Solving the Packing Problem in Solid-State Photon Upconversion

Credit:Percy Gonzalo Sifuentes-Samanamud / Tokyo University

Photochemical upconversion from visible light to UV has always worked better on paper — or more precisely, in solution — than it does in a stable, deployable material. A team at Kyushu University, led by Yoichi Sasaki and Nobuo Kimizuka, has now published a solid-state photon upconversion system that reaches a fluorescence quantum yield above 60% and an upconversion quantum yield of 1.9% under unconcentrated, ambient sunlight. The result, published in Nature Communications, is notable less for the efficiency number itself and more for what had to be solved to get there: a geometric constraint in organic solids that has kept triplet–triplet annihilation (TTA) upconversion well behind its solution-phase counterparts for over a decade.

Why Solution-Phase TTA Doesn't Translate to Solids

TTA upconversion relies on a sensitizer molecule absorbing a visible photon and undergoing intersystem crossing into a triplet excited state, a process of triplet formation that then transfers energy to a neighboring emitter molecule via the Dexter mechanism. When two triplets meet, they annihilate and release their combined energy as a single higher-energy UV photon — the delayed luminescence that defines UC emission. In solution, triplet exciton diffusion is straightforward — molecules diffuse freely, triplets collide at a useful rate, and the chemistry has been worked out for years. The problem is durability: solvent-based systems evaporate, degrade, and aren't something you build a stable device or coating around.

Moving the same chemistry into organic solids or organic glasses removes the solvent liability but introduces a different failure mode. Solids pack molecules close together by definition, and once packing gets tight enough, π-electron clouds from adjacent molecules start to overlap. That overlap opens non-radiative decay pathways, and triplets quench before they ever find a partner to annihilate with. The result is a system that looks fine on a bench in solution and falls apart the moment it's cast into a film.

"Molecules must be close enough for energy transfer but separated enough to prevent quenching of excitons." — Yoichi Sasaki, Kyushu University

This is the actual engineering problem: proximity and electronic overlap are normally coupled variables in a molecular solid. You can't easily tune one without dragging the other along with it. Most attempts at solid-state upconversion have effectively been trying to solve two competing constraints with one lever.

Decoupling Proximity from Overlap

The Kyushu team's approach was to change the lever. Working with an organic semiconductor scaffold called dihydroindenoindenedene (DHI), they attached alkyl chains to the molecule's sp³ carbon centers. These substituents don't meaningfully alter the π-system's electronic character — their job is purely spatial. By fixing the alkyl chain length and geometry, the team engineered an organic crystal with a controlled gap between neighboring DHI derivatives in the solid state: close enough to sustain efficient triplet energy transfer, wide enough to suppress the destructive electronic coupling that comes with tight π-stacking.

That distinction — using a steric substituent as a spacer rather than an electronic modifier — is what separates this from earlier attempts at solid-state TTA materials. It treats the packing problem as an exercise in crystal engineering first, rather than trying to solve it purely through electronic tuning of the chromophore system itself. The result was a solid-state photoluminescence quantum yield above 60%, and when paired with a donor molecule, an upconversion quantum yield of 1.9% under ordinary outdoor sunlight.

Roughly two UV photons generated for every hundred visible-light photons absorbed — using ambient sunlight alone, with no concentrated or artificial illumination required.

Why 1.9% Is a More Useful Number Than It Sounds

Efficiency figures in the low single digits don't sound like headline numbers, but the context matters more here than the value itself. Most solid-state upconversion systems carry a threshold excitation intensity well above what ambient sunlight provides — they require concentrated or artificial illumination to get anywhere close to their solution-phase analogs. A system that works under ordinary sunlight, without solvent, and without light concentration removes three of the biggest practical barriers to deployment simultaneously, even before efficiency is optimized further.

That combination points toward specific downstream uses: solar-driven photocatalysis, resin curing in 3D printing, UV-dependent gel hardening, and longer-term integration into solar cell devices and other photonic applications are all areas where the constraint has never really been theoretical efficiency ceilings — it's been the absence of a stable, solvent-free material that functions at real-world light levels. For materials researchers working in organic semiconductors and excited-state photophysics more broadly, the design principle — steric spacer control decoupled from electronic tuning — is likely to be more transferable than this specific DHI system.

A Longer Arc

The result also closes a research arc that started well before this paper. Kimizuka's group began working on photon upconversion via triplet energy migration in molecular self-assemblies back in 2012, with steady progress across solution and gel-phase systems over the following decade. The solid-state breakthrough came in May 2024, and the resulting manuscript was reportedly finished and handed to Kimizuka less than two weeks before his retirement from the university.

"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." — Nobuo Kimizuka, Professor Emeritus, Kyushu University

The material has been filed for patent protection, and the team points to straightforward synthesis and low-cost starting materials as practical advantages heading into further development.

If your work touches organic semiconductors, excited-state photophysics, or materials that depend on tightly controlled molecular packing, our team can help source or customize the materials your research requires. Contact us to discuss your project, follow us on LinkedIn for ongoing research coverage, or explore the full catalog at MSE Supplies.

Sources:

1. 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, 17(1). https://doi.org/10.1038/s41467-026-73898-0

2. Harvesting UV Light from sunlight just got 'solid’   | Research Results | KYUSHU UNIVERSITY. (n.d.). 九州大学（KYUSHU UNIVERSITY）. https://www.kyushu-u.ac.jp/en/researches/view/384