Excited-state acid-base chemistry of coordination complexes

Photoactivation of Ru(bpy)(2)dpp(2+) (dpp = 2,3-bis(2-pyridyl)pyrazine) in the presence of MC(6)(n-) (M = Pt(IV), Rh(III), Pd(IV)) leads to quantitative formation of the corresponding bimetallic *Ru(bpy)(2)(dpp)(2+) + MC(6)(n+) --> [Ru(bpy)(2)(dpp)MCl(4)](4-n) + 2Cl(-) Formation of the bimetallic does not occur by either electron- or energy-transfer processes. Instead, population of the MLCT state in Ru(bpy)(2)(dpp)(2+), localized on the dpp ligand, increases the electron density on peripheral nitrogens as evidenced by an order of magnitude greater than six, increase in their Bronsted basicity relative to that of the ground state. The increased electron density promotes an excited-state coordination chemistry, where changes in coordination, rather than energy or electron transfer, leads to emission quenching and subsequent formation of the bimetallic. The enhanced basicity exists only in the excited state, and coordination of the second peripheral nitrogen, i.e. the chelate effect, competitive with relaxation of the excited state achieves the observed, thermodynamically stable product. This article summarizes the excited-state proton-transfer chemistry of Ru(II) diimine complexes, and discusses the application of this chemistry to excited-state coordination chemistry and the formation of bimetallic complexes. (C) 2001 Elsevier Science B.V. All rights reserved.