Notable formation of a cubic phase from small bent-angle molecules based on the 1,7-naphthalene central core and alkylthio tails

Abstract

The mesomorphic properties of small angle, bent-shaped molecules [N(1,7)-S-n] consisting of a 1,7-naphthalene central core, typical Schiff-base side wings, and alkylthio tails with the carbon number n have been investigated, and a notable formation of a cubic (Cub) phase in addition to a distinct hexagonal columnar (Col_h) phase was observed. The Cub phase can form when the terminal chain length is longer than 22 and is stable over a wider temperature range (25 °C) as the length increases up to 30. The space symmetry of the Cub phase was determined as Pmn with a lattice size of 160–170 Å regardless of the terminal chain length. Assuming that the cubic unit cell is composed of a spherical assembly with a spatial electron density distribution, it can be described as containing eight spheres, each having a radius of 53 Å and comprising 300 molecules. This formation of a novel Cub phase, like the micellar assembly phases, was understood as an enclosed layer formation caused by layer deformation with significant curvature resulting from the asymmetric packing of bent molecules in combination with the longer alkylthio tail groups of N(1,7)-S-n. It is noteworthy that the present observation of a Cub phase with unequivocal experimental evidence establishing both its lattice structure and size is the first example of a novel self-assembly mode in a bent-core liquid crystal (LC) molecular system.