Excited-State Acid-Base Chemistry: Evidence for a Dissociative Excited State

Excitation (410 nm) of the bimetallic [(bpy)2Ru(CN)(mu-CN)Rh(NH3)4Br]^2+ produces the MLCT state localized on the (bpy)2Ru(CN)2 ligand. Photoinduced cleavage of the bimetallic occurs in the presence of [H+], and the dependence yields a Ka equivalent to that for ground-state cis-(bpy)2Ru(CN)2 implying separation of the bimetallic prior to relaxation. The pH dependence and the emissivity of a bimetallic composed of components that individually quench at a diffusion controlled rate suggest that rupture of the RuCNRh bond is due to the reduction in electron density at the cyano ligand that occurs on population of the MLCT state. Unlike known photoinduced metal ligand dissociations, where the excitation energy is consumed in the dissociation, the dissociated "(bpy)2Ru(CN)2 ligand" remains excited.