Directing electrons with molecular vibrations

Fundamental to many biomolecular reactions, such as photosynthesis, is the transfer of electrons from one part of a large molecule to another. Mimicking those reactions—for example, to produce solar fuels—requires the ability to control the electron-transfer processes. But conquering the complex energy landscapes of large, condensed-phase molecules has proved a challenge. Over the past decade, Duke University’s David Beratan and collaborators showed theoretically that electron transfer can be analogous to what happens in a double-slit experiment: If the electron’s initial and final states are connected by two different sequences of quantum orbitals, then the constructive or destructive interference between those paths determines the overall amplitude of electron transfer. Furthermore, the theorists postulated that exciting a molecular vibration that couples to just one of the paths could alter the interference and switch the electron transfer on or off. Now Julia Weinstein (Sheffield University, UK) and colleagues have experimentally demonstrated that switching. They designed an organometallic molecule made up of three parts: an electron donor D, an electron acceptor A, and a bridge B between them. As shown in the figure, exciting the molecule with UV light sends it to a charge-transfer excited state, denoted DB⁺A⁻. The molecule then returns to the ground state in one of three ways: directly, via a spin-triplet state DB³A, or via an electron transfer to a fully charge-separated state D⁺BA⁻. Normally, all three processes are present. But when Weinstein and colleagues excited a carbon–carbon triple-bond vibration in the bridge, they were able to switch off the electron transfer required to form the D⁺BA⁻ state. Although all the molecules return to the same ground state, such switching could eventually be used to steer a reaction between different sets of products. (M. Delor et al., Science 346, 1492, 2014 .)