Patterning of Functional Compounds by Multicomponent Langmuir

Sep 7, 2010 - Marion K. Brinks,‡ Armido Studer,*,‡. Harald Fuchs,†,§ and Lifeng Chi*,†,§. †Physikalisches Institut, and ‡Organisch-Chemi...
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Julia C. Niehaus,

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Patterning of Functional Compounds by Multicomponent Langmuir-Blodgett Transfer and Subsequent Chemical Modification Michael Hirtz, ,†,§ Marion K. Brinks,‡ Armido Studer,*,‡ Harald Fuchs,†,§ and Lifeng Chi*,†,§

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† Physikalisches Institut, and ‡Organisch-Chemisches Institut, Westf€ alische Wilhelms-Universit€ at, Corrensstrasse 40, 48149 M€ unster, Germany, and §Center for Nanotechnology (CeNTech), Heisenbergstrasse 11, 48149 M€ unster, Germany. M. Hirtz and J. C. Niehaus contributed equally to this work.

Received July 20, 2010. Revised Manuscript Received August 18, 2010 The multicomponent transfer of functional molecules by Langmuir-Blodgett (LB) technique onto solid substrates offers an interesting route for generation of functionalized patterns by self-assembly over large surface areas. In the present paper, we discuss LB transfer of mixed LB films containing different functional amphiphiles (an azide, an estrone derivate, a lithocholic acid derivative, or an alkoxyamine) in combination with dipalmitoylphosphatidyl choline (DPPC). The effect of the mixing ratio on pattern formation is discussed, and we provide some general design rules for the synthesis of functional molecules to be applicable for the multicomponent LB transfer process. We show that these functional compounds can be successfully transferred to oxidized Si wafers in stripe pattern. Covalent attachment of the functional entities is easily achieved, and the patterned surfaces are then ready for further chemical manipulation. This is demonstrated by site-specifically covalent attaching dye molecules applying the copper(l)-catalyzed alkyne azide click reaction (CuAAC), the thiol-ene reaction, and a surface-initiated radical polymerization.

Introduction Over the past decades, the modification of surfaces on microand nanoscales has become one of the most active research areas in the field of materials science. Structuring of surfaces is of high interest due to the fact that surface properties can be changed selectively, and in addition, local functionalization by chemical modification can be achieved. Strategies to pattern surfaces for later or in situ functionalization fall into two different categories: “top-down-strategies” like microcontact printing1,2 or electron beam lithography3 and “bottom-up-strategies” by using selfassembly approaches.4-6 An example belonging to the latter approach is the Langmuir-Blodgett (LB) lithography which will be used in the present study.7 The main template system applied in LB lithography is generated by self-organization of the phospholipid L-R-dipalmitoyl-phosphatidylcholine (DPPC) during LangmuirBlodgett transfer: a solid substrate (usually silicon or mica) is withdrawn from a DPPC-covered air/water interface, inducing a large-area pattern of parallel stripes and channel structures with mesoscale dimensions on the solid substrate with tunable feature size by adjusting of the transfer parameters.8 A detailed description of the pattern formation and some of the applications is given in the literature.7 The obtained DPPC stripe pattern was shown to *Corresponding authors. Lifeng Chi: Phone þ49-(0)251-83-33651, Fax þ49-(0)251-83-33602, e-mail [email protected]. Armido Studer: Phone þ49-(0)251-83-33291, Fax þ49-(0)251-83-36523, e-mail [email protected].

(1) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 550–575. (2) Jeon, L.; Choi, I. S.; Whitesides, G. M. Appl. Phys. Lett. 1999, 75, 4201–4203. (3) G€olzh€auser, A.; Eck, W.; Geyer, W.; Stadler, V.; Weimann, T.; Hinze, P.; Grunze, M. Adv. Mater. 2001, 13, 803–806. (4) Elemans, J. A. A. W.; Lei, S.; De Feyter, S. Angew. Chem., Int. Ed. 2009, 48, 7298–7332. (5) Ariga, K.; Lee, M. V.; Mori, T.; Yu, X.-Y.; Hill, J. P. Adv. Colloid Interface Sci. 2010, 154, 20–29. (6) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282–6304. (7) Chen, X.; Lenhert, S.; Hirtz, M.; Lu, N.; Fuchs, H.; Chi, L. Acc. Chem. Res. 2007, 40, 393–401. (8) Gleiche, M.; Chi, L. F.; Fuchs, H. Nature 2000, 403, 173–175.

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be useful as a template to guide self-assembly of nanoparticles9,10 or small molecules11 and can be modified for further chemical patterning12 enabling, e.g., etching of the pattern into silicon13,14 or electrodeposition of copper wires.15 All these patterning methods take place after the LB transfer of pure DPPC. On the other hand, some lipids (that are also normally part of phospholipid membranes in biological systems) can be directly transferred as a mixture with DPPC but not inhibit pattern formation.16 In experiments with admixtures of fluorescent-labeled dyes or even a pure dye (i.e., not bound to a lipid) to the DPPC monolayer, a phase separation leaving the admixed compound mainly in the liquid expanded (LE) phase was observed yielding fluorescent stripe patterns.17,18 Recently, we could demonstrate that LB transfer of mixed LB films containing DPPC and polymer initiators can be used for preparation of well-structured polymer brushes.19 These results raise the question whether other functional structures compatible with DPPC (especially nonlipid ones) can be site-specifically transferred to surfaces with this (9) Lu, N.; Chen, X.; Molenda, D.; Naber, A.; Fuchs, H.; Talapin, D. V.; Weller, H.; Muller, J.; Lupton, J. M.; Feldmann, J.; Rogach, A. L.; Chi, L. Nano Lett. 2004, 4, 885–888. (10) Chen, X.; Hirtz, M.; Rogach, A. L.; Talapin, D. V.; Fuchs, H.; Chi, L. Nano Lett. 2007, 7, 3483–3488. (11) Hao, J.; Lu, N.; Wu, Q.; Hu, W.; Chen, X.; Zhang, H.; Wu, Y.; Wang, Y; Chi, L. Langmuir 2008, 24, 5315–5318. (12) Lu, N.; Gleiche, M.; Zheng, J.; Lenhert, S.; Xu, B.; Chi, L.; Fuchs, H. Adv. Mater. 2002, 14, 1812–1815. (13) Lenhert, S.; Zhang, L.; Mueller, J.; Wiesmann, H. P.; Erker, G.; Fuchs, H.; Chi, L. Adv. Mater. 2004, 16, 619–624. (14) Lenhert, S.; Meier, M.; Meyer, U.; Chi, L.; Wiesmann, H. P. Biomaterials 2005, 26, 563–570. (15) Zhang, M.; Lenhert, S.; Wang, M.; Chi, L.; Lu, N.; Fuchs, H.; Ming, N. B. Adv. Mater. 2004, 16, 409–413. (16) Chen, X.; Lu, N.; Zhang, H.; Hirtz, M.; Wu, L.; Fuchs, H.; Chi, L. J. Phys. Chem. B 2006, 110, 8039–8046. (17) Chen, X.; Hirtz, M.; Fuchs, H.; Chi, L. Adv. Mater. 2005, 17, 2881–2885. (18) Chen, X.; Hirtz, M.; Fuchs, H.; Chi, L. Langmuir 2007, 23, 2280–2283. (19) Brinks, M. K.; Hirtz, M.; Chi, L.; Fuchs, H.; Studer, A. Angew. Chem. 2007, 119, 5324–5326. Brinks, M. K.; Hirtz, M.; Chi, L.; Fuchs, H.; Studer, A. Angew. Chem., Int. Ed. 2007, 46, 5231–5233.

Published on Web 09/07/2010

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Figure 1. Scheme for surface patterning by multicomponent LB transfer. Scheme 1. Synthesis of Azide 1

Figure 2. General structure of an appropriately functionalized compound for multicomponent LB transfer.

Scheme 2. Syntheses of Azide 2 and Olefin 3

Figure 3. Molecular structures studied: azide 1, lithocholic acid derivative 2, estrone derivative 3, and alkoxyamine 4.

approach, which would further generalize this “rapid” large surface area patterning technique. In the current study, we address this question by showing different functionalization options and some design considerations for compounds that are suitable for LB transfer as additives in combination with DPPC. The general scheme for LB transfer of a mixed LB film containing DPPC and a DPPC compatible structure A (mostly but not necessarily an amphiphile) to provide a stripe-patterned surface is given in Figure 1. Compound A is admixed to the DPPC monolayer and is enriched in the channels of the resulting stripe pattern after LB transfer. Importantly, compound A bears a reactive moiety to allow subsequent covalent attachment of the functionalized compound A to the surface. Physisorbed DPPC can be readily washed away by simple sonication in an organic solvent. The thus-obtained surface pattern may now be used for further chemical modification of the headgroup of compound A by surface wet chemistry, if compound A is appropriately functionalized. This elegant strategy for the generation of sitespecifically functionalized surfaces demands some special design elements on compound A fulfilling the following requirements: in the LB transfer, only molecules insoluble in water can be used (for stable monolayers, it is even desirable for the molecule to be amphiphilic, because this will enforce a more regular orientation Langmuir 2010, 26(19), 15388–15393

within the monolayer in combination with DPPC). For covalent attachment and the later wet chemical modification of the patterned surfaces, bifunctional molecules are needed. The anchor function for the covalent attachment to the surface must form the polar entity of the amphiphile. Due to the amphiphilic character of the molecule, the rest of the molecule has to be hydrophobic, including the functional headgroup for the later chemical modification. The general concept for the design of possible target structures is shown in Figure 2. We choose triethoxy silanes for the covalent attachment to oxidized silicon surfaces. Triethoxy silanes can be readily prepared from terminal double bonds by hydrosilylation. In comparison to the more reactive trichloro and trimethoxy silanes, they are more stable and easier to handle. Due to the necessary amphiphilic character of the molecules, the functional headgroup DOI: 10.1021/la102881r

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Figure 4. Obtained patterns after LB transfer with admixing of the estrone derivative 3 (insets indicate concentration with respect to DPPC, scale bar equals 2.5 μm, arrow denotes direction of withdrawal from LB trough).

for the later chemical modification has to be hydrophobic. As hydrophobic functional groups, unsaturated hydrocarbons like alkynes and alkenes and also relatively nonpolar azides or alkoxyamines will be investigated. Importantly, it has been previously shown that monolayers that are azide or alkyne terminated can be efficiently modified by using the copper(l)catalyzed alkyne azide click reaction (CuAAC).20,21 Also, the thiol-ene reaction on alkene-terminated monolayers has been successfully used to conduct chemistry at self-assembled monolayers (SAMs).22-25 Along with the “polar” anchor and the apolar headgroup, the targeted amphiphiles should bear a spacer entity that shows good interaction with the inner hydrophobic part of the DPPC layer. We thought that natural amphiphiles should provide a sound base as a lead structure for the spacer unit. Natural amphiphiles are found in biomembranes where they selfassemble to form bilayers. Therefore, we were confident that chemical structures based on these amphiphiles should form stable monolayers in combination with DPPC that will be used as major component in our multicomponent LB patterning.

Materials and Experimental Details On the basis of the design rules discussed above, we decided to study four different structures in the multicomponent LB lithography. The chemical formulas of the target compounds 1-4 are depicted in Figure 3. (20) Decreau, R. A.; Collman, J. P.; Yang, Y.; Yan, Y.; Devaraj, N. K. J. Org. Chem. 2007, 72, 2794–2802. (21) Ku, S.-Y.; Wong, K.-T.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 2392– 2393. (22) Kade, M. J.; Burke, D. J.; Hawker, C. J. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 743–750. (23) Jonkheijm, P.; Weinrich, D.; K€ohn, M.; Engelkamp, H.; Christianen, P. C. M.; Kuhlmann, J.; Maan, J. C.; N€usse, D.; Schroeder, H.; Wacker, R.; Breinbauer, R.; Niemeyer, C. M.; Waldmann, H. Angew. Chem. 2008, 120, 4493– 4496. (24) Bertin, A.; Schlaad, H. Chem. Mater. 2009, 21, 5698–5700. (25) Hensarling, R. M.; Doughty, V. A.; Chan, J. W.; Patton, D. L. J. Am. Chem. Soc. 2009, 131, 14673–14675.

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The synthesis of alkoxyamine 4 was previously reported.19 Azide 1 was synthesized starting from benzyl alcohol 5 via iodination to give 6, which was subjected to etherification with 9-decanol under standard conditions to afford 7 (Scheme 1). Hydrosilylation provided ester 8 that was further transformed via its alcohol 9 by using a Mitsunobu-type azidation to eventually give the targeted azide 1. A similar synthetic route was followed for the transformation of the readily available lithocholic acid methyl ester 10 into azide 2 (Scheme 2). The synthesis of olefin 3 started with estrone 11. Installation of the triethoxysilyl anchor functionality was conducted in analogy to the synthesis of 2, and the olefin moiety was introduced via a classical Wittig methenylation reaction. Yields obtained for the individual steps of these two syntheses and experimental details can be found in the Supporting Information. For each LB patterning experiment, a fresh solution of DPPC and the respective amphiphile in HLPC-grade chloroform at concentrations around 1 mg/mL was prepared. The fresh DPPC solution was divided into six parts, and to each part, an appropriate amount of the amphiphile 1, 2, 3, or 4 was added (2.5 mol %, 5.0 mol %, 7.5 mol %, 10.0 mol %, 12.5 mol %, and 15.0 mol % of the given amphiphile with respect to DPPC). As substrates, silicon wafers (300 nm oxide layer for the surfaces that were later labeled with fluorescent dyes; native oxide layer for all others) were cut into pieces of about 5  2 cm2 that were subsequently cleaned by sonication in chloroform, isopropanol, and pure water (DI, resistance of 18.2 MΩ cm) for 10 min each and finally exposed to a 300 W oxygen plasma at a pressure of 1 mbar for 2 min. The substrates were stored under pure water and used for the transfer within 3 h. The subphase temperature for all experiments was fixed at 25 °C; the relative humidity varied between 50% to 60% for the various experiments. After a substrate was submerged into the subphase, the respective solution was spread onto the airwater interface, and the chloroform was allowed to evaporate for about 10 min, an additional 50 min were then allowed for hydrolysis of the triethoxy anchors to enable a later covalent binding to the substrate. The monolayer was then compressed to a lateral pressure of 5 mN/m and another 10 min were allowed to pass so that the film could equilibrate. The substrate was then Langmuir 2010, 26(19), 15388–15393

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Figure 5. Summary of the width of stripes (squares) and channels (diamonds) and the overall periodicity (triangles) for the different compounds versus amount of admixing. lifted through the monolayer with a velocity of 10 mm/min. The substrate was kept under ambient conditions at room temperature for one day and was then studied by AFM to analyze the resulting pattern. For the monolayer obtained by using alkoxyamine 4 as a DPPC additive, additionally Brewster angle microscopy was performed. To this end, the solution containing a defined amount of 4 was spread onto a water subphase fixed at a temperature of 25 °C in a trough equipped with a Brewster angle microscope. After allowing the chloroform to evaporate for about 10 min, an isotherm was recorded and images were acquired from the Brewster angle microscope in regular intervals during compression. To further chemically modify the patterned wafers obtained after LB transfer, they were dried at 0.01 bar at 80 °C for 2 h to covalently bind the functionalized component by trans-silyletherification onto the wafer. Physisorbed DPPC was then removed by washing with CHCl3. The surface click reactions were conducted Langmuir 2010, 26(19), 15388–15393

by using a large excess of the corresponding reagents as will be described in detail below and in the Supporting Information.

Results and Discussion All additives 1-4 could be transferred together with DPPC without inhibiting pattern formation. A detailed compilation of images of a typical transfer for a given admixing of the estrone derivative 3 is shown in Figure 4. The width of stripes and channels, as well as the overall periodicty for all four compounds 1-4, is summarized in Figure 5. The height difference between the stripes and the channels was measured to be around 1 nm, which is consistent with earlier reports.10 There are some trends observed in all patterned films after transfer in the presence of an additive. All in all, the striped edges of the transferred patterns look more rugged as compared to those usually observed for pure DPPC transfers.8 This behavior concurs DOI: 10.1021/la102881r

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Figure 6. BAM images of LC phase domains in (a) a pure DPPC monolayer, (b) a DPPC monolayer admixed with 10 mol % of the alkoxyamine 4. The image width is 430 μm.

with the observation of irregularly shaped domains of DPPC in the liquid condensed (LC) phase during compression of mixed monolayers in BAM: The LC domains in pure DPPC films grow in roundish shapes with smooth edges, whereas the domains in the films with admixings of the alkoxyamine exhibit a widely irregular shape with jagged borders (Figure 6, further details given in Supporting Information). Importantly, the pattern dimensions become significantly smaller as compared to patterns obtained by the transfer of pure DPPC: In the current experiment, the overall periodicity varies from 1.2 μm down to under 400 nm depending on the amount and type of admixing, whereas transfers of pure DPPC usually yield periodicity way above 1 μm under similar transfer velocities. Under the current transfer parameters, all systems except for the alkoxyamine 4 lose the patterning with an admixing of 15 mol %, although patterning may be still possible by altering the transfer parameters to higher lateral pressures (instead of seeking optimal parameters for each transfer, we kept the transfer conditions constant over all admixings and for all compounds to ensure comparability). With admixing of 2.5 mol %, only the DPPC/ estrone 3 and the DPPC/alkoxyamine 4 mixture still exhibit regular striped pattern formation during film transfer. The other compounds 1 and 2 form more or less complete LC phase films as would be expected for pure DPPC under the applied transfer conditions. All transferred systems show a pronounced minimum in the overall periodicity (at 7.5 mol % for the estrone derivative 3, at 10.0 mol % for the alkoxyamine 4, the lithocholic acid 2, and the azide 1). With the exception of 4, in general the stripe size 15392 DOI: 10.1021/la102881r

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decreases while the channel size increases with increasing concentration of the additive, which concurs with the intuition that (since the admixing tends to accumulate in the LE phase channels) a higher concentration of additive should enlarge the channels. In the case of the lithocholic acid 2/DPPC mixed system, there is also a slight increase in channel size with decreasing admixing concentrations from 7.5 mol % to 5.0 mol %. By using the alkoxyamine 4, this counterintuitive behavior is even much more pronounced, since the stripe width stays nearly constant over all admixing percentages and the channel width increases clearly for low admixing concentrations. Similar trends were also observed in the previous experiments with dye admixing,14 and this leads to the conclusion that under some circumstances a lower concentration of admixing can have an even bigger effect on the condensation behavior than a higher admixing. This interesting result is currently not fully understood, and work to address this issue is ongoing in our lab. Another general observation is that a higher lateral transfer pressure is needed for patterning of films when DPPC is transferred in the presence of an additive. LB transfer of pure DPPC monolayers under the conditions applied herein at a pressure of ca. 5 mN/m would lead to a complete and unstructured LC phase coverage on the silicon substrates. This can be understood by the rise in the coexistence plateau of the isotherms with admixing (see Supporting Information). The condensation is hindered by the admixing, therefore requiring a rise in lateral pressure to occur. For successful wet chemical modification of the structured surfaces, the functional compounds have to be covalently bound to the Si wafer. To this end, the covered wafers obtained after LB transfer were dried at 0.01 bar at 60 °C for 2 h to covalently bind the functionalized component by trans-silyletherification onto the wafer. Physisorbed DPPC was then readily removed by washing with CHCl3. The thus obtained regular functional stable stripes were then further chemically modified. For example, stripes built up by the lithocholic acid derivatives 2 or amphiphile 1 containing the azide functionality as the headgroup at the surface were subjected to CuAAC using fluorescent LRA 12 as an alkyne (Figure 7A). The wafer was submerged at room temperature into an ethanol/toluene solution containing the alkyne-conjugated rhodamine derivative 12 (0.5 mM, 1 mL), which was readily synthesized from commercially available rhodamine sulfochloride (see Supporting Information), copper sulfate (0.1 equiv), and sodium ascorbate (0.2 equiv) for 12 h. After removal of the wafer, the surface was sonicated in DCM, acetone, ethanol, and water, respectively. The wafer was subsequently dried by passing an argon stream, and the structured surface was analyzed by fluorescence microscopy. Success of the chemical reaction was fully supported by “coloration” of the stripes. Importantly, the dimensions of the stripe pattern were not altered, showing that covalent attachment of the amphiphile to the wafer and also the subsequent wet chemical modification of the surface bound functional group did not destroy the pattern obtained by the initial LB transfer process. The alkene-terminated patterned surfaces obtained by using the estrone derivative 3 in the LB lithography were successfully further chemically modified by applying the thiol-ene reaction using fluorescent rhodamine thiol derivative 13. These surface radical reactions were best preformed by irradiating a solution of 13 in DMF (0.5 mM) containing the patterned wafer with a UV lamp (tungsten lamp, 500 W) for 5 h. After washing the wafer, fluorescent stripes were obtained proving successful thiol-ene type reaction at the patterned surface (Figure 7b). Finally, we used the alkoxyamine functionality at the surface to obtain the multicomponent LB transfer for controlled surface-initiated nitroxide mediated radical polymerization.19,26 Surface-initiated styrene polymerization (Figure 8) was conducted Langmuir 2010, 26(19), 15388–15393

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Figure 7. Modification of the patterned surfaces. (A) CuAAC on azide patterned surfaces using fluorescent LRA 12 as an alkyne component, fluorescent microscopy image (right) after reaction with LRA. (B) Thiol-ene reaction with rhodamine thiol 13 and fluorescence microscopy image after thiol-ene reaction (right).

Figure 8. Surface-initiated styrene polymerization of the alkoxyamine-terminated surface. AFM image after polymerization (right) (insets show cross section; scale bar, 2.5 μm; arrow denotes direction of withdrawal from LB trough).

in styrene at 125 °C for 24 h with 0.2 mol % 2,2,6,6-tetramethyl-1(1-phenyl-ethoxy)-piperidine as a sacrificial polymerization regulator as shown before.19 To remove physisorbed polystyrene, the brushes were rinsed with CH2Cl2. The AFM image shows the formation of regular stripes of polystyrene brushes (average stripe width (0.26 ( 0.03) μm, average stripe height (5.3 ( 0.2) nm).

Conclusion By transferring DPPC monolayers with admixture of different test compounds, we further established that this approach repre(26) Review on polymer brushes by NMP: Brinks, M. K.; Studer, A. Macromol. Rapid Commun. 2009, 30, 1043-1057. Reviews on NMP: Studer, A.; Schulte, T. Chem. Rec. 2005, 5, 27-35.Studer, A. Chem. Soc. Rev. 2004, 33, 267–273.

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sents a general and reliable method for patterning of various chemical compounds onto solid substrates. The LB transfer allows for generation of highly regular stripe patterns where the stripes contain highly interesting functional groups. The mixed layer LB transfer seems feasible for different types of molecules as long as they form stable monolayers at the air-water interface (i.e., are amphiphilic or insoluble in water). Herein, we presented the transfer of molecules bearing three different functionalities that are olefin, azide, and alkoxyamine. These functional groups can be further chemically modified by using wellestablished reactions that are the thiol-ene reaction, the Cucatalyzed azide/alkyne cycloaddition, and the nitroxide-mediated radical polymerization. Importantly, our method for in situ generation of functionalized patterned surfaces by LB transfer of multicomponent monolayers offers an easy route for large-area patterning. Acknowledgment. The authors thank the SFB858 (project B5), the Fonds der Chemischen Industrie (stipend to J. C. N.) and the TRR61 for funding. Supporting Information Available: Details on the syntheses of the compounds and experimental data for the novel compounds, additional AFM images of patterns with different amount of admixing, BAM images of DPPC/alkoxyamine monolayers on water, and corresponding isotherm data. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la102881r

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