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Article

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

Supporting Info

Dongil Lee,^† Robert L. Donkers,^‡ Gangli Wang, Amanda S. Harper, and Royce W. Murray*

Contribution from the Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599

J. Am. Chem. Soc., 2004, 126 (19), pp 6193–6199

DOI: 10.1021/ja049605b

Publication Date (Web): April 27, 2004

†

Present address: Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008-5413.

‡

Present address: Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, K1A 0R6, Canada.

In papers with more than one author, the asterisk indicates the name of the author to whom inquiries about the paper should be addressed.

, rwm@email.unc.edu

Abstract

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 CH₂Cl₂ 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(PEG₁₃₅S)₁₃(PhC₂S)₁₁, where PEG₁₃₅S is −SCH₂CH₂OCH₂CH₂OCH₃, 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.