The structure and decay dynamics of exciplexes derived from dibenzoylmethanatoboron difluoride and alkylbenzenes in cyclohexane

Dibenzomethanatoboron difluoride (DBMBF2) interacted with alkylbenzenes fro m its singlet excited state to form exciplexes ranging from weak polarity u p to contact radical ion pairs (CRIP); this exciplex series shows the chara cteristics in the Marcus "normal" region. In cyclohexane these exciplexes g ave intense fluorescence spectra and high quantum yields (Phi(ex)(proportio nal to)). The dipole moment of these exciple?tes calculated from the solvat ochromic shift of the fluorescence maximums (nu(max)) was used to estimate the coefficient ("a" and "c") of the CT and LE terms in the exciplex wavefu nction, On the basis of the measured lifetimes and Phi(ex)(proportional to) of these exciplexes. the radiative (k(t)(ex)) and nonradiative radiative ( k(NR)(ex)) rate constants were calculated. The former k(f)(ex) were also co mputed from a semi-empirical approach based on the assumption that the exci plex wavefunction could be adequately described by CT and LE stales, and th at *DBMBF2 primarily contributes to the probability of exciplex emission. T wo results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k(f)(ex) and k(NR)(ex) (or their logar ithmic value) against the LE contribution (c(2)) and transition energy gaps (h nu(max)) afford better correlation than those against -Delta G(-et). Th is indicates the role played by the LE contribution in generating the stabi lization energy (LI,) in these exciplexes through the /A(-)D(+)]= /*AD] res onance interaction: U-s, in turn, modifies - Delta G(-et) to afford the dec ay driving force h nu(max). Also, those plots from k(f)(ex) values (being d etermined directly from experiments) show better correlation than those fro m k(NR)(ex). In contrast to the GRIP type exciplexes in the Marcus "inverte d" region, these k(f)(ex) and k(NR)(ex) increase in the common trend with i ncreasing transition energy gaps. The k(NR)(ex) plots show less steep slope s and attains more quickly a minimum toward the GRIP region; the latter is identified as the turning point from the "normal" to inverted" region. Both the attenuation and reversal of the k(NR)(ex) value with increasing polari ty are believed to be generated by the emerging contribution of the intersy stem crossing process as an additional nonradiative process, which is induc ed by the increased spin - orbit coupling in highly polar exciplexes.