Quantum chemical investigation of biexcitons in conjugated polymers

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 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 suÆcient strength to lead to biexciton formation in the limit of long chains, and that their presence in shorter chains is due to con nement e ects. 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.