Influence of solvent polarity and temperature on dual fluorescence of 10,10′-dibromo, 9,9′-bianthryl

Fluorescence of 10,10′-dibromo, 9,9′-bianthryl (DBrBA) in solvents of different polarities (n-hexane, dibutyl ether, tetrahydrofuran, and acetone) has been investigated as a function of temperature. Changing of the solvent and variation of temperature modifies the ratio of local (LE) and charge transfer (CT) fluorescence quantum yields. From the basic fluorescence data (quantum yields, lifetimes, ratio of CT to LE fluorescence quantum yields) the temperature-dependent equilibrium constants for the charge transfer process in the excited singlet state were calculated and discussed on the basis of the modern electron transfer theories. It has been found that the intersystem crossing in DBrBA in nonpolar n-hexane, leading to the population of the lowest triplet state, proceeds via the third triplet state. It has been confirmed by the fluorescence measurements and quantum mechanical calculations. Surprisingly, the experimentally obtained intersystem crossing rate constants are very weakly dependent on temperature. Thus, the electron transfer reaction leading to the population of the molecular triplet state is probably an adiabatic reaction with a rate constant controlled by the dielectric relaxation of the solvent.