Three-dimensional covalent organic frameworks based on a π-conjugated tetrahedral node

Intervalence Charge Transfer in an Osmium(IV) Tetra(ferrocenylaryl) Complex

The functional properties of tetraaryl compounds, M(aryl)4 (M = transition metal or group 14 element), are dictated not only by their common tetrahedral geometry but also by their central atom. The identity of this atom may serve to modulate the reactivity, electrochemical, magnetic, and optical behavior of the molecular species, or of extended materials built from appropriate tetraaryl building blocks, but this has not yet been systematically evaluated. Toward this goal, here we probe the influence of Os(IV), C, and Si central atoms on the spectroelectrochemical properties of a series of redox-active tetra(ferrocenylaryl) complexes. We prepare these compounds through Negishi cross-coupling from brominated precursors and confirm, through single-crystal X-ray diffraction, they exhibit comparable molecular structures with only small differences attributable to their dissimilar M-aryl bond lengths. Solution voltammetric and spectroelectrochemical studies reveal that access to mixed-valence states is uniquely provided by the Os(IV) species, which also exhibits a near-IR absorption band that is characteristic of intervalence charge transfer processes. Together, this work serves to highlight the remarkable electrochemical stability and distinct electronic properties of the Os(aryl)4 unit, as well as the potential utility of these and other M(aryl)4 species as modular building blocks for three-dimensional polymers or functional molecular devices.

Photons, Excitons, and Electrons in Covalent Organic Frameworks

Covalent organic frameworks (COFs) are created by the condensation of molecular building blocks and nodes to form two-dimensional (2D) or three-dimensional (3D) crystalline frameworks. The diversity of molecular building blocks with different properties and functionalities and the large number of possible framework topologies open a vast space of possible well-defined porous architectures. Besides more classical applications of porous materials such as molecular absorption, separation, and catalytic conversions, interest in the optoelectronic properties of COFs has recently increased considerably. The electronic properties of both the molecular building blocks and their linkage chemistry can be controlled to tune photon absorption and emission, to create excitons and charge carriers, and to use these charge carriers in different applications such as photocatalysis, luminescence, chemical sensing, and photovoltaics. In this Perspective, we will discuss the relationship between the structural features of COFs and their optoelectronic properties, starting with the building blocks and their chemical connectivity, layer stacking in 2D COFs, control over defects and morphology including thin film synthesis, exploring the theoretical modeling of structural, electronic, and dynamic features of COFs, and discussing recent intriguing applications with a focus on photocatalysis and photoelectrochemistry. We conclude with some remarks about present challenges and future prospects of this powerful architectural paradigm.

Three-Dimensional Fully Conjugated Covalent Organic Frameworks for Efficient Photocatalytic Water Splitting

Covalent organic frameworks (COFs) are promising photocatalysts for water splitting, but their efficiency lags behind that of inorganic counterparts partly due to the limited charge transport and optical absorption properties. To overcome this limitation, we proposed to employ three-dimensional (3D) fully conjugated (FC) COFs with a topological assembly of cyclooctatetraene derivatives for photocatalytic water splitting. On the basis of first-principles calculations, we demonstrated that these 3D FC-COFs are semiconductors with exceptional charge transport and optical absorption properties. The carrier mobilities are comparable to those of inorganic semiconductors and superior to the record mobility observed in two-dimensional COFs. Additionally, the 3D FC-COFs exhibit broad visible light absorption with direct band gaps and high optical absorption coefficients. Among them, two 3D FC-COFs are identified for overall water splitting, while three others can facilitate the hydrogen evolution half-reaction. This study pioneers the design of 3D FC-COF photocatalysts, potentially advancing their applications in photocatalysis and optoelectronics.

Pd nanoparticle-decorated covalent organic frameworks for enhanced photocatalytic tetracycline hydrochloride degradation and hydrogen evolution

In this study, we synthesized novel bipyridine-based, sp²-carbon-linked COFs with the incorporation of ultra-small metal nanoparticles for enhanced photocatalytic tetracycline hydrochloride degradation and hydrogen evolution. The obtained photocatalyst exhibits strong visible light absorption and modulated electronic structure, owing to charge transfer between the metal and COFs, resulting in tuned proton absorption/desorption energy. As a result, the Pd-COFs exhibit remarkable photocatalytic activities for both tetracycline hydrochloride removal and hydrogen evolution. Specifically, the rate constant of photocatalytic tetracycline hydrochloride removal reaches 0.03406 min^-1 with excellent stability and the photocatalytic hydrogen evolution rate reaches 98.17 mmol g^-1 h^-1, outperforming the-state-of-the-art photocatalysts with noble Pt loading.

Three-Dimensional Covalent Organic Frameworks: From Synthesis to Applications

Three-dimensional covalent organic frameworks (3D COFs) with spatially periodic networks demonstrate significant advantages over their 2D counterparts, including enhanced specific surface areas, interconnected channels, and more sufficiently exposed active sites. Nevertheless, research on these materials has met an impasse due to serious problems in crystallization and stability, which must be solved for practical applications. In this Minireview, we first summarize some strategies for preparing functional 3D COFs, including crystallization techniques and functionalization methods. Hereafter, applications of these functional materials are presented, covering adsorption, separation, catalysis, fluorescence, sensing, and batteries. Finally, the future challenges and perspectives for the development of 3D COFs are discussed.

Conjugated Three-Dimensional High-Connected Covalent Organic Frameworks for Lithium-Sulfur Batteries

Developing conjugated three-dimensional (3D) covalent organic frameworks (COFs) still remains an extremely difficult task due to the lack of enough conjugated 3D building blocks. Herein, condensation between an 8-connected pentiptycene-based D_2h building block (DMOPTP) and 4-connected square-planar linkers affords two 3D COFs (named 3D-scu-COF-1 and 3D-scu-COF-2). A combination of the 3D homoaromatic conjugated structure of the former building block with the 2D conjugated structure of the latter linking units enables the π-electron delocalization over the whole frameworks of both COFs, endowing them with excellent conductivities of 3.2-3.5 × 10^-5 S cm^-1. In particular, the 3D rigid quadrangular prism shape of DMOPTP guides the formation of a twofold interpenetrated scu 3D topology and high-connected permanent porosity with a large Brunauer-Emmett-Teller (BET) surface area of 2340 and 1602 m² g^-1 for 3D-scu-COF-1 and 3D-scu-COF-2, respectively, ensuring effective small molecule storage and mass transport characteristics. This, in combination with their good charge transport properties, renders them promising sulfur host materials for lithium-sulfur batteries (LSBs) with high capacities (1035-1155 mA h g^-1 at 0.2 C, 1 C = 1675 mA g^-1), excellent rate capabilities (713-757 mA h g^-1 at 5.0 C), and superior cycling stability (71-83% capacity retention at 2.0 C after 500 cycles), surpassing the most of organic LSB cathodes reported thus far.

Sustained-release nanocapsule based on a 3D COF for long-term enzyme prodrug therapy of cancer

A well-designed three-dimensional (3D) covalent organic framework (COF) was constructed as a nanocapsule for the encapsulation of horseradish peroxidase (HRP), which could realize sustained release of HRP to prolong the duration of the therapeutic agents and promote long-term enzyme prodrug therapy.