Mechanism of Radical Cation Formation from the Excited States of Zeaxanthin and Astaxanthin in Chloroform

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ABSTRACT

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 p 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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