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J. Phys. Chem. 1996, 100, 10652-10657
Stabilization and Electron Spin Resonance Characterization of Ru(bpy)33+ in Silica Gel by Chemical Oxidation and Photoinduced Electron Transfer Kaoru Matsuura and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: October 18, 1995; In Final Form: March 12, 1996X
The stabilization of Ru(bpy)33+ from Ru(bpy)32+ at both 77 K and room temperature in silica gel by chemical oxidation with chlorine gas and by photoinduced electron transfer to suitable electron acceptors is reported. Stabilization at room temperature is of paticular interest for potential light energy storage systems. Paramagnetic Ru(bpy)33+ is characterized by electron spin resonance spectroscopy. The Ru(bpy)33+ yield formed by chlorine gas oxidation can be controlled by the silica gel pore size and decreases with increasing pore size. The Ru(bpy)33+ yield is 1 order of magnitude smaller in 14.0 nm pore silica gel compared to 2.5 nm pore silica gel. The Ru(bpy)33+ yield gradually decreases by preheating the silica gel to higher temperatures before impregnation with Ru(bpy)32+. These results suggest that the oxidation of Ru(bpy)32+ with chlorine gas takes place via a silica gel surface mediated reaction and that the direct reaction of adsorbed Ru(bpy)32+ with a chlorine molecule in the gas phase is less effective. Ru(bpy)33+ can also be stabilized in silica gel by photolysis in the presence of the electron acceptor S2O82- (persulfonate ion) with a 50% yield or tetrachloro-1,4benzoquinone (p-chloranil) with a lower yield.
Introduction The photochemistry of tris(2,2′-bipyridine)ruthenium(II), Ru(bpy)32+, has attracted considerable attention over the past two decades,1,2 since it was shown that the quenching of the excited state of Ru(bpy)32+ can proceed through an electron transfer reaction.1 Such reactions have been invoked in schemes for solar energy conversion into chemical energy. Absorption of visible light (450 nm) by Ru(bpy)32+ leads to a d-π* metalto-ligand charge transfer (MLCT) transition. The MLCT transition proceeds with unit quantum efficiency,3 and the lifetime of the excited state, Ru(bpy)32+*, is long (∼600 ns in fluid media4 and 740 ns in Vycor glass5) enough to be exploited in bimolecular reactions. The excited state of Ru(bpy)32+ is an efficient reductant (E0[Ru(bpy)33+/2+*] ) -0.88 V6) as well as an efficient oxidant (E0[Ru(bpy)32+*/+] ) 0.84 V7). It is energetically possible for Ru(bpy)33+, a redox product in the oxidative quenching of Ru(bpy)32+*, to split water into oxygen (E0[Ru(bpy)33+/2+] ) 1.26 V,8 E0[O2/H2O] ) 1.23 V2e). Formation of Ru(bpy)33+ was demonstrated by flash photolysis and by steady-state optical spectroscopy using Co complexes9 or persulfonate10 as electron acceptors. However, Ru(bpy)33+ formed by photolysis has not been stabilized in most cases because back electron transfer is rapid and usually exothermic. Heterogeneous systems such as micelles, vesicles, and microporous materials serve as good media for charge separation.11-18 Recombination can be controlled by means of hydrophobic-hydrophilic and electrostatic interactions with the interface. Recently, Borja and Dutta have reported16 that longlived charge separation can be achieved for zeolite Y in a ternary aqueous system in which an electron donor, Ru(bpy)32+, is synthesized and immobilized within the large supercages of zeolite Y;19 an electron transport agent, N,N′-tetramethylene2,2′-bipyridinium ion, is loaded into zeolite Y by ion exchange; and an electron acceptor, propylviologen sulfonate, is in the aqueous solution. However, Ru(bpy)33+ was also not detected in their system and presumed to react with water. To study the reaction of Ru(bpy)33+ with water, Ru(bpy)33+ was stabilized X
Abstract published in AdVance ACS Abstracts, June 1, 1996.
S0022-3654(95)03078-4 CCC: $12.00
within the supercages of zeolite Y by chlorine gas oxidation, and the mechanism of its reaction with water to form dioxygen was clarified.19,20 The present work demonstrates the room temperature stabilization of Ru(bpy)33+ in silica gel formed by oxidation with chlorine gas and by photoinduced charge transfer with strong electron acceptors, and its characterization by electron spin resonance (ESR). Silica gel was chosen because it can be obtained with various pore sizes, and long-lived photoinduced charge separation involving photoionizable aromatic amines has been achieved in silica gel with the yield dependent on the pore size.21 We have found that the yield of Ru(bpy)33+ formed by chlorine gas oxidation largely depends on the silica gel pore size so that yield control becomes possible. The formation of Ru(bpy)33+ by chlorine gas oxidation is suggested to involve reaction with silanol groups on the silica gel surface, and the pore size dependence reflects the local density of silanol groups. We have also shown that S2O82- and p-chloranil are sufficiently strong electron acceptors to enable stabilization of Ru(bpy)33+ in silica gel upon photoexcitation of Ru(bpy)32+. Experimental Section Tris(2,2′-bipyridine)ruthenium(II) chloride hexahydrate (Ru(bpy)3Cl2‚6H2O), Na2S2O8 (sodium persulfonate), tetrachloro1,4-benzoquinone (p-chloranil), anhydrous methyl alcohol, 4-hydroxy-TEMPO nitroxide radical (Aldrich), chlorine (Union Carbide), and silica gel powders with 2.5 (Sigma) and 6.0 and 14.0 nm (Aldrich) average pore diameters were used as received. The specific surface areas measured by the BET (BrunauerEmmett-Teller) method are 505, 480, and 300 m2/g for 2.5, 6.0, and 14.0 nm pore silica gels, respectively. Ru(bpy)32+ ions were adsorbed into silica gel by impregnation using 2.0 mL of silica gel powder in 4 mL of Ru(bpy)3Cl2 aqueous solution (4-100 mM) for 1 day or longer. The silica gel was filtered, washed with deionized water until washings became colorless, dried in air at 40 °C, and kept in a desiccator in the dark. The loadings of Ru(bpy)32+ adsorbed in silica gel were measured optically from the decrease of Ru(bpy)32+ in © 1996 American Chemical Society
Characterization of Ru(bpy)33+ in Silica Gel the liquid (λmax ) 452 nm, ) 14 300 cm-1 M-1). They varied from 5 to 80 µmol/g silica gel. For oxidation of Ru(bpy)32+ with chlorine gas, the sample powders were filled into 3.4 mm i.d. × 5.0 mm o.d. Suprasil quartz tubes to 20 mm in height. The loadings used to estimate the Ru(bpy)33+ yield are 10, 18, and 20 µmol/g silica gel with 2.5, 6.0, and 14.0 nm pores, respectively. These values correspond to almost the same loadings per volume of silica gel for each pore size. Thus, the number of Ru(bpy)32+ per sample is almost the same ((7.5-8.9) × 1017/sample). The samples were dehydrated at room temperature for 4 h (
