Theoretical Study of Long Oligothiophene Polycations as a Model for Doped Polythiophene

We investigated the different electronic states of oligothiophene polycations such as tri-, tetra-, hexa-, and octacations (nT3+, nT4+, nT6+, and nT8+) at the B3LYP/6-31G(d) level. 10-, 20-, 30-, and 50-mers of oligothiophene polycations were studied. This is the first time oligothiophene polycations have been studied using density functional theory (DFT). For relatively short (10- or 20-mer) oligothiophene polycations, the ground states are most likely the doublet for trications and the singlet for tetra-, hexa-, and octacations, while longer oligomer polycations (such as the 50-mer) exhibit degeneracy between different spin states. Using bond length alternation, charge distribution, and relative energies data, we showed that the electronic structure of sufficiently long tri- and tetracations (such as the 20- and 30-mer) and hexa- and octacations (such as the 50-mer) appears similar to that of the dications, with the oligothiophene chain separated into well-defined regions of cation radicals (polarons). Charge separation requires a chain length of at least about five thiophene rings per unit charge. Interestingly, one molecule of dopant per five thiophene rings is the typical doping level for polythiophene. Isodesmic reactions were used to assess the stability of oligothiophene polycations in the gas phase.