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 [Ag42(CBDT)15(TPP)4]2– (shortly Ag42) using a ligand-exchange induced structural transformation reaction starting from [Ag18H16(TPP)10]2+ (shortly Ag18). The formation of Ag42 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. Ag42 is the highest nuclearity silver nanocluster protected with carboranes reported so far. Although these clusters are thermally stable up to 200 °C in their solid state, light-irradiation of its solutions in dichloromethane results in its structural conversion to [Ag14(CBDT)6(TPP)6] (shortly Ag14). Single crystal X-ray diffraction of Ag14 exhibits Ag8–Ag6 core–shell structure of this nanocluster. Other spectroscopic and microscopic studies also confirm the formation of Ag14. Time-dependent mass spectrometry revealed that this light-activated intercluster conversion went through two sets of intermediate clusters. The first set of intermediates, [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 of [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 to Ag14 was complete. Density functional theory calculations reveal that the kernel-centered excited state molecular orbitals of Ag42 are responsible for light-activated transformation. Interestingly, Ag42 showed near-infrared emission at 980 nm (1.26 eV) with a lifetime of >1.5 μs, indicating phosphorescence, while Ag14 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 Ag42 is shown to be responsible for the core transformation.以碳硼烷（carboranes）——一种12顶点、近乎二十面体的硼碳骨架体系——保护的贵金属纳米团簇，因其独特的理化性质受到广泛关注。我们以[Ag18H16(TPP)10]2+（简称Ag18）为起始原料，通过配体交换诱导的结构转变反应，合成了邻碳硼烷-1,2-二硫醇（ortho-carborane-1,2-dithiol, CBDT）与三苯基膦（Triphenylphosphine, TPP）共保护的[Ag42(CBDT)15(TPP)4]2–（简称Ag42）。通过紫外-可见吸收光谱、质谱、透射电子显微镜、X射线光电子能谱、红外光谱及多核核磁共振光谱对Ag42的生成进行了确证。其具有多个特征性紫外-可见光学吸收特征，证实了其分子属性。Ag42是目前已报道的碳硼烷保护的银纳米团簇中核数最高的物种。该团簇在固态下可稳定至200 ℃，但其二氯甲烷溶液经光照后会发生结构转变，生成[Ag14(CBDT)6(TPP)6]（简称Ag14）。Ag14的单晶X射线衍射分析显示其具有Ag8–Ag6核壳结构的纳米团簇，其他光谱与显微研究也证实了Ag14的生成。时间分辨质谱研究表明，这一光触发的团簇间转化经历了两个阶段的中间团簇：光照8 h后生成第一组中间体[Ag37(CBDT)12(TPP)4]3–与[Ag35(CBDT)8(TPP)4]2–；光照16 h后生成第二组中间体[Ag30(CBDT)8(TPP)4]2–、[Ag26(CBDT)11(TPP)4]2–及[Ag26(CBDT)7(TPP)7]2–。至24 h时，转化完全完成。密度泛函理论计算表明，Ag42的核中心激发态分子轨道是其光触发转变的诱因。值得注意的是，Ag42在980 nm（1.26 eV）处表现出近红外发射，寿命超过1.5 μs，属于磷光；而Ag14在626 nm（1.98 eV）处呈现红色发光，寿命为550 ps，属于荧光。飞秒与纳秒瞬态吸收光谱揭示了其电子能级间的跃迁及相关载流子动力学过程。Ag42稳定激发态的形成被证实是其核结构转变的关键原因。