Quenching of Singlet Oxygen by Carotenoids via Ultrafast Superexchange Dynamics.

We analyze the quenching mechanism of singlet molecular oxygen (O) by carotenoids, namely lycopene, β-carotene, astaxanthin, and lutein, by means of quantum dynamics calculations and ab initio calculations. The singlet carotenoid (Car) and O molecules can form a weakly bound complex via donation of electron density from the highest occupied molecular orbital (HOMO) of the carotenoid to the π* orbitals of O. The Dexter-type superexchange via charge transfer states (Car/O) governs the O quenching. The Car/O states are substantially higher in energy (2-4 eV) than the initial Car/O states. The quantum dynamics calculations indicate an ultrafast O quenching on a timescale of subpicosecond owing to the strong electronic couplings in the carotenoid/O complexes. The superexchange mechanism via the Car/O states dominates the O quenching, although the direct two-electron coupling can also play a certain role.