Mechanism of Singlet Fission in Carotenoids from a Polyene Model System

Singlet fission (SF) is the process of formation of multiple excitons (triplet) from a locally excited singlet state. The mechanism of SF in polyacenes has been shown to proceed via a charge transfer intermediate state. However, carotenoids are not understood in the context of SF. This is possibly due to the complicated multireference nature of the low-lying excited states of carotenoids and the presence of a dark 2¹A_g state below the optically bright ¹B_u state. In this work, we show that the dark A_g state in polyenes and/or carotenoids, along with the charge transfer states, plays a pivotal role in the SF process. We notice that the relative importance of these states varies with a change in geometry and the overall presence of multiple pathways is crucial to the success of the SF process in carotenoid aggregates and disordered geometries.