Directional Electron Transfer in Chromophore-Labeled Quantum-Sized Au25 Clusters: Au25 as an Electron Donor

Novel Au-25(C6S)(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 form Au-25 clusters to pyrene. The rate of froward electron transfer from the Au-25 cluster to pyrene is ultrafast (similar to 580 fs), as observed with femtosecond fluorescence upconversion and transient absorption.