Ligand-Enhanced Optical Response of Gold Nanomolecules and Its Fragment Projection Analysis: The Case of Au-30(SR)(18)

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.