The C-40 xanthophylls zeaxanthin and astaxanthin were confirmed to form radical cations, Car· , in the electron-accepting solvent chloroform by direct excitation using subpicosecond time-resolved absorption spectroscopy in combination with spectroelectrochemical determination of the near-infrared absorption of Car· . For the singlets, the S2(1Bu ) state and most likely the Sx(3Ag−) state directly eject electrons to chloroform leading to the rapid formation of Car· on a timescale of ∼100 fs; the lowest-lying S1(2Ag−) state, however, remains inactive. Standard reduction potential for Car· was determined by cyclic voltametry to have the value 0.63 V for zeaxanthin and 0.75 V for astaxanthin from which excited state potentials were calculated, which confirmed the reactivity toward radical cation formation. On the other hand, Car· formation from the lowest triplet excited state T1 populated through anthracene sensitization is mediated by a precursor suggested to be a solute–solvent complex detected with broad near-infrared absorption to the shorter wavelength side of the characteristic Car· absorption. However, ground state carotenoids are able to react with a secondary solvent radical to yield Car· , a process occurring within 16 μs for zeaxanthin and within 21 μs for astaxanthin. Among the two xanthophylls together with lycopene and β-carotene, all having 11 conjugated double bonds, zeaxanthin ranks with the highest reactivity in forming Car· from either the S2(1Bu ) or the ground state. The effects of substituent groups on the reactivity are discussed.
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Vol. 82 • No. 2