Scalable Synthesis and Supramolecular Assembly of trans-A₂B₂ Porphyrins with Pendant Carboxylic Functional Groups

This article deals with facile synthesis of A₂B₂ porphyrins bearing carboxylate groups at 5,15- positions and different functional groups at 10,20- positions viz. 4-iodophenyl (a), 4-tert-butylphenyl (b), 3,5-dimethoxyphenyl (c) 3-carboxyphenyl (d), 2-thienyl (e) 4-nitrophenyl (f), 4-methoxy-2,3,5,6-tetrafluorophenyl (g) and 2-pyrenyl (h) as single major component in scalable yields. It relates also to the supramolecular assembly modes of these species. The use of stoichiometric amounts of trifluoroacetic acid in condensation of 5-(4-methoxycarbonylphenyl)dipyrromethane and different aldehydes led to the formation of trans-porphyrins as di-esters (1 a-1 h). These were transformed by hydrolysis to the corresponding di-acids (2 a-2 h). (1 a-1 h) and (2 a-2 h) were then core-metalated to afford the corresponding metalloporphyrin species (3 a-3 h) and (4 a-4 h). The absorption and emission spectra of the free base (2 a-2 h) and metallated porphyrins (4 a-4 h) were found greatly influenced by the substituents at 10,20-positions. SEM studies of solids deposited on surfaces from solution exhibited hydrogen-bonded block-shaped structures of 2 e and fibrous sheets of 2 f. Co-crystallization of 2 a and 2 g with 4,4′-bipyridyl yielded hetero-molecular structures stabilized by both hydrogen O−H⋅⋅⋅N, and halogen N⋅⋅⋅I bonds in 2 a, and by O−H⋅⋅⋅N hydrogen bonds and C−F⋅⋅⋅π interactions in 2 g.