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\ud the optical absorption spectra of three different\ud Au30(SR)18 monolayer-protected clusters (MPC):\ud Au30(StBu)18, Au30(SPh)18, and Au30(SPh-pNO2)18.\ud Au30(StBu)18 is known in the literature, and its crystal structure\ud is available. In contrast, Au30(SPh)18 and Au30(SPh-pNO2)18\ud are two species that have been designed by replacing the tertbutyl\ud organic residues of Au30(StBu)18 with aromatic ones so as\ud to investigate the effects of ligand replacement on the optical\ud response of Au nanomolecules. By analogy to a previously\ud studied Au23(SR)16\ud − anionic species, despite distinct differences\ud in charge and chemical composition, a substantial ligand\ud enhancement of the absorption intensity in the optical region\ud is also obtained for the Au30(SPh-pNO2)18 MPC. The use of\ud conjugated aromatic ligands with properly chosen electron-withdrawing substituents and exhibiting steric hindrance so as to also\ud achieve charge decompression at the surface is therefore demonstrated as a general approach to enhancing the MPC\ud photoabsorption intensity in the optical region. Additionally, we here subject the ligand-enhancement phenomenon to a detailed\ud analysis based on the fragment projection of electronic excited states and on induced transition densities, leading to a better\ud understanding of the physical origin of this phenomenon, thus opening avenues to its more precise control and exploitation

Time‐dependent density‐functional study of the photoabsorption spectrum of Au25(SC2H4C6H5)18 anion: Validation of the computational protocol

“…This analysis follows the lines of previous work, in which the enhancement in the absorption intensity of MPC systems (plasmon “re‐birth”) due to the interaction between the metal core, the Au‐S shell, and the ligands was investigated, with the difference that conjugation and delocalization effects due to aromatic ligands directly bound to the S atoms (aromatic thiolates) were considered. Clearly, the direct conjugation of the aromatic ligands to the Au‐S shell as investigated by Sementa and coworkers gives rise to a true electronic resonance which exalts the enhancement in absorption intensity, whence a plasmon “re‐birth” phenomenon. Such ligand effect on the optical response, especially that associated with π‐orbitals of aromatic ligands, appears as a promising tool of rational design in the field of optics of ligand‐protected metal nanoclusters.…”

Chirality in bare and ligand-protected metal nanoclusters

“…This is interesting from both a fundamental point of view and also in practical applications, as it can open avenues to a more precise control and exploitation of this complex phenomenon. To this purpose, here we extend to CD a fragment projection analysis, originally developed for photoabsorption [130], based on the fragment projection of electronic excited states and on induced transition densities. The aim of this analysis tool is to rationalize the interplay among excitations belonging to the metal cluster and coating shell in determining the dichroism of metal nanoclusters, and to achieve it by dissecting the system into two pieces or fragments that will be named M and L for convenience, and to express the CD as a sum of four terms, corresponding to M→M, M→L, L→M, L→L transitions.…”

Modulating photo-luminescence of Au2Cu6 nanoclusters via ligand-engineering

“…The EDGs tend to elevate both the HOMO and LUMO energies, and the HOMO-LUMO gap is enlarged because LUMO is more sensitive to the electronic effect of the substituents. 24,25 By contrast, both HOMO and LUMO energies of the NCs reduce when the EWGs are introduced, and the signicantly lowered LUMO energy (compared to that of HOMO energy) results in a reduced HOMO-LUMO gap. 24,25 The PL spectra of Au 2 Cu 6 NCs was characterized to verify the aforementioned inference of electronic effect.…”