  We show from many-body quantum mechanical calculations that there occur
structurally distinct triplet-pair eigenstates in the intramolecular singlet
fission (iSF) compound pentacene-tetracene-pentacene. Triplet excitons occupy
neigboring pentacene and tetracene monomers in the higher energy doubly
degenerate triplet-triplet multiexcitons, and terminal pentacene chromophores
in the lower energy multiexciton. The lowest energy multiexciton is reached by
ultrafast triplet migration within the triplet-triplet manifold, a result with
profound implication for the design of superior iSF compounds.
