Ultrafast Excited-State Dynamics of Carotenoids and the Role of the SX State

Carotenoids are natural pigments with multiple roles in photosynthesis. They act as accessory pigments by absorbing light where dilorophyll absorption is low, and they quench the excitation energy of neighboring chlorophylls under high-light conditions. The function of carotenoids depends on their polyene-like structure, which controls their excitedstate properties. After light absorption to their bright S-2 state, carotenoids rapidly decay to the optically dark S-1 state. However, ultrafast spectroscopy experiments have shown the signatures of another dark state, termed S-X. Here we shed light on the ultrafast photophysics of lutein, a xanthophyll carotenoid, by explicitly simulating its nonadiabatic excited-state dynamics in solution. Our simulations confirm the involvement of S-X in the relaxation toward S-1 and reveal that it is formed through a change in the nature of the S-2, state driven by the decrease in the bond length alternation coordinate of the carotenoid conjugated chain.