Light Exposure Promotes Degradation of Intermediates and Growth of Phosphine-Ligated Gold Clusters

Subnanometer clusters exhibit unique properties not observed at the bulk or nanometer scale. Many applications, however, require clusters to be monodisperse in size and composition. While substantial progress has been made empirically on producing specific-sized gold clusters using different ligands, the mechanisms of cluster formation remain insufficiently understood. Using high mass-resolution electrospray ionization mass spectrometry, we characterized the formation of cationic triphenylphosphine-ligated gold clusters [Au-x(PPh3)(y)(z+)] in solution. Our data provides evidence that small hydrogen-containing intermediate "building blocks" (i.e., Au-2(PPh3)(2)H+, Au-4(PPh3)(4)H+, and Au-7(PPh3)(7)H-5(2+)) are responsible for the growth of larger gold clusters (i.e., Au-6(PPh3)(6)(2+) and Au-8(PPh3)(7)(2+)) over several hours. In addition, intermediate degradation and cluster growth are shown to be dependent on irradiation of the reacting solution with light. Specifically, the rate of growth of large clusters is demonstrated to scale with the intensity of light exposure. Complementary theoretical calculations were also performed that provide molecular-level insight into the nature of the bonding between the hydrogens and gold atoms in the intermediate Au-phosphine clusters. Collectively, our joint experimental and theoretical findings advance the understanding of the factors at play in the size-selective synthesis of ligated gold clusters in solution.