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, Au₁₈(SR)₁₄ undergoes assembly into a tethered dimer, (Au₁₈)-(Au₂S₃)-(Au₁₈). This structural reorganization spatially separates the HOMO and LUMO over two Au₁₈ 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.