Adjustment of electronic and emissive properties of indolocarbazoles for non-doped OLEDs and cholesteric liquid crystal lasers

Abstract

Derivatives of indolo[3,2-b]carbazole containing differently substituted triphenyl ethenyl moieties were synthesized and characterized as multifunctional materials for optoelectronic devices. A structure-properties relationship of the compounds was studied. Alteration of the substituents of triphenyl ethenyl moiety (H-, Ph-, CN-) afforded to modify thermal, photophysical and charge-transporting properties of the compounds. The synthesized compounds demonstrated high thermal stability with the 5% weight loss temperatures reaching 453 °C. Indolo[3,2-b]carbazoles mono- and disubstituted with tetraphenyl ethenyl groups exhibited polymorphism with two melting peaks at 211/216 °C and 236/245 °C, respectively. Modification of the substitution pattern resulted in the changes of emission of the solid films from blue centered at 471 nm to yellow peaking at 583 nm. Addition of the tetraphenylethylene and triphenyl acrylonitrile fragments resulted in aggregation induced emission (AIE) or aggregation enhanced emission (AEE). The solutions of AIE/AEE-active compounds exhibited low photoluminescence quantum yields which did not reach 1%, while the solid films demonstrated the highly boosted values of 23–35%. Hole mobilities of ca. 10−3 cm2/V × s were observed for tri(tetra)phenyl ethenyl substituted indolo[3,2-b]carbazoles compounds. The compounds containing acrylonitrile moieties exhibited bipolar charge transport with drift mobilities of ca. 10−5 cm2/V × s. The synthesized compounds showed promising electroluminescent properties as fluorescent emitters. OLEDs containing these emitters showed maximum external fluorescent quantum efficiency up to 3.0% and maximum brightness up to 18 800 cd/m2. They also demonstrated lasing properties. Using the most promising compound dissolved in cholesteric liquid crystal, laser beam at 515 nm with full width at half maximum of ca. 3.5 nm was generated.