The photochemistry of α-arylcarboxylic anhydrides. Part 3. Photoreactions and luminescence spectra of 1- and 2-naphthylacetic anhydrides and 1,3-di-(1-naphthyl)propan-2-one

Abstract

The photochemistry of 1- and 2-naphthylacetic anhydride and the luminescence of these anhydrides and of two naphthyl-substituted propanones has been studied. 1-Naphthylacetic anhydride upon irradiation with λ 300 nm yields mainly 1,3-di-(1-naphthyl)propan-2-one. With λ 254 nm, the main product is 1,2-di-(1-naphthyl)ethane. 1,3-Di-(1-naphthyl)propan-2-one proved to be photostable for λ 300 nm; it is, however, photoreactive upon irradiation with λ 254 nm and then yields 1,2-di-(1-naphthyl)ethane. Essentially similar results have been obtained with 2-naphthylacetic anhydride. With both anhydrides a characteristic emission at λca. 400 nm has been observed which is ascribed to intramolecular excimer fluorescence. The excimer emission is more pronounced with the naphthylacetic anhydrides (with 6 σ bonds between the two naphthyl groups) than with 1,3-di-(1-naphthyl)propan-2-one (with 4 σ bonds between them). The assignment of the emission of the ketone at λ 400 nm to excimer fluorescence was made on the basis of a comparison with 1-(1-naphthyl)propan-2-one, which cannot form an intramolecular naphthyl excimer. Molecular model studies suggest that both the anhydrides and 1,3-di-(1-naphthyl)propan-2-one can adopt a sandwich type of conformation required for intramolecular excimer formation. The absence of fluorescence of the naphthalene monomer type for 1,3-di-(1-naphthyl)propan-2-one as well as its photostability (λ 300 nm) may be explained in terms of intramolecular transfer of singlet energy from the naphthyl to the carbonyl group, followed by intersystem crossing and subsequent intramolecular quenching of the carbonyl triplet by the naphthyl moiety.