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Unusual Solvent Effects on Optical properties of Bi-icosahedral Au Clusters Viraj Dhanushka Thanthirige, Ekkehard Sinn, Gary P. Wiederrecht, and Guda Ramakrishna J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b10948 • Publication Date (Web): 23 Jan 2017 Downloaded from http://pubs.acs.org on January 24, 2017
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The Journal of Physical Chemistry
Unusual Solvent Effects on Optical Properties of BiIcosahedral Au25 Clusters Viraj Dhanushka Thanthirige†, Ekkehard Sinn†, Gary P. Wiederrecht‡ and Guda Ramakrishna*† †
‡
Department of Chemistry, Western Michigan University, Kalamazoo, MI 49008
Center for Nanoscale Materials, Argonne National Laboratory, Lemont, IL 60439 *E-mail:
[email protected] Abstract Temperature-dependent and time-resolved absorption measurements were carried out to understand the influence of solvent hydrogen bonding on the optical properties of bi-icosahedral [Au25(PPh3)10(C6S)5Cl2]2+ (bi-Au25) clusters. Theoretical calculations have shown a low energy absorption maximum that is dominated by the coupling of the two Au13 icosahedra, as well as a high energy absorption arising from the individual Au13 icosahedra that make up the bi-Au25 clusters. Temperature-dependent absorption measurements were carried out on bi-Au25 in aprotic (toluene) and protic (ethanol and 2-butanol) solvents to elucidate the cluster-solvent hydrogen bonding interactions. In toluene, both the low and high energy absorption bands shift to higher energies consistent with electron-phonon interactions. However in the protic solvents, the low energy absorption shows a zig-zag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of
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toluene. Also at the temperature where the zig-zag trend was observed, new absorption peaks emerged at higher energy region. The results are attributed to the hydrogen bonding of the solvent with Au-Cl leading to a disruption of the coupling of icosahedra, which is reflected in unusual trends at the low energy absorption. However, at the transition temperature, the hydrogen bonding solvents distort the icosahedrons so much so that the symmetry of Au13 icosahedron is lifted leading to new absorption peaks at high energy. The transition happens at the dynamic crossover temperature where the solvent attains high density liquid status. Femtosecond time-resolved absorption measurements have shown similar dynamics for bi-Au25 in ethanol and toluene with slower vibrational cooling in ethanol. However, the nanosecond transient measurements show significantly longer lifetime for bi-Au25 in ethanol that suggest the solvent does have an influence on the exciton recombination.
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The Journal of Physical Chemistry
Introduction Optical, electrochemical, catalytic and magnetic properties of atomically precise thiolateprotected quantum sized gold clusters have received enormous research attention in recent years.1-4 Gold clusters with sizes less than 2 nm show a behavior different from plasmonic nanoparticles and are found to have amplified catalytic, nonlinear optical and luminescence properties.5-12 Experimental and theoretical studies are routinely employed to understand the unique properties of these clusters, as well as the fundamental cluster to nanoparticle transitions.5-23 The advent of accurate mass spectral analysis and crystal structures of the gold clusters has enabled both experimental and theoretical researchers to correlate structure-property relationships. Several gold clusters were synthesized that include Au25L18-, Au38L24, Au67L35, Au102L44, Au144L60 and so on, where L is a ligand of varying composition.17-43 Most gold clusters synthesized to date consisted of mainly core-gold passivated by shell-gold that comprises of either monomeric or dimeric staples (-SR-Au-SR- and -SR-Au-SR-Au-SR-). This unique coregold/shell-gold structure not only imparts exceptional stability to the clusters but also is crucial for their optical and catalytic properties.31-48 Among gold clusters having specific core-gold and shell-gold, different types of clusters like mixed triphenylphosphine and thiolate protected ones were synthesized that possess less shellgold. Phosphine-protected gold clusters are unique in the sense that their syntheses always produce the same cluster from the same starting materials and the crystal structures show no apparent staple motifs protecting the core-gold.49-54 The most investigated phosphine and thiolate protected gold cluster is [Au25(PPh3)10(SR)5Cl2]2+ (bi-Au25), with a unique crystal structure where icosahedra comprised of two Au13 units are coupled giving rise to unique optical properties. One of the first crystal structures to be reported on atomically precise gold clusters is 3 ACS Paragon Plus Environment
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for bi-Au25, by Tsukuda and co-workers.53 Later Nobusada and co-workers showed with their theoretical calculations that the low energy absorption arises out of the longitudinal axis connecting the two Au13 icosahedrons and the higher energy (
