Durene-decorated CBP derivatives as phosphorescent hosts and exciton-blocking materials for efficient blue OLEDs

Two novel durene-containing molecules, 1,4-bis-[4-(9-carbazolyl)-phenyl]-durene (CPD) and 1,4-bis-{4-[9-(3,6-(di-tert-butyl)carbazoyl)]-phenyl}-durene (t-BuCPD), which are derived from 4,4'-bis(9-carbazolyl)biphenyl (CBP) by inserting durene in its biphenyl core, are designed and synthesized for use as host materials for blue phosphors in organic light-emitting diodes (OLEDs). Inserting durene in biphenyl causes a right-angle torsion between the durene and the adjacent phenyl groups due to the strong steric hindrance effect of the durene group, confining the effective pi-conjugation on only one carbazole and one phenyl and increasing the triplet energies of CPD and t-BuCPD to over 3.0 eV. These durene-decorated molecules show higher thermal stabilities than many other CBP derivatives. Blue phosphorescent OLEDs were fabricated using CPD and t-BuCPD as triplet hosts and traditional iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C-2') picolinate (Firpic) as a dopant and excellent performances were achieved. In particular, peak efficiencies of 26.2 cd A(-1) and 14.8 lm W-1 were realized when CPD was used as both a host and exciton-blocking material. This is the first report using durene to tune the triplet energy levels of phosphorescent host materials.