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) Au-25 clusters. Optical absorption and electrochemistry results have shown that the Pt-doped cluster has a distinctly different HOMO-LUMO gap than that of Au-25, while the gap did not change much for Hg-doped Au-25. 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 Au-25 clusters. Interestingly, the peaks observed for Pt and Hg-doped clusters are very different from that of undoped Au-25 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 Au-25 clusters when compared to 43 +/- 6 meV for undoped Au-25. 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 Au-25 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 Au-12-icosahedron.