EPR of the first Fe(III)-containing spin-crossover metallomesogens

The low-spin (LS) to high-spin (HS) transition in two mesogenic Fe(III) complexes with alkyloxysalicyliden-N^′-ethyl-N-ethylendiamine as ligands is studied by electron paramagnetic resonance (EPR). The symmetrization of the crystal field around the Fe(III) ion under first heating of a polycrystalline sample from the room temperature was observed in the spectra of HS complexes. The line narrowing (from 45 to 15 mT) under the crystal-smectic phase transition is explained by the strengthening of the intermolecular exchange interaction, as a result of the structural reorganization of layers in the smectic phase. A feature of both mesogenic spin-transition systemsis an unusual field-induced spin instability which leads to the hysteresis of the HS-LS composition. This instability is detected by EPR and magnetic susceptibility methods. The alignment of systems by magnetic field in the mesophase and a complete or partial orientational order under cooling the mesophase to the glass state are the main reason for this hysteresis. The influence of EPR-silent LS complexes reveals itself in the line broadening of HS compounds’ when the temperature is lowered from 220 K.