3P029

Electrochemical Properties of Au:SR Clusters
Nirmalya K. Chaki,† Hironori Tsunoyama,† Yukatsu Shichibu,†Yuichi Negishi†, Tatsuya Tsukuda∗,†,‡†Research Center for Molecular-scale Nanoscience, Institute for Molecular Science, Myodaiji,
Okazaki 444-8585, Japan and ‡CREST, Japan Science and Technology Agency, Kawaguchi, Saitama
332-0012, Japan∗

Corresponding Author: tsukuda@ims.ac.jp

Thiol-protected gold (Au:SR) clusters have been attracted enormous interest in recent years
due to their versatile synthetic strategies, unique properties and several application possibilities,
especially in nanoscale devices [1]. The electrochemical measurements of size controlled Au:SRclusters is an ideal probe to understand the core-size dependent evolution of the electronic structures.
Indeed, electrochemistry has been proven to be one of the useful techniques to elucidate the electronic
properties of these materials [2]. The electrochemical determination of single-electron charging eventsis possible due to their tinny capacitive nature (sub-aF), termed as quantized double layer charging. Incontrast, when size approaches to certain threshold value, evolution of size dependent HOMO-LUMO
gap (HLG) becomes evident as a result of quantum size effect. In this presentation, we will present ourrecent progress on the synthesis and electrochemical measurement of Au38(SC12)24 and Au55(SC12)32
clusters to clearly demonstrate metal-to-nonmetal transition behavior and the variation of HLG withsize of Au:SR clusters.

Figure 1 Positive ion LDI mass spectra of (a) Au38(SC12)24 and (b) Au55(SC12)32 clusters. (c)Superimposed UV-Vis spectra of chromatographically separated Au38(SC12)24 (red) and Au55(SC12)32(blue) clusters.
We first prepared alkanethiolated gold clusters (Au:SCn; n =12) by thiolation of the preformed
Au clusters, which were weakly stabilized by poly(N-vinyl-2-pyrrolidone). Then, the Au:SCn clusters
thus prepared were fractionated into several components using recycling size exclusion
chromatographic method [3]. Mass spectrometric analysis revealed that two of the components werecomposed of clusters with a core mass of 8 and 11 kDa (Figure 1a and 1b). These clusters wereassigned to Au38(SC12)24 and Au55(SC12)32 after various characterizations, respectively. The
superimposed optical spectra are shown in Figure 1c, where several discrete electronic transitions areclearly visible. Further, these discrete electronic states have been identified using electrochemicalstudies. Our preliminary electrochemical results (Figure 2) reveal a significant molecular nature of1both (a) Au38(SC12)24 and (b) Au55(SC12)32 as reflected by the presence of 0.9 and 0.7 eV HLG
respectively. The results for Au38(SC12)24 are essentially the same as those reported for 8 kDa clusters
in ref. [2]. Formal potential spacing at various charge states and HLG variation of (a) Au38(SC12)24 and(b) Au55(SC12)32 clusters in dichloromethane are summarized in Table 1.

Figure 2 Differential pulse voltammetric (DPV) plots of (a) 0.5 mM Au38(SC12)24 and (b) 0.15 mM Au55(SC12)32
clusters in dichloromethane using 0.1 M tetrabutylammonium hexaflurophosphate on a Pt (1.6 mm diameter)
working electrode at -400C and at room temperature, respectively, under Ar atmosphere.

Table 1 Variation of formal potential spacing and HLG of Au38(SC12)24 and Au55(SC12)32 clusters
ΔE11
ΔE21
ΔE31
ΔE41
HOMO-LUMOClusters
Gap2 (eV)
(V)
(V)
(V)
(V)

1(a) Au38(SC12)240.29

1.25

0.41

0.42

0.9(b) Au55(SC12)320.35

1.04

0.31

0.42

0.7

ΔE (peak spacing as indicated in Figure 2), which is estimated from the averaged peak positions values ofcathodic and anodic going scans to minimize iRUNC distortion. 2 HOMO-LUMO Gap = [ΔE2 – (ΔE1 + ΔE3)/2]

In summary, we have demonstrated the isolation and electrochemical properties ofAu38(SC12)24 and Au55(SC12)32 clusters for the first time to understand the electronic property variations
as a function of core size. Systematic electrochemical measurements are underway for these clusters,
which will provide a strategy for tuning the electronic properties of gold clusters through precise
control of thiolate ligation as well as core size.
References[1] M.-C. Daniel and D. Astruc, Chem. Rev., 104, 293 (2004).
[2] S. Chen, R. S. Ingram, M. J. Hostetler, J. J. Pietron, R. W. Murray, T. G. Schaaff, J. T. Khoury, M.
M. Alvarez, and R. L. Whetten, Science, 280, 2098 (1998).
[3] H. Tsunoyama, Y. Negishi, and T. Tsukuda, J. Am. Chem. Soc., 128, 6036 (2006).

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