Absorption and fluorescence characteristics of some 2-alkyl- and 2-aryl-benzoxazoles in different solvents and at various acid concentrations

Abstract

Absorption and fluorescence spectral characteristics of some 2-alkyl- and 2-aryl-benzoxazoles have been studied in different solvents and at various acid concentrations. Dual fluorescence observed in the monocations of 2-alkylbenzoxazoles is due to π*→π and charge transfer (CT) transitions, indicating the near degeneracy of excited singlet states. 2-Arylbenzoxazoles and the corresponding monocations exhibit only a single fluorescence band due to π*→π transition. <em>pK</em>_a values for the monocation-neutral equilibria in both <em>S</em>₀ and <em>S</em>₁ states of the molecules have been determined. Pariser-Parr-Pople method for all the molecules and CNDO/S method for 2-methyl-, and 2-phenylbenzoxazoles have been employed to calculate transition energies, transition polarization and charge densities at different atoms in the <em>S</em>₀ and <em>S</em>₁ states to substantiate experimental data and, hence, to verify the applicability of the methods for heterocycles containing two hetero atoms.