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 spe ctra of seven benzo[a]phenothiazines, including 12H-benzo[a]phenothiaz ine, 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]phenothiaz ine, were determined at room temperature (298 K) in solvents of variou s polarity (cyclohexane, ethyl ether, ethyl acetate, tetrahydrofuran, ethanol, dimethylformamide, acetonitrile and dimethylsulphoxide). The effect of the solvent on the spectral characteristics was studied quan titatively. In combination with the ground state dipole moments of the se compounds, the spectral data of the benzo[a]phenothiazines were use d to evaluate their first excited singlet state dipole moments using t he solvatochromic shift method (Bakhshiev and Kawski-Chamma-Viallet co rrelations). The theoretical ground and excited singlet state dipole m oments of the benzo[a]phenothiazines were also calculated as the vecto r sum of the pi component (obtained by the Pariser-Parr-Pople method) and the sigma component (obtained from sigma bond moments). In most ca ses, a satisfactory agreement was found between the experimental and c alculated values of the dipole moments. For most benzo[a]phenothiazine derivatives under study, the experimental excited singlet state dipol e moments were higher than their ground state counterparts. The applic ation of the Kamlet-Abboud-Taft solvatochromic parameters to explain t he effect of the solvent on the spectral properties of benzo[a]phenoth iazines is discussed.