Time-resolved resonance Raman and surface-enhanced resonance Raman scattering study on monocation radical formation processes of heptylviologen at silver electrode surfaces

Abstract

Time-resolved resonance Raman and surface-enhanced resonance Raman scattering (RRS and SERRS) spectra were measured for the monocation radical of heptylviologen (HV'+) formed on a silver electrode surface as a function of time ( t ) after switching the electrode potential from -0.2 V (vs Ag/AgCl) to -0.65 V (the monocation radical formation potential). The intensity of the R R S band at 1530 cm-I due to HV'+ increases as a linear function of t 3 / 2 in the first step (0 to ca. 4 ms) and as a linear function of t l / * in the second (4-10 ms) and third steps (10-30 ms). The t312-dependence indicates an instantaneous nucleation mechanism, which consists of instantaneous formation of a nucleus and subsequent three-dimensional growth of the nuclei, and the t'i2-dependence indicates one-dimensional diffusion-controlled film growth processes. Measurement of transient currents confirmed the existence of the two kinds of one-dimensional diffusion-controlled film growth processes. Time-resolved SERRS spectral measurements proved that the monocation radical formation process in a few monolayers of the HV2+ molecules adsorbed at a roughened silver electrode surface proceeds following a mechanism similar to that of the first step observed for the smooth silver electrode surface. The time-resolved R R S and SERRS spectra indicated that at the beginning of the reduction process the radicals exist as a dimeric state at both the smooth and roughened silver electrode surfaces and that the conversion from the dimer to a radical salt begins when the radicals form a film of a thickness of at least 5 monolayers.

Interrogating Charge Storage on Redox Active Colloids via Combined Raman Spectroscopy and Scanning Electrochemical Microscopy

“…Coupling surface interrogation SECM (SI-SECM) to Raman spectroscopy enables us to comprehensively study the charge transfer mechanism in RAC monolayer films (Figure b). Viologen derivatives have multiple Raman-active peaks that allow structural changes to be observed during the reduction to the radical cation and neutral form. ,,− A strong resonance Raman signal from V + · arising from an absorption region (540 nm) near the excitation wavelength (532 nm), allows monitoring of the signal on non-noble electrode surfaces, circumventing the need for surface-enhancement . Raman spectroscopy has previously been coupled to shear force SECM, but this is the first time that SECM measurements have been used in situ with Raman spectroscopy with the intention of monitoring transient processes.…”

Novel Time-Resolved Surface-Enhanced (Resonance) Raman Spectroscopic Technique for Studying the Dynamics of Interfacial Processes: Application to the Electron Transfer Reaction of Cytochrome c at a Silver Electrode

“…20 Upon the synchronization of the laser pulses with triggering of a gatable photodiode array, the time resolution was limited by the laser pulse width. While this approach was restricted to m onitoring of photoinduced reactions on the electrode surface, Misono et al 17 studied potential jump-induced reactions by employing a pulsed laser to probe TR SE RR spectra with a time-resolution determined by the pulse width of the laser.…”

Facile Dimerization of Viologen Radical Cations Covalently Bonded to β-Cyclodextrin and Suppression of the Dimerization by β-Cyclodextrin and Amphiphiles

“…15,33 Radicals of bisviologens linked by flexible organic bridges form dimers of remarkable stability by intramolecular process. [30][31][32] Cyclodextrins (CDs) are cyclic oligosaccharides that possess hydrophobic cavities capable of forming inclusion complexes with a variety of organic molecules in aqueous solution. [34][35][36] Several investigators have reported the results of studies on the interaction of viologens with CDs and the effect of CDs on the dimerization of viologen radicals.…”

“…V •+ is known to dimerize in solution. − ,− The dimer formation affects the electrochromic properties of viologens 6,7,28 and further reactions where V •+ participates. ,, The dimers have been characterized by UV−vis, ,,,,− ,, IR, Raman, ,− and ESR 13 spectroscopies. The reported monomer−dimer equilibrium constant ( K D ) for dimethyl viologen radical cation (C 1 C 1 V •+ ) in aqueous solutions around 25 °C is in the range of 380−840 M -1 . ,,,, The ambiguity of the UV−vis spectroscopic parameters of the viologen dimer might be responsible for the wide range of the reported K D values.…”

“…The radical cations of unsymmetric viologens with long hydrocarbon chains 24-26 or symmetric viologens with relatively long hydrocarbon chains 27-30 form much more stable dimers than C 1 C 1 V •+ . However, the exact K D values for the viologens have not been reported, presumably due to the complications arising from adsorption of the viologens on electrode surface or glassware. , Radicals of bisviologens linked by flexible organic bridges form dimers of remarkable stability by intramolecular process. − …”