Theory of the dark state of polyenes and carotenoids

A theory is developed to describe the singlet dark state (usually labeled ${S}_{1}$ or $2{A}_{g}$) of polyenes and carotenoids. The properties this explain nonemissive linear polyenes, it responsible for photoprotection carotenoids in light harvesting complexes, because its triplet-pair character, thought be cause fission polyene-type systems. described here assumes that principle a combination triplet-pairs charge-transfer exciton. Crucially, these components only couple when triplet pair occupies neighboring C-C dimers, such an electron transfer between triplets creates nearest-neighbor excitation. This local coupling stabilizes $2{A}_{g}$ induces attraction triplets. In addition, electron-hole Coulomb exciton, increased probability dimers enhances triplet-triplet attraction: ``slaved'' also predicts as interaction increased, evolves from predominately exciton with small component character composed some character. Above critical there decoupling subspaces, becomes entirely unbound spin-correlated pairs. predictions binding energy ca. 0.4 eV are consistent density matrix renormalization group calculations Pariser-Parr-Pople (or extended Hubbard) model. It single-triplet 50%.