Temperature Dependence of Quenching Rates and Efficiencies of Net Forward and Reverse Electron Transfer in the Quenching of Protonated Triplet Methylene Blue by Complexes of Iron(II)

Article abstract of DOI:10.1021/j100392a014

The temperature dependence of k_q, the specific rate of quenching of protonated triplet methylene blue, ³MBH⁺, and of k_et and k_ret, the specific rates of net forward and reverse electron transfer, respectively, have been measured by laser flash photolysis-kinetic spectrometry using 1.0-J, 694.3-nm flashes from a Q-swiched ruby laser.The sum of the efficiencies of net forward and reverse electron transfer, F₁ and F₂, respectively, in the quenching of ³MBH²⁺ by Fe^II(CN)4bpy^2- in 30 vol percent CH3CN - 70 vol percent H2O has been found to be indistinguishable from unity at 291.0, 298.0, 305.5, 313.0, and 321.0 K.This result supports the prior conclusion that the only significant mechanism of this quenching reaction is reversible electron transfer.In contrast, for quenching of ³MBH²⁺ by Fe^II(H2O)6²⁺ in water over the same temperature range, F₁ + F₂ < 1, confirming that at least one other mechanism contributes significantly in this case.Thermodynamic quantities of activation, ΔG^*, ΔH^*, and ΔS^*, have been determined for these processes and their implications for mechanism are discussed.