Size Evolution Dynamics of Gold Nanoclusters at an Atom-Precision Level: Ligand Exchange, Growth Mechanism, Electrochemical, and Photophysical Properties.

Interpretation of size evolution is an essential part of nanocluster transformation processes for unraveling the mechanism at an atom-precision level. Here we report the transformation of a non-superatomic Au23 to a superatomic Au36 nanocluster via Au28 cluster formation, activated by the bulky 4-tert-butylbenzenethiol ligand. Time-dependent matrix-assisted laser desorption ionization mass spectrometry data revealed that the conversion proceeds through ligand exchange followed by the size focusing method, ultimately leading to size growth. We also validated this transformation through time-dependent ultraviolet-visible data. Density functional theory calculations predicted that the kernel of the Au28 cluster evolved through a linear combination of molecular orbitals of the fragment of 2e- units (Au42+ and Au3+) from the kernel of the Au23 cluster. Periodic growth of gold cores through continuous growth of Au4 tetrahedral unit leads to the formation of the Au36 cluster from the Au28 cluster. These results reinforce the plausibility of size evolution through the growth mechanism during the transformation process. Differential pulse voltammetry studies showed that the highest occupied molecular orbital-lowest unoccupied molecular orbital gap inversely varies with the kernel size of these clusters. Photophysical experiments support the molecular-like intersystem crossing rather than core-shell relaxation to these clusters. The trends of photoluminescence lifetime were found to be the reverse of those of the energy gap law. The increment of lifetimes for the larger cluster can be mainly due to the contribution of both hot carriers and band-edge carriers.