Competitive energy-transfer and reductive quenching of the CT excited states of copper(l) phenanthrolines

Abstract

For the charge-transfer excited states of three different copper phenanthrolines, decamethyl ferrocene is a significantly better quencher than ferrocene itself. In a more detailed investigation we have studied the quenching of photoexcited Cu(dpp) 2 + where dpp denotes 2,9-diphenyl-1,10-phenanthroline, by a series of ferrocene derivatives with varying numbers of methyl substituents. When the ferrocenes have relatively positive reduction potentials, the quenching rate is consistently around 10 8 M −1 s −1 due to an energy-transfer process. For the strongest reducing agents, the quenching rate constants are larger, but electron-transfer quenching does not become dominant until the driving force is about 0.3 V An innersphere reorganizational energy requirement of the copper system inhibits reductive quenching such that the effective self-exchange rate of the system is about 105 M-1 s1. In contrast, with oxidative quenchers the corresponding rate constant is about 10 10 M −1 s −1 . The former value is comparable to estimates obtained in previous studies of ground state processes that involve the population of the analogous da* molecular orbitals. Comparisons with literature data suggest that a significant Franck-Condon barrier inhibits energy-transfer quenching as well. It should be possible to avoid the various kinetic limitations by using phenanthroline ligands with bulkier substituents in the 2,9 positions; however, to develop more efficient photooxidants, it will also be necessary to enhance the excited-state reduction potential.