Au ₄₂ (PET) ₃₂ Nanocluster Sensitizer Unlocks the Annihilator Potential of Rubrene, Enabling High‐Performance NIR‐to‐Visible Photon Upconversion

Photon upconversion (UC) via triplet-triplet annihilation enables the conversion of near-infrared (NIR) photons into visible light, offering opportunities for solar energy harvesting, photocatalysis, and biophotonics. However, progress has been limited by the lack of triplet sensitizers capable of fully exploiting rubrene, the representative annihilator/emitter for NIR-to-visible UC. Here, we report Au₄₂(PET)₃₂ (Au₄₂; PET = 2-phenylethanethiolate), a highly anisotropic, needle-shaped gold nanocluster that unlocks the annihilator potential of rubrene, enabling high-performance NIR-to-visible UC. The Au₄₂/rubrene pair achieves record-setting UC quantum yields (Φ_UC, 50% maximum) of 16.5% (reabsorption-corrected quantum yield Φ_UCg of 21.4%) with a low threshold intensity (I_th) of 0.14 W cm^-2 under 808 nm excitation and 12.3% (Φ_UCg = 15.0%) under 936 nm excitation-over two orders of magnitude higher than previously reported values above 850 nm. Quantitative analysis revealed a high spin-statistical factor (f = 0.58) for rubrene, suggesting an attainable Φ_UC maximum of ∼30% in rubrene-based systems. The remarkable performances arise from the unique electronic structure of Au₄₂, which combines strong NIR absorption with high visible transparency, minimizing losses of the S₁ annihilator and the UC photons. These findings establish Au₄₂ as a benchmark sensitizer and exemplify a design principle for realizing highly efficient, low-threshold NIR-to-visible UC.