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/6-31G(d), 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.