Effect of tert-Butyl Functionalization on the Photoexcited Decay of a Fe(II)‑N‑Heterocyclic Carbene Complex

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-N-heterocyclic carbene complex, [Fe­(btbip)₂]²⁺ (btbip = 2,6-bis­(3-tert-butyl-imidazole-1-ylidene)­pyridine). The results demonstrate that a relatively minor structural change compared to its parent complex, [Fe­(bmip)₂]²⁺ (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 (^1,3MLCT) 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 (¹MC) states. This occurs because the tert-butyl functionalization stabilizes the ¹MC states, enabling the ^1,3MLCT → ¹MC 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.