Quantum chemical investigation of biexcitons in conjugated polymers

Abstract

The formation of biexcitons in conjugated polymers is examined within the Pariser–Parr–Pople (PPP) model of polyacetylene. The calculations are done using a scattering formalism that provides a size-consistent description of excited states containing both single and double electron–hole pair excitations. The excited-state absorption spectrum from the 1 1Bu exciton state was calculated and examined for signatures of biexciton formation. Calculations were performed on polyenes with up to 9 unit cells, and on long chains using periodic boundary conditions. While polyenes with 7 to 31 unit cells exhibit states with some of the features expected for biexcitons, such states are not seen in the limit of long chains. This suggests that, within the two-band model considered here, exciton–exciton interactions are not of sufficient strength to lead to biexciton formation in the limit of long chains, and that their presence in shorter chains is due to confinement effects. In the long-chain limit, transitions are found to states consisting of overlapping electron–hole pairs, but these states have energies above that required to make two free excitons. These states may result from scattering resonances between excitons.