Reviewing Extrapolation Procedures of the Electronic Properties on the π-Conjugated Polymer Limit

In this article, the extrapolation procedures of pi-pi* electronic transition energy on pi-conjugated oligomers are reexamined. Different models, including the simplest coupled oscillator, the free electron, the Huckel approach, the molecular exciton model, and some specific fitting-functions, are compared using the transition energies derived from theoretical calculations on three thiophene-based oligomer series. Specifically, oligomers of up to 30 repeating units have been considered to include the saturation effects as a function of chain length. The coupled oscillator model of W. Kuhn and the fitting-function of Hirayama are the models that present the better suit on the transition energy interpolation as a function of chain length. Using only the first four oligomers of the series (n = 2 up to 8) yields an estimation of the transition energy on the polymer limit with an average error of similar to 1.5%. The vertical and adiabatic ionization potential present a better fit with the Huckel model approach. Finally, implications of the environmental polarity on the electronic properties, molecular geometry, charge distribution, and aromaticity are shortly discussed.