- Email: [email protected]
Photochemistry of azobenzene in sol–gel systems
Journal of Non-Crystalline Solids 357 (2011) 100–104
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Photochemistry of azobenzene in sol–gel systems Tadashi Ide, Yuka Ozama, Kazunori Matsui ⁎ Department of Applied Material and Bio Science, College of Engineering, Kanto Gakuin University, Yokohama 236-8501, Japan
a r t i c l e
i n f o
Article history: Received 26 April 2010 Received in revised form 7 September 2010 Keywords: Sol–gel; Xerogel; Azobenzene; Photochromism
a b s t r a c t The photophysical and photochemical behavior of azobenzene incorporated into sol–gel systems was studied. Sols doped with azobenzene were prepared by the hydrolysis of tetramethoxysilane. The absorption spectra of azobenzene in sols with and without HCl as a catalyst showed photochromism; UV irradiation changes trans-azobenzene to cis-azobenzene, which returns to trans-azobenzene by successive visible light irradiation. The results indicate that azobenzene is not protonated in the sol prepared by the present conditions. The absorption spectra showed that the azobenzene-doped xerogel prepared without HCl is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption did not affect photochromic behavior and photo-reversible changes were observed in the xerogels. UV photolysis of the azobenzene-doped xerogel prepared with HCl so produced protonated benzo[c]cinnoline that photochromic behavior was deteriorated. Surface modified xerogels were prepared from the mixture of tetraethoxysilane and methyltriethoxysilane in order to make clear the effect of the surface silanol groups. It was shown that the formation of the protonated benzo[c]cinnoline is suppressed by the introduction of Si–CH3. These present results confirm that the acidic sites of Si–OH+ 2 of xerogels play an important role in the photochemical reaction of azobenezene in silica xerogels. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The photoisomerization of azobenzene (AB) and its derivatives has been of much interest from the viewpoints of the fundamental mechanism of photoisomerization and its potential applications in optical switches and memories, molecular machines, and nanodevices [1,2]. The following two mechanisms have been proposed for the photoisomerization of trans-AB; with excitation to the S1 (n,π*) state the isomerization proceeds via inversion about one nitrogen atom in the same molecular plane, while excitation to the S2 (π,π*) leads to isomerization via rotation around the N=N double bond [1]. Fujino and Tahara, however, have suggested that the inversion mechanism may take part also in the isomerization following the S2 (π,π*) excitation from the time-resolved Raman study [3]. Chang et al. have proposed that the rotation channel is operative for the photoisomerization of trans-AB in a nonviscous solvent, whereas the concerted inversion channel operates when the rotation channel is obstructed in a viscous solvent, in inclusion complexes or through chemical modification [1]. The photochemical studies of AB and its derivatives have been done in various environments such as Nafion membranes, cyclodextrins, zeolites, aluminophosphate, sol–gel glasses, lysozyme, and so on [4–15]. The inversion mechanism is considered to be operative in
⁎ Corresponding author. Tel.: +81 45 786 7157; fax: + 81 45 786 7098. E-mail address: [email protected] (K. Matsui). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.009
these environments, because the rotation channel is obstructed in such a restricted space. The AB systems follow two different reactions depending on the photolysis condition; upon excitation in a neutral medium, cis–trans photoisomerization occurs and photostationary mixtures of both isomers are obtained. When the photolysis is done in the acidic condition, AB changes to benzo[c]cinnoline (BC), and variable amounts of benzidine (BZ) are produced [16]. Photoirradiation of water-swollen Nafion-H+ membrane soaking AB results in BC, and is also produced if the number of AB molecules in each water cluster of the Nafion membrane is greater than 2 [5]. Corma et al. reported that photolysis of AB in the presence of a series of acidic zeolites gives rise to BC and BZ [6]. Lei et al. also reported that photolysis of AB in aluminophosphate leads to the formation of BC and BZ and the Lewis acid sites of aluminophosphate are considered to play an important role in the photocyclization chemistry [7]. Sol–gel glasses doped with organic molecules are promising materials for photoresponsive materials [17–19]. For example, morphological and optical properties including nonlinear optical performance have been studied for azo-polymer-based sonogel hybrid composites and azo-dye confined SiO2 sonogel films [20,21]. The sonogel host systems have provided thermal and mechanical stability protecting the activate chromophores and preserving their optical properties. Furthermore, photochromism becomes an excellent probe for the microstructure of matrices, because the reaction is sensitive to the microenvironmental factors such as polarity, viscosity, acid–base interactions, free volume and so on [10]. Therefore, photochromism of AB in sol–gel glasses is of considerable interest.
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
Fig. 1. Absorption spectra of azobenzene in a sol without HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
In this paper, we studied the photochemistry of AB in the sol–gel systems of tetramethoxysilane (TMOS), especially paying attention to the effect of acids. 2. Experimental section Chemicals. Azobenzene (AB, Aldrich), benzo[c]cinnoline (BC, Tokyo Kasei), tetramethoxysilane (TMOS, Tokyo Kasei), tetraethoxysilane (TEOS, Tokyo Kasei), methyltriethoxysilane (MTEOS, Tokyo Kasei), ethanol (Tokyo Kasei), and hydrochloric acid (Wako) were used as received. Water was deionized and distilled. Sample preparation. As a typical preparation condition, TMOS (5 mL), ethanol (5 mL, containing 10− 3 mol/L AB or BC) and water (2.5 mL) were mixed together. A slight amount of HCl was added as a catalyst in some solutions: 200 μL of 0.1 M or 1 M. These solutions were divided into two equally and placed in a plastic beaker sealed with a pin-holed film after stirring for 1 h. Gelation occurred after a few days and the gels were dried at an ambient temperature for a month. Modified xerogels were also prepared from the 5 mL mixed solution of TEOS (2.5 mL) and MTEOS (2.5 mL), ethanol (5 mL with 10− 3 mol/L AB ), water 2.0 mL, and 500 μL of 0.1 M HCl. The error was estimated for the sample preparation to be less than 5%. Measurements. Absorption spectra were recorded with a JASCO Ubest-50 spectrophotometer. Fluorescence spectra were taken with a JASCO FP-770 spectrophotometer. The accuracy of absorption measurement was ±0.3 nm and ±0.001 absorbance. The accuracy of fluorescence measurement was ±1.5 nm.
Fig. 2. Absorption spectra of azobenzene in the xerogel prepared without HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
101
Fig. 3. Absorbance change at 320 nm of the azobenzene-doped xerogels prepared without HCl (○) and with 200 μL of 1 M HCl (▲) under repeated irradiation of UV and visible light for 3 min.
For photochromic measurements, samples were irradiated with a high pressure Hg lamp (Ushio, 500 W) through filters (Toshiba UVD33s and IRA-25 S) for UV and (L-42 and IRA-25 S) for visible light. All measurements were done at room temperature. 3. Results Fig. 1 shows the absorption spectra of AB in a sol without HCl measured as prepared (a), after UV irradiation for 1 h (b), and after subsequent visible irradiation for 1 h (c). Fig. 1a shows the peaks at 228 and 315 nm along with the very weak band 431 nm, corresponding to those of trans-AB. UV irradiation induced new bands at 241 and 279 nm, and increased the absorbance of 431 nm with decrease in the absorbance of the trans-AB bands, as shown in Fig. 1b. After being subsequently exposed to visible light, the spectrum returned to the initial one (Fig. 1c). These spectral changes indicate the reversible trans–cis–trans reactions of AB in the sol [7]. Similar photochemical changes were observed in the xerogel prepared from a sol without HCl. Fig. 2a shows the absorption spectrum of AB in the xerogel. The absorption spectrum has the peaks at 229, 320, and 421 nm, which are similar to those of the sol. A closer look reveals the spectral changes such as the red shift of the 315 nm band to 320 nm, and the blue shift and slight increase of the 421 nm band in comparison with that at 431 nm from the sol to xerogel. After
Fig. 4. Absorption spectra of azobenzene in the xerogel prepared with 200 μL of 0.1 M HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
102
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
Fig. 5. Absorption spectra of azobenzene in the xerogel prepared with 200 μL of 1 M HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
Fig. 7. Fluorescence spectra of benzo[c]cinnoline and UV-irradiated azobenzene for 1 h in the acidic xerogels with 200 μL of 1 M HCl. The excitation wavelength was 360 nm.
UV irradiation for 1 h, new bands are observed at around 234 and 298 nm with a shoulder at 251 nm as shown in Fig. 2b, while the initial bands decrease. Visible irradiation of the UV-irradiated xerogel for 1 h alters the spectrum to the original one as shown in Fig. 2c. Fig. 3 shows the absorbance change at 320 nm of the xerogel prepared without HCl (○) for repeated irradiation of UV and visible light for 3 min. Reversible changes were observed over 10 cycles. Fig. 4 shows the absorption spectra of AB in the xerogel prepared with 200 μL of 0.1 M HCl. The sol showed similar reversible photochemical changes as that without HCl (not shown here). The as-grown xerogel (Fig. 4a) has a very similar spectrum to that prepared without HCl, indicating the no specific effect of acid on AB in the xerogel in this condition. After UV irradiation for 1 h, however, a strong band is observed at 249 nm along with a decreased absorbance at the 321 nm band (Fig. 4b). Upon visible irradiation for 1 h, the absorbance of the 321 nm band recovered to a half of its initial value, while that of the 249 nm band changed little as shown in Fig. 4c. From these results, the photochromic characteristic of AB is suggested to be deteriorated by HCl in the xerogel, while there is no effect in the sol. Fig. 5a shows the absorption spectrum of AB in the xerogel prepared with 200 μL of 1 M HCl. It should be noted here that the absorbance around 320 nm relatively decreases in comparison with that around 420 nm, indicating an increase in a fraction of the protonated trans-AB for the xerogel with ten times concentrated HCl [7,27]. A band of 250 nm appears with a doubled intensity of the 322 nm band after UV irradiation for 1 h (Fig. 5b) and spectral change
does not occur on successive visible irradiation for 1 h (Fig. 5c). Thus the results obtained from the xerogels with HCl (Figs. 4 and 5) indicate that UV photolysis of AB in the xerogel with HCl forms other species. Due to the effect of the photochemical reactions, photochromic behavior was deteriorated by the repeated irradiation of UV and visible light for the xerogel with 200 μL of 1 M HCl as shown in Fig. 3 (▲). The sol prepared with 200 μL of 1 M HCl showed a similar spectrum and photochromic behavior to that having no HCl (not shown here). Therefore, the environment of the AB molecules in the sol is considered to be analogous to that of AB molecules in methanolswollen Nafion-H+, where AB molecules are solubilized in the methanol pools, and their photochemical and photophysical behaviors are not intervened by the Nafion protons [5]. In order to identify the species, BC was dissolved in acidic xerogel. Fig. 6 shows the absorption spectrum of BC in the xerogel prepared from a sol with 200 μL of 1 M HCl. The bands observed at 252, 361, and 410 nm indicate the protonated BC (BCH+) [7,23]. The spectrum is quite similar to that of AB after UV irradiation in acidic xerogel (Fig. 5b). The absorption spectrum of BC in xerogel without HCl showed an absorption peak at 313 nm with a shoulder at 360 nm. These results indicate the almost complete formation of BCH+ from UV-irradiated AB in acidic xerogel. Fig. 7 shows the fluorescence spectra of BC in the acidic xerogel and UV-irradiated AB in acidic xerogel for 1 h. The fluorescence of BC in the acidic xerogel with a peak around 480 nm is very similar to that of BCH+ [23]. The fluorescence spectrum of UV-irradiated AB in the acidic xerogel also shows the same one as that of BCH+ in the acidic xerogel, whereas there was no fluorescence before UV
Fig. 6. Absorption spectrum of benzo[c]cinnoline in the xerogel prepared with 200 μL of 1 M HCl.
Fig. 8. Absorption spectrum of azobenzene in the xerogel prepared from 2.5 mL of TEOS and 2.5 mL of MTEOS with 200 μL of 1 M HCl.
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
103
Scheme 1. Photochemical reactions of azobenzene in xerogels prepared under acidic conditions.
irradiation. Therefore the fluorescence spectra also confirm the formation of BCH+ from UV-irradiated AB in acidic xerogel. In order to verify the surface effect, surface modified xerogels were prepared from the mixture of MTEOS and TEOS because the introduction of alkyl groups decreases the surface silanol groups of xerogels [22,25,26]. Fig. 8a shows the absorption spectrum of AB in xerogel prepared from TEOS (2.5 mL) and MTEOS (2.5 mL) with 500 μL of 0.1 M HCl. The band intensities decrease after UV irradiation for 1 h (Fig. 8b) and then almost recover to the initial ones (Fig. 8c). It is indicated that the formation of the BCH+ is suppressed by the introduction of Si–CH3 in comparison with the xerogel having 200 μL of 0.1 M HCl (Fig. 4). These present results confirm that the acidic sites of Si–OH+ 2 of xerogels play an important role in the photochemical formation of BCH+. 4. Discussion The absorption spectra of dopant molecules can give information concerning the local environment around the molecules in the sol–gel systems. The absorption spectra of AB showed the red shift of the 315 nm band to 320 nm, and the blue shift and slight increase of the 421 nm band in comparison with that at 431 nm from the sol to xerogel without HCl (Figs. 1 and 2). The red-peak shift indicates that the environment around AB in the sol is less polar than that in the xerogel as previously observed in the spectral shift from the Nafion membrane to water [5], corresponding well to the results probed by solvatochromic dye (Nile Red) [22]. The blue shifted n-π* band around 420 nm of the xerogel is ascribed to the formation of hydrogen bonding between the azo groups and silanol and/or water molecules on the silica surfaces [9,10]. It is known that if the azo nitrogens of trans-AB are protonated, the absorption band at 425 nm strongly appears and that at 315 nm considerably decreases [7,27]. As the positively charged surface of Si–OH+ 2 increases with an increase in HCl [24], the acidic sites of the Si–OH+ 2 of xerogels increase a fraction of the protonated trans-AB as seen in the xerogels in Fig. 5. For AB in the xerogels prepared without HCl, almost complete photochromism can be seen. The adsorption of AB on silica surfaces does not affect photochromic behavior, suggesting enough free volumes for the rotation channel and/or the concerted inversion mechanism for the photoisomerization of trans-AB. Photochromic behavior is deteriorated for the xerogels with HCl, which is explained in the following Scheme 1. Trans-AB is considered to be mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules as discussed above. As the acidic sites of Si–OH+ 2 increase with an increase in HCl, a fraction of AB adsorbed with the acidic sites increases. UV irradiation of protonated trans-AB produces protonated cis-AB with successive conversion to BCH+. It is also important to note that some portion of trans-AB adsorbed on the surface without the protonation changes to cis-AB by UV irradiation and the cis-AB can
promote unrestricted approach of the azo groups to the acidic sites, resulting in the BCH+ as observed in aluminophosphate [7]. The evidence of BZ formation was not apparently obtained from the absorption and fluorescence spectra. Considering that the AB molecules are dispersed in xerogels and AB molecules are immobilized on the xerogel surfaces, it is reasonable that BZ molecules are not formed in the xerogels. 5. Conclusion Azobenzene was successfully embedded in sol–gel xerogels prepared from alkoxysilanes. The absorption blue shift of the n-π* band around 420 nm reveals that azobenzene is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption does not affect photochromic behavior and photo-reversible changes are observed. By the addition of HCl as a catalyst into sol–gel systems, a fraction of azobenzene adsorbed on the acidic sites of xerogels increases because the positively charged surface such as Si–OH+ 2 increases with an increase in HCl. UV photolysis of azobenzene in the xerogel with HCl produces protonated benzo[c]cinnoline. Due to the effect of the photochemical reaction, photochromic behavior is deteriorated. Acknowledgment A part of this work was supported by MEXT. HAITEKU (2005– 2009). References [1] C.-W. Chang, Y.-C. Lu, T.-T. Wang, E.W.-G. Diau, J. Am. Chem. Soc. 126 (2004) 10109 and references therein. [2] V. Balzani, A. Credi, M. Venturi, in: Molecular Devices and Machines — A Journey into the Nano World, WILEY-VCH, Weinheim, 2003, pp. 177–199, pp. 206–209. [3] T. Fujino, T. Tahara, J. Phys. Chem. A 104 (2000) 4203. [4] P. Bortolus, S. Monti, J. Phys. Chem. 91 (1987) 5046. [5] C.-H. Tung, J.-Q. Guan, J. Org. Chem. 61 (1996) 9417. [6] A. Corma, H. García, S. Iborra, V. Martí, M.A. Miranda, J. Primo, J. Am. Chem. Soc. 115 (1993) 2177. [7] Z. Lei, A. Vaidyalingam, P. Dutta, J. Phys. Chem. B 102 (1998) 8557. [8] P.L. Gentili, U. Costantino, R. Vivani, L. Latterini, M. Nocchetti, G.G. Aloisi, J. Mater. Chem. 14 (2004) 1656. [9] M. Ueda, H.-B. Kim, T. Ikeda, K. Ichimura, J. Non-Cryst. Solids 163 (1993) 125. [10] M. Ueda, H.-B. Kim, K. Ichimura, Chem. Mater. 6 (1994) 1771. [11] J.D. Badjić, N.M. Kostić, J. Phys. Chem. B 105 (2001) 7482. [12] W. Que, X. Hu, X.L. Xia, L. Zhao, Opt. Express 15 (2007) 480. [13] M.L. Bossi, D.H. Murgida, P.F. Aramendia, J. Phys. Chem. B 110 (2006) 13804. [14] O. Ohtani, T. Itoh, Y. Monna, R. Sasai, T. Shichi, T. Yui, K. Takagi, Bull. Chem. Soc. Jpn 78 (2005) 698. [15] T. Inada, T. Terabayashi, Y. Yamaguchi, K. Kato, K. Kikuchi, J. Photochem. Photobiol. A Chem. 175 (2005) 100. [16] J. Griffiths, Chem. Soc. Rev. 1 (1972) 481. [17] B. Dunn, J.I. Zink, Chem. Mater. 9 (1997) 2280 and references therein. [18] D. Levy, Chem. Mater. 9 (1997) 2666 and references therein. [19] K. Matsui, in: S. Sakka (Ed.), Handbook of Sol–Gel Science and Technology, vol. 1, Kluwer Academic Publishers, Dordrecht, 2004, pp. 459–484.
104
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
[20] O.G. Morales-Saavedra, F.G. Ontiveros-Barrera, V. Torres-Zúñiga, J. GuadalupeBañuelos, R. Ortega-Martínez, E. Rivera, T. García, Smart Mater. Struct. 18 (2009) 085024. [21] V. Torres-Zúñiga, O.G. Morales-Saavedra, E. Rivera, J.O. Flores-Flores, J.G. Bañuelos, R. Ortega-Martínez, J. Modern Opt. 57 (2010) 65. [22] K. Matsui, K. Nozawa, Bull. Chem. Soc. Jpn 70 (1997) 2331. [23] J. Waluk, A. Grabowska, B. Pakuła, J. Lumin. 21 (1980) 277.
[24] J.C. Pouxviel, S. Parvaneh, E.T. Knobbe, B. Bunn, Solid State Ionics 32 (33) (1989) 646. [25] K. Matsui, M. Tominaga, Y. Arai, H. Satoh, M. Kyoto, J. Non-Cryst. Solids 169 (1994) 295. [26] C. Rottman, G.S. Grader, Y. DeHazan, D. Avnir, Langmuir 12 (1996) 5505. [27] H. Rau, Ber. Bunsenges. Phys. Chem. 71 (1967) 48.
Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Photochemistry of azobenzene in sol–gel systems Tadashi Ide, Yuka Ozama, Kazunori Matsui ⁎ Department of Applied Material and Bio Science, College of Engineering, Kanto Gakuin University, Yokohama 236-8501, Japan
a r t i c l e
i n f o
Article history: Received 26 April 2010 Received in revised form 7 September 2010 Keywords: Sol–gel; Xerogel; Azobenzene; Photochromism
a b s t r a c t The photophysical and photochemical behavior of azobenzene incorporated into sol–gel systems was studied. Sols doped with azobenzene were prepared by the hydrolysis of tetramethoxysilane. The absorption spectra of azobenzene in sols with and without HCl as a catalyst showed photochromism; UV irradiation changes trans-azobenzene to cis-azobenzene, which returns to trans-azobenzene by successive visible light irradiation. The results indicate that azobenzene is not protonated in the sol prepared by the present conditions. The absorption spectra showed that the azobenzene-doped xerogel prepared without HCl is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption did not affect photochromic behavior and photo-reversible changes were observed in the xerogels. UV photolysis of the azobenzene-doped xerogel prepared with HCl so produced protonated benzo[c]cinnoline that photochromic behavior was deteriorated. Surface modified xerogels were prepared from the mixture of tetraethoxysilane and methyltriethoxysilane in order to make clear the effect of the surface silanol groups. It was shown that the formation of the protonated benzo[c]cinnoline is suppressed by the introduction of Si–CH3. These present results confirm that the acidic sites of Si–OH+ 2 of xerogels play an important role in the photochemical reaction of azobenezene in silica xerogels. © 2010 Elsevier B.V. All rights reserved.
1. Introduction The photoisomerization of azobenzene (AB) and its derivatives has been of much interest from the viewpoints of the fundamental mechanism of photoisomerization and its potential applications in optical switches and memories, molecular machines, and nanodevices [1,2]. The following two mechanisms have been proposed for the photoisomerization of trans-AB; with excitation to the S1 (n,π*) state the isomerization proceeds via inversion about one nitrogen atom in the same molecular plane, while excitation to the S2 (π,π*) leads to isomerization via rotation around the N=N double bond [1]. Fujino and Tahara, however, have suggested that the inversion mechanism may take part also in the isomerization following the S2 (π,π*) excitation from the time-resolved Raman study [3]. Chang et al. have proposed that the rotation channel is operative for the photoisomerization of trans-AB in a nonviscous solvent, whereas the concerted inversion channel operates when the rotation channel is obstructed in a viscous solvent, in inclusion complexes or through chemical modification [1]. The photochemical studies of AB and its derivatives have been done in various environments such as Nafion membranes, cyclodextrins, zeolites, aluminophosphate, sol–gel glasses, lysozyme, and so on [4–15]. The inversion mechanism is considered to be operative in
⁎ Corresponding author. Tel.: +81 45 786 7157; fax: + 81 45 786 7098. E-mail address: [email protected] (K. Matsui). 0022-3093/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2010.09.009
these environments, because the rotation channel is obstructed in such a restricted space. The AB systems follow two different reactions depending on the photolysis condition; upon excitation in a neutral medium, cis–trans photoisomerization occurs and photostationary mixtures of both isomers are obtained. When the photolysis is done in the acidic condition, AB changes to benzo[c]cinnoline (BC), and variable amounts of benzidine (BZ) are produced [16]. Photoirradiation of water-swollen Nafion-H+ membrane soaking AB results in BC, and is also produced if the number of AB molecules in each water cluster of the Nafion membrane is greater than 2 [5]. Corma et al. reported that photolysis of AB in the presence of a series of acidic zeolites gives rise to BC and BZ [6]. Lei et al. also reported that photolysis of AB in aluminophosphate leads to the formation of BC and BZ and the Lewis acid sites of aluminophosphate are considered to play an important role in the photocyclization chemistry [7]. Sol–gel glasses doped with organic molecules are promising materials for photoresponsive materials [17–19]. For example, morphological and optical properties including nonlinear optical performance have been studied for azo-polymer-based sonogel hybrid composites and azo-dye confined SiO2 sonogel films [20,21]. The sonogel host systems have provided thermal and mechanical stability protecting the activate chromophores and preserving their optical properties. Furthermore, photochromism becomes an excellent probe for the microstructure of matrices, because the reaction is sensitive to the microenvironmental factors such as polarity, viscosity, acid–base interactions, free volume and so on [10]. Therefore, photochromism of AB in sol–gel glasses is of considerable interest.
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
Fig. 1. Absorption spectra of azobenzene in a sol without HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
In this paper, we studied the photochemistry of AB in the sol–gel systems of tetramethoxysilane (TMOS), especially paying attention to the effect of acids. 2. Experimental section Chemicals. Azobenzene (AB, Aldrich), benzo[c]cinnoline (BC, Tokyo Kasei), tetramethoxysilane (TMOS, Tokyo Kasei), tetraethoxysilane (TEOS, Tokyo Kasei), methyltriethoxysilane (MTEOS, Tokyo Kasei), ethanol (Tokyo Kasei), and hydrochloric acid (Wako) were used as received. Water was deionized and distilled. Sample preparation. As a typical preparation condition, TMOS (5 mL), ethanol (5 mL, containing 10− 3 mol/L AB or BC) and water (2.5 mL) were mixed together. A slight amount of HCl was added as a catalyst in some solutions: 200 μL of 0.1 M or 1 M. These solutions were divided into two equally and placed in a plastic beaker sealed with a pin-holed film after stirring for 1 h. Gelation occurred after a few days and the gels were dried at an ambient temperature for a month. Modified xerogels were also prepared from the 5 mL mixed solution of TEOS (2.5 mL) and MTEOS (2.5 mL), ethanol (5 mL with 10− 3 mol/L AB ), water 2.0 mL, and 500 μL of 0.1 M HCl. The error was estimated for the sample preparation to be less than 5%. Measurements. Absorption spectra were recorded with a JASCO Ubest-50 spectrophotometer. Fluorescence spectra were taken with a JASCO FP-770 spectrophotometer. The accuracy of absorption measurement was ±0.3 nm and ±0.001 absorbance. The accuracy of fluorescence measurement was ±1.5 nm.
Fig. 2. Absorption spectra of azobenzene in the xerogel prepared without HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
101
Fig. 3. Absorbance change at 320 nm of the azobenzene-doped xerogels prepared without HCl (○) and with 200 μL of 1 M HCl (▲) under repeated irradiation of UV and visible light for 3 min.
For photochromic measurements, samples were irradiated with a high pressure Hg lamp (Ushio, 500 W) through filters (Toshiba UVD33s and IRA-25 S) for UV and (L-42 and IRA-25 S) for visible light. All measurements were done at room temperature. 3. Results Fig. 1 shows the absorption spectra of AB in a sol without HCl measured as prepared (a), after UV irradiation for 1 h (b), and after subsequent visible irradiation for 1 h (c). Fig. 1a shows the peaks at 228 and 315 nm along with the very weak band 431 nm, corresponding to those of trans-AB. UV irradiation induced new bands at 241 and 279 nm, and increased the absorbance of 431 nm with decrease in the absorbance of the trans-AB bands, as shown in Fig. 1b. After being subsequently exposed to visible light, the spectrum returned to the initial one (Fig. 1c). These spectral changes indicate the reversible trans–cis–trans reactions of AB in the sol [7]. Similar photochemical changes were observed in the xerogel prepared from a sol without HCl. Fig. 2a shows the absorption spectrum of AB in the xerogel. The absorption spectrum has the peaks at 229, 320, and 421 nm, which are similar to those of the sol. A closer look reveals the spectral changes such as the red shift of the 315 nm band to 320 nm, and the blue shift and slight increase of the 421 nm band in comparison with that at 431 nm from the sol to xerogel. After
Fig. 4. Absorption spectra of azobenzene in the xerogel prepared with 200 μL of 0.1 M HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
102
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
Fig. 5. Absorption spectra of azobenzene in the xerogel prepared with 200 μL of 1 M HCl: (a) as prepared, (b) after UV irradiation for 1 h, (c) after subsequent visible irradiation for 1 h.
Fig. 7. Fluorescence spectra of benzo[c]cinnoline and UV-irradiated azobenzene for 1 h in the acidic xerogels with 200 μL of 1 M HCl. The excitation wavelength was 360 nm.
UV irradiation for 1 h, new bands are observed at around 234 and 298 nm with a shoulder at 251 nm as shown in Fig. 2b, while the initial bands decrease. Visible irradiation of the UV-irradiated xerogel for 1 h alters the spectrum to the original one as shown in Fig. 2c. Fig. 3 shows the absorbance change at 320 nm of the xerogel prepared without HCl (○) for repeated irradiation of UV and visible light for 3 min. Reversible changes were observed over 10 cycles. Fig. 4 shows the absorption spectra of AB in the xerogel prepared with 200 μL of 0.1 M HCl. The sol showed similar reversible photochemical changes as that without HCl (not shown here). The as-grown xerogel (Fig. 4a) has a very similar spectrum to that prepared without HCl, indicating the no specific effect of acid on AB in the xerogel in this condition. After UV irradiation for 1 h, however, a strong band is observed at 249 nm along with a decreased absorbance at the 321 nm band (Fig. 4b). Upon visible irradiation for 1 h, the absorbance of the 321 nm band recovered to a half of its initial value, while that of the 249 nm band changed little as shown in Fig. 4c. From these results, the photochromic characteristic of AB is suggested to be deteriorated by HCl in the xerogel, while there is no effect in the sol. Fig. 5a shows the absorption spectrum of AB in the xerogel prepared with 200 μL of 1 M HCl. It should be noted here that the absorbance around 320 nm relatively decreases in comparison with that around 420 nm, indicating an increase in a fraction of the protonated trans-AB for the xerogel with ten times concentrated HCl [7,27]. A band of 250 nm appears with a doubled intensity of the 322 nm band after UV irradiation for 1 h (Fig. 5b) and spectral change
does not occur on successive visible irradiation for 1 h (Fig. 5c). Thus the results obtained from the xerogels with HCl (Figs. 4 and 5) indicate that UV photolysis of AB in the xerogel with HCl forms other species. Due to the effect of the photochemical reactions, photochromic behavior was deteriorated by the repeated irradiation of UV and visible light for the xerogel with 200 μL of 1 M HCl as shown in Fig. 3 (▲). The sol prepared with 200 μL of 1 M HCl showed a similar spectrum and photochromic behavior to that having no HCl (not shown here). Therefore, the environment of the AB molecules in the sol is considered to be analogous to that of AB molecules in methanolswollen Nafion-H+, where AB molecules are solubilized in the methanol pools, and their photochemical and photophysical behaviors are not intervened by the Nafion protons [5]. In order to identify the species, BC was dissolved in acidic xerogel. Fig. 6 shows the absorption spectrum of BC in the xerogel prepared from a sol with 200 μL of 1 M HCl. The bands observed at 252, 361, and 410 nm indicate the protonated BC (BCH+) [7,23]. The spectrum is quite similar to that of AB after UV irradiation in acidic xerogel (Fig. 5b). The absorption spectrum of BC in xerogel without HCl showed an absorption peak at 313 nm with a shoulder at 360 nm. These results indicate the almost complete formation of BCH+ from UV-irradiated AB in acidic xerogel. Fig. 7 shows the fluorescence spectra of BC in the acidic xerogel and UV-irradiated AB in acidic xerogel for 1 h. The fluorescence of BC in the acidic xerogel with a peak around 480 nm is very similar to that of BCH+ [23]. The fluorescence spectrum of UV-irradiated AB in the acidic xerogel also shows the same one as that of BCH+ in the acidic xerogel, whereas there was no fluorescence before UV
Fig. 6. Absorption spectrum of benzo[c]cinnoline in the xerogel prepared with 200 μL of 1 M HCl.
Fig. 8. Absorption spectrum of azobenzene in the xerogel prepared from 2.5 mL of TEOS and 2.5 mL of MTEOS with 200 μL of 1 M HCl.
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
103
Scheme 1. Photochemical reactions of azobenzene in xerogels prepared under acidic conditions.
irradiation. Therefore the fluorescence spectra also confirm the formation of BCH+ from UV-irradiated AB in acidic xerogel. In order to verify the surface effect, surface modified xerogels were prepared from the mixture of MTEOS and TEOS because the introduction of alkyl groups decreases the surface silanol groups of xerogels [22,25,26]. Fig. 8a shows the absorption spectrum of AB in xerogel prepared from TEOS (2.5 mL) and MTEOS (2.5 mL) with 500 μL of 0.1 M HCl. The band intensities decrease after UV irradiation for 1 h (Fig. 8b) and then almost recover to the initial ones (Fig. 8c). It is indicated that the formation of the BCH+ is suppressed by the introduction of Si–CH3 in comparison with the xerogel having 200 μL of 0.1 M HCl (Fig. 4). These present results confirm that the acidic sites of Si–OH+ 2 of xerogels play an important role in the photochemical formation of BCH+. 4. Discussion The absorption spectra of dopant molecules can give information concerning the local environment around the molecules in the sol–gel systems. The absorption spectra of AB showed the red shift of the 315 nm band to 320 nm, and the blue shift and slight increase of the 421 nm band in comparison with that at 431 nm from the sol to xerogel without HCl (Figs. 1 and 2). The red-peak shift indicates that the environment around AB in the sol is less polar than that in the xerogel as previously observed in the spectral shift from the Nafion membrane to water [5], corresponding well to the results probed by solvatochromic dye (Nile Red) [22]. The blue shifted n-π* band around 420 nm of the xerogel is ascribed to the formation of hydrogen bonding between the azo groups and silanol and/or water molecules on the silica surfaces [9,10]. It is known that if the azo nitrogens of trans-AB are protonated, the absorption band at 425 nm strongly appears and that at 315 nm considerably decreases [7,27]. As the positively charged surface of Si–OH+ 2 increases with an increase in HCl [24], the acidic sites of the Si–OH+ 2 of xerogels increase a fraction of the protonated trans-AB as seen in the xerogels in Fig. 5. For AB in the xerogels prepared without HCl, almost complete photochromism can be seen. The adsorption of AB on silica surfaces does not affect photochromic behavior, suggesting enough free volumes for the rotation channel and/or the concerted inversion mechanism for the photoisomerization of trans-AB. Photochromic behavior is deteriorated for the xerogels with HCl, which is explained in the following Scheme 1. Trans-AB is considered to be mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules as discussed above. As the acidic sites of Si–OH+ 2 increase with an increase in HCl, a fraction of AB adsorbed with the acidic sites increases. UV irradiation of protonated trans-AB produces protonated cis-AB with successive conversion to BCH+. It is also important to note that some portion of trans-AB adsorbed on the surface without the protonation changes to cis-AB by UV irradiation and the cis-AB can
promote unrestricted approach of the azo groups to the acidic sites, resulting in the BCH+ as observed in aluminophosphate [7]. The evidence of BZ formation was not apparently obtained from the absorption and fluorescence spectra. Considering that the AB molecules are dispersed in xerogels and AB molecules are immobilized on the xerogel surfaces, it is reasonable that BZ molecules are not formed in the xerogels. 5. Conclusion Azobenzene was successfully embedded in sol–gel xerogels prepared from alkoxysilanes. The absorption blue shift of the n-π* band around 420 nm reveals that azobenzene is mostly adsorbed on silica surfaces by the hydrogen bonding between the azo groups and silanol and/or water molecules. The adsorption does not affect photochromic behavior and photo-reversible changes are observed. By the addition of HCl as a catalyst into sol–gel systems, a fraction of azobenzene adsorbed on the acidic sites of xerogels increases because the positively charged surface such as Si–OH+ 2 increases with an increase in HCl. UV photolysis of azobenzene in the xerogel with HCl produces protonated benzo[c]cinnoline. Due to the effect of the photochemical reaction, photochromic behavior is deteriorated. Acknowledgment A part of this work was supported by MEXT. HAITEKU (2005– 2009). References [1] C.-W. Chang, Y.-C. Lu, T.-T. Wang, E.W.-G. Diau, J. Am. Chem. Soc. 126 (2004) 10109 and references therein. [2] V. Balzani, A. Credi, M. Venturi, in: Molecular Devices and Machines — A Journey into the Nano World, WILEY-VCH, Weinheim, 2003, pp. 177–199, pp. 206–209. [3] T. Fujino, T. Tahara, J. Phys. Chem. A 104 (2000) 4203. [4] P. Bortolus, S. Monti, J. Phys. Chem. 91 (1987) 5046. [5] C.-H. Tung, J.-Q. Guan, J. Org. Chem. 61 (1996) 9417. [6] A. Corma, H. García, S. Iborra, V. Martí, M.A. Miranda, J. Primo, J. Am. Chem. Soc. 115 (1993) 2177. [7] Z. Lei, A. Vaidyalingam, P. Dutta, J. Phys. Chem. B 102 (1998) 8557. [8] P.L. Gentili, U. Costantino, R. Vivani, L. Latterini, M. Nocchetti, G.G. Aloisi, J. Mater. Chem. 14 (2004) 1656. [9] M. Ueda, H.-B. Kim, T. Ikeda, K. Ichimura, J. Non-Cryst. Solids 163 (1993) 125. [10] M. Ueda, H.-B. Kim, K. Ichimura, Chem. Mater. 6 (1994) 1771. [11] J.D. Badjić, N.M. Kostić, J. Phys. Chem. B 105 (2001) 7482. [12] W. Que, X. Hu, X.L. Xia, L. Zhao, Opt. Express 15 (2007) 480. [13] M.L. Bossi, D.H. Murgida, P.F. Aramendia, J. Phys. Chem. B 110 (2006) 13804. [14] O. Ohtani, T. Itoh, Y. Monna, R. Sasai, T. Shichi, T. Yui, K. Takagi, Bull. Chem. Soc. Jpn 78 (2005) 698. [15] T. Inada, T. Terabayashi, Y. Yamaguchi, K. Kato, K. Kikuchi, J. Photochem. Photobiol. A Chem. 175 (2005) 100. [16] J. Griffiths, Chem. Soc. Rev. 1 (1972) 481. [17] B. Dunn, J.I. Zink, Chem. Mater. 9 (1997) 2280 and references therein. [18] D. Levy, Chem. Mater. 9 (1997) 2666 and references therein. [19] K. Matsui, in: S. Sakka (Ed.), Handbook of Sol–Gel Science and Technology, vol. 1, Kluwer Academic Publishers, Dordrecht, 2004, pp. 459–484.
104
T. Ide et al. / Journal of Non-Crystalline Solids 357 (2011) 100–104
[20] O.G. Morales-Saavedra, F.G. Ontiveros-Barrera, V. Torres-Zúñiga, J. GuadalupeBañuelos, R. Ortega-Martínez, E. Rivera, T. García, Smart Mater. Struct. 18 (2009) 085024. [21] V. Torres-Zúñiga, O.G. Morales-Saavedra, E. Rivera, J.O. Flores-Flores, J.G. Bañuelos, R. Ortega-Martínez, J. Modern Opt. 57 (2010) 65. [22] K. Matsui, K. Nozawa, Bull. Chem. Soc. Jpn 70 (1997) 2331. [23] J. Waluk, A. Grabowska, B. Pakuła, J. Lumin. 21 (1980) 277.
[24] J.C. Pouxviel, S. Parvaneh, E.T. Knobbe, B. Bunn, Solid State Ionics 32 (33) (1989) 646. [25] K. Matsui, M. Tominaga, Y. Arai, H. Satoh, M. Kyoto, J. Non-Cryst. Solids 169 (1994) 295. [26] C. Rottman, G.S. Grader, Y. DeHazan, D. Avnir, Langmuir 12 (1996) 5505. [27] H. Rau, Ber. Bunsenges. Phys. Chem. 71 (1967) 48.