Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide

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Photoisomerization Reaction of Unsymmetrical Azobenzene Disulfide Self-Assembled Monolayers: Modification of Azobenzene Dyes to Improve Thermal Endurance for Photoreaction Kaoru Tamada,*,†,‡ Haruhisa Akiyama,† Tian-Xin Wei,† and Seung-Ae Kim§ National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan, Spatio-Temporal Function Materials Team, Frontier Research System, RIKEN, Wako, Saitama 351-01, Japan, Department of Chemistry, Hanyang University, Seoul 133-791, Korea Received April 20, 2002. In Final Form: November 22, 2002 Recently we have developed an unsymmetrical azobenzene disulfide with a short alkyl side chain, 4-hexyl-4′-(12-(dodecyldithio)-dodecyloxy)azobenzene (C6AzSSC12), aiming of a high efficiency in photoisomerization in SAMs on planar gold surfaces (Tamada, K.; et al. Langmuir 2002, 18, 5239). In this paper, we introduce an additional modification on the molecule to improve the thermal endurance for the photoreaction by attachment of a methyl group to the azobenzene ring, thus avoiding dye aggregation sterically. A “methyl-derivatized” azobenzenethiol (C6Az(Me)SH) SAM revealed a significant improvement in their photoreactivity compared with an unmodified azobenzenethiol (C6AzSH) SAM due to the steric effect of the methyl group. A “methyl-derivatized” unsymmetrical azobenzene disulfide (C6Az(Me)SSC12) SAM exhibited quite a similar photoresponse to that of C6AzSSC12 SAM before heat treatment owing to the free volume given by the unsymmetrical structure; however, only the C6Az(Me)SSC12 SAM could retain high photoreactivity in phase-segregated domains formed by annealing, unlike the C6AzSSC12 SAM. The C6Az(Me)SH and C6Az(Me)SSC12 SAMs exhibited a reaction kinetics different from that of C6AzSSC12 SAM due to a different quantum yield and the molecular tilt angle of the azobenzene unit. The C6Az(Me)SH SAM exhibited a typical character of “congested surface”, where the reaction rate from cis to trans was faster, while that from trans to cis was slower compared with those of C6Az(Me)SSC12 SAM.

1. Introduction Photoisomerization reactions in organic thin films with azobenzene functions, e.g., azopolymers,1 azosilanes,2-5 and LB and related films,6-10 have been studied widely in the past decade. However, azobenzene thiol SAMs on planar gold have been classified as unreactive surfaces against UV-vis photoirradiation, because the photoreaction in these SAMs is found to be hindered by their densely packed film structures.11-16 In a previous study, † National Institute of Advanced Industrial Science and Technology (AIST). ‡ RIKEN. § Hanyang University.

(1) Ichimura, K. Chem. Rev. 2000, 100, 1873. (2) (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. Liquid Cryst. 1996, 20, 423-435. (3) (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. (4) Xing, L.; Mattice, W. L. Langmuir 1996, 12, 3024. (5) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. (6) (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. (7) Velez, M.; Mukhopadhyay, S.; Muzikante, I.; Matisova, G.; Vieira, S. Langmuir 1997, 13, 870. (8) (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. (9) Seki, T.; Ichimura, K.; Fukuda, R.; Tanigaki, T.; Tamaki, T. Macromolecules 1996, 29, 9. (10) (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.

we have originally designed and synthesized unsymmetrical azobenzene disulfides with short alkyl side chains to achieve a highly efficient photoisomerization also on planar gold substrates.17,1817,18 In these SAMs, the free volume for the photoreaction of azobenzene moieties is guaranteed by 50% dilution of the dye functions on the surface, while densely packed SAM structures are kept at the bottom of the layers. The photoswitching reaction of these unsymmetrical azobenzene disulfide SAMs was characterized by contact angle measurement, surfaceinduced liquid crystal (LC) alignment,17 and surface plasmon resonance spectroscopy (SPR).18 In particular, we investigated the influence of the alkyl side chain length and of the contacting medium on the photoreaction by the SPR measurement. The SPR techniques have already been used widely to characterize the photoreaction of azobenzene-containing thin films;3,16 however, in the previous studies, all these measurements were performed in air. (11) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (12) 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. (13) 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. (14) (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. (15) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Elecro. Chem. 1997, 438, 213. (16) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436. (17) Akiyama, H.; Tamada, K.; Nagasawa, J.; Nakanishi, F.; Tamaki, T. Trans. Mater. Res. Jpn. 2000, 25, 425. (18) Tamada, K.; Akiyama, H.; Wei, T. X. Langmuir 2002, 18, 5239.

10.1021/la0258493 CCC: $25.00 © 2003 American Chemical Society Published on Web 02/05/2003

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Figure 1. Schematic of a phase segregation of unsymmetrical disulfide SAM, due to S-S bond cleavage by the reaction with gold.

Our study was rather unique since we performed on purpose all the SPR measurement in organic solvents, in order to enhance the photoresponse from the surface.18 In good solvents, the molecular tails of the trans azobenzenes are expected to be stretched out to the solvent phase by “solvation”, while the cis-azobenzenes possibly form a more condensed film with the shrunk chains, which gave an optimized contrast between the trans and cis form. This is the second report concerning photoreactive SAMs. In this study, we tested an additional modification on the unsymmetrical azobenzene disulfide molecules to improve their thermal endurance for photoreactions. It is known that disulfides turn to a pair of thiolates via S-S bond cleavage after the reaction with gold. Therefore, the unsymmetrical azobenzene disulfide may have a problem with long-term stability and thermal endurance during cycling.19,20 As shown in Figure 1, the two thiolates (azobenzene thiolate and alkyl thiolate) produced by S-S bond cleavage could possibly rearrange on the surface with time and form phase-segregated domains as a consequence of molecule-molecule interaction between the same molecular species (especially via the interaction between azobenzene rings).19,21,22 The diffusion coefficient of adsorbed thiolates must be quite slow, especially in the fully adsorbed SAMs; however, it would be accelerated upon heating. Then, once the phase segregation occurred, the photoreactivity of the SAMs could be lowered quite significantly. In our new azobenzene thiol and disulfide systems, a methyl group is directly introduced to the azobenzene ring in order to avoid dye aggregation by steric hindrance (C6Az(Me)SH and C6Az(Me)SSC12 in Figure 2). We studied the photoreaction of these “methyl-derivatized” azobenzene SAMs on planar gold substrates under thermal stress and compared this behavior with that of the unmodified azobenzene SAM systems (C6AzSH and C6AzSSC12 in Figure 2). 2. Experimental Section 2.1. Synthesis of Azobenzene Derivatives. The synthesis of azobenzenethiol (C6AzSH) and of unsymmetrical azobenzene disulfide (C6AzSSC12) is described elsewhere.11,17 The synthesis of the “methyl-derivatized” azobenzene thiol (C6Az(Me)SH), of the unsymmetrical disulfides (C6Az(Me)SSC12), and of their intermediate products were carried out as follows. 4-Hexyl-3′-methyl-4′-hydroxyazobenzene. An aqueous solution of 3.89 g of sodium nitrate in 11 mL of water was added to 5.6 mL of 5 N hydrochloric acid containing 10 g of 4-nhexylaniline at 5 °C. After stirring for 30 min, the solution was slowly added to 109 mL of methanol containing 6.10 g of o-cresol at 5 °C, and the mixture was neutralized with an aqueous solution of potassium carbonate. After the cool mixture was stirred for (19) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. (20) (a) Noh, J.; Hara M. Langmuir 2000, 16, 2045. (b) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (21) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (22) Stranick, S. J.; Parikh, A. N.; Tao, Y. T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636.

Figure 2. Molecular structures of unmodified azobenzene compounds (C6AzSH, C6AzSSC12) and “methyl-derivatized” azobenzene compounds (C6Az(Me)SH, C6Az(Me)SSC12). 2 h, the solution was stirred again for 6 h at room temperature. The appearing precipitate was filtered out and dissolved in diethyl ether. The ether solution was washed with water and then dried with magnesium sulfate. The solvent was removed by distillation. The crude solid was crystallized from hexane to give 9.97 g of orange crystals (60% yield), mp 87.5-88.5 °C. 1H NMR (CDCl3) δ ) 0.89 (3 H, t, CH3), 1.2-1.6 (24 H, m, CH2CH2CH2), 1.67 (2 H, t, t, Ar-CH2CH2), 2.30 (3 H, s, ArCH3), 2.67 (2 H, t, Ar-CH2), 6.90 (1 H, d, Ar-H), 7.29 (2 H, d, Ar-H), 7.75 (1 H, s, Ar-H), 7.779 (1 H, d, Ar-H), 7.79 ppm (2 H, d, Ar-H). 4-Hexyl-3′-methyl-4′-(12-bromododecyloxy)azobenzene. A mixture of 2 g of 4-hexyl-3′-methyl- 4′-hydroxyazobenzene, 0.93 g of potassium carbonate, 3.32 g of 1,12-dibromododecane, and 6.75 mL of N,N-dimethylformamide (DMF) was stirred for 6 h at 60 °C. The mixture was cooled to room temperature and diluted with ethyl ether. The ether solution was washed with water and then dried with magnesium sulfate, and the solvent was removed by distillation. The crude material was purified by column chromatography using a mixture of hexane and ethyl acetate (9:1) as the eluate to give 1.70 g of yellow crystals (46.7% yield). 1H NMR (CDCl3) δ 0.89 (3 H, t, CH3), 1.2-1.6 (22 H, m, CH2CH2CH2), 1.67 (2 H, t, t, Ar-CH2CH2), 1.82 (2 H, t, t, BrCH2CH2), 1.84 (2 H, t, t, Ar-OCH2CH2), 2.30 (3 H, s, Ar-CH3), 2.67 (2 H, t, Ar-CH2), 3.41 (2 H, t, BrCH2), 4.05 (2 H, t, Ar-OCH2), 6.91 (1 H, d, Ar-H), 7.30 (2 H, d, Ar-H), 7.75 (1 H, s, Ar-H), 7.779 (1 H, d, Ar-H), 7.79 ppm (2 H, d, Ar-H). 4-Hexyl-3′-methyl-4′-(12-(dodecylditio)dodecyloxy)azobenzene (C6Az(Me) SSC12). A solution of 0.64 g of sodium thiosulfate pentahydrate in 5 mL of water was added to 40 mL of DMF containing 1 g of 4-hexyl-3′-methyl-4′-(12-bromododecyloxy)azobenzene, and then the solution was stirred for 6 h at 60 °C for the preparation of the Bunte salt. A mixture of 0.41 g of dodecanethiol, 2 mL of methanol, 1 mL of water, and 0.088 g of sodium hydroxide was stirred for 1 h at a room temperature under Ar atmosphere. The DMF solution of the Bunte salt was added dropwise to this mixture under Ar atmosphere. After the mixture was stirred for 3 h, the objective material was extracted with hexane and then dried with magnesium sulfate. The solvent was removed by distillation and the obtained paste was purified by silica gel column chromatography using a mixture of hexane and ethyl acetate (19:1) as an eluate to give 1.01 g of yellow crystals (81.0%, yield), mp 31.5-33.0 °C. 1H NMR (CDCl3) δ 0.88 (3 H, t, CH3), 0.89 (3 H, t, CH3), 1.2-1.6 (40 H, m, CH2CH2CH2), 1.65 (4 H, t, t, SSCH2CH2), 1.67 (2 H, t, t, ArCH2CH2), 1.82 (2 H, t, t, ArOCH2CH2), 2.30 (3 H, s, ArCH3), 2.68 (4 H, t, SSCH2), 2.68 (2 H, t, Ar-CH2), 4.05 (2 H, t, Ar-OCH2), 6.91 (1 H, d, Ar-H), 7.30 (2 H, d, Ar-H), 7.75 (1 H, s, Ar-H), 7.77 (1 H, d, Ar-H), 7.79 ppm (2 H, d, Ar-H). Elemental anal. C:H:N:S: calcd 74.08:10.41: 4.02:9.20; found 74.37:10.71:4.05:9.26. 12-[2-Methyl-4-(4-hexylphenylazo)phenoxyl]dodecanethiol (C6Az(Me)SH). Hydrochroic acid, 3.5 g of 12 N, and 10 mL of water were added to a DMF solution of the Bunte salt prepared in the same manner as that described above. After stirring for 1 day at room temperature, the solution was neutralized with an aqueous solution of potassium hydroxide. The solution diluted with diethyl ether was washed with water

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and then dried with magnesium sulfate, and the solvent was removed by distillation. The crude solid was purified by silica gel column chromatography with a mixture of hexane and ethyl acetate (9:1) as the first elute and then carbon tetrachloride to give 0.26 g of yellow crystals (28.4% yield), mp 42.0-44.0 °C. 1H NMR (CDCl3) δ 0.89 (3 H, t, CH3), 1.2-1.6 (22 H, m, CH2CH2CH2), 1.65 (2 H, t, t, HSCH2CH2), 1.67 (2 H, t, t, Ar-CH2CH2), 1.84 (2 H, t, t, Ar-OCH2CH2), 2.30 (3 H, s, Ar-CH3), 2.52 (2 H, t,d, HSCH2), 2.67 (2 H, t, Ar-CH2), 4.05 (2 H, t, Ar-OCH2), 6.91 (1 H, d, Ar-H), 7.30 (2 H, d, Ar-H), 7.75 (1 H, s, Ar-H), 7.779 (1 H, d, Ar-H), 7.79 ppm (2 H, d, Ar-H). Elemental anal. C:H:N:S: calcd 74.95:9.74:5.64:6.45; found 74.78:9.86:5.58:6.54. 2.2. Sample Preparation and Gold Deposition. The 0.25 mM disulfide and 0.5 mM thiol solutions were prepared in hexane, with the sulfur concentration in solutions being kept constant for all the SAM formations. For the SPR measurement, the gold layers of about 48 nm in thickness were thermally deposited in a vacuum chamber (Edwards, ∼1 × 10-7 Torr, BIEMTRON, Co. Ltd., Ibaraki, Japan) on LaSFN9 glass slides (n ) 1.845 at λ ) 632.8 nm). Au(111)/mica substrates for the AFM imaging were prepared by epitaxial growth of 100-150 nm gold films onto freshly cleaved mica sheets in a vacuum chamber (BIEMTRON Co. Ltd., Ibaraki, Japan). Gold was thermally deposited on mica, which was prebaked at 550 °C for 3 h. The deposition was carried out at a rate of 1 Å/s with the substrate temperature at 430 °C under vacuum of 10-8 Torr. After the deposition, the substrates were annealed at 550 °C for ∼3 h. This procedure produced atomically flat Au(111) surfaces with single-crystal grains measuring 500-1000 nm in diameter. The Au(111)/mica substrates were removed from the vacuum chamber and immersed into freshly prepared solutions with the azobenzene derivatives. After 24 h immersion, the substrates were removed from the solutions, rinsed with absolute hexane, and dried in a stream of N2. The AFM imaging was performed within 1 day after the preparation of the SAMs. 2.3. Surface Plasmon Resonance Spectroscopy (SPR).23 The SPR setup, which is based on the configuration introduced by Kretschmann and Raether,24 and the experimental procedures are described in our previous report.25 The photoisomerization in the SAM was monitored in a liquid cell with the photoirradiation of UV and visible lights through the back glass window. An ultrahigh-pressure mercury lamp was used as the light source with color filters to obtain UV (364 nm, 2.44mW/cm2) and visible (440 nm, 2.70mW/cm2) light. The real-time photoresponse was observed by the “kinetics scan” mode, while the optical thickness of the trans and cis form SAMs (in the photostationary state) was determined from the shift of the SPR peak positions by the “angular-scan” measurement.18 The measurements were carried out in n-alkanes (the solvents with similar polarity but with different refractive index) and in air in order to investigate the influence of the contacting medium on the photoreaction. 2.4. AFM Imaging. The AFM images of the azobenzene derivative SAMs (C6AzSSC12, C6Az(Me)SSC12, C6AzSH, and C6Az(Me)SH) on Au(111)/mica were taken with a Nanoscope IIIa (Digital Instruments, Inc., Santa Barbara, CA) in the contact mode (30 µm scanner) in air at room temperature. A Si3N4 cantilever with a spring constant of 0.12 N/m was used with scan rates of 3-10 Hz. All images (400 × 400 pixels) were collected by the “height mode”. The applied force was minimized during the AFM imaging by adjusting the “set point voltage” to the lower limit. We could not find any visible damage of the surface by scanning at the condition utilized.11

3. Results and Discussion 3.1. Photoresponse Characterized by SPR. Figure 3 shows the real-time photoresponse of unmodified (23) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (24) (a) Kretschmann, E.; Raether, H. Z. Naturforsch. Teil 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. (b) Raether, H. Surface Plasmons on Smooth and Rough Surfaces and on Gratings. Springer Tracts in Modern Physics; Springer: Berlin, 1988; Vol. 111. (25) Tamada, K.; Ishida, T.; Knoll, W.; Fukushima, H.; Colorado, R., Jr.; Graupe, M.; Shmakova, O. E.; Lee, T. R. Langmuir 2001, 17, 1913.

Tamada et al.

Figure 3. Real-time photoresponse of the azobenzene SAMs on gold composed of (a)C6AzSH, (b)C6AzSSC12, (c)C6Az(Me)SH, and (d)C6Az(Me)SSC12.

azobenzene thiol and unsymmetrical disulfide (C6AzSH, C6AzSSC12) SAMs and that of “methyl-derivatized” (C6Az(Me)SH, C6Az(Me)SSC12) SAMs in hexane. The C6AzSSC12 exhibited more significant photoresponse compared with that for the corresponding thiol (C6AzSH) SAM as reported previously.18 The high photoreactivity in the C6AzSSC12 SAM can be interpreted as a consequence of the free volume around the dye functions provided by the short alkyl side chains (C12). The “methylderivatized” C6Az(Me)SSC12 SAM exhibited quite a similar photoresponse to that of a C6AzSSC12 SAM. It is understandable that the attachment of the methyl group did not result in a noticeable effect on the photoreaction for the case of the unsymmetrical disulfide, because they have already enough free volume for the photoreaction without methyl attachment. On the other hand, the “methyl-derivatized” C6Az(Me)SH SAM revealed a significant improvement in their photoreactivity compared with the C6AzSH SAM. It is remarkable that such a drastic change in their film property was produced by the attachment of one methyl group to the azobenzene ring. The photoresponse of these azobenzene SAMs in other solvents (pentane, heptane, and octane) are quite identical to that in hexane. These results are provided as supporting information. In Figure 4, we summarize the dielectric constant (;  ) n2, where n is the refractive index) of these azobenzene SAMs ((SAM)) in the trans and cis forms to indicate the “optical thickness” of each SAM layers.18 The (SAM) was determined from the peak shift in the SPR angular scan. For the calculation, the molecular length of the trans-C6AzSH SAM (d ) 3.6 nm) obtained in our previous study was utilized.11 The C6AzSH SAM exhibited the highest (SAM) in both the trans and cis forms compared with the other three SAMs due to its densely packed film structure. Reasonably, the (SAM)s of both disulfide SAMs (C6AzSSC12, C6Az(Me)SSC12) are almost identical. The C6Az(Me)SH SAM showed an (SAM) value intermediate between those of C6AzSH and the unsymmetrical disulfide (C6AzSSC12, C6Az(Me)SSC12) SAMs as expected from its molecular structure. The photoreactivity can be estimated by the difference of the (SAM) between the trans and cis isomers. As already suggested by the real-time photoresponse shown in Figure 3, the unsymmetrical disulfide (C6AzSSC12, C6Az(Me)SSC12) SAMs exhibited a much higher photoreactivity compared with that of the thiol (C6AzSH, C6Az(Me)SH) SAMs. The improvement of the photoreactivity by “methyl deriva-

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Figure 4. Dielectric constants of the azobenzene SAMs ((SAM)) determined from the SPR peak shift in various solvents. The filled symbols are (SAM) after 440-nm visible light irradiation (trans form), and the open symbols are E(SAM) after 364-nm UV light irradiation (cis form).

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Figure 6. First-order plots of the photoisomerization reaction of the azobenzene SAMs determined from the SPR data in Figure 3; C6Az(Me)SH (pluses), C6Az(Me)SSC12 (filled triangles), and C6AzSSC2 (open diamonds). (a) Cis to trans with visible light irradiation (440 nm, 2.7 mW/cm2); (b) trans to cis with visible light irradiation (364 nm, 2.44 mW/cm2).

in the azobenzene SAMs by use of a first-order plot.26 The detail of this analytical procedure is described in our previous report.18 Figure 6 is the ln(R) vs time plots of C6Az(Me)SH C6Az(Me)SSC12, and C6AzSSC12 SAMs in hexane (n-alkanes):

ln(R) ) ln[([t]0 - [t]∞)/([t] - [t]∞)] ) [[t]0/([t]0 [t]∞)]At ) [[t]0/[t]∞](B + K)t (1)

Figure 5. Dielectric constants of the azobenzene SAMs ((SAM)) in air and in n-alkanes to characterize the “swelling” effect. The filled symbols are (SAM) after 440-nm visible light irradiation (trans form), and the open symbols are (SAM) after 364-nm UV light irradiation (cis form).

tization” for the C6Az(Me)SH SAM is clearly demonstrated in the Figure 4 in comparison with that of the C6AzSH SAM. For all the cases, the trans form SAMs exhibited higher (SAM) than that for the cis form SAMs; i.e., the trans form SAMs are optically thicker than the cis form SAMs, in good agreement with our previous study.18 In Figure 5, we plotted the (SAM) values estimated in n-alkanes (the same data as shown in Figure 4) together with the data in air, to confirm the solvent effect.18 As shown in Figure 5, the optical thickness of these SAMs varied between in solvents and in air, as shown by the slope with the solid and dashed lines, where the SAMs with a small (SAM) exhibited a relatively large slope of the lines. In other words, the low density SAMs such as unsymmetrical disulfides were largely “solvated” in nalkanes, as expected from their film structures having “free volume”. In contrast with the unsymmetrical disulfide SAMs, the optical thickness of the thiol (C6AzSH, C6Az(Me)SH) SAMs were little influenced by the contacting solvents. From the slopes, we could assume the order of the film density as follows: (C6AzSH) > (C6Az(Me)SH) > (C6AzSSC12) > (C6Az(Me)SSC12). The mechanism of the enhancement of the photoreactivity by solvation was precisely discussed in our previous report.18 3.2. Reaction Kinetics: First-Order Plot. We characterized the kinetics of the photoisomerization reaction

where the ln (R) was determined by the change of reflectivity in the SPR measurement. For the case of the cis to trans photoisomerization with visible light irradiation at 440 nm, the reflectivities in the cis and trans isomers in the photostationary states were utilized as [t]0 and [t]∞, respectively.27 For the case of the trans to cis photoisomerization with UV light irradiation at 364 nm, eq 1 is also utilized by conversion of [t]0 to [t]∞ and [t]∞ to [t]0. As shown in Figure 6a, the cis to trans photoisomerization reaction proceeds following a “first-order kinetics” for all the SAMs, suggesting little steric effect in the reaction process. On the other hand, as shown in Figure 6b, the trans to cis photoisomerization reaction exhibits detectable deviations from the linear regression line as an evidence of steric effect during the photoreaction, which is typical of densely packed films.28,29 Interestingly, the deviation of “methyl attached” azobenzene SAMs (C6Az(Me)SH, C6Az(Me)SSC12) was not as clear as that of C6AzSSC12, reflecting the free volume around the dye functions. There is no influence of solvent on the reaction kinetics as long as n-alkanes are utilized. Figure 7 displays the reaction kinetics of the corresponding thiol or disulfide molecules dissolved in hexane (26) Mita, I.; Horie, K.; Hirao, K. Macromolecules 1989, 22, 558. (27) In this measurement, there is unavoidable errors in [t]0 and [t]∞ arising from the influence of cis azobenzene remaining in the SAM even after 440-nm light irradiation. However, since it is not possible to determine the real ratio of trans/cis azobenzenes by reflection UV absorption spectra (because the absorption peaks are too weak in the SAMs), we simply utilized the reflectivities in the photostationary states after 440- and 364-nm light irradiation as [t]0 and [t]∞, respectively. (28) Smets, G. Adv. Polym. Sci. 1983, 50, 17. (29) (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.

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Figure 7. First-order plots of the photoisomerization reaction of azobenzene derivatives in hexane solution (5 × 10-6 mol/L); C6AzSH (filled reversed triangles), C6AzSSC12 (filled squares), C6Az(Me)SH (open circles), C6Az(Me)SSC12 (open triangles). The [t]0 and [t]∞ values to calculate the Y-axis were determined by the UV-vis absorption spectra taken in a quartz liquid cell after several intermittent photoirradiations. (a) Cis to trans with visible light irradiation (440 nm, 1.0 mW/cm2); (b) trans to cis with UV light irradiation (364 nm, 1.0 mW/cm2).26

(5 × 10-6 mol/L). UV-visible absorption measurements were carried out on a UV-2500PC spectrometer (SHIMADZU) using a quartz cell (10 mm path), where photoisomerizations were stimulated upon irradiation with 364- and 440-nm light (1.0 mW/cm2) in the liquid cell at room temperature. The π-π* absorption band (λmax ∼ 353 nm) was used to examine the kinetics of the photoisomerization reaction. 26 As shown in Figure 7, the reaction rates of the thiols and the disulfides composed of the same azobenzene units were identical within the error bar; however, those with different azobenzene units were evidently different (with or without the “methyl derivatization”). The “methylderivatized” azobenzenes exhibited a 2-3 times faster initial rate of the reaction than that of the unmodified azobenzenes, suggesting a different quantum yield of the molecules. In addition, we characterized the molecular tilt angles of C6Az(Me)SSC12 and C6AzSSC12 SAMs on planar gold (trans form) with Fourier transform infrared reflection absorption spectroscopy (FTIR-RAS) and confirmed that the “methyl-derivatized” azobenzene SAMs exhibited a different tilt angle in the SAMs (the azobenzene group in the C6Az(Me)SSC12 SAM was more tilted compared with that for the C6AzSSC12 SAM).30 (30) Wei, T. X.; Akiyama, H.; Tamada, K., manuscript in preparation.

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Figure 8. Photoreactivity of unsymmetrical azobenzene disulfide (C6AzSSC12 and C6Az(Me)SSC12) SAMs after annealing. The SAMs were annealed at each temperature (70, 85, 100, 120, 140 °C) for 1 h, and the photoreactions were characterized by SPR alternately in hexane. Above, the realtime photoresponse of the SAMs; below, the estimated dielectric constants of the SAMs ((SAM)). After heat treatment, the photoreactivity (the difference in (SAM) between the trans and cis forms) reached to the level of corresponding single component azobenzenethiol SAMs (A, C6AzSH; B, C6Az(Me)SH).

Taking all these data into consideration, we extended detailed discussions of the initial rates of photoreaction only between the SAMs with the same azobenzene unit, i.e., between C6Az(Me)SH and C6Az(Me)SSC12 SAMs. For the case of the photoisomerization from cis to trans (Figure 6a), the reaction rate of the C6Az(Me)SH SAM was much faster than that of the unsymmetrical azobenzene SAMs (C6Az(Me)SSC12). In our previous study, we confirmed that the cis to trans isomerization was considerably faster in the unsymmetrical disulfide SAM with a longer alkyl side chain (C6AzSSC22) compared with that with a shorter alkyl side chain (C6AzSSC12).18 Both results can be reasonably interpreted by the influence of “surface stress”. The reaction inducing the reduction of the surface stress (e.g., the cis to trans isomerization) could be accelerated on the congested surface. On the other hand, for the case of the trans to cis photoisomerization (Figure 6b), the reaction rate of the C6Az(Me)SH SAM appeared to be slower than that of the C6Az(Me)SSC12 SAM. This result can be interpreted in analogy to the congested surface as well, i.e., as an example that the reaction was slowed when more stress was loaded by the reaction (e.g., the trans to cis isomerization).18 3.3. Photoreaction after Annealing. We characterized the change of the photoreactivity of unmodified and “methyl-derivatized” unsymmetrical azobenzene disulfide (C6AzSSC12, C6Az(Me)SSC12) SAMs before and after annealing.31 The SAMs were annealed in an oven (in air) for 1 h at each temperature (70, 85, 100, 120, 140 °C),

Reaction of Self-Assembled Monolayers

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Figure 9. AFM images of azobenzene SAMs before and after annealing (100 °C, 3 h): (a) C6AzSSC12, (b) C6Az(Me)SSC12, (c) C6AzSH, and (d) C6Az(Me)SH SAMs.

while the photoreaction was characterized alternately by the SPR measurement in hexane. As shown in Figure 8, the change of the photoreactivity was not so obvious for the C6AzSSC12 SAMs when the annealing temperature was less than 100 °C, a temperature at which the decomposition of the SAM (desorption of adsorbed thiolates) did not appear yet as shown by the constant film thickness of the trans form SAM in Figure 8. On the other hand, when the C6AzSSC12 SAM was annealing at T > 100 °C, the decomposition of the SAM could be observed (the film thickness of the trans form SAM became thinner), and the photoreactivity was drastically degraded. Finally the photoreactivity of the SAM reached the level of that for the corresponding thiol (C6AzSH) SAM, which is less than 20% of the original value, suggesting that phase segregation of the film by heat treatment occurred. It is quite reasonable to assume that the surface rearrangement leading to phase segregation is accelerated for the decomposed films, because the diffusion constant of adsorbed molecules must be directly correlated with the molecular density on the surface. The C6Az(Me)SSC12 SAM exhibited basically the same response as that of the C6AzSSC12 SAM upon heat treatment, though the decomposition of the SAM started at a slightly lower temperature. On the other hand, the photoreactivity of the C6Az(Me)SSC12 SAM was retained at about 50% of the original value, unlike in the C6AzSSC12 SAM. These results can be interpreted by the influence of methyl derivatization. The lower heat resistance of the C6Az(Me)SSC12 SAM against film decomposition can be interpreted by the weak interaction between the azobenzene groups due to the methyl de(31) (a) Delamarche, E.; Michel, B.; Kang H.; Gerber C. Langmuir 1994, 10, 4103. (b) Fukushima, H.; Seki, S.; Nishikawa, T.; Takiguchi, H.; Tamada, K.; Abe, K.; Colorado, R.; Graupe, M.; Shmakova, O. E.; Lee, T. R. J. Phys. Chem. B 2000, 104, 7417.

rivatization, while the persistence of the photoreactivity can be explained by the property of the single-component SAM (C6Az(Me)SH) resulting from the phase segregation after heat treatment (as shown in Figures 3 and 4, the C6Az(Me)SH SAM exhibits much higher photoreactivity compared with the C6AzSH SAM). Thus, the thermal endurance of the photoreactivity of unsymmetrical disulfide SAMs was largely improved by utilization of the modified azobenzene structures (methyl derivatization) in consideration with the steric effect. 3.4. AFM Images after Annealing. We performed AFM imaging of the azobenzene SAMs after annealing in order to confirm the formation of phase segregated domains by morphological method. Figure 9 shows AFM images of azobenzene SAMs (C6AzSSC12, C6Az(Me)SSC12, C6AzSH, and C6Az(Me)SH) before and after annealing at 100 °C for 3 h. The surfaces of all the SAMs looked structureless before annealing,18 while both of the disulfide (C6AzSSC12, C6Az(Me)SSC12) SAMs exhibited clear phase-segregated domains after annealing.21 The height difference between the two phases was ca. 3 Å for the both cases, suggesting that the single domains on the surface are composed of alkyl thiolates and azobenzene thiolates. In contrast to the disulfide SAMs, no morphological change could be observed on the thiol (C6AzSH, C6Az(Me)SH) SAMs upon annealing. The large defects indicating the decomposition of the thiolates (the pits with the depth of a molecular length) were not observed on all the SAMs. Conclusions The photoreaction of methyl-derivatized azobenzene (C6Az(Me)SH, C6Az(Me)SSC12) SAMs was studied by SPR in comparison with that of “unmodified” azobenzene (C6AzSH, C6AzSSC12) SAMs. The small modification of azobenzene dyes with methyl groups improved photore-

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activity of the thiol SAMs significantly by the steric control of the dye aggregation, which resulted in the thermal endurance for the photoreaction of disulfide systems; i.e., the C6Az(Me)SSC12 SAM could retain high photoreactivity even in the phase-segregated films formed by annealing, unlike the C6AzSSC12 SAM. The C6Az(Me)SH and C6Az(Me)SSC12 SAMs exhibited a reaction kinetics different from that of C6AzSSC12 SAM due to a different quantum yield and the molecular tilt angle of the azobenzene unit. The C6Az(Me)SH SAM exhibited a typical characteristic of a “congested surface”, where the reaction rate from cis to trans was faster, while that from trans to cis was slower compared with those of C6Az(Me)SSC12 SAM. This result is reasonably interpreted by “surface stress” induced by photoreactions. Acknowledgment. We thank Prof. W. Knoll of MaxPlanck-Institute fu¨r Polymer-forschung, Prof. M. Hara of

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Spatio-Temporal Function Materials Team, Frontier Research System, RIKEN, and Prof. Haiwon Lee of Hanyang University for their kind help. We also acknowledge Dr. K. Yase and Dr. T. Tamaki in AIST for their support in the project of “Harmonized Molecular Materials”. Tianxin Wei of AIST thanks Japan Science Technology Corporation (JST) for a STA fellowship, and SeungAe Kim of Hanyang University thanks “Winter Institute Program” in 2000, organized by KJCF/JKF. Supporting Information Available: Graphs of the photoresponse of C6Az(Me)SH and C6Az(Me)SSC12 in various solvents. This material is available free of charge via the Internet at http://pubs.acs.org. LA0258493]