The Cyano group as a polarity switch: Enhancing dielectric anisotropy in hydrogen-bonded nOBAF:CNPy complexes

Abstract

• New H-bonded LCs from fluoro benzoic acids and cyanopyridine were synthesized. • FT-IR confirmed supramolecular complex formation via O-H⋯N bonding. • All complexes exhibit enantiotropic nematic and smectic G phases. • Alkyl chain length modulates the thermal stability of nematic vs. smectic phases. • Supramolecular design yields a 4-fold enhancement in dielectric anisotropy. This study reports the preparation and characterization of a novel series of hydrogen-bonded liquid crystals formed between 4-cyanopyridine (acceptor) and 4-alkyloxy-2-fluorobenzoic acids (donor, alkyl chain length n = 9–14). The formation of the supramolecular O–H⋯N complex was unequivocally verified by FTIR spectroscopy. Thermal and mesomorphic investigations, conducted via DSC and POM, revealed that all complexes exhibit enantiotropic nematic and smectic G phases. The phase behavior was strongly dependent on the alkyl chain length; longer chains favored the SmG phase, while shorter chains stabilized the nematic phase over a wider range, as detailed in a constructed phase diagram. Crucially, dielectric properties measured in aligned cells showed a remarkable enhancement in dielectric anisotropy, with values exceeding Δε = 4.0 compared to Δε ≈ 0.6 for the pure acid. This dramatic increase is a direct result of the supramolecular structure, which aligns the strong molecular dipole of the cyano group along the director axis. These findings underscore the potential of hydrogen bonding for creating functional materials with tailored properties for electro-optic applications.