Solvatochromic fluorescence of 4-alkoxybenzoic acid liquid crystals: Ground and excited state dipole moments of monomer and dimer structures determined by solvatochromic shift methods

The absorption and fluorescence spectra of 4-ethoxybenzoic acid (4EOBA), 4-propoxybenzoic acid (4POBA) and 4-butoxybenzoic acid (4BOBA) liquid crystals have been recorded and evaluated comprehensively in a series of organic solvents. The fluorescence is observed as highly sensitive to solvent polarity, giving a synthetic rainbow of emission in different organic solvents. The ground state (μg) and excited state (μe) dipole moments of the monomer and dimer structures of the 4-alkoxybenzoic acid liquid crystals are determined by means of Bilot-Kawski, Lippert-Mataga, Bakhshiev, Kaski-Chamma-Viallet and Reichardt solvatochromic shift methods based on dielectric constant and refractive index function. Excited state dipole moments are found as larger than the ground state dipole moments both in monomer and dimer forms. Ground state dipole moments of dimer structures are observed to be higher than that of monomer structures. Consistency between experimental and theoretically computed ground state dipole moments indicates that molecules exist in dimer form. Solute-solvent interactions are analyzed by means of linear solvation energy relationships (LSERs) using Kamlet-Taft and Catalan parameters. Kamlet-Taft solvatochromic model indicates that dispersion-induction interaction has the stronger effect compared to dipolar-orientation interaction in controlling solute-solvent interactions. It is also found that solvatochromic behavior is dominantly controlled by hydrogen bond donor ability. Catalan model displays that the solvent of basicity is relatively affective to refer to the solvatochromic behavior. Both Kamlet-Taft and Catalan models designate positive solvatochromism for absorption and fluorescence spectra. HOMO, LUMO, molecular electrostatic potential (MEP) and solvent accessible surface (SAS) of monomer and dimer 4-alkoxybenzoic acids are calculated by using DFT-B3LYP method.