Covalent organic frameworks (COFs) are porous crystalline networks made from organic building blocks. Similarly to metal–organic frameworks (MOFs), they can be useful, e.g., in gas adsorption/separation, sensing, or catalysis. There is a subtype of flexible COFs that can undergo a reversible transformation between a porous and a non-porous state (similar to how a sponge can be compressed/decompressed to close/open its pores), so-called soft porous crystals (SPCs). SPCs can be useful in applications such as pressure-dependent gas adsorption.

Cheng Wang, Wuhan University, China, and colleagues have developed a COF (called FCOF-5) that shows a reversible expansion and contraction upon the adsorption of certain gases, i.e., it exhibits a gas-triggered gate-opening behavior. The team prepared FCOF-5 from tetra(p-aminophenyl)methane (TAPM) and 1,2,4,5-tetrakis[(4-formylphenoxy)methyl]benzene (TFMB, both pictured) via a condensation reaction. The C–O single bonds in the TFMB building block allow for changes in conformation and provide the framework with a certain flexibility.

In the absence of gases, FCOF-5 is structurally contracted and has very small pore sizes. The team found that these ultramicopores do not allow for the diffusion of even N₂. For CO₂, the team observed almost no adsorption at low pressure but a sudden increase in adsorption at higher pressures. This sudden increase provides evidence for a phase transformation of the COF from a contracted to an expanded state—i.e., the pores “open up” upon the adsorption of CO₂. According to the researchers, such a CO₂-triggered gate-opening behavior had rarely been observed in 3D COFs so far.

The team also investigated the adsorption of C₂H₂, C₂H₄, and C₂H₆, as well as of C₃ hydrocarbons (propyne, propylene, and propane) and found a similar guest-triggered gate-opening behavior. Due to differences in the gate-opening pressures, FCOF-5 can, for example, be used to separate trace amounts of C₃H₄ from C₃H₄/C3H₆ mixtures.

• Gas-Triggered Gate-Opening in a Flexible Three-Dimensional Covalent Organic Framework,
  Xiaoling Liu, Zhifang Wang, Ya Zhang, Na Yang, Bo Gui, Junliang Sun, Cheng Wang,
  J. Am. Chem. Soc. 2024.
  https://doi.org/10.1021/jacs.4c01331