Phosphine oxide functionalized pyrenes as efficient blue light emitting multifunctional materials for organic light emitting diodes

In a search for blue light emitting multifunctional materials, the electron transport enhancing diphenyl phosphine-oxide (Ph2P=O) group has been appended to blue light emitting pyrene derivatives. This design, we observe, leads to highly efficient electron transporting blue-emitters for non-doped organic light emitting devices (OLEDs) with good film formation characteristics. The superior performance is attributed to enhanced charge transport and formation of pyrene excimers assisted by thermally activated delayed fluorescence (TADF) in the device. We report the synthesis and characterization using experimental and computational methods of six such pyrene derivatives. Although three of these derivatives show quenching of luminescence in solvents at higher concentrations, in the thin film invariably all six of them exhibit typical pyrene excimer emission. X-ray crystal analysis reveals pi-pi stacking and the C-H center dot center dot center dot O interactions in the solid due to the P=O group. The measured electron mobilities for all the compounds are higher in comparison to the standard electron transport material, Alq(3). Non-doped OLEDs with the pyrene derivatives as emitters (multi-layer configuration) as well as electron transport cum emitters (bilayer configuration) exhibit excellent efficiencies. The derivatives as emitters display a performance with current efficiencies (eta(c)) in the range 21.1-30.1 cd A(-1), power efficiencies (eta(p)) 11.0-15.76 lm W-1, external quantum efficiencies (EQE) 7.2-9.1% and brightness 28 500-42 750 cd m(-2). In addition, the derivatives as electron transporting emitters demonstrate very good external quantum efficiencies in the range of 3.0-4.0%. These results demonstrate a successful strategy to obtain blue light emitting multifunctional materials for OLED applications.