Conformational Transitions of Phase-Separated Binary Molecules

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Interface-Rich Materials and Assemblies

Conformational Transitions of Phase-separated Binary Molecules Assisted by Surface Dehalogenation Hailong Zhu, Tianchao Niu, and Ang Li Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04220 • Publication Date (Web): 13 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Conformational Transitions of Phase-separated Binary Molecules Assisted by Surface Dehalogenation Hailong Zhu†,‡,§, Tianchao Niu†,§,⊥ and Ang Li*,†,§ †

State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of

Microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, People’s Republic of China ‡

University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of

China §

CAS Center for Excellence in Superconducting Electronics (CENSE), Shanghai

200050, People’s Republic of China KEYWORDS: melamine, HBB, self-assembly, phase transition

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ABSTRACT: Molecular devices have become an emergent branch of nanoscience and technology beyond traditional silicon-based electronic devices. The properties of these devices are intimately related to the molecular conformation and packing. In this article, three different conformations of melamine molecules are observed on Au(111) and a transition from lying-down to standing-up phase with long-range order is realized in melamine chains with the assistance of hexabromobenzene (HBB). We argue that it is the expanding of HBB domains from hexagonal to dimer phase due to the surface dehalogenation that facilitates the dehydrogenated of melamine to form a standing-up conformation. Similar transitions are also accomplished on Ag(111) surface. Our results provide an effective way to achieve up-standing molecular arrays with a long-range order on relatively less active metals. This may have a significant implication to fabricating organic thin film transistors.

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INTRODUCTION Molecular self-assembly of organic semiconductors has proved to be an effective way to fabricate functional electronic devices such as diodes, transistors, sensors, and photovoltaics.1 The properties of such devices are crucially determined by the molecular conformation and packing geometry. In general, most devices combine two or more species to build, for example, the donor/acceptor heterojuctions to complement the versatile functions of each ingredient and achieve superior performances in organic photovoltaics (OPVs) and organic thin film transistor (OFETs).2 The building blocks within the self-assembled molecular superstructures are interrelated and interact with each other, which can either form long-range ordered arrays stabilized by non-covalent interactions or phase separate into different domains. Particularly, some organic species can undergo the conformational switching or decomposition under certain external stimuli such as the electric field and heat. Controllable phase transition of self-assembled monolayer molecules has drawn considerable attentions during the past decades.3-4 Studying the precise packing conformation of molecules and its controllable evolution can help to understand the thermodynamics of self-assembled two dimensional molecular assembly and facilitate the development of desirable novel functionalities.5-6 High-refractive-index polymers based on brominated aromatic monomers such as the Poly(pentabromophenyl methacrylate) and its relatives are basic components in waveguide

or

light

emitting

diodes.

Other

brominated

aromatics

like

tetrabromopyrene are the building block for organic light-emitting diode (OLED) 3


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emitters.7 However, the bromine atoms can be easily dissociated at elevated temperatures generating molecule anchoring to the reactive surfaces. In addition, amino (-NH2) is one of the most commonly used anchoring groups in molecule junctions.8 The amino group terminated organic thin films are ideal functional chemical platform to host protein and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in the production of immunoassay-based diagnostic devices9 and solar cells10 respectively. In this article, we choose hexabromobenzene (HBB) and melamine (1,3,5-triazine-2,4,6-triamine) as the model system to study the cooperative effect of – NH2 group and bromine adatoms on the molecular conformational transition and packing geometry. The reactions of dehydrogenation of melamine11 and dehalogenation of HBB12-13 are depicted in Scheme 1. In spite of its planar molecular structure, melamine has a tendency to bond to the metal surface via two amino groups (-NH2) thus is a good candidate for realizing standing-up configuration. Typically, a densely packed up-standing phase of such molecules possesses higher thermal and mechanical stability. Ordered up-standing melamine species have been achieved on Ni(111)14 and Cu(111)15 substrates, but not on the more stable noble metals such as Au or Ag. How to establish the conformational control of melamine on less active substrate at (or near) ambient conditions is yet to be explored. By co-depositing HBB, we found that flat-lying melamine can be transferred to up-standing phase with the assistance of in-situ HBB dehalogenation on noble metal surface. Our finding may highlight a promising venue towards fabricating molecular devices by virtue of conformational engineering. 4


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Scheme 1. Reactions of dehalogenation of HBB and dehydrogenation of melamine.

EXPERIMENTAL SECTION The STM experiments were performed in ultrahigh vacuum (UHV) with a base pressure of 4×10-11 mbar. All STM images were recorded at 4.4 K in constant-current mode using an electrochemically etched tungsten tip. All Au and Ag single crystal substrates were cleaned by repeated cycles of 0.8 keV Ar+ sputtering and 600 °C annealing. Melamine and HBB were deposited onto the metal surface from pyrolytic boron nitride crucible in a dual-head Knudsen cell held at 90 and 82 °C respectively. The substrates were kept at room temperature (~20 °C) during evaporation. Melamine and HBB molecules were purchased from Sigma-Aldrich with a purity of 99%. RESULTS AND DISCUSSION Multiple Conformational Phases of Melamine on Au(111) Figure 1 summarizes the successive phases of melamine deposited on Au(111). In Figure 1A and B, each triangular unit stands for one single melamine molecule lying on Au surface. The honeycomb structure consisting of six melamine molecules in Figure 1A is the most common phase (phase I). Each flat-lying melamine molecule 5


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is stabilized by hydrogen bonds16-18 between neighboring melamine molecules. The unit cell contains two melamine molecules with lattice constants a=1.01 nm, b=1.03 nm. The orientation of these honeycomb lattice is defined by its hexagon centers which is about 14˚ to the [112$] direction of the Au(111) surface. The rectangular area highlighted in Figure 1B shows a more densely occupied state (phase II), where melamine molecules arrange themselves in a zipper-like structure. Figure 1E demonstrates the model of phase II, which appears as a boundary between two melamine hexagonal domains. Similar double-row melamine structure has been observed on Ag(111)19 and Au(111)20. Both phase I and II are obtained after room-temperature deposition. When we anneal the sample at 333 K, new configuration (phase III) with a Au atom locating in the hexagon center is widely observed (Figure 1C). The unit cell size becomes a=b=1.06 nm. As Mura et al. have calculated that it is energetically favorable for Au adatoms to reside inside the melamine hexagons21. In fact, the phase III is already occasionally visible after room temperature deposition (green circle in Figure 1B) and becomes dominant after annealing. When the annealing temperature is further increased to 363 K or above, the majority of melamine molecules are desorbed from the surface. Unlike the case on more reactive Cu or Ni, thermal energy alone is not enough to form a closely packed up-standing phase of melamine on Au(111) due to its weaker bonding to the noble metals. Assistance from other agent might be necessary to realize such conformational transitions.

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Figure 1. STM images showing the melamine molecules on Au(111) (A) and (B) as deposited at room temperature; (C) after annealing at 333 K respectively. (D)-(F) Models of melamine molecular structure corresponding to (A)-(C), the yellow balls represent Au atoms. Scanning parameters: (A) -0.8 V, 100 pA; (B) -0.2 V, 250 pA; (C) -0.2 V, 150 pA.

Dehalogenation Assisted Phase Transition on Au(111) and Ag(111) Figure 2 demonstrates the phase evolution of melamine on Au(111) in the presence of HBB and associated dehalogenation. After co-depositing melamine and HBB molecules at room temperature, we anneal the samples in steps up to 363 K. Figure 2A and B show the as-deposited melamine and HBB before annealing. Apparently two molecules are phase separated. The HBB-covered regions (one of which is marked with a yellow circle in Figure 2A) exhibit a hexagonal molecular arrangement (insert image). Melamine areas (red circle) show a mixture of phase I 7


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and II as described in the previous section, which are directly highlighted in Figure 2B. Following the herringbone pattern of Au(111) in HBB-covered regime, one can find that the location of phase II patches in melamine domains matches well with herringbone reconstruction. It means the domain walls of honeycomb melamine structure prefer to occupy the hcp regions of Au(111) surface. Figure 2C is a STM image of melamine and HBB molecules after annealing at 310 K. HBB molecules remain the same hexagonal phase whereas a large scale of long-range ordered phase II of melamine molecules is formed. The angle between 𝑎⃗ axis of phase II (defined in Figure 1B) and [112$] direction of Au(111) is measured to be 134˚.

Figure 2. STM images for melamine and HBB molecules (A) and (B) as deposited on Au(111); and after annealing at (C) 310 K; (D), (E) and (F) 333 K; (G), (H) and (I) 363 K, respectively. Scanning parameters: (A) -1.2 V, 111 pA; insert: -0.1 V, 1 nA; (B) 1 V, 250 pA; (C) 0.5 V, 250 pA; (D) -1 V, 200 pA; (E) -1 V, 1 nA; (F) -0.5 V, 300 pA; (G) 1 V, 100 pA; (H) -1 V, 300 pA; (I) -0.5 V, 8


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200 pA.

Figure 2D, E and F are the HBB and melamine topographies after 333 K annealing. Short pieces of bright chains appear in the vicinity of the boundaries between HBB and melamine domains, which are magnified in Figure 2F. The HBB molecules (yellow circle) near these bright chains exhibit a fourfold-symmetry arrangement22 (Figure 2E), and the melamine molecules in Figure 2D turn into phase III characterized by one bright spot in the center of each hexagon. There are two principal axes (red arrows) for the hexagon orientation in phase III, each being at 14˚ to the Au [112$] (yellow arrow). The models in Figure 1D and F have included these chiral packing modes, which are described in detail by Zhang et al.23 When the annealing temperature is further increased to 363 K (Figure 2G, H, I), there is no significant desorption occurred to melamine. Instead, a transition from flat-lying to up-standing phase (phase IV) with packed long-range order is accomplished. Chains of molecules are observed running along the 〈112$〉 crystallographic directions as shown in Figure S1. Similar melamine chains have been observed on the Cu and Ni surfaces, but never on Au(111) in the absence of HBB. Taking into account the fact that these chains start to form in close proximity to the HBB domains (Figure 2D), we believe the HBB molecules play a role in the development of phase IV. According to the previous study24, HBB molecules partially decompose into bromine and carbocyclic ring on Au(111) at 363 K. The resulting Br adatoms prefer to encircle the undecomposed HBB dimers, leading to an expanded unit cell as shown in Figure 2H. Hence the strong electronegativity25 of Br and the expanding of HBB 9


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network could assist the melamine molecules to stand upright and pack more closely. Hexagonal, tetragonal and dimer superstructures of HBB molecules upon different annealing can be further referred in Figure S2. The up-standing melamine shown in Figure 2G is 2 Å higher than the HBB molecules, thus ~2.8 Å higher compared to the flat-lying melamine molecules as the lying-down phase is about 0.8 Å lower than the HBB molecules in Figure 2A. The measured height difference agrees well with the estimated value from the molecular structural model. Ordinarily, melamine molecules desorb from the Au surface before overcoming the energy barrier of dehydrogenation to stand up straight. When co-deposited with HBB, as the HBB domains expand upon annealing, melamine molecules are compelled to make room. As a result, the up-standing phase of melamine can be realized on Au(111) with the assistance of HBB and associated dehalogenation. Figurer 3A is a magnified STM image of melamine molecules on Au(111) in the standing-up phase. It contains long-range ordered chains and each chain consists of two rows of bright dots. We propose that each bright dot represents one melamine molecule standing vertically and stacks to form a chevron type double-row chain. For the up-standing melamine, Greenwood et al. calculated the most stable geometry to be two NH groups bonding to the bridge sites of Au(111) and the azide N atom in-between bonding to the atop14. Based on the geometric information (intermolecular distances, angle, and atomic registry to the gold substrate) derived from our STM data, we model the surface structure exclusively as depicted in Figure 3D. The unit cell size is a=0.5 nm, b=1.45 nm, which can be described based on a Au *√3 × 5/ 10


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reconstruction cell with one molecule at the center. Line B in Figure 3A is along diagonal direction of the unit cell with spacing between adjacent rows in a chain of about 0.78 nm and line C is along the [112$] direction. The driving force for such a chain structure may be the attractive interaction between NH2-groups on adjacent row and π − π interaction between the aromatic rings along the chain. In general, melamine molecules self-assemble into flat-lying honeycomb structures stabilized by H-bonding interactions. Since the interconnection between aromatic rings and gold substrate is weak, melamine can be easily desorbed from Au surface at slightly elevated temperatures. With the assistance of HBB molecules, a closely packed melamine assembly with long-range order can be stabilized with N-Au junction on substrate after dehydrogenation, which could modify the electrical coupling among molecules as well as their coupling to the substrate26-27. In this sense, our finding is useful for the development of high-performance organic devices. Also, these results provide a way on studying the thermodynamics of self-assembly systems and call for further research on three-dimensional superstructures construction based on the coupling with the terminated amino-groups.

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Figure 3. (A) Magnified STM image of melamine up-standing chains. (B) and (C) Line profile along different directions marked in A. (D) Model of the melamine standing-up phase with long chains. (E) Models of a unit cell from top and side views. Scanning parameter: 0.5 V, 200 pA.

The following coverage dependence experiment further supports our argument that the expansion of HBB lattice upon dehalogenation facilitates the close-packing of the melamine. By decreasing the deposition time of melamine and HBB molecules, a partially covered Au surface is obtained. Figure 4 displays the surface configurations after 333K annealing. Melamine exhibits a phase III structure and HBB contains both hexagonal and fourfold-symmetry arrangements. Also, small pieces of bright chains appear near the boundaries of HBB and melamine. These are all the same as described in Figure 2D. But when further increasing the annealing temperature to 363 K or higher, we can not obtain large-scale long range ordered chains with up-standing phase. This proves that squeezing from the expansion of HBB to melamine plays an 12


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important role in a complete transition to upstanding phase of melamine molecules. For there are several bare surface areas after deposition, the free space is enough to accommodate the expanded HBB domains. The finding that the transition to the upright geometry does not take place for an incompletely covered sample is a strong evidence for our main argument that HBB assists the upright packing of melamine.

Figure 4. STM images of partially covered melamine and HBB on Au(111) after annealing at 333 K. Scanning parameters: (A) -1 V, 100 pA; (B) 1 V, 300 pA.

The phase transitions assisted by HBB dehalogenation is also verified on Ag(111), which is relatively more reactive than gold. After deposition at room temperature, phase III and IV of melamine coexist already (Figure 5A, insert is a magnified image of phase III). The yellow circles in Figure 5A and B mark the HBB covered area. Due to the activity of Ag, HBB molecules are already decomposed at room temperature. The produced Br adatoms with strong electronegativity and the dehalogenated HBB networks squeeze melamine molecules in space as we have discussed for Au(111). After 333 K annealing, transition from lying-down to standing-up is completed (Figure 5C). Figure 5D is a magnified image of phase IV, each circle represents a repeating unit with five up-standing molecules. The measured 13


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nearest intermolecular spacing along the [12$1] direction is 0.4 nm. Both up-standing melamine chains are obtained on Au(111) and Ag(111) with the assistance of HBB, but the periodic structures are not exactly the same. This may be associated with the special active sites of different metals, as Br atoms come from HBB dehalogenation also exhibit different arrangements on Au and Ag (Figure S3). Melamine prefers to bond to reactive metal vertically such as Cu and Ni while such a phase was never reported on Au or Ag before.28-30 Surface-dehalogenation assisted phase transition may provide an effective way to achieve up-standing melamine molecules on less active metals with a long-range ordered structure.

Figure 5. STM images for melamine and HBB molecules (A) and (B) deposited on Ag(111) at room temperature; and after annealing at (C), (D) 333 K. Scanning parameters: (A) 1 V, 100 pA; insert: -1.5V, 100 pA; (B) 1 V, 100 pA; (C) -1 V, 100 pA; (D) 1 V, 80 pA.

CONCLUSIONS We deposited melamine and HBB on Au(111) to study the phase evolution of 14


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molecular assembly with thermodynamic conditions. Four successive phases of melamine have been identified as the annealing temperature is increased. For as-grown samples, a honeycomb structure and the domain wall arrays can be found for the melamine molecules. After annealing at 313 K, those domain wall arrays enlarge to form long-range ordered zipper-like chains. Further annealing at 333 K leads to a large scale of honeycomb structure with Au adatoms filled in each hollow site. Most interestingly, the transition to an upright phase is accomplished at 363 K. During this process, HBB molecules gradually dehalogenate and expand in domain size, forcing the neighboring melamine molecules to change their conformation. The transition to the upright phase does not take place for melamine alone or partially covered samples. Our results demonstrate that squeezing from HBB expanding eventually assists the transition of melamine from lying-down to standing-up phase before desorption. On-surface dehalogenation assisted self-assembling provides an effective way to fabricate functional molecular network. The structures may have a significant application in high quality organic thin film transistors. SUPPORTING INFORMATION Supplementary STM images for the observed upright chains of large scale along the $ 〉 directions (Figure S1); the conformational phases of HBB molecules under 〈112 different annealing temperatures (Figure S2); and the different morphologies of Br atoms on Au(111) and Ag(111) (Figure S3). This material is available free of charge on the ACS Publications website.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Tel: +86-21-62511070 ext. 3301. Present address College of Materials Science & Engineering, Nanjing University of Science and

⊥

Technology, Nanjing 210094, China. ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China under contract No. 11227902, 21403282 and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences, Grant No. XDB04040300. A. L. is also supported by the One Hundred Talents Program of the Chinese Academy of Sciences. REFERENCES (1) Heath, J. R.; Ratner, M. A. Molecular Electronics. Phys. Today 2003, 56, 43-49. (2) Kang, J.; Shin, N.; Jang, D. Y.; Prabhu, V. M.; Yoon, D. Y. Structure and Properties of Small Molecule-Polymer Blend Semiconductors for Organic Thin Film Transistors. J. Am. Chem. Soc. 2008, 130, 12273-12275. (3) Shen, X. L.; Wei, X. D.; Tan, P. L.; Yu, Y. G.; Yang, B.; Gong, Z. M.; Zhang, H. M.; Lin, H. P.; Li, Y. Y.; Li, Q.; Xie, Y. S.; Chi, L. F. Concentration-Controlled Reversible Phase Transitions in Self-Assembled Monolayers on Hopg Surfaces. Small 2015, 11, 2284-2290. (4) Sun, K.; Shao, T. N.; Wang, J. Z. Orientation Transition of Pentacene Molecule 16


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C.

H.;

Ikonomov,

J.;

Sokolowski,

M.

Two

Commensurate

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Conformational Transitions of Phase-Separated Binary Molecules

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