Designing organic phosphine oxide host materials using heteroarmatic building blocks: inductive effects on electroluminescence

Phosphine oxide substitution of small molecules with high triplet exciton energies allows development of vacuum sublimable, electron transporting host materials for blue OLEDs. Heteroaromatic building blocks (carbazole, dibenzofuran and dibenzothiophene) with E_T ~ 3 eV were incorporated into phosphine oxide (PO) structures. External quantum efficiencies (EQEs) at lighting brightness (i.e., 800 cd/m²) reached as high as 9.8% at 5.2V for OLEDs using the heteroaromatic PO hosts doped with the sky blue phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C^2,) picolinate (FIrpic). Comparing device properties at a similar current density (i.e., J = 13 mA/cm²) showed the dibenzothiophene-bridged PO compound exhibits the highest EQEs and lowest operating voltages at all phosphor dopant levels. These results are explained with respect to the effects of the inductive phosphine oxide substituents on electrochemical, photophysical and electroluminescence properties of the substituted heteroaromatic building blocks.