Polar Effect on the Stacking Interaction between a Zinc Complex of an Anionic Porphyrin and Anionic Aromatics

Although 2-anthracenesulfonate did not interact with a zinc complex of 5,10,15,20-tetrakis(p-sulfonatophenyl)porphyrin (ZnTPPS), anthraquinone-2-sulfonate (AQS) and -2,6-disulfonate (AQDS) formed stable molecular complexes with ZnTPPS in aqueous solution. Such results suggest the polar effect on the stacking interaction between aromatic compounds. Molecular complexation of ZnTPPS was further studied using mono-substituted benzoate anions as the partners by means of 1H NMR spectroscopy. The electron-withdrawing groups of the benzoates stabilized the complexes, while the electron-donating groups restrained the complexation. These results suggest that a reduction of the electrostatic repulsion by electron-withdrawing group(s) is important to form stable molecular complexes of ZnTPPS and anionic aromatics via the London dispersion force. A similar mechanism could be applied to the formation of the AQS dimer, which takes a slipped edge-to-edge arrangement.