Novel functional materials based on triarylamines–synthesis and application in electroluminescent devices and photorefractive systems
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 (Tg) 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 Tg 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.