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

Systematic studies of electron-transfer processes in the ground states and excited triplet states of pyrrolidinofullerenes {C-60(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)(C-60(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; alpha = 0.6 in Delta log k(et)(G) = alpha Delta log K. This alpha value indicates that the activation energies of forward electron transfer in the ground states vary moderately with the thermodynamic stabilities of (C-60(C3H6N)R)(.-). Electron-transfer rate constants via (T)(C-60(C3H6N)R)*, which are close to the diffusion-controlled limit, do not show a large substituent effect (alpha' = 0), because of their highly exothermic processes. Such a linear free-energy relationship can be extended to other systems such as (T)(C-60(C3H6N)R)*/N,N-dimethylaniline, from which valuable information for electron-transfer processes can be obtained.