Photoswitching Behavior of Azobenzene-Containing Alkanethiol Self

Jul 27, 2010 - (6, 7) Self-assembled monolayers of azobenzene-containing substances .... Typically, an immersion time of 30 min was employed; however,...
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Photoswitching Behavior of Azobenzene-Containing Alkanethiol Self-Assembled Monolayers on Au Surfaces Ulrich Jung,† Olena Filinova,† Sonja Kuhn,† Dordaneh Zargarani,‡ Claudia Bornholdt,‡ Rainer Herges,*,‡ and Olaf Magnussen*,† † Institut f€ ur Experimentelle und Angewandte Physik, Christian-Albrechts-Universit€ at zu Kiel, Leibnizstr. 19, 24118 Kiel, Germany, and ‡Otto-Diels-Institut f€ ur Organische Chemie, Christian-Albrechts-Universit€ at zu Kiel, Otto-Hahn-Platz 3, 24098 Kiel, Germany

Received April 20, 2010. Revised Manuscript Received July 3, 2010 The photoisomerization of self-assembled monolayers of azobenzene-containing alkanethiols, as well as of mixed monolayers of these substances with n-alkanethiol spacer molecules on Au surfaces, was studied by photoelectrochemical measurements and surface plasmon resonance spectroscopy. A strong dependence on the molecular structure of the adsorbates was found, specifically on the linker between the azobenzene moiety and the alkanethiol: while molecules with an amide group were photoinactive, those with an ether group exhibited pronounced, reversible photoisomerization in pure and mixed adlayers. Both trans-cis and cis-trans isomerization followed first-order kinetics with time constants that suggest high quantum efficiencies for these processes.

Introduction Self-assembled monolayers (SAMs) of photo- and redoxswitchable organic compounds are of interest for a wide variety of applications and accordingly have received considerable attention.1-5 One of the most frequently studied class of switchable molecules are the derivatives of azobenzene, which exhibit cis-trans isomerism. Photoisomerization from the thermodynamically more stable trans isomer to the cis isomer and vice versa can be selectively induced by irradiation with either UV light of 365 nm or blue light of 435 nm.6,7 Self-assembled monolayers of azobenzene-containing substances on gold surfaces are commonly prepared by employing alkanethiols to which the azobenzene moiety is attached via an amide or ether linker. They have *E-mail: [email protected]; [email protected]. (1) Finklea, H. O. Self-assembled Monolayers on Electrodes. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John Wiley & Sons Ltd. : Chichester; pp 1-26. (2) Ulman, A. Chem. Rev. 1996, 96, 1533–1554. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–257. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103–1169. (5) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421–432. (6) Hartley, G. S. J. Chem. Soc. 1938, 633–642. (7) Hamelmann, F.; Heinzmann, U.; Siemeling, U.; Bretthauer, F.; Vor der Br€uggen, J. Appl. Surf. Sci. 2004, 222, 1–5. (8) Yu, H.-Z.; Wang, Y.-Q.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. J. Electroanal. Chem. 1995, 395, 327–330. (9) Wang, Y.-Q.; Yu, H.-Z.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Liu, Z.-F. Langmuir 1996, 12, 5466–5471. (10) Yu, H.-Z.; Wang, Y.-Q.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 2843–2848. (11) Lahav, M.; Ranjit, K. T.; Katz, E.; Willner, I. Chem. Commun. 1997, 259–260. (12) Yu, H.-Z.; Wang, Y.-Q.; Cai, S.-M.; Liu, Z.-F. Ber. Bunsen-Ges. Phys. Chem. 1997, 101, 257–264. (13) Zhu, T.; Yu, H. Z.; Cai, S. M.; Liu, Z. F. Mol. Cryst. Liq. Cryst. 1997, 294, 79–82. (14) Zhang, J.; Zhao, J.; Zhang, H. L.; Li, H. L.; Liu, Z. F. Chem. Phys. Lett. 1997, 271, 90–94. (15) Yu, H.-Z.; Zhang, J.; Zhang, H.-L.; Liu, Z.-F. Langmuir 1999, 15, 16–19. (16) Yu, H.-Z.; Zhang, H.-L.; Liu, Z.-F.; Ye, S.; Uosaki, K. Langmuir 1998, 14, 619–624. (17) Zhang, J.; Zhao, J.; Zhang, H. L.; Li, H. L.; Liu, Z. F. Thin Solid Films 1998, 327-329, 195–198. (18) Shao, H. B.; Li, D. Y.; Tu, J. S. Chin. Chem. Lett. 1999, 10, 145–146. (19) Wu, Z.; Dong, D.; Zhang, H.; He, H.; Liu, Z. Mol. Crys. Liq. Crys. 1999, 337, 305–308.

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been extensively studied by structure-sensitive and spectroscopic methods,8-54 revealing the formation of close-packed adlayers. However, the photoisomerization in these SAMs is usually (20) Yu, H.-Z.; Ye, S.; Zhang, H.-L.; Uosaki, K.; Liu, Z.-F. Langmuir 2000, 16, 6948–6954. (21) Yasuda, S.; Nakamura, T.; Matsumoto, M.; Shigekawa, H. J. Am. Chem. Soc. 2003, 125, 16430–16433. (22) Das, B.; Abe, S. J. Phys. Chem. B 2006, 110, 4247–4255. (23) Jung, U.; Baisch, B.; Kaminski, K; Krug, K.; Elsen, A.; Weineisen, T.; Raffa, D.; Stettner, J.; Bornholdt, C.; Herges, R.; Magnussen, O. J. Electroanal. Chem. 2008, 619-620, 152–158. (24) Jung, U.; M€uller, M.; Fujimoto, N.; Ikeda, K.; Uosaki, K.; Cornelissen, U.; Tuczek, F.; Bornholdt, C.; Zargarani, D.; Herges, R.; Magnussen, O. J. Colloid Interface Sci. 2010, 341, 366–375. (25) 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, 6071–6082. (26) Wolf, H.; Ringsdorf, H.; Delamarche, E.; Takami, T.; Kang, H.; Michel, B.; Gerber, C.; Jaschke, M.; Butt, H.-J.; Bamberg, E. J. Phys. Chem. 1995, 99, 7102–7107. (27) Jaschke, M.; Sch€onherr, H.; Wolf, H.; Butt, H.-J.; Bamberg, E.; Besocke, M. K.; Ringsdorf, H. J. Phys. Chem. 1996, 100, 2290–2301. (28) Zhu, T.; Yu, H. Z.; Wang, J.; Wang, Y. Q.; Cai, S. M.; Liu, Z. F. Chem. Phys. Lett. 1997, 265, 334–340. (29) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213–219. (30) Wang, R.; Iyoda, T.; Tryk, D. A.; Hasimoto, K.; Fujishima, A. Langmuir 1997, 13, 4644–4651. (31) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436–6440. (32) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Hara, M.; Knoll, W.; Ishida, T.; Fukushima, H.; Miyashita, S.; Usui, T.; Koini, T.; Lee, T. R. Thin Solid Films 1998, 327-329, 150–155. (33) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264–3271. (34) Zhang, H.-L.; Zhang, J.; Li, H.-Y.; Liu, Z.-F.; Li, H.-L. Mater. Sci. Eng., C 1999, 8-9, 179–185. (35) Li, H.; Lei, L.; Zhang, H.; Zhang, J.; Liu, Z. Spec. Lett. 2001, 34, 133–146. (36) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 2323–2331. (37) Mannsfeld, S. C. B.; Canzler, T. W.; Fritz, T.; Proehl, H.; Leo, K.; Stumpf, S.; Goretzki, G.; Gloe, K. J. Phys. Chem. B 2002, 106, 2255–2260. (38) Zhang, W.-W.; Li, H.-F.; Liu, L.; Xie, J.-L.; Lu, C.-S.; Zhou, Y.; Ren, X.-M.; Meng, Q.-J. J. Colloid Interface Sci. 2003, 261, 82–87. (39) Zhang, W.-W.; Ren, X.-M.; Li, H.-F.; Lu, C.-S.; Hu, C.-J.; Zhu, H.-Z.; Meng, Q.-J. J. Colloid Interface Sci. 2002, 255, 150–157. (40) Weber, R.; Winter, B.; Hertel, I. V.; Stiller, B.; Schrader, S.; Brehmer, L.; Koch, N. J. Phys. Chem. B 2003, 107, 7768–7775. (41) Zhang, W.-W.; Ren, X.-M.; Li, H.-F.; Xie, J.-L.; Lu, C.-S.; Zou, Y.; Ni, Z.-P.; Meng, Q.-J. J. Colloid Interface Sci. 2003, 268, 173–180. (42) Ito, M.; Wei, T. X.; Chen, P.-L.; Akiyama, H.; Matsumoto, M.; Tamada, K.; Yamamoto, Y. J. Mater. Chem. 2004, 15, 478–483.

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suppressed, which is commonly attributed to steric hindrance29,31,55 and electronic effects.36,43 Only in a few experiments photoinduced changes of pure azobenzene-derivatized alkanethiol SAMs on Au have been reported so far.11,30,31,36,42,51 An increase in the free volume of the azobenzene moieties within the SAMs generally improves the photoisomerization behavior, as was first demonstrated qualitatively in spectroscopic and photoelectrochemical studies employing mixed monolayers where the azobenzene derivatives were diluted by photoinactive shortchain spacer molecules21,31,56 or molecules with incorporated bulky groups.42,51,57-59 Detailed photoisomerization studies of such SAMs were performed by Tamada et al. using surface plasmon resonance (SPR) spectroscopy and focused on molecules with additional side groups to increase the free volume of the azobenzene moieties, such as unsymmetric disulfides with an azobenzenecontaining chain and an alkane spacer chain, methyl-derivatized azobenzenes, or azobenzene compounds with bulky carborane units in the anchor group.42,48,51 Indeed, introduction of these spacer groups was found to significantly improve the photoresponse, indicating reduced steric effects in these less densely packed SAMs. Quantitative studies revealed first-order kinetics for the cis-trans photoisomerization, whereas the trans-cis photoisomerization exhibited clear deviations, suggesting an additional slower process for azobenzene moieties incorporated in the SAMs. Furthermore, the effect of the solvent on the photoresponse of these adlayers was systematically investigated in these experiments. Quantitative photoreactivity studies of pure azobenzene-containing alkanethiol SAMs on colloidal Au clusters by UV/vis spectroscopy were reported by Zhang et al.36 They found photoisomerization quantum efficiencies for the SAMs that were much smaller than those of the dissolved substances and decreased exponentially with increasing length of the alkane chain. This behavior was attributed to excited-state quenching by the metallic core. However, due to the small diameter of the Au clusters used as substrate (∼2.5 nm) the packing density and consequently the photoisomerization behavior may substantially deviate from that of SAMs on planar substrates. The latter was explicitly demonstrated in studies of Au colloids capped with unsymmetric azobenzene disulfides.50 Similar studies of mixed monolayers prepared by coadsorption of azobenzene-containing alkanethiols and photoinactive (43) Micheletto, R.; Yokokawa, M.; Schroeder, M.; Hobara, D.; Ding, Y.; Kakiuchi, T. Appl. Surf. Sci. 2004, 228, 265–270. (44) Schmidt, R.; McNellis, E.; Freyer, W.; Brete, D.; GieSSel, T.; Gahl, C.; Reuter, K.; Weinelt, M. Appl. Phys. A: Mater. Sci. Proc. 2008, 267–275. (45) Weiss, P. S. Acc. Chem. Res. 2008, 41, 1772–1781. (46) Han, S. W.; Kim, C. H.; Hong, S. H.; Chung, Y. K.; Kim, K. Langmuir 1999, 15, 1579–1583. (47) Gahl, C.; Schmidt, R.; Brete, D.; McNellis, E. R.; Freyer, W.; Carley, R.; Reuter, K.; Weinelt, M. J. Am. Chem. Soc. 2009, 132, 1831–1838. (48) Tamada, K.; Akiyama, H.; Wei, T. X. Langmuir 2002, 18, 5239–5246. (49) Akiyama, H.; Tamada, K.; Nagasawa, J.’i.; Abe, K.; Tamaki, T. J. Phys. Chem. B 2003, 107, 130–135. (50) Manna, A.; Chen, P.-L.; Akiyama, H.; Wei, T.-X.; Tamada, K.; Knoll, W. Chem. Mater. 2003, 15, 20–28. (51) Tamada, K.; Akiyama, H.; Wei, T.-X.; Kim, S.-A. Langmuir 2003, 19, 2306–2312. (52) Onoue, M.; Han, M. R.; Ito, E.; Hara, M. Surf. Sci. 2006, 600, 3999–4003. (53) Nagahiro, T.; Akiyama, H.; Hara, M.; Tamada, K. J. Electron Spectrosc. Relat. Phenom. 2009, 172, 128–133. (54) Suda, M.; Kameyama, N.; Ikegami, A.; Einaga, Y. J. Am. Chem. Soc. 2009, 131, 865–870. (55) Willner, I.; Pardo-Yissar, V.; Katz, E.; Ranjit, K. T. J. Electroanal. Chem. 2001, 497, 172–177. (56) Jeoung, E.; Rotello, V. M. J. Supramol. Chem. 2002, 2, 53–55. (57) Pace, G.; Ferri, V.; Grave, C.; Elbing, M.; von H€anisch, C.; Zharnikov, M.; Mayor, M.; Rampi, M. A.; Samori, P. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9937–9942. (58) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samori, P.; Mayor, M.; Rampi, M. A. Angew. Chem. 2008, 120, 1–4. (59) Kondo, T.; Kanai, T.; Uosaki, K. Langmuir 2001, 17, 6317–6324.

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molecules (typically alkanethiols), a particular simple method to increase the free surface area of functional groups, are largely missing. Apart from the early qualitative study by Evans et al.,31 such systems were only employed in STM investigations of local photo- or tip-induced isomerization by the groups of Shigekawa and Weiss.21,60 However, in these experiments the concentration of the azobenzene derivatives in the SAMs was extremely low. Up to now, these investigations are only performed on structurally well-defined single-crystalline substrates, whereas in all other photoisomerization studies of azobenzene-containing SAMs, thin Au films or Au colloids were employed. Furthermore, apart from the studies mentioned above, the influence of the molecular structure on the photoisomerization behavior of the SAMs has received little attention up to now. In particular, the chemical structure of the ligands attached to the azobenzene moiety has not been addressed. This paper is part of a systematic study of azobenzene-containing molecular adlayers on Au(111) surfaces by structure-sensitive, spectroscopic, and electrochemical methods.23,24 Specifically, here we present photoelectrochemical and surface plasmon resonance (SPR) spectroscopy investigations of the photoinduced trans-cis isomerization in different SAMs of azobenzene-containing alkanethiols (Figure 1). The role of the linker between the azobenzene and the alkanethiol unit (amide or ether), as well as of coadsorbed n-alkanethiol spacer molecules, employed for reducing the azobenzene density within the SAMs, will be discussed. Furthermore, quantitative data on the photoisomerization kinetics will be presented.

Experimental Section Sample Preparation. The studied azobenzene-containing alkanethiols are shown in Figure 1. The synthesis and quality of these compounds was already discussed in our previous paper.24 A purity of all substances better than 99% was confirmed by NMR and mass spectroscopy. The (photo)electrochemical experiments were performed using Au(111) single crystals (MaTecK GmbH, J€ ulich, Germany) with diameters of 9 mm, which were oriented within (0.3°. For the SPR measurements, glass slides with 45-nm-thick Au films (XanTec bioanalytics GmbH, D€ usseldorf, Germany) were used. The gold crystals were cleaned by immersion in ”piranha” solution and subsequent extensive rinsing with 18.2 MΩ cm water (Elga LabWater), followed by annealing in a butane gas flame for 2-5 min. The self-assembled monolayers were prepared by immersion of the clean Au substrates into 1 mM solutions of the azobenzene-containing alkanethiols in dichloromethane (p.a., Merck KGaA, Darmstadt, Germany). Typically, an immersion time of 30 min was employed; however, in addition, SAMs with longer immersion times up to several hours were prepared, exhibiting identical reactivity. Mixed monolayers were produced in freshly prepared solutions of the azobenzene-containing alkanethiols with the following alkanethiol spacer molecules: n-propanethiol (Merck), n-butanethiol (Merck), n-hexanethiol (Acros Organics), n-octanethiol (Merck), n-dodecanethiol (Merck), or N-acetylcysteamine (Sigma-Aldrich). Mixing ratios of 0.96:0.04, 0.9:0.1, 0.8:0.2, 0.5:0.5, 0.2:0.8, 0.1:0.9, 0.02:0.98, or 0.01:0.99 mM were used. After self-assembly, the samples were extensively rinsed with dichloromethane and dried in air. On the basis of the data obtained in this work, as well as in our previous gapmode SERS study,24 highly reproducible SAMs of the pure and mixed compounds can be found, which are stable for at least several hours under the experimental conditions (e.g., in the SPR measurements, no differences in the photoswitching behavior were observed after a waiting time of more than 10 h). (60) Kumar, A. S.; Ye, T.; Takami, T.; Yu, B.-C.; Flatt, A.-K.; Tour, J. M.; Weiss, P. S. Nano Lett. 2008, 8, 1644–1648.

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Figure 2. Sketch of the photoelectrochemical cell. Kretschmann and Raether.64 On top of the sample, a flow cell with two reaction channels was installed, which were filled with ethanol (p.a., Merck). During the photoirradiation experiments, the sample in one of the channels was irradiated via a quartz glass window, whereas the sample in the other channel was kept in the dark. The SPR signal was monitored at a rate of 0.5 Hz as the difference in the index of refraction Δn near the Au surface between the two channels. STM measurements were performed in air using a PicoPlus SPM (Agilent, Inc., Santa Clara, USA) and mechanically cut Pt/Ir tips. Images were obtained at tunneling currents of 50 to 70 nA and a bias voltage of 700 mV.

Figure 1. Chemical structures of the azobenzene-containing alkanethiols used for preparation of the SAMs on Au (1a, N-(2-mercaptoethyl)-4-phenylazo-benzamide; 1b, 4-(4-iodo-phenylazo)-N-(2-mercapto-ethyl)-benzamide; 1c, 4-(4-hexyl-phenylazo)-N-(2-mercaptoethyl)-benzamide; 2a, 3-(4-(4-phenylazo)-phenoxy)-propane-1-thiol; 2b, 3-(4-(4-hexyl-phenylazo)-phenoxy)-propane-1-thiol; 2c, 6-(4-(4phenylazo)-phenoxy)-hexane-1-thiol; 2d, 6-(4-(4-hexyl-phenylazo)phenoxy)-hexane-1-thiol).

Instrumentation. For irradiation of the samples, two lightemitting diodes were employed: (i) a Shark OTLH-040-UV-LED (365 ( 5 nm, Laser Components GmbH, Olching, Germany) and (ii) an LED435-66-60 (435 ( 5 nm, Roithner Lasertechnik GmbH, Vienna, Austria). The (photo)electrochemical measurements were performed in a homemade photoelectrochemical cell (Figure 2) using an Autolab PGSTAT12 potentiostat (Eco Chemie, Utrecht, The Netherlands). The SAM-modified single-crystalline Au(111) working electrode was mounted in hanging-meniscus geometry. Counter electrode was a platinum wire and reference electrode a saturated calomel electrode (SCE), which was mounted in a separate compartment and connected to the photoelectrochemical cell via a salt bridge. As electrolyte, 0.1 M NaClO4 (suprapure, Sigma-Aldrich Chemie GmbH, Taufkirchen, Germany) with Britton-Robinson buffer at pH = 523 was used, which was deaerated with Ar (5.0 purity, Messer Griesheim). Photoelectrochemical experiments were performed by irradiating the working electrode from below through a 5-mm-thick quartz glass window and about 10 mm of electrolyte. The experiments were conducted at a scan rate of 20 mV/s. For each experiment, several independently prepared samples were employed. The microscopic surface area of the gold electrode was evaluated by integration of the charge of the gold oxide reduction.61-63 The roughness factor determined by this method was approximately 1.2 for freshly polished crystals. The SPR experiments were performed using a Reichert SR7000DC dual channel SPR instrument (Reichert Analytical Instruments, NY, USA). The setup is based on the configuration introduced by (61) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1–11. (62) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. Electrochimicia Acta 1986, 31, 1051–1061. (63) Angerstein-Kozlowska, H.; Conway, B. E.; Hamelin, A.; Stoicoviciu, L. J. Electroanal. Chem. 1987, 228, 429–453. (64) Kretschmann, E.; Reather, H. Z. Naturforsch., A: Phys. Sci. 1968, 23, 2135– 2136.

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Results Scanning Tunneling Microscopy. The structure of Au(111) surfaces covered with adlayers of azobenzene-containing alkanethiols was characterized by STM. Examples of the images obtained for pure SAMs of these molecules (compounds 1a and 2b) as well as mixed SAMs of 2b with n-butanethiol are displayed in Figure 3. All images show molecular structures with rather small domains. In between these domains, many monoatomically deep pits in the gold surface layer are observed (∼15% of the surface), which are similar to those for bare n-alkanethiol SAMs on gold and commonly attributed to a thiol-induced etching of the Au substrate.65 The results on pure 1a SAMs (Figure 3a) already have been reported in our previous publication23 and are reproduced here only for comparison. This compound forms small domains of rowlike structures with lattice constants of a = (10.5 ( 0.3) A˚ and b = (5.1 ( 0.2) A˚ at an angle of γ = (76.7 ( 1.3)°. The unit cell contains two molecules, resulting in a coverage of 2/7 ML (i.e., an area of 25.2 A˚2 per molecule). The row-like structures are most probably due to hydrogen bonds between the amide groups of adjacent molecules, since for azobenzene-containing alkanethiols with an ether linker distinctively different oblique structures are found (see below). The existence of hydrogen bonds in SAMs of azobenzenecontaining alkanethiols with an amide linker has been reported in several previous spectroscopy publications.14,16-18,20 The STM images obtained for pure 2b SAMs (Figure 3b,c) are in good agreement with results for similar azobenzene-containing alkanethiols with ether linkers.17,26,27,29,37,66 The domains in the molecular adlayer exhibit an oblique unit cell with lattice constants of a = (5.1 ( 0.3) A˚ and b = (6.0 ( 0.6) A˚. The angle in between the main lattice directions is γ = (83.6 ( 6.6)°, and the coverage corresponds to an area per molecule of 30.4 A˚2. Mixed SAMs of 2b and n-butanethiol (0.8:0.2 mM) (Figure 3d-f) also exhibit comparable small domains with a large number of etch pits in between. However, instead of the oblique structure observed for pure 2b SAMs, a stripe-like structure with a stripe separation of (9.8 ( 1.1) A˚ is identified (65) Sch€onenberger, C.; Sondag-Houethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611–614. (66) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 1005–1006.

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Figure 3. STM images of azobenzene-containing alkanethiol SAMs on Au(111). (a) Pure 1a SAM (15  15 nm2), from ref 23. (b) Large-scale

(40  40 nm2) and (c) high-resolution (15  15 nm2) images of a pure 2b SAM. STM images of a mixed 2b/n-butanethiol SAM (0.8:0.2 mM) showing (d) large scale topography (40  40 nm2), (e) high-resolution molecular structure (30  30 nm2), and (f) the presence of protusions inside the domains (30  30 nm2).

in high-resolution images (Figure √ 3e). This structure is in very good agreement with the (p  3), 8 e p e 10, superstructure previously found for n-butanethiol on Au(111),67 although the adlayer order is significantly lower in the mixed SAMs. In addition, many protusions, which are homogeneously distributed on the surface (∼4  1013 cm-2), can be identified in the STM images (Figure 3f). We attribute these protusions to individual 2b molecules coadsorbed within the butanethiol SAM. This interpretation is supported by the good agreement of the surface density obtained from the STM images and that derived from the electrochemical measurements (under assumption of a two-electron transfer, as is commonly done for azobenzene; see below). It is noted that it is difficult to clearly image in parallel the stripe-like structure associated with the butanethiol SAM and the protrusions assigned to the azobenzene derivative, which may be due to the rather different length of these two species. Overall, these STM results indicate that in the mixed SAMs the 2b molecules are randomly adsorbed within a surrounding matrix of the butanethiol molecules. Although phase separation cannot be completely excluded, no clear domains of the characteristic oblique structure as observed for pure 2b SAMs could be identified in the mixed monolayers, indicating that this is not an important effect for these SAMs. Furthermore, the concentration ratio of 2b to n-butanethiol on the substrate is significantly decreased as compared to that in solution. This has been observed in several previous studies2,4 and may be attributed to the larger diffusion coefficient of the considerably smaller n-butanethiol molecules in solution, leading to a much faster adsorption.4 Photoelectrochemical Measurements. We first briefly discuss the electrochemical properties of the azobenzene-containing SAMs as prerequisite for the photoelectrochemical studies of trans-cis isomerization, starting with molecules exhibiting an amide linker between the azobenzene moiety and the alkanethiol (substances 1a-1c; Figure 1). All SAMs of these compounds on single-crystalline Au(111) show a similar electrochemical behavior, exemplarily illustrated by cyclic voltammograms (CVs) of a 1a SAM23 (Figure 4a, solid line). A pair of redox peaks is observed in the double layer region, which can be clearly attributed to the (67) Poirier, G. E.; Tarlov, M. J.; Rushneier, H. E. Langmuir 1994, 10, 3383.

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reductive protonation or oxidative deprotonation of the azobenzene moieties, respectively.68 The charge density of these peaks (∼60 μC/cm2) is similar for pure 1a, 1b, and 1c SAMs, whereas the peak separation and the half-width of the redox peaks increase in this order (see Supporting Information). The former indicates similar coverages of all three substances in the range of 2/7 ML,23 the latter a lower reversibility of the redox reactions with increasing length of the headgroup due to pronounced steric hindrances of the proton transfer to the azobenzene moieties in the SAMs.9 To clarify the influence of the free volume of the azobenzene moieties on the SAMs properties, mixed monolayers of these compounds with thiol spacers (N-acetylcysteamine, n-butanethiol, n-hexanethiol, n-octanethiol, or n-dodecanethiol) were studied. As illustrated for a mixed SAM of 1a and N-acetylcysteamine, a similar pair of redox peaks is observed in the CVs (Figure 4a, dashed line). The facile control of the azobenzene surface concentration by this preparation method is demonstrated by the approximately linear change of the charge density of the redox peak with the mixing ratio in the preparation solution (Figure 4b). Furthermore, the observation that the electrochemical reactivity of azobenzene is independent of the surface concentration (i.e., since no pronounced steric effects were observed at higher coverages) indicates that the azobenzene-containing alkanethiols are distributed within a matrix of the alkane spacer molecules rather than to phase separate into larger domains, which is in agreement with the results of the STM measurements. For mixed SAMs with short-chain spacer molecules (in particular, N-acetylcysteamine), the peak separation typically is reduced, whereas for alkanethiol spacers with a chain length of g8, it is equal to or even larger than for pure 1a SAMs (Figure 4c). This is in good agreement with the redox behavior expected for close-packed mixed monolayers. Specifically, the larger reversibility of the redox reactions in mixed SAMs with short-chain spacers reflects an improved electrochemical accessibility of the azobenzene moieties and suggests a homogeneous distribution of the azobenzene moieties in these SAMs.21 Similar electrochemical studies of SAMs of 2a-2d, which exhibit an ether linker between the azobenzene moiety and the alkanethiol (68) Laviron, E.; Mugnier, Y. J. Electroanal. Chem. 1980, 111, 337–344.

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Figure 4. (a) Cyclic voltammograms of a pure 1a SAM and a mixed monolayer of 1a and N-acetylcysteamine (0.8:0.2 mM). (b) Oxidation peak charge density of mixed monolayers of 1a and N-acetylcysteamine as a function of the relative concentrations of these molecules in the solution used for SAM preparation. (c) Potentials of the redox peaks for pure 1a SAMs and mixed monolayers of 1a and different spacer molecules (0.2:0.8 mM, AC: N-acetylcysteamine).

(see Figure 1), reveal a slightly different behavior than that of the compounds with an amide group (substances 1a-1c). As illustrated in Figure 5a for a mixed SAM of 2b and n-butanethiol, a reduction peak with similar peak potential and charge density is observed also for these substances. However, the corresponding oxidation peak exhibits a much smaller charge density, and with successive potential cycling, the peaks irreversibly vanish until only the recharging of the electrochemical double layer capacity is observed. This deterioration of the SAMs can be largely avoided by restricting the negative potential limit of the voltammogram to the onset of the reduction peak (Figure 5b). In this case, the CV is highly reversible. A similar behavior was also found for the other ether-containing azobenzene derivatives. For this reason, most of the photoisomerization studies of the 2a-2d SAMs (see below) were performed in Langmuir 2010, 26(17), 13913–13923

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this limited potential range positive of the reduction peak. In addition, no oxidation peak was found in this potential range for pure 2b-2d SAMs, in agreement with a previous study of 2b SAMs on polycrystalline Au.14 In another experiment,38 for 2a and 2c SAMs redox reactions were observed, which exhibited considerably larger peak separations than identified here. Typical photoelectrochemical experiments are shown in Figure 6. The following procedure was employed in these measurements: In every experiment, a sequence of CVs, each consisting of several potential cycles, was recorded. In between this potential cycling (for typically 5 min) a rest potential positive of the redox peaks was applied, at which the sample was either kept in the dark, irradiated with UV light (365 nm) to induce trans-cis isomerization, or irradiated with blue light (435 nm) to induce cis-trans isomerization. The UV or blue light irradiation was also continued during the subsequent potential cycling. Initially, the azobenzene moieties in the SAMs are expected to be in an alltrans state, since the trans isomer is thermodynamical more stable than the cis isomer (see above). This is also in good agreement with the results of our previous gap-mode SERS study.24 Photoelectrochemical studies of the 2a-2d SAMs with an ether group show clear photoinduced effects. As directly visible in the according CVs (Figure 6a,c), illumination with UV light results in a positive shoulder of the reduction peak as compared to nonirradiated samples (see, e.g., cycles 1,2 and cycles 4,5 in Figure 6c, respectively), which can be attributed to the trans-cis isomerization of the azobenzene moieties in the SAMs and a different redox behavior of the cis isomer (see discussion below).29 The shift toward more positive potentials of the azobenzene reduction results in an increase of the charge density of the cathodic peak as well as of the charge density of the corresponding oxidation peak in the subsequent positive potential sweep. The latter is shown in Figure 6b,d for all potential cycles and clearly demonstrates reversible and reproducible photoinduced effects. Specifically, for a 2a SAM an increase of the charge density of 3.8 ( 0.5 μC/cm2 (i.e., by a factor of 0.9) is observed upon irradiation with UV light. In contrast, the charge density after irradiation with blue light is almost identical to that of the initial nonirradiated sample. As mentioned above, pure 2b, 2c, and 2d SAMs were not redox-active and consequently could not be studied by such photoelectrochemical experiments. However, for mixed monolayers of all compounds with an ether group (substances 2a-2d) and short-chain alkanethiol spacers (e.g., n-butanethiol), similar photoinduced changes of the electrochemical behavior were found as for pure 2a SAMs. This is illustrated in Figure 6c,d for a mixed monolayer of 2b and n-butanethiol. Here, even larger effects than for the pure 2a SAMs were identified, leading to an increase of the charge density of the oxidation peak by 3.2 ( 0.1 μC/cm2 (i.e., a factor of 3.7). Considering that the total charge of the peak is larger (18.2 μC/cm2 according to the CV in Figure 5a, first negative potential sweep), it is obvious that a substantial fraction of 2b molecules in the film have to participate in the photoisomerization reaction. Furthermore, a small increase in charge density is also observed upon irradiation with blue light, suggesting that also at 435 nm a fraction of the molecules is still isomerized from trans to cis state, albeit with a much lower cis state occupancy in photostationary equilibrium (a more quantitative estimation is difficult due to the large noise in these measurements). Indeed, UV/vis spectroscopy measurements of these compounds in solution indicate an incomplete cis-trans back-isomerization upon irradiation at 435 nm (see Supporting Information). Interestingly, the enhanced charge density is not only observed in the first potential cycle recorded after 5 min UV irradiation at the rest potential, but also in the potential cycles obtained DOI: 10.1021/la1015109

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Figure 5. Cyclic voltammograms of mixed monolayers of 2b and n-butanethiol (0.8:0.2 mM) showing (a) irreversible electrochemical reactivity upon cycling over the entire potential range of azobenzene reduction and (b) quasireversible behavior upon cycling only partly into the reduction regime.

immediately subsequently, which were recorded during continuous UV illumination (see Figure 6b,d). Since the electrochemical oxidation of hydrazobenzene results exclusively in trans azobenzene,68 this observation indicates a complete UV-induced restoration of the cis azobenzene equilibrium concentration during the ∼30 s between the end of the oxidation peak in the positive sweep and the onset of azobenzene reduction in the negative sweep, i.e., a rapid photoisomerization under the employed illumination conditions. In an analogous way, the slightly higher charge density of the first potential cycle recorded in the dark after UV irradiation (see, e.g., cycle 7 in Figure 6c,d) can be attributed to a trans-cis isomerization during the last ∼10 s of the preceding positive sweep (e.g., the potential range positive of the oxidation peak in cycle 6 of Figure 6c,d), which were still performed under UV light irradiation. To study the photoisomerization kinetics more quantitatively, a series of photoelectrochemical measurements with different UV irradiation times was performed. First, the sample was irradiated with UV light for different periods at the rest potential to (partly) convert the azobenzene moieties from trans to cis configuration. After this, the UV irradiation was terminated, and immediately (to minimize cis-trans back-isomerization by thermal relaxation), a cyclic voltammogram consisting of two potential cycles was recorded in the dark. The charge density of the oxidation peak in the first cycle then corresponds to a SAM, in which a fraction of the azobenzene moieties has been photoisomerized into the cis state, the second cycle to a SAM with the azobenzene moieties in an all trans state. The difference in the charge density of the oxidation peak between the two cycles Δσ(t) was normalized by the saturation value 13918 DOI: 10.1021/la1015109

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Δσ(600 s) and is plotted in Figure 7a as a function of irradiation time t. Obviously, the photostationary equilibrium is reached after less than one minute. Assuming first-order kinetics as indicated by the SPR measurements (see below), a time constant of ∼12 s is obtained from these data for pure 2a, as well as mixed 2b/n-butanethiol SAMs (see Table 1). In a similar way, the kinetics of the thermal cis-trans backisomerization was investigated (Figure 7b). Here, the sample was first brought into photostationary equilibrium by UV irradiation (for typically 5 min), then the UV irradiation was terminated and the sample was kept in the dark for different periods up to 10 min at the rest potential prior to recording the two potential cycles. The difference in charge density (in this case, normalized by Δσ(0 s), i.e., that obtained without additional waiting time) shows within the experimental errors no detectable variation with time. This indicates a time constant of the cis isomer that is considerably larger than 600 s, which is in agreement with the cis-trans relaxation behavior found for these molecules in solution. Analogous photoelectrochemical studies were performed for SAMs of the azobenzene-containing alkanethiols with an amide linker (substances 1a-1c). In contrast to SAMs of the compounds with an ether linker, only very weak photoinduced effects were found, even for mixed SAMs where the azobenzene surface density is reduced. As an example, photoirradiation experiments for a mixed SAM of 1a and N-acetylcysteamine are shown in Figure 8, where the CVs extended either over the entire potential range of the redox peaks (Figure 8a,b) or, as in the experiments on the ether-containing molecules, only partially into the reduction (Figure 8c,d). In both cases, the changes in the charge density of the oxidation peak due to UV or blue light irradiation are typically 10-3.71,72 Nevertheless, they are highly reversible and reproducible, as illustrated by multiple repetition of the irradiation sequence, and clearly result from photoisomerization of the azobenzene

(70) Kooyman, R. P. H. Physics of Surface Plasmon Resonance. In Handbook of Surface Plasmon Resonance; Schasfoort, R. B. M., Tudos, A. J., Eds.; Royal Society of Chemistry, 2008; pp 15-34.

(71) Subramanian, A.; Irudayaraj, J.; Ryan, T. Biosens. Bioelectron. 2006, 21, 998–1006. (72) Subramanian, A.; Irudayaraj, J.; Ryan, T. Sens. Actuators, B 2006, 114, 192–198.

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Table 1. Photoisomerization Parameters for the Photoinduced trans-cis and cis-trans Isomerization of Azobenzene-Containing SAMs Obtained in This Work and Previous Studies results compound

technique

λ/nm

I0/mW/cm2

τ/s

σeff/cm2

AzoOC3SH(2a) C6AzoOC3SH (2b)/ n-butanethiol 2b/n-butanethiol 2b/n-butanethiol

CV CV

365 365

5.67 5.67

11.7 ( 4.4 12.8 ( 6.2

(8.2 ( 3.9)  10-18 (7.5 ( 4.2)  10-18

SPR SPR

365 435

2.32 2.36

64.9 ( 23.6 80.2 ( 30.0

(3.6 ( 1.5)  10-18 (2.4 ( 1.0)  10-18

λ/nm

I0/mW/cm2

τ/s

σeff/cm2

SPR SPR STM

364 440 365

2.44 2.70 12

38.9 52.9 3240 ( 900

5.7  10-18 3.2  10-18 1.4  10-20

2PPES 2PPES

282 299

1.7 16.9

literature ref

compound

48,51 48,51 60

C6AzoOC12SSC12 C6AzoOC12SSC12 AzoC2OC4SH/ n-decanethiol TTBA TTBA

78 78

technique

4  10-21 8  10 -22

Figure 8. Photoelectrochemical behavior of SAMs of the azobenzene-containing alkanethiols with an amide group, exemplified by data for a mixed monolayer of 1a and N-acetylcysteamine (0.2:0.8 mM). The same methodology as in the experiment shown in Figure 6 was employed. The potential cycle extended either (a,b) over the entire azobenzene redox regime or (c,d) only partly into the reduction peak.

moieties in the SAMs. In contrast to the results obtained for the mixed SAMs, no photoinduced changes of the SPR signal (with exception of the step artifacts) were observed for pure 2b SAMs (Figure 9c). To determine the photoisomerization kinetics of the mixed 2b/ n-butanethiol SAMs, the photoinduced change in the SPR signal is shown on a semilogarithmic plot as a function of time (Figure 9d,e). No deviations from a linear relation are discernible for both UV and blue light irradiation, indicating that the observed changes can be described by first-order kinetics. The time constants obtained from fits of these data are (65 ( 24) s for the trans-cis and (80 ( 30) s for the cis-trans isomerization. In contrast, Tamada et al. reported a simple first-order kinetics only for cis-trans isomerization, whereas for trans-cis isomerization, 13920 DOI: 10.1021/la1015109

clear deviations were found.48,51 A more detailed comparison with the results of that study will be given below.

Discussion Our studies of the photoisomerization behavior of azobenzenecontaining alkanethiol SAMs on Au show pronounced differences between molecules where the azobenzene moiety is linked to the alkanethiol via an ether group and those where it is linked via an amide group, respectively. This interesting effect has up to now not been addressed explicitly in previous studies of these systems. Nevertheless, the reported photoisomerization behavior of such SAMs is in good agreement with our results. Almost all previous observations of reversible trans-cis photoisomerization employed Langmuir 2010, 26(17), 13913–13923

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Figure 9. Real-time SPR measurements of the photoinduced variations in the index of refraction for (a) a mixed monolayer of 2b and n-butanethiol (0.8:0.2 mM), (b) a pure n-butanethiol SAM, and (c) a pure 2b SAM on Au upon irradiation with UV (365 nm, 2.3 ( 0.5 mW/cm2) or blue (435 nm, 2.4 ( 0.5 mW/cm2) light. In addition, first-order plots of the (d) trans-cis and (e) cis-trans photosomerization kinetics are shown.

(mixed) SAMs of molecules with an ether linker between the azobenzene moiety and the alkanethiol. In particular, SAMs of similar molecules as 2a-2d (albeit with longer chain alkanethiols or different head groups) on very thin (polycrystalline) Au films or Au colloids were shown in photoelectrochemical, SPR, or UV/vis spectroscopy studies by several groups29,31,36,42,47,48,50,51,53,54 to exhibit clear photoisomerization effects. In contrast, no comparable data exist for SAMs of analogue molecules with an amide linker on Au, even though these systems have been studied to a similar extend by electrochemical, structure-sensitive, and spectroscopic techniques.8-24 The only exception is a note in a paper by Willner et al.55 mentioning briefly photoswitching in a mixed SAM containing derivatives of 1a with an attached diaminododecane end group. In this work, a very different photoelectrochemical response—almost complete suppression of the azobenzene redox peaks upon UV irradiation—was reported for such SAMs on Au wire electrodes. Despite intensive attempts using a wide variety of preparation methods and experimental conditions (e.g., different types of coadsorbed spacer molecules, different azobenzene surface coverages, different electrolytes), we were unsuccessful in reproducing such behavior for any of the molecules investigated. An interesting behavior was reported for SAMs of an R-lipoic acid derivative with an amide linker to the azobenzene moiety.73 There, no photoreactivity was observed when the molecules were absorbed in trans configuration, i.e., for SAMs prepared similarly as in our study, whereas molecules adsorbed in cis configuration were photoreactive. It was suggested that this behavior is caused by the lower packing density of the molecules in the latter case. However, in view of the very different structure of these molecules, in particular, the bulkier linker to the substrate and the reverse (73) Weidner, T.; Bretthauer, F.; Ballav, N.; Motschmann, H.; Orendi, H.; Bruhn, C.; Siemeling, U.; Zharnikov, M. Langmuir 2005, 24, 11691–11700.

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orientation of the amide moiety, the relevance of these findings for the systems studied in our work is unclear. Possible reasons for these differences between SAMs exhibiting an amide or an ether linker can be steric effects caused by differences in the SAM structure, as well as direct influences of these ligands on the azobenzene photoisomerization properties. We first discuss the role of the molecular structure of the SAMs, which influences the local environment of the azobenzene moieties and by this can affect its structural rearrangement during trans-cis photoisomerization. It was suggested that in SAMs a free area per azobenzene moiety of 45-50 A˚2 is a prerequisite for photoisomerization.49 For pure SAMs of the azobenzene-containing alkanethiols, relatively high coverages were obtained by structure-sensitive methods, corresponding to an area per molecule of ∼25-30 A˚2.17,23,26,27,29,37,66 Specifically, STM studies revealed for pure 1a SAMs a coverage of 2/7 ML on Au(111) corresponding to an area per molecule of 25.3 A˚2.23 For 4-(4-(4-phenylazo)-phenoxy)-butane-1-thiol, i.e., a molecule that differs from 2a only by its slightly longer alkane chain length (butane instead of propane), a coverage corresponding to an area per molecule of 25.3 A˚ was found.29 Since for SAMs of this class of compounds the coverage seems not to be significantly affected by the alkane spacer length according to previous STM/ AFM studies17,26,27,29,37,66 and also for pure 2b SAMs (i.e., a compound analogue to 2a, but with an additional hexyl headgroup) an area per molecule of 30.4 A˚2 was identified here, it is most probable that also 2a forms similar high-coverage SAMs. In addition, for pure 1a SAMs these observations are in good agreement with the coverage estimated from the electrochemical data (assuming a one-electron transfer).23 Hence, the area per azobenzene moiety is in principle too small to expect photoisomerization in both types of SAMs. Nevertheless, our photoelectrochemical experiments clearly revealed this to be possible in 2a SAMs, even for 2a SAMs on single-crystalline Au(111) substrates, whereas DOI: 10.1021/la1015109

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the amide-containing 1a SAMs are completely photoinactive. Obviously, this cannot be attributed to the azobenzene surface coverage, which is very similar in the pure SAMs of these two molecules. However, clear structural differences between 1a23 SAMs and SAMs of the azobenzene-containing alkanethiols with an ether linker17,26,27,29,37,66 were revealed by AFM/STM. More precisely, 1a SAMs exhibit stripe-like structures, whereas for all SAMs of the azobenzene-containing alkanethiols with an ether linker, oblique structures were found. These different structures of the SAMs with amide linkers are most probably due to cross-linking by hydrogen bonds between the amide groups. Such cross-linking was also confirmed in various spectroscopic studies14,16-18,20 and could inhibit the structural rearrangement associated with trans-cis isomerization. These steric effects should be less severe in mixed monolayers of the azobenzene-containing alkanethiols with short-chain alkanethiol spacer molecules. The much smaller charge densities obtained for these mixed SAMs indicate lower coverages than in the pure SAMs, and consequently, the steric hindrances should be significantly reduced. Furthermore, the electrochemical and STM data for mixed SAMs suggest that the azobenzene-containing compounds adsorb preferentially in a matrix of the spacer molecules (see above) and that phase separation into larger domains of the two different molecular species can be neglected during the experiments. This is also supported by a previous STM study of mixed 1a/n-octanethiol SAMs,21 although in that case, the 1a surface density was low. Therefore, the average area per azobenzene should be sufficient to allow photoisomerization and their spacing too large for significant cross-linking of the photoactive compounds (e.g., via hydrogen bonds between the amide groups). Indeed, the photoswitching properties of SAMs containing azobenzene alkanethiols with an ether group are significantly improved as compared to the corresponding pure SAMs, in good agreement with the results of previous studies.48,50,51 However, no indication for photoisomerization could be found for mixed SAMs of the amide-containing compounds, suggesting that steric effects are not primarily responsible for the lack of photoswitching in these SAMs. Instead, it is more likely that the latter originates from the electronic structure of these azobenzenecontaining alkanethiols. Specifically, the amide moiety is a strong electron acceptor. It is a well-known effect74-77 that substitution of free azobenzene at the 40 -position by an electron-withdrawing group causes an instability of the cis isomer against thermal backisomerization, i.e., a significant reduction of the lifetime of the cis isomer. The latter effect is not expected for azobenzene derivatives with an ether moiety, which is a much weaker electron acceptor. This is in good agreement with our experimental observations, since for 1a dissolved in CCl2H2 a lifetime of the cis isomer of only 3 h was found, whereas for 2b, the lifetime exceeded 35 h. In addition, our experiments provided quantitative data on the photoisomerization kinetics in the SAMs of azobenzene derivatives with an ether and a propanethiol group (2a and 2b), which are summarized and compared to results of previous studies in Table 1. To account for the different light intensities, I0, and photon energies in these experiments, we converted the measured time constant τ to the corresponding photoisomerization cross (74) Le Fevre, J. W.; Northcott, J. J. Chem. Soc. 1953, 867–870. (75) Wildes, P. D.; Pacifici, J. G.; Irick, G.; Whitten, D. G. J. Am. Chem. Soc. 1971, 93, 2004–2008. (76) Knoll, H. In CRC Handbook of Organic Photochemistry and Photobiology, 2nd ed.; Horspool, W., Lenci, F., Eds.; CRC press: Boca Raton, 2003; Chapter Photoisomerism of Azobenzene, pp 89-1-10. (77) Dokic, J.; Gothe, M.; Wirth, J.; Peters, M. V.; Schwarz, J.; Hecht, S.; Saalfrank, P. J. Phys. Chem. A 2009, 113, 6763–6773.

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sections σeff = (τnph)-1 = Eph (τI0)-1 = hc (τλI0)-1 (analogous to ref 78). Within the experimental errors, the cross sections obtained in our study are very similar (the slightly smaller values obtained by SPR spectroscopy may result from the different solvent and substrate employed in these measurements). Furthermore, they are in good quantitative agreement with the results of Tamada et al.,48,51 which is surprising in view of the much longer length of the alkanethiol spacer (dodecanethiol) used in that study. In UV/vis spectroscopy studies of this type of azobenzene derivatives on Au nanoclusters, a strong dependence of the photoisomerization quantum yields on the length of the alkane spacer unit was found, with a decrease by 2 orders of magnitude for butanethiol as compared to dodecanethiol.36 In contrast, our results suggest a negligible dependence on the spacer length down to propanethiol for these SAMs on planar substrates. Our results are also at variance to the STM observations by Kumar et al.,60 where for isolated azobenzene derivatives with a similar spacer length (albeit with a different position of the ether group) embedded in a alkanethiol matrix, much slower photoisomerization was observed. However, the presence of the STM tip in those measurements may have significantly reduced the UV intensity in the studied surface area, making a quantitative comparison difficult. Even smaller photoisomerization cross sections were found for tetra-tert-butyl-azobenzene molecules on Au(111) surfaces,78,79 where the planar adsorption geometry results in a distance of only a few angstroms between the azobenzene unit and the Au surface. Due to this, electronic quenching of the photoisomerization by the metal substrate is expected, which could account for the 3 orders of magnitude slower photoisomerization reactions. Interestingly, our data as well as those reported by Tamada42,48,51 indicate that the time constant for photoisomerization in pure azobenzene-containing SAMs does not substantially differ from that of mixed SAMs with alkanethiol spacer molecules, even for monolayers on single-crystalline substrates, where the pure SAMs are well-ordered. However, in both studies the amplitude of the measured signal was significantly enhanced in the presence of the spacer molecules. Hence, only the number of photoswitchable compounds is reduced at high azobenzene surface coverages, but not the kinetics of the remaining, active fraction, suggesting that those molecules are not (or only marginally) affected by their local environment, i.e., the surrounding SAM. The kinetic data allow us to roughly estimate the quantum efficiency of the trans-cis photoisomerization reaction in the SAMs. Since this reaction could be partly suppressed in pure SAMs, we concentrate on the mixed 2b/n-butanethiol SAMs, where these inhibition effects should be lower. The azobenzene surface concentration ΓAzo in the SAMs can be estimated from the coverage of pure 2c SAMs26,27,37 of (4.1 ( 0.2)  1014 cm-2 and the ratio of the redox peak charge densities of pure and mixed 2b SAMs (0.2) as (8.3 ( 0.4)  1013 cm-2. For 2b dissolved in CCl2H2, the molar extinction coefficient at 365 nm is ∼23 000 L mol-1 cm-1. Furthermore, cis-trans back-isomerization is neglected. With the reduction in intensity near the metal surface taken into account, which can be calculated via the Fresnel equations (providing a factor R of ∼0.4 for 365 nm at normal incidence), the absorption cross section results in σ = RI0(1-10-2εAzoΓAzo/NA). By comparison with the cross section for photoisomerization, a quantum efficiency of ∼0.1 is obtained, (78) Hagen, S.; Leyssner, F.; Nandi, D.; Wolf, M.; Tegeder, P. Chem. Phys. Lett. 2007, 444, 85–90. (79) Comstock, M. J.; Levy, N.; Kirakosian, A.; Cho, J.; Lauterwasser, F.; Harvey, J. H.; Strubbe, D. A.; Frechet, J. M. J.; Trauner, D.; Louie, S. G.; Crommie, M. F. Phys. Rev. Lett. 2007, 99, 1–4.

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i.e., similar to that for the molecules in solution.36 Although this analysis involves many assumptions, our results seem to be at variance with a decrease of the quantum efficiency for photoisomerization by several orders of magnitude, as suggested in the study by Zhang et al.36 This may be caused by a lower SAM packing density on the small Au clusters employed in the latter study, which could lead to a conformationally less rigid adlayer with smaller spacings of the azobenzene moieties to the metal surface. A similar explanation was already proposed36 to explain the rather low decay length of the quantum efficiency with spacer length.

Summary The photoisomerization reactions in pure self-assembled monolayers of azobenzene-containing alkanethiols as well as mixed SAMs with aliphatic spacer molecules were studied by two different experimental methods, photoelectrochemical measurements and surface plasmon resonance spectroscopy. The results obtained by these two techniques are in good agreement, indicating that both methods are well-suited for studying photoswitching of molecular adlayers on metallic substrates. A pronounced effect of the chemical group by which the azobenzene moiety is attached to the alkanethiol was found: While SAMs of molecules with an amide group did not exhibit significant photoisomerization, clear and highly reversible photoinduced effects were observed for SAMs of molecules with an ether group. This strongly different behavior was tentatively attributed to the

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electronic structure of the adsorbate molecules. For one of these ether-containing molecules, photoswitching could even be detected for the pure SAMs on single-crystalline Au(111) surfaces, indicating that steric hindrance of the photoisomerization reactions in these SAMs is smaller than anticipated. The trans-cis and cis-trans isomerization reactions followed first-order kinetics with time constants of approximately 1 min under the experimental conditions. Comparison with the results of Tamada et al.48,51 suggest that these reactions are rather independent of the length of the vertical spacer group down to chain lengths of n = 3. Furthermore, the corresponding photoisomerization quantum efficiencies were estimated to be of the same order of magnitude as those of the molecules in solution. Overall, these results demonstrate that even by employing a comparable simple preparation method self-assembled layers with good photoswitching properties can be obtained, which is of fundamental as well as practical importance in view of the substantial interest in these compounds for the preparation of photoactive nanosystems. Acknowledgment. We gratefully acknowledge financial support by the Deutsche Forschungsgemeinschaft to the Sonderforschungsbereich 677 ”Funktionen durch Schalten”. Supporting Information Available: Additonal data on electrochemical properties and photoswitching behavior. This material is available free of charge via the Internet at http://pubs.acs.org.

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