On direct photoexcitation, subpicosecond time-resolved absorption spectroscopy revealed that the 1Bu-type singlet excited state of all-trans-lycopene in chloroform was about seven times more efficient than all-trans-β-carotene in generating the radical cation. The time constant of radical cation generation from the 1Bu-type state was found to be ∼0.14 ps, a value that was comparable for the two carotenoids. On anthracene-sensitized triplet excitation, radical cation generation was found to be much less efficient for lycopene than for β-carotene. A slow rising phase (20–30 μs) in the bleaching of ground-state absorption was common for both lycopene and β-carotene in chloroform and was ascribed to an efficient secondary reaction with a solvent radical leading to the formation of carotenoid radical cations. The reverse ordering in the tendency of the excited states of different multiplicities for the two carotenoids to generate radical cations is discussed in relation to the two carotenoids as scavengers of free radicals.
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Vol. 80 • No. 2