New Charge Transporting Host Material for Short Wavelength Organic Electrophosphorescence:  2,7-Bis(diphenylphosphine oxide)-9,9-dimethylfluorene

We report the synthesis, crystal structure, and photophysical and electroluminescent properties of a new charge transporting host material for short wavelength phosphor-doped organic light emitting devices (OLEDs) based on 2,7-bis(diphenylphosphine oxide)-9,9-dimethylfluorene (PO6). The PO moiety is used as a point of saturation between the fluorene bridge and the outer phenyl groups so that the triplet exciton energy of PO6 is 2.72 eV, similar to that of a dibromo substituted fluorene, but it is more amenable to vacuum sublimation and has good film forming properties. Computational analysis (B3LYP/6-31G*) predicts the highest occupied molecular orbital and lowest unoccupied molecular orbital energies of PO6 to be lower by 1.5 and 0.59 eV, respectively, compared to a similar diphenylamino substituted derivative. In a simple bilayer OLED device, PO6 exhibits structured UV electroluminescence at a peak wavelength of 335 nm and structured lower energy emission with peaks at 380 and 397 nm, similar to the solid film and crystalline solid photoluminescence spectra. The longer wavelength peaks are attributed to aggregate formation via strong intermolecular interactions (PO···HC and edge-to-face CH···π contacts) and longer range electrostatic interactions between PO moieties leading to ordered regions in the film. Devices incorporating PO6 as the host material doped with iridium(III)bis(4,6-(difluorophenyl)pyridinato-N,C2)picolinate (FIrpic) exhibited sky blue emission with peak external quantum efficiency (η_ext,max) of 8.1% and luminous power efficiency (η_p,max) of 25.1 lm/W. At a brightness of 800 cd/m², generally considered to be sufficient for lighting applications, the η_ext and η_p are 6.7% and 11.8 lm/W and the operating voltage is 5.6 V, which is significantly lower than has been demonstrated previously using this dopant.