Nature of Charge Carriers in Long Doped Oligothiophenes: The Effect of Counterions

A series of oligothiophene dications doped with Cl3− ions were studied using density functional theory (DFT) at the B3LYP/6−31G(d) level. The balance between the bipolaron and polaron pair states was addressed by studying the closed-shell singlet, open-shell singlet, and triplet states of oligothiophene divalent salts using the relative energies of the different isomers, differences between the singlet and triplet states, bond length alternation analysis, charge distribution analysis, and isodesmic reactions. We found that contribution of polaron pair state to the electronic structure of the oligothiophene divalent salts is not observed in 8T2+(Cl3−)2 (short oligothiophene salts), appears in 12T2+(Cl3−)2 (medium-sized oligothiophene salts), and becomes the dominant in 20T2+(Cl3−)2 (long oligothiophene salts). Bipolarons are intrinsically unstable with respect to dissociation into polaron pairs regardless of the presence of counterions. Thus, even in the presence of counterions, we did not observe any bipolaron stabilization energy, however, doping ions localize polarons. The singlet and triplet states are energetically degenerate for long oligothiophene divalent salts, such as 20T2+(Cl3−)2.