Reversible Isomerization of Metal Nanoclusters Induced by Intermolecular Interaction

Abstract

Most inorganic nanoparticles are directly surface-terminated (and -stabilized) by protecting ligands, which could greatly affect the atomic packing and physical/chemical properties of their inorganic cores. Here, we show that the intermolecular interactions between the adsorbed molecules and surface ligands can also affect the core structure of inorganic nanoparticles. Through the coupling/decoupling of cationic surfactants (cetyltrimethylammonium cations, CTA+) and anionic surface ligands (para-mercaptobenzoic acid, p-MBA) in the aqueous phase, we have achieved a reversible transformation between two isomers of [Au25(p-MBA)18]− nanoclusters. The interconversion process between the two isomers shows the characteristics of a reversible nanocluster isomerization process, with a measured forward and backward activation barrier of 1.16 and 1.17 eV, respectively. One of the isomers has the structure similar to the single-crystal data of [Au25(SR)18]− known since 2008. The other isomer features a distinctly different optical absorption spectrum and color change (i.e., reddish brown to dark green) in the solution phase, and its structure is identified to be the one recently predicted and observed in gas phase through density functional theory (DFT) calculations. The two isomers are topologically connected via a simple rotation of the gold core without breaking any Au-S bonds at the metal-ligand interface. Molecular dynamics simulations suggest that the adsorbed CTA+ cations would interact with the p-MBA layer through the CH∙∙∙π interaction and stabilize the more open ligand shell of the new isomer in the forward isomerization process. Interestingly, this principle can also be extended to organic phases by using organo-soluble cations to stimulate the surface interactions. Our research proposes a new approach to control the structure of inorganic nanoparticles, which helps to customize their electronic and optical properties in the solution phase without changing their surface ligands.