Kinetic Fixation of Imine Bonds for Ultra-stable Crystalline Porous Aromatic Frameworks
The paper in Nature Communications is here: go.nature.com/2n1WOpm
Since the first report in 2005, COFs have been adopted as an auspicious platform for a plethora of applications by virtue of their high surface area, chemical diversity, and tunable functionality. The formation of COFs relies on the reversible covalent bond formation to connect multivalent monomers through thermodynamic equilibrium. However, this inherent reversibility of the linkages within COFs severely limits the stability towards solvents and chemicals and constrains their practical applications. Distinct from boroxine-linked COFs which are susceptible to water or protic solvents, imine-linked COFs represent the prevalent class of COFs that show improved hydrothermal stability. Due to the dynamic nature of imine, the imine-linked COFs still undergo hydrolysis under strongly acidic conditions or exchange with amines. To this end, a great deal of research efforts have been directed to the development of ultra-table COFs through keto-enol tautomerism, inter- and intra-layer H-bonding, interlayer interaction reinforcement by methoxy groups and etc. Despite the remarkable progress, the above-mentioned strategies require linkers with specific functionalities and the chemical stability is still far from satisfactory. As such, a facile methodology that enables the fabrication of chemically robust COFs is much desired.
In addition, imine-linked COFs are inferior in facilitating π electron delocalization between the linked units on account of the strongly polarized nature of the C=N bonds. Despite a few examples of crystalline π-extended 2D polymer layers obtained from the surface- or interface-assisted synthesis, and a most recent sp2 carbon-conjugated COF, a general strategy to reinforce strong covalent bonding and extended π-electron delocalization in the same framework remains largely unexplored.
To overcome the two aforementioned issues, we explored a post-synthetic modification strategy that can transform the dynamic imine linkages in COFs into more robust and conductive quinoline aromatic ring systems via aza-Diels-Alder cycloaddition. This framework-to-framework transformation offers a simple solution to the intrinsic instability associated with imine-linked COFs while retaining the framework’s crystallinity and permanent porosity. Our advanced transmission electron microscopic (TEM) studies provided direct evidence of the pore size changes concomitant with the framework-to-framework transformation. The as-formed, quinoline-linked COFs (denoted as MFs) display dramatically enhanced chemical stability over their imine-linked COF precursors, rendering them among the most robust COFs up-to-date that can withstand strong acidic, basic, redox environment and long-term stability test. Owing to the structural tunability of COFs and substrate diversity of the aza-DA reaction, the pores of COFs can be readily engineered to realize pre-designed surface functionality.
Overall, we envision that our succinct protocol paves a facile way to the synthesis of crystalline, porous aromatic frameworks that are difficult to obtain de novo, and it will facilitate practical applications of organic framework materials that require enhanced chemical stability, semiconducting properties, and pore surface functionality.