The Structure and Decay Dynamics of Exciplexes Derived from Dibenzoylmethanatoboron Difluoride and Alkylbenzenes in Cyclohexane

Dibenzomethanatoboron difluoride (DBMBF 2 ) interacted with alkylbenzenes from its singlet excited state to form exciplexes ranging from weak polarity up to contact radical ion pairs (CRIP); this exciplex series shows the characteristics in the Marcus “normal” region. In cyclohexane these exciplexes gave intense fluorescence spectra and high quantum yields (ϕ   ex ∞ . The dipole moment of these exciplexes calculated from the solvatochromic shift of the fluorescence maximums ( ν max ) was used to estimate the coefficient (“a” and “c”) of the CT and LE terms in the exciplex wavefunction. On the basis of the measured lifetimes and ϕ   ex ∞of these exciplexes, the radiative ( k   f exand nonradiative ( k   NR exrate constants were calculated. The former k   f exwere also computed from a semi‐empirical approach based on the assumption that the exciplex wavefunction could be adequately described by CT and LE states, and that *DBMBF 2 primarily contributes to the probability of exciplex emission. Two results agree with each other with small systematic deviations for those less polar exciplexes. The plots of k   f exand k   NR ex(or their logarithmic value) against the LE contribution ( c   2 ) and transition energy gaps ( h ν max ) afford better correlation than those against −Δ G −et . This indicates the role played by the LE contribution in generating the stabilization energy ( U s ) in these exciplexes through the |A − D + 〉⇌ |*AD〉 resonance interaction; U s , in turn, modifies −Δ G −et to afford the decay driving force h ν max . Also, those plots from k   f exvalues (being determined directly from experiments) show better correlation than those from k   NR ex . In contrast to the CRIP type exciplexes in the Marcus “inverted” region, these k   f exand k   NR exincrease in the common trend with increasing transition energy gaps. The k   NR explots show less steep slopes and attains more quickly a minimum toward the CRIP region; the latter is identified as the turning point from the “normal” to “inverted” region. Both the attenuation and reversal of the k   NR exvalue with increasing polarity are believed to be generated by the emerging contribution of the intersystem crossing process as an additional nonradiative process, which is induced by the increased spin‐orbit coupling in highly polar exciplexes.