The relaxation dynamics of two transition-metal complexes, [Ru(bpy) 3]2+ and [Ru(bpy)3(mcbpy)]2+, in ethanol solution and in poly(methyl methacrylate) (PMMA) and TiO2 films have been investigated with time-resolved emission and femtosecond transient absorption spectroscopy. The emission lifetime of a degassed [Ru(bpy)3]2+ solution in ethanol was determined to be 700 ns; to describe the self-quenching kinetics due to aggregation, three decay coefficients, 5.3, 70, and 220 ns, were obtained for the [Ru(bpy) 3]2+/PMMA film. The electron transfer through space in a [Ru(bpy)3]2+/TiO2 film competed with intrinsic intersystem crossing (∼100 fs) and vibrational relaxation (∼6 ps) in solid films. For the [Ru(bpy)2(mcbpy)]2+/TiO2 film, although the relaxation for electron transfer through bonds was more rapid than electron transfer through space, both processes occur on similar time scales. Through femtosecond transient absorption measurements, we provide important dynamical evidence for the interfacial electron transfer in both forward and backward directions. We conclude that in dye-sensitized solar-cell applications processes for interfacial electron transfer are significant not only through bonds but also through space.
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