Ligand Isomerism and Matrix Confinement Enable Near-Unity Quantum Yield in Ag17 Nanoclusters for Cryogenic Thermometry

Cryogenic, noncontact temperature sensing is essential for emerging technologies. Here, ligand isomerism modulates photophysical properties of atomically precise nanoclusters, enabling cryogenic optical thermometry. We synthesized emissive [Ag17(m1-CBT)12]3– (Ag17m1-CBT; m1-CBT = meta-carborane-1-thiol) and compared it with structurally analogous but nonemissive [Ag17(o1-CBT)12]3– (Ag17o1-CBT). Despite nearly identical structures and steric environments, the weaker electron-accepting meta-carborane ligand enhances radiative decay, increasing the solid-state quantum yield from <1% to 33% at room temperature. Embedding Ag17m1-CBT in a poly(methyl methacrylate) matrix yields a highly emissive film whose photoluminescence lifetime increases from 18.4 μs at 300 K to 392 μs at 80 K. A thermally equilibrated excited-state manifold enables optical thermometry over 80–140 K, with absolute and relative sensitivities of 3.8 μs K–1 and >0.97% K–1, respectively. The resulting films exhibit red-emission quantum yields of 59% at 300 K and 99% at 77 K, with negligible spectral shifts, establishing ligand isomerism as a powerful strategy for nanocluster-based cryogenic thermometry.