Dynamics in the isotropic and nematic phases of bent-core liquid crystals: NMR perspectives

Abstract

The dynamic properties of low-molecular-weight bent-core liquid crystals were revealed to be unusual with respect to the analogous rod-like mesogens. First pioneering investigations based on dynamic light scattering measurements were followed by intense and deep studies based on nuclear magnetic resonance (NMR) spectroscopy, mainly 2H NMR relaxation and line width analyses, 1H NMR relaxometry and diffusometry. These studies were of help in identifying the molecular motions responsible of the observed slow relaxation and the large 2H NMR spectral width. Viscosity and rheology measurements provided further elements confirming the slow dynamics of bent-core liquid crystals at a macroscopic level. All these experimental techniques indicate the presence of restricted dynamic motions as a distinctive feature of bent-core liquid crystals both in the isotropic and nematic phases. In this review, the case of several bent-core mesogens studied by different NMR methods is reported and the main results are discussed in view of the recent achievements concerning the tendency of bent-core molecules to form aggregates or temporary clusters both in the isotropic and nematic phases.