Binary Mixtures of 4-acetylpyridine and 4-n-alkyloxybenzoic Acids (4Ac.Py:nOBA): Investigations on Mesomorphism

Abstract The induction and stability of mesomorphism in new binary mixtures capable of forming intermolecular hydrogen-bonding (HB) interactions were investigated. The mixtures were prepared by treating 4-acetylpyridine, a nonmesogenic liquid, with mesogenic 4- n -alkyloxybenzoic acids ( n OBA, pentyl to undecyl homologues) in a 1:1 molar ratio. The vibrational absorption spectral peak shifts resulting from the interaction between the pyridyl nitrogen of 4-acetylpyridine and the carboxylic acid group of n OBA indicate that HB interactions exist between them in a mixture. Frontier molecular orbitals were assessed by geometrical optimization, and Density Functional Theory (DFT) calculations. Mesomorphic properties were studied using polarizing optical microscopy (POM) and differential scanning calorimetry (DSC). The results indicate that the mixtures exhibit enantiotropic nematic (N) and smectic C (SmC) phases, mirroring the phase types of n OBAs but significantly altering the thermal spans. Smectic G (SmG) phase was induced in some binary mixtures and was monotropic. Both the N and SmC mesophases were stabilized over a wide temperature range. Notably, the melting and clearing temperatures of the mixtures were lower than those of the n OBAs; however, they displayed a substantial enhancement in the mesomorphic thermal ranges. This behavior is fundamentally attributed to the robust nature of the intermolecular HB interactions. These findings reveal that pyridyl moieties can effectively tune the thermal spans and phase variants of the mixtures.