A new Schiff base compound, 3-(5-bromo-2-hydroxybenzylideneamino)phenol (abbreviated as BHAP) was synthesized and characterized by ¹H- and ¹³C- nuclear magnetic resonance and infrared spectroscopies. DFT/B3LYP/6-311++G(d,p) calculations were undertaken in order to explore the conformational space of both the E- and Z- geometrical isomers of the enol-imine and keto-amine tautomers of the compound. Optimized geometries and relative energies were obtained, and it was shown that the most stable species is the E-enol-imine form, which may exist in four low-energy intramolecularly hydrogen-bonded forms (I, II, V, and VI) that are almost isoenergetic. These conformers were concluded to exist in the gas phase equilibrium with nearly equal populations. On the other hand, the infrared spectra of the compound isolated in a cryogenic argon matrix (10 K) are compatible with the presence in the matrix of only two of these conformers (conformers II and V), while conformers I and VI convert to these ones by quantum mechanical tunneling through the barrier associated with the rotation of the OH phenolic group around the C–O bond. The matrix isolation infrared spectrum was then assigned and interpreted with help of the DFT(B3LYP)/6-311++G(d,p) calculated infrared spectra for conformers II and V. In addition, natural bond orbital (NBO) analysis was performed on the most stable conformer of the experimentally relevant isomeric form (E-enol-imino conformer V) to shed light on details of its electronic structure. This investigation stresses the fundamental structural relevance of the O–H···N intramolecular H-bond in o-hydroxyaryl Schiff base compounds.

The electronic transitions of 4-(decyloxy) benzoic acid (4DOBA), 4-undecyloxybenzoic acid (4UDOBA) and 4-(dodecyloxy) benzoic acid (4DDOBA) liquid crystals (LCs) have been investigated by using optical absorption and steady-state fluorescence spectra of in a series of different polarity organic solvents. Two electronic transition bands have been observed in steady-state fluorescence spectra. The solvatochromic properties, electronic transitions and electronic structure of LC compounds have been interpreted to relation with molecular structure. Solvatochromism determining as quantitative to solvent-solute interactions (linear solvent energy relationships) have done by using Kamlet-Taft and Catalan solvatochromism. The electrical dipole moments and electronic absorbance of the LCs molecules states were calculated by using B3LYP/6-311++G(d,p) method and basis set. The ground state experimental electrical dipole moments were determined by Bilot-Kawski method, when the excited state electric dipole moments were determined by Lippert-Mataga, Bakshiev, Kawski-Chamma-Viallet and Reichardt method, respectively. HOMO (Highest Occupied Molecular Orbital), LUMO (Lowest Unoccupied Molecular Orbital), MEP (Molecular Electrostatic Potential) and SAS (Solvent Accessibility Surface) shapes of molecules in both monomer and dimer structure have been drawn and they are proved to be dimers. GRAPHICAL ABSTRACT