Dimerization‐Enabled Thermally Activated Delayed Fluorescence and 500‐Fold Enhanced Emission in Gold Nanoclusters

Gold nanoclusters (Au NCs) are promising luminescent materials with broad applications in sensing, bioimaging, and photonics. However, highly emissive thiolate‐protected Au NCs remain difficult to access because conformational relaxation of ligands and flexible Au–thiolate motifs often promote nonradiative relaxation. Herein, we show that dimerization can switch on thermally activated delayed fluorescence (TADF) in non‐fluorescent Au NCs thus realizing the boost of quantum yield. Upon introduction of diphosphine ligands, Au 18 (SR) 14 undergoes assembly into a tethered dimer, (Au 18 )–(Au 2 S 3 )–(Au 18 ). This structural reorganization spatially separates the HOMO and LUMO over two Au 18 units, resulting in a small singlet–triplet energy gap (0.061 eV) and enabling reverse intersystem crossing. As a result, the non‐emissive monomer is converted into a highly emissive dimer with a photoluminescence quantum yield boosted from ∼0.1% to 50%. This work identifies dimerization as an effective strategy to activate the emission of dark NCs and establishes a design principle for TADF‐type metal nanoclusters.