In Search of the Quantum-Electronic Origin of Color Change: Elucidation of the Subtle Effects of Alloying with Copper on ≈1.8 nm Gold Nanoclusters

Experiments on the Au₁₄₄(SR)₆₀ class of cluster compounds have demonstrated the emergence of an absorption peak at 550 nm (2.25 eV) upon adding copper to the pure Au samples; in some cases, as little as one Cu atom seems to have this effect [ Chem. Commun. 2014, 50, 1722 and J. Phys. Chem. C 2015, 119, 10935 ]. These results are highly surprising as they appear to indicate that the combination of two metals that are not plasmonic in this size range produces a plasmon-like resonance. On the basis of calculations using time-dependent density-functional theory (TDDFT), it has recently been reported that "Copper Induces a Core Plasmon in Intermetallic Au_(144,145)-xCu_x(SR)₆₀ Nanoclusters" [ J. Phys. Chem. Lett. 2015, 6, 515 ]. In the present study we show that this effect is by no means general; systematic TDDFT calculations on both bare and ligand-covered clusters do not show the emergence of any clear resonance, plasmonic or other. Instead, suppression of intensity in the range of interest between 2.0 and 3.5 eV (620 to 350 nm) and small red shifts of weak spectral features may indeed change the color of the samples and may be seen as inducing a broad bump in the spectra. However, no clear increase in intensity if observed at the energy of interest (2.25 eV ≈ 550 nm) that would indicate the emergence of a plasmonic resonance. Other hypotheses will have to be explored to explain the experimental results, which show a strong resonance developing. (Figure Presented).