Interstaple Dithiol Cross-Linking in Au25(SR)18 Nanomolecules: A Combined Mass Spectrometric and Computational Study

Abstract

A systematic study of cross-linking chemistry of the Au₂₅(SR)₁₈ nanomolecule by dithiols of varying chain length, HS–(CH₂)_n–SH where n = 2, 3, 4, 5, and 6, is presented here. Monothiolated Au₂₅ has six [RSAuSRAuSR] staple motifs on its surface, and MALDI mass spectrometry data of the ligand exchanged clusters show that propane (C3) and butane (C4) dithiols have ideal chain lengths for interstaple cross-linking and that up to six C3 or C4 dithiols can be facilely exchanged onto the cluster surface. Propanedithiol predominately exchanges with two monothiols at a time, making cross-linking bridges, while butanedithiol can exchange with either one or two monothiols at a time. The extent of cross-linking can be controlled by the Au₂₅(SR)₁₈ to dithiol ratio, the reaction time of ligand exchange, or the addition of a hydrophobic tail to the dithiol. MALDI MS suggests that during ethane (C2) dithiol exchange, two ethanedithiols become connected by a disulfide bond; this result is supported by density functional theory (DFT) prediction of the optimal chain length for the intrastaple coupling. Both optical absorption spectroscopy and DFT computations show that the electronic structure of the Au₂₅ nanomolecule retains its main features after exchange of up to eight monothiol ligands.