Chiral Au₂₅ Nanospheres and Nanorods: Synthesis and Insight into the Origin of Chirality

Abstract

Chirality in nanoparticles is an intriguing phenomenon. Herein, we have devised a well-defined gold nanoparticle system for investigating the origin of chirality in nanoparticles. We have designed chiral thiols (R- and S-isomers) and synthesized chiral gold nanoparticles composed of 25 gold atoms and 18 ligands, referred to as Au(25)(pet)(18), where pet represents chirally modified phenylethylthiolate -SCH(2)CH(CH(3))Ph at the 2-position. These optically active nanoparticles are close analogues of the optically nonactive phenylethylthioalte-capped Au(25)(pet)(18) nanoparticles, and the latter's crystal structure is known. On the basis of the atomic and electronic structures of these well-defined Au(25) nanoparticles, we have explicitly revealed that the ligands and surface gold atoms of Au(25)(pet)(18) play a critical role in effecting the circular dichroism responses from the nanoparticles. Similar effects are also observed in chiral Au(25) rods. The mixing of electronic states of ligands with those of surface gold atoms constitutes the fundamental origin of chirality in such nanoparticles.

The fate of Au25(SR)18 clusters upon ligand exchange with binaphthyl-dithiol: interstaple binding vs. decomposition

“…Note that the CD spectra are quite different from the ones reported for Au 25 clusters covered by glutathione, camphorthiol or 1-methyl-2-phenylethylthiolate. [24][25][26] The CD spectra of the former two are quite similar. The optical activity of this cluster is thought to arise from the mixing of sulfur orbitals into the relevant cluster states.…”

First enantioseparation and circular dichroism spectra of Au38 clusters protected by achiral ligands

“…This indicates that (in this case) the chiral ligand does not have the dominant influence on the chiroptical properties of Au 38 clusters. We doubt that this finding can be generalized as chiral ligands can induce optical activity to clusters that are not intrinsically chiral (such as Au 25 (SR) 18 )1727. The good agreement between the CD spectra of Au 38 (SCH 2 CH 2 Ph) 24 and Au 38 ( L -SG) 24 strongly indicates that, in the latter case, one-handedness of the cluster (which dominated the CD spectra in the Au 38 case) is favoured over the other due to the presence of the chiral glutathionate-ligand.…”

“…Since then, numerous examples of more or less defined systems have been reported19. Protecting ligands include several derivatives of cysteine1020212223 as well as 'artificial' ligands such as binaphthyl systems242526 or other small organic thiolates27. Besides circular dichroism (CD) studies on the electronic transitions, the conformational analysis of the stabilizing ligand was demonstrated, using vibrational circular dichroism in the infrared202125.…”

Asymmetric Synthesis of Chiral Bimetallic [Ag28Cu12(SR)24]4– Nanoclusters via Ion Pairing

“…The majority of chiral metal nanoclusters known to date are of types (a) [27][28][29][30][31][32][33][34][35][36] and (b) 16,17,[37][38][39] , with a few belonging to types (c) and (d). [40][41][42] As prepared, many of these chiral metal nanoclusters (types b-d) are racemic mixtures that require separation and purification by various optical resolution methods.…”