Singlet fission, the molecular process in which a singlet excitation splits into two triplet excitons, not only holds great technological promise but also challenges our fundamental understanding of solids. It results from a complex interplay between electronic and vibrational degrees of freedom, yet most previous theoretical studies have focused on model systems with a compromised description for either the vibrational or electronic coordinates. As a result, present insights in singlet fission remain to be confirmed under more realistic conditions for the electronic-vibrational interaction. In this talk, I will present our recent efforts to address this issue by formulating a vibronic exciton model wherein a microscopic electronic description is combined with a nonperturbative quantum treatment of selected vibrational modes, using bulk pentacene as a prototypical fission material. The model is validated by holding simulations of linear and nonlinear spectroscopy to comparison with experiments. The mechanistic scenario of singlet fission that emerges from our calculations is one in which vibrational modes generate energetic resonances between singlet and triplet excitons, whereupon a thermodynamic drive promotes the fission process at ultrafast time scales.

Vibronic exciton modeling reveals how vibronic resonance and thermodynamics concertedly drive rapid singlet fission

256th National Meeting (2018)

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