Optimal arrangements of tetracene molecule pairs for fast singlet fission

Abstract

In search for a qualitative understanding of the effects of molecular packing on singlet fission (SF) rate, a simplified version of the frontier orbital model is described and illustrated on a pair of tetracene molecules. To identify all favorable physically accessible pair geometries, all significant local maxima of the square of the electronic matrix element for SF have been located within the six-dimensional space of possible arrangements of two rigid bodies, using a grid of over 4.7 © 108 pair geometries. Those at which the molecules interpenetrate were excluded using a hard-sphere model. The effects of intermolecular interaction on the SF energy balance and thus its rate constant kSF were approximated using Marcus theory at each of the maxima using the same simplified version of the frontier orbital model. Starting at these local maxima, the pair geometries were optimized for maximum kSF and the 21 best are reported along with their computed Davydov splitting and triplet biexciton binding energies. The optimal pair structures at the resulting maxima follow qualitative rules published previously and further elaborated here.