Spectroscopic and photophysical properties of thiophene-fluorene oligomers as well as their corresponding polyesters

A study of the spectroscopy and photophysics of thiophene-fluorene oligomers as well as the respective molecules incorporated in polyesters are reported. The same oligomers having carbonyl groups at both ends of the molecules have also been studied. These molecules provide a better correlation with their corresponding polyesters. The first absorption band of each derivative can be assigned to the S-1<--S-0 electronic transition computed from ZINDO/S calculations performed on the optimized geometries (HF/6-31G*). This transition corresponds mainly to the promotion of an electron from the HOMO to the LUMO and is strongly allowed and polarized along the long axis of the molecular frame. The insertion of alkyl lateral chains at the 3-position of the thiophene rings caused a torsion of the backbone of the oligomers, which induces a blue shift; of the absorption band. From fluorescence data, it is observed that a more planar conformation is favored in the relaxed excited states. The increase of the oligomer chain or the addition of carbonyl groups at both ends of the molecules induces a red shift of the spectra due to an increase in the electronic delocalization along the molecular frame. It was shown by HF/6-31G* ab initio calculations that the length of the oligomer chain and/or the presence of carbonyl groups do not significantly influence the ground state molecular conformation. In the polyesters, thiophene-fluorene units have about the same spectral positions as those of the corresponding esters indicating that the oligomers are well isolated in the polyester chain. Fluorescence quantum yields and lifetimes of methyl-substituted derivatives are smaller than those of the unsubstituted molecules. Similarly, these photophysical parameters are smaller for the polyesters compared to those of the respective esters. In these systems, the photophysical properties are mainly governed by nonradiative processes. However the luminescence of the polyesters remains relatively intense making them suitable for LED materials.