Photophysical behavior of some thymol based schiff bases using absorption and fluorescence spectroscopy

The absorption and fluorescence spectra of some biologically active thymol based Schiff bases have been studied at room temperature in a series of solvents of varying polarity. The solvent effect on the spectral properties of Schiff bases has been analyzed using Reichardt and Kawski equations and Kamlet-Taft and Catálan multi-parametric solvent polarity scales. The substituents and the solvents strongly influence the excited state behavior of these molecules owing to different electronic arrangements. The excited state dipole moments ( μe/μg and Δμ) of these molecules have been calculated using solvatochromic shift methods based on bulk solvent polarity functions f (ε,n) and φ (ε,n) and microscopic solvent polarity parameter ETN, respectively. The increase in excited state dipole moments (μe) indicates a more polar excited state and intramolecular charge transfer (ICT) characteristics of these molecules. The molecular orbitals (HOMO-1, HOMO, LUMO and LUMO+ 1) and molecular electrostatic potential (MEP) surfaces have been generated from their optimized geometries using semi-empirical (PM3) calculations to account for electronic excitations and changes in the overall charge distribution of Schiff bases. The Taft and Catalán multi-parametric equations used in the interpretation of the specific solute-solvent interactions reveal that the most important contribution to the solute-solvent interaction in the excited state comes from the hydrogen-bond acceptor capacity of the solvent.