How to use X-ray diffraction to elucidate 2D polymerization propagation in single crystals.

Covalent long-range ordered (crystalline) sheets called 2D polymers have recently been synthesized by irradiating single crystals of suitably packed monomers. To have such an action proceed successfully, billions of bond formation processes have to be mastered exclusively in two dimensions within 3D crystals. This raises questions as to how to elucidate the mechanism of these unusual polymerizations as well as their entire strain management. The article will show that single crystal X-ray diffraction based on both Bragg and diffuse scattering are powerful techniques to achieve such goal. The very heart of both techniques will be explained and it will be shown what can be safely concluded with their help and what not. Consequently, the reader will understand why some crystals break during polymerization, while others stay intact. This understanding will then be molded into a few guidelines that should help pave the way for future developments of 2D polymers by those interested in joining the effort with this fascinating and emerging class of 2D materials.

Enriching and Quantifying Porous Single Layer 2D Polymers by Exfoliation of Chemically Modified van der Waals Crystals.

2D polymer sheets with six positively charged pyrylium groups at each pore edge in a stacked single crystal can be transformed into a 2D polymer with six pyridines per pore by exposure to gaseous ammonia. This reaction furnishes still a crystalline material with tunable protonation degree at regular nano-sized pores promising as separation membrane. The exfoliation is compared for both 2D polymers with the latter being superior. Its liquid phase exfoliation yields nanosheet dispersions, which can be size-selected using centrifugation cascades. Monolayer contents of ≈30 % are achieved with ≈130 nm sized sheets in mg quantities, corresponding to tens of trillions of monolayers. Quantification of nanosheet sizes, layer number and mass shows that this exfoliation is comparable to graphite. Thus, we expect that recent advances in exfoliation of graphite or inorganic crystals (e.g. scale-up, printing etc.) can be directly applied to this 2D polymer as well as to covalent organic frameworks.

A Two-Dimensional Polymer Synthesized at the Air/Water Interface.

A trifunctional, partially fluorinated anthracene-substituted triptycene monomer was spread at an air/water interface into a monolayer, which was transformed into a long-range-ordered 2D polymer by irradiation with a standard UV lamp. The polymer was analyzed by Brewster angle microscopy, scanning tunneling microscopy measurements, and non-contact atomic force microscopy, which confirmed the generation of a network structure with lattice parameters that are virtually identical to a structural model network based on X-ray diffractometry of a closely related 2D polymer. The nc-AFM images highlight the long-range order over areas of at least 300×300 nm2 . As required for a 2D polymer, the pore sizes are monodisperse, except for the regions where the network is somewhat stretched because it spans over protrusions. Together with a previous report on the nature of the cross-links in this network, the structural information provided herein leaves no doubt that a 2D polymer has been synthesized under ambient conditions at an air/water interface.

A Two-Dimensional Polymer Synthesized through Topochemical [2 + 2]-Cycloaddition on the Multigram Scale.

The single-crystal-to-single-crystal (scsc) synthesis of a 2D polymer based on photochemically triggered [2 + 2]-cycloaddition is reported. Both monomer and polymer single crystals are analyzed by X-ray diffraction, which is the first case of a scsc two-dimensional polymerization based on this cycloaddition and the third ever case for a scsc synthesis of a 2D polymer. The product crystals at quantitative conversion are wet-exfoliated under mild conditions and afford countless features that are single and double layers as judged by their AFM heights of hAFM ≈ 1.2 ± 0.5 and 2.2 ± 0.5 nm, respectively. The X-ray-structure-based molecular weight of the 2D polymers and their degree of polymerization per µm2 are M = 360 MDa and Pn = 464 900, respectively. The sheet size is on the order of 5 × 5 µm2.

Decorating the Edges of a 2D Polymer with a Fluorescence Label.

This work proves the existence and chemical addressability of defined edge groups of a 2D polymer. Pseudohexagonally prismatic single crystals consisting of layered stacks of a 2D polymer are used. They should expose anthracene-based edge groups at the six (100) but not at the two pseudohexagonal (001) and (001̅) faces. The crystals are reacted with the isotopically enriched dienophiles maleic anhydride and a C18-alkyl chain-modified maleimide. In both cases the corresponding Diels-Alder adducts between these reagents and the edge groups are formed as confirmed by solid state NMR spectroscopy. The same applies to a maleimide derivative carrying a BODIPY dye which was chosen for its fluorescence to be out of the range of the self-fluorescence of the 2D polymer crystals stemming from contained template molecules. If the crystals are excited at λ = 633 nm, their (100) faces and thus their rims fluoresce brightly, while the pseudohexagonal faces remain silent. This is visible when the crystals lie on a pseudohexagonal face. Lambda-mode laser scanning microscopy confirms this fluorescence to originate from the BODIPY dye. Micromechanical exfoliation of the dye-modified crystals results in thinner sheet packages which still exhibit BODIPY fluorescence right at the rim of these packages. This work establishes the chemical nature of the edge groups of a 2D polymer and is also the first implementation of an edge group modification similar to end group modifications of linear polymers.

Exfoliation of two-dimensional polymer single crystals into thin sheets and investigations of their surface structure by high-resolution atomic force microscopy.

The acid-assisted wet-chemical and the adhesive-tape induced micromechanical exfoliation of differently sized single crystals of a 2D polymer (approx. 20 µm and 100 µm) is shown to result in thin sheet stacks. Tuning of the thickness is achieved via duration and frequency of the exfoliation, respectively. A color code is established that correlates interference colors of sheet stacks on SiO2(300 nm)/Si as observed under an optical microscope with their thicknesses measured by atomic force microscopy. This facilitates reliable monitoring of the exfoliation and quick identification of sheet stacks of a desired thickness. Furthermore, high resolution atomic force microscopy is applied to investigate the surfaces of starting crystals and both wet-chemically and micromechanically exfoliated sheet stacks aiming at exploring whether exfoliation proceeds with preservation of surface periodicity and with a low frequency of sheet rupturing. These investigations also aimed at uncovering possible point defects and domain (grain) boundaries in the surfaces. It appears that all investigated objects have a high molecular scale perfection and that both exfoliation methods proceed mild enough to largely preserve the molecular structure of the 2D polymer including the not covalently bonded template molecules being part of the crystal packing.