Investigation on the formation and decay of the N,N,N′,N′-tetramethylbenzidine radical cation using various oxidants and reductants by stopped-flow spectrophotometry

Abstract

N,N,N′,N′-Tetramethylbenzidine (TMB) is an aromatic amine that undergoes oxidation by various oxidizing agents such as Ce4+, MnO− 4, Cr2O2− 7; HSO− 5, S2O− 8, H2O2, Cl2, Br2 and I2, thereby serving as a reducing substrate. One-electron oxidation of TMB results in a radical cation (TMB˙+), and on further oxidation leads to the product dication (TMB++) were monitored by stopped-flow spectrophotometer at the absorption wavelength of TMB˙+(λmax; 460 nm). ESR data was also provided to confirm the formation of radical cation. The rates of both the formation and decay of TMB˙+ have been followed by a total second-order kinetics, a first-order dependence each on [TMB] (or) [TMB˙+] and [oxidant]. The kinetic and transition state parameters have been evaluated for the effects of pH and temperature on the formation and decay of TMB˙+ and discussed with suitable reaction mechanisms. Also, the rate constants for the reactions of the radical cation with various reducing agents such as sulfite (SO2− 3), thiosulfate (S2O2− 3), dithionite (S2O2− 4) and disulfite (S2O2− 5) and ascorbic acid (vitamin C, AH2 were determined. Besides these, this article also explains how TMB acts as a better electron relay than unsubstituted benzidine, even though both of them undergo one-electron oxidation and are used in the chemical routes to solar energy conversions. The observed rate constants for electron transfer were correlated theoretically using Marcus theory. The observed and calculated rate constants have good correlation.