Article pubs.acs.org/JPCC
Photochemical Glaser Coupling at Metal Surfaces Hong-Ying Gao,*,†,‡ Dingyong Zhong,†,‡ Harry Mönig,†,‡ Hendrik Wagner,§ Philipp-Alexander Held,§ Alexander Timmer,†,‡ Armido Studer,*,§ and Harald Fuchs*,†,‡,∥ †
Center for Nanotechnology, Heisenbergstraße 11, 48149 Münster, Germany Physikalisches Institut, Westfälische Wilhelms-Universität, Wilhelm-Klemm-Straße 10, 48149 Münster, Germany § Organisch-Chemisches Institut, Westfälische Wilhelms-Universität, Corrensstraße 40, 48149 Münster, Germany ∥ Institute for Nanotechnology, Karlsruhe Institute of Technology, 76344 Karlsruhe, Germany ‡
S Supporting Information *
ABSTRACT: On-surface synthesis is a powerful cutting-edge technology to build up covalently bound nanostructures directly at surfaces which enables the preparation of highly advanced one- or two-dimensional materials. In these processes, bond formation generally occurs by thermal activation of the precursor building blocks. In this Article we report the light-induced homocoupling of aryl-alkynes (Glaser coupling) at metal surfaces. Such photochemical approaches are of particular importance as potentially orthogonal processes to thermal on-surface reactions.
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INTRODUCTION In the past years, the on-surface synthesis as a tool to construct well-defined and covalently bonded nanostructures at surfaces has attracted significant attention from the research community due to its potential applications in materials science and molecular electronics.1−6 Up to now, the homocoupling of aryl halides7 at the surfaces was most commonly applied to build one-dimensional (1D) molecular wires,8−10 nanoribbons,11−13 or two-dimensional (2D) covalent organic networks.14−18 Other chemical reactions such as imine coupling,19,20 azide− alkyne cycloadditions,21,22 dehydration of boronic acids,23 dimerization of N-heterocyclic carbenes,24 acylation reactions,25,26 Bergman cyclizations,27 and C−H bond activation processes28−32 have also been reported. However, in all these approaches the interfacial bond formation has been achieved by thermal activation of the precursor molecules. Only in a strongly limited number of cases a photochemical induced bond formation has been described toward on-surface polymerization.33,34 Using this on-surface photochemical approach, not only conjugated nanostructures can be constructed in a novel fashion but also an understanding and controlling of on-surface photochemistry can be pursued.35,36 Recently the thermal dehydrogenative coupling of alkynes (Glaser coupling) was developed.29−32 On the basis of these results we were motivated to establish a novel photochemical activation mode for the construction of conjugated materials at the interface. In this Article, we present the photochemical coupling of aryl-alkynes, as an efficient reaction for the construction of oligomeric chains at metal surfaces as an alternative to the previously described thermal activation mode (Scheme 1).29−32 Herein, we demonstrate that it is possible to conduct the same © 2014 American Chemical Society
Scheme 1. Schematic Illustration of the Light-Induced OnSurface Glaser Coupling Reaction
chemical process either light induced or initiated by heat at the interfaces. Individual aryl-alkynes as well as the final products of the reactions are readily imaged by scanning tunneling microscopy (STM).
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METHODS Within this study, we chose the diethynyl-substituted π-system 1,4-diethynyl-2,5-dihexylbenzene (hereafter called aryl-alkyne, molecule 1 in Figure 1a; for its synthesis see previous report31). Experiments were performed with a low-temperature UHVSTM (Omicron), which works at 78 K under a base pressure of 1 × 10−10 mbar. STM imaging measurements were conducted in constant-current mode with the sample biased. Clean Received: December 4, 2013 Revised: March 5, 2014 Published: March 5, 2014 6272
dx.doi.org/10.1021/jp411889e | J. Phys. Chem. C 2014, 118, 6272−6277
The Journal of Physical Chemistry C
Article
than Au(111).37,38 Therefore, the molecular packing is more dense on the Ag(111) surface. For comparison, the dimer of the aryl-alkyne (molecule 2) was synthesized by classical solution-phase chemistry (see SI) and deposited at the Ag(111) surface. The STM highresolution image of the dimer shows the bisalkyne moiety with a measured center-to-center distance of the two arene entities of 0.95 ± 0.02 nm (Figure 1e, few impurities/fragments were also identified). This image clearly shows that the alkyne− alkyne bond in the dimer is well visualized and is very different from the image of the self-assembly structure of monomer 1. Therefore, STM analysis will readily allow us to distinguish whether homocoupling of aryl-alkyne monomers to the corresponding dimers at the surface occurred or not. According to the UV−vis adsorption spectrum of the arylalkyne monomers (Figure S2, SI), a UV laser source at 375 nm as well as laser sources of 488 and 630−650 nm were chosen, to avoid direct photodissociation.39−41 Importantly, thermal heating by the light source could be neglected since no significant temperature increase was measured under the applied conditions (temperature did not exceed 300 K). Along with the Au(111) and Ag(111) surfaces we also included the Cu(111) surface into these investigations. Initial photochemical homocoupling studies were conducted at the Cu(111) surface. We could observe the formation of aryl-alkyne dimers at this surface induced by UV irradiation at a wavelength of 375 nm (Figure S5, SI). The center-to-center distance of the dimers accounted for 0.95 ± 0.02 nm (theoretical value: 0.95 nm), which unambiguously proved the successful C−C bond formation. However, we also observed that the aryl-alkyne is tilted toward the Cu(111) surface within the monolayer, hence the probability of successful photochemical aryl-alkyne dimerization was not high (Figures S3−S6, SI). To optimize the photochemical Glaser coupling process toward higher efficiency, Au(111) and Ag(111) substrates were investigated next. Disappointingly, photochemical alkyne homocoupling was not observed at the Au(111) surface (see Figure S7, SI). We believe that the stronger confinement of the Au surface reduces the mobility of the aryl-alkynes and therefore suppresses the targeted photochemical reactions (diffusion of the aryl-alkyne at the surface is important for the coupling; see below). We were glad to find that at the Ag(111) surface the lightinduced on-surface Glaser coupling occurred rather efficiently. Figure 2a shows the high-resolution STM image of selfassembled aryl-alkynes at the Ag(111) surface before UV irradiation (few impurity molecules marked by red circles did not influence the photochemical process and are therefore not further considered herein). Upon UV light exposure for 2.5 h, the photochemical Glaser coupling reaction occurred within the ordered molecular islands (Figure 2b-i) as well as at the edge of the islands (Figure 2b-ii). A high-resolution inset image and the measured center-to-center distances 0.96 ± 0.02 nm (theoretical value: 0.95 nm) proved the success of the C−C bond formation. Moreover, the center-to-center distance is very similar to the distance measured for the ex situ prepared dimer, and the structure of the on-surface reaction product looks very similar (see above). It is highly likely that the observed coupling process is mediated by light: if there is free space for two proximate aryl-alkyne molecules to diffuse, subsequent intermolecular contact of the carbon triple bonds will lead to the formation of dimers driven by UV photons. For the photochemical events to occur within the ordered islands, the
Figure 1. (a) Molecular structure of the investigated aryl-alkyne. (b), (c) High-resolution STM images of the self-assembly structure of the aryl-alkyne at Au(111) (−0.5 V, 50 pA) and Ag(111) (−0.1 V, 100 pA), both in size 5 nm × 5 nm. (d), (e) Chemical structure of the presynthesized dimer (molecule 2) and high-resolution STM images of such dimers at the Ag(111) surface (−0.5 V, 10 pA, 8.5 nm × 8.5 nm).
surfaces of Cu(111), Au(111), and Ag(111) were obtained after several cycles of argon ion sputtering and annealing. The arylalkyne precursor, which exhibits a low vapor tension in vacuum, was deposited onto the metal surfaces by free diffusion using a custom-designed funnel within an isolated chamber. The deposition rate, ∼0.1 ML min−1, was calibrated via STM images. The dimer of aryl-alkyne (molecule 2, Figure 1d; for synthesis see Supporting Information (SI)), readily synthesized by solution-phase chemistry, was sublimated onto metal surfaces at 140 °C with a rate
