Quenching of n,pi*-excited states in the gas phase: variations in absolute reactivity and selectivity

The quenching of the n,pi*-excited azoalkane 2,3-diazabicyclo[2.2.2]oct-2-ene by 19 heteroatom-containing electron and hydrogen donors, that is, amines, sulfides, ethers, and alcohols, was investigated in the gas phase. Deuterium isotope effects were measured for 9 selectively deuterated derivatives. The data support the involvement of an excited charge-transfer complex, that is, an exciplex, for tertiary amines and sulfides, and a competitive direct hydrogen transfer from the C-H bonds of ethers or from the N-H or O-H bonds of secondary and primary amines or alcohols. The recently observed "inverted" solvent effect for the fluorescence quenching of azoalkanes by amines and sulfides in solution is supported by the observed rate constants in the gas phase, which are substantially larger than those in solution. A more pronounced inverted solvent effect for the weaker electron-donating sulfides and a presumably faster exciplex deactivation result in a switch-over in absolute reactivity relative to tertiary amines in the gas phase. Most importantly, the kinetic data demonstrate that the reactivity of the strongly dipolar O-H and N-H bonds in photoinduced hydrogen abstraction reactions shows a larger decrease upon solvation than that of the less polar C-H bonds. The azoalkane data are compared with previous studies on quenching of n,pi*-triplet-excited ketones in the gas phase.