In this paper we report how dielectric spectroscopy can help in creating of dual-frequency nematic liquid crystals. Dual-frequency nematic liquid crystals is new class of liquid crystal materials. Such mixture is usually formed by a combination of many components (even more than 10), which can be split into two groups: molecules having large transverse dipole moment and molecules with a large longitudinal dipole moment. The behavior of a base (parent) mixture, functional admixtures and nal dual-frequency nematic liquid crystals mixture is investigated by dielectric spectroscopy in wide frequency (100 Hz10 MHz) and temperature ranges. This allows us to nd out why the dual-frequency liquid crystal has an important feature: positive and negative dielectric anisotropy at dierent frequencies. We present parameters of molecular motions around short (S-mode) and long (L-mode) molecular axes observed in investigated materials and discuss how the creation of nal dual-frequency nematic liquid crystals mixture can modify molecular relaxations.

A new tolane-base liquid crystal, 4′-heptyl-3-fluoro-4-isothiocyanatotolane (7TOLF), and a biphenyl-base nematogen, 4′-heptyl-3-fluoro-4-isothiocyanatobiphenyl (7BF), have been investigated for the first time to determine their electro-optical behaviour and third order non-linearity using the static Kerr effect method. Both the nematic compounds have a fluorine atom attached to the phenyl-NCS moiety. They possess the same polar head group (-NCS) and alkyl tail (-C7H15). So the effect of the tolane group of nematogen on the electro-optical behaviour was investigated and compared. The temperature dependence of the electric Kerr constant in the isotropic phase and the pre-transitional behaviour has been investigated for these high birefringence compounds in the isotropic phase. Both the compounds have a positive and large Kerr constant which increases with decrease in temperature. The Landau–de Gennes model was obeyed for these compounds. For 7TOLF, the observed value of susceptibility, χ3 is about 228 times higher than that of CS2. The experimental Kerr effect data were also compared with those of the well-studied nematic liquid crystals.

Using the experimental data on the dielectric anisotropy, density, polarizability, dipole moment, and order parameter in the nematic phase of seven typical liquid crystalline substances, the applicability of the Maier-Meier theory for the description of the dielectric properties of nematics is checked. Substances with different polarity and different tendency to form associates in the nematic phase were studied. It is found that the Maier-Meier equations describe fairly well the dielectric permittivity components of nematics. The estimated values of the angle ß formed by the dipole moment with the long molecular axis are compared with those obtained in other studies.

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