Dependence of molecular polarizability on the orientational order in nematic liquid crystals

Abstract

The dispersion dependences of refractive indices in the visible range were used to obtain experimental values of the Lorentz tensor components L j and the mean molecular polarizability \( \bar \gamma \) for five nematic liquid crystals belonging to two homological series. The dependence of L j components on the homologue number, mesophase temperature, birefringence value, and the orientational order of molecules in the nematic phase and upon a nematic-smectic A phase transition was revealed. The effect of the isotropization of the Lorentz tensors and the local field tensor with decreasing birefringence and molecular polarizability anisotropy Δγ was confirmed. The quadratic dependence \( \bar \gamma \)(S) on the molecular orientational order parameter S in the nematic phase was found. It was invariant with respect to the nematic-smectic A transition. The dependences \( \bar \gamma \)(S) and Δγ(S) are explained within molecular statistical theory as consequences of the correlation between orientational and conformational degrees of freedom of molecules. These conformational degrees of freedom are related to the internal rotation of molecular fragments, which affects the electronic conjugation of the fragments and the oscillator strengths of molecular transitions.