Novel functional materials based on triarylamines–synthesis and application in electroluminescent devices and photorefractive systems

Abstract

A variety of new functional materials based on triarylamines, such as low molecular weight glasses which possess hole conducting/photoconductive properties as well as amorphous bifunctional materials which combine photoconductive and non-linear optical (NLO) properties in one compound, have been synthesized. The new hole transporting glasses belong to the class of 1,3,5-tris(triaryldiamino)benzenes (TTADB). The hyperbranched structure and the large aryl groups attached as substituents lead to high glass transition temperatures (T_g) of up to 141°C in these compounds. The TTADBs do not recrystallize upon cooling from the melt, but form stable glasses. Cyclic voltammetry studies reveal multi-oxidation stages for these compounds of which the first oxidation is reversible. The HOMO energy values determined from CV for TTADB-1 and TTADB-2 are -4.82 and -4.94 eV, respectively. Light emitting diodes with the structure ITO/TTADB-2/Alq3/Al (where ITO=indium tin oxide) show high efficiency and large current carrying capacity. Further, bifunctional compounds have been synthesized in which a photoconductive moiety such as bis(carbazolyl)triphenylamine or bis(diphenylamino)triphenylamine is covalently bound to different NLO chromophores. Some of these compounds are thermally and morphologically stable amorphous materials, possessing T_g in the range from 85 to 122°C. Cyclic voltammetry measurements reveal that the HOMO energy values are between -4.81 and -5.45 eV. In photorefractive measurements using 40 µm thick samples, a diffraction efficiency of 27%, which corresponds to a refractive index modulation (Δn) of 3.5×10^-3, a maximum two beam coupling gain coefficient (Γ) of 90 cm^-1 and a response time of 40 ms were obtained.