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