All-Alkynyl Protected Rod-Shaped Au₉(AgCu)₁₂₆ Nanocluster with Remarkable Photothermal Conversion

High-nuclearity intermetallic nanoclusters are important for investigating the evolution of alloy materials from atoms to plasmonic alloy nanoparticles. However, the synthesis of large-size alloy nanoclusters (∼2 nm) is still challenging. In this work, an all-alkynyl protected trimetallic nanocluster of unprecedented size, Au₉Ag_{126- x}Cu_x(PhCC)₆₈(BF₄)₅ (x = 0-20) (1) (PhCC = phenylacetylene), has been synthesized and its total structure determined by single crystal X-ray diffraction (SCXRD). The metal core of 1 is rod-like in structure, with a length of 1.92 nm and a width of 1.45 nm. Cluster 1 contains a concentric metal kernel in the manner of shell-by-shell arrangements of Au₃Ag₃₄@Au₆Ag₆₄@(AgCu)₂₈ protected by 68 PhCC ligands with 15 distinct alkynyl-metal binding configurations. Theoretic calculation reveals that 1 features a HOMO-LUMO energy gap of 0.29 eV. This suggests that 1 is situated at the boundary of the transition from a molecular to a metallic state. Remarkably, compared to other reported Au/Ag/Cu/Pd based nanoclusters, 1 exhibits significantly enhanced photothermal conversion capability. A substantial temperature rise of ∼51.5 °C within 5 min (λ_ex = 660 nm, 0.5 W cm^-2) and a record high photothermal conversion efficiency of 84.7% at 12 µM in N,N-dimethylformamide (DMF) were observed. Time-resolved transient absorption (TA) spectroscopy reveals that the electron-phonon coupling (τ_e-ph) of excited 1 occurs on the femtosecond timescale, resulting in an ultrafast electronic relaxation process and excellent photothermal performance. Cluster 1, when employed as a photothermal material, shows promise in biothermal therapy, photothermal catalysis, and photothermal imaging.