Ag7Au6: a 13-atom alloy quantum cluster.

Stable gold cluster molecules of the type Au11, [1a] Au13, [1b,c] and Au55, [1d] have been fascinating and were some of the early molecular nanosystems synthesized. Quantum phenomena such as Coulomb blockade were demonstrated with them. Unusually intense luminescence and chemical reactivity of molecular clusters or quantum clusters (QCs) of noble metals, for example gold have attracted intense interest in areas such as nanophotonics, bioimaging, catalysis, and others. In the past several years, some of these clusters have been characterized thoroughly, both theoretically and experimentally. Several reports of various core sizes of silver clusters are also known. However, synthesis of atomically precise alloy QCs of various compositions is a challenge. Murray et al. synthesized Pd@Au24(SR)18 which was further studied by Negishi et al. Negishi et al. isolated PdAu24(SR)18. [11b] AgnAu25 n(SR)18, and (AuAg)140(SR)60 (R=organic soluble alkyl/aryl groups) were obtained by simultaneously reducing metal salts in suitable conditions. There is also a family of well-characterized Ag/ Au/Cu cluster complexes that are different from the metal core clusters discussed herein. We present the very first experimental investigation of the water-soluble AgAu alloy system belonging to the 13-atom family of clusters by a new synthetic route that reveals a category of materials of structural and electronic complexity that is expected to attract interest in the coming years. The bimetallic 13-atom QC Ag7Au6, which is protected with mercaptosuccinic acid (H2MSA, or its dianion MSA upon ionization in solution), exhibits well-defined transitions in the absorption profile. This QC is prepared from the Ag7,8 cluster (containing a mixture of Ag7 and Ag8). [10d] Synthesis of alloy QCs involves three steps (for details, see the Supporting Information, S1). The various stages of synthesis are shown in Figure 1. First step is the synthesis of polydisperse Ag@H2MSA nanoparticles followed by the synthesis of Ag7,8 clusters by interfacial etching in the second step, as reported previously. Addition of an appropriate amount of 10 mm HAuCl4 to the as-synthesized Ag7,8 cluster in the third step yields the alloy cluster (for details see the Experimental Section).