Small symmetry-breaking triggering large chiroptical responses of Ag70 nanoclusters

Abstract

The origins of the chiroptical activities of inorganic nanostructures have perplexed scientists, and deracemization of high-nuclearity metal nanoclusters (NCs) remains challenging. Here, we report a single-crystal structure of Rac-Ag₇₀ that contains enantiomeric pairs of 70-nuclearity silver clusters with 20 free valence electrons (Ag₇₀), and each of these clusters is a doubly truncated tetrahedron with pseudo-T symmetry. A deracemization method using a chiral metal precursor not only stabilizes Ag₇₀ in solution but also enables monitoring of the gradual enlargement of the electronic circular dichroism (CD) responses and anisotropy factor g_abs. The chiral crystals of R/S-Ag₇₀ in space group P2₁ containing a pseudo-T-symmetric enantiomeric NC show significant kernel-based and shell-based CD responses. The small symmetry breaking of T_d symmetry arising from local distortion of Ag-S motifs and rotation of the apical Ag₃ trigons results in large chiroptical responses. This work opens an avenue to construct chiral medium/large-sized NCs and nanoparticles, which are promising for asymmetric catalysis, nonlinear optics, chiral sensing, and biomedicine.