Quantum Physics

Quantum Proton Shuttle Dramatically Boosts Energy Transfer Between Molecules

A proton briefly moves from one molecule to another and back again, acting as a temporary bridge that makes triplet energy transfer far faster and more efficient — a discovery with implications for solar cells, lasers, and catalysts.

• The mechanism, called proton shuttle-assisted triplet energy transfer (PS-TET), was observed as energy moved from ZnSe quantum dots to specially designed acceptor molecules attached to their surface.
• The proton's movement increases both the speed and efficiency of triplet energy transfer compared to equivalent molecules without the shuttle — and the process works at room temperature.
• The proton appears to travel through quantum mechanical tunneling rather than conventional heat-driven motion, meaning the effect depends on fundamental quantum behavior even at everyday temperatures.

Electrons and protons often work together in both living systems and engineered materials. The most familiar example is proton-coupled electron transfer (PCET), a process that underpins photosynthesis, cellular respiration, and nitrogen fixation. Scientists have long known that these tiny particles can coordinate their movements. But a team led by Professor Kaifeng Wu at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has now identified a related but distinct process that works specifically with triplet energy states — a finding that could give engineers a new way to control energy flow in advanced materials.

The process, published in Nature Materials, is called proton shuttle-assisted triplet energy transfer (PS-TET). The researchers observed it as energy moved from ZnSe-based colloidal quantum dots to phenol-pyridine dyadic acceptors attached to the quantum dots' surfaces.

When the ZnSe quantum dots absorb light, they enter an excited state. A hole (the absence of an electron) then moves from the quantum dot to the phenol molecule, while a proton simultaneously shifts from the phenol to the nearby pyridine. Next, an electron transfers from the quantum dot to the phenoxyl radical, and at the same moment the proton moves back from pyridinium to its original position. The proton ends up exactly where it started, but its temporary round trip has a dramatic effect: it greatly increases both the speed and efficiency of the overall triplet energy transfer.

The researchers found that adding a strongly electron-withdrawing group (trifluoromethyl) to the pyridine can even change the order in which the coupled steps occur, giving scientists a degree of control over the process.

Remarkably, the rate of PS-TET changed very little with temperature. This suggests the proton does not move through a conventional heat-driven process. Instead, it travels through quantum mechanical tunneling — a phenomenon in which particles pass through energy barriers they classically should not be able to cross. Calculations involving proton vibrational wavefunction overlap integrals supported this interpretation, showing that quantum effects steer the system toward efficient energy migration even at room temperature.

The discovery has practical implications for several technologies. Increasing triplet generation efficiency could improve photoredox and environmental catalysis. In organic optoelectronic devices such as solar cells and lasers, controlling triplet formation is critical — too much can degrade performance, while too little limits efficiency. The study suggests that scientists may be able to tune triplet formation as needed: creating a proton shuttle enhances the process, while removing the shuttle reduces or prevents it.

"The discovery of the PS-TET mechanism has profound implications for many modern molecular technologies involving the spin-triplet excited states of molecules," said Professor Wu. The finding opens a new avenue for controlling how energy moves at the molecular scale, using quantum effects that are available even at room temperature.