Spin-orbit coupling controlled spin chemistry of Ru(bpy)32+ photooxidation: Detection of strong viscosity dependence of in-cage backward electron transfer rate

Abstract

The magnetic field dependence of qce, the cage escape yield of radicals generated by photoelectron transfer between Ru(bpy)3zt and methylviologen (MV++) was investigated for a series of solvent mixtures of increasing viscosity, obtained by adding ethylene glycol (EGLY) to H20/ACN. From the experimental results the absolute values of the rate parameters characterizing the primary radical ion pair (kce for cage escape, zs for spin relaxation, and b e t for spin-allowed backward electron transfer) have been obtained. Particularly noteworthy is the strong viscosity dependence of b e t , which can be assigned to dielectric solvent relaxation as the rate limiting step of backward electron transfer in the EGLY containing solvents.

Diffusion, spin and reaction control in geminate reverse electron transfer

“…We will now sharpen our focus on the realistic ranges of the variable parameters D and suitable for the system in question. 22 The D range of k 0 experimental interest is indicated by the vertical lines in Fig. 1, the experimentally accessible range of values leads to a k 0 bunch of curves indicated by the broad curve in the Ðgure.…”

Magnetic field effect on the photooxidation efficiency of ferrocene

“…Due to the spin-orbit mixed nature of the primed states the pair states defined in Eqs. (8)(9)(10)(11) are not pure spin states. Nevertheless, this basis of states provides for the maximum possible concentration of singlet and triplet character on the individual states (cf.…”

“…we need a two-dimensional basis for the orbital part, the consequence of which will be that the set of radical pair states given in Eqs. (8)(9)(10)(11) are not fully transformable into a set of pure singlet and triplet substates. This consequence becomes evident when representing the pair states of Eq.…”

“…What can be learned from such experiments refers to the energetics of rather weak interactions and to kinetic processes with time constants down to some nanoseconds. In recent work about magnetic field effects on the photooxidation of RuU-trisdiimine complexes [4][5][6][7][8] we have shown that the concepts of the radical pair mechanism and the pertinent spin formalism can be generalized to systems with heavily perturbed spin states, namely to metal complexes with strongly spin-orbit coupled Kramers doublets from open-shell d-electron configurations. These systems require high magnetic fields to be affected but, in turn, evaluation of the magnetic field dependence affords access to reaction time constants and spin relaxation times in the ps time domain.…”

“…As we have shown in our previous investigations of Ru-complex spin chemistry [4][5][6][7][8], in metal centered d-electron radicals the rate of spin processes may be comparable to that of fast electron transfer processes, so that magnetic modulation of the spin processes will lead to detectable kinetic effects from which the details of the geminate kinetics can be determined. Here we will show that this method is also fully applicable to the case of the (Fc+..MB ) system.…”