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

Abstract

The second-order spin-orbit coupling is evaluated in two transition-metal complexes to establish the effect on the deactivation mechanism of the excited low-spin state in systems that undergo spin transitions under the influence of light. We compare the standard perturbational approach to calculate the second-order interaction with a variational strategy based on the effective Hamiltonian theory and show that the former one can only be applied in some special cases and even then gives results that largely overestimate the interaction. The combined effect of geometry distortions and second-order spin-orbit coupling leads to sizeable interactions for states that are nearly uncoupled in the symmetric (average) structure of the complex. This opens the possibility of a direct deactivation from the singlet and triplet states of the metal-to-ligand charge-transfer manifold to the final high-spin state as suggested from the interpretation of experimental data but so far not supported by theoretical descriptions of the light-induced spin crossover.

Spin‐inversion mechanisms in O2 binding to a model heme complex revisited by density function theory calculations

“…We did not calculate the spin–orbit couplings for the singlet–quintet spin inversion because the corresponding couplings are expected to be small. In this case, the direct spin–orbit coupling between the two electronic states is zero, and a non‐zero coupling can be introduced only from second‐order and higher‐order corrections, which are mediated by other electronic states . The spin–orbit coupling values are relatively large and are comparable to thermal energy.…”

Controlling the Lifetime of the Triplet MLCT State in Fe(II) Polypyridyl Complexes through Ligand Modification

“…The first point has been discussed extensively in the previous section, the vibrational modes and the corresponding frequencies were calculated from the optimised geometries used to calculate the adiabatic energy differences and the SO coupling was estimated following the procedure outlined in Refs. [19,20]. The spin-orbit couplings between the relevant electronic states were calculated as the expectation value of the spin-orbit operator using the CASSCF wave functions.…”

“…Effective spin-orbit couplings have been calculated by the effective Hamiltonian approach outlined in Ref. [19].…”

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

“…This perturbative approach can only be appliedt oa ddress the higher order coupling between the lowests tates of each spin multiplicity.I ts application to higherl ying electronic states (as has to be done in the study of deactivation processes of excited states)l eads to problems when the energy difference between state "b" and "m"b ecomes close to zero. Instead, the variational approachb ased on effective Hamiltoniant heory [122] does not show this limitation and can be applied to derive the effective coupling (direct plus all highero rder couplings) between all electronic states that play ar ole in the phenomenon under study.T he methodh as recently been applied to calculate the effective coupling between the quintet and the triplet MLCT states in [Fe(bpy) 3 ] 2 + ; [123,124] two high-lying excited states that play af undamentalr ole in the light-induced magnetism of this compound.…”

“…The methodh as been appliedt oF e II,III -polypyridylc omplexes, [47,123,124,128] aC u I complex [129] and severalO LED-related Ir III complexes, [92] among many other applications including organic molecules. As imilar approach, based on second-order perturbation theory,w as introduced by Peng et al [130] and has recently been appliedt os tudy the non-radiative decay processes in Ir III OLEDs.…”