Spectroscopic detection and reaction analysis of aromatic cation radicals using an electron-transfer stopped-flow method

For the purpose of elucidating and understanding the reaction dynamics of short-lived intermediates formed in the solution phase, it is very important to carry out detection and kinetic analysis in a homogeneous solution without being interfered by concentration gradients, which are usually unavoidable in electrochemical and pulse-radiolysis methods. Our group is proposing an electron-transfer stopped-flow (ETSF) method for the spectroscopic detection and reaction analysis of short-lived ion radicals formed in aprotic solvents. In the ETSF method, e.g., short-lived cation radicals are formed via electron transfer reactions with stable cation radicals in the mixing part of a stopped-flow apparatus, so that any changes in the absorption spectra are observed in an optical cell after stopping the solution flow. In this account, the principle and actual procedures of the ETSF method are described first. Then, some successful results obtained using the ETSF method in observing the absorption spectra and analyzing the reaction mechanisms are summarized for the reactions of poly-aromatic cation radicals and aromatic amine cation radicals. The former includes anthracene and pyrene derivative cation radicals formed in acetonitrile, and the latter includes aniline, N-methylaniline, N,N-dimethylaniline and diphenylamine derivative cation radicals formed in acetonitrile. Finally, as interesting examples that the ETSF analysis cannot give straightforward results, time-resolved absorption spectra observed for the oxidation processes of N,N-dimethyl-o-toluidine and N,N-dimethyl-m-toluidine in acetonitrile are shown and discussed.