The ‘size matching rule’ in di-, tri-, and tetra-cationic charged porphyrin/synthetic clay complexes: effect of the inter-charge distance and the number of charged sites

Abstract

Novel inorganic/organic hybrids formed from cationic porphyrins and synthetic clay were investigated. Di-, tri- and tetra-cationic charged porphyrins with different distances between adjacent cationic sites within the molecules were incorporated into the hybrids with synthetic clay. When tetra-cationic porphyrins were used, in which distances between adjacent cation sites were 1.05 and 1.31 nm, the porphyrin molecules adsorb on the clay surfaces with high density, neutralizing all of the negative charges of the clay surface, i.e. 100% adsorption vs. the cation exchange capacity (CEC), without aggregation, even though it is generally difficult to control or suppress the aggregation of organic molecules on inorganic surfaces. The mean center-to-center distance between porphyrin molecules in the hybrids was estimated to be 2.4 nm. When Δ l (the difference between the inter-cationic charge distance in the porphyrin molecule and the inter-anionic charge distance on the clay surface) was larger than ca. 0.2 nm, the porphyrin molecules adsorbed in amounts smaller than 100% vs. CEC. In the cases of di- and tri-cationic charged porphyrins, similar adsorption behavior was observed. The formation of these unique hybrids was rationalized by a size-matching of distances between the charged sites in the porphyrin molecule and on the clay surface (‘size-matching rule’).