Molecular Multistate Systems Formed in Two-Dimensional Porous Networks on Ag(111)

Host−guest interactions in porous supramolecular structures have been studied on surfaces using scanning tunneling microscopy, with anticipation of biochemical and sensor applications, but limited to cases of van der Waals interactions and hydrogen bonds. Here, we studied the intermolecular structures of 4,4′′-dibromo-p-terphenyl molecules self-caged in porous supramolecular structures with halogen bonds on Ag(111). The caged molecules hopped among six different configurations at higher than 50 K, showing a propeller-like pattern. At 30 K, they stayed at one of six states that were stabilized with Br···Br halogen bonds and Br···H hydrogen bonds with energy gains of 225, 197, and 163 meV, as revealed by our density functional theory calculations. The self-caged structure provides a model system to simulate multistate supramolecular memories. S porous structures have been actively studied due to their possible applications in biochemical devices. They are fabricated by self-assembly processes that are driven by molecule−molecule interactions. The forms of various twodimensional (2d) porous structures have been revealed on crystal surfaces using scanning tunneling microscopy (STM). The two-dimensional porous structures were used as molecular templates (host) to confine the second molecules (guest) in their periodic voids, and the interactions between host−guest molecules were examined due to their potential applications in conformational switching and chemical sensing. C60 molecules were trapped in the hexagonal voids made by perylene tetracarboxylic diimide and melamine on Ag/Si(111). Phthalocyanine or coronene molecules were placed into the tetragonal voids of 1,3,5-tris(10-carboxydecyloxy) benzene on a graphite surface. Porphyrin molecules have been nested on top of voids of 3,5-di(tert-butyl)phenyl in six equivalent directions on Cu(111). Sexiphenyl-based long molecules were self-caged into honeycomb voids on Ag(111). The rotation of a guest molecule has been observed in the porphyrin system, and switching between two rotational states has been observed in the sexiyphenyl-based system. In these examples, the binding mechanisms between host and guest molecules were limited to van der Waals interactions and hydrogen bonds. In this study, we report on the host−guest interactions in the self-caging system of 4,4′′-dibromo-p-terphenyl (DBTP) molecules, with halogen bonds on Ag(111) using STM. A rod-like DBTP molecule was caged in a hexagonal void and hopped among six stable configurations showing propeller-like patterns in the STM images above 50 K. The six configurations were stabilized by two interactions, Br···Br halogen bonds and Br···H hydrogen bonds. ■ RESULTS AND DISCUSSION Figure 1a shows the DBTP molecule structure of three phenyl rings and two Br terminations. The molecules had large enough surface diffusivity to form self-assembled network structures when DBTP was deposited at 150 K. After deposition, the sample was cooled to 80 K to perform the STM measurement, and a typical STM image obtained from DBTP on Ag(111) is shown in Figure 1b. The DBTP molecules formed porous twodimensional structures consisting of alternating rows of rectangles and hexagons, which covered ∼70% of the surface. The hexagon had two 90° angles and four 135° angles. The 90° angles formed when four molecules make a quartet node, whereas the 135° angles formed when three molecules make a triple node. Each node possessed chirality. Namely, quartet nodes were clockwise, whereas triple nodes were anticlockwise in Figure 1. The mirror structures in Figure 1 made of anticlockwise quartet and clockwise triple nodes were also Received: September 26, 2012 Revised: December 3, 2012 Published: December 11, 2012 Article