Effects of Added Salt on Photochemical Isomerization of Azobenzene

In Final Form: May 24, 1999 ... multilayer films of a cationic bipolar amphiphile and an anionic polyelectrolyte are ... film upon linearly polarized ...
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Langmuir 1999, 15, 6515-6521

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Effects of Added Salt on Photochemical Isomerization of Azobenzene in Alternate Multilayer Assemblies: Bipolar Amphiphile-Polyelectrolyte Jong-Dal Hong,* Eung-Soo Park, and Ae-Li Park Department of Chemistry, University of Inchon, 177 Dohwa-dong Nam-ku, 402-749 Inchon, South Korea Received March 5, 1999. In Final Form: May 24, 1999 The photochemical isomerization of azobenzene is investigated in systems in which symmetrically substituted chromophores are organized in a self-assembled molecular film on fused silica. Alternate multilayer films of a cationic bipolar amphiphile and an anionic polyelectrolyte are prepared by a selfassembly (SA) method based on layer-by-layer deposition using electrostatic attraction between oppositely charged ions. The bipolar amphiphile layer, which exists between polyelectrolyte layers, contains an azobenzene unit in order to allow for a trans-cis photoisomerization to take place. We have studied the effect of added salt (NaCl) to the bipolar amphiphile, i.e., 4,4′-bis((12-(trimethylammonio)dodecyl)oxy)azobenzene dibromide, BA-12, on the structure of the multilayer films (tilt angle of the chromophore, film total thickness) and on trans-cis or cis-trans photoisomerization. Optical dichroism is induced in the SA film upon linearly polarized light irradiation. This dichroism could be reversibly erased and rewritten by irradiation with light of an appropriate wavelength. Our study suggests that the level of birefringence and the photoisomerization in the SA multilayer films can be controlled to a certain degree by a systematic variation of the supramolecular ordering in the photochromic layers. The trans-cis photoisomerization in the SA films that are obtained from higher ionic strength occurs almost 10 times slower.

Introduction During the past several years, the layer-by-layer selfassembly technique based on electrostatic attraction between opposite charges1 has been devoted to realization of the variety of the multilayer heterostructures on a solid substrate. The assembly of cationic and anionic polyelectrolytes2 has been extended to the polyelectrolytes and DNA,3 polyelectrolytes and latex particles,4 polyelectrolytes and proteins,5 polyelectrolytes and delaminated clay platelets,6-9 polyelectrolytes and colloidal metal particles,10-12 polyelectrolytes and dyes,13-18 and polyelec* To whom correspondence should be addressed. E-mail: [email protected]. (1) Decher, G.; Hong, J.-D. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (2) Decher, G.; Hong, J.-D.; Schmitt, J. Thin Solid Films 1992, 210/ 211, 831. (3) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (4) Donth, E.; Walther, D.; Shilov, V. N.; Knippel, E.; Budde, A.; Lowack, K.; Helm, C. A.; Mo¨hwald, H. Langmuir 1997, 13, 5294. (5) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (6) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370. (7) Keller, S. W.; Kim, H. N.; Mollouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817. (8) Lvov, Y.; Ariga, Ichinose, K. I.; Kunitake, T. Langmuir 1996, 12, 3038. (9) Kotov, N. A.; De´ka´ny, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (10) Feldheim, D. L.; Crabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (11) Schmitt, J.; Decher, G.; Dressik, W. J.; Branduo, S. L.; Geer, R. E.; Shashidhal, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61. (12) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M J. Science 1995, 267, 1629. (13) Cooper, T.; Campbell, A.; Crane, R. Langmuir 1995, 11, 2713. (14) Yoo, D.; Lee, J.-K.; Rubner, M. F. Mater. Res. Soc. Symp. Proc. 1996, 413, 395. (15) Zhang, X.; Gao, M.; Kong, X.; Sun, Y.; Shen, J. J. Chem. Soc., Chem. Commun, 1994, 1005. (16) Araki, K.; Wagner, M. J.; Wrighton, M. S. Langmuir 1996, 12, 5393.

trolytes and electrically conductive or electroluminescent conjugated polymers.19,39 Bolaamphiphiles have been used as components of this technique.1,20 The layer formation (17) Sellergren, B.; Swietlov, A.; Amebrant, T.; Unger, K. Anal. Chem. 1996, 68, 402. Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T. Langmuir 1993, 9, 3461. Zhang, X.; Gao, M.; Kong, X.; Sun, Y.; Shen J. J. Chem. Soc., Chem. Commun. 1994, 1055. Saremi, F.; Tieke, B. Adv. Mater. 1995, 7, 378. Saremi, F.; Maassen, E.; Tieke, B. Langmuir 1995, 11, 1068. Saremi, F.; Lange, G.; Tieke, B. Adv. Mater. 1996, 8, 923. (18) Ariga, K.; Lvov, Y.; Kunitake, T. J. Am. Chem. Soc. 1997, 119, 2224. (19) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 958. Fou, C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F. J. Appl. Phys. 1996, 79, 7501. (20) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langumir 1995, 11, 942. (21) Hong, J.-D.; Park, E. S.; Park, A. L. Presented at the LB8 Conference, Asilomar, CA, 1997. (22) Saremi, F.; Tieke, B. Adv. Mater. 1998, 10, 388. (23) Hong, J.-D.; Park, E. S.; Park, A. L. Bull. Korean Chem. Soc. 1998, 19, 1156. (24) Rau, H., In Photochemistry and Photophysics; Rabek, J., Ed.; CRC Press Inc.: Boca Raton, FL, 1990; Vol. II, chapter 4. (25) Weigert, F., Verh. Dtsch. Phys. Ges. 1919, 21, 484. (26) Todorov, T.; Nicolova, L.; Tomova, N. Appl. Opt. 1984, 23, 4308. (27) Eich, M.; Wendorff, J. H.; Reck, B.; Ringsdorf, H. Makromol. Chem., Rapid Commun. 1987, 8, 59. (28) Eich, M.; Wendorff, J. M. Makromol. Chem., Rapid Commun. 1987, 8, 467. (29) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules 1992, 25, 2268. (30) Aoki, K.; Tamaki, T.; Seki, T.; Kawanishi, Y.; Ichimura, K. Langmuir 1992, 8, 1014. (31) Seki, T.; Fukuda, R.; Tamaki, T.; Ichimura, K. Thin Solid Films, 1994, 243, 675. (32) Paik, C. S.; Morawetz, H. Macromolecules 1972, 5, 17. Sung, C. S. P.; Gould, R.; Turro, N. T. Macromolecules 1983, 17, 1447. Yu, W. C.; Sung, C. S. P.; Robertson, E. Macromolecules 1988, 21, 355. Song, O.-K.; Wang, C. H.; Pauley, M. A. Macromolecules 1997, 30, 6913. (33) Siewierski, L. M.; Brittain, W. J.; Petrash, S.; Foster, M. D. Langmuir 1996, 12, 5838. Geue, Th.; Ziegler, A.; Stumpe, J. Macromolecules 1997, 30, 5729. (34) Katayama, N.; Ozaki, Y.; Seki, T.; Tamaki, T.; Iriyama, K. Langmuir 1994, 10, 1898. (35) Yokoyama, S.; Kakimoto, M.; Imai, Y. Langmuir 1994, 10, 4594. (36) Sekkat, Z.; Wood, J.; Geerts, Y.; Meskini, A. E.; Buechel, M.; Knoll, W. Synth. Met. 1996, 81, 281.

10.1021/la990265v CCC: $18.00 © 1999 American Chemical Society Published on Web 08/05/1999

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Figure 1. Schematic view of a self-assembled alternating multilayer film composed of a bipolar amphiphile BA-12 and an anionic polyelectrolyte PVS on a precoated fused silica substrate. The pretreated substrate contains 5 bilayers of PAH and PVS to obtain the maximum ionic concentration on the surface.

by these bipolar molecules produces charged surfaces upon their adsorption and allows the alternate assembly to continue. As can be also seen from the precedents of bolaamphiphiles and bilayer-forming amphiphiles, small molecules that tend to aggregate in water have been assembled as molecular layers in combination with oppositely charged polyions. Very recently, photoisomerization was demonstrated in the multilayer assemblies, in which the photoactive layers sandwiched between oppositely charged polyelectrolyte layers consist of bolaamphiphiles bearing azobenzene chromophores.21-23 The photoinduced cis-trans isomerization of azobenzene is a well-known photochemical phenomenon. 24 A remarkable consequence is the Weigert effect, in which birefringence or dichroism are induced on irradiation with polarized light.25 This has great potential specifically for polarization holography26 and more generally for a wide variety of optical information storage and switching applications.27-31 There are a number of reports in the literature on photoisomerization of azobenzene and its derivatives in solution24 and in doped32 and functionalized glassy polymers.33 Similar photoalignment has also been reported using Langmuir-Blodgett (LB) films of poly(vinyl alcohol) derivative with azobenzene side chains.34 Kakimoto and co-workers have studied photochemically inducible and erasable dichroism in which an azobenzene pendant unit in a polyimide LB film is molecular reoriented.35 Knoll and co-workers have reported dichroism induced in an azobenzene-silane monolayer self-assembled on a quartz glass substrate.36 There are a few recent reports of the Weigert effect in organized molecular structures.37,38 The layer-by-layer electrostatic deposition method has been widely applied for the fabrication of ultrathin, organized multilayers by using a variety of the charged polymeric materials over the past few years, as anticipated. However, the multilayer systems containing an internal ordering structure were rarely encountered in the literature. Therefore, we have constructed assemblies composed of a cationic bipolar amphiphile and an anionic polyelectrolyte on the precoated fused silica surface, shown schematically in Figure 1. Photoactive BA-12 layers exist between two polyelectrolyte layers. The presence of salt ions in the solution affects the conformation of adsorbing polyelectrolytes by shielding (37) Ichimura, K.; Hayashi, Y.; Akiyama, H. Langmuir 1993, 9, 3298. (38) Sekkat, Z.; Wood, J.; Geerts, Y.; Knoll, W. Langmuir 1995, 11, 2856. (39) Hong, J.-D.; Kim, D. S.; Char, K. H.; Jin, J. I. Synth. Met. 1997, 84, 815.

the charge on the polyion repeat units.45 These changes dictate the relative amounts of polymer that will adsorb on each chemical surface. In the previous works,2,39 we have shown that the thickness of the polyelectrolyte monolayer adsorbed on the oppositely charged surface can drastically be increased by addition of electrolyte to the polymer solution. These works were extended for studying ionic effects of sodium chloride on the selectivity of deposition for polyion adsorption on patterned surfaces.40 As discussed below, we will show that the variation of the salt content in solution can also affect the adsorbed amount and the thickness of the adsorbed layers of a bipolar amphiphile instead of polyelectrolytes. Thus, the orientation of the bipolar amphiphile adsorbed on the surface was significantly changed. The aim of this article is to study how the aggregate state formed in solution affects the structure of the multilayer assemblies transferred on solid substrate and also to investigate the dependence of the photoisomerization process and the level of the induced optical anisotropy in the SA multilayer on the morphological variations established via the change of preparation conditions. Experimental Section Materials. Poly(vinyl sulfate, potassium salt) (Mw ) 140 000), PVS, and poly(allylamine hydrochloride) (Mn ) 50 000-65 000), PAH, were purchased from Aldrich and used without further purification. The bipolar amphiphile 4,4′-bis((12-(trimethylammonio)dodecyl)oxy)azobenzene dibromide was synthesized, and the molecular structure was given in Figure 1. The bolaamphiphile, BA-12, was synthesized in three steps. First, a reductive coupling of p-nitrophenol with KOH leads to the 4,4′-dihydroxyazobenzene. Second, the 4,4′-dihydroxyazobenzene was converted to 4,4′-bis((12-bromododecyl)oxy)azobenzene by the Williamson reaction with 1,12-dibromododecane. Third, the 4,4′-bis((12-bromododecyl)oxy)azobenzene was quaternized with trimethylamine in toluene. 4,4′-Dihydroxyazobenzene. The reaction was carried out according to the literature.41 Yield: 11.4%. Mp: 216.5 °C. Anal. Calcd: C, 67.3; H, 4.7; N, 13.1. Found: C, 66.8; H, 4.7; N, 12.8. (40) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1977, 30, 7237. (41) Reck, B. Diploma, Mainz, 1985. (42) Okahata, Y.; Kunitake, T. J. Am. Chem. Soc. 1979, 101, 5231. (43) Mao, G.; Tsao, Y.; Tirrell, M.; Davis, H. T.; Hessel, V.; Ringsdorf, H. Langmuir 1993, 9, 3461. (44) Mao, G.; Tsao, Y.-H.; Tirrell, M.; Davis, H. T.; Hessel, V.; Esch, J. V.; Ringsdorf, H. Langmuir 1994, 10, 4174. (45) Cohen Stuart, M. A. J. Phys. (Paris) 1988, 49, 1001. Bo¨hmer, M. R.; Evers, O. A.; Scheutjens, J. M. H. M. Macromolecules 1990, 23, 2288.

Photochemical Isomerization of Azobenzene 1H NMR (DMSO, 60 MHz, ppm): δ 6.9 (d, phenyl ring, 4H ), 7.8 a (d, phenyl ring, 4Hb), 10.0 (s, phenyl-OH, 4Ha). 4,4′-Bis((12-bromododecyl)oxy)azobenzene. 4,4′-Dihydroxyazobenzene (4.9 g, 23 mmol) and 1,12-dibromododecane (30 g, 90 mmol) in ethanol were refluxed for 48 h. One minispatula of KI was added for the catalyst. When the mixture was cooled, the product precipitated as a yellow powder. The precipitates were filtered out and washed with H2O and with ethanol, respectively. The crude product was recrystallized. Yield: 52%. Mp: 109.5 °C. Anal. Calcd: C, 61.0; H, 8.0; N, 3.9; O, 4.5. Found: C, 63.0; H, 8.0; N, 3.9; O, 7.0. 1H NMR (CDCl3, 200 MHz, ppm): δ 1.2 (m, -(CH2)8CH2CH2Br, 40H), 1.9 (m, -OCH2CH2(CH2)8-, 4H), 3.2 (t, -(CH2)8CH2CH2Br, 4H), 4.0 (t, -OCH2CH2-, 4H), 7.1 (d, phenyl ring, 4Ha), 7.8 (d, phenyl ring, 4Hb). 4,4′-Bis((12-(trimethylammonio)dodecyl)oxy)azobenzene Dibromide. 4,4′-Bis((12-bromododecyl)oxy)azobenzene (1.4 g, 2 mmol) in toluene was allowed to react with 0.5 g (4 mmol) of trimethylamine in an ampule at 100 °C for 144 h. The product precipitated as yellow needles. The precipitates were filtered out and washed with warm toluene. Yield: 80%. Mp: 261 °C. Anal. Calcd: C, 61.0; H, 9.0; N, 6.8. Found: C, 58.0; H, 9.2; N, 6.9. 1H NMR (DMSO, 250 MHz, ppm): δ 1.3 (m, -(CH2)8CH2CH2NMe3, 40H), 1.7 (m, -OCH2CH2(CH2)8-, 4H), 3.1 (m, -CH2CH2N(CH3)3, 22H), 4.1 (t, -OCH2CH2-), 7.1 (d, phenyl ring, 4Ha), 7.8 (d, phenyl ring, 4Hb). Film Construction. Ultrapure water used for all experiments, and all cleaning steps was obtained by ion-exchange and filtration unit (Milli-Q, Millipore GmbH). The resistivity was better than 18.0 MΩ‚cm. The substrates for all adsorption experiments were fused silica slides of size 25 × 50 mm2. They were cleaned by ultrasonication in the mixture of H2SO4/H2O2 (7/3) and then heated in the mixture of H2O/H2O2/NH3 (5:1:1) at 80 °C for 1 h. The substrates were thoroughly washed with ultrapure water after both steps. The bipolar amphiphile and polyelectrolytes were deposited onto the negatively charged substrate as described previously.1,2 After each adsorption step the surface of the film was thoroughly rinsed and then blown dry with a stream of nitrogen gas. First, 5 bilayers of PAH and PVS were deposited by dipping in the PAH solution (11 unit mM, pH 4.0) and into the PVS solution (6 unit mM, pH 2.5) for 20 min each. Then, 10 bilayers of BA-12 and PVS were deposited by dipping the substrate alternately into the BA-12 (0.2 mM, pH 6.8) and into the PVS solution (6 unit mM, pH 2.5). The ionic strengths in BA-12 solution were adjusted to be 0.0, 0.1, 0.5, and 1.0 M by adding NaCl, respectively. The quantity of material deposited at each step was deduced from its absorption spectrum, which was determined on a Perkin-Elmer UV/visible spectrophotometer (Lambda 4B). Film Morphology. The thickness of the multilayered film on a silicon wafer was determined with an optical ellipsometer (Rudolph/Auto EL) equipped with a 632.8 nm He/Ne laser illuminating the sample at 70°. The orientation of the azobenzene chromophore in BA-12/PVS film was estimated from the polarized UV spectra obtained at 45° incidence angle. The polarized UV/visible spectra were taken with a HP 8452A diode array spectrophotometer. Photoisomerization. Photoisomerization of the azobenzene units was induced by irradiation in the ultraviolet (360 nm) for the trans-cis reaction and in the visible light (blue, 450 nm) for the cis-trans back-isomerization. The irradiating light was a high-pressure mercury lamp (Altech, 1 kW) equipped with glass filters (ultraviolet, UG1, Spindler & Hoyer; blue, GG10, Spindler & Hoyer) for UV light. The lamp power was adjusted to 2 mW/ cm2 for UV light and to 100 mW/cm2 for blue light. The surrounding temperature on the sample was adjusted to be ca. 30 °C using a cold plate (Stir-Kool model SK12, Aldrich Chem. Co.) to prevent thermal back-isomerization during UV irradiation. Polarized UV/visible spectra of the film before and after polarized UV irradiation were recorded on a Hewlett-Packard diode array spectrophotometer (model 8452A) with dichroic sheet polarizers. Dichroism measurements were performed by irradiating the samples with a linearly polarized UV light and immediately recording absorption spectra with the probe light polarization parallel (Abs||) and perpendicular (Abs⊥) to the initial UV polarization. The spectroscopic degree of order was calculated by

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S ) (Abs|| - Abs⊥)/(Abs|| + 2Abs⊥) where Abs|| ) UV absorbance of the probe light polarization parallel to the initial UV polarization and Abs⊥ ) UV absorbance of the probe light polarization perpendicular to the initial UV polarization.

Results and Discussion The multilayer assemblies of BA-12 and PVS on a precoated fused silica by the layer-by-layer deposition based on a SA method are distinguished by a vertical layered structure. The precoating of the substrate with 5 bilayers of PAH and PVS prior to the deposition of the active layers was designed to give the maximum surface charge, which also provided equal uniform surface conditions for the preparation of the multilayer films on the fused silica. The supramolecular structure of the SA films can be manipulated by changing adsorption conditions, i.e., by the control of ionic strengths in BA-12 solution. On irradiation the azobenzene moieties of BA-12 undergo cis-trans photoisomerization in solution and SA multilayers. Very recently, it was shown23 that on irradiation with linearly polarized light the tilted azobenzene groups in SA multilayers were reoriented, yielding optical in-plane anisotropy with an azimuthal angle of 90° toward the electric field vector of the incident light. The optical anisotropy is an induced birefringence and dichroism in the SA film resulting from a reorientation of the azobenzene moieties. Now, our study will be extended to learn how the structural factors affect the levels of the induced optical anisotropy and the photoisomerization process. UV Spectrometry Study of the Bolaamphiphile in Solution. Bolaform amphiphiles, recently known as bolaamphiphiles in the literature, are molecules containing two hydrophilic moieties connected by a hydrophobic chain. It has been shown that bolaamphiphiles have unusual aggregation behavior at the air-water interface and in lyotropic phases.42-44 Okahata et al. also reported that a cationic bolaamphiphile produced huge aggregates in dilute aqueous solution.42 We have investigated the formation of an aggregation of BA-12 in dilute aqueous solution depending on temperature and ionic strengths using UV/visible absorbance spectrometry instead of the common methods, i.e., surface tension measurement and polarization microscopy. Figure 2a shows UV/vis spectra of BA-12 in dilute aqueous solution (0.1 mM) depending on temperature. The absorption maximum at 357 nm by 58 °C was shifted about 17 nm to the blue of the maximum exhibited by 20 °C. In addition, the intensity of the absorbance strongly decreased at lower temperature. Both facts indicate an excitonic interaction of the aromatic chromophores forming H-aggregates at lower temperature, which are destroyed to the molecularly dispersed state around 58 °C. The absorption maxima versus temperature plot obtained from the spectral data presented in Figure 2a clearly shows spectral shifts of maximum absorbance at three different temperatures which indicate the phase transition of the aggregate (Figure 2b). The phase-transitions seem to occur at around 34, 48, and 58 °C. We guess that a lyotopic mesophase forms at 34 °C. On being heated to 48 °C, the mixture becomes a biphasic lyotropic/isotropic and finally isotropic at 58 °C. Our results are very similar to the report that the bolaform phenylenediacrylic acid derivative shows a lyotropic mesophase at 35.5 °C, an isotropic/lyotropic biphase at 48 °C, and an isotropic phase at 59 °C, which was obtained from the polarization microscopy measurement.43 In our case, the lyotropic mesophase was not

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Figure 4. (a) UV/visible absorption spectra of the selfassembled 10 bilayer BA-12/PVS on fused silica. The dipping solution of BA-12 is adjusted to different ionic strength by addition of NaCl: no salt (A); 0.1 M (B); 0.5 M (C); 1.0 M (D).

Figure 2. (a) UV/visible absorption spectra of BA-12 in aqueous solution (0.1 mM) depending on temperature. (b) Wavelength of the maximum absorbance vs temperature plot of 0.1 mM aqueous BA-12 solution (from data in part a).

Figure 3. (a) UV/visible absorbance of BA-12 in solution adjusted to different ionic strengths: no salt (A); 0.1 M (B); 0.5 M (C); 1.0 M (D). The maxima of absorbance are shown in parentheses (inset).

observed from the investigation with the polarization microscopy. An exact structure analysis can only be made by X-ray measurement. UV/visible absorbances of BA-12 in solution adjusted to the different ionic strengths are presented in Figure 3. The absorption maximum in solution at 341 nm was slightly blue-shifted to 337 nm by about 4 nm with increasing ionic strength to 0.1 M. The maximum was still more shifted to 336 and 335 nm with increasing ionic strength to 0.5 and 1.0 M, respectively. The results obtained from the spectrometry study in solution will provide the basis for understanding the morphology of the adsorbed layers on a solid substrate.

Figure 5. UV absorbance vs number of bilayers of BA-12/PVS (measured at 340 nm). The BA-12 solution is adjusted to four different ionic strengths by the addition of NaCl: no salt (b); 0.1 M ([); 0.5 M (2); 1.0 M (1).

Effect of Ionic Strength on Film Formation and on Multilayer Growth. Figure 4 shows UV/vis spectra of the self-assembled 10 bilayer BA-12/PVS on the precoated fused silica at different NaCl concentrations. The highest NaCl concentration we can use was 1.0 M, because particles precipitated above this concentration. The dipping solution of BA-12 is only adjusted to four different ionic strengths by addition of NaCl: 0 (no salt), 0.1, 0.5, and 1.0 M. Each of the samples are referred to as S0.0M, S0.1M, S0.5M, and S1.0M for a convenient description. The maximum absorbance of BA-12 was observed at 341 nm for no salt and at 337, 336, and 335 nm for 0.1, 0.5, and 1.0 M and shifts its absorbance to 345 nm (S0.0M) and to 332 nm (S0.1M, S0.5M, and S1.0M), respectively, when adsorbed on fused silica. When the maximum absorbance in solution (Figure 3) was compared with that of the adsorbed multilayer film, the maxima were observed in almost the same region of the absorbance. The results suggest that the formed aggregates in solution were directly adsorbed on the surface of the substrate. The intensity of maximum absorbance was drastically enhanced with the increasing ionic strength. This reveals that the adsorbed amount of BA-12 increased with higher ionic strength. The increased amount seemed to induce the additional aggregation and realignment of chromophores in the SA films, which were resulting in the observance of the blue-shift of the maximum absorbance. Figure 5 shows the UV absorbance of azobenzene chromophores at 340 nm as a function of the number of bilayers deposited. The adsorption occurs essentially uniformly, the same amount being deposited on each cycle. In the UV absorbances, it can be more clearly seen that the adsorbed amount of BA-12 was drastically increased with increasing ionic strength. This effect was often observed in the adsorption of polyelectrolytes onto the

Photochemical Isomerization of Azobenzene

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Table 1. Tilt Angle, Film Thickness, and Order Parameter of BA-12 in SA Films Prepared at Different Ionic Strengths samples

tilt angles (deg)

thickness of BA-12/PVS bilayer (Å)a

order params

S0.0M S0.1M S0.5M S1.0M

58.2 46.3 42.7 43.5

37.5 (16.8) 67.1 (46.4) 68.9 (48.2) 89.2 (68.5)

-0.033 -0.103 -0.097 -0.099

Scheme 1

a The values in the parentheses correspond to the thickness of a BA-12 monolayer sandwiched between PVS layers, calculated from the ellipsometry data.

surface. At higher ionic strength, the increased adsorbed amounts and thick adsorbed layers were found from the reduced intra- and intermolecular electrostatic repulsion because of the effective screening of charges on the polymer by the salt ions.45 Interestingly, it should be noted here that the added salt can affect the adsorption process of bipolar amphiphile on the oppositely charged surface. A linear fit of data yields an average increase of the optical density of 0.0177 (correlation coefficient R ) 0.997) for S0.0M, 0.0207 (R ) 0.998) for S0.1M, 0.0242 (R ) 0.999) for S0.5M, and 0.0279 (R ) 0.996) for S1.0M per layer of BA-12. Morphology of the SA Film. We have attempted to estimate the orientation of azobenzene moiety in the BA12/PVS film on the basis of the polarized UV spectra obtained at 45° incidence angle. This estimation was made by referring to the optoelectric equations reported by Vandevyver et al.46 The infinite dilution hypothesis was applied, and the ππ* transition dipole was assumed to be directed along the molecular axis of azobenzene. The refractive indices of the fused silica and the BA-12 layer were taken to be 1.47 and 1.43.46 On this basis, the average tilt angle of the azobenzene moiety with respect to the substrate normal was calculated to be 58.2, 46.3, 42.7, and 43.5° for S0.0M, S0.1M, S0.5M, and S1.0M, respectively. It has been shown that the tilt angle of the chromophore decreased markedly with increasing ionic strengths. This suggests that the azobenzene moiety stands perpendicular the surface with increasing ionic strength, but no significant differences at various salt concentrations were found. The results are in accordance with UV spectrometry. Table 1 summarizes the average tilt angle of the chromophore in SA multilayer at various ionic strengths and the estimated thickness of a bolaamphiphile layer sandwiched between the polyelectrolyte layers. Since the true refractive indices are unknown, we assumed the film refractive index to be 1.54 in analyzing the ellipsometric data. We have obtained an ellipsometric thickness of 41.5 Å for the bilayer of PAH/PVS and that of 37.5 Å (S0.0M), 67.1 Å (S0.1M), 68.9 Å (S0.5M), and 89.2 Å (S1.0M) for the bilayers of BA-12/PVS. According to the measurement, we can calculate the thickness of a bolaamphiphile layer to be 16.8, 46.4, 48.2, and 68.5 Å for S0.0M, S0.1M, S0.5M, and S1.0M, respectively, assuming the thickness of a PVS layer to be 20.7 Å which was estimated from the determined total thickness of the 5 bilayers of PAH/PVS. From a molecular modeling calculation, the molecular length of the compound BA-12, in fully stretched form, is estimated to be 46.2 Å (Scheme 1). Anticipated from the ellipsometry data, the azobenzene moiety should adopt a rather parallel orientation to the surface. However, the added salt has a dramatic effect on the orientation (thickness) of the bolaamphiphile. For the films prepared in the presence (46) Vandevyver, M.; Barraud, A.; Teixier, R.; Maillard, P.; Gianotti, C. J. Colloid Interface Sci. 1982, 85, 571

of 0.1 and 0.5 M NaCl, BA-12 seems to stand perpendicular to the surface. The thickness of BA-12 in S1.0M deviates slightly from the theoretical molecular length. This finding implies that a heterogeneous film was obtained in 1.0 M NaCl, adsorbed as the aggregate state formed in solution. Measurement of the polarized UV absorption spectrum of the films showed no preferred in-plane orientation of the azobenzene chromophores, even from the film prepared with the highest ionic strength. Structural Dependence of Azobenzene Photoisomerization. The influence of the initial order on the photoisomerization process of azobenzene is somewhat controversially discussed in the literature. On one hand, it was shown recently that the photoreorientation process can be restricted by the initial order of aligned liquid crystal polymer films and Langmuir-Blodgett multilayers.48,49 In such ordered systems the photoreorientation process is suppressed by the liquid-crystalline order. On the other hand, very recently, a different model was proposed in which the initial order of LB multilayers or liquid crystals plays an essential role for the lightstimulated reorientation process.50-52 The influence of the ordering of azobenzene layers sandwiched between two polyelectrolyte layers on the process is still lacking in the literature. An additional interest arises from the question as to whether and to what extent the photoreorientation will take place in the multilayer assemblies, in which the molecular mobility in the photochromic layers is restricted by the oppositely charged polyelectrolyte layers. Natansohn et al. reported that the level of optical anisotropy is seen to be directly proportional to the number of photons involved in the writing process, confirming the statistical nature of the process.29 It has been also shown that the structural factor plays a significant role in control over the levels of the induced birefringence; the levels of birefringence attainable in crystalline or liquid crystalline (47) Jones, C.; Day, S. Nature 1991, 351, 15. (48) Stumpe, J.; Geue, Th.; Fischer, Th.; Menzel, H. Thin Solid Films 1996, 284-285, 606. (49) Fischer, Th.; La¨sker, L.; Stumpe, J.; Kostromin, S. J. Photochem. Photobiol., A 1994, 80, 4531. (50) Janossy, I. Phys. Rev. E 1994, 49 (4), 2957. (51) Scho¨nhoff, M.; Chi, L. F.; Fuchs, H.; Lo¨sche, M. Langmuir 1995, 11, 163 (52) Scho¨nhoff, M.; Mertesdorf, M.; Lo¨sche, M. J. Phys. Chem. 1996, 100, 7558.

6520 Langmuir, Vol. 15, No. 19, 1999

Figure 6. Polarized UV/visible absorption spectra of the selfassmebled 10 bilayer BA-12/PVS (S0.1M) on fused silica before (b) and after 5 min (2, 1) of linearly polarized UV (360 nm) irradiation. Spectra are obtained for both parallel, 1, and perpendicular, 2, to the initial UV light polarization. For reasons of clarity, only absorbance (b) of the initial trans-azobenzene in the multilayer assemblies is shown.

polymer films are significantly higher than in amorphous polymer films.53 Figure 6 illustrates the dichroism observed in the SA multilayer film that was deposited on precoated fused silica from 0.1 M NaCl solution of BA-12. The linearly polarized light generates a reorientation of the azobenzene moieties perpendicular to the direction of the electric field vector of the incident light. This orientation process proceeds on continued irradiation within the steady state. This is demonstrated in these spectra, which were obtained after 5 min of irradiation with linearly polarized UV light. The polarized spectra were measured in directions both parallel and perpendicular to the initial UV light polarization. It can be clearly seen that the absorption (Abs⊥) is higher than the absorption (Abs||). This suggests that a significant number of trans-azobenzene molecules are reoriented and realigned perpendicular to the plane of photoirradiation after the trans-cis isomerization. This behavior is consistent with the photochemical reorientation of the azobenzene units previously reported in the literature.36,47 The degree of light-induced molecular orientation of BA-12 was determined by polarized UV spectroscopy. The order parameters, S ) (Abs|| - Abs⊥)/(Abs|| + 2Abs⊥) (as defined in ref 54), of the SA films were calculated at 338 nm from the anisotropic spectra and listed in Table 1. The order parameter S ) -0.033 for S0.0M decreased strongly to S ) -0.103 for S0.1M with increasing ionic strength but is not significantly changed between 0.1 and 1.0 M. In this experiment, it could be also demonstrated that the level of the optical anisotropy attainable in the multilayer assemblies depends strongly on the supramolecular ordering in the photochromic layers. The strong decrease of the order parameter resulted from the increased ordering of the photochromic layers that was achieved by increasing the ionic strengths. The UV-induced dichroism could be erased on irradiation (1 min) with unpolarized blue light (450 nm). Photoisomerization of the azobenzene units is induced by UV light (360 nm) for the trans-cis reaction and by visible light (blue, 450 nm) for the cis-trans backisomerization. Figure 7 shows the time dependence of the azobenzene trans-cis photoisomerization with respect to the parallel planes of polarized UV light. The maximum absorbance value of the trans form of the azobenzene unit (53) Ho, M.-S.; Natansohn, A.; Ronchon, P. Macromolecules 1996, 29, 44. (54) Jones, F.; Reeve, T. J. Mol. Cryst. Liq. Cryst. 1980, 60, 99.

Hong et al.

Figure 7. In-plane trans-cis photoisomerization of the selfassembled 10 bilayer BA-12/PVS on fused silica with respect to the parallel planes of the initial UV polarization. For the preparation of the different films, the ionic strength of BA-12 solution is adjusted to no salt (b), 0.1 M ([), 0.5 M (2), and 1.0 M (1), respectively, by the addition of NaCl.

Figure 8. In-plane photoisomerization of the self-assembled 10 bilayer BA-12/PVS on fused silica with respect to the parallel planes of the initial UV polarization. For the preparation of the different films, the ionic strength of the BA-12 solution is adjusted to no salt (b), 0.1 M ([), 0.5 M (2), and 1.0 M (1), respectively, by the addition of NaCl.

is found at 344 nm, where A0 and At are absorbance before (t ) 0) and after UV (t min), respectively. Spectral measurements were made immediately after photoirradiation. The time dependence of the azobenzene isomerization on the morphology of the multilayer films which were deposited from the different NaCl solution is shown in Figure 7. Clearly, there is a dramatic effect of the added salt on the photoisomerization characteristics. The transcis photoisomerization of the azobenzene unit in S0.0M occurs relatively large within 2 min (almost more than 90% with regard to the attained photostationary state) and completed after 5 min. In contrast to that, the increased amount of the added salt caused a much slower isomerization process, by almost 10 times. The effect of the different salt concentrations on isomerization was not so significant between 0.1 and 1.0 M. However, the transcis isomerization after 5 cycles in the case of the sample S0.1M occurs almost 5 times faster than the initial state. Figure 8 demonstrates the effect of the added salt to the BA-12 on cis-trans back-isomerization of azobenzene. It can be interestingly seen that, after ca. 50 s of blue light

Photochemical Isomerization of Azobenzene

irradiation, the back-isomerization to initial trans state was almost completed for all the samples investigated. In this case of back-isomerization, the salt effect does not seem to be included. No observance of the salt effect like the trans-cis isomerization seems to be resulted from the fast back-switching to the initial state and further from the strong intensity of blue light irradiation. However, it has been clearly seen that the amount of azobenzene which was switched back to the initial trans state strongly decreased depending on the amount of the added salt. As anticipated, we have found that the order parameter and the photoisomerization process are strongly dependent on the fabrication conditions. The slower photoisomerization of the films prepared at higher salt concentration could be caused either by changes in the aggregation state of the dye or by changes in the magnitude of the molecular tilt. The dependence of the photoisomerization on the dye aggregation has been extensively discussed in the literature.33,48-49 The molecular tilt can also affect the photoisomerization rates by changing the cross section for the absorption of the irradiating light. A lower rate would indicate that the molecules are standing more upright. The high value of the order parameter and the slower photoisomerization procedure in the film reflect the fact that the bolaamphiphile molecules are supraorganized to the ordered compact liquid crystal with increasing ionic strength. This is consistent with the results from the spectrometry study on UV absorbance of BA-12 in solution and in SA film on fused silica. Conclusions We have prepared the self-assembled alternate multilayers of a bipolar amphiphile, 4,4′-bis((12-(trimethylammonio)dodecyl)oxy)azobenzene dibromide, and an an-

Langmuir, Vol. 15, No. 19, 1999 6521

ionic polyelectrolyte, poly(vinyl sulfate, potassium salt), on fused silica in which the ionic strength in the solution of the bipolar amphiphile was varied from 0.1 to 1.0 M. The UV spectrometry study of BA-12 in dilute aqueous solution (0.1 M) showed that the molecules exist at room temperature as H-aggregates, considered as a lyotropic liquid crystal which is adsorbed directly onto the surface for the formation of the well-ordered film. Detailed characterization of the SA film morphology by polarized UV spectroscopy and ellipsometry has revealed that the tilt angle of the chromophore and the film thickness changed drastically with increasing ionic strength. We have also demonstrated that the level of the induced anisotropy and the photoisomerization in the SA multilayer film can be controlled to a certain degree by systematic variation of ionic strength in BA-12 solution. The in-plain dichroism induced with the polarized light could be reversibly written and erased in the SA film for several cycles without any observance of defect formations. Finally, we believe that the systematic manipulation of the photoinduced reorientation process in the multilayer assembly may be useful for anisotropically altering the structure and/or optical properties of ultrathin supramolecular structures containing azobenzene molecules. Acknowledgment. This paper was supported by the Non Directed Research Fund, Korea Research Foundation. We thank Dr. Dong-Young Kim in the Korea Institute of Science and Technology for support with the lightirradiation equipment. E.-S. Park expresses appreciation to Prof. Kwan Kim and to Mr. Sang-Jung Ahn in Seoul National University for allowing the use of a Rudolph/ Auto EL ellipsometer. LA990265V