Electrochemical and optical characteristics of the hole conducting material N,N′-diphenylbenzidine during oxidation in DMF

The electrochemical oxidation of the hole conducting material N,N′-diphenylbenzidine (DPB) in N,N-dimethylformamide (DMF) was investigated by cyclic voltammetry and spectroelectrochemistry. The results were compared to electro-oxidation in acetonitrile. Time-resolved UV–Vis spectroscopy, with spectra measured on a time scale of 20 ms by a diode array spectrometer, was used in conjunction with cyclic voltammetry to monitor the progress of electrochemical oxidation and reduction in situ and to obtain kinetic and structural information on reaction products. The oxidation of DPB in DMF is quasi-reversible at Esw<+1.2 V (vs. Ag/AgCl), while at higher switching potentials, the oxidation is followed by chemical reactions leading to the polymerization of DPB. The radical cation DPB+., which can be transformed into the neutral radical DPB· through the loss of a proton, was detected in the oxidation by in situ ESR spectroscopy. A reaction mechanism was proposed for the oxidation processes. Structural changes induced in DPB at high potentials are suspected to be accountable for device failure of DPB based light-emitting diodes (LEDs) in applications.