Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide

designed and synthesized for surface photoisomerization reactions. ... The profile of the trans to cis photoisomerization reaction by UV light irradia...
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Langmuir 2002, 18, 5239-5246

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Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide Self-Assembled Monolayers Studied by Surface Plasmon Spectroscopy: Influences of Side Chain Length and Contacting Medium Kaoru Tamada,*,†,‡ Haruhisa Akiyama,† and Tian Xin Wei† National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Spatio-Temporal Function Materials Team, Frontier Research System, RIKEN, Wako, Saitama 351-01, Japan Received December 27, 2001. In Final Form: April 16, 2002 We report the characteristics of azobenzene-containing self-assembled monolayers (SAMs) which are designed and synthesized for surface photoisomerization reactions. The photoreactive SAMs were composed of unsymmetrical azobenzene disulfides, in which the free volumes for photoreaction of azobenzene moieties are guaranteed by 50% dilution of dye functions at the molecular level on the surface. The photoswitching reaction was monitored in situ through the change of optical film thickness by means of surface plasmon spectroscopy. The influences of alkyl side chain length and contacting medium on the photoreaction were also investigated. The profile of the trans to cis photoisomerization reaction by UV light irradiation exhibited a clear deviation from first-order kinetics, suggesting a steric hindrance effect on the photoreaction process.

1. Introduction Gold thiol self-assembled monolayers (SAMs) have been studied extensively as a useful technique for surface fabrication.1-4 This technique gives well-ordered and longterm stabilized films by simple experimental procedures, for example, to immerse a gold substrate into a thiol solution. However, not only a high degree of structural order but also the dynamic response of the surface molecules would be expected for the further application of the SAMs. In this study, we propose a concept to fabricate a new type of dynamic surfaces with the selfassembling technique, where unsymmetrical disulfides were utilized to realize high photoreactivity in the densely adsorbed SAMs with azobenzene functions5,6 on gold. The structure and growth of the “hexylazobenzenethiol” (12-(4-((hexylphenyl)azo)phenoxy)dodecane-1-thiol, C6AzSH in Figure 1) SAM were investigated by several surface characterization techniques such as AFM, XPS, FTIRRAS, and reflection UV-vis absorption spectroscopy in our previous study,7 and the unique molecular ordering due to the intermolecular interaction was precisely determined and compared with those of other thiol SAMs.8-10 However, such densely packed azobenzene SAMs were hardly photoreactive against UV-vis pho* To whom correspondence should be addressed. E-mail: [email protected]. Telephone: +81-298-61-6305. Fax: +81-29861-6306. † National Institute of Advanced Industrial Science and Technology (AIST). ‡ Frontier Research System, RIKEN. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 481. (2) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (b) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (3) (a) Ulman, A. In An Introduction to Ultrathin Organic Films; Ulman, A., Ed.; Academic Press: Boston, MA, 1991. (b) Ulman, A. Chem. Rev. 1996, 96, 1533. (4) (a) Dubois, L. H.; Nuzzo, R. G. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4739. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (5) Griffiths, J. Chem. Soc. Rev. 1972, 1, 481. (6) Rau, H. In Photochemistry and Photophysics; Rabeck, J. F., Ed.; CRC Press: Boca Raton, FL, 1990; Vol.2, Chapter 4, pp 119-141. (7) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264.

Figure 1. Unsymmetrical azobenzene disulfides with different lengths of alkyl side chains (C6AzSSC12, C6AzSSC18, C6AzSSC22) and azobenzenethiol (C6AzSH).

toirradiation. Wang et al. performed electrochemistry of azobenzenethiol SAMs on gold and reported that only 5% or less of the cis form was achieved by UV light irradiation in such SAMs with 2D crystalline structure.11 Referring to the previous reports concerning azobenzene functionalized surfaces,12 for example, azosilanes13-16 and LB and related films,17-21 a monomeric dispersion of dye functions (8) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, Ch.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102. (9) 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. (10) (a) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 1005. (b) Wang, R.; Jiang, L.; Iyoda, T.; Trek, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (11) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.: Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213. (12) Ichimura, K. Chem. Rev. 2000, 100, 1873. (13) (a) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoi, A.; Aoki, K. Langmuir 1988, 4, 1214. (b) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Seki, T.; Suzuki, Y. Langmuir 1992, 8, 2601. (c) Ichimura, K.; Akiyama, H.; Kudo, K.; Ishizuki, N.; Yamamura, S. Liq. Cryst. 1996, 20, 423-435, 1996. (14) (a) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1995, 11, 2856. (b) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1996, 12, 2976. (15) Xing, L.; Mattice, W. L. Langmuir 1996, 12, 3024.

10.1021/la0157667 CCC: $22.00 © 2002 American Chemical Society Published on Web 05/18/2002

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with disordered chains seems to be necessary to induce high photoreactivity. For example, Sekkat et al. have succeeded in characterizing the photoreaction of an azosilane film with surface plasmon resonance (SPR) spectroscopy.14 They reported that the surface coverage in their films was quite low (about 30% as a surface coverage). Evans et al. utilized the mixed SAMs with short spacing molecules to introduce “free volume” into the system to facilitate the “photoswitching” reaction;22 however, it was not sufficient for the success. Coadsorption phenomena in mixed thiols are known to be quite complicated and difficult to control;23-26 for example, strong preferential adsorption of longer chain molecules was often observed, which results in island formation due to phase segregation in the SAMs.27,28 There is one report presenting highly photoreactive mixed monolayer systems at an airwater interface, where the spacer molecules with a negatively charged headgroup (DMPA) were utilized to ensure mixing with azobenzene derivatives with a positively charged headgroup at the molecular level.29 Recently, we designed and synthesized unsymmetrical azobenzene disulfides with alkyl side chains (C6AzSSC12, C6AzSSC18, C6AzSSC22 in Figure 1) and succeeded in achieving highly photoreactive SAMs on gold.30 In these SAMs, the free volume for photoreaction of azobenzene moieties is guaranteed by 50% dilution of dye functions on the surface, while densely packed SAM structures are kept in the bottom layers. In our previous report, we confirmed high photoreactivity of these SAMs by contact angle measurement (sessile drop method) and also observation of surface-induced liquid crystal (LC) alignment.30 These disulfide SAMs exhibited quite a different photoresponse because of the length of the alkyl side chains. The unsymmetrical disulfide SAM with a short alkyl chain (C6AzSSC12) exhibited the most drastic changes for contact angles and also for the control of LC alignment by UV-vis photoirradiation. These two methods are sensitive to the change of surface properties and are frequently utilized for the evaluation of the surface (16) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. (17) (a) Liu, Z.-F.; Morigaki, K.; Enomoto, T.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1992, 96, 1875. (b) Morigaki, K.; Liu, Z.-F.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1995, 99, 14771. (18) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (19) (a) Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama, K. Langmuir 1994, 10, 1898. (b) Sato, T.; Ozaki, Y.; Iriyama, K. Langmuir 1994, 10, 2368. (20) Seki, T.; Ichimura, K.; Fukuda, R.; Tanigaki, T.; Tamaki, T. Macromolecules 1996, 29, 9. (21) (a) Ichimura, K.; Fukushima, N.; Fujimaki, M.; Kawahara, S.; Matuzawa, Y.; Hayashi, Y.; Kudo, K. Langmuir 1997, 13, 6780. (b) Fujimaki, M.; Kawahara, S.; Matsuzawa, Y.; Kurita, E.; Hayashi, Y.; Ichimura, K. Langmuir 1998, 14, 4495. (22) Evans, S. D.; Johnson, S. R.: Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436. (23) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (24) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (25) (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (b) Forkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (26) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761. Offord, D. A.; John, C. M.; Linford, M. R.; Griffin, J. H. Langmuir 1994, 10, 883. (27) Tamada, K.: Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (28) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (29) Ahuja, R. C.; Maack, J.; Tachibana, H. J. Phys. Chem. 1995, 99, 9221. (30) Akiyama, H.; Tamada, K.; Nagasawa, J.; Nakanishi, F.; Tamaki, T. Trans. Mater. Res. Jpn. 2000, 25, 425.

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photoisomerization reaction with an azo unit; however, the information provided by these methods is rather semiquantitative, and we cannot obtain detailed information concerning the conformational change of individual molecules by photoreaction.12 In this study, we characterize the surface photoisomerization reaction with surface sensitive SPR spectroscopy, where the photoswitching reaction was monitored in situ in the liquid cell with organic solvents through the change of the optical film thickness.31 The SPR technique has already been used by several other research groups to characterize the photoreaction of azobenzene-containing polymer films,14,22 though all these measurements have been performed in air. In this study we performed the SPR measurement in organic solvents purposely, to enhance the optical contrast with photoreaction. In good solvents, the molecular tails of the trans-azobenzene are expected to be stretched out to the solvent phase by “solvation”, while cis-azobenzene possibly forms more condensed films with smaller chains, that may give the maximum contrast between the trans and cis forms to the system. The idea to use organic solvents as the contacting media in the SPR measurement was already proposed in our previous study.32 In the previous study, we succeeded in an independent determination of the refractive indices and the film thickness of semifluorinated alkanethiol SAMs,32 where the inertness (no solvation) of the fluorocarbon surface against the utilized organic solvents enabled the precise determination of the optical film thickness. In contrast to that case, the interaction between surface and solvents is actively used in this azobenzene system. 2. Experimental Section 2.1. Sample Preparation and Gold Deposition. The synthesis of unsymmetrical azobenzene disulfides (C6AzSSC12, C6AzSSC18, C6AzSSC22) and azobenzenethiol (C6AzSH) is described elsewhere.7,30 The 0.25 mM disulfide and 0.5 mM thiol solutions were prepared in hexane, so as to make the sulfur concentration in solutions constant for all the SAM formations. The gold layer about 430 nm in thickness was thermally deposited on LaSFN9 glass slides (high refractive index glass, n ) 1.845 at λ ) 632.8 nm) in a vacuum chamber (AUTO 306, Edwards, ∼1 × 10-6 mbar), to use for SPR measurement. Before use, the gold substrates were kept in absolute hexane to avoid surface contamination. 2.2. SPR Spectroscopy. The SPR setup, which is based on the configuration introduced by Kretschmann and Raether,33 is shown in Figure 2A.34 A p-polarized He Ne laser beam (λ ) 632.8 nm) was used as the light source, which was mechanically chopped in conjunction with a lock-in amplifier before entry into the prism. The intensity of the beam reflected at the gold surface was detected with a photodiode detector and recorded as a function of the incidence angle for “angular-scan” measurements (Figure 2B, left) or as a function of time at a fixed angle of incidence for “kinetics-scan” measurements (Figure 2B, right). All sample cells and the tubes were made of Teflon to be resistant against organic solvents. Prior to the all SPR experiments, the liquid cell with the bare gold substrate was filled with absolute hexane over 30 min. Both the dielectric constant and the thickness of the gold layer were determined from the SPR data (angular scan) in equilibrium by a curve fitting program.33,35 The SAM formation was initiated by injection of the disulfides or thiol solution into the cell, and the (31) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (32) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913. (33) Kretschmann, E.; Raether, H. Z. Naturforsch. 1968, 23, 2135. Raether, H. In Physics of thin films; Hass, G., Francombe, M. H., Hoffmann, R. W., Eds.; Academic: New York, 1977; Vol. 9, p 145. (34) Aust, E. F.; Ito, S.; Sawodny, M.; Knoll, W. Trends Polym. Sci. 1994, 9, 313.

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Figure 3. UV-vis absorption spectrum of C6AzSH in solution (CH2Cl2, dashed line) and reflection spectra of C6AzSH, C6AzSSC12, and C6AzSSC18 SAMs on Au(111)/mica.

Figure 2. (A) Schematics of the SPR experimental setup and photoreaction of the azobenzene SAM. (B) Peak shift by photoreaction in the SPR spectrum with “angular-scan” (left), and photoresponse of the SAMs monitored by the change of reflectivity with “kinetics scan” (right). adsorption process was monitored via the change of reflectivity by the kinetics-scan measurement (incidence angle: 66°). The reflectivity increased monotonically during the SAM formation, and it reached a constant value within a couple of hours. The adsorption was continued for at least 12 h, and then the surface was rinsed with absolute hexane. The photoisomerization reaction of the SAM was monitored in the same liquid cell with the photoirradiation of UV and vis light through the back glass window (Figure 2A). An ultrahigh-pressure mercury lamp with color filters was used as the UV (364 nm, 2.44 mW/cm2) and vis (440 nm, 2.70 mW/cm2) light source. The real-time observation of the photoresponse in azobenzene SAMs was carried out by the kinetics scan mode, while the optical thicknesses of the trans and cis form SAMs (in equilibrium) were determined from the shifts of SPR peak positions by the angular-scan measurements. These measurements were carried out in various media(air, n-alkanes, methanol, and water) to investigate the influence of the contacting medium on the photoreaction. In this process, one solvent was successively replaced by another miscible solvent (e.g., hexane f pentane f heptane f octane, or hexane f methanol f water). The refractive index of each solvent was determined from the critical angle for total internal reflection in the SPR data, to confirm the complete replacement of the solvent. 2.3. AFM Imaging and Reflection UV-vis Absorption Spectroscopy. The AFM images of the unsymmetrical disulfide SAMs on Au(111)/mica were taken with a Nanoscope IIIa (Digital Instruments, Inc., Santa Barbara, CA) in contact mode in air at room temperature. The procedure to make the Au(111)/mica is described elsewhere.7 We utilized exactly the same scanning conditions (a Si3N4 cantilever with a spring constant of 0.38 N/m; scanning rates of 20-30 Hz) as those we used for taking the 2D crystalline images with azobenzenethiol (C6AzSH) SAMs, to realize a direct comparison concerning the molecular ordering.7 Reflection UV-vis absorption spectra were measured on Au(111)/mica substrates with a Shimadzu UV-2500PC spectrometer by using the incident beam at an angle of incidence of 5° to the surface normal. The spectra were recorded at a sampling pitch of 0.1 nm in the 200-800 nm region.7 2.4. Measurement of Photoisomerization Reaction Kinetics of Azobenzene Derivatives in Solution. The reaction kinetics of azobenzenethiol and azobenzene disulfide derivatives (35) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings; Springer Tracts in Modern Physics, Vol. 111; Springer: Berlin, 1988.

dissolved in hexane (5 × 10-3 mM solutions) were determined via the UV-vis absorption spectra, to be compared with the reaction kinetics of the corresponding SAMs. The UV-vis absorption spectra were taken in a quartz liquid cell (1 cm thick) after several intermittent photoirradiations with UV (364 nm) and vis (440 nm) lights at room temperature. The peak intensities at 347 nm (λmax) are used to determine the rate of the isomerization reaction. The spectra in hexane solution (the trans and cis forms) are provided as Supporting Information.

3. Results and Discussion 3.1. Surface Morphology and UV-vis Absorption Spectrum. The surface morphologies of unsymmetrical disulfide SAMs (C6AzSSC12, C6AzSSC18, C6AzSSC22) prepared by 12 h of immersion in 0.25 mM hexane solutions were compared with that of azobenzenethiol SAM (C6AzSH) adsorbed under the same conditions. The AFM images of the disulfide SAMs revealed no crystalline structures on Au (111), unlike the case for C6AzSH SAMs (images are not shown).7 They both exhibited no detectable domain structures resulting from phase segregation of two thiolates, one is an azobenzenethiolate and another is an alkanethiolate, produced by S-S bond cleavage of unsymmetrical disulfide by adsorption. As shown in Figure 3, reflection UV-vis absorption spectra of both C6AzSSC12 and C6AzSSC18 SAMs exhibited a strong ∼240 nm peak but almost no peak at 360 nm, suggesting that the π-π* transition band of the long axes of trans-azobenzene is aligned about normal to the surface in these SAMs. These spectra are quite similar to that of the C6AzSH SAM.7 3.2. Photoresponse Characterized by SPR Techniques. Figure 4 is a real-time photoresponse of C6AzSH and C6AzSSC12 SAMs measured with SPR in hexane and in air, respectively. The C6AzSSC12 SAM exhibited a much larger photoresponse compared with that for the C6AzSH SAM both in hexane and in air. In agreement with the previous report with an azosilane film,14 the optical thickness tends to increase by irradiation with vis light (isomerization from cis to trans) and decrease by irradiation with UV light (isomerization from trans to cis). The reaction was enhanced in hexane for both C6AzSH and C6AzSSC12 SAMs. The C6AzSH SAM exhibited almost no optical response in air, while a slight change in reflectivity with photoirradiation was confirmed in hexane. A similar enhancement of photoreaction could be observed in other n-alkanes (pentane, heptane, and octane) as well. The SPR data of C6AzSH in pentane, heptane, and octane are provided as Supporting Information. The

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Figure 4. Comparison of the photoresponse of azobenzene SAMs in hexane and in air: (a) C6AzSH; (b) C6AzSSC12.

photoresponses of the other unsymmetrical disulfides, C6AzSSC18 and C6AzSSC22 SAMs, in hexane are shown in Figure 5. They exhibited almost identical photoresponses to that of the C6AzSSC12 SAM. Their data in air and in other n-alkanes (pentane, heptane, and octane) were also nearly the same as those of the C6AzSSC12 SAM. In Figure 6, we summarized the dielectric constant (;  ) n2, where n is the refractive index) of the azobenzene derivative SAMs ((SAM)) in the cis and trans forms, to indicate the “optical thickness” of each SAM layer. The (SAM) was determined from the peak shift in the SPR angular scan, with the thickness of the trans-C6AzSH SAM (d ) 3.6 nm) estimated in our previous study.7 The precision for determination of (SAM) is estimated to be about (0.02. The C6AzSH SAM exhibited the highest (SAM) in both the cis and trans forms compared with the values for the unsymmetrical disulfide SAMs (C6AzSSC12, C6AzSSC18, C6AzSSC22), according to its densely packed film structure. Among the disulfides, the molecules with longer side chains exhibited slightly higher (SAM) values. Photoreactivity can be estimated by the difference in optical thickness between the trans and cis forms. As presented in the real-time photoresponse in Figures 4 and 5, the disulfide SAMs exhibited much higher photoreactivity than that of the C6AzSH SAM. For all the cases, the trans form SAMs exhibited higher (SAM) values than the cis form SAMs; that is, the trans form SAMs are optically thicker than the cis form SAMs. 3.3. Solvent Effect in the Results of SPR. In Figure 7, we plotted the (SAM) estimated in n-alkanes (the same data shown in Figure 6) together with the data in air, to confirm the solvent effect. As shown with the solid lines in Figure 7, the optical thickness of the unsymmetrical disulfide SAMs (C6AzSSC12, C6AzSSC18, C6AzSSC22) varied significantly in n-alkanes and in air, for both the cis and trans forms, while it was not so obvious for the case of C6AzSH SAM. We plotted the data of the octadecanethiol (C18SH) SAM in Figure 7a (the dashed line) for comparison. Meaningfully, the C18SH SAM

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exhibited quite a similar slope to that of the C6AzSH SAM. This result reveals that the determinations of the SAM thicknesses of crystalline-like SAMs such as C18SH and C6AzSH are not largely influenced by contacting solvents, while the influence of solvent is not negligible for the case of unsymmetrical disulfide SAMs. In other words, the unsymmetrical disulfide SAMs are largely “solvated” in n-alkanes (n-alkane molecules are intercalated into the SAM layers), as expected from their film structures having a “free space” provided by the short alkyl chains. We performed one more experiment to confirm the solvation effect. We characterized the photoreaction of C6AzSH and C6AzSSC12 SAMs in methanol and water (Figure 8). The refractive indices of methanol and water are quite close (n(methanol) ) 1.328; n(water) ) 1.333); however, the solubilities of azobenzene derivatives are quite different. Both C6AzSH and C6AzSSC12 are soluble in methanol, while water has no affinity for them. As shown in Figure 8, the (SAM) in the cis form SAM in water was almost on the straight line joining the two data points in hexane and in methanol; however, the (SAM) in the trans form SAM showed a clear deviation from their data in methanol and hexane. This experimental result indicates that the optical thickness of the trans form SAMs was more influenced by contacting medium, compared with the cis form SAMs. On the basis of these SPR data, we can draw a schematic model for the enhancement of photoresponse by solvation as shown in Figure 9. In air, the optical thickness of the SAMs can be changed only by the changes of the electronic and structural properties induced by photoreaction. Since the hexyl terminal group in the C6AzSSC12 SAM is expected to be disordered in air (also in poor solvents) even for the trans form SAM,36,37 the change of optical thickness must arise only from the azobenzene dye unit and it should be quite small (e.g., ∼3 Å15). On the other hand, in good solvents, solvent molecules are intercalated into the film for both the trans and cis azobenzene SAMs, as confirmed in Figure 7, though the trans form SAMs can be solvated more than the cis form SAMs, as suggested in Figure 8. We presume that dynamic movement of solvent molecules at an interface contributes to the enhancement of the optical contrast during the photoreaction as shown in Figure 9. 3.4. Analysis of Reaction Kinetics with the FirstOrder Plot. The reaction kinetics of azobenzene dyes in thin solid films have been characterized extensively to discuss the differences of the photoreaction compared to that in solution, in correlation with the freedom of molecular motion due to the restriction of mobility.38 As a result, non-first-order progress of reactions (see below) is frequently observed in the solid films, due to the microscopically heterogeneous state of dye aggregation or less free volume distribution in the reaction media.39 For example, the photoisomerization reaction of azobenzene dyes bound in the polymer backbone or attached to the side chain is known to occur by a single rate process in dilute solution; however, that in solid films exhibits two separated processes: the first process as fast as that in dilute solution and the following slow process.40 The (36) We characterized the C6AzSSC12 SAM (trans form) with Fourier transform infrared-reflection absorption spectroscopy (FTIR-RAS). The shoulder peak of the υas(CH2) vibration mode appeared at 2924 cm-2, which corresponds to the liquid states (ref 37), suggesting alkyl chains in the C6AzSSC12 SAM are disordered unlike those in the C6AzSH SAM (ref 7). (37) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (38) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558. (39) Smets, G. Adv. Polym. Sci. 1983, 50, 17.

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Figure 5. Photoreaction of unsymmetrical disulfide SAMs in hexane: (a) C6AzSSC18; (b) C6AzSSC22.

Figure 6. Dielectric constants of the azobenzene SAMs ((SAM)) determined from the SPR peak shift in various solvents: C6AzSH (circles); C6AzSSC12 (reversed triangles); C6AzSSC18 (diamonds); C6AzSSC22 (triangles). The filled symbols are (SAM) after vis light irradiation (trans form), and the open symbols are (SAM) after UV light irradiation (cis form).

fraction of fast and slow processes is related to the amount of sites where the local free volumes are greater than a critical size necessary for the photoisomerization of the azobenzene group. The first-order plot is based on the following equation for the photoisomerization reaction. When trans-azobenzene with initial concentration [t]0 is exposed to 360 nm UV light, the rate of concentration change of transazobenzene to cis is given by eq 1.

-d[t]/dt ) A[t] -B[c] -K[c] ) (2.3 × 103)I0tftΦ tfc[t] - (2.3 × 103)I0cfcΦ cft[c] K[c] (1) where [t] and [c] are the concentrations of trans- and cisazobenzene, respectively ([c] ) [t]0 - [t]). A is the rate constant for the trans to cis photoisomerization, and B is that for the cis to trans photoisomerization. K is the rate constant for thermal recovery from cis to trans. I0 is the incident light intensity at 360 nm; t and c are the molar extinction coefficients. ft and fc are the correction factors for the absorption of light. Φ tfc and Φ cft are the quantum (40) (a) Sung, C. S. P.; Lamarre, L.; Chung, K. H. Macromolecules 1981, 14, 1839. (b) Lamarre, L.; Sung, C. S. P. Macromolecules 1983, 16, 1729. (c) Sung, C. S. P.; Gould, I. R.; Turro, N. J. Macromolecules 1984, 174, 1447. (d) Yu, W.; Sung, C. S. P.; Robertson, R. E. Macromolecules 1988, 21, 355.

Figure 7. Dielectric constants of the azobenzene SAMs ((SAM)) in air and n-alkanes: (a) after vis light irradiation (trans form); (b) after UV light irradiation (cis form). The data of the octadecanethiol SAM (C18SH) are plotted together for comparison (dashed line in Figure 7a).

yields for the trans to cis and the cis to trans photoisomerizations, respectively. For the case of the monomolecular films, eq 1 should be held as an approximation resulting in the first-order kinetics, since it is unnecessary to consider the contribution of film thickness. When the photostationary state is attained with the equilibrium value [t]∞, eq 1 can lead to the following relation,

A[t]∞ - B[c]∞ - K[c]∞ ) A[t]∞ - (B[t]0 - B[t]∞) (K[t]0 - K[t]∞) ) 0 (A + B + K)[t]∞ ) (B + K)[t]0 (2) From eq 2, the following first-order relation can be given,

ln(R) ) ln[([t]0 - [t]∞)/([t] - [t]∞)] ) [[t]0/([t]0 [t]∞)]At ) [[t]0/[t]∞](B + K)t (3)

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Figure 8. Comparison of dielectric constants of azobenzene SAMs ((SAM)) in water, methanol, and hexane; the filled symbols are the trans form, and open symbols are the cis form, respectively.

Figure 9. Schematic models for a “solvent effect”: a mechanism of enhancement in photoresponse with solvents for SPR measurement.

For the case of the cis to trans photoisomerization with vis light irradiation at 440 nm, eq 3 is also applicable by conversion of [t]0 to [t]∞ and [t]∞ to [t]0. Figure 10 shows the ln(R) versus time plots with C6AzSSC12 SAMs in various solvents (n-alkanes), where ln(R) was determined by the change of reflectivity in the SPR measurement. The equilibrium reflectivities in the trans and cis forms were utilized as [t]0 and [t]∞, respectively. As shown in Figure 10a, the cis to trans photoisomerization reaction proceeds almost following “firstorder kinetics”, suggesting little steric effect in the reaction process. On the other hand, as shown in Figure 10b, the trans to cis photoisomerization reaction exhibits a clear deviation from the first-order plot as evidence of a steric effect, that is typical of the densely packed film. There is no influence of solvent on the reaction kinetics as long as n-alkanes are utilized. The initial rate of photoreaction in the C6AzSSC12 SAM was compared with that of the corresponding molecules dissolved into hexane (5 × 10-3 mM solution), which was characterized with UV-vis absorption spectroscopy. The slope of the first-order plot (ln(R) versus time, ∆ ln(R)) for the cis to trans isomerization in the SAM was approximately ∆ ln(R)SAM ) 1.1 per minute under vis light irradiation (2.70 mW/cm-2, 440 nm), which is comparable to the reaction rate of the C6AzSSC12 molecules in hexane solution (∆ ln(R) solution ) 0.92 per minute with the same light intensity). On the other hand, the reaction rate for the trans to cis isomerization was much slower in the SAMs, ∆ ln(R) SAM ) 1.55 per minute under UV light irradiation (2.44 mW/cm-2, 364 nm), compared with the C6AzSSC12 molecules in hexane

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solution (∆ ln(R) solution ) 14.6 per minute with the same light intensity). This slow reaction for the trans to cis isomerization suggested the perpendicular alignment of the trans azobenzene in the SAM, as evaluated by reflection UV-vis absorption spectroscopy in our work (see Figure 3). The detailed analysis is described below. We estimated the averaged molecular orientation of the azo unit (the directions of the π-π* and n-π* transition bands in the trans and cis forms, respectively) from this kinetics data with SPR. The direction of the transition moment to the surface normal (θ)41 was estimated by the following equation, sin2 θ ) (2∆ ln(R)SAM)/ (3∆ ln(R)solution),42 where the photoisomerization process in the SAM was assumed to be identical to that in solution except for the averaged molecular orientation. As a result, the averaged direction of the π-π* transition moment at 364 nm is estimated to be 15° tilted to the surface normal. On the other hand, the average direction of the n-π* transition moment at 440 nm is estimated to be 63° tilted to the surface normal by this simple calculation. 43 It is known that the direction of the transition moment in trans-azobenzene does not agree with the direction of the molecular long axis.44 For example, the direction of the π- π* adsorption band in trans-azobenzene is approximately 17° tilted from the molecular long axis, while that of the n-π* adsorption band is 35° tilted from the molecular long axis. Following this value, the direction of the molecular long axis of trans-azobenzene can be estimated to be 2° ()17-15) tilted to the surface normal in our SAM (Figure 11). In contrast to the case of transazobenzene, no experimental data have been reported concerning the direction of transition bands in cisazobenzene; that is, it was actually impossible to estimate the exact direction of the molecular axis of the cisazobenzene from the data of the n-π* transition. However, considering the data in trans-azobenzene concerning the n-π* transition moment, which is 35° tilted to the molecular axis, the direction of the n-π* transition moment in cis-azobenzene is also assumed to be similarly different from its molecular axis. By this assumption, the obtained tilt angle of 63° for the direction of the n-π* transition moment can be led to a quite reasonable model for the molecular axis of cis-azobenzene,45 as schematically described in Figure 11. We have noticed that these analyses involve many assumptions and the possibility of large errors. For the case of the cis to trans isomerization under 440 nm of light irradiation, the largest error may arise from the influence of cis-azobenzene remaining in the SAM, which is too large to be ignored (see the UV-vis absorption spectra in the Supporting Information); however, we did not carefully consider this influence in our original calculation. For the case of the trans to cis isomerization (41) We also assumed that the directions of the absorption bands for both trans- and cis-azobenzene are identical in the SAM at the same wavelength (364 or 440 nm each), to apply the equation for the isotropic solution system to the SAM system. (42) The average orientation of transition moment (P) is considered to be P ) 2/3 in solution (space isotropic) and P ) 1 in monolayers with all dipoles parallel to the surface. Orrit, M.; Mo¨bius, D.; Lehmann, U.; Meyer, H. J. Chem. Phys. 1986, 85, 4966. (43) For the trans-azobenzene (at 440 nm): sin2 θ ) 1.10 × 2/(3 × 0.92) ) 0.8, sin θ ) 0.89, and θ = 63°. For cis-azobenzene (at 364 nm): sin2 θ ) 1.55 × 2/(3 × 14.6) ) 0.07, sin θ ) 0.26, and θ = 15°. (44) Uznanski, P.; Kryszenski, M.; Thulstrup, E. W. Spectrochim. Acta 1990, 46, 23. (45) The geometry of cis-azobenzene giving the most consistent interpretation of the electronic spectrum is known to be the “propellershaped” conformation, wherein the phenyl rings are rotated approximately 30° out of the plane. Beveridge, D. L.; Jaffe, H. H. J. Am. Chem. Soc. 1966, 88, 1948.

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Figure 10. First-order plots of the photoisomerization reaction of the C6AzSSC12 SAMsinfluence of contacting solvents (nalkanes) for reaction kinetics: (a) cis to trans with vis light irradiation (440 nm, 2.70 mW/cm2); (b) trans to cis with UV light irradiation (364 nm, 2.44 mW/cm2).

Figure 11. Conformational model of trans and cis azobenzenes placed on xyz-coordinate, where the direction of molecular long axis of trans azobenzene is configured to the y-axis (x,y,z) ) (0, 1, 0) and the two phenyl rings in trans azobenzene and the lower side of phenyl rings in cis azobenzene were placed on the xy plan (z ) 0). In this Figure, the molecular axis of cisazobenzene (the straight line joined the centers of the two phenyl rings) is about 46 degree tilted to the surface normal (the y-axis).

under 364 nm light irradiation, the influence of transazobenzene remaining in the SAM must be relatively small (see the UV-vis absorption spectra in the Supporting Information); however, the estimation of the initial rate of photoreaction (∆ ln(R)SAM) may include larger errors, since a clear deviation from the first-order plot is already confirmed, as shown in Figure 10b. We also need to keep in mind that the optical response with SPR does not necessarily need to be linear to the photoreaction of azo functions. The influence of contacting medium also needs to be considered. However, in the end, our kinetics data do not come into conflict with any other experimental

Figure 12. First-order plots of the photoisomerization reaction of unsymmetrical disulfide SAMssinfluence of alkyl side chain length on reaction kinetics: (a) cis to trans with vis light irradiation (440 nm, 2.70 mW/cm2); (b) trans to cis with UV light irradiation (364 nm, 2.44 mW/cm2).

results we have at all. It is rather surprising that the direction of the molecular long axis of trans-azobenzene in our kinetics model perfectly agreed with the result we obtained for the azobenzenethiol (C6AzSH) SAM in our previous study,7 in which the molecular long axis of transazobenzene was determined to be normal to the surface by several different techniques such as AFM, FTIR-RAS, and reflection UV-vis absorption spectroscopy. The last discussion in this paper is the influence of alkyl side chain length on the reaction kinetics. As shown in Figure 12, the C6AzSSC12 and C6AzSSC18 SAMs exhibited quite similar reaction kinetics; however, the C6AzSSC22 SAM showed a slightly different kinetics profile. The reaction rate of the C6AzSSC22 SAM from cis to trans was much faster than that of C6AzSSC12 and C6AzSSC18, while the reaction rate of the C6AzSSC22 SAM from trans to cis was slightly slower than those of

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the other two SAMs. This result is reasonably interpreted by the influence of “surface stress”. The surface of the C6AzSSC22 SAM is more congested with the long side chains (C22). The reaction from cis to trans would be accelerated on such a congested surface, since it is the direction to relax the surface. On the other hand, the reaction from trans to cis was slowed because it is the direction to load more stress. A similar phenomenon has been reported in azobenzene polymer systems.46 These data are quite reasonable when we remember that photoisomerization reaction of azobenzene is an equilibrium reaction.

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deviation from first-order plots was confirmed in the reaction kinetics for the trans to cis isomerization reaction, which can be reasonably interpreted by the steric effect. We estimated the averaged direction of the transition moment of both trans- and cis-azobenzene from the initial rate of photoreaction. The direction of the π-π* transition moment in trans-azobenzene (at 364 nm) is estimated to be 15° tilted to the surface normal, while that of the n-π* transition moment in cis-azobenzene (at 440 nm) is estimated to be 63° tilted to the surface normal. These data are not in conflict with the conformational model of the SAMs determined by the other surface characterization techniques.

4. Conclusions The photoreaction of unsymmetrical azobenzene disulfide SAMs (C6AzSSC12, C6AzSSC18, C6AzSSC22) was investigated precisely with the SPR technique and was compared with that of the azobenzenethiol (C6AzSH) SAM. The photoreaction monitored by in situ SPR measurement revealed high photoreactivity in the unsymmetrical disulfide SAMs. The optical contrast (the change of reflectivity) by photoreaction tends to be enhanced in good solvents (e.g., n-alkanes), and the influence of contacting medium was discussed. A clear (46) (a) Priest, W. J.; Sifain, M. M. J. Polym. Sci. A-1 1971, 9, 3161. (b) Ueda, M.; Kim, H. B.; Ikeda, T.; Ichimura, K. Chem. Mater. 1992, 4, 1229.

Acknowledgment. K.T. thanks Prof. W. Knoll, MaxPlanck-Institute fu¨r Polymer-forschung, and Prof. M. Hara, Spatio-Temporal Function Materials Team, Frontier Research System, RIKEN, for their kind help for SPR measurements. The authors acknowledge Dr. K. Yase and Dr. T. Tamaki in AIST for their support of the project of “Harmonized molecular Materials”. T.W. thanks Japan Science Technology Corporation (JST) for the STA fellowship. Supporting Information Available: Spectra of C6AzSSC12 in solution (2 pages). This material is available free of charge via the Internet at http://pubs.acs.org. LA0157667