Photochemical Creation of Covalent Organic 2D Monolayer Objects in Defined Shapes via a Lithographic 2D Polymerization

Recent advances in two-dimensional polymers: synthesis, assembly and energy-related applications

Two-dimensional polymers (2DPs) are a class of 2D crystalline polymer materials with definite structures, which have outstanding physical-chemical and electronic properties. They cleverly link organic building units through strong covalent bonds and can construct functional 2DPs through reasonable design and selection of different monomer units to meet various application requirements. As promising energy materials, 2DPs have developed rapidly in recent years. This review first introduces the basic overview of 2DPs, such as their historical development, inherent 2D characteristics and diversified topological advantages, followed by the summary of the typical 2DP synthesis methods recently (including "top-down" and "bottom-up" methods). The latest research progress in assembly and processing of 2DPs and the energy-related applications in energy storage and conversion are also discussed. Finally, we summarize and prospect the current research status, existing challenges, and future research directions of 2DPs.

Vertically Heterogeneous 2D Semi-Interpenetrating Networks Based on Cellulose Acetate and Cross-Linked Polybutadiene

This work reports the feasibility of polybutadiene (PB) cross-linking under UV irradiation in the presence of a linear polymer, cellulose acetate (CA), to form semi-interpenetrating polymer networks at the air-water interface. The thermodynamic properties and the morphology of two-dimensional (2D) CA/PB blends are investigated after UV irradiation and for a wide range of CA volume fractions. A contraction of the mixed Langmuir films is observed independent of the composition, in agreement with that recorded for the individual PB monolayer after cross-linking. The PB network formation is demonstrated by in situ sum-frequency generation spectroscopy on the equivolumic CA/PB mixed film. From Brewster angle microscopy observations, the PB network synthesis does not induce any morphology change at the mesoscopic scale, and all of the mixed films remain homogeneous laterally. In situ neutron reflectometry is used to probe the effect of PB cross-linking on the vertical structure of CA/PB mixed films. For all studied compositions, significant thickening of the films is evidenced, consistent with their contraction ratio. This thickening is accompanied by a partial expulsion of the PB toward the film-air interface, which is attributed to the hydrophobic character of the PB. This phenomenon is stronger for films rich in PB. In particular, the structure of the PB-rich film undergoes a transition from vertically homogeneous to inhomogeneous along the depth. 2D semi-interpenetrating polymer networks can thus be synthesized at the air-water interface with a morphology that is strongly influenced by the polymer-polymer and polymer-environment interactions.

In-situ nanospectroscopic imaging of plasmon-induced two-dimensional [4+4]-cycloaddition polymerization on Au(111)

Plasmon-induced chemical reactions (PICRs) have recently become promising approaches for highly efficient light-chemical energy conversion. However, an in-depth understanding of their mechanisms at the nanoscale still remains challenging. Here, we present an in-situ investigation by tip-enhanced Raman spectroscopy (TERS) imaging of the plasmon-induced [4+4]-cycloaddition polymerization within anthracene-based monomer monolayers physisorbed on Au(111), and complement the experimental results with density functional theory (DFT) calculations. This two-dimensional (2D) polymerization can be flexibly triggered and manipulated by the hot carriers, and be monitored simultaneously by TERS in real time and space. TERS imaging provides direct evidence for covalent bond formation with ca. 3.7 nm spatial resolution under ambient conditions. Combined with DFT calculations, the TERS results demonstrate that the lateral polymerization on Au(111) occurs by a hot electron tunneling mechanism, and crosslinks form via a self-stimulating growth mechanism. We show that TERS is promising to be plasmon-induced nanolithography for organic 2D materials.

Tessellation strategy for the interfacial synthesis of an anthracene-based 2D polymer via [4+4]-photocycloaddition

Inspired by the tessellation or tiling process in daily life, a rigid triangular macrocyclic molecule containing anthracene as a photo-active moiety was synthesized to realize pre-organization through π-π interactions. The successful preparation of a 2D polymer monolayer at the air/water interface was achieved through [4+4]-photocycloaddition.

Chemical kinetic mechanisms and scaling of two-dimensional polymers via irreversible solution-phase reactions

Two-dimensional (2D) polymers are extended networks of multi-functional repeating units that are covalently linked together but confined to a single plane. The past decade has witnessed a surge in interest and effort toward producing and utilizing 2D polymers. However, facile synthesis schemes suitable for mass production are yet to be realized. In addition, unifying theories to describe the 2D polymerization process, such as those for linear polymers, have not yet been established. Herein, we perform a chemical kinetic simulation to study the recent synthesis of 2D polymers in homogeneous solution with irreversible chemistry. We show that reaction sites for polymerization in 2D always scale unfavorably compared to 3D, growing as molecular weight to the 1/2 power vs 2/3 power for 3D. However, certain mechanisms can effectively suppress out-of-plane defect formation and subsequent 3D growth. We consider two such mechanisms, which we call bond-planarity and templated autocatalysis. In the first, although single bonds can easily rotate out-of-plane to render polymerization in 3D, some double-bond linkages prefer a planar configuration. In the second mechanism, stacked 2D plates may act as van der Waals templates for each other to enhance growth, which leads to an autocatalysis. When linkage reactions possess a 1000:1 selectivity (γ) for staying in plane vs rotating, solution-synthesized 2D polymers can have comparable size and yield with those synthesized from confined polymerization on a surface. Autocatalysis could achieve similar effects when self-templating accelerates 2D growth by a factor β of 10⁶. A combined strategy relaxes the requirement of both mechanisms by over one order of magnitude. We map the dependence of molecular weight and yield for the 2D polymer on the reaction parameters, allowing experimental results to be used to estimate β and γ. Our calculations show for the first time from theory the feasibility of producing two-dimensional polymers from irreversible polymerization in solution.

Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) Copolymer 2D Single Network at the Air-Water Interface

In this work, Langmuir monolayers based on poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEG-PPG-PEG) triblock copolymer were in situ stabilized at the air-water interface in the presence of a cross-linking agent, benzene-1,3,5-tricarboxaldehyde (BTC), in the aqueous subphase. The reaction takes place through acid-catalyzed acetalization between the terminal hydroxyl groups of the copolymer and aldehyde functions of the BTC molecules. Mean area per repeat unit measurements as a function of the reaction time show a significant monolayer contraction associated with an increase in its compressibility modulus. In addition, Brewster angle microscopy observations indicate the appearance of higher-density two-dimensional domains, irreversibly formed at constant surface pressure. This is also confirmed on a smaller scale by atomic force microscopy (AFM). These arguments, consistent with copolymer monolayer cross-linking in acidic medium, are supported in situ at the air-water interface by sum-frequency generation (SFG) spectroscopy. Furthermore, PEG-PPG-PEG monolayer cross-linking is not evidenced in alkaline medium, in coherence with the interfacial acid-catalyzed acetalization.

2D Photopolymerization of Liquid Crystalline Langmuir Monolayers: In Situ Observation by Reflected Polarizing Microscopy

Photopolymerization of Langmuir monolayers composed of bifunctional acrylic liquid crystalline (LC) compounds was observed in situ by polarizing optical microscopy. In a dark state, monolayers of the LC compounds formed at an air-water or liquid-liquid interface exhibited liquid-like fluidity and in-plane optical anisotropy because of the coherent molecular tilt from the surface normal. Irradiated by UV light, the in-plane anisotropy and the liquid fluidity gradually disappeared with time, indicating the formation of the polymerized monolayers. Because the constituent molecules possess polymerizable acryloyl groups, under UV light, they are combined by acrylic polymer chains grown on the interface, which decreases the intermolecular distance and disturbs the coherent molecular tilt, resulting in the evanescence of the in-plane optical anisotropy and the fluidity. In contrast to the classical model of radical polymerization, the time taken for the monolayers to be photopolymerized was inversely proportional to the UV intensity, which is ascribed to the ideal two dimensionality of the reaction field. Because the polymerization degree is quantitatively estimated from the in-plane optical anisotropy of the LC monolayers, the process is traced, from moment to moment, by in situ microscopy observation.

Don't Forget Langmuir-Blodgett Films 2020: Interfacial Nanoarchitectonics with Molecules, Materials, and Living Objects

Designing interfacial structures with nanoscale (or molecular) components is one of the important tasks in the nanoarchitectonics concept. In particular, the Langmuir-Blodgett (LB) method can become a promising and powerful strategy in interfacial nanoarchitectonics. From this viewpoint, the status of LB films in 2020 will be discussed in this feature article. After one section on the basics of interfacial nanoarchitectonics with the LB technique, various recent research examples of LB films are introduced according to classifications of (i) growing research, (ii) emerging research, and (iii) future research. In recent LB research, various materials other than traditional lipids and typical amphiphiles can be used as film components of the LB techniques. Two-dimensional materials, supramolecular structures such as metal organic frameworks, and biomaterials such as DNA origami pieces are capable of working as functional components in the LB assemblies. Possible working areas of the LB methods would cover emerging demands, including energy, environmental, and biomedical applications with a wide range of functional materials. In addition, forefront research such as molecular manipulation and cell fate control is conducted in LB-related interfacial science. The LB technique is a traditional and well-develop methodology for molecular films with a ca. 100 year history. However, there is plenty of room at the interfaces, as shown in LB research examples described in this feature article. It is hoped that the continuous development of the science and technology of the LB method make this technique an unforgettable methodology.

Construction of cell-plastics as neo-plastics consisted of cell-layer provided green alga Chlamydomonas reinhardtii covered by two-dimensional polymer

Green alga Chlamydomonas reinhardtii has gained interest as a sustainable resource because it can be easily grown using CO₂ as a carbon source owing to its high CO₂ assimilating activity. Although the robustness of the cell wall of C. reinhardtii makes it difficult to extract its intracellular products, such property is beneficial when using the cell as an ingredient to fabricate “cell-plastic” in this study. The cell layer, which is a component of the cell-plastic, was prepared with an intercellular filler to connect each cell because C. reinhardtii is a single-cell strain. The cell layers were then repeatedly piled to increase the strength of the cell-plastic. To avoid slippage between the cell layers, they were covered with a small amount of a two-dimensional polymer to maintain the flat surface structure of the cell-plastic. Based on the evaluation, the cell-plastic has the potential to be a novel, sustainable plastic using ubiquitous green algal cells in nature.

Infrared and Raman chemical imaging and spectroscopy at the nanoscale

The advent of nanotechnology, and the need to understand the chemical composition at the nanoscale, has stimulated the convergence of IR and Raman spectroscopy with scanning probe methods, resulting in new nanospectroscopy paradigms. Here we review two such methods, namely photothermal induced resonance (PTIR), also known as AFM-IR and tip-enhanced Raman spectroscopy (TERS). AFM-IR and TERS fundamentals will be reviewed in detail together with their recent crucial advances. The most recent applications, now spanning across materials science, nanotechnology, biology, medicine, geology, optics, catalysis, art conservation and other fields are also discussed. Even though AFM-IR and TERS have developed independently and have initially targeted different applications, rapid innovation in the last 5 years has pushed the performance of these, in principle spectroscopically complimentary, techniques well beyond initial expectations, thus opening new opportunities for their convergence. Therefore, subtle differences and complementarity will be highlighted together with emerging trends and opportunities.

Tip-enhanced Raman spectroscopy for structural analysis of two-dimensional covalent monolayers synthesized on water and on Au (111)

A two-dimensional (2D) covalent monolayer based on [4 + 4] cycloaddition reactions between adjacent anthracene units was synthesized at an air/water interface. For structural analysis, tip-enhanced Raman spectroscopy (TERS) provides direct evidence for the covalent bonds formed between monomer molecules. For the first time, progress of the photopolymerization reaction was monitored by irradiation (λ = 385 nm) of the monomer monolayer for different times, based on averaged TER spectra extracted from maps. In addition, a 2D polymerization on a Au (111) substrate was realized, which opens up new possibilities for such chemical transformations. This work uses TERS as a minimally invasive tool to investigate how the reaction conditions affect polymerization conversion. We show that the high sensitivity and the high spatial resolution of TERS can be used to estimate the crystallinity of 2D covalent monolayers, which is a key question in polymer synthesis.

Organic Radical-Linked Covalent Triazine Framework with Paramagnetic Behavior

The production of multifunctional pure organic materials that combine different sizes of pores and a large number of electron spins is highly desirable due to their potential applications as polarizers for dynamic nuclear polarization-nuclear magnetic resonance and as catalysts and magnetic separation media. Here, we report a polychlorotriphenylmethyl radical-linked covalent triazine framework (PTMR-CTF). Two different sizes of micropores were established by N₂ sorption and the presence of unpaired electrons (carbon radicals) by electron spin resonance and superconducting quantum interference device-vibrating sample magnetometer analyses. Magnetization measurements demonstrate that this material exhibits spin-half paramagnetism with a spin concentration of ∼2.63 × 10²³ spins/mol. We also determined the microscopic origin of the magnetic moments in PTMR-CTF by investigating its spin density and electronic structure using density functional theory calculations.