Exploiting aggregation & “heavy ligand effect” in photofunctional coordination compounds

Coordination compounds involving late transition metal elements (such as Re, Pd, Pt and Au) have found applications in optoelectronics, bioimaging and phototherapy. In all cases, the heavy atom effect plays a crucial role in modulating intersystem crossing rates as well as the radiative deactivation efficiencies from the lowest triplet state, which is known as phosphorescence. Usually, diffusion-controlled Dexter-type energy transfer to triplet dioxygen quenches long-lived excited triplet states while leading to the formation of singlet dioxygen, a highly reactive oxygen species. Notably, while aggregation phenomena typically lower the photoluminescence efficiencies due to intermolecular deactivation, planar d8-configured compounds can show red-shifted emission from dimeric units mediated by metal-metal coupling. Herein, the role of progressively heavier pnictogen-based ligands employed as monodentate ancillary units for photofunctional Pt(II) and Re(I) complexes will be discussed, while paying attention to excited state lifetimes, phosphorescence efficiencies and singlet dioxygen quantum yields. Besides introducing the “heavy ligand effect”, aggregation phenomena and the implementation of such coordination compounds as self-referenced oxygen sensors will be presented.