Anodic Oxidation of Novel Hole-Transporting Materials Derived from Tetraarylbenzidines. Electrochemical and Spectroscopic Characterization

Abstract

Fluorenylidene-linked triarylamines, potential hole-transporting materials, have been prepared by the palladium-catalyzed Hartwig-Buchwald amination. Their redox and spectral properties were investigated in solution, applying cyclic voltammetry, UV-VIS and EPR spectroscopy, and in situ spectroelectrochemical measurements. N , N , N' , N' -Tetraphenylbenzidine ( 1 ), N , N' -di(1-naphthyl)- N , N' -diphenylbenzidine ( 2 ), and triphenylamine ( 3 ) served as model substances in the study of the synthesized complex compounds 4 and 5 . In structure 4 , two triphenylamine centres are linked with a non-conjugated fluorene bridge; in structure 5 , two tetraarylbenzidine skeletons with two nitrogens are linked with a conjugated biphenyl-bridge system. In addition, structure 5 contains a non-conjugated fluorene bridge. The presence of the fluorene moiety in the molecular design has a significant influence on the investigated properties of the new materials. In the anodic oxidation of the tetraarylbenzidine-type compounds ( 1 , 2 , and 5 ), two well-defined reversible oxidation peaks were observed. However, the oxidation of the triphenylamine-type structures ( 3 and 4 ) is more complex, due to fast consecutive reactions. The dimer-like structures ( 4 and 5 ) are characterized by two independent oxidation centres that are simultaneously oxidized at approximately the same potentials. This was confirmed by quantitative cyclovoltammetric as well as UV-VIS investigations.