Here we investigate via first-principles simulations
the optical absorption spectra of three different
Au30(SR)18 monolayer-protected clusters (MPC):
Au30(StBu)18, Au30(SPh)18, and Au30(SPh-pNO2)18.
Au30(StBu)18 is known in the literature, and its crystal structure
is available. In contrast, Au30(SPh)18 and Au30(SPh-pNO2)18
are two species that have been designed by replacing the tertbutyl
organic residues of Au30(StBu)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 Au23(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 Au30(SPh-pNO2)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.
