Sammendrag
This study focuses on mechanisms and pathways of energy transfer in two carotenoid-pyropheophorbide dyads serving as an artificial light-harvesting antenna. The dyads contain carbonyl carotenoids peridinin (dyad 1) and fucoxanthin (dyad 2). In vitro studies of these carotenoids showed a pronounced dependence of the excited state lifetime on solvent polarity. This dependence was attributed to the presence of a state with charge transfer (CT) character in the excited state manifold. Here we measured carotenoid-pyropheophorbide energy transfer in solvents with different polarity. In both dyads, energy transfer efficiency varies with solvent polarity, reaching 80% in benzene, 69% in tetrahydrofuran and 22% in acetonitrile (dyad 1), and 27% in benzene, 19% in tetrahydrofuran and 13% in acetonitrile (dyad 2). The factors controlling this polarity dependence are 1) the competition of energy transfer rate with the S_{1/ICT} lifetime that is for carbonyl carotenoids significantly shorter in polar solvents; 2) the mutual orientation of the carotenoid and pyropheophorbide moieties, and 3) enhancement of the S_{1/ICT} dipole moment by increasing the ICT character of the S_{1/ICT} state in polar solvents. The possibility of tuning energy transfer through solvent polarity in combination with another spectroscopic feature of carbonyl carotenoids, efficient absorption of light in the spectral region close to the maximum of the solar irradiance curve (450-550 nm), makes these dyads attractive for potential application as artificial antenna.
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