Quenching of n, pi*-excited azoalkanes by amines: structural and electronic effects on charge transfer

The quenching of n,pi* singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene, a very weak electron acceptor, by 26 aliphatic and aromatic amines was invest igated in benzene. This photoreaction entails fluorescence quenching throug h exciplex formation with subsequent hydrogen atom abstraction from the N-H and alpha C-H bonds of the amines (photoreduction). The quenching rate con stants for aliphatic amines lie in the range 10(7)-10(9) M-1 s(-1), while t hose for the aromatic amines are generally higher and reach the diffusion-c ontrolled limit in some cases, e.g. 7.3 x 10(9) M-1 S-1 for N,N,N',N'-tetra methyl-p-phenylenediamine. A dependence of the magnitude of the fluorescenc e quenching rate constant on the adiabatic ionization potential reveals sig nificant scatter. Besides steric and stereoelectronic effects, variations i n the dissociation energies of the C-H and N-H bonds of the amines appear t o be responsible for the deviations, e.g., the faster quenching of secondar y amines compared with primary amines is presumably related to the weaker s econdary N-H bond dissociation energy. Solvent effects, deuterium isotope e ffects, and photoreaction quantum yields were determined. Copyright (C) 200 0 John Wiley & Sons, Ltd.