Shifting the MLCT of d6 metal complexes to the red and NIR

Light-active d 6 -coordination compounds hold great promise for light energy conversion, sensors and therapeutic applications. However, the activity in the red-to-NIR spectral region is highly desirable to convert solar light more efficiently, use low-cost red-light sources and activate these chromophores in biological tissue environment. Due to their versatility, tunability and broad intense absorption, d 6 -coordination compounds with metal-to-ligand charge transfer (MLCT) transitions are especially interesting. This review article offers a comprehensive collection of strategies to tune MLCT excited states in d 6 metal complexes and gives insights on group 6 to group 9 transition metals and their respective state-of-the-art MLCT engineering towards red-shifted absorption and emission properties with long-lived excited states. Strategies comprise lowering the π* level of the ligands, destabilizing and mixing of the metal-based HOMO, switching within a group of transition metals, matrix effects and insights into dealing with excited state deactivation in the context of the energy gap law. • Strong sigma donor ligands destabilize the HOMO and lead to a red-shift. • Extension of the ligand's π-system can result in a stabilized LUMO leading to a red-shift. • The HOMO of d 6 MLCT complexes of Co, Ir and Rh, often mix with ligand's, leading to Metal-Ligand to Ligand Charge Transfer. • The energy gap law is the biggest challenge for NIR and red excited states.