ao-Yan Qi

Highly thermally stable hydrogels derived from monolayered two-dimensional supramolecular polymers

It has been predicted that the properties of materials are dramatically influenced if their structures are confined to two-dimensional (2D) space. A representative example is graphene. However, for synthetic 2D materials, such influences have rarely been demonstrated. In this work, a rare example of how a 2D monolayer structure can impact the properties of bulk materials has been demonstrated by the construction of 2D supramolecular polymers (SPs) and their utilization in the fabrication of hydrogels. Maintaining the intrinsic 2D structures, the as-prepared hydrogels exhibited exceptional thermal stabilities (>180 °C), as revealed by an inversion test and a variable-temperature rheological study. The microstructures and morphologies of the 2D SPs have been extensively characterized by NMR spectroscopy, dynamic light scattering, small-angel X-ray scattering, transmission electron microscopy and atomic force microscopy. Furthermore, molecular dynamic simulations were also performed to shed light on the formation mechanism of the hydrogels.

The construction of single-layer two-dimensional supramolecular organic frameworks in water through the self-assembly of rigid vertexes and flexible edges

Single-layer two-dimensional supramolecular organic frameworks have been constructed in water through three-component self-assembly of rigid cross-shaped vertexes, flexible oligoethylene glycol edges, and cucurbit[8]uril (CB[8]) hosts, driven by CB[8]-encapsulation-enhanced donor–acceptor interaction between the viologen unit and 2,6-dihydroxynaphthalene segment which were peripherally incorporated into the vertex and edge, respectively.

Supramolecular radical polymers self-assembled from the stacking of radical cations of rod-like viologen di- and trimers

A series of π-conjugated oligomeric viologens (COVs) whose viologen segments are connected by phenyl or biphenyl units have been designed and synthesized. They exhibited interesting electrochemical and optical properties which are different from isolated viologen, as revealed by cyclic voltammetric and spectroscopic studies. The self-assembly behavior of the COV radical cations generated by treating COVs with sodium dithionite in aqueous media was studied systematically with UV-vis-NIR, electron paramagnetic resonance (EPR), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM), which revealed that they self-assembled into linear supramolecular radical polymers driven by the dimerization of radical cations. Comprehensive DFT calculations for the COVs and their radical cations were also performed and their structure–property relationships were revealed.

A Case Study on the Influence of Substitutes on Interlayer Stacking of 2D Covalent Organic Frameworks

Interlayer stacking of 2D covalent organic frameworks (COFs) plays a crucial role in determining not only the geometry of channels inside COFs but also the mobility of carrier transport between COF layers. However, though topological structures of 2D COFs monolayers can be precisely predicted through the structures of building blocks, factors affecting their interlayer stacking remain poorly understood. In this work, a truxene-based building block on which six methyl groups are introduced was designed. The condensation of it with 1,4-diaminobenzene or benzidine afforded 2D COFs with the methyl groups extending out-of-plane of the layers. A significant influence of the methyl groups on interlayer stacking of the COFs was revealed by the adoption of inclined packing of monolayers, which has never been experimentally observed before. This unprecedented stacking manner was confirmed by powder X-ray diffraction analysis, pore-size distribution analysis, and TEM investigation.

The construction of supramolecular polymers through anion bridging: from frustrated hydrogen-bonding networks to well-ordered linear arrays

Although it is not difficult to control the direction of a single noncovalent bond, to precisely control the orientation of noncovalent bonding arrays, which is a precondition for the construction of complicated supramolecular systems, is still a great challenge. In this work, we demonstrate the construction of one-dimensional (1D) supramolecular polymers by converting random hydrogen-bonding networks into well-ordered linear hydrogen-bonding arrays through an anion-bridging strategy. The supramolecular polymers were fabricated through the co-assembly of a propeller-like hexaazatriphenylene (HAT) derivative peripherally bearing six urea units with Cl− or Br− ions, driven by the formation of N–H⋯X (X = Cl−, Br−) hydrogen bonds between urea NH and halogen anions, a type of hydrogen bond that has been extensively used for selectively recognizing anionic species but remained unexplored to fabricate supramolecular polymers. These supramolecular polymers exhibited extremely low critical polymerization concentrations and were found to show response toward different stimuli including polarity of solvent, temperature, and metal ions.

A design strategy for the construction of 2D heteropore covalent organic frameworks based on the combination of C2v and D3h symmetric building blocks

While significant progress in covalent organic frameworks (COFs) has been made in recent years, the construction of COFs with heterogeneous porosity remains a great challenge. In this communication, we report two novel 2D heteropore COFs which bear two kinds of hexagonal micropores with different shapes and sizes. They have been constructed by the condensation of monomers with C2v- and D3h-symmetry, and their structures have been confirmed by PXRD studies, synchrotron SAXS experiments, theoretical simulations and pore size distribution analyses.

A thermally stable pH-responsive “supramolecular buckle” based on the encapsulation of 4-(4-aminophenyl)-N-methylpyridinium by cucurbit[8]uril

Supramolecular building blocks that can respond to external stimuli are the basis for the fabrication of responsive materials. However, those that can be used in aqueous media and at elevated temperatures are extremely limited. In this work, a cucurbit[8]uril(CB[8])-based host–guest system has been developed. It exhibits excellent response to the change of pH by forming a thermally stable 1:2 host–guest complex under neutral conditions and transforming into a 1:1 complex by releasing one guest molecule in the presence of an acid. This transformation makes this system serve as a useful “supramolecular buckle”, which can fasten and unfasten two components. These features endow it with great potential in fabricating pH-responsive materials in aqueous media over a wide temperature range.