Oligothiophenes substituted

In most of the recent literature on oligothiophenes however, substituted molecules become more and more favoured to those unsubstituted derivates since effects due to poor packing can be excluded much more easily and higher field-effect mobilities are usually easier to obtain. 

This synthetic strategy, however, leads in general to a mixture of isomers differing in the position of the -alkyl groups in certain thiophene rings. Thus e.g. alkyl-substituted 1,4-diketone 116 is cyclized with L.R. to terthiophene 117. On one hand this is bis-acylated to terthiophene 118 which is further converted into the bis-Mamiich base 120. On the other hand, terthiophene 117 is formylated under Vilsmeyer-Haack conditions to the terthiophene carbaldehyde 119. Both compounds are subjected to a Stetter reaction yielding the bis-1,4-diketone 121 which is cyclized by L.R. to the undecamer 122 [Eq. (50)]. 

In situ EPR spectra were taken from the paramagnetic sexithiophene radical cation and anion and allowed the determination of the spin density distributions. In accordance with the postulated 7r-dimerization, for the radical cation the intensity of the EPR signal decreases substantially with decreasing temperature and indicates a spin pairing to the 7r-dimer. This fact easily explains the puzzling observation that 

The difference between the two hexamer monolayers, however, is the spatial demand of the repeating units. For both compounds the area per unit cell contains one molecule. This area is 3.25 nm in the case of the non-regioregular sexithiophene and nonuniform spacing is found. In contrast, for the regioselectively substituted hexamer 129 the structure is much more uniform and the demand of area is much smaller (2.38 nm ). 

131 with NBS in chloroform/acetic acid provided the dibromo quaterthiophenes 

132 in 56-79% yield and were further coupled under Kumada conditions with the Grignard reagent of 2-bromo- or 5-trimethylsilyl-2-bromothiophene 133, 

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Figure 14-14. Chemical structure of unsubstituted (R = H) and end-substituted oligothiophene.

Only the nitro-substituted oligothiophenes display large bathochromic shifts, large Stokes shifts, high fluorescent quantum yields, and long lifetimes for excited states. As for the other substituents, the trend is mostly noticeable for the short oligomers like terthiophenes and seems to disappear for sexithiophenes. As can be inferred from their solvatochromic effect, an intramolecular charge transfer takes place in the excited states of these molecules. 

Table 14-1. Typical field-effect mobility (in cm V s ) of unsubstituted and dialkyl-substituted oligothiophenes.

The polaron model was also used by Waragai and Hotta [121] to analyze temperature-dependent data on methyl-substituted oligothiophenes. While their data did not cover a sufficient temperature range to discern a clear temperature dependence of the mobility, they noticed that [j. represented a drain voltage dependence, which they attributed to the dependence given by Eq. (14.69). Accordingly, the temperature-dependent mobility could be fitted to a thermally activated law of the form where Etot = fc/2 - ys/F, y being... 

Stranski-Krastanov growth mode 219 sub-micron channels 485 substituted oligothiophenes 474 substrates - passivation 442 substrates - Si02 substrate 429 surface... 

Assembly, Structure, and Performance of an Ultra-Thin Film Organic Field-Effect Transistor (OFET) Based on Substituted Oligothiophenes... 

Additionally, DHST molecules were oriented parallel to the substrate, whereas, a,o)-substituted oligothiophenes (e.g., a,o)-DHST) were vertically aligned leading to a higher mobility [11]. 

Here, we summarise results obtained on soluble swivel-cruciform oligothiophenes. The term swivel-cruciform reflects the fact that the single bond connection of the two non-substituted oligothiophenes at the central thienylene moieties permits, in contrast to the corresponding spiro-type dimers [21], a certain degree of rotation of the arms. This leads to a good solubility even without the attachment of solubilising alkyl chains. Initially, a series of swivel-cruci-... 

Crouch, D.J. et al.. Hexyl-substituted oligothiophenes with a central tetrafluorophe-... 

FIGURE 5.3.1 Chemical structure of alkyl-substituted oligothiophenes. 

The crystalline and molecular structure of several substituted oligothiophenes have been reported. Substitution may have a two-fold effect i) to make the longer oligomers more soluble and therefore more tractable for applications, and ii) to provide insight about the steric effects of the substituents for the molecular (and also the polymer) configurations. 

Here a complete overview is given on the structure of thin films of quinque- and sexithiophene, whereas films of smaller, longer, and substituted oligothiophenes are only stressed briefly. 

Figure 13.10. Schematic representation of different kinds of substituted oligothiophenes.

Dodccyl-substituted oligothiophenes on HOPG pack in lamellar structures where the oligothiophene units overlap slightly (Figure 13.12) [70,72,73]. This overlap... 

The polaron and bipolaron transitions are more or less reproduced by VEll calculations [172]. In summary, 4T is the shortest chain length which can stabilize one charge, 6T the shortest chain to bear two charges. This is not true for substituted oligothiophenes in which oligomers with n = 4 and 5 could also be doubly oxidized [33,207,210]. 

Trimethylsilyl-end-substituted oligothiophenes up to the octamer were studied in THF solution by Cheng et al. with THG experiments [285]. They found a considerably smaller change in y with a power of 2.8 without any evidence for saturation which they attributed to differences between THG and four-wave mixing with EFISH experiments (see above). 

There are only very few published data on substituted oligothiophene LEDs [67,312-315] and only two in which unsubstituted a6T is used as emitting layer [316-318]. 

Cyano-substituted oligothiophene 2.57 (Chart 1.13) was prepared by Suzuki-type coupling reaction between 5-bromo-5"-cyanoterthiophene and the bis-boronic acid of the inner terphenyl building block... 

Various diarylamino-capped bithiophenes were prepared by Pd-catalyzed coupling of dihalo-genated bithiophenes with diarylamines [158, 159]. Higher homologues of diphenylamino- and phenothiazino-substituted oligothiophenes 2.80 and 2.81 (Chart 1.18) were synthesized by Hartmann et al. in 13-45% yield by Pd-catalyzed reaction of stannylated diarylamino-capped smaller oligothiophenes and dibrominated thiophene-based central units [160]. 

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