Synthesis and Electrochemical and Spectroscopic Characterization of Biicosahedral Au25 Clusters

Abstract

The synthesis and electrochemical and spectroscopic characterization of biicosahedral Au₂₅ clusters with a composition of [Au₂₅(PPh₃)₁₀(thiolate)₅Cl₂]²⁺ are described. The biicosahedral Au₂₅ clusters protected with various types of thiol ligands including alkanethiols, 2-phenylethanethiol, 11-mercaptoundecanoic acid, and 11-mercapto-1-undecanol were synthesized in high yields using a one-step, one-phase procedure in which Au(PPh₃)Cl is reduced with tert-butylamine–borane in the presence of the thiol ligand in a 3:1 v/v chloroform/ethanol solution. All biicosahedral Au₂₅ clusters prepared exhibit characteristic optical absorption and photoluminescence properties. The emission energy is found to be substantially smaller than the optical absorption energy gap of 1.82 eV, indicating a subgap energy luminescence. The electrochemical HOMO–LUMO gap (∼1.54 eV) of the clusters is also substantially smaller than the optical absorption energy gap but rather similar to the emission energy. These electrochemical and optical properties of the biicosahedral Au₂₅ clusters are distinctly different from those of the Au₂₅(thiolate)₁₈ clusters.