New Solid Materials to Turn Visible light into UV Light

Jun 28

A breakthrough from Kyushu University solves a 14-year puzzle in photon upconversion, opening the door to solar-powered air purification and advanced manufacturing.

Imagine taking two lukewarm cups of water, pouring them together, and somehow getting a single cup of boiling water. In our macroscopic world, thermodynamics firmly forbids this. But at the quantum scale, light plays by different rules. Through a phenomenon known as photon upconversion, multiple low-energy photons can pool their energy to create a single, much higher-energy photon.

For over a decade, materials scientists have been chasing a specific flavor of this quantum trick: converting visible sunlight into ultraviolet (UV) light using solid-state materials. Now, a research team from Kyushu University has finally cracked the code, reporting their findings in Nature Communications . By meticulously engineering the spacing between organic molecules, they have developed a solid-state system that achieves this conversion under natural sunlight intensities—a breakthrough that could fundamentally change how we harness solar energy.

The UV Deficit in Solar Energy

While we often associate UV light with sunburns and the need for SPF 50, it is an incredibly useful, high-energy segment of the electromagnetic spectrum. We rely on UV light for curing resins in 3D printing, hardening dental composites, and driving photocatalytic reactions like water splitting and air purification using titanium dioxide (TiO₂) .

The problem is that UV light is scarce in natural sunlight. Only about 5% to 8% of the solar radiation reaching Earth's surface falls into the UV band, and only a fraction of that (the 300–400 nm range) is practically useful for these technologies . If we want to use the sun to power large-scale photocatalysis or advanced manufacturing, we need a way to "upgrade" the abundant visible light into the scarce but powerful UV light.

"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," explains Yoichi Sasaki, Associate Professor at Kyushu University's Faculty of Engineering and the study's corresponding author .

The Triplet-Triplet Annihilation Bottleneck

The specific mechanism the team utilized is called triplet-triplet annihilation upconversion (TTA-UC). The process involves two types of molecules: a sensitizer (or donor) and an annihilator (or acceptor).

First, the donor molecule—in this case, an iridium complex known as Ir(ppy)₃—absorbs a visible light photon. Through a process called intersystem crossing, the donor enters a high-energy "triplet" state. It then transfers this triplet energy to a nearby acceptor molecule via the Dexter electron transfer mechanism, which requires the electron clouds of the two molecules to physically overlap. When two of these triplet-excited acceptor molecules encounter one another, they "annihilate," pooling their energy to bump one of the molecules into an even higher-energy "singlet" state. As this singlet state relaxes, it emits a single, high-energy UV photon .

Scientists have known for years that TTA-UC works beautifully in liquid solutions, achieving quantum yields exceeding 16% . In liquids, molecules can freely diffuse, allowing triplets to easily find each other and collide. However, liquid systems are impractical for most real-world devices. They often require toxic solvents, they evaporate, and they are difficult to integrate into solid-state electronics or coatings.

Moving TTA-UC into the solid state has been notoriously difficult. "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," Sasaki notes . When aromatic molecules pack too closely, their excited states tend to "quench" or fizzle out non-radiatively before they can ever meet and annihilate. The central materials science challenge has been a spatial paradox: the molecules must be close enough to allow Dexter energy transfer (which requires orbital overlap), but separated enough to prevent the quenching of the delicate exciton states.

Solving the Spatial Paradox with sp³ Carbons

The breakthrough achieved by the Kyushu team, led by co-first authors Naoyuki Harada, Hayato Shoyama, and Nutnicha Boonmong, centers on a clever piece of molecular engineering. They focused on an organic semiconductor molecule called dihydroindeno[2,1-a]indene (DHI), which is highly fluorescent in dilute solutions but suffers severe quenching when crystallized.

To control the intermolecular spacing in the solid state, the researchers modified DHI by attaching bulky alkyl chains to its sp³-hybridized carbon atoms. Unlike the flat, sp²-hybridized carbons that make up the aromatic rings, sp³ carbons have a tetrahedral geometry, meaning their bonds point outward in three dimensions. By attaching isobutyl groups to these sp³ carbons, the team created a permanent steric shield above and below the flat π-conjugated plane of the DHI molecule .

This precise molecular design successfully solved the spatial paradox. The isobutyl chains act like molecular bumpers, preventing the π-planes of neighboring DHI molecules from getting too close and quenching each other. Yet, they still allow sufficient electronic interaction at the edges for rapid triplet energy migration.

The results speak for themselves. The engineered material, iBu-DHI, achieved a solid-state fluorescence quantum yield of over 60%—a massive improvement over the 10% yield of the unmodified DHI crystal. When paired with the Ir(ppy)₃ donor, the solid film achieved an absolute Vis-to-UV upconversion efficiency of 1.9% .

While 1.9% might sound modest, it is a monumental achievement for a solid-state system. More importantly, the system exhibits a threshold excitation intensity of just 1.2 mW/cm², which is below the irradiance of natural sunlight at the excitation wavelength (1.4 mW/cm²) . This means the material works efficiently under normal outdoor conditions, without the need for concentrated lasers. Furthermore, the dense packing of the solid film inherently protects the sensitive triplet states from oxygen quenching, a common failure mode in liquid TTA-UC systems.

A Collaborative and Personal Milestone

The research represents a highly collaborative effort across institutions. The precise tuning of the molecular energy levels was guided by density functional theory (DFT) calculations provided by Masahiro Ehara and Pei Zhao from the Institute for Molecular Science in Okazaki .

For the Kyushu University team, the publication is the culmination of a 14-year scientific journey. In 2012, Nobuo Kimizuka, now Professor Emeritus at Kyushu University's Research Center for Negative Emissions Technologies, began exploring photon upconversion in self-assembled molecular systems. Alongside Nobuhiro Yanai (now a Professor at the University of Tokyo), Kimizuka's group made steady progress in solutions and gels, but the solid-state remained elusive .

The breakthrough with iBu-DHI finally arrived in May 2024, less than a year before Kimizuka's retirement. The subsequent months were an intense push by the graduate students, alongside Sasaki and then-Assistant Professor Kiichi Mizukami, to finalize the research.

"We handed the draft to Professor Kimizuka just 11 days before he left the lab, which for us felt like a heartfelt retirement gift," Sasaki notes .

"This discovery is the culmination of over 14 years of our research and marks a major milestone in photon-upconversion and molecular self-assembly research," Kimizuka concludes .

Looking Forward

The Kyushu University team has filed a patent for the material, which benefits from straightforward synthesis using inexpensive starting materials. By successfully moving Vis-to-UV upconversion from fragile liquid systems into robust, oxygen-tolerant solid films, this research bridges a critical gap between fundamental photochemistry and practical device engineering.

As we look toward a future powered by renewable energy, materials like iBu-DHI could allow us to coat building windows with air-purifying photocatalysts that run purely on sunlight, or develop low-energy, solar-driven 3D printing technologies. It is a brilliant example of how manipulating matter at the nanoscale can unlock entirely new ways to interact with the world around us.