Search

Au-38(SR)(24) is one of the most extensively investigated gold nanomolecules along with Au-25(SR)(18) and Au-144(SR)(60). However, so far it has only been prepared using aliphatic-like ligands, where R = -SC6H13, -SC12H25 and -SCH2CH2Ph. Au-38(SCH2CH2Ph)(24) when reacted with HSPh undergoes core-size conversion to Au-36(SPh)(24), and existing literature suggests that Au-38(SPh)(24) cannot be synthesized. Here, contrary to prevailing knowledge, we demonstrate that Au-38(SPh)(24) can be prepared if the ligand exchanged conditions are optimized, under delicate conditions, without any formation of Au-36(SPh)(24). Conclusive evidence is presented in the form of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), electrospray ionization mass spectra (ESI-MS) characterization, and optical spectra of Au-38(SPh)(24) in a solid glass form showing distinct differences from that of Au-38(S-aliphatic)(24). Theoretical analysis confirms experimental assignment of the optical spectrum and shows that the stability of Au-38(SPh)(24) is not negligible with respect to that of its aliphatic analogous, and contains a significant component of ligand-ligand attractive interactions. Thus, while Au-38(SPh)(24) is stable at RT, it converts to Au-36(SPh)(24) either on prolonged etching (longer than 2 hours) at RT or when etched at 80 degrees C.

Here we investigate via first-principles simulations the optical absorption spectra of three different Au-30(SR)(18) monolayer-protected clusters (MPC): Au-30((SBu)-Bu-t)(18), Au-30(SPh)(18), and Au-30(SPh-pNO2)(18). Au-30((SBU)-B-t)(18) is known in the literature, and its crystal structure is available. In contrast, Au-30(SPh)(18) and Au-30(SPh-pNO(2))(18) are two species that have been designed by replacing the tertbutyl organic residues of Au-30((SBu)-Bu-t)(18) with aromatic ones so as to investigate the effects of ligand replacement on the optical response of Au nanomolecules. By analogy to a previously studied Au-23(SR)(16)(-) anionic species, despite distinct differences in charge and chemical composition, a substantial ligand enhancement of the absorption intensity in the optical region is also obtained for the Au-30(SPh-pNO(2))(18) MPC. The use of conjugated aromatic ligands with properly chosen electron-withdrawing substituents and exhibiting steric hindrance so as to also achieve charge decompression at the surface is therefore demonstrated as a general approach to enhancing the MPC photoabsorption intensity in the optical region. Additionally, we here subject the ligand-enhancement phenomenon to a detailed analysis based on the fragment projection of electronic excited states and on induced transition densities, leading to a better understanding of the physical origin of this phenomenon, thus opening avenues to its more precise control and exploitation.

We report the complete X-ray crystallographic structure as determined through single-crystal X-ray diffraction and a thorough theoretical analysis of the green gold Au-30(S-tBu)(18). While the structure of Au30S(S-tBu)(18) with 19 sulfur atoms has been reported, the crystal structure of Au-30(S-tBu)(18) without the mu(3)-sulfur has remained elusive until now, though matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) and electrospray ionization mass spectrometry (ESI-MS) data unequivocally show its presence in abundance. The Au-30(S-tBu)(18) nanomolecule not only is distinct in its crystal structure but also has unique temperature-dependent optical properties. Structure determination allows a rigorous comparison and an excellent agreement with theoretical predictions of structure, stability, and optical response.