Surface confined metallosupramolecular architectures: formation and scanning tunneling microscopy characterization

Abstract

Metallosupramolecular compounds have attracted a great deal of attention over the past two decades largely because of their unique, highly complex structural characteristics and their potential electronic, magnetic, optical, and catalytic properties. These molecules can be prepared with relative ease using coordination-driven self-assembly techniques. In particular, the use of electron-poor square-planar Pt(II) transition metals in conjunction with rigid, electron-rich pyridyl donors has enabled the spontaneous self-assembly of a rich library of 2D metallacyclic and 3D metallacage assemblies via the directional-bonding approach. With this progress in the preparation and characterization of metallosupramolecules, researchers have now turned their attention toward fully exploring and developing their materials properties. Assembling metallosupramolecular compounds on solid supports represents a vitally important step toward developing their materials properties. Surfaces provide a means of uniformly aligning and orienting these highly symmetric metallacycles and metallacages. This uniformity increases the level of coherence between molecules above that which can be achieved in the solution phase and provides a way to integrate adsorbed layers, or adlayers, into a solid-state materials setting. The dynamic nature of kinetically labile Pt(II)-N coordination bonds requires us to adjust deposition and imaging conditions to retain the assemblies' stability. Toward these aims, we have used scanning tunneling microscopy (STM) to image these adlayers and to understand the factors that govern surface self-assembly and the interactions that influence their structure and stability. This Account describes our efforts to deposit 2D rectangular and square metallacycles and 3D trigonal bipyramidal and chiral trigonal prism metallacages on highly oriented pyrolytic graphite (HOPG) and Au(111) substrates to give intact assemblies and ordered adlayers. We have investigated the effects of varying the size, symmetry, and dimensionality of supramolecular adsorbates, the choice of substrate, the use of a molecular template, and the effects of chirality. Our systematic investigations provide insights into the various adsorbate-adsorbate and substrate-adsorbate interactions that largely determine the architecture of each assembly and affect their performance in a materials setting. Rational control over adlayer formation and structure will greatly enhance the potential of these supramolecules to be used in a variety of applications such as host-guest sensing/diagnostic systems, molecular electronic devices, and heterogeneous stereoselective synthesis and catalysis.

Scheme 1

Schematic representation of the three techniques used in the self-assembly of metallosupramolecular adlayers on HOPG and Au(111) substrates: (A) solution deposition, (B) substrate immersion, (C) aqueous in-situ adsorption. RE = reference electrode, CE = count electrode, WE = working electrode.

Figure 1

Chemical structures and space-filling models of (A) “small” supramolecular rectangle 1, (B) “large” supramolecular rectangle 2, and (C) supramolecular square 3.

Figure 2

(A) Large scale and (B) High-resolution STM images showing the face down orientation of the adlayer of rectangle 1 on Au(111). (C) Unit cell and proposed structural model of the adlayer.

Figure 3

(A) Large scale and (B) High-resolution STM images showing the face down orientation of rectangle 2 on Au(111). (C) Unit cell and proposed structural model of the adlayer.

Figure 4

(A) Large scale and (B) High-resolution STM images of the adlayer of square 3 on Au(111). (C) Unit cell and proposed structural model of the adlayer.

Figure 5

(A) Large scale and (C) High-resolution STM images showing an edge down orientation of the adlayer of rectangle 2 on HOPG. (C) Proposed structural model.

Figure 6

(A) High-resolution STM images of the TCDB molecules adsorbed on HOPG. Unfilled ovals indicated by arrows highlight the existence of voids in the adlayer. (B) Chemical structure of TCDB as well as proposed structural model and unit cell of its adlayer on HOPG.

Figure 7

(A) Large scale and (B) High-resolution STM images of the adlayer formed upon templation of 1 by TCDB on HOPG. Not every TCDB void is filled, as indicated by the white arrow in (B). (C) Proposed structural model and unit cell of the mixed adlayer.

Figure 8

(A) Chemical structures of donor and acceptor building blocks (at left) that self-assemble to form a trigonal bipyramidal (TBP) metallacage as well as top and side views of the TBP X-ray suprastructure. (B) Chemical structure (at right) as well as top and side views of molecular models of the MMM and PPP enantiomers of a chiral trigonal prism (CTP).

Figure 9

(A) Large-scale and (B) High-resolution STM images of the adlayer of TBP on Au(111). (C) Proposed structural model and unit cell of the adlayer.

Figure 10

(A) Large-scale STM image of the adlayer found upon deposition of a racemic mixture of the CTP on Au(111). Separate chiral domains are indicated as I and II. (B) High-resolution STM image displaying regular rows of repeating units of a single enantiomeric domain.

Figure 11

(A) High-resolution STM image and (B) proposed model of the junction of two domains of opposite chirality on Au(111).