Simple Rule To Predict Boundedness of Multiexciton States in Covalently Linked Singlet-Fission Dimers

Because of the potential for increasing solar cell efficiencies, significant effort has been spent understanding the mechanism of singlet fission. We provide a simple connectivity rule to predict whether the through-bond coupling will be stabilizing or destabilizing for the 1 ( TT ) state in covalently linked singlet-fission chromophores. By drawing an analogy between the chemical system and a simple spin–lattice, one is able to determine the ordering of the multiexciton spin state via a generalized usage of Ovchinnikov’s rule. This allows one to predict (without any computation) whether the 1 ( TT ) multiexciton state will be bound or unbound with respect to the separated triplets in covalently linked singlet-fission dimers. To test our hypothesis, we have performed ab initio calculations on a systematic series of covalently linked singlet-fission dimers. Numerical examples are given, and the limitations of the proposed theory are explored.