Cobalt ortho- and para-substituted tetraphenylporphyrins inserted in SiO2 gels

The sol–gel method, generally, allows the trapping of important organic and biological systems in inorganic networks. Free and metallic porphyrins feature a wide range of important electrical, optical, and catalytic properties. To fix them in gels it is necessary to substitute an appropriate reactive group in the periphery of the macrocycle, which thus becomes more soluble in water or alcohols. The substituents more frequently employed have been –SO3−, –Na+, –COOH and pyridinium salts. We have tried to increase the number, type, and spatial position of such substituents, to evaluate their influence in the quality and properties of the final materials. In this work we report the results obtained with the Co(II) complexes of the meso-5,10,15,20-tetraphenylporphyrin macrocycle, in which the amino, –NH2(basic) or hydroxyl, –OH (acid) groups have been placed in the ortho- and para-positions of the phenyl groups. The insertion of cobalt porphyrins was made possible by the addition of small quantities of pyridine, methanol or dimethylformamide (DMF). Maxima in the transparency and strength of the materials were obtained with the para-amino substituted porphyrin, when methanol and pyridine were employed as the solvent media. Our results reveals that methanol has no influence on the morphology of the silica network, but DMF changes it drastically. The procedure here described has been successfully extended to trap other free and metallic porphyrinic systems.