The role of excited state molecular association in photoreactions of dibenzoylmethanatoboron difluoride with conjugated enones and en-esters: Anomalous excimer reaction
Yl. Chow et al., The role of excited state molecular association in photoreactions of dibenzoylmethanatoboron difluoride with conjugated enones and en-esters: Anomalous excimer reaction, RES CHEM IN, 26(7-8), 2000, pp. 643-666
The singlet excited state of the title borofluoride, *DBMBF2 reacted effici
ently with acyclic enones and en-esters but only feebly with the correspond
ing excimer in analogy to simple acyclic olefins to give cycloaddition prod
ucts, in spite of substantially different electronic character of the doubl
e 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 ex
cimer to cause the enone stimulated excimer dissociation which occurred mos
t likely by the intermediary of the enone exciplexes. The interaction of 2-
cyclopentenone (CP) by this process caused strong increases in *DBMBF2 fluo
rescence intensity, that was interpreted to arise from a fast reverse step
(k(x)) from the exciplex as shown by increasing *DBMBF2 fluorescence intens
ities upon addition of CP. Apparently the exciplexes from 2-cyclohexenone,
2-cyclooctenone and 2-cyclododecenone had a decreasing degree of reversibil
ity (i.e., relatively smaller reverse reaction rates): this trend caused Ph
i (a), increases and, correspondingly, lesser enhancement of *DBMBF2 fluore
scence 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 decrea
sed Phi (a) on increasing initial [DBMBF2] (i.e., higher excimer concentrat
ions); an hidden deactivation step was implicated in the excimer reaction w
ith these enones. The anomalous reaction patterns were assumed to be caused
by differences in the geometrical orientation of an enone within exciplexe
s and of enone attack on the excimer.