Revisiting the photophysical properties and excited singlet-state dipole moments of several coumarin derivatives

The solvent effects on the electronic absorption and fluorescence emission spectra of several coumarins derivatives, containing amino, N,N-dimethyl-amino, N,N-diethyl-amino, hydroxyl, methyl, carboxyl, or halogen substituents at the positions 7, 4, or 3, were investigated in eight solvents with various polarities. The first excited singlet-state dipole moments of these coumarins were determined by various solvatochromic methods, using the theoretical ground-state dipole moments which were calculated by the AM1 method. The first excited singlet-state dipole moment values were obtained by the Bakhshiev, Kawski–Chamma–Viallet, Lippert–Mataga, and Reichardt–Dimroth equations, and were compared to the ground-state dipole moments. In all cases, the dipole moments were found to be higher in the excited singlet-state than in the ground state because of the different electron densities in both states. The red-shifts of the absorption and fluorescence emission bands, observed for most compounds upon increasing the solvent polarity, indicated that the electronic transitions were of π–π* nature.

Graphical abstractThe solvent effects on the electronic absorption and fluorescence spectra of twelve coumarins derivatives were investigated in eight solvents with various polarities. The first excited singlet-state dipole moments of these coumarins were determined by various solvatochromic methods. The dipole moments were found to be higher in the excited singlet-state than in the ground state. Normalized fluorescence emission spectra of: (A) 7-bromo-4-trifluoromethylcoumarin (5 × 10−6 M) in various solvents at room temperature.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Solvent effects on the electronic absorption and fluorescence spectra of twelve substituted coumarins. ► Red-shifts of absorption and fluorescence bands upon increasing solvent polarity. ► Determination of ground-state and excited singlet-state dipole moments. ► Solvatochromic correlations. ► Excited singlet-state dipole moments are higher than the ground state ones.