Novel porphyrinoids, 15. Syntheses of novel expanded [26]porphyrins with conformational control by the “helical effect”

Novel [26]porphyrinogens[2]24–28 were conveniently prepared in few steps from the pyrrole building blocks 19–23. The selectivity of the cyclization is explained by a conformational helical effect due to steric congestion of the pyrrole β-substituents. The aromaticity of the [26]porphyrins[2]30–33, obtained by dehydrogenation of the porphyrinogens, is evident from the 1H-NMR spectra displaying 25-ppm shift differences for the inner and outer protons of the porphyrins. These pigments belong to the most intensive chromophores known so far, showing VIS absorptions at 544–550 nm with lg ϵ values up to 5.9.

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