Reservoir Effect in bichromophoric FeIII Complexes with Methylene Bridge

Abstract

Replacing noble metals in photocatalytic systems by their abundant counterparts to enhance sustainability aspects is in the focus of current research. Iron is still a dream candidate despite the usually short lifetimes of photoactive states. The multichromophoric approach, where an organic chromophore with long triplet lifetimes acts as an excited state reservoir when connected to an iron complex, is a highly promising strategy that gained recent attention. This effect cannot be realized by direct covalent connection of organic chromophores to the Fe^III complex [Fe(ImPH)₂][PF₆] (HImPH = 1,1′-(1,3-phenylene)bis(3-methyl-1-imidazol-2-ylidene) due to highly delocalized states enabling fast relaxation pathways. In this work, we present the effect of the shortest aliphatic chain bridge, i.e. methylene, on the electronic state decoupling in complexes combining the base complex [Fe(ImPH)₂][PF₆] with the chromophoric phenyl and anthracenyl moieties. Chromophore connection to [Fe(ImPH)₂][PF₆] was realized at the central phenyl ring as well as on the imidazole sides, resulting in four multichromophoric complexes. A thorough ground and excited state characterization was carried out in addition to extensive DFT calculations. The results clearly show an efficient decoupling utilizing the smallest possible space in a methylene bridge, realizing a reservoir effect for the anthracenyl substituted complexes leading to excited state lifetimes beyond 5 ns.