Electron-transfer mechanism in metalloporphyrin-quinone cyclophanes
Preparation and Characterization of Zinc Porphyrin-Quinone CyclophanesFor zinc complexation the respective metal-free porphyrin-quinone cyclophanes were heated with excess zinc acetate dihydrate in trichloromethane/methanol or in dichloromethanelmethanol. The zinc complexes were purified by MPLC on silica gel with cyclohexane/ethyl acetate and crys-
EtEt tallized from trichloromethane/methanol (for details see Ex-
For studying intramolecular electron-transfer reactions, three groups of double- and single-bridged porphyrin-quinone cyclophanes 1–4, 5–9, and 11–14, resp., with gradual variation of quinone acceptor strengths were synthesized. As key intermediates for building up the porphyrin-cyclophane skeleton the correspondingly 3,6-substituted 1,4-bis[4-(2-formylphenyl)-butyl]-2,5-dimethoxybenzenes 15–20 were synthesized. Condensation with pyrrole yielded the doubly bridged porphyrin cyclophanes 42–45; via the bis(dipyrrolylmethyl) derivatives 46–51 and 64–67 the single-bridged porphyrin cyclophanes 58–63 and 72–75 were obtained by acidic condensation with triethyl formate. Cleavage of the methoxy groups and oxidation yielded the corresponding porphyrin-quinone systems. Some magnesium and zinc complexes of these cyclophanes are described. Spectroscopic data (MS, 1H NMR) are reported in support of the proposed structures
Previous work on intramolecular electron‐transfer compounds of the quinone‐porphyrin cyclophane type with gradually increasing electron‐acceptor strength was now extended to acceptor‐porphyrin cyclophanes with 7,7,8,8‐tetracyanoquinodimethane (TCNQ) as an especially strong electron‐acceptor. The vertically stacked TCNQ‐porphyrin cyclophane 1 was prepared in a multi‐step synthesis. Physical properties related to structure and electron‐transfer processes of 1 are reported. To determine the distance dependence of the interaction between the electron‐acceptor TCNQ and the porphyrin, first results on the synthesis of the corresponding naphthalene‐spacered TCNQ‐porphyrin cyclophane 15 are presented.
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