Synthesis and Spectroscopic Characterization of Octaacetic Acid Tetraphenylporphyrins

Members of a new class of water-soluble, nonplanar porphyrins based on 5,10,15,20-tetraphenylporpyrin-2,3,7,8,-12,13,17,18-octaacetic acid (OAATPPs) have been synthesized, and their physical and chemical properties as well as their interactions with complexing agents have been investigated using UV-visible absorption, EXAFS, nuclear magnetic resonance and resonance Raman spectroscopies. Red shifts of 19-36 nm in the major absorption bands are noted for Ni derivatives of OAATPP and its octamethyl ester (OME), meso-tetrakis(p-nitrophenyl)-porphyrinoctaacetic acid octamethyl ester, and octakis(hydroxyethyl)tetraphenylporphyrin relative to nickel tetraphenylporphyrin. These red shifts are indicative of a high degree of nonplanarity. The short Ni–N distances of 1.92(2) Å for NiOAATPP-OME and 1.89(2) Å for NiOAATNPP-OME from EXAFS experiments corroborate the nonplanar nature of these porphyrins. The decreased ring current effect and temperature-dependent NMR measurements also confirm that the porphyrins are nonplanar in solution. Resonance Raman studies also indicate nonplanar conformations for NiOAATPP and its derivatives by the characteristic downshifts in the frequencies of the structure-sensitive marker lines. At low pH (<3) or in 4.5 M NaCl, conditions under which planar octaacid porphyrins like nickel uroporphyrin form π-π aggregates that give small characteristic upshifts in the structuresensitive Raman lines, NiOAATPP shows no significant changes in either the absorption or the Raman spectra. The π-π aggregates formed by planar Ni uroporphyrin do not form for NiOAATPP most likely because of the steric difficulty in stacking the highly nonplanar and sterically hindered OAATPPs. Finally, the affinity for axially coordinated nitrogenous bases is decreased by nonplanarity and increased by electron-withdrawing peripheral substituents.