Polarity-tuned energy transfer efficiency in artificial light-harvesting antennae containing carbonyl carotenoids peridinin and fucoxanthin

This study focuses on the 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). Studies of these carotenoids in solution showed a pronounced dependence of the excited-state lifetime on solvent polarity. This dependence was attributed to the presence of a state with intramolecular 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 scale of 31-44 ps for dyad 1, but it is nearly an order of magnitude slower for dyad 2 (195-280 ps). Energy transfer 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 S-1/ICT lifetime, which, for carbonyl carotenoids, is 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.