Understanding and
subsequently being able to manipulate the excited-state
decay pathways of functional transition-metal complexes is of utmost
importance in order to solve grand challenges in solar energy conversion
and data storage. Herein, we perform quantum chemical calculations
and spin-vibronic quantum dynamics simulations on the Fe-<i>N</i>-heterocyclic carbene complex, [Fe(btbip)<sub>2</sub>]<sup>2+</sup> (btbip = 2,6-bis(3-<i>tert</i>-butyl-imidazole-1-ylidene)pyridine).
The results demonstrate that a relatively minor structural change
compared to its parent complex, [Fe(bmip)<sub>2</sub>]<sup>2+</sup> (bmip = 2,6-bis(3-methyl-imidazole-1-ylidene)pyridine), completely
alters the excited-state relaxation. Ultrafast deactivation of the
initially excited metal-to-ligand charge transfer (<sup>1,3</sup>MLCT)
states occurs within 350 fs. In contrast to the widely adopted mechanism
of Fe(II) photophysics, these states decay into close-lying singlet
metal-centered (<sup>1</sup>MC) states. This occurs because the <i>tert</i>-butyl functionalization stabilizes the <sup>1</sup>MC states, enabling the <sup>1,3</sup>MLCT → <sup>1</sup>MC
population transfer to occur close to the Franck–Condon geometry,
making the conversion very efficient. Subsequently, a spin cascade
occurs within the MC manifold, leading to the population of triplet
and quintet MC states. These results will inspire highly involved
ultrafast experiments performed at X-ray free electron lasers and
shall pave the way for the design of novel high-efficiency transition-metal-based
functional molecules