Temperature-Dependent Absorption and Ultrafast Exciton Relaxation Dynamics in MAu₂₄(SR)₁₈ Clusters (M = Pt, Hg): Role of the Central Metal Atom

Temperature-dependent and ultrafast transient absorption measurements were carried out to probe the optical properties and exciton relaxation dynamics in metal-doped (Pt and Hg) Au₂₅ clusters. Optical absorption and electrochemistry results have shown that the Pt-doped cluster has a distinctly different HOMO-LUMO gap than that of Au₂₅, while the gap did not change much for Hg-doped Au₂₅. A decrease in temperature had resulted in much sharper absorption features as well as an increased number of absorption peaks, enhanced oscillator strength, and a shift in the energy maximum to higher energies for all metal-doped Au₂₅ clusters. Interestingly, the peaks observed for Pt and Hg-doped clusters are very different from that of undoped Au₂₅ cluster, suggesting that the altered structures play a crucial role on their optical properties. From the analysis of absorption peak shifts, higher phonon energies of 67 ± 8 meV were determined for Pt- and Hg-doped Au₂₅ clusters when compared to 43 ± 6 meV for undoped Au₂₅. The larger phonon energies suggest stronger coupling of core-gold and shell-gold and are explained by contraction of metal-doped clusters. Ultrafast transient absorption results have shown that Pt-doping lead to faster excited state relaxation, where more than 70% of the created electron-hole pairs recombine within 20 ps. However, Hg-doping and undoped Au₂₅ relax to shell gold and recombination takes a much longer time. The results are consistent with energy gap law, where the smaller energy gap for PtAu₂₄ led to faster exciton relaxation. An interesting correlation between the spin-orbit coupled transitions and bleach maximum was observed, which can be ascribed to exciton localization in Au₁₂-icosahedron.