Soluble 1D Coordination Polymers Based on Dendron-Functionalized

Publication Date (Web): September 8, 2005 ... As a result, it was found that the synthesized dendrimer forms a stable, soluble Pd(II) coordination pol...
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Soluble 1D Coordination Polymers Based on Dendron-Functionalized Bispyridine Ligand for Linking between Immobilized Molecules on Substrates Hideo Tokuhisa* and Masatoshi Kanesato Nanoarchitectonics Research Center, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan Received May 9, 2005. In Final Form: July 29, 2005 As a monomeric ligand for a soluble 1D coordination polymer, a benzyl-ether based dendrimer having a rigid 4,4′-bispyridine ligand at the focal point has been synthesized and the coordination chemistry with Pd(II) investigated by nuclear magnetic resonance, ultraviolet-visible and fluorescence spectroscopies, gel permeation chromatography measurement, and X-ray photoelectron spectroscopy. As a result, it was found that the synthesized dendrimer forms a stable, soluble Pd(II) coordination polymer with rough estimation of degree of polymerization of 10 in organic solvents. Furthermore, through the coordination polymer we attempted to link fourth-generation poly(amidoamine) dendrimers (PAMAM) individually immobilized on mica and confirmed the interconnection of the PAMAM through coordination polymers by atomic force microscopy.

Introduction Coordination polymers, that is, polymers based on monomer units that are held together by coordinative bonds, have received much attention for many years because of their potential use as magnetic, electronic, or photooptical molecular materials.1-5 The polymerization can be accomplished at room temperature in controlled self-assembly, since it takes place due to defined, directed interactions with relatively high binding constants.6 There are the overwhelming majority of papers about coordination polymers in the solid state, most of which do not exist outside the crystal lattice packing. In contrast, the number of coordination polymers that have been characterized in solution is surprisingly small. To gain solubility that allows us to pave the way for handling as a single coordination polymer, as well as to make polymer characterization, one possibility might consist of the introduction of bulky substituents,7 which prevents aggregation. Dendrimers, which are well-defined, monodisperse, branched polymers, have recently attracted great interest in the field of nanotechnology, mainly due to their synthetically controllable structure at a molecular level, size, shape, flexibility, surface chemistry, etc.8 Dendrons (branches of the dendrimer) are often introduced into molecules to enhance the solubilities and isolate them from each other.9-11 * To whom correspondence should be addressed. Tel: +81-29861-2442. Fax: +81-29-861-3029. E-mail: [email protected]. (1) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004, 43, 2334-2375. (2) Janiak, C. J. Chem. Soc., Dalton Trans. 2003, 2781-2804. (3) Moulton, B.; Zaworotko, M. J. Chem. Rev. 2001, 101, 1629-1658. (4) Harriman, A.; Raymond, Z. Chem. Commun. 1996, 1707-1716. (5) Schu¨tte, M.; Kurth, D. G.; Linford, M. R.; Co¨lfen, H.; Mo¨hwald, H. Angew. Chem., Int. Ed. 1998, 37, 2891-2893. (6) Brunsveld, L.; Folmer, B. J. B.; Meijer, E. W.; Sijbesma, R. P. Chem. Rev. 2001, 101, 4071-4097. (7) Kelch, S.; Rehahn, M. Macromolecules 1998, 31, 4102-4106. (8) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III Angew. Chem., Int. Ed. Engl. 1990, 29, 138-175. (9) Schluter, A. D.; Rabe, J. P. Angew. Chem., Int. Ed. 2000, 39, 864-883. (10) Tokuhisa, H.; Kubo, T.; Koyama, E.; Hiratani, K.; Kanesato, M. Adv. Mater. 2003, 15, 1534-1538. (11) Fre´chet, J. M. J. PNAS 2002, 99, 4782-4787.

In this study, we have synthesized a dendron-functionalized ligand based on a rigid 4,4′-bispyridine to form a 1D Pd(II) coordination polymer and characterized the resulting coordination polymer by nuclear magnetic resonance (NMR), ultraviolet-visible (UV-vis) and fluorescence spectroscopies, gel permeation chromatography (GPC) measurement, and X-ray photoelectron spectroscopy (XPS). Since the rigid, conjugated bispyridine, in which the extended π-electron conjugation runs along the molecular axis and bridges the gap between the metal centers, could serve as a conductive molecular wire, the coordination polymer bearing dendron side chains is expected to provide for a single molecular wire, which can bridge between amine- or pyridine-modified immobilizedmolecules or electrodes and possibly tune the electron conductivity by changing the molecular length through coordination chemistry. As a preliminary study, we have attempted to use the soluble coordination polymer for linking between immobilized molecules on a substrate as a single molecular wire and confirmed molecular networks based on intermolecular connections of the coordination polymers with the immobilized molecules by atomic force microscopy (AFM). To the best of our knowledge, there are few examples to visualize linking between immobilized molecules on surfaces through coordination chemistry, although some reports have already shown topographies of a single or bundle of coordination polymers themselves on substrates12 or very recently conjugated polymers bridging between Au nanoparticles, which were synthesized homogeneously in solution prior to deposition onto a substrate, by scanning probe microscopy (SPM) techniques.13 Experimental Section Materials and Instrumentation. Unless stated otherwise, all reagents and chemicals were purchased from commercial sources and used without further purification. Trimethylsilylacetylene, benzoyl chloride, bis(triphenylphosphine)palladium(12) Ikeda, C.; Fujiwara, E.; Satake, A.; Kobuke, Y. Chem. Commun. 2003, 616-617. (13) Nakashima, H.; Furukawa, K.; Ajito, K.; Kashimura, Y.; Torimitsu, K. Langmuir 2005, 21, 511-515.

10.1021/la051236p CCC: $30.25 © 2005 American Chemical Society Published on Web 09/08/2005

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Scheme 1. Synthesis of Dendrimer 3 and Model Compound 4

(II) dichloride, diisopropylamine, 2,5-dibromoaniline, 4-bromopyridine HCl, and triphenyl phosphite were obtained from TCI (Tokyo, Japan). N-Methylpyrrolidone and trans-bis(acetonitrile)palladium(II) chloride were received from Wako Pure Chemical (Osaka, Japan). Generation 4 PAMAM dendrimer, copper(I) iodide, dried triethylamine, and anhydrous pyridine were purchased from Aldrich (Milwaukee, WI). Tetrahydrofuran (dehydrated), HPLC grade CHCl3, and spectral grade CH2Cl2 were used as received from Kanto Chemical (Tokyo, Japan). The dendritic benzyl alcohols were synthesized according to the literature.14 1H NMR spectra were measured on a Bruker Avance 500 spectrometer for solutions in CDCl3 with Me4Si as an internal standard. UV-vis absorption and fluorescence spectra were recorded on a JASCO V-570 and a JASCO FP-750, respectively. GPC analysis was performed with a Shimadzu LC-10ADVP liquid chromatograph equipped with a Shimadzu SPD-10AVVP UVvis detector, relative to polystylene standards. XPS spectra were acquired using a Theromo VG Scientific Theta Probe system. XPS data acquisition was employed with a pass energy of 100 eV, a step increment of 0.05 eV, and a Mg anode power of 400 W. AFM topographies were recorded on a Digital Instruments NanoScope IIIa using the tapping mode in air at room temperature. For fluorescence titration experiments, 1.00 × 10-5 M CH2Cl2 solutions of dendrimer 3 were prepared accurately. The concentrations of trans-PdCl2(CH3CN)2 were varied from 0 to 1.75 × 10-5 M (total of eight measurements). The fluorescence spectra were measured upon excitation at 322 nm. The K value was obtained by curve fitting using the equations Ιcalc ) Ι0/[H]0{[H]0 - 1/2{b - (b2 - 4c)1/2}, b ) [H]0 + [G]0 + 1/K, and c ) [H]0[G]0, where I0 is the initial fluorescence intensity, [G]0 is the added concentration (M) of Pd(II), and [H]0 is 1.00 × 10-5 M. Synthesis. A 4,4′-bispyridine rigid ligand was synthesized according to Scheme 1. 1,4-dibromoaniline as a starting material was disubstituted with trimethylsilylacetylene using a typical Sonogashira coupling procedure.15 After removal of trimethylsilyl groups using KOH and methanol, the resulting 1,4-diethynylaniline was coupled with 4-bromopyridine to give the rigid ligand 1. A polyether dendron 2 was obtained by following the convergent-growth method.14 Then, the dendron focally substituted with a carboxylic acid was condensed with 1 using triphenyl (14) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638-7647. (15) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.; Jagessar, R. C.; Dirk, S. M.; Price, D. W.; Reed, M. A.; Zhou, C.-W.; Chen, J.; Wang, W.; Cambell, I. Chem. Eur. J. 2001, 7, 5118-5134.

phosphite and pyridine to give dendrimer 3. For a control experiment, model compound 4, which has a phenyl group instead of a large dendron, was also synthesized. The products were characterized mainly by 1H NMR and mass spectroscopy. 1,4-Diethynylaniline. 2,5-Dibromoaniline (10.0 g, 39.9 mmol), Pd(PPh3)2Cl2 (2.81 g, 4.00 mmol), and CuI (1.52 g, 7.98 mmol) were purged with N2, and then degassed THF (80.0 mL), trimethylsilylacetylene (10.2 g, 104 mmol), and triethylamine (80.0 mL) were added successively. The reaction was carried out at 50 °C for 24 h and at reflux temperature for another 24 h. Then the mixture was cooled to room temperature, evaporated, and extracted with CHCl3. After the extract was filtered and evaporated to dryness, the residue was chromatographed (silica, CHCl3/hexane, 1:4). The silyl-protected bis(ethynyl) derivative containing a small amount of impurities was collected and deprotected without further purification. The deprotection was performed by stirring the mixed solution containing the protected product, 1 M KOH aqueous solution (40 mL), and methanol (60 mL) at room temperature for 30 min. Then CH2Cl2 (100 mL) was added and stirred for another 1 h. The aqueous layer was removed, and the organic layer was dried (MgSO4), filtered, and evaporated. The residue was chromatographed (silica, CHCl3/hexane, 1:4) to afford 1,4-diethynylaniline as a yellow solid (5.41 g, 96%). 1H NMR (500 MHz, CDCl ): δ (ppm) ) 3.09 (s, 1H), 3.45 (s, 3 1H), 4.26 (br, 2H), 6.80 (dd, 1H, J ) 8.0, 1.4 Hz), 6.82 (d, 1H, J ) 1.4 Hz), 7.26 (d, 1H, J ) 8.0 Hz). 1,4-Bis(4′-pyridylethynyl)aniline (1). Diethynylaniline (3.40 g, 24.1 mmol), 4-bromopyridine-HCl (18.7 g, 96.4 mmol), Pd(PPh3)2Cl2 (0.849 g, 1.21 mmol), and CuI (0.459 g, 2.41 mmol) were added and dried under vacuum for 3 h. Then, degassed diisopropylamine (24.3 g, 241 mmol) was added and the mixture heated to 50 °C, followed by the addition of THF (100 mL). The mixture was stirred at 50 °C for 3 days. Then the reaction mixture was filtered and evaporated. The residue was chromatographed (NH-silica, CHCl3) to give a yellow solid (4.17 g, 59%). 1H NMR (500 MHz, CDCl ): δ (ppm) ) 4.37 (br, 2H), 6.923 6.93 (m, 2H), 7.36-7.38 (m, 5H), 8.61 (d, 2H, J ) 5.1 Hz), 8.62 (d, 2H, J ) 5.8 Hz). ESI-MS: (295.3) 296.1 (M + 1). Dendrimer (3). To fourth-generation dendron 2 (753 mg, 0.227 mmol) was added 1 (134 mg, 0.454 mmol), and the mixture was dried under vacuum for 1 h and purged with argon. Then N-methylpyrrolidone (NMP) (3 mL) and dried pyridine (0.220 mL, 2.72 mmol) were added, and the reaction mixture was heated to 100 °C. Triphenyl phosphite (0.357 mL, 1.36 mmol) in NMP (3 mL) was added and the mixture stirred at 100 °C overnight. The mixed solution was evaporated under vacuum, and the

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Figure 1. 1H NMR spectra of a 2.0 × 10-3 M CDCl3 solution of (a) dendrimer 3 and (b) model compound 4 containing different molar equivalents of PdCl2(CH3CN)2. Those marked with an asterisk represent durene as a standard. residue was purified by gel permeation chromatography and NHsilica column chromatograpy using 4:6 CHCl3/hexane as an eluent to give a yellow powder (133 mg, 16%). 1H NMR (500 MHz, CDCl ): δ (ppm) ) 4.85-5.00 (m, 60H), 3 6.49 (t, 4H, J ) 2.2 Hz), 6.52 (t, 8H, J ) 2.1 Hz), 6.57 (t, 2H, J ) 2.2 Hz), 6.58-6.66 (m, 28H), 6.74 (t, 1H, 2.1 Hz), 7.16 (d, 2H, 2.1 Hz), 7.22-7.41 (m, 85H), 7.46 (d, 1H, J ) 8.1 Hz), 8.34 (d, 2H, J ) 6.1 Hz), 8.59 (d, 2H, J ) 6.0 Hz), 8.78 (s, 1H), 8.82 (d, 1H, J ) 1.4 Hz). MALDI-TOF m/z analysis (calculated) 3589.7 (3590.0), [M + Li]+.

Model Compound (4). Benzoyl chloride (43 mg, 0.30 mmol), 1 (60 mg, 0.20 mmol), pyridine (79 mg, 1.0 mmol), and THF (5 mL) were combined and stirred at room temperature overnight. The mixture was washed with H2O, extracted with CHCl3, dried (MgSO4), and evaporated to dryness. The residue was chromatographed (NH-silica, CHCl3/hexane, 1:1) to give a yellow solid. 1H NMR (500 MHz, CDCl ): 7.33 (dd, 1H, J ) 7.9, 1.6 Hz), 3 7.39 (d, 2H, J ) 5.8 Hz), 7.41 (d, 2H, J ) 5.8 Hz), 7.54 (t, 2H, J ) 7.4 Hz), 7.58 (d, 1H, J ) 7.9 Hz), 7.63 (d, 1H, J ) 7.4 Hz), 7.96 (d, 2H, J ) 7.4 Hz), 8.64 (d, 2H, J ) 5.8 Hz), 8.68 (d, 2H, J ) 5.8 Hz), 8.80 (br, 1H), 8.89 (d, 1H, J ) 1.6 Hz).

Results and Discussion Self-Assembly through Metal Complexation in Solution. To investigate self-assembly through metal complexation of dendrimer 3 and model compound 4, titration experiments using NMR, UV-vis, and fluorescence spectroscopy were performed. Figure 1 shows 1H NMR spectra of 3 and 4 upon the addition of transPdCl2(CH3CN)2 in CDCl3 at the concentration of 2.0 × 10-3 M. Before the addition of Pd(II), both spectra showed characteristic signals due to aromatic, benzyl, and methylenic ether protons of the benzyl-ether dendron14 or aromatic protons of the phenyl group as well as welldefined signals due to bispyridine moieties. Interestingly, the chemical shift difference between the two doublets around 8.3-8.7 ppm arising from pyridine protons (Ha and Hb) become larger when a large dendron is introduced (δHa - δHb; 0.25 ppm for 3, 0.02 ppm for 4), indicative of a different magnetic environment in the two pyridines, probably due to an asymmetric shielding effect of the dendron. Addition of 1 equiv of Pd(II) leads to line broadening and a downfield shift of the signals arising from the bispyridine ligand of dendrimer 3, indicating that the pyridines can be complexed with Pd(II). It is noteworthy that the complex formation is also accompanied by the signal changes in the dendron region; signals arising from the dendrons become broader and shift to the upfield region, overall showing shielding effects due

Figure 2. UV-vis spectra of a 1.0 × 10-5 M CH2Cl2 solution of (a) dendrimer 3 and (b) model compound 4 containing different molar equivalents (0, 0.50, 1.0, 1.5, 2.0 equiv) of PdCl2(CH3CN)2.

to the interaction between the neighboring dendrons.16 These results suggest that the Pd addition causes polymerization through the Pd complexation of the linear, rigid, bidentate ligand. In the case of model compound 4, after addition of 1 equiv of Pd, the overall intensity of the spectrum significantly decreased compared to a peak at 6.91 ppm due to durene as a standard. Another equivalent of Pd(II) results in disappearance of all the signals. We found that this was because the Pd complex of 4 was completely precipitated out from CDCl3 in the NMR tube. In contrast, even after addition of 2 equiv of Pd(II), the complex of dendrimer 3 remained in solution without change in the intensity ratio of the dendrimer to the standard, indicating that the dendrons enhance the solubility of the coordination polymer significantly, as expected. The coordinative polymerization of 3 and 4 with Pd(II) was also investigated at the lower concentration of 1.0 × 10-5 M in CH2Cl2 by absorption and emission spectroscopy. Figure 2 shows the absorption spectral changes upon addition of PdCl2(CH3CN)2. In both cases, new red-shifted peaks, which can be assigned to the metal-to-ligand charge-transfer (MLCT) transitions, were observed, suggesting the formation of a Pd(II) complex. In the spectrum of dendrimer 3, the isosbestic points at 268 and 324 nm were well-defined, while for compound 4 they are not observed. This might suggest that only a single equilibrium between two species, namely free and complexed pyridine, occurs during the titration in the case of dendrimer 3. The stoichiometry of the complex formation between 3 and Pd(II) was determined by means of continuous-variation plots (Job plots) from UV spectroscopic studies at 375 nm, in which the total concentration of dendrimer 3 and Pd salt was kept constant at 1.0 × 10-5 M. Maximum complex formation occurred at a molar fraction of Pd of 0.5 relative to dendrimer 3 (i.e., 1:1 complexation). These results suggest that dendrimer 3 forms a 1:1-type coordination polymer with Pd(II). To estimate the binding constant, fluorescence titration experiments with PdCl2(CH3CN)2 were performed in CH2Cl2. Before the Pd addition, dendrimer 3 shows a strong fluorescence at 390 nm upon excitation at 325 nm. The (16) Percec, V.; Dulcey, A. E.; Balagurusamy, V. S. K.; Miura, Y.; Smidrkal, J.; Peterca, M.; Nummelin, S.; Edlund, U.; Hudson, S. D.; Heiny, P. A.; Duan, H.; Magonov, S. N.; Vinogradov, S. A. Nature 2004, 430, 764-768.

Soluble Coordination Polymer with Dendritic Ligand

Figure 3. Fluorescent spectral changes of a 1.0 × 10-5 M CH2Cl2 solution of dendrimer 3 by addition of PdCl2(CH3CN)2 upon excitation at 325 nm; the inset shows the integrated fluorescence area between 350 and 600 nm as a function of the Pd(II)/ dendrimer 3 ratio (Pd/Dn).

Figure 4. GPC curves of CHCl3 solution of dendrimer 3 containing different molar equivalents of PdCl2(CH3CN)2: (a) 0, (b) 1.0, (c) 1.5. Table 1. Gel Permeation Chromatography Dataa for Dendrimer (3) upon Addition of Pd(II) Pd/Dnb

Mn

Mw

Mw/Mn

0 1 1.5

2600 12500 8230

2710 35900 24100

1.04 2.88 2.93

a Calibrated with narrow-dispersity polystyrene standards. b The molar ratio of added Pd(II) to dendrimer 3.

fluorescence was quenched upon increasing the Pd concentration (Figure 3). The binding constant (K) was roughly determined by curve fitting with the least-squares method based on the assumption that the quenching is due to the 1:1 complexation,17 giving K ) 3 × 106 M-1, which is relatively higher compared to hydrogen-bonding systems applied for supramolecular polymerization.6 The degree of polymerization (DP) of the coordination polymer was assessed by GPC analysis using CHCl3 as an eluent. The GPC trace of dendrimer 3 shows a sharp peak with Mn ) 2600, and Mw/Mn ) 1.04, which were determined using polystyrene standards (Figure 4 and Table 1). Considering the fact that GPC is not sensitive to molecular weight but to hydrodynamic volume, the lower molecular weight (Mn ) 2600) compared with the calculated value (Mcal ) 3584) indicates that the dendrimer is more compact than the linear polystyrene with the same molecular weight, which have been often observed in branched polymers.14 In contrast, in the case of addition of 1.0 equiv of Pd(II), a significant shift of the GPC profile to higher molecular region was observed, to give an average DP of 10, which was taken from Mw ) 35 000. This might imply that even in unequivalent conditions during a GPC separation the coordination polymer is stable enough not to break down to the oligomeric or monomeric fragments. However, further addition of Pd (Pd/Dn ) 1.5:1) decreased overall peak intensities with the peak shifting to the lower (17) Lee, K.; Suh, M. P.; Suh, J. J. Polym. Sci. Polym. Chem. 1997, 35, 1825-1830.

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Figure 5. XPS spectra of the N 1s region for dendrimer 3 (a) and the Pd coordination polymer cast on Au substrates (b).

molecular weight region. This might indicate that excess Pd(II) shortens the coordination polymer to form the Pdended polymer, which tends to adsorb onto the stationary phase in the GPC column.18,19 From the above results, we can conclude that the fourth-generation (G4) dendrons prevent interpolymer aggregation, enhancing the solubility, and do not interfere with the Pd complexation to give a stable coordination polymer. Self-Assembly through Metal Complexation on Substrates. XPS measurements of the coordination polymers cast on a Au substrate have been performed to see whether the coordination bonds were retained even when the coordination polymers were dried on substrates. For reference, the XPS spectrum of dendrimer 3 on Au was also taken. Figure 5 demonstrates the N 1s core-level spectra. For dendrimer 3, the N 1s peak consists of two components, one for the two pyridine nitrogens appearing at 399.4 eV and the other for an amide nitrogen appearing at 400.8 eV, confirming the structure of the compound. In the case of the 1:1 coordination polymer, the N 1s peak can be curve-fitted with three component peaks, with two remaining peaks at 399.4 and 400.7 eV and a newly appearing one at 400.1 eV. The peak at 399.4 eV due to the free pyridine decreases significantly, while the higher energy peak at 400.1 eV due to the Pd-pyridine complexes dominates the signals arising from the pyridine nitrogen. Taken together with the fact that the Pd 3d signal is clearly present at 338 and 344 eV, these results reveal that the dendrimer firmly forms a 1:1 coordination polymer with Pd(II), even without solvents. This allows us to use the coordination polymer as a single, continuous molecular wire on substrates. One of our aims for this study is to link between designated areas through coordination polymers in a selfassembling manner and visualize the molecular network on surfaces. For the purpose, we have chosen individual amine-terminated poly(amidoamine) dendrimers (PAMAM) dispersed on mica as the designated area mainly for three reasons: (1) the amine-terminated dendrimers can coordinate with Pd(II) to adhere the coordination polymer; (2) the hydrophilic PAMAM dendrimer is expected to be stable on mica during the linking process using nonpolar halogenated solvents; (3) the dendrimer has a large enough size that we can distinguish the position of the dendrimer after coordination polymers are deposited on the surface by AFM. Accordingly, individual G4 PAMAM dendrimers were deposited on freshly cleaved mica by putting a few drops of 10-8 M methanolic solution of the PAMAM dendrimer (18) Dobrawa, R.; Lysetska, M.; Ballester, P.; Gru¨ne, M.; Wu¨rthne, F. Macromolecules 2005, 38, 1315-1325. (19) El-Ghayoury, A.; Schenning, A. P. H. J.; Meijer, E. W. J. Polym. Sci. Polym. Chem. 2002, 40, 4020-4023.

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Figure 6. Tapping-mode AFM images of individual PAMAM G4 dendrimers on mica (a), Pd coordination polymer with dendrimer 3 on the PAMAM dendrimer isolated surface before (b) and after (c) being treated with CH2ClCH2Cl solution containing pyridine. The coordination polymer bridging between PAMAM dendrimers is squared.

on the substrate, immediately followed by drying with N2 gas stream. The AFM height image of G4 PAMAM on mica reveals that the isolated G4 dendrimers were spread homogeneously over mica with the measured height of 0.35 ( 0.15 nm, though some aggregations with the height of about 1.0 nm were also observed (Figures 6a and S1). Considering of the ideal sphere diameter of 4.5 nm demonstrates that the G4 dendrimers are extensively deformed on the surface, as observed elsewhere.20-23 Then, the substrate was immersed into a 10-6 M CH2Cl2 solution containing the 1:1 coordination polymer for 5 min, followed by sonication in clean CH2ClCH2Cl to remove physisorbed species on mica for 15 min. As a result, single coordination polymers appeared as fibrous structures with almost homogeneous width (ca. 20 nm) and average height (ca. 1.3 nm) in the AFM image (Figures 6b and S1). In contrast, the apparent length shows a wide distribution from a dotlike structure to a 200-nm-long, wirelike structure. As a control experiment, the coordination polymer was deposited on mica without G4 PAMAM, revealing inhomogeneous deposition; i.e., there are aggregate coordination polymers on some areas and nothing observed on other large areas in the AFM image (not shown). So we think that the uniform spread of the coordination polymer in Figure 6 is responsible for the presence of PAMAM G4 dendrimer. By taking a close look at the AFM image, we can see ca. 1.8-nm-high, bright dots in the edge or in the middle of the fibrous structures, which might correspond to PAMAM G4 dendrimers (AFM average height 0.35 nm) overlapped with the coordination polymer (AFM average height 1.3 nm) through Pd complexation, although there seems to be some physisorbed coordination polymers with no such bright dot. From this point of view, some regions can be interpreted to show the bridging of the coordination polymer between PAMAM G4 dendrimers on mica (for example, one marked by a black square).

Interestingly, 5-min sonication of the coordination polymer surface in 2 mL of CH2ClCH2Cl containing 30 µL of pyridine removed all the fibrous structures from the surface, although sonication without pyridine did not show such a drastic change on the surface (Figure 5c). This might be caused by the preferable coordination of pyridine to Pd(II) over dendrimer 3, resulting in breaking of the coordination bonds to make free monomeric dendrimers in the solution. It suggests that dendrimer 3 forms tight coordination bonds with Pd(II) in an air and in noncoordinating solvent on the surface, which is consistent with the XPS results mentioned above.

(20) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492-4501. (21) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323-5324. (22) Mecke, A.; Lee, I.; Baker, J. R., Jr.; Holl, M. M. B.; Orr, B. G. Eur. Phys. J. E. 2004, 14, 7-16. (23) Betley, T. A.; Holl, M. M. B.; Orr, B. G.; Swanson, D. R.; Tomalia, D. A.; Baker, J. R. Langmuir 2001, 17, 2768-2773.

Supporting Information Available: Sectional views at the lines shown in AFM images of individual PAMAM G4 dendrimers on mica and Pd coordination polymer with dendrimer 3 on the PAMAM dendrimer isolated surface (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

Conclusions We synthesized and designed the dendron-functionalized ligand based on a rigid 4,4′-bispyridine as a monomeric ligand in order to bestow solubility on the resulting coordination polymer and found that it formed a wellsoluble 1:1 coordination polymer with Pd(II) in organic solvents by 1H NMR, UV-vis, and fluorescence titration experiments. The average DP was estimated to be about 10, which could be shown by GPC analysis. Linking between isolated PAMAM G4 dendrimers on mica through the linear 1D coordination polymer was visualized by AFM and found to be continuous in an air environment and noncoordinating solvent. Although the molecular linking takes place randomly at the moment, we think that interconnection of functional molecules through a molecular wire in a self-assembling manner could be an important tool to realize molecular electronic devices. Acknowledgment. We acknowledge Dr. W. Mizutani and Dr. R. Nagahata (National Institute of Advanced Industrial Science and Technology, Japan) for kindly allowing us to use the AFM instrument and MALDI-TOF mass spectrometer.

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