Construction of Metallacage-Cored Supramolecular Gel by Hierarchical Self-Assembly of Metal Coordination and Pillar[5]arene-Based Host-Guest Recognition.

In this work, a Pd2L4 metallacage 2 center dot([BF4](-))(4) with four pillar[5]arene units is first prepared and characterized by 1D multinuclear NMR (H-1, B-11, and F-19 NMR), 2D H-1-H-1 correlation spectra, H-1-C-13 heteronuclear single quantum coherence, and diffusion-ordered NMR spectroscopy, and electrospray ionization time-of-flight mass spectrometry. By the introduction of a ditopic guest molecule 3 into a chloroform solution of 2 center dot([BF4](-))(4), a supramolecular polymer network gel is successfully constructed based on the metal coordination interactions and host-guest recognition between the pillar[5]arene units of 2 center dot([BF4](-))(4) and neutral ditopic guest molecule 3. The temperature and pH responsivenesses of the supramolecular gel are studied, which are further employed for the controlled release of different cargos. As a demonstration, emodin and methylene blue are trapped in the cavities of the metallacage and in the pores of the supramolecular gel, respectively. Methylene blue is first released along with the gel-sol transition while emodin is then released by the further addition of acid to destroy the metallacage. This study explores the use of metallacage-cored supramolecular network gels for sequential controlled release and contributes to the development of smart and adaptive materials.