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

Non-covalent cofacial heterodimers and -trimers between meso-tetraphenyl-oc tacarboxylate-porphyrin and beta -tetracationic porphyrins have been prepar ed in bulk water. They are held together by Coulomb interactions between fo ur or eight beta -methylpyridinium and meso-phenylcarboxylate ion pairs. Th e observation of the UV-vis absorption titration indicated quantitative tri merization at concentrations >10(-6) M. The equilibrium constants in water were 2.3 x 10(6) M-1 for the dimerization and 1.7 x 10(7) M-1 for the conve rsion of the dimer to the trimer in water. Fluorescence of free base or zin c porphyrins was strongly diminished on heterodimerization and -trimerizati on by redox quenching. In the case of Mn(III) heterotrimer, electrochemistr y and spectroelectrochemistry showed that all three Mn(III) ions were oxidi zed simultaneously to Mn(IV) at a potential close to 0.20 V versus Ag \ AgC l 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 tr imers, namely Mn(II)P-Mn(III)P-Mn(II)P. The reduction of the central mangan ese 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. (C) 2001 Elsevier Science B.V. All rights reserved.