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Artificial photosynthetic antenna systems have been synthesized based on carotenoid polyenes and polymer-polyenes covalently attached to tetrapyrroles. Absorption of light in the blue/green region of the spectra excites the polyenes to their S2 state, and ultrafast singlet energy transfer to the tetrapyrroles occurs when the chromophores are in partial conjugation. The additional participation of other excited states of the polyene in the energy-transfer process is a requirement for perfect antenna function. Analogs of photosynthetic reaction centers consisting of tetrapyrrole chromophores covalently linked to electron acceptors and donors have been prepared. Excitation of these constructs results in a cascade of energy transfer/electron transfer which, in selected cases, forms a final charge-separated state characterized by a giant dipole moment (>150 D), a quantum yield approaching unity, a significant fraction of the photon energy stored as chemical potential, and a lifetime sufficient for reaction with secondary electron donors and acceptors. A new antenna-reaction center complex is described in which a carotenoid moiety is located in partial conjugation with the tetrapyrrole π-system allowing fast energy transfer (

Two approaches to eliciting photoelectrochemical reactions from carotenoids are being pursued. One uses LB films of amphipathic carotenoids deposited on semiconducting electrodes which are immersed in electrolytes containing electron donors or acceptors. Photocurrents are observed and their action spectra implicate the excited carotenoid pigment, possibly in an aggregated form, as the photoactive species. An example of the second approach is a molecular dyad consisting of a carotenoid covalently linked to Gjo. Excitation of the carotenoid moiety generates a charge-separated species in high yield. More complex systems include a molecular mad (C-P-Q) containing a porphyrin moiety (P) linked to a quinone (Q) and to a carotenoid (C). Triads of this type have been incorporated unidirectionally into lipid bilayer membranes. Time-resolved fluorescence experiments reveal that excitation of C-P-Q in the membrane leads to electron transfer from 1P to give C-P'+-Q". A subsequent electron transfer from C to P" yields C'+-P-Q'-. In this species the carotenoid pigment acts as a molecular wire to conduct charge across the membrane. In the presence of appropriate cofactors, this system translocates protons across the bilayer lipid membrane and generates proton motive force.