Aggregation-driven, re-entrant isotropic phase in a smectic liquid crystal material

ABSTRACT

An azobenzene-core chiral mesogen designed for a photoactive ferroelectric liquid crystal system with switchable polarisation displays a highly unusual phase sequence, with a re-entrant, optically isotropic, fluid phase found below smectic phases in mixtures with high enantiomeric purity. The re-entrant isotropic phase is found on the basis of X-ray scattering and freeze-fracture transmission electron microscopy experiments not to be a cubic or other highly ordered phase but instead a translationally disordered liquid. The material also forms a gel under a wide range of concentrations in 50:50 ethanol/chloroform solutions. Ultraviolet/visible and infrared spectroscopy and quantum chemistry calculations suggest that the primary unit in the re-entrant isotropic and gel phases is a dimer composed of molecules crossed by about 90°, which hinders the formation of crystal phases and forms tubules of helical aggregates in the gel phase.

Graphical Abstract

KEYWORDS:

• Smectic
• re-entrant phase transition
• gel
• chirality
• azobenzene dye

Acknowledgements

This work was supported by the Soft Materials Research Center under NSF MRSEC grants [No. DMR-0820579 and DMR-1420736]. Research was carried out in part at the National Synchrotron Light Source, supported by the U.S. Department of Energy, Divisions of Materials and Chemical Sciences.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This work was supported by the National Science Foundation [Grant Numbers DMR-0820579 and DMR-1420736].