Powerful helicity inducers: axially chiral binaphthyl derivatives

Axially chiral binaphthyl derivatives with highly twisting powers were synthesised by substituting phenylcyclohexyl (PCH) mesogenic moieties into 2,2' positions or 2,2',6,6' positions of binaphthyl rings. The di- and tetra-substituted binaphthyl derivatives were adopted as chiral dopants to induce chiral nematic liquid crystals (N*-LCs). The helical twisting power (beta(M)) of tetra-substituted binaphthyl derivative, D-3, having direct linkages between the PCH moieties and the binaphthyl rings at the 6,6' positions, was 449 mu m(-1). This is ca. 2.6 times larger than that (171 mu m(-1)) of di-substituted one, D-1. For systematic investigation of helical twisting power in the binaphthyl derivatives, several tetra-substituted binaphthyl derivatives (D-2-D-9) were synthesised by introducing different aromatic moieties into the 6,6' positions of the binaphthyl rings without methylene spacer. When the substituents group changed from p-hexaoxyphenyl to p-hexaoxybiphenyl, the helical twisting powers of the binaphthyl derivatives increased from 234 to 757 mu m(-1). Interestingly, D-3 and D-9 exhibited liquid crystallinity. Although the liquid crystallinity of the chiral dopant has no direct influence on the helical twisting power of the N*-LC, it plays a role in increasing the miscibility of the chiral dopant to host N-LC, leading to a raise of the upper limit in concentration of the chiral dopant. Consequently, both the high helical twisting power and the high miscibility of D-3 allowed us to prepare a highly twisted N*-LC, the helical pitch of which is in the nano-order, e.g. 270nm.