The Influence of Passivating Ligand Identity on Au₂₅(SR)₁₈ Spin-Polarized Emission

Magnetic circular photoluminescence (MCPL) spectra were collected following 3.1 eV excitation of two ligand-passivated Au₂₅(SR)₁₈ monolayer-protected clusters (MPCs). Both clusters generated spin-polarized emission; however, the degree of circular polarization noted for Au₂₅(SC₈H₉)₁₈, which was passivated with the aromatic phenylethanethiol ligand, was 5× that obtained for Au₂₅(SC3)₁₈, whose passivating ligand was aliphatic. Variable-magnetic field data were analyzed to determine Landé g-factors and spectroscopic term symbols for observable transitions contributing to the clusters’ MCPL spectra. For Au₂₅(PET)₁₈, transitions originated from one doublet and two quartet fine-structure superatomic electronic states; by comparison, the Au₂₅(SC3)₁₈ spectrum contained only two components, both of which arose from doublet superatomic electronic states. Additionally, Faraday B-term contributions, which report on field-induced mixing, were more pronounced for Au₂₅(SC3)₁₈ spectral components. Therefore, the decreased spin-polarized emission by Au₂₅(SC3)₁₈ was attributed to stronger coupling to nonradiative decay channels. These results suggest the Au₂₅(SR)₁₈ cluster’s passivating ligand can be used to tune the relative populations of emissive fine-structure states, the extent of mixing between radiative and nonradiative states, and the amplitude of spin-polarized emission in MPCs.