Quenching of n,π*-excited azoalkanes by amines: structural and electronic effects on charge transfer
✍ Scribed by Uwe Pischel; Werner M. Nau
- Publisher
- John Wiley and Sons
- Year
- 2000
- Tongue
- English
- Weight
- 90 KB
- Volume
- 13
- Category
- Article
- ISSN
- 0894-3230
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✦ Synopsis
The quenching of n,p* singlet-excited 2,3-diazabicyclo[2.2.2]oct-2-ene, a very weak electron acceptor, by 26 aliphatic and aromatic amines was investigated in benzene. This photoreaction entails fluorescence quenching through exciplex formation with subsequent hydrogen atom abstraction from the N-H and a C-H bonds of the amines (photoreduction). The quenching rate constants for aliphatic amines lie in the range 10 7 -10 9 M À1 s À1 , while those for the aromatic amines are generally higher and reach the diffusion-controlled limit in some cases, e.g. 7.3 Â 10 9 M À1 s À1 for N,N,N',N'-tetramethyl-p-phenylenediamine. A dependence of the magnitude of the fluorescence quenching rate constant on the adiabatic ionization potential reveals significant scatter. Besides steric and stereoelectronic effects, variations in the dissociation energies of the C-H and N-H bonds of the amines appear to be responsible for the deviations, e.g., the faster quenching of secondary amines compared with primary amines is presumably related to the weaker secondary N-H bond dissociation energy. Solvent effects, deuterium isotope effects, and photoreaction quantum yields were determined.