ARTICLE pubs.acs.org/JPCC
STM Investigation on the Formation of Oligoamides on Au{111} by Surface-Confined Reactions of Melamine with Trimesoyl Chloride S. Jensen,† J. Greenwood,† H. A. Fr€uchtl, and C. J. Baddeley* EaStCHEM School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, United Kingdom
bS Supporting Information ABSTRACT: The surface-confined coupling reaction between melamine (1,3,5-triazine-2,4,6-triamine) and trimesoyl chloride has been investigated on Au{111} by scanning tunneling microscopy. The reaction proceeds at room temperature with the formation of new diamide species at the boundaries between melamine domains. Annealing results in the production of longer oligoamides and the transformation of the melamine phase into a more tightly packed phase which has not been previously reported on Au{111} in ultrahigh vacuum experiments. The factors which may affect the formation of this phase are discussed.
’ INTRODUCTION The fabrication of nanoscale architectures on metallic and semiconductor surfaces has been greatly aided recently by research into the self-assembly of adsorbed organic precursors into long-range, periodic structures. A variety of structures have been realized on a range of surfaces. The fabrication of these nanoscale architectures has utilized noncovalent intermolecular forces, such as hydrogen bonds,1 5 van der Waals forces,6,7 and metal organic coordination,3,8,9 to produce ordered arrays. However, structures stabilized by these intermolecular forces tend to be of limited chemical and thermal stability and are therefore unsuitable for applications involving templating or catalysis. In recent years, a range of surface-supported 2D nanostructures have been fabricated by employing covalent reactions. Ideally, fully covalent structures would offer improved chemical and mechanical durability. The surface-confined polymerization of organic adsorbates by covalent reactions is proving to be a versatile area of surface chemistry. Reactions have been shown to proceed with single component systems such as octylamine coupling to form trioctylamine10 in which it is suggested that the Au{111} surface is a crucial component to the reaction mechanism. Also, the surface is vital to the mechanism of the radical coupling of tetra (mesityl) porphyrins on Cu{110}11 and to the Ullmann reaction for the coupling of diiodobenzene molecules in which the byproduct is copper iodide.12 A study on one-dimensional coordination polymers found that the Cu{111} surface provided adatoms necessary for the metal organic polymers to form promoting interactions between the Cu atoms and the pyridyl groups of the porphyrin derivative building blocks.13 There have been reports of surface-confined radical coupling reactions, including the thermally induced polymerization of 1,3,5-tris(4-bromophenyl)benzene14 and of tetra(4-bromophenyl) porphyrin, 15 both on Au{111}, as well as of 1,3,4-tris(4-bromo phenyl)benzene on r 2011 American Chemical Society
Cu{111} and Ag{110}16 all via debromination. Multicomponent reaction systems are also well documented in the literature, including the thermally induced reaction between a fullerene/ porphyrin binary nanostructure system on Ag{110}.17 Other examples include condensation reactions between aldehydes and amines 18 and between anhydrides and amines 19,20 as well as nucleophilic attack of amines at isocyanates.21 The urea linkages ( NH CO NH ) formed by the latter reaction enable intermolecular hydrogen bonding between the urea oligomers resulting in ordered structures of the reacted phase; the urea groups are also important in molecular recognition systems22 because of possible docking at the urea linkages. In addition, the NdC=O group is sufficiently reactive at the linear (sp) carbon in the center of the isocyanate to allow the reaction to proceed with minimal annealing. Generally, the formation of covalent interlinkages requires high thermal annealing temperatures to initiate the reaction.12 16,18 20 This restricts the choice of organic precursors as small molecules may desorb before the reaction is initiated at higher temperatures. Also, the precursors must be thermally stable at the required activation temperature. A way to overcome these issues is to select reactants that produce covalent interlinkages near room temperature. Schmitz et al.23 have reported on the reaction between an acyl chloride and amine resulting in polymerization at room temperature on the Ag{111} surface. This is an advantageous system because of the ease of nucleophilic substitution at the acyl chloride carbonyl group. As well as this, the polyamides are potentially capable of hydrogen bonding to guest species linking in with the theme of molecular recognition systems.24,25 Received: November 25, 2010 Revised: March 20, 2011 Published: April 08, 2011 8630
dx.doi.org/10.1021/jp111237q | J. Phys. Chem. C 2011, 115, 8630–8636
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Figure 1. Chemical structures of (a) melamine and (b) trimesoyl chloride.
In this article, we use scanning tunneling microscopy (STM) to investigate the reaction between trimesoyl chloride and melamine, the chemical structures of which are shown in Figure 1, on the Au{111} surface. Herein, we show that such an approach can be used to demonstrate the surface-confined polymerization on Au{111} forming oligoamides. We also show that oligoamide island formation at the melamine domain boundaries inhibits the desorption of melamine and enables a phase transition to a closepacked melamine phase which is normally not observed on Au{111} under ultrahigh vacuum (UHV) conditions but has been observed following deposition of melamine from hot solution by Zhang et al.26
’ EXPERIMENTAL SECTION The STM experiments were carried out in an Omicron UHV system with a base pressure of 1 10 10 mbar. The Au{111} sample was prepared by cycles of argon ion bombardment (1 kV) and annealing to 873 K until low-energy electron diffraction (LEED) and STM indicated the presence √ of a clean Au{111} surface exhibiting the characteristic ( 3 22) herringbone reconstruction.27 Melamine (Sigma-Aldrich) was heated to 418 K and was sublimed for 45 min at a pressure of 1 10 7 mbar onto the Au{111} substrate which was held at 300 K. Trimesoyl chloride was heated to 310 K before exposure to the sample for 10 min via a leak valve at a pressure of 1.0 10 7 mbar. Throughout this manuscript, we define 1 monolayer (ML) as 1 molecule per surface Au atom. Images of the surface were acquired by transferring under UHV conditions to the STM chamber where data were acquired in constant current mode using an electrochemically etched W tip with the sample at room temperature. STM images were processed using WSxM software.28 The proposed arrangement of melamine and diamide molecules on the Au{111} surface was modeled by periodic density functional theory (DFT) calculations using the SIESTA program29 employing the PBE functional,30 a numerical splitvalence PAO (pseudoatomic orbital31) basis set with polarization functions (SVP) and Troullier Martins pseudopotentials32 as available from the SIESTA website. On the basis of the assumption that the geometry of the adsorbate layer is predominantly determined by the H-bond network and other intermolecular interactions rather than by chemical interactions with the Au atoms, only the molecular layer was incorporated into the calculations. This allowed the optimization of the cell geometries on the basis of the intermolecular interactions. The unit cell length orthogonal to the plane of the molecular network was kept at 10 Å; all other lattice parameters were optimized. ’ RESULTS AND DISCUSSION The Au{111} surface was partially precovered by melamine at 300 K under which conditions the surface is characterized by domains of the characteristic hexagonal melamine arrangement.
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Under conditions of good resolution, each melamine molecule is typically observed as a single triangular feature because of the location and shape of its molecular orbitals.33 In the hexagonal phase, the aromatic rings form the points of a hexagon; each melamine is connected to a neighboring melamine species via two NH---N hydrogen bonds along the side of the hexagon.34,35 Imaging of the surface, carried out at room temperature immediately after depositing trimesoyl chloride, is shown in Figure 2a and shows the formation of lines of relatively bright contrast within the melamine domains. The absence of the herringbone reconstruction in regions between the melamine islands is likely to be indicative of a 2D gas of trimesoyl chloride. No evidence was found of ordered domains of unreacted trimesoyl chloride. The image also shows evidence of corrosion of the step edges and the formation of etch pits on the Au{111} surface after the adsorption of trimesoyl chloride; in addition, there are bright clusters imaged. These cannot be unambiguously identified but may be caused by the formation of clusters of gold chloride via etching of gold by trimesoyl chloride. We attribute the stripes through the melamine domains to the formation of new molecules analogous to the reaction of 1,4-phenylene diisocyanate with melamine.21 In this case, diamide molecules have been created by reaction of trimesoyl chloride molecules with two melamine molecules. We conclude this reaction stoichiometry on the basis that the two outer features of the newly formed molecule retain the appearance of melamine molecules and that the continued presence of two of the three NH2 functionalities on each melamine moiety facilitates intermolecular H-bonding interactions with neighboring (unreacted) melamine species. The diamide molecules, seen between adjacent melamine arrays in Figure 2b, appear as three adjoined lobes. The two terminal lobes are triangular in shape, analogous to melamine, and have been identified as triazine rings. These new molecules measure ∼15 Å in length, which is consistent with the calculated length of the conformation adopted by the molecule in this arrangement supporting the case for covalent bond formation. The terminal triazine rings of these diamide molecules each possess two amino groups, and thus, they can be readily incorporated without disruption of the hydrogen-bonding arrangement in the melamine domain. Under the imaging conditions used (i.e., tip biased positively), it is to be expected that the imaged diamide molecules will predominantly resemble the HOMO states in appearance with electrons tunneling from the occupied molecular states to the empty tip states. There is some evidence of asymmetry in the imaging of the oligomers with the highest contrast at one end of the features. This is consistent with the calculation of the HOMO of the oligomer which places the highest electron density on one terminal melamine moiety. Diamide formation appears to be limited to stripes between melamine domains. Similar behavior was observed in the adsorption of 1,4-phenyl diisocyanate21 on melamine-covered Au{111}. We believe trimesoyl chloride enters low-density domain walls between melamine domains upon which reaction occurs between the amino groups of melamine and the acyl chloride groups of trimesoyl chloride. To better understand the incorporation of these diamide molecules at the interface between melamine domains, DFT calculations were carried out on the intermixed phase. By placing the proposed diamide molecules among the typical hexagonal arrangement of melamine molecules, a repeating phase close to what is observed can be reproduced; the optimized arrangement 8631
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Figure 2. (a) Melamine domains featuring diamide species on the Au{111} surface after the deposition of trimesoyl chloride. The step edges have been reshaped, and the etch pits have been created by the presence of trimesoyl chloride (77.5 nm 77.5 nm, 1.5 V, 0.2 nA). (b) STM image showing the incorporation of diamide molecules into a melamine domain at 300 K (10 nm 10 nm, 1.5 V, 0.2 nA). The white unit cell represents the melamine arrangement. The green unit cell corresponds to the mixed melamine diamide phase modeled in c. (c) DFT optimized geometry of diamide molecules incorporated into melamine domains. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) for the diamide molecule are shown below, each interacting with melamine molecules via the triazine moieties.
is shown in Figure 2c. DFT reveals that the unit cell for this phase has sides measuring 10.32 Å and 23.14 Å separated by an angle of 76°. This compares with the experimentally measured values of 11 Å 22.7 Å with an included angle of 70°. This can be matched to the corresponding green unit cell drawn in Figure 2b. These diamide molecules are linear, and the chirality of the melamine domains on either side of the stripes of diamide molecules is maintained. The two triazine rings of the diamide molecules have to be mirror images of each other in order to comply with the chiral hydrogen bonding motif of the entire domain. The middle aromatic ring of the diamide molecules is calculated as being twisted slightly out of plane to maintain the optimal hydrogen
bonding interaction between their terminal triazine moieties and the melamine domain. Similar to the polyamide chains observed by Schmitz et al.,23 this reduces the electrostatic repulsions between H atoms of the amide groups and the central phenyl ring. In the optimized structure, the angle between the ring and the plane of the molecules is 20°. In the presence of the metal surface, this may be reduced in order to optimize the interaction of the molecule with the surface. Annealing the Au sample to 350 K removes the stripes, and the melamine domains undergo a phase transition to a higher density structure. In addition, longer oligoamide molecules are formed concentrated at the edges of the melamine domains as shown in 8632
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Figure 3. (a) STM image showing polyamide molecules conglomerated around the edges of melamine domains after the surface was annealed to 350 K (54.4 nm 36.7 nm, 1.5 V, 0.35 nA). Red arrows indicate pores in the melamine hexagonal array which remain unfilled after rearrangement. Green arrows indicate the unidentified domains. (b) Close-up view of the chlorine rows observed after annealing (9.7 nm 6.6 nm, 1.5 V, 0.3 nA). (c) Zoomed STM image showing the polyamide molecules around the edges of melamine domains (15.7 nm 15.7 nm, 1.5 V, 0.35 nA). Green arrows point toward molecules that have formed in a horseshoe shape. Red arrows indicate aromatic molecules that have reacted completely to form three amide linkages. Oligoamides similar to that shown above are thought to be represented by the horseshoe-shaped features in the STM image. (d) Close-up view of the close-packed melamine phase (5.8 nm 5.8 nm, 1.5 V, 0.35 nA). The unit cell is illustrated by the white parallelogram. An empty pore is highlighted by the red arrow, and a selection of the central, upright melamine species has been highlighted by green circles.
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Figure 3a. As well as the major melamine and oligoamide domains, some small domains are observed consisting of parallel rows of features separated by ∼4.3 Å. These are indicated by the green arrows in Figure 3a, and a close-up view is shown in Figure 3b. These features are not well-resolved along the rows. STM studies by Gao et al.36 have indicated that, √at low√coverage (
