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Optical Properties of Bi-Icosahedral Au25 and Au24 Clusters: Influence of Central Gold Atom

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Dr. Ekkehard Sinn



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Temperature-dependent absorption and ultrafast luminescence measurements of [Au24(PPh3)10(SC2H4Ph)5Cl2]+(bi-Au24) were studied and compared with [Au25(PPh3)10(C6S)5Cl2]2+ (bi-Au25). The investigations are carried out on these two clusters as they are structurally similar except for the absence of central Au atom for bi-Au24. The objective of the work is to probe the influence of the central gold atom on the optical transitions and electron-phonon interactions in bi-icosahedral nanoclusters. The investigations are focused both on the electron-phonon as well as hydrogen bonding interactions of axial Cl atoms with hydrogen bonding solvents and their influence on the vibronic transitions. It was theoretically shown that the absorption at 670 nm arises due to coupling of two icosahedrons which Au25 does have and Au24 does not. While for Au24, broad band at ∼560 nm was observed which was due to the interactions of the two units ( two Au12 units), 415 nm shoulder and 383 nm band that is due to the electronic transitions related to the localized electronic structures of the individual 12 Au atoms in the icosahedron. All these peaks becoming sharper and blue shifted when it goes to lower temperatures and it increases the oscillator strength and also it showed a new peak around 480 nm in lower temperatures. The electron-phonon interactions were obtained from fitting the band gap data as a function of temperature. Solvent dependent absorption measurements and ultrafast luminescence measurements show the hydrogen bonding formation of clusters with alcoholic solvents.

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