Ferroelectric metallomesogens composed of achiral spin crossover molecules

Abstract

Ferroelectric liquid crystals (FLCs) are fascinating functional materials that have a remnant and electrically invertible polarization. To date, typical FLCs have been mainly realized by molecular design such as the incorporation of chirality into a given molecular structure. Here, we report for the first time ferroelectricity induced by spin transition associated with a crystal – liquid crystal phase transition in achiral molecules. Iron(II) metallomesogens incorporating alkyl chains of type [Fe(3C_n-bzimpy)₂](BF₄)₂ (n = 8 (1), 10 (2), 12 (3), 14 (4), 16 (5) and 18 (6); bzimpy = 2,6-bis(benzimidazol-2′-yl)pyridine) that exhibit spin crossover (SCO) phenomena have been synthesized. Compounds 5 and 6 were each demonstrated to show SCO behaviour along with the occurrence of a phase transition between the crystalline (Cr) state and corresponding chiral smectic C (SmC*) state. The distortion of the coordination sphere in the high spin state is seen to trigger the generation of the SmC* state. The liquid crystalline compounds do not display ferroelectric behaviour in their Cr state but do exhibit ferroelectric hysteresis loops in their SmC* state. Ferroelectric switching was clearly confirmed by second harmonic generation (SHG) experiments involving the respective phases. These findings will undoubtedly lead to new strategies for the design of new FLCs based on metal-centred spin transitions.