Quenching of Singlet Oxygen by Carotenoids via Ultrafast Superexchange Dynamics

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