Furan-based porphyrins:: Tetraoxa[4n+2]porphyrin dications with 18π-, 22π-, or 26π-electron systems

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 D(4h)-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 C(2h) symmetry, shows a split Soret band with greatly reduced molecular extinction.