Conjugated alternating copolymers of fluorene and 2-pyridine-4-ylidenemalononitrile: synthesis, characterization and electroluminescent properties

A new series of light-emitting conjugated copolymers based on fluorene were synthesized via the Pd-catalyzed Suzuki coupling reaction. The copolymers were characterized by FT-IR, NMR, and elemental analysis. All the copolymers were readily soluble in common organic solvents and had substantially improved thermal properties. Cyclic voltammetry revealed that, with the incorporation of 2-[2,6-bis(2-arylvinyl)pyridine-4-ylidene]-malononitrile (BPM) donor-acceptor units in the polyfluorene backbone, these copolymers had low-lying LUMO energy levels ranging from -3.14 to -3.28 eV and raised HOMO energy levels ranging from -5.43 to -5.64 eV. They are thus promising candidates for charge balanced electroluminescence (EL) in light-emitting diodes (LEDs). The copolymer films emit strong orange-red photoluminescence (PL) with maxima at 570-598 nm. Single-layer LEDs with the configuration of ITO/PEDOT/copolymer/Ca/Al were efficient yellow to orange-red emitters, with external quantum efficiencies of 0.43-1.06%.