Fluorescence of 2,3-diazabicyclo[2.2.2]oct-2-ene revisited: Solvent-induced quenching of the n,pi*-excited state by an aborted hydrogen atom transfer

The fluorescence lifetimes of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) were d etermined in a large number of solvents and solvent mixtures, displaying va riations from 825 ns in CD3CN to 13 ns in CHCl3. Large deuterium isotope ef fects (8-11) and significant activation energies (generally 2-15 kJ mol(-1) ) for fluorescence quenching of DBO are observed, which support a novel sol vent-induced quenching mechanism via an aborted hydrogen atom transfer. In CCl4 a photoinduced electron-transfer quenching mechanism may compete. The fluorescence quenching by solvent mixtures is not proportional to the mole fraction of the two components, indicating a preferential solvation of the excited state.