Substitutional effects on the structural, spectroscopic, and photophysical properties of quaterthiophenes and sexithiophenes as well as on their corresponding polyesters

A conformational analysis and a study of the spectroscopy and the photophysics of alkylsubstituted tetrathiophenes and sexithiophenes as well as their corresponding polyesters are reported. The insertion of alkyl lateral chains caused a torsion of the backbone of the oligomers. This torsion induces a blue shift of the absorption bands and a decrease in their molar extinction coefficients. Semiempirical calculations (AM1 and PM3) performed on the tetramer show that dihedral angles between thiophene rings where alkyl chains are located are much twisted, whereas the torsional angle between thiophene rings without alkyl chains is similar to that found for unsubstituted oligothiophenes. From fluorescence data, we observed that a more planar conformation is favored in the relaxed excited state of all thiophene oligomers. The addition of two carbonyl groups at the ends of the molecules induces a red shift of the spectra due to an increase of the electronic delocalization along the molecular frame and an increase in the rigidity of the molecular backbone for short oligomers. Fluorescence quantum yields and lifetimes decrease compared to those of the unsubstituted oligothiophenes. In the corresponding polyesters, thiophene units have the same conformation as well as the same spectroscopic and photophysical properties as those of the isolated molecules.