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 (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 (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.