THE PHOTOCYCLOADDITION OF METHOXYNAPHTHALENES WITH ACETYLACETONE AND ACID-CATALYZED RETARDATION - REACTIONS OF NONFLUORESCENT EXCIPLEXES

Photolysis of either 1- or 2-methoxynaphthalene (MeONp) in the presenc e of acetylacetone (AA) in acetonitrile and methanol leads to cycloadd ition-ring opening to afford acetylacetonylnaphthalene derivatives reg ioselectively that may undergo the elimination of methanol. The quantu m yields of MeONp disappearance (Phi(c),) were retarded drastically by H2SO4 in the 0.001 mol dm(-3) range, whereas MeONp fluorescence spect ra were hardly affected under the acidic range. As singlet excited 2Me ONp 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 analys is', the nonfluorescent exciplex of singlet excited 2MeONp and AA is s hown, by the oxygen perturbation technique, to be formed reversibly, a nd to interact with H2SO4 promoting to ground state preferentially.