Optical and electron-deficient properties of poly[2,7-(9,9-dihexylfluorene)-Co-Bi-pyridine]: A combined experimental and theoretical study

A series of well-defined poly[2,7-(9,9-dihexylfluorenyl)-co-bipyridinyl] [F-(BPY)x] with varied BPY (bipyridine) unit contents from 0 to 50% were synthesized using palladium(0) catalyzed Suzuki cross-coupling reactions, and well characterized using 1H NMR, IR, and GPC. Their photophysical and electrochemical properties were investigated. These copolymers showed pure blue photoluminescence (440 nm) in dilute solutions with the fluorescence quantum yield >0.6. Electron-affinity potentials (LUMO levels) of these polymers estimated from cyclic voltammetry were significantly decreased with the increase of BPY content in the polymer backbone, and a level of 2.43 eV for alternating BPY-fluorene polymer (50% BPY) was achieved, making them good candidates for electron injection and transport materials in EL devices. A further great decrease in LUMO level up to 2.99 eV was found when Zn2+ was chelated into polymers. The experimental observations were supported by the quantum chemical calculations on these polymers, which provided the proof for the intuitive notion of an intramolecular charge transfer band in polymer chain.