| Abstract
| - This study focuses on the mechanisms and pathways of energy transfer in two carotenoid-pyropheophorbidedyads serving as an artificial light-harvesting antenna. The dyads contain carbonyl carotenoids peridinin (dyad1) and fucoxanthin (dyad 2). Studies of these carotenoids in solution showed a pronounced dependence ofthe excited-state lifetime on solvent polarity. This dependence was attributed to the presence of a state withintramolecular charge transfer (ICT) character in the excited-state manifold. Here we measured carotenoid-pyropheophorbide energy transfer in solvents with different polarity. Energy transfer occurs on a time scaleof 31−44 ps for dyad 1, but it is nearly an order of magnitude slower for dyad 2 (195−280 ps). Energytransfer efficiency varies with solvent polarity, reaching 80% in benzene, 69% in tetrahydrofuran, and 22%in acetonitrile for dyad 1 and 27% in benzene, 19% in tetrahydrofuran, and 13% in acetonitrile for dyad 2.The factors controlling this polarity dependence are (1) the competition of energy transfer rate with the S1/ICTlifetime, which, for carbonyl carotenoids, is significantly shorter in polar solvents, (2) the mutual orientationof the carotenoid and pyropheophorbide moieties, and (3) enhancement of the S1/ICT dipole moment by increasingthe ICT character of the S1/ICT state in polar solvents. The possibility of tuning energy transfer through solventpolarity in combination with another spectroscopic feature of carbonyl carotenoids, efficient absorption oflight in the spectral region close to the maximum of the solar irradiance curve (450−550 nm), makes thesedyads attractive for potential application as artificial antenna.
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