Acidification and assembly of porphyrin at an interface: counterion matching, selectivity, and supramolecular chirality.

The interfacial diprotonation and assemblies of a free-base achiral porphyrin, 5,10,15,20-tetrakis(3,5-dimethoxyphenyl)-21H,23H-porphine, on various acidic subphases were investigated. It has been shown that the compound could be diprotonated in situ on an acidic subphase and can form assemblies. The interfacially organized supramolecular assemblies were transferred onto a solid substrate, and the assemblies showed supramolecular chirality. Interestingly, the supramolecular chirality of the assemblies of the diprotonated species showed a counterion-dependent behavior. For the assemblies fabricated from the aqueous HCl subphases, a strong Cotton effect (CE) could be observed, although the porphyrin itself is achiral. When an aqueous HBr solution was used as the subphase, the assemblies showed a weak CE, whereas no CE could be detected for the assemblies formulated from the HNO3 or HI subphase. Interestingly, when a mixture of HBr and NaCl, or HNO3 and NaCl, was employed as the subphase, the formed assemblies displayed chiral features similar to those fabricated on the HCl subphase, suggesting that the Cl(-) could be preferentially visualized in terms of supramolecular chirality, although the system itself is composed of achiral species. On the basis of the experimental facts and a theoretical calculation, an explanation with regard to the different sizes of the counterions and the distinct binding affinities of the counteranions to the diprotonated porphyrin species has been proposed. Our findings provide new insights into the assembly of the diprotonated porphyrins as well as the interfacially occurring symmetry breaking.