Molecular Orbital Study on Electron Confinement Characteristics in Heteroaromatic Oligomers

A molecular orbital study on heteroaromatic oligomers has been performed to investigate their electron confinement characteristics. The semiempirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consistent-field molecular orbital) method was used to calculate the electronic structures of the oligomers as a function of the unit number for the three sets of heteroaromatic oligomers X n Y n X n with unit cells X and Y, n = 1−4. They are X = benzene, Y = pyrrole, X = benzene, Y = thiophene, and X = thiophene, Y = pyrrole, respectively. The electronic structures of the oligomers have been analyzed by means of the degree of occurrence of the electron confinement divided into the sequences of X units and of Y units. The analyses indicate that the oligomers with X = thiophene, Y = pyrrole can have the most promising tunable quantum-well structure by controlling the unit number in the oligomers studied. The effect of the confinement characteristics of the oligomers on the static second-order hyperpolarizability is also discussed.