Langmuir 1991,7, 2866-2868
2866
Luminescence of Tris(2,2’-bipyridine)ruthenium(11) in Sol-Gel Glasses Kazunori Matsui,’ Kei Sasaki, and Nobuyuki Takahashi College of Engineering, Kanto Gakuin University, Mutsuura, Kanazawa-ku, Yokohama 236, Japan Received May 14,1991. In Final Form: August 6, 1991 The luminescenceof tris(2,2’-bipyridine)ruthenium(II) (R2+)was studied in the sol-gel reaction system of tetraethyl orthosilicate. The spectrum showed a blue shift during the sol-gel stage, indicating R2+to be entangled by siloxane polymers and finally to become bound tightly to the sol-gel silica. Although the relative rates of the spectral shift differed depending on the preparation conditions,a very similar tendency was obtained on plotting the emission maximum against the viscosity for solutions with a different waterto-silane ratio at pH = 3.0. The relation between the emission maximum and the viscosity of a solution at pH = 1.0 differed from cases at pH = 3.0. Electrostatic interaction between the surface of the siloxane polymer and R2+is thus essential for stabilizing R2+on the surface with the charge varying with pH.
Introduction The photochemistry and photophysics of tris(2,2’-bipyridine)ruthenium(II) (R2+) have been extensively studied in various systems such as micelles and col1oids.l-3 In such systems, the photoinduced charge separation of R2+ molecules adsorbed on solid surfaces is of interest and useful from the viewpoints of photocatalytic reactions? solar energy conversion,5 and the like. Unlike semiconductors, in which charge separation occurs by absorbing light, even transparent glasses used as supporting materials modify the photochemical reactionsS6 For example, polymerized silica particles which incorporate R2+provide excellent charge separation.& Interactions between silica surfaces and R2+ are thus of considerable interest. The luminescence of R2+ has been used to determine the exact location of R2+both in surfactant aggregates in solution and in the adsorbed state on solids, i.e., hemimicelles, and to study interactions between R2+and the surface of solids.214 The luminescence of R2+is a sensitive probe for these studies. Organic molecules have been shown to be incorporated into silica-gel glasses by the sol-gel p r o ~ e s s .As ~ for R2+, fluorescent thin films containing the molecules have been (1) (a) Kalyanasundaram, K. Coord. Chem. Reu. 1982, 46, 159. (b) Nijs, H.; Friplat, J. J.; van Damme, H. J . Phys. Chem. 1983,87,1279. (c) Milosavljevic, B. M.; Thomas,J. K. J . Phys. Chem. 1985,89, 1830. (d) Ghosh, P. K.; Bard, A. J. J . Phys. Chem. 1984,88,5519. (e) Kuykendall, V. G.; Thomas, J. K. J . Phys. Chem. 1990,94,4224. (2) (a) Meisel, D.; Matheson, M. S.;Rabani, J. J. Am. Chem. SOC. 1978, ZOO, 117. (b) Hauenstein, B. L., Jr.; Dressick, W. J.; Buell, S. L.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. SOC.1983,105,4251. (c) Sato, H.; Kawasaki, M.; Haga, M.; Kasatani, K.; Ban, T.; Suenaga, H.; Kitamura, N. Nippon Kagaku Kaishi 1984, 51. (d) Sato, H.; Kasatani, H. Bull. Chem. SOC. Jpn. 1990,63,3678. (e) Kunjappu, J. T.; Somasundaran, P.; Turro, N. J. J . Phys. Chem. 1990,94, 8464. (3) (a) Hager, G. D.; Crosby, G. A. J . Am. Chem. SOC.1975,97,7031. (b) Barigelletti, F.; Belser, P.; von Zelewsky, A.; Juris, A,; Balzani, V. J . Phys. Chem. 1986,89,3680. (c) Kitamura, N.; Kim, H.-B.; Kawanishi, Y.;Obata, R.; Tazuke, S. J. Phys. Chem. 1986,90,1488. (d) Kim, H.-B.; Kitamura, N.; Tazuke, S. J . Phys. Chem. 1990,94, 7401. (4) For example: Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982,86, 4516, and references therein. (5) (a) ORegan, B.; Moser,J.; Anderson, M.; Gritzel, M. J. Phys. Chem. 1990,94,8720. (b) Nazeeruddin, M.; Liska, P.; Moser, J.; Vlachopoulos, N.; Griitzel, M. Helo. Chim. Acta 1990, 73, 1788. (6) (a) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982,86,4540. (b) Fan, J.; Shi, W.; Tysoe, S.; Strekas, T. C.; Gafney, H. D. J . Phys. Chem. 1989,93, 373. (c) Kamat, P. V.; Ford, W. E. J . Phys. Chem. 1989, 93, 1405. (d) Willner, I.; Eichen, Y.;Joselevich, E. J . Phys. Chem. 1990,94, 3092.
prepared by this process with the aid of a surface active agent, and the photophysics of the films have been studied in detail.7d In this study, R2+ was incorporated into solgel reaction systems and the luminescence of R2+was measured during the sol-gel process. Surfactants were added to a sol-gel solution and their effects on luminescence were studied. This study was conducted to provide some clarification of the mechanism by which R2+ is incorporated into silica and interactions between R2+ and silica.
Experimental Section Chemicals. Tris(2,2’-bipyridine)ruthenium(II)chloride (Rz+) from Aldrich, sodium dodecyl sulfate (SDS)of a biochemical
grade (Wako),tetraethylorthosilicate(TEOS)from Tokyo Kasei, and ethanol of a spectroscopic grade were used without further purification. All other chemicals were at least of reagent grade and used as received. The water used was deionized and distilled from a quartz still. Sample Preparation. A solution was prepared from TEOS, water containing R*+(8 X M) and ethanol. Stock solutions M) were used of SDS (2 mM and 10-l M) containingR2+(8 X instead of the above water when SDS was added to the solution. The solution was made acidic by HCl and stirred for 1h. Unless otherwise stated, the molar ratio of TEOS/water/ethanol was 1:6.2:3.8 and the pH of the solution was 3.0. Measurements. The emission and excitation spectra were taken with a JASCO FP-770 spectrofluorometerat room temperature. Viscosity was measured by a Cannon-Fenske viscometer.
Results Figure 1 shows the emission and excitation spectra of R2+as measured in the starting solution and xerogel. The emission maximum shifted from 602 nm in the solution to 588 nm in xerogel. However, the excitation spectra tracing the absorption profile do not change. Figure 2 shows changes in the emission maximum during sol-gel-xerogel stages for different preparation conditions. The emission maximum usually shifted to a shorter wavelength with time. However, the emission maximum virtually ceased to change after the gelation point. When (7) For example: (a) Kaufman, V. R.; Avnir, D. Langmuir 1986,2,717. (b) Pouxviel, J. C.; Parvaneh, S.; Knobbe, E. T.; Dunn, B. Sold State Ionics 1989,32/33,646. (c) Mataui, K.; Nakazawa, T. Bull. Chem. SOC. Jpn. 1990, 63, 11. (d) Avnir, D.; Kaufman, V. R.; Reisfeld, R. J . NonCryst. Solids 1985, 74, 395.
0743-7463/91f 2407-2866$02.50/0 0 1991 American Chemical Society
Langmuir, Vol. 7,No. 11, 1991 2867
Luminescence of Ru(bpy)2+in Sol-Gel Glasses
c
1
600 -
E $ a
+
-
595
-
590
-
Figure 1. Emission and excitation spectra of R*+in sol (- - -1 and xerogel (-). The excitation spectra were obtained by monitoring at 600 nm and the excitation wavelength for the
emission was 460 nm.
q ICP
the water-to-silane ratio ( r )changed at a constant pH value of 3.0, the final emission maxima were similar to each other (around 588 nm), although the rate was slower for r = 3.1 than for r = 6.2. However, when pH fell from 3.0 to 1.0 at a constant F value of 6.2, the final emission maximum became somewhat longer (around 592 nm). Figure 3 shows the emission maximum vs viscosity of the three solutions, which increased during the sol-gel reaction. Very similar curves were obtained for the different r values, while a different curve was observed when pH changed. The emission spectra of R2+ were measured in various solutions of water, ethanol, ethylene glycol, and glycerol and are plotted in Figure 3. The emission maximum was almost constant within the viscosity range tested. Figure 4 shows the effects of SDS on the emission spectrum of R2+. The maximum emission spectrum shifted from 602 nm in water to 580 nm when 2 X M SDS was added, indicating formation of a complex (R2+2DS-),,2 where DS- designates dodecyl sulfate. Following the addition of lo-' M SDS, the emission maximum shifted to 622 nm, indicating R2+ to have been solubilized by micelles.2 However, the addition of SDS into sol-gel systems at the typical concentrations had no effect on the emission spectra of R2+.
1
o3
Figure 3. Maximum emission wavelength of R2+as a function of viscosity of the sol-gel solutionsand a series of mixed solvents (see the text): 0, r = 6.2, pH = 3.0; A,r = 3.1, pH = 3.0; 0, r = 6.2,
Figure 2. Maximumemissionwavelengthof R2+over time during the sol-gel process under three preparation conditions: 0, r = 6.2, pH = 3.0; A, r = 3.1, pH = 3.0; 0,r = 6.2, pH = 1.0, where r = molar ratio of water/TEOS. The gelation point for each is indicated by an arrow.
1 o2
10
1
pH = 1.0; 0 , mixed solvents.
hlnm
Figure 4. Emission spectra of R2+ ([R2+]= 8 X 1od M) in water with and without SDS: -, [SDS] = 0 M,,,A = 602 nm; - - -, [SDS] = 2 X 10-9 M, X, = 580 nm; - -, [SDS] = 10-lM,, A = 622 nm.
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Discussion The luminescent excited state of R2+is assigned to the metal-to-ligand charge-transfer (MLCT) When R2+ is excited, solvent reorientation around the excited state molecules occurs to stabilize the MLCT excited state, which has a large dipole moment. Therefore, a blue shift of the emission is induced when motion of the solvent molecules is restricted, because the MLCT excited state is not completely stabilized within its lifetime.3c*d When R2+ was incorporated into colloidal silica, the emission showed a blue shift, indicating cationic R2+to be initially attached to the anionic Si02 particles and held rigidly in the system.6* The R2+ adsorbed on Ti02 also showed a blue shift due to stronger attachment to the ~ u r f a c e .The ~ relaxation mechanism above would explain these findings. Considering these findings together with the unchanged excitation spectra in Figure 1,the blue shift for R2+during the sol-gel reaction is concluded due to its attachment to polysiloxane polymers, which prevents the MLCT excited state from fully relaxing. This conclusion is supported by the fact that a larger water content ( r ) causes solutions to become more viscous in a short time8 and promotes the ~~
~~~~~
(8) Sakka, S.; Kamiya, K. J . Nora-Cryst. Solids 1982,48, 31.
Matsui et al.
2868 Langmuir, Vol. 7, No. 11, 1991
emission shift at pH = 3.0 (Figure 2). Although the rates of these reactions depended on the water content, the nature of the solutions should be the same if compared a t the same viscosity, which is determined by the degree of polymerization and polymer concentration.8 In fact, almost the same relation was obtained for the emission maximum vs the viscosity, as shown in Figure 3, even when the solutionsdiffered in their water content (r). The degree of a sol-gel reaction most likely determines the emission maximum. It should be noted here that R2+probes the microviscosity of the microenvironment around the polymers onto which R2+ is adsorbed. Therefore, the relationship between the emission maximum and observed viscosity of the sol-gel solutions is not directly comparable to that of the emission maximum and viscosity of homogeneous solutions. This is apparent from the results in Figure 3: the emission maximum of R2+ did not change for various homogeneous solutions within the range of viscosity tested. The emission maximum did not change after the gelation point, indicating R2+ to be so rigidly held to the polysiloxane polymers around the gelation point that further stabilization of the MLCT state cannot occur. This type of spectral change is completely different from previous findings for pyrene; that is, spectral changes occur mainly after gelation owing to change in solvent comp o ~ i t i o n This . ~ ~ can ~ ~ be ascribed to the different properties of these probe molecules, which will be discussed latter in relation to the effects of SDS. The effects of pH are complicated. The rate of change in the emission maximum decreased as pH decreased, due to changes in the kinetics of hydrolysis and polycondensation, just as in the case of the effects of r. However, in this case, the relation between the emission maximum and viscosity at pH 1.0 was quite different from those a t pH 3.0. The final maximum wavelength was 592 nm at pH = 1.0,while at pH = 3.0 it was 588 nm. If the solvent relaxation model is valid, these results imply that R2+ is less strongly held to polysiloxane polymers at pH = 1.0. Colloidal silica has an isoelectric point at pH = 2. This means that at a pH higher than 2, the surface charge of the silica is negative owing to the presence of SiO- groups on the silica surface, while a t a pH lower than 2, a positive charge is induced on the surface by SiOH2+ groups.7bA reasonable explanation of the effect of pH on the attachment of R2+to the silica polymers would thus be that R2+
is more rigidly held to the silica polymers with SiO-groups than to those with SiOH2+by electrostatic i n t e r a ~ t i o n . ~ ~ The emission spectra of R2+in the sol-gel solutions did not change when SDS was added, indicating that neither micelles nor complexes (R2+2DS-)nare formed in solution.2 This may be ascribed to the presence of ethan01.~The emission spectra in the xerogels showed a similar shift without dependence on SDS concentration, implying R2+ not to be incorporated into silica as a form solubilized by micelles or as a complex. This is consistent with the model of R2+incorporation into silica polymers described before. The same effect of SDS on the R2+emission was noted in colloidal silica, where R2+was also embedded in polymerized silica particles.6a The incorporation of R2+ is completed prior to micellization or complex formation. This differs from the case of pyrene? which was dissolved in the bulk phase of ethanol-rich solvents during the solgel stage and became progressively solubilized into micelles formed by the evaporation of ethanol during the gel-xerogel stage. This difference would be due to molecular properties: pyrene is a hydrophobic molecule dissolved in an ethanol-rich phase, while R2+is a hydrophilic cation located in a water-rich phase with a silica surface. Hemimicellization has been reported for the R2+-SDSalumina system.2dIe Surfactant aggregations are formed on the cationic surface of alumina by the adsorption of the anionic surfactant. The immobilization of R2+in hemimicelles was indicated by an emissionspectrum very similar to that for micelles, or shifted to a somewhat longer wavelength than that for micelles.2dve Under our experimental conditions, incorporation into hemimicelles did not occur, as shown that there was no difference in the emission spectra of R2+in the sol-gel systems with and without SDS. These differences can be ascribed to the negative surface charge of SiO- groups at pH = 3. The R2+ adsorbed on the silica surface was stabilized, while SD-was less so on the surface with negative charge. In conclusion, the emission spectra of R2+show R2+to be rigidly held to polysiloxane polymers during the solto-gelstage. This is affected by the surface charge of polysiloxane polymers (pH). (9) Mntaui, K.; Nnkezewn, T.; Morisnki, H. J. Phys. Chem. 1991,95, 976.
