In Situ STM Investigation of Aromatic Poly(azomethine) Arrays

Sep 6, 2012 - Vertically standing nanowalls of pristine poly(azomethine) on a graphite by chemical liquid deposition. Rintaro Higuchi , Ryota Tanoue ,...
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In Situ STM Investigation of Aromatic Poly(azomethine) Arrays Constructed by “On-Site” Equilibrium Polymerization Ryota Tanoue,† Rintaro Higuchi,† Kiryu Ikebe,† Shinobu Uemura,† Nobuo Kimizuka,‡,§ Adam Z. Stieg,∥,⊥ James K. Gimzewski,∥,⊥,# and Masashi Kunitake*,†,‡ †

Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan Core Research for Evolutional Science and Technology, Japan Science and Technology Agency (JST-CREST), Kawaguchi Center Building, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § Department of Chemistry and Biochemistry, Graduate School of Engineering, International Research Center for Molecular Systems (IRCMS), Kyushu University, 744 Moto-oka, Nishi-ku, Fukuoka 819-0395, Japan ∥ California NanoSystems Institute, 570 Westwood Plaza, Los Angeles, California 90095, United States ⊥ WPI Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan # Department of Chemistry and Biochemistry, University of CaliforniaLos Angeles, 607 Charles E. Young Drive East, Los Angeles, California 90095, United States ‡

S Supporting Information *

ABSTRACT: Two-dimensional (2D) arrays of π-conjugated aromatic polymers produced by surface-selective Schiff base coupling reactions between an aromatic diamine and an aromatic dialdehyde were investigated in detail using in situ scanning tunneling microscopy. Surface-selective coupling was achieved for almost all diamine/dialdehyde combinations attempted, although several combinations did not proceed even in homogeneous aqueous alkaline solution. Most of the combinations of an aromatic diamine and a dialdehyde, except the combinations of 4,4′-azodianiline with mono/bithiophenedicarboxaldehyde, formed highly ordered π-conjugated polymer arrays on an iodine-modified Au(111) surface in aqueous solution at a suitable pH. The simplest polymer of the various combinations tested, obtained from the combination of 1,4diaminobenzene with terephthaldicarboxaldehyde, gave a 2D array consisting of linearly connected benzene units. Poly(azomethine) adlayers caused a positive shift in the electrochemical potential of the butterfly shaped oxidative adsorption and reductive desorption of iodine. The acceleration of the reductive desorption of iodine suggests the existence of a weak interaction between the polymer layer and iodine. Not only the first polymer adlayers but also partially adsorbed secondary adlayers with “on-top” epitaxial behavior were frequently observed for all polymer systems. The alignment of the polymer chains in the adlayers possessed a certain regularity in terms of a regular interval between polymer chains because of repulsive interpolymer interactions.

1. INTRODUCTION Conjugated polymers (CPs) are widely used in commercial applications. Basic research on CPs is still active in terms of chemical design, synthesis, film preparation, and characterization to realize their full potential in electronic, magnetic, and optical applications.1−3 For a fundamental understanding of CPs, direct observation at the atomic scale of pristine CPs with well-defined mono- and multilayer structures on electrodes is required.4 If pristine CP can be synthesized and controlled in the higher-order structure, novel functions at the molecular level will become available as a result of the unique π-electron system. The preparation and molecular-scale visualization of well-defined mono- and multilayers of CPs have been achieved by casting of soluble modified CPs and surface polymerization, which can then be visualized by scanning tunneling microscopy © 2012 American Chemical Society

(STM). The visualization of oligoalkylthiophene adlayers containing bent moieties in the thiophene main chain, epitaxially aligned on highly oriented pyrolytic graphite (HOPG) by simple casting, has been reported.5−7 Surface polymerizations of CPs can be categorized as follows: (1) electrochemical polymerization,8−14 (2) tip-induced polymerization,15,16 (3) photopolymerization,17−19 (4) thermal polymerization under UHV,19−31 and (5) “on-site” polycondensation (chemical liquid deposition; CLD).33,34 Several examples of surface polymerizations that provide well-ordered CP monolayers, which are suitable for high-resolution Received: March 11, 2012 Revised: September 6, 2012 Published: September 6, 2012 13844

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preparation for each system are described in a previous article33 and in the Supporting Information. 2.3. In Situ STM Observations. A Nanoscope E microscope (Digital Instruments, Santa Barbara, CA) was used for electrochemical STM investigations. Electrochemically etched tungsten wires (Niraco, Tokyo, Japan) in 1.0 M KOH (Nacalai Tesque, Kyoto, Japan; 85%) were used as STM tips. The tips were coated with clear commercial nail polish to minimize the faradaic current. Au(111) single-crystal bead electrodes for the STM observations were prepared from Au wire (Tanaka Kikinzoku, Tokyo, Japan; 99.999%) by the Clavilier method according to a previous report.36 Iodine-modified Au(111) electrodes were prepared by immersion in an aqueous solution containing 10 mM KI (Kanto Chemical, Tokyo, Japan; 99.5%) for a few seconds. STM observations were carried out as follows. The iodine-modified electrode was transferred into an STM cell filled with pH-controlled reaction solution. All STM images were collected in constant-current mode. 2.4. Cyclic Voltammetry. Prior to the CV measurements, an iodine-modified Au(111) electrode was immersed for 30 s in an aqueous solution in the presence of ASB and TCA at pH 2.5 to prepare the Schiff-base-polymer-coated iodine-modified Au(111) sample. Cyclic voltammetry (CV) was carried out using a laboratory-built Au(111) cut electrode, which was prepared by mechanically polishing an Au single-crystal bead. CV measurements were performed using a potentiostat (Toho Technical Research, Tokyo, Japan) under a N2 atmosphere.

observation by STM, have been developed. Electrochemical polymerization of π-CPs such as polyacetylene, polyaniline, and polypyrrole is one of the most popular techniques used to prepare CP films. Generally, highly cross-linked insoluble amorphous films are prepared by applying a potential in an aqueous solution of the appropriate monomer.8,9 Sakaguchi and co-workers reported the first high-resolution STM imaging of linearly connected polythiophene.10,11 The surface electrochemical polymerization of an alkylthiophene adsorbed onto iodine-modified Au(111) substrates was achieved by applying a potential with a pulse sequence. Polymerization was delicately controlled using the pulse width, avoiding uncontrolled polymerization. Moreover, they achieved electrochemical block polymerization of two thiophene monomers bearing different substituents, indicating the possibility of building single-molecule superlattices on a surface through controlled electropolymerization. Yau and coauthors have reported electrochemical polymerization and high-resolution STM imaging12,13 of poly(3,4-ethylenedioxythiophene)35 as well as monolayer-level CP adlayers of polyaniline.14 Moreover, Aono and co-workers have demonstrated chain polymerization of diacetylene analogues on HOPG, initiated by an electronic pulse from an STM tip.15,16 This approach is significant as a fusion of “top-down” and “bottom-up” approaches. In addition, De Schryver17,18 and Harada19 have independently reported STM visualization of photochemical chain polymerization of diacetylene on a surface. As well as using these surface chain polymerization systems, successive polymerization has frequently been realized under UHV.20−32 Two-dimensional (2D) CP adlayers have been prepared using thermally initiated polycondensations20−25 such as Schiff base coupling21,22 and C−C coupling achieved by thermal annealing26−28 or an Ullmann reaction29−32 between monomers bearing multiple connection points. Very recently, we demonstrated a general substrate-mediated soft solution methodology for preparing extended π-conjugated polymeric nanoarchitectures in low dimensions by the use of π-conjugated covalent bonds.33 Based on thermodynamic control over equilibrium polymerization at the solid−liquid interface, where aromatic building blocks spontaneously and selectively link, close-packed arrays composed of one-dimensional (1D) aromatic polymers and 2D macromolecular frameworks were prepared and characterized using in situ STM. This methodology eliminates the need for the harsh reaction conditions and sophisticated equipment common to most current fabrication techniques and provides almost infinite possibilities for the preparation of robust materials with designed molecular architectures. Here, we compare π-conjugated linear polymer systems constructed by on-site polycondensation and selfassembly onto a solid−liquid interface and a Schiff base coupling reaction in a homogeneous aqueous solution in terms of catalytic reactivity. Also, the electrochemical potential dependence of the resulting nanostructures and epitaxial growth of multilayers are discussed.

3. RESULTS AND DISCUSSION 3.1. Catalytic Reactivity of Schiff Base Coupling for Various Building Block Combinations. Figure 1 shows a

Figure 1. (A) Schematic representation of surface-induced on-site polycondensation of a π-conjugated polymer and (B) aromatic amine and aromatic aldehyde units used for “on-site” polycondensation.

schematic representation of the concept of surface-induced polycondensation using Schiff base (also known as azomethine) coupling37−39 and the bifunctional molecular units used. As we reported previously,32 various polymer adlayers are formed by combination of aromatic diamine and aromatic dialdehyde molecules. The results of Schiff base reactions with different combinations of aromatic diamine molecules, namely 4,4diaminostilbene dihydrochloride (ASB), 4,4-azodianiline (ADA), and 1,4-diaminobenzene (AB) with aromatic dialdehyde molecules, namely terephthaldicarboxaldehyde (TPA), isophthaldicarboxaldehyde (IPA), phthaldicarboxaldehyde

2. EXPERIMENTAL METHODS 2.1. Chemicals. All chemicals were purchased from suppliers and used without further purification. 2.2. Preparation of Sample Solutions. The respective aromatic amines and aldehydes were dissolved in aqueous solutions of KI (1 mM), and then the pH was carefully adjusted by careful addition of aqueous solutions of HClO4 and/or NaOH. The details of sample 13845

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Table 1. Summary of the Results of Schiff Base Reactions of Different Combinations of Aromatic Diamine−Dialdehyde Molecules in Homogeneous Media and onto Surfacea

a

−: Not measured.

over the whole pH region for relatively low concentration conditions (typically 0.1 mM). Interestingly, polycondensation was achieved under acidic conditions onto a hydrophobic substrate, such as iodine-modified Au(111), although homogeneous reaction was prohibited across the entire pH range. The chemical structure of the diamine molecule predominantly rules the reactivity of the Schiff base coupling. However, the chemical structure of the dialdehyde building block also has an influence. In particular, differences in the substitution positions of the benzene dialdehyde molecules directly influence their reactivity. Although the coupling reaction was confirmed spectroscopically, no polymeric products of ASB− PA (ortho-substituted) were observed in homogeneous solutions, and no reaction products were observed on the substrate. The expected ASB−PA polymer could not possess a planar orientation because of steric hindrance. A planar orientation of a linear polymer in a 2D array would have advantages in terms of both the expansion of the π-conjugated system and the contact area between the aromatic moiety and the atomic surface of the substrate. The ASB−IPA (meta-substituted) system gave reaction products both in solution and at the interface. A comparison of TPA and IPA as reaction partners of ASB shows that the ASB−IPA system was less reactive than the ASB−TPA system. Therefore, a pH nearly equal to or slightly higher than the pKa was required to achieve the on-site reaction in the former system. 3.2. Structures of CP Adlayers Investigated by STM. Structural symmetry on a flat orientation was crucial in the structure of the polymer arrays on the surface. The most symmetrical molecular block, TPA, for all combinations with ASB, ADA, and AB, gave closely packed structures consisting of linear polymer chains, as mentioned in a previous paper.33 In contrast, the combination of meta-substituted IPA and ASB showed a zigzag structure. Lipton-Duffin et al. have reported STM imaging of 2D zigzag polymers prepared by a thermally initiated Ullmann coupling reaction between meta-substituted

(PA), 2,5-thiophenedicarboxaldehyde (TCA), and 2,2-bithiophene-5,5-dicarboxaldehyde (BCA), are summarized in Table 1. The reactivity of Schiff base coupling is predominantly ruled by combination of the amine and aldehyde molecules; the reaction is particularly affected by the pKa of the amine in homogeneous aqueous dilute solutions. Generally, an amine molecule with a higher pKa possesses higher reactivity because of the higher electronic density of the amine unit. For the three diamine aromatic molecules we used, the order of reactivity is ASB > AB > ADA. The pKa1 of ASB is 2.9, and no pKa2 is observed. The coupling reaction with ASB is thermodynamically prohibited at pH values lower than the pKa of the amines, and precipitation was observed above the pKa as polycondensation products with all the aldehyde molecules we used, except PA. The pKa1 of AB is observed at a pH very close to the pKa of ASB, but secondary protonation is observed at pH 6.0 as pKa2, indicating that the secondary protonation was suppressed by the first protonation. Precipitation in homogeneous aqueous media was observed at pH values above pKa2. In the pH region between pKa1 and pKa2, soluble oligomeric products would be formed but would not precipitate. As we have proved, the polycondensation proceeds on a hydrophobic surface even at a slightly lower pH than the pKa because of the accelerations of the reactions as a result of the hydrophobic nature of the surface. In the case of Schiff base coupling with ASB or AB as the diamine aromatic molecules, the azomethine polymer products were found on iodinemodified Au(111) for all combinations with dialdehydes, except the ortho-substituted dialdehyde PA. In the case of ADA, pKas were observed at lower pH values than in the case of AB, namely 2.1 and 3.9 as pKa1 and pKa2, respectively. Unlike the cases of systems with ASB or AB, no precipitation in homogeneous media was observed with ADA 13846

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Figure 2. In situ STM images of polymer adlayers acquired from combinations of TCA with ASB (A), AB (B), and ADA (C). (A′) Predicted model of an ASB−TCA azomethine polymer. The pHs selected for STM observations were 2.5, 5.0, and 3.0 for ASB−TCA, AB−TCA, and ADA−TCA, respectively.

diiodobenzene molecules, carried out under UHV.30 Besides, a disordered structure was also observed for the combination of meta-substituted IPA and AB. The AB connections are too short to allow a planar orientation at the solid−liquid interface. Thiophene-based TCA, which has connecting angles of ca. 150°, was able to form quasi-linear ordered structures from ASB and AB (Figure 2A,B). The ASB−TCA polymer array had neatly packed domain structures, showing that the coupling reaction proceeded almost exclusively on the iodine-modified Au(111) surface. Each spot in the array is attributed to a benzene and a thiophene unit. A relatively gently curved polymer structure based on TCA might have a planar orientation with a slightly tilted thiophene ring structure. This indicates an alternating arrangement of planar TCA moieties, as shown in the model in Figure 2A′. The highresolution image reveals clear boundaries of the ordered domains. Although the coupling reaction is reversible at the molecular level, dynamic changes in the domain structures, such as 2D Ostwald ripening,40−43 were not observed on the experimental time scale, namely 4−5 h. Interestingly, the ASB−TCA and ADA−TCA systems formed ordered and disordered arrays, respectively, as shown in Figures 2A and 2C, despite the similarities in their chemical structures. As already mentioned, ASB and ADA both gave ordered structures with TPA. In the case of the combinations of BCA, composed of two connected thiophene units, on-site reactions with ASB or ADA proceeded but did not give wellordered arrays, in spite of the higher symmetry of BCA (Figure 3). This might be because of the flexibility of the connections between the thiophene units in BCA. For the ASB−BCA system, in unclear images, string structures attributed to a single

polymer strand were observed. In contrast, no such structures were observed in the ADA−BCA system; some disordered adlayers were present. The combination of the smallest molecules, AB and TPA, also formed 2D polymer arrays consisting of a linear polymer, as shown in Figure 4A. The aligned spots are attributed to

Figure 4. Typical (A) wide-area and (B) molecular-resolution in situ STM images and (C) cross-sectional analysis of the adlayer consisting of a closely packed AB−TPA polymer array. The pH of the solution for STM observations was 2.5. Cross sections were obtained at the location marked by a solid line in (B).

individual benzene rings. The ordered domains were small, and the phase boundary was indistinct. In the arrays, the spots were frequently disconnected. This indicates a lower degree of polymerization than those in the ASB- or ADA-based polymers. 3.3. Formation of Epitaxial Secondary Adlayers. In fact, formation of second layers was frequently observed as a result of on-site polycondensation onto the polymer adlayers. In order to construct the polymer adlayers, which can be visualized by STM, very careful control of suitable solution conditions, especially for delicate pH control, is necessary. The selection of a slightly higher pH than that required for STM observation leads to “on-site” polycondensation and continuous

Figure 3. In situ STM images of polymer adlayers acquired from the combinations of BCA with ASB (A) and ADA (B). The pH selected for STM observations was 3.0 for ASB−BCA and ADA−BCA. 13847

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the polymer systems involving IPA and TCA. The stacking interactions between polymers bearing nonlinear chemical structures would be weaker than those in the case of polymer systems with a planar chemical structure. Nevertheless, molecular-scale periodicity was found even at the first stage of multilayer formation, as mentioned above, and atomic force microscopy studies of the CLD films revealed that their structure had a periodicity with a much longer span, instead of periodicity on the molecular scale.34 Propagation of secondary polymer adlayers and further deposition beyond secondary adlayers were not observed, in spite of the theoretical possibility, as a result of lack of controllability. However, the growing process might be logically explained using a combination of several images captured at different positions. Figure 6 shows two typical STM images of

deposition, and we have proposed it as a CLD method for spontaneous formation of thin polymeric layers.34 Figure 4B shows a typical secondary adlayer for the AB−TPA system. Interestingly, linear polymer chains consisting of bright benzene spots with two different contrasts were observed. The brighter lines are second polymer layers formed on top of the first polymer adlayers. In the high-resolution image (Figure 4B), all of the polymers, including the secondary polymer on top of a polymer adlayer, possessed the same propagation direction with side-by-side orientation, indicating on-top stacking of polymers. Cross-sectional analysis (Figure 4C) also supports this. The height of the secondary adlayer was twice that of the first. It was observed that all secondary polymers were propagated along the same direction as that of the underlying polymer, indicating that π−π interactions between stacked polymers are the dominant driving force promoting the formation of secondary adlayers. As another example, Figure 5 shows typical STM images of the ASB−TPA polymer system. A patch structure consisting of

Figure 6. In situ STM images (A, B) and corresponding models (A′, B′) of the highly ordered adlayer of ADA−TPA polymer arrays with different surface coverages. The pair of STM images was obtained from the same experiment, and the pH of the solution for on-site polycondensation was 3.0.

Figure 5. Typical (A) wide-area and (B) molecular-resolution in situ STM images and (B′) corresponding model and (C) cross-sectional analysis of the highly ordered adlayer consisting of a closely packed ASB−TPA polymer array. The pH of the reaction solution for on-site polycondensation was 2.5. The cross-sectional image was obtained at the locations marked by the solid lines in (B).

secondary adlayers of the ADA−TPA polymer with different surface coverages. Each propagation row of the secondary polymer was parallel at constant intervals. In the case of the secondary adlayer with relatively low coverage (Figure 6A), the interval was generally 1.6 ± 0.2 nm, which corresponded to three polymer rows, as shown in the model shown in Figure 6A′. The adlayer with higher coverage possessed empty canals at the same interval between the pairs of polymer rows. These results indicate that the weak repulsive interaction between the polymer chains (rows) cooperatively controls the structures of the polymer adlayers. It should be emphasized that the equilibrium reaction leads to a complex arrangement of the propagating polymer on polymer adlayers, based on thermodynamic control caused by weak repulsive interactions. Such cooperative alignment of polymer rows was frequently observed in the secondary adlayers for the ADA−TPA system, but not for ASB−TPA and AB−TPA, despite the structural similarity between ASB and ADA. Although the adlayer structure and images for ADA−TPA and ASB−TPA are similar, ADA−TPA tended to form larger domains than those in ASB−TPA.

distinct domain boundaries and relatively large domains with a striped pattern was observed, using the same contrast as that in Figure 4A. High-resolution images of the striped pattern revealed the formation of secondary adlayers. Similar to the AB−TPA system, the secondary polymers predominantly propagated along the underlying polymer. In addition, the degree of polymerization, especially for the second-layer polymer, was higher than that of the AB−TPA system. A similar alignment of the second layer of the polymer was also observed in the case of the ADA−TPA system. Ordered secondary adlayers were always observed only for the ADA−TPA, AB−TPA, and ASB−TPA systems, which formed parallel ordered polymer arrays consisting of linear aromatic polymers. No secondary adlayers were observed for 13848

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3.4. Electrochemical Potential Dependence of ASB− TCA Polymer Adlayer. CVs with or without a poly(azomethine) adlayer (ASB−TCA) on an iodine-modified Au(111) surface in 1 mM KI are presented in Figure 7A. The

potential region for ordered adlayers roughly matched the potential of the broader peak (0.7−1.0 V). From the slightly negative potential of 0.60 V, which is the limit potential of a double-layer region, the adlayer images became unclear, in spite of the existence of the iodine adlayers. Floating islands of ordered arrays were then frequently observed. These results indicate that the adsorption became weaker at negative potentials and that the polymer chains connected via azomethine bonds were destroyed because of the thermodynamically reversible reaction. The results prove that the reaction equilibrium is ruled not only by pH but also by surface concentrations of building block molecules. Generally, a negatively charged surface at a potential less than the OCP weakens adsorption and causes a decrease in surface coverage. At potentials more negative than those of the butterfly peaks, both the polymer and iodine adlayers disappeared entirely and only the Au(111) lattice was observed. The other combinations of reagents used to produce poly(azomethine) adlayers exhibited similar CVs and desorption behaviors.

4. CONCLUSIONS The catalytic reactivities of on-site polycondensations with various combinations of amine and aldehyde building blocks were described and compared with the reactivities in homogeneous solutions. The use of an iodine-modified Au(111) surface led to catalytic azomethine coupling onto the surface. The very simple preparation, in which CLD is conducted in a beaker filled with an aqueous solution at room temperature, is suitable for a diverse range of building blocks. Pristine aromatic polymers with simple chemical structures can be synthesized by this method but not by ordinary synthetic processes in homogeneous media because of their insolubility and unprocessability, caused by the lack of bulky side chains and ionic groups. The epitaxial nature of the polymer adlayers indicates a capacity to move beyond 2D molecular adlayers toward self-assembly of well-ordered 3D covalent nanoarchitectures using multiarmed building blocks. Surface-induced chemical liquid deposition represents a simple and low-energy method with strong potential for the development of sustainable and efficient industrial technology to support future organic electronics.

Figure 7. (A) Cyclic voltammograms of an unmodified iodinemodified Au(111) electrode (dotted line) and ASB−TCA-modified electrode (solid line) measured in 1 mM KI (pH 2.5). In situ STM images of ASB−TCA polymer adlayers in 1 mM KI (pH 2.5) in the presence of 1 mM ASB and 1 mM TCA, observed at (B) 0.85, (C) 0.60, and (D) 0.30 V vs RHE. The OCP is 0.70 V vs RHE.

typical butterfly-shaped redox peaks observed for the Au(111) electrode without the polymer adlayer are attributed to oxidative adsorption and reductive desorption of an iodine adlayer on the Au(111) surface at +0.3 V in pure KI (dashed line in Figure 7A). The polymer-modified sample showed a positive shift of the butterfly peaks, indicating the presence of polymer and iodine adlayers. This shift is caused by the weak interaction between the newly formed polymer layer and iodine, which accelerates reductive desorption of iodine. In addition, the peaks at ca. 0.9 V, which are related to isometric compression and phase transition to a rotated (√3 × √3)R30° phase,44 became broader after the formation of polymer adlayers. However, no structural change was observed when the potential was moved to where a rotated (√3 × √3)R30° phase would be formed (Figure 7B). This indicates that the polymer adlayers are not sensitive to the underlying iodine lattice. These CV results reveal that suitable, not too strong, interactions between the polymer and the iodine adlayer would catalyze an on-site azomethine coupling reaction to induce surface selectivity. The dependence of the electrode potential on the adlayer structure was investigated using STM. After observation of the ordered arrays at an open-circuit potential (OCP; ca. 0.8 V), the potential gradually moved negatively or positively. The ordered arrays (a typical example is shown in Figure 7B) were stable between 0.90 and 0.70 V vs a reversible hydrogen electrode (RHE). A more positive potential was avoided because of the risk of oxidation of the Au(111) surface. The

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details and a table summarizing the Schiff base polycondensations in terms of reactivity, including CLD. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly financially supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan.

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