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

Abstract

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.

Metal Doping of Au25(SR)18– Clusters: Insights and Hindsights

“…For both platinum and palladium, the direct synthesis of Au24M(SR)18 0 (R = C2Ph, C6, C12) has been consistently described to yield clusters doped at the center. [12][13][14][15][16][17][18][19] For R = C4, we followed the same synthetic and purification protocol described for platinum by Qian et al 12 and adapted for palladium by Kwak et al. 14 MALDI-TOF mass spectrometry clearly indicated that the purified clusters only contain one foreign-metal atom, as shown in Figure S2 for Au24Pt(SC4)18 0 , and no contamination from residual Au25(SC4)18 − or Au25(SC4)18 0 .…”

“…Modification of the metal composition has been studied for several clusters, but most research has focused on Au25(SR)18, which is an atomically precise cluster that has been long considered a convenient benchmark system for understanding properties and devising applications of gold nanoclusters. 10,11 Controlled doping of Au25(SR)18 − has been carried out with the noble metals platinum [12][13][14][15][16] and palladium, 14,[16][17][18][19] and less noble metals, such as cadmium [20][21][22] and mercury, 15,[20][21][22][23] also because of the ease by which monodoping could be achieved with these metals as opposed to, say, copper and silver. 9 Mass spectrometry and single crystal X-ray crystallography were extensively employed to interpret the doping results and, particularly, assign the specific locations where these single foreign-metal atoms go.…”

“…The platinum-doped clusters were prepared by direct synthesis, i.e., by reacting a mixture of tetrachloroauric and hexachloroplatinic acids with a given thiol, followed by sodium borohydride reduction. [12][13][14][15] The same procedure was applied to palladium 14,[16][17][18][19] and mercury. 15 Cadmium [20][21][22] and mercury [20][21][22][23] were introduced into preformed Au25(SR)18 − , and studied from several viewpoints.…”

Deepening the Understanding of Thiolate-Protected Metal Clusters Using High-Performance Liquid Chromatography

“…As shown in Figure 14 (Figure 16(a)) 148,286,507,568,603,706,721,767,768,791,795,813,826 and co-reduction method (Figure 16(b)). 43,58,174,219,222,241,262,300,305,311,346,356,431,536,587,601,614 In this study, we found that the isomer distribution also depends on the standing time in solution. As an example, Figure 17 shows the time dependence of the EI chromatogram of Au 25¹x Ag x (SC 4 H 9 ) 18 (x = 24) synthesized by the metalexchange method (Figure 16(a)).…”

Highly Luminescent Folate‐Functionalized Au22 Nanoclusters for Bioimaging

“…Recent advances in the synthesis of thiolate‐protected gold nanoclusters have led to stable clusters with atomically precise core–shell structures . They exhibit unique optical, electrochemical, and catalytic properties that set them apart from other larger gold nanoparticle systems . Their PL properties, in particular, have been the focus of recent research interests .…”