Free Triplets Versus Bound Triplet-Triplet Biexciton in Intramolecular Singlet Fission Materials: Structure-Property Correlations

Recent advances in singlet-fission research make it imperative that structure-property correlations that determine the 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 play in determining the rate and yield of free triplets. We also report many-body calculations of the spin gap, defined as 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 agrees with the trends that would be predicted from the computed biexciton binding energies.