In a remarkable breakthrough that challenges conventional perceptions of light and energy, scientists at Kyushu University in Fukuoka, Japan, have engineered a novel solid-state material capable of converting visible sunlight into ultraviolet (UV) light with unprecedented efficiency under everyday solar conditions. This cutting-edge development holds the promise of revolutionizing several industrial and environmental applications that rely heavily on UV light, which despite its critical importance, constitutes a mere 6% of solar radiation reaching the Earth’s surface.
This quantum leap in photonic manipulation hinges on a phenomenon known as photon upconversion, a process whereby two lower-energy photons—here, in the visible spectrum—combine to form a single higher-energy photon in the ultraviolet range. While seemingly counterintuitive in macroscopic terms, this effect exploits the quantum mechanical principle of triplet-triplet annihilation (TTA), where energy transfer between molecules facilitates the creation of more energetic light. The research team, led by Associate Professor Yoichi Sasaki of Kyushu University’s Faculty of Engineering, explored the intricacies of this process to overcome the longstanding challenge of achieving efficient upconversion in the solid state.
The mechanics of TTA involve a “donor” molecule absorbing visible light, thereby elevating its electrons into an excited triplet state. This energy is then transferred to a nearby “acceptor” molecule. When two excited triplets converge, they annihilate each other, releasing a photon of higher energy—in this case, UV light. Although this mechanism is well-established in liquid media due to molecular mobility facilitating frequent collisions, liquids are impractical in real-world applications because they often rely on volatile, toxic solvents and suffer from evaporation issues. Solid-state alternatives have remained elusive due to the tightly packed molecular architectures typical of solid materials, which often quench excited states before energy transfer can occur efficiently.
The Kyushu University team tackled this barrier by synthesizing an unprecedented organic semiconductor, dihydroindenoindenedene (DHI), chemically modified with alkyl chains attached to its sp³ carbon atoms. This structural modification induces precisely defined molecular spacing, maintaining an optimal balance where molecules remain close enough for efficient triplet energy transfer yet sufficiently separated to prevent premature quenching through excessive π-electron cloud overlap. The spatial control achieved here is a prime example of molecular engineering that preserves crucial electronic interactions while minimizing deleterious overlap effects.
This fine-tuned molecular arrangement culminated in material showing a fluorescence quantum yield exceeding 60% under solid-state conditions—a significant milestone, as it signals that the material can sustain long-lived excited states necessary for effective photon upconversion. When paired with an appropriate donor molecule, the hybrid system demonstrated a visible-to-UV upconversion efficiency of 1.9% under sunlight intensities typical of outdoor environments. While this percentage appears modest at first glance, it represents a radical advancement, as most previous solid-state systems failed to approach comparable efficiencies even when exposed to far more intense light sources.
The implications of this discovery extend far beyond the laboratory. UV light plays an essential role in a variety of applications, including air purification, resin curing technologies pivotal for additive manufacturing and 3D printing, as well as in dental and cosmetic industries involving gel hardening and nail art. The ability to harness ambient sunlight to generate UV photons, rather than relying on specialized ultraviolet light sources that consume considerable energy and complicate device design, offers a sustainable and potentially low-cost alternative. This breakthrough paves the way for solar-driven photocatalytic reactions and indoor environmental solutions where direct UV lamps are impractical or undesirable.
The achievement represents the culmination of over 14 years of dedicated research into photon upconversion and molecular self-assembly, with roots tracing back to pioneering work by Professor Nobuo Kimizuka, now Emeritus at Kyushu University. His early efforts focusing on photon upconversion via triplet energy migration laid the groundwork for this solid-state realization. The recent success was propelled by a dynamic collaboration involving graduate students Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, and Assistant Professor Kiichi Mizukami, who condensed years of incremental advances into this landmark discovery shortly before Professor Kimizuka’s retirement.
Beyond the scientific innovation, the project underscores the potent synergy between molecular engineering and quantum photophysics in crafting materials capable of sophisticated light manipulation. The use of sp³ carbon atom functionalization to dictate molecular packing challenges traditional paradigms that have, until now, constrained efforts to realize practical solid-state upconversion devices capable of operating under ambient conditions—a critical step toward real-world application.
While the current quantum yield and efficiency metrics represent a significant leap forward, ongoing research is poised to further optimize the molecular architectures and donor-acceptor combinations. Such advances may soon allow for finer control over energy transfer dynamics, enhanced durability of the solid films, and scalability for industrial production. Moreover, the straightforward chemical synthesis and utilization of cost-effective starting materials make this system particularly attractive for commercialization and broader deployment.
This discovery is not only a scientific triumph but also a strategic milestone for sustainable technology innovation. As societies worldwide seek to reduce energy consumption and develop greener technologies, the ability to convert abundant visible light into the more reactive UV spectrum on demand opens exciting pathways for clean manufacturing, environmental remediation, and renewable energy harvesting. The potential to integrate these materials into everyday devices and systems promises to catalyze disruptive technologies across multiple sectors.
The detailed findings, published in the prestigious journal Nature Communications, signal a new era in the study of photonic materials and quantum energy conversion. The research team’s profound understanding of molecular electronic interactions and meticulous control over microstructural assembly offer a blueprint for future exploration not only in the domain of photon upconversion but across diverse fields where control of light and energy at the molecular scale is paramount.
In sum, Kyushu University’s latest innovation crystallizes the ongoing evolution of photochemical science and molecular engineering. It elegantly illustrates how the fusion of quantum theory and skilled material design can usher in practical solutions to long-standing challenges, transforming ephemeral molecular phenomena into tangible societal benefits powered by the cleanest and most abundant energy source available: sunlight.
Subject of Research: Not applicable
Article Title: Sterically protected π-electron systems for efficient solid-state photon upconversion
News Publication Date: 23-Jun-2026
Web References:
- Kyushu University: https://www.kyushu-u.ac.jp/en/
- Faculty of Engineering, Kyushu University: https://www.eng.kyushu-u.ac.jp/e/
- Research Center for Negative Emissions Technologies: https://k-nets.kyushu-u.ac.jp/en/
- Yoichi Sasaki profile: https://hyoka.ofc.kyushu-u.ac.jp/html/100020203_en.html
References:
- Harada, N., Shoyama, H., Boonmong, N., Mizukami, K., Watanabe, Y., Zhao, P., Ehara, M., Sasaki, Y., Kimizuka, N. (2026). Sterically protected π-electron systems for efficient solid-state photon upconversion. Nature Communications. DOI: 10.1038/s41467-026-73898-0
Image Credits: Naoyuki Harada / Kyushu University
Keywords: photon upconversion, triplet-triplet annihilation, solid-state photonics, ultraviolet light, visible light conversion, molecular engineering, organic semiconductor, dihydroindenoindenedene, quantum yield, solar energy conversion, photochemical materials, quantum photophysics
