Polar and ionic liquid crystals based on the [closo-1-CB9H10]– and [closo-1-CB11H12]– boron clusters

Abstract

The subject of this dissertation is the development and evaluation of polar and ionic liquid crystals derived from boron clusters for electrooptical and anisometric ion transport applications. Liquid crystal is a state of matter characterized by properties intermediate between that of an ordinary liquid (fluidity, molecular disorder) and crystalline solid (long-range orientational and positional order) and is formed by anisometric rod-like, disc-like or bent-core molecules. The choice of the structural elements of the rigid core and flexible tails dictates the type of mesophase and its properties. Polyhedral boron clusters, possessing unusual molecular and electronic properties not offered by other organic molecules, are suitable core element for liquid crystalline materials for advanced applications. Thus, zwitterionic polar materials are derived from [closo-1-CB9H10]– and [closo-1-CB11H12]– anions by substitution with an onium fragment (pyridinium or sulfonium) and one electrically neutral fragment at antipodal positions. The ionic liquid crystals are designed as an ion pair, in which the anisometric anion is the major driver for induction of liquid crystalline properties. Within this context, practical and efficient synthetic methodologies were developed for functionalization and functional group transformations for anions [closo-1-CB11H12]– and [closo-1-CB9H10]– and a number of new zwitterionic and ionic materials for electroptical and anisometric ion transport applications have been prepared. The resulting zwitterionic and ionic materials were investigated in neat form and as additives to nematic hosts. Thermal, optical, dielectric, and powder X-ray methods were employed to characterize the new materials. They were found to be compatible with the nematic hosts, and they increased the dielectric response of the hosts due to high extrapolated dielectric anisotropy in a range of 18-70. Sulfonium-substituted zwitterions exhibit exclusively a nematic behavior, while pyridinium derivatives form either nematic or smectic phases. Ionic materials were found to have properties characteristic for ionic liquid crystals. Both polar and ionic materials were found to have properties promising for the desired applications.