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 × 107 to 25 × 107 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-...