Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Syntheses

Abstract In 1926 Hans Fischer and Bruno Walach synthesized the first porphyrins. H. Fischer, B. Walach, Justus Liebigs Ann. Chem. 1926 , 450 , 164–181. Currently more than 1400 new articles concerning the synthesis and uses of porphyrins are published every year. CAS Online search for 1994 . However, the strong interest in these compounds indicated by this is in sharp contrast to their restricted availability. This is reflected in the current price of up to 500 DM for 5 mg of the most important porphyrins used in research and other applications (see Scheme 3). Biomimetic syntheses offer possibilities for an improved approach to porphyrins. By following the example set by nature it is also possible to obtain novel porphyrinoids which are different from naturally occurring porphyrins. This is exemplified by N,N ′‐bridged porphyrinogens, which have cage structures, inverted porphyrinoids (N atoms in the outer periphery) and, in particular, porphyrins with expanded systems. Among the family of expanded porphyrins are superarenes with up to 34 π electrons. Their pronounced aromaticity is indicated by 1 H NMR spectra, bond length equivalence, planar structures, and electrophilic substitution. With their strong absorption bands, the strongest of which have ϵ values of > 1000000, a value which exceeds the absorption intensity of all other organic pigments observed until now, and their ability to act as efficient photosensitizers, the expanded porphyrins open interesting perspectives in the fields of photochemistry and photomedicine.