Chapter 11: Nitroxides in Liquid Crystals

This review article summarizes the individual features and suggests a general mechanism with respect to the unique spin glass-like superparamagnetic behaviour referred to as the positive magneto-LC effect, which was observed not only in the various chiral and achiral liquid crystalline (LC) phases formed by heating the original solid phases of all-organic nitroxide radical compounds with a negative dielectric anisotropy (Δε < 0), but also in their isotropic phases by further heating the LC phases and in the solid phases by cooling from the isotropic phases. Such a superparamagnetic behaviour could be interpreted in terms of the formation and/or enlargement of the superparamagnetic spins domains surrounded by ordinary paramagnetic spins by heating, and the preservation of the resulting increased superparamagnetic domains during further repeated heating and cooling processes, similarly to the case of metallic spin glass materials. Furthermore, to develop stable redox-resistant metal-free magnetic micro- and nano-sized carriers for a drug delivery system, etc., stable microemulsions and robust nano-sized mixed micelles were successfully prepared from simple nitroxide monoradical compounds. The resulting mixed micelles could encapsulate hydrophobic drugs and/or fluorescence agents to provide theranostic nanomedicines which are expected to be used as targeted drug delivery carriers visible by magnetic resonance imaging.