Synthesis and characterization of new red-emitting polyfluorene derivatives containing electron-deficient 2-pyran-4-ylidene-malononitrile moieties

A novel series of red-light-emitting copolymers derived from fluorene and 2-pyran-4-ylidene-malononitrile (PM) have been synthesized through a palladium-catalyzed Suzuki coupling reaction. The polymers were characterized by FT-IR, NMR, and elemental analysis. All these polymers are completely soluble in common organic solvents, such as THF, CH2Cl2, CHCl3, and toluene, and they have good thermal stability with onset decomposition temperature (T-d) of 406-407 degreesC and glass-transition temperature (T-g) of 73-186 degreesC. Cyclic voltanmetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from -3.53 to -3.57 eV and HOMO energy levels ranging from -5.77 to -5.79 eV, which indicated that they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes. These polymers in thin films can emit strong red photoluminescence (PL) around 641-662 nm with the corresponding additional peaks in the range 704-712 nm upon photoexcitation. Double-layer LEDs fabricated with the configuration of ITO/PEDOT/polymer/Ba/Al can emit red light with external quantum efficiencies of 0.21-0.38%. Preliminary electroluminescent (EL) results show that these polymers are novel promising candidates for red emissive materials in polymer light-emitting diodes.