Electronic spectroscopy and photophysics of phenylene-fluorene derivatives as well as their corresponding polyesters

In this study, we present spectroscopic and photophysical results on phenylene-fluorene oligomers as well as on their corresponding polyesters. To obtain a better correlation, oligomers having ester groups at both ends of the molecules have also been investigated. From ZINDO/S calculations performed on the optimized geometries (HF/6-31G(*)), the first absorption band of each derivative is assigned mainly to the electronic excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), which is strongly allowed and polarized along the long molecular axis (x). The increase of the oligomer length and/or the addition of carbonyl groups at both ends of the molecules induce a red shift of the absorption and fluorescence spectra due to an increase in the electronic delocalization along the molecular frame. From optimized geometries performed at the HF/6-31G(*) level, it is observed that the ground state molecular conformation is not much affected by the length of the oligomer and/or by the presence of end substituents (carbonyl groups). On the other hand, according to fluorescence data, all derivatives relax to more planar conformations in the first singlet excited state. Fluorescence quantum yields and lifetimes of ester derivatives are close to those of the corresponding compounds without carbonyl groups. In the corresponding polyesters, phenylene-fluorene units have about the same spectroscopic and photophysical properties as those of the isolated molecules. This clearly indicates that the oligomers are well isolated in the polyester chain. The luminescence of the polyesters is intense (phi(F) = 0.6) making these polymers promising materials for blue LED materials. (C) 2002 Elsevier Science B.V. All rights reserved.