A highly twisted triarylborane-based biphenyl as an efficient host for blue and green phosphorescent OLEDs

We report herein a novel triarylborane-based biphenyl derivative, o,o'-NPh2, which contains dimesitylboryl and diphenylamino groups at the lateral o,o'-positions of the biphenyl skeleton. This triarylborane-based biphenyl exhibits a high triplet energy (E-T = 2.57 eV), which is much higher than its normal linear regioisomer, p,p'-NPh2 (E-T = 2.28 eV), although their fluorescence spectra and thus singlet energies are very close. The high ET of o,o'-NPh2 is probably ascribed to its almost orthogonal conformation of the biphenyl skeleton, which would likely lead to good separation of the HOMO and LUMO and thus a small energy gap between the singlet and triplet excited states (Delta E-ST). The high E-T of o,o'-NPh2 has allowed incorporation of it as a host material in blue and green phosphorescent OLEDs. The devices hosted by o,o'-NPh2 can achieve high performances with maximum external quantum efficiencies (EQEs), current efficiencies (CEs) and power efficiencies (PEs) of 15.3% and 22.2%, 34.5 cd A(-1) and 84.2 cd A(-1), 31.4 lm W-1 and 76.6 lm W-1 for blue and green phosphorescent OLEDs, respectively. Moreover, the green phosphorescent OLED has extremely low efficiency roll off. The EQE remains almost unchanged at 1000 cd m(-2) and decreases by only 2.6% even at 10000 cd m(-2).