Electrochemical Reduction of Lucigenin on Mercury in Aqueous Media

Abstract

A more comprehensive study than those previously reported in the literature was carried out on the electrochemical reduction of 10,10'-dimethyl-9,9'-baicridinium ion (lucigenin) in aqueous solutions. Lucigenin (L') exhibits one, two, or three irreversible voltammetric reduction peaks, depending on its concentration and the scan rate. The first peak is due to the reduction of the first monolayer of molecules adsorbed on the electrode. This process involves two simultaneous one-electron transfers that yield a molecule (P), also immobilized at the electrode, which undergoes a conformational change to the form F'. In addition, the reagent molecules that reach the electrode by diffusion and the product (P) comproportionate to yield the intermediate radical R. New reactant molecules can displace the product because the monolayer is reversibly adsorbed. The transfer coefficient of the process and the activation energy of the conformational change were determined. The second peak is due to the reduction of a second monolayer of adsorbed lucigenin molecules. This takes place via the first monolayer of product molecules adsorbed on the electrode. Finally, the third peak arises from reduction of lucigenin molecules adsorbed on the previous monolayers. The overvoltage required to reduce lucigenin increases with increasing number of monolayers formed. The formation of new insoluble monolayers blocks the faradaic process altogether in the second scan at more positive potentials than those for the second or third peak. Infroduction 10,10'-Dimethyl-9-9'-biacridinium ion (L'), known as lucigenin and a member of the bipyridine family, has the structure CH3 + N Cl!3 Scheme L where the two tricyclic rings lie at an angle of 90°. There are several papers in the chemical literature dealing with the electrochemical reduction of lucigenin at various types of electrodes.'' However, the most significant contribution to knowledge on the electrochemical reduction of lucigenin in organic media was reported by Ahlberg et at.,' who used a Pt electrode and obtained a two-electron diffusion-controlled voltammetric peak at ca. -300 mV [vs. saturated calomel electrode (SCE)]. Both one-electron transfers take place simultaneously on species L' to yield reduction product P e e L2 P [1]The configuration of intermediate radical R, like that of species L', is such that the two tricyclic rings lie at an angle of 90°. However, the formation of a double bond between these groups in P pushes the molecule to a completely planar configuration, which is impossible owing to steric hindrance between the protons bonded to both rings. The most stable angle in this situation is ca. 45°.Ahlberg et at. found species P to undergo a conformational change to form F' (l0,10'-dimethyl-9,9'-biacridylene) (see Scheme II in Ref. 7), which has a more stable folded structure. The change also implies an alteration of the relative positions of the protons bonded to carbons 1 and 8, * Electrochemical Society Active Member. as well as carbons 1' and 8' in the tricyclic rings. In ...

Molecular Structure Dependence of the Phase Transition Spike Response of Viologens at an HOPG Electrode Using Bisviologen and Carboxylated Viologen

“…An adsorption layer of viologens on a well-defined electrode surface frequently exhibits a pair of spike voltammetric peaks corresponding to a one-electron redox reaction of the monolayer-level adsorption layer. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] This phenomenon has been understood as being a two-dimensional ͑2D͒ faradaic phase transition. One-electron reduction of viologen to monoradical monocation is accompanied by the formation of the condensed phase.…”

“…One-electron reduction of viologen to monoradical monocation is accompanied by the formation of the condensed phase. [3][4][5][6][7][8][12][13][14][17][18][19] The oxidized dication form of the viologen ͑V 2+ ͒ is usually highly soluble in water, while the one-electron reduced counterpart, monoradical monocation ͑V •+ ͒, is less or sparingly soluble in water. The reduced form tends to form a -stacked dimer.…”

“…Because the reduced form has higher adsorptivity on electrode surfaces, the redox reaction of the adsorption layer is observed at less negative potential than the bulk redox potential. This voltammetric wave is a sharp spike at Hg, HOPG, and some other electrodes, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] showing the nature of the first-order phase transition.…”

Electrochemiluminescence of Acridines

“…An adsorption pre‐wave accompanies the first step, while the second one is catalytic, owing to the chemical regeneration of the intermediate reduction product at the electrode surface . A more comprehensive study of the electrochemical reduction of lucigenin in aqueous solutions has been provided later by cyclic voltammetry at a static mercury drop electrode . This study shows that lucigenin exhibits one, two, or three irreversible voltammetric reduction peaks, depending on its concentration and the scan rate (Figure ).…”

“…Thel ucigenin electrochemicalr eduction reactions in aqueous solutionh ave been evaluatedf or mechanism of ECL by direct current polarography at adropping mercury electrode,b yc yclic voltammetry at an amalga- mated gold electrode and at ahanging mercury drop electrode,a nd by microcoulometry.I t has been found that lucigenin is reduced in two separate one-electron steps. An adsorption pre-wavea ccompanies the first step,w hile the second one is catalytic,o wing to the chemical regeneration of the intermediate reduction product at the electrode surface [17].Amore comprehensive study of the electrochemical reduction of lucigenin in aqueous solutions has been providedl ater by cyclic voltammetry at as tatic mercury drop electrode [18].T his study shows that lucigenin exhibits one,t wo,o rt hree irreversible voltammetric reduction peaks,d ependingo ni ts concentration and the scan rate ( Figure 2). Thef irst peak is attributed to the reduction of the first adsorbed monolayer on the electrode.T his processi nvolvest wo simultaneous one-electron transfers to produce the intermediate radical.…”