Optical Properties of PDAFs

The emission from PDAFs is violet-blue with a primary emission maximum at about 425 nm, and a secondary peak at about 445 nm. Well-defined oligomers of 85 have been prepared, and as a result the effective conjugation length for PDAFs has been determined to be about 12 imits (24 phenyl 

By contrast, the polymer 90 with branched alkyl chains shows only one nematic phase. Thus, 90 shows a phase transition at 167 °C upon heating with the reverse transition upon coohng being seen at 132 °C. Annealing of a film of 90 at 185 °C upon a rubbing-aligned polyimide layer followed by rapid cooling produced a film which showed polarised EL with a dichroic ratio of up to 20 [179,185,189,190]. 

The copolymer 94 also showed circularly polarised emission, but the degree of dissymmetry was much reduced [191-193]. This was attributed to the dioctylfluorene units preferring a planar conformation of the backbone while the chiral-substituted units adopt a helical structure. A combination of a helical backbone conformation and liquid crystallinity is thought essential for obtaining a high degree of circular polarisation. 

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