Polyfluorene Containing Diphenylquinoline Pendants and Their Applications in Organic Light Emitting Diodes

We present a short, efficient synthetic route for the preparation of a novel polyfluorene copolymer (PF-Q) containing two electron-deficient, 2,4-diphenylquinoline groups functionalized at the C-9 positions of alternate fluorene units that form a three-dimensional cardostructure. The presence of the rigid bulky pendent groups leads to & polyfluorene possessing a high glass-transition temperature (207°C) and very good thermal stability (5% weight loss observed at 460°C). A photoluminescence study revealed that the Förster energy transfer from the excited quinoline groups to the polyfluorene backbone is very efficient; it also demonstrated that the commonly observed aggregate/excimer formation in polyfluorenes is suppressed very effectively in this polymer, even after it has been annealed at 150°C for 20 h. A light emitting diode (LED) device prepared with PF-Q as the emitting layer exhibits a stable blue emission with a maximum brightness of 1121 cd/m² at 12 V and a maximum external quantum efficiency of 0.80% at 250 cd/m². We also used PF-Q, which contains diphenylquinoline units that behave as electron-transporting side chains, as a host material and doped it with 2.4 wt % of a red-emitting phosphorescent dye, Os(fppz), to realize a red electroluminescence with CIE color coordinates of (0.66, 0.34). The doped device exhibits a maximum external quantum efficiency of 6.63% (corresponding a luminance efficiency of 8.71 cd/A) at a current density of 47.8 mA/cm², together with a maximum brightness of 10457 cd/m².