Dimerization of organic radicals formed as a primary redox product is known for more than a half century. This often undesired deactivation of radicals recently came to the attention as a possible mode of storing binary information. Mechanism is called the 'reversible dimer switching' because potentials of the formation and re-oxidation are very different. Therefore a bistability is observed at a certain potential range. Recently we published several examples of dimer formation: pyridiniums, dodecylpyridinium, quinolinium, azoniahelicene and benzimidazolinium cations [1-4]. All compounds yield more or less similar cyclic voltammograms differing only in the potential range of bistability. Dimers are easily re-oxidized to the starting monomers and hence their preparation and identification is virtually impossible. Voltammograms like the one in Figure 1 may indicate a bimolecular dimerization but also can correspond to some other monomeric product formed by a first order chemical reaction. It is the order of redox induced subsequent chemical reaction, which offers a unique detection of dimers. Data analysis of the second order kinetics is hampered by non-existence of the inverse Laplace transformation of partial differential equations containing a quadratic term. This is circumvented by digital simulation of series of voltammograms at different concentrations and scan rates using the finite difference methods. Such a procedure involves handling a large number of adjustable parameters (like rate and equilibrium constants and redox potentials). Alternatively the evaluation of kinetic parameters from faradaic phase angle leads to a problem to include the surface concentration c(0,t) at a given potential. All communications using the phase angle approach suggest the estimation of c(0,t) by the finite difference technique as in the case of voltammetry. This duplicates the evaluation and offers no special advantage. We will demonstrate a substantial simplification by combining the convolution voltammetry and the electrochemical impedance spectroscopy. Our procedure works with convolution of a single cyclic voltammogram, yielding c(0,t), and a single set of impedance spectrum at the same bulk concentration. The simulation compares the fit or disagreement of two reaction schemes: the first and the second order chemical kinetics. Programs for both reaction schemes were written in software Mathematica (Wolfram Research, Inc,). We identified the dimer formation and estimated rate constants of bimolecular reactions for all compounds dodecylpyridinium, quinolinium, azoniahelicene and benzimid-azolinium cations. We will show the redox switching and monitoring of the stored binary information by UV-vis spectra or by electronic circular dichroism spectra in the case of chiral compounds.
Detection of Organic Dimers by Combination of Electrochemical Impedance Spectroscopy and Convolution Voltammetry
Clara LANZA
2018
Abstract
Dimerization of organic radicals formed as a primary redox product is known for more than a half century. This often undesired deactivation of radicals recently came to the attention as a possible mode of storing binary information. Mechanism is called the 'reversible dimer switching' because potentials of the formation and re-oxidation are very different. Therefore a bistability is observed at a certain potential range. Recently we published several examples of dimer formation: pyridiniums, dodecylpyridinium, quinolinium, azoniahelicene and benzimidazolinium cations [1-4]. All compounds yield more or less similar cyclic voltammograms differing only in the potential range of bistability. Dimers are easily re-oxidized to the starting monomers and hence their preparation and identification is virtually impossible. Voltammograms like the one in Figure 1 may indicate a bimolecular dimerization but also can correspond to some other monomeric product formed by a first order chemical reaction. It is the order of redox induced subsequent chemical reaction, which offers a unique detection of dimers. Data analysis of the second order kinetics is hampered by non-existence of the inverse Laplace transformation of partial differential equations containing a quadratic term. This is circumvented by digital simulation of series of voltammograms at different concentrations and scan rates using the finite difference methods. Such a procedure involves handling a large number of adjustable parameters (like rate and equilibrium constants and redox potentials). Alternatively the evaluation of kinetic parameters from faradaic phase angle leads to a problem to include the surface concentration c(0,t) at a given potential. All communications using the phase angle approach suggest the estimation of c(0,t) by the finite difference technique as in the case of voltammetry. This duplicates the evaluation and offers no special advantage. We will demonstrate a substantial simplification by combining the convolution voltammetry and the electrochemical impedance spectroscopy. Our procedure works with convolution of a single cyclic voltammogram, yielding c(0,t), and a single set of impedance spectrum at the same bulk concentration. The simulation compares the fit or disagreement of two reaction schemes: the first and the second order chemical kinetics. Programs for both reaction schemes were written in software Mathematica (Wolfram Research, Inc,). We identified the dimer formation and estimated rate constants of bimolecular reactions for all compounds dodecylpyridinium, quinolinium, azoniahelicene and benzimid-azolinium cations. We will show the redox switching and monitoring of the stored binary information by UV-vis spectra or by electronic circular dichroism spectra in the case of chiral compounds.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
