Variations in electronic states of coumarin hexanethiolate-labeled i-Au 25 and bi-Au 25 clusters
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Research Letter
Variations in electronic states of coumarin hexanethiolate-labeled i-Au25 and bi-Au25 clusters Angela Meola†, Nicole Hondrogiannis†, and Pierce Brown, Department of Chemistry, Towson University, Towson, MD 21252, USA Maksym Zhukovskyi, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA Zheng Zheng and Zeev Rosenzweig, Department of Chemistry and Biochemistry, University of Maryland Baltimore County, Baltimore, MD 21250, USA Keith Reber and Mary Sajini Devadas, Department of Chemistry, Towson University, Towson, MD 21252, USA Address all correspondence to Mary Sajini Devadas at [email protected] (Received 20 June 2019; accepted 8 August 2019)
Abstract
Au25 (C6 H14 S)18 − icosahedron and [Au25(PPh)10(C6H14S)5Cl2]2+ bi-icosahedron clusters were synthesized. Ligand exchange reactions were carried out with a new coumarin-derived fluorophore (Cou-SH) to label both clusters. Labeled and unlabeled Au25 were compared and the changes in the electronic structure were determined. The labeled clusters showed marked changes in electronic states, as evidenced by the quenching in the UV region and enhancement in the near infrared. The quantum yield from Cou-SH decreased and the quantum yield from the labeled Au25 increased. Second, the authors observed changes in the electrochemical band gap.
Introduction The electronic and optical properties of gold nanoparticles are integral in developing the technology for electronic warfare platforms for the obscuration and attenuation of light, medical detection technology for the treatment of cancer, nanoparticle sensors, and high-resolution biological imaging.[1–3] Variations in the nanoparticle size, stabilizing ligand, and geometry can influence the electrochemical potential, quantum yield, and nonlinear optical properties. The absorbance spectra of gold nanoparticles that display quantum confinement, electronic transitions between the ligand and the gold core in the higher frequency regions of the electromagnetic spectrum are observed.[4–6] The lower frequency transitions that are associated with the gold–gold interactions are seen in the near infrared (NIR) region.[1,7] The crystal structures of icosahedral and bi-icosahedral structures have generated an interest in the last decade. This is because, in the bi-structure, the majority of the Au atoms form the core as opposed to the shell. The icosoheral Au25 contains 13 core atoms, whereas the latter contains a dimer of 13 Au atoms, connected by a single vertex Au atom while retaining the same Au–Au distance. Both clusters also show a structured spectral profile analogous to molecules.[8,9] It is of general interest to compare the optical properties of these crystals (i- and bi-) with and without fluorophore labeling. This is because the quasicontinuous electronic bands in large nanoparticles (>5 nm) evolve to discrete levels as the size
† These authors contributed equally to this work.
decreases to 1.3 nm and below. This leads to superatom behavior.[10,11] Typically, the ligand
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