PDAFs

Scheme 2 Alkylation of fluorene to produce monomers for PDAFs...

This ready accessability of dialkylfluorene monomers is one of the major reasons for the extensive investigation of poly(dialkylfluorene)s. Three main methods have been used for the synthesis of the polymers. PDAFs can be made by oxidative coupling of the monomer 6 with iron(III) chloride. Poly(9,9-dihexyl-2,7-fluorene) (12) with a molecular mass of 5000 (n = ca. 20) was made in this way by Yoshino and co-workers (Scheme 3) [16,17], and used to make low-efficiency blue-emitting (A.max = 470 nm) devices [18-20]. 

In addition to the AA-BB cross-coupling method used by Dow, an AB-type Suzuki coupling of a 2-bromo-7-boronate 17 could also be used to make PDAFs. However, this method has so far been used only for preparing end-capped low molecular weight PF rods e.g., 18 as macroiniators in the synthesis of rod-coil copolymers (Scheme 6). 

Since PDAFs are wide bandgap materials, incorporation of other smaller bandgap chromophores as comonomers or substituents enables tuning of the emission colour due to efficient energy transfer from the fluorene to the other units. Random copolymers of fluorenes with other chromophores are readily prepared by the copolymerisation of the dihalochromophore with a dihalofluorene as exemplified in Scheme 9 for the synthesis of copolymers of dioctylfluorene with perylene dyes (1-5 mol%) [46,47]. 

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... 

Scheme 13) containing 0.03 mol % of an orange emitting heterocyclic comonomer. Incorporation of low concentrations of green and red-emitting silole comonomers into a PDAF has also been used to obtain pure white emission [56]. 

Scheme 14 PDAFs incorporating phosphorescent iridium complexes...

The blue emission from PDAFs is unstable, with the appearance of a strong emission band around 530 nm after annealing or upon running an EL device [14,15]. Initially this long wavelength emission band was believed to be due to emission from excimers [63,64], but subsequent work has shown that it is due to emission from fluorenone defects within the PDAF chain [15]. 

A route has been developed by Shu, Jen, and co-workers for the synthesis of a fluorene monomer 63 bearing two aryloxadiazole groups at the bridgehead by nucleophilic substitution of 4-fluorobenzonitrile with the fluorenyl anion, followed by conversion of the nitriles to oxadiazoles via tetrazole intermediates (Scheme 29) [98]. The EL efficiency of the alternating copolymer 64 in a single-layer device is considerably higher than for the PDAF homopolymer... 

Unfortunately, as with PDAFs, obtaining stable blue emission in the solid state has presented difficulties. The tetraoctylpolymer 87 was a green emitter in the solid state, with the PL and EL spectrum being dominated by a broad emission band at 560 nm [113]. Films of the polymer 88 with ethylhexyl substituents show blue PL (A.max = 429 450 nm). The EL from 88 was initially... 

Polyfluorenes (PFs), the simplest regular stepladder-type polyphenylenes, in which only every second ring is bridged, have been much studied in recent years due to their large PL quantiun efficiencies and excellent chemical and thermal stability, as evidenced by the number of recent reviews [162-164]. A further attractive feature of poly(9,9-dialkylfluorene)s (PDAFs) is the synthetic accessibility of the monomers, as alkylation and halogenation of fluorene proceed smoothly and in high yields. 

---