  We present a theory of the electronic structure and photophysics of 1:1
blends of derivatives of polyparaphenylenevinylene and fullerenes. Within the
same Coulomb-correlated Hamiltonian applied previously to interacting chains of
single-component $\pi$-conjugated polymers, we find an exciplex state that
occurs below the polymer's optical exciton. Weak absorption from the ground
state occurs to the exciplex. We explain transient photoinduced absorptions in
the blend, observed for both above-gap and below-gap photoexcitations, within
our theory. Photoinduced absorptions for above-gap photoexcitation are from the
optical exciton as well as the exciplex, while for below-gap photoexcitation
induced absorptions are from the exciplex alone. In neither case are free
polarons generated in the time scale of the experiment. Importantly, the
photophysics of films of single-component $\pi$-conjugated polymers and blends
can both be understood by extending Mulliken's theory of ground-state charge
transfer to the case of excited-state charge transfer.
