Energy gaps of α,α′-substituted oligothiophenes from semiempirical, ab initio, and density functional methods

Energy gaps have been estimated for -OMe and -NO2 alpha,alpha'-substituted oligothiophenes up to six monomers using semiempirical, Hartree-Fock and density functional methods. Scaled values calculated using noncorrelated methods are in good agreement with the experimental values, and so were nonscaled estimates predicted by density functional methods. Error bars are ca. 0.2 eV for all II oligothiophenes studied. The influence of the quality of the basis set on the energy estimates is discussed. The discrepancy observed for the -OMe- and -NO2-substituted sexithiophene result with respect to the experimental value is discussed and has been attributed to a charge transfer in the molecule. The Delta SCF approach has been found to be an alternative way to estimate energy gaps for molecular systems where Koopmans' theorem may not provide good results. Implications for predictions of HOMO-LUMO gaps of pi-conjugated systems are discussed and analyzed in terms of designing new materials with controlled properties.