3376
Langmuir 2006, 22, 3376-3380
Zinc Chelation and Photofluorochromic Behavior of Spironaphthoxazine Intercalated into Hydrophobically Modified Montmorillonite Hiromasa Nishikiori,*,† Ryo Sasai,‡,§ Katsuhiko Takagi,‡ and Tsuneo Fujii† Department of EnVironmental Science and Technology, Faculty of Engineering, Shinshu UniVersity, Wakasato, Nagano 380-8553, Japan, and Department of Crystalline Materials Science, Graduate School of Engineering, Nagoya UniVersity, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan ReceiVed December 1, 2005. In Final Form: January 24, 2006 Spironaphthoxazine (SNO) and Zn2+ were intercalated into montmorillonite interlayers hydrophobically modified by the alkyltrimethylammonium cation during UV light irradiation. The fluorescence spectra of the montmorillonite composites were observed to vary with an increase in the UV and visible light irradiation times. These composites exhibited two types of fluorescence emissions: F1, which originates from a new species, Xs, which is different from SNO (ring-closed form) and merocyanine (MC; ring-open form), and F2, which originates from the MC-Zn complex. With increasing UV light irradiation time, the F1 intensities decreased, whereas the F2 intensities increased. Xs, which is an intermediate species between SNO and MC, was transformed into MC and then coordinated with Zn2+ (i.e., MC-Zn complex) during the UV light irradiation. The reaction rate of the formation of the MC-Zn complex decreased for the hydrophobically modified montmorillonite due to a longer alkyl chain. The retrieval changes in the F1 and F2 intensities were observed with an increasing visible light irradiation time, implying the dissociation of the MC-Zn complex into Xs and Zn2+. The dissociation especially occurred for the hydrophobically modified montmorillonite with a longer alkyl chain. The formation and disappearance of Xs and the MC-Zn complex obeyed first-order kinetics, and therefore the interconversion between Xs and MC could be regarded as the rate-determining step of the whole reaction during the UV and visible light irradiations. The photoinduced reactions of the SNO species and Zn2+ were profoundly affected by the physicochemical environment provided by the clay interlayers. It is concluded that the present photoreactions can be controlled not only by the amounts of the intercalated SNO species and Zn2+, but also by the hydrophobic environment created by the surfactant molecules.
Introduction Clay minerals are naturally produced, widely distributed throughout the world, and available as materials that have little effect on the global environment. High-performance devices are required in the electronics fields for the sake of fast processing and recording large amounts of information, which are supported by quantum-directed materials controlled by photons. In this respect, photochromic functions can be applied to photonswitching devices as well as to the elements of displays.1-4 Spirooxazines (SOs) are widely investigated for their photochromic behavior, which is stable against repeated performance.2,4 The color change in the SOs is caused not only by the photoisomerization that transforms it into merocyanine (MC), but also by metal-chelation for coordination with specific metal ions.5,6 SOs also exhibit “photofluorochromism”; that is, the fluorescence intensity and maximum change by the light * Corresponding author. Address: Department of Environmental Science and Technology, Faculty of Engineering, Shinshu University, Wakasato, Nagano 380-8553, Japan. Phone: +81-26-269-5536. Fax: +81-26-2695550. E-mail:
[email protected]. † Shinshu University. ‡ Nagoya University. § Present address: Division of Environmental Research, EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. (1) Irie, M. Chem. ReV. 2000, 100, 1685. (2) Berkovic, G.; Krongauz, V.; Welss, V. Chem. ReV. 2000, 100, 1741. (3) Matsuda, K. Irie, M. J. Photochem. Photobiol. C 2004, 5, 169. (4) Yuan, W.; Sun, L.; Tang, H.; Wen, Y.; Jiang, G.; Huang, W.; Jiang, L.; Song, Y.; Tian, H.; Zhu, D. AdV. Mater. 2005, 17, 156. (5) Zhou, J.; Zhou, F.; Li, Y.; Zhang, F.; Song, X. J. Photochem. Photobiol. A 1995, 92, 193. (6) Preigh, M. J.; Lin, F.; Ismail, K. Z.; Weber, S. G. J. Chem. Soc., Chem. Commun. 1995, 2091.
irradiation and can potentially be applicable to various photofunctional materials. The photochromic behavior of SOs in heterogeneous solid systems, such as polymer matrixes,7-12 self-assembled films,13-16 sol-gel matrixes,17-22 and clay interlayers,23,24 have been well studied by a number of research groups. It is very important for the fluorescent and photoreaction properties of such SO species and their metal complexes to be investigated in solid matrixes or on solid surfaces to provide new light-emitting materials. (7) Sugiyama, K.; Nakano, H.; Ohga, K. Macramol. Chem. Phys. 1994, 195, 3915. (8) Atassi, Y.; Delaire, J. A.; Nakatani, K. J. Phys. Chem. 1995, 99, 16320. (9) Hu, A. T.; Wang, W.-H.; Lee, H.-J. J. Macromol. Sci. Pure Appl. Chem. 1996, A33, 803. (10) Arai, K.; Ohyama, T.; Shitara, Y. Polym. J. 1997, 29, 780. (11) Romani, A.; Chidichimo, G.; Formoso, P.; Manfredi, S.; Favaro, G.; Mazzucato, U. J. Phys. Chem. B 2002, 106, 9490. (12) Kim, S.-H.; Ahn, C.-H.; Keum, S.-R.; Koh, K. Dyes Pigm. 2005, 65, 179. (13) Kim, S.-H.; Lee, S.-M., Park, J.-H.; Kim, J.-H.; Koh, K.-N.; Kang, S.-W. Dyes Pigm. 2000, 45, 51. (14) Yagi, S.; Minami, N.; Fujita, J.; Hyodo, Y.; Nakazumi, H.; Yazawa, T.; Kami, T.; Ali, A. H. Chem. Commun. 2002, 2444. (15) Chen, H.; Li, Y.; Huo, F.; Wang, Z.; Zhang, X. Chem. Lett. 2003, 32, 1094. (16) Suk, S.; Suh, H.-J.; An, W. G.; Kim, J.-H.; Jin, S.-H.; Kim, S.-H.; Gal, Y.-S.; Koh, K. Mater. Sci. Eng. C 2004, 24, 135. (17) Biteau, J.; Chaput, F.; Boilot, J.-P. J. Phys. Chem. 1996, 100, 9024. (18) Hou, L.; Schmidt, H. J. Mater. Sci. 1996, 31, 3427. (19) Sun, X.; Fan, M.; Knobbe, E. T. Mol. Cryst. Liq. Cryst. 1997, 297, 57. (20) Schaudel, B.; Guermeur, C.; Sanchez, C.; Nakatani, K.; Delaire, J. A. J. Mater. Chem. 1997, 7, 61. (21) Ortica, F.; Favaro, G. J. Phys. Chem. B 2000, 104, 12179. (22) Lafuma, A.; Chodorowski-Kimmes, S.; Quinn, F. X.; Sanchez, C. Eur. J. Inorg. Chem. 2003, 221. (23) Nishikiori, H.; Sasai, R.; Arai, N.; Takagi, K. Chem. Lett. 2000, 1142. (24) Gentili, P. L.; Costantino, U.; Nocchetti, M.; Miliani, C.; Favara, G. J. Mater. Chem. 2002, 12, 2872.
10.1021/la053247o CCC: $33.50 © 2006 American Chemical Society Published on Web 03/02/2006
SNO Intercalated into Montmorillonite Scheme 1. Molecular Forms and Fluorescence Processes of SNO Species in Solvents.
Langmuir, Vol. 22, No. 7, 2006 3377 Table 1. Intercalation Properties of Alkyltrimethylammonium-Modified Montmorillonite Samples alkyl group Hex
Oct
Dec
Cet
d (before the SO intercalation)/Å 13.9 15.4 16.7 20.8 d (after the SO intercalation)/Å 14.2 15.2 15.9 18.0 amount of the intercalated SO/CEC% 0.10 0.96 2.70 4.16 amount of the intercalated Zn2+/CEC% 17.7 16.6 21.5 35.5
Experimental Section
The photochromism of a cationic-functionalized spiropyran intercalated into clays was studied by one of our groups.25 Nonionic spiropyran could also be intercalated into clays, the interlayers of which are modified by organic surfactants and made hydrophobic to control the photochromic behavior of the dyes.26-28 In our previous study, spironaphthoxazine (SNO) was introduced into surfactant-modified montmorillonite dispersed in acetone under UV illumination in the presence of zinc ions.23 It was reported that the fluorescence spectra of the samples exhibited photochromic behavior arising from the photoinduced interconversion between the Zn-untreated SNO itself and the Znchelated complex of the SNO-derived MC, by monitoring the fluorescence spectral changes of the Zn-complex of MC (MCZn complex). Recently, we observed three different fluorescence spectra originating from different excited species in various solvents at room temperature, as shown in Scheme 1.29 The only fluorescence from the excited states of SO was seen in the less polar solvents. In the polar protic solvents, however, a different fluorescence was seen from an excited intermediate species (Xh*), which was formed from the SO* to be hydrogen-bonded to the solvent molecules during the excited-state relaxation process. The structure of Xh or Xh* is suggested to be a nonplanar zwitterion formed by bond cleavage of the oxazine ring. In aprotic, highly polar solvents, a ground-state intermediate species (Xs) was transformed from some ground-state SO by strong solvation, and the excited species (Xs*) was observed to emit a fluorescence. The structure of Xs or Xs* is suggested to be similar to that of Xh or Xh*, but different from them at the point of interaction with the solvent molecules. In this study, the origins of the fluorescence and photochromic chelation properties of the SNO species and Zn2+ intercalated into the hydrophobic montmorillonite were investigated by measuring the fluorescence and excitation spectra. The spectroscopic and photoreaction properties significantly depend on the surrounding environment. The reaction of the species accelerated by UV and visible light irradiations will be discussed on the basis of the fact that the physicochemical properties of the microscopic environment in the clay interlayer space were modified by surfactants. (25) Takagi, K.; Kurematsu, T.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 2 1995, 1667. (26) Seki, T.; Ichimura, K. Macromolecules 1990, 23, 31. (27) Tomioka, H.; Itoh, T. J. Chem. Soc., Chem. Commun. 1991, 532. (28) Takagi, K.; Kurematsu, T.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 2 1991, 1517. (29) Nishikiori, H.; Tanaka, N.; Takagi, K.; Fujii, T. Res. Chem. Intermed. 2003, 29, 485.
SNO was synthesized and purified according to a procedure described elsewhere.30 ZnCl2 (Wako Chemicals, extra pure grade), acetone (Dojin Chemicals, spectro-grade), and hexyl- (C6H13), octyl(C8H17), decyl- (C10H21), and cetyl- (C16H33) trimethylammonium bromides (Tokyo Kasei) were used without further purification. Deionized and distilled water was used. The purified standard montmorillonite clay (Na0.33(Al1.67Mg0.33)Si4O10(OH)2‚nH2O), with a cation exchange capacity (CEC) of 119 mmol/100 g, was purchased from the Clay Science Society of Japan. The aqueous suspensions of montmorillonite (Mont) were magnetically stirred for 40 h together with the 100% CEC surfactant molecules, that is, hexyl- (Hex), octyl- (Oct), decyl- (Dec), and cetyl- (Cet) trimethylammonium bromides.31 These surfactantmodified clays were suspended in acetone and mixed with 50 mol % of SNO and excess amounts of ZnCl2 in the montmorillonite CEC. By stirring under UV light (Hg lamp, λ ) 350 ( 50 nm) for 48 h, the SNO and Zn2+, as the Mc-Zn complex, were adsorbed onto the hydrophobic clays (Hex-, Oct-, Dec-, and Cet-Monts), then completely dried at 60 °C, which are called Hex-, Oct-, Dec-, and Cet-Mont(S)s, respectively. The layer spaces of the clay composites were determined by Cu KR radiation operated at an applied voltage of 40 kV and current of 40 mA using an X-ray diffractometer (Rigaku RINT-2100). The amounts of the intercalated SNO and Zn2+ were estimated by measurement of the absorption spectra using both a spectrophotometer (JASCO V-550) and titration by chelation with EDTA, respectively. The fluorescence spectra of the hydrophobic montmorillonite containing SNO and Zn2+ were observed as a function of time with UV (350 ( 10 nm) or visible (450 ( 10 nm) light irradiation, and the photochromic behavior was studied using a fluorescence spectrophotometer (JASCO FP-750). These wavelengths, 350 and 450 nm, were selected because they can be strongly absorbed by SNO or Xs and the MC-Zn complex, respectively. The ratio of the irradiative intensity of 350 nm light to 450 nm light was 1:1.3. We could not observe significant changes in absorption or diffuse reflectance spectra for the clay samples during the UV or visible light irradiation because the amount of SNO was much smaller than that of the clay. On the other hand, the fluorescence spectroscopy was sensitive enough to observe the slight changes.
Results and Discussion Characterization of Surfactant-Modified Montmorillonite. Table 1 shows the layer spaces (d/Å) before and after the intercalation of SNO and Zn2+ together with the intercalation amounts of SNO and Zn2+. Since the layer space of the original Na+ montmorillonite is known to be 12.3 Å, the intercalation of the long alkyl-chained surfactants increases the separation between layers. The d values in Table 1 corresponded with those reported in previous papers.32,33 The net layer spaces of the clay composites, that is, Hex-, Oct-, Dec-, and Cet-Monts, are 4.3, 5.8, 7.1, and 11.2 Å, respectively, obtained by subtracting the clay layer thickness (9.6 Å) from the gross layer spaces. These (30) Chu, N. Y. C.; Photochromism: Molecules and Systems; Du¨rr, H., BouasLaurent, H., Eds.; Elsevier: Amsterdam, 1990; p 506. (31) Suito, E.; Arakawa, M.; Kondo, M. Kogyo Kagaku Zasshi 1963, 66, 1618. (32) Bala, P.; Samantaray, B. K.; Srivastava, S. K. Mater. Res. Bull. 2000, 35, 1717. (33) Jiang, J-.Q.; Cooper, C.; Ouki, S. Chemosphere 2002, 47, 711.
3378 Langmuir, Vol. 22, No. 7, 2006
Nishikiori et al.
Figure 1. (a) Fluorescence and (b) excitation spectra for Oct-Mont(S). The excitation wavelengths for the fluorescence spectra are (1) 350 and (2) 450 nm, and the emission wavelengths for the excitation spectra are (1) 450 and (2) 540 nm, respectively.
Figure 2. Changes in fluorescence spectra for Oct-Mont(S) during the (a) UV and (b) visible light irradiations. The spectra were observed upon excitation at 350 nm, after the light irradiations for (1) 0, (2) 5, (3) 15, (4) 30, and (5) 60 min.
results suggest that, depending on the alkyl chain length, the surfactant molecules are adsorbed on the clay layers with different configuration, which is the flat-lying mono- or bilayer, or the standing parallel monolayer.31 The results can also be interpreted as follows: all the surfactant molecules form parallel-oriented monolayer aggregates on the clay layers. On the basis of the length of each surfactant molecule and thickness of the clay layer, the orientation angles of the surfactant molecules toward the clay-layer plane were estimated to be 23.7, 25.9, 26.7, and 28.5° for the Hex-, Oct-, Dec-, and Cet-Monts, respectively. The layer spacing was slightly changed by the intercalation of SNO and Zn2+ that approached the layer distance (15.1 Å) of the Zn-exchanged montmorillonite, implying that some of the surfactant molecules were replaced by Zn2+. The amounts of SNO and Zn2+ intercalated in the surfactant-modified montmorillonite increased with increasing hydrophobicity in the clay interlayers. Only a small proportion of SNO to the montmorillonite CEC was intercalated into the montmorillonite because the intercalation was by a hydrophobic interaction. The intercalation efficiency of SNO, therefore, was improved with increasing the hydrophobicity of the interlayers. On the other hand, all the clay samples incorporated a much larger amount of Zn2+ than that of SNO. More hydrophobic surfactant molecules are more easily exchanged for Zn2+ on the surfactant-modified montmorillonite since the original layer surface is highly hydrophilic. Fluorescence and Excitation Spectra of the Montmorillonite Composites Containing SNO and Zn2+. Figure 1 shows the fluorescence (λex ) 350 and 450 nm) and excitation (λem ) 450 and 540 nm) spectra of the Oct-Mont(S), obtained by intercalating SNO and Zn2+ into the Oct-Mont during UV light irradiation. The fluorescence spectrum measured upon excitation at 350 nm exhibited two emission maxima centered at around 430 and 520 nm. Upon excitation at 450 nm, only one peak is observed at around 520 nm. The excitation spectra obtained by monitoring the fluorescence at 450 and 540 nm consisted of bands at around 370 and 480 nm, respectively. The fluorescence and the excitation
maxima at ∼430 and ∼370 nm, respectively, are reported to originate from Xs, referred to as F1.29 On the other hand, the fluorescence and the excitation maxima at ∼520 and ∼480 nm, respectively, originate from the MC-Zn complex, referred to as F2.23 These two types of fluorescences, F1 and F2, are also observed in the interlayers of the Hex-, Dec-, and Cet-Mont(S)s. The intercalated SNO molecules partly exist as Xs in the ground state. The formation of Xs means that the SNO molecules strongly interacted with the polar parts of the surfactant molecules. Particular attention was not paid to the F1 found in the wavelength region shorter than that of the MC-Zn complex in our previous study.23 On the other hand, as the MC-Zn complex could hardly be detected in the filtrate of the intercalated suspensions, it could be reasonably assumed that the zinc ions were chelated with MC under UV light irradiation in acetone, and most of the resulting MC-Zn complex was intercalated into the clay samples. The total amounts of SNO and Zn2+ intercalated in the surfactantmodified montmorillonite depend on the hydrophobicity in the interlayer, as shown in Table 1, whereas that of the intercalated MC-Zn complex is independent of it. Since the total amounts of SNO and Zn2+ include those of the components of the MCZn complex, the experimental results indicate that the amounts of the species intercalated in the clay interlayer as SNO and Zn2+(not the complex), depend on their hydrophobicity. Changes in the Fluorescence Spectra of the Montmorillonite Composites Containing SNO and Zn2+ under the Influence of Light Irradiation. Figure 2 shows the changes in the fluorescence spectra (λex ) 350 nm) of the Oct-Mont(S) sample observed during the (a) UV (350 nm) and (b) subsequent visible (450 nm) light irradiations. These incident UV and visible lights of 350 and 450 nm can be absorbed by SNO or Xs and the MC-Zn complex, respectively. The F1 intensities decreased, but the F2 intensities increased with the UV irradiation time. In addition, the retrieval changes were observed during the visible light irradiation. Similar spectral changes were seen in all the surfactant-modified clay composites in addition to the Oct-Mont(S). The fluorescence spectra of the samples remained unchanged
SNO Intercalated into Montmorillonite
Figure 3. Changes in fluorescence intensities and first-order fitting curves for Hex- ((), Oct- (9), Dec- (2), and Cet-Mont(S)s (b) during the UV and visible light irradiations. The excitation and emission wavelengths are (a) 350 and 420 nm and (b) 450 and 540 nm, respectively.
both in intensity and in the wavelength of the emission maximum when they were kept for 1 day at room temperature in the dark. The spectral changes indicate that the excited Xs was transformed into MC, which then interacted with Zn2+ to form the MC-Zn complex within the interlayers of the surfactant-modified montmorillonite during the UV light irradiation. On the other hand, the excited MC-Zn complex was observed to be dissociated into the components, MC and Zn2+, during visible light irradiation, the former of which immediately changed into Xs. In a previous paper, it was suggested that SNO, Zn2+, and the MC-Zn complex were intercalated into the surfactant-modified montmorillonite, and the SNO + Zn2+ reversibly changed into the MC-Zn complex.23 The present results indicate that the interconversion between Xs and the MC-Zn complex is more feasible rather than the previously reported interconversion between SNO and the MC-Zn complex.23 Upon dissociation of the zinc complex (MC-Zn) in the clay, the recovered Xs and Zn2+ remain close enough to react with each other and were expected to be reversibly subjected to photochromic behavior by the formation of the MC-Zn complex. Indeed, the present samples exhibit a photochromic behavior, although the SNO molecules are slightly dissolved in the clay interlayers. Figure 3 shows the changes in the intensities of F1 (λex ) 350 nm, λem ) 420 nm) and F2 (λex ) 450 nm, λem ) 540 nm) in the Hex-, Oct-, Dec-, and Cet-Mont(S)s with time during the UV and subsequent visible light irradiation. In all of the samples, the interconversion between Xs and the MC-Zn complex in the clay was observed because of the influence of the incident light. Stronger Xs (F1) and weaker MC-Zn complex (F2) fluorescence intensities were observed in the clay modified by employing the longer alkyl-chained surfactants, which is the more highly hydrophobic situation. The intercalated amount of SNO, which was much smaller than that of Zn2+, increased with
Langmuir, Vol. 22, No. 7, 2006 3379
the length of the alkyl chain of the surfactant-modified montmorillonite, as shown in Table 1. This is clearly reflected in the increase in the F1 intensities. The MC-Zn complex is more polar than Xs and can therefore be more stable within the more hydrophilic clay interlayers. Although Table 1 shows that the longer alkyl-chained montmorillonites more favorably accommodate the larger amount of Zn2+, this is presumably explained by the assumption that the MC-Zn complex formed by the chelation of MC with Zn2+under UV light irradiation during the initial stage was efficiently intercalated into the surfactantmodified clays. The MC-Zn complex as well as the Zn2+ were easily substituted with the more hydrophobic surfactant molecules in the clays. However, a larger proportion of the MC-Zn complex was probably dissociated into Xs and Zn2+ in the more hydrophobic clays. Therefore, the amount of the MC-Zn complex introduced into the clay interlayers could be considered to be restricted by the small amount of the SNO species and the hydrophobicity in the interlayers, despite the excess amount of Zn2+. Photoreaction Kinetics of SNO Species in SurfactantModified Montmorillonite. It can be assumed that the ringopening (Xs) and ring-closing (MC) interconversions of the SNO molecules are the rate-determining steps of the Zn-chelation of SNO during the UV and visible light irradiations. This is because the formation of the MC-Zn complex both in solvents and in the hydrophobic clay was found to be subject to a quasi firstorder reaction in previous studies.5,25,27 On the basis of this assumption, the changes in the fluorescence intensities of Xs and the MC-Zn complexes were plotted versus the irradiation time of the UV and visible lights. The plots were confirmed to obey first-order kinetics, as shown in Figure 3. The ring-opening and ring-closing reactions of the four systems, could therefore be estimated by the resulting apparent-rate constants, as summarized in Table 2. The rate constants of the consumption of Xs and the formation of MC-Zn under the influence of 350 nm light tend to decrease with the increasing hydrophobicity of the modifying surfactants. The values for F1 and F2 are within ∼0.09-0.18 and 0.05-0.09 min-1, respectively. On the other hand, the rate constants of the retrieval processes using 450 nm incident light roughly increased with an increase in the hydrophobicity. That is, the rate constants for F1 and F2 are within 0.017-0.023 and 0.004-0.011 min-1, respectively. These results indicate that the hydrophobic medium suppresses the ring-opening reaction of Xs but accelerates the ring-closure of the MC formed by the dissociation of the MCZn complex. The MC-Zn complex, having a higher dipole moment than Xs, favors the more hydrophilic environment. In conclusion, the reaction rates due to the 350 nm light are about 10 times faster than those due to the 450 nm light, although the irradiative intensity of 350 nm light is weaker than that of 450 nm light. This indicates that the quantum yield of the former reaction is obviously higher than that of the latter. The surfactantmodified clay interlayers provide a reaction environment suitable for the facile dissociation of the MC-Zn complex into Xs and Zn2+. The retrieval reaction rates in the environment are noted to be much lower than that of the chelation of Xs with Zn2+ forming the MC-Zn complex. However, the rate constants for the consumption of one species, Xs, or the MC-Zn complex were not comparable with those of the corresponding production of another species. Therefore, it can be concluded that the kinetic study of the fluorescent species Xs and the MC-Zn complex, is complicated by the reaction of the low-fluorescent SNO and the nonfluorescent MC-Zn complex in the present photochromic reactions. It has been reported that the nonfluorescent complex
3380 Langmuir, Vol. 22, No. 7, 2006
Nishikiori et al.
Table 2. Rate Constants (min-1) of the Photoinduced Reaction of Xs and the MC-Zn Complex in Alkyltrimethylammonium-Modified Montmorillonite Samplesa alkyl group Hexirradiation by 350 nm light irradiation by 450 nm light
Oct-
Dec-
Cet-
F1
F2
F1
F2
F1
F2
F1
F2
-0.176 0.017
0.089 -0.004
-0.114 0.019
0.077 -0.006
-0.102 0.023
0.062 -0.011
-0.085 0.020
0.051 -0.011
a
The rate constants were estimated by changes in the fluorescence intensities of Xs (F1) and the MC-Zn complex (F2) vs the irradiation time by the 350 and 450 nm lights. The positive and negative values denote the formation and consumption rates of these species, respectively.
was, in fact, observed in a solution containing SNO and metal ions.5,6 The fluorescence quantum yield of SNO is very low because the unshared electron pair on the imine N quenches the oxazine-ring fluorescence. The surfactant molecule or the metal ion occupies this electron pair on the Xs or MC, which can strongly emit fluorescence. Therefore, a type of MC-Zn complex in which the metal ion does not interact with the N is nonfluorescent. The rate constants estimated from the F1 changes are greater than those estimated from the F2 changes with both the 350 and 450 nm light irradiations. The photoinduced reaction probably involves conversions from Xs to the nonfluorescent MC-Zn complex and from the nonfluorescent MC-Zn complex to SNO. Thus, all the photoinduced reactions of the SNO species may depend on the physicochemical environment in the clay interlayers.
Conclusions Montmorillonite interlayers were modified by organic surfactants to control the hydrophobicity of the interlayers. Both SNO and Zn2+ could be intercalated into the hydrophobic montmorillonite under the influence of UV light. The fluorescence properties of SNO and the related species, that is, Xs (F1) and the Zn-MC complex (F2), intercalated into the hydrophobically modified montmorillonite have been investigated. Detailed studies of the fluorescence spectral changes of the above species have been conducted for various UV and visible light irradiation times from the viewpoints of the microscopic environments in the clay interlayers. The clay samples exhibited two types of fluorescences, F1 and F2, before light irradiation. The F1 decreased, but the F2 increased in intensity with an increase in the irradiation time by UV light, indicating that Xs was transformed into MC and coordinated with Zn2+ in the clays. The complex formation was
slower in the samples modified by the longer alkyl chain. In the case of the complex dissociation, the F1 increased and the F2 decreased with an increase in the visible light irradiation time. The complex dissociation, forming Xs and Zn2+, was as faster in the samples modified by the longer alkyl chain. The interconversion between Xs and MC could be regarded as the rate-determining step, obeying first-order kinetics for the formation and dissociation of the MC-Zn complex under UV and visible light irradiations. The nonionic species (SNO) is more easily intercalated into the more hydrophobic clay interlayers. At the same time, the ionic species (the MC-Zn complex and Zn2+) were also easily exchanged with the more hydrophobic surfactant molecules to be intercalated into the clays, although the complex was probably dissociated into Xs and Zn2+ after the intercalation. Therefore, the amount of the MC-Zn complex could be considered to be restricted by the small amount of SNO molecules and the hydrophobic surfactant molecules, despite the excessive adsorption of zinc ions into the clay interlayers. On the other hand, the rate constant for the consumption of one species (Xs or the MC-Zn complex) does not correspond with that for the formation of another species. Therefore, the interconversion between the fluorescent Xs with the MC-Zn complex is complicated by the reaction of the low-fluorescent SNO and the nonfluorescent MC-Zn complex under photochemical conditions. All the photoinduced reactions of the SNO species have been concluded to depend on the physicochemical environment in the clay interlayers. It is suggested that these photoreactions can be controlled by not only the amounts of the intercalated SNO species and Zn2+, but also the hydrophobic field of the surfactant molecules. LA053247O
