Organic derivatives of closo-boranes:: a new class of liquid crystal materials

closo-Boranes and heteroboranes have been investigated as structural elements for liquid crystals. This paper reviews our progress to date in developing structure-property relationships for these boron cluster mesogens and assesses the effects of both steric and electronic influences on the liquid crystal behavior. The observation that nematic phases are favored over smectic phases is attributed principally to the steric effect of the clusters. Electronic and stereoelectronic effects can lead to materials exhibiting unusual properties. For example, compounds may exhibit high refractive indices yet low birefringence, they may be W-transparent or, in some cases, exhibit strong charge transfer interactions with pi-aromatic systems. (C) 1999 Elsevier Science S.A. All rights reserved.