Syntheses of covalently‐linked porphyrin‐quinone complexes

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.