Deactivation of 9,9'-bianthryl in solution studied by photoacoustic calorimetry and fluorescence

The photophysics of singlet excited 9,9'-bianthryl has been studied in solution by stationary and time-resolved experiments using photoacoustic (PAC) and fluorescence methods. The results are interpreted under the assumption of a very fast solvent-dependent equilibrium between a pure local excited (LE) and a charge transfer (CT) state. The deactivation of the CT state is treated as nonadiabatic back electron transfer using the formalism developed by Could et al. for radiative and radiationless deactivation of radical ion pairs. Fluorescence spectra have been separated into LE and CT emission spectra. Calculated CT spectra yield reasonable values for the free enthalpy change of back electron transfer and for the LE reversible arrow CT equilibrium constant. The energy of the nonpolar triplet state as well as the quantum yields Q(isc) of intersystem crossing have been determined by the PAC method. The sum of Q(isc) and the fluorescence quantum yield Q(f) is near unity, independent of solvent polarity. A procedure for the quantitative correction of electrostriction effects in PAC is presented, which allows the determination of the enthalpy and entropy changes of the LE --> CT reaction and of the solvent reorganization reaction in the ground state. Intersystem crossing CT --> T-1 competes efficiently with CT fluorescence in strongly polar solvents due to spin-orbit coupling, the much smaller energy gap, and the large solvent reorganization energy.