Charges in oligothiophenes

Lischka showed in a recent paper that the extension of a bipolaron state decreased from 11 thiophene units without to 2 units in the presence of the counter-ion (Cr for p-, Li for n-doping) [280]. 

Both models will be discussed in parallel in the following subsections. However, in general discussions the band structure terms will be used as they are more common in literature. 

In any case defects arise as quinoid structural elements within the aromatic system (Fig. 42). However, as mentioned above, the defects are not as localized as Fig. 42 implies, but are extended over several double bonds. 

It is commonly accepted that in solution a-6T is oxidized in two steps, first to the radical cation, then to the dication. The cationic state is believed to exist as a 7r-dimer in which two radical cations couple via their 7r-systems and which has a nearly spin-less state [292-297]. This u-dimer forms as easier as longer the chain length and as lower the temperature is due to its exothermic building process. ESR measurements show that the mono-oxidized oligothiophene is a (nearly) free radical with an intense and narrow Lorentzian signal centered at g = 2.0023-2.0025 for unsubstituted [298] as well as alkyl-substituted a-nT [293], The dication formation is only possible for a-6T and longer oligomers, q-4T and a-5T either do not react or dimerize to octi- and decithiophene, respectively [299]. (For a review on results obtained in solution, compare [250].) 

In the solid state, however, the nature of the charge carriers remains unclear. The main reason is the very few investigations made with different materials, dopants, and characterization methods which do not allow for an easy interpretation. 

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As stated in section 5.1.3.3, charges in oligothiophenes are radical cations. Their transport is usually described with the help of polaron models. The determination of the carrier mobility, and of its dependence as a function of the temperature is therefore of crucial importance. 

Torsi, L. et al., Charge hansport in oligothiophene held-effect hansistors, Phys. Rev. 

In order to continue the study of photoinduced charge separation processes in oligothiophene-based materials, Otsubo et al. recently synthesized two dual oligothiophene-fullerene [Ceol triads 2.132 and 2.133 (Chart 1.26), in which a quaterthiophene and an octithiophene unit were separated by a propyl chain... 

Yi Y, Coropceanu V, Bredas J-L (2011) A comparative theoretical study of exciton-dissociation and charge-recombination processes in oligothiophene/fuUerene and oligothiophene/perylenediimide complexes for organic solar cells. J Mater Chem 21 1479... 

Figure 41. Schematic presentation of probable decay mechanisms in oligothiophene thin films. X X-traps k, k r, kf, kcr rate constants for radiative, non-radiative, and fluorescence decay and the transition into the charge-transfer state, respectively kT thermal energy E external or internal electric field [181],...

The photogeneration of charged excitations in oligothiophene thin films was only studied for a-6T in which the threshold for photoexcitation of charges has been located at around 2.2 eV by a comparison of the one photon excitation curve for radiative recombination and the photoconduction action spectrum [257]. 

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. 

Some dendrimers (389-391) have been reported. The thiophene-containing dendrimers are promising materials, due to the unique properties of oligothiophenes and their derivatives, such as high charge carrier mobility, efficient fluorescence, excellent stability in ambient conditions (even at elevated temperatures), and easy functionalization. 

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