Supramolecular Assemblies of Tetrahydroxyloligo ... - ACS Publications

Jun 10, 2010 - Figure 1c is a high-resolution STM image of the D2 domain. In comparison with the D1 domain, obviously, the molecules are differently ...
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Supramolecular Assemblies of Tetrahydroxyloligo(phenyleneethynylene) with Cross-Shaped Side Chains and Its Coadsorption with Diacids on Graphite Zhun Ma,† Yi-Bao Li,‡ Ke Deng,‡ Sheng-Bin Lei,*,§ Yan-Yan Wang,† Pei Wang,† Yan-Lian Yang,‡ Chen Wang,*,‡ and Wei Huang*,| Jiangsu Key Laboratory for Organic Electronics & Information Displays (KLOEID) and Institute of AdVanced Materials (IAM), Nanjing UniVersity of Posts & Telecommunications (NUPT), Nanjing 210046, P. R. China, The Academy of Fundamental and Interdisciplinary Science, Harbin Institute of Technology, Harbin, 150080, P. R. China, Institute of AdVanced Materials (IAM), Fudan UniVersity, Shanghai 200433, P. R. China, and National Center of Nanoscience and Technology, Beijing 100080, P. R. China ReceiVed: January 10, 2010; ReVised Manuscript ReceiVed: May 5, 2010

An oligo(phenyleneethynylene), with hydroxyl as end-groups of cross-shaped four alkoxy side chains (OHOPE), was designed and synthesized to investigate the effects of the symmetry of molecular structure as well as the hydrogen bonding and interaction between alkoxy side chains on its supramolecular assemblies by scanning tunneling microscopy (STM). OH-OPEs fabricate by themselves two distinct patterns on highly orientated pyrolytic graphite (HOPG) surface with dissimilar surface coverages, symmetries, and stabilities. The surface coverage of both patterns shows a clear dependence on the concentration of OH-OPE. Contrary to the general concept, in the present case, the D1 pattern with higher packing density is favored at low concentration, which is possibly due to the existence of specific interactions (H-bonds) between the molecules and the difference in the number of H-bonds in these patterns. Coadsorption of diacids of shorter alkyl chains could help to stabilize the low density, porous pattern, whereas diacids with too long alkyl chains destabilize it. Due to the flexibility of the side chains, the coadsorption of shorter diacids does not significantly change the packing of OH-OPE. Our results point to the conclusion that the coadsorption is due to host-guest accommodation rather than hydrogen bond complexation. Introduction Conjugated polymers are a novel class of organic semiconductors that combine the optical and electronic properties of semiconductors with the processing advantages and mechanical properties of polymers. They open a wide range of applications in organic electronics and optoelectronics such as electroluminescent devices,1 organic thin film transistors,2 solid-state plastic lasers and polymer-based photovoltaic cells.3,4 The self-assembly of π-conjugated macromolecules on solid substrates offers a strategy for the construction of well-defined and stable nanometer-sized structures with chemical functionalities and physical properties that are of potential use as active components in electronic devices.5-9 These processes will play a critical role in the so-called “bottom-up” strategy of nanofabrication. Therefore, understanding the organization of molecules on solid surfaces is essential, from both a fundamental and application perspective, for the construction of surface molecular nanostructures and nanodevices.10,11 In this context, due to their easier processability, oligomers are commonly investigated as model compounds of their related macromolecules.12 In particular, p-phenyleneethynylene derivatives have received special attention,13-16 besides their interesting optoelectronic properties, they exhibit a remarkable stiffness and * Corresponding authors. W.H.: tel, +86-25-8586 6008; fax, +86-258586 6999; e-mail: [email protected] or [email protected]. C.W.: tel/fax, +86-10-6256 2871; e-mail: [email protected]. S.B.L.: e-mail: [email protected]. † Fudan University. ‡ National Center of Nanoscience and Technology. § Harbin Institute of Technology. | Nanjing University of Posts & Telecommunications (NUPT).

linearity along the conjugated backbone17,18 that allows them to self-assemble into well-defined nanostructures19-24 and make them candidates for molecular nanowires in molecular scale electronic devices.25-27 The submolecular resolution capability of scanning tunneling microscopy (STM) makes it the technique of choice to investigate the molecular arrangements in the self-assembled monolayers (SAMs).28,29 In this work, we designed and synthesized an oligo(phenyleneethynylene) (OPE) to which crossshaped four alkoxy side chains with hydroxyl end groups were attached, and the effects of the molecular structure as well as the hydrogen bonding and interaction between alkoxy side chains on assembly patterns were explored by STM. Hydrogen bonding is one of the primary links among adjacent molecules and in many cases governs the resulting structure of surface assembly. With this specific system, by inserting flexible alkoxy chains between the hydroxyl group and rigid OPE backbone, we expect to tune the balance of the hydrogen bonding and close packing principle,30 which is critical in the assembling process. Herein we demonstrate some novel self-assembly structures of this conjugated multihydroxyl OPE (OH-OPE) directed by hydrogen bonds and alkoxy interactions, and also its coadsorption with diacids at the octylbenzene-graphite interface. Hopefully, this oligomer as an example may provide further information on OPE self-assembly at the interface dominated by molecular symmetry, host-guest accommodation, and side chains interactions. Experimental Section Materials and Measurements. Chemicals and reagents were purchased from Aldrich and Acros Chemical Co. and used

10.1021/jp1002353  2010 American Chemical Society Published on Web 06/10/2010

Assemblies of Tetrahydroxyloligo(phenyleneethynylene) without further purification unless otherwise stated. All of the solvents were used after purification according to conventional methods when required. 1H NMR spectra were collected on a Varian Mercury plus 400 spectrometer with chemical shifts being referenced against tetramethylsilane as the internal standard. MALDI-TOF (matrix-assisted laser desorption/ionization time-of-flight) mass spectra were carried out using a Shimadzu AXIMA-CFR plus time-of-flight mass spectrometer (Kratos Analytical, Manchester, U.K.). 2′,4′,6′-Trihydroxyacetophenone monohydrate (THAP) was selected as the matrix, and no salt was added for these conjugated oligomers. Synthesis of Oligomers. 1,4-Bis(phenoxyhexan-1-ol)-2,5diiodobenzene (1). 1,4-Diiodo-2,5-hydroquinone (1.0 g, 2.55 mmol) was dissolved in KOH/EtOH (0.343 g/10 mL), and the solution was stirred at room temperature. After 1 h, 3-bromo1-propanol (343 mg, 6.12 mmol) was added. The mixture was stirred at 50 °C. After 16 h, the mixture was cooled to room temperature, concentrated, and redissolved in EtOAc (20 mL). The solution was washed with NaOH (2 M, 4 × 10 mL), dried over anhydrous Na2SO4, and concentrated again to give the crude product. It was recrystallized in alcohol to afford colorless crystals (1.0 g, yield 70%). 1H NMR (CDCl3, ppm): δ 7.16 (s, phenyl H, 2H), 3.93 (t, phenyl-O-CH2-, 4H,), 3.66 (t, -CH2-OH, 4H,), 1.83-1.79 (m, phenyl-O-CH2-CH2-, 4H,), 1.63-1.24 (m, -CH2-, 12H). 3-(4-Iodophenoxy)propan-1-ol. 4-Iodophenol (2.21 g, 10 mmol) was dissolved in KOH/EtOH (0.684 g/10 mL), and the solution was stirred at room temperature. After 1 h, 3-bromo1-propanol (0.9 equiv) was added. The mixture was stirred at 50 °C for 16 h. Then, the mixture was cooled to room temperature, concentrated, and redissolved in EtOAc (20 mL). The solution was washed with NaOH (2 M, 4 × 10 mL), dried over anhydrous Na2SO4, and concentrated again to give the crude product. It was recrystallized in alcohol to afford colorless crystals (1.78 g, yield 71%). 1H NMR (CDCl3, ppm): δ 7.38 (d, phenyl H, 2H), 6.82 (d, phenyl H, 2H), 4.12 (t, phenyl-O-CH2-, 2H), 3.86 (t, -CH2-OH, 2H), 2.05 (m, -CH2-, 2H). 3-[4-(2-(Trimethylsilyl)ethynyl)phenoxy]propan-1-ol. To a solution of 3-(4-iodophenoxy)propan-1-ol (1.4 g, 5 mmol) and trimethyl(prop-1-ynyl)silane (0.735 g, 7.5 mmol) in 20 mL of diisopropylamine (DIPA) were added Pd(PPh3)4 (230 mg, 0.2 mmol) and cuprous iodide (38.4 mg, 0.2 mmol). The mixture was stirred overnight under nitrogen at 70 °C. The residue was treated with water and extracted with chloroform, and the extracts were dried over anhydrous Na2SO4. After the solvent was removed under reduced pressure, the residue was purified by column chromatography on silica gel with hexane/CH2Cl2 (1:1) to give the product as a yellow liquid (0.92 g, yield 74%). 1 H NMR (CDCl3, ppm): δ 7.38 (d, phenyl H, 2H), 6.82 (d, phenyl H, 2H), 4.12 (t, phenyl-O-CH2-, 2H), 3.86 (t, -CH2-OH, 2H), 2.05 (m, -CH2-, 2H), 0.24 (s, -Si(CH3)3, 9H). 3-(4-Ethynylphenoxy)propan-1-ol (2). To a solution of 3-[4(2-(trimethylsilyl)ethynyl)phenoxy]propan-1-ol (1.0 g, 4.026 mmol) in 5 mL of THF were added methanol (10 mL) and NaOH (2 mL, 5 M). The mixture was stirred at room temperature for 1 h. After the solvents were evaporated, the residue was poured into 100 mL of water and extracted with hexane twice. The combined hexane layer was washed with water and brine and dried over anhydrous Na2SO4. The pale yellow solid was obtained after the solvent was removed (0.69 g, yield 97%). 1H NMR (CDCl3, ppm): δ 7.42 (d, phenyl H, 2H), 6.84 (d, phenyl H, 2H), 4.12 (t, phenyl-O-CH2-, 2H),

J. Phys. Chem. C, Vol. 114, No. 26, 2010 11461 3.86(t, -CH2- OH, 2H), 3.00 (s, -CtH, 1H), 2.04 (m, -CH2-, 2H). OH-OPE. The target product OH-OPE was prepared by the Sonogashira cross-coupling reaction from monomer 1 (2 g, 3.56 mmol) and monomer 2 (1.57 g, 8.90 mmol), in the presence of 4% Pd(PPh3)4 (164 mg) and 4% CuI (27 mg) in 10 mL of DIPA at 60 °C for 2 days. The residue was treated with water and extracted with chloroform, and the extracts were dried over anhydrous Na2SO4. After the solvent was removed under reduced pressure, the residue was purified by column chromatography on silica gel with hexane/CH2Cl2 (1:4) to give OHOPE as a yellow solid and recrystallized in acetone to give the light yellow crystal (1.41 g, yield 60%). 1H NMR (CDCl3, ppm): δ 7.39 (d, phenyl H, 4H), 7.01 (s, phenyl H, 2H), 6.95 (d, phenyl H, 4H), 4.11 (t, phenyl-O-CH2-, 4H), 4.05 (t, phenylO-CH2-, 4H), 3.74 (t, -CH2-OH, 4H), 3.54 (t, -CH2- OH, 4H), 1.98 (m, -CH2-, b4H), 1.85-1.35 (m, -CH2-, 16H). MALDI-TOF MS (m/e): 658.7 (M•+, 100%). Purity: 99.0%. STM Observation. STM measurements were performed using a Multimode Nanoscope IIIa scanning probe microscope (Digital Instrument, Veeco Metrology Group, Santa Barbara, CA) with mechanically cut Pt/Ir (80/20) tips at ambient temperature. All images were recorded in the constant current mode. For the measurement at the solution-substrate interface, because of the poor solubility in pure octylbenzene, a mixed solvent (octylbenzene/acetone ) 3:1) was employed to dissolve OH-OPE (∼1 mg/mL) and then a droplet of the solution was deposited onto a freshly cleaved surface of highly orientated pyrolytic graphite (HOPG, Digital Instrument Co.), waiting for 10 min for the evaporation of acetone before STM measurement. For the concentration control experiments, the starting solution concentration is 3 × 10-3 mol/L, whereafter the sample solution was diluted by octylbenzene followed by the above-mentioned sample deposition and test procedure. The tunneling parameters are given in the corresponding figure captions. Different tips and samples were used to check the reproducibility and to exclude image artifact caused by the tips or sample. Computational Details. We performed theoretical calculation using density functional theory (DFT) provided by the DMol3 code.31 The Perdew and Wang parametrization32 of the local exchange-correlation energy are applied in the local spin density approximation (LSDA) to describe exchange and correlation. We expand the all-electron spin-unrestricted Kohn-Sham wave functions in a local atomic orbital basis. In such double-numerical basis set polarization is described. All calculations are all-electron ones and performed with the ExtraFine mesh. The self-consistent field procedure is done with a convergence criterion of 10-5 au on the energy and electron density. Results and Discussion 1. Synthesis and Structure Characterization. The synthetic route of OH-OPE is shown in Scheme 1. This oligo(phenyleneethylene) derivative was prepared according to the traditional Sonogashira cross-coupling reaction. The characterization of the target compound by usual NMR and mass spectrometry gave results in full agreement with the expected chemical structure. Furthermore, the final product OH-OPE was also characterized by HPLC for its purity. 2. Self-Assembly at the Interface. 2.1. Packing Structures of OH-OPE. Figure 1 shows STM images of a self-assembled monolayer (SAM) of OH-OPE at octylbenzene/HOPG interfaces. Figure 1a is a large-scale image in which two different domains (marked as D1 and D2, respectively) with strikingly different contrasts and symmetries can be discerned in the

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SCHEME 1: Synthesis of OH-OPEa

a

Key (i) EtOH, NaOH, 70 °C; (ii) DIPA, Pd(PPh3)4, CuI, 60 °C; (iii) CH3OH, THF, NaOH (5 M), room temperature.

Figure 1. Large scale STM image of OH-OPE molecule on HOPG (a) (101 × 101 nm2, Iset ) 335.7 pA, Vbias ) -680.8 mV) showing two different assembling patterns. High-resolution STM image of D1 (b) (15 × 15 nm2, Iset ) 709.9 pA, Vbias ) 716.6 mV) and D2 domain (c) (11.6 × 11.6 nm2, Iset ) 537.9 pA, Vbias ) 716.6 mV). (d) and (e) are the tentative molecular models for these two 2D packings. The definitions of some structural parameters are indicated in the images.

scanning area. Figure 1b shows a representative high-resolution STM image of the linear D1 pattern at the interface. The bright stripes and the dark area are alternating over the whole viewed area. The bright stripes correspond to the π-conjugated molecular backbones, which are characterized by a larger electronic density, and the dark parts can be assigned to the alkoxy chain moieties.20,22 Due to little difference in electronic density of alternating phenylene and ethynylene groups, they both appear with high contrast and we cannot distinguish one from the other in the STM image. From high resolution STM images, a unit cell with parameters of a ) 2.3 ( 0.1 nm, b ) 1.2 ( 0.1 nm, and R ) 70 ( 1° could be determined for the D1 pattern (Figure 1b). The assembly structures of linear type OPEs substituted with alkyl chains are most commonly observed to be dominated by alkyl chain interdigitation, which leads to linear lamellae-like packing where the intermolecular distance between OPEs along the lamellae is determined by the alkyl chain length. However, in the case of OH-OPE studied here, the 1.2 nm intermolecular distance between OH-OPEs along the lamellae is significantly

smaller than that expected from alkyl chain interdigitation (1.6 nm, Figure 2c). It is well-known that in the surface assembling of aliphatic alcohols the hydrogen bonds between hydroxyl groups play an important role, leading to herringbone or parallel packing; in both cases the molecules are packed head-to-head to favor hydrogen bond formation. Due to the existence of hydroxyl groups at the end of the alkyl substituents, hydrogen bonding must play a role in the surface assembly of OH-OPE. Several possible 2D arrangements with different extents of hydrogen bonding are listed in Figure 2a-c. In all these models OH-OPE is lying flat on the surface with its molecular plane parallel to the surface and the alkoxy substituents fully lie on the surface too. Figure 2b represents an arrangement with maximized alkoxy chain interdigitation, in the same time hydrogen bonds could also be formed between the propanol chains, while Figure 2c shows an arrangement in which hydrogen bonding is maximized. From the unit cell parameters of these arrangements (Table 1) it could be clearly seen that the packing density of the structure shown in Figure 2b,c is lower than that observed in the D1 pattern. From the high resolution STM image in Figure 1b, the width of the dark stripes (∆L, as indicated in the image) could be determined to be 0.7 nm, in agreement with the model in Figure 2b, where the propanol chains are supposed to interidigitate. But the 1.2 nm intermolecular distance along the lamellae could not allow interdigitation of the longer hexanol chains, thus we suppose the hexanol chains are not fully extended; instead gauche conformations are present, as shown in Figure 2a. This results in an intermolecular distance of 1.2 nm, agrees quite well with the STM observation. Besides increasing the surface density, this alignment also enables hydrogen bonding between the hydroxyl groups, further stabilizing the network. Our DFT simulations indicate the adsorption energy per surface area for the model in Figure 2a (10.31 (kcal/mol) · nm2) is significantly larger than that of the model shown in Figure 2b (7.26 kcal/ mol · nm2) and Figure 2c (3.56 (kcal/mol) · nm2). Figure 1c is a high-resolution STM image of the D2 domain. In comparison with the D1 domain, obviously, the molecules are differently arranged, and also the submolecular features could be visualized with more detail. Besides the bright aromatic backbone, the alkyl substituents can be revealed with two different orientations, resulting in two different spaces between neighboring molecular rows. In the image the narrower one is marked as D, while the whole repeating period is marked P. The values of D and P are measured to be 1.6 and 3.7 nm,

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Figure 2. Molecular models show the optimized model of OH-OPE in the D1 pattern (a), and packing of OH-OPE dominated by alkyl chain interdigitation (b) and hydrogen bonds (c).

TABLE 1: Summary of Structural Parameters of the 2D Networks Assembled by OH-OPE and Coadsorption of OH-OPE with Diacids sample OH-OPE (D1)

D (nm)a

P (nm)b

θ (deg)c

a (nm)

b (nm)

R (deg)

28 ( 1 30 ( 1 31 ( 1

2.3 ( 0.1 2.4 2.3 2.3 3.9 ( 0.1 3.6 ( 0.1 3.8 ( 0.1

1.2 ( 0.1 1.2 1.5 2.1 2.0 ( 0.1 2.0 ( 0.1 2.0 ( 0.1

70 ( 1 74 76 89 74 ( 1 70 ( 1 78 ( 1

d

OH-OPE (D2) OH-OPE/C12H24(COOH)2 OH-OPE/C6H12(COOH)2

1.6 ( 0.1 1.6 ( 0.1 1.8 ( 0.1

3.7 ( 0.1 3.7 ( 0.1 3.7 ( 0.1

a Distance between two OH-OPE molecules (see figures). b Period of the pattern (see figures). c Angle between OH-OPE chain axis and the packing orientation. d The first line corresponds to the unit cell parameters of D1 pattern measured from the STM images, and the three lines below correspond to unit cell parameters of molecular models shown in Figure 2a-c respectively.

respectively. The angle between the differently orientated alkyl chains, measuring about 120°, clearly indicates the alkyl chains are orientated according to the 3-fold symmetry of the graphite substrate. The unit cell parameters are determined to be a ) 3.9 ( 0.1 nm, b ) 2.0 ( 0.1 nm, and R ) 74 ( 1°. A tentative model of the D2 pattern is shown in Figure 1e. In this model, different from the expected cross-like conformation, one of the hexanol chains is orientated about 30° with respect to the OPE backbone. Two hydrogen bonds are formed between this tilted hexanol and propanol chain from another molecule and vice versa. Instead of interdigitation, the other hexanol, which orientated nearly perpendicular to the backbone, forms a hydrogen bond with the hexanol group from an adjacent molecule on the other side (see Figure 1e for details). This results in a porous network with lower surface density than for the D1 pattern. The D and P values in this model measuring 1.5 and 3.8 nm, with the unit cell parameters a ) 3.9 ( 0.1 nm, b ) 2.2 ( 0.1 nm, and R ) 80 ( 1°, agree quite well with the STM observations. In comparison with D1 domains, D2 networks are more stable against tip scanning. Considering the porous nature of this structure, the high stability is quite unusual. The cavities in the network must be filled by some other species, most probably the solvent, which serves to stabilize the network. 2.2. Concentration Dependent Measurements. At the solid-liquid interface, the relative surface coverage and stability of different phases can strongly depend on solute concentration.33-35 Since OH-OPE can form two patterns with different packing densities, it is rational that a concentration effect also exists for the self-assembly of OH-OPE. As shown in Figure 3, adjusting the concentration indeed affects the ratio of the two patterns on the surface. At high concentration (3 × 10-3 mol/L), the D2 phase is dominant; on the contrary, at low concentration (3 × 10-6 mol/L) the D1 pattern was exclusively observed. In the medium ranges, both patterns coexist and the ratio of D2 to D1 apparently drops with the decrease of solution concentration. In Figure 3, as the concentration decreases from 3 × 10-3 to 3 × 10-6 mol/L, the surface coverage of D1 varies from around 0 to 15%, 76%. and 100%, respectively. The concentration effect at the solid/liquid interface is directly related to the difference

Figure 3. Large scale STM images of OH-OPE molecule on HOPG at different solution concentrations: (a) 3 × 10-3 mol/L (78.9 × 78.9 nm2, Iset ) 506.6 pA, Vbias ) -500.2 mV); (b) 3 × 10-4 mol/L (115 × 115 nm2, Iset ) 506.6 pA, Vbias ) -500.2 mV); (c) 3 × 10-5 mol/L (129 × 129 nm2, Iset ) 335.7 pA, Vbias ) -680.8 mV); (d) 3 × 10-6 mol/L (102 × 102 nm2, Iset ) 396.7 pA, Vbias ) -590.5 mV).

in stability between the patterns and their respective packing densities. In general, a decrease in the concentration favors formation of the pattern with lower packing density. However, in the case of OH-OPE the D1 pattern with higher packing density is favored at low concentrations. Currently, we do not have an interpretation for this apparently unusual phenomenon, one possible reason is the existence of specific interactions (Hbonds) between the molecules and the difference in the number of H-bonds in these patterns.

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Figure 4. Large scale (a) and (c) and high resolution (b) and (d) STM images of the assembly resulting from coadsorption of OH-OPE with tetradecanedioic acid and octanedioic acid, respectively: (a) 157 × 157 nm2, Iset ) 317.4 pA, Vbias ) 699.8 mV; (b) 50.6 × 50.6 nm2, Iset ) 317.4 pA, Vbias ) 693.3 mV; (c) 87.3 × 87.3 nm2, Iset ) 455.8 pA, Vbias ) 604.0 mV; (d) 33.6 × 33.6 nm2, Iset ) 504.7 pA, Vbias ) 604.0 mV.

2.3. Coadsorption Structures of OH-OPE with Diacids. Surface confined 2D porous networks and their host-guest behavior toward proper guest molecules have attracted a lot of interest recently.36-40 These networks are built either with rigid building blocks connected by hydrogen bonding or coordination interactions or with flexible building blocks based on van der Waals interactions between flexible alkyl chains. However, 2D networks built with flexible blocks but via relatively strong hydrogen bonds will both enhance the stability of the network and enable it with flexibility to adjust itself in response to the accommodation of guest molecules. This triggers us to investigate the coadsorption of OH-OPE with diacids, involving tetradecanedioic acid, octanedioic acid, and docosanedioic acid [HOOC(CH2)nCOOH, n ) 12, 6, 20] dissolved in octylbenzene, to test how the flexible, hydrogen bonded network responds to the coadsorption of these linear guests. STM images of molecular adlayers formed from octylbenzene solutions of OH-OPEs with tetradecanedioic acid or octanedioic acid are shown in Figure 4. Now the linear D1 pattern with higher density does not appear at all; the whole surface is covered with the D2 type pattern. In the large-scale STM images (Figure 4a,c), the molecular network extends over the atomically flat terrace of the HOPG. And no structure resulting from the single-component monolayer of diacids was observed in both cases. The alternant bright and dark linear stripes are ascribed to conjugated backbones of OH-OPEs and alkyl chains on OHOPEs or diacids, respectively.41 The molecular orientation is also induced by the registry of alkyl chains to the graphite lattice. Comparable with those for the D2 pattern of OH-OPEs, the analogous parameters, the distance (D) between two OH-OPEs and the period (P) of the patterns of the OH-OPEs blending with diacids, are also listed in Table 1. Obviously, the coadsorption of diacids does not change the assembly structure significantly, but only slightly varies the spacing between neighboring molecules (intertape separation D). The role of the

Ma et al. diacids is to stuff the voids of the porous network and thus enable higher stability. The complete transformation into the D2 type pattern suggests that the cocrystallized host-guest D2 type architecture is thermodynamically more stable than the D1 pattern of OH-OPE and the lamellae pattern of diacids. From the high resolution images, the unit cell parameters of OH-OPE/C12H24(COOH)2 and OH-OPE/C6H12(COOH)2 complexes are determined to be a ) 3.6 ( 0.1 nm, b ) 2.0 ( 0.1 nm, R ) 70 ( 1° and a ) 3.8 ( 0.1 nm, b ) 2.0 ( 0.1 nm, R ) 78 ( 1°, respectively. Comparing the parameters of these two complexes with those of the pure OH-OPE monolayer, we see that the length of unit cells in direction b remains unchanged but the angle R and the value a change slightly with the interposition of diacids. Consequently, the interspace (D) varies from 1.6 to 1.8 nm; however, the distance of the period (P) of the two new patterns remains as 3.7 nm. Though submolecular resolution can be easily achieved on the host-guest complexes and many features with alkyl-chain-like characteristics were revealed, unfortunately due to the lack of functional groups that can help to distinguish them from each other (normally hydroxyl and carboxyl groups both appear with more or less darker contrast than alkyl chains in STM images), it is difficult to assign the accurate arrangement of each diacid molecule and therefore impossible to clarify the hydrogen bonding interaction between diacids and OH-OPEs that might contribute to the formation of OH-OPEs/CnH2n(COOH)2 (n ) 12, 6) complexes. However, considering the similarity of unit cell parameters, it is reasonable to assume that the tentative model in Figure 1e also applies to complexes, with minor variation of the conformation of OHOPEs. From the tentative model of the D2 pattern, two cavities with different sizes could be identified, as shown in Figure 1e. The larger one with a maximum length of 1.9 nm and the smaller one with a length of 1.5 nm are both parallel to the alkyl chains of OH-OPE in the pattern. According to the length of tetradecanedioic acid (1.8 nm) and octanedioic acid (1.0 nm), they could fill the larger or both cavities of the D2 pattern. One thing noticeable is the orientation of the hexanol chains on both sides of the OPE backbone could adjust their orientation to some extent in response to the coadsorption of the diacids. This kind of fitting behavior in response to accommodation of a guest has also been observed on other 2D nanoporous networks built with flexible building blocks.42-45 On the basis of the above results, we attributed the coadsorption of OH-OPE with diacids to accommodation of diacids by the OH-OPE porous network, rather than the hydrogen bonded complexation between carboxyl and hydroxyl groups. Codeposition of OH-OPE with diacid with increased length like docosanedioic acid [C20H40(COOH)2] did not show any regular pattern at the interface. We ascribe this to the too long length of docosanedioic acid (2.8 nm), which exceeds the size of the pores in the D2 pattern of OH-OPEs. It confirms our hypothesis that the coadsorption of diacids with OH-OPE is due to host-guest accommodation rather than hydrogen bond complexation. Conclusion In summary, an oligo(phenyleneethynylene) with crossshaped multihydroxyl side chains was synthesized, followed by the characterization of its morphologies and self-assembled patterns at the octylbenzene-HOPG interface by means of STM. Two distinctive patterns were observed at the solid/liquid interface, and their coverage showed a clear dependence on the solute concentration. The surface coverage of D1 increased

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