Fluorescence of 2,3-Diazabicyclo[2.2.2]oct-2-ene Revisited: Solvent-Induced Quenching of the n,π*-Excited State by an Aborted Hydrogen Atom Transfer

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The fluorescence lifetimes of 2,3-diazabicyclo[2.2.2]oct-2-ene (DBO) were determined in a large number of solvents and solvent mixtures, displaying variations from 825 ns in CD 3 CN to 13 ns in CHCl 3 . Large deuterium isotope effects (8−11) and significant activation energies (generally 2−15 kJ mol -1 ) for fluorescence quenching of DBO are observed, which support a novel solvent-induced quenching mechanism via an aborted hydrogen atom transfer. In CCl 4 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.