  We develop a theory of the electronic structure and photophysics of
interacting chains of $\pi$-conjugated polymers to understand the differences
between solutions and films. While photoexcitation generates only the exciton
in solutions, the optical exciton as well as weakly allowed excimers are
generated in films. Photoinduced absorption in films is primarily from the
lowest excimer. We are also able to explain peculiarities associated with
photoluminescence, including delayed photoluminescence and its quenching by
electric field.
