Gold-gold interaction directed self-assembly of polynuclear gold(I) clusters and ultrasmall gold nanoclusters : from cluster-to-cluster transformation to structure-dependent photophysics

Abstract

A series of functionalized polynuclear gold(I) μ3-sulfido clusters bearing Au(I)···Au(I) interactions has been synthesized using the chlorogold(I) precursors with unsaturated C=C bonds as active sites, including [(vdpp){AuCl}2], [(cis-dppee){AuCl}2] and [(trans-dppee){AuCl}2]. The identities of them have been fully characterized by NMR, electrospray-ionization mass spectrometry, elemental analysis, and single crystal X-ray diffraction analysis. The vdpp-based gold(I) sulfido clusters were found to undergo addition reaction-induced cluster-to-cluster transformation through post-modification of double bonds in the clusters; while photoinduced isomerization-driven structural reconfiguration was observed between the dppee-protected decagold(I) and octadecagold(I) clusters upon the cis-to-trans transformation of the dppee ligands. Diverse research methods, including UV-vis absorption spectroscopy, 1H NMR and 31P{1H} NMR spectroscopy, were employed to probe and monitor the transformation and assembly processes. Supported and driven by Au(I)∙∙∙Au(I) bonding interactions, the nuclearity and symmetry of these clusters were found to be largely different from each other, resulting in completely distinct photophysical features.

By employing axial chiral BINAP-based precursors, a new class of chiral gold(I)-sulfido clusters with unique structures has been constructed. Interestingly, pure enantiomers of the precursors were found to self-assemble into chiral hexa- and decanuclear clusters sequentially while a racemic mixture of them has resulted in heterochiral self-sorting of an achiral meso decanuclear cluster. Chirality has been demonstrated to determine not only the symmetry and structures, but also the photophysical behaviors of these clusters. The racemic mixture of decanuclear clusters was also shown to undergo rearrangement and heterochiral self-sorting to give a meso decanuclear cluster. The thermodynamic-driven heterochiral self-sorting of gold(I) clusters has provided a means to develop controlled self-assembly that may be of relevance to the understanding of chirality in nature.

A new class of mixed-valent ultrasmall gold nanoclusters of molecular characteristics has been constructed using 1,1-bis(diphenylphosphino)ferrocene (dppf) as the stabilizing ligand. The identities of the ultrasmall gold nanoclusters have been fully established by NMR, ESI mass spectrometry, elemental analysis, and single crystal X-ray diffraction analysis. Octa- and undecagold clusters were found to display different UV-vis absorption behavior. The ligation of the bidentate ligands and halides or pseudohalides has resulted in the symmetry breaking of these nanoclusters with C1 symmetry. The ultrasmall gold nanoclusters with different coordinating halides or pseudohalides showed distinct reactivity and stability in ligand or anion exchange experiments. The current research has provided insights into the formation and chirality origin of the diphosphine-stabilized ultrasmall gold nanoclusters, and hence the future design and synthesis of novel nanoclusters and cluster-based nanomaterials.