THEORY OF BIEXCITONS IN PI-CONJUGATED POLYMERS

Theoretical models of pi-conjugated polymers that emphasize the Coulom b electron-electron correlations predict that the lowest optical excit ation is to an exciton. We have extended our theoretical studies to an energy region that is considerably higher, to probe specifically the states that are reached by two-electron excitations from the ground st ate. Theoretical evidence for the biexciton, a bound state of two exci tons that occurs below the two-electron continuum, is found. The lowes t biexciton state is of even parity, can be reached by optical excitat ion from the 1B(u), exciton, and in principle also by two-photon excit ation from the ground state. A high energy photoinduced absorption see n in the picosecond time domain in poly (para-phenylenevinylene) (PPV) , as well as in other pi-conjugated polymers, is interpreted as a tran sition from the exciton to the biexciton.