Effect of Second-Order Spin-Orbit Coupling on the Interaction between Spin States in Spin-Crossover Systems

Area of Science:

• Computational chemistry
• Quantum mechanics
• Materials science

Background:

• Light-induced spin crossover (LCO) involves transitions between spin states in transition-metal complexes.
• Understanding excited state deactivation mechanisms is crucial for controlling LCO phenomena.
• Second-order spin-orbit coupling (SOC) plays a role in spin dynamics but is often challenging to compute accurately.

Purpose of the Study:

• To evaluate the impact of second-order SOC on the deactivation mechanism of excited low-spin states in LCO systems.
• To compare the standard perturbational approach with a variational strategy for calculating second-order SOC.
• To investigate the combined effects of geometry distortions and second-order SOC on spin state deactivation.

Main Methods:

• Calculation of second-order SOC using both standard perturbational and variational effective Hamiltonian methods.
• Theoretical analysis of two specific transition-metal complexes exhibiting LCO.
• Inclusion of geometry distortions in the theoretical models.

Main Results:

• The standard perturbational approach for second-order SOC often overestimates the interaction and is applicable only in specific cases.
• The variational strategy provides a more reliable calculation of second-order SOC.
• Combined effects of geometry distortions and second-order SOC create significant interactions between nearly uncoupled states.
• These interactions facilitate direct deactivation pathways from excited singlet and triplet states to the high-spin state.

Conclusions:

• Second-order SOC, particularly when combined with geometry distortions, is essential for understanding excited state deactivation in LCO.
• The findings support experimental interpretations of direct deactivation pathways that were previously lacking theoretical backing.
• Accurate theoretical methods are necessary for reliable predictions of spin dynamics in these systems.