| Summary: | Thesis (M.Sc.)--Memorial University of Newfoundland, 2000. Physics and Physical Oceanography Bibliography: leaves 162-182 The nature of the geometric and electronic transitions taking place in 7r-conjugated oligo(thiophene)s, oligo(cyclopentadiene)s, oligo(fulvene)s and their cyano based derivatives are investigated using an ab initio approach, including correlation effects. Theoretical evidence based on excited state electronic studies ofthe most stable forms of thiophene and cyclopentadiene based oligomers show that thecyano-derivatives of these polymers possess smaller intrinsic band gaps than their parents. The geometries of these neutral five-membered ring oligomers have been optimized using the ab initio restricted Hartree-Fock method followed by the single configuration interaction (CIS) technique with 3-2IG* basis set within the framework of Gaussian 94. It has been observed that the 3-2IG* basis set describes the qualitative geometric and electronic features reliably. Single substitution of electrons in the lowest unoccupied molecular orbitals (conduction band) from the highest occupied molecular orbitals (valence band) shows good agreements with the experimentally observed excited state energies (where available) for the six molecular systems investigated in this study. Of these, the lowest was for the tetramer of poly-(dicyanomethylene cyclopentadithiophene) (1.63 eV), followed by poly-(dicyanomethylene cyclopentadifulvene) (2.34 eV) and poly-(dicyano-methylene cyclopentadicyclopentadiene) (2.57 eV). The molecular geometric modifications in going from the ground to the lowest excited state show particular trends towards a full aromatic benzoid like structure with almost equal bond lengths along the molecular backbone. The heteroatomic substitutions and the geometry relaxation phenomenon show an efficient approach to band gap control. The geometry relaxation phenomena occurring in the singlet and triplet states show more pronounced and localized bond length alternations in the triplet states, confirming the more localized character of triplet states. Maximum planarity, weak interactions of the chain backbone with the bridging group, enhanced ir character of the highest occupied and the lowest unoccupied molecular orbitals along with charge transfer phenomenon also contribute to band gap lowering. For these oligomers, the evolution of the lowest energy, singlet-triplet transitions with chain length ranging from one to eight rings, has also been investigated. The lowest three singlet and three triplet states are characterized with the oscillator strengths in the oligomers along with the lattice distortions taking place due to the tz — ir* transitions. The inclusion of electron correlation results in a significant reduction in the band gap and shows excellent agreement with the experimental results. The calculated transition energies are within 0.1—0.4 eV of the experimental results.
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