Photoisomerization of Azobenzene Chromophores in Organic

Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 ... that arise from cis isomers experiencing different degrees of stra...
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Langmuir 2000, 16, 7449-7456

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Photoisomerization of Azobenzene Chromophores in Organic/Inorganic Zirconium Phosphonate Thin Films Prepared Using a Combined Langmuir-Blodgett and Self-Assembled Monolayer Deposition Aiping Wu† and Daniel R. Talham* Department of Chemistry, University of Florida, Gainesville, Florida 32611-7200 Received March 16, 2000. In Final Form: June 21, 2000 The photoinduced trans to cis isomerization of two new azobenzene-derivatized phosphonic acids, 4-(4′(3′′-pentyloxy)phenyl)azophenylbutylphosphonic acid (2a) and 4-(4′-propoxyphenyl)azophenylbutylphosphonic acid (2b), has been studied in zirconium phosphonate thin films. Multilayer films were assembled using a three-step combined Langmuir-Blodgett (LB) and monolayer self-assembly procedure that forms a layered metal phosphonate lattice in the polar region of asymmetric bilayers which are composed of octadecylphosphonate and the azobenzene chromophore. Multilayer films containing 2a and 2b undergo rapid and reversible photoisomerization. For both molecules, thermal relaxation of the cis form shows both fast- and slow-relaxing components that arise from cis isomers experiencing different degrees of strain in the films. The relaxation rates and the ratio of fast/slow relaxing components can be changed through synthetic modification of the chromophore and by altering the thin-film architecture. When the cis form of 2a is deposited directly, virtually all of the fast-relaxing component is eliminated, and the thermal half-life for cis to trans isomerization is greater than 11 h.

Introduction Photoisomerization of azobenzene chromophores is a well-known phenomenon that involves the reversible change of conformation between the more stable trans isomer and its less stable cis isomer upon light irradiation (Scheme 1).1-3 This isomerization results not only in a change of geometry and dipole moment but also in a change in its optical properties. As a result, azobenzene-derivatized materials have attracted significant interest due to potential applications in areas that include optical information storage, photochemical switching, nonlinear optics, and liquid crystal alignment.4-12 Many of the possible uses of azobenzene chromophores are best manifested in thin films, and several attractive approaches to multilayer thin films having a controllable thickness and molecular-level organization have recently * To whom correspondence should be addressed. † Current address: Nanovation Technologies, Inc., 1801 Maple Avenue, Evanston, Illinois 60201. (1) Nuyken, O.; Scherer, C.; Baindl, A.; Brenner, A. R.; Dahn, U.; Gartner, R.; Kaiser-Rohrich, S.; Kollefrath, R.; Matusche, P.; Voit, B. Prog. Polym. Sci. 1997, 22, 93. (2) Kumar, G. S.; Neckers, D. C. Chem. Rev. 1989, 89, 1915. (3) Rau, H. In Photoisomerization of Azobenzenes in Photochemistry and Photophysics; Rabek, J. F., Eds.; CRC Press: Boca Raton, FL, 1990; Vol. II, pp 119-141. (4) Lin, W.; Lin, W.; Wong, G. K.; Marks, T. J. J. Am. Chem. Soc. 1996, 118, 8034. (5) Roberts, G. G., Ed. Langmuir-Blodgett Films; Plenum Press: New York, 1990. (6) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658. (7) Ikeda, T.; Tsutsumi, O. Science 1995, 268, 1873. (8) Tachibana, H.; Nakamura, T.; Matsumoto, M.; Komizu, H.; Manda, E.; Niino, H.; Yabe, A.; Kawabata, Y. J. Am. Chem. Soc. 1989, 111, 3080. (9) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (10) Wang, X.; Balasubramanian, S.; Li, L.; Jiang, X.; Sandman, D. J.; Rubner, M. F.; Kumar, J.; Tripathy, S. K. Macromol. Rapid Commun. 1997, 18, 451. (11) Seki, T.; Sakuragi, M.; Kawanishi, Y.; Suzuki, Y.; Tamaki, T.; Fukuda, R.; Ichimura, K. Langmuir 1993, 9, 211. (12) Ichimura, K.; Hayashi, Y.; Akiyama, H.; Ishizuki, N. Langmuir 1993, 9, 3298.

Scheme 1. Conformational Change of Azobenzene

been developed. These multilayer deposition schemes can be distinguished according to the nature of the driving force for multilayer growth. Traditional LangmuirBlodgett (LB) multilayers are based on alternating van der Waals interactions between the hydrophilic and the hydrophobic regions of suitable amphiphiles.13,14 Several thin-film “self-assembly” procedures have been developed that rely on specific chemical interactions between molecular functional groups and an active surface14-16 that result in spontaneous adsorption from solution. Further modification of the surface, either by surface reaction or by adding a different complementary layer, allows the deposition cycle to be repeated, eventually leading to multilayer assemblies. Examples of interactions used for multilayer self-assembly include silane condensation,16 formation of siloxane polymers,4,17 metal organophosphonate linkages,18-21 and hydrogen bonding.22 Related are (13) Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 57, 964. (14) Ulman, A. Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (15) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (16) Tillman, N.; Ulman, A.; Penner, T. L. Langmuir 1989, 5, 101. (17) Kakkar, A. K.; Yitzchaik, S.; Roscoe, S. B.; Kubota, F.; Allan, D. S.; Marks, T. J.; Lin, W.; Wong, G. K. Langmuir 1993, 9, 388. (18) Cao, G.; Hong, H.; Mallouk, T. E. Acc. Chem. Res. 1992, 25, 420. (19) Katz, H. E.; Scheller, G.; Putvinski, T. M.; Schilling, M. L.; Wilson, W. L.; Chidsey, C. E. D. Science 1991, 254, 1485.

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layer-by-layer depositions based on electrostatic interaction between oppositely charged polyelectrolytes.23 Each of these deposition procedures allows control over issues such as layer thickness and layer composition while providing various degrees of control over the intralayer organization of active functional groups. Aggregation of azobenzene chromophores is commonly observed in well-organized and closely packed LB multilayers, which restricts the free volume available for trans to cis isomerization.24-27 Strategies aimed at increasing free volume to facilitate trans to cis isomerization include modification of the chromophores with bulky substituents28,29 or attaching them in pendant polymers.30-32 Other strategies rely on modification of the LB film structure, such as including the azobenzene in the cavity of amphiphilic cyclodextrins33 or forming complexes with polyions,34-37 which provide more free volume as the size of the polyion increases. Similar strategies have been employed to effect trans to cis isomerization in thin films prepared using various of the self-assembly deposition procedures.38-42 Because the extent of photoisomerization observed in azobenzene-containing thin films depends on the available free volume,33,35,36,41 it is important to find ways to control the molecular organization, orientation, and structure of the photoactive chromophores. In this article, we report on the photoisomerization of azobenzene chromophores in metal phosphonate-based thin films that are prepared using a combination of LB and monolayer self-assembly methods. We have previously described43-47 routes to a new class of Langmuir-Blodgett (20) Katz, H. E. Chem. Mater. 1994, 6, 2227. (21) Katz. H. K.; Bent, S. F.; Wilson, W. L.; Schilling, M. L.; Ungashe, S. B. J. Am. Chem. Soc. 1994, 116, 6631. (22) Maoz, R.; Sagiv, J.; Degenhardt, D.; Mohwald, H.; Quint, P. Supramol. Sci. 1995, 2, 9. (23) Decher, G.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (24) Nakahara, H.; Fukuda, K. J. Colloid Interface Sci. 1983, 93, 530. (25) Kawai, T.; Umemura, J.; Takenaka, T. Langmuir 1989, 5, 1378. (26) Song, X.; Perlstein, J.; Whitten, D. G. J. Am. Chem. Soc. 1997, 119, 9144. (27) Petruska, M. A.; Talham, D. R. Chem. Mater. 1998, 10, 3672. (28) Anzai, J.; Sugaya, N.; Osa, T. J. Chem. Soc., Perkin Trans. 1994, 2, 1897. (29) Moss, R. A.; Jiang, W. Langmuir 1995, 11, 4217. (30) Seki, T.; Ichimura, K. Polym. Commun. 1989, 30, 109. (31) Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1994, 10, 4594. (32) Seki, T.; Ichimura, K. Thin Solid Films 1989, 179, 77. (33) Yabe, A.; Kawabata, Y.; Niino, H.; Tanaka, M.; Ouchi, A.; Takahashi, H.; Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1988, 1. (34) Hall, R. A.; Hara, M.; Knoll, W. Langmuir 1996, 13, 2551. (35) Matsumoto, M.; Miyazaki, D.; Tanaka, M.; Azumi, R.; Manda, E.; Kondo, Y.; Yoshino, N.; Tachibana, H. J. Am. Chem. Soc. 1998, 120, 1479. (36) Tachibana, H.; Azumi, R.; Tanaka, M.; Matsumoto, M.; Sako, S.; Sakai, H.; Abe, M.; Kondo, Y.; Yoshion, N. Thin Solid Films 1996, 284-285, 73. (37) Nishiyama, K.; Kurihara, M.; Fujihira, M. Thin Solid Films 1989, 179, 477. (38) Ichimura, K.; Suzuki, Y.; Seki, T.; Hosoki, A.; Aoki, K. Langmuir 1988, 4, 1214. (39) Sekkat, Z.; Wood, J.; Greets, Y.; Knoll, W. Langmuir 1995, 11, 2856. (40) 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. (41) Saremi, F.; Tieke, B. Adv. Mater. 1998 10, 388. (42) Dante, S.; Advincula, R.; Frank, C. W.; Stroeve, P. Langmuir 1999, 15, 193. (43) Byrd, H.; Pike, J. K.; Talham, D. R. Chem. Mater. 1993, 5, 709. (44) Byrd, H.; Whipps, S.; Pike, J. K.; Ma, J.; Nagler, S. E.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 295. (45) Byrd, H.; Pike, J. K.; Talham, D. R. J. Am. Chem. Soc. 1994, 116, 7903. (46) (a) Seip, C. T.; Byrd, H.; Talham, D. R. Inorg. Chem. 1996, 35, 3479. (b) Seip, C. T.; Granroth, G. E.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 1997, 119, 7084.

Wu and Talham Scheme 2

film in which the polar regions of the LB bilayers form a metal phosphonate continuous lattice, which is analogous to structures found in solid-state chemistry. Similar layered assemblies can be prepared by using an LB monolayer to form an active surface layer (the template layer) at which a second layer (the capping layer) can be self-assembled by the adsorption of organophosphonate molecules from solution.44 Multilayers are prepared by applying another LB template layer and repeating the cycle. This three-step deposition procedure (Scheme 2) is quite versatile. It allows preparation of well-defined layers of molecules that could not normally be deposited in LB films and is an easy way to prepare asymmetric assemblies. The inorganic network in both types of film makes them extremely stable.43,44,47 Metal phosphonate LB films are insoluble under most organic or aqueous conditions. Also, the continuous-lattice network prohibits the organic amphiphiles from spontaneously rearranging, as can happen in traditional LB films. Furthermore, the inorganic network can be used to add typically solid-state functions, such as magnetism, to the thin-film assemblies.46 In earlier studies, we incorporated azobenzene chromophores into metal phosphonate-based films as part of our efforts to develop function in the organic region of mixed organic/inorganic LB systems.27 The azobenzene core was derivatized using a phosphonic acid headgroup and a long 14-carbon alkyl tail in order to make a wellbehaved LB amphiphile, 1. Optical spectroscopy was used to learn how the organic molecules organize within the LB layers, given the constraints imposed by the rigid inorganic metal phosphonate network. In that work,27 attempts to effect trans to cis isomerization of the azobenzene showed that the chromophores were too tightly packed and without sufficient free volume to undergo a change in conformation. To demonstrate reversible trans to cis isomerization in metal phosphonate thin films, we (47) Petruska, M. A.; Talham, D. R. Langmuir 2000, 16, 5123.

Azobenzene Chromophores in Thin Films Scheme 3. Synthetic Route to Azobenzene Chromophores 2a and 2b

have now synthesized two new azobenzene chromophores possessing a phosphonic acid hydrophilic portion and either a branched alkoxy (2a) or short linear alkoxy (2b) hydrophobic portion (Scheme 3). These molecules do not form stable Langmuir monolayers, but they can be deposited using the three-step combined LB and selfassembly process. Molecules 2a and 2b were designed to control the free volume available to the azobenzene chromophore in the assembled films and to probe the impact of their structure on molecular organization and photoisomerization. Rapid and highly efficient trans to cis photoisomerization is demonstrated for multilayers of both 2a and 2b. The efficiency of the isomerization reaction and the lifetime of the cis form can be changed through modifications in the deposition procedure.

Experimental Section Synthesis. General. All reagents were purchased from either Aldrich Chemical Co. (Milwaukee, WI) or Fisher Scientific (Pittsburgh, PA) and used without further purification, unless otherwise indicated. Dichloromethane was dried over P2O5 overnight and distilled under N2 atmosphere prior to use. Triethyl phosphite was dried and distilled over sodium. Triethylamine and pyridine were dried over CaH2. Acetone was dried and distilled over molecular sieves. NMR spectra were obtained on a Varian VXR-300 spectrometer. Elemental analysis and mass spectrometry were performed by the University of Florida Spectroscopic Services laboratory. Melting points were measured

Langmuir, Vol. 16, No. 19, 2000 7451 using a Thomas-Hoover capillary melting point apparatus and were uncorrected. UV-visible spectra were recorded on a Hewlett-Packard 8452A diode-array spectrophotometer. The compounds 4-(4′-hydroxybutyl)aniline, 4, and 4-hydroxy-4′-(4′′hydroxybutyl)azobenzene, 5, were prepared as described previously.27,48 4-(3′-Pentyloxy)-4′-(4′′-hydroxybutyl)azobenzene (6a) and 4-Propoxy-4′-(4′′-hydroxybutyl)azobenzene (6b). A mixture of 5 (2.5 g, 9.3 mmol), 1.5 equiv of alkyl bromide (3-bromopentane for 6a, 1-bromopropane for 6b), and potassium carbonate (2.57 g, 2 equiv) in 75 mL of acetone was refluxed over two nights. After the reaction cooled, 50 mL of diethyl ether and 50 mL of water were added, forming two layers. The organic layer was washed twice with 50 mL of 2 M aqueous NaOH solution, once with 50 mL of water, dried over NaSO4, and concentrated in vacuo. Purification by column chromatography on silica, eluting with a mixture of hexanes/ethyl acetate (ratio ) 1:1, Rf ) 0.45 on TLC), gave an orange liquid product for 6a with 62.6% yield: 1H NMR (CDCl3) δ 7.81 (d, 2H), 7.78 (d, 2H), 7.30 (d, 2H), 7.00 (d, 2H), 4.25 (t, 1H), 3.47 (t, 2H), 2.67 (t, 2H), 1.50-1.67 (m, 8H), 0.95 (t, 6H). HRMS (FAB) found m/z 341.2228, calcd for C21H29N2O2 m/z 341.2229 (M+). Recrystallization of the residue from hexanes twice gave an orange solid product 6b with 84.1% yield: mp 83-85 EC; 1H NMR (CDCl3) δ 7.86 (d, 2H), 7.78 (d, 2H), 7.38 (d, 2H), 7.06 (d, 2H), 4.05 (t, 2H), 3.72 (t, 2H), 2.72 (t, 2H), 1.58-1.84 (m, 6H), 1.06 (t, 3H). HRMS (FAB) found m/z 313.1918, calcd for C19H25N2O2 m/z 313.1916 (M+). 4-(3′-Pentyloxy)-4′-(4′′-bromobutyl)azobenzene (7a) and 4-Propoxy-4′-(4′′-bromobutyl)azobenzene (7b). In a round-bottom flask, p-toluenesulfonyl chloride (0.77 g) was dissolved in 7 mL of pyridine and chilled in an ice/salt bath for 30 min. To it, compound 6 (3 mmol) dissolved in 14 mL of pyridine was added dropwise from an addition funnel over 30 min. After stirring in the ice/salt bath for an additional 15 min, the flask was capped and stored in the freezer overnight. The next day, the orange solution was poured into a mixture of 90 mL of ice/70 mL of concentrated HCl. The solution was extracted twice using 350 mL of chloroform. The organic layer was washed once with 140 mL of a 50% ice/ HCl solution and twice with 350 mL of saturated NaHCO3 solution, was dried over MgSO4, and was concentrated in vacuo. To this tosylated product, 7.8 g of LiBr and 70 mL of acetone were added; then it was refluxed overnight. After cooling to room temperature, 350 mL of water was added. The mixture was extracted twice using 350 mL of diethyl ether, was dried over MgSO4, and was concentrated in vacuo. Purification of the residue by column chromatography on silica, eluting with a mixture of hexanes/ethyl acetate (ratio ) 2/1, Rf ) 0.71 on TLC), gave an orange liquid product, 7a, with 61.2% yield: 1H NMR (CDCl3) δ 7.82 (d, 2H), 7.78 (d, 2H), 7.33 (d, 2H), 7.01 (d, 2H), 4.27 (t, 1H), 3.46 (t, 2H), 2.67 (t, 2H), 1.60-1.80 (m, 8H), 0.95 (t, 6H). HRMS (FAB) found m/z 403.1393, calcd for C21H28N2OBr m/z 403.1385 (M+). Purification by column chromatography on silica, eluting with a mixture of ether/CHCl3 (ratio ) 1/2, Rf ) 0.8 on TLC), gave a yellow-orange solid product, 7b, with 73.4% yield: mp 57-59 EC; 1H NMR (CDCl3) δ 7.86 (d, 2H), 7.78 (d, 2H), 7.38 (d, 2H), 7.06 (d, 2H), 4.07 (t, 2H), 3.71 (t, 2H), 2.72 (t, 2H), 1.58-1.84 (m, 6H), 1.06 (t, 3H). HRMS (FAB) found m/z 375.1036, calcd for C19H24N2OBr m/z 375.1072 (M+). Diethyl (4-(4′-(3′′-Pentyloxy))phenylazophenyl)butylphosphonate (8a) and Diethyl (4-(4′-Propoxy)phenylazophenyl)butylphosphonate (8b). Compound 7 (1.44 mmol) was heated and stabilized at 140 °C. To it, excess triethyl phosphite (0.86 mL, 3.5 equiv) was added. The ethylbromide that formed was allowed to distill from solution during the reaction. After the reaction was continued overnight, the excess triethyl phosphite was removed by distillation under vacuum. Purification of the residue by column chromatography on silica, eluting with a mixture of ether/acetone (ratio ) 2/1, Rf ) 0.47 on TLC), gave an orange liquid product 8a with 87.4% yield: 1H NMR (CDCl3) δ 7.85 (d, 2H), 7.78 (d, 2H), 7.35 (d, 2H), 7.03 (d, 2H), 4.28 (t, 1H), 4.01 (t, 4H), 2.68 (t, 2H), 1.60-1.80 (m, 10H), 1.26 (t, 6H), 0.95 (t, 6H); HRMS (FAB) found m/z 461.2579, calcd for C25H38N2O4P m/z 461.2569 (M+). Purification of the residue by column chromatography on silica, eluting with a mixture of ether/acetone (ratio ) 2/1, Rf ) 0.5 on TLC), gave an orange liquid product 8b with 82.4% yield: 1H NMR (CDCl3) δ 7.86 (d, 2H), 7.78 (d, 2H), 7.38 (d, 2H), 7.06 (d, 2H), 4.07 (t, 2H),

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4.01 (t, 4H), 2.72 (t, 2H), 1.58-1.84 (m, 8H), 1.26 (t, 6H), 1.06 (t, 3H); HRMS (FAB) found m/z 433.2235, calcd for C23H34N2O4P m/z 433.2256 (M+). 4-(4′-(3′′-Pentyloxy)phenylazophenyl)butylphosphonic Acid (2a) and 4-(4′-Propoxyphenylazophenyl)butylphosphonic Acid (2b). To a solution of 8 (0.34 mmol) in 12 mL of dichloromethane, triethylamine (0.19 mL, 4 equiv) and bromotrimethylsilane (0.18 mL, 4 equiv) were added under N2 atmosphere. After being stirred overnight, the solution was concentrated under vacuum, and 20 mL of methanol was added to the residue. After stirring overnight, two-thirds of the methanol was removed, and 50 mL of water was added. The solution was extracted using 50 mL of diethyl ether three times. The organic layer was concentrated in vacuo, yielding a yellowish solid. Recrystallization of the solid from a mixture of hexanes/ethyl ether gave an orange-yellow solid product 2a with 61% yield: mp 117-118 EC; 1H NMR (CD3OD) δ 7.86 (d, 2H), 7.78 (d, 2H), 7.34 (d, 2H), 7.03 (d, 2H), 4.31 (5, 1H), 2.72 (t, 2H), 1.66-1.72 (m, 10H), 0.98 (t, 6H); HRMS (FAB) found m/z 405.1943, calcd for C21H30N2O4P m/z 405.1972 (M+). Anal. Calcd for C21H29N2O4P: C, 60.38%; H, 6.93%; N, 7.18%. Found: C, 59.96%; H, 6.96%; N, 7.09%. UV λmax (ethanol): 242 nm (, 10 300 M-1 cm-1), 352 nm (, 25 000 M-1 cm-1), 448 nm (, 2300 M-1 cm-1). Recrystallization of the solid from a mixture of ether/ hexanes/methanol gave a yellowish solid product 2b with 60% yield: mp 162-163 EC; 1H NMR (CD3OD) δ 7.86 (d, 2H), 7.78 (d, 2H), 7.34 (d, 2H), 7.03 (d, 2H), 4.06 (t, 2H), 2.72 (t, 2H), 1.661.82 (m, 8H), 1.03 (t, 3H); HRMS (FAB) found m/z 377.1626, calcd for C19H26N2O4P m/z 377.1630 (M+). Anal. Calcd for C19H25N2O4P: C, 60.64%; H, 6.65%; N, 7.45%. Found: C, 60.62%; H, 6.71%; N, 7.44%. UV λmax (ethanol): 240 nm (, 13000 M-1 cm-1), 350 nm (, 23000 M-1 cm-1), 444 nm (, 2000 M-1 cm-1). Deposition of Multilayers. General. Octadecylphosphonic acid (OPA, Alfa Aesar), zirconyl choride octahydrate (ZrOCl2, 98%, Aldrich), and octadecyltrichlorosilane (OTS, 95%, Aldrich) were commercially available and used without further purification. Solvents, such as chloroform (HPLC grade, Acros) and ethanol, were used as received. Quartz microscope slides were purchased from Chemglass (Vineland, NJ) and were used as substrates for optical measurements. Prior to use, quartz substrates were cleaned using the RCA procedure and were made hydrophobic by treatment with OTS.27 Langmuir-Blodgett and Self-Assembly Deposition. The Langmuir-Blodgett experiments were performed using a KSV Instruments (Stratford, CT) 3000 system. A homemade Telflon trough (10.8 cm × 67 cm) was modified to operate with double barriers. Surface pressure was measured using a platinum Wilhelmy plate suspended from a KSV microbalance. For all experiments, water having a resistance of 18 MΩ cm was produced from a Barnstead E-pure purification system. For the LB monolayer preparations, an OPA solution, 0.3 mg/mL in chloroform, was spread onto the water surface and compressed to form a monolayer at the air/water interface, which was then transferred by dipping the substrate down through the film held at a constant pressure of 20 mN/m. Adsorption of the transition metal ion, Zr4+, was accomplished by exposing the OPA-coated substrate to an aqueous solution of ZrOCl2 for 20 min, then rinsing with water and drying over N2. Solutions for self-assembly of the azobenzene chromophores 2 were prepared in EtOH/H2O (90/ 10) mixed solvent with concentrations that varied from 10-3 to 10-4 mol/L. The metalated substrates were exposed to the azobenzene solutions for 20 min at room temperature. After rinsing the substrate with ethanol and dipping the azobenzenecovered substrate into ethanol for additional 30 min, the substrates were dried over N2. Measurements. UV-vis spectra were recorded on a HewlettPackard 8452A diode-array spectrophotometer. The incident beam was aligned perpendicular to the substrate. Photoirradiation was performed using a 450-W (medium pressure) 7825 immersion lamp (ACE Glass). An LG350 filter combined with a Corning 7-54 filter was used to produce UV light (approximately 340-410 nm) for the trans to cis isomerization. An LG 450-nm filter was used for the cis to trans isomerization, blocking light with wavelengths < 440 nm. To avoid heating, samples were placed 25 cm away from the light source.

Wu and Talham

Figure 1. UV-visible spectra of 1 × 10-6 M 2a in EtOH before (solid line) and after (dashed line) UV irradiation.

Results and Discussion Synthesis of Azobenzene Derivatives. The azobenzene derivatives, 2, were synthesized following the procedures shown in Scheme 3. All steps represent wellknown reactions9,22,48,49 and are similar to the routes we used previously to prepare alkylphosphonic acid substituted azobenzenes.27 Briefly, the parent azobenzene was obtained by coupling a substituted aniline and phenol in the presence of nitrite. The alkoxy groups were incorporated by reaction with alkyl bromide in the presence of a base. Bromination of the alcohol and the subsequent Arbuzov reaction resulted in phosphonate ester derivatives. The esters were hydrolyzed by reaction with bromotrimethylsilane, followed by treatment with methanol and water to yield the phosphonic acid derivatives in analytically pure form. Photoisomerization in Solution. Photoisomerization of compounds 2 was observed in chloroform solutions, as illustrated by the UV-visible spectrum of 2a in Figure 1, which is similar to that of 2b (data not shown). Typical of azobenzene derivatives, the trans form is more stable and is the dominant isomer before irradiation by UV light. The characteristics of the trans form of 2a are a strong π-π* transition at 352 nm (, 25 000 M-1 cm-1) and a weak n-π* band at 448 nm (, 2300 M-1 cm-1). The absorption at 242 nm (, 13 000 M-1 cm-1) is attributed to a π-π* transition whose moment is roughly parallel to the short axis of the trans isomer.32,42 The corresponding bands for the trans form of 2b are found at 350 nm (, 23 000 M-1 cm-1), 444 nm (, 2000 M-1 cm-1), and 240 nm (, 13 000 M-1 cm-1), respectively. Upon UV irradiation, the π-π* band shifts to shorter wavelength and the n-π* band becomes more intense, indicating that the trans form is converted to the cis form in both solutions. This trans to cis isomerization in solution is nearly quantitative and is reversible. The cis form is short-lived, completely reverting to the trans form when treated with visible irradiation for a few minutes or after thermal relaxation for about 3 h. The half-life for thermal relaxation is several minutes in both cases (data not shown). Multilayer Assembly and Characterization. Scheme 2 shows the three-step combined LB and self-assembly procedure used for the layer-by-layer depositions.44 In the first step, an OPA LB monolayer was transferred onto a hydrophobic substrate by dipping the substrate down through the Langmuir monolayer at the air/water interface and into a vial immersed in the subphase. The vial (48) Jones, R.; Tredgold, R. H.; Hoorfar, A.; Allen, R. A.; Hodge, P. Thin Solid Films, 1985, 13, 57. (49) Bhattacharya, A. K.; Thyagarajan, G. Chem. Rev. 1982, 81, 415.

Azobenzene Chromophores in Thin Films

containing the OPA-coated substrate was then removed from the trough, together with water. In the second step, aqueous ZrOCl2 was added to the vial, resulting in metalation of the phosphonate surface. After adsorption for 20 min, the substrate was rinsed with water and dried under N2. In the third step, the substrate was placed in a solution containing 2 for 20 min, allowing for adsorption of the azobenzene chromophores onto the metalated surface. The substrate was extensively treated with ethanol to remove physisorbed azobenzene molecules that were not bound to the metal phosphonate network, and then dried under N2. For further buildup, this three-step procedure can be repeated, and in this manner, organic/ inorganic hybrid multilayers were assembled which possess a layered metal phosphonate lattice between the OPA and azobenzene layers. This bilayered structure is denoted as OPA/Zr/2, in which the azobenzene chromophores are 2a or 2b. For the studies reported here, up to 15 bilayers were routinely deposited. The combined LB/self-assembly process provides several advantages. In previous work, we showed that well-defined layers are formed with sufficient structural coherence to give rise to X-ray diffraction.44 Even if defects arise in the self-assembly step, they do not perpetuate, because the LB layer smooths over the surface, redefining each layer. Non-centrosymmetric, alternate-layer assemblies are easily prepared, and the active molecules need not form stable Langmuir monolayers. Importantly, the metal phosphonate network renders these films insoluble and stable to elevated temperatures.43,47 Even though the interactions between bilayers are hydrophobic van der Waals contacts, the ionic-covalent in-plane binding crosslinks the chromophores and greatly enhances film stability. The deposition process is monitored by qualitatively observing changes in surface wettability following each step. The surface of the substrate is initially hydrophobic and changes to hydrophilic after the metalation step. It then changes to hydrophobic again after the third step as a result of the coordination of the azobenzene chromophores to the metal phosphonate network. The alkoxy groups on the azobenzene then provide the hydrophobicity required to repeat the process. The LB transfer ratio for the OPA layer was 1.0 ( 0.1 for each cycle. The UV-visible spectra of the films after each threestep cycle provide a more quantitative measure of the multilayer assembly. Figure 2 shows UV-visible spectra of OPA/Zr/2a and OPA/Zr/2b multilayers after each of the first five deposition cycles. In both multilayers, the absorption bands characterize the trans form of the azobenzene chromophores. The linear relationship between the absorbance at λmax and the number of bilayers, as shown in the inset, indicates that the same amount of azobenzene was assembled during each three-step cycle. Two differences can be seen in the UV-visible spectra of the two azobenzene multilayer systems. First, the π-π* band of the trans form is centered at 350 nm in the OPA/ Zr/2a multilayers, which is about the same as the value seen in solution. In the OPA/Zr/2b multilayers, however, the π-π* band of the trans form is located at 340 nm, shifted 10 nm blue from that in solution. Second, the absorbance of the band near 240 nm is less than that at 350 nm in the OPA/Zr/2a multilayers, and the relative intensities are reversed in the OPA/Zr/2b multilayers. The electronic structure of the molecules is quite similar, and these differences result from the molecules packing differently in the two films. Molecular Organization and Orientation. It is known that the optical absorption of azobenzene chro-

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Figure 2. UV-visible spectra of multilayers of (a) OPA/Zr/2a and (b) OPA/Zr/2b after each of five deposition cycles. The absorbance intensity increases with an increasing number of layers. Inset: absorbance at λmax versus number of bilayers.

mophores is very sensitive to interchromophore interactions.24-27 As explained by the exiton coupling model,50 a blue shift occurs in the π-π* transition when adjacent azobenzene chromophores are H-aggregated, with a parallel alignment of the transition dipoles. A red shift occurs when the adjacent azobenzene chromophores are J-aggregated, or with an in-line alignment. H-aggregates of azobenzene chromophores are commonly observed in well-organized and closed-packed LB films.24-27 For example, a 50-nm blue shift of the λmax of compound 1, compared to that in solution, was observed when 1 was incorporated into a zirconium phosphonate LB film.27 When the λmax of the π-π* band of 2a in solution (352 nm) is compared to that in the multilayer systems, a nearly negligible shift of about 2 nm is observed. In contrast, a 10-nm blue shift was observed in the OPA/Zr/2b multilayers. Partial H-aggregation occurs in multilayers of 2b, which does not happen in 2a-based multilayer systems. The structural difference between 2a and 2b indicates that the branched 3-pentyloxy group in 2a hinders closepacking of the azobenzene chromophore. The n-propoxy group of 2b allows some degree of aggregation, but to a smaller extent than that which is observed in LB films of 1. The transition dipole of the 242-nm band is roughly parallel to the short axis of the trans isomer, while that of the 350-nm band is parallel to its long axis.3 Therefore, (50) McRae, E. G.; Kasha, M. Physical Processes in Radiation Biology; Academic Press: New York, 1964, p 23.

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Figure 3. Spectral changes of a film of 2 before and after irradiation by UV and visible light. (a) five-bilayer film of OPA/ Zr/2a, and (b) five-bilayer film of OPA/Zr/2b. Solid line, before UV irradiation; dashed line, after irradiation with UV light for 5 min; dash-dotted line, after irradiation with 450-nm light for 10 min.

comparison of the intensities of the absorption bands near 240 and 350 nm gives an indication of the orientation of the azobenzene chromophores in the films.32,42 The spectra were measured in transmission with the incident beam perpendicular to the substrate. If the long axis of the azobenzene chromophore is perpendicular to the substrate, the ratio of absorbances, A240/A350, is large and decreases as the molecules tilt away from the film normal. Results clearly show that A240/A350 in the OPA/Zr/2b multilayer film (1.18) is large as compared to that for the OPA/Zr/2a multilayer system (0.88), demonstrating that the azobenzene chromophores in the 2b layers are oriented closer to perpendicular than the molecules in films of 2a. The branched tail on 2a prevents the molecules from closepacking and also leads to a larger tilt of the azobenzene chromophore. Photoisomerization in Assemblies. The photoisomerization of the azobenzene chromophores in each film was investigated. The as-prepared multilayers are in the trans form, as shown by their UV-visible spectra in Figure 3. The intense π-π* transition band around 350 nm (340 nm for 2b) and the shoulder near 450 nm corresponding to the n-π* band are both characteristic of trans azobenzenes. Upon UV irradiation, the π-π* band gradually shifts to blue and becomes less intense, while the n-π* band near 450 nm increases in intensity, signaling formation of the cis isomer. This change is almost completely reverted by irradiation with visible light at 450 nm. The reversible spectral changes show that both multilayers undergo trans to cis isomerization by UV irradiation and cis to trans isomerization by visible irradiation. The trans to cis reaction reaches a photostationary state after UV irradiation for 5 min, and the cis form converts back to the trans form after visible irradia-

Wu and Talham

Figure 4. Thermal relaxation of the photogenerated cis form in multilayers of 2. (a) Solid circles, five-bilayer film of OPA/ Zr/2a; solid triangles, five-bilayer film of OPA/Zr/2b. (b) The cis-deposited film of 2a. The solid lines in each plot are fits to eq 1 using the parameters listed in Table 1.

tion for 10 min. The reproducibility of the isomerization reactions was monitored over five cycles without significant change in the UV-visible spectra. The aggregation of azobenzene chromophores, either H-type or J-type, restricts photoisomerization due to the limited free volume available within the aggregated assemblies.24-27 For example, photoisomerization was suppressed in metal phosphonate LB films of 1 because of strong H-aggregation.27 However, by combining a change in the molecular structures with an alternative assembly procedure, control over molecular organization, and thereby control over photoisomerization, has been achieved. Neither compound 2a nor 2b forms long-range aggregates, although films of 2b do show evidence of some degree of H-aggregation. The 10-nm blue shift in the π-π* band is characteristic of short-range aggregation on the order of dimers or trimers.26 However, the photoisomerization reaction is not greatly affected because the UVvisible spectra the cis forms of the two films are nearly identical. Thermal Relaxation in Assemblies. Because the cis form of azobenzene is the less stable isomer, it thermally converts back to the trans form. The thermal relaxation of the isomerized films was studied by monitoring the return of the π-π* band of the trans form after UV irradiation produced the photostationary state. The cis multilayers were kept in the dark and at room temperature, and the UV-visible spectra were periodically monitored over the course of several days. Figure 4a shows the increase in the absorbance at λmax of the trans isomer (350 and 340 nm, respectively) versus time for the multilayers of OPA/Zr/2a and OPA/Zr/2b. The gradual increase in absorbance reflects the isomerization from the

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Table 1. Kinetics Results of Thermal Relaxation of the cis Form of 2 Produced in Multilayersa

a

multilayers

R

k1 (min-1)

k2 (min-1)

R2

OPA/Zr/2a OPA/Zr/2b OPA/Zr/(2a50/OPA50) OPA/Zr/2a-cis

0.60 ( 0.05 0.45 ( 0.04 0.40 ( 0.06 0.029 ( 0.006

3.4E-3 ( 2E-4 6.1E-3 ( 8E-4 7.2E-3 ( 8E-4 3.2E-3 ( 4E-4

5.2E-4 ( 5E-5 7.5E-4 ( 9E-5 9.1E-4 ( 7E-5 5.0E-4 ( 7E-5

0.9994 0.9975 0.9957 0.9979

The rate constants and mole fraction, R, correspond to eq 1.

cis back to the trans forms. In both multilayer systems, the half-life of the cis form was several hours and complete recovery of the trans form took about 4 days. The relaxation could not be fit to simple first-order kinetics. As has been seen in other examples of azobenzene isomerization in condensed media,2 the relaxation is better described as the sum of two processes composed of slow- and fastrelaxing components. After the photochemical isomerization, some cis species are caught in a highly strained environment and relax back to trans more quickly than molecules that are not strained.51 The relaxation data were better fit to a sum of two exponential terms that correspond to the slow- and fast-relaxing components (eq 1).

A∞ - At ) Re-k1t + (1 - R)e-k2t A∞ - A0

(1)

The absorbance of the π-π* band of the trans form is monitored immediately after trans-cis isomerization (A0), during the course of the reaction (At) and after reaching steady state (A∞). The rate constants, k1 and k2, are listed in Table 1, along with the parameter “R”, which is the mole percent of the fast-relaxing component. In both the OPA/Zr/2a and the OPA/Zr/2b films, there are nearly equal amounts of the fast- and slow-relaxing components. The rate constants for the fast components are roughly an order of magnitude larger than those of the slow components in each case. The film of 2a, in which the 3-pentyloxy substituent appears to limit the organization of the azobenzene chromophores, relaxes more slowly than the analogous films of 2b, in which the n-propoxy substituent allows some degree of aggregation. Mixed Multilayers. To control the packing density of azobenzene chromophores, and perhaps to influence the photoisomerization reactions, we have explored alternative deposition procedures. The first is mixed monolayers based on molecule 2a. In this system, the azobenzene solution used for self-assembly was diluted with an equal amount of octadecylphosphonic acid, and the resulting film is denoted OPA/Zr/(2a50/OPA50). UV-visible studies in Figure 5 show that multilayers assembled from this 50% 2a and 50% OPA solution were reproducible, and the λmax of the π-π* band of the trans azobenzene is not shifted relative to that of the films containing monolayers of pure 2a. There is no guarantee that OPA and 2a adsorb from solution at the same rate, but the optical spectra show that the absorbance of a six-bilayer mixed film (Figure 5) is not much different from that of a three-bilayer pure film (Figure 2a). However, the relative intensity of the 240- and 350-nm bands (1.26) is inverted in the mixed film when compared to the pure film, indicating that the azobenzene chromophores are oriented closer to the normal in the mixed layer. The OPA matrix serves to align the azobenzene units. Reversible photoisomerization is also observed in the mixed multilayers, but the OPA matrix appears to influence the cis isomer lifetime. Trans to cis isomerization (51) Imai, Y.; Naka, K.; Chujo, Y. Macromolecules 1999, 32, 1013.

Figure 5. Optical spectra of a mixed-multilayer film of OPA/ Zr/(2a50/OPA50) after each deposition cycle. The inset shows the absorbance at 352 nm versus the number of bilayers.

can be induced with UV irradiation for 5 min, and nearly complete recovery of the trans form results from either visible irradiation or thermal relaxation. The thermal relaxation data in Table 1 show that there are both fastand slow-relaxing components but that the rate constants for both components are approximately twice the values observed for the films containing layers of pure 2a. The close packing of adjacent OPA molecules may reduce the free volume available to the azobenzenes and produce more highly strained cis isomers that relax more quickly. Cis Form Assembled Multilayers. If strain on the cis isomers is primarily responsible for fast relaxation, then the cis-trans isomerization might be slowed if intermolecular packing was optimized for the cis form. This optimal packing might be achieved by depositing the cis isomer directly. To form a monolayer of the cis form, a solution of 2a was photolyzed during step 3 of the deposition process (Scheme 2). To minimize thermal relaxation, the azobenzene solution and substrate were exposed to UV light during solution self-assembly. Figure 6a shows that the cis conformation of 2a dominates after deposition from solution. The linear increase of absorbance at the λmax of the n-π* transition band versus the number of bilayers demonstrates that a highly reproducible amount of cis azobenzene is deposited after each LB and self-assembly cycle. After standing in the dark for 4 days, the cis-assembled multilayers converted back to the trans form (Figure 6b). After thermal relaxation, the λmax and intensity of the trans-form π-π* transition is similar to that of the trans-deposited films (Figure 2). However, the ratio A240/A350 in the OPA/Zr/2a-cis films (0.72) is less than that in the trans-deposited films, indicating a less-upright orientation of the azobenzene. Because the cis form requires more free volume,26,33,35,41 the molecules are likely less tightly packed and tilt farther from the normal. As shown in Figure 6b, UV-induced trans to cis isomerization of the cis-deposited film is highly efficient. Relaxation back to the trans form is slow, and analysis of the decay using eq 1 (Figure 4b and Table 1) shows that

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cis form optimizes the free volume of the azobenzene chromophore and reduces strain on the cis isomer after it is formed by photolysis. Conclusions

Figure 6. Optical spectra of the cis-deposited multilayer film of OPA/Zr/2a-cis. (a) UV-visible spectra after each deposition cycle with the inset showing the absorbance at 450 nm versus the number of bilayers. (b) Spectral changes before and after photoisomerization. The solid line is the UV-visible spectrum of the trans form of the five-bilayer film in a, and the dashed line is the same film after UV irradiation for 5 min.

there is only 3% of the fast-relaxing component in this film. The slow-relaxing component has the same rate constant as the slow component of the trans-deposited film, OPA/Zr/2a. The half-life for the trans to cis isomerization is now more than 11 h. Clearly, deposition of the

We have demonstrated that metal-phosphonate-based multilayer films containing azobenzene chromophores can be prepared that undergo highly efficient trans to cis photoisomerization reactions. Film formation involves a three-step process that combines LB and self-assembled monolayer deposition steps to yield organized mixed organic/inorganic films that are stable in both organic and aqueous solvents. Previous studies showed that while well-organized metal phosphonate LB films of a substituted azobenzene could be prepared, photoisomerization was inhibited in those films by the close-packing of the chromophores.27 The combined LB and self-assembly deposition method permits non-LB-active molecules to be deposited in layered assemblies, allowing structural modification of the azobenzene molecules to deter aggregation and enhance photoisomerization. The photoisomerization reaction and the rate of thermal relaxation can be controlled through a combination of synthetic modification of the azobenzene chromophore and variations in film architecture. The relaxation kinetics revealed that the films are composed of two components that relax at different rates. The fast-relaxing component is likely due to cis isomers that are in a more strained environment than the slow-relaxing component. The relaxation rates for each component can be changed by structural modification of the azobenzene or by altering the free volume in mixed films. The fast-relaxing component is nearly eliminated when the cis isomer is deposited directly, leading to films with longer cis lifetimes. Acknowledgment. We thank Professor Kirk S. Schanze for assistance with the photoisomerization experiment and the University of Florida Major Analytical Instrumentation Center for instrument time. Acknowledgment is made to the National Science Foundation for financial support (CHE-9618750). LA000407H