| Abstract
| - A series of oligothiophene dications (from the sexithiophene dication to the 50-mer oligothiophene dication,nT2+, n = 6−50) were studied. Density functional theory (DFT) at the B3LYP/6-31G(d) level and, in somecases, also at BLYP/6-31G(d), was applied to study the singlet and triplet states of the whole series. Wefound that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that thesinglet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure groundstate for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state isthe more important state, with only a small contribution made by the polaron pair state. For medium sizeoligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to theelectronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contributionfrom the polaron pair state becomes dominant, and these molecules can be considered as consisting of twoindependent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability ofoligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophenedications (n = 6−12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The majorconclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regardingthe relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.
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