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Abstract
Fluorescence of naphthanthrone,6H-benzo[cd]pyren-6-one, in degassed organic solution increases on light irradiation, but it stays almost constant in aerated solution. To clarify this phenomenon, the absorption and emission spectra of naphthanthrone were measured in several organic solvents as a function of irradiation time at room temperature and 77 K. The fluorescence enhancement was observed only in solvents containing hydrogen such as ethanol and benzene, but not observed in non-hydrogen-containing solvents. On the basis of the experimental data we proposed the mechanism of the fluorescence enhancement. Naphthanthrone excited to the triplet state on irradiation attracts a near solvent molecule around its carbonyl group to form an association species, which provides strong fluorescence near the wavelength region of the fluorescence of naphthanthrone. The absorption energy and absorptivity calculated for the compound by the ZINDO method were consistent with the experimental results.