Self-Assembled Layers Based on Isomerizable Stilbene and

High-resolution spectra were fitted using Avantage (Thermo VG software package) peak-fitting algorithms. All spectra have been normalized to the Au 4f...
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Langmuir 2008, 24, 2479-2486

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Self-Assembled Layers Based on Isomerizable Stilbene and Diketoarylhydrazone Moieties Jan Marten,† Andreas Erbe,‡,§ Kevin Critchley,‡,| Jonathan P. Bramble,‡ Edwin Weber,*,† and Stephen D. Evans*,‡ Technische UniVersita¨t Bergakademie Freiberg, Institut fu¨r Organische Chemie, Leipziger Strasse 29, 09596 Freiberg, Germany, and School of Physics and Astronomy, UniVersity of Leeds, Leeds LS2 9JT, United Kingdom ReceiVed October 8, 2007. In Final Form: December 7, 2007 The ability to form self-assembled layers on gold (Au) using five organosulfur compounds that contain isomerizable groups has been investigated. The isomerizable groups are either stilbene or diketoarylhydrazone derivatives. To anchor them on a gold surface, the isomerizable groups have been combined with sulfur-containing groups (disulfide, 1,2-dithiolane, and thiophene). The resulting thin films assembled on gold were characterized by X-ray photoelectron spectroscopy (XPS), infrared (FTIR) reflectance spectroscopy, ellipsometry, and water contact angle measurements. Though all substances have the potential to form self-assembled monolayers (SAMs), only two of them, disulfanediylbis(ethane-2,1-diyl) bis(4-styrylbenzoate) (1) and 4-[(2,4-dioxo-3-pentylidene)diazane-2,2,1-triyl]phenyl thioctate (4), yield the expected structure, the latter one showing the possibility to incorporate diarylketohydrazone moieties into SAMs. The compound 4-[(2,4-dioxo-3-pentylidene)diazane-2,2,1-triyl]phenyl thiophene-2-carboxylate (5) does not self-assemble on gold, but 4-styrylphenyl thioctate (3) presumably forms multilayers. In the case of disulfanediylbis(ethane-2,1-diyl) bis[4-(p-nitrostyryl)benzoate] (2), we propose a structure with a fraction of the molecules bound to gold via the nitro group. The results show that the propensity of organosulfur compounds to self-assemble on gold not only is determined by the sulfur-containing group but also is affected by the complete molecule.

1. Introduction Over the past two decades, self-assembled monolayers (SAMs) have become a valuable tool with which to influence the properties of surfaces.1,2 Such surfaces with tailor-made properties are promising for application in many fields where surface properties play a crucial role in the function.3-5 In recent years, there has been the desire to incorporate more and more functionality into the layers.6 The trend of preparing SAMs with more complex functions implies the involvement of more complex molecules and consequently more complex intermolecular interactions than for the simple functionalized hydrocarbon chains used in early work. As a result, layers with reactive or cleavable groups have been prepared and used to pattern surface properties.7,8 Layers of molecules that can reversibly switch between two states with different properties have been produced and characterized.4 As a macroscopic property, the wettability of a SAM could be switched by the potential applied to the electrode9 or by changing the pH of a solution.10 Experimental work has also * To whom correspondence should be addressed. E-mail: edwin.weber@ chemie.tu-freiberg.de, [email protected]. † TU Bergakademie Freiberg. ‡ University of Leeds. § Current address: Academia Sinica, Institute of Physics, 128 Academia Road, Section 2, Nangang, Taipei 11529, Taiwan. | Current address: Department of Chemical Engineering, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109. (1) Dubois, L. H.; Nuzzo, R. G. Annu. ReV. Phys. Chem. 1992, 43, 437-463. (2) Ulman, A. Chem. ReV. 1996, 96, 1533-1554. (3) Whitesides, G. M. Chem. ReV. 2005, 105, 1171-1196. (4) Liu, Y.; Mu, L.; Liu, B.; Kong, J. Chem.sEur. J. 2005, 11, 2622-2631. (5) Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. AdV. Mater. 2000, 12, 1315-1328. (6) Crivillers, N.; Mas-Torrent, M.; Perruchas, S.; Roques, N.; Vidal-Gancedo, J.; Veciana, J.; Rovora, C.; Basabe-Desmonts, L.; Ravoo, B. J.; Crego-Calama, M.; Reinhoudt, D. N. Angew. Chem. 2007, 119, 2265-2269. (7) Ofir, Y.; Zenou, N.; Goykhman, I.; Yitzchaik, S. J. Phys. Chem. B 2006, 110, 8002-8009. (8) Critchley, K.; Zhang, L.; Fukushima, H.; Ishida, M.; Shimoda, T.; Bushby, R. J.; Evans, S. D. J. Phys. Chem. B 2006, 110, 17167-17174.

inspired simulation studies of conformational changes in SAMs.11 Many of these macroscopic changes are caused by changes in the structure of molecules constituting the SAM. An important way to trigger these changes is by illumination with light of a suitable wavelength.12,13 Such irradiation may trigger reversible photochemical intramolecular transformations.14,15 However, substances that exhibit a reversible conformational or configurational change are more prominent. Azobenzenes and stilbenes are one important class of such substances, which are well known for their light-induced E-Z isomerization.16 Azobenzenes have been attached as thiolates to Au surfaces,17-19 and Au particles,20 or as silanes to silicon surfaces.21,22 In multilayers and mixed (9) Lahann, J.; Mitragotri, S.; Tran, T.-N.; Kaido, H.; Sundaram, H.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (10) Jiang, Y.; Wang, Z.; Xu, H.; Chen, H.; Zhang, X.; Smet, M.; Dehaen, W.; Hirano, Y.; Ozaki, Y. Langmuir 2006, 22, 3715-3720. (11) Vemparala, S.; Kalia, R. K.; Nakano, A.; Vashishta, P. J. Chem. Phys. 2004, 121, 5427-5433. (12) Klessinger, M.; Michl, J. Lichtabsorption und Photochemie Organischer Moleku¨le; VCH: Weinheim, Germany, 1991. (13) Becker, H. G. O. Einfu¨hrung in die Photochemie; Deutscher Verlag der Wissenschaften: Berlin, 1991. (14) Kudernac, T.; van der Molen, S. J.; van Wees, B. J.; Feringa, B. L. Chem. Commun. 2006, 3597-3599. (15) Zareie, M. H.; Barber, J.; McDonagh, A. M. J. Phys. Chem. B 2006, 110, 15951-15954. (16) Du¨rr, H.; Bouas-Laurent, H., Eds.; Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2006. (17) Muzikante, I.; Gerca, L.; Fonavs, E.; Rutkis, M.; Gustina, D.; Markava, E.; Stiller, B.; Brehmer, L.; Knochenhauer, G. Mater. Sci. Eng. C 2002, 22, 339-343. (18) Stiller, B.; Knochenhauer, G.; Markava, E.; Gustina, D.; Muzikante, I.; Karageorgiev, P.; Brehmer, L. Mater. Sci. Eng. C 1999, 8-9, 385-389. (19) Evans, S. D.; Johnson, S. R.; Ringsdorf, H.; Williams, L. M.; Wolf, H. Langmuir 1998, 14, 6436-6440. (20) Zhang, J.; Whitesell, J. K.; Fox, M. A. Chem. Mater. 2001, 13, 23232331. (21) Wen, Y.; Yi, W.; Meng, L.; Feng, M.; Jiang, G.; Yuan, W.; Zhang, Y.; Gao, H.; Jiang, L.; Song, Y. J. Phys. Chem. B 2005, 109, 14465-14468. (22) Hamelmann, F.; Heinzmann, U.; Siemeling, U.; Bretthauer, F.; vor der Bru¨ggen, J. Appl. Surf. Sci. 2004, 222, 1-5.

10.1021/la703109x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/08/2008

2480 Langmuir, Vol. 24, No. 6, 2008 Scheme 1. Molecular Structures of the Bifunctional Stilbene Compounds

SAMs, the E-Z isomerization has been observed, but because of the steric hindrance in pure SAMs, no switching could be observed.19 To overcome the problem of steric hindrance that prevents the molecules from changing their configuration, lateral spacers have been introduced.21 A problem intrinsic to azobenzenes is the lack of stability of the Z configuration, which will revert to the E form after some time.16 Stilbenes can be used to overcome that problem. Though their absorption is at a lower wavelength, this can be modified with a suitable substituent. Stilbene-containing SAMs have been prepared on Au surfaces, but switching their configuration was possible only in multilayer thin films.23,24 On Au particles, the isomerization has been found to depend on the particle size.25 The examples mentioned so far incorporate one function into a SAM. There are, however, molecules, that can react to several different environmental effects. Diketoarylhydrazones are one example because they exhibit a pH-dependent structure,26 can complex metal ions,27 can be involved in hydrogen bonding to neighboring molecules, and may also react to light.28,29 To the best of our knowledge, this class of multifunctional molecules has not yet been incorporated into SAMs. In this work, two classes of compounds (Schemes 1 and 2) have been studied regarding their ability to form self-assembled layers on Au. One class is based on the stilbene motif (1-3), and the other class is based on the diketoarylhydrazone moiety (4, 5). The resulting adsorbed layers have been characterized by X-ray photoelectron spectroscopy (XPS), ellipsometry, reflectance infrared (IR) spectroscopy, and water contact angle measurements. The results show that with more complex interactions within the layers adsorbed to Au the resulting structures may also become more complex. 2. Experimental Section 2.1. Synthesis. 2.1.1. Instrumentation and Materials. Melting points were determined with a hot-stage microscope (VEB Dresden Analytik) and are uncorrected. 1H and 13C NMR spectra were recorded using a Bruker Avance DPX 400 (400 MHz) instrument. The chemical (23) Wolf, M. O.; Fox, M. A. Langmuir 1996, 12, 955-962. (24) Sudesh Kumar, G.; Neckes, D. C. Chem. ReV. 1989, 89, 1915-1925. (25) Zhang, J.; Whitesell, J. K.; Fox, M. A. J. Phys. Chem. B 2003, 107, 6051-6055. (26) Barchiesi, E.; Bradamante, S.; Carfagna, C.; Farraccioli, R. J. Chem. Soc., Perkin Trans. 2 1988, 8, 1565-1572. (27) Marten, J.; Seichter, W.; Weber, E. Z. Anorg. Allg. Chem. 2005, 631, 869-877. (28) McVie, J.; Alastair, D.; Sinclair, R. S.; Truscott, T. G. J. Chem. Soc., Perkin Trans. 1980, 2, 286-290. (29) Courtot, P.; Pichon, R.; Le Saint, J. J. Chem. Soc., Perkin Trans. 1981, 22, 19.

Marten et al. Scheme 2. Molecular Structures of the Monofunctional Stilbene and Diketoarylhydrazone Compounds

Scheme 3. Components of the Condensation Reactions

shifts (δ) are reported as ppm relative to SiMe4. Mass spectra were recorded with an EI-MS (Finnigan MAT 8200) or an ESI-MS (Bruker Daltonik, ESQUIRE-LC ion trap). The course of the reactions was monitored using TLC (Merck silica gel 60-F256-coated plates). Column chromatography was performed with silica gel 60 (0.0640.100 mm). Reagents and chemicals, including 10-12 (Scheme 3), were obtained from commercial sources (Fluka and Merck). The solvents used were purified or dried according to literature procedures.30 4-Styrylbenzoic acid (6), 4-(p-nitrostyryl)benzoic acid (7), 4-styrylphenol (8), and 4-[(2,4-dioxo-3-pentylidene)diazane-2,2,1triyl]phenol (9) (Scheme 3) were prepared following published procedures.31-34 (30) Leonard, J.; Lugo, B.; Procter, G. Praxis der Organischen Chemie; VCH: Weinheim, Germany, 1994. (31) Bell, F.; Waring, D. H. J. Chem. Soc. 1948, 1024-1026. (32) Carella, A.; Castaldo, A.; Centore, R.; Fort, A.; Sirigu, A.; Tuzi, A. J. Chem. Soc,. Perkin Trans. 2002, 2, 1791-1795.

Self-Assembled Layers 2.1.2. Compounds 1 and 2: General Procedure. N,N′-Dicyclohexylcarbodiimide (DCC) was added to a stirred solution containing the respective stilbenecarboxylic acid, 2,2′-(disulfanediyl)diethanol, and 4-(dimethylamino)pyridine (DMAP) in dichloromethane at 0 °C. The mixture was stirred for 1 day at this temperature, allowed to warm to room temperature, and filtered. The filtrate was washed with water, separated, and dried (MgSO4). Evaporation of the solvent under reduced pressure and column chromatography on silica gel (CH2Cl2 eluent) yielded the pure products. Specific details for each compound are given below. 2.1.2.1. Disulfanediyl-bis(ethane-2,1-diyl) Bis(4-styrylbenzoate) (1). DCC (1.13 g, 6.93 mmol), 4-styrylbenzoic acid (6) (1.41 g, 6.3 mmol), 2,2′-(disulfanediyl)diethanol (10) (486 mg, 3.15 mmol), and DMAP (78 mg, 6.3 mmol) in 10 mL of dichloromethane were used to yield the product as white needles (1.2 g, 67%). 1H NMR (400 MHz, CDCl3) δ: 3.10 (s, 4H, CH2S), 4.60 (s, 4H, CH2O), 7.11 (m, 2H, HCdC), 7.17 (m, 2H, CdCH), 7.26 (m, 4H, Ar-H), 7.31 (m, 3J 3 H-H ) 7.2 Hz, 4H, Ar-H), 7.37 (m, JH-H ) 7.4 Hz, 2H, Ar-H), 7.53 (s, 3JH-H ) 8.6 Hz, 4H, Ar-H), 8.00 (m, 3JH-H ) 8.4 Hz, 4H, Ar-H). 13C NMR (100.6 MHz, CDCl3) δ: 37.54 (CH2S), 62.77 (CH2O), 126.36 (Ar), 126.82 (Ar), 127.52 (CdC), 128.28 (CdC), 128.6 (C-COO), 128.79 (Ar), 130.15 (Ar), 131.38 (Ar), 136.72 (C-CdC), 142.06 (CdC-C), 166.09 (COO). MS (EI) 566.9 ([M + H]+ , 2.5), 343.1 (66), 225.4 (100). 2.1.2.2. Disulfanediyl-bis(ethane-2,1-diyl) Bis[4-(p-nitrostyryl)benzoate] (2). DCC (1.13 g, 6.93 mmol), 4-(p-nitrostyryl)benzoic acid (7) (1.7 g, 6.3 mmol), 2,2′-(disulfanediyl)diethanol (10) (486 mg, 3.15 mmol), and DMAP (78 mg, 6.3 mmol) in 10 mL of dichloromethane were used to yield the product as a yellow powder (1.3 g, 63%). 1H NMR (400 MHz, CDCl3) δ: 3.18 (s, 4H, CH2S), 4.53 (s, 4H, CH2O), 7.47 (m, 2H, HCdC), 7.48 (m, 2H, CdCH), 7.72 (m, 3JH-H ) 8.4 Hz, 4H, Ar-H), 7.81 (m, 3JH-H ) 8.8 Hz, 4H, Ar-H), 7.94 (m, 3JH-H ) 8.0 Hz, 4H, Ar-H), 8.17 (s, 3JH-H ) 8.4 Hz, 4H, Ar-H). 13C NMR (100.6 MHz, CDCl3) δ: 37.13 (CH2S), 63.00 (CH2O), 124.29 (Ar), 127.55 (Ar), 127.99 (Ar), 129.27 (CdC), 129.39 (CdC), 130.03 (Ar), 132.14 (C-COO), 141.38 (C-CdC), 143.66 (CdC-C), 146.86 (C-NO2), 165.53 (COO). MS (EI) 656.13 ([M]+, 0.5), 296.2 (100), 269.1 (26). 2.1.3. Compounds 3-5: General Procedure. N,N′-Dicyclohexylcarbodiimide (DCC) was added to a stirred solution of the respective phenol and corresponding carboxylic acid in dry THF. The mixture was heated to reflux for 12 h, allowed to cool to room temperature, and filtered. Evaporation of the solvent under reduced pressure and column chromatography (1:1 CH2Cl2/n-hexane eluent) yielded the pure products. Specific details for each compound are given below. 2.1.3.1. 4-Styrylphenyl Thioctate (3). DCC (1.65 g, 8 mmol), thioctic acid (2.65 g, 8 mmol), and 4-styrylphenol (8) (0.78 g, 4 mmol) in 20 mL of dry THF were used to yield the product (0.8 g, 52%) as a white powder. 1H NMR (400 MHz, CDCl3) δ: 1.55 (s, 2H, CH2), 1.71 (s, 2H, CH2), 1.88 (s, 2H, CH2), 1.43 (s, 2H, CH2), 2.55 (s, 2H, CH2), 3.12 (s, 1H, CH-S), 3.57 (s, 2H, CH2-S), 7.04 (m, 2H, HCdCH), 7.05 (m, 2H, Ar-H), 7.24 (m, 3JH-H ) 7.4 Hz, 1H, Ar-H), 7.33 (m, 3JH-H ) 7.2 Hz, 2H, Ar-H), 7.47 (m, 2H, Ar-H), 7.50 (m, 2H, Ar-H). 13C NMR (100.6 MHz, CDCl3) δ: 24.61 (CH2), 28.66 (CH2), 34.12 (CH2-COO), 34.56 (CH2-CH), 38.48 (CH2-S), 40.20 (CH2-CH), 56.29 (CH), 121.74 (Ar), 126.49 (Ar), 127.38 (Ar), 127.64 (CdC), 127.67 (CdC), 128.66 (Ar), 128.89 (C-CdC), 135.04 (Ar), 137.15 (Ar), 150.09 (C-O), 171.79 (COO). MS (ESI) 384.9 ([M + H]+, 50). 2.1.3.2. 4-[(2,4-Dioxo-3-pentylidene)diazane-2,2,1-triyl]phenyl Thioctate (4). DCC (4.86 g, 22.7 mmol), thioctic acid (4.68 g, 22.7 mmol), and 4-[(2,4-dioxo-3-pentylidene)diazane-2,2,1-triyl]phenol (2.5 g, 11.36 mmol) in 60 mL of dry THF were used to yield the product as a yellow solid (3.2 g, 69%). 1H NMR (400 MHz, CDCl3) δ: 1.58 (m, 2H, CH2), 1.79 (m, 2H, CH2CH), 1.95 (m, 2H, CH2), 2.49 (s, 3H, CH3), 2.58 (m, 2H, CHCH2), 2.60 (m, 2H, CH2COO), 2.61 (s, 3H, CH3), 3.15 (m, 1H, CH-S), 3.61 (m, 2H, CH2-S), 7.13 (33) Friedrich, K.; Henning, H. G. Chem. Ber. 1959, 92, 2944-2952. (34) Mohan, M.; Arora, C. P.; Gupta, H. K.; Jha, N. K. Acta Pharm. Jugosl. 1986, 36, 37-45.

Langmuir, Vol. 24, No. 6, 2008 2481 (m, 3JH-H ) 9.2 Hz, 2H, Ar-H), 7.41 (m, 3JH-H ) 8.8 Hz, 2H, Ar-H), 14.75 (m, 1H, NH). 13C NMR (100.6 MHz, CDCl3) δ: 24.62 (CH2), 26.36 (CH3), 28.70 (CH2), 31.61 (CH2), 34.10 (CH2-COO), 34.60 (CH2-CH), 38.53 (CH2-S), 40.26 (CH2-CH), 56.31 (CH), 117.07 (Ar), 122.86 (Ar), 133.39 (C-N), 139.22 (C-O), 148.38 (CdN), 171.84 (COO), 196.93 (CdO), 197.98 (CdO). MS (ESI) 409.1 ([M + H]+, 100). 2.1.3.3. 4-[(2,4-Dioxo-3-pentylidene)diazane-2,2,1-triyl]phenyl Thiophene-2-carboxylate (5). DCC (4.7 g, 28.8 mmol), thiophene2-carboxylic acid (3.3 g, 22.7 mmol), and 4-[(2,4-dioxo-3-pentylidene)diazane-2,2,1-triyl]phenol (2.5 g, 11.36 mmol) in 60 mL of dry THF were used to yield the product as yellow needles (2.7 g, 72%). 1H NMR (400 MHz, CDCl3) δ: 2.50 (s, 3H, CH3), 2.62 (s, 3H, CH3), 7.20 (m, 3JH-H ) 4.0 Hz, 1H, thiophene), 7.28 (m, 3JH-H ) 8.8 Hz, 2H, Ar-H), 7.45 (m, 3JH-H ) 8.8 Hz, 2H, Ar-H), 14.81 (m, 1H, NH). 13C NMR (100.6 MHz, CDCl3) δ: 26.50 (CH3), 31.51 (CH3), 116.97 (Ar), 122.86 (Ar), 128.01 (CH-CH-S), 132.41 (CH-C-S), 133.28 (CH-S), 133.67 (CdN), 134.77 (S-C-COO), 139.25 (C-O), 148.11 (C-N), 160.35 (COO), 196.82 (CdO), 197.88 (CdO). MS (ESI) 331.0 ([M + H]+, 100). 2.2. Preparation of Self-Assembled Layers. Glass slides were cleaned in a mixture of 96% H2SO4 and 30% H2O2 (70:30 v/v) for 10 min. The slides were then carefully rinsed with deionized water and dried in a stream of nitrogen. An Auto 306 thermal evaporator (BOC Edwards, Crawley, U.K.) was used to deposit a 5 nm Cr adhesion layer onto the glass substrate, onto which an ∼150 nm Au (99.99%; Goodfellow, Huntington, U.K.) layer was evaporated. Evaporation was done at with a deposition rate of 0.1 nm s-1 at 10 cm - 1) for the ester CdO stretching mode. This may indicate some involvement of the carbonyl groups in the formation of the multilayers.

4. Conclusions The results of the studies of substances 1-5 on Au surfaces are summarized in Table 4. Though all components have been designed to form classic self-assembled monolayers when interacting with gold, only 1 and 4 do. Whereas compound 5 is another example of a thiophene derivative that does not adsorb to Au surfaces, 2 forms monolayers where fractions of the molecules are bound to the Au surface via the nitro group, and 3 presumably forms multilayers via a mechanism that is not clear. Comparing the sulfur-containing moieties, the influence on the self-assembly behavior of the other parts of the molecule becomes obvious. Compounds 1 and 2 are both disulfides, the only difference between them being the nitro group in 2. Nevertheless, 1 forms “simple” SAMs whereas for 2 there is evidence of the formation of a structure as depicted in Figure 4b. Here, the introduction of one functional group changes the structure of the obtained assembly on gold. However, comparing 3 and 4, both 1,2-dithiolanes, shows that this is not a general trend. Compound 4, which has more possibilities to be involved in intra- and intermolecular interactions than 3, forms a SAM, whereas 3 very likely forms multilayers. Furthermore, this work shows the possibility of forming multifunctional SAMs with a diketoarylhydazone moiety that (72) Kim, K.; Jeon, W. S.; Kang, J.-K.; Lee, J. W.; Jon, S. Y.; Kim, T.; Kim, K. Angew. Chem., Int. Ed. 2003, 42, 2293-2296.

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has the potential to switch under different external triggers. The possibilities of this class of compounds in SAMs still needs to be explored. Acknowledgment. We thank Hans G. Bo¨rner for facilitating this collaboration. The U.K. side of this research was supported

Marten et al.

by the BBSRC (24/JF/19090) and EPSRC (grant GR/S59826/ 01). Financial support (FOR 335) by the Deutsche Forschungsgemeinschaft (DFG) is gratefully acknowledged. In addition, A.E. thanks Professor Chiafu Chou for his support. LA703109X