Molecular Engineering of Atomically Precise Silver Clusters into 2D and 3D Framework Solids

Abstract

Two- and three-dimensional (2D and 3D) atomically precise nanocluster (NC)-based metal–organic frameworks (MOFs) with properties richer than those of NCs themselves are emerging materials. However, fabricating such materials with good stability has not been easy. In this work, a facile synthetic strategy was employed for the creation of silver NC–MOFs starting from [Ag12(TBT)7(TFA)4(CH3CN)6]+, facilitated by heterocyclic amines, 4,4′-bipyridine (bpy) and pyrazine (pyz), via metal–metal and metal-sulfide rearrangement reactions, where TBT and TFA are tertiarybutylthiolate and trifluoroacetate, respectively. In one of the reactions, the pyz ligand facilitates the formation of a 2D framework with a trigonal crystal system, which exhibits high stability and emits bright green luminescence at low temperatures. Owing to its facile synthesis, good stability, efficient luminescence, uniform porosity, and layered structure, the resultant hexagonal 2D nanosheets can be efficiently exfoliated from parent crystals. The 2D nanosheets are structurally similar to graphene. A top-down approach was employed for the exfoliation of stable 2D nanosheets with lateral dimensions in the range of 0.156 μm. In another case, the bpy ligand induces the construction of a 3D framework with an orthorhombic crystal system. Owing to its interpenetrated AB···AB structure, robustness, and efficient green luminescence at room temperature, the resultant 3D MOF is capable of functioning as a high-performance luminescent sensor for selective detection of explosive analogues, 2-nitrotoluene and 2,4-dinitrotoluene, with excellent recyclability. However, in the absence of the heterocyclic amines, a pristine AgNC was formed. Time-dependent density functional theory calculations were employed to understand the mechanism of energy transfer in AgNC-MOFs. Our strategy offers an unprecedented approach in which heterocyclic amines facilitate intramolecular rearrangement reactions, resulting in 2D and 3D atomically precise NC framework materials. This work not only demonstrates the creation of 2D and 3D materials but also provides new insights into the critical surface coordination chemistry controlled by heterocyclic amines for defining the morphology and properties of cluster frameworks.