Water Processable Polythiophene Nanowires by Photo-Cross-Linkingand Click-FunctionalizationHyeong Jun Kim,†,‡ Matthew Skinner,‡ Hojeong Yu,§,⊥ Joon Hak Oh,§ Alejandro L. Briseno,‡

Todd Emrick,‡ Bumjoon J. Kim,*,† and Ryan C. Hayward*,‡

†Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon,305-701, Korea‡Department of Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts 01003, United States§Department of Chemical Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk 790-784, South Korea⊥School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, SouthKorea

*S Supporting Information

ABSTRACT: Replacing or minimizing the use of halogenatedorganic solvents in the processing and manufacturing of conjugatedpolymer-based organic electronics has emerged as an importantissue due to concerns regarding toxicity, environmental impact, andhigh cost. To date, however, the processing of well-orderedconjugated polymer nanostructures has been difficult to achieveusing environmentally benign solvents. In this work, we report thedevelopment of water and alcohol processable nanowires (NWs)with well-defined crystalline nanostructure based on the solutionassembly of azide functionalized poly(3-hexylthiophene) (P3HT-azide) and subsequent photo-cross-linking and functionalization ofthese NWs. The solution-assembled P3HT-azide NWs weresuccessfully cross-linked by exposure to UV light, yielding goodthermal and chemical stability. Residual azide units on the photo-cross-linked NWs were then functionalized with alkyneterminated polyethylene glycol (PEG−alkyne) using copper catalyzed azide−alkyne cycloaddition chemistry. PEGfunctionalization of the cross-linked P3HT-azide NWs allowed for stable dispersion in alcohols and water, while maintainingwell-ordered NW structures with electronic properties suitable for the fabrication of organic field effect transistors (OFETs).

KEYWORDS: Solution-assembled nanowires, photo-cross-linking, click-functionalization, water processable organic electronics

Conjugated polymers have been evaluated extensively asthe active component in cost-effective, lightweight, and

flexible electronic devices due to their promising optoelectronicproperties.1−5 Over the past decade, significant academic andindustrial efforts have been devoted to the development ofefficient organic electronic devices. As a result, conjugatedpolymer-based organic field effect transistors (OFETs) withcharge carrier mobilities comparable to amorphous silicon havebeen demonstrated,6−8 and polymer solar cells have beenfabricated with power conversion efficiencies that exceed thethreshold for successful commercialization.9−11 Solutionprocessing of high efficiency organic electronic devices,however, almost exclusively involves the use of halogenatedsolvents, such as chloroform and chlorobenzene.12−16 Thetoxicity, environmental impact, and high energy cost associatedwith these halogenated solvent-based processes make their useunsustainable.17,18 Furthermore, stricter regulation on the massproduction of halogenated solvents,18 with the eventual goal ofeliminating the use of these chemicals, strongly supports theneed for the development of new electro-active materials and

processes compatible with environmentally benign solvents.Despite the importance of, and growing interest in, “green”solvents for processing organic electronics, only limited successhas been demonstrated to date.14−18

Solvents such as alcohols and water represent attractivealternatives to halogenated solvents due to reduced environ-mental impact, toxicity, and cost. In particular, the developmentof an aqueous platform for processing of organic electronicsrepresents an important step toward the goal of sustainabledevice manufacturing. Prior to this work, two major strategiesfor the synthesis of water soluble conjugated polymers havebeen reported. One approach involves the incorporation ofionic moieties to the polymer backbone, which providesolubility in alcohols and water.19−21 These charged groups,however, can quench charge carriers and influence the energylevel of the polymers, resulting in poor device performance

Received: March 26, 2015Revised: July 30, 2015Published: August 20, 2015

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when these polymers are used in the active layer.15,17 In thesecond approach, conjugated polymers are synthesized withnonionic, hydrophilic pendent groups12,22,23 or hydrophilicblocks (i.e., polyethylene glycol, PEG).24,25 For example, Duanet al.12 recently synthesized alcohol-soluble, narrow band gapconjugated polymers functionalized with pendent amino groupsand demonstrated their use as an active layer in PSCs. Despitetheir promising solubility characteristics, however, the presenceof amino side groups in these conjugated polymers led todegradation of device performance. The introduction of bulkyor extended hydrophilic side chains to improve aqueoussolubility can also disturb the formation of a well-definedcrystalline nanostructure, an important characteristic indetermining the optoelectronic properties of many conjugatedpolymers.17,26 Ensuring the fidelity of well-ordered crystallinenanostructure after introduction of solubilizing groups is animportant step toward achieving efficient green-solventprocessable conjugated polymer devices.Solution assembly of conjugated polymers into one-dimen-

sional crystalline nanowires (NWs) represents an attractiveroute for processing of organic optoelectronic devices due tothe excellent optoelectronic properties and long-range chargetransport pathways offered by these well-organized structureswith nanometer-scale cross-sectional dimensions.27−33 Forexample, conjugated polymer NWs have been used to enhancecharge carrier mobility,34−36 control bulk heterojunctionmorphology in solar cells,37−39 produce percolated conductingstructures in blends with insulating polymers,40,41 and providehybrid structures with inorganic semiconductors.42−44 Surfacemodification of preassembled conjugated polymer NWs withhydrophilic moieties would seem to represent a simpleapproach to achieve conjugated polymers that retained well-ordered crystalline nanostructures and good optoelectronicproperties while allowing for subsequent processing fromaqueous suspensions. For this strategy to succeed, maintainingstability (against both dissolution and aggregation) of the NWsduring functionalization is a key challenge.In this work, we present a strategy to chemically modify

solution assembled conjugated polymer NWs, based on azide-functionalized poly(3-hexylthiophene) (P3HT-azide), whichallows for photo-cross-linking with UV light45,46 to providestability and subsequent functionalization with alkyne-termi-nated molecules using click chemistry47,48 (Scheme 1). Photo-cross-linked P3HT-azide NWs are shown to be highly stableagainst temperature and solvent treatments, retaining theirnanostructural integrity and electronic performance. Thesephoto-cross-linked NWs are then successfully functionalizedwith alkyne-terminated PEG (PEG-alkyne) via the coppercatalyzed cycloaddition reaction using residual azide units onthe surfaces of the NWs. The resulting functionalized P3HT-PEG NWs can be stably dispersed in a variety of alcohols andwater, while maintaining effective charge transporting nano-

structures and electronic properties, enabling the processing ofefficient OFETs from environmentally benign solventswaterand alcohols.P3HT-azide copolymers were synthesized following a

previously reported procedure.45,47,48 Random copolymeriza-tion of 3-(azidohexyl)thiophene and 3-hexylthiophene wasconducted with feed ratios of 20 and 80 mol %, respectively.The actual content of 3-(azidohexyl)thiophene in thecopolymer product was determined to be 21 ± 1 mol %using 1H NMR (Figure S1). The polymerization conditionswere carefully optimized to yield highly regioregular P3HT withrelatively high molecular weight; these characteristics allow theformation of crystalline NW structures with long-range chargetransport pathways and optimal optoelectronic properties. TheP3HT-azide copolymers were highly regioregular (>98%), asdetermined by 1H NMR, with a number-average molecularweight (Mn) of 35 000 g/mol and dispersity (Đ) of 1.4, relativeto polystyrene stands, as measured using size exclusionchromatography (SEC).Solution assembly of the P3HT-azide copolymer was

induced by a mixed solvent method.49,50 Briefly, the additionof the poor solvent 1-butanol (BuOH, δ = 11.30 cal1/2 cm−3/2)to a solution of P3HT-azide in the good solvent tetrahydrofur-an (THF, δ = 9.52 cal1/2 cm−3/2) drives growth of crystallineNWs. These solvents were chosen because they have low UVcut-offs (THF = 212 nm, BuOH = 215 nm), a criticalconsideration for azide photo-cross-linking. As the relativeconcentration of BuOH in the P3HT-azide solutions wasincreased, a progressive decline in the intensity of theabsorption peak associated with dissolved P3HT (λ = 450nm) was observed by UV−vis spectroscopy. Simultaneously,vibronic bands at 564 and 611 nm appeared and increased inintensity, indicating the π-electron delocalization of P3HT-azide chains associated with NW formation (Figure 1a).51 Theoptimal solvent composition for NW self-assembly wasdetermined to be 7:1 (v/v) THF/BuOH. Under this condition,well-defined NWs were formed that had lateral dimensions of18−20 nm and lengths of several micrometers, as visualizedusing transmission electron microscopy (TEM) (Figure 1c).Remarkably, incorporation of 20 mol % azide in the P3HT-azide copolymers does not appear to substantially disrupt themolecular packing or self-assembly of the copolymers, as well-defined NWs similar to those typically formed from P3HThomopolymers30,31,52 were obtained.Photo-cross-linking of P3HT-azide NWs was carried out

using solutions in quartz cuvettes under nitrogen atmosphere,with exposure to UV light (λ = 254 nm, 0.76 mW cm−2) drivingdecomposition of azides into highly reactive nitrenes, whichleads to cross-linking via C−H insertion.53 The conversion ofazide groups was determined using Fourier transform infraredspectroscopy (FT-IR) by monitoring the reduction of the azidestretch at 2100 cm−1 (Figure S2). Figure 1b demonstrates the

Scheme 1. Schematic Illustration of Photo-Cross-Linking and Click-Functionalization of Solution-Assembled P3HT-Azide NWs

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relative amount of residual azide units after different UVexposure times; the azide peak intensity decreases withextended UV irradiation, indicating the successful conversionof the azides in solution. Interestingly, the morphology ofphoto-cross-linked P3HT-azide NWs was unchanged, regard-less of the extent of cross-linking, as shown using TEM (Figure1d,e). Photo-cross-linked P3HT-azide NWs irradiated for 60min (60% azide conversion, Figure 1d) and 120 min (90%azide conversion, Figure 1e) have a similar NW structurecompared to the uncross-linked NWs. This photo-cross-linkingmethod is particularly attractive both because it does notrequire the use of additional chemical reagents and because itavoids the extensive aggregation of NWs typically found exceptunder very dilute conditions for previously reported chemicalcross-linking methods.54−56

The thermal stability and solvent resistance of the photo-cross-linked P3HT-azide NWs were examined by monitoringthe absorbance of vibronic bands in the UV−vis spectra(Figures S3−4). The relative intensity of the vibronic peak at564 nm is plotted as a function of the solution temperature(Figure 2a) and volume equivalents of added chlorobenzene(CB) (Figure 2b). Increasing the temperature from 25 to 50 °Cleads to complete dissolution of uncross-linked P3HT-azideNWs, as shown by the disappearance of the vibronic peaks. Incontrast, the photo-cross-linked NWs retained significantabsorption at 564 nm, with those irradiated for 60 minshowing almost no reduction in intensity after thermaltreatment, indicating a high degree of structural stability. Asimilar trend was observed with regards to solvent resistance.Addition of an equal volume of CB, which is one of the bestsolvents for P3HT, to the uncross-linked NWs resulted in

complete dissolution, whereas NWs irradiated for 60 minretained nearly the original vibronic band peak intensity afteraddition of an equal volume of CB. Interestingly, only 60%azide conversion (60 min of UV irradiation) was sufficient toyield highly stable NWs, leaving 40% of residual azide groupsavailable as reactive sites for further functionalization.To examine the effect of photo-cross-linking on the electrical

properties of the P3HT-azide NWs, the hole transportmobilities of OFET devices were measured, as summarized inTable 1. A series of devices based on P3HT-azide NWs treated

with UV exposure times of 0, 30, 60, and 90 min werefabricated in a top-contact, bottom-gate geometry. The surfaceof a highly n-doped Si wafer with a thermally grown SiO2 (300nm Ci = 10 nF cm−2) was modified with an octadecyltrime-thoxysilane (OTS) monolayer following a literature proce-dure.57 Gold electrodes were deposited with a channel length of50 μm and a channel width of 1000 μm. Without cross-linking,P3HT-azide NWs exhibited a hole mobility of 7.7 × 10−3 cm2

V−1 s−1, which agrees with previously reported hole mobilitiesof highly ordered P3HT films.45,58 We observed similar holemobilities of 6.5 × 10−3 cm2 V−1 s−1 and 6.0 × 10−3 cm2 V−1 s−1

for P3HT-azide NW devices that had been subjected to 30 and

Figure 1. (a) UV−vis absorption spectra of P3HT-azide in differentsolvent mixtures (THF/BuOH (v/v), 1 mg/mL). (b) Residual azideas a function of UV exposure time as determined from FT-IR spectra(inset), which show a decrease in intensity of the azide peak (2100cm−1) with UV exposure. TEM images of P3HT-azide NWs in 7:1 (v/v) THF/BuOH mixtures after (c) 0 min, (d) 60 min, and (e) 120 minof exposure to UV light.

Figure 2. (a) Thermal and (b) solvent resistance of P3HT-azide NWsin 7:1 THF−BuOH (v/v) solution with different UV exposure times:0, 10, 30, and 60 min. NW stability was monitored using UV−visspectroscopy to measure the peak intensity of the vibronic band at 564nm.

Table 1. OFET Characteristics of P3HT-Azide NWsSubjected to Different UV Exposure Times

mobility(× 10−3 cm2 V−1 s−1)

on/offratio Vt (V)

UV 0 min 7.7 ± 2.8 >103 −13.9 ± 10.9UV 30 min 6.5 ± 1.9 >103 2.7 ± 5.3UV 60 min 6.0 ± 1.5 >103 −1.8 ± 10.8UV 90 min 3.2 ± 0.9 >103 −9.6 ± 13.9

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60 min of photo-cross-linking, respectively. While degradationin performance was observed for devices exposed to furtherirradiation (hole mobility of 3.2 × 10−3 cm2 V−1 s−1 at 90 min),devices cross-linked for 60 min showed similar mobility andtransistor characteristics as uncross-linked devices (Figure S5),demonstrating that photo-cross-linking of azide-functionalizedconjugated polymers is an effective route to produce stableNWs that retain their electronic properties.A distinct advantage of the P3HT-azide system is that

residual azide groups on the photo-cross-linked NWs can besubsequently functionalized with alkyne-terminated moleculesby copper catalyzed azide-alkyne cycloaddition. We chose PEGas a model system for functionalization because of its excellentaqueous solubility (Figure 3a). Alkyne-terminated monomethyl

ether PEG (PEG-alkyne, Mn = 3.0 kg/mol, Đ = 1.05, FigureS6a), was synthesized following a modified literature procedure(Scheme S1).59 Fidelity of the alkynyl end-group of the PEG-alkyne was confirmed by MALDI-TOF (Figure S6b) and 1HNMR (Figure S7) analyses. PEG-alkyne was then coupled tophoto-cross-linked P3HT-azide NWs (60 min UV exposure) ina 7:1 THF/BuOH solution in the presence of a CuBr catalyst.Following the click reaction, the azide peak disappeared fromthe FT-IR spectrum, indicating complete reaction of residualazides with PEG-alkyne (Figure S8). Excess PEG-alkyne wasremoved by centrifugation, and the resulting product wastreated with CupriSorb to remove residual copper. TEMimages (Figure 3b) revealed that P3HT-PEG NWs retainedessentially the same morphology as pristine P3HT-azide NWs.In contrast, functionalization of uncross-linked P3HT-azideNWs with PEG led to an almost complete destruction of NWstructure, as evidenced by a dramatic decrease in the intensityof the vibronic bands in UV−vis spectra (Figure S9) and a lossof NW morphology in TEM images (Figure 3c). To determinethe areal density of PEG chains on the P3HT-PEG NWs, weadopt the monoclinic structural model of P3HT crystals ofBrinkmann et al.,60 which gives an area per thiophene unit of

0.15 nm2 along the NW surfaces, corresponding to (100)crystal planes. However, only half of these have alkyl chainsfacing toward the nanowire surface, yielding an areal density of3.3 nm−2. Since 21 mol % of hexyl units are initially azidefunctionalized, and 60% of these are consumed during cross-linking, the areal chain density of remaining azides, andtherefore grafted PEG chains, is calculated to be 0.28 nm−2.This is several times larger than the surface overlap densityestimated for 3 kg/mol PEG chains, 1/(π × Rg

2) ≈ 0.08 nm−2,with radius of gyration Rg ≈ 2 nm, suggesting that the graftedPEG layer may provide effective steric stabilization of the NWsin polar solvent environments.To gain insight into possible changes in the molecular

packing of P3HT-azide NWs following PEG functionalization,wide-angle X-ray scattering (WAXS) of cross-linked P3HT-azide and P3HT-PEG NWs was performed (Figure 4a). Photo-cross-linked P3HT-azide NWs exhibited two distinct peakswith d-spacings of 1.69 and 0.37 nm, consistent with literaturevalues for the distance between polythiophene backbonesseparated by interdigitated side chain (100) and π−π stackingdistance (010), respectively.26 After PEG functionalization, thedistances between adjacent P3HT-azide chains were essentiallyunchanged at 1.69 and 0.37 nm, indicating that the molecularpacking within P3HT-azide NWs, a critical factor indetermining optoelectronic properties, was not altered bysurface functionalization with PEG-alkyne. We can concludethat P3HT-azide NWs provide an excellent platform for addingdiverse functionality on the surface of NWs using facile clickchemistry, while the molecular packing structure of the NWscan be preserved due to photo-cross-linking.To determine the effect of PEG functionalization on the

electrical performance of NWs, OFET devices were fabricatedby casting films using 7:1 THF/BuOH as a solvent. Devicesbased on P3HT-PEG NWs showed shifted threshold voltages,and device operation became hysteric (Figure S10), whichmight be a result of the polarity of the PEG side chains that caninduce p-type doping in the active layer.23,46 In addition, theability of PEG chains to coordinate cationic species, particularlyLi+, K+, and Na+, could yield higher levels of salt impurities infilms of the functionalized polymers, further affecting perform-ance. While the hole mobility for P3HT-PEG NWs (7.2 × 10−4

cm2 V−1 s−1) was reduced by an order of magnitude relative tothat for the UV-cross-linked NWs, it remained 2 orders ofmagnitude larger than that for uncross-linked P3HT NWsfunctionalized with PEG (4.3 × 10−6 cm2 V−1 s−1), clearlydemonstrating that photo-cross-linking of P3HT-azide NWsprovide sufficient stability to allow additional functionality to beadded to the NW surface while largely preserving structural andelectronic properties.Figure 4b−d shows the structure of P3HT-PEG NWs

dispersed in a variety of polar solvents. To demonstrate thesuitability of these functionalized NWs for processing usingenvironmentally benign solvents, suspensions in 7:1 THF/BuOH were centrifuged to isolate the P3HT-PEG NWs,followed by dispersion in BuOH (δ = 11.30 cal1/2 cm−3/2),MeOH (δ = 14.28 cal1/2 cm−3/2), and water (δ = 23.50 cal1/2

cm−3/2). In all cases, P3HT-PEG NWs formed dispersions thatwere stable against aggregation, while nonfunctionalized P3HT-azide NWs remained highly aggregated. To demonstrate thepotential of these materials for organic electronics processedfrom high polarity solvents, OFET devices were prepared fromP3HT-PEG NWs suspended in each solvent. The holemobilities for devices based on P3HT-PEG NWs dispersed in

Figure 3. (a) Functionalization of photo-cross-linked P3HT-azideNWs with PEG-alkyne using copper-catalyzed azide−alkyne cyclo-addition chemistry. Following PEG functionalization, (b) photo-cross-linked P3HT-azide NWs maintained their structure, whereas (c) thestructure of uncross-linked P3HT-azide NWs was disrupted.

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BuOH, MeOH, and water were found to be 6.5 × 10−4, 5.7 ×10−4, and 2.1 × 10−4 cm2 V−1 s−1, respectively (Figure 4e,Figures S11 and S12). The slightly lower mobility of the water-based device was most likely a result of high surface tension,which resulted in a relatively nonuniform film. It should benoted, however, that this is the first demonstration of a water-processed P3HT-based organic electronic device, thus demon-strating the excellent potential of P3HT-azide for formation ofstable and functional NW structures. Furthermore, theextension of this approach to conjugated polymers withinherently higher mobilities than P3HT may provide a futureroute to water-processable NW structures with even moreattractive electronic properties.Conclusions. In this work, we have demonstrated a

platform for producing chemically and thermally stable,functionalized conjugated polymer NWs that facilitate deviceprocessing with high polarity and environmentally benignsolvents. Copolymers based on P3HT and containing 20 mol %azide-functionalized thiophene units undergo crystallization-driven solution assembly into NWs similar to those of P3HThomopolymers. Irradiation with 254 nm UV light drives cross-linking, with a conversion of 60% of azide units providingexcellent stability against both thermal and solvent treatments.Surface modification of cross-linked NWs with PEG-alkyneusing the residual azide groups allowed for the stable dispersionof functionalized NWs in alcohols and water, enablingprocessing of P3HT-based OFET devices from environ-mentally benign solvents. We anticipate that this approach tohighly stable and functionalized NWs will provide a valuable

and flexible tool for the processing of other types of conjugatedpolymer-based devices, including photovoltaics and sensors.

Methods. Preparation of Solution-Assembled P3HT-AzideNWs. P3HT-azide was first dissolved in tetrahydrofuran (THF)at 10 mg/mL with heating and sonication. The solution wasthen passed through a PTFE syringe filter (0.45 μm), and 1-butanol (BuOH) was slowly added into the solution withvigorous stirring to induce homogeneous nucleation of theNWs. The resulting solution with a concentration of 1 mg/mLand a THF/BuOH volume ratio of 7:1 was aged for 48 h withstirring at room temperature.

Photo-Cross-Linking and Click-Functionalization. P3HT-azide NW solutions were placed in a quartz cuvette undernitrogen atmosphere and exposed to UV light (λ = 254 nm)from a low power hand-held lamp (UVP UVGL-25, 0.76 mWcm−2), with gentle stirring. For the click reaction, all procedureswere performed under nitrogen atmosphere to avoid oxidationof the Cu catalyst. After photo-cross-linking, an excess amountof PEG-alkyne (1.5 eq, 0.015 mmol, 35 mg) was added to the60 min photo-cross-linked P3HT-azide NW solution (10 mL,0.01 mmol of azide units). Next, a solution containing 10 mg ofCuBr (0.07 mmol) and 14.55 μL of PMDETA (0.07 mmol) in1 mL of 7:1 THF/BuOH was prepared, and then 0.14 mL (1eq, 0.01 mmol) of this CuBr/PMDETA solution was addedinto the P3HT-azide and PEG-alkyne solution to initiate theclick reaction. Prepared solutions were stirred overnight at 40°C. Afterward, CupriSorb (1 mg) was added to remove thecupper catalyst, and then excess PEG-alkyne was removed bythe centrifugation. Collected suspensions of P3HT-PEG NWs

Figure 4. (a) X-ray scattering of photo-cross-linked P3HT-azide NWs and P3HT-PEG NWs. TEM images of P3HT-PEG NWs dispersed in (b)BuOH, (c) MeOH, and (d) water, with insets showing a lack of dispersion for P3HT NWs (left) and excellent dispersion for P3HT-PEG NWs(right). (e) Hole mobilities measured for OFET devices prepared from P3HT-PEG NWs dispersed in various solvents (7:1 THF/BuOH, BuOH,MeOH, and water) were compared.

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were redispersed in THF/BuOH, BuOH, MeOH, or water at aconcentration of 1 mg/mL.Electronic Device Fabrication and Measurement. OFET

devices were fabricated on heavily doped n-type silicon waferswith a 300 nm thick thermally grown SiO2 layer (Ci = 10 nFcm−2) as the substrate and dielectric. The surface of Si/SiO2substrates was then modified with OTS self-assembledmonolayer (SAM) as previously reported.57 The contactangle on the hydrophobic OTS-modified wafer with D.I.water droplet was found out to be >106°. The OTS-treatedsubstrates were washed sequentially with toluene, acetone, andisopropyl alcohol and dried under a nitrogen stream.Suspensions containing 1 mg/mL of photo-cross-linkedP3HT-azide NWs in 7:1 THF/BUOH or P3HT-PEG NWsin 7:1 THF/BuOH, BuOH, MeOH, or water were drop-castonto the substrates, followed by heating for 10 min at 150 °Cto remove solvent. Au electrodes (40 nm) were thermallyevaporated onto the substrates with a 50 μm channel lengthand a channel width of 1000 μm using a shadow mask. Theelectrical performance was measured in a N2-filled gloveboxusing a Keithley 4200 SCS and Micromanipulator 6150 probestation.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.nano-lett.5b01185.

Detailed experimental procedures and additional charac-terization data (PDF)

■ AUTHOR INFORMATION

Corresponding Authors*E-mail: hayward@umass.edu.*E-mail: bumjoonkim@kaist.ac.kr.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

This research was supported by the US Department of Energy,Office of Basic Energy Sciences through Grant DE-SC0006639(self-assembly, photo-cross-linking, and functionalization stud-ies) and a Korea Research Foundation Grant from the KoreanGovernment (2013R1A2A1A03069803; polymer synthesis,support for H.J.K.).

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