Unsubstituted oligothiophenes

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

A recent publication by the group of Barbarella has disclosed the rapid preparation of poorly soluble unsubstituted and modified a-quinque- and sexithiophenes by the extensive use of bromination/iodination steps and microwave-assisted Suzuki and Sonogashira cross-couplings (Scheme 6.16) [42]. Suzuki reactions were either carried out under solvent-free conditions on a strongly basic potassium fluoride/ alumina support for the synthesis of soluble oligothiophenes, or in solution phase for the preparation of the rather insoluble a-quinque- and sexithiophenes. In both cases, 5 mol% of [l,l -bis(diphenylphosphino)ferrocene]dichloropalladium(II)... 

Data from <1996JPC18683> for the unsubstituted oligothiophenes in dioxane at room temperature. 

The conformational analysis of oligothiophenes by use of a combined molecular dynamics (MD)/NMR spectroscopic protocol has been carried out. A series of MD simulations were performed for 2-(2-thienyl)-3-hexylthiophene 173, 2,5-bis(3 -hexyl-2 -thienyl)thiophene 176, and 2,5-bis(4 -hexyl-2 -thienyl)-thiophene 177, with a new MM2 torsional parameter set developed earlier for unsubstituted and methyl-substituted 2,2 -bithiophene. 

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

Still, unsubstituted a-nT is a model system used in recent literature. One disadvantage towards application of the unsubstituted oligothiophenes however is the poor solubility of the rigid rods [34]. 

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. 

A similar trend was reported for unsubstituted oligothiophenes [48]. The highest measured mobility was 0.12 cmWs for DFI7T in the linear regime [22, 40]. 

The absorption maxima of the oligothiophene dimers in solution range from 361 nm (BT3) and 426 nm (BT5) to 451 run (BT7). These values are similar to the optical properties of the corresponding unsubstituted linear oligothiophenes T3-T6 [28]. There is only a minor redshift of max of ca. 9 nm in BT3 compared to T3 and BT5 compared to T5, which is probably due to a weak intramolecular electronic interaction of the arms in BT3 and BT5. Comparison of the solution and solid state spectra (Figure 6.2a, b) shows a distinct red shift of the films spectra, which is more pronounced for the more extended dimers... 

In the following, results obtained for thin films prepared from unsubstituted oligothiophenes, are reviewed in detail. The influence of substituents is briefly mentioned where it is important. Results of oligothiophenes in solution and on polythiophenes are... 

Figure 13.1. Schematic repre.sentation of the geometric structure of unsubstituted oligothiophenes (anT).

There are several wet deposition procedures, including sedimentation, spin coating, electrochemical deposition, self assembly, and the Langmuir-Blodgett technique. Common for all these methods is that no ordered films of unsubstituted pure oligothiophenes can be prepared. 

Large differences in the geometric structure occur between substituted and unsubstituted oligothiophenes. As substituents mainly alkyl-chains or alkyl-rings are used to enhance solubility (Figure 3.10). End-substi-... 

Alkyl-substitution influences the absorption spectra in thin films mainly by their influence on the co-planarity of the molecules, particularly in the excited state, and only to a minor extent by their electron pushing (inductive) effect (see [181]). The latter should result in a red shift which is not observed in absorption spectra of thin films but can be detected in the fluorescence spectra. In absorption spectra, major blue shifts of the absorption peaks occur if larger differences in the torsion angle between different rings are induced by the substituents. As long as the co-planarity is not distorted, quite similar absorption spectra are observed if compared to unsubstituted oligothiophenes [13,182]. 

ESR measurements show that the mono-oxidized oligothiophene is indeed a (nearly) fiee radical with an intense and narrow Lorentzian signal centred at g = 2.0023-2.0025 for unsubstituted [198] as well as alkyl-substituted anT [192]. 

The influence of different organic semiconductors is evident from Section 5.1 in which already the field effect mobilities and conductivities were summarized. 6T shows the highest mobility of unsubstituted oligothiophenes, also much higher than polythiophene, and it is therefore used in most devices. Alternatively end-alkylsubstituted 6T can be used which shows even higher mobilities due to its enhanced order. 

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]. 

AE(S,-T4) is the S1-T4 energy difference (in eV) calculated, at the INDO/MRD-CI level, for the unsubstituted oligothiophenes the values between parentheses have been extrapolated from a linear relationship between the excitation energies and the inverse number of rings. < )f is the fluorescence yield Icnr denotes the nonradiative decay rate measured in solution and the value calculated on the basis of simple assumptions (see text). 

Quantitative investigations of the kinetics of these polymerization a-coupling steps have been performed by fast-scan voltammetry on ultramicroelectrodes in order to estimate the lifetime of the transient species. The rate constants for the dimerization of thiophene are greater than 10 M s, while the lifetime of oligothiophene radical cations increases with chain length the rate constant for unsubstituted thiophene tetramer is close to 10 and that of the pentamer is below 10 M s [18]. These studies have... 

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