Fluorescence quenching of n,π*-excited azoalkanes by amines:: What is a sterically hindered amine?

The fluorescence quenching of two azoalkanes, the weak acceptor 2,3-diazabicyclo [2.2.2]oct-2-ene and its less reactive derivative 4-methyl-1-isopropyl-2,3-diazabicyclo [2.2.2]oct-2-ene, by a series of aliphatic amine donors was examined by time-resolved spectroscopy in benzene. The fluorescence quenching rate constants ranged from 0.13 x 10(7) to 25 x 10(7) M-1 s(-1), while the activation energies were found to be 1.3 and 3.0 kcal mol(-1) for the best (N-ethyldicyclohexylamine) and the poorest (N,N-diisopropyl-3-pentylamine) donor. The oxidation potentials of the amines and the reduction potentials of the azoalkanes were measured. Deviations from the expected dependence of the quenching rate constants on the energetics of electron transfer were observed. Steric effects on the kinetics of quenching, namely a reduction by up to a factor of 50, became significant for amines with three secondary alkyl substituents, triisopropylamine, and N,N-diisopropyl-3-pentylamine, while a stereoelectronic effect was found for 1,4-diazabicyclo[2.2.2]octane (reduction by a factor of 2). The experimental data are interpreted within a general kinetic scheme, which involves both exciplex formation and electron transfer, by assuming a steady-state kinetics for quenching. The steric effects are attributed to the hindrance toward exciplex formation, based on the experimental results and semiempirical calculations (PM3). Factors governing steric hindrance by amines, differences between ketone and azoalkane accepters, and the persistence of the corresponding amine radical cations are discussed.