EXCIPLEXES OF (DIBENZOYLMETHANATO)BORON BENZENES - THE CONTROL OF EXCIPLEX ELECTRONIC-STRUCTURE

Singlet excited (dibenzoylmethanato)boron difluoride ((DBM)BF2) interacted with a series of substituted benzenes (SB) to give strongly fluorescent exciplexes, which exhibited a wide range of emission maxima, h nu(max) (5500 cm(-1)), in cyclohexane depending on SB electron-donating ability. Exciplex dipole moments were determined by solvatochromic shifts, which ranged from the limiting value of 13.8 for the contact radical ion pair (GRIP) to 1.1 D; these were correlated with the oxidation potential difference between (DBM)BF2 and SB. These observations were explained in terms of relative contributions of charge transfer (\A(-)D(+)]) and locally excited (\*AD]) states. The formation of these exciplexes are shown to be driven by an electrochemical force from charge transfer as well as the stabilization energy from \A(-)D(+)] <----> \*AD] resonance interaction. The latter is shown to be the highest at the vicinity of a lowest HOMO-HOMO energy gap and becomes negligible at values greater than 0.7 eV, which can be shown from calculations. A plot of fluorescence maxima against the redox potential differences reveals electrochemical driving force, stabilization energy, and \*AD]-dominated emission. The exciplex formation pattern is also rationalized with the frontier orbital theory.