Effects of Intraligand Electron Delocalization, Steric Tuning, and Excited-State Vibronic Coupling on the Photophysics of Aryl-Substituted Bipyridyl Complexes of Ru(II)

The synthesis and photophysical characterization of a series of aryl-substituted 2,2‘-bipyridyl complexes of RuII are reported. The static and time-resolved emission properties of [Ru(dpb)3](PF6)2, where dpb is 4,4‘-diphenyl-2,2‘-bipyridine, have been examined and are contrasted with those of [Ru(dmb)3](PF6)2 (dmb = 4,4‘-dimethyl-2,2‘-bipyridine). It is shown through analysis of electrochemical data and detailed fitting of the emission spectrum that the unusually large radiative quantum yield for [Ru(dpb)3](PF6)2 in CH3CN solution at room temperature is due to reduction of the degree of geometric distortion along primarily ring-stretch acceptor mode coordinates relative to other molecules in this class. It is proposed that the 3MLCT excited state of [Ru(dpb)3]2+ is characterized by a ligand conformation in which the 4,4‘-phenyl substituents are coplanar with the bipyridyl fragment, leading to extended intraligand electron delocalization and a smaller average change in the C−C bond length upon formation of...