Quantum Clusters in Cavities: Trapped Au₁₅ in Cyclodextrins

We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime components of 28, 71, and 24 ps for Au15@αCD, Au15@βCD, and Au15@γCD, respectively. The clusters also are strongly luminescent in the solid state. Both in the solution and in the solid state, the luminescence is sensitive to solvents/vapors. The clusters adhere to glass plates, and the solvent dependency of luminescence was used to create patterns that are erased upon gradual evaporation of the solvent. This self-erasing property was further demonstrated with clusters supported on a thin layer chromatography (TLC) plate. Selective detection of metal ions using the luminescence of the clusters is reported. Evaporation of the cluster solutions leads to luminescent gel-like materials

Quantum Clusters in Cavities: Trapped Au₁₅ in Cyclodextrins

We have prepared Au15 quantum clusters anchored to α-, β-, and γ-cyclodextrin (CD) cavities. The synthesis process involves the core etching of larger clusters and the simultaneous trapping of the clusters formed inside the CD cavities. The clusters were characterized by various tools, such as optical absorption and luminescence spectroscopies, electrospray ionization−mass spectrometry (ESI-MS), X-ray photoelectron spectroscopy (XPS), circular dichroism spectroscopy, and two-dimensional nuclear magnetic resonance (2D NMR) spectroscopy. Trapping of the cluster in the CD cavity was proven by circular dichroism and also by rotational Overhauser effect spectroscopy (ROESY), in terms of the distinct cross peak between proton “e” of the glutathione (−SG) ligand and the “H3” proton of CD. Dynamic light scattering (DLS) studies showed a hydrodynamic diameter of ∼3−4 nm, indicating one CD molecule per cluster with an extension of one water of hydration. The clusters are intensely luminescent, with major lifetime components of 28, 71, and 24 ps for Au15@αCD, Au15@βCD, and Au15@γCD, respectively. The clusters also are strongly luminescent in the solid state. Both in the solution and in the solid state, the luminescence is sensitive to solvents/vapors. The clusters adhere to glass plates, and the solvent dependency of luminescence was used to create patterns that are erased upon gradual evaporation of the solvent. This self-erasing property was further demonstrated with clusters supported on a thin layer chromatography (TLC) plate. Selective detection of metal ions using the luminescence of the clusters is reported. Evaporation of the cluster solutions leads to luminescent gel-like materials

Light-Triggered Reversible Supracolloidal Self-Assembly of Precision Gold Nanoclusters

Monolayer thiol-protected noble metal nanoclusters are attractive nanoscale building blocks for well-defined colloidal superstructures. However, achieving facile reversible self-assembly of nanoclusters using external stimuli is still in its infancy. Herein, we report the synthesis and photon-assisted reversible self-assembly of thiolated azobenzene-stapled Au25 nanoclusters. Photoactivation of functionalized nanoclusters in dichloromethane by irradiating ultraviolet light at 345 nm results in a visual change and formation of disc-like colloidal superstructures (d ∼ 100–1000 nm). The superstructures readily disassemble into individual nanoclusters upon irradiating with visible light at 435 nm. Systematic changes in both the electronic absorption bands and nuclear magnetic resonance spectra of chromophores in solution suggest that the photoisomerization of surface ligands drives the self-assembly. High-resolution transmission electron microscopy, electron tomographic reconstruction, dynamic light scattering, and small-angle X-ray powder diffraction show that the disc-like superstructures contain densely packed nanoclusters. Long-range self-assembly and disassembly under ultraviolet and visible light, respectively, demonstrate reversible photoswitching in nanoclusters

Photofabrication of Fullerene-Shelled Quantum Dots Supramolecular Nanoparticles for Solar Energy Harvesting

Quantum dots-based electron donor-acceptor systems play a rising role in the design of renewable and carbon-free energy harvesting technologies. In this article, we discuss the photofabrication of fullerene-shelled quantum dots supramolecular nanoparticles, in which the fullerene shell acts as not only a well-defined electron acceptor but also a robust protecting layer against the photocorrosion of the quantum dot core. We evaluate the ensemble and single-molecule electron transfer from the core to the shell in the nanoparticles and the photocurrent response of a photoelectrochemical cell constructed using the nanoparticles. The supramolecular nanoparticle has been prepared by the covalent tethering of a fullerene-thiol monolayer to the quantum dot followed by the photochemical reactions of free fullerene-thiol to the tethered monolayer. The nanoparticles are characterized using scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy. Correlated single-photon emission and the two-state ON-OFF photoluminescence show that single quantum dots are included in the supramolecular nanoparticles. The fullerene-shells suppress the blinking of single quantum dots by acting as well-defined electron traps, without allowing the transfer of Auger electrons to unknown traps. Electron transfer from the quantum dot-core to the fullerene-shell is apparent from the short ON and OFF durations in the photoluminescence intensity trajectories of single quantum dots, quenching of the photoluminescence intensity and lifetime of quantum dots at the ensemble level, and the characteristic transient absorption band of the anion radical of fullerene. We next construct a photoelectrochemical cell using the supramolecular nanoparticles, and the transferred electron is externally driven in the cell to generate ∼400 μA/cm2 photocurrent. Electron transfer from the highly stable quantum dots to the protecting fullerene-shells places the supramolecular nanoparticles among the most promising antenna systems for the construction of cost-effective and stable next generation solar energy harvesting systems