Covalent Organic Frameworks with trans-Dimensionally Vinylene-linked π-Conjugated Motifs

Ratiometric Fluorescent Detection of Chromium(III) Based on One-Dimensional Imine-Linked Covalent Organic Framework

Chromium (Cr3+) as a highly toxic pollutant has aroused much attention due to its wide industrial applications. Covalent organic frameworks (COFs) have been considered as one of the most promising metal ion sensors due to their open pore channels and abundant adsorption sites. Herein, a novel luminescent one-dimensional COF (Py-An COF) was constructed by the condensation of 5'-(anthracen-9-yl)-[1,1':3',1″-terphenyl]-4,4″-dicarbaldehyde (An-2CHO) and 1,3,6,8-tetrakis(p-aminophenyl) pyrene (PyTTA). The resulting COF showed high crystallinity, comparative high surface area, and good thermal stability, which can be utilized for Cr3+ fluorescent sensors with high sensitivity and selectivity. Furthermore, the coordination between Schiff-base N atoms and Cr3+ inhibits the photoinduced electron transfer (PET) process, resulting in the enhanced fluorescence intensity of chromophores (436 nm) and decreased the fluorescence intensity of Py-An COF (512 nm) simultaneously, which eventually realizes a highly efficient ratiometric fluorescent sensor for trace Cr3+. Compared with other porous fluorescent materials, Py-An COF possesses a lower detection limit, higher sensitivity, and better selectivity. This work provides strategic guidance for the design of COFs as ratiometric fluorescence sensors.

Narrow-Pore Engineering of Vinylene-Linked Covalent Organic Frameworks with Weak Interaction-Triggered Multiple Responses

Vinylene-linked covalent organic frameworks (COFs) are emerging as promising crystalline materials, but their narrow pore engineering is severely impeded by the weak reversibility of the carbon-carbon double bond formation reaction, which has led to less exploration of their ultramicroporous structures and properties. Herein, we developed a single aromatic ring-based tetratopic monomer, tetramethylpyrazine, which undergoes a smooth Knoevenegal condensation at its four arylmethly carbon atoms with linear aromatic dialdehyde monomers upon the self-catalyzed activation of pyridine nitrogen-containing monomers in the presence of an organic anhydride. This has resulted in the formation of two vinylene-linked COFs, which both crystallized in orthorhombic lattices, and layered in AA stacking fashions along the vertical directions. They exhibit high surface areas and well-tailored ultramicropore sizes up to 0.5 nm. The unique cross-linking mode at two pairs of para-positions of each pyrazine unit through carbon-carbon double bonds afford them with π-extended conjugation over the in-plane backbones and substantial semiconducting characters. The resultant COFs can be well-dispersed in water to form stable sub-microparticles with negative charges (zeta potentials: ca. -30 mV), and exhibiting tunable aggregation behaviors through protonation/deprotonation. As a consequence, they exhibit pore-size-dependent colorimetric responses to various anions with different pKa values in high selectivity.

Hexatopic Vertex-Directed Approach to Vinylene-Linked Covalent Organic Frameworks with Heteroporous Topologies

A D_3h-symmetric hexatopic monomer was first prepared by attaching the three-fold ditopic moiety 2,6-dimethylpyridine to the meta-positions of a phenyl ring. It was further condensed at its six pyridylmethyl carbons with linear ditopic aromatic dialdehydes, resulting in two vinylene-linked COFs with heteroporous topologies, as revealed by powder X-ray diffraction (PXRD), nitrogen sorption, and pore-size distribution analyses, as well as transmission electron microscopy (TEM) image. The linear- and cross-conjugations, respectively, arising from the 2,6-linked pyridines and meta-linked phenylenes in the hexatopic nodes rendered the resultant COFs with well-patterned π-delocalization, allowing for efficiently catalyzing the bromination of aromatic derivatives with the pore-size-dependent conversion yields and regioselectivity under the irradiation of green light.