Facile transformation of imine covalent organic frameworks into ultrastable crystalline porous aromatic frameworks

Abstract

The growing interest in two-dimensional imine-based covalent organic frameworks (COFs) is inspired by their crystalline porous structures and the potential for extensive π-electron delocalization. The intrinsic reversibility and strong polarization of imine linkages, however, leads to insufficient chemical stability and optoelectronic properties. Developing COFs with improved robustness and π-delocalization is highly desirable but remains an unsettled challenge. Here we report a facile strategy that transforms imine-linked COFs into ultrastable porous aromatic frameworks by kinetically fixing the reversible imine linkage via an aza-Diels-Alder cycloaddition reaction. The as-formed, quinoline-linked COFs not only retain crystallinity and porosity, but also display dramatically enhanced chemical stability over their imine-based COF precursors, rendering them among the most robust COFs up-to-date that can withstand strong acidic, basic and redox environment. Owing to the chemical diversity of the cycloaddition reaction and structural tunability of COFs, the pores of COFs can be readily engineered to realize pre-designed surface functionality.

Facile Fabrication of Redox‐Responsive Covalent Organic Framework Nanocarriers for Efficiently Loading and Delivering Doxorubicin

“…The particle size of both F68@SS‐COFs ( Figure A) and F68@n‐COFs (Figure S2A, Supporting Information) almost remained the original size without precipitation or phase separation after incubation in the weak acidic environment for 48 h. Their stability may be due to the benefit of PEGylated shell. In addition, some previous studies have demonstrated that the imine‐based COFs, at least for a time period, are relatively stable under weak acidic condition . However, F68@SS‐COFs possessed a disulfide‐rich COFs core in the internal space and thus F68@SS‐COFs are expected to be able to disintegrate in the presence of GSH.…”

Light-promoted synthesis of highly-conjugated crystalline covalent organic framework

“…To obtain a crystalline product in continuously linked molecular system, this reversible covalent bond formation is important because it allows continuous covalent bond formation/deformation process that is essential to obtain highly symmetric, stable and homogenous products 21 . Reversibility is especially important in the Schiff's base reaction [25][26][27] , because amorphous or poorly crystalline materials are easily observed in polymers such as aza-CMP that containing imines 13,28 . The presence of water and the pH of precursor solution are important factors to obtain crystalline imine-based COF by an efficient reversible imine-condensation reaction 20 .…”

Fully Conjugated Two‐Dimensional sp2‐Carbon Covalent Organic Frameworks as Artificial Photosystem I with High Efficiency

“…Despite the fact that the extended π‐conjugation can propagate through the ‐N=C‐ double bonds in the Schiff base structure, the imine‐linkage displays relatively poor stability and weak electron delocalization. To overcome this limitation, researchers have been actively exploring new reactions to construct fully conjugated COFs with an optimal electron delocalization . However, examples of fully conjugated imine‐free COFs with sp 2 ‐carbon double bond structures are still rare because it is extremely difficult to form ‐C=C‐ bonds via reversible coupling reactions and simultaneously gain the desired conjugation as well as the refined crystallinity within the COF structure.…”