Pyrophosphonate Covalent Organic Frameworks

Abstract

Herein, we report a new family of covalent organic frameworks (COFs), namely pyrophosphonate-COFs, constructed via pyrophosphonate linkages. Pyrophosphonate-COFs can be synthesized via a single-step condensation reaction of the charge-assisted hydrogen-bonded organic framework (HOF) GTUB5, which is constructed from phenylphosphonic acid and 5,10,15,20‐tetrakis[p‐phenylphosphonic acid] porphyrin. The reported pyrophosphonate-COF, which we call GTUB5-COF was synthesized by simply heating its two-linker HOF precursor GTUB5 without using chemical reagents. GTUB5-COF exhibits good water and water vapor stability during the gas sorption measurements. Furthermore, GTUB5-COF exhibits exceptional electrochemical stability in 0.5 M Na2SO4 electrolyte in water. The formation of pyrophosphonate bonds upon heating was confirmed by magic angle spinning nuclear magnetic resonance spectroscopy, Fourier-transform infrared spectroscopy, and mass spectrometry coupled with thermal analysis. The condensed product pyrophosphonate-COF can efficiently adsorb CO2. It has a more favorable heat of adsorption value for CO2 capture at lower pressures than water vapor, making it a suitable candidate for selective CO2 capture in the presence of water vapor. The absorption and emission of GTUB5-COF are governed by localized transitions (Soret and Q bands) within the porphyrin unit, which results in broad-banded fluorescence in the near-infrared range at around 800 nm.