Imagine pouring two cups of warm water together and somehow ending up with one cup of boiling water. It doesn’t happen in everyday life, but something similar is possible at the quantum level. Multiple low-energy light particles can combine their energy to create a single particle with much higher energy.
Researchers at Kyushu University have developed a solid-state molecular material that can convert visible sunlight into ultraviolet (UV) light under normal outdoor conditions. The new material achieved a photo upconversion efficiency of 1.9%, according to research published in the journal June 23. nature communications.
Why is UV rays important?
Although many people associate UV rays with sunburn and skin damage, UV rays play an important role in many technologies. UV light is also used in applications such as air purification, curing resins in 3D printing, curing gels in dental fillings, and even treating nails.
Despite its usefulness, UV light only accounts for about 6% of sunlight that reaches the Earth’s surface. Still, only a portion of that ultraviolet light is practical for technological applications.
“What we’re doing here is ‘adding’ the energy from two visible light photons to create one ultraviolet photon. This is an interesting process called optical upconversion,” explains Yoichi Sasaki, associate professor at Kyushu University’s Faculty of Engineering and corresponding author of the study.
Convert visible light to ultraviolet light
This process relies on a phenomenon known as triplet-triplet annihilation (TTA). In this approach, a molecule known as a donor absorbs visible light and enters a high-energy triplet state. That energy is then transferred to a nearby acceptor molecule.
When two triplet states encounter each other, they combine and release energy as a single UV photon.
Scientists have long known that TTA works effectively in liquids because the molecules can move freely and interact easily. However, liquid systems often require toxic solvents and can evaporate over time, limiting their practicality. As a result, researchers have spent years searching for reliable solid-state alternatives.
“In solids, the molecules are tightly packed, and the π-electron clouds (electron-dense regions above and below each molecular plane) can overlap,” Sasaki says. “Then the triplets easily disappear before they can meet. The molecules need to be close enough that energy can be transferred, but far enough apart to prevent exciton annihilation.”
New solid-state solution
The team’s breakthrough came with an organic semiconductor called dihydroindenoindedene (DHI).
The researchers modified DHI by adding an alkyl chain to the sp3 carbon atom with four bonds oriented in a fixed 3D direction. This design created carefully controlled spacing between adjacent molecules. The molecules remained close enough to transfer energy efficiently while avoiding strong electronic interactions that could inhibit performance.
The resulting material exhibited strong luminescence, long-lived excited states, and highly efficient energy transfer. A solid-state fluorescence quantum yield of over 60% was achieved.
When combined with donor molecules, the system reached an upconversion efficiency of 1.9%.
“This means that for every photon of visible light that is absorbed, approximately two UV photons are produced,” Sasaki added. “It may sound low, but this only works with natural sunlight. Most solid-state materials cannot achieve this, even at much higher light intensities.”
Potential applications for solar-powered UV lights
The researchers have filed a patent application for the material.
In addition to its performance, this material also has practical advantages. It is relatively easy to synthesize and is made from inexpensive starting materials. The research team believes it could eventually be used in photocatalysis for solar power generation, indoor air purification systems, and low-brightness 3D printing technology.
14 years of scientific journey
For the researchers involved, this result represents more than a technological advance.
In 2012, Nobuo Kimizuka, currently professor emeritus at the Kyushu University Minus Emission Technology Research Center, began research on photon upconversion by triplet energy transfer in self-assembled molecular systems. His goal was to establish a form of molecular systems chemistry in which self-assembly could perform useful functions.
Over the next few years, his group made steady progress using solution-based and gel-based systems. However, achieving efficient solid-state upconversion remained difficult.
In May 2024, less than a year until Kimizuka’s retirement, a major breakthrough finally arrived.
The next few months were an intense effort to complete the project. Graduate students Naoyuki Harada, Hayato Shoyama, and Bunmon Nutonitsha collaborated with Sasaki and then-Assistant Professor Kiichi Mizukami of Kyushu University’s Faculty of Engineering to compile years of research into a final publication.
“We handed over the draft to Professor Kimizuka just 11 days before he left the lab, and it was like a heartfelt retirement gift for us,” says Sasaki.
“This discovery is the culmination of more than 14 years of our research and represents a major milestone in the study of photon upconversion and molecular self-assembly,” Professor Kimizuka concluded.
