A reference example for the significance of molecular symmetry in UV/Vis spectroscopy is shown by the spectra of the three planar octaethyltetraoxa[4n+2]porphyrin dications 1 – 3 (n=4 – 6). Whereas the D4h‐symmetric 18π and 26π dications 1 and 3 (the symmetry refers to the ring framework) exhibit very sharp Soret bands with record values for the molar extinction coefficients ε, the 22π dication 2, which only possesses C2h symmetry, shows a split Soret band with greatly reduced molecular extinction.

geometry of the copper complex (Fig. 3a).ii3f Moreover. the EPR spectrum is indicative of a type-I1 copper binding site in a similar ligdnd environment with g , , = 2.27 and A , , = 173 (cm-x lo4). Peisach and Blumberg analysis[141 is

Record extinctions ε of 1.6×106 and 1.0×106 for the UV/Vis Soret bands, as well as extreme chemical shifts for the inner and outer perimeter protons in the 1H NMR spectra, characterize the two closely related 26π porphyrinoids 1 and 2, respectively. The dichotomy that 1 and 2 differ markedly in the extinctions of the Soret bands (appearing at nearly the same wavelength) yet almost agree—within the limits of comparison—in the 1H NMR spectra is explained convincingly on the basis of the molecular structures of the two compounds.

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