Molecular Design Principles for Photoactive Transition Metal Complexes: A Guide for "Photo-Motivated" Chemists

Luminescence and photochemistry involve electronically excited states that are inherently unstable and therefore spontaneously decay to electronic ground states, in most cases by nonradiative energy release that generates heat. This energy dissipation can occur on a time scale of 100 fs (∼10^-13 s) and usually needs to be slowed down to at least the nanosecond (∼10^-9 s) time scale for luminescence and intermolecular photochemistry to occur. This is a challenging task with many different factors to consider. An alternative emerging strategy is to target dissociative excited states that lead to metal-ligand bond homolysis on the subnanosecond time scale to access synthetically useful radicals. Based on a thorough review at the most recent advances in the field, this article aims to provide a concise guide to obtaining luminescent and photochemically useful coordination compounds with d-block elements. We hope to encourage "photo-motivated" chemists who have been reluctant to apply their synthetic and other knowledge to photophysics and photochemistry, and we intend to stimulate new approaches to the synthetic control of excited state behavior.