Optical Activity of Chiral Metal Nanoclusters

ConspectusThe emerging chirality in nanomaterials represents one of the most dynamic areas in modern science. Although many novel chiral nanomaterials have been reported, the origin of their chirality and related optical activity have been not unveiled unambiguously. Thanks to the fast development of noble metal cluster chemistry, the structures of many chiral nanoclusters (CNCs) have been precisely determined and their chirality origin has been decoded as four different levels including chiral ligand, helix staple motif, asymmetry metal kernel, and chiral arrangement of organic ligands. Generally, the strong optical activity associated with the electron transitions of the noble metal core is popularly reported among CNCs protected by chiral ligands, following the so-called "outside-in" chirality transfer theory, namely, from organic ligand to metal core. Exceptionally, inherent chiral structures are discerned inside CNCs consisting of achiral ligands, such as the helix staples found in Au38(SR)24 and Au102(SR)44 (SR = thiolate) and the chiral metal kernel existing in Au20(PP3)4Cl4 (PP3 = tris(2-(diphenylphosphino)ethyl) phosphine). These chiral nanostructures induce distinct optical activity and even present reversed chirality transformation in the case of Au38(SR)24 (i.e., from chiral core structure to organic ligand). In the past decade, our group has carried out extensive research work on preparation, enantioseparation, optical activity, and application of chiral inorganic nanostructures. As representatives, enantiopure right-handed and left-handed Au20(PP3)4Cl4 clusters of intrinsic kernel chirality were acquired through an innovative supramolecular self-assembly method and their circular dichroism (CD) feature involving only the metal core was systematically studied; Au3[R/S-Tol-BINAP]3Cl (R/S-Tol-BINAP: R/S-2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl) clusters with the smallest metal-atom number among the reported CNCs were designed and synthesized by using the privileged chiral Tol-BINAP ligand, which exhibited not only strongly enhanced CD signal also remarkable circular polarized luminescence (CPL) response as self-assembled into monodispersed nanocubes.In this Account, we aim at reviewing the fast development of CNCs featuring strong chiral attributes and optical activity. We will briefly introduce the preparation methods of CNCs, such as direct synthesis, ligand exchange, and enantioseparation. In the following parts, the commonly used tools for characterizing the chirality of CNCs are summarized, including CD, vibrational circular dichroism (VCD), CPL, single X-ray diffraction, nuclear magnetic resonance (NMR), and theoretical prediction. Then, the optical activity of CNCs will be systematically discussed, especially their CD, VCD, and CPL activity along with their chirality origin. Finally, future strategies for fabricating CNCs possessing strong optical activity as well as potential chirality-related applications will be proposed. We believe that this Account will trigger more research interest to not only study the amazing optical activity of CNCs but also employ them in many fields.