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 selfconsistent-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 quantumwell 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.
The refractive index of a novel silicon-germanium ladder copolymer synthesized by partially substituting silicon in the main chain of polyphenylsilsesquioxane by germanium could be controlled between 1.558 and 1.573 by changing the content of germanium up to 10 mol%, without losing thermal stability over 500 °C.
Quasi-diabatic band theory has been developed by using the localized crystal orbital (LCO) transformation to study the electronic structure representing the diabatic behavior of electrons. The theory has been applied to nondoped alternating trans-polyacetylene (t-PA) to understand better the electronic structure from this new standpoint. The results show that the π-(π*-) band of the newly constructed crystal orbital becomes higher (lower) than that of the canonical crystal orbital. The occupation number analysis also indicates that the π-bands near the Fermi energy level play a significant role in the electron transport. These results have been discussed in terms of the LCOs representing the biradical and zwitterionic states.
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