Luminescent liquid crystals based on 2,1,3-benzoxadiazole: conducive heterocycle or poor cousin of benzothiadiazole?

Abstract

ABSTRACT Benzothiadiazole derivatives are widely investigated due to their well-known photophysical properties, including attractive liquid crystalline behavior. However, although their molecular cousin benzoxadiazole has similar characteristics, being a strong acceptor and good emitter, it has not been properly explored in materials science applications. In fact, no reports on its mesomorphic behavior could be found in the literature. Therefore, in this study a novel series of fluorescent calamitic liquid crystals derived from the 4,7-diphenyl-2,1,3-benzoxadiazole core, with varying numbers of alkoxy chains, were designed and successfully synthesized. All of the final compounds presented calamitic liquid crystal phases (nematic and smectic). Electrochemical studies showed oxidation and reduction patterns similar to those of benzochalcogendiazole analogs, however with higher band gaps. The compounds displayed yellow photoluminescence under UV light excitation in solution and in the solid state. In solution, the fluorescence quantum yields of the benzoxadiazole-based compounds were significantly higher than those previously described for similar benzothiadiazoles (ΦF = 0.38–0.45). Graphical Abstract