Anodic oxidation of novel hole-transporting materials derived from tetraarylbenzidines. Electrochemical and spectroscopic characterization

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 spectroelectr ochemical measurements. N,N,N',N'-Tetraphenylbenzidine (1), N,N'-di(1 -naph thyl)-N,N'-diphenylbenzidine (2), and triphenylamine (3) served as model su bstances in the study of the synthesized complex compounds 4 and 5. In stru cture 4, two triphenylamine centres are linked with a non-conjugated fluore ne bridge; in structure 5, true tetraarylbenzidine skeletons with two nitro gens are linked with a conjugated biphenylbridge system. In addition, struc ture 5 contains a non-conjugated fluorene bridge. The presence of the fluor ene moiety in the molecular design has a significant influence on the inves tigated properties of the new materials. In the anodic oxidation of the tet raarylbenzidinetype compounds (1, 2, and 5), two well-defined reversible ox idation peaks were observed. However, the oxidation of the triphenylamine-t ype 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 sam e potentials. This was confirmed by quantitative cyclovoltammetric as well as UV-VIS investigations.