Redox Reactions of Isolated Ferrocene Derivatives Observed By Electrochemical Scanning Tunneling Microscope
Authors: Kobayashi Yuzu , Hong Misun , Wong Raymond , Yokota Yasuyuki , Takeya Jun , Kim Yousoo 
Published: July 2022

Redox reactions on electrode surfaces have been widely applied to fundamental technologies such as energy conversion and storage, metal refining, and electronics. Recently, it has become clear that local structures at the single-atom and single-molecule level affect the efficiency and selectivity of redox reactions [1]. EC-STM, a combination of scanning tunneling microscopy (STM) and electrochemical (EC) measurements, allows direct observation of electrochemical interfaces with high spatial resolution [2], and we aimed to observe the redox reactions of single molecules using this technique. As a target molecule, ferrocene (Fc) was coupled to a tripodal base molecule [3], which was known to form an ordered self-assembled monolayer (SAM) on a gold electrode. By preparing mixed SAM with the molecules with and without Fc moieties, we could stably deposit Fc species on the Au (111) electrode in an isolated and dispersed state, which enables us to observe clear changes in the EC-STM images upon the redox reaction of the single Fc derivative.
A mixture of 8,13-trimercaptotriptycene (Trip) and its Fc derivative (Fc-Trip) was deposited on an Au(111) electrode (the molecules were provided by the Fukushima Lab., Chemical Biology Laboratory, Tokyo Institute of Technology, and the Suzuki Lab., Graduate School of Science, Hokkaido University). Cyclic voltammetry (CV) was performed with a potentiostat (HZ-7000, Hokuto Denko) at a sweep rate of 100 mV/s in 0.1 M HClO4 solution. STM measurements were performed using an MS-10 STM (Bruker), controlled by a NanoScope V (Bruker). A Pt/Ir wire coated with Apiezon wax was used as a probe in the home-made electrochemical cell. The sample potential (Esample) was varied from 0.2 V (vs Ag/AgCl) to 0.4 V while the potential difference between the tip and the sample was kept at 0.3 V.
The CV results are shown in Fig. 1 (a), where the peak originating from the redox reactions of Fc-Trip is observed (dotted line is only for Trip), indicating that the redox potential of Fc-Trip is ~0.3 V. Since the full width at half maximum of the peak is about 0.1 V, Fc-Trip do not interact with each other and are considered to be isolated and dispersed on the Au(111) electrode [4]. Corresponding EC-STM images are shown in Fig. 1 (c)-(e). While the bright spots derived from Fc-Trip are observed at Esample = 0.2 V (the tip potential Etip is 0.5 V), as indicated by white circles, the bright spots almost disappeared at Esample = 0.4 V (Etip = 0.7 V). We confirmed that the change of the height is reversible with respect to the applied potentials, as shown in Fig. 1(c) and 1(e). The height of the bright spot in the EC-STM image reflects the electron transfer rate through the redox molecule between the tip and the sample.
When the sample potential is set to 0.2 V, Fc-Trip is formally in the reduced state, but the electrons are readily transferred to the tip regulated at 0.5 V. On the other hand, when the sample potential is set to settle the oxidized state (0.4 V), the molecule cannot pass electrons to the high potential tip. Therefore, in the case of Fig. 1(d), the current between the tip and the sample is not increased with the presence of the Fc moiety, leading to the apparent disappearance of spots. We noticed that some bright spots did not disappear at Esample = 0.4 V (an arrow in Fig. 1(d)), which suggests that there is heterogeneity in the redox reactivity. In the future, we plan to investigate the heterogeneity by combining spectroscopic techniques with the EC-STM measurement.
[1] J. H. K. Pfisterer et al., Nature
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[2] Y. Yokota et al., J. Phys. Chem. C, 111, 7561 (2007)
[3] F. Ishiwari et al., J. Am. Chem. Soc., 141, 5995 (2019)
[4] C. E. D. Chidsey et al., J. Am. Chem. Soc., 112, 4301 (1990)

Figure 1