Surface Grafting of Functionalized Poly(thiophene)s Using Thiol−EneClick Chemistry for Thin Film StabilizationK. Lindsey Martin, Yannick Nyquist, Edmund K. Burnett, Alejandro L. Briseno, and Kenneth R. Carter*

Conte Polymer Center for Polymer Research, University of MassachusettsAmherst, 120 Governors Drive, Amherst, Massachusetts01003, United States

*S Supporting Information

ABSTRACT: Regioregular poly[(3-hexylthiophene)-ran-(3-undecenylthiophene)] (pP3HT) and vinyl terminated poly(3-hexylthiophene) (xP3HT) were synthesized by the McCullough method and surface grafted to thiol modified silicon dioxidewafers using thiol−ene click chemistry. Utilizing this method, semiconducting, solvent impervious films were easily generated.Thiol−ene click chemistry is convenient for film stabilization in electronics because it does not produce side products that couldbe inimical to charge transport in the active layer. It was found through grazing incidence wide-angle X-ray scattering (GIWAXS)that there is no change in microstructure between as-spun films and thiol−ene grafted films, while there was a change after thethiol−ene grafted film was exposed to solvent. Organic field-effect transistors (oFETs) were fabricated from grafted films that hadbeen swelled with chloroform, and these devices had mobilities on the order of 10−6 cm2 V−1 s−1, which are consistent withpoly(thiophene) monolayer devices.

KEYWORDS: conjugated polymers, click-chemistry, polythiophene, surface grafting, microstructure

■ INTRODUCTION

The emergence of conjugated polymers in the field ofelectronics has inspired considerable research on their proper-ties and their incorporation into various device structures.1 As aresult of these studies, conjugated polymers can be syntheticallytuned to have specific electronic and solubility properties.2

Presently, constructing thin films of conjugated materials overan assortment of large area substrates is facile and cost-effectiveand can be accomplished through a variety of methods,including dip-coating, spin-coating, and roll-to-roll process-ing.3−5

However, conjugated polymer films are susceptible to vaporor solvent vitiation, limiting useable device architectures anddevice performance.6 Devices requiring the deposition ofmultiple layers of polymer films can be difficult to fabricate,as the sequential solutions may disturb the initial film, throughswelling, polymer chain reorganization, or by removal of theunderlying film.7,8 Solvent vapors can change the filmmorphology, in some cases inducing phase segregation of thetwo components to such a degree that charge transport is nolonger viable.9

Grafting conjugated polymers to substrate surfaces presents aunique solution to the above-mentioned thin film instabilityissues. Grafted polymers are chemically bound to a surface,which enables the films to be solvent resistant and tolerateexposure to solvent vapors without significant swelling.10

Polymers can be grafted to a surface by functionalizing thepolymer and the substrate with complementary chemicalmoieties, and introducing the polymer to the substrate byeither solution or melt processes, and this method is termed“grafting-to”. The ultimate thickness of these grafted films, asmeasured by ellipsometry or AFM, is dictated by how well themodified polymers diffuse to the surface, where they chemicallyreact with the surface functionalities, and the molecular weightof the polymer.11−15 Polymers can also be grafted from asurface.12,16,17 In this case, an initiator is immobilized on thesurface and exposed to reactive monomers, allowing the growthof densely packed, uniform polymer chains.18 In addition to

Received: July 14, 2016Accepted: October 24, 2016Published: October 24, 2016

Research Article

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© 2016 American Chemical Society 30543 DOI: 10.1021/acsami.6b08667ACS Appl. Mater. Interfaces 2016, 8, 30543−30551

rendering the film resistant to solvents, grafting allows forprecision over the density, orientation, and location ofpolymers on substrates.19,20 This can enable specializedmorphologies, different from the spin coated films, whichimpact charge transport through semiconducting polymerfilms.21,22

Many different chemistries have been used to create graftedfilms, including condensation and nitrene insertion reactions, aswell as various click chemistry strategies.7 Click chemistry isparticularly advantageous for grafting-to applications because itdoes not yield byproducts, can be achieved under mildconditions, is nearly quantitative, and has fast reactiontimes.23,24 The most commonly employed click chemistryused for grafting-to applications is the azide−alkyne cyclo-addition reaction.25−27 Conjugated polymer films graftedthrough azide−alkyne click chemistry have been shown to besolvent impervious and have charge carrier mobilitiescomparable to nongrafted devices.13,28 Thiol−ene chemistryhas also been used for grafting purposes, detailing thephotopolymerization of nonconjugated difunctionalized mono-mers from a functionalized surface, or the anchoring ofbiocompatible materials to substrates.29,30 It has also beenshown that grafting thiol functionalized small molecules andpolymers to vinyl functionalized polythiophene films throughthiol−ene click chemistry can substantially impact the chargetransport through the films without changing the surfacemorphology.31

Thiol−ene click chemistry has primarily been used tostabilize conjugated polymer networks through cross-link-ing.10,32,33 In one report, functionalized poly(dialkyl fluorene)swere grafted to modified substrates using thiol−ene clickchemistry.12 These grafted films maintained the sameoptoelectronic properties as the analogous spin-coated films,even after rinsing the film with good solvents for the linearpolymers. In this study, we extend the application of thiol−eneclick grafting to two distinctly functionalized poly(thiophenes).We analyze the solid state structure of grafted films, before andafter rinsing, for both functionalized poly(thiophenes), andcompare these to the morphology of the spin-coated films.Additionally, the grafted film thicknesses were monitored withrespect to UV and solvent exposure, and the electronicproperties were also measured.

■ RESULTS AND DISCUSSION

Synthesis of Alkene Functionalized Poly(thiophene)s.Alkene functionalized poly(thiophene)s were synthesized totest the efficacy of thiol−ene click chemistry as a covalentbinder between the polymer and a thiol modified silicon

dioxide wafer. Poly(3-pentenylthiophene) (P3PNT), a polymercontaining terminal alkene moieties on the side chains, wassynthesized by polymerizing 2-bromo-5-chlorozinc-3-pente-nylthiophene with a nickel catalyst using the McCulloughmethod.23,34 The polymerization resulted in a polymer thatfluoresced a bluish white color when exposed to UV light, instark contrast to the bright yellow/orange fluorescencecharacteristic of low dispersity and high molecular weightpoly(thiophene)s. The MALDI-TOF spectrum of P3PNTrevealed that the polymer consisted of highly dispersedmolecular weights. Mechanistically, it has been hypothesizedthat the active Ni(0) can coordinate with the pendant alkenespresent in the monomer and polymer. This alkene coordinationcompetes with the polymer forming reaction by breaking itscomplex with the thiophene ring, which limits the effectivenessof the polymerization.23,35,36 Although this polymer was notused for subsequent studies in electronic devices, it was used inthiol−ene click surface reactions (vide infra).To circumnavigate the polymerization complication, a

random copolymer of 3-undecenylthiophene and 3-hexylth-iophene (pP3HT), with a molar ratio of 2:3, was synthesized bythe McCullough method (Figure 1a). It was anticipated thatthe long undecene chain would be less likely to interact withthe Ni(0) catalyst, but after allowing the solution to react for 14h, 3-undecenylthiophene was only incorporated at 14%(determined by 1H NMR). Mn was determined to be about5460 g/mol via MALDI-TOF, while Đ was 1.02.To study the reactivity of chain-end−ene functionalized

polymers, vinyl end-capped P3HT (xP3HT) was synthesizedusing the McCullough method from 2-bromo-5-chlorozinc-3-hexylthiophene (Figure 1b). The reactions were set up andallowed to proceed at room temperature for 5 min and thencooled to 0 °C and allowed to react for 10 additional minutes.The growing polymer chains were terminated by adding anexcess of vinyl-Grignard reagent. Cooling the reaction beforeintroducing the Grignard reagent was critical to reduce thepresence of hydrogenated end groups, formed by magnesium-halogen exchange.37 The vinyl monocapped P3HT wascharacterized by 1H NMR and MALDI-TOF to confirm thesuccess of the vinyl-Grignard capping. Attempts to substitutethe bromide end group of the vinyl monocapped P3HT werefirst made using Kumada coupling. The mono functionalizedpolymer and Ni(dppp)Cl2 were cooled to 0 °C, and vinyl-MgClsolution was added dropwise. The MALDI-TOF analysisshowed that the resulting polymer was terminated by hydrogenrather than the desired vinyl group. The vinyl substitution wassuccessfully accomplished using Stille coupling to make atelechelic vinyl polymer by reacting the monocapped P3HT

Figure 1. (a) Synthetic schematic for pP3HT, where R1 is 7 and R2 is 3. (b) Synthetic schematic for xP3HT where R2 is 3.

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with tributyl(vinyl)stannane in toluene at 100 °C for 24 h,using a catalytic amount of Pd[PPh3]4. The product wascharacterized by 1H NMR and MALDI-TOF. The Mn of thetelechelic polymer was found to be 4575 g/mol with a Đ ≈1.01.Thiol−Ene Surface Grafting of Polythiophenes. Heav-

ily doped p-type silicon substrates, with a 200 nm thermallygrown silicon dioxide layer, were sequentially sonicated inacetone, ethanol, and isopropanol to remove contaminants,followed by immersion in a piranha cleaning solution (amixture of 3:1 sulfuric acid to hydrogen peroxide) for 4 h andrinsing with water. This process produced a hydrophilic surfacewith a nondiscernable water contact angle (WCA) less than 5°.The cleaned silicon wafers were then functionalized with (3-mercaptopropyl)triethoxysilane (MPTS), through vapor phasedeposition and concurrent condensation of the MPTS with thenative SiO2 surface. This was accomplished by placing thesilicon wafers in a Petri dish overnight at 100 °C next to ashallow glass dish containing 0.1 mL of MPTS on a hot plate.Successful silanation of the surface was probed by measuringthe change in WCA. The WCA of the treated surfaces variedbetween 73° to 87°. The higher contact angles of the MPTSmonolayer may be due to less oriented monolayers (FigureS11).38 The thiol modified wafers were transferred to aglovebox, where 5 mg/mL solutions of functionalized poly-(thiophene) in chlorobenzene were spin-coated at 1000 rpmfor 60 s. The spin-coated resulting films had WCAs greater than90°. The wafers were annealed at 150 °C for 5 min and thenexposed to 254 nm UV light inside the glovebox for apredetermined amount of time to graft the functionalizedpoly(thiophene)s to the surface. The grafted samples wereintroduced to chloroform using one of three procedures (videinfra), to remove any unreacted polymer. They were reheatedto 150 °C for 5 min to remove any residual solvent (Figure 2).Using the procedure described above, P3PNT was grafted in

air to a MPTS monolayer, resulting in a film with a WCA of101°. After immersion soaking in chloroform for 8 h, the filmwas dried to reveal an essentially unchanged WCA of 103°. TheXPS of this surface is shown in Figure S10 and the C 1s peakincludes a shoulder which could be deconvoluted into fourpeaks. The principle integration at 284.71 eV represents 70% of

bonds to carbon. An adjacent integration, at 285.94 eV,represents 15% of the bonds to carbon, and is indicative ofhydroxyl groups bonded to the α carbon of the thiophene ring.This position is easily oxidized upon exposure to radical oxygenspecies. The smallest integrations at 287.25 and 288.93 eVcompromise 16% of bonds to carbon and represent carbonyland carboxyl groups, respectively, signifying further oxidation ofP3PNT.39

A series of control experiments were performed to confirmthat a thiol−ene click reaction was the most effective methodfor the formation of insoluble thin films. These experimentsused nonfunctionalized P3HT and P3PNT to show that boththe vinyl groups in P3PNT, and the MPTS coated substratewere required for grafting. The UV-initiated grafting reactionswere performed in air in order to partially oxidize the P3PNT,as described above. The XPS of oxidized poly(thiophene)s havea distinctive C 1s peak, with the largest integration at 284.75 eVdue to the methylene carbons and aromatic bonds betweencarbon. Three smaller integrated curves protrude like ashoulder from the 284.75 eV peak, which represent variousdegrees of poly(thiophene) oxidation. This characteristic peakcan be used as a label to determine if P3PNT is present on thewafer, which would indicate that thiol−ene click grafting wassuccessful (Figure S7).39−41

The first control experiment involved spin-coating P3PNTonto a clean SiO2 substrate and exposing it to 254 nm UV lightat 150 °C for 1 min. The WCA of the spin-coated film was 93°.The film was immersion soaked in chloroform for 8 h. Afterdrying, the contact angle decreased noticeably to 63°. The C 1speak for this sample did not possess the oxidation shouldercharacteristic of oxidized poly(thiophene)s. The small amountof carbon content detected is likely due to residual organiccompound contamination either from the chloroform treat-ment or adsorbed air during handling (Figure S8). Thisexperiment showed that there was not any residual P3PNT onthe silicon wafer after the chloroform immersion soak,suggesting that UV exposure and heat are insufficient to induceradical cross-linking or fixing of the polymer to the SiO2 surfaceon this time scale.In the second control experiment, poly(3-hexylthiophene)

was spin-coated onto a MPTS modified substrate and received

Figure 2. Thiol−ene click grafting schematic. (1) MPTS vapor deposition to make monolayer, (2) spin coat substrate with vinyl functionalizedpoly(thiophene), (3) expose sample to UV light and heat, and (4) wash sample with CHCl3.

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the same UV-grafting procedure as described in the first controlexperiment. The WCA of the resulting film was 98°. The filmwas then immersion soaked in chloroform for 8 h. After drying,the WCA was observed to decrease to 86°, which is within therange associated with a nonsubstituted MPTS monolayer. TheC 1s peak is symmetric and can be deconvoluted into twointegrals, one at 284.75 eV and one at 286.10 eV, with the284.75 eV peak dominating. This spectrum is comparable toreported spectra of MPTS monolayers, representing methylenecarbons (284.75 eV), as well as oxygen bonds to carbon fromethoxy groups in the film (286.10 eV; Figure S9).38,42 Thiscontrol experiment indicated that the MPTS monolayerremained on the wafer, but immersion soaking in chloroformremoved all of the poly(3-hexylthiophene).From the controls it seems evident that both the thiol and

vinyl moieties are necessary for insoluble polythiophenes filmformation. The XPS and WCA measurements support theclaim that thiol−ene click chemistry is the mechanism ofinsoluble film formation for these conditions, and no otherchemistry, namely vinyl cross-linking, is responsible foranchoring P3PNT to the substrate surface. In Figure 3, a bargraph depicts the elemental composition of each sample for thegrafted P3PNT film, the first control, the second control, andthe rinsed grafted P3PNT film.

Morphology/Microstructure. Both xP3HT and pP3HTwere grafted to MPTS monolayers in a N2 glovebox for 10 minusing the previously described procedure. After grafting, somesamples were rinsed with chloroform, which entails dropping0.3 mL of chloroform on the surface of the grafted film andthen immediately spinning it off at 2000 rpm for 60 s, in theglovebox at room temperature. This was done to removepolymer that did not covalently graft to the surface. XPS wasperformed on spin-coated, grafted, and grafted and rinsedsamples of xP3HT and pP3HT (Figures S14 and S15).To investigate the microstructure of these films, grazing-

incidence wide-angle X-ray scattering (GIWAXS) experimentswere performed on the spin-coated, grafted, and rinsed samplesof xP3HT and pP3HT (Figure 4). The angle was changed toprobe the microstructure throughout the film, and themicrostructure was observed to be constant from the interfaceof the substrate and the graft to the surface of the film. TheGIWAXS from the as spun samples show similar orientation forboth polymers. The (100) reflection, along with higher-orderreflections, is centered at q = 0.34 Å−1 (d = 18.5 Å) along theqz-axis. The (010) reflection is present at q = 1.70 Å−1 (d = 3.70

Å) along the qxy-axis. These reflection positions are character-istic of an edge-on orientation with the lamella layer structureoriented normal to the film and the π−π interchain stacking inthe plane of the film. After exposure to UV-light and heat toinduce grafting, no morphological differences could be resolvedfrom the scattering. This indicates that the grafting proceduredoes not effect the microstructure of the poly(thiophene) films.After the films were rinsed, a clear rearrangement could be seenin the structure of the films. The pP3HT samples showed achange in preferred orientation with the (100) reflectionpresent in the plane. Higher-order reflections were absent inthis sample, indicating that the film is less ordered after rinsing.xP3HT shows a similar change in orientation, but a moreordered system, as the (100)-axis is oriented in the plane andthe (010) reflection is present normal to the film. This film alsoshows a dual texture with the (100)-axis also present normal tothe film, but the main reflection positions are representative ofa face-on orientation.43

Film Thicknesses and Grafting Density. Both xP3HTand pP3HT were grafted to MPTS surfaces with certainprocedural variations. The grafting times were varied between1, 5, and 10 min. Additionally, the grafted films were exposed tochloroform using three different methods to remove residualunreacted polymer. The first method was rinsing, as describedabove, and the film was only exposed to chloroform for a fewseconds. The second method, immersion soaking, requiredplacing the grafted film and 10 mL of chloroform into ascintillation vial, and then setting the vial on a vortex mixer for24 h. The last method, puddle and rinse, involved dropping 0.3mL of chloroform onto a grafted film, full wetting the surface,resting on the spin-coater chuck. After 30 s of the chloroformswelling the grafted film, the spin-coater ramped to 1000 rpmover a period of 5 s, and then spun consistently for 60 s, toremove the unreacted polymer that dissolved in the chloroformdroplet. These three methods vary the time that the graft wasexposed to chloroform. The WCAs of the grafted films weremeasured after chloroform exposure for each of the graftingtimes. AFM and profilometry were used to determine thethicknesses of the films. In Figure 5 the thicknesses of thegrafted and exposed films are plotted against grafting times foreach solvent exposure method. For both pP3HT and xP3HT,rinsing was the least effective method for removing unreacted,interpenetrated polymer, while puddle and rinse was the mosteffective method. Previous reports describe the calculation ofgrafting density for both silane monolayers and the polymergrafts.11,13 The MPTS monolayer thickness was around 0.4 nm(Figure S11), corresponding to a grafting density of 1molecule/nm2. Consequently, it should not be possible forthe grafted polymer films to have a grafting density higher thanthe MPTS density. Grafting densities greater than 1 chain/nm2

are suggestive of residual interpenetrated polymer, or thebeginning of vinyl cross-linking. As the puddle and rinsemethod removed the interpenetrated polymer most efficientlyfor both xP3HT and pP3HT, it seems that the high graftingdensities for xP3HT for the 10 min graft times could be a resultof secondary vinyl cross-linking reactions. This is likely due tothe well-known diradical formation between two styrenic endgroups upon exposure to heat. The diradical has an extremelyshort lifetime, allowing it to quickly ring close. This is one ofthe possible reactions from the Flory mechanism of styrenethermally initiated autopolymerization. Aliphatic alkenes do notundergo this reaction and are not apt to undergo these kinds ofthermally initiated reactions as readily as styrenic materials. For

Figure 3. Bar graph depicting the elemental composition (%) of bothcontrols compared to the grafted film and the grafted and rinsed film,determined by XPS.

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example, a previous report on the thermal cross-linking ofpoly(3-(5-hexenyl)thiophene), describes needing 2 h of 150 °Cheating to create a cross-linked network of this polymer.44,45

The high grafting densities from the rinse method are mostlikely due to interpenetrated polymer.Electronic Properties. Organic field effect transistors

(oFETs) were fabricated with a bottom gate, top contactarchitecture using both grafted pP3HT and xP3HT films as theactive layer, and the native SiO2 as the dielectric. Active layerswere grafted for 1, 5, and 10 min, followed by puddle and rinsewith chloroform. The hole mobilities were calculated using theslope of √IDS versus VGS line, in the saturation regime (FiguresS12 and S13). The channel length of the electrodes was 100μm, while the width was 2000 μm. The dielectric capacitance ofSiO2 is 1.7 x10−8 F/cm2.46 The devices turned on undernegative gate voltages, demonstrating hole transport throughthe grafted and swelled film. The transfer curves generated forxP3HT and pP3HT based devices are shown in Figure 6. Thelinear hole mobilities were calculated to be on the order of 10−6

cm2V−1s−1 for both pP3HT and xP3HT grafted and swelledactive layers, for each grafting time. The low mobility for thesefilms could be a result of many factors, including the face-onorientation, the thinness of the films, and high contactresistance. Previous reports indicate that face-on orientationleads to hole mobilities 100x lower than edge-on P3HT films.Charge transport in the direction of the pi-pi stacking is moreeffective in the architecture of typical oFETs when the film hasedge-on orientation.47,48 It has also been shown that inmonolayers of P3HT, oFET mobilities are typically on theorder of 10−5−10−4 cm2 V−1 s−1, even for monolayersexhibiting edge-on orientation. In this report, it is explainedthat ultrathin films may be strained due to their contact withtwo interfaces (air, substrate) causing them to lose thecrystalline order that a bulk film might possess. It is alsopossible that interfacial defects between the monolayer and thesubstrate can hinder charge transport entirely, where in a bulkfilm, the upper layers of polymer would help assist chargetransport across such defects.48,49

■ CONCLUSIONS

This study showed thiol−ene click chemistry effectively graftsfunctionalized poly(thiophene)s to MPTS modified SiO2surfaces. Grafting times between 5 and 10 min were enoughto achieve high grafting densities for both end and pendantfunctionalized poly(thiophene)s. Puddle and rinsing the graftedfilms with chloroform was the most effective method forremoving interpenetrated, nongrafted polymer. The success ofthe grafting experiments was reliably tested by contact angleand X-ray photoelectron spectroscopy (XPS). Two controlswere used to validate that the fabricated films were a result ofalkene functionalized poly(thiophenes) grafting to the sub-strates by thiol−ene click chemistry, as opposed to the alkenefunctionalities radically cross-linking to create imperviousnetworks. Unexpectedly, the grafting procedure did not effectthe microstructure of the film, maintaining an edge-onorientation from the as spun film to the grafted film. Thegrafts that were rinsed in chloroform showed a significantchange in microstructure to a face-on orientation, with thexP3HT graft being more ordered than the pP3HT graft. To ourknowledge, this is the first example of an oriented film made bya grafting-to method. Even after swelling with chloroform, thesefilms were electrically active, demonstrating hole transport infield-effect transistors. The mobilities were low for poly-(thiophene) derivatives, but inline with ultrathin films, whichmay experience interfacial strain and defects to a greater degreethan bulk films do. Thiol−ene click surface grafting offers asimplistic method to fabricate well oriented, solvent resistant,electroactive, ultrathin films.

■ EXPERIMENTAL SECTIONMaterials. All materials were purchased from Sigma-Aldrich or

VWR and used without further purification. Tetrahydrofuran (THF)was dried over sodium/benzophenone and distilled before using.Silicon substrates (P-doped, 0.01−0.018 ohm·cm, 200 nm thermallygrown SiO2) were purchased from University Wafers. Gold for metalevaporation was purchased from Kurt Lesker.

Instrumentation. All nuclear magnetic resonance (NMR) spectrawere acquired on a Bruker 500 (500 MHz) spectrometer andinternally referenced via residual solvent signal (CHCl3:

1H 7.26 ppm).

Figure 4. Grazing incidence wide-angle X-ray scattering (GIWAXS) where, (top) 2D image maps of spin-coated, grafted, and grafted and rinsedpP3HT films; and (bottom): 2D image maps of spin-coated, grafted, and grafted and rinsed xP3HT films.

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MALDI-TOF was performed on a MicroFlex LRF MALDI-TOF fromBruker Daltonics (Billerica, MA). Mass spectral data were obtained atthe University of Massachusetts Mass Spectrometry Center. Dithranol(40 mg/mL in THF) was used as a matrix for both polymers (10 mg/mL in THF). Dithranol solution was mixed with poly(thiophene)solution at a 1:1 ratio and spotted on a target. XPS was performed on aQuantum 2000 Scanning ESCA Microprobe from Physical Electronics.Deconvolutions of Cs1 peaks were made on MultiPak software using aGuassian Lorentzian mixed function ratio with a Shirley background.fwhm for the integrations ranged between 1.5 and 2. Film thicknesseswere measured by a Dektak 150 profilometer or a DI 3000 AFM fourin the Scanning Probe Microscopy Facility at University ofMassachusetts Amherst.Synthesis of 3-Undecenylthiophene. The procedure for this

Kumada coupling is based off of ref 23. A clean multineck round-bottom flask, fitted with rubber septa, a condenser, and an oval shapedstir bar, was dried by alternating pulling vacuum and purging with N2

three times while being heated with a heat gun. While the apparatuswas drying, 3-bromothiophene (0.95 equiv), Ni(dppp)Cl2 (0.0042equiv), and 11-bromo-1-undecene (1 equiv) were being purged withN2 in separate vials. 1.2 equiv of magnesium turnings were added tothe dried round-bottom and the drying procedure was repeated untilthe magnesium was very shiny and staticky. After the round-bottomcooled, a crystal of iodine was added. Then vacuum was pulled on theapparatus for 10 min. The apparatus was flushed with N2 and 15 equivof ethyl ether or 2-methyltetrahydrofuran were added, resulting in adark red solution. (Tetrahydrofuran is not a suitable solvent for thisreaction.) The 11-bromo-1-undecene was removed from its vial by adry syringe and added dropwise over the course of 45 min to the mainapparatus at room temperature. As the Grignard forms, the solutionconsecutively turns orange, yellow, pale yellow, white, then clear andbegins to bubble and heat up. This is followed by a slow darkening ofthe solution to gray. A dark gray solution is a good indicator that theGrignard has formed. After the 11-bromo-1-undecene is added in full,

Figure 5. (top) Thicknesses of grafted films after different solvent exposure methods, the calculated grafting densities and WCAs for pP3HT. Thethickness of the spin-coated film was 56 ± 1 nm. (bottom) Thicknesses of grafted films after different solvent exposure methods, the calculatedgrafting densities, and WCAs for xP3HT. The thickness of the spin-coated film was 57 ± 2 nm.

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the reaction is heated to 29 °C for 4 h. 3-bromothiophene wastransferred from its vial with a dry syringe and added dropwise to thesolution over 30 min at 29 °C. Then while the reaction flask was underpositive N2 pressure, the septum was removed and Ni(dppp)Cl2 wasadded in bulk to the solution. The solution turns red-ish. The reactionwas heated to reflux overnight. After 12 h, the reaction is a red/blackcolor. If the reaction is run in ethyl ether, it is easy for the solvent toevaporate off during the reflux, which may lead to a greyish sludge after12 h. The round-bottom is placed in an ice bath and 12 M HCl (1:1ratio of total reaction moles to HCl) is added to the solution. Thesolution is extracted three times with ether, followed by three washeswith water. The organic phase is dried over MgSO4, filtered, andcondensed through rotary evaporation. Thin layer chromatographyshows two spots before purification, the topmost spot is the product,while the bottom spot is bisthiophene. The product is purified on asilica column with hexanes. The final product is a clear oil. The bestyield achieved was 52%.1H NMR (CDCl3, 500 MHz) Reference 7.26ppm: 7.23 (m, 1H), 6.93 (d,1H), 6.91 (m, 1H), 5.83 (m, 1H), 4.96(m, 2H), 2.63 (t, 2H), 2.03 (m, 2H), 1.60 (m, 4H), 1.29 (m, 14H)Synthesis of 2-Bromo-3-undecenylthiophene. This procedure

for the lithium halogen exchange is based off of ref 23. A multi neckround-bottom flask was dried in the same way as reported for the 3-undecenylthiophene synthesis. The round-bottom was placed in a dryice/acetone bath at −78 °C. Fifteen eq of dry tetrahydrofuran (THF)were added to the round-bottom by a dry syringe. 1.06 eq. of t-butyllithium were added to the cold round-bottom dropwise using a verydry syringe and the solution turned yellow. Before addition of t-butyl

lithium the syringe was attached to a N2 line and purged with N2 for15 min. One eq of N2 purged 3-undecenylthiophene was addeddropwise to the solution over the course of 15 min. The reaction wasrun at −78 °C for 1 h. The solution was a light orange/yellow color.2.1 eq. of carbon tetrabromide were dissolved in a minimum amountof THF and purged with N2. This was added by a dry syringe to thereaction vessel dropwise over the course of 15 min at −78 °C. Thesolution turned orange. The solution was reacted for 1 h at roomtemperature. The solution was quenched with ice, worked up, andpurified in the same manner as 3-undecenylthiophene. If the solutionturns black, it has failed due to the introduction of oxygen. The yieldsranged between 40 and 60%.1H NMR (CDCl3, 500 MHz) Reference7.26 ppm: 6.88 (d, 1H), 6.79 (d,1H), 5.81 (m, 1H), 4.97 (m, 2H),2.54 (t, 2H), 2.04 (m, 2H), 1.61 (m, 4H), 1.26 (m, 14H)

Synthesis of Poly(3-hexylthiophene-ran-3-undecenylthio-phene). This procedure for the McCullough method is based off ofref 23. In a dry scintillation vial, 1.66 eq. of 2-bromo-3-hexylthiopheneand 1 eq. of 2-bromo-3-undecenylthiophene were added and purgedwith N2. The total amount of moles for this mixture was set as 1 eq. forthe following polymerization. In a multi neck round-bottom flask, thatwas dried as previously reported, 11 eq. of dry THF were added. Thereaction vessel was cooled to −78 °C. One eq of diisopropylamine(DIPA) was added to the cold reaction vessel. The solution was lightyellow. The DIPA was never purged before addition to the reactionvessel, but it would not hurt to do that. Following the DIPA, 1 eq. of n-butyl lithium was added with a dry syringe. The reaction was stirred for1 h at −78 °C. Then the thiophene mixture (1 equiv) was added to thereaction vessel dropwise over the course of 30 min and stirred for 1 hat −78 °C. The solution turned dark yellow. 1.3 eq. of dried zincchloride dissolved in a minimal amount of dry THF was addeddropwise to the reaction vessel over the course of 15 min and reactedat −78 °C for 1 h. The solution turned bright orange. The reactionvessel was warmed to room temperature and 0.009 eq. of Ni(dppp)Cl2was added to the reaction vessel. The solution turned red. As thereaction ran, the solution turned dark red, almost looking black. Thereaction was run for 14 h at room temperature while covered inaluminum foil, and then precipitated into cooled methanol. Theproduct was filtered and purified by Soxhlet extraction using methanol,hexanes, and chloroform. Each solvent was run for 8 h, while heating at100 °C. The final product appears to be golden/purple flakes in thesolid state. When dissolved, the polymer fluoresces bright yellow.1HNMR (CDCl3, 500 MHz) Reference 7.26 ppm: 6.98 (s, 1H), 5.79 (m,1H), 4.93 (dd, 2H), 2.80 (t, 2H), 2.01 (m, 0.22), 1.69 (m, 2H), 1.34(m, 8H), 0.89 (t, 3H)

Synthesis of Vinyl Monocapped P3HT. The synthesis for thevinyl capped P3HT is the same as the synthesis for poly(3-hexylthiophene-ran-3-undecenylthiophene) with the substitution of2-bromo-3-hexylthiophene as the sole monomer and has similar colorchanges. After Ni(dppp)Cl2 was added to the reaction vessel, thereaction proceeded for 5 min before being placed in an ice bath for 10min. The amount of vinyl magnesium chloride that was added wascalculated from a 30:1 ratio to the moles of nickel catalyst. Vinylmagnesium chloride was added quickly to the reaction vessel bysyringe, and then the entire solution was immediately poured intocooled methanol. The polymer was purified in the same way as poly(3-hexylthiophene-ran-3-undecenylthiophene). MALDI-TOF indicatesthat the polymer is terminated with bromide and vinyl groups.

1H NMR (CDCl3, 500 MHz) Reference 7.26 ppm: 6.97 (s, 1H),5.49 (d, 1H), 5.12 (d, 2H), 2.81 (t, 2H), 1.69 (m, 2H), 1.44 (m, 2H),1.33 (d, 4H), 0.90 (t, 3H)

Synthesis of telechelic vinyl dicapped P3HT. The mono-capped P3HT (1 equiv) was dried and placed in a scintillation vialwith a septum and a small stir bar. It was moved into a glovebox wherePd(PPh3)4 (0.04 equiv) was added to the scintillation vial. The vial wascapped, moved back to the hood and put under a nitrogen line. Drytoluene (6 equiv) was added to the scintillation vial. N2 purgedtributyl(vinyl)stannane (60 equiv) was added to the solution and thereaction was heated to 100 °C and reacted for 24 h. Since theequivalents of the catalyst and the stannane group are in excess inorder to cap the bromine end group of the polymer, they are

Figure 6. (top) Transfer curves for pP3HT(dark green) and (bottom)xP3HT (dark purple). √IDS vs VGS on second axis, light green forpP3HT and light purple for xP3HT.

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malleable. The solution was precipitated into methanol and purified inthe same way as poly(3-hexylthiophene-ran-3-undecenylthiophene).MALDI-TOF indicates the polymer is terminated by vinyl groups.1HNMR (CDCl3, 500 MHz) Reference 7.26 ppm: 6.98 (s, 1H), 6.44 (q,1H), 6.14 (dd, 1H), 5.66 (dd, 1H), 2.81 (t, 2H), 1.71 (m, 2H), 1.43(m, 2H), 1.34 (d, 4H), 0.91 (t, 3H)Organic Field Effect Transistor Fabrication. Si/SiO2 (200 nm)

substrates were cleaned by sonication for 10 min in a series of solvents,which included acetone, ethanol, and isopropyl alcohol (IPA). IPA isthe cleanest solvent and therefore should always be used last. Thesubstrates were dried with a stream of N2 and placed in piranha acidfor 4 h. The substrates were individually placed in scintillation vialscontaining DI water to rinse off remaining piranha acid. To do this, DIwater was flushed into the scintillation vials repeatedly and poured out.The substrates were dried by a stream of N2 and those being used forgrafted films were modified with MPTS. The substrates were moved tothe glovebox. Five mg samples of functionalized poly(thiophene) weremade and moved to the glovebox. Solutions of 5 mg/mL offunctionalized poly(thiophene) was made with dry, glovebox qualitychlorobenzene. These solutions stirred at 55 °C overnight, covered inaluminum foil. The solutions were spun onto the substrates through a0.45 μm PTFE syringe filter at 1000 rpm for 60 s. The substrates wereannealed and then grafted as described above. Then the substrateswere exposed to chloroform and then dried for 5 min at 150 °C. Ausource drain were thermally evaporated through a shadow mask (W/L= 16) at 5 × 10−6 Torr. The devices were measured in ambientenvironment outside the glovebox.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b08667.

1H NMR spectra, MALDI-TOF mass spectra, XPSspectra (DOCX)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: krcarter@polysci.umass.edu.Author ContributionsThis manuscript was written through contributions of allauthors. All authors have given approval of the final version ofthe manuscript.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSAuthors K.L.M., Y.N., and K.R.C. would like to thank theNational Science Foundation (DMR Polymers 1506968) forfinancial support. N.E.B. and A.L.B. acknowledge the Office ofNaval Research under Award Number N000141110636

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