Balancing Noncovalent Interactions in the Self-Assembly of Nonplanar Aromatic Carboxylic Acid MOF Linkers at the Solution/Solid Interface: HOPG vs Au(111).

This study explores directed noncovalent bonding in the self-assembly of nonplanar aromatic carboxylic acids on gold and graphite surfaces. It is the first step in developing a new design strategy to create two-dimensional surface metal-organic frameworks (SURFMOFs). The acid molecules used are tetraphenylethene-based and are typically employed in the synthesis of three-dimensional (3D) MOF crystalline solids. They include tetraphenylethene tetracarboxylic acid, tetraphenylethene bisphenyl carboxylic acid, and tetraphenylethene tetrakis-phenyl carboxylic acid. The two-dimensional structures formed from these molecules on highly ordered pyrolytic graphite (HOPG) and Au(111) are studied by scanning tunneling microscopy in a solution environment. The process of monolayer formation and final surface linker structures are found to be strongly dependent on the combination of the molecule and substrate used and are discussed in terms of intermolecular and molecule-substrate interactions, bonding geometry, and symmetry of the acid molecules. In the case of linker self-assembly on HOPG, the molecule-substrate interactions play a significant role in the resulting surface structure. When the acid molecules are adsorbed on Au(111), the intermolecular interactions tend to dominate over the weaker molecule-substrate bonding. Additionally, the interplay of π-π interactions and hydrogen bonding that directs the surface self-assembly on different supports can be modified by varying the linker concentration. This is particularly applicable for the case of the acid molecules adsorbing on the Au(111) substrate. Precise control over predesigned surface structures and orientation of the nonplanar aromatic carboxylic linkers open up an exciting prospect for manipulating the direction of SURFMOF growth in two dimensions and potentially in 3D.