Takamasa Sagara

A redox-active monolayer on an optically transparent electrode constitutes a typical platform for spectroelectrochemical sensing. The necessity for its sophistication arises from the availability of multi-dimensional sensing signals. Simultaneous monitoring of the redox current and color change synchronized with the oxidation state change significantly enhances sensitivity and selectivity. This study aimed to elucidate the modification of an indium tin oxide (ITO) electrode with a viologen monolayer with an ordered orientation. Novel methods were developed to immobilize a viologen molecule bearing a carboxyl group to form assembled monolayers through a condensation reaction using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide with N-hydroxy-succinimide (EDC/NHS). In the two methods of immobilization, one utilizes a two-step process to firstly form an aromatic siloxane base layer and subsequently attach the viologen derivative through an amide linkage by post-amidation. The other employs a direct ester linkage between the hydroxyl groups of the ITO surface and the carboxyl group of the viologen derivative. The latter method was also applied to immobilize a ferrocenyl group at a very short distance from the ITO surface. Potential-modulated UV-visible transmission absorption spectral measurement techniques with oblique incidence of plane-polarized light were employed to determine the orientation of the longitudinal axis of the reduced form of the viologen. The frequency dependence data of the potential-modulated transmission absorption signals were utilized to analyze the electron transfer kinetics. The performance of the two viologen-modified electrodes was compared to that of an ITO modified by post-amidation to the most commonly used base layer prepared with 3-aminopropyl triethoxysilane.

Entities showing dynamic processes on an electrode surface range from small molecules to millimetersized soft materials. The dynamics is driven by the potential dependence of the interaction of ions and dipoles with the electrode surface charge as well as the lateral interactions among the molecules. Regulated reversible movements of molecular assemblies at electrified interfaces result in new interfacial functions and emergent syntheses. To understand what are really changing at the electrode/solution interface while dynamic processes are undergoing, it is essential to have facile tools to analytically track the interfacial dynamics, as exemplified by potential-modulated spectroscopies. Typical dynamic behavior and spectroelectrochemical methods are broadly reviewed.

In redox-active ionic liquids (RAILs), either or both of the constituent ions are redox-active. Because of the high concentration of the ions, RAILs exhibit not only ion conduction but also electron conduction through the bimolecular electron self-exchange reaction. Because neat RAILs do not contain any supporting electrolyte, migration of the redox active ions results in enhancement or diminishment of the redox current at an electrode. To treat the migration effect for electrochemical analysis, a limiting current correction was theoretically derived by Oldham, Hyk, and Stojek (Oldham, K. J. Electroanal. Chem. 1992, 337, 91-126; Hyk, W.; Stojek, Z. Anal. Chem. 2002, 747, 4805-4813) for the steady-state voltammetry. Although steady-state voltammetry is a robust method in electrochemistry, the actual measurement is time-consuming and cannot be always made because of the instability of the electrochemical system. To overcome the problem, we propose the use of cyclic voltammetry to evaluate the diffusion coefficient of the redox-active ion that constitutes RAIL. The peak currents were analyzed by the purely diffusional framework of the Aoki-Matsuda-Osteryoung equation (Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H.; Osteryoung, J. J. Electroanal. Chem. 1984, 171, 219-230.) in the range from several mV s^-1 to several ten mV s^-1, and the migration correction to the near-steady-state limiting current was applied on the basis of the Oldham-Hyk-Stojek theory to scale the diffusion coefficient. As an example of RAILs, [FcC₆ImC₁][TFSI], which exhibits charge increase reaction with the same sign (S⁺ - e^- ⇌ P²⁺), was used and the cyclic voltammograms were recorded at various sizes of the microdisk electrodes and various scan rates. The peak currents obeyed the Aoki-Matsuda-Osteryoung equation with the scaled diffusion coefficient, which has the same value as determined by the steady-state voltammogram. Our approach can be used to evaluate the diffusion coefficient of redox-active ions that constitute the RAIL with the charge increase reaction with the same sign.

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We investigated the binding of anionic surfactants of lower concentrations than their critical micelle concentrations (cmcs) to the cationic redox-active viologen site in the interior of a self-assembled monolayer (SAM) on a polycrystalline Au electrode. We embedded the viologen site in the midway of the alkyl chain to facilitate the ion-pairing binding, which depends on the oxidation state of the viologen. We found that the binding of anionic surfactants and inorganic anions causes a negative shift of the formal potential of the redox couple of the viologen radical cation/viologen dication in line with the binding equilibrium. In contrast, the anion binding was weak and trivial when viologens are located at the SAM surface, indicative of the enhancement of the binding by the electrostatic interaction in the microenvironment with the low dielectric constant. The negative shift of the formal potential of viologen in the interior was greater for the surfactants with longer alkyl chain lengths, indicative of the efficacy of the alkyl chain–chain interaction. The chain-length-dependent potential shift followed the linear Traube rule but with a smaller slope than that in the original rule. We also demonstrated that the conjugated layer of the viologen SAM with dodecyl sulfate at a lower concentration than the cmc completely blocks the direct electron transfer (ET) from the Au electrode to solution-phase Fe(CN)₆^3– but allows mediated ET around the formal potential of the viologen.

Viologen is among the most attractive and easiest‐to‐use organic redox active group in many functional molecular assemblies. It plays crucial roles as an electron transfer mediator in the artificial photo‐energy conversion systems and electron‐transfer protein assemblies and as a building block of supramolecules. Its features include electrochemically reversible redox activity and stability. Strong blue color and tendency to dimerization of the one‐electron reduced form, viologen mono‐radical mono‐cation, are remarkable. In this Account, we describe the use of viologen to give a powered movement of small molecules and motion of millimetre‐sized macroscopic soft‐matters and the use of viologen ionic liquid as electrochromic materials. Attractivities of the use of viologen units for powering and coloring are demonstrated and discussed. In particular, we highlight driving of mechanical movements by π–π stacking dimerization, incorporation in a hydrogel to attain highly deformable material, induction of 2D phase transformation, and sharp color change of very thin ionic liquid layer in a compartment‐less electrochromic display.

A highly deformable hydrogel, viologen pendant poly-L-lysine-based glutaraldehyde cross-linked hydrogel (PLLV-GA-gel), was characterized using surface-enhanced Raman scattering (SERS) signals under electrochemical control on a roughened Au electrode surface. We found that one-electron reduced forms remain near the electrode surface, preferentially as a dimer, even at the potentials at which all the viologens should be of a further reduced state. Implications of the SERS data were discussed in terms of heterogeneity and dimerization of one-electron reduced form viologens.

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