The key steps in the photosynthetic conversion of light to chemical potential energy include not only photodriven charge separation, but also prevention of the back-reaction (charge recombination). Although the first of these steps has been achieved in several biomimetic solar energy conversion systems, retarding the back-reaction has proved more difficult. This may be accomplished by rapidly moving the electron, the hole, or both away from the site of excitation to more stabilizing environments. In photosynthetic membranes, the electron is transferred sequentially over several closely coupled molecules, including tetrapyrroles and quinones1–3. In semiconductor/liquid interfacial systems both the electron and the hole migrate following excitation4,5. We now report that substantial slowing of the back-reaction has been achieved with a tripartite molecule in which a long-lived photodriven charge-separated state of relatively high potential is formed from an excited singlet state in accordance with the above principles. This molecular triad (compound I) consists of a tetraarylporphyrin covalently linked to both a carotenoid and a quinone. In solution, excitation of the porphyrin moiety by visible light results in the rapid (<100 ps) formation of a transient species C+.−P−Q−−., with a lifetime on the μs time scale and an energy more than 1 eV above the ground state.

Photodriven charge separation in a carotenoporphyrin–quinone triad

This review highlights recent studies of synthetic, covalently linked multicomponent molecular devices which mimic aspects of photosynthetic electron transfer. After an introduction to the topic, some of the salient features of natural bacterial photosynthetic reaction centers are described. Elementary electron transfer theory is briefly discussed in order to provide a framework for the discussion which follows. Early work with covalently linked photosynthetic models is then mentioned, with references to recent reviews. The bulk of the discussion concerns current progress with various triad (three-part) molecules. Finally, some even more complex multicomponent molecules are examined. The discussion will endeavor to point out aspects of photoinitiated electron transfer which are unique to the multicomponent species, and some of the considerations important to the design, synthesis and photochemical study of such molecules.

Photosynthetic model systems

Model reactions in photosynthesis

Lead structures for applications in photodynamic therapy. Part 1: Synthesis and variation of m-THPC (Temoporfin) related amphiphilic A2BC-type porphyrins

Quantum mechanical effects in inorganic and bioinorganic electron transfer

Syntheses of covalently‐linked porphyrin‐quinone complexes

— The fluorescence properties of a covalently-linked porphyrin-quinone complex and its zinc derivative were studied in a variety of organic solvents The kinetics of fluorescence decay for both the quinone and hydroquinone oxidation states were measured in acetonitrile, dichloromethane, dimethyl-formamide, and pentane The fluorescence yield and kinetics of decay at room temperature were little affected in the porphyrin or zinc porphyrin complexes when the attached quinone was reduced However, for these complexes the fluorescence yield and lifetimes were both substantially decreased in acetonitrile and dichloromethane when the quinone was in its oxidized state These latter decay kinetics were not explainable by a process having a single exponential decay On the other hand, little fluorescence quenching or lifetime shortening was observed in dimethylformamide or pentane, indicating unique solvent dependencies for the quenching process Evidence was obtained for photoproduced charge separation from EPR measurements on the covalently-linked zinc porphyrin-quinone complex The EPR data showed equivalent concentrations of a Zn porphyrin cation radical and a benzoquinone anion radical in acetonitrile or dichloromethane at both room temperature and 77 K The charge separated state rapidly decayed at room temperature (in sub-millisecond times) but was quite stable at 77 K It is concluded that light-induced charge separation in acetonitrile and dichloromethane at room temperature may occur from the excited singlet state with a high quantum efficiency A photoproduced charge separated state also occurred when the covalently-linked complexes were incorporated into egg yolk phosphatidylcholine liposomes The quantum yield for radical formation in this latter system was 01 and the lifetimes of the radical species formed were many minutes

Photochemical characterization of covalently‐linked porphyrin‐quinone complexes*

Covalently-linked porphyrin quinone complexes as rc models

Model Systems for the Primary Photochemical Events of Photosynthesis and Electron Transfer in Bioenergetic Membranes

The information explosion has made it impossible for biologists to keep abreast of research in specialties other than their own. To bring a sampling of the best of basic research in biology, BioScience publishes these expanded abstracts of research reports, deemed especially worthy of wide dissemination by the editors of AIBS member society journals, soon after their publication. BioScience intends to stimulate interest in research by providing the essence of recent findings; therefore, these condensations are not citable. Researchers wishing to cite these papers in their own work should refer to the original report in the primary journal.

Josephine L. Y. Kong

Papers

Syntheses of covalently‐linked porphyrin‐quinone complexes

Photochemical characterization of covalently‐linked porphyrin‐quinone complexes*

Covalently-linked porphyrin quinone complexes as rc models

Model Systems for the Primary Photochemical Events of Photosynthesis and Electron Transfer in Bioenergetic Membranes

Covalently Linked Porphyrin-Quinone Complexes