MOLECULAR-ORBITAL STUDY ON ELECTRON CONFINEMENT CHARACTERISTICS IN HETEROAROMATIC OLIGOMERS

A molecular orbital study on heteroaromatic oligomers has been perform ed to investigate their electron confinement characteristics. The semi empirical MNDO-SCF-MO (modified neglect of diatomic overlap self-consi stent-field molecular orbital) method was used to calculate the electr onic 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 cell s X and Y, n = 1-4. They are X = benzene, Y = pyrrole, X = benzene, Y = thiophene, and X = thiophene, Y = pyrrole, respectively. The electro nic structures of the oligomers have been analyzed by means of the deg ree of occurrence of the electron confinement divided into the sequenc es of X units and of Y units. The analyses indicate that the oligomers with X = thiophene, Y pyrrole can have the most promising tunable qua ntum-well structure by controlling the unit number in the oligomers st udied. The effect of the confinement characteristics of the oligomers on the static second-order hyperpolarizability is also discussed.