Fluorescence of Scopoletin Including its Photoacidity and Large Stokes Shift.

Scopoletin is highly fluorescent in water and acts as a photoacid exhibiting excited-state proton transfer, ESPT, competitive with fluorescence. Its absorbance and emission spectral characteristics yield ground-state and excited-state pKa values of 7.4 ± 0.1 and 1.4 ± 0.1, respectively. The pKa* implies an ESPT rate constant an order of magnitude smaller than that for umbelliferone. This report provides quantum yield measurements in water that are comparable to quinine sulfate, and fluorescence lifetime values that are on a par with other similar coumarins yet provide insight into the ESPT process. The scopoletin anion is observed in tetrahydrofuran by reaction with a strong base. The Stokes shift of aqueous scopoletin is >100 nm in the pH range 3 to 7 due in part to its action as a photoacid. Modeling by density functional theory methods provides reasonable support for the experimental results.