Impact of enantiomeric ligand composition on the photophysical properties of chiral Ag29nanoclusters

Abstract

Chiral surface ligands have often been employed to impart optical activity to metal nanoclusters, metal nanoparticles and semiconductor nanocrystals. They are considered to form a chiral structure to a certain degree whereas the effect of such chiral structure on the global physicochemical properties apart from chiroptical ones has been unexplored. We herein demonstrate the impact of optical purity of chiral surface ligand on the emission property of silver nanocluster (NCs). Chiral bidentate a-dihydrolipoic acid (DHLA) with varied enantiomeric excess (ee) values was employed as a surface capping ligand to prepare a series of silver NCs, displaying identical absorption and emission profiles typical for Ag29 NCs. Interestingly, the emission quantum yields exhibited a clear dependence on the enantiopurity of DHLA. The more enantiopure DHLA afforded more emissive NCs. This |ee|-dependent emission efficiency was discussed in association with the orientation of ligands on the Ag29 NCs. The surface structures of Ag29(dithiolate)12 models composed of enantiopure and racemic ligand systems were compared with the aid of DFT calculations, suggesting that the enantiopure surface is more stable with one-handed ligand orientation. Two-dimensional NMR technique also supported the observation that well-defined ligand orientations depend on the enantiomeric composition of chiral ligand.