Double-charge-transfer molecule may help improve solar fuels

For decades, scientists have been trying to use the sun’s energy to turn water and carbon dioxide into fuel. While nature makes photosynthesis seem effortless, it requires choreographing multiple complex charge-transfer steps.

To study these charge-transfer processes and better understand how to harness them for productive chemistry, researchers often turn to model systems such as donor-photosensitizer-acceptor complexes. These structures are made up of a metal complex flanked by an electron-accepting molecular piece and an electron-donating piece such that when the metal center absorbs light energy, the acceptor and donor can store it in an electron-hole pair.

University of Basel chemist Oliver Wenger and his graduate student Mathis Brändlin have now created a molecule that can capture and store up to two electron-hole pairs at once (Nat. Chem. 2025, DOI: 10.1038/s41557-025-01912-x). Previous researchers had succeeded in jamming one electron and two holes into a single molecule, or two electrons and one hole, but two electrons and two holes remained elusive.

Wenger and Brändlin’s approach was deceptively simple: just add a second donor group and a second acceptor group to either end of a donor-photosensitizer-acceptor complex. The trick was to choose the right molecular pieces and arrange them so that they’d efficiently shuttle charges away from the central ruthenium photosensitizer.

The final molecule is large and complicated, and Wenger says making it was “a heroic effort” on Brändlin’s part.

The researchers characterized the complex extensively using a suite of methods including cyclic voltammetry and ultraviolet–visible spectroscopy. The “final experiment that nailed it” involved firing two lasers—one continuous, the other pulsed—at the molecule and monitoring its spectroscopic signal to confirm that it could indeed hold two electrons and two holes at once, Wenger says.

From the spectroscopic data, the researchers determined that the molecule can hold one electron-hole pair for about 120 µs and two pairs for a little less than 1 µs—which may sound vanishingly short, but it would be plenty long enough to do chemistry, Wenger says.

Daniel G. Nocera of Harvard University, an expert in artificial photosynthesis who was not involved in the work, calls it “an interesting fundamental study” in molecular energy storage. Practical charge transfer for solar fuels is better accomplished using semiconductors, he says, but Brändlin and Wenger’s cleverly designed complex nevertheless provides useful insights into charge-transfer mechanisms.

The researchers have not yet used the complex to drive any chemical reactions. But that would be the natural next step, Wenger says—that is, if he can find someone to take over the project after Brändlin defends his dissertation later this year.