Atomically Precise Metal Nanoclusters: Emerging Materials for Near-Infrared II Photoluminescence

ConspectusAtomically precise metal nanoclusters (NCs) are a class of nanomaterials composed of a specific number of metal atoms stabilized by well-defined organic ligands. These NCs exhibit molecular-like electronic states and offer exceptional control over their optoelectronic properties. Recent advancements have extended their photoluminescence deep into the near-infrared II (NIR-II) window (950-1700 nm), a spectral region that provides significant advantages for biomedical imaging and photonic applications, including reduced tissue scattering, minimal autofluorescence, and enhanced penetration depth. In comparison to conventional quantum dots and larger plasmonic nanoparticles, atomically precise metal NCs offer unprecedented tunability in terms of emission wavelength, quantum yield, and photostability, facilitated by the modulation of size, composition, and ligand shell chemistry.In this Account, we highlight cutting-edge strategies, including ligand engineering, core-shell engineering, and alloying, which enable fine-tuning of NIR-II photoluminescence in metal NCs. We also explore the photophysical mechanisms underlying NIR-II emission, such as core-ligand charge transfer, metal-centered transitions, and the role of surface electronic states in radiative recombination efficiency. Advanced spectroscopic techniques, such as time-resolved photoluminescence and transient absorption, are discussed for their ability to probe excited-state lifetimes and energy transfer processes that control the emission properties. Finally, we critically address the current limitations in quantum yield enhancement, long-term photostability, and biocompatibility while outlining future directions for developing hybrid materials and multifunctional NC platforms and advancing NIR-II photonic technologies. Our Account aims to offer molecular-level insights and guide the rational design of next-generation atomically precise metal NCs as versatile materials for advanced NIR-II photoluminescence.