Colour Control by Copolymerisation

These materials were designed so that by efficient FOrster energy transfer from the fluorene to the dye units, efficient emission across the whole visible spectrum could be obtained. Perylene dyes were chosen as the chro- 

If a very low mole fraction of a red or orange-emitting chromophore is incorporated into a PDAF, then energy transfer is incomplete so that emission from both the fluorene and the comonomer is observed. By carefully balancing the amounts of emission form the fluorene and comonomer, white emission can be obtained. This approach has been used very successfully by Wang and coworkers to obtain white-emitting copolymers [55]. The highest efficiency for white emission (9 cd/A), was obtained from copolymer 

These approaches have been adopted more recently to incorporate phosphorescent chromophores into PF in order to make use of the fact that a large proportion (up to 75%) of all excitons formed in LEDs are triplet states, whose energy can only be harvested by using phosphorescent units. The first fluorene copolymers with phosphorescent units 34-35 were made by Holmes and coworkers who added monobrominated red- or green-emitting iridium complexes to an AA-BB Suzuki polycondensation [57]. With short fluorene chains, only emission from the iridium complexes are observed, but with longer fluorene chains some blue emission is also seen. Other groups have since incorporated different phosphorescent units such as platinum [58] or zinc salen [59] units or porphyrins [60,61 ]. 

White EL has been obtained from a copolymer containing low mole fractions of a green fluorescent (0.005-0.05 mol %) and a red phosphorescent (0.1-0.5 mol%) comonomer with efficiencies of up to 6.1 cd/A [62], 

Copolymers 113 and 114 containing tetracene and pentacene units show emission from the acene units to give green (A.max = 520 nm) and red (A-max = 623 nm) PL and EL, respectively [220]. However, due to rapid degradation of the acenes by oxidation their emission rapidly turns blue and the emission efficiency drops markedly. The pentacene copolymer 114 shows a lower turn-on voltage and higher efficiency with 1 mol % as opposed to 10 mol % of pentacene. 

Since the efficiency of energy transfer between the fluorene and the dye is dependent upon the overlap of the fluorene emission spectrum and the absorption spectrum of the dye, it was anticipated that energy transfer to the 

The PL spectra of these materials in solution showed emission from both components roughly proportional to their relative mole ratios, but in films energy transfer occurred so that the PL spectriun was dominated by emis- 

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