Atomistic insight into the aggregation of [Au₂₅(SR)₁₈] nanoclusters

Atomically precise nanoclusters have been proven to give solid state aggregates with intriguing optical properties. However, the mechanism that regulates this aggregation remains unclear. Here, the aggregation of two Au₂₅ nanoclusters in solution is investigated through enhanced sampling molecular dynamics simulations. To understand how the free energy of the systems depends on the nanocluster features, calculations were performed on three nanocluster pairs which differ in charge states and substituent nature and dimension. Our results show that the choice of the ligands heavily affects the free energy profile of the systems when the structures are nearby and, in some cases, the formation of a dimeric phase is observed. This phase is particularly stable in long-chain substituted nanoclusters, where the long alkane chains can generate bundles and the gold cores are closer compared to the short-chain ligands. We found a remarkable agreement between our calculations and the literature-available solid-state structures, especially for the orientation of the interacting nanoclusters. Moreover, some of the dimeric structures are prodromal to the formation of the aurophilic intercluster bond observed in the crystal structures, meaning that the dimer can act as a precursor and can drive the whole crystallization mechanism toward the formation of stable crystal species.