Intramolecular electron transfer in cyanide-bridged ruthenium dimers

The absorption, luminescence, and transient-absorption spectra of [(bpy)(tpy)Ru-II(CN)](+) (bpy = 2,2'-bipyridine; tpy = 2,2':6'2 ''-terpyridine) and [(bpy)(tpy)(RuCNRuII,III)-C-II(NH3)(5)](3+,4+) have been measured. Metal-to-ligand charge-transfer (MLCT) excitation of the monomer produces an excited state that decays with a rate constant of 1.3 x 10(8) s(-1). Picosecond excitation of [(bpy)(tpy)Ru(II)CNRU(III)(NH3)(5)](4+) does not lead to any detectable transients, indicating that relaxation rate constants are >10(12) s(-1). A transient attributable to [(bpy)(tpy(.-))Ru(II)CNR(III)(NH3)(5)](3+) is observed following picosecond excitation of [(bpy)(tpy)RuCNRuII(NH3)(5)](3+); this intermediate decays via intramolecular electron transfer with a rate constant of 3.8 x 10(10) s(-1). Quantum-mechanical descriptions of the nuclear reorganization accompanying electron transfer are required to explain the observed kinetics.