Imaging Successive Intermediate States of the On-Surface Ullmann Reaction on Cu(111): Role of the Metal Coordination Sören Zint,† Daniel Ebeling,*,† Tobias Schlöder,‡ Sebastian Ahles,§ Doreen Mollenhauer,‡ Hermann A. Wegner,§ and André Schirmeisen† †
Institute of Applied Physics (IAP), Justus Liebig University Giessen, Heinrich-Buff-Ring 16, 35392 Giessen, Germany Institute of Physical Chemistry and §Institute of Organic Chemistry, Justus Liebig University Giessen, Heinrich-Buff-Ring 17, 35392 Giessen, Germany
‡
S Supporting Information *
ABSTRACT: The in-depth knowledge about on-surface reaction mechanisms is crucial for the tailor-made design of covalently bonded organic frameworks, for applications such as nanoelectronic or -optical devices. Latest developments in atomic force microscopy, which rely on functionalizing the tip with single CO molecules at low temperatures, allow to image molecular systems with submolecular resolution. Here, we are using this technique to study the complete reaction pathway of the on-surface Ullmann-type coupling between bromotriphenylene molecules on a Cu(111) surface. All steps of the Ullmann reaction, i.e., bromotriphenylenes, triphenylene radicals, organometallic intermediates, and bistriphenylenes, were imaged with submolecular resolution. Together with density functional theory calculations with dispersion correction, our study allows to address the long-standing question of how the organometallic intermediates are coordinated via Cu surface or adatoms. KEYWORDS: on-surface Ullmann coupling, reaction pathway, atomic force microscopy, density functional theory, CO tip, submolecular resolution imaging
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the frequently studied on-surface Ullmann coupling (see e.g., refs 15 and 16 for recent reviews), i.e., a coupling reaction between aryl halides, parts of the reaction pathway are still not well understood. In general, the Ullmann reaction is composed of two steps, which correspond to the homolytical scission of the C−X bonds (where X is a halogen, cf. green step in Figure 1) and the subsequent coupling of the resulting (adsorbed) radicals under formation of a covalent C−C bond (red step in Figure 1). Depending on the surface material and the molecular species, the latter can include an organometallic intermediate state where the two organic moieties are connected via a C− Metal−C (in our case C−Cu−C) bond (yellow step in Figure 1). Different types of organometallic intermediates have been proposed in literature, which can be divided into two classes: (1) Tilted/bent adsorption structures where the molecules are coordinated via a metal atom, which resides inside the metal surface (i.e., metal surface atom);19,20 and (2) planar adsorption
he controlled assembly of one and two-dimensional structures from individual molecular building blocks1−3 represents a seminal step toward the fabrication of nanoscale devices. This is, in particular, crucial for applications such as molecular electronics4−6 or optical devices.7 In 2007, Grill et al.3 used the on-surface Ullmann-type coupling8−10 for building covalently bonded molecular structures in a controlled manner. This has led to a whole research field in which researchers are exploring different types of on-surface reactions to design molecular architectures (see e.g., refs 11−16). It was recently demonstrated that the Ullmann-type coupling can even be used to build rather complex structures such as graphene nanoribbons17 or multicomponent networks.18 In order to predict product structures and thus to be able to rationally optimize the corresponding reaction conditions, it is vital to gain a deep fundamental understanding of the coupling reaction. In particular, the complete reaction pathway with all its intermediate steps should be known to actively design assemblies with particular features. The interesting parameters to be studied for each step are the reaction temperatures, surface diffusion, adsorption structures, etc. However, even for © 2017 American Chemical Society
Received: February 16, 2017 Accepted: March 27, 2017 Published: March 27, 2017 4183
DOI: 10.1021/acsnano.7b01109 ACS Nano 2017, 11, 4183−4190
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Figure 1. Ullmann reaction pathway from BTP to Bis-TP with intermediate reaction products on Cu(111). First reaction step (green): Dehalogenation of BTP and creation of (adsorbed) TP-radicals. Second reaction step (yellow): Formation of organometallic intermediates. Two possible kinds of organometallic intermediates are depicted: (1) lifted/nonlifted surface atom and (2) lifted/nonlifted adatom. Final reaction step (red): Bis-TP is formed via the creation of covalent carbon−carbon bonds.
Figure 2a−c depicts a molecular model, a constant height AFM scan, and a STM topography image of BTP on Cu(111). In order to achieve submolecular resolution, all measurements have been performed at 5 K with CO-functionalized tips.26,42 While the AFM image unambiguously resolves the chemical structure of the BTP molecule, i.e., the position of the bromine atom can be determined exactly, the STM contrast rather indicates the overall shape of the molecule. Additionally, the AFM image reveals a planar adsorption of the BTP molecule on the Cu(111) surface. In general, chemical bonds appear in the AFM frequency shift images as bright lines due to repulsive tip−sample forces, which lead to positive frequency shifts of the oscillating tuning fork sensor. This repulsive tip−sample interaction stems from Pauli repulsion26 and short-range electrostatic forces.43−46 Two characteristic features can be observed in AFM scans with CO tips due its flexibility. This flexibility provokes a forcedependent tilting of the CO molecule, which leads to a sharpening effect on the imaged bonds on one hand and an image distortion on the other hand.29,47,48 After clearly identifying the initial state, the debromination reaction has been triggered by controlled sample heating to a temperature of 120 K. Corresponding images of a TP-radical are shown in Figure 2d−f. Both the AFM and the STM images allow for a discrimination of the two different types of molecules (BTP vs TP-radical), since the overall shape of the radical is more symmetric. The chemical structure and the almost planar adsorption of the radical are revealed by the AFM scan. It was shown in previous investigations of adsorbed aryl halides that the discrimination between a molecule and the corresponding radical can be difficult. In two studies of aryl diradicals on NaCl(2Ml)/Cu(111), only slight deformations could be observed for the adsorbed anthracene and naphtho[1,2,3,4-g,h,i]perylene diradicals. By contrast, both molecules showed significantly bent structures when adsorbed on clean Cu(111) surfaces, strongly indicating a higher reactivity of the latter.36,49 For the TP-monoradicals, we do not observe such a strong bending of the molecule. Though, in our case, one of the three carbon rings (see red arrows and inset in Figure 2e) appears to be slightly darker than the other two, which implies that this ring is slightly bent down toward the surface. Since the observed bending is rather weak and an asymmetric dark halo is observed around the molecule, an influence of a tilted CO molecule cannot be ruled out. However, we have observed this feature systematically for four different radicals, which have been imaged with three different CO-tips (see Supporting
structure where the metal adatom resides in the molecular plane (i.e., lifted metal adatom).21 At a closer look, these two limiting cases can be subdivided into further cases where the metal atom is partially lifted out of the surface plane19,22 or where it assumes the position of a nonlifted adatom23−25 (cf. Figure 1). Since the existence of any kind of intermediate state during a reaction can influence the obtained molecular assembly (e.g., by changing energy barriers, diffusion, etc.), it is necessary to study the whole reaction pathway in detail. Here, we investigate the whole pathway for the Ullmann reaction from bromotriphenylene (BTP) to bistriphenylene (Bis-TP) on a Cu(111) surface. In order to follow each of the individual reaction steps (cf. Figure 1), the sample has been heated in a controlled fashion to 100−660 K. Using atomic force microscopy (AFM) at 5 K, the chemical structure of the reactants during each reaction step can be imaged. The AFM technique, which is required for this purpose has been introduced a few years ago by Gross et al. and relies on functionalizing the AFM tip with single CO molecules at low temperatures.26 Using this method, several researchers were able to perform high-resolution studies of adsorbed molecules on flat surfaces and to explore reaction mechanisms (see e.g., refs 27−39). Based on the example of 2D molecular islands and polymeric chains on Ag(111), this technique has been applied to gain insights into parts of the on-surface Ullmann reaction.40 Using the AFM technique in combination with density functional theory (DFT) calculations with dispersion correction, we are able to shed more light on the precise adsorption structure of the organometallic intermediates on Cu(111). This is a crucial step toward resolving the recent debate about the nature of the metal coordination (see e.g., refs 19−21, 24, and 25) during the on-surface Ullmann-type coupling.
RESULTS AND DISCUSSION The first step of the reaction is the debromination of BTP, with formation of triphenylene (TP) radicals, which we characterized directly after sublimation of the BTP molecules. Keeping the sample at a temperature below 100 K during sublimation allows us to image the BTP molecules in their initial state (cf. Methods for further details). Previously, we observed that the intramolecular interactions between TP molecules have significant influence on their orientation for coverages of 0.1−1.0 monolayer.41 Hence, low surface coverages (
