Adlayer Structure of Shape-Persistent Macrocycle Molecules

Mar 19, 2014 - School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences (UCAS), 19A Yuquanlu, Beijing,. 100049 ...
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Adlayer Structure of Shape-Persistent Macrocycle Molecules: Fabrication and Tuning Investigated with Scanning Tunneling Microscopy Wei Huang,† Tian-Yue Zhao,‡ Ming-Wei Wen,† Zhi-Yong Yang,*,† Wei Xu,*,‡ Yuan-Ping Yi,*,‡ Li-Ping Xu,*,§ Zhi-Xiang Wang,† and Zhan-Jun Gu∥ †

School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences (UCAS), 19A Yuquanlu, Beijing, 100049, People’s Republic of China ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100090, People’s Republic of China § Research Center for Bioengineering and Sensing Technology, University of Science & Technology Beijing, Beijing 100083, China ∥ CAS Key Laboratory for Biomedical Effects of Nanomaterials and Nanosafety, National Center for Nanosciences and Technology of China and Institute of High Energy Physics, Chinese Academy of Sciences (CAS), Beijing, China ABSTRACT: The assembling structure of square and triangular macrocycle molecules constructed with diethynylcarbazole units was investigated by scanning tunneling microscopy (STM) on a graphite surface. STM observation revealed that the square macrocycle molecule (M1) forms a multilayer on the graphite surface. In the first layer, M1 assembles into medium-sized domains with a few defects and dislocations, whereas, for the second layer, most of M1 are dispersed on the first layer separately. A tentative stacking mode of the bilayer structure is provided in this paper on the basis of information given by STM experiments. Considering the interlayer distance given by the crystal data on the similar molecules and the length of the M1 alkyl chains, we think that it is possible that part of the top M1 side chains adsorbs on the cavity area of the bottom M1 and probably plays a dominant role in stabilizing the second layer. This postulation is verified by a control experiment in which coronene is filled in the cavity of M1 and no bilayer structure of M1 is found. The triangular molecule (M2) organizes into a single layer with larger and less defect domains. Two M2 are paired together in parallel, but opposite-oriented style, and are responsible for the serrate edge of the molecular row. The alkyl chains of M2 adopt rather diverse arrangements without disturbing the assembly of M2 core parts. When the solution contains both coronene and M2, no M2−coronene complex is observed and the adlayer characteristics of M2 are essentially the same as those of only M2 in solution. The results may help us to learn the stacking behavior of macrocycle molecules with different shapes, understand surface self-assembling principles, and develop high-performance devices based on related materials.

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INTRODUCTION Shape-persistent macrocycles (SPMs) with a rigid π-conjugated backbone and an internal nanoscale cavity have been paid a rising attention in varied fields of chemical, physical, and material science due to their unique structure and related optical and electronic properties.1,2 It is a well-known fact that the molecular stacking in SPM film influences the properties of the film and corresponding devices largely. Thus, the importance of investigating the relationship between SPM molecular structure and stacking behavior becomes obvious.3,4 Realizing the point, great efforts have been devoted to prepare, characterize, and tune the film structure of SPMs. To reveal the spatial arrangement at the atomic level, X-ray diffraction (XRD) is one of the most powerful tools.5−9 Previous crystallographic studies on carbazole−ethynylene macrocycles with different side chains showed that the alkyl chain length has an important effect on the stacking structure.3 SPMs containing terpyridine units were synthesized, and their © 2014 American Chemical Society

XRD data indicated that side alkyl chains can change both the crystal packing and the molecular conformation.8 However, crystallographic investigation of SPMs is extremely challenging for several reasons: crystallization of macrocycles is dependent largely on luck; disorder of alkyl chains leads to a poor diffraction pattern, which brings great difficulty to the following structure refinement; crystal structure collapses, caused by losing solvent molecules trapped in the macrocycle cavity when taking the crystal out of solution.3,5 Aside from X-ray crystallography, scanning tunneling microscopy (STM) is another powerful structure-characterizing tool with submolecular resolution.10−13 In particular, the samples for STM experiments are much easier to prepare than those for XRD. Therefore, most of SPM solid films were Received: November 26, 2013 Revised: February 20, 2014 Published: March 19, 2014 6767

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Scheme 1. Illustrative Structure Models of Square Diethynylcarbazole Macrocycle M1 (a) and Triangular Diethynylcarbazole Macrocycle M2 (b), Where R is n-C16 H33, and Coronene (c)

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inspected by STM rather than XRD.2,4,6,7,14−24 STM experiments demonstrated that giant spoked-wheel-like SPMs with a size of 5 nm or even larger can form stable ordered solid film on the highly oriented pyrolytic graphite (HOPG) surface.6,15,16 The molecular backbone is imaged clearly with STM, and the adlayer arrangement is acquired straightforwardly based on STM imaging. 5-fold molecules are very unlikely to form three-dimensional (3D) crystals because their symmetry is mismatched with the translational order of a classical crystal lattice. However, their film structure can be studied with STM easily. Studies show that pentagonal SPMs with proper alkyl chains form ordered domains on both HOPG and Au(111) surfaces.18 The two-dimensional (2D) crystallization process is affected by sorts of factors, including SPM core size, side chains (types, length, positions etc.), concentration, substrate, solvent, etc. Therefore, the resulted 2D structure varies with the subtle balance of all interactions.22,25−31 STM experiments can provide direct inspection of interested systems. Considering the nanosized cavity of SPMs, it is natural to choose SPMs to construct hybrid film. The other component could fill in the cavity,32−34 stick on the macrocycle backbone,35,36 or adsorb on top of the SPM core part.37 Among these studies, the SPM matrix in hybrid film is not changed by additional ingredients. However, in some cases, SPM stacking could be affected by other components indeed. The dense-packing structure of dehydrobenzo-[12]annulene derivatives is adjusted to porous honeycomb by a single guest molecule or assembling unit made by other components.38−40 The assembling pores formed by host molecules are not fixed: size, shape, and symmetry, etc., change with different guest units.40 Moreover, it was found that the guest molecule can help dehydrobenzo-[12]annulene with different alkyl chains mixing better.41 In this report, assembling behaviors of the square and triangular SPM molecules made of diethynylcarbazole units were investigated with STM. We found that square SPM (M1, Scheme 1a) assembles into a multilayer on the HOPG surface. When coronene (Scheme 1c) is introduced into the system, the cavity of M1 is filled with coronene and only the monolayer of the M1−coronene complex is observed. For the triangular SPM (M2, Scheme 1b), only a single layer is found independent of the existence of coronene. No M2−coronene complexes are formed because the cavity of M2 is not large enough to accommodate coronene. Carbazole-based materials are widely used in organic electronics because of their hole-transporting properties and excellent air stability.42 The results in our research may help to understand the formation of SPM film, develop the principle of surface patterning, and optimize the performance of organic devices based on carbazole SPMs.

EXPERIMENTAL METHODS Diethynylcarbazole macrocycles M1 and M2 (Scheme 1, a and b, respectively) were synthesized according to ref 43. Coronene (Scheme 1c) and 1-phenyloctane (solvent) were purchased from Aldrich and used without further purification. The STM sample was prepared by depositing a drop of solution containing the interested molecules on a freshly cleaved HOPG surface. The STM tip was immersed in solution when scanning. All STM experiments were carried out on a Nanoscope IIIa scanning tunneling microscope (Digital Instruments Co., CA) with mechanically cut Pt/Ir (90/10%) tips. The tunneling conditions are provided in the figure captions.

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RESULTS AND DISCUSSION Assembly of M1. Figure 1 presents a typical large-scale STM image of the M1 adlayer formed on the HOPG surface.

Figure 1. Large-scale STM image of M1 adlayer on HOPG. Tunneling conditions: Vbias = 857 mV, Itip = 492 pA.

The distinguishing feature shown in this image is that bright circles (some are pointed out with black arrows) are distributed on the top of the bottom layer which is made of ordered small spots. Bottom layer domains are not easily discerned in Figure 1 because their contrast is much lower than bright circles. Polygonal black lines are drawn to indicate boundaries of several bottom layer domains. M1 is the only compound in this sample besides 1-phenyloctane solvent. Therefore, it is safe to say that both the bottom layer and bright circles are formed by M1 molecules. Repeated experiments show that bottom domains are usually medium sized with a certain number of defects and the second layer is always made of dispersed M1. No dense-packing top layer is found, although it cannot be completely excluded. Most of M1 in the second layer adsorb on 6768

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ordered domains, but not all. Scanning lines in the image are caused by dynamic movement of M1 in the top layer. In Figure 1, we cannot get much detailed information on the M1 bottom adlayer. Therefore, the STM tip was brought to the sample more closely by adjusting tunneling conditions to swipe off top M1 molecules so that we can get high-quality STM images of the bottom layer. In the bottom layer, M1 stacks densely in ordered domains with varied sizes, as shown in Figure 2a. A single M1 is imaged

Figure 3. (a) Medium-sized STM image of M1 double adlayer. Tunneling conditions: Vbias = 1.04 V, Itip = 323 pA. (b) Top (upper part) and side (bottom part) views of tentative arranging model of M1 double layer. Top layer M1 was colored as green for clarity. Note that, in this model, the physical separation between two layers is a qualitative drawing. The alkyl chains pointing up and down only indicate that they direct toward the solution phase and bend down correspondingly. The tilting angles shown in this model do not mean the sole and exact angle in the real case.

Figure 2. Large-scale (a) and high-resolution (b) STM images of M1 bottom layer. (c) Tentative arranging model of M1 bottom layer. Tunneling conditions: (a) and (b) Vbias = −600 mV, Itip = 657 pA.

imposed in Figure 3a to demonstrate the square constructed by four neighboring dots. The size of the square is about 2.3 nm, which is close to the ring size measured in images like Figure 2. Therefore, a group of four neighboring dots is attributed to one M1 molecule. We have examined the density distribution of the M1 frontier orbitals, including the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and the orbitals close to its frontier orbitals, such as HOMO-1, HOMO-2, LUMO+1, and LUMO+2, but none of them are featured as a four-dot morphology. Further experiments need to be done to explain this interesting topography. In solid film, the interlayer stacking is an essential point because of its direct influence on film properties and device performance in turn. The big and small half circles in Figure 3a were employed to mark out the top M1 and the right bottom medium-contrast dot, respectively. According to the above analysis, a single dot corresponds to one corner of the M1 macrocycle. Therefore, it can be concluded that top M1 locates at one corner of the bottom M1 molecule. The similar displacing arrangement has also been found in the crystal of carbazole−ethynylene macrocycles.3 A tentative model of the M1 bilayer is proposed in Figure 3b (upper part, top view; bottom part, side view). In this model, top layer M1 are colored as green for clarity and the alkyl chains of bottom M1 hidden in Figure 2c are shown also to get an overview of the bilayer stacking. The bottom M1 arranging in Figure 3b is the same as that in Figure 2c since Figure 2 was acquired after the top layer was swiped off by the STM tip. The physical distance between two M1 layers cannot be measured directly from STM images due to the intrinsic STM working mechanism. However, the interlayer distance given by XRD data on the macrocycle molecules, which is also constructed with carbazole and ethynylene units, can be a valuable reference.3 In this research, the value of interlayer distance varies from 0.3 to 0.8 nm with the different side chain length, but never larger than 1 nm. Therefore, it is reasonable to think that the interlayer separation in our system is less than 1 nm since the molecular structure in their system is very similar to M1. The side alkyl chain of M1, n-C16 H33, could be 2 nm long when it extends completely. Considering the layer distance and side chain length, it is acceptable to think that some of the side chains of M1 in the

as a high-contrast ring centered with a dark hole. A typical highresolution image is provided in Figure 2b. This image reveals that each bright ring is actually a quasi square made of four short bright rods (marked with white bars in Figure 2b). The ring appearance is highly consistent with the macrocycle shape of M1. The unit cell was deduced and is outlined in Figure 2b with the parameters determined as a = 2.5 ± 0.2 nm, b = 3.4 ± 0.2 nm, α = 65° ± 3°. Side alkyl chains are not observed due to their low contrast and dense packing of macrocycles. d1, referred to as the distance between neighboring M1 rows, is measured as 0.9 ± 0.2 nm, which is too short to accommodate the n-C16H33 chain whose theoretical length is about 2.2 nm. Inspecting Figure 2b carefully, it can be found that there exists large space room between two M1 molecules located at the long diagonal line of the unit cell. The length of this space, defined as d2 in Figure 2b, is measured as 2.3 ± 0.2 nm, which is roughly close to the theoretical length of n-C16H33 in the complete extending conformation. Two M1 models were superimposed in Figure 2b to show the most possible orientation of M1 in the bottom layer. One pair of side chains at the opposite macrocycle apex adsorbs along the direction of the long diagonal line of the unit cell, while the other pair of alkyl chains may direct toward the solution phase due to lacking enough space for it to lie on the HOPG surface. Previous studies on self-assembly also report similar phenomena that alkyl chains in one molecule may adopt different conformations to form a denser adlayer or reduce steric hindrance.44−48 On the basis of analysis of the domain structure, a tentative arranging model of the M1 bottom layer is given in Figure 2c. The side chains away from the HOPG surface are cut off in the molecular model for clarity, except the M1 pointed out by the black arrow is shown as a whole structure. After finishing the study on the bottom layer, the STM tip was retracted to a higher position and moved to other areas to observe the 3D structure of the M1 assembly. Figure 3a is a medium-sized STM image showing bottom and top layers simultaneously. Interestingly, the bottom layer in images showing both layers, like Figures 1 and 3a, consists of medium-contrast dots rather than rings. A black square was 6769

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top layer will bend down and part of the side chains will adsorb on the unoccupied surface area in the cavity of bottom M1. Two arrows in Figure 3b point out the bending-down chains of top M1. Other alkyl chains of top M1 may stand upward to avoid overcrowding with the tilting-up chains of bottom M1. Surface adsorption of the top M1 side chains may play a key role in stabilizing isolated top M1 molecules. To verify this judgment, a control experiment was carried out. In control experiment, coronene was added to M1 solution which was used to prepare samples of Figures 1−3. A drop of mixing solution was deposited on the fresh surface of HOPG. Figure 4a is a typical large-sized STM image of this sample. Figure 5. Large-scale (a) and high-resolution (b) STM images of M2 adlayer. Tunneling conditions: (a) Vbias = 769 mV, Itip = 502 pA; (b) Vbias = 1.08 V, Itip = 470 pA.

stacking of the M2 pairs is responsible for the sawtoothed appearance of stripe boundaries. The complementary alignment in the M2 pair is favorable for forming a dense assembly, which is more stable due to a high surface covering ratio. Two triangles were superimposed in Figure 5b to help illustrate the molecular pair. In contrast to the fixed arranging of the M2 macrocycle part, the alkyl chains demonstrate flexible surface conformations. They align in parallel or fishbone style in two sides of the M2 macrocycle, like the molecular rows marked by “I” and “II”, respectively. Moreover, two styles can switch freely without disturbing the structure of bright lines constructing by M2 macrocycles. No unit cell can be outlined because of the varied assembling structure of side alkyl chains. However, the distance between two neighboring M2 pairs (d3 in Figure 5b) and the distance between two neighboring bright lines (d4 in Figure 5b) is determined as 3.8 ± 0.2 and 3.7 ± 0.2 nm in sequence. Several M2 models were provided to show the adsorbing structure in local area. During all experiments, the concentration of M2 solution is roughly equal to that of M1. Considering the tunneling conditions of Figure 5b (Vbias = 1.08 V, Itip = 470 pA) and that of Figure 1 (Vbias = 857 mV, Itip = 492 pA), we can know that the STM tip is more far away from the surface when acquiring Figure 5b, which means that the multilayer of M2 is very unlikely to be swiped off if existing. Moreover, no multilayer structure of M2 is observed in repeated experiments. When coronene was added into M2 solution, no M2−coronene complex is found either. The cavity of M2 may be too small to hold coronene. For each M2 molecule, all three alkyl chains adsorb on the HOPG surface, whereas, for each M1, only two alkyl chains lie on surface, which leads to that the interaction of M2−substrate is stronger than that of M1−substrate. Therefore, M2 forms a single layer with much less defects. In contrast, defects and dislocations are frequently observed in the M1 bottom layer and the average domain size is smaller. The forming of the M1 second layer depends on the existence of stabilizing interaction, that is, surface adsorption of part of the top M1 alkyl chains on the cavity area of the bottom M1 molecules. As a consequence, the M1 double adlayer could be adjusted to a single layer by filling the cavities with properly sized guest molecules. The research in this paper is helpful in learning the assembly mechanism, fabricating surface patterns, and optimizing film properties prepared by related materials.

Figure 4. Large-scale (a) and high-resolution (b) STM images of the sample containing M1 and coronene. Tunneling conditions: (a) Vbias = 600 mV, Itip = 631 pA; (b) Vbias = 600 mV, Itip = 509 pA.

Most of the M1 cavity is filled with coronene. Unfilled M1 can also be observed, as indicated by white arrows in Figure 4a. The high-resolution image (Figure 4b) reveals that a single coronene is observed as a small ring with slight deforming and fuzziness caused by dynamic moving of the coronene in the cavity. One M1−coronene model was superimposed in Figure 4b to show the host−guest complex. The unit cell parameters of hybrid film are the same as those of the pure M1 structure within experimental errors. No double layer of M1 is found on the sample of mixing solution, although repeated experiments were performed on numerous areas and several samples with varied tunneling parameters. In this system, coronene molecules occupy the surface space of the M1 cavity in the bottom layer, which excludes the possibility of surface adsorption of the top M1 side chains. The main stabilizing factor of the second layer does not exist any more. Hence, the second layer cannot be found in our experiments. The results given by the control experiment confirm our initial speculation on the forming of the M1 second layer. Assembly of M2. For comparison, the adlayer of M2 on the HOPG surface was studied with STM also. Figure 5a gives one typical large-scale STM image of the M2 adlayer showing a highly ordered domain with few defects or amorphous structures. Bright lines are attributed to the macrocycle part of M2, while low-contrast trenches are occupied by the interdigitated alkyl chains. Inspecting Figure 5a carefully, it can be found that the edge of high-contrast stripes is a serrate edge instead of a smooth one. The single alkyl chains and triangular macrocycles could be recognized clearly in the submolecular image Figure 5b, which provided direct evidence for inferring the adlayer arrangement. In each bright line, the basic constructing unit is the molecular pair formed by two parallel, but opposite-oriented, M2 molecules and the periodical 6770

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CONCLUSION The adlayer structure of SPM molecules M1 and M2 consisting of carbazole and diethynylene units was investigated with STM on a graphite surface. M1 assembles into a bilayer when the system contains M1 only. Second layer M1 sits on the corner position of the bottom M1, which allows part of the top M1 side chains adsorbing on the cavity area of bottom M1. The surface adsorption of the top M1 side chains is thought as the dominant factor in stabilizing the second layer. When the cavity of M1 is filled with coronene, no second layer of M1 is found in repeated experiments. Thus, our research shows that the assembling structure of M1 can be tuned from double layer to single layer by guest molecules. Moreover, the role of top molecular side chains in the surface assembly is revealed, which is rarely realized in previous studies on surface self-assembly. For M2, it forms a single layer regardless of the existence of coronene. The results are of importance in understanding the relationship between arranging behavior and molecular structure of SPMs, fabricating predesigned surface patterns, and developing high-performance organic devices.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Z.-Y.Y.). Tel: +86-1088256978. *E-mail: [email protected] (W.X.). *E-mail: [email protected] (Y.-P.Y.). *E-mail: [email protected] (L.P.X.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported, in part, by the President Fund of UCAS, Starting Fund of UCAS, Open Fund of Beijing National Laboratory for Molecular Sciences, National Basic Research Programs of China (973 program, 2012CB932504), National Natural Science Foundation of China (grant 21303200) and Beijing Natural Science Foundation (Grant No. 2122038), and Chinese Academy of Sciences is gratefully acknowledged.

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dx.doi.org/10.1021/jp4115964 | J. Phys. Chem. C 2014, 118, 6767−6772