The movement of electrons and protons often occurs simultaneously, both in biological systems and in engineered materials. One of the most well-known examples is the coupled transfer of a proton and an electron (PCET), which plays a crucial role in bioenergetics, cellular respiration, photosynthesis, and nitrogen fixation. This process is also utilized in the development of artificial materials for energy conversion and storage. Recently, another related process was discovered—coupled singlet energy transfer involving a proton (PCEnT).

Exploring New Mechanisms of Energy Transfer

Building on previous research into PCET and PCEnT, a research team has investigated another little-studied process: triplet-state energy transfer associated with proton movement. Triplet-state energy transfer is a primary means of energy transmission in both natural and synthetic systems, but it differs from singlet-state energy transfer. Understanding how proton movement influences this process could lead to new methods for controlling energy flows in advanced materials.

In a recent publication, the team described a previously unknown mechanism, now termed proton-shuttle-assisted triplet energy transfer (PS-TET). This process was observed during energy transfer from ZnSe-based colloidal quantum dots to phenol-pyridine acceptors anchored on their surface.

The Mechanism of Proton-Shuttle-Assisted Energy Transfer

When quantum dots of ZnSe absorb light, they enter an excited state. Next, a hole moves from ZnSe to the phenol group, while simultaneously a proton shifts from phenol to pyridine. After this, an electron transfers from ZnSe to the phenoxyl radical, and the proton returns from pyridinium to its original position. These interconnected steps enable the overall transfer of triplet-state energy from ZnSe quantum dots to phenol-pyridine dyads. Although the proton ultimately returns to its starting point, its temporary movement significantly impacts the process. This "shuttle" mechanism greatly increases both the speed and efficiency of triplet-state energy transfer compared to systems without a proton shuttle. It was also found that adding a strongly electron-accepting trifluoromethyl group to pyridine can alter the sequence of coupled proton, electron, and hole transfer steps.

Quantum Mechanical Features of the Process

The rate of PS-TET remained nearly unchanged with temperature variation, indicating that the proton likely moves not by conventional thermal means, but rather through quantum mechanical tunneling. Calculations that considered the overlap of proton vibrational wavefunctions supported this hypothesis. Such integrals help determine the preferred relaxation pathways of the excited state and guide the system toward efficient triplet energy migration. The results show that quantum effects can be harnessed to control charge and energy transfer in complex materials, even at room temperature.

Significance of the Discovery for Technology

The discovery of the PS-TET mechanism is highly significant for modern molecular technologies involving excited triplet states of molecules. Enhancing the efficiency of triplet-state generation can improve photocatalysis and environmentally friendly catalysis. In other technologies, however, the formation of triplet states may be undesirable. Organic optoelectronic devices, such as solar cells and lasers, operate more efficiently when unwanted triplet states are suppressed. The research demonstrates that the formation of triplet states can be tuned as needed: creating a proton shuttle enhances this process, while its removal reduces or prevents it.