Exciton Delocalization and Triplet Diffusion in Singlet Fission Materials: A Many-Electron Tight-Binding and Non-Orthogonal CI Study

A many-electron tight-binding (METB) approach is presented to use the results of nonorthogonal configuration interaction with fragments (NOCI-F) calculations on small ensembles for the study of much larger (or even periodic) systems. The METB approach is validated against NOCI-F for a stack of 15 ethene molecules, small enough to be handled in full by the NOCI-F approach and large enough to extract relevant information from METB treatment. Thereafter, METB is used to show how the singlet fission coupling depends on the degree of delocalization of the singlet excitonic state. Finally, METB is applied to extract the dispersion of some many-electron states in the tetracene crystal. The insight gained from these METB calculations based on NOCI-F parametrization can be used to further study the fundamentals of singlet fission and the subsequent fate of the biexciton.