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

Abstract 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, Au 9 Ag 126‐ x Cu x (PhCC) 68 (BF 4 ) 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 Au 3 Ag 34 @Au 6 Ag 64 @(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.