Vibrations Drive Ultrafast Intersystem Crossing of a Photoexcited Cr(III) Complex

The Cr(III) coordination complex serves as an archetypical 3d transition metal system for probing ultrafast excited-state dynamics with spin conversion due to its intrinsic intersystem crossing (ISC) pathway, 4T2g → 2Eg, upon photoexcitation. Here, we conduct ab initio molecular dynamics simulations in the 4T2g state of a model Cr(III) coordination complex, followed by analyses of multireference electronic structure properties. Across 50 trajectories, the compound retains a persistent Jahn–Teller distortion in the excited state, while exhibiting prominent symmetric metal–ligand bond stretching vibrations with frequencies of 219 and 465 cm–1. State-averaged complete active space self-consistent field (SA-CASSCF) calculations obtain two corresponding normal modes at 225 and 487 cm–1 with symmetric stretching character. The lower-frequency twisting/scissoring mode strongly modulates the 4T2g/2Eg energy gap, periodically zeroing the energy gap, whereas spin–orbit coupling is essentially invariant to vibrational motion (≈60–80 cm–1). A Landau–Zener analysis shows that every trajectory reaches its first 4T2g/2Eg crossing within 100 fs, with a transition probability at that crossing of 74% on average, in agreement with the sub-100 fs ISC inferred from experiment. Furthermore, calculations of single-point excited-state absorption from 2Eg to a higher ligand-to-metal charge-transfer (LMCT) state indicate that the coherences previously observed in transient absorption spectra arise from nuclear motion on the 2Eg surface. These results provide insights into the correlation between vibrational motion and electronic transitions, which can facilitate rational molecular design of transition metal complexes with desired excited-state properties by leveraging ligand versatility.