No abstract

Covalent organic frameworks (COFs), formed by reversible condensation of rigid organic building blocks, are crystalline and porous materials of great potential for catalysis and organic electronics. Particularly with a view of organic electronics, achieving a maximum degree of crystallinity and large domain sizes while allowing for a tightly π-stacked topology would be highly desirable. We present a design concept that uses the 3D geometry of the building blocks to generate a lattice of uniquely defined docking sites for the attachment of consecutive layers, thus allowing us to achieve a greatly improved degree of order within a given average number of attachment and detachment cycles during COF growth. Synchronization of the molecular geometry across several hundred nanometers promotes the growth of highly crystalline frameworks with unprecedented domain sizes. Spectroscopic data indicate considerable delocalization of excitations along the π-stacked columns and the feasibility of donor-acceptor excitations across the imine bonds. The frameworks developed in this study can serve as a blueprint for the design of a broad range of tailor-made 2D COFs with extended π-conjugated building blocks for applications in photocatalysis and optoelectronics.

Traditionally, the properties and functions of covalent organic frameworks (COFs) are defined by their constituting building blocks, while the chemical bonds that connect the individual subunits have not attracted much attention as functional components of the final material. We have developed a new series of dual-pore perylene-based COFs and demonstrate that their imine bonds can be protonated reversibly, causing significant protonation-induced colour shifts towards the nearinfrared, while the structure and crystallinity of the frameworks are fully retained. Thin films of these COFs are highly sensitive colorimetric acid vapour sensors with a detection limit as low as 35 µg L-1 and a response range of at least four orders of magnitude. Since the acidochromism in our COFs is a cooperative phenomenon based on electronically coupled imines, the COFs can be used to determine simultaneously the concentration and protonation strength of non-aqueous acid solutions, in which pH electrodes are not applicable, and to distinguish between different acids. Including the imine bonds as functiondetermining constituents of the framework provides an additional handle for constructing multifunctional COFs and extending the range of their possible applications.

Covalent organic frameworks (COFs) are an emerging class of highly tuneable crystalline, porous materials. Here we report the first COFs that change their electronic structure reversibly depending on the surrounding atmosphere. These COFs can act as solid-state supramolecular solvatochromic sensors that show a strong colour change when exposed to humidity or solvent vapours, dependent on vapour concentration and solvent polarity. The excellent accessibility of the pores in vertically oriented films results in ultrafast response times below 200 ms, outperforming commercially available humidity sensors by more than an order of magnitude. Employing a solvatochromic COF film as a vapour-sensitive light filter, we demonstrate a fast humidity sensor with full reversibility and stability over at least 4000 cycles. Considering their immense chemical diversity and modular design, COFs with fine-tuned solvatochromic properties could broaden the range of possible applications for these materials in sensing and optoelectronics.

Most covalent organic frameworks (COFs) to date are made from relatively small aromatic subunits, which can only absorb the high-energy part of the visible spectrum. We have developed near-infrared-absorbing low bandgap COFs by incorporating donor-acceptor-type isoindigo- and thienoisoindigo-based building blocks. The new materials are intensely colored solids with a high degree of long-range order and a pseudo-quadratic pore geometry. Growing the COF as a vertically oriented thin film allows for the construction of an ordered interdigitated heterojunction through infiltration with a complementary semiconductor. Applying a thienoisoindigo-COF:fullerene heterojunction as the photoactive component, we realized the first COF-based UV- to NIR-responsive photodetector. We found that the spectral response of the device is reversibly switchable between blue- and red-sensitive, and green- and NIR-responsive. To the best of our knowledge, this is the first time that such nearly complete inversion of spectral sensitivity of a photodetector has been achieved. This effect could lead to potential applications in information technology or spectral imaging.

Two-dimensional covalent organic frameworks (2D-COFs) are crystalline, porous materials comprising aligned columns of π-stacked building blocks. With a view toward the application of these materials in organic electronics and optoelectronics, the construction of oligothiophene-based COFs would be highly desirable. The realization of such materials, however, has remained a challenge, in particular with respect to laterally conjugated imine-linked COFs. We have developed a new building block design employing an asymmetric modification on an otherwise symmetric backbone that allows us to construct a series of highly crystalline quaterthiophene-derived COFs with tunable electronic properties. Studying the optical response of these materials, we have observed for the first time the formation of a charge transfer state between the COF subunits across the imine bond. We believe that our new building block design provides a general strategy for the construction of well-ordered COFs from various extended building blocks, thus greatly expanding the range of applicable molecules.

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.

A major challenge in modern society is the reduction of microplastics created by polymers having stable C–C backbones. The chemistry of radical ring-opening copolymerization of cyclic ketene acetals (CKAs) with vinyl monomers in introducing degradable ester units into the C–C backbone is highly promising. Although the corresponding reaction in an aqueous medium should provide biodegradable primary dispersions, the bottleneck is the hydrolytic instability of CKAs. Therefore, in-depth hydrolysis kinetics of CKA (2-methylene-1,3-dioxepane, MDO) at different pH values and temperatures under homogenous and heterogenous conditions are studied to get a “sweet spot” under which emulsion polymerization of MDO might be possible. Depending on the pH, the hydrolysis of MDO undergoes three different mechanisms with slowed hydrolysis kinetics under alkaline conditions. Besides 4-hydroxy-1-butylacetate (4-HBA), other co-hydrolysis products were detected, leading to the autocatalysis effect. The fast MDO hydrolysis during emulsion copolymerization with vinyl acetate led to the formation of polymers with extremely less incorporation of ring-opened MDO units. Degradation tests of the corresponding emulsion copolymers compared with copolymers prepared in solution confirmed the low incorporation ratio of MDO. The results and discussion presented in this work will be a strong guideline for future emulsion copolymerizations of CKAs.