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 have 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 2Ag state below the optically bright 1Bu state. In this work, we show that the dark Ag state in polyenes / carotenoids, along with the charge transfer states, play a pivotal role in the SF process. We notice that the relative importance of these states vary with change in geometry and the overall presence of multiple pathways is crucial to the success of SF process in carotenoid aggregates and disordered geometries.