Shifting the MLCT of d6 metal complexes to the red and NIR
Dieter Sorsche, M. Lima, Nicolas Meitinger, Krishna Prasad, Subrata Mandal, Ksenija D. Glusac, Sven Rau, Andrea Pannwitz
Abstract
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.