Journal of Physical Chemistry, Vol.99, No.49, 17558-17565, 1995
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.
Keywords:RADICAL-ION-PAIRS;DIBENZOYLMETHANATOBORON DIFLUORIDE DBMBF2;1;3-DIKETONATOBORON DIFLUORIDES;SOLVENT REORGANIZATION;PICOSECOND DYNAMICS;LASER PHOTOLYSIS;CONTACT;FUMARONITRILE;STATE;PHOTOCYCLOADDITION