Modeling Photofunctional Transition Metal Complexes

Abstract: Transition metal complexes play a crucial role in solar energy conversion. These coordination compounds have promising applications in dye-sensitized solar cells and photocatalysis, with particular interest in solar fuel production. However, many of the photoactive transition metal complexes used in light-harvesting applications are still based on rare and expensive metals from the transition metal block. As an alternative to the popular Ru or Ir transition metal complexes, new research lines are emerging to solve the challenges in utilizing first-row transition metals, including Fe and Co. Density functional theory (DFT), has been broadly utilized to understand the underlying photophysics and quenching mechanisms of earth-abundant metal complexes excited states.An optimal match of metal and ligand is essential for achieving photoactive metal complexes. The tuning effect of interchanging the metal in a d6 hexa-carbene complex has shown the tight relationship between the metal charge and the final photoproperties. The charge induces a structural compression of the excited states favoring the occurrence of high-energy metal-centered states. The studied Co(III) complex displayed luminescence with an impressive long lifetime from a triplet metal-centered state, proving the functionality of these states. The population of triplet metal-centered states in low-spin d6 and quasi-octahedral complexes promotes several near-degenerate states with distinct structural distortions. Ab initio molecular dynamics in a Rh(III) complex, with dual emission from ligand-centered and metal-centered states, indicate that the entropy gain by geometry distortions drives the crossover reaction. Systems without metal-centered states, such as d0 titanocenes and scandocenes, also encounter distorted ligand-to-metal charge transfer states, which triggers radical ligand formation. Photochemical investigations on ligand-to-metal charge transfer states have been further extended to a Fe(III) carbene complex. Our results indicated that this complex shows a fast photoinduced charge disproportionation at high concentration, and the charge recombination occurs in the inverted Marcus regime. Diffusion of charge-separated species is also relevant to suppress recombination. The control of ion-pairing by solvent interactions promotes the hole migration of a donor molecule after quenching this Fe(III) complex. These active species can subsequently catalyze reactions, such as hydrogen production. A DFT protocol of (photo)redox potentials can assist the selection of optimal photosensitizer, donor, and proton reduction catalyst at a low computational cost. DFT also serves as a useful tool to assess multiple mechanistic reaction paths to generate hydrogen by proton reduction catalysts.