Article pubs.acs.org/JPCC
A Contoured Network of Anionic Trimesic Acids on Au(111) and Its Host−Guest Chemistry Jandee Kim,† Choong K. Rhee,*,† Hyun-J. Koo,‡ Emine E. Kasapbasi,§ and Myung-H. Whangbo⊥ †
Department of Chemistry, Chungnam National University, Daejeon 305-764, Korea Department of Chemistry and Research Institute for Basic Science, Kyung Hee University, Seoul 103-701, Korea § Food Engineering Department, Istanbul Aydin University, Istanbul, Turkey ⊥ Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ‡
S Supporting Information *
ABSTRACT: This work describes a trimesic acid (TMA) network formed at the open-circuit potential (OCP, ∼0.13 V) on Au(111), which we investigate with electrochemical scanning tunneling microscopy (STM). The unit cell of the TMA network at the OCP was (5√3 × 5√3)R30°−4TMA, while that of a well-known honeycomb network of flat-lying TMAs at −0.1 V was (6 × 6)R30°−2TMA. The most significant difference between the two networks is the fraction of hydrogen bonds appearing as bright spots in the STM images: it is 1/3 in the former but 1 in the latter. Additional differences were the shapes and lengths of the STM spots and the packing density of TMAs. A contoured network composed of crownlike anionic TMA hexamers was proposed to explain all the differences. The TMAs in the crownlike hexamer were rotated along the molecular axis and were tilted up from the surface by ∼30° to have hydrogen bonds with reasonable O−H···O distance, and the hexamers were interconnected to a contoured network. The existence of the anionic TMA hexamer was evidenced by selective host−guest interaction of Cu2+ ions and 1-mercaptohexane with the contoured network. At a higher potential than the OCP, simple arrays of anionic TMA dimers with much higher packing densities were observed, depending on the availability of TMAs from solution. The potential dependence of the structure of the TMA adlayer is discussed in terms of a transition from molecular adsorption to anionic adsorption.
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INTRODUCTION A two-dimensional (2D) crystal engineering relies on the tendency for organic supramolecules to self-assemble at liquid− solid or ultrahigh vacuum (UHV)−solid interfaces.1−5 The supramolecules are primary building blocks interconnected via interactions between the functional moieties attached to their central portions to form 2D layers. These bottom-up architectures of 2D organic monolayers are based mainly on noncovalent bonds (e.g., hydrogen bonds), metal−ligand coordinations, and van der Waals interactions between the building blocks. In addition to the binding forces between the primary building blocks, the adsorbate−surface interaction anchoring the adsorbate and the solvent delivering the adsorbing species to the substrate surfaces contribute significantly to the structures of 2D organic layers. Thus, the information on the structures of organic monolayers, mainly obtained by scanning tunneling microscopy (STM) investigations, is indispensable in understanding the interactions between the primary building blocks and their useful properties such as nanopores in regular patterns, surface chiralities, and reactivities. Trimesic acid (TMA) is one of the intensively studied primary building blocks forming 2D self-assembled monolayers of supramolecules on solid surfaces. Three carboxylic acids attached to a benzene ring in trigonal symmetry allow © XXXX American Chemical Society
directional hydrogen bonds in dimer or trimer synthons to form nanoporous network monolayers on solid surfaces. There are two nanoporous structures of flat-lying TMAs on graphite surface known as the honeycomb (or chicken wire) and flower structures.6,7 The nanopores of the honeycomb structure were demonstrated to host C60 buckminsterfullerenes, which are transferable among adjacent nanopores.8 The occurrence of the two phases depends on the chain lengths of the employed alkanoic acid solvents.9 On Au(111), several adstructures of TMA including network structures have been observed as the potential changed in aqueous electrolytes.10−12 Ishikawa et al. have reported four phase transitions during a potential shift in 0.1 M HClO4 solution containing 0.1 mM TMA: desorption (
