Charge-transfer processes in metal complexes enable luminescence and memory functions

Molecular transition-metal–ligand complexes are emerging as useful paradigms in materials science. Transition metal complexes have diverse metal d electron configurations, oxidation states, coordination numbers and geometries such that they can undergo a diverse array of electronic transitions. Metal-to-ligand charge-transfer transitions and their associated excited states are especially attractive given their rich redox properties and robustness. This chemistry is accessible by appropriate choice of low-valence metal centres and strong π-acceptor ligands. An in-depth fundamental understanding of their charge-transfer, assembly and structure–property relationships is important to allow us to rationally design complexes and tune their characteristics for an intended application. In addition to their attractive light-harvesting and photocatalytic applications, this Perspective describes recent developments in the use of transition metal complexes as materials in phosphorescent organic light-emitting diodes and resistive memory devices. Metal–ligand complexes undergo diverse charge-transfer processes when stimulated by light or electric fields. This Perspective describes how these processes can be exploited in photosensitizers, luminophores and resistive memory materials.