Energy transfer and spin polarization of the carotenoid triplet state in synthetic carotenoporphyrin dyads and in natural antenna complexes

A series of carotenoporphyrin dyads, in which the carotenoid is covalently linked to a tetraarylporphyrin at the ortho, meta or para position of one of the meso aromatic rings, has been studied using Time-Resolved Electron Paramagnetic Resonance (TREPR) spectroscopy. In parallel, an investigation has been carried, on two different photosynthetic antenna systems, the B800–B850 complex ofR. acidophila and the LHCII complex of higher organisms. The initial spin polarization of the carotenoid triplet-state, populated indirectly by laser excitation, has been detected. It has been demonstrated that the initial polarization is not a characteristic property of the carotenoid triplet-state, as previously stated, but depends on the donor-acceptor mutual orientation. The triplet energy transfer to the carotenoid from a chlorophyll or porphyrin triplet state is discussed on the basis of the observed spin polarization.