A Contribution to the Study of the Nematic—Isotropic Transition. Influence of Polar Terminal Groups

Abstract

Correlation studies of the T _N-1 of fifteen 4′-substituted 2-hydroxy-4-n-octyloxy-α,α'-dimethylbenzalazines and empirical parameters for the polar end groups [MR (molar refractivity) related to the electronic polarizability and μ (dipole moment)] are presented.

The suitability of the combination of both parameters is checked in order to predict the clearing temperatures in this series.

The validity of these correlations is tested against another series: 4-X-phenyl 4-(trans-4-decylcyclohexyl)benzoates (X:—I,—Cl,—Br,—F,—CHO,—COCH₃,—NCS,—CN,—NO₂) taken from the literature.

In order to carry out these studies, twelve new 2-hydroxy-4-n-octyloxy-4′-X-α,α'-dimethylbenzalazines with substituents X:—H,—OH,—NH₂,—SH,—COCH₃,—COOCH₃,—COOH,—OCOCH₃,—OCOC₆H₅,—NCS,—CF₃,—NH₃ ⁻Cl⁻ and their analogues: α,α'-dimethyl-2-hydroxy-4-n-octyloxy-benzylidene-4′-pyridylmethy-lenchydrazinc and its N-oxide, were synthesized and their mesomorphic properties studied.

The azines with groups X:—NH₂,—SH,—OCOCH₃,—OCOC₆H₅,—COCH₃ and—COOCH₃ only show a nematic phase, that with X:—NCS shows nematic and S_A phases, and those with X:—CF₃ and—COOH smectic phases alone.