MECHANISTIC STUDIES ON THE DUAL REACTION PATHWAYS OF SINGLET EXCITED DIBENZOYL(METHANATO)BORON DIFLUORIDE (DBMBF(2)) - REACTIONS OF THE EXCIMER AND EXCIPLEXES

The mechanism of the interaction of singlet excited dibenzoyl(methanat o)boron difluoride (DBMBF(2) = *A) with olefins and substituted benze nes (SB) was investigated by the determination of effective quenching distances (R(eff)) from fluorescence monitors in acetonitrile as analy zed by the Smoluchowski-Collins-Kimball equation and by solvent effect s on the dichotomy of the formation of cycloadducts and [4 + 2] dimers from 1,3-cyclohexadiene (CHD) and 1,3-cyclooctadiene (1,3-COD). The c orrelation of R(eff) with Delta G(et)(o) directly demonstrates that th e reactants have to get closer to affect electron transfer from these substrates as the driving force become more positive and coalesce on e xciplexes. Both photophysical and photochemical probes converge on the partition of encounter pairs to the SSRIP (the k(et) pathway which le ads to radical ion reactions and nonfluorescent) and exciplexes (the k (ex) pathway which leads to cycloadducts and fluorescent species); the former pathway was promoted by low redox energies of donor-acceptor p airs and by high solvent polarity. The reaction with 1,3-cylcohexadien e gave high quantum yields of the [4 + 2] dimers in acetonitrile and o f the cycloadduct in nonpolar solvents; the latter quantum yields were correlated with the solvent polarity parameter E(T)(30) in a smooth d istribution regardless of pure non-SE or SE solvents or mixture solven ts (including benzene), in spite of the fact that the reactive interme diates in the presence of SB are DBMBF(2)-SB exciplexes. It is propos ed that in the exciplex the locally excited state provides the driving force in the cycloaddition and the CT state regulates its regiospecif icity. The quantum yield of the dimers from CHD was greatly enhanced, reaching their maximum at 12 mol % of p-xylene in acetonitrile; the en hancement was attenuated with the addition of toluene and benzene in t hat order. This shows that the p-xylene exciplex underwent extraordina ry facile substitution with CHD by electron transfer in acetonitrile p robably promoted by superexchange interactions. The DBMBF(2) excimer reacted with CHD and 1,3-COD to give higher yields of cycloadducts but not the dimers, which must mean that the excimer substitution with CH D proceeds to give the new exciplex but not the radical ion pair. The latter failure as well as that of the benzene exciplex was suggested t o arise from the insignificant CT content of the excimer and exciplex. The frontier MO scheme was utilized to rationalize the enhanced react ivity of the exciplexes and excimer on the bases of increased electron ic and orbital interactions, respectively.