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

摘要

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 ~3MLCT_(prox) or ~3MLCT_(dist) 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 Zn ~(II) ion to the open coordination site of the laminate acceptor ligand is observed to significantly lower the energy of the ~3MLCT _(dist) 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 ~3MLCT_(dist) state. In tpphz complexes, Zn II coordination can reorder the energy of the ~3MLCT _(prox) and ~3MLCT_(dist) 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 ? and possess exceptionally long lifetimes by equilibration with other triplet states. Ru enlightened: Long-lived, directional photoinduced charge separation in Ru~(II) complexes with laminate polypyridyl ligands is demonstrated. Charge-separated states are exhibited by complexes bearing laminate acceptor and bithienyl donor ligands (see scheme). Unusual excited-state lifetimes reflect equilibration of ~3MLCT _(prox) or ~3MLCT_(dist) states with a ~3LC state or a ~3ILCT state that formally localizes a hole on the bithiophene and an electron on the laminate acceptor ligand.