EXCIPLEX BINDING-ENERGY AND KINETIC RATE CONSTANTS OF THE INTERACTION BETWEEN SINGLET EXCITED-STATE (DIBENZOYLMETHANATO)BORON DIFLUORIDE AND SUBSTITUTED BENZENES

For exciplex formations of singlet excited (dibenzoylmethanato)boron difluoride (*(DBM)BF2) with a series of substituted benzenes (SB), their heat of formation (binding energy, -Delta H-ex) and kinetic rate constants were measured. In the vicinity of room temperature, the exciplexes from less substituted benzenes including xylenes were formed reversibly, as shown by oxygen perturbations of fluorescence intensity and time-resolved fluorescence analysis. For these exciplex formations, temperature dependent intensity changes were utilized to determine -Delta H-ex, which permits the calculation of the repulsive energy of the Franck-Condon ground state complex R(e) from exciplex energy. For other related exciplexes from irreversible formation at room temperature, these values were evaluated by extrapolations of the correlation with steric factors. The heat of formation becomes larger, the lifetime of the exciplexes becomes longer, and k(-ex) becomes slower, as E(ox)- (SE) becomes lower, i.e., more electron donating. The heat of formation is correlated with the stabilization energy of exciplexes depending on the extent of CT (charge transfer) and LE (locally excited) contributions; that is, either a much greater than c or c much greater than a in the exciplex wave function. In the high-CT region, Delta H-ex + R(e) originates primarily from the redox potential gap. An additional stabilization energy is developed for the exciplexes from trimethylbenzenes to toluene as the HOMO-HOMO gap becomes closer. In the weak CT (or high LE) contribution region, the (Delta H-ex + R(e)) term becomes nearly a constant, and the exciplex fluorescence peaks all appears red-shifted about 0.25 eV from the *(DBM)BF2 peak.