Theory of biexcitons in Φconjugated polymers

Theoretical models of Φconjugated polymers that emphasize the Coulomb electron-electron correlations predict that the lowest optical excitation is to an exciton. We have extended our theoretical studies to an energy region that is considerably higher, to probe specifically the states that axe reached by two-electron excitations from the ground state. Theoretical evidence for the biexciton, a bound state of two excitons that occurs below the two-electron continuum, is found. The lowest biexciton state is of even parity, can be reached by optical excitation from the 1BU exciton, and in principle also by two-photon excitation from the ground state. A high energy photoinduced absorption seen in the picosecond time domain in poly (yaro-phenylenevinylene) (PPV), as well as in other Φconjugated polymers, is interpreted as a transition from the exciton to the biexciton.