Langmuir 2007, 23, 8021-8027
8021
Adlayer Structures of Aza- and/or Oxo-Bridged Calix[2]arene[2]triazines on Au(111) Investigated by Scanning Tunneling Microscopy (STM) Cun-Ji Yan,† Hui-Juan Yan, Li-Ping Xu,† Wei-Guo Song, Li-Jun Wan,* Qi-Qiang Wang,† and Mei-Xiang Wang* Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing 100080, China ReceiVed January 7, 2007. In Final Form: April 26, 2007 The adlayers formed by a series of aza- and/or oxo-bridged calix[2]arene[2]triazines on Au(111) surfaces were investigated by scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. 1,3-Alternate configurations of these molecules are preserved on gold surfaces as in their three-dimensional crystals. STM images show that the cavity sizes of these molecules are finely tuned by substituting the bridging nitrogen atom with oxygen atoms, which change the strengths and densities of the intermolecular hydrogen bonds. Hydrogen bond interaction influences the molecular orientation and conformation in the adlayers, and it plays a key role in the formation of these two-dimensional supramolecular architectures. Coadsorption of calix[2]arene[2]triazine with 1,3,5-tris(5-carboxyamyloxy)benzene (TCAB) intervenes with the intermolecular hydrogen bond formations among the calix[2]arene[2]triazine molecules and consequently causes a conformational transition of the calixarene molecules from rhombic to square. These results demonstrate the role of hydrogen bonds in molecular assembly formations.
Introduction Calixarene type molecules have been essential parts in supramolecular chemistry.1 They are employed as hosts for a wide variety of neutral,2-6 cationic,7-9 and anionic guests10,11 with high selectivity for specific guest molecules. Such high selectivity mostly results from the conformation and cavity structure of the calixarene molecules,12 and it may be advantageous for molecular recognition. To enhance specific hostguest associations, many strategies, such as using the bowlshaped arrangement of the four aromatic groups for the cavity,3,13 linking two or more calixarene units covalently as one receptor,2,5,9 constructing a “handle” on the top of the calixarene “basket”,10,14 or replacing the phenol units with other heteroaromatic units, have been developed to tune the cavity.15,16 Recently, calix[2]* To whom correspondence should be addressed. E-mail: wanlijun@ iccas.ac.cn (L.J.W.);
[email protected] (M.X.W.). † Also in Graduate School of CAS, Beijing, China. (1) Lumetta, G. J., Rogers, R. D., Gopalan, A. S. Calixarenes for Separation; ACS Symposium Series 757; American Chemical Society: Washington, DC, 2000. (2) Haino, T.; Matsumoto, Y.; Fukazawa, Y. J. Am. Chem. Soc. 2005, 127, 8936-8937. (3) Leyton, P.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Domingo, C.; CamposVallette, M.; Saitz, C.; Clavijo, R. E. J. Phys. Chem. B 2004, 108, 17484-17490. (4) Zadmard, R.; Schrader, T. J. Am. Chem. Soc. 2005, 127, 904-915. (5) Zadmard, R.; Schrader, T. Angew. Chem., Int. Ed. 2006, 45, 2703-2706. (6) Nielsen, K. A.; Cho, W. S.; Jeppesen, J. O.; Lynch, V. M.; Becher, J.; Sessler, J. L. J. Am. Chem. Soc. 2004, 126, 16296-16297. (7) Nabeshima, T.; Saiki, T.; Sumitomo, K. Org. Lett. 2002, 4, 3207-3209. (8) Kim, S. K.; Lee, S. H.; Lee, J. Y.; Lee, J. Y.; Bartsch, R. A.; Kim, J. S. J. Am. Chem. Soc. 2004, 126, 16499-16506. (9) Webber, P. R. A.; Beer, P. D.; Chen, G. Z.; Felix, V.; Drew, M. G. B. J. Am. Chem. Soc. 2003, 125, 5774-5785. (10) Lee, C. H.; Na, H. K.; Yoon, D. W.; Won, D. H.; Cho, W. S.; Lynch, V. M.; Shevchuk, S. V.; Sessler, J. L. J. Am. Chem. Soc. 2003, 125, 7301-7306. (11) Nishiyabu, R.; Anzenbacher, P. J. Am. Chem. Soc. 2005, 127, 82708271. (12) Ikeda, A.; Shinkai, S. Chem. ReV. 1997, 97, 1713-1734. (13) Credi, A.; Dumas, S.; Silvi, S.; Venturi, M.; Arduini, A.; Pochini, A.; Secchi, A. J. Org. Chem. 2004, 69, 5881-5887. (14) Cameron, B. R.; Loeb, S. J.; Yap, G. P. A. Inorg. Chem. 1997, 36, 54985504. (15) Avarvari, N.; Mezailles, N.; Ricard, L.; Le Floch, P.; Mathey, F. Science 1998, 280, 1587-1589.
arene[2]triazines with their cavities fine-tuned by bridging heteroatoms have been synthesized.17 The combination of the electronic, conjugative, and steric effects of different heteroatoms strongly affects the cavity size. These molecules also readily form ordered self-assembly structures because of multiple intermolecular hydrogen bond interactions. To use calixarenes in molecular recognition, it is essential to immobilize calixarene molecules on an electrode surface, the ability of which relies on the conformation and cavity size of the calixarene molecules on the surface. Thus, understanding the adsorption and adlayer structure of the calixarene molecules on the gold surface, which is frequently used as an electrode, is of prime importance. Scanning tunneling microscopy (STM) is a powerful tool for studying adlayer structures with atomic or molecular resolution on an electrode surface.18-20 The adlayers can be formed by many molecules with steric structures, such as proteins,21 metallamacrocyclic supramolecular assemblies,22,23 fullerenes,24 cyclodextrins,25,26 hydrocarbon cages,27 and calixarenes.28-30 Several STM investigations of calixarene (16) Bucher, C.; Zimmerman, R. S.; Lynch, V.; Kral, V.; Sessler, J. L. J. Am. Chem. Soc. 2001, 123, 2099-2100. (17) Wang, M. X.; Yang, H. B. J. Am. Chem. Soc. 2004, 126, 15412-15422. (18) Wan, L. J. Acc. Chem. Res. 2006, 39, 334-342. (19) Itaya, K. Prog. Surf. Sci. 1998, 58, 121-247. (20) Xu, S.; Szymanski, G.; Lipkowski, J. J. Am. Chem. Soc. 2004, 126, 1227612277. (21) Chi, Q.; Zhang, J.; Nielsen, J. U.; Friis, E. P.; Chorkendorff, I.; Canters, G. W.; Andersen, J. E. T.; Ulstrup, J. J. Am. Chem. Soc. 2000, 122, 4047-4055. (22) Yuan, Q. H.; Wan, L. J.; Jude, H.; Stang, P. J. J. Am. Chem. Soc. 2005, 127, 16279-16286. (23) Gong, J. R.; Wan, L. J.; Yuan, Q. H.; Bai, C. L.; Jude, H.; Stang, P. J. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 971-974. (24) Shirai, Y.; Osgood, A. J.; Zhao, Y.; Yao, Y.; Saudan, L.; Yang, H.; Yu-Hung, C.; Alemany, L. B.; Sasaki, T.; Morin, J. F.; Guerrero, J. M.; Kelly, K. F.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 4854-4864. (25) Miyake, K.; Yasuda, S.; Harada, A.; Sumaoka, J.; Komiyama, M.; Shigekawa, H. J. Am. Chem. Soc. 2003, 125, 5080-5085. (26) Ohira, A.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. J. Am. Chem. Soc. 2003, 125, 5057-5065. (27) Fujii, S.; Akiba, U.; Fujihira, M. J. Am. Chem. Soc. 2002, 124, 1362913635. (28) Yoshimoto, S.; Abe, M.; Itaya, K.; Narumi, F.; Sashikata, K.; Nishiyama, K.; Taniguchi, I. Langmuir 2003, 19, 8130-8133.
10.1021/la070039i CCC: $37.00 © 2007 American Chemical Society Published on Web 06/15/2007
8022 Langmuir, Vol. 23, No. 15, 2007
adlayers on metal surfaces have been previously reported. In an electrolyte solution, p-tert-butylcalix[4]arene-1,3-dithiol adlayers with two different packing arrangements and the internal molecular structure on a Au(100)-(1 × 1) surface are revealed.28 Well-ordered arrays of a calix[8]arene derivative, OBOCMC8, and the guest-host complex C60/OBOCMC8 are constructed on a Au(111) surface, and the C60 molecules in the cavity of the calix[8]arene derivative are distinctly resolved by STM.30 In the present paper, we report the well-defined adlayer structures of a series of calix[2]arene[2]triazine molecules with bridging heteroatoms on Au(111) surfaces. High-resolution STM images show the cavity structures of the different calix[2]arene[2]triazine molecules at the submolecular level. Density functional theory (DFT) computation results provide in depth details about the conformational structure of the molecules on the surface and the hydrogen bond interactions in the self-assembled adlayers. A novel composite molecular adlayer with calix[2]arene[2]triazine molecules and 1,3,5-tris(5-carboxyamyloxy)benzene (TCAB) molecules is also reported. STM images show a molecule shape transformation from a rhombic shape for each molecule in the adlayer of tetraazacalix[2]arene[2]triazine into a square shape with the coadsorption of the TCAB molecules. The conformational changes from different calix[2]arene[2]triazine molecules as well as calix[2]arene[2]triazine/TCAB complexes are attributed to the changes in the intermolecular hydrogen bond interactions, and DFT calculation results support this assertion.
Yan et al.
Figure 1. Crystal structures (top view and side views) of (a) molecule a, tetraazacalix[2]arene[2]triazine, and (b) molecule c, diazadioxocalix[2]arene[2]triazine: red, O; cyan, C; blue, N; white, H; pink, Cl. Scheme 1 . Chemical Structures of Aza- and/or Oxo-Bridged Calix[2]arene[2]triazines and 1,3,5-Tris(5-carboxyamyloxy)benzene (TCAB)
Experimental Section A well-defined Au(111) surface on gold beads was prepared by the flame-annealing method.18 The molecular adlayers were formed by immersing a Au(111) crystal into a spectroscopic grade ethanol solution containing less than 1 µM heteroatom-bridged calix[2]arene[2]triazine for ∼3 min. The Au(111) electrode was then mounted in a Tefion electrochemical cell filled with 0.1 M HClO4. In situ STM experiments were carried out with a Nanoscope E microscope (Digital Instruments Inc., Santa Barbara, CA) in 0.1 M HClO4. The typical electrode potentials were 0.55 V (versus a reversible hydrogen electrode (RHE)), which is in the double layer region. The tunneling tip was prepared by electrochemically etching a W wire (0.25 mm in diameter) in 0.6 M KOH. The side walls of the tips were sealed with transparent nail polish to minimize the faradic current. All the images were collected in the constant-current mode. Theoretical calculations on periodic structures were carried out with DMol 3 in Materials Studio 3.1 (Accelrys, San Diego, CA). DFT calculations were performed using the PW91 generalized gradient approximation (GGA). DFT semilocal pseudopotentials were used to describe the electron-core interactions. The k points were obtained from the Monkhorst-Pack scheme with medium mesh. The convergence criterion of the self-consistent field (SCF) procedure was set to be 10-5 au on the energy. DIIS (direct inversion in an iterative subspace) and thermal smearing were applied to promote SCF convergence. The sizes of the unit cells are in accordance with the periodicity of the Au(111) surface based on the STM measurements. The gold surface was mimicked by three layers of Au atoms (for the mixed adlayer of tetraazacalix[2]arene[2]triazine and TCAB, the gold surface was represented by one layer of Au atoms) constrained during the geometry optimization process.
Results and Discussion Three adlayers of aza- and/or oxo-bridged calix[2]arene[2]triazines were prepared on a Au(111) surface. The chemical (29) Pan, G. B.; Bu, J. H.; Wang, D.; Liu, J. M.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. J. Phys. Chem. B 2003, 107, 13111-13116. (30) Pan, G. B.; Liu, J. M.; Zhang, H. M.; Wan, L. J.; Zheng, Q. Y.; Bai, C. L. Angew. Chem., Int. Ed. 2003, 42, 2747-2751.
structures of these molecules are shown in Scheme 1. Based on their crystal structures, the molecules are composed of two isolated phenyl rings and two triazine segments with different combinations of aza(NH) groups and oxygen atoms as bridging linkages, and they adopt the 1,3-alternative configuration because of the lack of intra-annular hydrogen bonds (Figure 1).17 On the other hand, the intermolecular hydrogen bonds between the triazinyl rings and the bridging aza(NH) groups have a profound influence on the molecular packing patterns for the calix[2]arene[2]triazines. 1. Tetraazacalix[2]arene[2]triazine (Molecule a) Adlayer. Figure 2a is a typical large-scale STM image of the adlayer of molecule a on Au(111). The molecules adsorb on the substrate and form a well-defined structure. Owing to the adsorption of the adlayer, the image corresponds to an unreconstructed Au(111) surface. On the next two adlayers formed by molecules b and c, the same Au(111) surfaces can be seen. According to the Au(111) lattice, we find that the molecule a adlayer grows along the A and B directions, which are parallel to the 〈121〉 direction, showing a hexagonal symmetry. Figure 2b is a high-resolution STM image showing the structural details of the molecular adlayer. Each molecule can be discerned as a rhombic shape and appears as a set of four bright spots originating from the phenyl rings and triazine parts. The nearest molecular distances along the A and B directions are measured to be 9.8 ( 1 Å and 9.7
Adlayer Structures of Calixarenes on Au(III)
Langmuir, Vol. 23, No. 15, 2007 8023
Figure 2. (a) Typical STM image of the tetraazacalix[2]arene[2]triazine adlayer formed on Au(111). (b) High-resolution STM image of the adlayer. (c) Optimized structural model for the tetraazacalix[2]arene[2]triazine adlayer. (d) Hydrogen bonds between the tetraazacalix[2]arene[2]triazine molecules indicated by black dashed lines.
( 1 Å, respectively. The angle between the A and B directions is 60 ( 2°. According to the intermolecular distance and adlayer symmetry, a (2x3 × 2x3)R30° structure is deduced with a unit cell defined in Figure 2b. Careful examination of the STM images also reveals that an angle of 30 ( 1° exists between the longer diagonal and the molecular rows. To understand the molecular conformation and orientation on Au(111), we performed theoretical calculations using density functional theory. The formation of a molecular adlayer on a surface is dominated by molecule/molecule interactions and molecule/substrate interactions. As for the adlayer of molecule a, the hydrogen bond plays an important role in the formation of the two-dimensional ordered arrangements, similar to that in its 3D crystal structure.22 Theoretical calculations reveal that the molecule preserves its 1,3-alternative conformation on the Au(111) surface. The phenyl rings of the molecules are nearly vertical to the surface and the triazine parts are tilted on the surface in different orientations in favor of the formation of a hydrogenbonded network, which cause the difference in brightness in the STM image for the two spots. In addition, it is possible that the chloride atoms in the molecule can interact with the Au(111) substrate, although the details of such an interaction are unclear from the computation results. From the theoretical calculations, a structural model for the molecule a adlayer is shown in Figure 2c. The model agrees well with the STM images. Figure 2d
shows the results of geometry optimization. From the optimized model, it can be seen that one molecule interacts with its neighboring four molecules, forming four pairs of hydrogen bonds. The length of the hydrogen bonds is 2.4 ( 0.2 Å in the A direction and 1.8 ( 0.2 Å in the B direction, marked by black dashed lines in Figure 2d. 2. Oxotriazacalix[2]arene[2]triazine (Molecule b) Adlayer. The chemical structure of the molecule b adlayer is almost identical to that of molecule a. The only difference is the replacement of one NH group with an oxygen atom in its bridge site. The STM image of the molecule b adlayer is shown in Figure 3a. From this image, an ordered monolayer of molecule b can be seen on Au(111). Compared with the underlying Au(111) lattice, it is found that all molecular rows align along the 〈110〉 direction. The molecular rows in directions A and B cross each other at an angle of 60 ( 2°. The high-resolution STM image in Figure 3b shows that the molecular feature in the STM image is almost the same as that of molecule a. Each molecule appears in a set of four bright spots, as marked by circles, forming a rhombic configuration. Each bright spot corresponds to either a phenyl ring or a triazine part. A unit cell for the molecular adlayer is deduced and shown in Figure 3b. The intermolecular distances in directions A and B are the same at 11.5 ( 1 Å. The adlayer symmetry and measured results indicate a (4 × 4) structure.
8024 Langmuir, Vol. 23, No. 15, 2007
Yan et al.
Figure 3. (a) Typical STM image of the oxotriazacalix[2]arene[2]triazine adlayer formed on Au(111). (b) High-resolution STM image of the adlayer. (c) Optimized structural model for the oxotriazacalix[2]arene[2]triazine adlayer. (d) Hydrogen bonds formed in the adlayer indicated by black dashed lines.
A theoretical computation was carried out for the molecule b adlayer on Au(111). Figure 3c shows the calculated result, which is consistent with the STM observation. The 1,3-alternate configuration is also maintained on the surface (similar to what is observed for molecule a), and the triazine rings demonstrate a larger affinity to Au(111). However, owing to the structural difference, two triazine rings of molecule b lay nearly flat on the surface. Two phenyl units in the optimized structure on the surface adopt different orientations on Au(111). Such molecular conformation favors the intermolecular hydrogen bonds. The details of the hydrogen bonds are illustrated in Figure 3d. The lengths of the hydrogen bonds are approximately 2.8 and 3.1 Å. The longer hydrogen bond lengths indicate that the hydrogen bond interaction for molecule b is weaker than that for molecule a. 3. Dioxodiazacalix[2]arene[2]triazine (Molecule c) Adlayer. Figure 4a is a typical STM image acquired of the adlayer of molecule c on the Au(111) surface. The molecular rows align along the 〈110〉 direction of the underlying Au(111) lattice and cross each other in directions A and B at an angle of 60 ( 2°. Individual molecules can be discerned in the image by a depression in the center of the near-square shaped molecule image. The high-resolution STM image (Figure 4b) shows that each molecule can be seen as a set of four bright spots marked by red circles. The bright spots are attributed to the phenyl rings and triazine
parts of the molecules. Although the major feature of the molecule is almost the same as that of molecules a and b, a near-square conformation, rather than a rhombic one, is adopted by molecule c. The adlayer shows a hexagonal symmetry with an intermolecular distance of 11.5 ( 1 Å (∼4 times that of the Au-Au atom distance in the Au(111) lattice). Based on these results, a (4 × 4) structure is determined for the adlayer. A structural model is proposed and optimized by DFT as shown in Figure 4c. In this model, two triazinyl units in a molecule keep nearly flat on the surface, while two benzene rings stand on the surface. The calculated result reveals that there are identical hydrogen bond interactions between the molecules in both the A and B directions. The lengths of the hydrogen bonds are ∼2.9 Å, similar to those in the adlayer of molecule b. However, the number of hydrogen bonds is decreased, indicating weaker hydrogen bond interactions between the molecule c molecules than those between the molecule a and b molecules. Figure 4d is an expanded illustration for the hydrogen bonds in molecule c. A near-square shape for each molecule in the model is consistent with the STM image. These results demonstrate that intermolecular hydrogen bonds (bond length and density) in the molecular adlayers play an important role in structural formation. From molecule a to c, owing to the changes of the chemical structures in the three
Adlayer Structures of Calixarenes on Au(III)
Langmuir, Vol. 23, No. 15, 2007 8025
Figure 4. (a) Typical STM image of the dioxodiazacalix[2]arene[2]triazine adlayer on Au(111). (b) High-resolution STM image of the adlayer. (c) Optimized structural model for the adlayer. (d) Hydrogen bonds formed among the molecules indicated by black dashed lines.
molecules at the bridge heteroatom positions, intermolecular hydrogen bonds change and cause different conformations and cavities of the calixarene molecules from rhombic to near-square. 4. Coadsorption of Molecule a and TCAB. For a desired host-guest specification, it is important to control and obtain a desirable conformation of calixarene molecules. We attempted to promote the conformation transition for the molecule a adlayer by coadsorbing TCAB molecules with molecule a. The TCAB molecules were designed to act as molecular clips to modulate calixarene adsorption on the Au(111) surface. Figure 5 shows the effect of TCAB coadsorption. Figure 5a is a typical STM image acquired of the adlayer of molecule a and TCAB on Au(111) showing molecular self-organization. The molecules form ordered rows along the 〈110〉 and 〈121〉 directions of the underlying Au lattice (indicated by A and B, respectively), which are almost perpendicular to each other with an angle of 90 ( 2° between them. The intermolecular distances in the A and B directions are measured to be a ) 17.3 ( 1 Å and b ) 19.9 ( 1 Å, respectively. A (6 × 4x3) structure is then defined in the adlayer. A unit cell is superimposed on the images in Figure 5a and b. A remarkable feature in the STM image is the periodic nearsquares with bright lines. Careful examination indicates that each square is a molecule a molecule. Since calix[2]arene[2]triazine type molecules are flexible and their conformations will be
influenced by weak intermolecular interactions such as hydrogen bonds as discussed above, the square shape indicates a different conformation from the rhombic conformation in Figure 2. The high-resolution STM image in Figure 5b shows that there are four bright spots (marked by four red circles) in a square. Based on the molecular chemical structure, the four bright spots in the STM image are assigned to two phenyl rings and two triazine segments in molecule a, providing direct evidence of the conformational transition of molecule a from rhombic to square. The conformational transition is due to the existence of TCAB in the adlayer. TCAB molecules, which can be identified in Figure 5b, appear as bright spots around the calixarene molecules. The electronic structure of the TCAB molecule is calculated by DFT to explain the TCAB-induced conformational transition. The highest occupied molecular orbital (HOMO) and HOMO+1 of the TCAB molecule are shown in Figure 6. HOMO+1 is composed of four degenerated orbitals, one of which is located in the central benzene ring that corresponds to the large bright spot (marked by the red hexagon in Figure 5b), and the other three are mainly located in the three carboxyl groups, which give rise to the small bright spots (marked by the blue-filled circles in Figure 5b). Based on the above analysis, a structural model for the coadsorbed adlayer is proposed in Figure 5c. Each molecule
8026 Langmuir, Vol. 23, No. 15, 2007
Yan et al.
Figure 5. (a) Large scale STM image of the tetraazacalix[2]arene[2]triazine and TCAB coadsorbed adlayer on Au(111). (b) High-resolution STM image of the coadsorbed adlayer. (c) Structural model for the adlayer. (d) Hydrogen bonds formed in the coadsorbed adlayer marked by black dashed lines and the conformation of tetraazacalix[2]arene[2]triazine in the coadsorbed adlayer obtained from DFT.
Figure 6. HOMO+1 and HOMO of the TCAB molecule calculated by DFT.
a shows a near-square shape and is surrounded by two TCAB molecules. Intermolecular hydrogen bonds exist between the TCAB molecules and molecule a, and they constitute a 2D network with large-size cavities. The details of the molecular interactions of TCAB and molecule a are also investigated by a theoretical simulation. The results show that hydrogen bond interactions exist between molecule a and the surrounding TCAB molecules in the coadsorbed adlayer. The length and direction
of these hydrogen bonds are different from those between the molecule a molecules in their pure adlayer. Two TCAB molecules behave like a clip to coadsorb with a molecule a molecule and force the conformation transition of molecule a from rhombic to square. The central benzene ring of a TCAB molecule is parallel to the Au(111) surface. Two pairs of hydrogen bonds are expected to form between TCAB and the calixarene molecules, as indicated by the black dashed lines in Figure 5d. Based on STM observations and computational analyses, we conclude that the calixarene type molecules can self-organize and form ordered 2D adlayers on a Au(111) surface. These results are consistent with those in the literature.30 The 1,3-alternate configuration in molecules a, b, and c is preserved on Au(111) with different cavity appearances. From molecule a to molecule c, a rhombic conformation is gradually transformed to nearsquare with the chemical structure change in these molecules. In the structure formation and cavity conformation transition, intermolecular hydrogen bonds play an important role. The changes in the heteroatoms in the bridge positions of the molecules induce changes in the intermolecular hydrogen bonds, resulting in different intermolecular reactions and cavities in the molecules and making it possible to take advantage of different intermo-
Adlayer Structures of Calixarenes on Au(III)
Langmuir, Vol. 23, No. 15, 2007 8027
lecular interactions to regulate the adlayer structures of calixarenes. The coadsorption of molecule a and TCAB is a typical example. With the coadsorption of TCAB molecules, the adlayer symmetry, the arrangement, and the conformation of the molecule are tuned. This method may offer a new approach to desirably immobilize molecules with special conformations on solid surfaces, and it is significant in designing functional electrodes for molecular recognition.
configuration with different cavities on a Au(111) surface. Hydrogen bond interaction between bridging aza(NH) groups and triazine rings greatly influences the formation of the molecular architectures. Different hydrogen bonds yield different molecular packing patterns. With the coadsorption of TCAB, the molecular conformation of the calix[2]arene[2]triazine cavity can be tuned from rhombic to square. The results are important in host-guest chemistry, surface chemistry, and nanodevice fabrication.
Conclusions
Acknowledgment. Financial support from the Natural Science Foundation of China (Nos. 20575070, 20121301, and 20673121), the National Key Project on Basic Research (No. 2006CB806100), and the Chinese Academy of Sciences is gratefully acknowledged.
Well-defined adlayers of three heteroatom-bridged calix[2]arene[2]triazines on a Au(111) surface were prepared and investigated by STM and DFT calculations. The STM images provide direct structural evidence for molecular self-assembly and demonstrate that the molecules keep their 1,3-alternate
LA070039I
