Electronic Structure of Thiophene Oligomer Dications: An Alternative Interpretation from the Spin-Unrestricted DFT Study

The geometric and electronic structures of oligothiophene dications (with 6 to 12 monomers) have been revisited using the spin-unrestricted broken symmetry hybrid density functional B3LYP method. It is found that there exists a transition region of bipolaron to two-polaron structure conversion in the moderately sized oligomers, as that had been reported earlier in an AM1-CI calculation. According to our calculation, the transition region should be from hexamer to octamer. TD-DFT simulation led to a different rationalization of the experimental UV/visible spectra, which suggested the coexistence of bipolaron and two-polaron state in the transition region.