Electrochemistry and Optical Absorbance and Luminescence of Molecule-like Au₃₈ Nanoparticles

This paper describes electrochemical and spectroscopic properties of a well-characterized, synthetically accessible, 1.1 nm diam Au nanoparticle, Au₃₈(PhC₂S)₂₄, where PhC₂S is phenylethylthiolate. Properties of other Au₃₈ nanoparticles made by exchanging the monolayer ligands with different thiolate ligands are also described. Voltammetry of the Au₃₈ nanoparticles in CH ₂Cl₂ reveals a 1.62 V energy gap between the first one-electron oxidation and the first reduction. Based on a charging energy correction of ca. 0.29 V, the indicated HOMO-LUMO gap energy is ca. 1.33 eV. At low energies, the optical absorbance spectrum includes peaks at 675 nm (1.84 eV) and 770 nm (1.61 eV) and an absorbance edge at ca. 1.33 eV that gives an optical HOMO-LUMO gap energy that is consistent with the electrochemical estimate. The absorbance at lowest energy is bleached upon electrochemical depletion of the HOMO level. The complete voltammetry contains two separated doublets of oxidation waves, indicating two distinct molecular orbitals, and two reduction steps. The ligand-exchanged nanoparticle Au₃₈(PEG ₁₃₅S)₁₃(PhC₂S)₁₁, where PEG ₁₃₅S is -SCH₂CH₂OCH₂CH ₂OCH₃, exhibits a broad (1.77-0.89 eV) near-IR photoluminescence band resolvable into maxima at 902 nm (1.38 eV) and 1025 nm (1.2 eV). Much of the photoluminescence occurs at energies less than the HOMO-LUMO gap energy. A working model of the energy level structure of the Au₃₈ nanoparticle is presented.