Spectroscopic studies and theoretical analysis of some selected heterocycles

Abstract

Seven derivatives of heterocycles benzimidazole, benzoxazole and benzothiazole were studied, five of which were synthesized and characterized. The molecular geometry and newlinespectroscopic data of the compounds in the ground state were calculated using the density functional theory (DFT/B3LYP) method with the 6-311++G(d,p) basis set. A comparison between the experimental and calculated data was attempted. Molecular electrostatic potential (MEP) and global reactivity parameters were deduced using theoretical calculations. HOMO-LUMO energy gap for each compound was determined by DFT and cyclic voltammetry. The cyclic voltammograms were recorded in acetonitrile solvent using lithium perchlorate as the supporting electrolyte. For all the compounds experimentally determined HOMO LUMO energy gap in polar solvent was lesser than that from DFT calculated energy gap. Using the HOMO-LUMO energy gap, global reactivity parameters were calculated. The effect of solvents of varying polarity on the absorption and emission spectra of the compounds was studied. Large excitation and emission energy differences were observed for all the selected heterocycles. The excitation and fluorescence spectra of selected heterocycles were recorded in eight solvents of different solvent polarity. It is evident from the excitation spectra that on increase of the solvent polarity, a bathochromic shift takes place for and#960;-and#960;* transition, and this is attributed to the high influence of solvent polarity in the excited state of heterocycle newlinecompared to its ground state. The dipole moments in the ground and the first excited state of heterocycle derivatives were newlinecalculated using Lippert-Mataga and Kawski-Chamma-Viallet methods. Guggenheim-Debye method was adopted to calculate ground state dipole moment. The dipole moments of the compounds were also calculated using Time Dependent-Density Functional Theory (TD-DFT). The dipole moment values of the compounds suggested that the excited state has more charge separation and thus becomes more polar.