Alkyl effects on the optoelectronic properties of bicarbazole/cyanobenzene hybrid host materials: Double delayed fluorescent host/dopant systems in solution-processed OLEDs

Three 3,3'-bicarbazole derivatives, 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(2-methylbenzonitrile) (pCNBCzmMe, 1), 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diAbis(3-(trifluoromethyl)benzonitrile) (pCNBCzoCF3, 2) and 4,4'-(9H,9'H-[3,3'-bicarbazole]-9,9'-diyl)bis(2-(trifluoromethyl)benzonitrile) (pCNBCzmCF3, 3) were designed and synthesized through a simple one-step catalyst-free C-N coupling reaction, by using 9H,9'H-3,3'-bicarbazole and alkyl substituted fluorocyanobenzene as starting materials. Compounds 1-3 exhibit high thermal stabilities with T-d above 400 degrees C and T-g from 134 to 165 degrees C. They show thermally activated delayed fluorescence (TADF) characteristics with microsecond scale long lifetimes, similar triplet energy of similar to 2.65 eV whereas different absorption and photoluminescence behavior according to the substitution of CH3 or CF3 alkyl units at different meta- or ortho-positions. Low singlet -triplet band gaps (Delta E-ST) of 0.30, 0.19 and 0.14 eV are observed for 1-3, respectively. By partially blocking the electroactive sites at the 3,3'-position of carbazole to prevent electropolymerization, these 3,3'-bicarbazole derivatives perform favorable electro-oxidative stability. From both experimental and theoretical results, the introduction of electron-withdrawing CF3 in 2 and 3 lowers both of the HOMO and LUMO levels compared to the weak electron-donating CH3-substituted 1. The hole and electron transport properties can also be tuned through different alkyl on different ortho- or meta-positions. It is interesting that the electron-donating CH3 meta-structured 1 exhibit one order higher electron mobility than the strong electron-withdrawing CF3 ortho-positioned 2 and meta-structured 3, while ortho-CF3 linked compound 2 with more twisted geometry showed the poorest hole-transport properties. By using the three TADF materials as hosts to conduct double host/dopant TADF systems for solution processed green TADF devices, maximum power efficiencies are achieved at 29.9, 22.7 and 30.9 Im/W for 1, 2 and 3 based devices respectively, in simple structure of ITO/PEDOT:PSS (40 nm)/1-3:4CzCNPy (40 nm)/TmPyPB (60 nm)/LiF (0.8 nm)/Al (100 nm). (C) 2016 Elsevier Ltd. All rights reserved.