Electron and proton motion often work together in both living systems and engineered materials. The most familiar example is proton-coupled electron transfer (PCET), a process that plays a central role in bioenergetics, cellular respiration, photosynthesis, and nitrogen fixation. PCET has also influenced the design of many artificial materials used for energy conversion and storage. More recently, scientists identified another related process known as proton-coupled singlet energy transfer (PCEnT).
Building on earlier studies of PCET and PCEnT, a team led by Prof. Kaifeng Wu at the Dalian Institute of Chemical Physics of the Chinese Academy of Sciences investigated another important but poorly understood process: triplet energy transfer linked to proton movement.
Triplet energy transfer is a major pathway for moving energy in both natural and synthetic systems, but it operates differently from singlet energy transfer. Understanding how proton motion influences this process could open new ways to control energy flow in advanced materials.
In a study published in Nature Materials, the researchers reported a previously unknown mechanism called proton shuttle-assisted triplet energy transfer (PS-TET). The process was observed as energy moved from ZnSe-based colloidal quantum dots (QDs) to phenol-pyridine dyadic acceptors attached to their surfaces.
How the Proton Shuttle Moves Energy
When the ZnSe QDs absorb light, they enter an excited state. A hole then moves from ZnSe to phenol while a proton simultaneously shifts from phenol to pyridine.
Next, an electron transfers from ZnSe to the phenoxyl radical. At the same time, the proton moves back from pyridinium to its original location. Together, these linked steps produce the overall movement of spin-triplet energy from the ZnSe QDs to the phenol-pyridine dyads.
The proton ultimately ends up where it started, but its temporary movement has a major effect. The shuttle greatly increases both the speed and efficiency of triplet energy transfer compared with a methylated analog that does not contain the proton shuttle.
The team also found that adding a strongly electron-withdrawing trifluoromethyl substituent to pyridine can change the order in which the proton-coupled electron and hole transfer steps occur.
Quantum Tunneling at Room Temperature
The rate of PS-TET changed very little with temperature. This suggests that the proton does not move through a conventional heat-driven process. Instead, it appears to travel through quantum mechanical tunneling.
Calculations involving proton vibrational wavefunction overlap integrals supported this interpretation. These integrals help determine which excited-state relaxation pathways are favored and steer the system toward efficient triplet energy migration.
The findings show that quantum effects can be used to control charge and energy transfer in complex materials even at room temperature.
Potential Uses in Solar Cells, Lasers, and Catalysis
“The discovery of the PS-TET mechanism has profound implications for many modern molecular technologies involving the spin-triplet excited states of molecules,” Prof. Wu noted.
Increasing triplet generation efficiency could improve photoredox and environmental catalysis. In other technologies, however, triplet formation may need to be limited. Organic optoelectronic devices such as solar cells and lasers can perform better when unwanted triplet states are suppressed.
The study suggests that scientists may be able to tune triplet formation as needed. Creating a proton shuttle could enhance the process, while removing the shuttle could reduce or prevent it.