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.

Fig. 1

Post-synthetic modification of COFs via aza-DA reaction. a The reaction scheme showing the transformation of one lattice unit of COF-1 into MF-1a-e. b The simulated modeled structure showing the extended frameworks of COF-1 and MF-1a. The yellow and cyan sphere indicate pore diameter of ~2.9 and ~2.6 nm. Note that the illustrated structure of MF-1 contains fully converted quinolines and does not represent the actual degree of transformation

Fig. 2

Characterization of original COF-1 (black) and post-synthetically modified MF-1a (red) and MF-1b(blue). a FT-IR spectra. b Solid-state ¹³C CP-MAS NMR spectra. c PXRD patterns showing retention of crystallinity after the Povarov reaction. d N₂ sorption isotherm curves

Fig. 3

HRTEM characterization of COF-1 and MF-1a. a Low-dose, high-resolution TEM image of COF-1 (scale bar, 10 nm). b The Fourier-filtered image of selected red square areas (scale bar, 5 nm), Inset: Fast Fourier Transform (FFT) from the red square on the COF-1. c Low-dose, high-resolution TEM image of MF-1a (scale bar, 10 nm). d The Fourier-filtered image of the selected red square area (scale bar, 5 nm), Inset: FFT from the red square on the MF-1a

Fig. 4

Chemical stability of MF-1a and COF-1. PXRD patterns of MF-1a (a) and COF-1 (b) after treatment with 12 M HCl at 50 °C for 8 h (green), 98% triflic acid at ambient temperature for 3 days (black), 14 M NaOH in H₂O/MeOH solution at 60 °C for 1 day (red), 5 equiv. of NaBH₄ in MeOH at 65 °C for 1 day (blue), and 5 equiv. of KMnO₄ in H₂O/CH₃CN solution at ambient temperature for 1 day (purple)

Fig. 5

Structural characterization and surface properties of MF-1 series. a Powder XRD patterns of COF-1 (black), MF-1a (red), MF-1c (green), MF-1d (blue), and MF-1-e (purple). b FT-IR spectra of COF-1 and MF-1a, c-e. The peaks highlighted in cyan are characteristic vibrations from these functional groups. c Water contact angles (CA) of water droplet on the pressed pellet of COF-1 and MF-1a-e. MF-1b′ is the NaHCO₃-treated MF-1b that undergoes partial ester hydrolysis