Synthesis, redox behavior and electrodeposition of biferrocene-modified gold clusters

www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 473 (1999) 113 – 116

Synthesis, redox behavior and electrodeposition of biferrocene-modified gold clusters Tetsuo Horikoshi, Motoaki Itoh, Masato Kurihara, Kenya Kubo, Hiroshi Nishihara * Department of Chemistry, School of Science, The Uni6ersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113 -0033, Japan Received 31 October 1998; received in revised form 9 February 1999; accepted 26 February 1999

Abstract Biferrocene-modified Au clusters, comprising a 2.2 90.3 nm-diameter Au core covered with 20 biferrocene-terminated thiolates and 75 octyl thiolates on average, were synthesized by a substitution reaction of octanethiol-modified clusters with biferrocene-terminated alkanethiol, (h5-C5H5)Fe(C10H8)Fe(h5-C5H4CO(CH2)7SH) (1). The biferrocene-modified cluster undergoes two-step oxidation reactions in NBu4ClO4 +CH2Cl2 and the second oxidation process leads to the formation of a uniform redox-active Au cluster film on electrode. The surface plasmon absorption of the cluster film depends on the oxidation state of the redox active species on the cluster surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Gold cluster; Biferrocene; Electrodeposition; Surface plasmon

1. Introduction The modification of a gold surface by self-assembled monolayers (SAM) of a functional alkanethiol has attracted continuous attention. Recently, Au clusters stabilized with alkanethiols were created and have initiated a new research area on the metal molecule interface [1 – 6]. Murray et al. have reported the synthesis and electrochemistry of Au clusters modified with ferroceneterminated alkanethiols [7 – 9]. On the other hand, the unique characteristics of p-conjugated ferrocene oligomers have been clarified lately. We have analyzed the redox behavior of oligo(1,1%-dihexylferrocenylene) based on the theory for the linearly combined multi-redox system with internuclear interaction presented by Aoki and Chen [10]. This analysis indicates that the mixed-valence states can be expressed roughly as a linear combination of reduced 

Presented at the International Symposium on Electrochemistry of Ordered Interfaces, Sapporo, Japan, 11–12 September, 1998. * Corresponding author. Tel.: +81-3-38122111 (ext. 4346); fax: +81-3-58006890. E-mail address: [email protected] (H. Nishihara)

and oxidized sites, and the order of charge distribution depends on the oxidation state [11–14]. This study aims at functionalization of the metal cluster surface by multi-step redox active molecules. As the first step we synthesized a biferrocene-modified Au cluster using a biferrocene-terminated alkanethiol, 1 and investigated its redox behavior. The first example of electrodeposition of redox-active clusters and the novel redox behavior of the electrode film are presented in this paper.

2. Experimental The biferrocene derivative, 1, was synthesized by a method similar to that for the ferrocene derivative described in the literature [15]. At first, Bfc-CO(CH)7Br (where Bfc is 1-biferrocenyl) was prepared with a yield of 13% by the reaction of biferrocene with Br(CH2)7COCl · AlCl3 in CH2Cl2 at room temperature for 12 h. It was reacted with thiourea in EtOH at reflux for 40 h, followed by a treatment with 0.02 M NaOH(aq) to give 1 in a yield of 55%. 1H-NMR (CDCl3): d 4.576 (2H, t, J= 1.8 Hz), 4.349 (2H, t, J=1.8 Hz), 4.343 (2H, t, J= 1.8 Hz), 4.304 (2H, t,

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J= 1.8 Hz), 4.204 (4H, t, J = 1.8 Hz), 3.970 (5H, s), 2.534 (2H, tt, J=7.2 Hz), 2.432 (2H, t, J = 7.2 Hz), 1.4 – 1.2 (12H, m), 0.921 (1H, t, J= 6.9 Hz). Octanethiol-modified Au clusters, 2, have been prepared by the method in the literature [7,8]. Successive reaction of HAuCl4 · H2O (0.772 g) in water (60 cm3) with N(C8H17)4Br (4.56 g) in toluene (200 cm3), C8H17SH (0.274 g), 0.5 mol dm − 3 NaBH4(aq) (40 cm3) at room temperature followed by concentration of the organic layer to 20 cm3, pouring into ethanol (800 cm3), standing overnight at − 17°C, filtration and rinsing gave 2 in a yield of 446 mg (95%). A typical procedure to prepare the biferrocenemodified gold cluster, 3, is as follows. A mixture of octanethiol modified Au clusters (2, 203 mg) and 1 (40.0 mg) in 1.0 cm3 toluene was stirred at room temperature for 48 h. The solution was poured into 300 cm3 ethanol and stored in a refrigerator (−17°C) for one day. After filtration with a membrane filter (pore size: 25 mm), the residue was washed thoroughly with ethanol, dissolved again in CH2Cl2, filtered and dried in vacuo to afford 3 as a dark purple powder. Yield: 0.190 g, 85%. Anal. Found: C, 16.92; H, 2.26%. C1160H1895Au309Fe40O20S95 [(C8H17S)75(C28H31Fe2OS)20Au309]. Calc.: C, 16.90; H, 2.48%. 1H-NMR (CD2Cl2): d 4.5–3.8 (17H, m, br, cyclopentadienyl groups), 2.6–0.9 (ca. 60H, m, br, CH2), 0.9 – 0.7 (12 H, br, CH3). IR, UV-vis-NIR, and 1H-NMR spectra were recorded with a Shimadzu FT-IR 8100M, JASCO V570, and a JEOL EX270 spectrometer, respectively. AFM images of the cluster films were recorded in the tapping mode with a Shimadzu SPM-9500 spectrometer. Cyclic voltammetry was carried out in a standard three-compartment cell under an argon atmosphere at 25°C equipped with a Pt wire counter electrode and an Ag Ag + reference electrode (10 mmol dm − 3 AgClO4 in 0.1 mol dm − 3 NBu4ClO4 +MeCN, E 0%(ferrocenium/ ferrocene)=0.20 V vs. Ag Ag + ) with a BAS CV-50W voltammetric analyzer. A three-compartment quartz cell was used for the spectroelectrochemical measurements.

estimated from the 1H-NMR spectrum and the elemental analysis data is 20 as average. The UV-vis spectra of both 2 and 3 in CH2Cl2 exhibit an absorption band at 520 nm with a molar absorption coefficient, omax = 7.7× 105 mol − 1 dm3 cm − 1 due to the surface plasmon of Au [18–21]. A small increase is observed in the UV region of the spectrum for 3 compared to 2 due to p–p* transition of biferrocenyl moieties. Cyclic voltammograms of 3 at glassy carbon in 0.1 mol dm − 3 NBu4ClO4 + CH2Cl2 for consecutive potential scans are given in Fig. 1(a). At the first scan, two reversible oxidation waves appear at E 0%= (Ep,a −Ep,c)/ 2= 0.105 and 0.517 V versus Ag Ag + . These E 0% values are similar to those of 1 dissolved in solution. When the voltammetry is carried out at different scan rates using a fresh electrode in each experiment, the peak-to-peak separation, DEp = Ep,a − Ep,c is small (20 mV) and constant at low scan rates ( B 1 V s − 1) and the peak current increases almost linearly with the sweep rate, implying that the modified cluster is adsorbed on the electrode surface. The coverage of biferrocene sites at glassy carbon evaluated from the peak area of the cyclic voltammogram, Gbiferrocene is 3.7 × 10 − 10 mol cm − 2, and thus the surface coverage of the cluster, Gcluster = 1.9× 10 − 11 mol cm − 2 as twenty biferrocene units locate on a cluster. This value corresponds to a few monolayers of clusters since the monolayer coverage of clusters with the diameter of 5 nm is calculated to be 7.6×10 − 12 mol cm − 2. But this adsorption is weak because no redox activity appears when the electrode once dipped in a solution of 3 is rinsed with CH2Cl2 and immersed again in NBu4ClO4 + CH2Cl2 without 3, as has been reported for ferrocene-modified Au clusters [7].

3. Results and discussion Gold clusters modified with biferrocene-terminated alkanethiol, 1, were prepared by the substitution method of octanethiol-stabilized Au clusters, 2, developed by Murray et al. [7,8] as shown in Scheme 1. A TEM image of 2 indicates that the core size of the clusters is 2.29 0.3 nm, corresponding to 309 Au atoms. It is proposed that the number of alkyl thiolate units on one cluster is 95 as reported for this cluster size [16,17], the number of biferrocene on the cluster 3

Scheme 1.

T. Horikoshi et al. / Journal of Electroanalytical Chemistry 473 (1999) 113–116

Fig. 1. Cyclic voltammograms of 3 (6.1 mmol dm − 3) at a glassy carbon disk (3 mm diameter) (a) and at an ITO plate (2 cm2 area) (b) in 0.1 mol dm − 3 NBu4ClO4 + CH2Cl2 for consecutive potential scans at 0.1 V s − 1. Numbers in the figure refer to those of cyclic scans.

When the cyclic potential scans are carried out in the solution of 3 at glassy carbon in a potential range between − 0.3 and 0.3 V vs. Ag Ag + where only the first-step oxidation occurs, a steady-state voltammogram is observed after several scans, implying no occurrence of electroactive film formation at the electrode. However, as is shown in Fig. 1(a), when the positive potential limit of the cyclic scan is 0.9 V versus Ag Ag + , more positive than the second oxidation potential, redox waves increase in size with the scan number, denoting electrodeposition of a redox-active film. This electrooxidative deposition takes place at common electrode substrates such as Pt, Au or indium-tin oxide (ITO) (see Fig. 1(b)), indicating that inter-cluster attractive interaction is the requirement for the deposition. Interestingly, the shape of the voltammogram changes gradually during the potential scans, and it converges finally into one redox wave as displayed in Fig. 1(a). The film formed by the electrodeposition at ITO exhibits a characteristic absorption band due to the surface plasmon of Au clusters at 520 nm as shown in Fig. 2(a). This indicates that the nature of the cluster core is not influenced significantly by the deposition compared with that when the cluster particles are dispersed individually in solution. The surface coverage of the clusters

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estimated from the absorbance at 520 nm and omax of the cluster as noted above, Gcluster(vis) equals 1.7× 10 − 9 mol cm − 2, corresponding to ca. 220 monolayers of the clusters and ca. 1.1 mm in thickness. An AFM image of the electrodeposited film of 3 on ITO has indicated that the roughness of the film is within 3 nm for a 100 ×100 nm2 area. This implies the monolayer-level smoothness of the film. The cluster film formed by electrochemical oxidation of 3 shows a reversible redox wave in NBu4ClO4 + CH2Cl2 without biferrocene-modified Au clusters, 3, and the shape and magnitude of the redox wave are almost unchanged against repeated potential scans. This implies that the film once formed is strictly adsorbed on the electrode surface and that a strong attractive force remains between the clusters in the reduced state. It should be noted that Au clusters modified with ferroceneterminated alkanethiol, FcCO(CH2)7SH (Fc is ferrocenyl), undergo no film formation even when the positive potentials causing the electrodeposition of a biferrocene derivative 1 are applied. Possible rationales for this strong inter-cluster interaction would be the positive charge accumulation on clusters enforcing aggregation due to like-charge attractions [22], the van der Waals interaction between biferrocene units because such interaction between ferrocene units affecting the redox properties has been observed for ferrocene-terminated alkanethiol self-assembled monolayers [23], and the chemical bond formation between biferrocene units on different clusters by oxidative decomposition of the biferrocene units since ferrocenium ion decomposes in a second order reaction in aprotic media [24,25]. The surface coverage of the 1e − redox sites estimated from the cyclic voltammogram for the film in Fig. 2(a) is 1.1× 10 − 8 mol cm − 2. This indicates that the surface coverage of the clusters, Gcluster(CV) is 2.7 × 10 − 10 mol

Fig. 2. (a) UV-vis-NIR spectra of the electrodeposited film of 3 at ITO under the conditions given in Fig. 1(b) at − 0.3 V (reduced state) and 0.7 V (oxidized state) vs. Ag Ag + . (b) The differences in the spectra at given potentials from the one at −0.3 V vs. Ag Ag + in 0.1 mol dm − 3 NBu4ClO4 +CH2Cl2. Numbers in the figure refer to the potentials vs. Ag Ag + .

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cm − 2, if one cluster undergoes redox reactions of twenty biferrocene units and thus 40 electrons. A large discrepancy between Gcluster(vis) and Gcluster(CV) supports the occurrence of chemical decomposition of biferrocene units under the electrodeposition conditions. At present, however, the relation of the change in the shape of the voltammogram and the mechanism of electrodeposition is not understood, and a further study requisite in order to draw a conclusion on the electrodeposition mechanism is currently in progress in our laboratory. Spectro-electrochemical measurements of the electrodeposited film of 3 formed at ITO indicate that the absorption at 520 nm decreases according to the oxidation of redox active sites in the film as shown in Fig. 2(b). This spectral change occurs reversibly with change of the potential. The molar absorption coefficient, based on the number of 1e − redox active units estimated from the change in absorbance and the amount of charge passed during the oxidation, is 4× 104 mol − 1 dm3 cm − 1, much larger than o for the d – d transition of ferrocene appearing around 450 nm (102) [26]. It is thus reasonable to suppose that the spectral change is not attributed to the absorption of the redox active units attached to the cluster surface but due to the surface plasmon. It is probable that the change in oxidation state of the redox-active adsorbates on clusters leads to the provision or withdrawal of additional electron density at the interface, causing the change in intensity of the surface plasmon band [21]. These results demonstrate that the electronic properties of Au clusters can be controlled by the oxidation states of the surface-attached redox-active molecules. In conclusion, we have presented a new method to form a metal cluster assembly using electrooxidation of redox-active surface modifiers. This technique might be useful to fabricate molecular electronic devices.

Acknowledgements This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (No. 09237101 on Priority Area Research of ‘Electrochemistry of Ordered Interfaces’ and No. 09440226), and The Asahi Glass Foundation.

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