Ligand-Enhanced Optical Response of Gold Nanomolecules and Its Fragment Projection Analysis: The Case of Au₃₀(SR)₁₈

Abstract

Here we investigate via first-principles simulations the optical absorption spectra of three different Au 30 (SR) 18 monolayer-protected clusters (MPC): Au 30 (S t Bu) 18, Au 30 (SPh) 18, and Au 30 (SPh- p NO 2 ) 18 . Au 30 (S t Bu) 18 is known in the literature, and its crystal structure is available. In contrast, Au 30 (SPh) 18 and Au 30 (SPh- p NO 2 ) 18 are two species that have been designed by replacing the tert -butyl organic residues of Au 30 (S t Bu) 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- p NO 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.