Energy gaps of alpha,alpha '-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 de nsity functional methods. Scaled values calculated using noncorrelated meth ods are in good agreement with the experimental values, and so were nonscal ed estimates predicted by density functional methods. Error bars are ca. 0. 2 eV for all II oligothiophenes studied. The influence of the quality of th e 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 transf er in the molecule. The Delta SCF approach has been found to be an alternat ive way to estimate energy gaps for molecular systems where Koopmans' theor em may not provide good results. Implications for predictions of HOMO-LUMO gaps of pi-conjugated systems are discussed and analyzed in terms of design ing new materials with controlled properties.