Sub-ångström bond length tuning enhances photoluminescence quantum yield in copper nanoclusters

Understanding how atomic-scale structure dictates light emission in metal nanoclusters is central to designing efficient luminophores. Despite decades of intensive investigation into their photoluminescence, a clear quantitative link between metal-metal bonding and emission efficiency is still lacking. Here we show that quantitatively modulating Cu-Cu bond distances during crystallization of Cu₆(SR)₆ nanoclusters enables a direct correlation between structure and emission performance. By synthesizing a series of Cu₆(SR)₆ nanoclusters with quantitatively modulated Cu-Cu bond lengths, we reveal an exponential relationship between bond distance and photoluminescence quantum yield (PLQY), and a linear correlation with emission energy. Density functional theory (DFT) calculations and ultrafast spectroscopy demonstrate that the enhanced PLQY arises from reduced HOMO-LUMO overlap induced by extended Cu-Cu distances, which promotes greater orbital localization. Simultaneously, the associated widening of the electronic gap suppresses non-radiative decay via the energy-gap law, further contributing to the increase in PLQY. This work establishes a quantitative relationship between Cu-Cu bond distance and quantum yield in Cu clusters, providing a general design framework for achieving high-efficiency emitters through quantitative bond-length engineering.