Chinese Science Bulletin

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www.scichina.com www.springerlink.com Chinese Science Bulletin | July 2007 | vol. 52 | no. 13 | 1856-1859

Halogen bonded two-dimensional supramolecular assemblies studied by high resolution scanning tunneling microscopy

YANG XunYu1, WANG Fang1, CHEN QiuXia1, WANG LiYan1† & WANG ZhiQiang2† 1 Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, China; 2 Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing

100084, China

We described the formation of self-organized two-dimensional (2D) assemblies of N-(2,3,5,6-tetrafluoro- 4-iodophenyl)hexadecylamine and 1-dodecyl-imidazole at the liquid/HOPG interface. The two-dimen- sional assemblies showed a fishbone-like pattern structure as revealed by high-resolution scanning tunneling microscopy. Although different interactions can drive the formation of 2D assemblies, as far as we know, this is the first report on halogen bond-driven 2D assemblies.

two-dimensional assemblies, halogen bond, scanning tunneling microscopy (STM)

Halogen bonding is a non-covalent interaction between halogen atoms (Lewis acid) and atoms containing lone pairs (Lewis base). It has drawn more and more atten-tion from chemistry, material science, and bioorganic chemistry[1], due to its well-defined directionality and moderate strength somewhere between chemical bonds and van der Waals interactions[2], as the cement to as-semble molecules into supramolecular architectures[3]. Since Guthrie reported the formation of NH3···I2 com-plex in 1863[4], several analytical methods including X-ray diffraction[5], NMR[6], UV/Vis[7,8] and so on[2], have been used to characterize the structure of the halo-gen bonding. However, it is still not easy to study the halogen bonding by these methods, and there are few reports on especially employing imaging analytical methods such as TEM, SEM, AFM, STM to visualize the structure directly.

Among the different imaging analytical methods, scanning tunneling microscopy (STM) is a powerful tool to investigate the 2D self-assembly of organic molecule at the liquid/solid interface on the sub-molecular scale[9]. The related research has been extended from one-com- ponent, such as alkanes, alcohols, carboxylic acids[10], and so on[11], to two-component systems[12]. Although

different interactions such as van der Waals interac-tions[13,14], hydrogen bonding[15,16], coordination bond-ing[17,18], even “covalent bonding”[19,20] have been used to drive the formation of 2D assemblies, halogen bond-ing is seldom mentioned.

It should be noted that the more electron withdrawing the atom, or the moiety, bound to the halogen is, the stronger the halogen bonding to which it gives rise[21] will be. In so doing, we designed and synthesized a pair of building blocks, N-(2,3,5,6-tetrafluoro-4-iodoph- enyl)hexadecylamine (TFIPh-16A) as an acceptor and 1-dodecyl-imidazole as a donor (Scheme 1):

N-(2,3,5,6-tetrafluoro-4-iodophenyl)hexadecylamine: A mixture of pentafluoroiodobenzene (1.0 g, 3.40 mmol) and 1-hexadecylamine (0.7 g, 2.8 mmol) was refluxed at 80 °C for 42 h in the presence of potassium carbonate. The crude product was diluted with water and the or-ganic product was extracted with chloroform. The Received March 21, 2007; accepted April 16, 2007 doi: 10.1007/s11434-007-0265-2 †Corresponding authors (email: wangliyan@vip.sina.com, wangzhiqiang@mail. tsinghua.edu.cn) Supported by the National Natural Science Foundation of China (Grant Nos. 20334010 and 20674028), the National Basic Research Program of China (Grant Nos. 2007CB808000 and 2005CB724400), and the Ministry of Education of China

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collected organic layer was dried with sodium sulphate and evaporated. The product N-(2,3,5,6-tetrafluoro-4- iodophenyl)hexadecylamine was purified by silica gel column using petroleum ether as the eluent. 1H NMR (CDCl3, 500 MHz, ppm): δ 0.88 (3H, CH3-), 1.26 (26H, aliphatic), 1.59 (2H, -CH2-CH2NH-), 3.40 (2H, -CH2NH-).

Scheme 1 The building blocks for the halogen bonded 2D assemblies.

1-dodecyl-imidazole: A mixture of 1-bromododecane (3.37 g, 13.5 mmol), imidazole (1.08 g, 15.9 mmol) and potassium carbonate (2.50 g, 18.1 mmol) in dry THF (25 mL) was refluxed with stirring for 24 h. After the re-moval of salts by filtering and THF by evaporating, the product was then dissolved in chloroform and the re-mained salts and imidazole were extracted by water. The crude product in chloroform was then dried by sodium sulphate for 12 h. The product 1-dodecyl-imidazole was purified by silica gel column using ethyl acetate/ methanol (10:1) as the eluent. 1H NMR (CDCl3, 500 MHz, ppm): δ 7.50, 7.06, 6.91 (3H, imidazole); 3.93 (2H, -CH2-N-), 1.77 (2H, -CH2-CH2N-), 1.25 (18H, aliphatic), 0.88 (3H, CH3-).

If N-(2,3,5,6-tetrafluoro-4-iodophenyl)hexadecyla- mine can form halogen bonding with 1-dodecyl-imida- zole, it will be beneficial to enhance the interaction be- tween the adsorbate and the highly oriented pyrolytic graphite (HOPG) surface, thus facilitating the formation of stable 2D supramolecular assemblies. In order to con- firm this assumption, we mixed 1-dodecyl-imidazole with TFIPh-16A in 1-phenyloctane with a concentration of ~ 1 mg/mL and a 1:1 molar ratio. A drop of the solu- tion was applied onto the HOPG surface. As shown in Figure 1, the STM observation indicates that mixture of 1-dodecyl-imidazole and TFIPh-16A forms stable 2D assemblies after adsorption on HOPG. There are several domains that contain different orientated stripes, and the angle between the stripes belonging to every two adja- cent domains is either 60° or 120°. These angles are consistent with the hexagonal structure of graphite sur-

face, suggesting the effect of the HOPG substrate lattice on the formation of 2D assemblies on HOPG surface[10]. The domains are composed of alternative bright stripes and dim stripes extending over several donzen nanome- ters to several hundred nanometers.

Figure 1 The large scale STM image of the halogen bonding complex on HOPG. 218.6 nm×218.6 nm; U = 800 mV; I = 80.0 pA.

More subtle structures can be visualized in the high resolution STM image, as shown in Figure 2. The dim stripes are composed of interdigitated alkyl chains of the adsorptive molecules, whose width is 2.0±0.2 nm, marked by arrow b in Figure 2. The fishbone-like bright stripes are corresponding to the ordered array of conju- gate moieties of 1-dodecyl-imidazole and TFIPh-16A. The width of the bright bars marked by arrow a in Fig- ure 2 is 1.7±0.2 nm, which refers to the extended conju- gate structure of the two adsorptive molecules connected through halogen bonding of the two complementary headgroups. The figure is however a little bigger than the calculated value, but it is reasonable to consider that there is an enlarging effect of the tip. As indicated in Figure 2, there are no unoccupied holes in the 2D as- semblies. This effect suggests that the two building blocks complement very well, although they contain alkyl chains of different lengths. In other words, the two building blocks take the interdigitated conformation and well form a lamellar structure on the basis of halogen bonding between them and their interaction with sub- strate, with possible model shown in Figure 3.

We have done the following control experiment to confirm that the halogen bonding plays a leading role in

1858 YANG XunYu et al. Chinese Science Bulletin | July 2007 | vol. 52 | no. 13 | 1856-1859

Figure 2 The high resolution STM image of the halogen bonding com-plex on HOPG. 27.77 nm×27.77 nm; U = 700 mV; I = 80.0 pA.

Figure 3 The tentative molecular model of the conformation of the halogen bonding complex adsorbed on HOPG. the formation of stable 2D assemblies. Taking the 2D assemblies of pure 1-dodecyl-imidazole adsorbed on HOPG as an example, it is not stable enough to be well

Figure 4 STM image of pure 1-dodecyl-imidazole on HOPG/1-pheny-loctane interface. 74.29 nm×74.29 nm; U = −800 mV; I = 100.0 pA.

visualized by STM, as shown in Figure 4. One plausible reason could be that the alkyl chain of the 1-dode- cyl-imidazole is not long enough. Therefore, the interac-tion between 1-dodecyl-imidazole molecule and HOPG surface is not strong enough to immobilize the molecule. As a result, only curvature stripes are observed but no high resolution image can be obtained. The fact that sta-ble 2D assemblies can only be formed by the mixture of 1-dodecyl-imidazole and TFIPh-16A clearly indicates that the two halogen bonded building blocks have en-hanced significantly their interaction with the substrate, allowing for high order of the 2D assemblies.

In summary, we have designed and synthesized two new building blocks for halogen bonded 2D su-pramolecular assemblies. STM can provide direct struc-tural details for the halogen-bond driven assemblies. Moreover, this line of research can enrich the family of low-dimensional supramolecular assemblies by intro-ducing another driving force, such as halogen bonds.

Special thanks go to Prof. Zhang Xi of Tsinghua University for his helpful discussion and suggestive comments.

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EDITOR

XU Guangxian (Hsu, Kwang-Hsien) College of Chemistry and Molecular Engineering Peking University Beijing 100871, China

AIMS AND SCOPE Science in China Series B: Chemistry, an academic journal cosponsored by the Chinese Academy of Sciences and the National Natural Science Foundation of China, and published by Science in China Press and Springer, is committed to publishing high-quality, original re-sults in both basic and applied research. Science in China Series B: Chemistry is published bimonthly in both print and electronic forms. It is indexed by Science Citation Index.

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