Effect of hydroxyl group on the photophysical properties of benzo[a]xanthenes – Solvatochromic studies and estimation of dipole moment

Absorption and emission spectra of three substituted benzo[a]xanthenes were studied at 298 K in solvents of different polarity for the first time. The fluorescence of 2-hydroxy-9,9-dimethyl-12-(pyridin-2-yl)-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one was weak compared to 3-hydroxy-9,9-dimethyl-12-(pyridin-2-yl)-9,10-dihydro-8H-benzo[a]xanthen-11(12H)-one. It was observed that an increase in the solvent polarity led to an increase in the Stokes shift. The solvent effect on the spectral properties of benzo[a]xanthenes has been investigated by using the Lippert–Mataga and Reichardt–Dimroth methods. Bakhshievʹs and Kawski–Chamma–Vialletʹs correlations were used to determine the excited state dipole moment, ground state dipole moment and their ratio. The nature and extent of solute–solvent interactions were described by multi-linear correlation using the three-parameter Kamlet–Taft and the new four-parameter Catalán polarity scales. Multiple regression analysis indicates that both non-specific solute–solvent interactions and specific solute–solvent interactions play an important role in the position of the Stokes shift. The polarizable continuum model using IEF-PCM was considered to calculate excitation energies in methanol. The HOMO and LUMO energies have been performed by TD-DFT (B3LYP/6-311G (d, p)) approach.