High-Efficiency Iron Photosensitizer Explained with Quantum Wavepacket Dynamics

Fe(II) complexes have long been assumed unsuitable as photosensitizers because of their low-lying nonemissive metal centered (MC) states, which inhibit electron transfer. Herein, we describe the excited-state relaxation of a novel Fe(II) complex that incorporates N-heterocyclic carbene ligands designed to destabilize the MC states. Using first-principles quantum nuclear wavepacket simulations we achieve a detailed understanding of the photoexcited decay mechanism, demonstrating that it is dominated by an ultrafast intersystem crossing from ¹MLCT–³MLCT proceeded by slower kinetics associated with the conversion into the ³MC states. The slowest component of the ³MLCT decay, important in the context of photosensitizers, is much longer than related Fe(II) complexes because the population transfer to the ³MC states occurs in a region of the potential where the energy gap between the ³MLCT and ³MC states is large, making the population transfer inefficient.