Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements: experimental methodology and results

Abstract

The electron density distribution of a light-induced molecular excited state, i.e. the high spin metastable state of [Fe(phen)₂(NCS)₂], was determined from steady-state photocrystallographic measurements. We defined the experimental conditions under which the accuracy of the measured diffraction data is compatible with an electron density analysis. These include: (i) a large structural and electronic contrast between high spin (HS) and low spin (LS) states, (ii) an efficient photoconversion under light irradiation and (iii) slow relaxation of the HS metastable state. Multipolar modeling of the electron density yielded a deformation density and 3d-orbital populations for Fe(II) characteristic of a high spin (t_2g⁴e_g²) electron configuration and support the assumption of significant σ-donation and π-backbonding of the Fe—N interactions. The electron density distribution in the intermolecular regions confirms anisotropic intermolecular interactions with possibly a layer topology parallel to the orthorhombic (ab) plane, related to the system cooperativity.