Scientists at Yokohama National University, in collaboration with RIKEN and other leading research institutions in Japan and Korea, have made a groundbreaking discovery that could revolutionize the way we understand and manipulate electron behavior in molecules. This discovery, recently published in the journal Science, opens up new possibilities for controlling molecular states at ultrafast timescales, with far-reaching implications for electronics, energy transfer, and chemical reactions.
At the heart of this study is a novel approach to controlling the movement and distribution of electrons within molecules. Traditionally, understanding and controlling how electrons behave at such a microscopic level has been incredibly challenging, particularly on the timescales required to make meaningful advancements in fields like electronics and energy technology. However, the researchers’ discovery could enable rapid, precise manipulation of electrons using light pulses in the terahertz frequency range—a form of light far less commonly used in scientific experiments due to its low energy compared to visible light.
Atoms and molecules are made up of a central nucleus, positively charged, and negatively charged electrons that orbit around it. The distribution of these electrons is crucial for determining how the molecule behaves in various contexts, whether it be its ability to absorb light, conduct electricity, or participate in chemical reactions. This electron arrangement is highly significant in numerous applications, ranging from energy harvesting systems like solar cells to electronic components like light-emitting diodes (LEDs).
In processes like solar energy conversion, light striking a molecule can excite an electron, raising it to a higher energy level. This process can leave behind a “hole,” creating an exciton, which is a type of energy packet that moves through the molecule. These excitons are pivotal in technologies like solar cells, where their movement is harnessed to convert sunlight into electricity, and in LEDs, where they help to release energy as visible light.
However, there are many other critical molecular states beyond these common excitons. These include charged states, where a molecule either gains or loses an electron, and charged excited states, where both an electron is excited to a higher energy level and the molecule also experiences a charge change. Both of these states play important roles in processes such as energy transfer and catalysis. Despite their importance, until now, controlling these states, especially on ultrafast timescales, had been exceedingly difficult.
In typical experimental setups, visible light is often used to manipulate electron states in molecules. However, visible light alone does not have sufficient energy to alter the charge of a molecule or manipulate electron numbers directly. This limitation has been a key challenge in advancing certain fields of molecular electronics and energy technologies. To address this challenge, the Yokohama researchers turned to terahertz light—light with a much lower frequency than visible light. Though invisible to the human eye, terahertz light offers a unique ability to induce electron movement between molecules and other materials, thanks to its ability to interact with electrons without significantly raising their energy levels, as visible light does.
The researchers used specialized equipment to direct terahertz light pulses at individual molecules through a cutting-edge microscope, a device capable of manipulating single molecules at the atomic level. The terahertz pulses enabled the team to add or remove electrons from molecules with remarkable precision, allowing them to explore new ways of controlling not just excitons, but a wide range of other molecular states that are essential for chemical reactions and energy transfer processes.
One of the most striking results of the study was the discovery that terahertz pulses can also convert invisible terahertz light into visible light within molecules, a phenomenon previously unexplored. This occurs through a process where the terahertz light induces a shift in molecular energy states, transforming one form of light into another. This could lead to innovations in light-based technologies, potentially opening the door to smaller, more efficient optical devices.
Professor Ikufumi Katayama, one of the study’s corresponding authors and a professor in the Faculty of Engineering at Yokohama National University, highlighted the significance of this discovery, saying, “While excitons typically form when light is absorbed by a material, our findings reveal they can also be created through charged states using these specially designed terahertz pulses. This opens new possibilities for controlling how charge moves within molecules, which could lead to better solar cells, smaller light-based devices, and faster electronics.”
Perhaps the most notable achievement of this research is the ability to control exciton formation at the single-molecule level. This is an incredibly difficult task because, in typical conditions, the creation of excitons occurs randomly and unpredictably. However, by applying terahertz pulses, the team was able to precisely control when and how excitons were formed, ensuring that the molecular reactions followed a specific, desired path.
Professor Jun Takeda, another corresponding author from the Faculty of Engineering at Yokohama National University, further explained the significance of this achievement: “By precisely controlling how electrons move between a single molecule and the metal tip of the specialized microscope, we were able to guide exciton formation and the chemical reactions that follow. These processes usually happen randomly, but with terahertz pulses, we can determine exactly when and how reactions occur at the molecular level. This could lead to breakthroughs in nanotechnology, advanced materials, and more efficient catalysts for energy and industry.”
This unprecedented control of molecular states at such an intricate level has vast implications for the future of technology. One of the key applications of this discovery could be in the development of advanced materials and catalysts. By controlling the formation of excitons and charged states with such precision, it may be possible to design materials that exhibit enhanced performance in areas like energy storage, conversion, and catalysis. For example, the ability to control the electron movement in molecules could lead to more efficient solar cells, reducing energy loss and increasing the efficiency of converting sunlight into usable electricity.
The researchers also see potential applications in the development of faster and more powerful electronics. By controlling the electron distribution in molecules with terahertz light, it may be possible to design smaller, more efficient light-based devices, paving the way for the next generation of optoelectronics and computing technologies. In particular, the ability to manipulate electrons with such precision could lead to faster transistors and memory devices, accelerating the development of high-performance electronics.
Furthermore, the study’s findings also have potential implications for the field of quantum computing. The ability to manipulate molecular states and excitons on such a fine scale could lead to breakthroughs in quantum information processing, where the precise control of quantum states is essential. Quantum computers rely on the ability to control and manipulate the quantum states of atoms and molecules, so the techniques developed by the Yokohama researchers could play a crucial role in advancing this emerging field.
More information: Kensuke Kimura et al, Ultrafast on-demand exciton formation in a single-molecule junction by tailored terahertz pulses, Science (2025). DOI: 10.1126/science.ads2776. www.science.org/doi/10.1126/science.ads2776