Sterically hindered triarylamines as building blocks for organic electronic materials

Abstract

ABSTRACT Organic electronic materials have been investigated for many years and have undergone massive development. OLEDs have become state of the art in the production of e.g. displays and TV screens. One of the major problems of the first OLED material generation (e.g. oligothiophenes) was the affinity to crystallize, an obstacle in the process of making amorphous, glass like materials. A major step forward was the introduction of hole transporting triarylamine caps on the electron transporting thiophene linker systems. The modification of this system basically removed this obstacle and revealed not only a great number of new possible substitution patterns but also a possibility to fine tune the properties. The main areas for substitution are the two phenyl groups on the outside of the system which allow ortho-, meta-, and para-substitution as well as a planarization of the whole cap. As para disubstituted and planarized caps have been investigated in former works, the goal of this work was to investigate the scope and limitations in the synthesis of symmetrically and asymmetrically ortho substituted triarylamines. In order to find a reliable synthetic route for many different substances, various synthetic protocols have been evaluated to find that Buchwald Hartwig Amination followed by aromatic substitution gave the best results. This approach proved to be applicable for most moieties with only slight variations. Even though this route provided a reliable method, the addition of the third phenyl group via aromatic substitution turned out to be the crucial step considering sterical hindrance. The scope of possible substitution patterns was revealed by systematical investigation of methyl substituted diphenylamines showing, that more than one ortho substituent on each phenyl group provides too much sterical hindrance to further synthesize triarylamines. The triarylamines themselves provide a valuable starting point for a wide variety of building blocks for organic electronic materials that can be synthesized in only a few steps by introduction of functional groups such as halides, azides or boronic esters. These modifications allow not only various coupling reactions (Suzuki, Sonogashira, et. al.) but also triazole and isoxazole synthesis.