Construction of Supramolecular Polymer Gels Cross-linked by Two Types of Discrete Well-defined Metallacycles through Self-sorting

A dipyridyl donor D1 decorated with pillar[5] arene unit and a dipyridyl donor D2 containing neutral guest moiety were successfully synthesized. Through the X-ray single crystal analysis, it was found that the angle between the two pyridine unit in D1 was about 128 degrees and the distance between two nitrogen atoms was about 1.48 nm, while the angle between the two pyridine units of D2 was about 118 degrees and the distance between the two nitrogen atoms was about 0.98 nm, much shorter than that in D1. Thus, a metallacycle containing three pillar[5] arene units (H) and a metallacycle possessing three neutral guest units (G) were successfully constructed in one pot coordination-driven self-assembly from dipyridyl donors (D1 and D2) and 120 degrees di-Pt(II) acceptor through size-control self-sorting strategy. Moreover, driven by the host-guest interaction between metallacycles H and G, a new family of supramolecular polymer, cross-linked by these two kinds of discrete well-defined metllacycles H and G, was gradually constructed with the increase of concentration. By concentration-dependent H-1-NMR measurement, it was found that the typical signals of metllacycle G were gradually disappeared with the increase of concentration, indicating the enhanced host-guest interaction between metallacycles H and G. Meanwhile, the analysis of 2-D diffusion ordered spectroscopy (DOSY) tests of H superset of G showed that the diffusion coefficient D gradually decreased with the increase of concentration, and D-1 mmol/(L)/D-16 mmol/(L) value was ca. 31.6, also demonstrating the formation of supramolecular polymer. Scanning electron microscope (SEM) was also used to investigate the morphology change during the concentration increasing process. More intuitionally, cross-linked networks were observed with the increase of concentration of H. G. Interestingly, the resultant supramolecular polymer could transforme into supramolecular polymer gels when the concentration was increased to a relatively high value (9.9 wt%). Moreover, due to the existence of discrete well-defined metallacycles and the dynamic nature of host-guest hierarchical self-assembly, reversible gel-sol transitions were successfully realized through the disassembly and reassembly of such cross-linked supramolecular polymer networks stimulated by various external stimuli such as temperature, competitive guest, and halide, etc.