Electronic absorption and fluorescence spectra of 2-phenyl-substituted benzothiazoles: study of excited-state proton transfer reactions

Abstract

The absorption and fluorescence spectral properties of 2-(3′-hydroxyphenyl)-, 2-(4′-hydroxyphenyl)-, and 2-(3′,4′-dihydroxyphenyl)-, and 2-(4′-hydroxy-3′-methoxyphenyl), 2-(3′-hydroxy-4′-methoxyphenyl)-, and 2-(3′-methoxyphenyl)benzothiazoles have been studied in a number of solvents of varying polarity. The ionization constants (pKa) for various prototropic reactions of these molecules in both S0 and S1 states are determined. The effect of substitution on the spectral properties and on the pKa values are discussed. The molecules have been found to undergo biprotonic phototautomerism in dilute acid solutions. On the basis of the fluorimetric titration behaviour of the molecules (except 2-(3′-methoxyphenyl)benzothiazole), the existence of monocation–zwitterion equilibrium in the S1 state is proposed. PPP-SCF-MO-CI method has been used to calculate charge densities on the heteroatoms. Key words: spectra, proton transfer.