Boron Clusters as the Centerpiece of Advanced Liquid Crystals: Fundamental Chemistry and Properties

Abstract

Liquid crystals incorporating boron clusters are of interest for electro-optics, ion transport, and fundamental structure-property relationship studies. A liquid crystal is a fluid possessing orientational and positional order between that of the lattice of a solid (long-range orientational and long-range positional order) and the random disorder (no orientational or positional order) of a liquid. Balance between the rigidity of the molecular core and flexible periphery dictates the type of liquid crystalline phase and its stability. Anisometric molecules, typically rods or discs, form liquid crystalline phases. closo-Boron clusters are inorganic structures characterized by highly delocalized bonding and high chemical, oxidative, and thermal stability. These clusters can exist as highly symmetrical molecules that are either neutral or negatively charged.

Within this framework, a negatively charged boron cluster, [closo-1-CB9H10]-, was exploited as the centerpiece of both zwitterionic and ionic liquid crystalline materials. Access to these materials was limited by the lack of synthetic methodology and understanding of reactive intermediates of the [closo-1-CB9H10]- anion. Therefore, a systematic approach was taken to advance the synthetic and physical-organic chemistry of the [closo-1-CB9H10]- anion within the context of incorporating it into liquid crystalline structures. Once this stepwise approach was completed, the newly discovered methodology was employed in the preparation of advanced liquid crystalline materials containing the [closo-1-CB9H10]- anion.

Both the zwitterionic and ionic materials were studied for liquid crystalline properties using thermal, optical, and in some cases dielectric and XRD methods. The zwitterionic materials posses large longitudinal dipole moments and were utilized as additives to other liquid crystals, which caused large increases in the dielectric properties of the bulk material. These findings are promising for electro-optical applications. The ionic materials displayed typical liquid crystalline behavior expected for ionic architectures. However, the design of such materials is unique in that the anisometric anion is the driving force behind the organization of the molecules in the fluid phase. These types of materials are promising for photo-physical effects and the potential transport of ions for energy storage or delivery.