Self-quenching and Cross-quenching Reactions of Excited Platinum(II) Diimine Complexes

Abstract

Neutral platinum(II) diimine complexes undergo self-quenching in fluid solution, characterized by a linear increase in the emission decay rate with concentration. The corresponding quenching rates (∼10 9 -10 10 M −1 s −1 ) are nearly diffusion limited. The accumulated evidence suggests that an excited platinum complex, M*, reacts with a ground-state complex, M, to form an excimer, M 2 *, which rapidly relaxes to give two ground-state complexes. The reaction mechanism and the nature of the excimer are discussed in context of the electronic structures, spectroscopy, and quenching kinetics of these complexes. Steady-state cross-quenching experiments suggest that an excited platinum complex also can react rapidly (∼10 9 -10 10 M −1 s −1 ) with a different ground-state platinum diimine complex (Q), presumably to form an exciplex, MQ*. In contrast, aromatic molecules and platinum(II) complexes lacking a diimine ligand are relatively ineffective quenchers. These results suggest that both the diimine ligand and platinum center are important for quenching.