Highly Charged Cations from N,N,N′,N′-Tetrakis(4-aminophenyl)benzidine and Its N,N,N′,N′-Tetrakis(4-methoxyphenyl)-Substituted Homologue Studied by Thin-Layer in Situ Electron Spin Resonance/UV-Vis-NIR Spectroelectrochemistry

The redox behavior of N,N,N',N'-tetrakis(4-aminophenyl)benzidine (A) and its N,N,N',N'-tetrakis(4-methoxyphenyl)-substituted analogue (B) was studied in detail by a new designed in situ thin layer electron spin resonance (ESR)/UV-vis-NIR spectroelectrohemical cell. The spectroelectrochemical studies of cation radicals, dications, and tetracations indicate a strong difference in stability of higher ions of two model compounds with different phenyl substitution. In cyclovoltammetry the small peak separation of the first two oxidation peaks for both compounds points to a small energetic difference in the first two electron transfers, while the peak separation of the second and third peak is quite large. A well resolved ESR spectrum of the B center dot+ cation radical with dominating splittings from two nitrogen atoms is observed and an ESR silent product was determined at the third oxidation peak for both compounds, confirming the formation of a tetracation by a two electron transfer at the third voltammetric peak. In contrast to structure A a more complex redox behavior was found for B under voltammetric cycling. The reaction mechanism indicates the transformation of B in a new product P with a carbazole moiety. This compound can form a highly stabilized cation radical P center dot+ A theoretical study based on density functional theory calculations has clarified the role of charging in changes to the structures of both triarylamine derivatives A and B.