Understanding the Ligand-Dependent Photoluminescent Mechanism in Small Alkynyl-Protected Gold Nanoclusters

Alkynyl-protected gold clusters have recently gained attention because they are more structurally versatile than their thiolate-protected counterparts. Despite their flexibility, however, a higher photoluminescent quantum yield (PLQY) has been observed experimentally compared to that of organically soluble thiolate-protected clusters. Previous experiments have shown that changing the organic ligand, or R group, in these clusters does not affect the geometric or electronic properties of the core, leading to a similar absorption profile. This article serves as a follow-up to those experiments in which the geometric, optical, and photoluminescent (PL) properties of Au₂₂(ETP)₁₈ are pieced together to find the photoluminescence mechanism. These properties are then compared between Au₂₂(C≡CR)₁₈ clusters where the ligand is changed from R = ETP to PA and ET (ETP = 3-ethynylthiophene, PA = phenylacetylene, and ET = 3-ethynyltoluene). As the theoretical results do not reproduce the same absorption profile among the different ligands as in the experiment, this article also presents a supplementary benchmark of the geometric and optical properties among the three ligands for different levels of theory. The calculations show that the photoluminescence mechanism with the ETP ligand results in ligand-to-metal-to-metal charge transfer (LMMCT), while PA and ET are likely a result of core-dominated fluorescence. The changes are the result of the Au(I) ring atoms as well as how the aromatic groups are connected to the cluster. Additionally, dispersion, solvent, and polarization functions are all important to creating an accurate chemical environment, but the most useful tool in these calculations is the use of a long-range-corrected exchange-correlation functional.