Effect of fluorenone units on the property of polyfluorene and oligofluorene derivatives: Synthesis, structure-properties relationship, and electroluminescence

A series of well-defined 9,9'-dihexylfluorene-co-fluorenone copolymers with various fluorenone contents and a set of monodisperse oligofluorenes in the chain center have been developed through the palladium-catalyzed Suzuki coupling reaction. The structure-property relationships, especially the effect of fluorenone moieties on the photophysical and electroluminescent properties of these materials, are systematically investigated to elucidate the exact origin of the low-energy emission in polyfluorenes. On the basis of our substantial studies of the steady-state photoluminescence (PL) and PL decay dynamics of the fluorenone-containing oligomers and copolymers both in dilute solutions and in thin films, the origin of the controversial low-energy emission band might be due to the interaction between intrachain fluorenone moieties in polyfluorenes instead of the intermolecular aggregates or excimers. We further show that fluorene pentamer with a central fluorenone unit ( oligomer 2) is more appropriate to represent the actual chromophore responsible for the green emission in the copolymers. The green emission of the copolymers in the solid states is well described by single-exponential decay and has a lifetime ranging from 4.21 to 8.15 ns, depending on fluorenone content. The substantially enhanced intensity of the green emission with increasing the intermolecular interaction in solid states results from the contribution of both the intramolecular and the more efficient intermolecular energy transfer from the fluorene segments to fluorenone moieties. We also find that the emission wavelength and the lifetime of the green emission for the copolymer with the lowest fluorenone content (PHF-FO0.1) in solid states are very similar to those of the emerging low-energy emission for poly(9,9'-dihexylfluorene) (PDHF) after high-temperature thermal annealing in air. Bright yellow electroluminescence (EL) is achieved from double-layer LEDs based on these materials with an emission peak of 540-579 nm that steadily red-shifted with increasing fluorenone contents. The better luminance and external quantum efficiencies are probably due to improved electron injection and carrier transporting as well as efficient charge trapping and recombination at the fluorenone sites.