Potential modulation reflectance of self-assembled naphthoquinone monolayers on gold electrodes

Potential modulation reflectance (PMR) was measured as a function of potential and wavelength of light for self-assembled monolayers (SAMs) formed from dialkyl disulfide-linked naphthoquinones with different spacer alkyl chain on a gold electrode. On the cyclic voltammogram a redox reaction between naphthoquinone and naphtho-hydroquinone in the SAMs is clearly observed the oxidation and reduction peak currents are strongly dependent on the spacer alkyl chain length. On the PMR spectra two peaks are observed: one at 470 nm wavelength originates from the redox change between the naphthoquinone and naphtho-hydroquinone and the other peak at 530 nm wavelength may be due to formation of a charge transfer complex between naphthoquinone and naphtho-hydroquinone; From the phase retardations of PMR and differential capacitance, it is supposed that relatively slow processes like a configuration change of the naphthoquinone derivative accompanied by proton transfer are included in the redox reaction.