The photocycloaddition of methoxynaphthalenes with acetylacetone and acid-catalysed retardation: reactions of non-fluorescent exciplexes

Abstract

Photolysis of either 1- or 2-methoxynaphthalene (MeONp) in the presence of acetylacetone (AA) in acetonitrile and methanol leads to cycloaddition–ring opening to afford acetylacetonylnaphthalene derivatives regioselectively that may undergo the elimination of methanol. The quantum yields of MeONp disappearance (Φ_c) were retarded drastically by H₂SO₄ in the 0.001 mol dm^–3 range, whereas MeONp fluorescence spectra were hardly affected under the acidic range. As singlet excited 2MeONp is known to lack interaction with H₂SO₄, the photocycloaddition of 2MeONp was used as a model to search for the intermediate responsible for the acid catalysis. Using ‘extended fluorescence quenching analysis’, the nonfluorescent exciplex of singlet excited 2MeONp and AA is shown, by the oxygen perturbation technique, to be formed reversibly, and to interact with H₂SO₄ promoting to ground state preferentially.