# Au42(PET)32 Nanocluster Sensitizer Unlocks the Annihilator Potential of Rubrene, Enabling High‐Performance NIR‐to‐Visible Photon Upconversion

## TL;DR

A new gold nanocluster enables efficient conversion of near-infrared light to visible light using rubrene, achieving record performance in photon upconversion.

## Contribution

Au42(PET)32 nanocluster is introduced as a highly effective sensitizer for rubrene-based photon upconversion with record quantum yields.

## Key findings

- Au42(PET)32 enables 21.4% upconversion quantum yield under 808 nm excitation.
- The system achieves 15.0% upconversion quantum yield under 936 nm excitation.
- The high spin-statistical factor of rubrene suggests a potential maximum upconversion efficiency of ~30%.

## 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 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 S
1 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.

A needle‐shaped gold nanocluster, Au42(PET)32, serves as a powerful triplet sensitizer, enabling rubrene to fully express its intrinsic annihilator capability (spin statistical factor f = 0.58). This sensitizer–annihilator pair achieves record‐high upconversion internal quantum yields of 21.4% (808 nm excitation) and 15.0% (936 nm excitation), showcasing the emerging role of gold nanoclusters in triplet–triplet annihilation photon upconversion.