  A theory is developed to describe the singlet dark state (usually labeled S1
or 2Ag) of polyenes and carotenoids. The theory assumes that in principle this
state is a linear combination of a singlet triplet-pair and an odd-parity
charge-transfer exciton. Crucially, these components only couple when the
triplet-pair occupies neighboring dimers, such that an electron transfer
between the triplets creates a nearest-neighbor charge-transfer excitation.
This local coupling stabilises the 2Ag state and induces a nearest neighbor
attraction between the triplets. In addition, because of the electron-hole
attraction in the exciton, the increased probability that the electron-hole
pair occupies neighboring dimers enhances the triplet-triplet attraction: the
triplet pair is `slaved' to the charge-transfer exciton. The theory also
predicts that as the Coulomb interaction is increased, the 2Ag state evolves
from a predominately odd-parity charge-transfer exciton state with a small
component of triplet-pair character to a state predominately composed of a
triplet-pair with some exciton character. Above a critical Coulomb interaction
there is a decoupling of the triplet-pair and charge-transfer exciton
subspaces, such that the 2Ag state becomes entirely composed of an unbound
spin-correlated triplet pair. The predictions of this theory are qualitatively
consistent with high-level density matrix renormalization group calculations of
the Pariser-Parr-Pople (or extended Hubbard) model.
