Appearance of induced chiral nematic phase in solutions of 4-n-alkyloxy-4′-cyanobyphenyles with symmetric camphorsubstituted hemiporphyrazines

The synthesis of chiral symmetric camphorsubstituted hemiporphyrazine ((+)HPA) and its racemic mixture)HPA) was carried out by crossover condensation of corresponding camphoradicyanopyrazines with 1,3-phenylenediamine under microwave radiation. By the method of polarizing microscopy, a chiral nematic phase formation of the 4-(n)alkoxy-4'-cyanobiphenyls (n = 3,4,5,6,7,8) nematic mixtures (CB6) was observed under the influence of the symmetric camphorsubstituted hemiporphyrazine, the clearing temperatures and helix pitch were measured, and the twisting power value 13 was calculated. The degree of destabilizing effect of the racemic mixture (+/-)HPA on the mesophase was shown to be higher than that of the chiral dopant. Under study the electric permittivity of the CB6 + (+/-)HPA and CB6 + (+)HPA binary systems the dielectric anisotropy of the achiral composition was found to weakly depend on the dopant concentration, whereas in the case of the spiral mesophase, there is a significant decrease in anisotropy with an increase in the (+)HPA content. At high concentrations of the chiral dopant, jumps in the temperature dependences of the electric permittivity components in the mesophase were detected. Such behavior has been interpreted in the framework of the notions of competition between the dopant and the magnetic field, which attempts to untwist the spiral mesophase. A non-monotonic change in the kinematic viscosity was found on the temperature dependences in the chiral nematic region. In this case, the twisting of the mesophase with a chiral dopant is counteracted by the orientation of the molecules by the flow of a liquid crystal. The equally weak effect of both the chiral dopant (+)HPA and its racemic mixture on the birefringence of LCs is shown, which is caused by the coincidence of the local orientational order in the nematic phase and in the quasinematic layers of the chiral nematic. (C) 2019 Elsevier B.V. All rights reserved.