Coordination-Controlled One-Dimensional Molecular Chains in Hexapodal Adenine-Silver Ultrathin Films

Growth of a silver coordination polymer of a C₃-symmetric hexaadenine ligand is studied on highly oriented pyrolytic graphite (HOPG), using high-resolution atomic force microscopy (AFM). This unusual ligand offers 6-fold multidentate coordination sites, and consequently, a multidimensional growth of coordination polymer is expected. Notably, each discrete hexapodal unit is bridged by two silver ions along one of the crystallographic directions, resulting in high interaction energy along this direction. When the polymer was deposited on an HOPG surface from a dilute solution, we observed abundant one-dimensional (1D) coordination polymer chains, with a minimum width of approximately 4.5 nm. The single-crystal structure using X-ray analysis is compared with the surface patterns to reconcile and understand the structure of the 1D polymer on an HOPG surface. The energy levels of Ag-L1 within the proposed model were calculated, on the basis of the X-ray crystal structure, and compared to the ligand states to gain information about the electronic structure of ligand upon Ag coordination. On the basis of the wave functions of a few molecular orbitals (MOs) near the Fermi energy, it is surmised that unfilled MOs may play a crucial role in the transport properties of the Ag-L1 adlayer.