Zhikun Zheng

Synthesis of Free‐Standing, Monolayered Organometallic Sheets at the Air/Water Interface

The recent discovery of how to obtain and utilize individual layers of graphite, that is, graphene, and other inorganic monolayered sheets has turned the spotlight brighter than ever on a basically ignored field of chemistry, the rational synthesis of two-dimensional polymers. While there have been numerous reports on the synthesis of monolayered polymer films with irregularly networked internal structures since the pioneering work by Gee in 1935, little is known to date about the synthesis of a free-standing, 2D network with an ordered internal structure. This paucity is contrasted by the richness of fragments of such networks that were obtained by approaches such as iterative organic synthesis, selfassembly, or on-surface polymerization. As of now, the lateral dimensions of these fragments are too small to expect sheet-like properties; furthermore, they cannot yet be isolated and manipulated. Considering the huge application potential for structurally well-defined, free-standing 2D networks, which ranges from ultrasensitive membranes, molecular sieves, and devices based on high charge carrier mobility to materials with outstanding mechanical strength and the like, we felt the need to establish a synthesis program aiming at filling the gap between the one-dimensional (linear synthetic and biological polymers, carbon nanotubes, etc.) and the three-dimensional structures (hyperbranched and cross-linked bulk polymers, laminar crystals such as graphite, diamond, etc.) by providing access to structurally defined 2D polymers. Herein we report the synthesis of a free-standing monolayer sheet consisting of the hexafunctional terpyridine (tpy)-based D6h-symmetric monomer 1 (Figure 1) which was designed for the present purpose 12] and is to be held together by metal ion complexes between ideally all six tpy units of one monomer with one of the tpy units of each of the six neighboring monomers. This mode of polymerization is in principle reversible and could allow for dynamic bond formation. It has often been used for construction of complex but well-defined compounds as well as supramolecular assemblies. 15, 16] Figure 1 shows a targeted network. To avoid any three-dimensional growth of the coordination network during polymerization, monomer 1 was confined to two dimensions prior to polymerization by spreading it at the air/water interface on a Langmuir–Blodgett (LB) trough. The main advantages of using the air/water interface for the present synthesis instead of solid substrates include 1) the flat and uniform surface on a large length scale, 2) the availability of the water subphase as a pool of reagents and catalysts, 3) the straightforward preparation and facile isolation of single sheets by transfer onto solid substrates and supports of all sorts, 20] 4) the possibility to preset the lateral surface pressure and lateral concentration of monomers prior to the polymerization, and 5) the ease in performing polymerization under ambient conditions. A sub-monolayer of monomer 1 was spread at the air/ water interface from chloroform solution and compressed to a pressure of 30 mNm . The compression process was monitored by Brewster angle microscopy up to 10 mNm 1 and found to be fully reversible (Figure S1 in the Supporting Information) and to provide thin layers that are homogenous at the resolution of micrometers (Figure S2 in the Supporting Information). The point of inflection of the corresponding surface pressure–area isotherm (Figure 2a) was observed at a pressure of approximately 10 mNm , from which a mean molecular area of approximately 520 2 is estimated. This preliminary value is in good agreement with the formation of a dense monolayer in which the monomers lie flat on the interface. This arrangement was supported by AFM contactmode scratching and tapping-mode imaging experiments after vertical transfer of the compressed monolayer (at 10 mNm ) onto a mica substrate. Figure 2 b shows the scratched area and the corresponding height profile, providing the apparent height happ 0.8 nm. While happ values obtained by ambient-condition AFM are known to not accurately reflect real heights, the value of approximately 0.8 nm nevertheless suggests a monolayer. Not only is it in a reasonable range for a conjugated structure, parts of which may significantly deviate from coplanarity, but also a [*] T. Bauer, Dr. Z. Zheng, Dr. J. Sakamoto, Prof. A. D. Schl ter Department of Materials, Institute of Polymers Swiss Federal Institute of Technology, ETH Z rich HCI J 541, 8093 Z rich (Switzerland) E-mail: schlueter@mat.ethz.ch sakamoto@mat.ethz.ch

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Stretchable and Semitransparent Conductive Hybrid Hydrogels for Flexible Supercapacitors

Conductive polymers showing stretchable and transparent properties have received extensive attention due to their enormous potential in flexible electronic devices. Here, we demonstrate a facile and smart strategy for the preparation of structurally stretchable, electrically conductive, and optically semitransparent polyaniline-containing hybrid hydrogel networks as electrode, which show high-performances in supercapacitor application. Remarkably, the stability can extend up to 35,000 cycles at a high current density of 8 A/g, because of the combined structural advantages in terms of flexible polymer chains, highly interconnected pores, and excellent contact between the host and guest functional polymer phase.

Synthesis of two-dimensional analogues of copolymers by site-to-site transmetalation of organometallic monolayer sheets

Monolayer sheets have gained attention due to the unique properties derived from their two-dimensional structure. One of the key challenges in sheet modification/synthesis is to exchange integral parts while keeping them intact. We describe site-to-site transmetalation of Zn(2+) in the netpoints of cm(2)-sized, metal-organic sheets by Fe(2+), Co(2+), and Pb(2+). This novel transformation was done both randomly and at predetermined patterns defined by photolithography to create monolayer sheets composed of different netpoints. All transmetalated sheets are mechanically strong enough to be spanned over 20 × 20 μm(2) sized holes. Density functional theory calculations provide both a model for the molecular structure of an Fe(2+)-based sheet and first insights into how transmetalation proceeds. Such transmetalated sheets with random and patterned netpoints can be considered as two-dimensional analogues of linear copolymers. Their nanoscale synthesis presents an advance in monolayer/polymer chemistry with applications in fields such as surface coating, molecular electronics, device fabrication, imaging, and sensing.

Square-micrometer-sized, free-standing organometallic sheets and their square-centimeter-sized multilayers on solid substrates.

Oligofunctional terpyridine-based monomers are spread at an air/water interface, where they are connected with transition metal salts such as Fe(II) into mechanically coherent monolayer sheets of macroscopic dimension. The conversions of these processes are determined by XPS for several monomer/metal ion combinations. The sheets are transferred onto TEM grids, the 20 × 20 square micrometer sized holes of which can be spanned. AFM indentation experiments provide in-plane elastic moduli which are compared with naturally occurring sheets such as graphene. The new organometallic sheets are also used to create multilayer assemblies on square centimeter length scales on solid substrates. Finally some directions are provided where this research can lead to in future and where its application potential lies.

Unusual Ultra‐Hydrophilic, Porous Carbon Cuboids for Atmospheric‐Water Capture

There is significant interest in high-performance materials that can directly and efficiently capture water vapor, particularly from air. Herein, we report a class of novel porous carbon cuboids with unusual ultra-hydrophilic properties, over which the synergistic effects between surface heterogeneity and micropore architecture is maximized, leading to the best atmospheric water-capture performance among porous carbons to date, with a water capacity of up to 9.82 mmol g(-1) at P/P0 =0.2 and 25 °C (20% relative humidity or 6000 ppm). Benefiting from properties, such as defined morphology, narrow pore size distribution, and high heterogeneity, this series of functional carbons may serve as model materials for fundamental research on carbon chemistry and the advance of new types of materials for water-vapor capture as well as other applications requiring combined highly hydrophilic surface chemistry, developed hierarchical porosity, and excellent stability.

Approaching two-dimensional copolymers: photoirradiation of anthracene- and diaza-anthracene-bearing monomers in Langmuir monolayers.

By using structurally similar amphiphilic monomers, it is shown that compressed monolayers of varying amounts of such monomers at the air/water interface can be converted by photo-irradiation into the corresponding covalently connected monolayer sheets. Since one of the monomers carries three anthracene units and the other three 1,8-diaza-anthracene units, the growth reaction is proposed to take place through photochemically achieved [4+4]-cycloaddition between pairs of these units that are co-facially (face-to-face) arranged, to furnish the corresponding covalent dimers. While evidence for both homodimers is amply available, the existence of the heterodimer needs to be established with the help of a model reaction to support the conceptual aspect of this work, copolymerization in two dimensions. The sheet copolymers exhibit substantial robustness in that they can be spanned over 20 × 20 μm(2)-sized holes without rupturing under their own weight. X-ray photoelectron spectroscopy (XPS) studies reveal that the monomers are incorporated into the sheet copolymers according to feed. These results establish existence of the first covalent sheet copolymer, which is considered a step ahead towards novel 2D materials.