Just as a round stone rolls faster on a steep slope than on a gentle one, chemical processes speed up as they become more energetically favorable. At least, that’s what usually happens. But 60 years ago, when Rudolph Marcus developed his theory of electron transfer—the basis for all oxidation and reduction chemistry—he found that under certain circumstances, the transfer should actually slow down when it’s made more exoergic.
That counterintuitive prediction—the so-called Marcus inverted region—was experimentally confirmed in 1984, and in 1992 Marcus was awarded the Nobel Prize in Chemistry for his theory (see Physics Today, January 1993, page 20). Now James Mayer and Sharon Hammes-Schiffer (both at Yale University), Leif Hammarström (Uppsala University in Sweden), and their colleagues have observed the signature of a Marcus inverted region in a different type of reaction. Rather than the transfer of a single electron, their reaction involves the simultaneous transfer of an electron and a proton.
The researchers designed the charge-transfer reaction to take place entirely within a single molecule, as shown in the figure. When the molecule is prepared in a charge-separated state—with an extra proton bound to the nitrogen atom in the upper right and an extra electron on the trio of fused benzene rings in the lower left—it relaxes back to the ground state through a concerted transfer of the two charged particles to the middle of the molecule.
Marcus’s theory stems from the insight that charge transfer can proceed only when the initial and final states have both the same energy and the same geometry. To satisfy that requirement, the solvated molecule system must wander away from its initial-state equilibrium, at the bottom of the green parabola in the figure, to the point where the green and purple parabolas cross. The figure illustrates the conditions of a Marcus inverted region, in which lowering the energy of the final state (progressively darker purple parabolas) raises the energy of the crossing point. Because the system must surmount a higher energy barrier, the charge transfer slows down.
It’s difficult to design charge-transfer processes that are identical in every respect except for the energy difference between the initial and final states. The Yale and Uppsala researchers managed it, to a good approximation, through synthetic chemistry: By placing different chemical groups at the position marked “R” in the molecule, they tuned the free energy of the reaction over a few tenths of an electron volt, more than enough to see a change in the rate of concerted charge transfer. Sure enough, the molecules with the greatest change in energy also showed the slowest charge transfer.
Concerted proton–electron transfer reactions are important in, among other things, materials for harvesting solar energy. Excitation by solar photons creates charge-separated states whose energy can be converted to electric current or chemical fuel. If the charges rapidly recombine, the energy is wasted. Slowing the rate of recombination by engineering the material in a Marcus inverted region could enhance the energy-extraction efficiency. (G. A. Parada et al., Science, 2019, doi:10.1126/science.aaw4675.)
