Abstract
The fluorescence properties of 7-substituted coumarins were studied on the bases of the following two points; energy difference between nπ* and nπ* states (ΔE) and nature of the fluorescent state. The energy difference (ΔE) increases with the increase in electron donating power of the substituent, X. For derivatives with small ΔE the fluorescence yield increases with the increase in ΔE, indicating that nπ* character of the fluorescent state plays an important role in the emission properties. Further increase in ΔE, however, results in the decrease in the fluorescence yield. It was found that this drastic change in the fluorescence yield is closely related to the solvent shift (Δνsolv) of absorption spectrum and is also related to the contribution of charge transfer configuration arrising from electron transfer from styryl to the carbonyloxy group (CTso) to the fluorescent state. Therefore, it seems probable that the CTso-character of the fluorescent state is another factor controlling the fluorescence properties of 7-substituted coumarins.
