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

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 10(5) M(-1) s(-1). 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 d sigma* 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.