Unusual Solvent Effects on Optical Properties of Bi-Icosahedral Au25 Clusters

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• 作者：Viraj Dhanushka Thanthirige ; Ekkehard Sinn ; Gary P. Wiederrecht ; Guda Ramakrishna
• 刊名：Journal of Physical Chemistry C
• 出版年：2017
• 出版时间：February 16, 2017
• 年：2017
• 卷：121
• 期：6
• 页码：3530-3539
• 全文大小：699K
• ISSN：1932-7455

文摘

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 zigzag trend with decreasing temperature. In contrast, the high energy absorption in protic solvents shifts monotonically to higher energy similar to that of toluene. Also at the temperature where the zigzag 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.