Bimolecular reactivity of 3d metal-centered excited states (Cr, Mn, Fe, Co)

Abstract

Metal-centered (MC) excited states (ESs) of 3d transition metal complexes (TMCs) often possess rather low energies so that these represent the lowest energy ESs. Additionally, MC states are often strongly distorted, hence they efficiently decay non-radiatively to the ground state. As bimolecular photoinduced electron transfer (PET) and energy transfer (EnT) processes require contact to the substrate, the metal confinement of the ES wavefunction of MC states makes these processes challenging. Consequently, MC states are considered less useful as compared to long-lived charge transfer states of higher energy with wavefunctions extending onto the ligands. Despite these supposed drawbacks, some classes of TMCs can successfully engage in bimolecular PET and EnT processes with MC states being the photoactive states. We discuss these initial examples of MC ES reactivity covering chromium, manganese, iron, and cobalt complexes with the aim to gain a deeper understanding of these processes and to identify the decisive key parameters. Finally, we present catalytic photoredox and energy transfer processes using photosensitizers with suitable MC ESs.