Saturation of the optical band gap and properties of five-membered heteroaromatic oligomers

The saturation behavior of the optical band gap and dynamic (hyper)polarizabilities of five-membered heteroaromatic oligomers - oligothiophene, oligofuran and oligopyrrole - has been studied by means of ab initio response theory in the random phase approximation. The calculated optical band gap for the oligomers and the extrapolated band gap for the polymers are found to be within 0.2 eV of available experimental data. The calculated saturation lengths of the band gaps are in good agreement with experimental determinations. The saturation of the polarizabilities is found to be slower than that of the band gaps, especially when dispersion is taken into account. The calculations indicate that previous experimental observation of fast convergence of the optical properties of oligothiophenes may be due to disorder in the samples.