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 phot ophysics of alkylsubstituted tetrathiophenes and sexithiophenes as wel l as their corresponding polyesters are reported. The insertion of alk yl lateral chains caused a torsion of the backbone of the oligomers. T his torsion induces a blue shift of the absorption bands and a decreas e in their molar extinction coefficients. Semiempirical calculations ( AM1 and PM3) performed on the tetramer show that dihedral angles betwe en thiophene rings where alkyl chains are located are much twisted, wh ereas the torsional angle between thiophene rings without alkyl chains is similar to that found for unsubstituted oligothiophenes. From fluo rescence 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 shif t of the spectra due to an increase of the electronic delocalization a long the molecular frame and an increase in the rigidity of the molecu lar backbone for short oligomers. Fluorescence quantum yields and life times decrease compared to those of the unsubstituted oligothiophenes. In the corresponding polyesters, thiophene units have the same confor mation as well as the same spectroscopic and photophysical properties as those of the isolated molecules.