We have synthesized polyfluorene copolymers containing bis(2,2-diphenylvinyl)fluorene pendent groups attached orthogonally to the C-9 positions of fluorene units. These polymers possess high glass transition temperatures and good thermal stability. The results from PL studies indicate that most excitons formed in the polyfluorene backbone by direct photoexcitation are likely to migrate to lowerenergy pendent groups, from which emission occurs. An organic light-emitting device using the copolymer PF4-DPVF as the emitting layer exhibits a voltage-independent and stable blue emission with color coordinates (0.15, 0.17) at 11 V, with a maximum brightness of 3137 cd/m 2 at 9 V (262 mA/cm 2 ) and a maximum external quantum efficiency of 1.06%. In addition, we blended PF4-DPVF as the host material with 0.5 wt % of MEH-PPV to realize a white electroluminescence having CIE coordinates of (0.29,0.34) and a maximum brightness of 3258 cd/m 2 (119 mA/cm 2 ). We demonstrate that both Förster energy transfer and direct charge trapping/recombination on the MEH-PPV guest are responsible for the observed EL.
We have synthesized a novel polyfluorene copolymer polyfluorene–bis[4‐(diphenylamino)styryl]fluorene (PF–DPAS) by orthogonally attaching an amino‐substituted distyrylarylene dye bis[4‐(diphenylamino)styryl]fluorene, onto the C9 position of a fluorene unit. We have investigated this polymer's thermal properties, electronic properties (viz., absorption and photoluminescence), and electrochemical behavior. Photoluminescence studies indicate that color tuning can be achieved through efficient Förster energy transfer from the higher‐energy polyfluorene backbone to the lower‐energy pendant DPAS units. We have fabricated light‐emitting diodes with the structure indium tin oxide (ITO)/poly(3,4‐ethylenedioxythiophene) (PEDOT)/emitting layer/1,3,5‐tris(N‐phenylbenzimidazol‐2‐yl)benzene (TPBI)/Mg:Ag. The devices, based on blends of PF–DPAS in polyfluorene–triphenylamine–oxadiazole (PF–TPA–OXD), exhibit significant improvements in device performance relative to that of the pure PF–TPA–OXD device; we attributed this improvement to both a red‐shift of the electroluminescence (EL) spectra and an enhancement in quantum efficiency. At a blend ratio of 1:20, the EL spectrum is voltage‐independent and stable, and exhibits the characteristic emission of a DPAS moiety: a peak at 461 nm and Commission Internationale de l'Eclairage (CIE) coordinates of (0.15, 0.18). The maximum external quantum efficiency is 2.08 % (2.87 cd A–1) at a bias of 9 V (86.1 mA cm–2) with a brightness of 2467 cd m–2; the maximum brightness (6916 cd m–2) occurred at an applied voltage of 13 V and a current density of 361 mA cm–2.
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 a 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. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 859–869, 2005
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