Tetrakis(oxadiazolyl)benzenes and -pyrazines : novel fluorescent cruciform liquid crystals

Abstract

This study investigates the fluorescent and mesomorphic properties of tetrakis(oxadiazolyl)benzenes (TOBEs) and -pyrazines (TOPYs), novel cruciform liquid crystals. The impact of a set of four side chains on optical and mesomorphic properties is reported. TOBE absorbs in the UV range (~360 nm), while substituting the central benzene ring by pyrazine (TOPY) shifts the absorption maximum to 396 nm but lowers the fluorescence quantum yield (TOBE: 72%; TOPY: 21%; in toluene). Fluorescence in the green-to-orange range is influenced by solvent polarity. Mesophase analysis shows quite narrow phases (~20 K) for TOBE. As the pyrazine core lowers the melting point and elevates the clearing temperature, huge mesophase ranges (~80–90 K) are detected for TOPY. Side chain variations further influence material properties: TOBE with linear alkyl chains exhibits multiple crystal–crystal transitions, while branching enhances mesophase stability and alters fluorescence characteristics. The Huisgen reaction provides a cost-effective synthetic route for these fluorescent mesogens, offering high yields and efficiency.