Visualizing dynamic competitive adsorption processes between iodine and methyl iodide within single covalent organic framework crystals.

Covalent organic frameworks (COFs) are important porous adsorbents for volatile iodine (I) and methyl iodide (CHI). In situ monitoring of the dynamic adsorption process of single COF crystals toward I and CHI is a critical and fundamental issue for understanding the reaction mechanism and improving the sorption performance. Here, we report operando real-time dark-field optical microscopy (DFM) imaging of visually studying the dynamic adsorption behavior of LZU-111 (LZU=Lanzhou University) COFs in the I/CHI binary gaseous mixture at the single-crystal level. Time-lapse imaging shows that the uptake of CHI and I results in the R intensity increase and B intensity decrease of the DFM images. Employing the R and B intensities as two indicators, we find an unusual blinking of R/B intensities from single LZU-111 crystals, which is attributed to the intermittent sorption-desorption processes of CHI and I within the LZU-111 framework. The visualization of the dynamic reaction process provides clear evidence that the competitive adsorption between CHI and I goes through a multi-time and oscillatory reaction pathway instead of a successive procedure. Combined with theoretical calculations, the difference in the migration capability and initial pressure is identified for initiating the intermittent blinking events.