Light-Activated Intercluster Conversion of an Atomically Precise Silver Nanocluster

Noble metal nanoclusters protected with carboranes, a 12-vertex, nearly icosahedral boron-carbon framework system, have received immense attention due to their different physicochemical properties. We have synthesized ortho-carborane-1,2-dithiol (CBDT) and triphenylphosphine (TPP) coprotected [Ag-42(CBDT)(15)(TPP)(4)](2-) (shortly Ag-42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)(10)](2+) (shortly Ag-18). The formation of Ag-42 was confirmed using UV-vis absorption spectroscopy, mass spectrometry, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and multinuclear magnetic resonance spectroscopy. Multiple UV-vis optical absorption features, which exhibit characteristic patterns, confirmed its molecular nature. Ag-42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 degrees C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag-14(CBDT)(6)(TPP)(6)] (shortly Ag-14). Single crystal X-ray diffraction of Ag-14 exhibits Ag-8-Ag-6 core-shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag-14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [Ag-37(CBDT)(12)(TPP)(4)](3-) and [Ag-35(CBDT)(8)(TPP)(4)](2-) were formed after 8 h of light irradiation, and the second set comprised of [Ag-30(CBDT)(8)(TPP)(4)](2-), [Ag-26(CBDT)(11)(TPP)(4)](2-), and [Ag-26(CBDT)(7)(TPP)(7)](2-) were formed after 16 h of irradiation. After 24 h, the conversion to Ag-14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag-42 are responsible for light-activated transformation. Interestingly, Ag-42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 mu s, indicating phosphorescence, while Ag-14 shows red luminescence at 626 nm (1.98 eV) with a lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond transient absorption showed the transitions between their electronic energy levels and associated carrier dynamics. Formation of the stable excited states of Ag-42 is shown to be responsible for the core transformation.