Core-Only Calamitic Liquid Crystals: Molecular Design and Optoelectronic Properties

Abstract

Thermotropic liquid crystals are typically thought of as requiring a rigid core and flexible side-chains to exhibit liquid crystallinity, but precedent does exist for retaining liquid crystalline nature without these side-chains in highly conjugated, para-substituted aromatic structures, albeit at relatively high temperatures (>300 °C). Hence, this work aims to establish molecular design criteria for promoting liquid crystallinity in heteroaromatic, fully conjugated, core-only calamitic liquid crystals at sufficiently low phase transition temperatures. The synthesis and characterization of an extensive library of calamitic target structures (32) has been accomplished, where primarily enantiotropic smectic A and nematic phases were observed. Comparison of the mesomorphic properties between different cores and with different terminal functional groups was done and rationalization of the observed differences is attempted through single crystal and computational data. These all-aromatic compounds also display strong fluorescence in solution and in the solid-state that extends over most of the visible spectrum and displays quantum yields between 3 % and 95 %.