Long-lived, directional photoinduced charge separation in RuII complexes bearing laminate polypyridyl ligands

Abstract

RuII complexes incorporating both amide-linked bithiophene donor ancillary ligands and laminate acceptor ligands; dipyrido[3,2-a:2',3'-c]phenazine (dppz), tetrapyrido[3,2-a:2',3'-c:3'',2''-h:2''',3'''-j]phenazine (tpphz), and 9,11,20,22-tetraazatetrapyrido[3,2-a:2',3'-c:3'',2''-l:2''',3''']-pentacene (tatpp) exhibit long-lived charge separated (CS) states, which have been analyzed using time-resolved transient absorption (TA), fluorescence, and electronic absorption spectroscopy in addition to ground state electrochemical and spectroelectrochemical measurements. These complexes possess two electronically relevant ³MLCT states related to electron occupation of MOs localized predominantly on the proximal "bpy-like" portion and central (or distal) "phenazine-like" portion of the acceptor ligand as well as energetically similar ³LC and ³ILCT states. The unusually long excited state lifetimes (τ up to 7 μs) observed in these complexes reflect an equilibration of the ³MLCTprox or ³MLCTdist states with additional triplet states, including a ³LC state and a ³ILCT state that formally localizes a hole on the bithiophene moiety and an electron on the laminate acceptor ligand. Coordination of a ZnII ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ³MLCTdist state by decreasing the magnitude of the excited state dipole and resulting in much shorter excited state lifetimes. The presence of the bithiophene donor group is reported to substantially extend the lifetime of these Zn adducts via formation of a ³ILCT state that can equilibrate with the ³MLCTdist state. In tpphz complexes, ZnII coordination can reorder the energy of the ³MLCTprox and ³MLCTdist states such that there is a distinct switch from one state to the other. The net result is a series of complexes that are capable of forming CS states with electron-hole spatial separation of up to 14 Å and possess exceptionally long lifetimes by equilibration with other triplet states.