Proton-induced Quenching of Methoxynaphthalenes Studied by Laser Flash Photolysis and Inclusion Effect of β-Cyclodextrin on the Quenching

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Abstract
Proton-induced quenching in both excited singlet and triplet states of methoxynaphthalenes in H2O–CH3CN mixtures has been studied by means of laser flash photolysis. Inclusion effect of β-cyclodextrin (βCD) on the proton-induced quenching has been also examined. It is shown that the proton-induced quencing (the rate constant 1kq) of 1-methoxynaphthalene (1RH) occurs effectively in the excited singlet state, but not in the triplet state on the basis of the Tn←T1 absorption measurements. There is no enhancement of triplet formation via the proton-induced quenching since the 1kq value obtained by the Tn←T1 absorption measurements agrees with that obtained by fluorimetry. It was unsuccessful to observe a protonated σ-complex produced by the excited-state protonation to the naphthalene ring. The 1:1 inclusion complex between 1RH and βCD is produced with the association constant Kg (=27 mol−1 dm3 at 300K). In the presence of βCD, the 1Lb character of 1RH increases in the fluorescent state and the proton-induced quenching of 1RH markedly decreases, which may be due to inclusion and environmental effects of βCD upon the behavior in the excited state of 1RH. As for 2RH having little or no intramolecular CT character (1La), there is no proton-induced quenching.

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