In situ observation of absorption spectra of the heptylviologen cation radical (HV +·) was performed by slab optical waveguide (SOWG) spectroscopy utilizing indium-tin-oxide (ITO) electrodes. On application of potential sweep and potential step techniques from −300 to −600 mV (vs. Ag|AgCl), we observed the initial adsorption of HV +· on the ITO electrode. Two specific absorption bands were found at around 380 and 400 nm, which indicated that HV +· existed as both monomer and dimer on ITO with a monolayer or a few deposition layers. The change in the intensity and the peak position of the absorption spectra from adsorption species of HV +· was correlated with the electrode potential. These two absorption bands became incorporated in one broad band after a 30 s interval at −600 mV.
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