Thermally conductive ultra-low-k dielectric layers based on two-dimensional covalent organic frameworks.

As the features of microprocessors are miniaturized, low-dielectric-constant (low-k) materials are necessary to limit electronic crosstalk, charge build-up, and signal propagation delay. However, all known low-k dielectrics exhibit low thermal conductivities, which complicate heat dissipation in high-power-density chips. Two-dimensional (2D) covalent organic frameworks (COFs) combine immense permanent porosities, which lead to low dielectric permittivities, and periodic layered structures, which grant relatively high thermal conductivities. However, conventional synthetic routes produce 2D COFs that are unsuitable for the evaluation of these properties and integration into devices. Here, we report the fabrication of high-quality COF thin films, which enable thermoreflectance and impedance spectroscopy measurements. These measurements reveal that 2D COFs have high thermal conductivities (1 W m-1 K-1) with ultra-low dielectric permittivities (k = 1.6). These results show that oriented, layered 2D polymers are promising next-generation dielectric layers and that these molecularly precise materials offer tunable combinations of useful properties.

Trends in the thermal stability of two-dimensional covalent organic frameworks.

Two-dimensional covalent organic frameworks (2D COFs) are synthetically diverse, layered macromolecules. Their covalent lattices are thought to confer high thermal stability, which is typically evaluated with thermogravimetric analysis (TGA). However, TGA measures the temperature at which volatile degradation products are formed and is insensitive to changes of the periodic structure of the COF. Here, we study the thermal stability of ten 2D COFs using a combination of variable-temperature X-ray diffraction, TGA, diffuse reflectance infrared spectroscopy, and density functional theory calculations. We find that 2D COFs undergo a general two-step thermal degradation process. At the first degradation temperature, 2D COFs lose their crystallinity without chemical degradation. Then, at higher temperatures, they chemically degrade into volatile byproducts. Several trends emerge from this exploration of 2D COF stability. Boronate ester-linked COFs are generally more thermally stable than comparable imine-linked COFs. Smaller crystalline lattices are more robust to thermal degradation than chemically similar larger lattices. Finally, pore-functionalized COFs degrade at significantly lower temperatures than their unfunctionalized analogues. These trends offer design criteria for thermally resilient 2D COF materials. These findings will inform and encourage a broader exploration of mechanical deformation in 2D networks, providing a necessary step towards their practical use.

Phenazine-Based Covalent Organic Framework Cathode Materials with High Energy and Power Densities.

Redox-active covalent organic frameworks (COFs) are promising materials for energy storage devices because of their high density of redox sites, permanent and controlled porosity, high surface areas, and tunable structures. However, the low electrochemical accessibility of their redox-active sites has limited COF-based devices either to thin films (<250 nm) grown on conductive substrates or to thicker films (1 µm) when a conductive polymer is introduced into the COF pores. Electrical energy storage devices constructed from bulk microcrystalline COF powders, eliminating the need for both thin-film formation and conductive polymer guests, would offer both improved capacity and potentially scalable fabrication processes. Here we report on the synthesis and electrochemical evaluation of a new phenazine-based 2D COF (DAPH-TFP COF), as well as its composite with poly(3,4-ethylenedioxythiophene) (PEDOT). Both the COF and its PEDOT composite were evaluated as powders that were solution-cast onto bulk electrodes serving as current collectors. The unmodified DAPH-TFP COF exhibited excellent electrical access to its redox sites, even without PEDOT functionalization, and outperformed the PEDOT composite of our previously reported anthraquinone-based system. Devices containing DAPH-TFP COF were able to deliver both high-energy and high-power densities, validating the promise of unmodified redox-active COFs that are easily incorporated into electrical energy storage devices.

Acid Exfoliation of Imine-linked Covalent Organic Frameworks Enables Solution Processing into Crystalline Thin Films.

Covalent organic frameworks (COFs) are highly modular porous crystalline polymers that are of interest for applications such as charge-storage devices, nanofiltration membranes, and optoelectronic devices. COFs are typically synthesized as microcrystalline powders, which limits their performance in these applications, and their limited solubility precludes large-scale processing into more useful morphologies and devices. We report a general, scalable method to exfoliate two-dimensional imine-linked COF powders by temporarily protonating their linkages. The resulting suspensions were cast into continuous crystalline COF films up to 10 cm in diameter, with thicknesses ranging from 50 nm to 20 µm depending on the suspension composition, concentration, and casting protocol. Furthermore, we demonstrate that the film fabrication process proceeds through a partial depolymerization/repolymerization mechanism, providing mechanically robust films that can be easily separated from their substrates.

Chemical Control over Nucleation and Anisotropic Growth of Two-Dimensional Covalent Organic Frameworks.

Two-dimensional covalent organic frameworks (2D COFs) are composed of structurally precise, permanently porous, layered polymer sheets. 2D COFs have traditionally been synthesized as polycrystalline aggregates with small crystalline domains. Only recently have a small number of 2D COFs been obtained as single crystals, which were prepared by a seeded growth approach via the slow introduction of monomers, which favored particle growth over nucleation. However, these procedures are slow and operationally difficult, making it desirable to develop polymerization methods that do not require the continuous addition of reactants over days or weeks. Here, we achieve the rapid growth of boronate ester-linked COFs by chemically suppressing nucleation via addition of an excess of a monofunctional competitor, 4-tert-butylcatechol (TCAT), into the polymerization. In situ X-ray scattering measurements show that TCAT suppresses colloid nucleation, which enables seeded growth polymerizations in the presence of high monomer concentrations. Kinetic Monte Carlo simulations reveal that TCAT limits oligomers to sizes below the critical nucleus size and that in-plane expansion is restricted compared to out-of-plane oriented attachment of oligomers. The simulations are consistent with transmission electron micrographs, which show that the particles grow predominantly in the stacking direction. This mechanistic insight into the role of the modulators in 2D polymerizations enables the size and aspect ratio of COF colloids to be controlled under operationally simple conditions. This chemically controlled growth strategy will accelerate the discovery and exploration of COF materials and their emergent properties.

Improved synthesis of ß-ketoenamine-linked covalent organic frameworks via monomer exchange reactions.

ß-Ketoenamine-linked covalent organic frameworks (COFs) offer excellent structural versatility and outstanding aqueous stability, but their stability complicates obtaining samples with high crystallinity and surface areas. In contrast, imine-linked COFs are often isolated with superior materials quality. Here we synthesize several ß-ketoenamine-linked COFs, including two unreported structures, with unmatched crystallinity and high surface areas by preparing the corresponding imine-linked COF and exchanging its triformylbenzene monomers with triformylphloroglucinol.

Seeded growth of single-crystal two-dimensional covalent organic frameworks.

Polymerization of monomers into periodic two-dimensional networks provides structurally precise, layered macromolecular sheets that exhibit desirable mechanical, optoelectronic, and molecular transport properties. Two-dimensional covalent organic frameworks (2D COFs) offer broad monomer scope but are generally isolated as powders comprising aggregated nanometer-scale crystallites. We found that 2D COF formation could be controlled using a two-step procedure in which monomers are added slowly to preformed nanoparticle seeds. The resulting 2D COFs are isolated as single-crystalline, micrometer-sized particles. Transient absorption spectroscopy of the dispersed COF nanoparticles revealed improvement in signal quality by two to three orders of magnitude relative to polycrystalline powder samples, and suggests exciton diffusion over longer length scales than those obtained through previous approaches. These findings should enable a broad exploration of synthetic 2D polymer structures and properties.

Equilibration of Imine-Linked Polymers to Hexagonal Macrocycles Driven by Self-Assembly.

Macrocycles based on directional bonding and dynamic covalent bond exchange can be designed with specific pore shapes, sizes, and functionality. These systems retain many of the design criteria and desirable aspects of two-dimensional (2D) covalent organic frameworks (COFs) but are more easily processed. Here we access discrete hexagonal imine-linked macrocycles by condensing a truncated analogue of 1,3,5-tris(4-aminophenyl)benzene (TAPB) with terephthaldehyde (PDA). The monomers first condense into polymers but eventually convert into hexagonal macrocycles in high yield. The high selectivity for hexagonal macrocycles is enforced by their aggregation and crystallization into layered structures with more sluggish imine exchange. Their formation and exchange processes provide new insight into how imine-linked 2D COF simultaneously polymerize and crystallize. Solutions of these assembled macrocycles were cast into oriented, crystalline films, expanding the potential routes to 2D materials.

Synthesis of 2D Imine-Linked Covalent Organic Frameworks through Formal Transimination Reactions.

Covalent organic frameworks (COFs) are crystalline, permanently porous, two-dimensional or three-dimensional polymers with tunable topology and functionality. COFs linked with imines or ß-ketoenamines are more chemically stable than their boron-linked counterparts, making them more promising for a broad range of applications, including energy storage devices, proton-conductive membranes, and catalyst supports. We report a general and scalable method for synthesizing imine- and ß-ketoenamine-linked COFs based on the formal transimination of N-aryl benzophenone imines. These substrates are often the synthetic precursors of traditional polyfunctional aryl amine monomers and are more stable, soluble, and easy to handle and purify. The imine- and ß-ketoenamine-linked COFs obtained from this approach show excellent materials quality, as characterized by X-ray diffraction and surface area analysis. The most optimized COF exhibited a Brunauer-Emmett-Teller surface area (>2600 m2/g) very close to its theoretical value (2830 m2/g). This method is amenable to both conventional solvothermal conditions and microwave heating, providing similar or even improved materials quality with shorter reaction times. The high materials quality, scalability, and availability of benzophenone imine monomers are all attractive features of this approach.

Dearomatization Approach to 2-Trifluoromethylated Benzofuran and Dihydrobenzofuran Products.

A mild dearomatization enabled ortho-selective replacement of an aromatic C-H bond with a hexafluoroacetylacetone (hfacac) substituent has been developed. This reaction is dependent on a hypervalent iodine generated phenoxonium intermediate, a critical choice of solvent, and reagent addition order. The fluorinated dihydrobenzofuran product can be transformed into dihydrobenzofuran and benzofuran products decorated with a 2-trifluoromethyl group. The 3-trifluoromethylacyl substituted benzofurans rapidly form hydrates, which can be reduced to the corresponding alcohols.

Metal-Free Synthesis of Fluorinated Indoles Enabled by Oxidative Dearomatization.

Nitrogen heterocycles are found in a majority of approved small-molecule pharmaceuticals, and the number of approved fluorinated drugs is increasing each decade. Therefore, new approaches for accessing fluorinated nitrogen heterocycles are of great significance. A novel, scalable, and metal-free method for accessing a wide range of fluorinated indoles is described. This oxidative-dearomatization-enabled approach assembles 2-trifluoromethyl NH-indole products from simple commercially available anilines with hexafluoroacetylacetone in the presence of an organic oxidant. The nature of the aniline N-capping group is critical for the success of this new reaction. Furthermore, the indole products contain a 3-trifluoroacetyl group, which can be exploited to access a plethora of useful functional groups.

Formation of Fused Aromatic Architectures via an Oxidative Dearomatization - Radical Cyclization Rearomatization Approach.

A new mild C-C bond forming cyclization approach of catechol derivatives is reported. This approach relies on an initial dearomatization step using lead (IV) acetate followed by a carefully controlled radical cyclization step, which under the reaction conditions also facilitates rearomatization. Triethylborane is the key to the success of this reaction as it enables the reaction to proceed at low temperatures and is also believed to aid rearomatization. The amount and ratio of triethylborane and reducing agent (tributyltinhydride) that is employed as well as the concentration the reaction is run at are all essential to the success of this new approach.

Analysis of the structural diversity, substitution patterns, and frequency of nitrogen heterocycles among U.S. FDA approved pharmaceuticals.

Nitrogen heterocycles are among the most significant structural components of pharmaceuticals. Analysis of our database of U.S. FDA approved drugs reveals that 59% of unique small-molecule drugs contain a nitrogen heterocycle. In this review we report on the top 25 most commonly utilized nitrogen heterocycles found in pharmaceuticals. The main part of our analysis is divided into seven sections: (1) three- and four-membered heterocycles, (2) five-, (3) six-, and (4) seven- and eight-membered heterocycles, as well as (5) fused, (6) bridged bicyclic, and (7) macrocyclic nitrogen heterocycles. Each section reveals the top nitrogen heterocyclic structures and their relative impact for that ring type. For the most commonly used nitrogen heterocycles, we report detailed substitution patterns, highlight common architectural cores, and discuss unusual or rare structures.

Data-mining for sulfur and fluorine: an evaluation of pharmaceuticals to reveal opportunities for drug design and discovery.

Among carbon, hydrogen, oxygen, and nitrogen, sulfur and fluorine are both leading constituents of the pharmaceuticals that comprise our medicinal history. In efforts to stimulate the minds of both the general public and expert scientist, statistics were collected from the trends associated with therapeutics spanning 12 disease categories (a total of 1969 drugs) from our new graphical montage compilation: disease focused pharmaceuticals posters. Each poster is a vibrant display of a collection of pharmaceuticals (including structural image, Food and Drug Administration (FDA) approval date, international nonproprietary name (INN), initial market name, and a color-coded subclass of function) organized chronologically and classified according to an association with a particular clinical indication. Specifically, the evolution and structural diversity of sulfur and the popular integration of fluorine into drugs introduced over the past 50 years are evaluated. The presented qualitative conclusions in this article aim to promote innovative insights into drug development.