Chiral Functionalization of an Atomically Precise Noble Metal Cluster: Insights into the Origin of Chirality and Photoluminescence

Abstract

We probe the origin of photoluminescence of an atomically precise noble metal cluster, Ag₂₄Au₁(DMBT)₁₈ (DMBT = 2,4-dimethylbenzenethiolate), and the origin of chirality in its chirally functionalized derivatives, Ag₂₄Au₁(<i>R</i>/<i>S</i>-BINAS)_<i>x</i>(DMBT)_18-2<i>x</i>, with <i>x</i> = 1-7 (<i>R</i>/<i>S</i>-BINAS = <i>R</i>/<i>S</i>-1,1'-[binaphthalene]-2,2'-dithiol), using chiroptical spectroscopic measurements and density functional theory (DFT) calculations. Combination of chiroptical and luminescence spectroscopies to understand the nature of electronic transitions has not been applied to such molecule-like metal clusters. In order to impart chirality to the achiral Ag₂₄Au₁(DMBT)₁₈ cluster, the chiral ligand, <i>R</i>/<i>S</i>-BINAS, was incorporated into it. A series of clusters, Ag₂₄Au₁(<i>R</i>/<i>S</i>-BINAS)_<i>x</i>(DMBT)_18-2<i>x</i>, with <i>x</i> = 1-7, were synthesized. We demonstrate that the low-energy electronic transitions undergo an unexpected achiral to chiral and back to achiral transition from pure Ag₂₄Au₁(DMBT)₁₈ to Ag₂₄Au₁(<i>R</i>/<i>S</i>-BINAS)_<i>x</i>(DMBT)_18-2<i>x</i>, by increasing the number of BINAS ligands. The UV/vis, luminescence, circular dichroism, and circularly polarized luminescence spectroscopic measurements, in conjunction with DFT calculations, suggest that the photoluminescence in Ag₂₄Au₁(DMBT)₁₈ and its chirally functionalized derivatives originates from the transitions involving the whole Ag₂₄Au₁S₁₈ framework and not merely from the icosahedral Ag₁₂Au₁ core. These results suggest that the chiroptical signatures and photoluminescence in these cluster systems cannot be solely attributed to any one of the structural components, that is, the metal core or the protecting metal-ligand oligomeric units, but rather to their interaction and that the ligand shell plays a crucial role. Our work demonstrates that chiroptical spectroscopic techniques such as circular dichroism and circularly polarized luminescence represent useful tools to understand the nature of electronic transitions in ligand-protected metal clusters and that this approach can be utilized for gaining deeper insights into the structure-property relationships of the electronic transitions of such molecule-like clusters.