Speaker
Description
Electron transfer reactions are arguably the simplest chemical reactions but they have not yet ceased to intrigue researchers. Charge-separation and charge-recombination reactions are at the core of life-sustaining processes, molecular electronics and solar cells. Intramolecular electron donor-acceptor systems capture the essential features of these reactions and enable their fundamental understanding. We investigated intramolecular electron transfers in weakly polar solvents covering a range of 100 kcal mol–1 in exothermicities and showed that their rates increase, then decrease, and finally increase again with the driving force of the reactions [1]. Concomitantly, apparent activation energies change from positive, to negative and finally to positive. Reactions with positive activation energies are found to be faster than analogous reactions with negative effective activation energies. More recently, we investigate the same reactions in polar solvents and in a series of alcohols with dielectric constants ranging from 10 to 33. Both charge-separation and charge recombination reactions revealed a strong dependence on the driving-force of the reaction, in contrast with the observations published for weakly polar solvents (Figure 1). Interestingly, charge recombinations in 1-octanol and dichloromethane differ by one order of magnitude although their dielectric constants and exothermicities are very similar. These reactions occur in the Marcus inverted region. The medium effect is very strong in these reactions but it is not simply related to the solvent polarity and exothermicity of the reaction. A revision of the existing theories of electron transfer is proposed.