Excited-State Electron Transfer in Ligand-Bridged Dimeric Complexes of Osminum

Article abstract of DOI:10.1021/j100401a038

The photophysical properties of the unsymmetrical ligand-bridged Os dimers, <(bpy)2(CO)Os^II(L)Os^II(phen)(dppe)(Cl)>³⁺ and <(bpy)2(CO)Os^II(L)Os^III(phen)(dpppe)(Cl)⁴⁺ (L = 4,4'-bipyridine (4,4'bpy) or 1,2-bis(4-pyridyl)ethane (bpa), bpy = 2,2'-bipyridine, phen = 1,10-phenanthroline, and dppe = 1,2-bis(diphenylphosphino)-cis-ethene), have been investigated by using a variety of techniques.In the (II,II) dimers, two independent metal to ligand charge transfer (MLCT) chromofores are present, based upon localized d?(Os) -> phen and d?(Os) -> 2,2'-bpy transitions at the respective metal centers.The results of transient absorption and emission experiments suggest that quenching of the higher energy, Os(bpy) localized MLCT state does not occur by Os(bpy) MLCT -> Os(phen) MLCT energy transfer which is energetically favorable.Rather, the quenching may occur by reductive electron transfer from the adjacent Os^II(phen) site.Rate constants for the intramolecular quenching have been measured for the bpa-bridged dimer at low temperatures in a glassy solvent.The excited-state behvior of the (II,II) dimers (L = 4,4'-bpy or bpa) is similar in frozen solutions.However, from steady-state transient emission and transient absorption data, past the glass to liquid transition the 4,4'-bpy bridging ligand in the 4,4'-bpy dimer flattens and acts as an electron acceptor.The flattening and concomitant intermolecular quenching process in fluid media are rapid compared to the lifetime of the initially populated CT excited state.In the mixed-valence (II,III) dimers the spectroscopically observed Os(bpy) MLCT excited state is relatively unperturbed by the adjacent Os^III(phen) site and there is no evidence for quenching by any of a number of energetically favorable pathways.