Photoinduced intramolecular electron-transfer (ET) reactions in donor-bridge-acceptor molecules such as dimethylphenylenediamine-proline-coumarin343 were examined in two ionic liquids (ILs) and in acetonitrile and methanol. For this system, many key ET parameters differ significantly between the ILs and the neutral solvents, and the distribution of observed ET rates is much broader for the ILs.  While the photoinduced forward ET reaction is in the Marcus inverted regime in each case, the free energy barrier is about twice as large for the ILs than for the two neutral solvents.Bimolecular ET rates are frequently explained in terms of solvent shear viscosity.  However, we observe that photoinduced bimolecular ET rates in ILs do not track the self-diffusion coefficients of the reactants (and IL cation and anion) obtained from NMR diffusion measurements. We will explain how this results from two effects. The first is that bimolecular rates for photo-induced reactions are normally in the early-time limit of the full Smoluchowski equation.  The second is that the anionic electron donor has a self-diffusion coefficient that is 2.5 times smaller than for the analogous neutral donor.This work was supported by the US-DOE Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences under contracts DE-FG02-09ER16118 at Rutgers and DE-AC02-98CH10886 at BNL.

Photoinduced electron-transfer reactions in ionic liquids

243rd National Meeting (2012)

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