Triplet–triplet annihilation photon upconversion (TTA-UC) in solvent-free solid films is the form needed for practical photon management, yet converting visible sunlight into ultraviolet (UV) light in the solid state has long been limited by the mutual exclusion of high fluorescence quantum yield and fast triplet exciton diffusion. A 2026 breakthrough showed that dihydroindeno[2,1-a]indene (DHI) derivatives carrying alkyl chains above and below the π-plane (sterically protected π-electron systems) lift this exclusion: an isobutyl-substituted DHI film sensitized by Ir(ppy)₃ reaches an absolute upconversion quantum yield of 1.9% at a threshold of 1.2 mW cm⁻², below solar spectral irradiance. The open directions consolidated here — the first of which the source work itself explicitly commits to studying in depth, and which later literature leaves unsolved — are: replacing the expensive, UV-reabsorbing iridium sensitizer with metal-free thermally activated delayed fluorescence (TADF) sensitizers in a systematic solid-state study; suppressing reabsorption losses at the upconverted UV wavelength; sustaining long-term upconversion in air without inert sealing; and extending the steric-protection design principle, together with design guidelines for uniform donor dispersion, to chromophore families beyond DHI.

Background

Photon upconversion converts two low-energy photons into one photon of higher energy. The most efficient molecular route at low light intensity is triplet–triplet annihilation (TTA): a sensitizer molecule absorbs a visible photon and, after intersystem crossing (a spin-forbidden transition from the excited singlet to the triplet state), transfers its triplet energy to an acceptor (also called annihilator); two diffusing acceptor triplets then collide and annihilate, leaving one acceptor in its emissive singlet state, which fluoresces at shorter wavelength than the light absorbed. Key figures of merit are the absolute upconversion quantum yield ΦUC (upconverted photons emitted per photon absorbed, with 100% meaning one UV photon out per two visible photons in) and the threshold excitation intensity Ith, the intensity above which the upconverted emission switches from a quadratic to a linear dependence on excitation power; TTA-UC is efficient only near or above Ith, so a low Ith — at or below the solar spectral irradiance (about 1.4 mW cm⁻² near 445 nm) — is required for sunlight-driven operation. In solution, TTA-UC systems with visible-to-UV output reach quantum yields around 10%, but in solvent-free solids performance collapses: dense chromophore packing quenches both singlet and triplet excitons through excimers (bound dimeric excited states that decay non-radiatively), crystalline defects, and aggregation, while the loose packing that would prevent quenching slows the triplet diffusion on which TTA depends. High fluorescence quantum yield and fast triplet exciton diffusion are therefore hard to achieve simultaneously in the solid state. Harada, Shoyama, Boonmong, Mizukami, Watanabe, Zhao, Ehara, Sasaki, and Kimizuka (Nature Communications 17, 5134, 2026; first posted on ChemRxiv in April 2025) resolved this trade-off for visible-to-UV conversion with a family of dihydroindeno[2,1-a]indene (DHI) derivatives — rigid, UV-emitting (around 370 nm) polycyclic chromophores — functionalized with alkyl chains fixed above and below the π-conjugated plane from the sp³ bridgehead carbons. These chains set a precisely controlled intermolecular distance and π-orbital overlap, so the best derivative, isobutyl-substituted iBu-DHI, keeps a high fluorescence quantum yield (about 90% in solution), a long triplet lifetime, and fast triplet diffusion in spin-coated crystals; sensitized by the triplet donor Ir(ppy)₃ (tris(2-phenylpyridine)iridium(III), a standard heavy-metal complex whose strong spin–orbit coupling enables efficient triplet formation), the film shows a solid-state record for visible-to-UV TTA-UC: absolute ΦUC of 1.9% after self-absorption correction at Ith = 1.2 mW cm⁻². Practical and conceptual gaps remain. First, the sensitizer: Ir(ppy)₃ is an expensive precious-metal complex, and its absorption overlaps the UV emission of iBu-DHI, causing reabsorption losses, which the source suggests could be further suppressed by a sensitizer with minimal UV absorption (the related singlet back energy transfer is measured to be small in this benchmark, the acceptor fluorescence lifetime being essentially unchanged on doping, 1.9 ns neat versus 1.8 ns doped); the authors demonstrated only preliminarily (supplementary figure) that the metal-free thermally activated delayed fluorescence (TADF) sensitizers 4CzIPN and DABNA — purely organic emitters whose tiny singlet–triplet energy gap lets thermal energy drive population of the triplet state — can sensitize the film, promising in-depth studies in future work. Second, stability: the film upconverts in air for over an hour under intense irradiation but photodecomposes unless sealed under inert atmosphere, whereas long-term (over 100 h) unsealed air stability has already been achieved for other visible-to-UV upconversion films prepared by solvent-free melt processing (Enomoto and Murakami, 2023). Third, understanding: the rationalization of why iBu-DHI packing gives such a low threshold rests on quantum-chemical calculations of triplet energy transfer and TTA rate constants for molecular pairs found in the crystals, plus qualitative comparison with the methyl and ethyl-butyl analogues; a predictive framework that takes a molecular structure and packing arrangement as input and outputs the expected upconversion performance does not exist — the source states only that its π-protection design principle 'will be widely extended to various chromophores' and that molecular design guidelines enabling uniform dispersion of the donors within the crystalline assembly without phase separation are desired — and contemporaneous work (Lowe et al., 2026) shows that solution-phase screening fails to predict solid-state upconversion behavior.

Problem Statement

Realize a practical, sunlight-driven, solvent-free solid-state visible-to-ultraviolet TTA photon-upconversion film based on sterically protected π-chromophores, and establish the design rules that make it rationally transferable. Concretely, starting from the demonstrated iBu-DHI/Ir(ppy)₃ benchmark (absolute ΦUC = 1.9% at Ith = 1.2 mW cm⁻²), the problem is to develop the system further along the following open directions: (1) metal-free sensitization — systematic characterization and optimization of solid-state visible-to-UV TTA-UC in which the triplet sensitizer is a purely organic TADF emitter (such as 4CzIPN or DABNA, or improved alternatives) whose triplet energy lies above that of the acceptor, replacing the expensive iridium complex; the source demonstrates this only preliminarily (supplementary figure) and defers in-depth studies of such metal-free systems to future work. (2) Suppressed reabsorption — following the source's own suggestion, a sensitizer with minimal absorption at the upconverted UV emission wavelength (around 370 nm for DHI) would reduce reabsorption losses relative to the Ir(ppy)₃ benchmark; singlet back energy transfer in that benchmark is already measured to be small (acceptor fluorescence lifetime 1.9 ns neat versus 1.8 ns doped) and is therefore not an open loss channel to be reduced further. (3) Sunlight-level operation in air without sealing — the film should retain a threshold excitation intensity at or below the solar spectral irradiance (about 1.4 mW cm⁻² near 445 nm), as the Ir(ppy)₃ benchmark already does, and should sustain its upconverted emission under continuous irradiation in ambient atmosphere for at least 100 h, well beyond the roughly one-hour window of the current spin-coated films; the source reports photodecomposition in air under intense irradiation (λdt = 370 nm, Iex = 2.0 W cm⁻²) that is suppressed by sealing in an inert atmosphere, and sets no duration target, while over-100-hour unsealed air stability has been demonstrated for visible-to-UV films of a different, melt-processed materials class (Enomoto and Murakami, 2023). (4) Transferable design rules — an understanding of how intermolecular distance and packing geometry control triplet energy transfer, annihilation, and diffusion and hence ΦUC and Ith, in a form that can be applied to systems not yet measured, together with the design guidelines the source itself calls for (uniform dispersion of the donors within the crystalline molecular assembly without phase separation, and high quantum yield unaffected by the formation of crystalline defects) and with extension of the π-protection principle beyond the DHI family, which the source asserts but does not demonstrate.

These directions share one practical objective — low-cost, air-tolerant, sunlight-driven UV photon generation from solvent-free solid films — which the current benchmark does not yet meet. The objective is a single research program, not four alternative tasks, and is complete only when all four directions are jointly resolved in one system: a film sensitized by a metal-free TADF emitter (direction 1) that reduces reabsorption of its ~370 nm upconverted emission relative to the Ir(ppy)₃ benchmark (direction 2), meets the quantitative targets stated in direction (3), and is explained by design rules that predict the performance of a chromophore system not yet measured (direction 4). Progress on any subset of these directions advances, but does not by itself complete, the objective.

References

  1. Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, Kiichi Mizukami, Yuya Watanabe, Pei Zhao, Masahiro Ehara, Yoichi Sasaki, Nobuo Kimizuka, 'Sterically protected π-electron systems for efficient solid-state photon upconversion', Nature Communications 17, 5134 (2026). DOI: 10.1038/s41467-026-73898-0, https://doi.org/10.1038/s41467-026-73898-0
  2. Naoyuki Harada, Hayato Shoyama, Nutnicha Boonmong, Kiichi Mizukami, Yuya Watanabe, Pei Zhao, Masahiro Ehara, Yoichi Sasaki, Nobuo Kimizuka, 'Sterically Protected π-Electron Systems Enable Efficient, Sunlight-Driven Solid-State Photon Upconversion', ChemRxiv preprint (2025). DOI: 10.26434/chemrxiv-2025-mmh9d-v2, https://doi.org/10.26434/chemrxiv-2025-mmh9d-v2
  3. Lukas Naimovicius, Liwia Wolek, Simon K. Zhang, Judy E. Kim, Michael J. Tauber, Andrew B. Pun, 'Activating Solid-State Triplet–Triplet Annihilation Upconversion via Bulky Annihilators', Journal of the American Chemical Society 148, 3811–3819 (2026). DOI: 10.1021/jacs.5c21298, https://doi.org/10.1021/jacs.5c21298 (preprint: 10.26434/chemrxiv-2025-pwf89)
  4. Masaki Miyata, Kodai Matsumoto, Kiichi Mizukami, Yoichi Sasaki, Nobuo Kimizuka, 'Enhancing photon upconversion properties in anthracene derivatives through meticulous control of excluded volume around π-systems', Nanoscale 18, 10158–10164 (2026). DOI: 10.1039/d5nr05481h, https://doi.org/10.1039/d5nr05481h
  5. Nutnicha Boonmong, Yoshiki Ishii, Shun Watanabe, Takuma Yasuda, Yoichi Sasaki, Nobuo Kimizuka, 'Optimizing Three-Dimensional, π-Orbital Accessibility for Molecular Diffusion-based Photon Upconversion', ChemRxiv preprint (2026). DOI: 10.26434/chemrxiv.15003771/v1, https://doi.org/10.26434/chemrxiv.15003771/v1
  6. Seamus S. Lowe, Jesse A. Wisch, Isaac Walton, Tuo Hu, Chiao-Jung Su, John E. Anthony, Barry P. Rand, 'Low-Threshold Visible-to-Ultraviolet Solid-State Triplet-Fusion Upconversion', ACS Energy Letters 11, 5112–5119 (2026). DOI: 10.1021/acsenergylett.6c01224, https://doi.org/10.1021/acsenergylett.6c01224
  7. Riku Enomoto, Yoichi Murakami, 'Solvent-free temperature gradient melt formation of efficient visible-to-UV photon upconversion organic films with subsolar threshold and over 100 h photostability in air', Journal of Materials Chemistry C 11, 1678–1683 (2023). DOI: 10.1039/d2tc04578h, https://doi.org/10.1039/d2tc04578h
  8. Riku Enomoto, Megumi Hoshi, Hironaga Oyama, Hideki Agata, Shinichi Kurokawa, Hitoshi Kuma, Hidehiro Uekusa, Yoichi Murakami, 'van der Waals solid solution crystals for highly efficient in-air photon upconversion under subsolar irradiance', Materials Horizons 8, 3449–3456 (2021). DOI: 10.1039/d1mh01542g, https://doi.org/10.1039/d1mh01542g
  9. Xin Zhang, Rongxin Zhang, Lei Wang, Feng Chen, Shan He, Zihao Xu, Zhigang Xia, Guijie Liang, 'High-yield visible-to-ultraviolet upconversion by ZnSe quantum dots with dynamic triplet transfer', Nature Communications 17, 8777 (2026). DOI: 10.1038/s41467-026-75613-5, https://doi.org/10.1038/s41467-026-75613-5
  10. Kuniyuki Miwa, Tsubasa Okamoto, Yasuhiro Kobori, Akihito Ishizaki, 'Influence of Molecular Orientation and Exciton Transfer Rates on Solid-State Triplet–Triplet Annihilation Photon Upconversion', The Journal of Physical Chemistry C 129, 12904–12915 (2025). DOI: 10.1021/acs.jpcc.5c02994, https://doi.org/10.1021/acs.jpcc.5c02994