Methylene-linked liquid crystal dimers and the twist-bend nematic phase

The transitional properties of three methylene-linked liquid crystal dimers are reported, namely, 1,5-bis(4-cyanoanilinebenzylidene-4'-yl)pentane (CN-5-CN), 1,5-bis(4-methoxyanilinebenzylidene-4'-yl)pentane (1O-5-O1), and 1,5-bis(4-ethoxyanilinebenzylidene-4'-yl)pentane (2O-5-O2). Each dimer exhibits two monotropic mesophases. The higher temperature mesophase is a normal nematic phase while the lower temperature phase is assigned as a twist-bend nematic phase. The assignment of the twist-bend nematic phase was based on the strong similarities in the optical textures observed to those reported recently for a structurally similar dimer. The complete miscibility of the mesophases exhibited by CN-5-CN and 1O-5-O1 has been established. The analogous hexamethylene-linked dimers exhibit only the normal nematic phase as do the corresponding ether-linked dimers. A review of the literature reveals another five methylene-linked odd-membered dimers that exhibit a nematic-nematic transition and, in each, the lower temperature nematic phase exhibits similar properties to those reported for the twist-bend nematic phase. The formation of this new nematic phase has been attributed to a negative bend elastic constant which results from the bent geometry of methylene-linked odd-membered dimers.