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

Abstract

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.

Deactivation of Excited States in Transition‐Metal Complexes: Insight from Computational Chemistry

“…Even at low temperatures, as pinc rossover model system quickly breakst he Born-Oppenheimer (BO) approximation due to the strongv ibronic motiont riggered by the photoexcitation. [145] The following decay may reach or avoid the minimum of the HS state depending on the strengtho ft he tunnelling constant arisingf rom spin-orbit coupling. Furthermore, time-dependent calculations on the ultrafast spin crossover (20 fs) of aF e-Co Prussian blue showedt hat spin-orbit coupling can rapidlyv ary duringt he deactivation process.…”

“…Spin crossover systems are characterized by strong vibronic coupling between the states involved in the deactivation process, especially in proximity of conical intersections. Even at low temperatures, a spin crossover model system quickly breaks the Born‐Oppenheimer (BO) approximation due to the strong vibronic motion triggered by the photoexcitation . The following decay may reach or avoid the minimum of the HS state depending on the strength of the tunnelling constant arising from spin–orbit coupling.…”

Spatio-temporal Investigations of the Incomplete Spin Transition in a Single Crystal of [Fe(2-pytrz)2{Pt(CN)4}]·3H2O: Experiment and Theory

“…From the microscopic point of view, the spin transition phenomenon has been considered as a vibronic problem [22,23] in which the electronic and vibrational structures of the molecule are strongly coupled, to the extent that in some cases, the Born-Oppenheimer approximation is not valid [24,25]. The microscopic changes of the magnetoelastic properties of the SCO solids at the transition, accompanied with large volume changes resulting from the constructive interferences of the molecular volume changes which deploy at long-range through elastic interactions, cause important variations in the physical properties of the SCO materials, which show thermo-chromic features at the transition as well as significant rigidity changes.…”

The fate of molecular excited states: modeling donor–acceptor dyes

“…In this work, we expand the analysis to further validate the approach, discussing different system-bath coupling Hamiltonians as well as the effects of different choices for the bath spectral density. Specifically, we will show that some of the approximations adopted in our previous work, 46,49 and often adopted in the literature, 27,[50][51][52][53] need careful consideration. We then address the molecular model and discuss the robustness of the results in the multimode case.…”