60th Annual Report on Research 2015

Overview

The overall goal of the project is to probe electron-phonon interactions in atomically precise-gold cluster/graphene oxide nanocatalysts. Quantum-sized gold clusters possess unique crystal structure where in the core-gold is passivated by a shell-gold with staple motifs made up of –S-Au-S- (either monomeric or dimeric or trimeric). The electron-phonon interactions of core-gold and shell-gold influence their optical and catalytic properties. To accomplish the objective of monitoring electron-phonon interactions in gold cluster/graphene oxide nanocomposites, following specific tasks are carried out.

(i)     Synthesis , characterization and electron-phonon interaction in Amine terminated bi-icosahedral Au₂₅ (bi-Au₂₅) clusters,

(ii)   Electron-phonon interactions in Au₆₇, Au₁₀₂ and Au₁₄₄ clusters

(iii)  Ultrafast and temperature-dependent luminescence measurements on ultrabright Au₂₂ clusters.

Significant Results

(i)     Synthesis, characterization and Electron-phonon interactions in Amine-terminated Au₂₅ Clusters The approach followed to synthesize gold clusters covalently attached to graphene oxide was to prepare amine-terminated thio-capped gold clusters that can readily bind to carboxylate terminated graphene oxide via amide bond formation. To accomplish this, several amine-terminated thiols were used for synthesizing bi- Au₂₅ clusters and spherical Au₂₅ clusters that included (i) cysteamine, (ii) hexylamine and (iii) butyl-ethyl amine. (see Figure 1A for structures) We have successfully synthesized bi-Au₂₅ clusters capped with amine-terminated thiols. The synthetic procedure was similar to what was reported for hexane-thiol capped bi-Au₂₅ with slight modifications in the ratio of reducing agent to gold phosphine and stirring conditions. ESI-MS was used to characterize the clusters and one such mass-spectrum (Figure 1B) was shown for butylethylamine thiol-capped bi-Au₂₅ that has shown pure clusters with little contamination from other sizes. The core-shell electron-phonon interactions were measured for the synthesized clusters using temperature-dependent absorption and luminescence measurements.(Figure 1C) The electron-phonon interactions were determined change in the lowest energy maximum as a function  of temperature that gave a phonon energy of 20 ± 10 meV which is similar to hexanethiol passivated bi-Au₂₅ clusters. The measurements have shown negligible influence of changing the ligand on electron-phonon interactions. Also, the results have revealed unique solvent-dependent low temperature absorption that can be better explained by invoking hydrogen bonding of the solvent with chlorine at the end. This observation was originally observed for hexane thiol capped bi-Au₂₅ and further confirmed with thiolated amine passivated bi-Au₂₅ clusters. The investigations are currently underway to attach the amine terminated clusters with carboxylated graphene oxide that was synthesized via modified Hummers method.

Figure 1. (A) Cartoon structures of bi-Au₂₅ clusters capped with thiolated amines. (B) ESI-MS of [Au₂₅(PPh₃)₁₀(C₄NHC₂S)₅Cl₂]²⁺ in methanol and (C) Temperature-dependent absorption spectra of [Au₂₅(PPh₃)₁₀(NH₂C₆S)₅Cl₂]²⁺ in ethanol.

(ii) Electron-phonon interactions in Au₆₇, Au₁₀₂ and Au₁₄₄ clusters

Research attempts are underway to attach larger quantum-sized gold clusters such as Au₆₇, Au₁₀₂ and Au₁₄₄ to graphene oxide to prepare novel nanocatalysts. As a preliminary study, electron-phonon interactions in hexane thiol passivated Au₆₇, Au₁₀₂ and Au₁₄₄ clusters (Figure 2A) were monitored via temperature-dependent absorption and ultrafast luminescence dynamics measurements. The idea is to understand the influence of cluster’s size, structure and symmetry on the electron-phonon interactions in gold clusters. Interesting temperature-dependent absorption spectral features were observed with decreasing temperature for clusters of different sizes. (Figure 2B-D) No new structural features were observed for Au₆₇ and Au₁₀₂ at low temperatures unlike smaller clusters such as Au₂₅ and Au₃₈ where sharper vibronic transitions appeared. Interestingly, more spectral information was observed for Au₁₄₄ clusters when compared to Au₆₇ and Au₁₀₂ clusters. This unique result is ascribed to higher symmetry of Au₁₄₄ when compared to low symmetric Au₆₇ and Au₁₀₂. Also the temperature-dependent optical absorption spectra were modeled with electron-phonon and exciton-phonon interactions and electron-phonon energy for Au₁₄₄ (> 50 meV) is in the range of smaller clusters but much lower energy is observed for Au₆₇ and Au₁₀₂ (~20 meV). The differences observed were attributed to the differences in the symmetry of crystal structures where Au₆₇ and Au₁₀₂ possess low symmetry while Au₁₄₄ has highest icosahedreon symmetry with all 60 ligands in a symmetric environment. In addition, exciton-phonon coupling was determined and the results have shown a decrease with increasing cluster size that was ascribed to lower exciton-phonon coupling strength for lower symmetry clusters. However, larger exciton-phonon coupling was observed for Au₁₄₄ when compared to Au₆₇ and Au₁₀₂ clusters that was again assigned to the symmetry of the cluster. Ligand exchange reactions would be used to couple these clusters to graphene oxide to prepare hybrid nanoclusters that can be used for catalysis.

Figure 2. (A) Structures of Au₆₇, Au₁₀₂ and Au₁₄₄ clusters. Temperature-dependent absorption spectra of (B) Au₆₇(C₆S)₃₅, (C) Au₁₀₂(C₆S)­₄₄ and (D) Au₁₄₄(C₆S)₆₀ in methylcyclohexane.

(ii) Time-resolved and temperature-dependent measurements to probe the origin of ultrabright luminescence from Au₂₂(SG)₁₈ clusters

In addition to temperature-dependent absorption, temperature-dependent luminescence measurements as a function of temperature are being used to probe electron-phonon in gold clusters. In one such study, temperature-dependent steady-state and time-resolved photoluminescence (PL) measurements were carried out to understand the enhanced PL in Au₂₂(SG)₁₈ clusters when they are bound to tetraoctylammonium bromide. (Figure 3A) We have Hiprobed the origin of PL in Au₂₂(SG)₁₈ clusters with combined time-resolved and temperature-dependent luminescence techniques.  Temperature-dependent steady-state PL measurements (Figure 3B) have shown the presence of highly luminescent PL state below the freezing point. The measurements were further confirmed by temperature-dependent time-resolved PL studies that has shown long-lived PL state below the freezing point. From the results, it was concluded that the luminescence arises from the ligand-to-metal-metal charge transfer state of the shell-gold and also the presence of high quantum-yield triplet state in frozen media. As it was possible to observe ultrabright emission from these clusters in frozen media, Au₂₂(SG)₁₈ was bound to bulky TOA to rigidify the Au(I)-thiolate shell. The luminescence efficiency of the rigidified Au₂₂ clusters enhanced remarkably and exhibits high quantum-yield luminescence ( >60%) at room temperature.

Figure 3. (A) Schematic diagram showing the luminescent enhancement in Au₂₂(SG)₁₈ clusters via rigidification of the shell-gold. (B) Temperature-dependent luminescence spectra of Au₂₂(SG)₁₈ in water/glycerol mixture showing sudden transition around the freezing temperature and (C) Temperature-dependent time-resolved luminescence decay traces at different temperatures.