EXCITED-STATE ACID-BASE CHEMISTRY - A NEW QUENCHING MECHANISM

Most photochemical reactions degrade a metal complex, but recent exper iments in this laboratory reveal a series of unusual photochemical rea ctions that increase the metal content of the complex. A 450 nm excita tion of bis(2,2'-bipyridine) (2,3-bis(2-pyridyl) pyrazine)ruthenium(II ), Ru(bpy)(2)dpp(2+), complexes in the presence of PtCl62-, PdCl62- an d RhCl63- results in quantitative formation of the corresponding bimet allic [Ru(bpy)(2)(dpp)MCl4](2+). The excited state energies and therma l redox potentials of the chlorides and the specificity of the reactio n suggest that formation of the bimetallic is not the consequence of e nergy transfer or electron transfer quenching. Instead; we propose tha t these reactions are the result of an excited state acid-base chemist ry that manifests itself as changes in coordination and nucleophilicit y. The emissive MLCT state in Ru(bpy)(2)(dpp)(2+) is localized on dpp, and H+ quenching indicates that population of the emissive MLCT state increases the basicity of at least one of the peripheral nitrogens by at least four orders of magnitude relative to that in the ground stat e. Population of the MLCT state (bpy)(2)Ru(CN)(2), on the other hand, shifts charge density to the bpy ligand thereby decreasing the basicit y of the cyanide ligand by ca. four orders of magnitude. The pH depend ence of the quantum yield of decomposition of trans-[(bpy)(2)Ru(CN)(mu -CN)Rh(NH3)(4)Br](2+) suggests that this immense change in basicity le ads to a dissociative excited state. In complexes that exhibit excited slate acid-base properties, the data presented here suggest that quen ching is not necessarily limited to electron or energy transfer, but a lso occurs by an excited state acid-base chemistry that leads to chang es in coordination and/or nucleophilicity. (C) 1998 Elsevier Science S .A.