Effect of <i>tert</i>-Butyl Functionalization on the Photoexcited Decay of a Fe(II)-<i>N</i>-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-<i>N</i>-heterocyclic carbene complex, [Fe­(btbip)₂]²⁺ (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)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 <i>tert</i>-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

Perspective: Preservation of coherence in photophysical processes

Coherence is one of the most important phenomena in ultrafast sciences. We give our perspective on the terminology, observation, and preservation of coherence in photophysical processes with some glimpses to the past and some looking-head to what may pave the way for scaling one of the last bastions in ultrafast science, namely, that of mode specific chemistry where it will be possible to break any specific bond by tailoring the pulse, an accomplishment that obviously would be the dream of any chemist