Monolayer reactions of protected Au nanoclusters with monothiol tiopronin and 2,3-dithiol dimercaptopropanesulfonate.

The novel thiol bridging "staple motif RS-Au-SR" discovered at the Au-thiolate interface has tremendously advanced the structural understanding of monolayer protected Au clusters (AuMPCs). In this paper, multidentate dithiol ligands are introduced into the monolayer of the Au clusters. The impacts of dithiols on the Au-monothiolate interfacial bonding and related physical properties are explored. A correlation is established of the near-IR luminescence with Au-tiopronin monothiol interactions that are constrained by the dithiol molecule structures. Two types of monolayer reaction are studied: (1) monothiol tiopronin AuMPCs with dithiol molecule 2,3-dimercaptopropanesulfonate (DMPS) and (2) DMPS Au dithiol clusters (AuDTCs) with tiopronin monothiol ligands. Upon the addition of excess DMPS molecules into tiopronin MPC solution, tiopronin molecules are efficiently liberated from the original AuMPCs monitored by proton NMR. The process is accompanied by the decrease of near-infrared luminescence of the tiopronin AuMPCs. A slower enhancement of the 282 nm absorption band is observed, a signature of DMPS Au4DTCs characterized by mass spectrometry. The analysis of the reaction kinetics reveals a two-step mechanism: a facile ligand replacement followed by a sluggish core etching process. The reverse approach, tiopronin molecules reacting with DMPS DTCs, results in the addition of tiopronin into DMPS monolayer instead of ligand exchange. Near-IR luminescence intensifies with the monolayer addition of tiopronin.