Multilayer Assembly and Patterning of Poly(p-phenylenevinylene)s via

Department of Materials Science and Engineering, The Pennsylvania State ... The observed complex assembly behavior suggests that both covalent and ...
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Langmuir 2004, 20, 9600-9606

Multilayer Assembly and Patterning of Poly(p-phenylenevinylene)s via Covalent Coupling Reactions Ziqi Liang and Qing Wang* Department of Materials Science and Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802 Received May 11, 2004. In Final Form: August 5, 2004 Poly(p-phenylenevinylene)s with amines and pentafluorophenyl esters on side chains were synthesized and assembled on solid substrates by sequential layer-by-layer (LBL) deposition. This approach enables the creation of robust multilayer thin films via in-situ covalent coupling reactions between successive layers. The buildup of the multilayers was followed by UV/vis absorption spectroscopy and ellipsometry. The observed complex assembly behavior suggests that both covalent and hydrogen-bonding interactions are involved in the formation of multilayer films. The organized structure and surface morphology of resultant multilayers were investigated by reflectance Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and atomic force microscopy. This covalent LBL method was further applied to generate conjugated polymer micropatterns using microstamped self-assembled monolayers as templates.

* To whom correspondence should be addressed: e-mail [email protected]; Fax 1-814-865-2917.

carrier mobility and luminescence quantum yield.10,11 It is important to explore new methods to assemble these materials to form well-defined films. It is also interesting and challenging to construct multifunctional nanostructured films with a supramolecular architecture in which molecules with different functionalities are incorporated into individual layers. A potent alternative to the conventional thin-film fabrication approaches, such as spin-coating, LangmuirBlodgett, electrochemical, and chemical vapor depositions, is the layer-by-layer (LBL) technique introduced by Decher et al.12 In the LBL deposition technique, multilayer thin films are prepared by alternately dipping a pretreated substrate into aqueous solutions containing positively and negatively charged polyelectrolytes. The LBL assembly has emerged as a versatile and facile method for the preparation of functional thin films on planar or curved surfaces with molecular-level controlled thickness and composition.13,14 A broad range of materials systems have been incorporated into organized LBL assemblies, such as synthetic polymers, inorganic nanoparticles, nanotubes, clays, and biomolecules.15 Aside from electrostatics, many other intermolecular forces including hydrogen bonding, covalent bonds, metal-ligand coordination, and chargetransfer interactions have been employed for the LBL assembly as well.16 Recently, this technique has been adapted to the manipulation of conjugated polymers into multilayers with molecular dimensions by Rubner and other groups.17,18 The resultant multilayer thin films

(1) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43. (2) Reed, M. A.; Tour, J. M. Sci. Am. 2000, 282, 86. (3) Tour, J. M. Acc. Chem. Res. 2000, 33, 791. (4) Reed, M. A., Lee, T. Eds.; Molecular Nanoelectronics; American Scientific Publishers: Stevenson Ranch, CA, 2003. (5) Mahler, G.; May, V.; Schreiber, M. Molecular Electronics: Properties, Dynamics, and Applications; Marcel Dekker: New York, 1996. (6) Hadziioannou, G., van Hutten, P. F., Eds.; Semiconducting Polymers: Chemistry, Physics and Engineering; Wiley-VCH: New York, 2000. (7) Skotheim, T. A., Elsenbaumer, R. L., Reynolds, J. R., Eds.; Handbook of Conducting Polymers, 2nd ed.; Marcel Dekker: New York, 1998. (8) Zakhidov, A. A.; Yoshino, K. Synth. Met. 1995, 71, 18754. (9) (a) Kim, J.; Levitsky, I. A.; McQuade, D. T.; Swager, T. M. J. Am. Chem. Soc. 2002, 124, 7710. (b) Kuroda, K.; Swager, T. M. Macromol. Symp. 2003, 201, 127.

(10) Schwartz, B. J. Annu. Rev. Phys. Chem. 2003, 54, 141. (11) Cornil, J.; Beljonne, D.; Calbert, J.; Bre´das, J. L. Adv. Mater. 2001, 13, 1053. (12) (a) Decher, G.; Hong, J. D. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (b) Decher, D.; Hong, J. D.; Schmitt, J. Thin Solid Films 1992, 210, 831. (c) Decher, G. Science 1997, 277, 1232. (13) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (14) Decher, G., Schlenoff, J. B., Eds.; Multilayer Thin Films; WileyVCH: Weinheim, 2003. (15) (a) Kaschak, D. M.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 4222. (b) Hempenius, M. A.; Pe´ter, M.; Robins, N. S.; Kooij, E. S.; Vancso, G. J. Langmuir 2002, 18, 7629. (c) Fendler, J. H. Chem. Mater. 1996, 8, 1616. (d) Caruso, F. Adv. Mater. 2001, 13, 11. (e) Rouse, J. H.; Lillehei, P. T. Nano Lett. 2003, 3, 59. (f) Mamedov, A.; Ostrander, J.; Aliev, F.; Kotov, N. A. Langmuir 2000, 16, 3941. (g) Jin, Y.; Shao, Y.; Dong, S. Langmuir 2003, 19, 4771.

Introduction The research efforts of many groups have shown that under special conditions the electronic properties of organic materials can be comparable to those of inorganic materials used in modern electronic devices.1,2 Enjoying advantages such as ease of processing, versatility of molecular structures and tunable physical properties, moleculebased wires, gates, transistors, and storage devices are promising in the future of the technology revolution.3-5 Among various conducting molecular materials, π-conjugated polymers offer us a great opportunity to explore new concepts in a variety of thin film devices ranging from electronic and optoelectronic devices to chemical and biomedical sensors.6,7 Many of these applications require the controlled assembly of conducting polymers as multilayer structures on solid substrates. For example, heterogeneous structures formed by conducting polymers with different electronic affinities and band gaps are being sought for applications in light harvesting due to the highly efficient charge separation at the layer interfaces.8 The efficient energy migration in conducting polymer multilayer films has been utilized to amplify sensory response in chemosensors.9 Controlling the solid-state organization of conjugated polymer thin films has a crucial impact on the macroscopic properties of the materials such as charge-

10.1021/la048828c CCC: $27.50 © 2004 American Chemical Society Published on Web 09/21/2004

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Scheme 1. Synthesis of Poly(p-phenylenevinylene)s Containing Amine Side Chains

composed of electronically conducting polyanions and polycations have found greatest utility in light-emitting diodes, photovoltaic cells, electrochromic switches, and optical sensors.19 We have previously reported a hydrogenbonding-driven LBL assembly approach for the generation of conjugated polymer multilayer films in organic solvents.20 The ability to fabricate multilayer thin films in organic media broadens the range of the LBL techniques and opens a viable way to incorporate functional materials with poor water solubility into LBL assemblies. In this work, we describe the sequential deposition of conjugated polymers based on the coupling of the reactive groups on the polymer side chains. This approach permits in-situ covalent cross-linking of the polymer interlayers, leading to organized multilayer thin films with great (16) (a) Chen, W.; McCarthy, T. J. Macromolecules 1997, 30, 78. (b) Stockton, W. B.; Rubner, M. F. Macromolecules 1997, 30, 2717. (c) Wang, L.; Wang, Z.; Zhang, X.; Shen, J. Macromol. Rapid Commun. 1997, 18, 509. (d) Wang, L.; Fu, Y.; Wang, Z.; Fan, Y.; Zhang, X. Langmuir 1998, 15, 1360. (e) Sukhishvili, S. A.; Granick, S. Macromolecules 2002, 35, 301. (f) Zhang, Y.; Guan, Y.; Liu, J.; Xu, J.; Cao, W. Synth. Met. 2002, 128, 305. (g) Hamada, K.; Serizawa, T.; Kitayama, T.; Fujimoto, N.; Hatada, K.; Akashi, M. Langmuir 2001, 17, 5513. (h) Shimazaki, Y.; Mitsuishi, M.; Ito, S.; Yamamoto, M. Langmuir 1997, 13, 1385. (i) Chun, L.; Kagan, C. R. J. Am. Chem. Soc. 2003, 125, 336. (17) (a) Ferreira, M.; Rubner, M. F. Macromolecules 1995, 28, 7107. (b) Fou, A. C.; Rubner, M. F. Macromolecules 1995, 28, 7115. (c) Ferreieira, M.; Cheung, J. H.; Rubner, M. F. Thin Solid Films 1994, 244. 806. (18) (a) Zhu, L.; McCullough, R. D. Adv. Mater. 2002, 14, 901. (b) Ram, M. K.; Salerno, M.; Adami, M.; Faraci, P.; Nicolini, C. Langmuir 1999, 15, 1252. (c) Lukkari, J.; Saloma¨ki, M.; Viinikanoja, A.; A ¨ a¨ritalo, T.; Paukkunen, J.; Kocharova, N.; Kankare, J. J. Am. Chem. Soc. 2001, 123, 6083. (d) Chan, E.; Lee, D.; Ng, M.; Wu, G.; Lee, K.; Yu, L. J. Am. Chem. Soc. 2002, 124, 12238. (e) Li, H.; Li, Y.; Zhai, J.; Cui, G.; Xiao, S.; Lu, F.; Jiang, Lei, Zhu, D. Chem.sEur. J. 2003, 9, 6031. (f) Zotti, G.; Zecchin, S.; Schiavon, G.; Vercelli, B.; Groenendaal, L. Chem. Mater. 2003, 15, 2222. (19) (a) Fou, A. C.; Onitsuka, O.; Ferreira, M.; Rubner, M. F.; Hsieh, B. R. J. Appl. Phys. 1996, 79, 7501. (b) Baur, J. W.; Kim, S.; Balanda, P. B.; Reynolds, J. R.; Runber, M. F. Adv. Mater. 1998, 10, 1452. (c) Mattoussi, H.; Rubner, M. F.; Zhou, F.; Kumar, J.; Tripathy, S. K.; Chiang, L. Y. Appl. Phys. Lett. 2000, 77, 1540. (d) Lvov, Y.; Yamada, S.; Kunitake, T. Thin Solid Films 1997, 300, 107. (e) Boehme, J. L.; Mudigonda, D. S. K.; Ferraris, J. P. Chem. Mater. 2001, 13, 4469. (f) DeLongchamp, D. M.; Kastantin, M.; Hammond, P. T. Chem. Mater. 2003, 15, 1575. (g) Tokuhisa, H.; Hammond, P. T. Adv. Funct. Mater. 2003, 13, 831. (h) Wang, X.; Kim, Y.; Drew, C.; Ku, B.; Kumar, J.; Samuelson, L. A. Nano Lett. 2004, 4, 331. (20) Liang, Z.; Cabarcos, O. M.; Allara, D. L.; Wang, Q. Adv. Mater. 2004, 16, 823.

stability. Poly(p-phenylenevinylene)s, a well-known class of conjugated polymer, are functionalized with amino and pentafluorophenyl ester groups on side chains. While the facile condensation between amines and activated esters provides the main driving force, hydrogen-bonding interactions are proposed to contribute to the assembly process as well. The interplay between covalent and hydrogen-bonding interactions results in a complex assembly behavior first found in nonpolyelectrolyte LBL systems. We further demonstrate the patterning of the covalent-bonded conjugated polymer multilayers using microstamped surfaces as templates in this study. Results and Discussion Synthesis and Structural Characterization. Poly(p-phenylenevinylene)s (PPVs) bearing complementary reactive functional groups were synthesized and employed to fabricate the LBL assembly. The PPVs containing amino and pentafluorophenyl ester groups were prepared according to Schemes 1 and 2. The reactions of 1,4-diiodo2,5-hydroquinone with 5-[(tert-butoxycarbonyl)amino]-1pentyl bromide and ethyl 7-bromoheptanoate afforded monomers I and II, respectively. A typical Heck reaction condition was applied to the preparation of polymers I and III.21 The polymerization was carried out in DMF in the presence of a catalytic amount (3 mol %) of Pd(OAc)2 with the tertiary amine and triarylphosphine under a nitrogen atmosphere. Polymers I and III are soluble in organic solvents such as THF, chloroform, and DMF. Gel permeation chromatography (GPC) analysis shows that polymers I and III possess weight-average molecular weights of approximately 27 kDa with a polydispersity of 2.1. The Boc protecting group in polymer I was removed by the treatment with trifluoroacetic acid, leading to polymer II having amines on side chains. The hydrolysis of polymer III, followed by acidic workup, yielded polymer IV with carboxylic acid side chains. The absence of the broad signal at about 1.2 ppm corresponding to methyl protons in the 1H NMR spectra confirms the deprotection reactions. The carboxylic acid groups were further converted to pentafluorophenyl esters via the reaction with pentafluorophenol and 1-ethyl-3-(dimethylaminopropyl)(21) Bao, Z.; Chen, Y.; Cai, R.; Yu, L. Macromolecules 1993, 26, 5281.

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Scheme 2. Synthesis of Pentafluorophenyl Ester-Substituted Poly(p-phenylenevinylene)s

carbodiimide (EDC). It is known that pentafluorophenyl esters possess a reactivity that is about 10 times greater than N-hydroxysuccinimidyl esters.22 The high reactivity of pentafluorophenyl esters toward amines ensures that polymer V is efficiently chemically bound to polymer II in the LBL assembly process. The 19F NMR spectrum of polymer V clearly shows the signals from pentafluorophenyl groups at 153, 158, and 162 ppm, indicating the presence of the reactive esters in the polymer. The 1H NMR spectra of polymers II and V generally feature the chemical shifts of the dialkoxyl-substituted PPVs. The chemical shifts due to the vinyl protons appear around 7.1 ppm, while the signals at about 7.3 and 7.5 ppm correspond to the aromatic protons on the phenyl rings. These polymers show green fluorescence with an emission maximum at about 505 nm. Characteristic UV/vis absorption maximum due to the π-π* transition of the conjugated backbone was observed at around 450 nm in THF. The features of the FTIR spectra are also consistent with the results reported in the literature.21 An intense signal due to the ether linkage appears at 1220 cm-1, and the absorption peak at 970 cm-1 is attributed to the trans-vinylene CdC stretching. LBL Assembly of PPV Multilayers. The first step in building up the LBL assemblies involved the modification of gold surface with self-assembled monolayers (SAMs) containing carboxylic acid end groups. The carboxylic acidderivatized gold substrates were then activated with pentafluorophenol to generate reactive SAMs terminated with pentafluorophenol esters.23 PPV functionalized with amine side chains, polymer II, reacted with the pentafluorophenol esters on the surface to yield the first layer of the polymer. Subsequently, PPV derivatized with pentafluorophenol ester groups, polymer V, reacted with the residual amino groups of the immobilized polymer II to give the second polymer layer. Repeated deposition of amines and pentafluorophenol esters containing PPV polymers resulted in the corresponding multilayer films. (22) Kovacs, J.; Mayers, G. L.; Johnson, R. H.; Cover, R. E.; Ghatak, V. R. J. Org. Chem. 1970, 35, 1810. (23) Lahiri, J.; Ostuni, E.; Whitesides, G. M. Langmuir 1999, 15, 2055.

Figure 1. Dependence of absorbance at 472 nm on the deposition time for polymers II and V in DMF.

The film formation process was followed by UV/vis absorption spectroscopy. The solid-state absorption peak at 472 nm originating from π-π* transition was utilized to monitor the LBL deposition process. Figure 1 illustrates the results of absorption kinetics studies, where the absorbance at 472 nm is shown as a function of the deposition time. Specifically, we examined the adsorption of polymers II onto SAM-coated substrates to form first polymer layer and deposition of polymers V and II on the corresponding polymer layers to develop second and third layers, respectively. The results follow the general path commonly observed in LBL polyelectrolytes, in which the UV/vis absorbance increases with the deposition time initially and then levels off after a certain time. Similar dynamic processes were observed in the immobilization of polymers V and II onto the polymer surfaces. Thus, the deposition times were set as 45, 30, and 35 min for the initial adsorption of polymer II on the SAMs and subsequent deposition of polymers II and V on the polymer layers, respectively. Figure 2 exhibits a typical series of UV/vis absorption spectra for the formation of PPV multilayer films. The absorption intensity at 472 nm against the layer number is depicted in Figure 3. Rather than a linear dependence of absorbance on the number of deposited layers, which

Poly(p-phenylenevinylene)s

Figure 2. UV/vis absorption spectra of sequentially deposited polymers II/V multilayers.

Figure 3. Evolution of absorbance at 472 nm as a function of the number of layers. The dotted lines illustrate the absorptiondesorption phenomena.

Figure 4. Ellipsometric thickness of polymers II/V multilayers as a function of layer number.

is typical for most LBL-assembled polymer films, the polymers II/V system demonstrates a zigzag growth pattern. The absorption increases upon adsorption of polymer V on polymer II surface but unexpectedly decreases upon immobilization of polymer II onto polymer V. Despite this zigzag trend, the absorption of bilayer increases uniformly with the number of bilayers. The same trend has been revealed by ellipsometry measurements indicated in Figure 4. The bilayer thickness was observed to grow linearly, while the film thickness vs the number of layer shows a similar zigzag pattern. The resultant films are on the order of ∼0.8 nm per layer calculated from the ellipsometry measurements. Similar zigzag behavior has been recently reported in the LBL assembly

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Figure 5. Grazing-angle FTIR spectra from 1500 to 1800 cm-1 of PPV multilayers: (a) a cast film of polymer V; (b) first layer; (c) second layer.

of polyelectrolytes, which are attributed to the existence of a complicated adsorption-desorption phenomenon that dominates film growth.24,25 In the present case, this result suggests that formation of the multilayers is not solely governed by the covalent interactions between amines and active ester groups. There is another attractive interaction also responsible for the layer growth. It is known from our previous work that the PPVs containing carboxylic acids and amines interact strongly by hydrogenbonding and the resultant interactions are able to induce multilayer assembly.20 Thus, we infer that hydrogenbonding interactions between carbonyl groups of polymer V and amines of polymer II are also involved in the buildup process aside from covalent coupling reactions. A likely explanation for the observed complex assembly behavior is that polymer V tends to absorb on polymer II surface via both covalent and noncovalent interactions. Desorption of hydrogen-bonded parts takes place to some extent with subsequent layer deposition. As also shown in Figures 3 and 4, the lines connecting the data points before and after polymer V adsorption are almost parallel, indicating a constant amount of polymer V removal after each adsorption step of polymer II. To avoid cross-contamination of solutions due to desorption of polymer V, fresh polymers II and V solutions were used for deposition of each polymer layer in our experiments. Grazing-angle FTIR was used to establish evidence of the interactions for this assembly process. The IR spectra of polymers II and V show characteristic bands associated with alkoxyl-substituted PPVs: 1600 and 1494 cm-1 (Cd C stretching vibrations in the benzene ring), 1464 cm-1 (CH2 scissoring mode of long polymethylene hydrocarbon chains), and 1422 cm-1 (scissoring mode of the CH2 connected to oxide atom). The N-H stretching peak in a cast film of polymer II appears at 3364 cm-1. For polymer V, the band centered at 1732 cm-1 is attributed to carbonyl groups from pentafluorophenol esters. Figure 5a-c shows the IR spectra from 1500 to 1800 cm-1 of a cast film of polymer V, the first immobilized layer of polymer II, and a bilayer film comprising polymers II/V, respectively. As illustrated in Figure 5b, the deposition of polymer II on activated SAMs leads to a new peak appearing at 1705 cm-1. This peak is assigned to CdO stretching (amide I) from the amide bonds formed by the reaction between pentafluorophenol esters of SAMs and amines of polymer II. The amide II peak (N-H mixed mode) is believed to overlap with the CdC stretching of benzene at around (24) Tedeschi, C.; Caruso, F.; Mohwald, H.; Kirstein, S. J. Am. Chem. Soc. 2000, 122, 5841. (25) (a) Khopade, A. J.; Caruso, F. Langmuir 2002, 18, 7669. (b) Esumi, K.; Akiyama, S.; Yoshimura, T. Langmuir 2003, 19, 7679.

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Figure 6. XPS survey spectra of PPV multilayer thin films.

1600 cm-1. The intensities of these two peaks at 1705 and 1600 cm-1 increase noticeably in the subsequent layers formed by the immobilization of polymer V onto polymer II. These results confirmed that the multilayer film was alternately assembled mainly based on covalent coupling of amines and activated esters. The proposed hydrogenbonding interactions were not detected from the FTIR spectra, seemingly due to the presence of dominating covalent bonds and also the broad nature of spectroscopic peaks of polymers. Further investigation of the surface structure was provided by X-ray photoelectron spectroscopy (XPS) study. As shown in Figure 6, the XPS spectra display the presence of all expected characteristic elements: C1s (284.5 eV), N1s (398.1 eV), and O1s (531.0 eV). The observed additional peak at 287.7 eV is assigned to C1s from -CONH-, further proving the formation of amide linkage between the layers of the film.26 The intensities of photoelectron lines of C1s (284.5 and 287.7 eV) increase with the number of deposited layers, suggesting consecutive deposition of PPVs. In the 10-layered film with polymer V as the outmost layer (Figure 6), a F1s peak appearing at 684.9 eV indicates the presence of free pentafluorophenol esters on the top multilayer polymer surface. These excess functional groups allow the covalent attachment of the next polymer layers or other functional materials with complementary reactive groups, thus enabling the construction of complex microstructures suitable for electronic and optoelectronic applications. The covalent cross-linking nature of polymers II/V multilayer films has also been verified in film stability experiments, in which LBL films were dipped into THF for several hours under sonication. No significant changes have been observed in both film thickness and UV/vis spectra. On the contrary, the hydrogen-bonding-based PPV multilayer films were found to dissociate quickly under the same conditions in our parallel experiments.20 Micropatterned SAMs as LBL Deposition Templates. This covalent LBL assembly was further applied (26) Lee, J. K.; Lee, K.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141.

Figure 7. AFM height image and cross-section profile of patterned PPV multilayers.

to create robust patterned multilayer films. Micropatterning of conjugated polymers is one of the most critical issues in the realization of organic electronics.27 One approach to generating patterned structures is based on molecular templates created by microcontact printing (µCP) of SAMs.28 µCP is a soft lithographic method of depositing patterned SAMs of alkanethiolates onto Au surfaces and siloxanes onto SiO2 substrates.29,30 The printed SAMs chemically modify the substrate surface and have been utilized as templates for living polymerization, chemical etching, metal deposition, cell growth, and protein adhesion.31 Hammond et al. have extensively studied selective deposition of polyelectrolytes and colloids onto chemically patterned templates through electrostatic interactions.32 We present here the covalent LBL assembly of conjugated polymers on µCP patterned SAMs. Patterned 16-mercaptohexadecanoic acids were introduced on gold substrates through an oxidized poly(dimethylsiloxane) (PDMS) stamp with 10 µm wide line features. After the activation of the carboxylic acid end groups of the SAMs, polymers II and V were alternately deposited onto patterned SAMs. Figure 7 presents a tapping-mode atomic force microscopy (AFM) topography image and cross(27) Holdcroft, S. Adv. Mater. 2001, 13, 1753. (28) Liang, Z.; Rackaitis, M.; Li, K.; Manias, E.; Wang, Q. Chem. Mater. 2003, 15, 2699. (29) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (30) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550. (31) (a) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597. (b) Huck, W. T. S.; Yan, L.; Stroock, A.; Hagg, R.; Whitesides, G. M. Langmuir 1999, 15, 6862. (c) Hidber, P. C.; Nealey, P. F.; Helbig, W.; Whitesides, G. M. Langmuir 1996, 12, 5209. (d) Amirpour, M. L.; Ghosh, P.; Lackowski, W. M.; Crooks, R. M.; Pishko, M. V. Anal. Chem. 2001, 73, 1560. (e) Chen, C. S.; Mrksich, M.; Huang, S.; Whitesides, G. M.; Ingber, D. E. Biotechnol. Prog. 1998, 14, 356. (32) (a) Jiang, X.; Hammond, P. T. Langmuir 2000, 16, 8501. (b) Chen, K. M.; Jiang, X.; Kimerling, L. C.; Hammond, P. T. Langmuir 2000, 16, 7825. (c) Jiang, X.; Clark, S. L.; Hammond, P. T. Adv. Mater. 2001, 13, 1669. (d) Tokuhisa, H.; Hammond, P. T. Langmuir 2004, 20, 1436.

Poly(p-phenylenevinylene)s

section analysis of a 10-layered PPV thin film assembled on printed SAMs. It is revealed in height image that the light lines with clear edges indicative of patterned PPVs are spaced by the dark regions representative of unpatterned bare gold substrate. The average cross-section analysis indicates that the patterned PPV stripes are approximately 10 µm wide and 9 nm thick, which are in excellent agreement with original feature of the stamp used for patterning and film thickness measured by ellipsometry. Conclusions Functionalized PPVs have been prepared and assembled on solid substrates via a LBL sequential manner. The approach based on in-situ covalent coupling reaction results in well-controlled thin films with great stability. Multiple interactions among layers are proposed to account for the observed complex assembly behavior. Although there exists a complicated adsorption-desorption phenomenon, the deposition process is still reproducible from layer to layer, since the film thickness after desorption process still increases linearly with the layer number. An efficient approach to patterned conjugate polymer multilayer thin films was also demonstrated. Robust PPV micropatterns were generated using printed SAMs as templates to direct the multilayer assembly. The functional groups on polymer films introduce nearly unlimited opportunities for the further attachment of various functional species atop the polymer thin films, which, in turn, opens up the possibility of constructing complex multicomponent structures with cooperative electronic and photonic properties. Experimental Section Materials. All reagents were purchased from Aldrich and used without further purification. All the solvents used for the synthesis were HPLC grade. Tetrahydrofuran (THF) was distilled from sodium benzophenone ketyl under nitrogen prior to use. All chemical reactions were performed under a nitrogen atmosphere using oven-dried glassware. Diiodohydroquinone,33 divinylbenzene,34 and 5-(N-Boc-amino)-1-pentyl bromide35 were synthesized according to literature procedures. Synthesis of Monomer I. Potassium hydroxide (0.89 g, 15.89 mmol) was added to a solution of 1,4-diiodo-2,5-hydroquinone (1.9 g, 5.25 mmol) and 5-[(tert-butoxycarbonyl)amino]-1-pentyl bromide (4.19 g, 15.74 mmol) in 25 mL of DMSO. The mixture was stirred overnight at room temperature, then extracted with CH2Cl2, and organic extracts were washed with water. The organic layer was dried over anhydrous MgSO4, and the removal of solvent gave a crude product. The product was further purified by chromatography using dichloromethane/hexane (2:1 v/v) as eluents (3.18 g, 83%). 1H NMR (CDCl3, ppm): δ 1.40 (s, 18H, -CH3), 1.50-1.60 (m, 8H, -CH2-), 1.84 (m, 4H, -CH2-), 3.16 (m, 4H, -CH2-N-), 3.93 (t, 4H, -CH2-O-), 4.61 (br, 2H, -NH), 7.18 (s, 2H, aromatic protons). Anal. Calcd for C26H42O6N2I2: C, 42.6; H, 5.74; N, 3.83. Found: C, 42.5; H, 5.78; N, 3.80. Monomer II. In a similar procedure as described for monomer I, monomer II was obtained from 1,4-diiodo-2,5-hydroquinone and ethyl 7-bromoheptanoate in 78% yield. 1H NMR (CDCl3, ppm): δ 1.20 (m, 6H, -CH3), 1.32 (m, 4H, -CH2-), 1.39 (m, 4H, -CH2-), 1.58 (m, 4H, -CH2-), 1.78 (m, 4H, -CH2-), 2.23 (t, 4H, -CH2-CO-), 3.91 (t, 4H, -O-CH2-), 4.12 (d, 4H, -COOCH2-), 7.17 (s, 2H, aromatic protons). Anal. Calcd for C24H36O6I2: C, 42.7; H, 5.34. Found: C, 42.4; H, 5.37. Polymerization. To a stirred mixture of monomer I (0.62 g, 0.84 mmol), p-divinylbenzene (0.107 g, 0.84 mmol), palladium (33) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 7017. (34) Strey, B. T. J. Polym. Sci., Part A 1965, 3, 265. (35) Egbertson, M.; Chang, C.; Duggan, M.; Gould, R.; Halczenko, W.; Hartman, G.; Laswell, W.; Lynch, J.; et al. J. Med. Chem. 1994, 37, 2537.

Langmuir, Vol. 20, No. 22, 2004 9605 acetate (7.21 mg, 0.032 mmol), and tri-o-tolylphosphine (48.7 mg, 0.16 mmol) in 15 mL of DMF was added tri-n-butylamine (0.50 mL, 2.09 mmol). The reaction mixture was refluxed at 90 °C with stirring for 6 h under a nitrogen atmosphere and then poured into methanol. The precipitate was collected, redissolved in chloroform, and filtered to remove the catalyst residue. The filtrate was concentrated and precipitated into methanol, followed again by filtration and reprecipitation. The crude polymer was further purified by extraction in a Soxhlet extractor with methanol for 24 h and then dried under a vacuum at 25 °C for 24 h. 0.40 g of orange solid of polymer I was obtained. 1H NMR (CDCl3, ppm): δ 1.29 (broad, 18H, -CH3), 1.53 (broad, 8H, -CH2), 1.83 (broad, 4H, -CH2-), 3.11 (broad, 4H, -CH2-N-), 4.02 (broad, 4H, -CH2-O-), 4.54 (broad, 2H, -NH-), 7.07 (broad, 4H, vinyl protons), 7.19 (broad, 2H, aromatic protons), 7.47 (broad, 4H, aromatic protons). Anal. Calcd for C36H50N2O6: C, 71.3; H, 8.24; N, 4.62. Found: C, 72.6; H, 8.04; N, 4.50. Polymer III. 1H NMR (CDCl3, ppm): δ 1.20 (broad, 6H, -CH3), 1.42 (broad, 4H, -CH2-), 1.51 (broad, 4H, -CH2-), 1.63 (broad, 4H, -CH2-), 1.82 (broad, 4H, -CH2-), 2.24 (broad, 4H, -CH2CO-), 3.92-4.10 (broad, 8H, -CO2-CH2-, -O-CH2-), 7.07 (broad, 4H, vinyl protons), 7.20 (broad, 2H, aromatic protons), 7.47 (broad, 4H, aromatic protons). Anal. Calcd for C34H44O6: C, 74.5; H, 8.02. Found: C, 72.9; H, 7.84. Synthesis of Polymer II. To a mixture of polymer I (250 mg) in 10 mL of CH2Cl2 was added 2 mL of trifluoroacetic acid via syringe. The solution was stirred at room temperature overnight under a nitrogen atmosphere and quenched in methanol. The precipitate was collected by filtration, redissolved in a minimum amount of hot chloroform, and precipitated into methanol. The resulting polymer was washed with methanol by using a Soxhlet extractor and dried under vacuum at room temperature for overnight. 150 mg of dark orange solid was obtained in 60% yield. 1H NMR (CDCl3, ppm): δ 1.36 (broad, 4H, -CH2-), 1.47 (4H, -CH2-), 1.94 (broad, 4H, -CH2-), 3.19 (broad, 4H, -CH2N-), 4.08 (broad, 4H, -CH2-O-), 4.62 (broad, 4H, -NH2), 7.17 (broad, 4H, vinyl protons), 7.24 (broad, 2H, aromatic protons), 7.51 (broad, 4H, aromatic protons). Anal. Calcd for C26H34N2O2: C, 76.8; H, 8.37; N, 6.89. Found: C, 73.7; H, 8.13; N, 6.56. Synthesis of Polymer IV. To a 10 mL THF solution of polymer III (200 mg) was added potassium tert-butoxide (20 mg) in one portion. The mixture was stirred overnight at room temperature under a nitrogen atmosphere. The reaction was neutralized with trifluoroacetic acid and then quenched in methanol. The solid polymer was washed with methanol for 24 h and dried under vacuum. 120 mg of dark orange solid of polymer IV was obtained in 60% yield. 1H NMR (CDCl3, ppm): δ 1.44 (broad, 4H, -CH2-), 1.49 (broad, 4H, -CH2-), 1.62 (broad, 4H, -CH2-), 1.79 (broad, 4H, -CH2-), 2.20 (broad, 4H, -CH2-CO), 4.01 (broad, 4H, -O-CH2-), 7.10 (broad, 4H, vinyl protons), 7.19 (broad, 2H, aromatic protons), 7.48 (broad, 4H, aromatic protons). Anal. Calcd for C30H36O6: C, 73.2; H, 7.31. Found: C, 71.5; H, 6.98. Synthesis of Polymer V. To a 10 mL dry DMF solution of polymer IV (100 mg) was added a mixture of pentafluorophenol (5 mg) and 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (3 mg). The mixture was stirred at room temperature overnight under a nitrogen atmosphere. A 50 mg dark orange solid of polymer V was produced after a similar workup procedure as polymer II in 50% yield. 1H NMR (CDCl3, ppm): δ 1.45 (broad, 4H, -CH2-), 1.49 (broad, 4H, -CH2-), 1.60 (broad, 4H, -CH2-), 1.79 (broad, 4H, -CH2-), 2.30 (broad, 4H, -CH2CO-), 4.08 (broad, 4H, -O-CH2-), 7.12 (broad, 4H, vinyl protons), 7.17 (broad, 2H, aromatic protons), 7.46 (broad, 4H, aromatic protons). 19F NMR (CDCl3, ppm): δ 153 (d, 4F, aromatic), 158 (t, 4F, aromatic), 162 (m, 2F, aromatic). Fabrication. Substrate Preparation. The gold-coated silicon substrates were formed by thermal deposition of ∼100 Å of Cr followed by ∼1500 Å of Au on top of a clean 2 in. silicon wafer. The substrate were then cleaned in a freshly prepared piranha solution of 30% H2O2/H2SO4 (1:3 v/v) (caution: piranha solution is very corrosive and must be treated with extreme caution; it reacts violently with many organic materials and must not be stored in tightly closed vessels) for 15 min, rinsed thoroughly with Millipore water, further rinsed with absolute ethanol, and dried under a stream of dry N2.

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Multilayer Assembly. The self-assembled monolayer was prepared by dipping a Au-coated silicon wafer into 1 mM absolute ethanol solution of 16-mercaptohexadecanoic acid overnight and then rinsing it with absolute ethanol followed by a blow dry of N2. The formed acid-terminated SAM was first activated into pentafluorophenol ester ended SAM by immersing it into a solution of DMF containing 0.1 M 1-ethyl-3-(dimethylaminopropyl)carbodiimide and 0.2 M pentafluorophenol for 20 min, followed by washing with absolute ethanol and then drying under N2. The activated SAMs were exposed to polymer II in DMF solution (1 mg/mL) and then washed thoroughly with CH2Cl2 to remove physisorbed and weakly bound materials present on the surface, followed by briefly drying in a flow of N2, giving rise to the first layer of PPV. The first layer was immersed into a DMF solution of polymer V (1 mg/mL) with the same washing and drying procedure, resulting in the second layer of PPV. The multilayer was then constructed by repeating the above steps in a cyclic fashion. Formation of Micropatterned Multilayer. The microcontact printing method for alkanethiols on gold was followed as described by Kumar et al.29 The stamp was fabricated by casting a 10:1 mixture of Sylgard 184 elastomer/curing agent (Dow Corning, Midland, MI) over a photolithographically prepared silicon masters. The PDMS stamp was first treated by air plasma for 1 min to create a highly hydrophilic surface. A saturated 1 mM absolute ethanol solution of 16-mercaptohexadecanoic acid was then used to ink oxidized PDMS stamp. After evaporation of the solvent, the PDMS stamp was briefly dried with a gentle stream of N2 and then brought into contact with the substrate for 1 min at room temperature. The sample was rinsed with absolute ethanol to remove the excess mercaptohexadecanoic acid and dried with N2. The substrate was then dipped into the solution of pentafluorophenol and 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride for 20 min, rinsed with ethanol, and dried with N2. The generated patterned SAMs with pentafluorophenyl ester groups were utilized as templates to direct the assembly. Multilayer thin films were prepared by simply repeating the basic LBL deposition process. Characterization. The 1H and 19F NMR spectra were recorded on a Bruker AM 300 spectrometer. Molecular weights

Liang and Wang and distribution of polymers were determined by using gel permeation chromatography with a Waters Associates liquid chromatograph equipped with a Waters 510 HPLC pump. THF was used as the eluent, and polystyrenes were used as the standard. UV/vis spectra were recorded by using a Varian Cary 100 UV-vis spectrophotometer together with a DRA-CA-30I sphere accessory for thin film reflectance measurements. Incremental thickness measurements were taken from a Gaertner Scientific Corp. model L2W26D ellipsometer with a He-Ne laser operating at an incidence angle of 72° and a wavelength of 632.8 nm. An isotropic refractive index value of 1.50 and 1.55 was assigned to the monolayer and polymer thin film, respectively. Grazing-angle FTIR spectra were obtained in single reflection mode using Digilab Fourier transform infrared spectrometer (Biorad, Cambridge, MA). The p-polarized light was incident at 86° relative to the surface normal of the substrate, and a liquid N2 cooled wide-band MCT detector was used to detect the reflected light. A spectrum of a SAM of n-hexecanethiolate-d18 (C12D25SAMs and C16H33-SAMs) on gold was taken as a reference. Spectra were averaged over 400 scans at 4 cm-1 resolution against a background of pure SAM. X-ray photoelectron spectroscopy analyses were performed on a Kratos Axis Ultra spectrometer equipped with a monochromatic Al KR source. A pass energy of 80 eV and an energy step of 0.1 eV were used for the survey of sample with titled 45°. Surface topography of patterned multilayer thin films was mapped with Digital Instruments Nanoscope IIIa Multimode tapping-mode atomic force microscopy. A silicon-etched tip (Nanotips, DI) with a resonance frequency of ∼300 kHz and spring constant of ∼40 N m-1 was used for tapping mode SFM imaging. The images were acquired in at a scan rate of 0.5-1 Hz with a resolution of 512 × 512 pixels.

Acknowledgment. We gratefully acknowledge the financial support of the Pennsylvania State University and the Commonwealth of Pennsylvania through the Lehigh/Penn State Center for Optical Technologies. LA048828C