A research team from Kyushu University in Japan recently announced that they have developed a new type of solid-state molecular material that can convert visible light into ultraviolet light under natural sunlight conditions. It has achieved a visible-to-ultraviolet light upconversion efficiency of 1.9% under outdoor sunlight, which is considered an important milestone in the field of solid-state photon upconversion and molecular self-assembly research. The relevant results were published in the journal Nature Communications on June 23, 2026.
The researchers vividly pointed out that this process is similar to "in the quantum world, pouring two cups of warm water together to get a cup of boiling water": things that are impossible to happen in macroscopic daily life can be realized through quantum processes at the microscopic photon level. In this work, two low-energy visible light photons can "join forces" to form a higher-energy ultraviolet photon, thereby achieving "upgraded utilization" of light energy.
Ultraviolet light plays a key role in areas such as air purification, 3D printing resin curing, dental filling materials, and nail light curing. However, in natural sunlight, ultraviolet light only accounts for about 6% of the total solar radiation reaching the earth's surface, and only a small part of it can be utilized by technology. The goal of the Kyushu University team is to use "photon upconversion" technology to convert the originally abundant visible light resources into ultraviolet light with more application value, providing a cheaper and safer light source for a variety of technologies that rely on ultraviolet light.
This research used a photon upconversion mechanism called "triplet-triplet annihilation" (TTA). Specifically, in the system, the "donor" molecule first absorbs visible light, and the electrons transition to a high-energy triplet state; then, the energy is transferred to the nearby "acceptor" molecule, forming a triplet state excitation of the acceptor; when the two triplet states meet in space and "annihilate", the superimposed energy is released in the form of a beam of ultraviolet light photons. This solution is relatively easy to implement in a liquid system, because molecules can move freely in the solution, which is more conducive to triplet collisions. However, liquid systems often rely on toxic solvents and have volatilization problems, making it difficult to meet the needs of practical applications. Therefore, efficient solid-state materials have always been the "Holy Grail" in this field.
In the solid state, molecules are tightly arranged, and the π electron clouds above and below the molecular plane are prone to strong overlap, causing the excited state energy to be quenched before up-conversion is achieved, causing the system's luminous efficiency to drop significantly. To solve this problem, the research team selected the organic semiconductor molecule dihydroindenoindenedene (DHI) and introduced an alkyl chain to its sp3 carbon atoms with tetrahedral alignment to precisely control the spacing and relative orientation between molecules through steric hindrance. This molecular design allows adjacent molecules to be close enough to efficiently transfer energy between molecules, but remain moderately "separated" to avoid overcoupling of the π electron cloud and triggering exciton quenching.
Thanks to this structural engineering, the new material exhibits bright luminescence, long-lived excited states and efficient energy transfer in the solid state, with a solid-state fluorescence quantum yield exceeding 60%. After pairing with an adapted donor molecule, the system achieved a visible-to-UV upconversion efficiency of 1.9% under natural sunlight, meaning that out of one hundred visible photons absorbed, about two were ultimately converted into UV photons. The research team pointed out that although this number does not sound "dazzling", it has surpassed the level that most similar systems can achieve under high light intensity conditions without the need for concentrated light, relying entirely on natural sunlight, and being a solid material.
In terms of application prospects, the team has submitted a patent application for this material. The synthesis route of this material is relatively simple and the starting raw materials it relies on are cheap, laying the foundation for future large-scale preparation and industrialization. Researchers believe that this solid-state upconversion platform is expected to play a role in solar-driven photocatalysis, indoor air purification, and low-light intensity 3D printing, converting ordinary sunlight into a more "processing-capable" ultraviolet light source.
This breakthrough is also the culmination of a research plan that has lasted for more than ten years. As early as 2012, Nobuo Kimizuka, currently an honorary professor at the "Negative Emission Technology Research Center" of Kyushu University, began to explore the use of self-assembly systems to achieve triplet energy migration and photon upconversion, hoping to give materials new functions through molecular self-assembly. In the subsequent years of research, he led his team to achieve a series of progress in solution and gel systems, but they were still unable to overcome the key difficulty of efficient solid-state systems.
The turning point will occur in May 2024. Graduate students Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, and Kiichi Mizukami, who was an assistant professor in the Faculty of Engineering at Kyushu University at the time, and others joined forces with Yoichi Sasaki to integrate years of research accumulation in a short period of time and finally completed this work. Team members recalled that they handed over the final draft of the paper to Professor Kimitsuka only 11 days before his retirement. This result is also like a meaningful "retirement gift" to the laboratory.
Professor Kimitsuka said that this discovery is not only the culmination of more than 14 years of research work by his team, but also marks a new stage in the research on photon upconversion and molecular self-assembly. With the help of this new solid-state system, the vision of using ordinary sunlight to obtain an "upgraded" version of ultraviolet light is gradually moving from laboratory concepts to practical applications.