Electronic structures of neutral, cationic and dicationic states of the low band gap polymers. Polythieno[3,4-b]benzene, a case study

The electronic structures of neutral cationic and dicationic states of oligothieno[3,4-b]benzene containing up to 20 repeating units have been studied using broken symmetry B3LYP/3-21G* method. The model predicts that short oligomers (up to decamers) have closed shell aromatic ground state, while the ground state of longer oligomers possesses multireference character and cannot be described within a framework of one determinant methods. It has been found that the ground state has significant contributions from triplet state. The S0 → S1 energy calculated for longest of polythieno[3,4-b]benzene (PITN) oligomer containing 20 repeating units is in excellent agreement with experiment. Unlike polythiophene where the ground state of dications is open shell polaron pair state dicationic ground state of calculated oligomers is closed shell bipolaronic state (at least up to oligomers containing 20 repeating units) which was confirmed by stability calculations.