Theoretical Prediction of Optical Absorption and Emission in Thiolated Gold Clusters.

Although the photoluminescence of gold clusters has been extensively studied so far, there are still questions on the origin of the emission in these materials. In this work, we report time-dependent density functional theory calculations on the absorption and emission spectra of the well-studied Au ₂₅ (SR) ₁₈ ^- cluster, the lowest energy isomer of the Au ₃₈ (SR) ₂₄ cluster, and five isomers of the Au ₂₂ (SR) ₁₈ cluster. Good agreement between the calculated and measured absorption spectra, as well as with the lowest-energy emission values for these clusters, was demonstrated, verifying the accuracy of the theoretical methods employed. Our results for Au ₂₅ (SR) ₁₈ ^- explain a newly observed feature in the absorption peak, also rationalizing the optical response in terms of the superatom model. The analysis of the absorption and emission characteristics of the Au ₂₅ (SR) ₁₈ ^- and Au ₃₈ (SR) ₂₄ clusters provides an estimate of the spectral regions, where fluorescence or phosphorescence is predicted to occur. Interestingly, we find that for Au ₂₂ (SR) ₁₈ , one of the five proposed structures could be present at a significant concentration in the sample, even though it is not the lowest in energy structure, which can be explained, in part, by solvent effects.