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Chem. Mater. 1997, 9, 2588-2591
Luminescence Properties of Tris(2,2′-bipyridine)ruthenium(II) in Sol-Gel Systems of SiO2 Kazunori Matsui* and Fumitoshi Momose Department of Chemistry, College of Engineering, Kanto Gakuin University, Mutsuura-cho, Kanazawa-ku, Yokohama 236, Japan Received April 2, 1997. Revised Manuscript Received September 5, 1997X
The luminescence spectra and the lifetime of tris(2,2′-bipyridine)ruthenium(II), Ru(bpy)32+, were studied in the sol-gel reaction system of tetramethoxysilane (TMOS) at room temperature. The luminescence peaks and lifetime were considerably altered by the nature of the sol-gel matrix. These findings are discussed in connection with adsorption of Ru(bpy)32+ on gels and lower temperature measurements at 77 K.
Introduction Tris(2,2′-bipyridine)ruthenium(II), Ru(bpy)32+, has received the attention of many researchers because of its unique properties such as strong luminescence, moderate excited-state lifetime, energy and electrontransfer reactions and chemical stability.1,2 The luminescent excited state of Ru(bpy)32+ is assigned to the metal-to-ligand charge-transfer (MLCT) state. The luminescence properties are very sensitive to polarity and viscosity of solvent because of the MLCT character. The dynamic properties of the excited state of Ru(bpy)32+ and related complexes adsorbed on semiconductors have been intensively investigated for understanding dye sensitization and practical applications.3-5 For example, the electron-transfer rate constant was obtained for Ru[bpy(COOH)2]32+ in TiO2 colloidal suspension.3a The luminescence dynamics were studied for adsorbed Ru(II) complexes on powdered TiO2.5a In addition to semiconductors, there is a number of studies on Ru(bpy)32+ adsorbed on nonconducting materials such as SiO2, which can actually modify photochemical reactions.6,7 The sol-gel process provides an advantageous method for doping of organic dyes and coordination compounds * Corresponding author. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Kalyanasundaram, K. Photochemistry of Polypyridine and Porphyrin Complexes; Academic Press: London, 1992. (2) Balzani, V.; Barigelletti, F.; De Cola, L. In Photoinduced Electron Transfer II; Mattay, J., Ed.; Top. Curr. Chem. 158; Springer-Verlag: London, 1990; p 31. (3) (a) Desilvestro, J.; Gra¨tzel, M.; Kavan, L.; Moser, J.; Augustynski, J. J. Am. Chem. Soc. 1985, 107, 2988. (b) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1988, 110, 1216. (c) Nazeeruddin, Md. K.; Liska, P.; Moser, J.; Vlachopoulos, N.; Gra¨tzel, M. Helv. Chim. Acta 1990, 73, 1788. (d) O’Regan, B.; Moser, J.; Anderson, M.; Gra¨tzel, M. J. Phys. Chem. 1990, 94, 8720. (e) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (4) (a) Bignozzi, C. A.; Chiorboli, C.; Indelli, M. T.; Scandola, M. A. R.; Varani, G.; Scandola, F. J. Am. Chem. Soc. 1986, 108, 7872. (b) Amadelli, R.; Argazzi, R.; Bignozzi, C. A.; Scandola, F. J. Am. Chem. Soc. 1990, 112, 7099. (c) Furlong, D. N.; Wells, D.; Sasse, W. H. F. J. Phys. Chem. 1986, 90, 1107. (5) (a) Kajiwara, T.; Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1982, 86, 4516. (b) Hashimoto, K.; Hiramoto, M.; Sakata, T.; Muraki, H.; Takemura, H.; Fujihira, M. J. Phys. Chem. 1987, 91, 6198. (c) Hashimoto, K.; Hiramoto, M.; Kajiwara, T.; Sakata, T. J. Phys. Chem. 1988, 92, 4636. (6) (a) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982, 86, 4540. (b) Thomas, J. K.; Wheeler, J. J. Photochem. 1985, 28, 285. (c) Kamat, P. V.; Ford, W. E. J. Phys. Chem. 1989, 93, 1405.
S0897-4756(97)00186-5 CCC: $14.00
in inorganic oxides.8,9 Dopant molecules are well dispersed in the sol-gel matrixes. Therefore, the doped sol-gel system is a simple and appropriate method for studying the interaction between molecules and oxide surface. The photophysics and photochemistry of Ru(bpy)32+ in SiO2 and TiO2 xerogels prepared by the solgel process are naturally of interest as expected from the above context.10-16 However, little is known about the immobilization mechanism of Ru(bpy)32+ in these gels. Here we report the steady-state and dynamic properties of Ru(bpy)32+ in SiO2 sol-gel systems prepared under different catalytic conditions. Changes in the spectra and lifetimes were measured at room temperature during the sol-gel reactions. The luminescence spectra and lifetimes of sols and xerogels were also obtained at 77 K and compared with those at room temperature. Experimental Section Chemicals. Tris(2,2′-bipyridine)ruthenium dichloride (Ru(bpy)3Cl2, Aldrich) and tetramethoxysilane (TMOS, Tokyo Kasei) were used without additional purification. Methanol was of a spectroscopic grade. Water was deionized and distilled. Sample Preparation. TMOS (10.0 mL) and methanol (10.0 mL) containing 10-4 M Ru(bpy)32+ were mixed together, and then 5.0 mL of water was added. A slight amount of HCl or NH4OH was added as a catalyst in some solutions. The molar ratio of TMOS:water:methanol:catalyst was 1:4.1:3.7: (7) (a) Shi, W.; Wolfgang, S.; Strekas, T. C.; Gafney, H. D. J. Phys. Chem. 1985, 89, 974. (b) Kennelly, T.; Gafney, H. D.; Braun, M. J. Am. Chem. Soc. 1985, 107, 4431. (c) Fan. J.; Shi, W.; Tysoe, S.; Strekas, T. C.; Gafney, H. D. J. Phys. Chem. 1989, 93, 373. (8) (a) Avnir, D.; Braun, S.; Ottolenghi, M. In Supramolecular Architecture: Synthetic Control in Thin Films and Solids; Bein, T., Ed.; ACS Symposium Series No. 499; American Chemical Society: Washington, DC, 1992; p 384. (b) Avnir, D. Acc. Chem. Res 1995, 28, 328. (9) Zink, J. I.; Dunn, B. S. J. Ceram. Soc. Jpn. 1990, 99, 878. (10) Avnir, D.; Kaufman, V. R.; Reisfeld, R. J. Non-Cryst. Solids 1985, 74, 395. (11) Modes, S.; Lianos. P. Chem. Phys. Lett. 1988, 153, 351. (12) Matsui. K.; Sasaki. K.; Takahashi, N. Langmuir 1991, 7, 2866. (13) Castellano, F. N.; Heimer, T. A.; Tandhasetti, M. T.; Meyer, G. J. Chem. Mater. 1994, 6, 1041. (14) Papoutsi, D.; Lianos, P.; Yianoulis, P.; Koutsoukos, P. Langmuir 1994, 10, 1684. (15) Castellano, F. N.; Stipkala, J. M.; Friedman, L. A.; Meyer, G. J. Chem. Mater. 1994, 6, 2123. (16) Castellano, F. N.; Meyer, G. J. J. Phys. Chem. 1995, 99, 14742.
© 1997 American Chemical Society
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Chem. Mater., Vol. 9, No. 11, 1997 2589
Figure 2. Luminescence peak wavelength of Ru(bpy)32+ at 298 K over time during the sol-gel process under three catalytic conditions: acidic (4); neutral (b); basic (0). Figure 1. Luminescence spectra of Ru(bpy)32+ in basic sols just after preparation, measured at 298 (a) and 77 K (b), and in basic xerogels after 1 month, measured at 298 (c) and 77 K (d). 0.03 (HCl) or 1:4.1:3.7:0.003 (NH4OH). The apparent pH values were 2.1 and 8.0 for initial mixing. These solutions were placed in 150-mL plastic beakers sealed with a pinholed film after stirring for 1 h. Measurements. Absorption spectra were recorded with a JASCO Ubest-50 spectrophotometer. Emission and excitation spectra were taken with a JASCO FP-770 spectrophotometer. The excitation wavelength was 450 nm. Luminescence decay profiles were obtained with a N2 laser (Usho AN-200) and photomultiplier (Hamamatsu Photonics R0955) system assembled by Atago Bussan Co. Unless stated otherwise, these measurements were done at ambient temperature (298 K) for aerated samples. The specific surface areas of xerogels were estimated by the BET method of nitrogen gas adsorption (Coluter OMNISORP 100CX). Adsorption isotherms were obtained by exposing 0.10 g of nondoped SiO2 xerogel powders to 10 mL aqueous solutions of Ru(bpy)32+ with different concentrations for 24 h followed by centrifugation and spectrophotometric determination of unadsorbed Ru(bpy)32+ concentrations.
Results and Discussion Figure 1 shows the luminescence spectra of Ru(bpy)32+ in the basic TMOS sols measured at 298 (a) and 77 K (b). On cooling, the luminescence spectra exhibited a blue-shifted peak from 600 to 580 nm and a vibrational structure around 620 nm as previously observed in other solvents.17 Figure 1 also shows the luminescence spectra of Ru(bpy)32+ in the basic TMOS xerogels (after 1 month) measured at 298 (c) and 77 K (d). The emission peak shifted from 600 nm in the sols to 581 nm in the xerogels at 298 K. The emission maximum showed a further shifted peak at 574 nm with a vibrational structure around 614 nm at 77 K. The excitation spectra for the sols and the xerogels measured at 298 and 77 K changed little except for a vibrational structure seen at 77 K. These results and previous findings lead us to conclude that the blue shifts of the emission maxima from sols to xerogels and from 298 to 77 K are due to the (17) (a) Barigelletti, F.; Belser, P.; Zelewsky, A. V.; Juris, A.; Balzani, V. J. Phys. Chem. 1985, 89, 3680. (b) Kitamura, N.; Kim, H. B.; Kawanishi, Y.; Obata, R.; Tazuke, S. J. Phys. Chem. 1986, 90, 1488. (c) Kim, H. B.; Kitamura, N.; Tazuke, S. J. Phys. Chem. 1990, 94, 7401.
increased rigidity of the surrounding medium.6a,12,17 That is because the luminescent excited state of Ru(bpy)32+ with a polar MLCT character cannot be completely stabilized within its lifetime when “solvent” reorientation is prevented in rigid medium. Here “solvent” changes from a mixed solution to gel and xerogel when the sol-gel reaction proceeds. Similar spectral shifts were observed for the acidic and neutral (no catalyst) systems of TMOS. Figure 2 summarizes changes in the emission λmax during the sol-gel processes for the different preparation conditions. The emission λmax generally shifted to a shorter wavelength with time. The change in the emission λmax was slower for the acidic system than for the basic system. Gelation times were a half day, 2 days, and 6 days from the initial mixing for the basic, neutral, and acidic samples. The large emission shift was observed on the next day after preparation for the basic sample while the shift was completed more than 10 days after preparation for the acidic sample. The neutral TMOS system showed an intermediate situation. The emission peak shifts seem to be directly related to the gelation for the basic and neutral samples except the acidic sample. The luminescence decays from Ru(bpy)32+ were measured at 298 K during the sol-gel reactions. The luminescence decays can be adequately described by a single-exponential decay or by a sum of two exponential decays given by 2
I(t) )
Ri exp[-(t/τi)] ∑ i)1
(1)
and the average lifetimes are calculated by13 2
〈τ〉 )
∑ i)1
2
Riτi2/
Riτi ∑ i)1
(2)
The luminescence lifetime was properly fitted by a single-exponential decay time for the three starting solutions at 298 K. However, the decay profile of the basic systems became a double exponential from the next day. The neutral TMOS systems also showed a double-exponential profile for 1-6 days after preparation and then a single-exponential decay again. On the contrary, the acidic sample could apparently be fitted
2590 Chem. Mater., Vol. 9, No. 11, 1997
Matsui and Momose
Table 1. Luminescence Lifetimes of Ru(bpy)32+ in Sols (Just Prepared) and Gels (after 1 month) Measured at 298 and 77 K sols 298 K acidic systems
neutral systems
gels 77 K
280 ns
2.93 µs (0.45)
280 ns
6.54 µs (0.55) 〈τ〉 ) 5.57 µs 3.05 µs (0.50) 6.88 µs (0.50) 〈τ〉 ) 5.70 µs
basic systems
285 ns
459 ns
basic systemsd
440 ns (0.20) 1130 ns (0.80) 〈τ〉 ) 1.07 µs
EtOH-MeOH 4:1 (v/v)e a
by a single-exponential decay during the whole stage of the sols to xerogels. Figure 3 shows an example of decay profiles for the acidic xerogels after 1 month fitted by a single- and double-exponential curves as well as residuals. The luminescence decay can be fitted a little bit better by a sum of two-exponential decays than a single-exponential one as judged by the fitting residuals. Therefore a double-exponential fit is more suitable for more precise analysis, although we regarded the lifetime as a single-exponential decay. Figure 4 shows evolution of the emission lifetime of Ru(bpy)32+ during the sol-gel reactions for the different catalytic conditions. Average lifetimes were plotted for the double-exponential cases. The luminescence lifetime was adequately described by a single-exponential time constant τ ≈ 280 ns for the three starting solutions. The lifetime of the basic sample quickly increased to 1.6 µs within a day and gradually decreased to a stable lifetime of 1.1 µs. On the contrary, the lifetime of the acidic sample increased slowly and became constant at 850 ns after more than 10 days. The neutral sol-gel system showed intermediate behavior. Interestingly,
2.70 µs (0.35)
835 690 ns (0.23) 890 ns (0.77) 〈τ〉 ) 850 ns 955 nsa 620 ns (0.27) 1.09 µs (0.73) 〈τ〉 ) 1.01 µs 645 ns (0.30)
6.73 µs (0.65) 〈τ〉 ) 6.01 µs 2.88 µs (0.38) 6.95 µs (0.62) 〈τ〉 ) 6.13 µs 2.05 µs (0.23) 6.66 µs (0.77) 〈τ〉 ) 6.27 µs
5.2 µs
Single-exponential fit. b Concentration qeunching were observed. Taken from ref 10. c Taken from ref 16. from ref 23.
Figure 3. Luminescence decay profiles of Ru(bpy)32+ in acidic xerogels measured at 298 K and residuals: (a) fitted by a single-exponential curve; (b) fitted by a double-exponential curve.
77 K
nsa
1.38 µs (0.70) 〈τ〉 ) 1.26 µs 0.9-1.9 µs 0.503 µs (0.27) 1.26 µs (0.72) 〈τ〉 )1.16 µs 610 ns (0.90) 1600 ns (0.10) 〈τ〉 ) 833 ns 629 ns (0.88) 1700 ns (0.12) 〈τ〉 ) 917 ns
SiO2 xerogelsb SiO2 xerogelsc acidic systemsd
298 K
d
Taken from ref 11. e Taken
Figure 4. Luminescence lifetime of Ru(bpy)32+ at 298 K over time during the sol-gel process under three catalytic conditions: acidic (4); neutral (b); basic (0). The right axis shows a change in the relative weight of the acidic sol-gel systems.
we can see a very similar relation between the changes in the lifetime and emission over time. Some of the decay parameters are tabulated in Table 1 as well as those measured at 77 K and taken from references. The results in this experiment are almost consistent with previous one. The luminescence lifetime of Ru(bpy)32+ decreases by the following factors in this experimental system: oxygen quenching (samples were not degasses), collisional quenching with the solvent molecules, and concentration quenching.10,11,14,18 Here we should be reminded that the change in the lifetime looks like that in the emission λmax, which was explained by the immobilization of Ru(bpy)32+ in sol-gel matrixes. Naturally we can expect that diffusion-controlled processes would be suppressed such as oxygen quenching and collisional quenching with the solvent molecules as the sol-gel reactions proceed. Apparent two exponential decays observed at (18) Samuel, J.; Ottolenghi, M.; Avnir, D. J. Phys. Chem. 1992, 96, 6398.
Sol-Gel Systems of SiO2
Chem. Mater., Vol. 9, No. 11, 1997 2591 Table 2. BET Surface Areas of Three Types of Xerogels and Ru(bpy)32+ Adsorption Parameters for the Langmuir Isotherm Γ ) ΓsβC/(1 + βC) on Them
acidic xerogels neutral xerogels basic xerogels
Figure 5. Adsorption isotherm of Ru(bpy)32+ at 298 K on SiO2 xerogels prepared from different catalysis: acidic (4); neutral (b); basic (0).
298 K are possibly ascribed to different microenvironments of the trapped Ru(bpy)32+. A gel model with two microdomains was previously considered from dynamic fluorescence anisotropy experiments of rhodamine 6G in TMOS-derived gels.13,19 The decays were not sensitive to the observed wavelength; this finding may suggest that Ru(bpy)32+ has similar emission spectra in the microdomains. The lifetimes of Ru(bpy)32+ in the sols and xerogels were fitted by a double exponential decay curve at 77 K, while some of them were fitted by a single exponential decay at 298 K. This is possibly caused by the existence of two microdomains in which two decay components of Ru(bpy)32+ emissions could not be clearly resolved from the time resolution at 298 K. As discussed above, the blue shift in the emission λmax and the increase in luminescence lifetime are related to the rigidity of the matrixes. Most of Ru(bpy)32+ for the basic and neutral samples is expected to be adsorbed and immobilized in the siloxane polymers and/or the surface after gelation, because of the electrostatic interaction of Ru(bpy)32+ and the negatively charged surface of the siloxane polymers at these pH. This differs from the case of the acidic sample, in that the negativity charged sites decrease in comparison with the above cases and even a positively charged surface may be formed if the pH was lower than 2-2.5.20 Thus Ru(bpy)32+ is not expected to be easily immobilized in the acidic siloxane polymers after gelation. To examine this hypothesis, adsorption isotherms of Ru(bpy)32+ in aqueous solutions on the xerogels were measured (Figure 5). The adsorption isotherms for the basic and neutral xerogels could be analyzed by the Langmuir isotherm Γ ) ΓsβC/(1 + βC) as found for rhodamine 6G on SiO2.21 On the contrary, the acidic xerogels did not adsorb Ru(bpy)32+ as a measurable amount within our experimental condition. The saturation coverage Γs values were estimated to be 9.8 × 10-9, (19) Narang, U.; Wang, R.; Prasad, P. N.; Bright, F. V. J. Phys. Chem. 1994, 98, 17. (20) Pouxviel, J. C.; Parvaneh, S.; Knobbe, E. T.; Dunn, B. Solid State Ionics 1989, 32/33, 646.
BET surface area (m2/g)
Γs (mol/m2)
β (M-1)
368 426 445
0 7.5 × 10-9 9.8 × 10-9
7.3 × 104 4.3 × 104
7.5 × 10-9, and ≈0 mol/m2 for the basic, neutral, and acidic xerogels, while the BET surface areas were almost the same (Table 2). These results support the electrostatic adsorption mechanism of Ru(bpy)32+ on the basic and neutral systems for explaining the changes in the spectra and lifetime; i.e., Ru(bpy)32+ binds to the highly branched silica clusters with negative charge and becomes immobilized when gelation occurs.22 It still remains uncertain by which mechanism Ru(bpy)32+ is trapped in the acidic xerogels. The weight change of the sol-gel systems gives a clue (right axis in Figure 4). It showed a pronounced decrease initially due to methanol evaporation, followed by a slow decrease after 10 days. It is noted that the lifetimes and emission peaks, which are sensitive to the environment, became almost constant after 10 days at which time the weight became nearly stable. Therefore, we can explain the trapping mechanism for the acidic systems as follows: Under the acid-catalyzed conditions, and especially with low additions of water (H2O:Si e 5) employed here, Ru(bpy)32+ is dissolved in a methanol solvent pool with water within the primarily linear or randomly branched siloxane polymers entangled.22 The microviscosity around Ru(bpy)32+ is expected to increase by the solvent evaporation, inducing gradual changes in emission peaks and lifetimes. Finally Ru(bpy)32+ becomes immobilized by deposition on the silica pore surfaces because the solvent was almost evaporated to dryness. Conclusions We have measured the luminescence spectra and lifetime of Ru(bpy)32+ in sol-gel systems of TMOS prepared under different catalysts. The luminescence spectra showed a blue shift as the sol-gel reactions proceeded and measured temperature decreased. This is ascribed to the restricted stabilization of the MLCT excited state within the excited-state lifetime. The lifetime increased in a manner similar to the spectral shift as the sol-gel reaction proceeded. These spectral and lifetime changes are induced by the immobilization of Ru(bpy)32+ by silica gels. The amounts and rates of the changes were dependent on the catalyst shown in the following order: basic > neutral > acidic. The catalysis effect is mainly explained by the adsorption of Ru(bpy)32+ on the silica surface of which the charge is determined by pH. CM970186+ (21) Anderson, C.; Bard, J. A. J. Phys. Chem. 1995, 99, 9882. (22) Brinker, C. J.; Scherer, G. W. J. Non-Cryst. Solids 1985, 70, 301. (23) Elfring, Jr. W. H.; Crosby, G. A. J. Am. Chem. Soc. 1981, 103, 2683.
