Visualizing Kinetic Control and Thermodynamic Selection in Nanocluster Structural Evolution

Abstract

It is extremely challenging to clarify the precise atomic-level evolution of nanoclusters from a dynamically trapped state to a thermodynamically selected product. Various factors play distinctly different roles in the thermodynamic and kinetic control of structural transformation reactions. In this study, we used the system Ag14(SPh)12(DPPH)4 → Ag23(SPh)18(DPPH)4 as a homologous model system, and combined single-crystal structural analysis, time-resolved electrospray ionization mass spectrometry (ESI-MS), ultraviolet–visible absorption spectroscopy, and photoluminescence measurements for correlation analysis. Crystallographic studies revealed that the Ag23 framework could be geometrically decomposed into two substructures related to Ag14, both sharing the metal and thioimine groups, providing a structural basis for the bimolecular fusion hypothesis. Time-dependent spectroscopic techniques revealed the consumption process of Ag14 and the growth process of Ag23, while temperature-dependent experiments showed a kinetic window within which the conversion rate accelerated, but higher thermal input would promote breakage/decomposition and form Ag10. Due to the ability of ESI-MS to generate gas-phase fragments, ions containing Ag5- and Ag9 were regarded as candidate species detected by ESI, rather than confirmed solution intermediates. Therefore, the core breakthrough was not the isolated Ag14 or Ag23 nuclear numbers, but the mechanistic connection of the two structurally similar cluster states in a single reaction network and the identification of thermal activation as the key factor guiding the evolution of the cluster states.