We have developed a fully ab initio version of the Frenkel-Davydov exciton model, in which collective excited states of a molecular crystal or aggregate are expanded as direct products of monomer excitations. These basis states, and the couplings between them, can be computed in a distributed and highly efficient manner. We have also implemented analytic derivative couplings that describe how the site energies and energy transfer integrals are modulated by vibrational coordinates, and which amount to ab initio versions of the excitation-phonon coupling parameters in the phenomenological Holstein-Peierls Hamiltonian. Surprisingly, we find that the largest excitation-phonon couplings correspond to intramolecular vibrational modes with frequencies of 1430 and 1540 cm–1. We use these calculations to parameterize a model Hamiltonian for singlet fission in crystalline tetracene, in which vibronic coupling can be turned on or off, and similarly charge-transfer diabatic states (which are occassionally invoked in discussions of the singlet fission mechanism) can be included or not. Redfield dynamics simulations with this model predict rapid population of the "multi-exciton" state, in which triplet excitations on neighboring monomers are spin-coupled to an overall singlet state, when just one of the aforementioned vibrational modes is included. We conclude that the endothermic nature of singlet fission in tetracene is illusory, and that the process is in fact spontaneous when viewed in a vibronic basis.

Vibronic coherence in singlet fission: Results for crystalline tetracene from an ab initio exciton model

256th National Meeting (2018)

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