Influence of Moieties and Chain Length on the Abundance of Orthogonal and Tilted Phases of Linear Hydrogen-Bonded Liquid Crystals, Py16BA:nOBAs

Abstract

A novel series of linear hydrogen-bonded (HB) liquid crystalline (LC) complexes (HBLCs), namely Py16BA:noBAs, is reported. The series was prepared by the complexation of a novel non-mesogenic proton acceptor moiety, namely (4-pyridyl)-benzylidene-4′-n-hexadecyl aniline (Py16BA), with a mesogenic proton donor, p-n-alkoxy benzoic acid (noBAs for n = 3, 4, 5, 6, 7, 8, 9, and 12) moieties. Formation of HB complexes is confirmed through FTIR spectra. The LC phases exhibited by Py16BA:noBAs, the transition temperatures (Tc), and the heat of transition (enthalpy) (ΔH) are determined by polarizing optical microscope (POM) and differential scanning calorimetry (DSC) techniques. A comparative study of phase abundance is presented (with respect to the HB complexes of pure noBAs). An orthogonal smectic-A phase is induced by quenching the nematic phase in the lower homologs, while a quasi-2D tilted smectic-F phase is induced in the intermediate (or higher) homologs Py16BA:noBAs. The odd–even effect is observed across the isotropic–LC phase and LC–solid phase transitions. Overall mesogenic stability is enhanced. Optimization of the tilted smectic phase stability is discussed in the wake of the configuration of hydrogen bonding and the steric intrusion due to the increase in length of the flexible end-chain. Based on the extrapolation of the odd–even effect, an ACBF multi-critical point is predicted in the phase diagram in the vicinity of intermediate homologs, possibly in a binary of Py16BA:5oBA and Py16BA:6oBA.