Triplet–triplet annihilation (TTA) photon upconversion has reached record photon energies approaching the UVC range — about 4.33 eV, corresponding to roughly 286 nm — using a benzene-derived annihilator sensitized by a high-triplet-energy fluorinated iridium complex, and, in parallel, TIPS-benzene emitters paired with an organic heptazine sensitizer. However, these demonstrations operate only in a transient, pulsed-laser regime with UVA excitation, suffer severe reabsorption (filter) effects in the UVB/C region, and remain inefficient. The open problem is the practical route: steady-state, low-intensity incoherent (LED- or solar-driven) upconversion into the UVC range. Three coupled design fronts characterize this goal: benzene-derived annihilators with enlarged singlet–triplet energy gaps near the thermodynamic condition 2ET1≈ES12E_{\mathrm{T1}}\approx E_{\mathrm{S1}} that retain high fluorescence yield and photostability; sensitizers combining high triplet energy, strong visible/UVA absorption, and negligible UVB/C absorptivity; and media or solid-state architectures that mitigate reabsorption. A February 2026 frontier review still names the efficiency of visible-to-UV upconversion as a key open challenge, and no steady-state UVC-range demonstration has been reported.

Background

Photon upconversion combines two low-energy photons into one photon of higher energy. The most practical route for low-intensity, incoherent light — as from LEDs or the sun — is sensitized triplet–triplet annihilation upconversion (sTTA-UC). A sensitizer molecule absorbs the incident photon and, after intersystem crossing (a radiationless spin flip into the triplet manifold), transfers its triplet excitation to an annihilator molecule by Dexter-type triplet energy transfer (a short-range electron-exchange mechanism). Two diffusing annihilator molecules in their lowest triplet states (T1T_1) then collide and undergo triplet–triplet annihilation (TTA): one returns to the ground state while the other is promoted to its lowest excited singlet state (S1S_1), which emits a photon of higher energy than the one absorbed — an anti-Stokes emission. Because two triplets each carry energy ET1E_{\mathrm{T1}}, the energetics require 2ET1≥ES12E_{\mathrm{T1}}\geq E_{\mathrm{S1}}; an annihilator with a large singlet–triplet gap ES1−ET1E_{\mathrm{S1}}-E_{\mathrm{T1}} therefore permits a larger anti-Stokes energy gain. The ultraviolet spectrum is conventionally divided into UVA (315–400 nm), UVB (280–315 nm) and UVC (100–280 nm); a bulky triisopropylsilylethynyl (TIPS-ethynyl) substituent is a widely used structural motif that tunes annihilator photophysics, and TIPS-naphthalene is the benchmark UV annihilator.

Blue-to-UV sTTA-UC was first demonstrated in 2006 with an iridium sensitizer and a pyrene annihilator, and the TIPS-naphthalene benchmark later enabled quantum yields beyond 10%. The Kerzig group pushed the accessible energies upward: purely organic vis-to-UV upconversion with an annihilator singlet state beyond 4 eV (Zähringer et al., 2022) and blue-to-UVB upconversion enabled by a benzene-based annihilator (Zähringer et al., 2023), with benzene derivatives attractive because their high-lying triplet states make them suitable for very large anti-Stokes shifts. The state of the field up to 2023 is reviewed by Uji et al. (Angew. Chem. Int. Ed. 2023).

In 2025 the limit was pushed towards the UVC range from two directions: Zähringer, Heusel, Schmitz, Glorius and Kerzig combined a benzene-based annihilator with a high-triplet-energy fluorinated iridium sensitizer and, exciting at 355/375 nm in the UVA, generated upconverted emission at about 4.33 eV (≈286 nm) — the highest-energy upconverted photons reported to date, approaching the UVC region; and Uji et al. combined TIPS-benzene-based emitters with an organic sensitizer based on heptazine (a nitrogen-rich aromatic heterocycle) that absorbs in the visible/UVA and has a high triplet energy, to reach UVB/UVC-range energy generation. Both records, however, are far from practical: emission below 365 nm suffers an enormous efficiency falloff, the chromophores' own large molar extinction coefficients in the UVB/C cause unavoidable reabsorption (filter) losses in diffusion-limited solution systems, photostability under sustained UV irradiation is poor, commercial high-power LEDs below 365 nm do not exist, and only pulsed-laser transient operation has been achieved. The unresolved target is therefore the whole practical route: steady-state upconversion into the UVC range under low-intensity, incoherent LED or solar illumination, which can be decomposed into three coupled design fronts — annihilator molecular design (enlarged ES1−ET1E_{\mathrm{S1}}-E_{\mathrm{T1}} gap at retained fluorescence yield and photostability, which would also allow sensitization by the broader class of lower-triplet-energy sensitizers), a spectrally matched sensitizer (high triplet energy, strong absorption in the visible/UVA excitation window around 355–370 nm, negligible UVB/C absorptivity, adequate lifetime), and reabsorption mitigation (media engineering, solid-state architectures, or optical filtering).

Problem Statement

Design and demonstrate a sensitizer–annihilator system (including, where needed, an engineered medium or solid-state architecture) that achieves triplet–triplet annihilation photon upconversion with emission in the UVC range proper — wavelengths of 280 nm and below (photon energies of about 4.43 eV and above), beyond the current record of about 4.33 eV (≈286 nm), approaching the UVC region — driven by steady-state (continuous-wave), low-intensity visible or near-UV excitation relevant to incoherent LED or solar operation, at solar-relevant irradiance (up to about 1 sun ≈ 100 mW cm⁻², or the equivalent incoherent LED irradiance), rather than by pulsed-laser transient excitation.

Three coupled design fronts characterize this objective. These are research directions and open challenges rather than conjunctive acceptance requirements: any sensitizer–annihilator–medium combination that meets the UVC-emission target under steady-state low-intensity drive answers the open problem, whatever the annihilator family or the reabsorption-mitigation strategy, provided the demonstrated system is characterized as specified in the deliverable below. The three fronts are: (1) benzene-derived annihilators whose singlet–triplet energy gap ES1−ET1E_{\mathrm{S1}}-E_{\mathrm{T1}} is enlarged toward the thermodynamic condition 2ET1≈ES12E_{\mathrm{T1}}\approx E_{\mathrm{S1}}, while preserving high ES1E_{\mathrm{S1}}, high fluorescence quantum yield and photostability — a gap near the limit would also open sensitization by the broader class of lower-triplet-energy sensitizers; (2) identification or synthesis of a matched sensitizer that simultaneously provides a triplet energy high enough to populate the high-lying annihilator triplet, strong absorption in the visible/UVA excitation window (around 355–370 nm or at longer wavelengths), negligible absorptivity in the UVB/C emission window to avoid reabsorption losses, and an adequate excited-state lifetime; and (3) a system architecture or medium (media engineering, solid-state host, or optical filtering) that mitigates the reabsorption losses which currently preclude steady-state operation in diffusion-based solution systems.

The deliverable is a demonstrated upconversion system together with the quantitative characterization of its operation under steady-state, low-intensity illumination: upconverted emission spectrum; upconversion quantum yield, with the convention stated (external versus internal, the spin-statistical limit used, and whether the value is corrected for filter/reabsorption effects); the operating and threshold excitation intensities, together with the excitation source type (LED, solar simulator, or cw laser used as a low-intensity proxy), reported quantitatively against LED/solar-relevant scales; and photostability under sustained cw drive. Progress on a single design front (for example, an annihilator or sensitizer meeting its property combination) constitutes at most a partial answer, accompanied by the photophysical characterization showing which of the coupled constraints it discharges.

References

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