Abstract
Electron transfer reactions are arguably the simplest chemical reactions but they have not yet ceased to intrigue chemists. 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. Here, we report intramolecular electron transfers covering a range of 100 kcal mol−1 in exothermicities that show an increase, then a decrease, and finally an increase in rates with the driving force of the reactions. 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. The increase of the reorganization energy with the driving force of the reactions can explain the peculiar free-energy relationship observed in this work.
Highlights
Electron transfer reactions are arguably the simplest chemical reactions but they have not yet ceased to intrigue chemists
Electron donor–acceptor moieties covalently linked by a rigid spacer provide valuable insight on Electron transfer (ET) reactions because they allow for precise control of both electronic coupling (V) and driving force (ΔG0)[11,12,13], and are relevant for the fabrication of solar energy conversion and organic electronics devices[14].The ability to explore the free-energy dependence of very exothermic ET reactions, often believed to be deep in the Marcus inverted region and slow, depends on the design of systems that can be investigated over wide ΔG0 and T ranges without changes in reaction mechanism or electronic coupling
The nitrile groups are rotated by 82° in 31± with respect to 11±, while the C = C bond increases from 1.36 Å in 1 and in 11, 1.43 Å in 11± to 1.47 Å in 31±
Summary
Electron transfer reactions are arguably the simplest chemical reactions but they have not yet ceased to intrigue chemists. 1234567890():,; Electron transfer (ET) reactions are implicated in fundamental processes, and have been scrutinized since Marcus related ET rates to solvent (λs) and molecular vibration (λv) reorganization energies[1,2] Their two most distinctive features are a free-energy dependence characterized by an increase in the rates as their exothermicities increase followed by their decrease for very exothermic reactions (the Marcus “inverted” region)[2], and the observation of fast rates even when electron donor and acceptor moieties are separated by long and rigid spacers[3,4]. We show that exothermic rate restrictions have a limit, that dielectric continuum models overestimate λs in weakly and moderately polar solvents (i.e., dielectric constants ε between 2.6 and 16), and that the activationless rates in the inverted region are followed by higher rates with positive activation energies (Ea) at higher exothermicities These findings can be explained by the increase of the reorganization energy with the driving force of the reactions
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