Au67(SR)35 Nanomolecules: Characteristic Size-Specific Optical, Electrochemical, Structural Properties and First-Principles Theoretical Analysis

ABSTRACT: The preparation of gold nanomolecules with sizes other than Au25(SR)18, Au38(SR)24, Au102(SR)44, and Au144(SR)60 has been hampered by stability issues and low yields. Here we report a procedure to prepare Au67(SR)35, for either R = −SCH2CH2Ph or-SC6H13, allowing high-yield isolation (34%, ∼10-mg quantities) of the title compound. Product high purity is assessed at each synthesis stage by rapid MALDI−TOF mass-spectrometry (MS), and high-resolution electrospray-ionization MS confirms the Au67(SR)35 composition. Electronic properties were explored using optical absorption spectroscopy (UV− visible−NIR regions) and electrochemistry (0.74 V spacing in differential-pulsed-voltammetry), modes of ligand binding were studied by NMR spectroscopy (13C and 1H), and structural characteristics of the metal atom core were determined by powder X-ray measurements. Models featuring a Au17 truncated-decahedral inner core encapsulated by the 30 anchoring atoms of 15 staple-motif units have been investigated with first-principles electronic structure calculations. This resulted in identification of a structure consistent with the experiments, particularly, the opening of a large gap (∼0.75 eV) in the (2−) charge-state of the nanomolecule. The electronic structure is analyzed within the framework of a superatom shell model. Structurally, the Au67(SR)35 nanomolecule is the smallest to adopt the complete truncated-decahedral motif for its core with a surface structure bearing greater similarity to the larger nanoparticles. Its electronic HOMO−LUMO gap (∼0.75 eV) is nearly double that of the larger Au102 compound and it is much smaller than that of the Au38 one. The intermediary status of the Au67(SR)35 nanomolecule is also reflected in both its optical and electrochemical characteristics