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.