Synthesis and Characterization of a New Series of Paramagnetic Ferroelectric Liquid Crystalline Nitroxide Radicals

Abstract

A new series of all-organic liquid-crystalline (LC) chiral radical compounds, trans-1-alkoxyphenyl-4-[(4-(4-alkoxyphenyl)-2,5-dimethylpyrrolidine-1-oxy-2-yl)-benzylideneamino]benzenes (1) with various alkyl chains, containing a chiral nitroxide unit in the mesogen core were synthesized and their LC properties were fully characterized. The enantiomerically enriched compounds (2S,5S)-1 showed N*, TGBA*, SmA* and SmC* phases, while the racemates (±)-1 exhibited N, SmA and SmC phases. The phase transition behavior and the ferroelectric properties of the SmC* phase of (2S,5S)-1 differed from those of previously reported analogues with an ester group as a substitute for the imino group of (2S,5S)-1. Some of the (2S,5S)-1 series showed a temperature-dependent spontaneous polarization PS (θ) inversion. We discuss the origin of this difference in terms of their molecular structures optimized by molecular orbital calculations. The phase transition behavior supports the hypothesis that the large ϕ, which is defined as the angle between the molecular long axis (one of the principal axes of inertia) and the direction of the dipole moment in each molecule, results in the stabilization of the SmA* (SmA) phase.