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

Covalent organic frameworks (COFs) are important porous adsorbents for volatile iodine (I₂) and methyl iodide (CH₃I). In situ monitoring of the dynamic adsorption process of single COF crystals toward I₂ and CH₃I 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 I₂/CH₃I binary gaseous mixture at the single-crystal level. Time-lapse imaging shows that the uptake of CH₃I and I₂ 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 CH₃I and I₂ within the LZU-111 framework. The visualization of the dynamic reaction process provides clear evidence that the competitive adsorption between CH₃I and I₂ 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.