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 MC6n- (M = Pt(IV), Rh(III), Pd(IV)) leads to quantitative f ormation of the corresponding bimetallic *Ru(bpy)(2)(dpp)(2+) + MC6n+ --> [Ru(bpy)(2)(dpp)MCl4](4-n) + 2Cl(-) Formation of the bimetallic does not occur by either electron- or energy-tr ansfer processes. Instead, population of the MLCT state in Ru(bpy)(2)(dpp)( 2+), localized on the dpp ligand, increases the electron density on periphe ral nitrogens as evidenced by an order of magnitude greater than six, incre ase in their Bronsted basicity relative to that of the ground state. The in creased electron density promotes an excited-state coordination chemistry, where changes in coordination, rather than energy or electron transfer, lea ds to emission quenching and subsequent formation of the bimetallic. The en hanced basicity exists only in the excited state, and coordination of the s econd peripheral nitrogen, i.e. the chelate effect, competitive with relaxa tion of the excited state achieves the observed, thermodynamically stable p roduct. This article summarizes the excited-state proton-transfer chemistry of Ru(II) diimine complexes, and discusses the application of this chemist ry to excited-state coordination chemistry and the formation of bimetallic complexes. (C) 2001 Elsevier Science B.V. All rights reserved.