Theoretical study of thiophene oligomers: Electronic excitations, relaxation energies, and nonlinear optical properties

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Abstract
Using Hartree–Fock ab initio or semiempirical Austin model 1 (AM1) techniques to estimate geometries and an intermediate neglect of differential overlap/configuration interaction (INDO/CI) technique to calculate electronic transition energies, we examine the nature of the lowest singlet excited states in thiophene oligomers containing up to eight rings. The evolution of the ordering of the lowest two (1B and 2A) excited states is investigated as a function of chain length. The geometry relaxation phenomena in the 1B and 2A states are modeled on the basis of either bond‐order/bond‐length relationships or the formation of bipolaron‐type defects. The chain‐length dependence of the third‐order polarizability γ is evaluated from a sum‐over‐states approach. All the results are discussed in comparison to those obtained in polyenes of similar sizes.