Fluorescence quenching of fluorenone by alcohols

The absorption and fluorescence spectra of fluorenone have been observed in various solvents. The fluorescence intensity increases with increasing solvent polarity, except in alcoholic solvents. The relative fluorescence intensity of fluorenone in acetonitrile is about 14-times greater than that in methanol. There are isosbestic points in the absorption spectra of fluorenone in mixed solvents comprising ethanol-cyclohexane and ethanol-acetonitrile, indicating the formation of a 1 : 1 hydrogen-bonded complex between fluorenone and ethanol. The equilibrium constants for the complex formation are estimated to be 2.0 +/- 0.5 and 0.6 +/- 0.2 mol(-1) dm(3) in ethanol-cyclohexane and ethanol-acetonitrile solvents, respectively. The results indicate that the complex formation changes the relative energy of the singlet and triplet states, which have n-pi* and pi-pi* characters, respectively. The complex formation causes a strong fluorescence quenching of fluorenone in the alcohols. A plot of the fluorescence quenching also shows that there is a route for excited-state quenching. It is concluded that the S-1 state has mainly an n-pi* character in both nonpolar and alcoholic solvents, while a pi-pi* character in the S-1 state increases in polar and non-hydrogen bonding solvents. CNDO/S calculations support the experimental results.