Molecular organisation within the smectic C mesophase of the liquid crystal 4,4′-di-n-heptyloxyazoxybenzene

Abstract

The long axes of the molecules in the mesophase of a smectic C liquid crystal are tilted with respect to the normal to the smectic planes. The magnitude of this tilt angle for the smectic C phase of 4,4′-di-n-heptyloxyazobenzene is determined, using electron resonance spectroscopy, to be 38.5 ± 1° in agreement with both optical and X-ray studies. The accuracy of the present study stems from our ability to grow a monodomain sample of the smectic phase by employing magnetic and surface forces when the smectic phase is formed from the nematic phase. The uniform distribution of the molecules within the monodomain sample can only be deformed by a magnetic field if its strength exceeds some critical value. When this deformation does occur the existing surface forces appear to be destroyed although new image forces are created which serve to maintain the deformed distribution. If, however, the liquid crystal is contained in a conventional glass tube then it proves impossible to obtain a monodomain sample because of the unimportance of surface forces. The investigation also suggests that the orientational order in the smectic mesophase is only slightly in excess of the order for the preceding nematic phase. Finally a study of the dynamics of the molecular realignment by a magnetic field reveals that this process may not be described by a single exponential decay.