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

Abstract

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, $^{4}T_{2g} \rightarrow ^{2}E_{g}$, upon photoexcitation. Here we conduct ab initio molecular dynamics simulations in the $^{4}T_{2g}$ 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 cm$^{-1}$ and 465 cm$^{-1}$. State-averaged complete active space self-consistent field (SA-CASSCF) calculations obtain two corresponding normal modes at 225 cm$^{-1}$ and 487 cm$^{-1}$ with symmetric stretching character. The lower-frequency twisting/scissoring mode strongly modulates the $^{4}T_{2g}/^{2}E_{g}$ energy gap, periodically zeroing the energy gap, whereas spin-orbit coupling is essentially invariant to vibrational motion ($\approx$ 60-80 cm$^{-1}$). Furthermore, calculations of single-point excited-state absorption from $^{2}E_{g}$ 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 $^{2}E_{g}$ 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.