Synthesis and performance evaluation of fluorinated biphenyl diluters for high birefringence liquid crystals

Liquid crystal (LC) mixtures with high birefringence (∆n) and low rotational viscosity play an essential role for fast-response LC devices. However, high birefringence and low rotational viscosity are a pair of contradictory performance parameters. In this work, two high-∆n LC diluters are proposed to achieve a fine balance between them. These two terminal olefin diluters containing a rigid biphenyl core with a lateral fluorine substituent, a buty-3-enyl terminal group and a flexible n-alkyl chain were designed and synthesised through classical organic synthetic reactions. Meanwhile, their comprehensive properties were further evaluated in a LC mixture P02-F containing fluoro-tolane and a LC mixture P12-ISO containing isothiocyano-tolane. The results show that the biphenyl diluters 2BBF2V and 3BFB2V could effectively decrease the Tm of LC mixture P12-ISO to below −25°C, while maintain high ∆n and large Δε, which are suitable for the high-∆n LC mixtures containing isothiocyano-tolane. Furthermore, DFT calculations of molecular conformation and polarisability components were utilised to correlate the experimental findings.