Voltammetric and Electroreflectance Study of Thiol-Functionalized Viologen Monolayers on Polycrystalline Gold:  Effect of Anion Binding to a Viologen Moiety

Abstract

The monolayers of thiol-functionalized viologens, i.e., dihexafluorophosphate salts of N-butyl-N‘-(4-mercaptobutyl)-4,4‘-bipyridinium (C4V2+C4SH) and N-pentyl-N‘-(5-mercaptopentyl)-4,4‘-bipyridinium (C5V2+C5SH), on a polycrystalline gold electrode were characterized in several aqueous electrolyte solutions using cyclic voltammetry and electroreflectance (ER) spectroscopy. The voltammetric response for the viologen dication/radical cation couple (V2+/V•+) in Na2SO4, K2SO4, KPF6, or KF solution was found to be electrolyte anion dependent. However, considerable differences due to the electrolyte anion or the alkyl chain length in the fractional content of dimer forms in the V•+ state and the average orientation of the V•+ moiety, both estimated from the results of ER spectral measurements, were not observed. A large voltammteric peak width was observed in KF solution. Because the voltammetric measurements suggested a strong and weak binding to the viologen moiety respectively for PF6 - and F- ions, the dependence of voltammetric response on the concentration of PF6 - was investigated quantitatively for a monolayer of C4V2+C4SH in the mixed solutions of KPF6 and KF at a constant ionic strength. It was found that one PF6 - ion additionally binds to a viologen moiety upon the oxidation of V•+ to V2+. The ratio of the binding constant to V2+ and V•+ was estimated to be approximately 1.2 × 104 M-1 at an ionic strength of electrolyte of 0.4 M. The absence of considerable dependence of the PF6 - binding on the ionic strength was suggested by voltammetric measurements in the solutions containing solely KPF6. The electron-transfer rate constants for the monolayers were estimated to be no less in the order of magnitude than 1 × 104 s-1.

Two-dimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment

“…Comparing the potentials of the characteristic redox-processes V 21 2 V 1d and V 1d 2 V for HS-6V6-SH and HS-6V6-H under the present experimental conditions with literature data [51][52][53][54][56][57][58][59][60]63 indicates that co-adsorbed bromide ions are partially replaced by perchlorate ions, which are in excess in the supporting electrolyte chosen in this study.…”

“…75 This qualitative discussion points to a tilted orientation of the viologen moiety with respect to the surface normal as suggested by Sagara. 59 Attempts to estimate a rate constant from the variation of the peak separation on the scan rate and based on chronoamperometric experiments 72 with microelectrodes indicated that k o (at E P1/P1 0 ) 4 150 s À1 . 76 The second pair of peaks P2 (E ¼ À0.920 V) and P2 0 (E ¼ À0.900 V) is assigned to V 1d 2 V. 51,52 The peak-to-peak separation is constant up to 1 V s À1 and the peak height scales linearly with the scan rate.…”

Electrochemical formation of viologen nano/microsized wires and tubes by potential sweep technique combined with micellar disruption method

“…Such anion replacement has been already reported by some earlier workers [44][45][46][47][48]. The DDV 2+ nano/microstructures remained unchanged for more than two years when they are stored as dry samples in air, which was checked by the SEM and micro-FT-IR measurements.…”

“…This hydrophobic nature makes possible its film formation using the micellar disruption method and prevents the dissolution of the film during the redox treatment. Additionally, the hydrophobic environment in the film provides the site at which the above counter ion exchange occurs [44][45][46][47][48]. (iii) Though not evidenced experimentally, the material may be required to possess a functional moiety which interacts with the others to form the ordered structures.…”

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

“…While the absorbance at 600 nm started to decrease, that at 400 nm further increased as cathodic current of the second cathodic peak started to flow. Electrochemistry of viologen has been studied for long time [17][18][19][20] and it is well known that viologen dication (V 2+ ) is subject to the two step electrochemical reduction to yield the neutral form of viologen, V 0 , via radical cation form of viologen,…”