  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
intrachain exciton in solutions, the optical exciton as well as weakly allowed
charge-transfer excitons are generated in films. We extend existing theories of
the lowest polaronpair and charge-transfer excitons to obtain descriptions of
the excited states of these interchain species, and show that a significant
fraction of ultrafast photoinduced absorptions in films originate from the
lowest charge-transfer exciton. Our proposed mechanism explains the
simultaneous observation of polaronlike induced absorption features peculiar to
films in ultrafast spectroscopy and the absence of mobile charge carriers as
deduced from other experiments. We also show that there is a 1:1 correspondence
between the essential states that describe the photophysics of single chains
and of interacting chains that constitute thin films.
