Heterodimers and -trimers of meso-tetra-(isophthalicacid)-porphyrin octaanions with meso- and β-tetramethylpyridinium-porphyrin tetracations and their manganese complexes in water. Electrochemistry, spectroelectrochemistry and fluorescence quenching

Non-covalent cofacial heterodimers and -trimers between meso-tetraphenyl-octacarboxylate-porphyrin and β-tetracationic porphyrins have been prepared in bulk water. They are held together by Coulomb interactions between four or eight β-methylpyridinium and meso-phenylcarboxylate ion pairs. The observation of the UV–vis absorption titration indicated quantitative trimerization at concentrations >10−6 M. The equilibrium constants in water were 2.3×106 M−1 for the dimerization and 1.7×107 M−1 for the conversion of the dimer to the trimer in water. Fluorescence of free base or zinc porphyrins was strongly diminished on heterodimerization and -trimerization by redox quenching. In the case of Mn(III) heterotrimer, electrochemistry and spectroelectrochemistry showed that all three Mn(III) ions were oxidized simultaneously to Mn(IV) at a potential close to 0.20 V versus Ag ∣ AgCl at pH 12. Electroreduction of the peripheral cationic Mn(III) porphyrins was achieved at −0.20 V versus Ag ∣ AgCl and gave the first multivalent trimers, namely Mn(II)P–Mn(III)P–Mn(II)P. The reduction of the central manganese porphyrin finally occurred at −0.60 V yielding Mn(II)P–Mn(II)P–Mn(II)P trimers. Heterotrimers Mn(III)P–H2P–Mn(III)P and H2P–Mn(III)P–H2P were also studied as reference compounds for electrochemical measurements.