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Surface Separation and In-situ Structural Regulation of Photosensitive Oligomer in Flexible Template Xiao-Yang Zhu, Yanfang Geng, Hongjun Xiao, Linxiu Cheng, Hongyu Shi, Fengying Zhao, Kandong Hu, Xuan Guo, Bin Tu, and Qingdao Zeng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b04293 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Surface Separation and In-situ Structural Regulation of Photosensitive Oligomer in Flexible Template Xiaoyang Zhu1‡ ,Yanfang Geng,1‡ Hongjun Xiao1, Linxiu Cheng,1 Hongyu Shi,1 Fengying Zhao,2* Kandong Hu,2 Xuan Guo, 2 Bin Tu,1* Qingdao Zeng1* 1

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

for Excellence in Nanoscience, National Center for Nanoscience and Technology, No. 11 Beiyitiao, Zhongguancun, Beijing 100190, P. R. China. 2

Jiangxi College of Applied Technology, Ganzhou 341000, P. R. China.

‡

These authors contributed equally to this work.

ABSTRACT: Surface selective adsorption and separation are very important for the application of surface functional materials. In this study, a photosensitive diazo-macrocycle has been synthesized by solvent method with very low yield, which can adsorb onto the substrate surface modified with template molecule. By using this flexible template on the graphite surface, a simple separation strategy for macrocyclic molecule with specific shape and size from reaction mixtures was developed. Additionally, one of two azo units in this trapped photosensitive macrocycle could convert from trans to cis conformation under the UV irradiation due to the

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steric effect. Our results provide a new way to construct functional nano-devices using surface flexible template as the separation and regulation medium.

KEYWORDS: separation, in-situ, photosensitive, scanning tunneling microscopy

INTRODUCTION Molecular recognition based on weak interactions between substrates and receptors is an important process in chemical and biological systems. Two-dimensional (2D) nanoporous networks physisorbed on diverse solid surfaces have been extensively investigated in field of catalysis, patterning and separation due to the capability of immobilizing a variety of guest molecules through molecular recognition.1-3 In addition to the cavities comprised of one or more host molecules through weak interaction among the reported numerous systems, the shape and size of many cavities are limited to the internal structures of macrocyclic compounds, such as cyclophanes, calixarenes and so on.4-8 The former cavities possess capabilities of flexibly tuning their conformations upon external stimuli like pressure, electric filed, light, etc, leading to betterdefined cavities, and thus enhancing its strength and specificity to the recognition of guest molecules.9 The latter ones, however, usually have frozen cavities due to the stable conformation of macrocycles, which limits their applications especially in molecule selectivity and recognition. Although molecular recognition technique is scientifically rigorous in the potential chemical and material applications, there is no report especially on molecule separation from a mixture containing several types of compounds with different sizes and shapes by using 2D supramolecular template. The traditional multicomponent separation and purification methods such as rectification, extraction, and exchange chromatograph are usually employed in solution


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according to their chemical or physical properties like mass, density, size, shape, or affinity etc. Instead, the separation efficiency of 2D networks on surfaces mainly depends on the match of incoming molecules in size and shape to that of nanopores, in which the intermolecular and molecule-surface interactions should also be considered. Surface separation strategy can simplify the separation process after bulk reaction and it is also a fast way for the separation of trace compound in mixture. In addition, surface separation can be considered as a nanoscale traditional separation strategy. Thus this study on the surface separation based on selective adsorption would be helpful for explaining the principle of conventional phase separation in solution. Macrocyclic compounds have received a lot of attention due to their broad range of potential applications in material science.10-13 Among those, azo-based macrocycles have been widely investigated because of their unique photoresponsive switchable characteristics which greatly expand their applications in the fields of molecular machines.14-19 Upon the stoichiometry effect, macrocycles containing multi-azo units display different conformations after irradiation with UV light.20 Hence, the synthesis and regulation of switchable macrocycles would help to construct a variety of functionalized molecular machines. However, the step-by-step reaction of the aromatic macrocycles from the precursor monomers often results in the production of by-products such as larger macrocycles and linear oligomers which is difficult to be separated and purified via traditional methods.21 1,3,5-tris(10-carboxydecyloxy)-benzene (TCDB) assemblies on surfaces are a well-studied molecular template in our group for recognition of various receptor molecules (Figure S1).20, 22 The moderate hydrogen bond and flexible alkyl chain have provided TCDB the distinctive flexible structures on surfaces, which can be tuned by the incoming molecules. We would like to explore a method to separate and directly characterize one kind of macrocycles that possess


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azobenzene moiety but difficultly dissolve in common solvents. The target macrocycle can adopt two configurations via azo unit: a planar trans (t) form and a nonplanar cis (c) form on surface. We further investigated the adsorption geometries of these two azobenzene groups in macrocycles. The result shows that only one trans-cis transformation can be observed, and this transformation behavior is energetically preferred in cavity on HOPG surface. These kinds of complex separation relying on specific molecular recognition at a molecule level were detected by scanning tunneling microscope (STM). RESULTS AND DISCUSSION The synthetic route is shown in Figure 1a. Firstly, 4-[(4-aminophenyl)diazenyl]phenylamine (A) (318 mg, 1.5 mmol) and isophthaloyl chloride (B) (304 mg, 1.5 mmol), each dissolved in Nmethyl-2-pyrrolidone (NMP) (15 mL), were added concomitantly to a flask containing Et3N (2 mmol) at 0 oC. The mixture was stirred until it gradually warmed up to room temperature (4-6 h), and was then heated under reflux for 12 h, after which the reaction was quenched by addition of acetyl chloride followed by methanol. There could be several by-products mainly exhibiting linear (L) oligomer and macrocylic (M) oligomer structures based on this chemical reaction (Figure 1). After removing solvent and other volatile fractions, the remaining solid residue 450 mg was examined by FT-IR, H NMR and MALDI-TOF MS, as presented in Figure 1c, Figure 1d and Figure 1e, respectively. According to the literature, the FT-IR spectrum was analyzed.23-24 The detailed assignment of peaks was explained in supporting information. The band observed in the region 1400-1446 cm-1 can be assigned to C-N stretching vibrations absorption for aromatic amines. The out-of-plane wagging of N-H is moderately active with a broad band in the region 806-854 cm-1. The NH2 twisting vibration is calculated to be 1101 cm-1. Although the amide peak can be observed, there


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are still unreacted reactants. The result has been proved by H NMR spectrum, in which the peaks at around 8.0 ppm can confirm the formation of -CONH-. And then, MALDI-TOF MS was used to determine the molecular weight of the mixture. The MALDI-TOF MS revealed a couple of peaks, indicating a mixture of products. No peak accounts for higher molecular weight oligomers in the solid residue. Upon preliminary analysis, the peaks appearing at m/z 684.2 and 1368.5 can be corresponded to M[2A+2B] and M[4A+4B] macrocycles. The peak at 606.9 and 660.4 might be the intermediate L[A+2B] with different number of water. The peak at 1734.8 might be designated to the linear oligomer L[5A+5B]. The other peaks might be designated to linear oligomers, which might adsorb water or other small molecules due to the terminal group -NH2 and -COCl. (a)

N

H N

O

(b)

N N H2N

NH2

N

O

n

NH

O

A NMP

O

O

O Cl

N N

H2 N

Cl

M[nA+nB] H N

(c)

L[nA+nB]

(d)

NH

NH

N N

H N

N N

NH

O

B

O

O

COCl n

M[2A+2B] (e) 606.9

921 1011 893

Intensity

653 1651 1592

1150

1303 1247 1498 1522

2000

680

1404 1320

1500 1000 -1 Wavenumber (cm )

% Intensity

660.4

1101

1442

Intensity

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590.8 684.2 716.2

698

831.1

847

1396.2 1368.5 1734.8

548

500

0

1

2

3

4 5 6 7 8 9 10 11 12 chemical shift (ppm)

600

800

1000 1200 1400 1600 1800 m/z

Figure 1. (a) Reaction route and chemical structure of cyclic and linear oligomers. (b) Chemical structure of macrocycle M[2A+2B]. FT-IR (c), H NMR(d) and MALDI-TOF MS (e) spectrum of untreated non-volatile residue from the reaction route.


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A freshly cleaved HOPG was immerged into the pyridine solution containing the residue for a while (Figure 2a), HOPG substrate was taken out and left for a while. After the pyridine volatilization was complete, a drop of 1-phenyloctane (PO) was added. STM images were recorded under ambient conditions in constant current mode. Unfortunately, only flat HOPG surface could be observed as shown in Figure 2b, which indicates the residue mixture cannot self-assemble into regular structure on HOPG surface. While TCDB monolayer expectedly formed on HOPG surface when the graphite substrate was immerged into TCDB solution in toluene. Afterwards, the templated HOPG substrate was immerged into the residue solution followed by STM scanning and the high-resolution assembly structure of macrocycle/template is shown in Figure 2c. It is clear that featured quadrilateral-like molecules adsorbed into the TCDB cavities. From the large scale of the STM image, the TCDB cavities were occupied by the macrocyclic molecule. However, there were still unoccupied TCDB cavities in some other areas of the images. Therefore, it can be deduced that the occupancy of macrocycle in the rectangle TCDB cavity is not 100%.


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Figure 2. (a) Schematic separation process of macrocycles in mixture by TCDB molecules. (b) STM image of untreated non-volatile residues on HOPG surface (I = 289.0 pA, V = 594.0 mV). (c) Large-scale STM image after depositing mixture onto the TCDB template on HOPG surface (I = 329.6 pA, V = 600.0 mV). High-resolution STM image of TCDB/macrocycle system is shown in Figure 3a. The spot as well as the bright line can be assigned to the benzene ring and alkyl side chain of TCDB molecule, respectively, as marked with red spot and red line. In combination with previous reports, it is not difficult to find out the rectangle cavity is formed by TCDB molecules through hydrogen bonding from the terminal carboxyl group. On the basis of the size and topographic consistency, the entrapped cyclic molecule should be designated to one component of the products. In this case, the self-assembled TCDB monolayer serves as a separation template to selectively entrap one certain macrocycle from the mixture into cavities. And then, the sample


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was irradiation under 365 nm. Large-scale image for the system after irradiation is shown in Figure 3b. After careful analyzing the STM image, it can be found that the molecular conformations in domain A and B are different. The photoisomerism reaction happened after preliminary judgment due to the presence of two azo units. But, there is still some region marked with A without conformation conversion. Therefore, the conversion from trans to cis was not 100%. It is worth noting that there are two azo units making the analysis more complex. The question is how do these two azo groups respond to light irradiation? And then, high-resolution STM images before and after irradiation were recorded, as shown in Figure 4.

Figure 3. (a) High-resolution STM image of TCDB/macrocycle system (I = 329.6 pA, V = 600.0 mV). (b) Large-scale STM image of TCDB/macrocycle after irradiation under 365 nm (I = 299.1 pA, V = 600.0 mV; (c) High-resolution STM images of TCDB/macrocycle after irradiation (I = 289.0 pA, V = 594.0 mV). The high-resolution STM image provided in Figure 4a visibly shows the details of the entrapped molecules. The bright spot and lines around each quadrilateral respectively correspond to the benzene core and side chain of TCDB molecule, respectively. The two sides L1 and L2 of bright parallelogram is measured to be 0.7 nm × 1.2 nm with angle of 45o, which is consistent with the size of M[2A+2B] macrocycle. The unit cell parameters are measured to be a = 3.5 ± 0.2 nm, b = 1.8 ± 0.2 nm, and α = 76 ± 2o. The different electron density of the quadrilateral ring


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might be attributed to the nonplanar structure of M[2A+2B] induced by the two azobenzene moieties. In combination with the density functional theory (DFT) calculations (Table 1), both two azobenzene units of M[2A+2B] macrocycle display trans (t) configuration, which is thermodynamically favored. The self-assembled molecular model of M[2A+2B](t,t)/TCDB was proposed and displayed in Figure 4c. The oxygen atom in the side chain of TCDB interacts with the hydrogen in benzene ring of M[2A+2B](t,t) through -O···H hydrogen bonding. These two bidiagonal hydrogen bonding might be the main driving force which accounts for the stabilization of non-planar adsorption configuration for M[2A+2B](t,t) in the cavity.

Figure 4. High-resolution STM image on HOPG surface before (a) and after (b) irradiation (I = 289.0 pA, V = 594.0 mV). (c) and (d) Proposed self-assembled models corresponding to (a) and (b). To further confirm the entrapped target, the sample was then exposure to 365 nm UV light with a high pressure mercury lamp and glass filter. The temperature of sample was kept around 25 oC with the help of circulating cooling water. High-resolution STM image is shown in Figure 4b. The size and shape of the entrapped macrocycle slightly changes after UV light irradiation but again conform to that of M[2A+2B](t,c). The unit cell parameters are measured to be a = 3.7


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± 0.2 nm, b = 2.0 ± 0.2 nm, and α = 78 ± 2o. The electron density of the quadrilateral changed dramatically after irradiation with 365 nm light, which could be corresponding to the configuration change of M[2A+2B](t,t) caused by the conformation transition of azo units. According to the exhibition of gas-phase molecule, that is, the benzene ring connected with – C=O is planar to surface and exhibits as a bright spot while the benzene ring attached to azo unit is much thinner due to the vertical configuration,16 it is possible for the M[2A+2B](t,t) case that only one trans azo unit changed to cis conformation, leaving the other one’s conformation remaining unchanged. Additionally, there is no clear change after longer exposure time upon 365 nm UV light, therefore, the unchanged trans conformation did not switch to cis conformation, as schematic explain in Figure 5. Upon irradiation at 435 nm light, the structure of macrocycle in cavity can be reversibly transformed to the conformation before exposure to 365-nm-irradiation, as shown in Figure 4. Meanwhile, the assembled TCDB structure can be adjustable to the conformation change of the trapped macrocycle. These results indicate that the conformation of M[2A+2B] is controlled by the flexible TCDB template upon irradiation with different light wavelength, as shown in Figure 5.

Figure 5. Molecular conformation transition between M[2A+2B](t,t) and M[2A+2B](t,c).

The stability of M[2A+2B](t,t)/TCDB and M[2A+2B](t,c)/TCDB complexes is analyzed and compared by using density functional theory. Table 1 shows the calculated results of the unit cell


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parameters and energy in the two arrangement modes. Ea-a indicates interaction between adsorbates; Ea-s indicates the interaction between adsorbates and substrate; Ecell indicates the total interaction energy in cell; E indicates the total interaction energy per unit area. DFT calculations indicate both structures are energetically similar and thermodynamically stable after competitively adsorbing from the residue mixture.

Table 1.

Calculated unit cell parameters and energies of M[2A+2B](t,t)/TCDB and

M[2A+2B](t,c)/TCDB two assembly structures.

a (nm) b (nm) α (o) Ea-a (kcalmol-1) Ea-s (kcalmol-1) Ecell (kcalmol-1) E (kcalmol-1Å-2)

M[2A+2B](t,t)/TCDB

M[2A+2B](t,c)/TCDB

1.94 3.46 74 -42.671 -408.567 -461.905 -0.575

2.06 3.66 79 -50.201 -432.725 -496.998 -0.650

There have been some examples in which the formed cavity of TCDB on surface can adsorb various molecules with different size and shape.22, 26 It needs to be noted that the TCDB network was exposed to single component solution in most cases, where there is no competitive adsorption to the cavity. When different guest molecules were successively dropped onto the TCDB monolayer, however, there is indeed selective effect depending on the guest structures, along with the appropriate change of TCDB pattern in a certain degree.22 Towards practice application, there are still so many parameters needed to be optimized. Firstly, the coverage of TCDB network on the defined area of HOPG surface is difficult to determine. In addition, although TCDB template can accommodate different guests into the cavity in combination previous results, there is still some strict rules of the molecular size and shape. Therefore, much more templates should be developed for the specific molecules. In addition, TCDB is comprised


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of flexible alkyl chains, which are the key point for tunable cavity, however, the flexibility would induce instability of the systems. In the future, the stability and adjustability of template should be investigated in order to satisfy the application. CONCLUSIONS In summary, the synthesized M[2A+2B] by solution method was specifically extracted from a residue mixture by the virtue of TCDB templates on HOPG substrate. Although the separation efficiency is not complete resulting in some unoccupied regions, the results indicate the selective separation of trace molecules from mixture would be possibly achieved by using monolayer medium. In a sense, this method would be significant for the organic synthesis with low yield. A large amount of molecules are expected to be purified by increasing the specific surface area of porous material modified by the separation monolayer. Note that the molecular size and shape play a significant role in the selective separation system. In addition, the trapped macrocycle M[2A+2B] showed reversible response to the ultraviolet light radiation at 365 nm and visible light radiation at 435 nm due to the presence of photosensitive azo units. However, only one azo unit could convert between trans and cis under irradiation, resulting in two molecules M[2A+2B](t,t) and M[2A+2B](t,c) with different conformations. This kind of nanostructure induced by photoisomerisation would be helpful to construct diverse functionalized molecular machines in the process of separation. ASSOCIATED CONTENT Supporting Information. Sample preparation, measurements, calculation, chemistry structure and STM images of TCDB. AUTHOR INFORMATION


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Corresponding Author * E-mail: [email protected]; * E-mail: [email protected]; * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Basic Research Program of China (2016YFA0200700), The National Natural Science Foundation of China (Nos. 21472029 and 21773041) and Engineering Research Center of Nano Geomaterials of Ministry of Education (No. NGM2016KF010 and NGM2017KF014) are also gratefully acknowledged.

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22. Shen, Y. T.; Zeng, L. J.; Lei, D.; Zhang, X. M.; Deng, K.; Feng, Y. Y.; Feng, W.; Lei, S. B.; Li, S. F.; Gan, L. H.; Zeng, Q. D.; Wang, C. Competitive adsorption and dynamics of guest molecules in 2D molecular sieves. J. Mater. Chem. 2011, 21 (24), 8787-8791. 23. Panicker, C. Y.; Varghese, H. T.; Ushakumari, L.; Ertan, T.; Yildiz, I.; Granadeiro, C. M.; Nogueira, H. I. S.; Mary, Y. S. FT-IR, FT-Raman, SERS spectra and computational calculations of 4-ethyl-N-(2'-hydroxy-5'-nitrophenyl)benzamide. J. Raman Spectrosc. 2010, 41 (4), 381-390. 24. Arunagiri, C.; Arivazhagan, M.; Subashini, A.; Maruthaiveeran, N. Theoretical and experimental calculations, Mulliken charges and thermodynamic properties of 4-chloro-2nitroanisole. Spectrochim. Acta A 2014, 131, 647-656. 25. Sundaraganesan, N.; Kalaichelvan, S.; Meganathan, C.; Joshua, B. D.; Cornard, J. FT-IR, FT-Raman spectra and ab initio HF and DFT calculations of 4-N,N'-dimethylamino pyridine. Spectrochim. Acta A 2008, 71 (3), 898-906. 26. Shen, Y. T.; Deng, K.; Zhang, X. M.; Lei, D.; Xia, Y.; Zeng, Q. D.; Wang, C. Selective and Competitive Adsorptions of Guest Molecules in Phase-Separated Networks. J. Phys. Chem. C 2011, 115 (40), 19696-19701.

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