Reactions of Excited Metal Complexes in Solid Matrices and Liquid Solutions

3 Reactions of Excited Metal Complexes in Solid Matrices and Liquid Solutions 

Electron transfer from the excited states of Fe(Jl) to the H30+ cation in vitreous aqueous solutions of H2S04 which results in the formation of Fe(III) and of H atoms has been studied in Refs. [48,49], The quantum yield of the formation of Fe(III) in 5.5 M H2S04 at 77 K has been found to be only two times smaller than at room temperature. Photo-oxidation of Fe(II) is also observed at 4.2 K. The actual, very weak dependence of the efficiency of Fe(II) photo-oxidation on temperature indicates the tunneling mechanism of this process [48,49], A detailed 

Due to the complicated kinetics for both processes no attempt was made in Ref. [53] to treat the data quantitatively. It was estimated, however, that the back electron transfer reaction is slower by about 3 orders of magnitude than that of the forward electron transfer. At the same time, the free energy change for the forward reaction (AG° = —0.4 eV) is smaller than for the back electron transfer (AG° = —1.7 eV). This decrease of the reaction rate at large exothermicity J = — AG° was attributed [53] to the decrease of the Franck-Condon factors with increasing J in the situation when J exceeds the reorganization energy Er. 

Quantitative investigations of the photoinduced electron transfer from excited Ru(II) (bpy)3 to MV2 + were made in Ref. [54], in which the effect of temperature has been studied by steady state and pulse photolysis techniques. The parameters ve and ae were found in Ref. [54] by fitting the experimental data on kinetics of the excited Ru(II) (bpy)3 decay with the kinetic equation of the Eq. (8) type. It was found that ae did not depend on temperature and was equal to 4.2 + 0.2 A. The frequency factor vc decreased about four orders of magnitude with decreasing the temperature down to 77 K, but the Arrhenius plot for W was not linear, as is shown in Fig. 9. 

Electron transfer was interpreted in Ref. [54] in terms of the nonradiative decay process [20-24,44]. For an up-to-date review of theoretical works on electron transfer see the relevant chapter in this volume (R. A. Marcus — Recent developments in fundamental concepts of PET in biological systems). 

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