Investigation of ultrafast intermediate states during singlet fission in lycopene H-aggregate using femtosecond stimulated Raman spectroscopy Available to Purchase

The singlet fission process involves the conversion of one singlet excited state into two triplet states, which has significant potential for enhancing the energy utilization efficiency of solar cells. Carotenoid, a typical π conjugated chromophore, exhibits specific aggregate morphologies known to display singlet fission behavior. In this study, we investigate the singlet fission process in lycopene H-aggregates using femtosecond stimulated Raman spectroscopy aided by quantum chemical calculation. The experimental results reveal two reaction pathways that effectively relax the S₂ (1¹B_u⁺) state populations in lycopene H-aggregates: a monomer-like singlet excited state relaxation pathway through S₂ (1¹B_u⁺) → 1¹B_u⁻ → S₁ (2¹A_g⁻) and a dominant sequential singlet fission reaction pathway involving the S₂ (1¹B_u⁺) state, followed by S* state, a triplet pair state [¹(TT)], eventually leading to a long lifetime triplet state T₁. Importantly, the presence of both anionic and cationic fingerprint Raman peaks in the S* state is indicative of a substantial charge-transfer character.