Vibronic Theory of Ultrafast Intersystem Crossing Dynamics in a Single Spin-Crossover Molecule at Finite Temperature beyond the Born--Oppenheimer Approximation.

Quantum density matrix theory is carried out to study the ultrafast dynamics of the photoinduced state in a spin-crossover (SC) molecule interacting with a heat bath. The investigations are realized at finite temperature and beyond the usual Born-Oppenheimer (BO) approach. We found that the SC molecule experiences in the photoexcited state (PES) a huge internal pressure, estimated at several gigapascals, partly released in an "explosive" way within ∼100 fs, causing large bond length oscillations, which dampen in the picosecond time scale because of internal conversion processes. During this regime, the BO approximation is not valid. Depending on the tunneling strength, the ultrafast relaxation may proceed through the thermodynamic metastable high-spin state or prevent it. Interestingly, we demonstrate that final relaxation toward the low-spin state always follows a local equilibrium pathway, where the BO approach is valid. Our formulation reconciles the nonequilibrium and the equilibrium properties of this fascinating phenomenon and opens the way to quantum studies on cluster molecules.