A solvatochromic study of new benzo[a]phenothiazines for the determination of dipole moments and specific solute-solvent interactions in the first excited singlet state

The electronic absorption and fluorescence excitation and emission spectra of seven benzo[a]phenothiazines, including 12H-benzo[a]phenothiazine, 9-methyl-benzo[a]phenothiazine, 10-methyl-benzo[a]phenothiazine, 11-methyl-benzo[a]phenothiazine, 5-oxo-5H-benzo[a]phenothiazine, 5-oxo-6-methyl-benzo[a]phenothiazine and 5-oxo-6-hydroxy-benzo[a]phenothiazine, were determined at room temperature (298 K) in solvents of various polarity (cyclohexane, ethyl ether, ethyl acetate, tetrahydrofuran, ethanol, dimethylformamide, acetonitrile and dimethylsulphoxide). The effect of the solvent on the spectral characteristics was studied quantitatively. In combination with the ground state dipole moments of these compounds, the spectral data of the benzo[a]phenothiazines were used to evaluate their first excited singlet state dipole moments using the solvatochromic shift method (Bakhshiev and Kawski-Chamma-Viallet correlations). The theoretical ground and excited singlet state dipole moments of the benzo[a]phenothiazines were also calculated as the vector sum of the pi component (obtained by the Pariser-Parr-Pople method) and the sigma component (obtained from sigma bond moments). In most cases, a satisfactory agreement was found between the experimental and calculated values of the dipole moments. For most benzo[a]phenothiazine derivatives under study, the experimental excited singlet state dipole moments were higher than their ground state counterparts. The application of the Kamlet-Abboud-Taft solvatochromic parameters to explain the effect of the solvent on the spectral properties of benzo[a]phenothiazines is discussed.