Cross-Linked Supramolecular Polymer Gels Constructed from Discrete Multi-pillar[5]arene Metallacycles and Their Multiple Stimuli-Responsive Behavior

A new family of discrete hexakis-pillar[5]arene metallacycles with different sizes have been successfully prepared via coordination-driven self-assembly, which presented very few successful examples of preparation of discrete multiple pillar[n]arene derivatives. These newly designed hexakis-pillar[5]arene metallacycles were well characterized with one-dimensional (1-D) multinuclear NMR (1H and 31 P NMR), two-dimensional (2-D) 1H–1H COSY and NOESY, ESI-TOF-MS, elemental analysis, and PM6 semiempirical molecular orbital methods. Furthermore, the host–guest complexation of such hexakis-pillar[5]arene hosts with a series of different neutral ditopic guests G1–6 were well investigated. Through host–guest interactions of hexakis-pillar[5]arene metallacycles H2 or H3 with the neutral dinitrile guest G5, the cross-linked supramolecular polymers H2⊃(G5)3 or H3⊃(G5)3 were successfully constructed at the high-concentration region, respectively. Interestingly, these cross-linked supramolecular polymers transformed into the stable supramolecular gels upon increasing the concentrations to a relatively high level. More importantly, by taking advantage of the dynamic nature of metal–ligand bonds and host–guest interactions, the reversible multiple stimuli-responsive gel–sol phase transitions of such polymer gels were successfully realized under different stimuli, such as temperature, halide, and competitive guest, etc. The mechanism of such multiple stimuli-responsive processes was well illustrated by in situ multinuclear NMR investigation. This research not only provides a highly efficient approach to the preparation of discrete multiple pillar[n]arene derivatives but also presents a new family of multiple stimuli-responsive "smart" soft matters.