SATURATION OF THE OPTICAL BAND-GAP AND PROPERTIES OF 5-MEMBERED HETEROAROMATIC OLIGOMERS

The saturation behavior of the optical band gap and dynamic (hyper)pol arizabilities of five-membered heteroaromatic oligomers-oligothiophene , oligofuran and oligopyrrole-has been studied by means of ab initio r esponse theory in the random phase approximation. The calculated optic al band gap for the oligomers and the extrapolated band gap for the po lymers are found to be within 0.2 eV of available experimental data. T he calculated saturation lengths of the band gaps are in good agreemen t with experimental determinations. The saturation of the polarizabili ties is found to be slower than that of the band gaps, especially when dispersion is taken into account, The calculations indicate that prev ious experimental observation of East convergence of the optical prope rties of oligothiophenes may be due to disorder in the samples.