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Polyelectrolyte layered assemblies containing azobenzene-modified polymer and anionic cyclodextrins
Materials Science and Engineering C 23 (2003) 579 – 583 www.elsevier.com/locate/msec
Polyelectrolyte layered assemblies containing azobenzene-modified polymer and anionic cyclodextrins Iwao Suzuki a, Katsuhiko Sato a, Masaki Koga a, Qiang Chen b, Jun-ichi Anzai a,* a
Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan b Life Sciences College, Nankai University, Weijin Road 94, Tianjin 300071, China Accepted 26 June 2003
Abstract Sulfonated cyclodextrins (s-a-CyD and s-h-CyD) were employed for constructing photosensitive thin films containing azobenzene residues. Azobenzene-modified poly(allylamine hydrochloride) (Az-PAH) and s-a-CyD or s-h-CyD were deposited alternately on the surface of a quartz slide to prepare multilayer thin films. Az-PAH formed inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-hCyD film. The Az-PAH isomerized from E- to Z-isomer in the films under UV light irradiation, and the original E-form was recovered under visible light irradiation or thermally in dark. The thermal isomerization followed the first-order kinetics in the swelled films though the kinetics deviated from the first-order plot in the dry films. The host – guest complexation of Az-PAH and s-a-CyD in the film affected significantly on the photochemical and thermal isomerization. D 2003 Elsevier B.V. All rights reserved. Keywords: Azobenzene; Photoresponse; Polyelectrolyte multilayer film; Cyclodextrin
1. Introduction The development of photosensitive polymetric materials has been a focal subject in materials science and technology. Azoaromatic polymers which contain azobenzene chromophores are widely used to develop photosensitive molecular assemblies including monolayers [1], Langmuir – Blodgett films [2], and organic –inorganic hybrid films [3]. Recently, polyelectrolyte multilayer (PEM) films have attracted much attention because of the simplicity of the technique and the versatility in the film design [4– 6]. In this context, it is interesting to construct polyelectrolyte multilayer assemblies containing azobenzene chromophores. From this viewpoint, different types of azobenzene-modified polyelectrolytes have been employed for constructing photosensitive PEM films in which the azobenzene residues exhibit photoisomerization [7 –10]. In this context, we have reported photoresponse of PEM films composed of an azobenzene-modified poly(allylamine
* Corresponding author. Tel.: +81-22-217-6841; fax: +81-22-2176840. E-mail address: [email protected] (J. Anzai). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0928-4931(03)00050-X
hydrochloride) (Az-PAH), which were prepared by a layerby-layer deposition of Az-PAH and polyanion [11]. The Az-PAH exhibited photoisomerization from E- to Z-form in the film upon UV light irradiation and recovered the original E-form reversibly in dark or by visible light irradiation. In the thermal isomerization of Az-PAH from Z- to E-form in dark, the kinetics deviated from the firstorder plot due to strained environment in the film or a lack of free volume around the azobenzene residues. We also prepared photosensitive PEM films by chemical modification of PEM films with an active ester of azobenzene carboxylate. In this modified films, however, azobenzene residues formed aggregates and the photoresponse was suppressed significantly [12]. In this situation, it is interesting to use negatively charged cyclodextrin (CyD) as an anionic counterpart of cationic Az-PAH for constructing photosensitive PEM films. CyD might afford free volume around the azobenzene residues through host – guest complexation in the film, which probably enables the azobenzene residues to isomerize reversibly according to the first-order kinetics. Heretofore, PEM films composed of azobenzene polymer and ionic CyD have been prepared but photoisomerization was not reported [13,14].
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2. Experimental part 2.1. Materials An aqueous solution (20%) of poly(allylamine hydrochloride) [PAH; average molecular weight (MW), ca. 10,000] was purchased from Nittobo (Tokyo, Japan). AzPAH was prepared according to the reported procedure [11]. The content of azobenzene residue in the Az-PAH was determined to be 3.5 mol% (azobenene residues vs. total primary amino groups in the polymer chain) using UV – visible spectrophotometry. Sulfonated a-cyclodextrin (s-aCyD) and h-cyclodextrin (s-h-CyD), in which 8 –9 and 12– 14 primary and secondary hydroxyl groups in parent a-CyD and h-CyD are sulfonated, were obtained from Aldrich Chemical (Milwaukee, WI). The chemical structures of Az-PAH and CyD derivatives used are depicted in Fig. 1. All other chemicals used are of highest grade available. 2.2. Apparatus UV –visible absorption spectra were measured using a Shimadzu UV-3100PC spectrophotometer (Kyoto, Japan) for monitoring the formation of PEM films and photoresponse. A quartz-crystal microbalance (QCM) (QCA 917 system, Seiko EG&G, Tokyo, Japan) was employed for the gravimetric analysis of swelling property of the films.
for 30 min to deposit the first layer through a hydrophobic force of attraction. After being rinsed in PBS for 10 min to remove any weakly adsorbed Az-PAH, the quartz slide was immersed in s-a-CyD or s-h-CyD solution (100 Ag ml 1 PBS ) for 30 min to deposit the second layer. This process provides both sides of the quartz slide with an AzPAH/s-CyD layer. The deposition was repeated to prepare multilayer films. After each deposition, the absorbance of the quartz slide around 330 nm, originating from the k – k* transition of the abenzene chromophore, was monitored. The preparation of the films was carried out at ca. 20 jC. The water uptake of the films upon swelling was evaluated by means of QCM, where the resonance frequency of the film-coated quartz resonator (9 MHZ) was measured in air for the dry and swelled films. 2.4. Photoirradiation A 500-W xenon lamp was used as light source for photoirradiation. The UV (320 < k < 380 nm) and visible light (k>450 nm) was isolated using Corning 7-37 and Toshiba Y-45 glass filters, respectively. The content of Zisomer in the irradiated samples was calculated from the decrease in absorbance at the k – k* absorption maximum, assuming that the absorbance of the Z-isomer at this wavelength is negligibly small compared with that of the E-isomer [15].
2.3. Preparation of polyelectrolyte multilayer films 3. Results and discussion The PEMs were prepared on the surface of a quartz slide (5 1 0.1 cm). The quartz slide was first treated in dichlorodimethylsilane (10% solution in toluene) overnight at room temperature to make the surface hydrophobic, and was washed with toluene, acetone, and distilled water. The silylated quartz slide was immersed in an Az-PAH solution (100 Ag ml 1, in a phosphate-buffered saline, PBS, pH 5.0)
Fig. 1. The chemical structures of Az-PAH and sulfonated CyDs used.
3.1. Preparation of Az-PAH/s-a-CyD and Az-PAH/s-b-CyD films The preparation of Az-PAH multilayer films was evaluated by measuring UV – visible absorption spectra of the films. Fig. 2 shows typical absorption spectra of Az-PAH/ s-a-CyD and Az-PAH/s-h-CyD films and the change in absorbance of the films as a function of the number of depositions. Both spectra exhibited a clear absorption maximum (kmax) arising from k – k* transition of E-azobenzene chromophores, and the intensity of the absorption band was enhanced linearly with increasing number of depositions. These results suggest that a constant amount of Az-PAH is adsorbed upon each deposition to form a layered assembly. Thus, s-a-CyD and s-h-CyD were successfully built into the Az-PAH multilayer films as anionic counterparts. It should be noted here that the kmax of the Az-PAH/s-a-CyD film is found at 335 nm while the AzPAH/s-h-CyD film exhibited absorption maximum at 326 nm. This might arise from the host – guest complexation of the azobenzene residues with s-a-CyD in the film because an aqueous solution of Az-PAH exhibited the kmax value at 326 nm. To confirm the complex formation, absorption spectra of Az-PAH were measured in aqueous solution in
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Fig. 2. UV – visible absorption spectra of Az-PAH/s-a-CyD (a) and Az-PAH/s-h-CyD films (b). The inset plots absorbance at kmax in k – k* transition region as a function of the number of layers.
the presence of s-a-CyD and s-h-CyD (Fig. 3). The kmax value of Az-PAH exhibited a red shift in the presence of sa-CyD whereas the addition of s-h-CyD induced no shift in kmax, suggesting the complexation of Az-PAH with s-aCyD. We have recently reported that s-a-CyD forms inclusion complex with methyl orange while s-h-CyD does not [16]. The loading of azobenzene residues per deposition on the quartz slide was estimated to be 8.3 10 8 mol cm 2 in Az-PAH/s-a-CyD film and 5.7 10 8 mol cm 2 for AzPAH/s-h-CyD film from the absorbance data, using a molar extinction coefficient of 2.5 104 M 1 cm 1 for the azobenzene unit. In any event, Az-PAH can be built into PEM film by a layer-by-layer deposition with s-a-CyD and s-h-CyD. The azobenzene residues in Az-PAH form inclusion complex with s-a-CyD in the PEM film.
Fig. 3. UV – visible absorption spectra of Az-PAH in solution in the presence and absence of s-CyDs. The concentration of s-CyD; none (a), 1 10 5 M s-a-CyD (b), and 1 10 5 M s-h-CyD (c). Az-PAH; 10 Ag/ml in 10 mM acetate – NaOH buffer (pH 5.0).
3.2. Photoresponse of the films Photoisomerization of azobenzene chromophores in condensed media depends significantly on the local environments such polarity, viscosity, and free volume distribution around the chromophores [1– 3,11,12]. Fig. 4 shows UV – visible absorption spectra of Az-PAH/s-a-CyD film (10 bilayers on both surfaces of the slide) before and after UV light irradiation in air. UV light irradiation induced a decrease in intensity of absorption band originating from k – k* transition of E-azobenzene residue, showing the formation of the mixture of E- and Z-form of azobenzene residues in the film. The isomerization reaction reached a photostationary state after ca. 30-min irradiation, in which the content of Z-form of the azobenzene residues was 16% in the film, and the resultant Z-isomer was converted nearly completely to the original E-form by visible light irradiation for 1 min. The photochemical reactions of the film can be
Fig. 4. UV – visible absorption spectra of (Az-PAH/s-a-CyD)10 film. (a) Before irradiation, (b) after UV light irradiation and (c) after visible light irradiation.
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induced repeatedly several times without any deterioration of the spectra. The Az-PAH/s-h-CyD film showed an almost the same behavior in the isomerization reactions. Thus, the isomerization reactions of azobenzene residue in the films were demonstrated to be reversible and reproducible. We studied the photoresponse of the films in air and in water (i.e., dry and swelled films). The contents of Z-form at the photostationary state in dry films were lower than those in swelled films. This may be caused by a compact packing of the polyelectrolyte chains in the dry films and, as a result, the free volume available around the azobenzene chromophores is limited in the films. It has been reported that photoisomerization of azobenzene is suppressed or inhibited in polymer films due to the close packing of the polymer chains and consequently the lack of free volume [3,11,12,17]. It is clear that host – guest complexation in the Az-PAH/s-a-CyD film did not improve the photoresponse in dry state. On the other hand, the content of Zisomer at the photostationary state was enhanced after swelling for both films. The QCM study showed that the water content of the swelled films was 61% and 55% for the Az-PAH/s-a-CyD and Az-PAH/s-h-CyD films, respectively. (The water content is defined as the weight of water in the film over the total weight of water plus film materials in the swelled film.) The effects of swelling on the content of Zisomer were much significant for the Az-PAH/s-a-CyD film; the content of Z-isomer in the swelled film was 49% as compared with 16% in the dry film. For the Az-PAH/s-hCyD film, the content of Z-isomer was 30% in the swelled sample and 20% in dry. This might be originating from the host – guest complexation of the azobenzene residues in this film, in view of the fact that CyD complexation often enables azobenzene derivatives to isomerize upon photoirradiation in condensed media where otherwise photoisomerization is severely suppressed or prohibited [18,19]. 3.3. Thermal isomerization reaction from Z- to E-form The thermal Z- to-E reverse isomerization was studied in dark by monitoring the recovery of the k –k* absorption band after the photostationary state had been obtained under UV light irradiation. It is known that the thermal Z to E isomerization usually follows first-order kinetics in solution [20]. In practice, the first-order plot for the isomerization of Az-PAH in an aqueous solution gave a straight line (the first-order rate constant was calculated to be 0.33 10 3 min 1 at 40 jC). Fig. 5 shows that the first-order plot deviated from a straight line for the dry films. The isomerization proceeded faster than in solution in the beginning and was followed by a slow process. This type of decay kinetics of azobenzene derivatives is often observed for film samples, and the fast process can be attributed to trapping of some portion of Zisomers in a strained environment in the film [2,3]. In the fast process of the decay, the isomerization rate in the AzPAH/s-a-CyD film was slower than that in the Az-PAH/s-h-
Fig. 5. First-order plots for the thermal Z to E isomerization of azobenzene residues in the films at 40 jC: (Az-PAH/s-a-CyD)10 film in dry (a) and swelled state (b); (Az-PAH/s-h-CyD)10 films in dry (c) and swelled state (d). The first-order plot was obtained according to the equation ln(C0/ C) = kt, where C0 and C denote the content of Z-isomer at time zero and time t, respectively, and k is the first-order rate constant.
CyD film, which may be ascribable to the fact that Az-PAH forms inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-h-CyD film. It has been reported that the rate of thermal isomerization of azoaromatic dyes decreased upon complexation with CyD [21]. In other words, s-a-CyD probably released the azobenzene residues from the strained environment to some extent through complexation in the Az-PAH/s-a-CyD film. On the contrary, the rate of decay followed first-order kinetics in the swelled films. The rate constants were 0.33 10 3 and 0.63 10 3 min 1 at 40 jC in the Az-PAH/s-a-CyD and Az-PAH/s-hCyD films, respectively. The rate of decay in the Az-PAH/sa-CyD film is slower than that in the Az-PAH/s-h-CyD film. Interestingly, the rate of decay in the swelled Az-PAH/ s-a-CyD film is comparable to that in solution. These observations suggest that azobenzene residues are free from any strained environment which accelerates the E to Z isomerization in the swelled films. This is not the case for Az-PAH/PVS films, in which the decay process did not follow the first-order kinetics even after swelling [11].
4. Conclusions Az-PAH/s-a-CyD and Az-PAH/s-h-CyD films were prepared successfully by a layer-by-layer deposition of cationic Az-PAH and the sulfonated CyDs. Az-PAH formed inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-h-CyD film. The azobenzene residues isomerized from E- to Z-isomer in the films, and the original Eform was recovered in dark or under visible light irradiation reversibly. The complexation of Az-PAH and s-a-CyD in the film had a significant influence on the photochemical and thermal isomerization of azobenzene residues. It became apparent that s-a-CyD and s-h-CyD are useful mate-
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rials for improving photoresponse of azobenzene-containing polyelectrolyte films.
References [1] T. Seki, T. Fukuchi, K. Ichimura, Langmuir 16 (2000) 3564. [2] C. Tadeschi, M. Polli, M.P. Fontana, O. Pieroni, Thin Solid Films 284 – 285 (1996) 174. [3] Y. Imai, K. Naka, Y. Chujo, Macromolecules 32 (1999) 1013. [4] G. Decher, J.D. Hong, J. Schmitt, Thin Solid Films 210 – 211 (1992) 831. [5] G. Decher, Sciences 277 (1997) 1232. [6] T. Farhat, G. Yassin, S.T. Dubas, J.B. Schlenoff, Langmuir 15 (1999) 6621. [7] S. Dante, R. Advincula, C.W. Frank, P. Stroeve, Langmuir 15 (1999) 193. [8] F. Saremi, B. Tieke, Adv. Mater. 10 (1998) 288. [9] J.D. Hong, E.S. Park, A.L. Park, Langmuir 15 (1999) 6515.
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[10] A. Toutianoush, B. Tieke, Makromol. Chem., Rapid Commun. 19 (1998) 591. [11] I. Suzuki, T. Ishizaki, T. Hoshi, J. Anzai, Macromolecules 35 (2002) 577. [12] I. Suzuki, T. Ishizaki, H. Inoue, J. Anzai, Macromolecules 35 (2002) 6470. [13] P. Fischer, M. Koetse, A. Laschewsky, E. Wiscgerhoff, L. Jullien, A. Persoons, T. Verbiest, Macromolecules 33 (2000) 9471. [14] M. Koetse, A. Laschewsky, A.M. Jonas, T. Verbiest, Colloids Surf., A 198 – 200 (2002) 275. [15] G.S. Paik, H. Morawetz, Macromolecules 5 (1972) 171. [16] K. Sato, I. Suzuki, J. Anzai, Colloid Polym. Sci. (2003) (in press). [17] H. Nakahara, K. Fukuda, J. Colloid Interface Sci. 93 (1994) 1879. [18] J. Anzai, N. Sugaya, T. Osa, J. Chem. Soc., Perkin Trans. 2 (1994) 1879. [19] M. Ueda, H.-B. Kim, T. Ikeda, K. Ichimura, Chem. Mater. 4 (1992) 1229. [20] C.D. Eisenbach, Ber. Bunsenges. Phys. Chem. 84 (1980) 680. [21] A.M. Sanchez, R.H. de Rossi, J. Org. Chem. 61 (1996) 3446.
Polyelectrolyte layered assemblies containing azobenzene-modified polymer and anionic cyclodextrins Iwao Suzuki a, Katsuhiko Sato a, Masaki Koga a, Qiang Chen b, Jun-ichi Anzai a,* a
Graduate School of Pharmaceutical Sciences, Tohoku University, Aramaki, Aoba, Sendai 980-8578, Japan b Life Sciences College, Nankai University, Weijin Road 94, Tianjin 300071, China Accepted 26 June 2003
Abstract Sulfonated cyclodextrins (s-a-CyD and s-h-CyD) were employed for constructing photosensitive thin films containing azobenzene residues. Azobenzene-modified poly(allylamine hydrochloride) (Az-PAH) and s-a-CyD or s-h-CyD were deposited alternately on the surface of a quartz slide to prepare multilayer thin films. Az-PAH formed inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-hCyD film. The Az-PAH isomerized from E- to Z-isomer in the films under UV light irradiation, and the original E-form was recovered under visible light irradiation or thermally in dark. The thermal isomerization followed the first-order kinetics in the swelled films though the kinetics deviated from the first-order plot in the dry films. The host – guest complexation of Az-PAH and s-a-CyD in the film affected significantly on the photochemical and thermal isomerization. D 2003 Elsevier B.V. All rights reserved. Keywords: Azobenzene; Photoresponse; Polyelectrolyte multilayer film; Cyclodextrin
1. Introduction The development of photosensitive polymetric materials has been a focal subject in materials science and technology. Azoaromatic polymers which contain azobenzene chromophores are widely used to develop photosensitive molecular assemblies including monolayers [1], Langmuir – Blodgett films [2], and organic –inorganic hybrid films [3]. Recently, polyelectrolyte multilayer (PEM) films have attracted much attention because of the simplicity of the technique and the versatility in the film design [4– 6]. In this context, it is interesting to construct polyelectrolyte multilayer assemblies containing azobenzene chromophores. From this viewpoint, different types of azobenzene-modified polyelectrolytes have been employed for constructing photosensitive PEM films in which the azobenzene residues exhibit photoisomerization [7 –10]. In this context, we have reported photoresponse of PEM films composed of an azobenzene-modified poly(allylamine
* Corresponding author. Tel.: +81-22-217-6841; fax: +81-22-2176840. E-mail address: [email protected] (J. Anzai). 0928-4931/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0928-4931(03)00050-X
hydrochloride) (Az-PAH), which were prepared by a layerby-layer deposition of Az-PAH and polyanion [11]. The Az-PAH exhibited photoisomerization from E- to Z-form in the film upon UV light irradiation and recovered the original E-form reversibly in dark or by visible light irradiation. In the thermal isomerization of Az-PAH from Z- to E-form in dark, the kinetics deviated from the firstorder plot due to strained environment in the film or a lack of free volume around the azobenzene residues. We also prepared photosensitive PEM films by chemical modification of PEM films with an active ester of azobenzene carboxylate. In this modified films, however, azobenzene residues formed aggregates and the photoresponse was suppressed significantly [12]. In this situation, it is interesting to use negatively charged cyclodextrin (CyD) as an anionic counterpart of cationic Az-PAH for constructing photosensitive PEM films. CyD might afford free volume around the azobenzene residues through host – guest complexation in the film, which probably enables the azobenzene residues to isomerize reversibly according to the first-order kinetics. Heretofore, PEM films composed of azobenzene polymer and ionic CyD have been prepared but photoisomerization was not reported [13,14].
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2. Experimental part 2.1. Materials An aqueous solution (20%) of poly(allylamine hydrochloride) [PAH; average molecular weight (MW), ca. 10,000] was purchased from Nittobo (Tokyo, Japan). AzPAH was prepared according to the reported procedure [11]. The content of azobenzene residue in the Az-PAH was determined to be 3.5 mol% (azobenene residues vs. total primary amino groups in the polymer chain) using UV – visible spectrophotometry. Sulfonated a-cyclodextrin (s-aCyD) and h-cyclodextrin (s-h-CyD), in which 8 –9 and 12– 14 primary and secondary hydroxyl groups in parent a-CyD and h-CyD are sulfonated, were obtained from Aldrich Chemical (Milwaukee, WI). The chemical structures of Az-PAH and CyD derivatives used are depicted in Fig. 1. All other chemicals used are of highest grade available. 2.2. Apparatus UV –visible absorption spectra were measured using a Shimadzu UV-3100PC spectrophotometer (Kyoto, Japan) for monitoring the formation of PEM films and photoresponse. A quartz-crystal microbalance (QCM) (QCA 917 system, Seiko EG&G, Tokyo, Japan) was employed for the gravimetric analysis of swelling property of the films.
for 30 min to deposit the first layer through a hydrophobic force of attraction. After being rinsed in PBS for 10 min to remove any weakly adsorbed Az-PAH, the quartz slide was immersed in s-a-CyD or s-h-CyD solution (100 Ag ml 1 PBS ) for 30 min to deposit the second layer. This process provides both sides of the quartz slide with an AzPAH/s-CyD layer. The deposition was repeated to prepare multilayer films. After each deposition, the absorbance of the quartz slide around 330 nm, originating from the k – k* transition of the abenzene chromophore, was monitored. The preparation of the films was carried out at ca. 20 jC. The water uptake of the films upon swelling was evaluated by means of QCM, where the resonance frequency of the film-coated quartz resonator (9 MHZ) was measured in air for the dry and swelled films. 2.4. Photoirradiation A 500-W xenon lamp was used as light source for photoirradiation. The UV (320 < k < 380 nm) and visible light (k>450 nm) was isolated using Corning 7-37 and Toshiba Y-45 glass filters, respectively. The content of Zisomer in the irradiated samples was calculated from the decrease in absorbance at the k – k* absorption maximum, assuming that the absorbance of the Z-isomer at this wavelength is negligibly small compared with that of the E-isomer [15].
2.3. Preparation of polyelectrolyte multilayer films 3. Results and discussion The PEMs were prepared on the surface of a quartz slide (5 1 0.1 cm). The quartz slide was first treated in dichlorodimethylsilane (10% solution in toluene) overnight at room temperature to make the surface hydrophobic, and was washed with toluene, acetone, and distilled water. The silylated quartz slide was immersed in an Az-PAH solution (100 Ag ml 1, in a phosphate-buffered saline, PBS, pH 5.0)
Fig. 1. The chemical structures of Az-PAH and sulfonated CyDs used.
3.1. Preparation of Az-PAH/s-a-CyD and Az-PAH/s-b-CyD films The preparation of Az-PAH multilayer films was evaluated by measuring UV – visible absorption spectra of the films. Fig. 2 shows typical absorption spectra of Az-PAH/ s-a-CyD and Az-PAH/s-h-CyD films and the change in absorbance of the films as a function of the number of depositions. Both spectra exhibited a clear absorption maximum (kmax) arising from k – k* transition of E-azobenzene chromophores, and the intensity of the absorption band was enhanced linearly with increasing number of depositions. These results suggest that a constant amount of Az-PAH is adsorbed upon each deposition to form a layered assembly. Thus, s-a-CyD and s-h-CyD were successfully built into the Az-PAH multilayer films as anionic counterparts. It should be noted here that the kmax of the Az-PAH/s-a-CyD film is found at 335 nm while the AzPAH/s-h-CyD film exhibited absorption maximum at 326 nm. This might arise from the host – guest complexation of the azobenzene residues with s-a-CyD in the film because an aqueous solution of Az-PAH exhibited the kmax value at 326 nm. To confirm the complex formation, absorption spectra of Az-PAH were measured in aqueous solution in
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Fig. 2. UV – visible absorption spectra of Az-PAH/s-a-CyD (a) and Az-PAH/s-h-CyD films (b). The inset plots absorbance at kmax in k – k* transition region as a function of the number of layers.
the presence of s-a-CyD and s-h-CyD (Fig. 3). The kmax value of Az-PAH exhibited a red shift in the presence of sa-CyD whereas the addition of s-h-CyD induced no shift in kmax, suggesting the complexation of Az-PAH with s-aCyD. We have recently reported that s-a-CyD forms inclusion complex with methyl orange while s-h-CyD does not [16]. The loading of azobenzene residues per deposition on the quartz slide was estimated to be 8.3 10 8 mol cm 2 in Az-PAH/s-a-CyD film and 5.7 10 8 mol cm 2 for AzPAH/s-h-CyD film from the absorbance data, using a molar extinction coefficient of 2.5 104 M 1 cm 1 for the azobenzene unit. In any event, Az-PAH can be built into PEM film by a layer-by-layer deposition with s-a-CyD and s-h-CyD. The azobenzene residues in Az-PAH form inclusion complex with s-a-CyD in the PEM film.
Fig. 3. UV – visible absorption spectra of Az-PAH in solution in the presence and absence of s-CyDs. The concentration of s-CyD; none (a), 1 10 5 M s-a-CyD (b), and 1 10 5 M s-h-CyD (c). Az-PAH; 10 Ag/ml in 10 mM acetate – NaOH buffer (pH 5.0).
3.2. Photoresponse of the films Photoisomerization of azobenzene chromophores in condensed media depends significantly on the local environments such polarity, viscosity, and free volume distribution around the chromophores [1– 3,11,12]. Fig. 4 shows UV – visible absorption spectra of Az-PAH/s-a-CyD film (10 bilayers on both surfaces of the slide) before and after UV light irradiation in air. UV light irradiation induced a decrease in intensity of absorption band originating from k – k* transition of E-azobenzene residue, showing the formation of the mixture of E- and Z-form of azobenzene residues in the film. The isomerization reaction reached a photostationary state after ca. 30-min irradiation, in which the content of Z-form of the azobenzene residues was 16% in the film, and the resultant Z-isomer was converted nearly completely to the original E-form by visible light irradiation for 1 min. The photochemical reactions of the film can be
Fig. 4. UV – visible absorption spectra of (Az-PAH/s-a-CyD)10 film. (a) Before irradiation, (b) after UV light irradiation and (c) after visible light irradiation.
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induced repeatedly several times without any deterioration of the spectra. The Az-PAH/s-h-CyD film showed an almost the same behavior in the isomerization reactions. Thus, the isomerization reactions of azobenzene residue in the films were demonstrated to be reversible and reproducible. We studied the photoresponse of the films in air and in water (i.e., dry and swelled films). The contents of Z-form at the photostationary state in dry films were lower than those in swelled films. This may be caused by a compact packing of the polyelectrolyte chains in the dry films and, as a result, the free volume available around the azobenzene chromophores is limited in the films. It has been reported that photoisomerization of azobenzene is suppressed or inhibited in polymer films due to the close packing of the polymer chains and consequently the lack of free volume [3,11,12,17]. It is clear that host – guest complexation in the Az-PAH/s-a-CyD film did not improve the photoresponse in dry state. On the other hand, the content of Zisomer at the photostationary state was enhanced after swelling for both films. The QCM study showed that the water content of the swelled films was 61% and 55% for the Az-PAH/s-a-CyD and Az-PAH/s-h-CyD films, respectively. (The water content is defined as the weight of water in the film over the total weight of water plus film materials in the swelled film.) The effects of swelling on the content of Zisomer were much significant for the Az-PAH/s-a-CyD film; the content of Z-isomer in the swelled film was 49% as compared with 16% in the dry film. For the Az-PAH/s-hCyD film, the content of Z-isomer was 30% in the swelled sample and 20% in dry. This might be originating from the host – guest complexation of the azobenzene residues in this film, in view of the fact that CyD complexation often enables azobenzene derivatives to isomerize upon photoirradiation in condensed media where otherwise photoisomerization is severely suppressed or prohibited [18,19]. 3.3. Thermal isomerization reaction from Z- to E-form The thermal Z- to-E reverse isomerization was studied in dark by monitoring the recovery of the k –k* absorption band after the photostationary state had been obtained under UV light irradiation. It is known that the thermal Z to E isomerization usually follows first-order kinetics in solution [20]. In practice, the first-order plot for the isomerization of Az-PAH in an aqueous solution gave a straight line (the first-order rate constant was calculated to be 0.33 10 3 min 1 at 40 jC). Fig. 5 shows that the first-order plot deviated from a straight line for the dry films. The isomerization proceeded faster than in solution in the beginning and was followed by a slow process. This type of decay kinetics of azobenzene derivatives is often observed for film samples, and the fast process can be attributed to trapping of some portion of Zisomers in a strained environment in the film [2,3]. In the fast process of the decay, the isomerization rate in the AzPAH/s-a-CyD film was slower than that in the Az-PAH/s-h-
Fig. 5. First-order plots for the thermal Z to E isomerization of azobenzene residues in the films at 40 jC: (Az-PAH/s-a-CyD)10 film in dry (a) and swelled state (b); (Az-PAH/s-h-CyD)10 films in dry (c) and swelled state (d). The first-order plot was obtained according to the equation ln(C0/ C) = kt, where C0 and C denote the content of Z-isomer at time zero and time t, respectively, and k is the first-order rate constant.
CyD film, which may be ascribable to the fact that Az-PAH forms inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-h-CyD film. It has been reported that the rate of thermal isomerization of azoaromatic dyes decreased upon complexation with CyD [21]. In other words, s-a-CyD probably released the azobenzene residues from the strained environment to some extent through complexation in the Az-PAH/s-a-CyD film. On the contrary, the rate of decay followed first-order kinetics in the swelled films. The rate constants were 0.33 10 3 and 0.63 10 3 min 1 at 40 jC in the Az-PAH/s-a-CyD and Az-PAH/s-hCyD films, respectively. The rate of decay in the Az-PAH/sa-CyD film is slower than that in the Az-PAH/s-h-CyD film. Interestingly, the rate of decay in the swelled Az-PAH/ s-a-CyD film is comparable to that in solution. These observations suggest that azobenzene residues are free from any strained environment which accelerates the E to Z isomerization in the swelled films. This is not the case for Az-PAH/PVS films, in which the decay process did not follow the first-order kinetics even after swelling [11].
4. Conclusions Az-PAH/s-a-CyD and Az-PAH/s-h-CyD films were prepared successfully by a layer-by-layer deposition of cationic Az-PAH and the sulfonated CyDs. Az-PAH formed inclusion complex in the Az-PAH/s-a-CyD film but not in the Az-PAH/s-h-CyD film. The azobenzene residues isomerized from E- to Z-isomer in the films, and the original Eform was recovered in dark or under visible light irradiation reversibly. The complexation of Az-PAH and s-a-CyD in the film had a significant influence on the photochemical and thermal isomerization of azobenzene residues. It became apparent that s-a-CyD and s-h-CyD are useful mate-
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rials for improving photoresponse of azobenzene-containing polyelectrolyte films.
References [1] T. Seki, T. Fukuchi, K. Ichimura, Langmuir 16 (2000) 3564. [2] C. Tadeschi, M. Polli, M.P. Fontana, O. Pieroni, Thin Solid Films 284 – 285 (1996) 174. [3] Y. Imai, K. Naka, Y. Chujo, Macromolecules 32 (1999) 1013. [4] G. Decher, J.D. Hong, J. Schmitt, Thin Solid Films 210 – 211 (1992) 831. [5] G. Decher, Sciences 277 (1997) 1232. [6] T. Farhat, G. Yassin, S.T. Dubas, J.B. Schlenoff, Langmuir 15 (1999) 6621. [7] S. Dante, R. Advincula, C.W. Frank, P. Stroeve, Langmuir 15 (1999) 193. [8] F. Saremi, B. Tieke, Adv. Mater. 10 (1998) 288. [9] J.D. Hong, E.S. Park, A.L. Park, Langmuir 15 (1999) 6515.
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[10] A. Toutianoush, B. Tieke, Makromol. Chem., Rapid Commun. 19 (1998) 591. [11] I. Suzuki, T. Ishizaki, T. Hoshi, J. Anzai, Macromolecules 35 (2002) 577. [12] I. Suzuki, T. Ishizaki, H. Inoue, J. Anzai, Macromolecules 35 (2002) 6470. [13] P. Fischer, M. Koetse, A. Laschewsky, E. Wiscgerhoff, L. Jullien, A. Persoons, T. Verbiest, Macromolecules 33 (2000) 9471. [14] M. Koetse, A. Laschewsky, A.M. Jonas, T. Verbiest, Colloids Surf., A 198 – 200 (2002) 275. [15] G.S. Paik, H. Morawetz, Macromolecules 5 (1972) 171. [16] K. Sato, I. Suzuki, J. Anzai, Colloid Polym. Sci. (2003) (in press). [17] H. Nakahara, K. Fukuda, J. Colloid Interface Sci. 93 (1994) 1879. [18] J. Anzai, N. Sugaya, T. Osa, J. Chem. Soc., Perkin Trans. 2 (1994) 1879. [19] M. Ueda, H.-B. Kim, T. Ikeda, K. Ichimura, Chem. Mater. 4 (1992) 1229. [20] C.D. Eisenbach, Ber. Bunsenges. Phys. Chem. 84 (1980) 680. [21] A.M. Sanchez, R.H. de Rossi, J. Org. Chem. 61 (1996) 3446.