High birefringence laterally fluorinated terphenyl isothiocyanates: Structural, optical and dynamical properties

High birefringence liquid crystals are important for fast switching display and various other applications. One such singly fluorinated isothiocyanato terphenyl compound 2TP-3'F-4NCS which exhibits very broad range nematic (68 °C) and a narrow range smectic phase have been studied by X-ray, dielectric and optical methods. Optical birefringence and order parameters of its doubly fluorinated homologue 2TP-3',3F-4NCS have also been determined. Contrary to common perception weak antiparallel correlation of molecules is observed in both the compounds. Observed dielectric anisotropy (around 11), though less than cyano compounds, is strong enough to ensure a driving voltage suitable for thin (<2 μm) TFT based LC display cell. Measured splay elastic constant suggests that singly fluorinated compound will exhibit faster response. Flip-flop mode relaxation is observed lowest at 510 kHz and 2.81 MHz respectively in the doubly and singly fluorinated compound. Thus the singly fluorinated compound excludes the possibility of undesirable energy absorption below MHz range applications. Above all both the compounds exhibit high birefringence, highest values being 0.373 and 0.357 respectively in the singly and doubly fluorinated compounds. Although ∆n decreases with temperature it is found to be greater than 0.3 till 164 °C in 2TP-3'F-4NCS and 136 °C in 2TP-3',3F-4NCS. In addition, both the compounds exhibit high value of order parameters in low temperature region. These two compounds are, therefore, expected to be very suitable for formulating high birefringence nematic mixtures for fast switching displays.