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

A study of the spectroscopy and photophysics of thiophene-fluorene oligomer s as well as the respective molecules incorporated in polyesters are report ed. 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 derivativ e can be assigned to the S-1<--S-0 electronic transition computed from ZIND O/S calculations performed on the optimized geometries (HF/6-31G*). This tr ansition corresponds mainly to the promotion of an electron from the HOMO t o 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 o f the thiophene rings caused a torsion of the backbone of the oligomers, wh ich induces a blue shift; of the absorption band. From fluorescence data, i t is observed that a more planar conformation is favored in the relaxed exc ited 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 du e 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 o ligomer chain and/or the presence of carbonyl groups do not significantly i nfluence the ground state molecular conformation. In the polyesters, thioph ene-fluorene units have about the same spectral positions as those of the c orresponding esters indicating that the oligomers are well isolated in the polyester chain. Fluorescence quantum yields and lifetimes of methyl-substi tuted derivatives are smaller than those of the unsubstituted molecules. Si milarly, these photophysical parameters are smaller for the polyesters comp ared to those of the respective esters. In these systems, the photophysical properties are mainly governed by nonradiative processes. However the lumi nescence of the polyesters remains relatively intense making them suitable for LED materials.