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On-surface synthesis of self-assembled monolayers of benzothiazole derivatives studied by STM and XPS Hongyu Shi, Yuhong Liu, Jian Song, Xinchun Lu, Yanfang Geng, Junyong Zhang, Jingli Xie, and Qingdao Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00674 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 16, 2017

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On-surface synthesis of self-assembled monolayers of benzothiazole derivatives studied by STM and XPS Hongyu Shi,1,2 Yuhong Liu,1 Jian Song, 1 Xinchun Lu,1,* Yanfang Geng,2,* Junyong Zhang,3 Jingli Xie,3,* and Qingdao Zeng2,*

1

2

State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China.

CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for

Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China.

3

College of Biological, Chemical Science and Engineering, Jiaxing University, Jiaxing 314001,

China.

ABSTRACT：：On-surface synthesis has gradually become a prevalent approach to construct two-dimensional functional monolayers on various substrates. In the present work, the synthesis of self-assembled monolayers (SAMs) of benzothiazole derivatives was conducted at the liquid/solid interface for the first time. Two kinds of nanostructures were achieved on the highly oriented pyrolytic graphite (HOPG) surface via the condensation reaction between aromatic

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aldehyde derivatives and 2-aminothiophenol (ATP). The formation of thiazole-based self-assemblies was revealed by scanning tunneling microscopy (STM) and further confirmed by X-ray photoelectron spectroscopy (XPS). The successful synthesis of the benzothiazole derivatives not only extends the scope of on-surface reactions but also can be applied in designing multifunctional SAMs at the interface.

KEYWORDS: On-surface synthesis, benzothiazole, self-assembled monolayers (SAMs)


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INTRODUCTION

On-surface synthesis has been regarded as an important avenue for constructing novel and stable nanostructures at the interface.1-5 In contrast with the synthetic systems occurring in the bulk phase, the “bottom-up” synthesis on the surface could create new compounds with low dimensions and innovational properties. For example, the surface covalent organic frameworks (SCOFs) with backbones containing π conjugation, mostly achieved via the “bottom-up” synthesis strategy,6, 7 may have enhanced carrier mobility, which has been corroborated on some two-dimensional (2D) materials like graphene.8 These properties would extend potential applications of synthetic on-surface functional films in optoelectronic devices and nanosensors.9, 10

Supramolecular self-assembly could form large-scale nanostructures through non-covalent interactions such as van der Waals force, hydrogen bond and halogen bond.11-16 Especially in recent years, constructing porous structures using star-shaped molecules has received much attention.17-20 A typical example is the hydrogen bond induced networks of trimesic acid (TMA) and 1,3,5-benzenetribenzoic acid (BTB). However, the stability of the structures driven by non-covalent interactions is restricted. In comparison, on-surface reaction involves the breakage and formation of robust covalent bonds, thus leading to thermodynamically stable nanostructures. Similarly, porous nanostructures stabilized by covalent bonds are being pursued by researchers. For example, star-shaped molecules with halogen atoms could form porous


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networks via condensation reaction on the surface.16, 21, 22 Compared with reactions in solution, surface activity and precursors could have a significant effect on the pathways and products of on-surface reactions.23, 24 As a tool for characterization, scanning tunneling microscopy (STM) makes it possible to gain direct insight into the on-surface reaction in situ at the molecular scale.25, 26 The combination of surface reaction and STM characterization has expanded the scope of traditional organic synthesis. Over the past decade, a wide range of chemical reactions have been performed on various surfaces to synthesize new compounds, such as C-C coupling (e.g. Ullmann coupling and Glaser coupling) reaction,23, 24, 27, 28 Schiff-base coupling reaction,29, 30 boronic anhydridation reaction,31 photochemical reaction,32 Knoevenagel reaction33 and so on. Even so, efforts are under way to explore more types of chemical reactions on various surfaces. Benzothiazole is a kind of aromatic heterocyclic compound, which consists of a benzene ring and a five-membered thiazole ring containing both sulfur and nitrogen atoms. Its derivatives have played a variety of roles in many areas, including medicine, pesticide, analytical chemistry and optical materials.34-36 These extensive applications have aroused researchers’ great interest in the synthesis of novel benzothiazole derivatives. One of the most commonly reported synthetic strategies of benzothiazole derivatives is via the condensation of 2-aminothiophenol (ATP) with aldehydes,37, 38 carboxylic acids,39 amines,36

esters,40 etc. For example, Zhou et al. applied the

visible-light-responsive photoredox reaction to the synthesis of 2-aryl benzothiazoles based on ATP and amines, and the non-metal boron-dipyrromethene was used as the catalyst under a moderate condition.36 In response to the demand for green chemistry, Bahrami et al. developed a


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catalytic redox cycling approach to synthesize 2-substituted benzothiazoles by using ATP and a variety of aromatic aldehydes as precursors, and the H2O2 with Fe(NO3)3 system ensured the synthetic rate and efficiency, which has been regarded as both low-cost and environmentally friendly.41 As illustrated in Figure 1a, the condensation reaction includes three processes: initial formation of the intermediate Schiff base by amino and aldehyde group; cyclization of the Schiff base and sulfhydryl group; further oxidation to form benzothiazole group.34 It is plausible to envision that on-surface synthesis of benzothiazole derivatives could open a promising pathway to design molecular devices. So far, however, little attention has been paid to the synthesis of benzothiazole functional self-assembled monolayers (SAMs) on the surface. In this work, we synthesized benzothiazole derivatives via the condensation of ATP and two aromatic aldehydes, 4-dodecyloxybenzaldehyde (DBA) and 1,3,5-tri(4′-formylphenyl)benzene (TFB), on the highly oriented pyrolytic graphite (HOPG) surface. The self-assembled monolayers of benzothiazole derivatives after the in situ reactions as well as aromatic aldehydes prior to the reactions were revealed by STM. Meanwhile, the samples were further characterized by X-ray photoelectron spectroscopy (XPS), which provided convincing evidence for the formation of the benzothiazole group.


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a R

HS

H

HS

O

H

H

+

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S

S R

H2N

R

R N H

N

N

ATP

Benzothiazole

b O

O

HS +

H

S H2N N

O

DBA H

c

ATP

DBA-BT

O S

N

HS + H2N

O

H H

TFB

O

N

ATP

S S

N

TFB-BT

Figure 1. (a) Schematic presentation for the synthesis mechanism of benzothiazole derivatives via the condensation of aldehyde and ATP; (b, c) on-surface reaction routes of aldehyde precursors DBA or TFB with ATP to form benzothiazole derivatives DBA-BT or TFB-BT, respectively.

EXPERIMENTAL SECTION Materials. 4-Dodecyloxybenzaldehyde (DBA, 98%), one of the aromatic aldehydes used as precursors,

was

purchased

from

the

Alfa

Aesar

Company.

The

other

one,

1,3,5-Tri(4′-formylphenyl)benzene (TFB) was synthesized according to our previous report.30


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The reactant 2-aminothiophenol (ATP, 98%) was purchased from the Acros Company. All the materials above were used without any further purification. On-surface synthesis of benzothiazole derivatives. For the synthesis of the surface benzothiazole derivatives, a droplet of 1-phenyloctane/ethanol (1:1) mixed solution containing aromatic aldehydes DBA or TFB (1 µL with a concentration of 10-4 M) was first deposited onto the freshly cleaved HOPG (grade ZYA, NTMDT, Russia) surface. Then, a droplet of ATP (1 µL) was added to the surface. Subsequently, the samples were annealed at 60 oC for 8 hours. During the heating process, the 1-phenyloctane/ethanol (1:1) solution was added to the sample ceaselessly to guarantee that the reaction happened at the liquid/solid interface. After cooling to room temperature, a droplet of 1-phenyloctane (0.6 µL) was deposited onto the surface before STM measurement. Characterization. STM measurements were individually carried out at the liquid/solid interface before adding ATP to the sample and after the surface reaction, by using a Nanoscope IIIA system (Bruker, USA) under ambient conditions. All STM images were captured with mechanically formed Pt/Ir (80/20) tips in the constant-current mode. The specific tunneling conditions (i.e. tunneling current and bias voltage) are stated in the corresponding figure captions. The drift for all the STM images was calibrated by using an atomic resolution HOPG lattice as a reference. The HyperChem software package was used as a tool to build assembly models based on the intermolecular distances and angles. To further confirm the formation of the benzothiazole group, the reacted samples that have been characterized with STM, as well as the


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precursor ATP, were measured by XPS after the evaporation of the solvent, using an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) equipped with a monochromatized Al Kα X-ray source (1486.6 eV). All the binding energy values of obtained spectra were referenced to the C 1s neutral-carbon peak at 284.8 eV. The XPS spectra underwent a background subtraction using the Shirley method before the deconvolution of the peaks by the XPSPEAK software.

RESULTS AND DISCUSSION Formation of DBA-BT SAM. DBA is a tadpole-like aromatic aldehyde with the aldehyde group on one side. Prior to the synthetic reaction, the assembly structure of pristine DBA on the HOPG surface was characterized using STM, as shown in Figure 2a. It is revealed that DBA molecules adopts a regular lamellar structure. From the high-resolution image in Figure 2b, it can be observed that each lamella consists of four rows of tadpole-like molecules. The angle (α) between the neighboring rows is 134 ± 1°, as labeled in Figure 2b. The average chain length of the assembled molecules was measured as 1.3 ± 0.1 nm (L1, L2), which coincides well with the theoretical chain length of DBA as calculated by density functional theory (DFT) to be 1.34 nm. Besides, the edges of the rows are composed of linearly arranged spots with the size of 0.5 ± 0.1 nm, which could be attributed to the benzene rings of DBA molecules. Therefore, the lamellar structure was formed by the aggregation of tadpole-like DBA molecules, as illustrated in the molecular model in Figure 2c. There are two kinds of bindings, namely head to head and head to tail, as circled in red and blue, respectively. Both the bindings are driven by the hydrogen


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bond formed between the oxygen atom in the aldehyde group and the hydrogen atom. The unparallel feature of the chains in neighboring rows may be due to the orientation of the hydrogen bond. Besides, the distance between neighboring chains is measured as 0.5 ± 0.1 nm, indicating that the alkyl chains could interact with each other via van der Waals force.42, 43

b

a

c α L1 L2

10 nm

3 nm

Figure 2. (a) Large-scale and (b) high-resolution STM images of pristine DBA assembly structure at the 1-phenyloctane/HOPG interface, overlaid with the molecular model of one lamella to guide the eye. Tunneling conditions: Iset = 289.9 pA, Vbias = 594.5 mV. (c) Suggested molecular model for the pristine DBA assembly with circles marking two kinds of bindings, namely head to head and head to tail.

After characterization of the pristine DBA assembly, the synthetic reaction was performed by adding ATP onto the sample. The reaction route is illustrated in Figure 1b. After annealed at 60 o

C for 8 hours and then cooled down to room temperature, the sample was imaged at the

1-phenyloctane/HOPG interface by using STM. As shown in Figure 3a, the assembly structure still adopts a lamellar feature. Different from the lamellar structure of DBA, each lamella


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consists of two rows of tadpole-like molecules and the width of the lamella is roughly half of DBA. The neighboring lamellae are arranged in different directions, as marked by two opposite arrows. As labeled in Figure 3b, the angle (α) between the neighboring lamellae is 134 ± 1°. Compared with the angle in DBA assembly structure, it can be inferred that part of the lamellar configuration remained unchanged during the in situ transformation. Another difference lies in the brighter and wider edges of the rows than the edges in DBA assembly structure. The detailed structure could be observed in the high-resolution image in Figure 3b. It is revealed that the bright edges are aggregations of the heads of the tadpole-like molecules. The average size of the heads was measured as 1.0 ± 0.1 nm, about twice the head size of DBA, which coincides well with that of the DBA-BT molecule. Compared with DBA, the heads of DBA-BT are arranged in staggered feature, as illustrated by the molecular model overlaid on the STM image. During the reaction process, the enlargement of the heads resulted in the intercalation of benzothiazole groups. From the molecular model, it can be inferred that the head to head binding is derived from the C−H···N hydrogen bonding. Due to the intercalation of benzothiazole groups, the alkyl chains separated to some extent. The distance between neighboring chains is measured as 0.9 ± 0.1 nm, thus van der Waals force unlikely exists among the alkyl chains. Actually, it is quite possible that the solvent molecules have taken part in the assembly through interacting with the adsorbate molecules to improve the stability of assembly structures.44-46 Besides, the chain length (L) was measured as 1.3 ± 0.1 nm, which indicates that the chains of the reactant molecules


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remained unchanged after the reaction. Hence, it could conceivably be hypothesized that the benzothiazole derivative DBA-BT has been successfully synthesized on the surface as expected. b

a

L α

10 nm c

Ⅰ

Ⅱ

5 nm Ⅰ

Ⅱ

10 nm

Figure 3. (a) Large-scale and (b) high-resolution STM images of the assembly structure of the synthesized benzothiazole derivative DBA-BT at the 1-phenyloctane/HOPG interface, overlaid with the molecular model to guide the eye. (c) Two types of the DBA-BT assembly structures and the corresponding suggested molecular models. Tunneling conditions: Iset = 289.9 pA, Vbias = 594.5 mV.

It should be pointed out that there could be two types of nanoarrays of the benzothiazole derivative DBA-BT, as marked as type I and II in Figure 3c. Type I is the main array with double-row lamellae. By contrast, type II contains single-row lamellae. The weak head-tail interaction may explain why this type of the DBA-BT assembly structure is rare compared to type I.


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As stated in the experimental section, the synthetic reaction was performed at the liquid/solid interface. At present, the STM observation could hardly provide real-time information about the assembly switching process or the intermediate state, thus it cannot be unambiguously determined whether the reaction occurred on the surface or in the supernatant liquid phase.47 Although there is no direct evidence to demonstrate that the reaction occurred on the surface, it can still be inferred indirectly by analyzing the molecular characteristics and imaging features.48, 49

For example, the dynamic process of the formation and decomposition of covalent organic

frameworks observed by STM was a powerful evidence for the on-surface synthesis at the liquid/solid interface.50 As for the synthesis of benzothiazole derivative DBA-BT, it is less likely that the condensation reaction occurred in the liquid phase, because most of the bulk-phase syntheses of benzothiazole derivatives require catalysts or other rigorous conditions as stated above. Even if the reaction could occur in the liquid phase, it is energetically unfavorable that DBA-BT molecules exchange from solution into the absorbed monolayer to take the place of DBA. Formation of TFB-BT SAM. Different from the tadpole-like DBA, TFB is a star-shaped trialdehyde with three-fold symmetry. The STM image of the pristine TFB assembly structure at the 1-phenyloctane/HOPG interface is shown in the left STM image in Figure 4a. Featured by triangular shape, the TFB molecules align in a close-packed nanoarchitecture. The result is different from the dimer feature reported in the previous study,30 which might result from the solvent effect on the assembly structures.44 Actually, this close-packed arrangement is a typical


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assembly structure of star-shaped molecules and no specific intermolecular interactions are required for this.51-53 The average size of the triangles is measured to be 1.1 ± 0.1 nm, which is in agreement with the theoretical size of the TFB molecule. The parameters of the unit cell overlaid on the STM image of the TFB assembly were measured as follows: a = 1.5 ± 0.1 nm, b = 1.5 ± 0.1 nm, α = 60 ± 1°. a HS H2N

b α a

3 nm

b′ α′ a′

5 nm

b

Figure 4. (a) High-resolution STM images of the assembly structures of pristine TFB and the synthesized benzothiazole derivative TFB-BT at the 1-phenyloctane/HOPG interface, overlaid with the corresponding molecular models to guide the eye. Tunneling conditions: Iset = 289.9 pA, Vbias = 594.5 mV. (c) Suggested molecular model for the structural transformation of the on-surface synthetic reaction.


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Upon the addition of ATP onto the preformed TFB assembled monolayer to induce the synthetic reaction, the sample was characterized by STM after heating at 60 oC for 8 hours. As shown in the right STM image in Figure 4a, the product molecules assemble into a less compacted pattern than TFB. With close observation, it can be found that the assembled molecules adopt a rough triangular feature. Compared with the triangular TFB molecules, the product molecules were measured to be nearly twice larger in size (1.9 ± 0.1 nm), which is in good agreement with the expected size of TFB-BT. Moreover, the dimensions of the unit cell declare complete disparity after the surface reaction, which were measured as a′ = 2.9 ± 0.1 nm, b′ = 2.9 ± 0.1 nm, α′ = 60 ± 1°. It is acknowledged that supramolecular assembly is a dynamic process based on the principle of minimum energy, which involves ceaseless adsorption and desorption of assembled molecules. From TFB to TFB-BT, the enlargement in molecular size means increased space requirement. To reduce steric hindrance, the assembled molecules are rearranged with a larger unit cell after the reaction, as illustrated by the molecular model overlaid on the STM image. Due to the steric effect, there is no specific strong interaction between assembled molecules.51 Therefore, the instability induced by the lack of intermolecular interaction makes it easier to be disturbed by STM tip when scanning, which may be the reason why the shapes of some molecules appear to be irregular in the STM image. It should be pointed out that despite the absence of intermolecular interaction, the structure hardly changed with time, which could be attributed to the bonding between TFB-BT and the substrate that could keep the molecules from diffusing around to some extent. Moreover, from the energy perspective, the less


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instability of TFB-BT than TFB further proved that the synthetic reaction occurred on the surface instead of in the supernatant liquid phase, because the exchange of TFB-BT from solution into the absorbed monolayer to replace TFB is energetically unfavorable. During the reaction process, the enlargement of the molecular moieties caused the in situ transformation from close-packed structure to porous structure. Consequently, the structural transformation could provide strong evidence for the synthesis of the benzothiazole derivative following the reaction route illustrated in Figure 1c. The molecular model for the structural transformation of the condensation between the trialdehyde and ATP is displayed in Figure 4b. XPS analysis. Although the assembly structural transformation after the surface reaction could be revealed by STM, the chemical information is limited and additional analysis technique for product identification is necessary. To further confirm the synthesis of the benzothiazole derivatives, XPS measurements were performed to investigate the molecular structure. In view of the structural characteristics of the benzothiazole group, which is featured by the imine bond and the endocyclic sulfur, high-resolution scans of the precursor ATP and reacted samples were obtained for the N 1s and S 2p photoelectron regions, as shown in Figure 5.


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b

Figure 5. (a) N 1s and (b) S 2p XPS deconvoluted spectra of pristine ATP and the on-surface synthesized benzothiazole derivatives, namely DBA-BT and TFB-BT.

In order to achieve a better observation of the molecular structural transformation, the XPS spectra of pristine ATP were measured before the condensation reaction. In Figure 5, both the N 1s and S 2p spectra of pristine ATP are featured by single band with the peaks at 399.3 eV and 163.4 eV, which correspond to N in the amine group (N−H) and S in the sulfydryl group (S−H), respectively.54, 55 After the condensation reaction, two reacted samples were measured for the N 1s and S 2p regions. It is worth noting that the reacted samples were characterized with STM prior to XPS measurements, so that the data consistency and reliability can be guaranteed. As for


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the N 1s region, the spectra of the two samples can be deconvoluted as two bands, as shown in Figure 5a. The binding energy located at 399.3 eV could be ascribed to N in the amine group (N−H) of ATP, which indicates that some of the ATP molecules remained unreacted.54 The other band with the peak at 398.7 eV corresponds to N in the newly generated imine bond (C=N) of the benzothiazole group.56, 57 At the same time, the S 2p spectra for the two reacted samples were obtained. As shown in Figure 5b, the S 2p spectra can be deconvoluted as two bands as well. Apart from the peak at 163.4 eV indicating S in the sulfydryl group (S−H) of unreacted ATP molecules,55 the new band with the peak at 164.4 eV could be assigned to the endocyclic sulfur of the benzothiazole group.58, 59 Therefore, by comparing the XPS results before and after the condensation reaction, the existence of the imine bond (C=N) and the endocyclic sulfur in the compounds after the reaction provides powerful evidence for the successful synthesis of the benzothiazole derivatives. According to the element conservation laws, the proportions of newly generated C=N and endocyclic sulfur should be equal. It is generally known that the integration of the XPS peaks could be taken to define the percentage of various elements. In Figure 5, for the N 1s and S 2p spectra of the same sample, the roughly equal ratios between the peak areas of purple and green spectra provide a further quantitative proof for the realization of the benzothiazole group.

CONCLUSIONS


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In summary, we successfully synthesized the benzothiazole derivatives at the liquid/HOPG interface by the condensation of ATP and two aromatic aldehydes, namely tadpole-like DBA and star-shaped TFB. High-resolution STM images revealed the lamellar or close-packed assembly structural transformation from aldehydes to benzothiazole derivatives, mediated by intermolecular hydrogen bonds. Moreover, the N 1s and S 2p XPS spectra provided further support for the formation of the benzothiazole group containing imine bond and endocyclic sulfur. This research achieved a new surface reaction and could pave the way to the extension of traditional organic synthesis to surface synthesis. More importantly, based on the applications of benzothiazole derivatives, this work also provides a promising way to construct functional nanodevices.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Large-scale STM images of the assembly structures of pristine TFB and the synthesized benzothiazole derivative TFB-BT at the 1-phenyloctane/HOPG interface.

AUTHOR INFORMATION

Corresponding Authors *E-mail: [email protected].


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*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected].

Author Contributions Hongyu Shi and Yuhong Liu contributed equally to the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS

This work was financially supported by the National Basic Research Program of China (Nos. 2016YFA0200700, 2013CB934203) and the National Natural Science Foundation of China (Nos. 51522504, 21472029).

REFERENCES

(1) Perepichka, D. F.; Rosei, F. Chemistry Extending Polymer Conjugation into the Second Dimension. Science 2009, 323, 216-217. (2) Zwaneveld, N. A. A.; Pawlak, R.; Abel, M.; Catalin, D.; Gigmes, D.; Bertin, D.; Porte, L. Organized Formation of 2D Extended Covalent Organic Frameworks at Surfaces. J. Am. Chem.


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Table of Contents HS

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On-Surface Synthesis of Self-Assembled ... - ACS Publications

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