| Summary: | Poly(dialkylfluorene)s are one of the most important classes of blue-light emitting conjugated polymers for use in polymer light-emitting diodes. Despite their advantages of ready synthetic accessibility, and high luminance, they suffer a major problem in colour stability with an emission band in the green about 530 nm appearing very rapidly upon operation of a device. The primary source of these defects is thought to be the oxidation of monoalkylfluorenes to the corresponding fluorenone, the former being present as impurities in the dialkylfluorene monomers. (Fig. 16660) The synthesis and properties of monoalkylfluorene derivatives are described in Chapter 2. The degradation pathways of monoalkylfluorene derivatives in air were investigated using GC-MS. The thermal degradation of 9-<i>tert</i>-butylfluorene was also investigated. Fluorenone formation was suppressed and 2,7-disubstituted-9-tert-butyl-9H-fluorene derivatives exhibited enhanced thermal stability. A new synthetic route to 9,9-dioctyl-9<i>H</i>-fluorene without monoalkylfluorene defects is described in Chapter 3. Light emitting devices of polyfluorenes, prepared by Yamamoto and Suzuki polymerisation of the defect-free monomers, exhibited minimal green emission attributable to fluorenone formation. Incorporation of as little as 0.06 mol% of 9-octyl-9<i>H-</i>fluorene comonomer in polyfluorene led to evolution of green fluorescence in the PL and EL emission spectra. The 3-dimensional configuration of oligomeric polyfluorenes carrying twisted biphenyl comonomers is studied in Chapter 4. A study of the X-ray crystal structures of the model oligomers provided useful insight into how the twisted biphenyls effectively modify the planarity of polyfluorene backbone and suppress potential interchain interactions.
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