TEM images of MPC samples were obtained with a JEOL transmission electron microscope (JEM-1230). MPC samples were prepared by dipping a Formvar/carbon-coated copper grid (400C-FC, EMS) in 1 mg/mL MPC in CH 2 Cl 2 for the hexanethiolate MPC and water in the case of the glutathione MPC and citrate nanoparticle. Three typical regions of each sample were imaged at 600 K magnification and 200 K magnification for the Au-Citrate sample. Core-size histograms were read from digitized photographic images using ImageJ software [http://rsb.info.nih.gov/ij/].

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Novel Au 25 (C 6 S) 17 PyS clusters (pyrene-functionalized Au 25 clusters) showing interesting electrochemical and optical properties are synthesized and characterized. Significant fluorescence quenching is observed for pyrene attached to Au 25 clusters, suggesting strong excited-state interactions. Time-resolved fluorescence upconversion and transient absorption measurements are utilized to understand the excited-state dynamics and possible interfacial electron-and energy-transfer pathways. Electrochemical investigations suggest the possibility of electron transfer from Au 25 clusters to the attached pyrene. Fluorescence upconversion measurements have shown faster luminescence decay for the case of pyrene attached to Au 25 clusters pointing toward ultrafast photoinduced electron/energy-transfer pathways. Femtosecond transient absorption measurements have revealed the presence of the anion radical of pyrene in the excited-state absorption, suggesting the directional electron transfer from Au 25 clusters to pyrene. The rate of forward electron transfer from the Au 25 cluster to pyrene is ultrafast (∼580 fs), as observed with femtosecond fluorescence upconversion and transient absorption. SECTION Nanoparticles and Nanostructures

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Temperature-dependent absorption and ultrafast luminescence dynamics of [Au25(PPh3)10(SC6)5Cl2 ]2+ (Au25-rod) was studied and compared with [Au25(SC6)18]− (Au25-sphere) and Au38(SC2Ph)24 (Au38-rod) to understand the influence of the crystal structure on the optical properties of monolayer protected gold clusters. The temperature-dependent absorption of Au25-rod shows a shift in the absorption maximum to high energies and a small increase in the oscillator strength with decrease in temperature. The energy shift was modeled via the O’Donnell and Chen relationship, which yielded average phonon energy of 160 ± 80 cm–1, quite smaller than the 350 cm–1 observed for Au25-sphere and Au38-rod. There is an increase in the oscillator strength with a decrease in temperature of about 40% for Au25-rod while it is nearly 250% for Au25-sphere and more than 180% for Au38-rod. The oscillator strength increase is attributed to the coupling of core-gold exciton and shell-gold phonons. The smaller increase in the oscillator strength for Au25-rod is consistent with its structure that possesses no shell-gold. Femtosecond luminescence measurements carried out on Au25-rod clusters show wavelength-independent ultrafast luminescence decay traces. The lifetimes from the analysis are consistent with the relaxation of higher energy states. In contrast, Au25-sphere and Au38-rod clusters show specific wavelength-dependent luminescence growth and decay, representing the relaxation of core-gold states to shell-gold states.