New solid-state material converts blue sunlight into UV at record efficiency, enabling solar-driven chemistry
Sunlight is abundant, but most of its energy arrives as visible and infrared photons, not the high-energy ultraviolet light needed to drive many chemical reactions, cure resins, or purify water. A team at Kyushu University has developed an organic solid-state material that solves this problem by converting two low-energy blue photons into one higher-energy UV photon, achieving an efficiency that makes the process practical under natural sunlight for the first time.
The research, published June 23 in Nature Communications, caps a 14-year effort by the laboratory of Professor Emeritus Nobuo Kimizuka, handed to him as a final draft just 11 days before his retirement.
Not your typical upconversion
Most photon upconversion systems work either through nonlinear optical crystals (requiring intense laser pulses) or through lanthanide-doped nanoparticles (efficient but limited to specific wavelengths). The Kyushu team’s approach uses an entirely different mechanism: triplet-triplet annihilation (TTA) photon upconversion in an organic semiconductor.
The system has two components:
- Ir(ppy)₃ (tris(2-phenylpyridine)iridium(III)), an organometallic complex that absorbs blue light at approximately 445 nm and efficiently converts it to a long-lived triplet state through intersystem crossing.
- iBu-DHI (tetraisobutyl-substituted 5,10-dihydroindeno[2,1-a]indene), an organic semiconductor engineered with alkyl side chains that create precisely controlled intermolecular spacing of approximately 0.4 nm, close enough for the Dexter electron-exchange mechanism to transfer triplet energy efficiently, but far enough apart to prevent the exciton quenching that has doomed previous solid-state TTA systems.
How it works
The energy cascade proceeds in four steps:
1. Absorption: Ir(ppy)₃ absorbs a blue photon (~445 nm), exciting an electron to a singlet state, which rapidly undergoes intersystem crossing to a triplet state.
2. Transfer: The triplet energy hops to a neighboring iBu-DHI molecule via Dexter electron exchange, a quantum-mechanical process requiring direct orbital overlap.
3. Annihilation: Two iBu-DHI molecules in triplet states diffuse together and collide. Their triplets combine and annihilate, promoting one of the molecules to a higher-energy singlet state.
4. Emission: That singlet state decays radiatively, emitting a single UV photon whose energy is approximately the sum of the two absorbed visible photons.
The calculated triplet energy transfer time for iBu-DHI is 1.25 microseconds, compared with 42 milliseconds for a bulkier derivative (2-EtBu-DHI), a difference of four orders of magnitude that explains why the molecular design is critical.
Record performance metrics
The material achieved an absolute upconversion quantum yield of 1.9% in the solid state, the highest ever reported for a room-temperature, visible-to-UV TTA-UC system operating below an excitation threshold of 10 mW/cm². The threshold intensity itself, the minimum light level needed to sustain upconversion, is just 1.2 mW/cm² for spin-coated films and 0.7 mW/cm² for drop-cast films. For context, solar irradiance at 445 nm is approximately 1.4 mW/cm², meaning the system operates below the intensity of natural sunlight.
The material also shows remarkable defect tolerance: the crystalline iBu-DHI retains 69–83% fluorescence quantum yield in the solid state versus 88% in solution, whereas the unsubstituted parent compound (DHI) drops from 96% to just 10% when crystallized, a near-total collapse that has historically frustrated solid-state TTA-UC efforts.
The triplet lifetime in the solid state is 4.0 milliseconds, long enough for triplet diffusion and annihilation to occur efficiently before the energy is lost.
Applications
The ability to generate UV photons from natural sunlight without external power opens several practical uses:
- Solar-driven photocatalysis, UV drives chemical reactions including water splitting and pollutant degradation that visible light cannot
- Air and water purification, UV inactivates pathogens and breaks down volatile organic compounds
- 3D printing resin curing, photopolymer resins can be cured using concentrated sunlight rather than high-powered UV lamps
- Dental fillings and gel nail coatings, light-cured materials that currently require specialized UV sources
The iBu-DHI material can be synthesized in a straightforward one-pot reaction from inexpensive starting materials without toxic solvents, and the team has filed a patent on the technology. Notably, iBu-DHI also works with organic TADF (thermally activated delayed fluorescence) sensitizers like 4CzIPN, which avoids the need for expensive iridium complexes.
Corresponding author Yoichi Sasaki, associate professor at Kyushu’s Faculty of Engineering, noted that the real breakthrough is in the molecular design principle, not just the specific material. “The steric protection strategy, using alkyl chains on sp³ carbons to control spacing without blocking orbital overlap, can be applied to other organic semiconductor systems,” he said. “This is a general solution to the solid-state quenching problem.”
Sources:
1. Harada, N., Shoyama, H., Boonmong, N. et al. “Sterically protected π-electron systems for efficient solid-state photon upconversion.” Nature Communications 17, 5134 (2026). DOI: 10.1038/s41467-026-73898-0
2. Kyushu University. Press release via ScienceDaily, June 23, 2026.
3. Sasaki, Y. & Kimizuka, N. Kyushu University. Additional reporting by Phys.org and SciTechDaily.