Exceptional Excited-State Lifetime of an Iron(II)-N-Heterocyclic Carbene Complex Explained

Earth-abundant transition-metal complexes are desirable for sensitizers in dye-sensitized solar cells or photocatalysts. Iron is an obvious choice, but the energy level structure of its typical polypyridyl complexes, featuring low-lying metal-centered states, has made such complexes useless as energy converters. Recently, we synthesized a novel iron-N-heterocyclic carbene complex exhibiting a remarkable 100-fold increase of the lifetime compared to previously known iron(II) complexes. Here, we rationalize the measured excited-state dynamics with DFT and TD-DFT calculations. The calculations show that the exceptionally long excited-state lifetime (∼9 ps) is achieved for this Fe complex through a significant destabilization of both triplet and quintet metal-centered scavenger states compared to other Fe(II) complexes. In addition, a shallow (3)MLCT potential energy surface with a low-energy transition path from the (3)MLCT to (3)MC and facile crossing from the (3)MC state to the ground state are identified as key features for the excited-state deactivation.