Solution structure of porphyrin aggregates determined by ¹H NMR ring current shifts. II. Conformations of dimers and higher aggregates of water‐soluble porphyrins

Abstract

Abstract The aggregation of the water‐soluble porphyrins tetra(4‐carboxyphenyl)porphyrin free base (H 2 TPPC), tetra(4‐ N ‐methylpyridyl)porphyrin free base (H 2 TMPyP) and their zinc derivatives (ZnTPPC and ZnTMPyP) was studied by 1 H NMR and optical absorption spectroscopy. In the solvent H 2 OCH 3 OH (4:1), mixing of the anionic H 2 TPPC or ZnTPPC with the cationic H 2 TMPyP or ZnTMPyP yields various types of “hetero” aggregates. Only H 2 TPPC forms “homo” aggregates in this solvent. Addition of 1 m KCl results in an increase in the aggregation of H 2 TPPC and also enhances the aggregation of ZnTPPC. The structures of the “hetero” aggregates are stacks of the oppositely charged porphyrins in an alternating order. However, when one of the constituents is H 2 TPPC, aggregates having structures of the “homo” type are also formed. The 1 H NMR ring current shifts are accounted for by identical conformations of dimers, and dimer structures in trimers and higher aggregates of the “hetero” type.