In a major advancement for UV energy research, scientists at Kyushu University have developed a solid material that converts visible light into high-energy UV light with 1.9% efficiency, eliminating the need for hazardous solvents.
Revealed in a recent paper published in Nature Communications, the Kyushu team created precisely controlled gaps between molecules in a solid material by attaching alkyl chains to the sp³ carbon atoms of an organic molecule.
Unlike at the macro scale, where adding two separate cups of warm water together doesn’t combine their energy levels to produce a boil, at the quantum scale, combining low-energy photons can result in a single, higher-energy photon, which is essential to the new energy conversion process.
Upgrading Light Energy
UV light may be harsh on human skin, but its high energy density makes it valuable for a variety of applications, including air purification and resin curing in 3D printing. Although ultraviolet radiation accounts for only about 6% of the sunlight that reaches Earth’s surface, only a fraction of that can be effectively harnessed. The new material developed at Kyushu University converts ordinary visible light into ultraviolet light, potentially increasing the availability of this valuable energy source.
“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,” explained co-author Yoichi Sasaki, Associate Professor at Kyushu University’s Faculty of Engineering.
Annihilating Light Photons
Triplet-triplet annihilation is a key mechanism behind photo upconversion, a process in which two excited molecules interact to produce a single higher-energy UV photon. It begins when a donor molecule absorbs light, exciting its electrons into a high-energy triplet state. That energy is then transferred to a neighboring molecule, after which the two excited states interact and combine to generate a new ultraviolet photon.
Although scientists have understood this process for years, practical limitations have hindered its widespread use. Traditionally, triplet-triplet annihilation occurs in liquids that can evaporate and often require toxic solvents, prompting the Kyushu team to search for a solid-state alternative.
“In solids, molecules are packed tightly, and the π electron clouds—regions of high electron density hovering above and below each molecular plane—can overlap,” says Sasaki. “When that happens, triplets easily fizzle out before they ever meet. Molecules must be close enough for energy to transfer but separated enough to prevent quenching of excitons.”
A Solid Solution
The researchers found their answer in dihydroindenoindenedene (DHI), an organic semiconductor. By attaching alkyl chains, they were able to precisely control the spacing between molecules, creating an arrangement that allowed efficient energy transfer while minimizing unwanted electronic interactions.
The resulting alkyl-optimized DHI demonstrated efficient energy transfer, long-lived excited states, and strong light emission, achieving an upconversion efficiency of 1.9%.
“This means roughly two UV photons are produced for every hundred visible-light photons absorbed,” Sasaki adds. “It may sound low, but it runs on natural sunlight alone. Most solid-state materials cannot realize this even at much higher light intensity.”
With its relatively easy synthesis and low material cost, the team says their work provides a crucial new solution for harnessing UV light. A patent for the material is currently pending.
The work was the culmination of the career of pioneering upconversion researcher Nobuo Kimizuka, now Professor Emeritus at Kyushu University’s Research Center for Negative Emissions Technologies, who retired 11 days after the paper was completed.
“This discovery is the culmination of over 14 years of our research,” Kimizuka said, adding that it “marks a major milestone in photon-upconversion and molecular self-assembly research.”
The paper, “Sterically Protected π-Electron Systems for Efficient Solid-State Photon Upconversion,” appeared in Nature Communications on June 23, 2026.
Ryan Whalen covers science and technology for The Debrief. He holds an MA in History and a Master of Library and Information Science with a certificate in Data Science. He can be contacted at ryan@thedebrief.org, and follow him on Twitter @mdntwvlf.