[Ag62S12(CBT)32]4+: A 2.2 nm Two-Electron Superatomic Carborane-Thiolated Silver Nanocluster Exhibiting Multilayer Charge Separation

Abstract

Investigating large metal nanoclusters decorated with three-dimensional molecular cages, with complete structural characterization, is challenging; however, their detailed understanding is important to study electronic confinement and associated properties. We have structurally resolved a nearly spherical 2.2 nm silver cluster with the molecular composition [Ag₆₂S₁₂(CBT)₃₂]⁴⁺, altogether having 842 atoms, solely protected with meta-carborane-thiolates (CBTs). This is the largest nanocluster with carborane-based molecular cages reported so far. Abscission of the cluster reveals that it has a face-centered-cubic Ag₁₄ inner core encapsulated with 12 sulfides, which is further surrounded by an outer scaffold of Ag₄₈–S₃₂ shell, protected with 32 meta-carboranes. The silver-sulfide skeleton of the nanocluster showed an assembly of multilayered polyhedra, having a mixture of Platonic and Archimedean solids. High-resolution mass spectrometric analyses and other spectroscopic studies further confirmed the molecular composition. This nanocluster exhibits characteristic molecular multiband optical absorption features along with a weak near-infrared (NIR) emission band. Ultrafast femtosecond transient absorption studies revealed stable photoexcited states linked to interlayer electron mobility between the neutral Ag₁₄ core and the positively charged Ag₄₈ shell, which are surrounded by negatively charged sulfide (S₁₂ and S₃₂) layers. Computational analysis shows that this cluster behaves as a two-electron superatom with a band gap of 1.77 eV, which is associated with the energy difference between the 1S symmetric and 1P nonsymmetric states. Successful structural characterization and associated optical properties of the nanocluster suggested that other larger metal nanoclusters encapsulated by three-dimensional molecular cages may be suitable for single-particle photonic and optoelectronic applications.