The Role of Functional Groups in Substituted Benzoic Acids Used as Dopants in Liquid Crystal Mixtures on the...

Thermotropic nematic liquid crystals (LC) are orientationally ordered liquids composed of mesogenic molecules that become isotropic liquids at the nematic-isotropic transition temperature (T_NI), at which the order disappears. Dissolving a nonmesogenic substance in an LC usually produces a disruption of the molecular order that leads to a decreasing of the T_NI. However, some nonmesogenic substances enhance the molecular order, yielding increments of the T_NI. Rigid aromatic carboxylic acids like benzoic acid possess this order-enhancing capacity due to the formation of elongated dimers, which align with the uniaxial nematic environment. In the present research, the thermodynamics of the dimerization of 20 substituted benzoic acids dissolved in the nematic LC 4-n-pentyl-4'-cyanobiphenyl (5CB) and the phase behavior of the mixtures were assessed. The substituents include amino, fluoro, chloro, bromo, iodo, and nitro groups linked to the aromatic ring of the benzoic acid at ortho, meta, and para positions. Both the geometry and the electron-withdrawing or donating effect of the substituent have an influence on the ordering capability of the substituted benzoic acid. In general, electron-donating groups at the para position in benzoic acids have a large order-enhancing power. Electron-withdrawing groups promote the ionization of the substituted benzoic acids, limiting the dimerization capability of these dopants. Regarding the substitution position of the groups, the para substitutions lead to the formation of long and slim calamitic dimers with a large aspect ratio length/diameter. These calamitic dimers align with the nematic solvent and act as scaffolds in the uniaxial solvent. The nematic LC mixtures were analyzed by polarized optical microscopy, differential scanning calorimetry, and infrared spectroscopy. Theoretical calculations aided to ascertain the most probable dimers in amphoteric dopants like aminobenzoic acids. A thermodynamic model was developed to analyze the nematic-isotropic transitions. Two dopants, 4-aminobenzoic acid and 4-N-methylaminobenzoic acid, exhibit extraordinary order-enhancing power, yielding increments of the nematic-isotropic transition temperature up to 12 K with minor concentration of the dopants.