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

closo-Boranes and heteroboranes have been investigated as structural elemen ts for liquid crystals. This paper reviews our progress to date in developi ng structure-property relationships for these boron cluster mesogens and as sesses the effects of both steric and electronic influences on the liquid c rystal behavior. The observation that nematic phases are favored over smect ic phases is attributed principally to the steric effect of the clusters. E lectronic and stereoelectronic effects can lead to materials exhibiting unu sual properties. For example, compounds may exhibit high refractive indices yet low birefringence, they may be W-transparent or, in some cases, exhibi t strong charge transfer interactions with pi-aromatic systems. (C) 1999 El sevier Science S.A. All rights reserved.