Due to the possibility of improving photovoltaic efficiencies, significant effort has been directed towards understanding the singlet fission mechanism. Although accurate quantum chemical calculations would provide a detail-rich view of the singlet fission mechanism, this is complicated by the multiexcitonic nature of one of the key intermediates, the <sup>1</sup>(TT) state. Being described as two simultaneous and singlet-coupled triplet excitations on a pair of nearest neighbor monomers, the <sup>1</sup>(TT) state is inherently a multielectronic excitation. This fact renders most single-reference quantum chemistry methods incapable of providing accurate results. In this talk I will discuss our recent work in exploring the mapping of simple (single-excitation) ab initio calculations onto a spin lattice model, which can be solved by exact diagonalization to yield the low-energy biexciton states. In addition to creating a simple computational approach to analyzing certain (but not all) aspects of the biexciton states, the simple form of the spin-Hamiltonian offers a relatively transparent view of these states, which has allowed us to make predictions about exciton binding energies in covalently linked chromophores. We will discuss the model, it’s applications, it’s limitations, and possibilities for extensions, such as a derivation of an underlying excitonic Hamiltonian coming from a QDPT treatment of the spin Hamiltonian.

Modeling singlet-fission biexciton states as an ab initio spin model: Justifications and applications

2019 Spring National Meeting

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