Spectroelectrochemical evidence of the role of viologen moiety as an electron transfer mediator from ITO substrate to a Pt complex acting as a confined molecular catalyst for hydrogen evolution reaction

• Confinement of Pt complex in viologen monolayer on ITO accelerates HER.
• Spectroelectrochemical measurements clarify the redox states of viologen monolayer.
• Electron transfer from ITO to Pt complex via viologen during HER was confirmed.

Spectroelectrochemical measurements at indium tin oxide (ITO) electrode modified with viologen monolayer showed two redox peaks corresponding to viologen dication/radical cation (V2 +/V+) and viologen radical cation/neutral form (V+/V0) in cyclic voltammogram (CV), and stable spectra corresponding to radical cation (V+) and neutral form (V0) of viologen at potentials between V2 +/V+ and V+/V0 redox peaks and those more negative than V+/V0 redox peak, respectively. On the other hand, at viologen monolayer modified ITO electrode with Pt complex confined within the monolayer by ion exchange reaction, no redox peaks but large current due to hydrogen evolution reaction (HER) were observed in CV and no absorption peaks corresponding to V+ or V0 were observed in the UV/visible spectra obtained during the potential scan. Time-resolved spectroelectrochemical measurements, however, showed that V+ is formed upon the potential step to the potentials more negative than V2 +/V+ redox potential and disappeared within ca. 1 ms, showing that electron is transferred from ITO electrode to proton to form hydrogen via viologen moiety and Pt complex.

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