Fluorescence Quenching and the Charge Transfer in the Excited State. II. Dimethylnaphthylamine-Dimethyl Phthalate in Various Solvents

Abstract

Abstract As an extension of a previous paper (Bull. Chem. Soc. Japan, 39, 2588 (1966)), the quenching action of p-methyl toluate, ethyl benzoate, dimethyl esters of m-, o-, p-phthalic acids on the fluorescence of N, N-dimethyl-2-naphthylamine has been studied in several solvents. The quenching constants of the above substances (in the above sequence) in cyclohexane and dimethylformamide are respectively (0.5, ∼1, 65, 85, 93) and (41, 58, 103, 85, 124). The order, toluate<benzoate<phthalate, is consistent with the charge transfer mechanism of quenching in which fluorescer and quencher act respectively as a donor and an acceptor. In slightly or nonpolar solvents a new emission attributed to an excited CT complex has been found; its intensity has been the strongest in the case of dimethyl m-phthalate. Using this substance as a quencher, solvent effect on the quenching efficiency and the CT emissivity has been studied. It has been found that in weakly polar solvents, these two quantities decrease similarly with the increasing solvent polarity, whereas in highly polar solvents the former is large and the latter is negligible. The results in less polar solvents suggest that the reorientation of the solvated molecules is an essential process for both the quenching and the CT emission. Another evidence for this view has been found in the experiments with mixed solvents. In highly polar solvents, the production of free radical ions is most plausible but the possibility of the formation of a solvated ion pair with no emissivity, cannot be ruled out.