Spectroscopic Observation of the Dimerization Reactions of the 9-Phenylcarbazole Cation Radical in Acetonitrile

Abstract

Abstract The dimerization reaction processes of the 9-phenylcarbazole cation radical (PCZ•+) in acetonitrile (AN) were observed using an electron-transfer stopped-flow method. Although the lifetime of PCZ•+ in AN is less than a few hundred milliseconds, the dynamic transformation of the absorption spectra of PCZ•+ was successfully observed in AN for the first time with the proposed method. By observing the changes in the absorption spectra in the visible region, the decrease in PCZ•+ and the concurrent increase in the dimer cation radical were observed with isosbestic points. In addition, in the presence of neutral PCZ, the acceleration of the dimerization of PCZ•+ was clearly confirmed. Consequently, in the presence of PCZ, the rate law was determined to be −d[PCZ•+]/dt = 3Kk′[PCZ•+]2[PCZ], where 3Kk′ is 1.4 × 109 M−2 s−1. This means that the reaction mechanism involves a two-step interaction with the catalytic effect of neutral PCZ. Although the difference in the reactivity had already been known between PCZ•+ and the triphenylamine cation radical, the reaction mechanism has been clarified to be different in both cases.