Structural and thermodynamic consequencies of the interaction of conformational degrees of freedom of azomethines in the nematic phase

The interaction of the conformational degrees of freedom of azomethines in the nematic phase, which is induced by electronic donor-acceptor properties of the terminal substituents of the benzylideneaniline core is studied. These degrees of freedom related to the rotation angles φ k around the bonds between the substituent and the aniline ring (φ 1 ) and also between the CH=N bridge and the aniline ring (ω 2 ) are characterized by the parameters Q k = 〈cos 2 φ k 〉. It is found that the interaction of these degrees of freedom is manifested in the linear dependence Q 2 ( Q 1 ). Within the phenomenological theory the effect of this interaction on changes δ k in the Q k values during the nematic liquid crystal-isotropic liquid phase transition is revealed along with the temperature T NI and character of this transition. The derivation of previously established empirical dependences T NI ( Q k ) is presented in the presence of direct and indirect steric effects of side substituents affecting the Q k values. A diverse combination of δ k signs in the nematic phase, which is a prerequisite for the conformational polymorphism of the nematic phases of azomethines, is shown.