Fucoxanthin-Pyropheophorbide and Zeaxanthin-Pyropheophorbide Dyads as New Models for Study on Carotenoid-Chlorophyll Excited State Interactions

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Abstract
Fucoxanthin-pyropheophorbide dyads, 3c and 4c, and zeaxanthin-pyropheophorbide dyads, 3d and 4d, were prepared as the first example of natural carotenoid-linked pyropheophorbide. Singlet–singlet energy transfer from carotenoid to pyropheophorbide is more efficient in fucoxanthin-pyropheophorbide dyads than the corresponding zeaxanthin-pyropheophorbide dyads, while the singlet excited state of the pyropheophorbide is quenched more strongly in the zeaxanthin-linked molecules. Marked differences in carotenoid-pyropheophorbide singlet excited-state interactions between fucoxanthin and zeaxanthin strongly suggest their different roles in vivo; antenna function for fucoxanthin and energy-dissipation for zeaxanthin. Both carotenoids similarly quench the triplet excited state of the pyropheophorbide through triplet–triplet energy transfer with rates which depend on temperature and the linkage.

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