Influence of bipyridine isomers and some structurally related compounds on nematic liquid crystal orientation at the aqueous interface

Abstract

In this paper, orientational anchoring transition of nematic liquid crystal (LC) 4-cyano-4′-pentylbiphenyl (5CB) at the LC-aqueous interface in the presence of bipyridine isomers (2,2′-bipyridine (BPY22) and 4,4′-bipyridine (BPY44)) and some structurally related compounds (1,10-phenanthroline (PANTH110) and aldrithiol-4 (ALD4)) in the aqueous phase is presented. The addition of BPY22, BPY44 and PANTH110 into the aqueous phase causes 5CB to undergo concentration dependent planar-to-homeotropic transition at the LC-aqueous interface. In contrast, ALD4 on addition into the aqueous phase depending on its concentration diminishes the interface between 5CB and the aqueous phase. Furthermore, the LC ordering transitions are related to pH of the solutions containing BPY22 or BPY44 or PANTH110 as revealed by the cross-polarized optical images and the changes in the tilt angle. Density functional theory (DFT) has been used to evaluate stabilization energy and hydrogen bonding strength for understanding the mechanism of LC ordering at the aqueous interface. Our findings provide a new set of compounds that cause homeotropic alignment of 5CB at the LC-aqueous interface, which is a prerequisite for the development LC based biosensors.