Synergistic Intramolecular Charge Transfer Promotes Au Nanoclusters with Enhanced NIR-II Photoluminescence

Gold nanoclusters (Au NCs) protected by molecular ligands represent a new class of second-generation near-infrared (NIR-II) luminescent materials that have been widely studied. However, the photoluminescence efficiencies of most NIR-II emitting Au NCs in aqueous solution are generally lower than 0.2%, and to fully exploit the advantages of AuNCs in the NIR-II region, improving their photoluminescence efficiency has become an urgent need. Considering the holistic nature of the core-shell structure of Au NCs, herein, we propose a synergistic intramolecular charge transfer (ICT) strategy to enhance the luminescence. The NIR-II fluorescence quantum yield of Au NCs was increased 6-fold to 5.59% by the synergistic effect of heteroatomic copper doping and ligand p-MBA deprotonation. Experimental characterization results show that the strong p-pi conjugation between d10 metal and the deprotonated p-MBA enhances the charge transfer between the metal core and ligand. The synergistic ICT process strongly suppressed the nonradiative process, thereby enhancing the emission intensity. Our findings provide a facile method for understanding the integrity of the core-shell structure of Au NCs and regulating their photoluminescence properties.