Electronic structure evolution of neutral and dicationic states of conjugated polymers with their band gap

The electronic structures of neutral and dicationic states of 8 conjugated oligomers were studied at UB3LYP/6-31G* level as models of conjugated polymers with different band gaps (Eg). The evolution of the electronic structure for polymers with experimental Eg ranging from 3.0 to 0.0 eV has been investigated for both neutral and dicationic species. For polymers with Eg > 1.1 eV the ground state of neutral oligomers was found to be a closed shell singlet (S0) while the dicationic state was a polaron pair open shell singlet (OSS) state. For polymers with lower Eg, OSS becomes the ground state for both dicationic and neutral states. The energy difference between OSS and S0 state increases with closure of Eg for neutral oligomers. However, in dications the S0–OSS energy difference becomes less than 1 kcal/mol for the low band gap polymers. Both, the charge distribution and the bond length alternation pattern suggest that bipolaronic state and not polaron pair state dominates in dications of the low band gap polymers (Eg < 1.1 eV).