Effect of chiral dopant on a helical Sm1 phase of banana-shaped N-n-O-PIMB molecules

Abstract

The chiral helical structures of the Sm1 phase of naphthalene-2,7-diyl bis[4-(4-alkoxyphenyliminomethyl)benzoates] (N-n-O-PIMB) occur with equal probability for right- and left-handed structures. In this report, we investigated the effect of chiral dopants on the formation of these structures. Two chiral homologues, N-4-O-PIMB2* with (S)-(+)-2-methylbutoxy tails and N-8-O-PIMB6* with (S)-(+)-6-methyloctyloxy tails, were synthesized for this purpose. We found that the chiral agent induces an imbalance between the two chiral domains such that the area of one of the two domains increases with chiral dopant content. The position of the chiral centre of the doping agent plays an important role in controlling the helical sense as well as the strength of the effect. A stronger effect was observed for N-4-O-PIMB2* in which the asymmetric carbon is closer to the central core. The helical pitch is not affected by the amount of chiral dopant as observed from the wavelength maximum in the CD curve. Further, it is interesting that N-4-O-PIMB2* induces preferentially left-handed helical structures, while N-8-O-PIMB6* forms right-handed structures. These unusual effects of the chiral dopant are discussed with a comparison with the conventional chiral phases such as the chiral nematic and SmC phases.