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The ability to grow covalent organic framework (COF) films allows for studying their properties as solid layers and enables the incorporation of these materials into a variety of functional devices...

Covalent organic frameworks (COFs) are a highly versatile group of porous materials constructed from molecular building blocks, enabling deliberate tuning of their final bulk properties for a broad range of applications. Understanding their excited-state dynamics is essential for identifying suitable COF materials for applications in electronic devices such as transistors, photovoltaic cells, and water-splitting electrodes. Here, we report on the ultrafast excited-state dynamics of a series of fully conjugated two-dimensional (2D) COFs in which different molecular subunits are connected through imine bonds, using transient absorption spectroscopy. Although these COFs feature different topologies and chromophores, we find that excited states behave similarly across the series. We therefore present a unified model in which charges are generated through rapid singlet-singlet annihilation and show lifetimes of several tens of microseconds. These long-lived charges are of particular interest for optoelectronic devices, and our results point toward the importance of controlling the singlet-singlet annihilation step in order to increase the yield of separated charges.

Covalent organic frameworks (COFs), consisting of covalently connected organic building units, combine attractive features such as crystallinity, open porosity and widely tunable physical properties. For optoelectronic applications, the incorporation of heteroatoms into a 2D COF has the potential to yield desired photophysical properties such as lower band gaps, but can also cause lateral offsets of adjacent layers. Here, we introduce dibenzo[g,p]chrysene (DBC) as a novel building block for the synthesis of highly crystalline and porous 2D dual-pore COFs showing interesting properties for optoelectronic applications. The newly synthesized terephthalaldehyde (TA), biphenyl (Biph), and thienothiophene (TT) DBC-COFs combine conjugation in the a,b-plane with a tight packing of adjacent layers guided through the molecular DBC node serving a specific docking site for successive layers. The resulting DBC-COFs exhibit a hexagonal dual-pore kagome geometry, which is comparable to COFs containing another molecular docking site, namely 4,4′,4″,4‴-(ethylene-1,1,2,2-tetrayl)-tetraaniline (ETTA). In this context, the respective interlayer distances decrease from about 4.60 Å in ETTA-COFs to about 3.6 Å in DBC-COFs, leading to well-defined hexagonally faceted single crystals sized about 50-100 nm. The TT DBC-COFs feature broad light absorption covering large parts of the visible spectrum, while Biph DBC-COF shows extraordinary excited state lifetimes exceeding 10 ns. In combination with the large number of recently developed linear conjugated building blocks, the new DBC tetra-connected node is expected to enable the synthesis of a large family of strongly p-stacked, highly ordered 2D COFs with promising optoelectronic properties.

Light-driven water electrolysis at a semiconductor surface is a promising way to generate hydrogen from sustainable energy sources, but its efficiency is limited by the performance of available photoabsorbers. Here we report the first time investigation of covalent organic frameworks (COFs) as a new class of photoelectrodes. The presented 2D-COF structure is assembled from aromatic amine-functionalized tetraphenylethylene and thiophene-based dialdehyde building blocks to form conjugated polyimine sheets, which π-stack in the third dimension to create photoactive porous frameworks. Highly oriented COF films absorb light in the visible range to generate photoexcited electrons that diffuse to the surface and are transferred to the electrolyte, resulting in proton reduction and hydrogen evolution. The observed photoelectrochemical activity of the 2D-COF films and their photocorrosion stability in water pave the way for a novel class of photoabsorber materials with versatile optical and electronic properties that are tunable through the selection of appropriate building blocks and their three-dimensional stacking.

The building industry makes a great effort to reduce energy consumption. The use of nanotechnology is one of the approaches to surpassing the properties of conventional insulation materials. In this work, an industrial cost‐effective method to manufacture highly porous materials with excellent thermal insulation properties is described. The materials are prepared from polystyrene recovered from the building sector and electrospun as nanofiber‐based sheets. Varying electrospinning parameters allow controlling the morphology of the produced materials. The materials are obtained with differences in interfiber and inner‐fiber porosity and morphology. The thermal conductivity of the freestanding and compressed materials is evaluated. Those differences affect the insulation performance: the materials with higher interfiber porosity show better thermal insulation in the freestanding state. An increase of the inner‐fiber porosity leads to better insulation in the compressed samples. Insertion of carbon nanomaterials reduces the effects of the infrared Radiation. Nanofiber‐based insulation materials from the recycled expanded polystyrene (EPS) show thermal conductivity values of 20 to 25 mW/mK (ie, 20% to 30% below the thermal conductivity of the commercial EPS). The effect of integrating polystyrene nanofiber sheets into conventional wall‐building materials is also investigated in terms of thermal insulation. The nanofiber insulation sheets are sandwiched between two pieces of the building materials resulting in a drastic increase of the insulation effect. The materials have a great potential in using, for example, as thermal insulation for the restoration of historic buildings in the narrow central parts of the old towns.

Two-dimensional covalent organic frameworks (2D COFs) attract great interest owing to their well-defined pore structure, thermal stability, high surface area, and permanent porosity. In combination with a tunable chemical pore environment, COFs are intriguing candidates for molecular sieving based on selective host-guest interactions. Herein, we report on 2D COF structures capable of reversibly switching between a highly correlated crystalline, porous and a poorly correlated, nonporous state by exposure to external stimuli. To identify COF structures with such dynamic response, we systematically studied the structural properties of a family of two-dimensional imine COFs comprising tris(4-aminophenyl)benzene (TAPB) and a variety of dialdehyde linear building blocks including terephthalaldehyde (TA) and dialdehydes of thienothiophene (TT), benzodithiophene (BDT), dimethoxybenzodithiophene (BDT-OMe), diethoxybenzodithiophene (BDT-OEt), dipropoxybenzodithiophene (BDT-OPr), and pyrene (Pyrene-2,7). TAPB-COFs consisting of linear building blocks with enlarged π-systems or alkoxy functionalities showed significant stability toward exposure to external stimuli such as solvents or solvent vapors. In contrast, TAPB-COFs containing unsubstituted linear building blocks instantly responded to exposure to these external stimuli by a drastic reduction in COF layer correlation, long-range order, and porosity. To reverse the process we developed an activation procedure in supercritical carbon dioxide (scCO2) as a highly efficient means to revert fragile nonporous and amorphous COF polymers into highly crystalline and open porous frameworks. Strikingly, the framework structure of TAPB-COFs responds dynamically to such chemical stimuli, demonstrating that their porosity and crystallinity can be reversibly controlled by alternating steps of solvent stimuli and scCO2 activation.

Covalent organic frameworks (COFs) formed by connecting multidentate organic building blocks through covalent bonds provide a platform for designing multifunctional porous materials with atomic precision. As they are promising materials for applications in optoelectronics, they would benefit from a maximum degree of long-range order within the framework, which has remained a major challenge. We have developed a synthetic concept to allow consecutive COF sheets to lock in position during crystal growth, and thus minimize the occurrence of stacking faults and dislocations. Hereby, the three-dimensional conformation of propeller-shaped molecular building units was used to generate well-defined periodic docking sites, which guided the attachment of successive building blocks that, in turn, promoted long-range order during COF formation. This approach enables us to achieve a very high crystallinity for a series of COFs that comprise tri- and tetradentate central building blocks. We expect this strategy to be transferable to a broad range of customized COFs. Covalent organic frameworks (COFs) are attractive multifunctional porous materials that can be generated with atomic precision. However, endowing them with long-range order—desirable for applications—has remained challenging. Now, propeller-shaped building units have been used that allow consecutive layers to lock in position, resulting in highly crystalline COFs.

Crystallinity and porosity are of central importance for many properties of covalent organic frameworks (COFs), including adsorption, diffusion, and electronic transport. We have developed a new method for strongly enhancing both aspects through the introduction of a modulating agent in the synthesis. This modulator competes with one of the building blocks during the solvothermal COF growth, resulting in highly crystalline frameworks with greatly increased domain sizes reaching several hundreds of nanometers. The obtained materials feature fully accessible pores with an internal surface area of over 2000 m(2) g(-1). Compositional analysis via NMR spectroscopy revealed that the COF-5 structure can form over a wide range of boronic acid-to-catechol ratios, thus producing frameworks with compositions ranging from highly boronic acid-deficient to networks with catechol voids. Visualization of an -SH-functionalized modulating agent via iridium staining revealed that the COF domains are terminated by the modulator. Using functionalized modulators, this synthetic approach thus also provides a new and facile method for the external surface functionalization of COF domains, providing accessible sites for post-synthetic modification reactions. We demonstrate the feasibility of this concept by covalently attaching fluorescent dyes and hydrophilic polymers to the COF surface. We anticipate that the realization of highly crystalline COFs with the option of additional surface functionality will render the modulation concept beneficial for a range of applications, including gas separations, catalysis, and optoelectronics.