Mesophase study of pure and doped cyanobiphenyl liquid crystals with salen-type systems

ABSTRACT

Two salen Schiff base ligands derived from the condensation of ethylendiamine on salicylaldehyde and 5-chlorosalicylaldehyde, namely N,N′-bis(salicylidene)ethylene-diamine (L1) and N,N′-bis(5-chlorosalicylidene)ethylene-diamine (L2) as well as two of their iron(III) and nickel(II) complexes, were prepared and then used as doping agents of two thermotropic liquid crystals (LCs) of cyanobiphenyl type, namely 4-cyano-4′-n-pentyl-biphenyl (5CB) and 4-cyano-4′-n-octyl-biphenyl (8CB). The study of the mesophase of pure and doped LCs was carried out by UV–visible spectroscopy equipped with a heating compartment for precise temperature control, differential scanning calorimetry and polarised optical microscopy. The characteristic nematic/isotropic and smectic-A/nematic transition temperatures of 5CB- and 8CB-based systems were measured and then compared to those of the literature concerning pure 5CB and 8CB. Optical microscopy results revealed the existence of Schlieren and focal conic textures of the nematic and smectic states, respectively, both of pure and doped LCs. The homogeneity of the obtained guest–host systems was proven by the linear evolution of their transition temperatures as function of the solute concentration, with correlation factors close to unity.

GRAPHICAL ABSTRACT

KEYWORDS:

• Schiff base
• guest–host
• liquid crystal
• phase transition
• nematic state
• smectic-A state
• texture of mesogenic systems

Acknowledgements

This work has been accomplished in the framework of an international research programme. The authors gratefully acknowledge the support of the Algerian Ministry of Higher Education and Scientific Research (MESRS), the University of Sciences and Technology of Oran (USTO-MB)/Algeria, the French Ministry of Higher Education and Research (MENESR), the CNRS and the University Lille 1 – Sciences and Technologies/France. The authors are thankful to Dr. Christophe Beyens for experimental support.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental materials

Supplemental data for this article can be accessed here.