Nanoscale Self-Hosting of Molecular Spin-States in the Intermediate Phase of a Spin-Crossover Material

A new spin-crossover (SC) complex [(FeH2L2-Me)-H-II][AsF6](2) has been synthesized, in which H2L2-Me denotes the chirogenic hexadentate N-6 Schiff-base ligand bisa{[(2-methylimidazol-4-yl)methylidene]-3-aminopropyl}ethylenediamine. This complex has revealed a rich variety of phases during its two-step thermal crossover, as well as photoinduced spin-state switching. A high-symmetry high-spin (HS, S=2) phase, a low-symmetry low-spin (LS, S=0) phase, an intermediate phase characterized by an unprecedented lozenge pattern of 12 predominantly HS molecular crystallographic sites confining 18 predominantly LS molecular crystallographic sites, and a photoinduced low-symmetry HS phase have been accurately evidenced by temperature-dependent magnetic susceptibility, Mossbauer spectroscopy, and crystallographic studies. This variety of phases illustrates the multi-stability of this system, which results from coupling between the electronic states and structural instabilities.