Linear free-energy relationship for electron-transfer processes of pyrrolidinofullerenes with tetrakis(dimethylamino)ethylene in ground and excited states

Abstract

Systematic studies of electron-transfer processes in the ground states and excited triplet states of pyrrolidinofullerenes {C₆₀(C₃H₆N)R [R=H (1), p-C₆H₄NO₂ (2), p-C₆H₄CHO (3), p-C₆H₅ (4), p-C₆H₄OMe (5), p-C₆H₄NMe₂ (6)]} with tetrakis(dimethylamino)ethylene (TDAE) have been carried out by steady-state and transient absorption measurements in the visible–NIR region. Analyses of the equilibria of the electron-transfer processes in the ground states indicate that free ion radicals are produced in polar solvents. Photoinduced electron-transfer processes via^T(C₆₀(C₃H₆N)R)* were observed by applying a perturbation to the equilibria of the electron-transfer reactions in the ground states by laser flash photolysis. Based on the relationship of the thermodynamic data and kinetic data, the electron-transfer rate constants in the ground states (k_et^G) can be evaluated. The k_et^G values are affected by the substituents to a smaller extent compared with the equilibrium constants (K) in polar solvents; α=0.6 in Δ log k_et^G=α Δ log K. This α value indicates that the activation energies of forward electron transfer in the ground states vary moderately with the thermodynamic stabilities of (C₆₀(C₃H₆N)R)^-. Electron-transfer rate constants via^T(C₆₀(C₃H₆N)R)*, which are close to the diffusion-controlled limit, do not show a large substituent effect (α′=0), because of their highly exothermic processes. Such a linear free-energy relationship can be extended to other systems such as ^T(C₆₀(C₃H₆N)R)*/N,N-dimethylaniline, from which valuable information for electron-transfer processes can be obtained.