Observation of positive and negative magneto-LC effects in all-organic nitroxide radical liquid crystals by EPR spectroscopy

Abstract

The generation of spin glass-like inhomogeneous magnetic interactions (the average spin–spin interaction constant: [J with combining macron] > 0 or [J with combining macron] < 0), which we refer to as positive or negative “magneto-LC effects”, respectively, was observed in the bulk liquid crystalline (LC) phases of chiral all-organic radical compounds 2 with a positive dielectric anisotropy (Δε > 0) under weak magnetic fields. The sign and magnitude of [J with combining macron] depended on the type of LC phase or superstructure; stronger positive interactions ([J with combining macron] > 0) operated in the chiral smectic A (SmA*) phase of (2S, 5S)-2b (89% ee) than in the chiral nematic (N*) phase of (2S, 5S)-2a (96% ee), whereas weak negative interactions ([J with combining macron] < 0) were observed in the achiral nematic (N) phase of (±)-2a. The origin of the positive magneto-LC effects operating in the SmA* and N* phases was interpreted in terms of the generation of ferromagnetic head-to-tail spin–spin dipole interactions, whereas antiferromagnetic interactions arising from the formation of the RS magnetic dipolar interaction were responsible for the negative magneto-LC effects in the N phase.