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Abstract
Fluorescence of naphthanthrone, 6H-benzo[cd]pyren-6-one, in degassed organic solution increases by a factor of 10–100 on strong irradiation and the spectrum is blue-shifted. To clarify the mechanism of the fluorescence enhancement, we investigated effects of intensity of irradiation, deuteration of solvents, and concentration of naphthanthrone on the enhancement. The fluorescence was more rapidly enhanced with stronger irradiation, though there was a latent period of several minutes. The enhancement was, however, reduced to 30–70% by deuteration of the solvents. The spectra of enhanced fluorescence showed more defined vibrational structure and larger blue-shift as the concentration was decreased from 10−4 to 10−6 M. These experimental results were interpreted on the basis of the associative complex model: the association of naphthanthrone with a solvent molecule proceeds step-wisely in a solvent cage by several times of excitation and undergoes kinetic isotope effects. At lower concentrations, the movement of the associative complex is more strongly suppressed, leading to enhanced fluorescence spectra with clearer vibrational structure and larger blue-shift.
This work was supported in part by KAKENHI (21550025) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.