New methods for the accurate determination of extinction and diffusion coefficients of aromatic and heteroaromatic radical anions in N,N-dimethylformamide

Two electrochemical methods are presented that allow an easy access to highly accurate UV-visible spectra of aromatic and heteroaromatic radical anions with life times longer than 50 s in N,N-dimethylformamide. The uncertainty on the extinction coefficients obtained is estimated to be as low as 5% which is a significant improvement in accuracy compared with other methods. The study has involved the radical anions of benzophenone, fluorenone, 1,4-naphthoquinone, anthraquinone, 1,2-benzanthraquinone, (E)-stilbene, anthracene, 9,10-diphenylanthracene, fluoranthene, perylene, benzonitrile, p-tolunitrile, 1,4-diacetylbenzene, 1,4-dicyanobenzene, 9,10-dicyanoanthracene, (E)-azobenzene, (E)-4-methoxycarbonylazobenzene, quinoxaline and phenazine. The first method takes advantage of a galvanostatic generation of the radical anions from the parent compounds combined with the recording of the absorbance at specific wavelengths by means of a so-called dip probe. The second technique described is based on the continuous and complete electroreduction of the aromatic compounds in an efficient electrochemical flow cell placed just ahead of a quartz cuvette in a traditional diode-array spectrometer. The successful strategy of combining a galvanostatic generation of aromatic radical anions with the measurement of a characteristic physical property has also been employed in the determination of diffusion coefficients. The recording device is here a rotating disc electrode. (C) 1998 Elsevier Science S.A. All rights reserved.