Influence of Substituents on Two-Dimensional Ordering of Oligo

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Langmuir 2004, 20, 8892-8896

Influence of Substituents on Two-Dimensional Ordering of Oligo(phenylene-ethynylene)ssA Scanning Tunneling Microscopy Study Zhongcheng Mu, Xunyu Yang, Zhiqiang Wang, and Xi Zhang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China, and Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130021, People’s Republic of China

Jinling Zhao and Zhishan Bo* State Key Lab of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China Received June 14, 2004. In Final Form: July 26, 2004 Two-dimensional (2D) assemblies of alkoxy-substituted oligo(phenylene-ethynylene)s bearing different substituents adsorbed on highly oriented pyrolytic graphite (HOPG) were studied by using scanning tunneling microscopy. It was found that the introduction of different endgroups or a biethynylene linkage into oligo(phenylene-ethynylene)s can significantly change their 2D ordering on HOPG. The carboxylic endgroups can direct the conjugated oligomers to form ordered lines through intermolecular hydrogen bonding. The possibility of controlling the 2D assemblies of conjugated molecules is of importance in designing organic optoelectronic devices.

Introduction π-Conjugated oligomers and polymers, such as oligothiophene1,2 and p-phenylenethynylene derivatives,3 present unique electronic properties which open a wide range of applications both in optoelectronics and photonics.1,4 Specifically, besides their interesting optoelectronic properties, p-phenylenethynylene derivatives show a remarkable stiffness and linearity along the conjugated backbone, which allows them to self-assemble into welldefined nanostructures5 and become candidates for molecular nanowires in molecular-scale electronic devices.6 * To whom correspondence should be addressed at Tsinghua University. Tel.: +86-10-62796283. Fax: +86-10-62773155. Email: [email protected]. (1) Garnier, F.; Hajlaoui, R.; Yassa, A.; Srivastava, P. Science 1994, 265, 1684. (2) (a) Biscarini, F.; Zamboni, R.; Samorı´, P.; Ostoja, P.; Taliani, C. Phys. Rev. B 1995, 52, 14868. (b) Ba¨uerle, P.; Fischer, T.; Bidlingmeier, B.; Stabel, A.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 303. (3) (a) Kondo, K.; Okuda, M.; Fujitani, T. Macromolecules 1993, 26, 7382. (b) Tour, J. M. Chem. Rev. 1996, 96, 537. (c) Giesa, R. J. Macromol. Sci. Rev. Chem. Phys. 1996, C36, 631. (d) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605. (4) (a) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539. (b) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (c) Stabel, A.; Herwig, P.; Mu¨llen, K.; Rabe, J. P. Angew. Chem., Int. Ed. Engl. 1995, 34, 1609. (d) Lidzey, D. G.; Bradley, D. D. C.; Alvarado, S. F.; Seidler, P. F. Nature 1997, 386, 135. (e) Friend, R. H.; Gymer, R. W.; Holmes, A. B.; Burroughes, J. H.; Marks, R. N.; Taliani, C.; Bradley, D. D. C.; Dos Santos, D. A.; Bre´das, J. L.; Lo¨gdlund, M.; Salaneck, W. R. Nature 1999, 397, 121. (5) (a) Kim, J.; McHugh, S. K.; Swager, T. M. Macromolecules 1999, 32, 1500. (b) Samorı´, P.; Francke, V.; Mu¨llen, K.; Rabe, J. P. Chem. Eur. J. 1999, 5, 2312. (c) Samorı´, P.; Severin, N.; Mu¨llen, K.; Rabe, J. P. Adv. Mater. 2000, 12, 579. (d) McQuade, D. T.; Kim, J.; Swager, T. M. J. Am. Chem. Soc. 2000, 122, 5885. (e) Steffen, W.; Bunz, U. H. F. Macromolecules 2000, 33, 9518. (f) Kim, J.; Swager, T. M. Nature 2002, 411, 1030. (g) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31, 52.

It is well-known that a fine-tuning of the performance of molecular-based devices depends on the spatial arrangement of the molecules. Therefore, it is a very active topic to fabricate highly ordered, reproducible, and stable supramolecular structures through interfacial self-assembly of functional molecules on flat solid substrates. Scanning tunneling microscopy (STM) has become an essential tool to study two-dimensional (2D) orientation and defects of molecules at an interface on the submolecular scale. The molecular conductivity can also be measured directly with STM by probing the resistance within the self-assembled organic monolayers.6a,7 Samorı´ et al. have investigated the self-assembly of end-functionalized poly(para-phenyleneethynylene)s on flat solid crystalline substrates5b,5c and studied the solid-state structures of functionalized phenyleneethynylene trimers, including the single crystals and monolayers at the solidliquid interface, by means of X-ray diffraction and STM, respectively.8 Gong et al. have observed morphologies and structures of self-assembled monolayers of two conjugated oligo(phenylene-ethynylene)s by STM,9 and Kim et al. explored the structure-property correlations of monolayers and multilayers formed by conjugated poly(phen(6) (a) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1075. (b) Dhirani, A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319. (c) Cygan, M. T.; Dunbar, T. D.; Arnold, J. J.; Bumm, L. A.; Shedlock, N. F.; Burgin, T. P.; Jones, L., II; Allara, D. L.; Tour, J. M.; Weiss, P. S. J. Am. Chem. Soc. 1998, 120, 2721. (7) (a) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506, 2857. (b) Datta, S.; Tian, W.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. Phys. Rev. Lett. 1997, 79, 2530. (8) Samorı´, P.; Francke, V.; Enkelmann, V.; Mu¨llen, K.; Rabe, J. P. Chem. Mater. 2003, 15, 1032. (9) Gong, J.-R.; Zhao, J.-L.; Lei, S.-B.; Wan, L.-J.; Bo, Z.-S.; Fan, X.-L.; Bai, C.-L. Langmuir 2003, 19, 10128.

10.1021/la048538w CCC: $27.50 © 2004 American Chemical Society Published on Web 08/28/2004

2D Ordering of Oligo(phenylene-ethynylene)s Chart 1. Chemical Structures of Alkoxy-Substituted Oligo(phenylene-ethynylene)s

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In the present article, we want to investigate the selfassembly of oligo(phenylene-ethynylene)s with different groups located either at two ends or in the middle of π-conjugated backbones (Chart 1) on highly oriented pyrolytic graphite (HOPG), leading to understanding the effect of substituents on 2D ordering. Hopefully, our results provide valuable information for further design and synthesis of π-conjugated polymers toward application in fabricating organic optoelectronic devices on solid surfaces. Experimental Section

ylene-ethynylene)s by using the Langmuir-Blodgett deposition technique.5a These investigations aimed at obtaining direct or indirect 2D ordering information of π-conjugated polymers and oligomers at the liquid-solid interfaces which can provide useful models to interpret the confined structure and conformation of functional polymers or oligomers at interfaces.7,10

Synthesis of Alkoxy-Substituted Oligo(phenylene-ethynylene)s. The chemical structures of oligo(phenylene-ethynylene)s 1-5 used for STM studies are shown in Chart 1. The synthesis of oligo(phenylene-ethynylene)s 2-5 is outlined in Scheme 1. Compound 2 was brought about by cross-coupling of compounds 611 and 7.12 This reaction was carried out in tetrahydrofuran (THF) at 50 °C with diisopropylethylamine as a base and Pd(PPh3)2Cl2/CuI as cocatalyst precursors. After purification, 2 was obtained in a yield of 55%. Cleavage of the trimethylsilyl (TMS) endgroups of compound 2 in THF with 2.5folds of tetrabutylammonium fluoride (TBAF) afforded compounds 3 in a yield of 81%. Compound 4 was synthesized in an 87% yield by homocoupling of compound 6 under Glaiser reaction

Scheme 1. Routes for Synthesizing Alkoxy-Substituted Oligo(phenylene-ethynylene)sa

a (a) Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, N2, and 60 °C for 24 h; (b) TBAF, THF, and room temperature for 2 h; (c) Pd(PPh3)2Cl2, CuI, THF, diisopropylamine, and 60 °C for 24 h.

Figure 1. STM images of 1 adsorbed on HOPG in 1-phenyloctane. (a) A large-scale image (31.8 nm × 31.8 nm, U ) 1.078 V, I ) 67.5 pA); (b) the high-resolution STM image (14.1 nm × 14.1 nm, U ) 1.078V, I ) 67.5 pA).

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Figure 2. STM images of 2 adsorbed on HOPG. (a) A large-scale image (56.7 nm × 56.7 nm, U ) 1.072 V, I ) 60.0 pA); (b) the high-resolution STM image (20.0 nm × 20.0 nm, U ) 1.072 V, I ) 113.9 pA).

Figure 3. STM image (29.4 nm × 29.4 nm, U ) -600 mV, I ) 100.0 pA) of 3 adsorbed on HOPG.

conditions with diisopropylethylamine as a base, Pd(PPh3)2Cl2/ CuI as cocatalyst precursors, and oxygen as an oxidant. Deprotection of 4 gave 5 in a 78% yield. Compounds 1, 2, and 4 exhibited good solubility in common organic solvents such as THF, chloroform, CH2Cl2, and 1-phenyloctane; compounds 3 and 5 showed limited solubility in solvents such as chloroform, CH2Cl2, dimethylformamide, dimethyl sulfoxide (DMSO), and 1-phenyloctane but good solubility in THF. All reagents were obtained from commercial suppliers and used without further purification unless otherwise indicated. THF was distilled from Na-benzophenone under nitrogen. Diisopropylamine and triethylamine (TEA) were distilled from CaH2. The preparation of compounds 1 and 6 were reported elsewhere.11 Thin-layer chromatography analysis was performed using silica gel HSG (F254) plates, and the eluted plates were observed under a UV detector. Chromatographic purifications were performed by flash chromatography on silica gel (200-300 mesh). 1H NMR spectra were obtained at 300 MHz using chorofrom-d1 or DMSO-d6 as the solvent. 13C NMR spectra were recorded at 75 MHz using chorofrom-d1 as the solvent. Matrixassisted laser desorption ionization time-of-flight mass spectra were recorded on a BIFLEXIV mass spectrometer.

Compound 2. To a reaction flask were charged compound 6 (57 mg, 0.0797 mmol), compound 7 (25 mg, 0.0369 mmol), Pd(PPh3)2Cl2 (0.6 mg, 0.0008 mmol), CuI (0.5 mg, 0.0026 mmol), and THF (10 mL)/diisopropylamine (4 mL) as the solvent. The mixture was stirred under N2 at 60 °C for 24 h. The solvent was removed by rotary evaporation, and the residue was purified by silica gel flash chromatography eluting with hexane/ethyl acetate (20:6, v/v) to provide 38 mg (55%) of 2 as a yellow solid.1H NMR δ 8.01 (d, 4H), 7.58 (d, 4H), 7.02 (m, 6H), 4.44 (t, 4H), 4.04 (m, 12H), 1.85 (m, 12H), 1.53 (m, 12H), 1.25 (m, 96H), 1.15 (t, 4H), 0.88 (m, 18H), 0.097 (s, 18H). 13C NMR δ 165.1, 152.7, 152.4, 152.3, 130.2, 128.7, 128.2, 126.8, 116.1, 116.0, 115.8, 113.8, 113.1, 112.1, 92.7, 90.7, 90.3, 87.8, 68.6, 68.5, 68.4, 62.2, 30.7, 28.5, 28.3, 28.2, 24.9, 24.9, 24.8, 21.5, 16.3, 12.9, -2.6. Compound 3. To a solution of 2 (20 mg, 0.011 mmol) in THF was added TBAF (0.02 mL, 0.020 mmol). The mixture was stirred at room temperature for 2 h, then poured onto a short pad of silica gel, and eluting with CH2Cl2 increasing to CH2Cl2/CH3OH (7:1, v/v) to afford 3 as a yellow solid (15 mg, 81%). MS m/z calcd for C112H166O10, 1672.5; found, 1672.0. Compound 4. To a reaction flask were charged compound 6 (30 mg, 0.042 mmol), Pd(PPh3)2Cl2 (0.3 mg, 0.00040 mmol), CuI (0.3 mg, 0.0016 mmol), THF (6 mL), and diisopropylamine (3 mL). The mixture was stirred at 60 °C for 24 h. The solvent was removed by rotary evaporation, and the residue was purified by silica gel flash chromatography eluting with hexane/ethyl acetate (4:1, v/v) to provide 4 as a yellow solid (26 mg, 87%). 1H NMR δ 8.01 (d, 4H), 7.57 (d, 4H), 7.00 (m, 4H), 4.43 (t, 4H), 4.01 (m, 8H), 1.83 (m, 8H), 1.53 (m, 8H), 1.25 (m, 64H), 1.15 (t, 4H), 0.88 (m, 12H), 0.097 (s, 18H). 13C NMR δ 166.2, 155.0, 153.7, 131.4, 130.1, 129.4, 127.9, 117.7, 117.1, 114.6, 94.7, 88.8, 79.6, 77.3, 69.9, 69.6, 63.4, 31.9, 29.7, 29.7, 29.4, 29.4, 29.3, 29.3, 29.1, 26.1, 25.9, 22.7, 17.5, 14.1, -1.5 Compound 5. To a solution of 4 (15 mg, 0.011 mmol) in THF was added TBAF (0.022 mL, 0.022 mmol). The mixture was stirred at room temperature for 2 h, then poured onto a short pad of silica gel, and eluted with CH2Cl2 and then CH2Cl2/CH3OH (7:1). A total of 10 mg (78%) of 3 was obtained as a yellow solid. MS m/z calcd for C82H114O8, 1227.8; found, 1227.7. In Situ STM Observation. STM measurements were performed using a well-known commercial Multimode Nanoscope (R) IV scanning tunneling microscope (Digital Instrument, Veeco Metrology Group, Santa Barbara, CA) with mechanically cut Pt/Ir (90:10) tips at ambient temperature. All images were recorded in the constant current mode. For the measurement at (10) Tour, J. M. Trends Polym. Sci. 1994, 2, 332. (11) Zhao, J. L.; Bo, Z. S. Eur. J. Org. Chem., submitted for publication. (12) Swager, T. M.; Gil, C. J.; Wrighton, M. S. J. Phys. Chem. 1995, 99, 4886.

2D Ordering of Oligo(phenylene-ethynylene)s

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Figure 4. STM images of 4 adsorbed on HOPG in 1-phenyloctane. (a) A large-scale image (200.0 nm × 200.0 nm, U ) 1.072 V, I ) 90.4 pA); (b) the high-resolution STM image (20.4 nm × 20.4 nm, U ) -1.080 V, I ) 90.0 pA).

Figure 5. STM image (22.8 nm × 22.8 nm, U ) -600 mV, I ) 150.0 pA) of 5 adsorbed on HOPG. the solution-substrate interface, a solution of alkoxy-substituted oligo(phenylene-ethynlene) with TMS endgroups in pure 1-phenyloctane was dropped onto a freshly cleaved surface of HOPG (Digital Instrument Co.). Because of the poor solubility of the alkoxy-substituted oligo(phenylene-ethynylene) with carboxyl groups in pure 1-phenyloctane, we used a mixed solvents (1phenyloctane/THF ) 1:2) to dissolve compound 3 or 5 and then dropped the solution onto a freshly cleaved surface of HOPG (Digital Instrument Co.), waiting for about 10 min for the evaporation of THF before in situ STM measurement. Measurement conditions are given in the corresponding figure captions. Different tips and samples were used to check the reproducibility and to ensure no image artifact caused by the tips or sample. Flattening of the images was carried out to compensate for tilting of the substrate and scan line artifacts, and a low-pass filtered transform was employed to remove scanning noise in the STM images.

Results and Discussion Alkoxy-substituted oligo(phenylene-ethynlene) bearing a single TMS group (molecule 1) forms a stable monolayer after adsorption on HOPG, as shown in Figure 1. A large-

scale STM image shows polycrystalline structure which consists of domains with different molecular orientations (Figure 1a). The bright stripes and the dark area correspond to the π-conjugated molecular backbones and the alkyl chain moieties, respectively. Bright and dim stripes align alternatively, which can be attributed to the different electronic densities of different moieties of a molecule. From the high-resolution STM image (Figure 1b), one can identify individual molecules more clearly. The length of one individual bright stripe is 2.60 nm, consistent with the length of π-conjugated molecular backbones. The alkyl chains between two adjacent bright stripes are interdigitated, supported by a distance (∆L) between two adjacent bright rows of 1.26 nm. Moreover, every two molecules within a row are dislocated to a certain extent, as highlighted by black lines in Figure 1b, and this dislocation is due to the introduction of one TMS endgroup in the molecular backbone. Similarly, molecule 2 bearing two TMS endgroups also forms a stable stripe structure after it adsorbs on HOPG (Figure 2). In contrast to Figure 1, it is shown that molecule 2 can form a uniform monolayer with molecular orientations extending over hundreds of square nanometers (Figure 2a). From Figure 2b, the high-resolution STM image, we can find that the adjacent molecules within a row are dislocated with their TMS groups interdigitating each other. Considering that the only difference between molecule 1 and molecule 2 is the number of TMS endgroups, we conclude that their different 2D ordering results from the influence of endgroups. From these results we could think that the features of molecular structure can determine the molecular arrangement at the solidliquid interfaces, leading to energy-favorable surface patterns. To further understand the influence of endgroups on 2D ordering, we designed and synthesized molecule 3. This molecule keeps the same backbone structure as molecule 2, but two TMS endgroups are replaced by two carboxyl groups, as shown in Chart 1. The in situ STM observation indicates that molecule 3 forms a uniform and stable monolayer on HOPG (Figure 3). The alkyl chains of 3 interdigitate alternatively in adjacent stripes. In addition, the two carboxyl groups on 3 can form hydrogen bonding with adjacent carboxyl groups; as a

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result, the molecular backbones within a stripe arrange into a line without dislocation. To clarify if the introduction of a biethynylene group in the middle of the molecule backbone can influence the 2D ordering, we designed and synthesized molecules 4 and 5. Molecule 4 contains only four alkyl chains, and two of them are spaced by biethynylene; hence, its interaction with the HOPG substrate and interaction among the molecules are not as strong as those of molecule 2. Consequently, molecule 4 forms a monolayer with many domains (Figure 4a). As shown in Figure 4b, the highresolution STM image clearly indicates that each molecular backbone dislocates to some extent in the same stripe containing two TMS endgroups. Moreover, the introduction of biethynylene provides enough space for the alkyl chains in the adjacent molecule to interdigitate one by one. For the same backbone structure bearing two carboxyl endgroups, instead of two TMS endgroups, molecule 5 forms a similar monolayer but without dislocation because of the formation of hydrogen bonding (Figure 5). This finding further supports our assumption in the discussion of molecule 3 that the directionality of the hydrogen bond is favorable for the alignment of the 2D assemblies.

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Conclusions We synthesized alkoxy-substituted oligo(phenyleneethynlene) derivatives bearing different substitutes and characterized their 2D assembly structures on HOPG by STM. The results showed that the change of the chemical structure of the oligo(phenylene-ethynlene) derivatives can significantly monitor their 2D ordering. The possibility of controlling the 2D assemblies of conjugated molecules is of importance for further design and synthesis of organic electronic materials for the fabrication of low-dimensional devices. Acknowledgment. We thank the Major State Basic Research Development Program (Grant G2000078102 and 2002CB613401), the Ministry of Education and Natural Science Foundation of China (20334010, 50225313, 20225415, and 20374053), and High-Tec 863 Project (2003AA302140) for financial support. LA048538W