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The development of synthetic two-dimensional crystalline polymers (2DCPs), such as 2D covalent-organic polymers and 2D metal-organic polymers, is receiving increasing attention due to their intriguing chemistry and unique properties, as well as potential role in wide ranging applications, such as electronics, sensing, catalysis, separation, and energy storage and conversion. Complementary to the top-down exfoliation towards the preparation of 2DCPs, bottom-up interface-assisted synthesis is advantageous in the 2D dynamic arrangement of the molecules or precursors, offering the chance to generate ultra-thin structures with large lateral sizes. This article provides guidelines on the preparation of free-standing, single-layer, or multi-layer 2DCPs via liquid-interface-assisted synthesis, mainly involving polymerization at the air–water and liquid–liquid interfaces, as well as the Langmuir-Blodgett method. Insight into the advantages and challenges of synthesis strategies and chemistry methodologies are provided for the future development of interfacial synthesis of 2DCPs with diverse structural and functional control.

Angewandte Chemie / International edition International edition 59(15), 6028 - 6036 (2020). doi:10.1002/anie.201915217, Single‐layer and multi‐layer 2D polyimine films have been achieved through interfacial synthesis methods. However, it remains a great challenge to achieve the maximum degree of crystallinity in the 2D polyimines, which largely limits the long‐range transport properties. Here we employ a surfactant‐monolayer‐assisted interfacial synthesis (SMAIS) method for the successful preparation of porphyrin and triazine containing polyimine‐based 2D polymer (PI‐2DP) films with square and hexagonal lattices, respectively. The synthetic PI‐2DP films are featured with polycrystalline multilayers with tunable thickness from 6 to 200 nm and large crystalline domains (100–150 nm in size). Intrigued by high crystallinity and the presence of electroactive porphyrin moieties, the optoelectronic properties of PI‐2DP are investigated by time‐resolved terahertz spectroscopy. Typically, the porphyrin‐based PI‐2DP 1 film exhibits a p‐type semiconductor behavior with a band gap of 1.38 eV and hole mobility as high as 0.01 cm2 V−1 s−1, superior to the previously reported polyimine based materials., Published by Wiley-VCH, Weinheim

Despite rapid progress in recent years, it has remained challenging to prepare crystalline two-dimensional polymers. Here, we report the controlled synthesis of few-layer two-dimensional polyimide crystals on the surface of water through reaction between amine and anhydride monomers, assisted by surfactant monolayers. We obtained polymers with high crystallinity, thickness of ~2 nm and an average crystal domain size of ~3.5 μm2. The molecular structure of the materials, their grain boundaries and their edge structures were characterized using X-ray scattering and transmission electron microscopy techniques. These characterizations were supported by computations. The formation of crystalline polymers is attributed to the pre-organization of monomers at the water–surfactant interface. The surfactant, depending on its polar head, promoted the arrangement of the monomers—and in turn their polymerization—either horizontally or vertically with respect to the water surface. The latter was observed with a surfactant bearing a carboxylic acid group, which anchored amine monomers vertically through a condensation reaction. In both instances, micrometre-sized, few-layer two-dimensional polyamide crystals were grown. It is difficult to prepare 2D polymers that are crystalline over large areas. Now, few-layer 2D polyimides and polyamides with good crystallinity on the micrometre scale have been synthesized on a water surface. A surfactant monolayer is used to organize amine monomers before their polymerization with anhydride moieties.

One of the key challenges in two-dimensional (2D) materials is to go beyond graphene, a prototype 2D polymer (2DP), and to synthesize its organic analogues with structural control at the atomic- or molecular-level. Here we show the successful preparation of porphyrin-containing monolayer and multilayer 2DPs through Schiff-base polycondensation reaction at an air–water and liquid–liquid interface, respectively. Both the monolayer and multilayer 2DPs have crystalline structures as indicated by selected area electron diffraction. The monolayer 2DP has a thickness of∼0.7 nm with a lateral size of 4-inch wafer, and it has a Young's modulus of 267±30 GPa. Notably, the monolayer 2DP functions as an active semiconducting layer in a thin film transistor, while the multilayer 2DP from cobalt-porphyrin monomer efficiently catalyses hydrogen generation from water. This work presents an advance in the synthesis of novel 2D materials for electronics and energy-related applications., Forming 2D polymers in a controlled manner on the atomic and molecular level is difficult. Here, Feng and others have used Schiff-base polycondensation reactions at an air-water or liquid-liquid interface to form porphyrin containing monolayer and multilayer 2D polymers.

A transmission electron microscopy study reveals grain boundaries and their formation mechanisms in 2D polymers., Two-dimensional (2D) polymers hold great promise in the rational materials design tailored for next-generation applications. However, little is known about the grain boundaries in 2D polymers, not to mention their formation mechanisms and potential influences on the material’s functionalities. Using aberration-corrected high-resolution transmission electron microscopy, we present a direct observation of the grain boundaries in a layer-stacked 2D polyimine with a resolution of 2.3 Å, shedding light on their formation mechanisms. We found that the polyimine growth followed a “birth-and-spread” mechanism. Antiphase boundaries implemented a self-correction to the missing-linker and missing-node defects, and tilt boundaries were formed via grain coalescence. Notably, we identified grain boundary reconstructions featuring closed rings at tilt boundaries. Quantum mechanical calculations revealed that boundary reconstruction is energetically allowed and can be generalized into different 2D polymer systems. We envisage that these results may open up the opportunity for future investigations on defect-property correlations in 2D polymers.