Coupled-cluster approach for studying the optical properties of charged π-conjugated oligomers

We have developed a size-consistent correlated quantum-chemical approach, the coupled-cluster equation of motion (CC-EOM) method, to describe the charged excited states of conjugated materials. We apply this formalism to study a variety of conjugated oligomers: polyenes, oligophenylenevinylenes, oligothiophenes, and oligophenylenes. The transition energies and absorption intensities of the positively charged species are calculated. In all cases, two subgap absorption features are found to dominate the optical spectrum, which are the characteristic optical signatures for polarons in conjugated materials. The relative intensity of these two bands and the dependence on chain length and chemical structure are analyzed. Excellent agreement is found with experimental data.