Thiophene 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. 

Copolymers of Thiophenes with Other Conjugated Moieties 2.4.9.1 Thiophene Copolymers with Aromatic Moieties... 

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. 

Tuning the Properties of Substituted Phenylene-Thiophene Copolymers... 

A low band-gap (Eg 1.6 eV) conjugated thiophene copolymer 468 with pyrrole and BT units was synthesized by Stille coupling [568]. They showed emission in the NIR region (AEL 800 nm) with turn-on voltage below 4 V but with very low efficiency (Chart 2.113). 

In order to quantify the transition metal ion concentration, Jones et al. [107] developed a highly sensitive fluorescent chemosensor in the form of dialkoxy-phenyleneethynylene-thiophene copolymers 68/69. The PAEs were functionalized on the thiophene unit with terpyridine (68), and included 2,2 -bipyridine (69) as a Lewis acid receptor. The terpyridine polymers [108] were found to respond quantitatively to transition metal ions at concentrations as low as 4x10 M (NP, Hg, Cr ", and Co " ). The additionally used bpy-PAE demonstrates that variation in the chelation at the receptor site is an important variable in tuning selectivity. The observed dynamic quenching mechanism, combined with the solubility of this material, provides the opportunity to extend these initial investigations to thin solid films for use in real-time monitoring applications. 

A summary of thiophene copolymer repeat units and corresponding M s are provided in Table 1. 

Because their oxidation potentials are similar, substituted pyrroles can be copolymerized with pyrrole, allowing the limiting conductivity of the fully-doped polymer to be varied 194,19S). The oxidation potentials of the monomers, and hence their reactivity ratios, are sensitive to the substituent196). Inganas et al.197) reported the synthesis of pyrrole-thiophene copolymers starting from terthiophene, whose oxidation potential is similar to that of pyrrole. Sundaresan et al.198) copolymerized pyrrole with 3-(pyrrol-l-yl)propanesulphonate to give a polymer in which the sulphonate counter-ion is a part of the polymer structure. 

Because of the problems associated with copolymerizations of monomers of very different reactivity, many authors have looked at an alternative approach which is to synthesise appropriate sections of the desired polymer chain, then couple them electrochemically to get the final polymer. Naitoh et al.199) synthesised the dimer, 2,2 -thienylpyrrole and used this as a monomer to prepare the alternating pyrrole-thiophene copolymer. They claimed that the copolymer film obtained with HSO as the counter-ion is more conductive than either of the corresponding homopolymers by a factor of 10 to 20. McLeod et al. 200) synthesised 2,5-dithienylpyrrole and polymerized it electrochemically with silver p-toluenesulphonate as the electrolyte. They obtained films of polymer whose conductivity could be varied in the range 10 8 to 0.1 Scm-1. Surprisingly, some low conductivity films were soluble in acetone or acetonitrile and evaporation of the solvent gave a powder of similar conductivity. Based on the shift in the absorption maximum in the visible spectrum on polymerization, it was concluded that the soluble films were polymers with molecular weights of 4000. 

Ferraris and Skiles 201) have also described a range of thiophene copolymers synthesised in a similar way. 

Silole-thiophene copolymers, with varying silole thiophene ratios from 1 2 to 1 4, have been obtained by a palladium cross-coupling reaction173, as outlined in Scheme 28. 

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). 

Phenylene-thiophene copolymers 562b and 562a emit blue light at ca. 450-475 nm, with somewhat different reported ELex values of 0.2% (96AM982) and 0.03% (97MM4608) respectively, for ITO/polymer/Ca configurations. 

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. 

Cao and coworkers also prepared random 3,6-carbazole-benzothiadia-zole-thiophene copolymers 601 (02MI709). Copolymer 601 emitted saturated red light (from 660 to 730 nm, depending on the stoichiometry). 

Dithienosilole-thiophene copolymers 21 (Fig. 10) can show excellent FET performances.38 With the copolymers as the active layer, remarkably high hole mobility from 0.02 to 0.06 cm2/(V s) can be achieved. Furthermore, the FET devices possess high current on/off ratios of 105—106. The FET devices also display impressive stabilities under repeated on/off cycles up to 2000 in air. 

FIGURE 14. Plot of the bandgaps (Iif) determined from absorption edge versus silole/thiophene ratios for a series of silole-thiophene copolymers. Reproduced by permission of Wiley-VCH from Reference 45... 

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