  We present a generic theory of primary photoexcitations in low band gap
donor-acceptor conjugated copolymers. Because of the combined effects of strong
electron correlations and broken symmetry, there is considerable mixing between
a charge-transfer exciton and an energetically proximate triplet-triplet state
with an overall spin singlet. The triplet-triplet state, optically forbidden in
homopolymers, is allowed in donor-acceptor copolymers. For an intermediate
difference in electron affinities of the donor and the acceptor, the
triplet-triplet state can have stronger oscillator strength than the
charge-transfer exciton. We discuss the possibility of intramolecular singlet
fission from the triplet-triplet state, and how such fission can be detected
experimentally.
