SYNTHESES OF NOVEL EXPANDED [26]PORPHYRINS WITH CONFORMATIONAL CONTROL BY THE HELICAL EFFECT

Novel [26]porphyrinogens[2] 24-28 were conveniently prepared in few st eps from the pyrrole building blocks 19-23. The selectivity of the cyc lization is explained by a conformational helical effect due to steric congestion of the pyrrole beta-substituents. The aromaticity of the [ 26]porphyrins[2] 30-33, obtained by dehydrogenation of the porphyrinog ens, is evident from the H-1-NMR spectra displaying 25-ppm shift diffe rences for the inner and outer protons of the porphyrins. These pigmen ts belong to the most intensive chromophores known so far, showing VIS absorptions at 544-550 nm with 1g epsilon values up to 5.9.