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

The generation of spin glass-like inhomogeneous magnetic interactions (the average spin-spin interaction constant: (J) over bar > 0 or (J) over bar < 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 (Delta epsilon > 0) under weak magnetic fields. The sign and magnitude of (J) over bar depended on the type of LC phase or superstructure; stronger positive interactions ((J) over bar > 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) over bar < 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.