Thiophene-fluorene copolymers

An increase in the PL QE of the fluorene-thiophene copolymers can be achieved by introduction of -oxidized thiophene units (although no efficient EL from such materials was reported). This aspect and the chemical structures of thiophene-iS,5 -dioxide-fluorene copolymers are discussed in more detail in Section 2.4. 

Beaupre and Leclerc reported fluorene-thiophene copolymers in which fluorene and thiophene-S,S-dioxide fragments 558 and 559 were separated by one or two thiophene units (02MI192). 

Vamvounis and Holdcroft prepared a series of fluorene-thiophene copolymers 579 with a varying ratio of conjugated (2,5-thienylene) and nonconjugated (3,4-thienylene) thiophene moieties in the polymer chain (04AM716). The PL efficiencies in films are low (6 and 7%, respectively). Substantial increases in solid-state PL efficiencies were observed reaching a value of 43% for copolymer 579e. 

Ravirajan P., Haque S. A., Durrant J. R., Poplavskyy D., Bradley D. D. C. and Nelson J. (2004), Hybrid nanocrystalline TiOi solar cells with a fluorene-thiophene copolymer as a sensitizer and hole conductor , J. Appl. Phys. 95, 1473-1480. 

Pattison, L.R. et ah. Temperature dependence of the ordering of semiconducting fluorene-thiophene copolymers on rubbed polyimide alignment layers, in Organic photovoltaics VI, ed. Z.H. Kafafi and P.A. Lane, SPIE, Bellingham, WA, 2005, 143-150. 

Polyfluorenes are an important class of LEP with high thermal, photo and environmental stability and efficient bright blue emission. This stimulated a number of researchers to develop fluorene-thiophene copolymers for light-emitting applications. In addition to an expected increase in PL quantum etSciency, such a combination of electron-rich thiophene units with relatively electron-deficient fluorene units should modify the bandgap of the material (and thus tune the emission) and improve the charge injection/transport balance, compared with fluorene homopolymers. 

Scheme 19.6 Synthesis of fluorene-thiophene copolymers 64a-fand the effect of the ratio of thienylene isomers in the copolymers on the PL efficiency. Numbers in the abbreviations of copolymers denote the portion of A in the mixture of A-E B...

While retaining much of the substituted PT character (e.g., good hole-transport properties and stability), these materials exhibit significantly improved fluorescence efficiency in the solid state (cl>Pi up to 29%) that leads to (hllof UP to 0.1% for ITO/453/Ca PLED (Table 2.6). Other widely studied thiophene copolymers with aromatic 9,9-disubstituted fluorene units were already described above in Section 2.3. 

B. Liu, W.-L. Yu, Y.-H. Lai, and W. Huang, Synthesis, characterization, and structure-property relationship of novel fluorene-thiophene-based conjugated copolymers, Macromolecules, 33 8945-8952, 2000. 

Fluorene-thiophene-phenylene copolymers 592-593 clearly demonstrate the effect of the exact position of CN groups in the vinylene fragment on the emission of the materials (04MM5265). 

Handbook of Thiophene-based Materials Applications in Organic Electronics and Photonics Table 4.6 Electrochemical properties of fluorene-based copolymers PFTORT, PFTTORTT and PFTCNVT... 

Recently, Kim et al. reported that a FET using the copolymer of fluorene and selenophene, poly(9,9 -dioctylfluorene-a/t-biselenophene) (51), as an active layer exhibited a high hole mobiUty of 0.012 cm V s which is better than the performance of the fluorene and thiophene copolymer counterpart [104]. It is speculated that the high mobility of 51 is a result of the close molecular packing and the well-defined orientation of selenophene-containing polymer chains. 

A regioregular homopolymer of 3-nonylthieno[3,2-Z>]thiophene was also prepared (Table 17.1, 10) however the molecular weight was low and no evidence of thermal transitions was observed by DSC [51]. Charge transport characteristics were not reported. The homopolymer of 3,6-dimethoxythieno[3,2-f>]thiophene [53] and copolymers of 3,4-dialkoxythieno[2,3-f>]thiophene [54] have recently been reported as a possible alternative to the conducting polymer ethylenedioxythiophene (PEDOT). Co-polymers of thieno[3,2-Z>]thiophene [55-57] and thieno[2,3-f>]thiophene [58] with 9,9-dialkyl fluorene have also been reported for both transistor and OLED applications. In the case of the thieno[3,2-b]thiophene copolymer, the polymer exhibited thermotropic liquid crystalline behavior with a nematic phase at high temperature [59]. p-Type field effect mobilities in transistor devices were on the order of 1 x 10 cm V s . Although the value is on the low side of acceptable performance, a comparison to the well-studied... 

Huang s group has systematically studied the structure-property relationships of fluorene-thiophene-based conjugated polymers 57-60 [95, 96], In contrast to poly thiophene homopolymers, the regiochemistry of substitution in bithiophene fragments in the studied copolymers shows little effect on the optical bandgap (59 and 60, respectively Eg = 2.49 and 2.58 eV [96] or 2.57 and 2.60 eV [97, 98]) or the emission maxima, but the head-to-head copolymer 60 was significantly more thermally stable. 

Hillmyer and co-workers [141] developed Isothianaphthalene (TTN)—thiophene copolymer using distannyl-isothianaphthene (obtained via lithiation of isothianaphthalene in presence of n-butyl lithium in tetramethylethylenediamine (TMEDA) and THF followed by reaction with trimethyltin chloride). It was found that this molecule could be used as a precursor to prepare the ITN-fluorene copolymer/ITN-thiophene through Stille coupling with 9,9-dihexyl-2,7-dibromofluorene/ bis(bromothienyl)-isothianaphthene. Fig. 24. 

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