A hallmark of supramolecular chemistry is the use of well-defined molecules or macromolecules and intermolecular forces to create larger, more complex chemical systems with new and unique properties. This noncovalent synthetic strategy affords a variety of structures ranging from host-guest complexes and lipid organizations to linear metallopolymers through the use of, for example, hydrogen bonding, van der Waals forces, or metal-ligand bonds. Of the various supramolecular assemblies known, networks are particularly interesting since their macroscopic properties can be significantly different than the properties of the individual building blocks. The creation of a supramolecular network requires two different molecular structures whereby one structure possesses at least two and the other three or more complementary molecular recognition groups. Such a strategy has been applied to prepare supramolecular polymer networks using hydrogen and metal-ligand bonding. We have recently reported the discovery of "supramolecular ionic networks" created using multi-cationic and multi-anionic molecules.
Specifically, we chose to prepare an ionic network using a phosphonium dication, P2+, and a tetraanion, ethylenediaminetetraacetate (EDTA4-) and the idealized network structure is shown in the figure. Ionic liquid P2+:2Cl- possess a viscosity of 1000 Pa•s . The substitution of the mono anion, Cl-, for the tetraanion, EDTA4- while maintaining the charge ratio (1:1) with P2+, affords a significant increase in the viscosity to approximately 12,000 Pa•s. The enhanced viscosity is a consequence of the network formed between the dication and tetraanion. To confirm this ionic network formation, several control experiments were performed. Compositions P2+:DDA2-, P+:Cl-, 2P+:DDA2-, 4P+:EDTA4-, and P+:EE all possess significantly less viscosities.
Importantly, the connectivity that is present in an ionic network enables the preparation of macroscopic materials with specific sizes and shapes. To illustrate this feature, we prepared ionic materials from geminal phosphonium dication, P2+, with either para-tetracarboxy-5,10,15,20-tetraphenyl-21H,23H-porphine, H2TPP4-, or as a control, 5,10,15,20-tetraphenyl-21H,23H-porphine, H2TPP (Figure A). The former is expected to form an ionic network as it possesses four carboxylate residues available for ionic bonding. Ionic liquid 2P2+:H2TPP4- has a viscosity of ≈ 106 Pa•s at 25 oC. The mixture of 2P2+ and H2TPP4- was heated to 160 oC and a fiber was hand-pulled from the solution. The fiber was dark purple in color, flexible, moldable into a coil, and was approximately 1 mm in diameter and 10 cm in length (Figure B). Under the same conditions, if P2+ is mixed with H2TPP, which lacks the anionic carboxylate groups for ionic bonding, a low viscous P2+:2Cl- ionic liquid with dissolved H2TPP is obtained (≈1000 Pa•s; Figure C).