A Trimetallic Mixed Ru(Ⅱ)/Fe(Ⅱ) Terpyridyl Complex with A Long-Lived Excited State in Solution at Room Temperature

摘要

The development of chromophores for light harvesting and visible light sensitization of photochemical processes has evolved considerably. Complex and effective systems have been developed consisting of multiporphyrin arrays, metal diimine and triimine complexes, and a variety of other related chromophores. Among the most widely studied transition metal complexes have been low-spin d~6 transition metal complex derivatives of bipyridyl and terpyridyl ligands. These complexes are of interest because they absorb a significant portion of the visible spectrum, have relatively long-lived excited states ( > 1 μs), are often stable following one-electron oxidation and reduction, and exhibit good photochemical stability. The equilibrated excited state of these complexes has been assigned as a metal-to-ligand charge transfer (MLCT) state. First-row transition metal complexes in this series (e.g., Fe(Ⅱ)) have not proved to be effective sensitizers because low-energy ligand field excited states on the metal provide non-radiative decay pathways that significantly decrease excited-state lifetimes and lead to facile complex decomposition. Recently, we have prepared a variety of phenylene—vinylene-substituted terpyridyl ligands that, when coordinated to Ru(Ⅱ), have long-lived excited states that are localized on the phenylene—vinylene-containing ligand (~3IL). In these complexes the lowest-energy absorption transition is MLCT; intersystem crossing leads to a long-lived triplet excited-state localized on the ligand (~3IL) that serves as the reactive excited state for energy- and electron-transfer reactions. In this report we describe the photophysical behavior of two trimetallic terpyridyl complex light-harvesting ensembles containing two Ru(Ⅱ) centers and an Fe(Ⅱ) center. Through control of the phenylene-vinylene moiety an Fe(Ⅱ)-containing complex having a long-lived excited state can be prepared. To our knowledge this is the first example of an Fe(Ⅱ) diimine- or triimine-containing complex with an excited-state lifetime long enough for bimolecular reactions.