The role of excited state molecular association in photoreactions of dibenzoylmethanatoboron difluoride with conjugated enones and en-esters: Anomalous excimer reaction

The singlet excited state of the title borofluoride, *DBMBF2 reacted efficiently with acyclic enones and en-esters but only feebly with the corresponding excimer in analogy to simple acyclic olefins to give cycloaddition products, in spite of substantially different electronic character of the double bonds. Cyclic enones, on the contrary, reacted with both *DBMBF2 and the excimer with increasing fluorescence quenching constants and adduct quantum yields as the rigidity of enone rings relaxes. They interacted with the excimer to cause the enone stimulated excimer dissociation which occurred most likely by the intermediary of the enone exciplexes. The interaction of 2-cyclopentenone (CP) by this process caused strong increases in *DBMBF2 fluorescence intensity, that was interpreted to arise from a fast reverse step (k(x)) from the exciplex as shown by increasing *DBMBF2 fluorescence intensities upon addition of CP. Apparently the exciplexes from 2-cyclohexenone, 2-cyclooctenone and 2-cyclododecenone had a decreasing degree of reversibility (i.e., relatively smaller reverse reaction rates): this trend caused Phi (a), increases and, correspondingly, lesser enhancement of *DBMBF2 fluorescence in the same order. The fast enone, while mainly reacted with excimer, showed almost the same reactivity and Phi (a) pattern of acyclic enones. In contrast to cyclic olefins, the last three enones reacted to show decreased Phi (a) on increasing initial [DBMBF2] (i.e., higher excimer concentrations); an hidden deactivation step was implicated in the excimer reaction with these enones. The anomalous reaction patterns were assumed to be caused by differences in the geometrical orientation of an enone within exciplexes and of enone attack on the excimer.