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Porphyrin substituent regiochemistry, conformation and packing - the case of 5,10-diphenylporphyrin
(2015)
5,10-Disubstituted porphyrins are more recent additions to the family of meso-substituted porphyrins. A crystallographic comparison of 5,10-diphenylporphyrin with the regioisomeric 5,15-disubstituted system reveals striking differences in their conformation. In the free base porphyrins the former uses mainly out-of-plane distortion to alleviate steric strain while in-plane core elongation predominates in the latter. In contrast, the structure of the Cu(II) complex is planar and forms strong p-p aggregates with very small lateral shifts. Macroscopically, the packing is similar to that of porphyrin sponges of the 5,10,15,20-tetraphenylporphyrin type.
Current applications in porphyrin chemistry require the use of unsymmetrically substituted porphyrins. Many current industrial interests in optics and biomedicine require systems with either pushpull (electron-donating and -withdrawing groups) or amphiphilic systems (hydrophobic and hydrophilic groups). In this context we present the class of 5,10-A2B2-type porphyrins for which two different substituents are positioned in diagonally opposite meso positions. Thus, the intramolecular dipole moment in these tetrapyrroles is positioned along a beta-beta vector passing through two pyrrole rings. This is opposite to the situation of the frequently used 5,15-A2BC porphyrins for which the dipole moment is oriented along a mesomeso axis. We have elaborated syntheses of the 5,10-A2B2 porphyrins by using transition-metal-catalyzed transformations of 5,10-A2 porphyrins or direct substitutions reactions thereof; this gives the target molecules in 2277% overall yields. The compounds exhibit interesting structural, spectroscopic, and optical features and can serve as building blocks for new porphyrin arrays and applications.