wolf and Fox photoreaction, the mixture darkened slightly. The crude product was purified by chromatography on silica using ethyl acetate/ hexanes (1:10) to remove residual starting material. The mixture of photoproducts were eluted using ethyl acetate/hexanes (1:1). Two regiosisomeric photoproducts were carefully separated as oils using chromatography (silica, ethyl acetate/hexanes (1:3)).

Compound 5a. 1H NMR: 7.40 (d, 4 H, J ) 8.4 Hz), 7.13 (d, 4 H, J ) 8.4 Hz), 6.92 (d, 4 H, J ) 8.7 Hz), 6.67 (d, 4 H, J ) 8.7 Hz), 4.37 (m, 4 H), 3.83 (t, 4 H, J ) 6.7 Hz), 2.83 (t, 4 H, J ) 7.4 Hz), 2.29 (s, 6 H), 1.70 (quintet, 4 H, J ) 7.6 Hz), 1.53 (quintet, 4 H, J ) 7.4 Hz), 1.4 -1.2 (m, 24 H). 13C NMR: 196.0, 157.8, 146.1, 131.8, 130.8, 128.8, 128.7, 118.9, 114.3, 109.8, 67.9, 47.8, 46.5, 30.6, 29.4, 29.4, 29.3, 29.3, 29.2, 29.1, 29.0, 28.7, 26.0. MS (CI): m/z 871 (94), 642 (2), 435 (100), 231 (3.6), 182 (48). HRMS (CI): m/z calcd for C54H66N2O4S2 (M + H)+, 871.4542; found, 871.4530. Compound 5b. 1H NMR: 7.42 (d, 4 H, J ) 8.3 Hz), 7.14 (d, 4 H, J ) 8.3 Hz), 6.93 (d, 4 H, J ) 8.7 Hz), 6.68 (d, 4 H, J ) 8.7 Hz), 4.37 (m, 4 H), 3.83 (t, 4 H, J ) 6.6 Hz), 2.84 (t, 4 H, J ) 7.4 Hz), 2.30 (s, 6 H), 1.70 (quintet, 4 H, J ) 7.4 Hz), 1.54 (quintet, 4 H, J ) 7.6 Hz), 1.4-1.2 (m, 24 H). 13C NMR: 196.0, 157.7, 145.9, 132.0, 131.1, 128.9, 128.6, 118.6, 114.3, 110.3, 68.0, 47.8, 46.8, 30.6, 29.5, 29.4, 29.4, 29.3, 29.3, 29.1, 29.1, 28.8, 26.0. MS (CI): m/z 871 (18), 436 (70), 337 (100). HRMS (CI): m/z calcd for C54H66N2O4S2 (M + H)+, 871.4542; found, 871.4524. Gold Substrates. Thick substrates for electrochemical measurements were prepared by argon plasma sputtering of ∼100 Å of Cr followed by ∼2000 Å of Au onto single crystal Si wafers (Silicon Sense). Surfaces were either used immediately or cleaned by immersion in freshly prepared 30% H2O2/H2SO4 (3:1) prior to derivatization. Optically transparent substrates for absorption experiments were prepared by argon plasma sputtering of ∼30 Å of Cr followed by ∼100 Å of Au onto quartz slides (Quartz Plus, Inc.). These substrates were used immediately. Monolayer Preparation. Monolayers were prepared by immersion for 2-24 h of the gold substrates in 1-5 mM ethanolic solutions of thiol 1 or 2. After soaking, the samples were rinsed with ethanol, acetone, and water and were dried under a stream of nitrogen. Thin films of 1 and 2 were prepared by casting CH2Cl2 solutions onto quartz slides and allowing the solvent to evaporate. Preparative scale irradiation of 3 was carried out by placing a solid sample in a Petri dish on a hot plate and slowly heating it to its melting point. Heating was continued throughout the irradiation. Quantum yields were calculated by determining the photon flux with a benzophenone-benzhydrol actinometer.42 The extents of isomerization and dimerization of 1 in the solid state were determined by integration of the 1H NMR spectra after irradiation for a specified period. For monolayer samples, the (42) Moore, W. M.; Ketchum, M. J. Am. Chem. Soc. 1962, 84, 13681371. (43) Curtin, L. S.; Peck, S. R.; Tender, L. M.; Murray, R. W.; Rowe, G. K.; Creager, S. E. Anal. Chem. 1993, 65, 386. (44) Groat, K. A.; Creager, S. E. Langmuir 1993, 9, 3668-3675. (45) Hickman, J. J.; Ofer, D.; Zou, C.; Wrighton, M. S.; Laibinis, P. E.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 1128.

Photochemistry of Stilbenes on Gold irradiation time required for complete photodimerization was determined by monitoring the absorption spectrum. The surface coverage was estimated by the method of Porter.49,50 Photochemistry. Irradiation of thin films, monolayers, solids, and solutions with a 450 W Hanovia medium-pressure mercury lamp were all carried out under identical conditions. The lamp output was passed through a cooled water filter to remove IR radiation and through a Schott BG40 filter to remove ultraviolet light of λ < 350 nm. Thin films of 1 and 2 were prepared by casting CH2Cl2 solutions onto quartz slides and allowing the solvent to evaporate. Preparative scale irradiation of 3 was carried out by placing a sample in a Petri dish on a hot plate and slowly heating the sample until it melts. The sample was heated throughout the irradiation. Quantum yields were calculated by determining the photon flux using a benzophenonebenzhydrol actinometer.42 The extents of isomerization and dimerization of 1 in the solid state were determined by integration of the 1H NMR spectrum after irradiation for a given amount of time. For monolayer samples, the irradiation time required for complete photodimerization was determined by monitoring the absorption spectrum. The surface coverage was estimated by the method of Porter.49,50 Electrochemistry. Cyclic voltammetry was carried out on a BAS-100 electrochemical analyzer interfaced to a personal computer for data workup and storage. [n-Bu4N]PF6 (Aldrich) was recrystallized three times from absolute ethanol; reagent grade KOH (Mallinkrodt) was used as received. Acetonitrile (Aldrich anhydrous) was used as received. Experiments were done in a three electrode cell with a platinum mesh counter electrode and a silver wire quasi-reference electrode. For blocking experiments, decamethylferrocene (Strem) was dissolved in 0.1 M [n-Bu4N]PF6 in CH3CN and the working electrode was a bare or modified gold sample. For stripping experiments, 0.1 M aqueous KOH was used as electrolyte. Surface roughness was determined by the method of Oesch.39

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Figure 1. Absorption spectra of thin films of 1 on quartz (a) before irradiation, (b) after 5 min irradiation, and (c) after 20 min irradiation with a filtered (λ < 350 nm) medium-pressure mercury lamp.

Results and Discussion Photochemistry of Thin Films of 1 and 2. In order to examine the photochemical reactivity of 1 and 2 in the solid state, we cast films of these compounds from CH2Cl2 onto quartz slides and followed the progress of the photoreaction by absorption spectroscopy. A very smooth, clear film was obtained from 1, whereas 2 was cast as a cloudy film. Absorption spectra of the films of 1 and 2 are shown in Figures 1a and 2a, respectively. The spectrum of a thin film of 1 is very similar to that obtained in a CH2Cl2 solution (Figure 3a). The thin film spectrum of 2, however, differs significantly from the corresponding solution spectrum of 1 (Figure 3b). Both the high- and low-energy bands in 1 are slightly red-shifted in the thin film (λmax ) 248 and 332 nm) compared with the solution (λmax ) 246 and 326 nm). The spectrum of the thin film of 2 contains a broad, low-energy absorption centered at approximately 338 nm (blue-shifted from the solution λmax at 346 nm) and a higher energy band at λmax ) 244 nm. None of the thin film or solution spectra exhibit significant absorption beyond 500 nm. The blue-shift observed for 2 is likely the result of intermolecular interactions enhanced by aggregation in the solid state. Such behavior is well-known in Langmuir-Blodgett (LB) films of other chromophores.1 Irradiation of the thin films of 1 and 2 with a IR-filtered medium-pressure mercury lamp results in significant (46) Hickman, J. J.; Laibinis, P. E.; Auerbach, D. I.; Zou, C. F.; Gardner, T. J.; Whitesides, G. M.; Wrighton, M. S. Langmuir 1992, 8, 357. (47) Tsutsumi, H.; Furumoto, S.; Morita, M.; Matsuda, Y. J. Electrochem. Soc. 1992, 139, 1522. (48) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (49) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (50) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359.

Figure 2. Absorption spectra of thin films of 2 on quartz (a) before irradiation, (b) after 5 min irradiation, and (c) after 20 min irradiation with a filtered (λ < 350 nm) medium-pressure mercury lamp.

changes in the absorption spectra of the films. After 5 min irradiation, a shoulder appears at ∼275 nm in the spectrum of 1 with a slight blue-shift of the high-energy band to 238 nm and a red-shift of the low-energy band to 348 nm (Figure 1b). After irradiation for 20 min (Figure 1c), no additional features appear, but slight intensity changes are observed for some of the bands. Irradiation of a thin film of 2 for 5 min results in a shift of λmax for the low-energy band to 350 nm, the appearance of a shoulder at 275 nm, and a blue-shift of the high-energy band to 240 nm (Figure 2b). Further irradiation (for 20 min) results in increases in the intensity of these new

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Figure 3. Absorption spectra of (a) 1 and (b) 2 in CH2Cl2.

features, but no new bands. The similarity between the spectra obtained after 20 min of irradiation of 1 and 2 suggests that the photoproducts are similar. In order to determine the structure of the photoproducts, we carried out photolyses of solid samples of 3 and 4. We chose to use the (acetylthio) derivatives for these experiments since they are stable to air, whereas the corresponding thiols undergo slow air oxidation to the disulfides. The use of 3 and 4 as models for 1 and 2 is reasonable, as the photochemical behavior of the stilbene moiety in 1 and 2 , on a small scale, is the same as that observed for 3 and 4 (vide infra), except that parallel geometric isomerization and photodimerization disulfide formation occurs concomitantly with dimerization in the thiols. This results in a number of different products, namely, those arising from both stilbene photochemistry and thiol oxidation. Since we are primarily interested in the photoreactivity of the stilbene moiety as a model for the photoreactivity of a monolayer of 1 or 2, we did not separate the complex mixture obtained from photolysis of solid 1 or 2. Instead, we pursued the cleaner photoreactions of 3 and 4, in which the complicating oxidative chemistry is blocked by the presence of the inert thioacetate. Irradiation of solid 3 in air for 2 h, followed by analysis of the product by 1H NMR, indicated partial formation of a new compound, the NMR spectrum of which contained several complex multiplets at approximately 4.4 ppm. These features are consistent with a cyclobutane structure, presumably formed by this stilbene dimerization. We could accelerate the formation of this dimer by irradiation of a melt of 3; in this way we prepared enough material to purify and characterize. The 1H NMR spectrum of the crude product obtained from irradiation of the melt was identical to that obtained by solid state irradiation of 3. Since cis-trans isomerization and dimerization are both observed during the solid state irradiation, there are a total of 11 possible cyclobutane isomers deriving from cisand trans-stilbenes substituted with two different groups.40,41 Using column chromatography, we were able to separate two isomeric products from the irradiation of 3 but could not separate stereoisomers despite repeated attempts under a range of conditions. The 500 MHz 1H NMR spectra of these two fractions (5a and 5b) showed very different patterns in the region 4.3-4.5 ppm, where cyclobutyl ring H’s are expected to appear (Figure 4). The symmetry of these peaks suggests that both isomers likely arise from dimerization of either two cis or two trans isomers. The complexity of the 1H NMR spectra made it difficult to unambiguously assign the structures of the two isomers. The CI mass spectra of 5a and 5b are useful in partial assignment of their structures. The spectrum

Figure 4. Cyclobutyl H region in the 1H NMR spectra of (a) 5a and (b) 5b.

of 5a contains peaks at m/z 642 and 231, consistent with fragmentation of the cyclobutane ring into [RCHdCHR], where R is O(CH2)10SCOCH3 or CN. In addition, there is a peak at 436 corresponding to the [RCHdCHR′] fragment, (R ) O(CH2)10SCOCH3, R′ ) CN). This isomer must therefore have the A regioisomeric structure. The mass spectrum of 5b, on the other hand, does not have significant peaks at 642 or 231 and, thus, is likely to be stereoisomer B.

The absorption spectra of thin films of 5a and 5b (Figure 5) are useful in analyzing the progress of the photoreaction of 1 and 2, and by monitoring the progress of the solidstate photoreaction of 3 and 4 simultaneously by absorption spectra and by 1H NMR spectroscopy, we could establish the course of the photoreactions taking place. During the irradiation of solid samples of 3 and 4 (250 mg), small aliquots (10 mg) of the irradiated solid were removed at 30 min intervals. The composition of each aliquot was determined by analysis of its 1H NMR spectrum in the aromatic region. Integration of the unique resonances of 3, 4, 5a, and 5b permit a quantitative delineation of the composition of the aliquot. The 1H NMR spectra contained resonance other than those assigned to 3, 4, 5a, and 5b. The irradiation of 3 resulted in isomerization to 4 in addition to the concomitant formation of 5a and 5b (Figure 6). A small amount of dimer 5 is

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Figure 7. Absorption spectra of monolayers of (a) 1 and (b) 2 on transparent Au films.

Figure 5. Absorption spectra of 5a as (a) a thin film on quartz and (b) in CH2Cl2 and of 5b as (c) a thin film on quartz and (d) in CH2Cl2.

Figure 6. Plot of the composition of an irradiated solid state sample of 3 as a thin film on quartz irradiated with a filtered (λ < 350 nm) medium-pressure mercury lamp as a function of irradiation time. Percentage of 3 (4), 4 (0) and 5a and 5b (]) present determined by 1H NMR integration.

observed even at very short irradiation times. Since isomerization has already occurred at this point, we cannot conclude that 5a or 5b is formed exclusively by dimerization of 3. Secondly, isomerization continues throughout the irradiation time. At the end of the experiment less than 20% of 3 remains, whereas nearly 70% of 4 remains. Under otherwise identical conditions, solutions of 1 and 2 in benzene achieve a photostationary state which is ∼80% cis isomer, with no evidence of cyclodimerization. Irradiation of solid 4 under the same conditions yielded only the slow formation of 5a and 5b with no formation of 3.

Photochemistry of Monolayers of 1 and 2. In order to examine the photoreactions occurring within SAMs of 1 and 2 on Au, we used optically transparent Au substrates, prepared by sputter deposition of Au onto a thin layer of Cr on quartz slides.18 The substrates absorbed between 50 and 75% of the incident light between 190 and 800 nm. SAMs of 1 and 2 prepared on these substrates by immersion in 1-5 mM ethanol solutions overnight exhibit the absorption spectra shown in Figure 7. The spectrum of 1 as a monolayer is very similar to that observed as a solution and thin film. In addition, a broad low-energy absorption with λmax ) 570 nm is observed. Significantly, this band is not present in either the solution or thin film spectra. Compounds such as 1 and 2, in which stilbene is substituted with an electron donor on one ring and an electron acceptor on the other ring, frequently exhibit charge-transfer transitions in their electronic spectra.41,42 It is possible that this transition is enhanced in the monolayer by the local dipolar interactions enhanced by the ordered structure of the monolayer, an effect which has been observed previously in non-centrosymmetric structures.1 The spectrum of 2 (Figure 7b) is also similar to its solution and thin film spectra; however, a blue-shift is observed for the low-energy band to approximately 300 nm (Figure 8b). This band appears to be asymmetric, with significant lower energy absorption. This is likely the result of aggregation similar to that observed in the thin film spectrum of 1 (vide supra). The monolayer spectrum of 2 also contains the broad low-energy absorption observed for 1. Irradiation of a monolayer of 1 results in significant changes in the absorption spectrum (Figure 8). After 0.5 h irradiation, the lower energy absorption broadens slightly with increased absorption at ∼300 nm. The higher energy band is slightly blue-shifted to λmax ) 240 nm. After longer irradiation, a shoulder to the band at 240 nm appears, the high-energy band shifts to λmax ) 235 nm, and a new lower energy band appears at λmax ) 350 nm (Figure 8c-e). These results are consistent with the isomerization of 1 to 2 in the monolayer and with the formation of one or more cyclobutanes by photodimer-

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Figure 8. Absorption spectra of monolayers of 1 (a) before irradiation with a filtered (λ < 350 nm) medium-pressure mercury lamp; (b) after 0.5 h irradiation; (c) after 1 h irradiation; (d) after 2 h irradiation; and (e) after 3 h irradiation.

ization. After 3 h irradiation (Figure 8e), most of the monolayer has been converted to cyclobutane products (cf. Figure 5). Because of the low quantum yield in the monolayer, a determination of an action spectrum for surface dimerization is difficult, but excitation into the UV absorption maxima for 1 and 2 was required for the observed conversions. It is clear from these results that the quantum yield for isomerization and dimerization in the monolayer is low. Using a benzophenone-benzhydrol actinometer,42 we estimated the light intensity between 300 and 400 nm to be approximately 5 × 1016 photons/s cm2. Assuming an approximate surface coverage of ∼10-9 mol/cm2 (vide infra), and a conversion time to dimer of 3 h, the quantum yield for cyclodimerization in the monolayer is approximately 1 ( 0.5 × 10-6. Irradiation of solid, microcrystalline samples of 1 and 2 had significantly higher quantum yields for conversion to photoproducts [Φcisftrans ) 4 ( 1 × 10-3, Φcisfdimer ) 6 ( 2 × 10-4]. It is likely that differences in either intermolecular ordering or the proximity of the metal substrate results in the lower quantum yield observed in the monolayer than in the solid state. The absorption spectrum of an irradiated monolayer of 2 is shown in Figure 9. After 1 h irradiation, a new band at 285 nm appears along with a shoulder at 350 nm. With longer irradiation these bands become more clearly defined (Figure 9c-d). The similarities between the spectra of 1 and 2 after 3 h irradiation suggest that both photoreactions result in a similar distribution of cyclobutane products. We conclude that irradiation of monolayers of 1 and 2 results in photoisomerization and slower photodimerization. Thus, the changes in the contact angle of water with these monolayers observed upon irradiation38 are the result of these photochemical processes. Since irradiation of a monolayer of 2 resulted in no discernible change in

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Figure 9. Absorption spectra of monolayers of 2 (a) before irradiation with a filtered (λ < 350 nm) medium-pressure mercury lamp, (b) after 1 h irradiation, (c) after 2 h irradiation, and (d) after 3 h irradiation.

the contact angle with water but absorption spectral changes demonstrate photodimerization, we conclude that 2 and the photodimer interact with the probe liquid similarly. Irradiation of a monolayer of 1, which causes a decrease in the contact angle with water,38 clearly results in photodimerization. Although unequivocal spectroscopic evidence of isomerization in the monolayer of 1 is obscured by the competing photodimerization, it is likely that this process is indeed occurring since the solid-state irradiation of 1 results in both isomerization and dimerization. Electrochemical Blocking. Previously, we reported that there is a difference in the extent of blocking observed of the electrochemical response for oxidation of a solution of Fe(CN)4- at 1- or 2-derivatized electrodes.38 We attributed this difference to a possible difference in either the structural order or the average thickness of the SAMs. Recently, we have begun exploring the use of redox couples soluble in organic solvents such as CH3CN to determine the blocking characteristics of SAMs. There are only a few reports of using organic solvents to determine blocking characteristics of monolayer derivatized electrodes.43-47 Creager found that n-alkanethiol monolayers block charged redox couples such as Fe(CN)64- in aqueous solution more completely than neutral couples such as ferrocene in propylene carbonate.44 Remarkably, dodecanethiol monolayers block virtually none of the redox activity of decamethylferrocene in propylene carbonate. We examined the blocking characteristics of monolayers of 1 on gold in decamethylferrocene (DMFc) in CH3CN with 0.1 M [n-Bu4N]PF6 added electrolyte (Figure 10). Surprisingly, a monolayer of 1 blocks the redox behavior of DMFc very well (Figure 10b) between -0.5 and +0.5 V vs Ag wire. We examined monolayers of n-alkanethiols on gold under identical conditions and found that, as expected, little blocking is observed for DMFc in CH3CN. These results suggest that the structure of the monolayer

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Figure 11. Cyclic voltammograms in 0.1 M KOH of (a) a 1-derivatized gold electrode, (b) the second (s) and third (- - -) scans of the same 1-derivatized gold electrode, and (c) bare gold. Scan rate ) 100 mV/s. All electrodes have identical surface areas.

Figure 10. Cyclic voltammograms of decamethylferrocene in 0.1 M [n-Bu4N]PF6 in CH3CN at 25 °C at (a) bare gold, (b) the initial scan of a 1-derivatized gold electrode between -0.5 and +0.5 V vs Ag wire (c) the second scan of a 1- derivatized gold electrode between -0.5 and 1.0 V, and (d) the third scan of a 1-derivatized gold electrode between -0.5 and +0.5 V. Scan rate ) 100 mV/s. All electrodes have identical surface areas.

of 1 is significantly more stable in CH3CN than that of a monolayer of n-alkanethiol under comparable conditions. On the second scan to +1.0 V, a broad oxidative wave and a corresponding reduction wave appear (Figure 10c). On the third scan to +0.5 V, the DMFc wave is clearly present, albeit with a larger peak-to-peak splitting and reduced anodic and cathodic current than for a bare gold electrode of comparable area (Figure 10a). These results suggest that the monolayer of 1 is disrupted at moderate overpotentials (between +0.5 and +1.0 V), possibly by partial desorption of the adsorbed molecules for the surface. These results are consistent with experiments on SAMs of p-HS(CH2)11OC6H4NdNC6H5,48 which indicate that monolayers of planar aromatic systems form dense, stable structures. In this study, the authors propose a structural model incorporating azobenzene groups in a well-ordered array with an underlying disordered array of alkyl tethers. The structural similarities between molecules 1 and 2 and this substituted azobenzene suggest that a similar model may apply to the monolayers of 1 and 2, and may explain the differences in the blocking behavior between 1 and dodecanethiol monolayers in CH3CN. Electrochemical Stripping. Previous work by Porter et al. has demonstrated that SAMs of alkyl thiols may be electrochemically stripped at negative potentials in aqueous KOH.49,50 The relevant redox reaction involved is believed to be that shown in eq 1.

AuSR + e- f Au(0) + RS-

(1)

The area under the stripping wave may then be used to determine surface coverage. The coverages thus obtained are only approximate, but the shape and potential of the reduction wave yield qualitative information about the desroption process. We examined SAMs of 1 (Figure

Figure 12. Cyclic voltammograms in 0.1 M KOH of (a) a 2-derivatized gold electrode, (b) the second (s) and third (- - -) scans of the same 2-derivatized gold electrode, and (c) bare gold. Scan rate ) 100 mV/s. All electrodes have identical surface areas.

11) and 2 (Figure 12) in this fashion. The first cycle for a monolayer of 1 shows several broad reduction waves, with a very small return wave at approximately -0.6 V vs Ag (Figure 11a). This behavior is unlike that observed for SAMs of alkyl thiols on Au/mica, where a single sharp reduction wave was observed under these stripping conditions.49,50 This difference in behavior suggests that SAMs of 1 either have a different structure or may exist in several different structural forms, thus giving rise to several separate stripping waves. Monolayers of n-alkyl thiols show a distinct chain length dependence for the reductive stripping wave as a result of differences in the degree of order in monolayers of various chain length. The second and third scans for monolayers of 1 demonstrate that most of the material is stripped irreversibly in the first scan. However a small, sharp wave at -0.9 V remains (Figure 11b). This feature disappears completely if the potential is held at -1.2 V for 30 s. This

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wave may be due to a small amount of reduced 1 which precipitates after stripping. Integration of the area under the curve observed in the initial scan (Figure 11a), corrected for surface roughness, corresponds to an approximate surface coverage of 1 × 10-9 mol/cm2. This is comparable to the surface coverage calculated for dodecanethiol on Au.49 The stilbene head group thus appears to have little effect on the surface coverage; however, monolayers of ferrocenyl-terminated alkyl thiols also have comparable surface coverages.49 In contrast to the results observed for 1, reductive stripping of monolayers of 2 (Figure 12) is quite different. A single, sharp stripping peak is observed during the first scan (Figure 12a). Subsequent scans (Figure 12b) show a single peak shifted to slightly lower potential than in the first scan, and several corresponding broad oxidation peaks. 2 is thus more difficult to remove from the surface than 1, indicating that 2 may form more stable structures on the surface. In addition, since only a single sharp peak is observed, it is likely that most of the molecules are in a similar environment in the monolayer and thus are desorbed at the same potential. Determination of the surface coverage of 2 by integration of the peak area on the first cycle gives a coverage of 3 × 10-10 mol/cm2. Since later cycles still have a relatively strong reductive stripping peak, it is possible that the entire monolayer is not stripped on the first scan and that the calculated coverage is inaccurate. Conclusions The photochemistry of SAMs of substituted stilbenes 1 and 2 on Au surfaces is similar to that observed in the solid state for analogous compounds 3 and 4. Although the quantum yield for isomerization and dimerization in

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the SAMs is low, we were able to demonstrate changes in hydrophobicity38 of the surface resulting from these reactions. The quantum yields for these processes in the solid state are also low, suggesting that in a sterically hindered environment reactions involving nuclear motion are not favored. Significantly, the difference between the quantum yield in the monolayer and the solid state indicates that the metal substrate may partially deactivate the excited state on the surface. Nonetheless, complete quenching of the excited state by the metal does not occur, since observable photochemistry takes place within the monolayer. In addition to changes in molecular properties, changes in the supramolecular structure of the monolayer are also important in controlling photochemistry within a SAM. The observed differences in the reductive stripping voltammograms of 1 and 2 on Au suggest that there are differences in the monolayer structure which may influence reactions within the monolayers. Absorbance spectroscopy indicates that isomerization and dimerization occur upon irradiation of monolayers of 1 and 2, but the mechanism by which these processes occur within the environment of the monolayer is unknown. The sensitivity of these reactions to their environment provide an unique opportunity to control condensed phase photochemistry. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences. M.O.W. gratefully acknowledges a postdoctoral fellowship from the Natural Sciences and Engineering Research Council of Canada. We thank Mark F. Arendt for preparation of the gold surfaces used in this work. LA9509256