Light-Activated\nIntercluster Conversion of an Atomically\nPrecise Silver Nanocluster

Abstract

Noble\nmetal nanoclusters protected with carboranes, a 12-vertex,\nnearly icosahedral boron–carbon framework system, have received\nimmense attention due to their different physicochemical properties.\nWe have synthesized <i>ortho</i>-carborane-1,2-dithiol (CBDT)\nand triphenylphosphine (TPP) coprotected [Ag₄₂(CBDT)₁₅(TPP)₄]^2– (shortly Ag₄₂) using a ligand-exchange induced structural transformation reaction\nstarting from [Ag₁₈H₁₆(TPP)₁₀]²⁺ (shortly Ag₁₈). The formation of Ag₄₂ was confirmed using UV–vis absorption spectroscopy, mass\nspectrometry, transmission electron microscopy, X-ray photoelectron\nspectroscopy, infrared spectroscopy, and multinuclear magnetic resonance\nspectroscopy. Multiple UV–vis optical absorption features,\nwhich exhibit characteristic patterns, confirmed its molecular nature.\nAg₄₂ is the highest nuclearity silver nanocluster protected\nwith carboranes reported so far. Although these clusters are thermally\nstable up to 200 °C in their solid state, light-irradiation of\nits solutions in dichloromethane results in its structural conversion\nto [Ag₁₄(CBDT)₆(TPP)₆] (shortly Ag₁₄). Single crystal X-ray diffraction of Ag₁₄ exhibits\nAg₈–Ag₆ core–shell structure of\nthis nanocluster. Other spectroscopic and microscopic studies also\nconfirm the formation of Ag₁₄. Time-dependent mass spectrometry\nrevealed that this light-activated intercluster conversion went through\ntwo sets of intermediate clusters. The first set of intermediates,\n[Ag₃₇(CBDT)₁₂(TPP)₄]^3– and [Ag₃₅(CBDT)₈(TPP)₄]^2– were formed after 8 h of light irradiation, and the second set comprised\nof [Ag₃₀(CBDT)₈(TPP)₄]^2–, [Ag₂₆(CBDT)₁₁(TPP)₄]^2–, and [Ag₂₆(CBDT)₇(TPP)₇]^2– were formed after 16 h of irradiation. After 24 h, the conversion\nto Ag₁₄ was complete. Density functional theory calculations\nreveal that the kernel-centered excited state molecular orbitals of\nAg₄₂ are responsible for light-activated transformation.\nInterestingly, Ag₄₂ showed near-infrared emission at 980\nnm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence,\nwhile Ag₁₄ shows red luminescence at 626 nm (1.98 eV) with\na lifetime of 550 ps, indicating fluorescence. Femtosecond and nanosecond\ntransient absorption showed the transitions between their electronic\nenergy levels and associated carrier dynamics. Formation of the stable\nexcited states of Ag₄₂ is shown to be responsible for the\ncore transformation.