Photophysical Investigation of Iron(II) Complexes Bearing Bidentate Annulated Isomeric Pyridine-NHC Ligands

The possibility of achieving luminescent and photophysically active metal–organic compounds relies on the stabilization of charge transfer states and kinetically and thermodynamically blocking non-radiative dissipative channels. In this contribution, we explore the behavior of bidentate iron complexes bearing N-heterocyclic carbene ligands with extended conjugation systems by a multidisciplinary approach combining chemical synthesis, ultrafast time-resolved spectroscopy, and molecular modeling. Lifetimes of the metal-to-ligand charge transfer and metal-centered states reaching up to ≈20 ps are evidenced, while complex decay mechanisms are pointed out, together with a possible influence of the facial and meridional isomerism. The structural degrees of freedom driving the non-radiative processes are highlighted, and their rigidification is suggested as an effective way to further increase the lifetimes.