  Recent advances in singlet-fission research make it imperative that
structure-property correlations that determine optical signatures of the
triplet-triplet spin biexciton as well as its binding energy be understood
precisely. We report many-body calculations of excited state absorptions from
the triplet exciton and the triplet-triplet biexciton from two transversally
linked dimers of pentacene derivatives. Comparison of experiment against theory
leads to new interpretations of experiments performed earlier. We show that in
the para-linked isomer the triplet-triplet does not dissociate to free triplets
through the duration of the measurements. In contrast, even as calculated and
experimental transient absorptions agree in the meta-isomer, the experimental
observations here are more difficult to interpret, indicating the strong role
structural variations can play in determining the rate and yield of free
triplets. We also report many-body calculations of the spin gap, the energy
difference between the spin quintet versus spin singlet triplet-triplet, as
well as the binding energy of the spin singlet triplet-triplet, defined as the
energy difference between two free triplets and the bound biexciton. The spin
gap and the binding energy of the spin singlet triplet-triplet are different
quantities in all but coupled two-level systems. The experimental behavior in
the transversally linked dimers as well as previously studied longitudinally
linked dimers agree with the trends that would be predicted from the computed
biexciton binding energies.
