Correlation between the Molecular Structure and Photoresponse in

Sep 27, 2008 - ReceiVed July 30, 2008. ReVised Manuscript ReceiVed August 26, 2008. We have ... These films were prepared on Au(111) from solutions of...
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Langmuir 2008, 24, 11691-11700

11691

Correlation between the Molecular Structure and Photoresponse in Aliphatic Self-Assembled Monolayers with Azobenzene Tailgroups Tobias Weidner,*,†,# Frauke Bretthauer,‡ Nirmalya Ballav,† Hubert Motschmann,§ Horst Orendi,§ Clemens Bruhn,‡ Ulrich Siemeling,‡ and Michael Zharnikov*,† Angewandte Physikalische Chemie, UniVersita¨t Heidelberg, 69120 Heidelberg, Germany, Institut fu¨r Chemie and Center for Interdisciplinary Nanostructure Science and Technology (CINSaT), UniVersita¨t Kassel, 34109 Kassel, Germany, and Max Planck Institute of Colloids and Interfaces, 14476 Golm/Potsdam, Germany ReceiVed July 30, 2008. ReVised Manuscript ReceiVed August 26, 2008 We have compared the structural and photoisomerization properties of self-assembled monolayers (SAMs) comprising either the trans or cis isomers of azobenzene terminated dithiolane with in-chain amide unit, viz., 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide (1). These films were prepared on Au(111) from solutions of both isomers. Structure and composition of the SAMs were studied by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy. The photoresponse of the films was monitored in real time by ellipsometry. SAMs fabricated from the trans isomer were found to be densely packed and highly ordered. These films did not show any discernible photoresponse upon irradiation with UV light, which, under favorable conditions, triggers the trans-cis isomerization. In contrast, films prepared from solutions containing predominantly the cis isomer were loosely packed and mostly disordered but exhibited reversible photoreactivity. The results confirm that steric effects, i.e., available free volume, play a dominant role for the photoresponse of aliphatic SAMs bearing the photoactive azobenzene group. The crystal structure of 1 (trans isomer) exhibits a row-like aggregation of neighboring molecules by weak hydrogen bonds and can be taken as a model for the arrangement of 1 in the monolayer films. Further, in addition to the surface coordination behavior, we have also mimicked the chemisorption of the 1,2-dithiolane moiety onto the gold substrate in molecular coordination chemistry in oxidative addition reactions with the zero-valent platinum complex [Pt(PPh3)4].

* Corresponding authors. E-mail: [email protected] (T.W.); [email protected] (M.Z.). † Universita¨t Heidelberg. ‡ Universita¨t Kassel. § Max Planck Institute of Colloids and Interfaces. # Present address: National ESCA and Surface Analysis Center for Biomedical Problems (NESAC/BIO), University of Washington, Seattle, WA 98195.

external stimuli. In this context, SAMs bearing photoactive functional units are very attractive for providing surfaces with reversible photoswitchable properties. Among the different candidates for such units, azobenzenes, with their unique photoisomerization capability,5 are very promising moieties. Ultraviolet light of a suitable wavelength can trigger isomerization of azobenzenes from the thermodynamically more stable transto the cis-form. The isomerization can be reversed by visible light or thermal relaxation. Many applications have been proposed to utilize the trans-cis isomerization of azobenzene, including data storage,6 light-powered molecular machines,7 and cavities with switchable dimensions.8 The trans-cis isomerization of azobenzene requires a free volume, since the molecular structure changes significantly. This requirement is usually met if the photoisomerization is performed in solution, but is not always fulfilled if azobenzene-bearing molecules are assembled on a suitable support as a SAM, with this assembly being a prototype of a future molecular electronics device. In particular, quite promising SAM systems, i.e., films of azobenzene-substituted alkanethiols on gold, are densely packed and usually highly ordered due to strong van der Waals interactions between the adjacent aliphatic chains and π-stacking of the aromatic units.9 The dense packing limits the free volume which is needed for the isomerization of the tailgroups and, in such a way, restricts photoswitching in these films. The only exception is a cooperative transformation of whole domains from

(1) (a) Ulman, A. An Introduction to Ultrathin Organic Films: From LangmuirBlodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (b) Ulman, A. Chem. ReV. 1996, 96, 1533. (c) Ulman, A., Ed. Self-Assembled Monolayers of Thiols; Thin films; Academic Press: San Diego, CA, 1998; Vol. 24. (d) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (e) Schreiber, F. J. Phys.: Condens. Matter 2004, 16, R881. (f) Ulman, A. Acc. Chem. Res. 2001, 34, 855. (2) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (3) Wohltjen, H.; Snow, A. W. Anal. Chem. 1998, 70, 2856. (4) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103.

(5) Rabek, J. F., Ed. Photochemistry and Photophysics; CRC Press: Boca Raton, 1988. (6) (a) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (b) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (7) (a) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (b) Hugel, T.; Holland, N. B.; Cattani, A.; Moroder, L.; Seitz, M.; Gaub, H. E. Science 2002, 296, 1103. (c) Ferri, V.; Elbing, M.; Pace, G.; Dickey, M. D.; Zharnikov, M.; Samorı`, P.; Mayor, M.; Rampi, M. A. Angew. Chem., Int. Ed. 2008, 47, 3407. (8) Shinkai, S. Pure Appl. Chem. 1987, 59, 425.

1. Introduction Control over properties of surfaces and interfaces is an extremely important issue for both scientific and industrial communities. To a certain extent, this demand can be met by self-assembled monolayers (SAMs), which are 2D polycrystalline monomolecular films of rod-like molecules, covalently attached to suitable substrates and giving the respective surfaces a new chemical identity.1 Molecules capable of spontaneously forming SAMs usually consist of three basic building blocks: a headgroup for covalent attachment to the substrate, a functional group that defines the chemical or functional properties of the outer film surface (tailgroup), and a spacer unit that separates the head- and tailgroup and drives the self-assembly by lateral interactions between adjacent molecules. SAMs have been extensively used to tailor the properties of surfaces as well as for applications in such diverse fields as, e.g., molecular electronics,2 chemical vapor sensing,3 and nanotechnology.4 Another interesting issue is preparation of smart functional surfaces capable of reacting to

10.1021/la802454w CCC: $40.75  2008 American Chemical Society Published on Web 09/27/2008

11692 Langmuir, Vol. 24, No. 20, 2008

one isomer to the other as was recently shown for highly ordered films of aromatic azo-biphenyls on Au and Pt.7c,10 However, such a cooperative transformation has not been achieved so far in aliphatic SAMs. Instead, much effort has been made to fabricate azobenzenebased aliphatic SAMs with sufficient free volume. In particular, photoswitching has been observed in mixed SAMs where azobenzene-bearing molecules were incorporated into a matrix of short alkane thiols.11 The ratio of both components in solution, however, did not correlate with the stoichiometry of the resulting mixed SAMs.12 This problem has also been addressed by using asymmetrically substituted disulfides having an unsubstituted and an azobenzene-bearing aliphatic chain part.13 However, both the above two-component strategies can still lead to phase separation within the monolayer film and to aggregation of the azobenzene-carrying adsorbate molecules, limiting the photoisomerization to molecules adsorbed at domain boundaries.9g,14 To overcome the free volume problem and to avoid phase separation, Ito et al. used an alternative single-component strategy and prepared azobenzene-containing SAMs with pronounced photoisomerization capacity from molecules bearing spherical para-carborane as a spacer unit in the alkyl chain.15 An alternative single-component strategy is the use of binding units, which cover a surface area whose diameter ideally exceeds that of the functional azobenzene unit. Adsorbate molecules with multiple anchors have been investigated in this context. However, dipodal headgroups bound via two short alkyl chains did not lead to the anticipated improved photoswitching behavior.16 A more rigid, three-legged molecule with rod-like aromatic anchor groups showed photoswitching, but has not been tested in SAMs so far.17 A further promising route to generate enough free volume for the isomerization processes in azobenzene-containing SAMs is the preparation of films from solution under UV irradiation. Under these conditions, the equilibrium between the two isomers in the solution is strongly shifted toward the cis form, so that films with more free volume can be fabricated.18 However, no rigorous analysis of the correlation between the molecular structure and the molecular arrangement in the azobenzene-bearing aliphatic SAMs on one side and their photoisomerization capability on the other side has been performed so far. We decided to study (9) (a) Tamada, K.; Nagasawa, J.; Nakanishi, F.; Abe, K.; Ishida, T.; Hara, M.; Knoll, W. Langmuir 1998, 14, 3264. (b) 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. (c) 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. (d) Wang, R.; Iyoda, T.; Jiang, L.; Hashimoto, K.; Fujishima, A. Chem. Lett. 1996, 25, 1005. (e) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (f) Wang, R.; Iyoda, T.; Jiang, L.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. J. Electroanal. Chem. 1997, 438, 213. (g) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436. (10) Pace, G.; Ferri, V.; Grave, C.; Elbing, M.; von Ha¨nisch, C.; Zharnikov, M.; Mayor, M.; Rampi, M. A.; Samorı`, P. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 9937. (11) (a) Yasuda, S.; Nakamura, T.; Matsumoto, M.; Shigekawa, H. J. Am. Chem. Soc. 2003, 125, 16430. (b) Kumar, A. S.; Ye, T.; Takami, T.; Yu, B.-C.; Flatt, A. K.; Tour, J. M.; Weiss, P. S. Nano Lett. 2008, 8, 1644. (12) (a) Tamada, K.; Hara, M.; Sasabe, H.; Knoll, W. Langmuir 1997, 13, 1558. (b) Stranick, S. C.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (13) Akiyama, H.; Tamada, K.; Nagasawa, J.; Nakanishi, F.; Tamaki, T. Trans. Mater. Res. Jpn. 2000, 25, 425. (14) Tamada, K.; Akiyama, H.; Wei, T. X. Langmuir 2002, 18, 5239. (15) Ito, M.; Wei, T. X.; Chen, T.-L.; Akiyama, H.; Matsumoto, M.; Tamada, K.; Yamamoto, Y. J. Mater. Chem. 2005, 15, 478. (16) (a) Gustina, D.; Markava, E.; Muzikante, I.; Stiller, B.; Brehmer, L. AdV. Mater. Opt. Electron. 1999, 9, 245. (b) Cook, M. J.; Nygård, A.-M.; Wang, Z.; Russell, D. A. Chem. Commun. 2002, 22, 1056. (17) Takamatsu, D.; Yamakoshi, Y.; Fukui, K. J. Phys. Chem. B 2006, 110, 1968. (18) Kondo, T.; Kanai, T.; Uosaki, K. Langmuir 2001, 17, 6317.

Weidner et al. Scheme 1. Target Molecules of This Studya

a

See text for details.

this correlation using SAMs prepared by chemisorption of 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxamide (1) on gold (see Scheme 1). 1 is a derivative of the commercially available R-lipoic acid (also known as thioctic acid) and contains a 1,2-dithiolane functionality for anchoring to the substrate. Note that such dithiolane headgroups offer several practical advantages. They are chemically more robust and easier to handle than other sulfur-based binding units like thiols and thioacetates, and they are straightforward to work with from a preparative point of view. Furthermore, they are well-known to bind particularly strongly to gold owing to the surface chelate effect.19 Their chemisorption to gold involves the cleavage of the S-S bond and concomitant formation of two thiolate-gold bonds, which corresponds to an oxidative addition to zero-valent Au. The amide group incorporated in the flexible aliphatic chain of the lipoic acid has already been shown to increase the lateral stability in SAMs due to hydrogen bonding between adjacent spacers.20 In this study, we have compared the structural and photoisomerization properties of SAMs adsorbed from solutions containing either exclusively the trans or predominantly the cis isomer of 1. In addition, we investigated the influence of hydrogen bonding on the SAM structure by comparing the SAMs prepared from 1 to those prepared from its analogue, 4-(phenyldiazenyl)phenyl-4-(1,2-dithiolane-3-yl)-butylcarboxylate (2), in which the amide group in the spacer is replaced by an ester moiety (see Scheme 1). The structural characterization of the SAMs was carried out by X-ray photoelectron spectroscopy (XPS) and nearedge X-ray absorption fine structure (NEXAFS) spectroscopy. Photoswitching was studied in real time by ellipsometry. Additionally, we have also mimicked the chemisorption of the 1,2-dithiolane moiety of 1 and 2 to the gold substrate in molecular coordination chemistry in oxidative addition reactions with a zero-valent precious metal complex. The following section describes the preparative approach and experimental techniques. The experimental data are presented and preliminarily discussed in section 3. A thorough discussion of the results is given in section 4, followed by a summary in section 5. (19) (a) Kunze, J.; Leitch, J.; Schwan, A. L.; Faragher, R. J.; Naumann, R.; Schiller, S.; Knoll, W.; Dutcher, J. R.; Lipkowski, J. Langmuir 2006, 22, 5509. (b) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E.; Hussey, C. L. Langmuir 1998, 14, 124. (c) Willey, T. M.; Vance, A. L.; Bostedt, C.; van Buuren, T.; Meulenberg, R. W.; Terminello, L. J.; Fadley, C. S. Langmuir 2004, 20, 4939. (d) Marten, J.; Erbe, A.; Critchley, K.; Bramble, J. P.; Weber, E.; Evans, S. D. Langmuir 2008, 24, 2479. (20) (a) Sabapathy, R. C.; Bhattacharyya, S.; Leavy, M. C.; Cleland, W. E., Jr.; Hussey, C. L. Langmuir 1998, 14, 124. (b) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 5319. (c) Tam-Chang, S.-K.; Biebuyck, H. A.; Whitesides, G. M.; Jeon, N.; Nuzzo, R. G. Langmuir 1995, 11, 4371.

Aliphatic SAMs with Azobenzene Tailgroups

2. Experimental Section 2.1. Synthetic Work. Synthetic work involving air-sensitive compounds was performed under an atmosphere of dry nitrogen by using standard Schlenk techniques or a conventional glovebox. Solvents were appropriately dried and purified. Chemicals were procured from standard commercial sources and used as received. NMR spectra were recorded with a Varian Unity INOVA 500 instrument operating at 500.13 MHz for 1H. MALDI mass spectra were obtained with a Bruker Daltonics BiFlex IV instrument (N2 laser, 337 nm; DCTB matrix). ESI and ACPI mass spectra were obtained with a ThermoQuest Finnigan LCQ Deca instrument. Elemental analyses were performed by the microanalytical laboratory of the University of Halle. a. A solution of DL-R-lipoic acid (1.00 g, 4.85 mmol), N,N′dicyclohexylcarbodiimide (DCC, 1.00 g, 4.85 mmol), 1-hydroxybenzotrizol (HOBt, 0.66 g, 4.85 mmol), and 4-aminoazobenzene (0.98 g, 4.85 mmol) in dichloromethane (30 mL) was stirred for 72 h at room temperature. The precipitate was removed by filtration and washed with acetone (2 × 2 mL). The combined filtrate and washings were reduced to dryness in vacuo to afford the crude product, which was subsequently purified by column chromatography (silica gel, dichloromethane/methanol 30/1). Recrystallization from ethyl acetate afforded analytically pure 1 as orange crystals. Yield 0.80 g (42%). 1H NMR (CDCl ): δ (ppm) ) 1.52 (m, 2 H), 1.74 (m, 2 H), 1.90 3 (m, 2 H), 2.43 (m, 3 H), 3.10 (m, 1 H), 3.17 (m, 1 H), 3.57 (m, 1 H), 7.33 (s, 1 H, NH), 7.44 (m, 1 H), 7.49 (m, 2 H), 7.68 (m, 2 H), 7.88 (m, 4 H). 13C NMR (CDCl3): δ (ppm) ) 25.3, 28.9, 34.9, 37.7, 38.6, 40.4, 56.4, 119.8, 122.8, 124.1, 129.1, 130.8, 140.4, 149.1, 152.7, 171.1. MS/MALDI(+): m/z (%) ) 385 (100) [M]+. Anal. Calcd for C20H23N3OS2 (385.1): C, 61.99; H, 6.11; N, 10.81; S, 16.39%. Found: C, 62.32; H, 5.97; N, 10.91; S, 16.62%. b. DL-R-lipoic acid (1.00 g, 4.85 mmol) and N,N′-dicyclohexylcarbodiimide (DCC, 1.00 g, 4.85 mmol) were dissolved in dichloromethane (30 mL) with stirring. After 10 min, 4-hydroxyazobenzene (0.96 g, 4.85 mmol) was added. The stirred mixture was refluxed for 48 h and was subsequently cooled and stored at 4 °C. The precipitate was filtered off and washed with cold dichloromethane (2 × 2 mL). The combined filtrate and washings were reduced to dryness in vacuo to afford the crude product, which was dissolved in a minimal amount of dichloromethane/methanol 30/1 and subjected to purification by column chromatography (silica gel, dichloromethane). The product was obtained as orange crystals. Yield 1.36 g (71%). 1H NMR (CDCl ): δ (ppm) ) 1.57 (m, 2 H), 1.78 (m, 4 H), 1.91 3 (m, 1 H), 2.47 (m, 1 H), 2.60 (m, 2 H), 3.13 (m, 1 H), 3.18 (m, 1 H), 3.59 (m, 1 H), 7.23 (m, 2 H), 7.49 (m, 3 H), 7.89 (m, 2 H), 7.95 (m, 2 H). 13C NMR (CDCl3): δ (ppm) ) 24.5, 28.7, 34.1, 34.6, 38.5, 40.2, 56.3, 122.2, 122.8, 124.0, 129.1, 131.0, 150.2, 152.5, 152.6, 171.6. MS/ACPI(+): m/z (%) ) 387 (100) [M]+. Anal. Calcd for C20H23N2O2S2 (386.2): C, 62.16; H, 5.70; N, 7.25; S, 16.58%. Found: C, 62.15; H, 5.72; N, 7.26; S, 15.98%. c. A solution of [Pt(PPh3)4] (50 mg, 0.04 mmol) and 1 (15 mg, 0.04 mmol) in C6D6 (2 mL) was stored at room temperature in a 5 mm NMR tube. The progress of the reaction was monitored by 31P{1H} NMR spectroscopy. After 12 h, a small amount of insoluble material was removed by filtration. The filtrate was layered with n-hexane (4 mL), which afforded the product as a yellow, crystalline solid after 2 d. The crystals were isolated by filtration, washed with a small amount of benzene, and dried in vacuo. Yield 20 mg (45%). The single crystal used for the X-ray diffraction study was not dried in vacuo and was found to contain 5 molecules of benzene per formula unit. 1H NMR (C D ): δ (ppm) ) 1.52 (m, 6 H), 1.91-2.34 (m, 4 H), 6 6 3.25 (m, 1 H), 3.84 (m, 2 H), 6.89 (m, 12 H), 7.03 (m, 12 H), 7.20 (m, 2 H), 7.39 (m, 7 H), 8.07 (m, 5 H), 8.17 (m, 2 H). 31P{1H} NMR (C6D6): δ (ppm) ) 23.0 (d, 2JP,P ) 21.0 Hz), 25.6 (d, 2JP,P ) 21.0 Hz). 195Pt NMR (C6D6): δ (ppm) ) -4682 (dd, 1JP,Pt ) 2762 and 2862 Hz). MS/MALDI(+): m/z (%) ) 1104 (100) [M]+. Anal. Calcd for C56H53N3OP2PtS2 · C6H6 (1183.9): C, 62.93; H, 4.99; N, 3.55; S 5.41%. Found: C, 62.88; H, 5.49; N, 3.16; S, 4.53%.

Langmuir, Vol. 24, No. 20, 2008 11693 d. A yield of 15 mg (34%) was obtained from the reaction of [Pt(PPh3)4] (50 mg, 0.04 mmol) and 2 (15 mg, 0.04 mmol) by a procedure analogous to that described for 3. 1H NMR (C D ): δ (ppm) ) 1.47 (m, 6 H), 1.95-2.42 (m, 4 H), 6 6 3.30 (m, 1 H), 3.85 (m, 2 H) 6.90 (m, 12 H), 7.02 (m, 12 H), 7.12, (m, 5 H), 7.41 (m, 6 H), 7.98 (m, 2 H), 8.03 (m, 2 H). 31P{1H} NMR (C6D6): δ (ppm) ) 24.6 (d, 2JP,P ) 21.1 Hz), 27.1 (d, 2JP,P ) 21.1 Hz). 195Pt NMR (C6D6): δ (ppm) ) -4681 (dd, 1JP,Pt ) 2748 and 2851 Hz). MS/MALDI(+): m/z (%) ) 1105 (100) [M]+. Anal. Calcd for C56H52N2O2P2PtS2 (1105.2): C, 60.80; H, 4.70; N, 2.53; S, 5.79%. Found: C, 60.11; H, 5.33; N, 2.30; S, 5.79%. 2.2. X-ray Crystallography. For each data collection, a single crystal was mounted on a glass fiber, and all geometric and intensity data were taken from this sample. Data collection using Mo KR radiation (λ ) 0.71073 Å) was made on a Stoe IPDS2 diffractometer equipped with a two-circle goniometer and an area detector. Absorption correction for 3 was done by integration using X-red.21 The data sets were corrected for Lorentz and polarization effects. The structures were solved by direct methods (SHELXS97) and refined using alternating cycles of least-squares refinements against F2 (SHELXL97).22 All non-H atoms were found in difference Fourier maps and were refined with anisotropic displacement parameters. H atoms were placed in constrained positions according to the riding model with the 1.2-fold isotropic displacement parameters except the H atom involved in the hydrogen bridge in 1, which was refined freely with an isotropic displacement parameter. Graphical representations were made using ORTEP-3 win.23 X-ray crystal structure analysis of 1: Empirical formula C20H23N3OS2, M ) 385.53, T ) 298(2) K, monoclinic, P 21/c, a ) 5.3153(6), b ) 16.7055(19), c ) 22.204(3) Å, β ) 93.845(10)°, V ) 1967.2(4) Å3, Z ) 4, Fcalc ) 1.302 g/cm-3, µ ) 0.285 mm-1, F(000) ) 816, θ ) 1.53° to 25.00°, Index ranges -5fhf6, -19fkf19, -26flf26, 10 180 collected, 3471 independent (Rint ) 0.1117) and 1894 observed reflections [I > 2σ(I)], 3471 data, 239 refined parameters, S ) 1.000, R1 ) 0.1026, wR2 ) 0.2426, max. residual electron density 0.464 and -0.385 e Å-3. X-ray crystal structure analysis of 3: Empirical formula C86H83N3OP2PtS2, M ) 1495.70, T ) 153(2) K, triclinic, P -1, a ) 10.2245(10), b ) 18.390(2), c ) 21.869(2) Å, R ) 108.640(8), β ) 100.170(8), γ ) 100 785(8)°, V ) 3702.1(7) Å3, Z ) 2, Fcalc ) 1.342 g/cm-3, µ ) 2.043 mm-1, F(000) ) 1536, crystal dimensions 0.48 × 0.26 × 0.09 mm3, Tmin/Tmax ) 0.51/0.84, θ ) 1.84° to 25.00°, Index ranges -11fhf11, -21fkf21, -26flf26, 24 127 collected, 12 246 independent (Rint ) 0.1365) and 6730 observed reflections [I > 2σ(I)], 12 246 data, 772 refined parameters, S ) 0.854, R1 ) 0.0740, wR2 ) 0.1819, max. residual electron density 1.199 and -1.564 e Å-3. 2.3. Photoisomerization in Solution. The photoresponse of 1 dissolved in dichloromethane was determined by UV-visible absorption spectroscopy using a home-built spectrometer.24 The spectra were taken in a UV-grade quartz cell with an optical beam path of 1 cm before and after irradiation with UV light (364 nm) at room temperature. 2.4. Film Preparation. The gold substrates for film fabrication were prepared by thermal evaporation of 100 nm gold (99.99% purity) onto polished single-crystal silicon (111) wafers (Silicon Sense) primed with a 5 nm Ti layer for adhesion promotion. The resulting films were polycrystalline with a grain size of 20-50 nm and predominantly possessed (111) orientation.25 The films were formed by immersion of freshly prepared gold substrates in 10 µM solutions of 1 and 2 in ethanol at room temperature for 18 h. The immersion occurred either under “standard” conditions in the dark (21) X-red Ver. 1.06, Program for numerical absorption correction; Stoe & Cie: Darmstadt, 2004. (22) Sheldrick, G. M. SHELXS 97 and SHELXL 97, Programs for crystal structure solution and refinement, University of Go¨ttingen, Germany, 1997. (23) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837. (24) Hendrich, C.; Bosbach, J.; Stietz, F.; Hubenthal, F.; Vartanyan, T.; Tra¨ger, F. Appl. Phys. B: Laser Opt. 2003, 76, 869. (25) Heister, K.; Zharnikov, M.; Grunze, M.; Johansson, L. S. O. J. Phys. Chem. B 2001, 105, 4058.

11694 Langmuir, Vol. 24, No. 20, 2008 or under UV light irradiation resulting in the formation of the films comprising predominantly trans or cis isomers of 1, respectively; these films will be denoted below as 1-trans and 1-cis. After immersion, the samples were carefully rinsed with copious amounts of ethanol, blown dry with nitrogen, and kept in plastic containers filled with nitrogen until characterization. 2.5. Near-Edge X-ray Absorption Fine Structure Spectroscopy. NEXAFS measurements were performed at the HE-SGM beamline of the synchrotron storage ring BESSY II in Berlin, Germany. The spectra were collected at the C K-edge and N K-edge, with retardation voltages of -150 and -300 V, respectively. Linearly polarized light with a polarization factor of ∼0.82 was used. The energy resolution was approximately 0.4 eV, and the incidence angle of the X-ray light was varied from 90° to 20° in 10-20° steps. Raw NEXAFS spectra were normalized to the incident photon flux by division through a spectrum of a clean, freshly sputtered gold sample. The photon energy scale was referenced to the prominent π1* resonance of freshly cleaved highly oriented pyrolytic graphite at 285.38 eV.26 Further, the spectra were reduced to the standard form by subtracting linear pre-edge background and normalizing to the unity edge jump determined by a horizontal plateau 40-50 eV above the absorption edge. 2.6. X-ray Photoelectron Spectroscopy. The XPS measurements were carried out under UHV conditions at a base pressure better than 1.5 × 10-9 mbar. The experiments were performed using an Al KR X-ray source and an LHS11 analyzer. The energy resolution was ca. 0.9 eV. The X-ray source was operated at a power of 260 W and positioned about 1.5 cm away from the samples. The energy scale was referenced to the Au 4f7/2 peak of alkanethiol-coated gold at a binding energy (BE) of 84.0 eV.27 XPS spectra were fitted by symmetric Voigt functions and either Shirley-type or linear background. To fit the S 2p doublets, we used a pair of such peaks with the same fwhm, a branching ratio of 2:1 (S 2p3/2/S 2p1/2), and spin-orbital splitting of 1.18 eV verified by fit.27 The fits were carried out self-consistently: the same peak parameters were used for identical spectral regions. 2.7. Real-Time Ellipsometry. The photoisomerization in the 1-trans and 1-cis films was monitored in real time by ellipsometry with a Multiskop (Optrel) ellipsometer.28 The measurements were carried out under ambient conditions. The wavelength of the incident light was 633.8 nm; it was shined in at an angle of 75° against the surface normal. The evolution of the phase shift ∆ of the reflected light with respect to the incident one was followed during irradiation with UV (360 nm) and visible (450 nm) light. The light for the photoreaction was generated by a high-pressure Hg lamp using color glasses.

3. Results 3.1. Synthesis and Characterization of Compounds. Amide 1 was prepared by condensation of R-lipoic acid and 4-aminoazobenzene utilizing a standard protocol (DCC, HOBt, dichloromethane solvent, room temp.). Essentially the same approach was chosen for the ester derivative 2 (DCC, dichloromethane solvent, reflux). The crystal structure of 1 was determined by single-crystal X-ray diffraction. The respective molecular structure is shown in Figure 1. Bond parameters are unexceptional and compare well with those of pristine R-lipoic acid.29 In particular, the S-S bond length of 1 of 2.049(3) Å is indistinguishable within experimental error from the corresponding value of 2.053(4) Å reported for R-lipoic acid. Very similar values have also been observed for the carboxylato complexes [M(lip)2(H2O)2] [M ) Zn, S-S (26) Batson, P. E. Phys. ReV. B 1993, 48, 2608. (27) Moulder, J. F.; Stickle, W. E.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Chastian, J.; Ed.; Perkin-Elmer Corp.: Eden Prairie, MN, 1992. (28) Harke, H.; Teppner, R.; Schulz, O.; Motschmann, H.; Orendi, H. ReV. Sci. Instrum. 1997, 68, 3133.

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Figure 1. Molecular structure of 1 in the 3D crystal. The aggregation to one-dimensional chains by amide-mediated hydrogen bonding is indicated for a pair of neighboring molecules.

2.056(3) Å; M ) Cd, S-S 2.047(3) Å)] (lip ) κ2-lipoato).30 The two six-membered rings of the azobenzene moiety are in a coplanar arrangement. The molecules of 1 are aggregated as one-dimensional chains through intermolecular NsH · · · O interactions along the crystallographic a axis. The N · · · O and H · · · O distances of ca. 3.16 and 2.39 Å, respectively, and the NHO angle of 161.5° are compatible with a weak hydrogen bond.31 The chemisorption of 1 and 2 to a gold substrate can be viewed as an oxidative addition of the 1,2-dithiolane moiety to zerovalent precious metal (vide supra). This surface-chemical process was mimicked in molecular coordination chemistry by the reactions of 1 and 2 with the zero-valent platinum complex [Pt(PPh3)4] (Scheme 2).32 The reactions were performed on an NMR tube scale and were conveniently monitored by 31P{1H} NMR spectroscopy. They proved to proceed smoothly and swiftly at room temperature over the course of several hours in C6D6 solvent, affording the respective product (3, 4) in essentially quantitative yield. Two doublets (2JP,P 21.0 Hz) are observed for the two chemically inequivalent phosphorus atoms in the 31P{1H} NMR spectrum in each case with δ(31P) between ca. 23 and 27 ppm. Theses two doublets exhibit the expected 195Pt satellites with 1JP,Pt values of ca. 2760 and 2860 Hz in each case, while the corresponding 195Pt NMR signal appears as an ill-resolved doublet of doublets at ca. -4680 ppm. These NMR spectroscopic values are in accord with data obtained for closely related six(29) Stroud, R. M.; Carlisle, C. H. Acta Crystallogr., Sect. B 1972, 28, 304. (30) Strasdeit, H.; von Do¨llen, A.; Duhme, A.-K. Z. Naturforsch. B: Chem. Sci. 1997, 52, 17. (31) Review: Taylor, R.; Kennard, O. Acc. Chem. Res. 1984, 17, 320. (32) Seminal paper: Zanella, R.; Ros, R.; Graziani, M. Inorg. Chem. 1973, 12, 2736. See also: (b) Beck, W.; Schorpp, K.; Stetter, K. H. Z. Naturforsch. B: Chem. Sci. 1971, 26, 684.

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Scheme 2. Reactions of 1 and 2 with the Zero-Valent Platinum Complex [Pt(PPh3)4] Resulting in the Formation of 3 and 4a

a

This reaction mimics the adsorption of 1 and 2 on the Au(111) substrates.

Figure 3. UV-visible absorption spectra of 1 in dichloromethane before (black line) and after (gray line) UV irradiation.

Figure 2. Molecular structure of 3 in the 3D crystal.

membered platinacycles.33,34 The crystal structure of the amide derivative 3 was determined by single-crystal X-ray diffraction. The respective molecular structure is shown in Figure 2. Bond parameters compare well with those of closely related compounds.34 The platinum atom resides in a distorted squareplanar coordination environment. The PPtP and SPtS bond angles are 98.63(10)° and 92.65(11)°, respectively, while the two PPtS bond angle values are 81.69(11)° and 87.15(19)°. The sum of angles around Pt is 360.12°. The Pt-P bond lengths are almost identical [2.285(3) and 2.308(3) Å], whereas the Pt-S bond lengths differ slightly [2.310(3) and 2.361(3) Å]. In contrast to 1, the platinum complex 3 derived from it does not exhibit coplanar C6 rings in the crystal and also shows no intermolecular NsH · · · O interactions. Instead, neighboring molecules are connected by an NsH · · · S interaction, whose parameters (H · · · S 2.63 Å, N · · · S 3.47 Å, NHS (33) See, for example: (a) Aucott, S. M.; Kilian, P.; Robertson, S. D.; Slawin, A. M. Z.; Woollins, J. D. Chem. Eur. J. 2006, 12, 895. (b) Weigand, W.; Bosl, G.; von Dielingen, B.; Gollnick, K. Z. Naturforsch. B: Chem. Sci. 1994, 49, 513. (34) See, for example, Aucott, S. M.; Milton, H. L.; Robertson, S. D.; Slawin, A. M. Z.; Walker, G. D.; Woollins, J. D. Chem. Eur. J. 2004, 10, 1666.

159.8°) are indicative of an NsH · · · S hydrogen bond of medium strength according to commonly accepted criteria.35 3.2. UV-visible Absorption Spectra. Figure 3 shows the UV absorption spectra of 1 in dichloromethane before and after irradiation with UV light. The trans isomer exhibits a strong π-π* absorption peak at 350 nm, a weak (forbidden) n-π* band around λ) 450 nm, and an additional band at 255 nm.36 Upon UV irradiation, the dominant π-π* band is blue-shifted to 316 nm and decreases in intensity, the n-π* becomes more intense, and the low wavelength band is now found at 236 nm. This suggests that 1 has been transferred into a cis isomer.37 Note that the cis form can be converted back to the trans isomer by thermodynamically driven back-isomerization or by irradiating the solution with visible light. Note also that the spectra in Figure 3 were not mathematically corrected for the noncomplete conversation; they are characteristic of the trans derivative (before UV irradiation) and the photostationary state with enriched cis content (upon UV irradiation). 3.3. XPS. Normalized S 2p, C 1s, N 1s, and O 1s XPS spectra of 1-trans and 1-cis SAMs are presented in Figure 4, along with the corresponding fits. The S 2p spectra of both films are very similar. They exhibit a single S 2p3/2,1/2 doublet. The BE position of this doublet (∼162.2 eV) is characteristic of thiolate-type sulfur bound to metal surfaces,25,38 suggesting that, upon the adsorption of 1 on Au, the covalent S-S bond of the dithiolane headgroup is cleaved through an oxidative addition mechanism and that two thiolate-type sulfur-gold bonds are subsequently formed.19d The similarity of the spectra for both films and the absence of the features characteristic of oxidized sulfur species in the spectra of 1-cis in particular show that the UV irradiation during the adsorption process did not change the sulfur-gold binding chemistry and did not result in the oxidation of sulfur. The C 1s spectra of 1-trans and 1-cis SAMs are also very similar with a slightly lower intensity of the observed features for 1-cis film. The main peak at 284.6 eV is assigned to the carbon backbone and a small shoulder at higher BE (287.6 eV) is related to the carboxyl species.27 The N 1s and O 1s spectra (35) Siemeling, U.; Bretthauer, F.; Bruhn, C. Z. Anorg. Allg. Chem. 2006, 632, 1027, and references cited therein. (36) (a) Rau, H. Angew. Chem., Int. Ed. Engl. 1973, 12, 224. (b) Na¨gele, T.; Hoche, R.; Zinth, W.; Wachveitl, J. Chem. Phys. Lett. 1997, 272, 489. (37) (a) Wu, A.; Talham, D. R. Langmuir 2000, 16, 7449. (b) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955. (38) (a) Sun, F.; Castner, D. G.; Grainger, D. W. Langmuir 1993, 9, 3200. (b) Castner, D. G.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (c) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733.

11696 Langmuir, Vol. 24, No. 20, 2008

Figure 4. C 1s, S 2p, N 1s, and O 1 s XPS spectra (open circles) of SAMs prepared from 1 in either the trans (1-trans) or cis (1-cis) form, along with the corresponding fits (solid lines). Note that, whereas the C 1s, N 1s, and O 1s emissions are singlets, the S 2p emission is a doublet comprising two components with a definite spin-orbit splitting and branching ratio (see section 2). These components are shown in the S 2p spectra (gray solid lines).

of both 1-trans and 1-cis films exhibit single emissions at 399.8 and 531 eV, respectively. These BEs can be assigned to the amide and carboxyl groups in the spacer units.27 In summary, the XPS data allow the conclusion that 1-trans and 1-cis isomers form well-defined and contamination-free SAMs on gold, with the constituents covalently attached to the substrate by the “double” thiolate anchors. No oxidation or chemical modification of the molecules due to UV irradiation could be detected. Apart from the spectra analysis, the effective thickness of 1-trans and 1-cis films could be determined on the basis of the intensities of the C 1s and the Au 4f emissions.39 The thickness values were derived from the IC1s/IAu4f intensity ratios using the attenuation lengths reported in ref 40 (28.07 Å and 30.99 Å for the C 1s and Au 4f photoelectrons, respectively). Following this procedure, we obtained values of 18.4 Å and 15.9 Å for 1-trans and 1-cis films, respectively. These values suggest a substantially lower packing density for the 1-cis film compared to 1-trans one. Note that the former value correlates very well with the expected film thickness of ca. 18 Å, considering a molecular length of 20.9 Å and a tilt angle of ca. 30° (see the NEXAFS results below). 3.4. NEXAFS Spectroscopy. Additional information about the identity, chemical composition, and structure of the target films is provided by NEXAFS spectroscopy. This technique gives chemical information on the adsorbed species by sampling the electronic structure of their unoccupied molecular orbitals. Furthermore, it provides insight into the average orientation of these species using the linear dichroism effects in X-ray absorption, i.e., the dependence of the absorption resonance intensities on the orientation of the electric field vector of the incident synchrotron light with respect to the probed molecular orbital.41 These dichroic effects can be easily monitored by plotting the difference of the spectra recorded at normal (90°) and grazing (20°) angles of X-ray incidence. In contrast, the spectra collected at the so-called magic angle of X-ray incidence (39) Thome, J.; Himmelhaus, M.; Zharnikov, M.; Grunze, M. Langmuir 1998, 14, 7435. (40) Lamont, C. L. A.; Wilkes, J. Langmuir 1999, 15, 2037. (41) Sto¨hr, J. NEXAFS spectroscopy; Springer Series in Surface Science 25; Springer-Verlag: Berlin, 1992.

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Figure 5. C K-edge NEXAFS spectra of 1-trans (left panel) and 1-cis (right panel) SAMs acquired at X-ray incidence angles of 90°, 55°, and 20°. The bottom curves represent the difference between the 90° and 20° spectra. The dotted lines correspond to zero.

(55°) are not affected by any effects related to the orientation of the adsorbed species and thus give only information about their chemical identity. C K-edge spectra of 1-trans and 1-cis films acquired at X-ray incidence angles of 90°, 55°, and 20° are presented in Figure 5, along with the difference between the 90° and 20° spectra. All spectra in Figure 5 exhibit an absorption edge related to the excitation of the C 1s electrons into the continuum states and the characteristic absorption resonances. The spectra are dominated by the pronounced π1* resonance of phenyl ring near 285.2 eV, which is accompanied by a Rydberg resonance (R*) near 287.1 eV, a π* resonance assigned to the CdO group at 288.5 eV, and several broad σ* resonances at higher photon energies. The above assignments were made in accordance with refs 41 and 42. The spectra of 1-trans films (Figure 5, left panel) exhibit a pronounced linear dichroism, which is additionally highlighted by the respective 90° - 20° difference curve. This is a clear signature of orientational order. In addition, the positive sign of the observed difference peaks for the π* resonances implies an upright orientation of the molecular constituents in the 1-trans SAMs. In contrast, the spectra of the 1-cis films (Figure 5, right panel) show a much smaller linear dichroism, suggesting that the orientational order in the films is rather poor. This agrees well with the lower packing density observed for these films by XPS. This conclusion is further corroborated by the N K-edge NEXAFS spectra of 1-trans and 1-cis films. The spectra acquired at X-ray incidence angles of 90°, 55°, and 20° are shown in Figure 6, along with the respective 90° - 20° curves. They are dominated by a distinct π* resonance at 398.4 eV related to the NdN unit of the azobenzene moiety. This resonance is accompanied by several weak π*-like features in the 400-405 eV range and several broad σ* resonances at higher photon energies.43 Analogous to the C K-edge data, the N K-edge spectra (42) (a) Bagus, P. S.; Weiss, K.; Schertel, A.; Wo¨ll, C.; Braun, W.; Hellwig, C.; Jung, C. Chem. Phys. Lett. 1996, 248, 129. (b) Scho¨ll, A.; Fink, R.; Umbach, E.; Mitchell, G. E.; Urquhart, S. G.; Ade, H. Chem. Phys. Lett. 2003, 370, 834. (c) Frey, S.; Stadler, V.; Heister, K.; Eck, W.; Zharnikov, M.; Grunze, M.; Zeysing, B.; Terfort, A. Langmuir 2001, 17, 2408. (d) Zharnikov, M.; Grunze, M. J. Phys.: Condens. Matter 2001, 13, 11333. (e) Fuxen, C.; Azzam, W.; Arnold, R.; Witte, G.; Terfort, A.; Wo¨ll, Ch. Langmuir 2001, 17, 3689. (f) Paik, M. Y.; Krishnan, S.; You, F.; Li, X.; Hexemer, A.; o, Y.; Kang, S. H.; Fischer, D. A.; Kramer, E. J.; Ober, C. K. Langmuir 2007, 23, 5110. (43) Zubavichus, Y.; Shaporenko, A.; Grunze, M.; Zharnikov, M. J. Phys. Chem. A 2005, 109, 6998.

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observed in crystalline 1 (vide supra), is an essential prerequisite for the formation of ordered SAMs of 1-trans on gold. Since 1-trans films exhibit a high orientational order and consist of upright-oriented molecules, we decided to perform a quantitative analysis of the NEXAFS spectra to determine the average tilt angle of the azobenzene unit. The tilt angles R of the π* orbitals with respect to the surface normal were analyzed using the most prominent π* resonances at the C and N K-edge spectra. The intensities of these resonances were monitored as a function of the incidence angle Θ and evaluated according to the theoretical expression for a vector-type orbital.41

{ 31 [1 + 21 (3 cos Θ - 1)(3 cos R - 1)] + (1 - P)(1 ⁄ 2)sin R} (1)

I(R,Θ) ) A P ×

2

2

2

Figure 6. N K-edge NEXAFS spectra of 1-trans (left panel) and 1-cis (right panel) SAMs acquired at X-ray incidence angles of 90°, 55°, and 20°. The bottom curves represent the difference between the 90° and 20° spectra. The dotted lines correspond to zero.

Figure 7. C K-edge NEXAFS spectra of a SAM prepared from 2 (under the standard conditions) acquired at X-ray incidence angles of 90°, 55°, and 20°. The bottom curve represents the difference between the 90° and 20° spectra. The dotted line corresponds to zero.

of the 1-trans film (Figure 6, left panel) reveal a significant linear dichroism, as, in particular, is highlighted by the respective difference spectrum, whereas the dichroism in the spectra of the 1-cis film is significantly lower. This supports our previous statement that the orientational order in 1-trans SAMs is significantly higher than that in 1-cis films. To demonstrate the importance of the in-chain functional unit in the aliphatic part of the adsorbate species for the overall film quality, SAMs prepared from 2, in which the amide group of 1 is replaced by an ester unit, were prepared and investigated. The C K-edge NEXAFS spectra for 2 adsorbed in trans orientation (i.e., 2-trans films) recorded at X-ray incidence angles of 90°, 55°, and 20° are shown in Figure 7, along with the 90° - 20° difference spectrum. While the spectra exhibit basically the same resonance pattern as 1-trans and 1-cis films, they show almost no linear dichroism, as is highlighted by the difference spectrum. This implies a very low orientational order in the 2-trans film. On the basis of this result, we conclude that lateral hydrogen bonding between the adjacent amide units in the aliphatic chains, similar to what is

where A is a constant and P is the polarization factor of the synchrotron light. The transition dipole moments (TDMs) of the respective π* orbitals are oriented perpendicular to the ring plane and to the azo linkage. The average tilt angles of the π* orbitals related to the phenyl and azo moieties have been determined to be 64.2° ( 3° and 61.2° ( 3°, respectively. The close values for these moieties suggest that the phenyl rings are almost coplanar in the SAM and that the orientation of the π*-orbitals in the azo unit are parallel to those of the phenyl rings. Note that the above coplanar arrangement agrees quite well with the molecular conformation of 1-trans in the respective 3D crystals as was determined by X-ray diffraction (see section 3.1). On the basis of the above angles for the π* orbitals, the average molecular tilt angle in the SAM can be estimated according to a general formalism.41 For this analysis, not only the tilt but also the twist of the molecules must be taken into account.44 Assumptions about the twist angle are not straightforward to make because small changes to the spacer composition reportedly can make a significant impact on the twist angle of azobenzene units in SAMs and, e.g., hexagonal (ca. 30° twist)9c as well as oblique (ca. 45° twist) 2D structures9f have been observed. For our analysis, we assumed a herringbone molecular packing in the 1-trans SAMs, similar to the crystal structure of the bulk compound (v.s.). Mimicking the crystal structure of 1-trans, we assumed a twist angle of 34.3°, which is close to the values observed for 3D crystals of other aromatic compounds45 and other aromatic SAMs.46 The resulting average tilt angle of the azobenzene moiety is 31.6°. The accuracy of this value is about (3°, which is not a result of a statistical analysis but just the general accuracy of the NEXAFS experiment and the data evaluation procedure (including systematic errors). Note that the above average tilt angle for 1-trans SAMs is remarkably close to that of biphenyl-substituted alkanethiols47 and alkaneselenols48 on gold, with an odd number of methylene units in the alkyl chain (like in 1-trans). 3.5. Real-Time Ellipsometry. The photoreactivity of the 1-trans and 1-cis SAMs was investigated by real-time ellip(44) (a) Shaporenko, A.; Adlkofer, K.; Johansson, L. S. O.; Tanaka, M.; Zharnikov, M. Langmuir 2003, 19, 4992. (b) Zharnikov, M.; Ku¨ller, A.; Shaporenko, A.; Schmidt, E.; Eck, W. Langmuir 2003, 19, 4682. (45) (a) Cruickshank, D. W. J. Acta Crystallogr. 1956, 9, 915. (b) Trotter, J. Acta Crystallogr. 1961, 14, 1135. (c) Kitaigorodskii, I. A. Organic Chemical Crystallography; Consultants Bureau: New York, 1961. (d) Charbonneau, G.-P.; Delugeard, Y. Acta Crystallogr. 1976, B32, 1420. (46) Ballav, N.; Schu¨pbach, B.; Dethloff, O.; Feulner, P.; Terfort, A.; Zharnikov, M. J. Am. Chem. Soc. 2007, 129, 15416. (47) (a) Zharnikov, M.; Frey, S.; Rong, H.; Yang, Y.-J.; Heister, K.; Buck, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 3359. (b) Rong, H.-T.; Frey, S.; Yang, Y.-J.; Zharnikov, M.; Buck, M.; Wu¨hn, M.; Wo¨ll, C.; Helmchen, G. Langmuir 2001, 17, 1582. (48) Shaporenko, A.; Mu¨ller, J.; Weidner, T.; Terfort, A.; Zharnikov, M. J. Am. Chem. Soc. 2007, 129, 2232.

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sometry. Figures 8 and 9 display the real-time photoresponse of the 1-trans and 1-cis films, respectively, measured with ellipsometry in air; the same vertical scaling was used for both figures. In particular, the phase shift ∆ of the reflected light was recorded during alternating irradiation with UV and visible light. As known, the phase shift ∆ in ellipsometry is related to the ellipsometric parameter F by49

F ) Rs ⁄ Rp tan Ψ exp(i∆)

(2)

where Rp and Rs are the effective complex reflection coefficients of the interface for p- and s-polarized light, respectively; and tan Ψ is the absolute value of the ratio of Rp and Rs. Generally, the value of ∆ shows a complicated dependence on the optical thickness which, in its turn, depends on such factors like the packing density, film thickness, its exact structure, and film morphology.49 So, even though the photoisomerization in 1-trans and 1-cis films does not involve any changes in the effective packing density, there are significant structural changes, which should be accompanied by the respective changes of ∆. Thus, the behavior of this parameter can be used to monitor the photoresponse of the azobenzenecontaining films. In other words, in our experiment, ellipsometry can basically provide a measure of the trans-cis photoisomerization. As shown in Figure 9, ∆ increases abruptly upon irradiation of the 1-cis SAMs with UV light and achieves a plateau. Further, the phase shift decreases back to the baseline level after irradiation of the film with visible light. This behavior can be consecutively reproduced, as shown in Figure 9, which means that the isomerization reaction within the 1-cis films can be repeated many times with only a minimal loss of photoresponse. Note that

Figure 8. Real-time photoresponse of a 1-trans film monitored by ellipsometry; the variation of the phase shift ∆ is shown. UV and visible light irradiation was started in turns several times during the time frame shown in the figure.

Figure 9. Real-time photoresponse of a 1-cis film monitored by ellipsometry; the variation of the phase shift ∆ is shown. Start of UV and visible light irradiation is also shown.

the general linear increase of ∆ overimposed onto the lightinduced step-like behavior was most likely related to a drift of the background signal; a similar drift was also observed for the 1-trans SAMs (Figure 8). On the basis of the above data, we conclude that the 1-cis SAMs exhibit reversible photoisomerization capabilities. In contrast to that, 1-trans SAMs did not show noticeable photoreactivity (Figure 8).

4. Discussion The XPS, NEXAFS, and dynamic ellipsometry results give a consistent picture of the chemical identity, structure, and photoreactivity of 1-trans and 1-cis SAMs, as well as of the interplay between the latter two parameters. All experimental data suggest that 1 forms well-defined and contamination-free SAMs on Au(111) for both trans and cis isomers. The molecules are attached to the substrate via thiolate anchors, with both S atoms of the former dithiolane group being simultaneously bound. This “double anchor” bond is formed after cleavage of the S-S bond in the dithiolane unit, which occurs via an oxidative addition mechanism. This kind of attachment is in agreement with results previously reported for dithiolane-based SAMs on gold.19d The film structure was found to be strongly dependent on the form (trans or cis) of the molecular adsorbates. According to the NEXAFS data, the films fabricated from the precursors in the trans form are characterized by the upright-oriented and well-ordered azobenzene moieties, while the analogous films prepared from the cis isomers turned out to be mostly disordered. This finding is further supported by the film thickness values derived on the basis of the XPS datasmuch smaller film thickness was found for the 1-cis films as compared to 1-trans ones. Significantly, according to the XPS data, this thickness difference is not somehow related to any degradation of the 1-cis molecules during the film preparation, since there was no evidence for UV-induced damage or decomposition in the respective SAMs. On the basis of the above results, we conclude that the sterically more demanding 1-cis species takes up more free volume at the interface and, in such a way, reduces the molecular density of the film. Upon thermodynamic relaxation into the trans form, which presumably occurs after the 1-cis film is taken out of the solution, the available packing density is too low to form a well-aligned SAM structure, so that both the aliphatic ”feet” and azobenzene units become disordered. Whether these films are initially ordered before they relax into the trans form is not clear at the moment. Note that our findings agree well with an electrochemical study performed by Kondo et al.,18 who found that the charge transfer rate in ferrocene-containing azobenzene-terminated SAMs was increased for films assembled from molecules in the cis form. They assumed that the lower packing density in those films made the surface more accessible for the charge carriers. Comparison of 1-trans SAMs to the films prepared from 2, where the amide in the spacer unit is replaced by an ester moiety and no hydrogen bonding occurs, is also very insightful in this context. According to the NEXAFS data, the latter films did not show significant ordering. On the basis of this finding, we can conclude that the additional hydrogen bonding mediated by the amide groups in the adjacent chains represents an important factor for the assembly of well-ordered films of 1-trans. However, this promoting effect presumably does not work in the case of 1-cis films because of the larger average distance between the (49) Azzam, R. M. A. Ellipsometry and polarized light; North-Holland: Amsterdam, 1987.

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Figure 10. A schematic drawing illustrating the correlation between the structure of 1-trans (top) and 1-cis (bottom) SAMs and their photoimerization behavior. The available free volume seems to be the decisive parameter.

neighboring molecules. The results of crystallization experiments performed with 1-trans, 1-cis, and 2 are in agreement with this hypothesis. While 1-trans readily formed bulk crystal structures via hydrogen bonds between adjacent molecules, this was not the case for 1-cis and 2, indicating, in the case of 2, that a lack of hydrogen bonding hinders the formation of ordered structures even in the bulk state, in which the molecules have far more degrees of freedom for arrangement and packing as compared to the 2D phase. Molecule 1 showed reversible photoswitching in solution. After arranging in the SAMs, a pronounced correlation between the film stucture and photoreactivity was found as shown schematically in Figure 10. The dynamic ellipsometry measurements demonstrated that 1-cis films were reversibly photoresponsive. At the same time, 1-trans SAMs did not show any evidence of photoreactivity. This behavior can be directly correlated with the very different packing densities and orientational order observed for the respective SAMs: 1-trans films are densely packed and well-ordered, whereas 1-cis films are loosely packed and have a low orientational order. The photoisomerization reaction seems to be sterically hindered by the comparably small available free volume per molecule in the densely packed 1-trans SAMs. Note that the observed correlation between the available free volume and photoresponse is in accordance with the results by Tamada et al.14 and Ito et al.,15 who claim that dense packing limits photoresponse of azobenzene-containing SAMs with

aliphatic spacers. In purely aromatic SAMs, however, significant orientational order in combination with good photoreactivity has been observed for both trans and cis adsorbed azobenzenecontaining SAMs.7c,10 This effect was explained by cooperative switching of whole domains, a process that entails less free volume and even requires densely packed and highly ordered SAM structures. So far, it is not clear whether such cooperative switching can be achieved in densely packed aliphatic SAMs as well.

5. Conclusions In summary, we have used the 4-(phenyldiazenyl)phenyl4-(1,2-dithiolane-3yl)-butylcarboxamide (1) precursor to prepare well-defined aliphatic SAMs bearing the photoactive azobenzene tailgroups in either trans or cis orientation and shown that the photoresponse of these SAMs is correlated with their packing density and orientational order. Whereas the films fabricated from the molecules in the trans form were found to be densely packed and highly ordered, which was additionally mediated by in-chain amide moieties, loosely packed and mostly disordered films were obtained from the molecules adsorbed in the cis state. The former films did not show any evidence for photoisomerization, whereas the less densely packed 1-cis films underwent reversible trans-cis isomerization upon consecutive irradiation with UV and visible light. Since the chemical composition of the films adsorbed from the cis and trans isomers was identical, we conclude that

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the packing density and molecular alignment within the film played the deciding role for the photoresponse capability of the film. The molecules adsorbed in the cis orientation take up more free volume at the interface during the film formation, leaving enough free space for the photoswitching in the film. Acknowledgment. T.W., N.B., and M.Z. thank M. Grunze for the support of this work, Ch. Wo¨ll and A. Nefedov for technical

Weidner et al.

cooperation at BESSY II, and the BESSY II staff for the assistance during the NEXAFS experiments. We are grateful to F. Vogel and F. Tra¨ger for providing the experimental setup for the UV-visible absorption spectroscopy measurements. This work has been supported by the German BMBF (05KS4VHA/4) and DFG (ZH 63/9-2). LA802454W