EXCITONS AND BIEXCITONS IN POLY(PARA-PHENYLENEVINYLENE)

The Huckel model fails to describe optical absorption in PPV and its d erivatives, and explicit inclusion of moderate electron-electron inter actions is essential for a proper theoretical description. The lowest optical state is an exciton with a binding energy estimated as 0.90 +/ - 0.15 eV, in excellent agreement with experiment. Such a large excito n binding energy strongly suggests stable biexcitons. The concept of b iexcitons in pi-conjugated polymers is introduced, and picosecond phot oinduced absorption is explained as excited state absorption from the exciton to the biexciton.