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′-diyl)bis(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[sbnd]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 °C and T_g from 134 to 165 °C. They show thermally activated delayed fluorescence (TADF) characteristics with microsecond scale long lifetimes, similar triplet energy of ∼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 (△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 lm/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).