Insights into the Distance Dependence of the Electron Transfer Rate through the Monolayer Protecting Au25 Nanoclusters

The monolayer that protects small gold nanoparticles is often represented as the 3D equivalent of 2D self-assembled monolayers (SAMs) on extended gold surfaces. However, despite the growing relevance of monolayer-protected clusters (MPCs) in important applied areas, such as catalysis and nanomedicine, our knowledge of the structure of these 3D SAMs is quite limited. This is particularly true for the MPCs displaying molecule-like properties (core diameters of less than 1.6 nm). We prepared a large series of monodisperse Au₂₅(SC_nH_2n+1)₁₈ clusters and studied how electrons tunnel through these monolayers. Heterogeneous electron-transfer results, nicely supported by ¹H NMR spectroscopy and molecular dynamics results, provide an interesting picture of the effective structure of the 3D monolayers protecting ultrasmall gold clusters. The distance dependence of the electron-transfer rate in solution is also compared with that of electron transport in the solid state.