REDUCTION OF TRIPLET TETRAPHENYLPORPHYRIN DICATION BY ARYL AMINES AND HYDROQUINONES - KINETICS AND PRIMARY RADICAL YIELDS

The photophysics and one-electron reduction of triplet tetraphenylporphyrin dication by homologous series of substituted triphenylamines and hydroquinones in acetone solution have been studied by nanosecond flash photolysis. Triplet yield is similar to 0.3 and direct internal conversion from the singlet is a major decay pathway. Quenching rates (10(9) - 10(6) M(-1) s(-1)) for reductants covering a range of Delta G(0') similar to 0.4 V lead to reorganization energies lambda similar to 0.6 +/-0.1 eV. Radical yields (similar to 0.38 for the amines) are remarkably constant for recombination free energies between 1.02 and 1.45 V. A rate-limiting step in the back reaction is suggested, involving spin-orbit relaxation of the primary radical pair.