An intermediate state of the triphenylamine cation radical revealed using an electron-transfer stopped-flow method

Abstract

Using an electron-transfer stopped-flow method, the triphenylamine cation radical (TPA·+) was generated in acetonitrile via the electron transfer between TPA and the tris(p-bromophenyl)amine cation radical (TBPA·+), and the changes in the absorption spectra were observed. The well-known decay process of TPA·+ was observed in the absence of TPA, reflecting the dimerization reaction of TPA·+ and the consecutive electron-transfer reactions of the dimer compound, tetraphenylbenzidine (TPB). When TBPA·+ was mixed with a large excess amount of TPA, new absorption peaks appeared around 400 and 850 nm. No time changes were observed for these peaks, while the decrease in TPA·+ and the increase in TPB·+ were observed overlapping on these peaks. The new absorption spectrum can be assigned to that of (TPA)2·+, or, more presumably, (TPA)n·+. Although (TPA)n·+ has not been recognized as an intermediate in the electrochemical oxidation of TPA, it has been ascertained that (TPA)n·+ is electrochemically generated under certain conditions, e.g., during the initial stage of the oxidation of TPA in a stirred solution containing a sufficient amount of TPA.