INFLUENCE OF ELECTRONIC DELOCALIZATION IN METAL-TO-LIGAND CHARGE-TRANSFER EXCITED-STATES

In the metal-to-ligand-charge transfer (MLCT) excited states of the ligand-bridged complexes [(dmb)2Ru(mu-bbpe)-Ru(dmb)2](PF6)4 and [(bpy)2Os(mu-bbpe)Os(bpy)2](PF6)4) (bpy is 2,2'-bipyridine, dmb is 4,4'-dimethyl-2,2'-bipyridine, bbpe is trans-1,2-bis-(4-(4'-methyl)-2,2'-bipyridyl)ethene) bbpe acts as the acceptor ligand. This conclusion is based on transient UV-visible and resonance Raman measurements, which also reveal that the excited electron in the Ru complex is delocalized over the bbpe ligand. Compared to related complexes having comparable energy gaps, the lifetimes of [(dmb)2Ru(mu-bbpe)Ru(dmb)2]4+* (tau = 1.31 mus in CH3CN at 298 K) and [(dmb)2Ru(bbpe)]2+* (tau = 1.15 mus in CH3CN at 298 K) are unusually long. The extended lifetimes are a delocalization effect caused by decreased bond displacement changes in the excited state. This decreases vibrational overlap between states, and the rate constant for nonradiative decay. Delocalization disperses the excited electron over the molecular framework of the acceptor ligand, decreasing changes in local bond displacements compared to bpy. These results have important implications for the design of complexes which are broad visible light absorbers and yet retain accessible excited state lifetimes.