Masaaki Mitsui, Shinjiro Takano, Tatsuya Tsukuda
ABSTRACT 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 Au 42 (PET) 32 ( Au 42 ; 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 Au 42 /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 Au 42 , which combines strong NIR absorption with high visible transparency, minimizing losses of the S 1 annihilator and the UC photons. These findings establish Au 42 as a benchmark sensitizer and exemplify a design principle for realizing highly efficient, low‐threshold NIR‐to‐visible UC.