Toward Anion-π Interactions Directed Self-Assembly with Predesigned Dual Macrocyclic Receptors and Dianions

Realizing anion-π interaction induced self-assembly with charge-neutral π receptors as building components is extremely challenging. We designed and synthesized a series of bisoxacalix[2]arene[2]triazines 7-11 in which two macrocyclic motifs are linked in diverse branching angle and rigidity. Crystal structures showed the use of rigid linkers is able to control the orientation of the two macrocyclic cavities. The interplay between the two cavities was revealed by binding studies of 8-11 with chloride in solution. Whereas 180°- and 120°-branched hosts 8 and 9 possess dual complexation ability, 60°-branched and flexibly linked hosts 10 and 11 only form 1:1 complex with chloride. Association and self-assembly of these bismacrocyclic building units with dianionic naphthalene-1,5-disulfonate were systematically investigated. The formation of oligomeric self-assemblies and large aggregates in solution was suggested by ¹H NMR titrations, concentration- and temperature-variable ¹H NMR, diffusion-ordered spectroscopy (DOSY), ESI-MS, and dynamic light scattering (DLS). The anion-π induced long-distance self-assembly with coherent particle formation was revealed by SEM, TEM, cryo-TEM, and AFM techniques. The systematic studies allowed us to draw the conclusion that the dianion served to bridge the initial host aggregates through anion-π interactions and was responsible for the coherent particle formation. The cavity orientation of the bismacrocycle components was found to have a significant influence on the coherent particle morphology.