Photoreduction of 4,4'-bipyridine by amines in acetonitrile-water mixtures: influence of H-bonding on the ion-pair structure and dynamics.
The photoreduction of 4,4'-bipyridine (44BPY) by diazabicyclo[2.2.2]octane and triethylamine (TEA) is investigated by using picosecond transient absorption and time-resolved resonance Raman spectroscopy in various acetonitrile-water mixtures. The results are interpreted on the basis of a preferential solvation effect resulting from the presence of a specific interaction between 44BPY and water by hydrogen bonding. Below 10% water, the free 44BPY species is dominant and leads upon photoreduction to a contact ion pair that undergoes efficient intrapair proton transfer if TEA is the amine donor. Above 10% water, most of the 44BPY population is H-bonded and leads upon photoreduction to a hydrated ion pair in which the intrapair proton transfer is inhibited. Instead, the 44BPY(-*) species is protonated by water through the hydrogen bond with a rate constant that increases by more than 3 orders of magnitude on going from 10% to 100% water. The dependence of this rate constant on the solvent mixture composition suggests that the reaction of intracomplex proton transfer is controlled by the hydration of the residual OH(-) species by three molecules of water, leading to a trihydrated HO(-)(H(2)O)(3) species.
Poizat
Olivier
O
Laboratoire de Spectrochimie Infrarouge et Raman, UMR 8516 de l'Université et du CNRS, Centre d'Etudes et de Recherches Lasers et Applications, Université des Sciences et Technologies de Lille, Villeneuve d'Ascq, France. poizat@univ-lille1.fr
Buntinx
Guy
G
Boilet
Laurent
L