Imaging of Guest Molecule Adsorption onto 2D Covalent Organic Frameworks by Scanning Tunneling Microscopy

Porous materials play an important role in molecular adsorption and separation. However, understanding the mechanisms of molecular adsorption and separation into the pores remains a challenge. Herein, the adsorption of 1,3,5-tris(4-iodophenyl)benzene (TIPB) molecules onto a two-dimensional (2D) covalent organic framework (COF) monolayer is studied by low-temperature scanning tunneling microscopy (LT-STM) and density functional theory calculations. The COF monolayers are synthesized via the on-surface Ullmann reaction. The COF films have six types of porous morphology with different pore sizes, namely, tetragon, pentagon, hexagon, heptagon, octagon, and nonagon lattices. The adsorption behavior of guest TIPB molecules into the host COF's pores is in situ probed by high-resolution LT-STM. Our results reveal that the in-plane adsorption of TIPB molecules is pore-size-dependent and influenced by the interaction with the substrate, with tetragon and pentagon lattices showing no molecule adsorption due to their small pore size. Hexagon and heptagon lattices can adsorb one TIPB molecule through I-H bonding, while octagon and nonagon lattices are able to accommodate two TIPB molecules through both I-H and I-I bonding. The substrate affects the in-plane adsorption. After annealing, the adsorbed molecules can form new covalent bonds with the COF lattice, resulting in the pore-size-dependent grafting of TIPB molecules at the COF branched chains. These findings help to understand the pore-size-dependent adsorption mechanisms and the covalent grafting of guest molecules onto 2D COFs.