Mesophase behaviour of binary mixtures of bent-core and calamitic compounds

TL;DR

This study investigates the mesophase behavior of binary mixtures of bent-core and calamitic liquid crystals, revealing phase interactions, lowered melting points, and the induction of a monotropic SmC phase, supported by experimental and quantum-chemical analysis.

Contribution

It provides new insights into the phase behavior of mixed bent-core and calamitic liquid crystals, including the unexpected formation of a monotropic SmC phase.

Findings

Nematic and smectic phases do not coexist in mixtures.

Mixtures exhibit lowered melting temperatures.

Induction of monotropic SmC phase at low bent-core concentration.

Abstract

The mesophase behaviour of binary mixtures of bent-core and calamitic liquid crystals is presented. The nematogenic 4,6-dichloro-1,3-phenylene bis[4'-(10-undecen-1-yloxy)-1,1'-biphenyl-4-carboxylate] (I) was the banana-shaped component. As the calamitic compound ethyl 4'-(9-decen-1-yloxy)-1,1'-biphenyl-4-carboxylate (II), similar to one arm of the bent-core molecule, was used which exhibits smectic phases in a wide temperature range. A total of 6 mixtures with different compositions were prepared and studied by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction on non-oriented samples. In the mixtures a nematic phase is not concomitant with smectic A phase, and the temperature range of both phases highly depends on the concentration of the comprising compounds. Lowered melting temperatures have been observed for all mixtures with respect to that of the pure compounds. Unforeseen finding is the induction of a monotropic SmC phase in mixtures with lowest concentration of the bent-core compound. Semi-empirical quantum-chemical calculations have also been performed. Based on the calculated molecular conformation, as well as on collected X-ray diffraction data, a model for a possible self-assembly of the banana-shaped and calamitic compounds is proposed.