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

The fluorescence quenching of two azoalkanes, the weak acceptor 2,3-diazabi cyclo [2.2.2]oct-2-ene and its less reactive derivative 4-methyl-1-isopropy l-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 que nching rate constants ranged from 0.13 x 10(7) to 25 x 10(7) M-1 s(-1), whi le the activation energies were found to be 1.3 and 3.0 kcal mol(-1) for th e best (N-ethyldicyclohexylamine) and the poorest (N,N-diisopropyl-3-pentyl amine) donor. The oxidation potentials of the amines and the reduction pote ntials of the azoalkanes were measured. Deviations from the expected depend ence of the quenching rate constants on the energetics of electron transfer were observed. Steric effects on the kinetics of quenching, namely a reduc tion by up to a factor of 50, became significant for amines with three seco ndary alkyl substituents, triisopropylamine, and N,N-diisopropyl-3-pentylam ine, while a stereoelectronic effect was found for 1,4-diazabicyclo[2.2.2]o ctane (reduction by a factor of 2). The experimental data are interpreted w ithin a general kinetic scheme, which involves both exciplex formation and electron transfer, by assuming a steady-state kinetics for quenching. The s teric effects are attributed to the hindrance toward exciplex formation, ba sed on the experimental results and semiempirical calculations (PM3). Facto rs governing steric hindrance by amines, differences between ketone and azo alkane accepters, and the persistence of the corresponding amine radical ca tions are discussed.