Two-Dimensional Self-Assembled Structures of Melamine and Melem

Dec 2, 2010 - (1, 17-25) For instance, artistic nanoarchitectures such as rosettes and ribbons composed of melamine and perylene bisimides (PBI) have ...
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Two-Dimensional Self-Assembled Structures of Melamine and Melem at the Aqueous Solution-Au(111) Interface† Shinobu Uemura,‡ Masashi Aono,‡ Tamikuni Komatsu,§ and Masashi Kunitake*,‡,

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan, §High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan, and JST-CREST, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan )

‡

Received October 1, 2010. Revised Manuscript Received November 14, 2010 Self-assembled structures of melamine and the condensed melamine derivative melem were investigated at aqueous solution-Au(111) interfaces by cyclic voltammetry and in situ scanning tunneling microscopy (STM) observation. The adsorption/desorption behaviors of both molecules on Au(111) surfaces could be controlled by varying the electrochemical potential and solution concentration. In the negative potential region, self-assembled structures of melem and melamine were constructed by double hydrogen bonding systems between nitrogen atoms of triazine rings and amine groups. In addition, melem formed a closely packed structure at potentials of between -0.3 and -0.15 V or in solutions at higher concentrations.

Introduction In recent years, the construction and investigation of selfassembled nanoarchitectures composed of organic molecules have been key areas in nanoscience and nanotechnology research.1-3 Among the 2D self-assembled structures, porous network structures have frequently been reported because of their versatile applications as templates and receptors.4,5 Although these porous structures are unfavorable thermodynamically because of the low density of the cavities in 2D and 3D structures, multiple complementary intermolecular interactions such as hydrogen bonds are employed. For example, in the field of crystal engineering, trimesic acid is known to form porous 3D crystal structures through hydrogen bonds between carboxylic groups.6,7 In a similar fashion, 2D porous structures at utltrahigh vacuum (UHV)-highly oriented pyrolytic graphite (HOPG),8 fatty acid solutions-HOPG,9,10 and aqueous solution-Au(111)11 interfaces have been observed by scanning tunneling microscopy † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding author. E-mail: [email protected]. Tel: þ81-96-342-3673. Fax: þ81-96-342-3679.

(1) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Angew. Chem., Int. Ed. 2009, 48, 7298–7333. (2) Wang, D.; Wan, L.-J. J. Phys. Chem. B 2007, 111, 16109–16130. (3) Barth, J. V. Annu. Rev. Phys. Chem. 2007, 58, 375–407. (4) Kudernac, T.; Lei, S.; Elemans, J. A. A. W.; De Feyter, S. Chem. Soc. Rev. 2009, 38, 402–421. (5) Bonifazi, D.; Mohnani, S.; Llanes-Pallas, A. Chem.—Eur. J. 2009, 15, 7004– 7025. (6) Duchamp, D. J.; Marsh, R. E. Acta Crystallogr. 1969, B25, 5–19. (7) Zaworotko, M. J. Chem. Commun. 2001, 1–9. (8) Griessl, S.; Lackinger, M.; Edelwirth, M.; Hietschold, M.; Heckl, W. M. Single Mol. 2002, 3, 25–31. (9) Lackinger, M.; Griessl, S.; Heckl, W. M.; Hietschold, M.; Flynn, G. W. Langmuir 2005, 21, 4984–4988. (10) Kampschulte, L.; Lackinger, M.; Maier, A.-K.; Kishore, R. S. K.; Griessl, S.; Schmittel, M.; Heckl, W. M. J. Phys. Chem. B 2006, 110, 10829–10836. (11) Ishikawa, Y.; Ohira, A.; Sakata, M.; Hirayama, C.; Kunitake, M. Chem. Commun. 2002, 2652–2653. (12) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. H. J. Phys. Chem. B 2004, 108, 11556–11560. (13) Griessl, S. J. H.; Lackinger, M.; Jamitzky, F.; Markert, T.; Hietschold, M.; Heckl, W. M. Langmuir 2004, 20, 9403–9407. (14) Ivasenko, O.; MacLeod, J. M.; Chernichenko, K. Yu.; Balenkova, E. S.; Shpanchenko, R. V.; Nenajdenko, V. G.; Rosei, F.; Perepichka, D. F. Chem. Commun. 2009, 1192–1194.

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(STM). Using the cavities, the construction of host-guest structures and guest manipulations by STM have also been reported.12-14 Two-dimensional porous network structures are constructed using building blocks with highly symmetrical features and functional groups for multiple intermolecular interactions with neighboring molecules. Recently, 2D porous structures without hydrogen bonds at the liquid-solid interfaces have also been reported as a result of steric hindrance.15,16 Melamine (1,3,4-triazine-2,4,6-triamine) is often used as a building block in supramolecular chemistry because of its triangular shape with three functional groups at the apexes.1,17-25 For instance, artistic nanoarchitectures such as rosettes and ribbons composed of melamine and perylene bisimides (PBI) have been constructed from complementary triple hydrogen bonds between NH 3 3 3 O and CdO 3 3 3 N.17,21,22 Naturally, the hydrogen bonds in pure melamines are formed in the bulk crystal26,27 and at various medium-solid interfaces.28-34 (15) Lei, S.; Tahara, K.; De Schryver, F. C.; Van der Auweraer, M.; Tobe, Y.; De Feyter, S. Angew. Chem., Int. Ed. 2008, 47, 2964–2968. (16) Uemura, S.; Sengupta, S.; W€urthner, F. Angew. Chem., Int. Ed. 2009, 48, 7825–7828. (17) Whitesides, G. M.; Simanek, E. E.; Mathias, J. P.; Seto, C. T.; Chin, D. N.; Mammen, M.; Gordon, D. M. Acc. Chem. Res. 1995, 28, 37–44. (18) Sijbesma, R. P.; Meijer, E. W. Chem. Commun. 2003, 5–16. (19) Ajayaghosh, A.; George, S. J.; Schenning, A. P. H. J. Top. Curr. Chem. 2005, 258, 83–118. (20) Yagai, S. J. Photochem. Photobiol., C 2006, 7, 164–182. (21) Theobald, J. A.; Oxtoby, N. S.; Phillips, M. A.; Champness, N. R.; Beton, P. H. Nature 2003, 424, 1029–1031. (22) Hoeben, F. J. M.; Zhang, J.; Lee, C. C.; Pouderoijen, M. J.; Wolffs, M.; W€urthner, F.; Schenning, A. P. H. J.; Meijer, E. W.; De Feyter, S. Chem.—Eur. J. 2008, 14, 8579–8589. (23) Madueno, R.; R€ais€anen, M. T.; Silien, C.; Buck, M. Nature 2008, 454, 618– 621. (24) Palma, C.-A.; Bjork, J.; Bonini, M.; Dyer, M. S.; Llanes-Pallas, A.; Bonifazi, D.; Persson, M.; Samori, P. J. Am. Chem. Soc. 2009, 131, 13062–13071. (25) Gardener, J. A.; Shvarova, O. Y.; Briggs, G. A. D.; Castell, M. R. J. Phys. Chem. C 2010, 114, 5859–5866. (26) Hughes, E. W. J. Am. Chem. Soc. 1941, 63, 1737–1752. (27) Cousson, A.; Nicolaı¨ , B.; Fillaux, F. Acta Crystallogr. 2005, E61, o222– o224. (28) Perdigao, L. M. A.; Perkins, E. W.; Ma, J.; Staniec, P. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. B 2006, 110, 12539–12542. (29) Staniec, P. A.; Perdigao, L. M. A.; Rogers, B. L.; Champness, N. R.; Beton, P. H. J. Phys. Chem. C 2007, 111, 886–893.

Published on Web 12/02/2010

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Figure 1. Chemical structures of melamine (left) and melem (right).

Melem (2,5,8-triamino-1,3,4,6,7,9,9b-heptaazaphenalene) (Figure 1) is a condensed melamine derivative.35,36 Recently, melem and its polymeric derivatives have attracted attention because of properties such as photocatalysis and fluorescence with rare metals.37,38 Melem has potential as a building block for supramolecular architectures because of these features. To our limited knowledge, the self-assembly of melem at interfaces has not yet been reported. In this article, we report an electrochemical and STM investigation of melamine and melem self-assembled structures at the aqueous solution-Au(111) interface. Throughout the electrochemical investigations and in situ STM observations at the various concentrations, polymorphism of the self-assembled structures was controlled by the concentration and electrochemical potential control.

Experimental Methods Melamine (TCI Co. Ltd., Japan) was purchased and used without further purification. Melem was synthesized according to previous reports.35 Weakly basic solutions were used as the electrolyte solutions for electrochemistry and electrochemical STM to avoid the protonation of primary amine group (melamine pKa = 5.0).39 The sample solutions were prepared as follows. Melamine solutions were prepared by dissolving melamine in NaClO4 (Nacalai Tesque Inc., Japan) aqueous solution, and the pH and concentration were adjusted by the addition of 0.1 M NaOH (Nacalai Tesque Inc., Japan). Melem solutions with concentrations ranging from nearly saturated to 1/40 of the saturated concentration were prepared by the dilution of a saturated melem solution with a NaClO4 aqueous solution. The pH and concentration of those solutions were adjusted by the addition of 0.1 M NaOH. The concentration of melem in a saturated solution in ultrapure water was determined to be 1.8  10-5 M (ε 237 = 1.5  104) by UV-vis spectroscopy (Cary Bio 300, Varian Technologies Japan Ltd.). Au(111) facets formed on singlecrystal beads were used directly in electrochemical STM. The singlecrystal beads were mechanically polished to form Au(111) cut (30) Xu, W.; Dong, M.; Gersen, H.; Rauls, E.; Vazquez-Campos, S.; CregoCalama, M.; Reinhoudt, D. N.; Stensgaard, I.; Laegsgaard, E.; Linderoth, T. R.; Besenbacher, F. Small 2007, 3, 854–858. (31) Silly, F.; Shaw, A. Q.; Castell, M. R.; Briggs, G. A. D. Chem. Commun. 2008, 1907–1909. (32) Silly, F.; Shaw, A. Q.; Castell, M. R.; Briggs, G. A. D.; Mura, M.; Martsinovich, N.; Kantorovich, L. J. Phys. Chem. C 2008, 112, 11476–11480. (33) Zhang, X.; Chen, T.; Chen, Q.; Wang, L.; Wan, L.-J. Phys. Chem. Chem. Phys. 2009, 11, 7708–7712. (34) Zhang, H.-M.; Pei, Z.-K.; Xie, Z.-X.; Long, L.-S.; Mao, B.-W.; Xu, X.; Zheng, L.-S. J. Phys. Chem. C 2009, 113, 13940–13946. (35) Komatsu, T. Macromol. Chem. Phys. 2001, 202, 19–25. (36) Sattler, A.; Pagano, S.; Zeuner, M.; Zurawski, A.; Gunzelmann, D.; Senker, J.; M€uller-Buschbaum, K.; Schnick, W. Chem.—Eur. J. 2009, 15, 13161–13170. (37) Wang, X.; Maeda, K.; Thomas, A.; Takanabe, K.; Xin, G.; Carlsson, J. M.; Domen, K.; Antonietti, M. Nat. Mater. 2009, 8, 76–80. (38) Ishii, A.; Habu, K.; Kishi, S.; Ohtsu, H.; Komatsu, T.; Osaka, K.; Kato, K.; Kimura, S.; Takata, M.; Hasegawa, M.; Shigesato, Y. Photochem. Photobiol. Sci. 2007, 6, 804–809. (39) Dixon, J. K.; Woodberry, N. T.; Costa, G. W. J. Am. Chem. Soc. 1947, 69, 599–603. (40) Ohira, A.; Sakata, M.; Taniguchi, I.; Hirayama, C.; Kunitake, M. J. Am. Chem. Soc. 2003, 125, 5057–5065. (41) Uemura, S.; Taniguchi, I.; Sakata, M.; Kunitake, M. J. Electroanal. Chem. 2008, 623, 1–7.

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Figure 2. (a, b) Typical STM images, (c) the corresponding molecular model, and (d) CV of melamine self-assembled structures at the aqueous solution-Au(111) interface. (a) Es = -0.10 V, Ebias = -0.20 V, and It = 0.45 nA. (b) Es = -0.20 V, Ebias=-0.1 V, and It = 0.80 nA. samples, and these were used for cyclic voltammetry (CV). Electrode preparation was the same as described earlier.40,41 The Au(111) electrodes were annealed with a hydrogen flame and then immediately cooled by immersion in ultrapure water. Annealed Au(111) was transferred into an electrochemical STM cell filled with a prepared solution. Electrochemical STM investigations were carried out in aqueous solutions using a Nanoscope E (Digital Instruments, Santa Barbara, CA). A tungsten tip for STM was prepared by electrochemical etching in 0.1 M KOH and was then covered with clear nail polish to minimize residual faradic currents. Platinum wires were used as the reference and counter electrodes. All potential values reported are referenced to the saturated calomel electrode (SCE). The STM images were obtained in constant-current mode. CV was carried out using a common potentiostat with the hanging meniscus method in a three-compartment electrochemical cell under a N2 atmosphere. Platinum wires were used as the reference and the counter electrodes. All potential values reported are referenced to the SCE.

Results and Discussion Melamine. Figure 2 shows typical STM images of the melamine self-assembled structure at the interface between the 100 μM aqueous solution and the Au(111). A honeycomb network structure covered the Au(111) surfaces with a few domain boundaries and some brighter aggregates. The high-resolution image (Figure 2b) revealed that the honeycomb structure consisted of bright triangularly shaped spots, which were attributed to melamine molecules. The unit cell in the corresponding molecular model (Figure 2c) was a rhombus with parameters of a = b = 1.0 ( 0.1 nm and R = 60 ( 5°. The observed honeycomb structures were formed by the double hydrogen bond systems between neighboring melamine molecules at the aqueous solutionAu(111) interface. Additionally, some cavities in the honeycomb structures were obviously filled with brighter spots (indicated by arrows in Figure 2b). The spots in these cavities might be due to additional melamine molecules. These results are consistent with most studies, which have reported honeycomb structures for 2D DOI: 10.1021/la103948n

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self-assembled melamine structures prepared by vapor deposition28-30,32 and drop casting33,42 onto Au(111) and HOPG. Additional melamine molecules in the cavities of the honeycomb structure have also been observed in another study.42 An exception is the report by Silly et al. in which both honeycomb and closely packed structures were observed for melamine 2D structures on Au(111) in UHV.32 The self-assembled structures could be changed from the honeycomb structure to a closely packed structure with an increase in melamine surface coverage. These results indicate that melamine self-assembled structures can be prepared regardless of the preparation procedures and substrates but their structures may depend on the surface coverage. The adsorption and desorption of melamine molecules onto Au(111) surfaces were managed under electrochemical potential control. The honeycomb structure was observed at potentials from -0.1 to -0.35 V versus SCE, which is close to the open circuit potential. At -0.4 V and more negative potentials, the melamine molecules were desorbed from the Au(111) surface and the bare Au(111) surface was observed. In contrast, aggregates were observed on the surface at more positive potentials (>0.0 V). This STM observation of the potential dependency at the interface is consistent with the CV study. Typical CV results for the melamine solution are shown in Figure 2d. The oxidative and reductive peak couple was observed at -0.02 V, which was in contrast to the lack of peaks in the CV of the free melamine solution (dotted line). The potential region (-0.35 to -0.1 V) for the melamine self-assembled structures was located on the negative side of this peak. Excess melamine adsorption and aggregation were observed at the peak potential (-0.02 V). Melem. In the nearly saturated aqueous solution of melem (1.5  10-5 M), STM images revealed closely packed selfassembled structures at the interface between the solution and Au(111) (Figure 3a,b). Bright spots were densely ordered over the entire surface with few domain boundaries and structural defects (Figure 3a). In the high-resolution image (Figure 3b), melem molecules were clearly observed as bright triangular spots. As shown in the tentative model (Figure 3c), the melem molecular spots were aligned linearly and the trianglar spots in each row were orientated in the opposite direction to neighboring rows. Two types of hydrogen bond systems would facilitate the formation of this structure. First, melem molecules in different rows would hydrogen bond in a side-to-side manner, which is similar to the double hydrogen bond system in the melamines. Second, melem molecules within a row would also hydrogen bond in a head-to-tail fashion. The unit cell parameters for the melem structure were a = 0.85 ( 0.04 nm, b = 1.55 ( 0.05 nm, and R = 90 ( 5°. Considering that the molecular size of melem is 0.75 nm between nitrogen atoms of amino groups as calculated by MOPAC, these parameters agree with the expected distance of the hydrogen-bonded structures. Interestingly, STM images showed melem self-assembled into a slightly different structure with the 2  10-6 M solution (Figure 4a), in which the concentration was 10 times more dilute than that of the nearly saturated solution. The same structure was observed with the 5  10-7 M solution, which contained 1/40 of the melem concentration in the nearly saturated solution. In these structures, the triangular spots were arranged in a similar honeycomb structure to the melamine self-assembled structure (Figure 2b). As shown in the tentative model in Figure 4b, single melem molecules in the honeycomb structures were stabilized by three side-to-side double hydrogen bonds with neighboring

melem molecules, which is again similar to melamine (Figure 4b). The unit cell (indicated by the rhombus in Figure 4b) parameters were a = b = 1.37 ( 0.04 nm and R = 60 ( 5°. The parameters of the melem honeycomb structure were approximately 0.3 nm larger than those for melamine. In addition, the estimated cavity diameter from the tentative models was larger for melem (0.9 nm) than for melamine (0.6 nm). The different hydrogen bond systems in the two ordered structures of melem are interesting. In the honeycomb structure, each melem molecule is connected uniformly to the surrounding three molecules by three double hydrogen bond systems in a sideto-side fashion with rotational symmetry. The rotational symmetry of the double hydrogen bonds on the sides of the melem molecules has also been reported in the bulk crystal.43,44 However, the neighboring melem molecules are lower than the center melem plane, and the honeycomb structure does not appear in the bulk crystal. In contrast, the closely packed model (Figure 3b) included two side-to-side and two head-to-tail hydrogen bond systems. The fact that the honeycomb structure is preferred over the closely packed structure in a dilute solution indicates that the hydrogen bond system in the honeycomb structure is stronger than that in the closely packed structure. This preference may arise from the different fashion of hydrogen bonding and/or stabilization of the symmetrical structure. At higher concentrations, the closely packed structure is advantageous because it gives a higher surface coverage. Many studies have reported 2D chirality in the structure built from achiral components, and this is due to the confinement of molecules to the surface.45-47 In melamine honeycomb structures,

(42) Zhang, H.-M.; Xie, Z.-X.; Long, L.-S.; Zhong, H.-P.; Zhao, W.; Mao, B.-W.; Xu, X.; Zheng, L.-S. J. Phys. Chem. C 2008, 112, 4209–4218.

(43) J€urgens, B.; Irran, E.; Senker, J.; Kroll, P.; M€uller, H.; Schnick, W. J. Am. Chem. Soc. 2003, 125, 10288–10300. (44) Sattler, A.; Schnick, W. Z. Anorg. Allg. Chem. 2006, 632, 238–242.

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Figure 3. (a, b) Typical STM images and (c) a tentative model of a closely packed melem structure at the aqueous solution-Au(111) interface. (a) Es=-0.15 V, Ebias = -0.20 V, and It = 0.95 nA. (b) Es=-0.15 V, Ebias = -0.2 V, and It = 1.0 nA. The concentration was 1.5  10-5 M.

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Figure 5. STM images and molecular models of the melem honeycomb network structure with different chiralities at the interface between the lower-concentration solution (2  10-6 M) and Au(111). STM conditions: Es=-0.15 V, Ebias=-0.20 V, and It = 1.5 nA.

Figure 4. (a) Typical STM image and (b) a tentative model of melem honeycomb network structures at the interface of the lowerconcentration solution with Au(111). (a) Es = -0.60 V, Ebias = 0.15 V, and It=1.4 nA. The solution concentration was 2  10-6 M.

2D chirality has also been reported because of tilting of the double hydrogen bond system from the molecular centers.29,42 We also observed 2D chirality in the honeycomb structures of melamine and melem. The ring units of the honeycomb structures result from a propeller-type arrangement with either clockwise or counter clockwise rotation (Figure 5). No 2D chirality was observed in the closely packed structure, which suggests that the chiral rotational arrangements originate from the side-to-side hydrogen bonding. In trimesic acid, which can have a honeycomb structure, 2D chirality is not observed because hydrogen bonds are arranged in a head-to-tail fashion.8-11 Domains for both clockwise and counter clockwise rotational arrangements were observed in the STM image (Figure 5). The potential dependency of melems at the interface was also investigated and compared with CVs. Figure 6 shows the CVs and typical STM images for the potential dependence of melem adsorption in the approximately half-saturated solution (1  10-5 M). (45) Katsonis, N.; Lacaze, E.; Feringa, B. L. J. Mater. Chem. 2008, 18, 2065– 2073. (46) Elemans, J. A. A. W.; De Cat, I.; Xu, H.; De Feyter, S. Chem. Soc. Rev. 2009, 38, 722–736. (47) Raval, R. Chem. Soc. Rev. 2009, 38, 707–721.

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Similar to that for melamine, the reductive and oxidative peak couple was observed at -0.11 V, which was very close to the peak couple of melamine at -0.02 V. Whereas a more positive potential than the peak couple at -0.11 V (such 0.1 V) was applied in the STM investigation, large aggregates were randomly observed on the surface, suggesting that the peak couple at -0.11 V in the CV was due to excess adsorption (Figure 6e). The oxidative shoulder and broad reductive peak were found near -0.60 V, although the peak couple of melamine was not observed in this potential region. In a negative potential region lower than the reductive peak at -0.6 V, melem molecules were desorbed from the surface and were not observed by STM. The small peak couple should be attributed to the adsorption/desorption of melem molecules. The two ordered structures—honeycomb and closely packed— were essentially observed in the middle potential region between the regions of desorption and excess adsorption (-0.60 to -0.15 V), which was located in the potential gap in two peak couples. We note that the ordered structures of melem were present in a wider potential range than those for melamine (-0.35 to -0.1 V). When the potential was changed gradually from -1.0 V to a positive potential, the honeycomb structure was observed from -0.6 to -0.3 V (Figure 6b). The closely packed structure was observed from -0.3 to -0.15 V (Figure 6d). Around -0.3 V, the mixed structure in which the honeycomb structure and closely packed structure coexisted was frequently observed (Figure 6c). These results obviously indicate that the adsorption and selfassembled structure of melem are controlled by the concentration and the electrochemical potential. In other words, the melem adsorbed structure is selected from two ordered structures by means of the surface coverage, which is thermodynamically controlled by the solution concentration and/or electrochemical potential. A similar potential dependence of the adsorption behavior including an order-to-order phase transition has been observed with trimesic acid.11 DOI: 10.1021/la103948n

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Figure 6. (a) Typical cyclic voltammogram (CV) of a melem aqueous solution using the Au(111) electrode. The dotted line in the CV is for free melem solution, and the solid line is for a 1  10-5 M melem aqueous solution. (b-e) Corresponding STM images. (b) E = -0.60 V, (c) E = -0.30 V, (d) E = -0.15 V, and (e) E = -0.05 V.

At lower concentrations or in the lower potential region, the honeycomb structure is preferred because of stabilization by the rotationally symmetrical double hydrogen bond system. At higher concentrations, the closely packed structure is preferred. The unit cell areas divided by the number of molecules per unit cell in (48) Palermo, V.; Samori, P. Angew. Chem., Int. Ed. 2007, 46, 4428–4432. (49) Uemura, S.; Tanoue, R.; Yilmaz, N.; Ohira, A.; Kunitake, M. Materials 2010, 3, 4252–4276.

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the closely packed and honeycomb structures were estimated to be 0.66 and 0.81 nm2, respectively. This phase transition of melem at the interface indicated that the polymorphism of melem is controlled by various external stimuli. Polymorphism that is dependent on surface coverage and the solution concentration has been observed in other studies.9-11,15,16,24 The application of external factors such as the electrochemical potential also results in polymorphism because interactions among molecules, substrates, and media play significant roles at interfaces.4,48,49 These results then indicate that the interactions around melem can easily be controlled at the aqueous solution-Au(111) interface. In a similar manner to that of the self-assembled structures of melem, melamine on Au(111) in UHV has been observed to have both honeycomb and closely packed structures.32 Like melem, the self-organized structures of melamine might also be thermodynamically rather than kinetically managed. In addition, the melamine closely packed structure may have an intermediate phase between the honeycomb structures and disordered adsorption. However, we did not observe this in the research. Both melamine and melem possesses similar polymorphism, and their self-assembled structures can be prepared regardless of the preparation procedures and substrates. In addition, these structures depend on the surface coverage.

Conclusions We investigated the self-assembly of triangular molecules at aqueous solution-Au(111) interfaces under electrochemical control. Both melem and melamine self-assembled structures were dependent on the electrochemical potential. The polymorphism of melem and melamine as triangular building blocks is promising for the preparation of novel supramolecular structures by external stimuli. This is the first report of the 2D self-assembly of melem, and the results will aid nanoscale investigations of other melamine derivatives.

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