Turning 3D Covalent Organic Frameworks into Luminescent Ratiometric Temperature Sensors

Impact of Postsynthetic Modification on the Covalent Organic Framework (COF) Structures

Covalent organic frameworks (COFs) have emerged as a versatile class of materials owing to their well-defined crystalline structures and inherent porosity. In the realm of COFs, their appeal lies in their customizable nature, which can be further enhanced by incorporating diverse functionalities. Postsynthetic modifications (PSMs) emerge as a potent strategy, facilitating the introduction of desired functionalities postsynthesis. A significant challenge in PSM pertains to preserving the crystallinity and porosity of the COFs. In this study, we aim to investigate the intricate interplay between PSM strategies and the resulting crystalline and porous structures of the COFs. The investigation delves into the diverse methodologies employed in PSMs, to elucidate their distinct influences on the crystallinity and porosity of the COFs. Through a comprehensive analysis of recent advancements and case studies, the study highlights the intricate relationships among PSM parameters, including reaction conditions, precursor selection, and functional groups, and their impact on the structural features of COFs. By understanding how PSM strategies can fine-tune the crystalline and porous characteristics of COFs, researchers can harness this knowledge to design COFs with tailored properties for specific applications, contributing to the advancement of functional materials in diverse fields. This work not only deepens our understanding of COFs but also provides valuable insights into the broader realm of PSM strategies for other solid materials.

Ionic Covalent Organic Framework as a Dual Functional Sensor for Temperature and Humidity

Visual sensing of humidity and temperature by solids plays an important role in the everyday life and in industrial processes. Due to their hydrophobic nature, most covalent organic framework (COF) sensors often exhibit poor optical response when exposed to moisture. To overcome this challenge, the optical response is set out to improve, to moisture by incorporating H-bonding ionic functionalities into the COF network. A highly sensitive COF, consisting of guanidinium and diformylpyridine linkers (TG-DFP), capable of detecting changes in temperature and moisture content is fabricated. The hydrophilic nature of the framework enables enhanced water uptake, allowing the trapped water molecules to form a large number of hydrogen bonds. Despite the presence of non-emissive building blocks, the H-bonds restrict internal bond rotation within the COF, leading to reversible fluorescence and solid-state optical hydrochromism in response to relative humidity and temperature.

Analysis of COF-300 synthesis: probing degradation processes and 3D electron diffraction structure

Although COF-300 is often used as an example to study the synthesis and structure of (3D) covalent organic frameworks (COFs), knowledge of the underlying synthetic processes is still fragmented. Here, an optimized synthetic procedure based on a combination of linker protection and modulation was applied. Using this approach, the influence of time and temperature on the synthesis of COF-300 was studied. Synthesis times that were too short produced materials with limited crystallinity and porosity, lacking the typical pore flexibility associated with COF-300. On the other hand, synthesis times that were too long could be characterized by loss of crystallinity and pore order by degradation of the tetrakis(4-aminophenyl)methane (TAM) linker used. The presence of the degradation product was confirmed by visual inspection, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). As TAM is by far the most popular linker for the synthesis of 3D COFs, this degradation process might be one of the reasons why the development of 3D COFs is still lagging compared with 2D COFs. However, COF crystals obtained via an optimized procedure could be structurally probed using 3D electron diffraction (3DED). The 3DED analysis resulted in a full structure determination of COF-300 at atomic resolution with satisfying data parameters. Comparison of our 3DED-derived structural model with previously reported single-crystal X-ray diffraction data for this material, as well as parameters derived from the Cambridge Structural Database, demonstrates the high accuracy of the 3DED method for structure determination. This validation might accelerate the exploitation of 3DED as a structure determination technique for COFs and other porous materials.

Europium(III) Functionalized Covalent Organic Framework as Sensitive and Selective Fluorescent Switch for Detection of Uranium

Uranium poses severe health risks due to its radioactivity and chemical toxicity if released into the environment. Therefore, there is an urgent demand to develop sensing materials in situ monitoring of uranium with high sensitivity and stability. In this work, a fluorescent Eu3+-TFPB-Bpy is synthesized by grafting Eu3+ cation onto TFPB-Bpy covalent organic framework (COF) synthesized through Schiff base condensation of monomers 1,3,5-tris(4-formylphenyl)benzene (TFPB) and 5,5'-diamino-2,2'-bipyridine (Bpy). The fluorescence of Eu3+-TFPB-Bpy is enhanced compared with that of TFPB-Bpy, which is originated from the intramolecular rotations of building blocks limited by the bipyridine units of TFPB-Bpy coordinated with Eu3+. More significantly, Eu3+-TFPB-Bpy is a highly efficient probe for sensing UO22+ in aqueous solution with the luminescence intensity efficiently amplified by complexation of UO22+ with Eu3+. The turn-on sensing capability was derived from the resonance energy transfer occurring from UO22+ to the Eu3+-TFPB-Bpy. The developed probe displayed desirable linear range from 5 nM to 5 μM with good selectivity and rapid response time (2 s) for UO22+ in mining wastewater. This strategy provides a vivid illustration for designing luminescence lanthanide COF hybrid materials with applications in environmental monitoring.