Synthesis of structure-defined conjugated polymers directly bound to porphyrins

5-(3-Thienyl)-10,15,20-triethyl-21H,23H-porphine was prepared by the ring closure of pyrrole and a 3:1 mixture of propionaldehyde and 3- thiophenecarbaldehyde. Oxidative polymerization of the porphyrin with typical oxidants such as FeCl₃ gave a polythiophene derivative bearing porphyrins directly bound to the β-position of the main chain. The very bulky β-substituents allowed the polymerization to proceed regioselectively even at room temperature, resulting in the formation of the β-substituted polythiophene with an HT-HT content of more than 90%. The porphyrin substituents aligned cofacially along the main chain, which served as the catalytic site for the four-electron reduction of O₂ to H₂O (O₂ + 4e^- + 4H⁺ = 2H₂O) when cobalt ions were introduced into the porphyrins. A novel platinum-free catalyst for the reduction of O₂ for fuel cell cathodes, 1/CB (M = Co^II), was thus prepared by the electropolymerization of the porphyrin at the surface of carbon black (CB), which reduced O₂ with four electrons at more poritive potentials than the standard two-electron reduction potential of O₂ to H₂O₂.