Unraveling the Geometrical Effects on Singlet Fission of Carotenoids: A Model Perspective

Singlet fission (SF) is a phenomenon that generates multiple excitons (triplets) on different chromophores from a single exciton (singlet) on one chromophore. Owing to the strong electronic correlation and a complicated excited state manifold of carotenoids (polyenes), the SF mechanism in carotenoids is different from acenes shown in J. Phys. Chem. Lett., 2022, 13, 6800–6805. However, the mechanism is expected to have significant effects of the geometry in the excited state and strong vibronic couplings between these low-lying excited states. Employing high-level state-of-the-art electronic structure methods, we show that the dark A_g states and charge transfer components play a major role in the SF process. The success of the process is strongly dependent on the relative orientation of the monomers. We have also shown that the high-frequency modes involving changes in bond length alternation are strongly coupled to the excited electronic states. These nuclear vibrational modes facilitate the SF process.