Langmuir 1992,8, 673-675
673
Complex Formation between Pyrene and 8-Cyclodextrin in Sol-Gel Glasses Kazunori Matsui College of Engineering, Kanto Gakuin University, Mutsuura, Kanazawa-ku, Yokohama 236, Japan Received March 25, 1991.In Final Form: October 7, 1991 The fluorescenceof pyrene was measured in the sol-gel reaction system of tetraethyl orthosilicate with 8-cyclodextrin (D-CD). At low 8-CD concentrations, the fluorescence intensity ratio of the third peak to the first peak (Z3/Z1) decreased during desiccation, indicating an increase in the environmental polarity. At high 8-CD concentrations, however,13/11increased during desiccation up to values similar to those for inclusion complexes formed from pyrene and 8-CD,which has a relatively hydrophobic cavity. From these results it is concluded that inclusion complexes made from pyrene and 8-CD are trapped in the sol-gel glasses.
Introduction There has recently been a growing interest in organic molecule doped inorganic materials prepared by the solgel process. The photophysics and photochemistry of such systems have been extensively studied, viz., the photochemical hole burning of 1,4-dihydroxyanthraquinone,l the excimer fluorescence of pyrene,213the fluorescence of 7-azaindole,4a proton-transfer reaction of ~ y r a n i n epho,~~~ to~hromism,~J' a photocatalytic reaction? and so on. It has been shown that changes in polarity during the sol-gel process can be probed by using the monomer fluorescence of pyrene.2J0 This is based on the fact that the intensity ratio, I3/Z1, of the third vibronic band to that of the 0-0 band decreases as the solvent polarity increases.11J2 In a previous study, we used this intensity ratio and suggested that the micellar formation of sodium dodecyl sulfate (SDS) occurs in a sol-gel silica;13that is, we found that 13/11 in xerogels with high SDS concentrations had values similar to those for micellar solutions of SDS. However, the details were not completely elucidated. There have been several fluorescence studies on inclusion complexes formed from pyrene and 8-cyclodextrin (8-CD).l4-l7 These results indicate that pyrene forms inclusion complexes with P-CD, which has a hydrophobic cavity. Therefore, by analogy to the micelles, these ~~
(1) Tani, T.; Namikawa, H.; Arai, K.; Makishima, A. J . Appl. Phys. 1985,58, 3559. (2) Kaufman, V. R.; Avnir, D. Langmuir 1986, 2, 717. (3) Mataui, K.; Usuki, N. Bull. Chem. SOC.Jpn. 1990, 63, 3516. (4) Mataui, K.; Matauzuka, T.; Fujita, H. J . Phys. Chem. 1989, 93, 4991. (5) Kaufman, V. R.;Avnir, D.; Pines-Rojanski, D.;Huppert, D. J . NonCryst. Solids 1988, 99, 379. (6) Pouxviel, J. C.; Parvaneh, S.;Knobbe, E. T.; Dunn, B. Solid State Ionics 1989,32133, 646. ( 7 ) Levy, D.; Avnir, D. J.Phys. Chem. 1988,92,4734. (8) Mataui, K.; Morohoshi, T.; Yoshida, S. Proceedings of the MRS Internotional Meeting on Advanced Materials, Tokyo; Materials Research Society: Pittaburg, 1989; Vol. 12, p 203. (9) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. J. Phys. Chem. 1989, 93, 7544. (10) Matsui, K.; Nakazawa, T. Bull. Chem. SOC.Jpn. 1990, 63, 11. (11) Nakajima, A. Bull. Chem. SOC.Jpn. 1971, 44, 3272. (12). Kalvanasundaram. . K.:. Thomas. J. K. J . Am. Chem. SOC.1977.99. . . 2039. (13) Mataui, K.; Nakazawa, T.; Morisaki, H. J . Phys. Chem. 1991,95, 976. (14) Edwards, H. E.; Thomas, J. K. Carbohydr. Res. 1978,65,173. (15) Y o m u , T.; Hoshino, M.; Imamura, M. J.Phys. Chem. 1982,86, 4426. (16) Nakajima, A. Spectrochim. Acta 1983,39A, 913. (17) Nakajima, A. Bull. Chem. SOC.Jpn. 1984,57, 1143.
0743-7463/92/2408-0673$03.00/0
inclusion complexes should also become trapped in solgel glasses. In addition, this material system is not so complicated as the micellar system. Thus, a study of the inclusion complexes in sol-gel glasses might provide a clue to the mechanism by which micelles are trapped in such glasses. In this work, the monomer fluorescencespectra of pyrene in sol-gel systems containing 8-CD were studied and the formation of inclusion complexes between pyrene and 8CD in the sol-gel glasses was confirmed.
Experimental Section Chemicals. Pyrene (Aldrich)was recrystallized several times from ethanol. Tetraethylorthosilicate (TEOS)from Tokyo Kasei was used without additional purification. 8-cyclodextrin (8-CD) from Wako was recrystallized from water. Ethanol was of a spectroscopic grade. Water was deionized and distilled. Sample Preparation. The sol-gel glass was prepared by the acidic hydrolysis of TEOS in ethanol. Ethanol containing 1 0 - 6 M pyrene, a solution of 8-CD in water, and TEOS were mixed together. The molar ratio of TE0S:water:ethanol was usually 1:6.2:3.8. The solutions were adjusted to a pH of 3 with HC1, and then stirred for 1h. Those for measurements of time dependence were simultaneouslyprepared. The solutionswere usually placed in plastic beakers sealed with a pinholed film. Some of the solutionswere placed in glass bottles with screw caps to prevent the evaporation of volatile components. Instrumentation. The fluorescencespectra were taken with a JASCO FP-770spectrofluorometerin rectangular excitation at room temperature. Results Figure 1shows examples of the fluorescence spectra of pyrene as measured in the starting solution (A), xerogel without 8-CD (B),and xerogel with 8-CD (C). The intensity ratio of the third peak (384 nm) to the first peak (373nm) was around 0.76 in the solutions regardless of the addition of 8-CD. 13/11 decreased from the 0.76 of solution (A) to 0.58 in the xerogel without 8-CD (B) aa previously observed,1° indicating an increase in the environmentalpolarity. When 8-CD was added,ZS/Il showed a dramatic increase to around 1.3 (C), indicating that the addition of 8-CD decreases the environmental polarity of the pyrene molecules in xerogels.11J2 Figure 2 shows the changes in 13/11during the sol-gelxerogel stages with and without 8-CD. The weight of the sol-gel systems was measured, and the normalized weight (wt/wt.d was plotted. Experiments were conducted in two 0 1992 American Chemical Society
Matsui
674 Langmuir, Vol. 8,No. 2, 1992
9
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Figure 3. Z3/Z1over time during the sol-gel process at a molar ratio of ethanoLTEOS = 1.9 and water:TEOS = 6.2: 0,without 8-CD; 0 , with 8-CD. Each gelation point is indicated on the abscissa.
400
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Figure 1. Fluorescence spectra of pyrene in the sol-gelsystem: (A) sol-gelsolution without 8-CD; (B)xerogel without LI-CD; (C) xerogelwith8-CD. TE0S:HzO:ethanol = 1:6.2:3.8 (molar ratio); [pyrene] = 10-6 M in ethanol; [b-CD] = 10-3 M in water.
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Figure 4. Zs/Zl over time during the sol-gel process at a molar ratio of ethanoLTEOS = 7.6 and water:TEOS = 6.2: 0,without 8-CD; 0 , with 8-CD. Each gelation point is indicated on the abscissa.
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took twice as long as for the completely open system (see the abscissa in Figure 2). After the vessel was opened at 300 h, 13/11initially decreased very gradually, and showed a relatively sharp decrease for the sample without 8-CD (A)and a rapid increase for the sample with 8-CD (A) after passing 900 h. These changes occurred during the steady decrease in weight (0) at a point where it reached around 45 % of its initial value. The final weight was 19% of the initial value for the initially closed system and 18% for the completely open system. The above results indicate that the same reactions occurred in both systems, although the rates were different, and that the evaporation of the solvents induced the changes in 13/11. Figures 3 and 4 show how 13/11changes over time for different ethano1:TEOS molar ratios in open vessels. By comparing the results in Figures 2-4 with each other, it can be seen that the drastic changes in 13/11 occurred later for higher ethanol levels. This also indicates that the evaporation of ethanol plays an important role in the changes in &/I1. The final values of 13/11after about 900 h are plotted in Figure 5 versus the 8-CD concentration in water used for the sol-gel process. The 13/11value increased with the 8-CD concentration, and had a final value of 1.3 for M. concentrations above The 1 3 / 1 1 values were measured after ethanol solutions containing pyrene ( M) and water solutions containing 8-CD M) were mixed together and stirred for 2 h. Figure 6 shows the results. When a solution did not contain
Complex Formation between Pyrene and /3-Cyclodextrin
Langmuir, Vol. 8, No. 2, 1992 675
The above results explain mostly the changes in '5 during the sol-gel process. In the starting sol-gel solution, 13/11
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Figure 5. Effect of p-CD concentration on13/11in xerogels. The concentration is given in terms of that in the water used for the sol-gel process.
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Figure 6. 13/11versus ratio of ethanokwater: [pyrene] = 10" M,0. M; [B-CD] = 0, 0;[B-CD] = M, D; [B-CD] =
@-CD,13/11decreased as the ethanokwater ratio decreased owing to an increase in polarity.l0 However, there was a significant increase in 13/11up to around 1.3-1.4 when the solution contained @-CD. One possible reason for this is that the change in the ethanokwater ratio also changes the pyrene and @-CDconcentrations. Even so, the increase in 13/11with the decrease in the ethanokwater ratio can mainly be attributed to the effect of the solvent composition, not to changes in the concentrations of pyrene and @-CD,because roughly similar tendencies were observed for two different @-CDconcentrations and M), although the 13/11 values were larger for the more concentrated solution of @-CD. These results indicate that the pyrene molecules become progressively solubilized in the cavities of @-CD,the environmental polarity of which is low, as the ethanol level decreases and the 8-CD concentration increases.
Discussion Previous studies have indicated that inclusioncomplexes are formed between pyrene molecules and @-CDs,and the equilibrium of the reaction is thought to be pyrene + /3-CD s pyrene-8-CD ~J~ where pyrenep-CD denotes the 1:l ~ o m p l e x . ~The estimated 1 3 / 1 1 value of the complex is 1.47.16 Since the fluorescence spectrum of pyrene in an ethanol-water-pCD system is the composite of that from the complex and that from the bulk phase of ethanol and water, 13/11should vary from 0.56 (in water without p-CD) to 1.47 (complex) as the solvent composition changes. The results in Figure 6 thus indicate that most of the pyrene is dissolved in the bulk phase at higher ethanol levels. However, the equilibrium shifts to the complex when the ethanokwater ratio becomes smaller than around 1. Finally)13/11approaches 1.47 at smaller ethanol levels. Further associations of the complexes) which would be revealed by an increase in Is/ I 1 up to around 2.0 in some cases,14have not been found under our experimental conditions.
pyrene is dissolved in the bulk phase, which consists of ethanol, water, and TEOS. While the hydrolysis and polymerization reactions proceed, the ethanol evaporates. Consequently) 13/11 decreases due to an increase in the environmental polarity when no @-CDis present.1° On the other hand, when the starting solution contains @-CD, pyrene gradually forms inclusion complexes with @-CD during the evaporation of the ethanol, as shown in Figure 6. Therefore, in contrast to the situation without @-CD, 13/11 increases as the sol-gel reaction proceeds. The increase in 13/11occurred simultaneously with the decrease in 13/11for the system without @-CD,further confirming that the change in the solvent composition causes these effects. The inclusion complexes are most probably 1:l complexes, not further aggregations of the complexes,l4 judging from the results in Figures 5 and 6.16 A remaining problem is why 13/11initially changes very slowly and then changes very rapidly at some point during the sol-gel process even though the ethanol continues to evaporate steadily. This is in contrast to the change in 13/11for the ethanol-water system (Figure 6). One possible explanation is as follows. The pyrene molecules in the sol-gel system are probably not confined uniformly throughout the medium, but in a relatively ethanol rich phase,6J0which is induced by the adsorption of water on the silica surface and the hydrophobic nature of pyrene. In addition, the polar silanol groups on the silica surface will also have an effect upon the environmental polarity of pyrene, particularly, when the gel becomes relatively dry. Therefore, the behavior of 13/11in an inhomogeneous sol-gel system may be different from that in a homogeneous solvent system. The explanation of the change in 13/11for a sol-gelsystem with 8-CD is basically the same as that for a sol-gel system with SDS mi~e1les.l~This suggests that the changes in 13/11during a sol-gel process that includes additives such as B-CD and SDS can be explained without considering special interactions between the additives and the siloxane polymers. In other words, the surface charge of the silica is probably negative owing to the presence of SiOgroups on the silica surface at pH 3: and so the electrostatic interaction between the silica surface and negatively charged dodecyl sulfate or naturally neutral 8-CD becomes less important. As seen above, the molecular environment becomes considerably more hydrophobic due to the inclusions than is the case with xerogels which have physisorbed water.l* Thus, it should be possible to modify the excited-state properties of organic molecules in sol-gel silica by adding p-CD.lS2l Such a variability will extend the potential applications of organic molecule doped silica.
Acknowledgment. We express our thanks to Messrs. Y. Arai, M. Tominaga, T. Kawata, T. Koganei, Y. Miura, and K. Mogi for their help with the experiments. Registry No. 8-CD pyrene complex, 83565-38-2. (18)Hench, L. L.; West, J. K. Chem. Reu. 1990,90,33.
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