Light-induced excited spin state trapping: ab initio study of the physics at the molecular level

This paper provides a qualitative analysis of the physical content of the low-energy states of a spin-transition compound presenting a light-induced excited spin state trapping (LIESST) phenomenon, namely, [Fe(dipyrazolpyridine)2](BF4)2, which has been studied using the wave function-based CASPT2 method. Both the nature of the low-energy states and the relative position of their potential energy wells as a function of the geometry are rationalized from the analysis of the different wave functions. It is shown that the light-induced spin transition occurring in such systems could follow several pathways involving different excited spin states. In an ideal octahedral geometry, the interconversion from the excited singlet state to the triplet of lower energy, which is usually seen as an intermediate state in the LIESST mechanism, is quite unlikely since there is no crossing between the potential energy curves of these two states. On the contrary, in lower-symmetry complexes, the geometrical distortion of the coordination sphere due to ligand constraints is responsible for the occurrence of a crossing between these two states in the Franck-Condon region, leading to a possible participation of this triplet state in the LIESST mechanism. In the reverse LIESST process, a crossing between the potential energy curves of another triplet state and the excited quintet state occurs in the Franck-Condon region as well.