Article pubs.acs.org/JPCC
Effect of Metal Surfaces in On-Surface Glaser Coupling Hong-Ying Gao,†,‡,⊥ Jörn-Holger Franke,†,‡,⊥ Hendrik Wagner,§ Dingyong Zhong,†,‡ Philipp-Alexander Held,§ Armido Studer,*,§ and Harald Fuchs*,†,‡,∥ †
Physikalisches Institut, Universität Münster, Wilhelm-Klemm-Strasse 10, 48149 Münster, Germany Center for Nanotechnology, Heisenbergstrasse 11, 48149 Münster, Germany § Organisch-Chemisches Institut, Universität Münster, Corrensstrasse 40, 48149 Münster, Germany ∥ Institute for Nanotechnology, Karlsruhe Institute of Technology, 76344 Karlsruhe, Germany ‡
S Supporting Information *
ABSTRACT: The homocoupling of alkynes at metal surfaces, which was disclosed recently, is a promising reaction for efficient construction of conjugated nanostructures at metal surfaces. However, the role of the metal substrate as well as the mechanistic course of this process have not been investigated. The metal surface could act cooperatively (a) for two-dimensional confinement to properly orient the organic reactant and (b) also as an active mediator in the C−C bond-forming reaction. Herein we report covalent coupling of the dimers of 1,4-diethynylbenzene at various metal surfaces. The model reaction was investigated experimentally by STM and also by theoretical DFT calculations. Detailed statistical analysis and the theoretical results strongly support the involvement of the metal surface in the C−C bond-forming process. On the basis of these investigations, a model with two possible reaction pathways is suggested to describe the process: C−C coupling via direct CH activation and C−C coupling via alkynyl activation by π-complex formation. tures becomes feasible,3,25,26 which is often hampered by solubility problems occurring in classical solution-phase polymerization and characterization problems. In some cases insulator surfaces are suitable for covalent bond-forming processes, which indicates that the surface in such cases simply acts as a template for orientation of precursor molecules prior to covalent bond formation.25,26 While in most cases the metal substrate likely mediates the coupling reaction, experimental mechanistic studies are difficult to design and to conduct and are therefore often missing in this field.27 In an excellent work, the catalytic role of stepped metal edges has been proved for the Ullman coupling very recently.28 The understanding of the mechanism of on-surface reactions with respect to the participation of interfacial metal atoms is highly important for further optimization of such processes. Herein we present a combined experimental and computational study on the covalent coupling of bisalkynes at metal surfaces addressing the role of the metal substrate in the C−C bond-forming process (Scheme 1).29−31
1. INTRODUCTION The direct and covalent interconnection of individual precursor molecules at surfaces has emerged as a valuable tool for the “bottom-up” synthesis of nanostructures within the field of nanotechnology. This novel methodology has allowed preparing various fascinating materials.1−3 Such on-surface chemical reactions at interfaces can be analyzed conveniently by ultrahigh-vacuum (UHV) scanning tunneling microscopy (STM). By using on-surface synthesis, the structure of a superlattice can be simply programmed by the specific design of the corresponding monomers as well as by the chosen type of chemical reaction.4,5 However, the number of applicable bondforming processes at interfaces is limited to a few reaction types, such as the Ullman coupling,1,2,6−10 dehydration and esterification of boronic esters,11 imine formation,12,13 acylation reactions,14,15 dimerization of N-heterocyclic carbenes,16 carbon−metal−carbon interactions,17 and dehydrogenative coupling of alkanes.18 These reactions conducted at the interface provided well-ordered structures by covalent bondforming events at surfaces.2,9−12,14,16 Formation of interfacial networks can also be achieved by noncovalent intermolecular interactions, such as van der Waals forces,19 hydrogen bonding,20 π−π stacking,21 dipolar interaction,22 metal−organic coordination,23 and other substrate-mediated interactions.24 Compared to network formation processes using classical solution-phase chemistry, complex structure formation by covalent bond-forming processes at the interface shows advantages. For example, preparation of rigid, planar, as well as semiconducting π-conjugated polymers or network struc© 2013 American Chemical Society
2. EXPERIMENTAL AND COMPUTATIONAL DETAILS 2.1. Experimental Details. Experiments were performed with an Omicron low-temperature ultrahigh vacuum (UHV) STM at a base pressure 1 × 10−10 mbar, which operates at 78 K in the constant current topographic mode with sample biased. Received: July 11, 2013 Revised: August 15, 2013 Published: August 15, 2013 18595
dx.doi.org/10.1021/jp406858p | J. Phys. Chem. C 2013, 117, 18595−18602
The Journal of Physical Chemistry C
Article
used throughout in conjunction with the PAW method. Plane waves up to a cutoff energy of 400 eV were used to represent the wave functions. The Au(111) and Ag(111) substrates were represented by a (6 × 8) supercell three layers thick with the upmost two free to relax. K-point sampling was limited to the Gamma point only, and Gaussian broadening of 0.1 eV was used. Molecules are adsorbed on one side of the slab only, and dipole corrections to the energy were carried out accordingly. Ionic relaxation was carried out until all forces were smaller than 10 meV/A, with the wave functions converged to energy changes below 10−5 eV. Transition states were found with the climbing images nudged elastic band method37,38 converged to forces smaller than 20 meV/A on all images.
Scheme 1. On-Surface Glaser Coupling of Arylalkynes
The atomic flat noble metal surfaces of Au(111) and Ag(111) (Mateck company) were cleaned by cycles of sputtering and annealing. The precursor molecules (monomer 1, from SigmaAldrich) were evaporated in ultrahigh vacuum by sublimation from a Knudsen cell at ∼70 °C. Most of the dimer molecules (dimer 2) were formed inside the Knudsen cell during sublimation of the monomers, and they were deposited at a sublimation temperature of ∼105 °C onto metal surfaces. The deposition rate, ∼ 0.1 ML/min (monolayer per minute), was calibrated by STM images. Subsequent annealing of metal surfaces covered with the dimers up to ∼125 °C for 30 min, monitored by an IR-thermometer, led to oligomerization of the alkyne functionalities. The success of the C−C bond formation was proved by swinging oligomer chains with the STM tip without destroying the connections between the individual dimers. The center to center distance measurements of the conjugations further unambiguously proved the successful C− C bond formation. Statistical analysis for the observed chemical reaction products at metal surfaces was applied. DFT calculations were also performed to demonstrate the role of the metal substrate in on-surface synthesis. 2.2. Simulation Methods. Density functional theory (DFT) calculations were performed using the plane-wave implementation VASP.32−34 The oPBE-vdW functional35,36 was
3. RESULTS AND DISCUSSION 3.1. Dimers of 1,4-Diethynylbenzene at the Metal Surface. As a precursor for the on-surface Glaser coupling we chose 1,4-diethynylbenzene (1, Figure 1a). It is feasible that the C−C coupling as well as other reactions may occur inside the quartz crucible during the heating process.29 We therefore conducted thermal deposition of the monomers at sublimation temperatures as low as possible to suppress thermal reactions in the crucible (70, 80, and 90 °C, for more details see Supporting Information-1 (SI-1)) and identified along with dimers a few individual rotating species at the metal surface,39 as illustrated in Figure 1b-i. Swinging in an arc type mode (half rotors) was also observed as shown in the top structure of Figure 1b-ii. The diameter of these molecular rotors was experimentally measured to lie at (2.05 ± 0.05) nm (edge to edge distance of the rotor, as marked in Figure 1b-ii and iii), which is slightly more than twice the backbone length of monomer 1 (calculated gas-phase bond length of 1: 0.81 nm). This indicates that at least one Au atom must be involved to build up these rotors.
Figure 1. Monomers and dimers of precursor C10H6 molecules at the Au(111) surface. (a) Chemical structure of monomer 1 and (b) the illustration of the possible structure (i) and STM image (ii) (−2 V, 5 pA, 8.6 nm × 8.6 nm) of individual molecular rotors with a corresponding line prolife (iii) over one rotor. (c) The chemical structure of dimer 2 and (d) STM high-resolution images of dimers in order (i) (−1 V, 50 pA, 6 nm × 6 nm) and disorder (ii) (−0.5 V, 50 pA, 6 nm × 6 nm) on the Au(111) surface. The line profile (iii) along the dimer alkyne axis indicates the covalent coupling between two monomers. 18596
dx.doi.org/10.1021/jp406858p | J. Phys. Chem. C 2013, 117, 18595−18602
The Journal of Physical Chemistry C
Article
We suggest an alkynyl−Au complex as the possible structure for the experimentally observed rotating molecules as shown in Figure 1b-i. Indeed, the calculated bond length for the alkynyl− Au complex which is 0.99 nm agrees well with the experimentally determined rotor diameter. It is likely that the rotor might be an intermediate (Au-activated alkyne) on the way to the dimer, and we assume that for the Glaser coupling at the Au surface Au metals are involved in the C−C bondforming step. Upon increasing the molecular sublimation temperature to more than 90 °C (up to 115 °C), rotor structures were barely visible, and clean monolayer formation resulting from the deposition of dimeric molecules of 1,4-diethynylbenzene at the Au(111) surface was observed (Figure 1c). Typically, the dimers showed both ordered and disordered structures at the Au(111) surface, especially close to a coverage of 1 monolayer (ML) (Figure 1d, i and ii). The center-to-center distance between two phenyl rings within the dimer was measured to be (0.96 ± 0.03) nm (Figure 1d-iii), which matches well the theoretical bond length of a model bisalkynyl compound obtained by DFT calculation in the gas phase (theoretical value: 0.95 nm). At relatively low coverage (
