Triplet Energy Transfer and Photon Upconversion from Metal Nanocluster with Near‐Unity NIR Emission Quantum Yield

Abstract Metal nanoclusters have emerged as new triplet sensitizers of photo‐upconversion, while the strategy of engineering their electronic properties for efficient utilization of triplet energy remains elusive. Au ₁₆ Cu ₆ ( ^t BuPhC≡C) ₁₈ (Au ₁₆ Cu ₆ ) metal nanocluster shows near‐unity near‐infrared (NIR) phosphorescent quantum yield at room temperature and near‐unity intersystem crossing quantum yield, which is an ideal platform for understanding the mechanism of triplet energy transfer (TET). Here, a model system is designed to probe the TET from Au ₁₆ Cu ₆ to different organic acceptors and obtained k _TET from Stern–Volmer plots. Transient absorption spectroscopy clearly shows the formation of T ₁ signals in organic molecules accompanied by the decay of T ₁ signals in Au ₁₆ Cu ₆ , which indicates a direct TET mechanism. Among four organic molecules, the TET between Au ₁₆ Cu ₆ and perylene possesses the highest TET quantum yield. Moreover, a relatively small threshold intensity of 0.4 W cm ⁻² is obtained with 532 nm excitation and a large anti‐Stokes shift of 0.68 eV with 635 nm excitation in triplet‐triplet annihilation (TTA) upconversion (UC). The efficient TET and UC in Au ₁₆ Cu ₆ nanoclusters and the underlying mechanism will pave the way for optimizing their performance and expanding their utilization in optoelectronic devices.