Quenching of Singlet Oxygen by Carotenoids via Ultrafast Superexchange Dynamics.

We analyze the quenching mechanism of singlet molecular oxygen (1O2) by carotenoids, namely lycopene, β-carotene, astaxanthin, and lutein, by means of quantum dynamics calculations and ab initio calculations. The singlet carotenoid (1Car) and 1O2 molecules can form a weakly bound complex via donation of electron density from the highest occupied molecular orbital (HOMO) of the carotenoid to the πg* orbitals of 1O2. The Dexter-type superexchange via charge transfer states (Car•+/O2•-) governs the 1O2 quenching. The Car•+/O2•- states are substantially higher in energy (2-4 eV) than the initial 1Car/1O2 states. The quantum dynamics calculations indicate an ultrafast 1O2 quenching on a timescale of subpicosecond owing to the strong electronic couplings in the carotenoid/O2 complexes. The superexchange mechanism via the Car•+/O2•- states dominates the 1O2 quenching, although the direct two-electron coupling can also play a certain role.