Abstract: | The configuration of the lowest excited state of acenaphthenone, S1(π, π*) or T1(π, π*), depending on the solvent, dominates photoluminescence. The T1(n, π*) state in aprotic organic solvents is responsible for the phosphorescence of acenaphthenone. The wavelengths of the phosphorescence measured in benzene are 576 nm and 635 nm (vibronic) with 3.3 × 10?4 quantum efficiency. However, the S1(π, π*) state in protic solution which dominates the fluorescence emission depending upon acidity is the most distinctive feature of acenaphthenone. The wavelengths of the emissions are 446 nm under water solvation with 0.185 quantum efficiency and 538 nm with 0.097 quantum efficiency under high acidity. The emission at 446 nm is assigned from a H-bonded keto-form excited state, whereas the emission at 538 nm is probably due to the excited state of protonated keto-form. The pKa value in aqueous solution measured by diminution of fluorescence in basic solutions is 12.5 ± 0.4. |