Push-pull biphenyl and tolane derivatives as novel luminescent liquid crystals: synthesis and properties

Addressing the current challenge of high melting points and long π-conjugated skeletons in luminescent nematic liquid crystals (LCs), this study explores the incorporation of push-pull dyes with biphenyl and tolane skeletons to extend fluorescence emission wavelengths into the visible light region. We synthesised push-pull-type biphenyl and tolane derivatives incorporating alkylamino groups as donors and cyano groups as acceptors. The thermal behaviour was investigated using optical polarisation microscopy, differential scanning calorimetry and X-ray diffraction. Notably, some molecules exhibited a nematic phase. The photophysical properties of the solution and solid states were also investigated. Furthermore, we formulated a room-temperature luminescent LC by blending several LCs. Importantly, the fluorescence in the nematic phase exhibited a red-shift compared to that observed in the solution and solid states. This study demonstrates the efficacy of the push-pull structure in the design of luminescent LCs. The findings of this study hold considerable implications for the development of advanced optoelectronic devices, particularly those requiring luminescent materials with tailored properties.