Relaxation frequencies for flip-flop rotation of three- and four-ring compounds and dual frequency addressing mixtures with high birefringence

Abstract

Relaxation frequencies fr characterising the molecular rotation around the short axes (flip-flop movement) of 45 three- and four-ring polar compounds with high birefringence (mainly cyclohexyl, phenyl and biphenyl tolanes and quaterphenyls terminated with F, OCF3, NCS group) have been measured. These compounds exhibit a large positive dielectric anisotropy Δε = ε║ − ε⊥ in the nematic phase and relatively low fr values (in the range of dozens kHz). The dependence of fr upon molecular structures and physical properties of systems (molecular rigid core length, polarity and viscosity) is discussed. Mixtures for dual frequencies addressing display with the crossover frequency fco of the order of ~1 kHz have been prepared by joining the above compounds with a mixture consisted of two, three and four fluoro-substituted in the lateral position tolanes and phenyltolanes showing negative dielectric anisotropy. It was stated that relaxation frequency is correlated mainly with their length and polarity.