Au₆₇(SR)₃₅ nanomolecules: Characteristic size-specific optical, electrochemical, structural properties and first-principles theoretical analysis

The preparation of gold nanomolecules with sizes other than Au ₂₅(SR)₁₈, Au₃₈(SR)₂₄, Au ₁₀₂(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 =-SCH₂CH ₂Ph 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.