On-Surface Synthesis of Ligands to Elaborate Coordination Polymers on an Au(111) Surface

Abstract

On-surface metal-organic polymers have emerged as a class of promising 2D materials. Here, we propose a new strategy to obtain coordination polymers by transforming supramolecular networks into coordination polymers by surface-assisted cyclo-dehydrogenation of organic building blocks. All nanostructures are fully characterized by using scanning tunneling microscopy under ultra-high vacuum on a gold surface. We demonstrated that the balance between molecule-molecule interaction and molecule-substrate interaction can be drastically modified by a strong modification of the geometry of the molecules thanks to a thermal annealing. This new way is an efficient method to elaborate on-surface coordination polymers.

Keywords: coordination polymers; on-surface synthesis; scanning tunnelling microscopy.

Figure 1

CPK models of 10,10′-di-(4″-pyridyl)-9,9′-bianthryl (PBA) and 10,10′-di-(4″-cyanophenyl)-9,9′-bianthryl (CPBA) molecules respectively.

Figure 2

STM images of self-assembled molecules on Au(111) surface. (a) PBA supramolecular network (Vs = −1.5 V, It = 10 pA, 30 nm × 30 nm, inset 4 nm × 4 nm) (b) CPBA supramolecular network (Vs = 1.5 V, It = 10 pA, 20 nm × 20 nm, inset 3 nm × 3 nm).

Figure 3

(a) STM image (Vs = −1.5 V, It = 10 pA, 8 nm × 8 nm) of PBA supramolecular network with corresponding superimposed model and unit cell vectors: U_PBA = 2.25 nm, V_PBA = 2.25 nm, ϴ_PBA = 60°. (b) Supramolecular model of PBA molecules on Au(111) surface with the inset of magnified image representing three adjacent PBA molecules, where two adjacent pyridyl rings are separated by a distance of 0.42 nm (blue arrow) and two nitrogen atoms point towards the centre of the anthracenyl ring of a third PBA molecule (highlighted by two red arrow, length: 0.41 nm).

Figure 4

(a) STM image (Vs = 1.5 V, It = 10 pA, 8 nm × 8 nm) of CPBA supramolecular network with corresponded superimposed supramolecular model and unit cell vectors: U_CPBA = 2.58 nm, V_CPBA = 1.45 nm, ϴ_CPBA = 94°. (b) 3D model of CPBA/Au(111). Inset highlights the interaction between one nitrogen atom pointing out the center of anthracenyl ring of the adjacent molecule, which are separated by 0.45 nm (red arrow).

Figure 5

STM images of formed nanostructures observed on Au(111) surface after thermal annealing of supramolecular networks up to 673 K. (a) PBA (Vs = −1.7 V, It = 10 pA, 30 nm × 30 nm) inset 3 nm × 3 nm, (b) CPBA (Vs = −2.0 V, It = 7 pA, 60 nm × 60 nm, inset 3.5 nm × 3.5 nm).

Figure 6

Thermal-induced cyclodehydrogenation of PBA and CPBA molecules. The starting PBA and CPBA molecules have a 3D cross-shaped core (i.e., bianthryl core), the corresponding bisanthene cores (Flat-PBA and flat-CPAB) are flattened.

Figure 7

CPK model of PBA and flat-PBA highlighting the decreasing of molecule-surface distance by 0.24 nm due to the flattening of PBA induced by thermal annealing.

Figure 8

(a–c) STM images (Vs = −1.8 V, It = 10 pA) of PBA supramolecular nanostructures with three different configurations obtained after thermal annealing: (a) Chain-like nanostructures of protrusions (7 nm × 3 nm), (b) Y-shaped trimer nanostructures (4 nm × 4 nm) and (c) Cross-shaped of nanostructures (4 nm × 4 nm). (d) Molecular models of the three different type of coordination (red dot: Au ad-atom surrounded by organic moieties).

Figure 9

(a–c) STM images (Vs = −2.0 V, It = 7 pA) of three nanostructures obtained after thermal annealing of CPBA: (a) Rod-like protrusions (7 nm × 3 nm) (b) Y-shaped trimer of nanostructures (4 nm × 4 nm) and (c) Cross-shaped nanostructures (5 nm × 5 nm). (d) Molecular models of three different type of coordination (Red dot: Au ad-atom surrounded by organic moieties).