Polyfluorenes polymerization

DuPont is an active player in OLED technology. Polymers used in devices as emitting materials are poly(p-phenylenevinylene), poly(arylenevinylene)s, poly(p-phenylene), poly(arylene)s, polyquinolines, and polyfluorenes. In some cases, an anionic surfactant such as lithium nonylphenoxy ether sulfate was added to the above-mentioned polymeric emitters... 

Beinhoff M, Appapillai AT, Underwood LD, Frommer JE, Carter KR. Patterned polyfluorene surfaces by functionalization of nanoimprinted polymeric features. Langmuir 2006 22 2411-2414. 

Polyfluorene was first synthesized by Fukuda et al. via oxidative polymerization of huorene monomers using ferric chloride as a catalyst.2,11 Both mono- and dialkyl-substituted polyfluorenes were synthesized. Figure 10.1 shows the repeat unit of poly(9,9/-dialkyl-huorene-2,7-diyl). The polymers are soluble in common solvents such as chloroform, dichloromethane, and toluene. Figure 10.2 shows the absorption and fluorescence spectra of a solution of poly(9,9/-dihexylfluorene-2,7-diyl) (PDHF) in chloroform.11 The onset of the tt-tt absorption is at 420 nm, rising to a peak at 380 nm, yielding an optical gap of 2.95 eV. The fluorescence spectrum contains vibronic peaks at 417 and 440 nm and a shoulder at 470 480 nm. 

The first quantitative study of the device efficiency of polyfluorene LEDs was reported by Grice and co-workers in 1998.5 The LED used ITO-coated glass as the anode, a hole-transport layer of a polymeric triphenyldiamene (poly-TPD), an emissive layer of PFO, and a calcium cathode. A schematic energy-level structure for the device is inset in Fig. 10.14. The injection barrier for holes into poly-TPD is approximately 0.3 eV, much less than that for PFO (0.8 eV), and the injection barrier for electrons into PFO is approximately 0.1 eV. Poly-TPD had excellent film-forming properties, was transparent to PFO emission, and was insoluble in... 

Low fixed charge density and interface states have been reported for this material. A recent report using BCB as a dielectric states -type conduction has been observed for semiconducting polymers such as polyfluorene and poly(p-phenyle-nevinylene), which have otherwise been considered exclusively p-type semiconductors [24], It is proposed that n-type conduction in these polymeric semiconductors has not been observed previously due to trapping of electrons at the dielectric-semiconductor interface due to hydroxyl groups. This study highlights the critical nature of the dielectric-semiconductor interface and its role in modifying the performance of the semiconductor. 

Another popular category of polymeric semiconductor materials is the polyfluorenes, which have the general structure shown in Fig. 3.5 (a) [23]. The variant F8T2, which is shown in Fig. 3.5 (b) has attracted the greatest attention because of its easily accessible nematic transition at 280° C and high solubility due to its alkyl chains. 

Another interesting starting compound is the spirobifluorene, which contains two almost perpendicular electropolymerizable units [293], thus able to polymerize in two directions. Inserting a vinylogous tetrathiafulvalene (TTF) moiety into the spirobifluorenyl framework enables to design a new three-dimensional (3D) copolymer with the structure represented below [294] (Scheme 18.9) the copolymer exhibits both electroactivities due to oxidation of TTF and p-doping of polyfluorene. 

Many synthetic routes are utilized to generate 6,12-functionalized dihydroindeno[l,2-h]fluorenes, typically from 22 or 26. One direct method is base-promoted alkylation of the methylene carbons of 26. For example, lithiation of 26 followed by treatment with 1-bromooctane gave tetrasubstituted 101 (Scheme 29) [80]. Subsequent bromination at the 2- and 8-positions with CuBr/ alumina furnished 102, which could then be polymerized to form polyindeno-fluorene 103 linked at the 2- and 8-positions rather than at the 6- and 12-positions as seen in Scherf s polyindenofluorene 85. Muellen and co-workers foxmd that 103 showed a bathochromic shift in fluorescence compared to polyfluorene (into the visible region) and it formed a liquid crystalline phase at high temperature, possibly making a suitable LED component. 

Three principal types of Suzuki polymerizations for polyfluorenes have been developed for homopolymers and copolymers by Dow, Cambridge Display Technology (CDT), and Covion, which was merged with Merck. Early detailed experimental procedures were reported in the patent literature (Scheme 5.5 and Table 5.2). These and other groups have used these polymerizations for numerous poly fluorene derivatives (Table 5.3). 

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