A comparative study of homological series of oligothiophenes: the influence of oligomer length and conformation on some structural, chemical, physical properties

Résumé

Two homological series of oligothiophenes were studied with the aid of DFT (B3P86, PBE0 and B3LYP) in the aim of exploring the influence of the conformation and conjugation length of oligomers on a number of properties. From the geometrical structure viewpoint, the results have shown that the linear series remains planar, independently on the length of the oligomer, while in the nonlinear series, the planarity is very depend on the number (n) of the thiophene rings: the planarity remains for n ≤ 5 to give place to a helical conformation for n ≥ 6. Upon oxidation, the linear series conserve the planarity; the other series, while remaining helical is somehow altered. From the electronic point of view, some of the electronic and physical properties characterizing a good electron-donating (ED) and charge-transporting (CT) capacity such as the frontier molecular orbital  energies (EHOMO and ELUMO), the ionization potential (IP) and the intramolecular reorganization energy were calculated and comparison made to those of C16S8 and some other already used OFET materials. Some of the studied derivatives were found to display a comparable/much lower h, higher IP and ELUMO than those for some of the already well known organic field-effect transistor (FET) materials such as pentacene, anthracene, and DT-TTF. We recommend a further investigation of these for FET use issue.