Temperature-dependent spin relaxation : a major factor in electron backward transfer following the quenching of *Ru(bpy)3 2+ by methyl viologen

Abstract

The magnetic-field dependence of the cage escape efficiency (φce) of [Ru(bpy)3]3+ and methyl viologen radicals (MV+ ) from the primary redox pair formed upon quenching of photoexcited [Ru(bpy)3]2+ by MV2+ was measured by laser flash spectroscopy in aqueous solution as a function of the magnetic field (0-2.85 T) in the temperature range from 5 to 69 °C. Furthermore, the 1H NMR T1 times of the paramagnetic [Ru(bpy)3]3+ were measured between -40 and 42 °C. The kinetic data were analyzed in terms of a kinetic model that takes into account spin conservation in the forward reaction between the 3MLCT state of [Ru(bpy)3]2+ and the electron acceptor MV2+ yielding a triplet spin-correlated radical pair (RP) and the in-cage backward electron transfer requiring singlet character of the RP. The triplet-to-singlet spin conversion of the geminate RP is explicitly treated by the stochastic Liouville equation formalism. By theoretical simulation of the observed magnetic field dependence of φce, the temperature dependent absolute values of the rate constants kce (cage escape), kbet (backward electron transfer in singlet RPs), and kTS (magnetic-field independent triplet-to-singlet interconversion) could be assessed. The temperature dependence of kce exhibits a very good proportionality to the solvent viscosity. The values obtained for kTS are in good agreement with the results on the electron spin relaxation time of [Ru(bpy)3]3+ derived by the Solomon relation from the 1H NMR T1 times. The effective rate of backward electron transfer in the geminate RP turns out to be close to spin-controlled, i.e., it is determined by the rate constant kTS of the triplet-singlet spin conversion process. The true rate constant kbet, varying from 5.5 × 1010 s-1 to 1.2 × 1011 s-1, is about seven times larger than the effective value for the total backward electron transfer comprising spin conversion and spin-allowed backward electron transfer.