Synthesis and Characterization of Electropolymerized Nanostructured Aminophenylporphyrin Films

Porphyrins substituted at meso positions with aminophenyl groups undergo oxidative electropolymerization in a process analogous to the formation of polyaniline. Porphyrins that successfully generate polymer films on the electrode include those tetrasubstituted with four p-aminophenyl groups, trisubstituted, or transdisubstituted, but not cis-disubstituted or monosubstituted. The polymerization process is monitored independently by cyclic voltammetry, absorption spectroscopy, and an electrochemical quartz crystal microbalance. The mechanism is considered analogous to aniline polymerization, except that attack of electrophilic nitrogens must occur at ortho positions of another aminophenyl group. Reflectance FT-IR and resonance Raman spectroscopy detect the presence of diphenylamine, dihydrophenazine, and phenazine linkages in the porphyrin polymer film from tetra(4-aminophenyl)porphyrin (TAPP). TAPP polymerized in dichloromethane (DCM) with added pyridine gradually passivates; i.e., electronic conductivity diminishes and polymer growth levels off (the films are light yellow). Without added pyridine, electronic conductivity is sustained and film growth continues to a thick black film. Diphenylamine and dihydrophenazine linkages were common in films whose electroactivity remained constant throughout the film growth process while phenazine linkages were prevalent in films where passivation and loss of electrochemical activity had occurred. It is proposed that overoxidation to the phenazine structures leads to loss of electronic conductivity, analogous to formation of pernigraniline in polyaniline. The morphology of poly-TAPP is a highly interconnected nanofibrous network, with fiber diameters in the range 40-100 nm, with somewhat different structures depending on polymerization conditions.