Bulky t-Butyl Thiolated Gold Nanomolecular Series: Synthesis, Characterization, Optical Properties, and Electrocatalysis

Here we report the synthesis and size-dependent optical study of a series of bulky t-butyl thiolated monodisperse nanomolecules (NMs): Au-23(S-tBu)(16), Au-30(S-tBu)(18), AU(46)(S-tBU)(24), and Au-65(S-tBu)(29). We further employ this series of NMs to address how the NM core size affects electrocatalytic oxygen reduction reactivity of these NMs in alkaline media. Though Au-23(S-tBu)(16) and Au-30(S-tBu)(18) have been reported before, the expansion of this series is brought about by the discovery, through the synthesis and characterization, of two new and larger species, in the bulky t-butyl thiolated NM series: namely Au-46(S-tBu)(24) and Au-65(S-tBu)(29). Furthermore, we take an in-depth look at the ligand effects in the bulky thiolated NM series and ligand dictation over the gold atoms to surface ligands. Assignments of molecular formulae are based on results obtained in high-resolution nanoelectrospray ionization mass spectrometry (nESI-MS). Purity was confirmed with matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF-MS). Further analysis of the new bulky thiolated NMs, Au-46(S-tBu)(24) and Au-65(S-tBu)(29), was conducted using UV-vis-NIR spectroscopy to observe their unique optical properties. With the expansion of this series, we conclude that the bulky t-butyl thiolated series is unique from those observed with aliphatic and aromatic ligands; that is, how the bulky ligands dictate the size, composition, and structure of the NM is distinct from those of aliphatic and aromatic ligands. This provides insight into the tunability and core size manipulation of gold thiolate NM (i.e., we can make different sizes of NMs using specific types of ligands by varying the gold to thiol ratio). The largest NM Au-65(S-tBu)(29) was found to facilitate 80% OH- production, while the smallest size NM Au-23(S-tBu)(16) produced 53% OH-. From the optical measurements we determined that the excited state lifetimes of this series of t-butyl thiolate protected gold NMs follow the energy gap law except for Au-30(S-tBu)(18) which can be ascribed to its symmetry.