Prof. ZHOU Meng’s research team from the University of Science and Technology of China (USTC), collaborating with Prof. WANG Quanming’s team from Tsinghua University (THU) achieved near-unity room-temperature photoluminescence quantum yield (PLQY) (>99%) in the near-infrared (NIR) emission of metal nanoclusters in solution. Their work was published in Science on January 19^th.

Gold nanoclusters (Au NCs) as NIR-emissive materials hold potential in biomedical applications. However, the PLQY of Au NCs in NIR region is typically low, often less than 10%. To address this problem, synthesized Au₂₂(^tBuPhC≡C)₁₈(Au₂₂) and its copper-doped counterpart, Au₁₆Cu₆(^tBuPhC≡C)₁₈ (Au₁₆Cu₆) and study their photophysical properties. Single-crystal x-ray diffraction analysis revealed that Au₂₂ and Au₁₆Cu₆ share similar structures. Au₂₂ showed an emission peak at 690 nm and Au₁₆Cu₆ at 720 nm. The absolute PLQY of Au₂₂ and Au₁₆Cu₆ in air were 9% and 95%, respectively. In deaerated solution, the PLQY of Au₁₆Cu₆ reached 100% measured by both absolute and relative methods. Time-correlated single-photon counting measured the photoluminescence lifetimes of Au₂₂ and Au₁₆Cu₆ to be 485 ns and 1.64 μs, respectively.

Molecular structures of Au₂₂ and Au₁₆Cu₆ and structure anatomy of Au₁₆Cu₆. (Image by Prof. ZHOU’s team)

Further investigation of NCs’ excited-state dynamics through transient-absorption spectroscopy revealed that both NCs' luminescent states originated from the triplet state (T₁), with distinct dynamic processes observed in femtosecond transient-absorption spectroscopy. Under 380 nm excitation, Au₂₂ showed a slow rise of 148 ps, while Au₁₆Cu₆ showed a rapid relaxation of 0.5 ps. Triplet sensitization experiments confirmed that these processes are attributed to ultrafast intersystem crossing (ISC) from singlet state (S₁) to T₁. Due to copper doping, Au₁₆Cu₆ has smaller ∆E_st, significantly accelerating its ISC rate. As a result, Au₁₆Cu₆ ultimately showcases PLQY close to 100%.

Ultrafast excited-state dynamics results of Au₂₂ and Au₁₆Cu₆. (Image by Prof. ZHOU’s team)

The approach to achieve near-unity PLQY could enable the development of highly emissive metal cluster materials. Specifically, this work demonstrates that near-unity PLQY can be attained with an alloy of gold-copper nanoclusters even in solution at room temperature, which will enable applications ranging from biological imaging to luminescent devices.

Paper link: https://doi.org/10.1126/science.adk6628

(Written by WU Yuyang, edited by ZHANG Yihang, USTC News Center)