The dual pathway in photocycloaddition of 1,3-diketonatoboron difluorides: excimer reactions

The lowest singlet excited state of dibenzoylmethanatoboron difluoride DBMBF2, a model compound of the BF2 complexes of 1,3-diketones, reacted with various simple olefins to give regiospecific and stereospecific photocycloadducts of 1,5-diketones similar to those from the de Mayo type reaction. DBMBF2 in acetonitrile exhibited two discrete fluorescences at 398 and 416 nm for the monomer and at 522 nm for the excimer; they were both quenched, but in different proportions, by a simple olefin. An "oxygen test" showed that the excimer of DBMBF2 is formed irreversibly in acetonitrile. The quantum yields of the photocycloaddition were shown to be proportional not only to olefin concentrations but also to DBMBF2 concentrations. Kinetic analysis has established that the total quantum yield is the sum of those arising from the interactions of the singlet excited DBMBF2 and its excimer, respectively, with an olefin, i.e., the sum of the quantum yields of exciplex and triplex pathways. The contributions from the two pathways are determined by the type of olefins and the range of DBMBF2 concentrations. For endocyclic olefins, the triplex pathway is more important and the corresponding photocycloaddition becomes very efficient as soon as the excimer starts to form in [DBMBF2] > 0.001 M. For the monosubstituted olefins, on the contrary, the exciplex pathway is always more important than the triplex pathway; they react primarily from the singlet excited state of DBMBF2. Key words: singlet state photocycloaddition, irreversible excimer formation, excimer cycloaddition, triplex and exciplex reactions.