Microscopic Observation of the Oligomerization Process of HCOOH Molecules Confined by a 2D Metal-Organic Network on Cu(111).

Porous 2D metal-organic networks (2D-MONs), formed from metal atoms and organic linker molecules on a solid surface, contain various guest adatoms and molecules in their pores as a host thin film. Aggregation of the guest atoms/molecules within the pore results in the formation of a cluster/oligomer, and the size of the cluster/oligomer is controlled by the pore. In this study, the host 2D-MON consists of 1,3,5-tris(pyridyl)benzene molecules and Cu atoms on Cu(111). The guest adsorbates are HCOOH molecules that form 1D zigzag chains by the intermolecular hydrogen bond on Cu(111). By scanning tunneling microscopy (STM) measurements and density functional theory (DFT) calculations, it is revealed that HCOOH tetramers (chains of four molecules) appear in the pores most frequently. The two-coordinate Cu atom in the 2D-MON attracts an HCOOH molecule, in which van der Waals interactions play an important role. A zigzag chain grows from this HCOOH molecule, and the end of the tetramer forms the attractive electrostatic interaction with the H atom of the 2D-MON, stabilizing the tetramer in the pore. Thus, the oligomerization process of HCOOH molecules in the pore are controlled by the host-guest interaction as well as the intermolecular hydrogen-bonding.