A synthetic route for the preparation of covalently‐linked porphyrin‐quinone and metalloporphyrin‐quinone complexes as models for the phototrap in bacterial photosynthesis is described. 5‐(4‐Carboxyphenyl)‐10,15,20‐tritolylporphyrin, prepared by a mixed aldehyde approach, was attached to a benzoquinone center with a propanediol bridge by means of ester linkages. The starting point for the benzoquinone moiety was 2,5‐dihydroxyphenylacetic acid, whose hydroquinone function was first protected by preparing its dimethyl ether. The spacing between the two centers of the complex could be altered simply by varying the length of the bridging group (a diol) employed. Boron tribromide was used to unmask the quinol derivatives in the final coupled products. The zinc(II) derivative of porphyrin‐quinone complex was prepared by addition of a saturated solution of zinc acetate in methanol to a solution of the corresponding porphyrin‐hydroquinone complex in dichloromethane at room temperature. The structures of these complexes were confirmed by nmr spectroscopy, uv‐visible absorption, and mass spectroscopy. Oxidation of the quinol moiety in the covalently‐linked complex to its corresponding quinonoid derivative was accomplished by treating a solution of the complex in dichloromethane with a stoichiometric amount of 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone, a high potential benzoquinone.

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, dimethylformamide, 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 0.1 and the lifetimes of the radical species formed were many minutes.

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Aus Hydrochinonessigsäure (Ia) entsteht nach Methylierung zu (Ib) weiter durch Umesterung ein Ester (II), der mit Porphyrin‐carbonsäurechloriden wie (IIIa) zum Derivat (IIIb) reagiert, das schrittweise an den beiden Phenolethergruppen demethyliert werden kann.