Researchers at Kyushu University have developed a solid-state molecular material that can convert visible light into ultraviolet light under ordinary outdoor sunlight, opening the way for potential solar-powered applications in air purification, photocatalysis and low-intensity 3D printing.
The study reported a visible-to-UV conversion efficiency of 1.9%. According to the university, the material works through a process called photo upconversion, in which the energy from two lower-energy visible light photons is combined to produce one higher-energy ultraviolet photon.
“What we do here is ‘add together’ the energy from two visible light photons to make one ultraviolet photon. It’s a fascinating process called photo upconversion,” said Yoichi Sasaki, Associate Professor at Kyushu University’s Faculty of Engineering and corresponding author of the study.
Although UV light makes up about 6% of the sunlight reaching Earth’s surface, only part of it is practically usable. UV light is widely used in areas including air purification, resin curing for 3D printing, dental filling materials and nail art.
One known route for achieving upconversion is triplet-triplet annihilation, or TTA. In this process, a donor molecule absorbs visible light and transfers energy to an acceptor molecule. When two excited triplet states meet, they can combine and release their energy as a UV photon. The process works well in liquid systems, where molecules move freely, but those systems can depend on toxic solvents and may evaporate.
Sasaki said solid materials present a different challenge because tightly packed molecules can cause excited states to lose energy before they interact usefully. The team addressed this by using an organic semiconductor called dihydroindenoindenedene, or DHI, and attaching alkyl chains to its sp³ carbon atoms. This created controlled gaps between neighboring molecules, allowing energy transfer while reducing unwanted electronic interactions.
The optimized material showed a solid-state fluorescence quantum yield above 60%. When combined with a donor molecule, it achieved 1.9% upconversion efficiency.
“This means roughly two UV photons are produced for every hundred visible-light photons absorbed,” Sasaki said. “It may sound low, but it runs on natural sunlight alone. Most solid-state materials cannot realize this even at much higher light intensity.”
The material has been filed for a patent. The team said it also offers potential practical benefits, including straightforward synthesis and low-cost starting materials.
Nobuo Kimizuka, Professor Emeritus at Kyushu University, said the discovery “is the culmination of over 14 years of our research and marks a major milestone in photon-upconversion and molecular self-assembly research.”
Article source: Kyushu University
