Thiophene-thiophene versus phenyl-phenyl coupling in 2-(Diphenylamino)-Thiophenes: an ESR-UV/Vis/NIR spectroelectrochemical study.

The oxidation of several 2-diphenylamino-substituted thiophenes and N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines with different substitution patterns in the 5-position of their thiophene moieties was studied by cyclovoltammetric and spectroelectrochemical measurements (ESR, UV/Vis/NIR). These measurements revealed both the structure of the oxidation products obtained and that of their cationic intermediates, as well as the pathway of their formation and follow-up reactions. Thus, the formation of the radical cations in the first electrochemical oxidation step of the target molecules was demonstrated. Depending on the substitution pattern, these radical cations give rise to several consecutive processes to a different extent. Thus radical dimers, dehydrodimers with a 2,2-bithiophene, 3,3'-bithiophene, or benzidine structure, as well as radical monocationic and dicationic products can be formed. For N,N'-bis(2-diphenylamino-5-thienyl)-substituted phenylene-1,4-diamines rather stable radical cations and dications were primarily formed and unambiguously identified. These species were transformed into 2,2'-bithiophene dimers and oligomers when the 5-position of the thiophene moiety was unsubstituted. By in situ spectroelectrochemistry the influence of the substituents on the type of coupling reactions can be demonstrated and followed in detail.