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Quenching of n,π*‐excited azoalkanes by amines: structural and electronic effects on charge transfer
Author(s) -
Pischel Uwe,
Nau Werner M.
Publication year - 2000
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/1099-1395(200010)13:10<640::aid-poc273>3.0.co;2-9
Subject(s) - chemistry , photochemistry , quenching (fluorescence) , kinetic isotope effect , excited state , reaction rate constant , steric effects , bond dissociation energy , dissociation (chemistry) , singlet state , deuterium , fluorescence , stereochemistry , kinetics , physics , quantum mechanics , nuclear physics
The quenching of n,π* 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 α C—H bonds of the amines (photoreduction). The quenching rate constants for aliphatic amines lie in the range 10 7 –10 9 M −1 s −1 , while those for the aromatic amines are generally higher and reach the diffusion‐controlled limit in some cases, e.g. 7.3 × 10 9 M −1 s −1 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. Copyright © 2000 John Wiley & Sons, Ltd.