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By incorporating electron-accepting benzimidazole and electron-donating indolo3,2-bcarbazole into one molecule, two novel donor–acceptor bipolar host materials, TICCBI and TICNBI, have been synthesized. The photophysical and electrochemical properties of the hybrids can be tuned through the different linkages (C- or N-connectivity) between the electronic donor and acceptor components. The promising physical properties of these two new compounds made them suitable for use as hosts doped with various Ir or Os-based phosphors for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). PhOLEDs using TICCBI and TICNBI as hosts incorporated with Ir-based emitters such as green (PPy)2Ir(acac), yellow (Bt)2Ir(acac), and two new red emitters (35dmPh-6Fiq)2Ir(acac) (i3) and (4tBuPh-6Fiq)2Ir(acac) (i6) accomplished high external quantum efficiencies ranging from 14 to 16.2%. Nevertheless, the red PhOLED device incorporating TICNBI doped with the red emitter osmium(II) bis3-(trifluoromethyl)-5-(4-tert-butylpyridyl)-1,2,4-triazolatedimethylphenylphosphine Os(bpftz)2(PPhMe2)2 achieved a maximum external quantum efficiency, current efficiency, and power efficiency of 22%, 28 cd A−1, and 22.1 lm W−1, respectively, with CIE coordinates of (0.65,0.35). The external quantum efficiency remained high (20%) as the brightness reached to 1000 cd m−2, suggesting balanced charge fluxes within the emitting layer, rendering devices with limited efficiency roll-off.
Ting et al. (2012) studied this question.