Synthesis and liquid-crystalline properties of methacrylate monomers carrying a p-terphenyl laterally substituted with one or two cyano groups

Abstract

Two methacrylate monomers, carrying a p-terphenyl laterally substituted with one or two cyano groups, were synthesized and studied from the liquid crystal and optical points of view. The synthetic route involved several reactions, including the Suzuki reaction that allows the coupling of aryl groups. The chemical structure of these new monomers, as well as that of their corresponding precursors, was confirmed by 1H and 13C NMR spectroscopies. The mesomorphic behavior of both monomers and precursors was studied by differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and X-ray diffraction (XRD) analyses. Molecules bearing one cyano group melt at lower temperature and display a mesomorphic behavior over larger temperature intervals as compared to those bearing two cyano groups. The four compounds (two monomers and two precursors) showed only non-ordered smectic phases. By X-ray diffraction it was determined that molecules stack in single layers within the smectic phase, except for one of the monomers that stacks in double layers. The electrochemical characterization revels that these molecules behaves as p type materials, and that their electronic nature (band gap) can be modulated by introducing CN groups. The molecules absorb and emit in the blue spectral range, a region of high energy light. The large Stokes' shifts indicate that the geometry of the molecules changes from aromatic in the electronic ground state to a more coplanar quinoid structure in the first excited state. The fluorescence quantum yield is high (70–80%) suggesting that these new monomers can be used as precursors for the synthesis of fluorescent liquid crystal polymers, which is a work in progress.