Energy Gap Law for Exciton Dynamics in Gold Cluster Molecules.

The energy gap law relates the nonradiative decay rate to the energy gap separating the ground and excited states. Here we report that the energy gap law can be applied to exciton dynamics in gold cluster molecules. Size-dependent electrochemical and optical properties were investigated for a series of n-hexanethiolate-protected gold clusters ranging from Au25 to Au333. Voltammetric studies reveal that the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps of these clusters decrease with increasing cluster size. Combined femtosecond and nanosecond time-resolved transient absorption measurements show that the exciton lifetimes decrease with increasing cluster size. Comparison of the size-dependent exciton lifetimes with the HOMO-LUMO gaps shows that they are linearly correlated, demonstrating the energy gap law for excitons in these gold cluster molecules.

Au21S(SAdm)15: An Anisotropic Gold Nanomolecule. Optical and Photoluminescence Spectroscopy and First-Principles Theoretical Analysis.

We introduce a class of gold nanomolecules exhibiting anisotropy as a major feature by reporting steady-state and time-resolved photoluminescence and anisotropy measurements and in-depth theoretical analysis of energetics and optical response of a recently synthesized Au21S(SAdm)15 nanomolecule (SAdm = adamantanethiol). Starting from single-crystal X-ray data showing that Au21S(SAdm)15 exhibits a symmetry-broken structure, we unambiguously demonstrate how this translates into a striking anisotropy of its properties, for example, of its (chiro)optical absorption spectrum of great promise for sensing, optoelectronic, and electrochemical applications, and argue about the abundance and general significance of this class of compounds.

Enhanced luminescence of Au22(SG)18 nanoclusters via rational surface engineering.

We report design strategies for the preparation of highly luminescent Au22(SG)18 clusters, where SG is glutathione, by the functionalization of the cluster shell. In these strategies, the cluster shell was covalently modified with small aromatic molecules and pyrene chromophores that led to a 5-fold PL enhancement by rigidifying the shell-gold. Highly luminescent water-soluble gold clusters with a PL quantum yield of 30% were obtained at room temperature. To further enhance the luminescence, the pyrene chromophores in the functionalized Au22 clusters were photoexcited at 350 nm to induce energy transfer from pyrene to the Au22 cluster. Steady-state and time-resolved PL measurements have shown evidence of enhanced rigidity with increased PL lifetimes for the functionalized Au22 clusters. However, the energy transfer efficiency was found to be only 14% because of the competing electron transfer deactivation pathway as evidenced by the formation of the pyrene anion radical revealed in the ultrafast transient absorption measurements. To suppress the electron transfer pathway, the pyrene functionalized Au22 clusters were ion-paired with tetraoctylammonium (TOA) cations that could break the electron transfer pathway, leading to a dramatic 37-fold increase in PL brightness with the resonance energy transfer efficiency of ca. 80%. This work presents effective design strategies for the preparation of highly luminescent gold clusters by the combination of rigidifying effect and energy transfer sensitization.

Ultrabright Luminescence from Gold Nanoclusters: Rigidifying the Au(I)-Thiolate Shell.

Luminescent nanomaterials have captured the imagination of scientists for a long time and offer great promise for applications in organic/inorganic light-emitting displays, optoelectronics, optical sensors, biomedical imaging, and diagnostics. Atomically precise gold clusters with well-defined core-shell structures present bright prospects to achieve high photoluminescence efficiencies. In this study, gold clusters with a luminescence quantum yield greater than 60% were synthesized based on the Au22(SG)18 cluster, where SG is glutathione, by rigidifying its gold shell with tetraoctylammonium (TOA) cations. Time-resolved and temperature-dependent optical measurements on Au22(SG)18 have shown the presence of high quantum yield visible luminescence below freezing, indicating that shell rigidity enhances the luminescence quantum efficiency. To achieve high rigidity of the gold shell, Au22(SG)18 was bound to bulky TOA that resulted in greater than 60% quantum yield luminescence at room temperature. Optical measurements have confirmed that the rigidity of gold shell was responsible for the luminescence enhancement. This work presents an effective strategy to enhance the photoluminescence efficiencies of gold clusters by rigidifying the Au(I)-thiolate shell.