Theoretical study of long oligothiophene dications: bipolaron vs polaron pair vs triplet state

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 some cases, also at BLYP/6-31Gd, was applied to study the singlet and triplet states of the whole series. We found that the singlet state is the ground state for all oligothiophene dications up to the 20-mer, and that the singlet and triplet states are degenerate for longer oligomers. Thus, the triplet state is never a pure ground state for these dications. We found that, for short oligothiophenes dication (e.g., 6T2+), the bipolaron state is the more important state, with only a small contribution made by the polaron pair state. For medium size oligothiophene dications (e.g., 14T2+), both the bipolaron state and the polaron-pair state contribute to the electronic structure. Finally, in long oligothiophene dications, such as 30T2+ and 50T2+, the contribution from the polaron pair state becomes dominant, and these molecules can be considered as consisting of two independent cation radicals or a polaron pair. Results from isodesmic reactions show that the stability of oligothiophene cation radicals over dications is inversely proportional to chain length. Small oligothiophene dications (n = 6-12) were studied at the CASSCF(m,m)/6-31G(d) (m = 4, 6, and 10) level. The major conclusions of this paper regarding the relative energy of the singlet state versus the triplet state and regarding the relative stability of the bipolaron versus the polaron pair were also supported by CASSCF calculations.