Tailoring Carbon Tails of Ligands on Au 52 (SR) 32 Nanoclusters Enhances the Near-Infrared Photoluminescence Quantum Yield from 3.8 to 18.3.

One of the important factors that determine the photoluminescence (PL) properties of gold nanoclusters pertain to the surface. In this study, four Au 52 (SR) 32 nanoclusters that feature a series of aromatic thiolate ligands (-SR) with different bulkiness at the para -position are synthesized and investigated. The near-infrared (NIR) photoluminescence (peaks at 900-940 nm) quantum yield (QY) is largely enhanced with a decrease in the ligand's para -bulkiness. Specifically, the Au 52 (SR) 32 capped with the least bulky p -methylbenzenethiolate ( p -MBT) exhibits the highest PLQY (18.3% at room temperature in non-degassed dichloromethane), while Au 52 with the bulkiest tert -butylbenzenethiolate (TBBT) only gives 3.8%. The large enhancement of QY with fewer methyl groups on the ligands implies a nonradiative decay via the multiphonon process mediated by C-H bonds. Furthermore, single-crystal X-ray diffraction (SCXRD) comparison of Au 52 ( p -MBT) 32 and Au 52 (TBBT) 32 reveals that fewer methyl groups at the para -position lead to a stronger interligand π···π stacking on the Au 52 core, thus restricting ligand vibrations and rotations. The emission nature is identified to be phosphorescence and thermally activated delayed fluorescence (TADF) based on the PL lifetime, 3 O 2 quenching, and temperature-dependent PL and absorption studies. The 1 O 2 generation efficiencies for the four Au 52 (SR) 32 NCs follow the same trend as the observed PL performance. Overall, the highly NIR-luminescent Au 52 ( p -MBT) 32 nanocluster and the revealed mechanisms are expected to find future applications.