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 <i>J. Phys. Chem. Lett.</i>, <b>2022</b>, <i>13</i>, 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 <i>A</i>_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.