Electronic excited polythiophenes

BT), they also act as models of the conjugated organic polymers polyparaphenylene (PPP) and polythiophene (PT), respectively. As is discussed in detail elsewhere [96], the results for the electronic excited states of the cation and anion of BP can be used for interpreting the polaron states in doped PPP. The main features of the spectra are reviewed briefly below. 

The exact relative locations of the lowest excited Bu and Ag states are difficult to predict on a theoretical basis. Indeed, they sensitively depend on the interplay between electron correlation effects and bond-length alternation effects, as shown for instance by Soos and his co-workers23. Strong effective bond alternations favour the 1BU state as the S, state this is the case in polyparaphenylene and PPV due to the presence of phenylene rings. The effective bond alternation is much weaker in polyacetylene while it is intermediate in polythiophene where the 2Ag state is found to lie slightly above the 1BU state. 

We mentioned above that in real materials, disorder of any kind localizes the excitations over a limited length on the chain, generating effective conjugation lengths so that, for instance, the optical properties are those of a finite, often quite small, molecule. It may well be that excitons are very important in such cases. This has been proposed, for instance, for substituted PPV and polythiophene [83]. This is not by itself a proof that were it possible to prepare the corresponding perfect chain, the excitons would still be important there, although this is precisely what is found in PDAs, because electron correlations have different intensities on different CPs. 

Polythiophenes usually form disordered layers, in particular if electrochemical deposition is used [55-58], Preparation of plasma-polymerized polythiophene leads to disordered layers with comparable structure to elcctrochemically prepared films [59]. However, if the substrate is put near to the RF source where the concentration of electrons and excited Argon species is high, the concentration of thiophene monomers is low, and the fragmentation probability of the monomer is high, platelet structures with clear grain boundaries could be observed. But these platelets are likely to consist of products other than polythiophene due to the above-mentioned preparation conditions,... 

Raman spectra of doped polythiophene exhibit also modifications provided excitation wavelengths are taken in the red range. The main Ag mode is shifted to 1411 cm. An analysis of these spectra in the frame of our method, i.e. by modifying the main force constants associated to the bonds of the polymer backbone, leads also to a good fit of the modified Raman spectra. In contrary to doped PPP or doped PPV, it is not obvious whether the quinoid structure appears clearly. Instead, we do need to modify the C-S force constant, putting in evidence that the S atom plays a certain role in the electronic structure modification. Further details on the calculations will also be published elsewhere. 

Polythiophene has a narrow band gap and electrons can readily be excited by photons, so allowing the polymer to exhibit photoconductivity. Because of this, it is finding appHcations in solar cells and organic transistors. 

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