Au ₇₆ (SC ₆ H ₄ - <i>p</i> -CH ₃ ) ₄₂ Square Quantum Platelet: One-Dimensional Growth of Quantum Rods Turns 90 Degrees

The growth pattern of atomically precise nanoclusters (NCs) is of fundamental interest, and the structural effect on their photoluminescence (PL) is important owing to their PL in the second near-infrared (NIR-II) range (900-1700 nm optoelectronically or 1000-1700 nm biologically) that holds great promise for optoelectronic and biomedical applications. Because of the small energy gaps required for NIR-II emission, the PL performance of NIR-II luminophores is largely limited by nonradiative processes. In this work, we discovered a Au₇₆(<i>p</i>-MBT)₄₂ (<b>Au</b>_<b>76</b>) (<i>p</i>-MBTH = <i>p</i>-methylbenzenethiol) nanocluster featuring a face-centered cubic (fcc) core in a square shape (edge length: 1 nm). This square quantum platelet can be viewed as a side-facet (010) growth of the Au₅₂(<i>p</i>-MBT)₃₂ (<b>Au</b>_<b>52</b>) rod, as opposed to the (001) facet growth. We found that <b>Au</b>_<b>76</b> exhibits bright emission centered at 970 nm with a PL quantum yield (PLQY) of 30% in solution under ambient conditions, which can be further enhanced to 40% when the solution is deaerated. X-ray crystallography analysis coupled with time-resolved spectroscopy revealed that the nearly doubled PLQY compared to <b>Au</b>_<b>52</b> (18.3%) was resulted from shorter Au-Au bond lengths in <b>Au</b>_<b>76</b> (average 2.835 Å) than that in <b>Au</b>_<b>52</b> (3.04 Å). This work provides important insights into the design of highly luminescent NCs, which are promising for photovoltaics, photocatalysis, and optoelectronic applications.