A QUANTITATIVE STUDY OF THE EFFECT OF SOLVENT ON THE ELECTRONIC ABSORPTION AND FLUORESCENCE-SPECTRA OF SUBSTITUTED PHENOTHIAZINES - EVALUATION OF THEIR GROUND AND EXCITED SINGLET-STATE DIPOLE-MOMENTS

Electronic absorption and fluorescence excitation and emission spectra of five phenothiazines (phenothiazine, promethazine, thionine, methylene blue and Azure A) were determined at room temperature (293 K) in several solvents of various polarities (cyclohexane, dioxane, ethyl ether, chloroform, ethyl acetate, 1-butanol, 2-propanol, ethanol, methanol, acetonitrile, dimethylformamide and dimethyl sulfoxide). The effect of the solvents upon the spectral characteristics was studied. In combination with the ground state dipole moments of these phenothiazines, the spectral data were used to evaluate their first excited singlet-state dipole moments by means of the solvatochromic shift method (Bakhshiev's and Kawski-Chamma-Viallet correlations). The theoretical ground and excited singlet-state dipole moments for phenothiazines were calculated as a vector sum of the pi component (obtained by the Pariser-Parr-Pople method) and the sigma component (obtained from sigma-bond moments). A reasonable agreement was found with the experimental values. For most phenothiazines under study, excited singlet-state dipole moments were found to be significantly higher than their ground-state counterparts. The application of the Kamlet-Abboud-Taft solvatochromic parameters to the solvent effect on spectral properties of phenothiazines is discussed.