Intramolecular Charge Transfer and Solvation Dynamics of Thiolate-Protected Au₂₀(SR)₁₆ Clusters Studied by Ultrafast Measurement

Abstract

It is accepted that the monolayer ligand shell in monolayer-protected gold nanoclusters (MPCs) plays an important role in stabilizing the metal core structure. Previous reports have shown that the core and shell do not interact chemically, and very few studies investigating the intramolecular charge transfer (ICT) between the core and ligand shell in clusters have been reported. The underlying excited state relaxation mechanisms about the influence of solvents, the optically excited vibration, and the roles of the core and shell in charge transfer remain unknown to a large extent. Here we report a femtosecond transient absorption study of a Au20(SR)16 (R = CH2CH2Ph) cluster in toluene and tetrahydrofuran. The ICT from the outside shell to the inside core upon excitation in Au20(SR)16 is identified. The observed solvation-dependent oscillations in different solvents further confirm the photoinduced ICT formation in Au20(SR)16. The results provide a fundamental understanding of the structure-property relationships about the solvation-dependent core-shell interaction in Au MPCs.

Polarization Resolved Measurements of Individual DNA‐Stabilized Silver Clusters

“…[ 20,21 ] Recent experimental works [ 22,23 ] indicate that fl uorescent Ag:DNA contains a neutral silver core that is surrounded by base-bound silver ions. This arrangement is analogous to the known structure of gold clusters that are stabilized by small organic ligand molecules, [24][25][26] which possess both transitions associated with the gold core, and transitions involving charge transfer between the gold core and the surrounding ligands plus their directly attached gold atoms. [ 25,26 ] For Ag:DNAs, the specifi c mode of cluster-DNA binding is unknown, but given the existence of a neutral silver core one might expect both core-centered transitions and charge transfer transitions between the core and baseattached silver ions.…”

“…This arrangement is analogous to the known structure of gold clusters that are stabilized by small organic ligand molecules, [24][25][26] which possess both transitions associated with the gold core, and transitions involving charge transfer between the gold core and the surrounding ligands plus their directly attached gold atoms. [ 25,26 ] For Ag:DNAs, the specifi c mode of cluster-DNA binding is unknown, but given the existence of a neutral silver core one might expect both core-centered transitions and charge transfer transitions between the core and baseattached silver ions.In the case of such silver core-ligand transitions, small variations in the specifi c conformation of the DNA might be expected to affect directions of transition dipole moments. Thus it is of great interest to study individual Ag:DNAs under conditions that minimize environmental fl uctuations, using techniques that can probe such orientational effects.…”

“…[ 31 ] The anisotropic shape results in more than one collective excitation: a low-energy longitudinal mode and a higher energy transverse mode. [ 26 ] The development of sensitive photothermal imaging techniques has enabled direct measurement of the dependence of the absorbance of gold nanorods on excitation polarization, for rods with dimensions of ∼25 nm × 75 nm, corresponding to ∼10 6 Au atoms. [ 34 ] For these nanorods, single particle measurements at the energy of the longitudinal mode fi nd modulation indexes M very close to 1 for rods with aspect ratios (length to diameter) of ∼3, while measurements at the energy of the transverse mode fi nd M << 1.…”

Solvent Dependent Excited State Behaviors of Luminescent Gold(I)–Silver(I) Cluster with Hypercoordinated Carbon

“…As changes in the nature of the solvent may have strong effect on the excited state processes of metal clusters, 33 we have investigated the excited state relaxation of I in more polar CH 3 CN. As seen in Figure 4A Figure 4B), suggesting that energy level of the long-lived state is almost unaffected by changing the solvent polarity.…”

Ultrafast Relaxation Dynamics of Luminescent Rod-Shaped, Silver-Doped AgxAu25–x Clusters

“…Coherent oscillations have been observed experimentally in ligand protected nanoclusters such as Au 20 (with thiolate and phosphine ligands),35, 49 Au 25 (both spherical and rod-shaped),19,44 Au 144 and Au 309 . 50 After a careful examination of the kinetic traces, we observed coherent oscillations in both Au25 and Au 12 Ag 13 clusters at almost all wavelengths (seeFigure 6).The Fourier transformations on the data gave an oscillation frequency of 26 cm -1 (0.78 THz) in Au 12 Ag 13 and 28 cm -1 (0.84 THz) in Au 25 rod (seeFigure S5in Supporting Information) and no higher vibration frequency oscillations were observed here.…”