Conjugated Polymers with Switchable Carrier Polarity

Article pubs.acs.org/Macromolecules

Conjugated Polymers with Switchable Carrier Polarity Colin R. Bridges,† Chang Guo,‡ Han Yan,† Mark B. Miltenburg,† Pengfei Li,† Yuning Li,*,‡ and Dwight S. Seferos*,† †

Department of Chemistry, Lash Miller Chemical Laboratories, University of Toronto, 80 Saint George Street, Toronto, Ontario M5S 3H6, Canada ‡ Department of Chemical Engineering and Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Ave West ON, Waterloo, N2L 3G1, Canada S Supporting Information *

ABSTRACT: Stimuli responsive polymers can change their properties as a result of their environment. Factors such as temperature, light, pH, or solvent can all trigger a polymer response. We present conjugated polymers with switchable carrier polarity, accomplished using a functional group that can be converted from electron donating to electron withdrawing. The polymers presented herein are polyselenophenes containing α-ketal side-chains. The α-ketal side-chains can be converted to electron withdrawing α-ketone side-chains postpolymerization. Since the starting monomer is relatively electron-rich, it can be polymerized using chain growth methods. Switching the electron donating ability of the side chain postpolymerization is an effective way to synthesize electron-deficient conjugated polymers from electron-rich monomers. Whereas the α-ketal polymer has optoelectronic properties that are consistent with other electron-rich (p-type) polymers, the αketo polymer features a broad red optical-absorption, narrow HOMO−LUMO gap (∼1.5 eV), low-lying HOMO and LUMO levels, and ambipolar charge transport properties. We support these observations using density functional theory calculations.

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INTRODUCTION

recently been utilized in smart windows, displays, and eyewear.16−21 When considering the charge-transport properties of conjugated polymers, there are three classes of materials: ptype (hole conducting), n-type (electron conducting) and ambipolar (both hole and electron conducting). Typically, a polymer will only exhibit a single type of transport, however there are a few reports of polymers exhibiting high and balanced ambipolar transport.22−25 While p-type conjugated polymers are now common and often easy to synthesize, there are relatively few structural motifs that exhibit n-type behavior. Electron transport polymers are usually based on moieties with electron withdrawing functional groups such as rylene diimides or diketopyrrolopyrroles.26−29 Developing synthetic routes and structural motifs for n-type organic materials is an area that requires further research. Given the large number of available ptype polymers, the ability to switch a polymer from electronrich to electron-deficient could be an effective method to develop new classes of ambipolar or n-type materials,30 or materials with responsive carrier mobility. Here we describe a conjugated polymer with switchable carrier polarity. We chose to design the polymer with a

Applying an external stimulus can trigger changes in a polymer’s mechanical properties, optical properties, solvophobicity, molecular uptake and release, or phase behavior. This change in properties is usually a result of changes in the polymer’s backbone or side chain conformation, swelling, protonation, or oxidation state. Materials that exhibit this property are denoted “switchable”, and can be triggered by environmental stimuli such as temperature, solvent, pH, pressure, electrical bias, or the presence of a certain reagent or target molecule.1−11 Switchable polymers have been used for biomedical applications as materials for selective drug release or therapeutics. These polymers typically do not have a conjugated backbone, and are most commonly switched between corresponding states by temperature or pH. Conjugated small molecules and conjugated polymers have also been applied as switchable materials, taking advantage of their unique optoelectronic characteristics. The photoisomerization of azobenzene is an example of light-triggered geometric switching from the cis to trans isomer.12−15 The most readily switched property in conjugated polymers is oxidation state. Conjugated polymers absorb strongly in the visible range, however oxidizing or reducing the polymer shifts the optical absorbance to near-infrared range, thus switching the polymer from opaque to transparent. This electrochromic behavior has © XXXX American Chemical Society

Received: June 5, 2015 Revised: July 17, 2015

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DOI: 10.1021/acs.macromol.5b01225 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis and Postpolymerization Switching of P-ketal to P-keto

Figure 1. Geometry and electronic structure (HOMO and LUMO diagrams) for hexamers of P-ketal and P-keto calculated using DFT.

Scheme 2. 2,5-Dibromo-3-[2-tridecyl-1,3-dioxolan-2-yl]selenophene

B


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dibromo-3-tetradecan-1-oneselenophene. The α-ketone is not compatible with the Grignard metathesis activation step required for CTP, so it was protected using ethylene glycol, yielding the target monomer. CTP generally requires two steps: activation followed by polymerization. We activated 2,5-dibromo-3-[2-tridecyl-1,3dioxolan-2-yl]selenophene using 1 equivalent of isopropylmagnesium chloride lithium chloride complex. The Grignard equilibrium creates a 65:35 mixture of isomers in the 2- and 5-positions, respectively (see the Supporting Information). The steric bulk of the protecting group prevents monomers activated in the 2-position from participating in the polymerization. Our attempts to selectively activate the 5-position by using a di-iodinated monomer or more bulky Grignard reagents were not successful. Nonetheless, P-ketal can be synthesized by adding Ni(dppp)Cl2 to a solution of the activated monomer. The nonselective activation step causes low yields of the final polymer, however we still observe a molecular weight dependence on catalyst loading and the narrow dispersities expected in controlled CTP (Table 1).41−45

selenophene backbone because certain polyselenophenes have exhibited efficient conduction of both electrons and holes.31−34 The polymer must also have a functionality capable of switching from electron donating to electron withdrawing. To achieve this, we propose to take advantage of the acid-catalyzed reversible conversion of the ketal to the ketone functional group. Therefore, we targeted poly(3-[2-tridecyl-1,3-dioxolan2-yl]selenophene) (P-ketal), a selenophene polymer with a relatively electron-rich conjugated backbone and an α-ketal side-chain. P-ketal should be able to be converted to the electron-deficient analogue, poly(3-[tetradecan-1-one]selenophene) (P-keto), using postpolymerization conversion (Scheme 1). We study the effect that switching the polymer between the two forms has on the molecular orbital levels, morphology, crystallinity, optical properties, and more importantly, the charge carrier transport properties of these materials in organic thin film transistors.

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RESULTS AND DISCUSSION First, we conducted density functional theory (DFT) calculations to probe the electronic and structural differences between the two forms of the polymer. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels of conjugated polymers are related to the type of transport they exhibit. Since conjugated organic materials generally have high-lying HOMO and LUMO levels, conjugated polymers are typically efficient hole-transport materials. Electron transport can be improved by lowering the energy of the LUMO, which can be achieved by creating an electron-deficient backbone.35−38 The B3LYP 6-31G (d) basis set was used to model the optimized geometries of P-ketal and P-keto hexamers. In both cases, the C13H27 chain was replaced with a methyl group. Both polymers possess delocalized HOMO and LUMO diagrams (Figure 1), with P-keto having a slightly more planar backbone than P-ketal. Importantly, the HOMO and LUMO levels are lowered by 0.6 eV in P-keto compared to P-ketal, indicating that the electron withdrawing α-ketone functional group has a significant effect on the frontier energy levels. In P-keto, the HOMO is also delocalized across the ketone, which extends the π-conjugation into the side-chain and decreases the HOMO−LUMO gap. The maximum absorption (λmax) from time-dependent DFT is predicted to occur at 667 nm for P-ketal and 698 nm for P-keto (see the Supporting Information). The much lower HOMO and LUMO levels in P-keto indicates the charge transport characteristics may switch from p-type to n-type. Similar behavior has been observed in other conjugated molecules.35,38 P-keto can be synthesized by postpolymerization conversion of P-ketal, so only a single monomer is needed to synthesize both polymers. We chose 2,5-dibromo-3-[2-tridecyl-1,3dioxolan-2-yl]selenophene (Scheme 2) as our target monomer and hypothesized that it could be polymerized using controlled catalyst transfer polymerization (CTP) methods, which are generally chain-growth polymerizations that lead to narrow dispersity polymers. The target monomer was synthesized starting from 3-iodoselenophene and N-methoxy-N-methyltetradecanamide. 3-Iodoselenophene was synthesized according to literature procedures.39,40 N-Methoxy-N-methyltetradecanamide was prepared in one step from myristoyl chloride. Treating 3-iodoselenophene with isopropylmagnesium chloride followed by N-methoxy-N-methyltetradecanamide yielded 3tetradecan-1-oneselenophene. This compound was subsequently treated with N-bromosuccinimide to afford 2,5-

Table 1. Molecular Weight, Dispersity, and Yield of P-Ketal and P-Keto polymer

loadinga (mol %)

Mn (kDa)b

Đb

yield (%)

P-ketal P-keto P-ketal P-keto P-ketal P-keto

0.5 − 1 − 2 −

21.6 21.5 10.9 10.4 4.46 3.58

1.26 1.35 1.60 1.80 1.40 1.31

29 − 30 − 13 −

a

Ni(dppp)Cl2 was used as the catalyst for all polymerizations bMn and Đ determined using size exclusion chromatography in 1,2,4trichlorobenzene at 140 °C.

P-ketal was converted to P-keto by treating it with excess para-toluenesulfonic acid followed by precipitation in methanol (Scheme 1). This polymer conversion reaction was complete and efficient, as determined by 1H NMR and size exclusion chromatography. After conversion the signals corresponding to the four protons on the ethylene bridge of P-ketal are absent, and both the aryl proton and α-methylene protons are shifted downfield due to the more shielding and electron withdrawing ketone functional group (Figure 2). Importantly, the SEC trace for the protected and deprotected polymer have nearly identical profiles. This indicates that the polymer is not fragmenting or cross-linking during deprotection. Small changes in molecular weight and dispersity are observed, likely due to differences in each polymers hydrodynamic radius, secondary interactions with the stationary phase, and optical absorbance properties. Until recently, it has been difficult to synthesize electrondeficient polymers using controlled CTP methods.46−49 In addition to P-keto being an interesting example of an electrondeficient polyselenophene, using a postpolymerization conversion to produce P-keto from P-ketal is unique. Since the modification occurs on the solubilizing chain this method is very likely amenable to other polymers, making it a useful way to synthesize electron-deficient polymers from electron-rich monomers using CTP. While the conversion between ketal and ketone functional groups is generally reversible, we found that reverting P-keto to P-ketal did not occur due to unfavorable steric interactions between the protecting group and polymer C


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Figure 2. 1H NMR spectroscopy (left) and SEC analysis (right) for P-ketal and P-keto.

backbone. We hereafter refer soley to the conversion of P-ketal to P-keto as switchable. One of the most marked differences after switching are the striking changes in the optical properties: in solution, P-ketal appears orange and P-keto appears dark purple. P-ketal exhibits an absorption onset of 580 and 690 nm in solution and solid state, respectively (corresponding to an Eg = 1.80 eV). After annealing, the spectra exhibits a distinct shoulder commonly attributed to π-stacking in poly(3-alkylthiophenes) and other semicrystalline conjugated polymers.50−52 After converting Pketal to P-keto, both the solution and thin film absorbance redshift significantly. In the solid state, the absorption onset and λmax for P-keto are 840 and 655 nm, respectively (corresponding to an Eg = 1.48 eV), and a shoulder is observed. The solution absorption spectra significantly red-shifts upon cooling to 25 °C and begins to show the same features as the solid state absorption spectra. This suggests possible self-assembly or aggregation in solution. The 0.3 eV narrower HOMO−LUMO gap observed for P-keto was predicted by DFT calculations and is a result of the ketone extending the π-conjugated backbone (Figure 3). Converting P-ketal to P-keto should also have an effect on the HOMO and LUMO level positions and we measured this using cyclic voltammetry (CV). In CV, the oxidation onset (10% of max current) can indicate the HOMO level, and by adding the HOMO−LUMO gap (determined by the thin film onset of absorption) to the HOMO we can estimate the LUMO level (see the Supporting Information). P-ketal has an oxidation onset at +500 mV vs ferrocene/ferrocenium (Fc/Fc+) (corresponding to a HOMO level at −5.30 eV; all values reported relative to vacuum unless otherwise noted).53 This is similar to the HOMO levels typically reported for electron-rich polymers and provides evidence that P-ketal is relatively electron-rich.54−56 The CV exhibits a prominent quasireversible oxidation and only minimal current from reduction. Compared to P-ketal, P-keto is more difficult to oxidize and exhibits an onset of oxidation at +950 mV vs Fc/Fc + (corresponding to a HOMO level at −5.73 eV). This highlights the effect introducing an electron withdrawing ketone has on the π-conjugated system. Based on the onset of absorption the LUMO levels for P-ketal and P-keto are estimated to occur at −3.50 eV and −4.25 eV, respectively. The relative differences in electron affinity for these two polymers are also evident by the reduction onset in the CV, which occurs at −1900 mV and

Figure 3. Solution and thin film absorbance of P-ketal (top). Absorbance of P-keto as a thin film and as a solution in chlorobenzene at 25 °C and at 100 °C (bottom)..

−1150 mV vs Fc/Fc+ for P-ketal and P-keto, respectively. This corresponds to a LUMO level at −2.90 eV and −3.85 eV for Pketal and P-keto, respectively. In contrast to P-ketal, P-keto exhibits a reduction in addition to an oxidation. P-keto has a low-lying LUMO level (Table 2) that is in the range of electron-transport conjugated polymers.26−29,35−38 The optical Table 2. Energy Levels for P-Ketal and P-Keto polymer

HOMOa (eV)

LUMOopticalb (eV)

LUMOE‑chema

Egc

P-ketal P-keto

−5.30 −5.75

−3.50 −4.25

−2.90 −3.65

1.80 1.48

a

Estimated using cyclic voltammetry relative to the Fc/Fc+ redox couple and referenced to the vacuum according to ref 53. bDetermined by adding the optical Eg to the HOMO level. cDetermined from the onset of thin film absorption. D


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Figure 4. 1D and 2D XRD patterns of P-keto (top) and P-ketal (bottom; dashed line is as cast, and solid line is after annealing at 100 °C).

type I and type II lamellar spacing61 at 2θ = 2.91°, 4.50° (30.4 Å) and 3.61°, 7.37° (24.5 Å), as well as a prominent π-stacking reflection at 2θ = 24.7° (3.61 Å). P-keto also has a peak corresponding to the distance between ketone groups along the polymer backbone at 2θ = 10.8° (8.18 Å). 2D XRD reveals that there is no preferential orientation with respect to the substrate for these polymers, indicating an anisotropic distribution of crystallites. Highly crystalline polymers tend to have high charge carrier mobility,50−52,58,64−69 thus these polymers may be favorable for electronic applications. The morphology determined by AFM sheds light on the nature of the apparent self-assembly of these polymers. P-ketal adopts similar structures seen in polyalkylthiophenes and selenophenes. A 25 °C solution of P-keto shows distinct fibril formation upon casting, indicating some self-assembly already has occurred in solution (evidenced by the difference in optical absorbance between the 25 and 100 °C solutions). These fibrils are 10−20 nm wide and several hundred nm long, and are reminiscent of the self-assembly behavior of poly(3-alkylthiophene) polymers and copolymers (see the Supporting Information).70−73 To investigate the electrical properties of the polymers we first measured the transport properties of single carrier (holeonly) devices. Our initial results showed that P-ketal and P-keto exhibited hole mobilities of μh = 5.6 × 10−4 cm2 V−1 s−1 and μh = 6.9 × 10−5 cm2 V−1 s−1, respectively, in space-charge limited current measurements (see the Supporting Information). This is similar to structurally related conjugated polymers such as poly(3-hexylthiophene) (P3HT), and inspired us to test these materials in organic thin film transistors. Here we tested electron and hole mobility in n-channel and p-channel operations, respectively, using bottom-gate bottom-contact geometry. P-ketal displays moderate hole mobility (μh = 1.6 × 10−5 cm2 V−1 s−1) but no electron mobility. This is expected

gap is notably lower than the electrochemical gap, an observation that is consistent with other conjugated polymers. Nonetheless both methods for determining the LUMO level consistently show that P-keto is more easily reduced than Pketal. Both the sharp, endothermic melting transitions (see the Supporting Information) and vibronic structure observed in the solid-state absorption spectra are suggestive of the crystalline nature of these polymers. XRD was used to examine the molecular packing of P-ketal and P-keto. 1D and 2D XRD patterns of films of P-ketal and P-keto exhibit several strong diffraction peaks (Figure 4) showing the high crystallinity of both polymers. The pattern of P-ketal exhibits peaks corresponding to lamellar spacing as well as strong side-chain and π-stacking peaks. The distance for lamellar packing is related to the length of the alkyl chain. Given the length of the alkyl chain on P-ketal (C13), we expect the lamellar spacing to be around 32 Å. The first order peak for this reflection is out of range for our detectors, however the shoulder of this peak near the detector cut off, and we observe the second and third order reflections for a 32.8 Å spacing at 2θ = 5.4° and 10.7°, respectively. The peak at 2θ = 14.2° corresponding to a spacing of ∼6.3 Å is attributable to the regioregular spacing between the protecting group along the polymer backbone and is consistent with the optimized geometry of this polymer. Finally, the weak reflection at 2θ = 22.1° corresponding to a distance of 4.04 Å is attributed to the π-stacking distance.57−63 After annealing this film at 100 °C (solid line), all the reflections become significantly sharper and stronger, particularly the π-stacking band, indicating a high degree of crystallinity as the polymers reorganize. Additionally, reflections at 2θ = 12.6°, 26.1° (7.02 Å) and 17.3° (5.12 Å) emerge, and we attribute them to crystallinity within the side-chain. P-keto has similar crystalline behavior, exhibiting reflections for both a E


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Figure 5. Transfer curves for bottom-gate bottom-contact OTFT devices after annealing at 100 °C (P-ketal) and 150 °C (P-keto) for 20 min. Device dimensions: Channel width (W) = 1 mm; channel length (L) = 30 μm.

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as the HOMO and LUMO energy levels of this polymer are relatively high, and similar to those of other p-type materials such as P3HT. P-keto is more electron-deficient, and has a sufficiently deep LUMO level to facilitate electron injection and stabilize electron transport. Indeed, this polymer exhibits electron transport performance with electron mobility of up to 1.0 × 10−4 cm2 V−1 s−1 in the n-channel operation mode (Figure 5). This demonstrates the ability to switch transport properties using functional group conversion. In the p-channel operation mode, we also observed hole transport behavior with mobility of up to 4.9 × 10−4 cm2 V−1 s−1. The ambipolar charge transport characteristics of this P-keto polymer suggests that the LUMO and HOMO energy levels are favorably positioned for efficient injection and transport of both electrons and holes. It is interesting that P-keto exhibits balanced electron and hole mobility and possibly shows the synergistic effects incorporating an electron-rich heterocycle (selenophene) with an electron-deficient side chain (alkylketone) have on the transport properties of π-conjugated polymers.

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EXPERIMENTAL SECTION

General Considerations. All reagents were used as received unless otherwise noted. Solvents THF and toluene were purchased from Caledon Laboratories Ltd., degassed, stored under nitrogen and dried over molecular sieves prior to use. Grignard reagents were titrated with salicylaldehyde phenylhydrazone to determine concentration immediately prior to use.74 Methanol was purchased from EMD. Hexanes, dichloromethane, and ethyl acetate were purchased from Caledon Laboratories Ltd. 1,2,4-trichlorobenzene (spectrophotometric grade) was purchased from Fisher Scientific. Chloroform, myristoyl chloride, triethylamine, N,O-dimethylhydroxylamine hydrochloride, isopropylmagnesium chloride solution (1.6 M), Nbromosuccinimide, ethylene glycol, p-toluenesulfonic acid, and Ni(dppp)Cl2 were purchased from Sigma-Aldrich. 3-Iodoselenophene was synthesized according to literature procedures.39,40 Instrumentation. Absorption spectra were recorded using a Varian Cary 5000 spectrometer. Film absorption measurements were made using a glass substrate, polymer was spin coated from 10 mg/mL chloroform solution. NMR spectra were recorded on a Varian Mercury 400 spectrometer (400 MHz). Masses were determined on a Waters GCT Premier ToF mass spectrometer (EI). Polymer molecular weights were determined using a Viscotek HT-SEC module 350A (1,2,4-trichlorobenzene stabilized with butylated hydroxytoluene, 140 °C) with narrow weight distribution polystyrene standards using absorption at 486 nm. Electrochemistry was conducted using a BASi Epsilon EC potentiostat with polymer films drop cast onto a platinum button electrode from a 10 mg/mL solution in chloroform using 0.1 M TBAPF6 in acetonitrile as the electrolyte solution. 2D diffraction data were collected at the MAX Diffraction Facility at McMaster University, Hamilton, ON, Canada, using the following instrumentation: Bruker Smart6000 CCD area detector, 3-circle D8 goniometer, Rigaku RU200 Cu Kα̅ rotating anode and Göebel cross-coupled parallel focusing mirrors. 1D diffraction data was collected on a Rigaku MiniFlex 600 diffractometer using a 2.0 kW Cu X-ray tube. AFM imaging was carried out on a Bruker Dimension Icon AFM. Films were cast on a glass substrate from a 10 mg/mL solution in chloroform. Synthesis of N-Methoxy-N-methyltetradecanamide. Triethylamine (8.80 g, 87 mmol) was added dropwise to a solution of N,Odimethylhydroxylamine hydrochloride (4.23 g, 43.4 mmol) and myristoyl chloride (10.71 g, 43.4 mmol) in 225 mL of dichloromethane at 0 °C. The reaction mixture was stirred at 0 °C for 1 h and warmed to room temperature. The reaction was quenched with 200 mL of saturated aqueous sodium bicarbonate, washed twice with 200 mL of water and 200 mL of saturated aqueous sodium chloride, dried over MgSO4, and filtered. The filtrate was concentrated to give NMethoxy-N-methyltetradecanamide as a pale yellow oil (11.00 g, 40.6 mmol, 93%) which was used without further purification. 1H NMR (CDCl3, 400 MHz): δ 3.66 (s, 3H), δ 3.16 (s, 3H), δ 2.39 (t, J = 7.6 Hz, 2H) δ 1.60 (quint, J = 6.8 Hz, 2H) δ 1.32−1.20 (br m, 20H) δ

CONCLUSIONS

We have synthesized a polyselenophene that can convert from electron rich to electron deficient using an efficient postpolymerization conversion strategy. We also synthesize an electron-deficient conjugated polymer using controlled CTP methods from an electron-rich conjugated polymer precursor, which is an interesting strategy for the preparation of welldefined electron-deficient materials. Both analogues of the polymers are highly crystalline. P-ketal features a wide HOMO−LUMO gap (∼1.8 eV) and energy levels that are typical of p-type polymers. P-keto exhibits low-lying HOMO and LUMO levels, a broad, deep-red optical absorption spectrum, and a narrow HOMO−LUMO gap (∼1.5 eV). While P-ketal exhibits solely p-type transport, the LUMO of Pketo is low enough to allow for electron injection and transport. This shows that transport properties of conjugated polymer can be switched using postpolymerization conversion. P-keto is especially interesting because it exhibits nearly balanced electron/hole transport, and there are few single structural motifs with this behavior. The α-ketoselenophene could therefore be a useful structural motif for the design of other p-type or n-type electronic materials. F


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Macromolecules 0.86 (t, J = 6.4 Hz, 3H). 13C NMR (CDCl3, 400 MHz): δ 61.28, 32.05, 29.79, 29.77, 29.76, 29.74, 29.65, 29.58, 29.55, 29.40, 24.78, 22.80, 14.21. HRMS-ESI: calcd, 272.25895; found, 272.25908; Δ = 0.46 ppm Synthesis of 3-Tetradecan-1-oneselenophene. A flame-dried Schlenk flask containing 3-Iodoselenophene (3.58 g, 14.0 mmol) in 140 mL dry, degassed THF was cooled to 0 °C under an argon atmosphere. Isopropylmagnesium chloride solution (1.67 M in THF as determined by titration with salicylaldehyde phenylhydrazone, 9.13 mL, 14.6 mmol) was added dropwise and the reaction mixture was stirred at 0 °C for 1 h. N-methoxy-N-methyltetradecanamide (4.13 g, 15.2 mmol) was added via syringe and the reaction mixture was stirred for 1 h at 0 °C, then warmed to ambient temperature and stirred for 16 h. The reaction was quenched with 200 mL saturated aqueous ammonium chloride, diluted with 200 mL of ethyl acetate, washed three times with 200 mL of water and once with 200 mL of saturated aqueous sodium chloride, dried over MgSO4, and filtered. The filtrate was concentrated and purified via column chromatography using 95:5 hexanes: ethyl acetate as eluent to give 3-tetradecan-1-oneselenophene as a white solid (3.56 g, 10.4 mmol, 74%). 1H NMR (CDCl3, 400 MHz): δ 8.82 (tdd, 1H, 2JSe−H = 44 Hz, 4JH−H = 2.4, 1.2 Hz), δ 7.97 (tdd, 1H, 2JSe−H = 56 Hz, 3JH−H = 5.6, 4JH−H = 2.4 Hz), 7.85 (dd, 1H, 3 JH−H = 5.6, 4JH−H = 1.2 Hz), δ 2.86 (t, J = 7.6 2H) δ 1.71 (quint, J = 6.8 Hz, 2H) δ 1.50−1.20 (br m, 20H) δ 0.88 (t, J = 6.4 Hz, 3H). 13C NMR (CDCl3, 400 MHz): δ 195.41, 145.33, 138.75, 130.98, 129.50, 39.90, 32.06, 29.82, 29.80, 29.79, 29.77, 29.65, 29.62, 29.53, 29.50, 24.63, 22.83, 14.26. HRMS-ESI: calcd, 343.15401; found, 343.15404; Δ = 0.08 ppm. Synthesis of 2,5-Dibromo-3-tetradecan-1-oneselenophene. A flask containing 3-tetradecan-1-oneselenophene (2.00 g, 5.90 mmol) and 60 mL DMF was heated to 40 °C. N-Bromosuccinimide (2.60 g, 14.7 mmol) was added portionwise over 1 h. The reaction was stirred at 40 °C for 3 h and monitored using thin-layer chromatography. Aliquots of NBS (0.260 g, 1.47 mmol) were added each hour until the starting material and reaction intermediates (2-bromo-4-tetradecan-1oneselenophene) were consumed. The reaction was quenched with 50 mL saturated aqueous sodium thiosulfate and diluted with 100 mL ethyl acetate. The organic phase was separated and washed with water, saturated aqueous sodium chloride, dried over MgSO4, and filtered. The filtrate was concentrated and purified on a silica plug using 95:5 hexanes: ethyl acetate as eluent to give 2,5-dibromo-3-tetradecan-1oneselenophene (3.00 g, 5.9 mmol, quantitative) as a pale yellow oil. 1 H NMR (CDCl3, 400 MHz): δ 7.51 (s, 1H), δ 2.88 (t, J = 7.6 Hz, 2H) δ 1.69 (m, 2H) δ 1.40−1.20 (br m, 20H) δ 0.89 (t, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 400 MHz): δ 194.94, 142.76, 134.13, 121.65, 115.24, 42.24, 32.07, 29.82, 29.79, 29.75, 29.68, 29.62, 29.60, 29.57, 29.52, 29.50, 29.34, 24.10, 22.84, 14.27. HRMS-ESI: calcd, 498.97504; found, 498.97467; Δ = −0.74 ppm. Synthesis of 2,5-Dibromo-3-[2-tridecyl-1,3-dioxolan-2-yl]selenophene. A flask containing 2,5-dibromo-3-tetradecan-1-oneselenophene (3.00 g, 5.9 mmol), ethylene glycol (1.86 g, 30 mmol), and ptoluenesulfonic acid (28 mg, 0.1 mmol) dissolved in 60 mL of toluene was outfitted with a dean-stark trap and a condenser. The reaction mixture was refluxed for 16 h, cooled to room temperature and quenched with solid sodium carbonate. The organic phase was washed with 100 mL water, 50 mL saturated aqueous sodium chloride, dried over MgSO4, and filtered. The filtrate was concentrated and purified via column chromatography using 97:3 hexanes:ethyl acetate as eluent to give 2,5-dibromo-3-[2-tridecyl-1,3-dioxolan-2-yl]selenophene as a pale yellow oil. (2.41 g, 4.4 mmol, 75%) 1H NMR (CDCl3, 400 MHz): δ 7.15 (s, 1H), δ 4.01 (m, 2H), δ 3.83 (m, 2H) δ 1.95 (m, 2H) δ 1.40−1.20 (br m, 20H) δ 0.87 (t, J = 6.8 Hz, 3H). 13C NMR (CDCl3, 400 MHz): δ 145.29, 134.23, 113.91, 111.42, 109.84, 64.71, 38.50, 32.08, 29.85, 29.81, 29.79, 29.75, 29.65, 29.53, 29.28, 25.21, 23.73, 22.86, 22.02, 14.30. HRMS-ESI: calcd, 543.00125; found, 543.00074; Δ = −0.95 ppm. General Procedure for the Synthesis of P-ketal. A flame-dried Schlenk flask containing 337.2 mg (0.620 mmol) of 2,5-dibromo-3-[2tridecyl-1,3-dioxolan-2-yl]selenophene in 6.0 mL of dry, degassed THF was sparged with argon. Isopropylmagnesium chloride lithium chloride complex [0.310 mL of a 1.03 M solution in THF (0.620

mmol); determined by titration with salicylaldehyde phenylhydrazone] was added dropwise, and the mixture was stirred for 1 h at ambient temperature under argon. The reaction mixture was transferred to a Schlenk flask containing the desired amount of Ni(dppp)Cl2. The polymerization was stirred for 20 h at 60 °C under argon, and was quenched by precipitation into methanol. P-ketal synthesized using 2 mol % or 1 mol % Ni(dppp)Cl2 was purified by washing in a Soxhlet apparatus with methanol and then extracted with chloroform. P-ketal synthesized using 0.5 mol % Ni(dppp)Cl2 was purified by washing in a Soxhlet apparatus with methanol followed by ethyl acetate, and then extracted with chloroform. The product, a dark purple solid, was isolated from the chloroform fraction by concentration under vacuum. 1 H NMR (CDCl3, 400 MHz): δ 7.50 (br s, 1H), 4.06 (br m, 2H), 3.94 (br m, 2H), 1.97 (br, 2H), 1.50−1.06 (br m, 20H), 0.87 (br, 3H). Yields and molecular weights are provided in Table 1. General Procedure for the Synthesis of P-keto. P-ketal (40 mg) was stirred in 20 mL of chloroform with 200 mg (500 wt %) ptoluenesulfonic acid for 4 h. The reaction was precipitated into methanol and filtered to give a dark blue solid. The solid was washed with methanol, collected and dried under vacuum. This process was repeated two times before the polymer was purified by washing in a Soxhlet apparatus with methanol followed by hexanes and extracted using chloroform. The product, a dark blue solid, was isolated from the chloroform fraction by concentration under vacuum (32 mg, 95%). 1H NMR (CDCl3, 400 MHz): δ 8.06 (br s, 1H), 2.91 (br, 2H), 1.80−1.06 (br m, 22H), 0.865 (br, 3H). Yields and molecular weights are provided in Table 1. Polymer DFT Calculations. Calculations were carried out using the B3LYP hybrid functional in Gaussian 09.75−77 Energy levels of the polymer were determined using a 6-31G(d) basis set for both geometry optimization and energy levels.56 Alkyl chains were replaced with methyl groups to reduce the computation time. Transistor Fabrication and Characterization. The semiconducting performance of the polymers was characterized by a bottom-gate, bottom-contact organic thin film transistor structure (W = 1 mm; L = 30 μm, W/L = 33). Heavily n-doped Si wafer with a 300 nm thick SiO2 layer was used as the substrate, where the conductive Si layer and the SiO2 layer function as the gate and the dielectric, respectively. The source/drain electrode pairs were prepared by thermal evaporation of gold through a shadow mask. The substrates were cleaned using an ultrasonic bath with deionized water, rinsed with acetone and isopropanol and then immersed in a dodecyltrichlorosilane solution (3% in toluene) for 15 min. The semiconducting films with a thickness of ∼30−50 nm were deposited on the substrate by spin coating solution of P-ketal or P-keto in chloroform (5 mg mL−1) at 3000 rpm for 60 s and subsequently annealed at 80, 100, or 150 °C for 20 min. All the measurements were performed in a nitrogen atmosphere in the absence of light by Agilent B2912A Semiconductor Analyzer. Procedure for Space Charge Limited Current (SCLC) Measurements. The device architecture for p-type SCLC mobility test was ITO/PEDOT:PSS(30 nm)/Polymer(100 nm)/Au(100 nm). Indium−tin oxide (ITO)-coated glass substrates (Colorado Concept Coatings LLC) were cleaned successively with aqueous detergent, deionized water, methanol, and acetone for 5 min each, and then treated in an oxygen-plasma cleaner for 15 min. PEDOT:PSS was filtered through a 0.45 μm syringe filter, spin coated at 3000 rpm, and then annealed at 150 °C for 10 min in an ambient atmosphere. Each polymer was spun cast from o-dichlorobenzene (10 mg/mL) at 700 rpm for 60 s onto the PEDOT layer, and annealed at 120 °C for 15 min. Gold (100 nm) was thermally evaporated using an Angstrom Engineering (Kitchener, ON) Covap II metal evaporation system at 1 × 10−6 Torr.

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(20) Granqvist, C. G.; Azens, A.; Hjelm, A.; Kullman, L.; Niklasson, G. A.; Ronnow, D.; Mattsson, M. S.; Veszelei, M.; Vaivars, G. Sol. Energy 1998, 63, 199. (21) Mortimer, R. J.; Dyer, A. L.; Reynolds, J. R. Displays 2006, 27, 2. (22) Zhou, X.; Ai, N.; Guo, Z.-H.; Zhuang, F.-D.; Jiang, Y.-S.; Wang, J.-Y.; Pei, J. Chem. Mater. 2015, 27, 1815. (23) Chen, Z.; Lee, M. J.; Shahid Ashraf, R.; Gu, Y.; Albert-Seifried, S.; Meedom Nielsen, M.; Schroeder, B.; Anthopoulos, T. D.; Heeney, M.; McCulloch, I.; Sirringhaus, H. Adv. Mater. 2012, 24, 647. (24) Nakano, M.; Osaka, I.; Takimiya, K. Macromolecules 2015, 48, 576. (25) Lee, J. S.; Son, S. K.; Song, S.; Kim, H.; Lee, D. R.; Kim, K.; Ko, M. J.; Choi, D. H.; Kim, B.; Cho, J. H. Chem. Mater. 2012, 24, 1316. (26) Tilley, A. J.; Guo, C.; Miltenburg, M. B.; Schon, T. B.; Yan, H.; Li, Y.; Seferos, D. S. Adv. Funct. Mater. 2015, 25, 3321. (27) Kozycz, L. M.; Gao, D.; Tilley, A. J.; Seferos, D. S. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3337. (28) Sun, B.; Hong, W.; Yan, Z.; Aziz, H.; Li, Y. Adv. Mater. 2014, 26, 2636. (29) Vasimalla, S.; Senanayak, S. P.; Sharma, M.; Narayan, K. S.; Iyer, P. K. Chem. Mater. 2014, 26, 4030. (30) Wei, S.; Xia, J.; Dell, E. J.; Jiang, Y.; Song, R.; Lee, H.; Rodenbough, P.; Briseno, A. L.; Campos, L. M. Angew. Chem., Int. Ed. 2014, 53, 1832. (31) Chen, Z.; Lemke, H.; Albert-Seifried, S.; Caironi, M.; Nielsen, M. M.; Heeney, M.; Zhang, W.; McCulloch, I.; Sirringhaus, H. Adv. Mater. 2010, 22, 2371. (32) Shahid, M.; McCarthy-Ward, T.; Labram, J.; Rossbauer, S.; Domingo, E. B.; Watkins, S. E.; Stingelin, N.; Anthopoulos, T. D.; Heeney, M. Chem. Sci. 2012, 3, 181. (33) Chang, H.-H.; Tsai, C.-E.; Lai, Y.-Y.; Liang, W.-W.; Hsu, S.-L.; Hsu, C.-S.; Cheng, Y.-J. Macromolecules 2013, 46, 7715. (34) Chung, D. S.; Kong, H.; Yun, W. M.; Cha, H.; Shim, H.-K.; Kim, Y.-H.; Park, C. E. Org. Electron. 2010, 11, 899. (35) Nicolai, H. T.; Kuik, M.; Wetzelaer, G. A. H.; de Boer, B.; Campbell, C.; Risko, C.; Bredas, J. L.; Blom, P. W. M. Nat. Mater. 2012, 11, 882. (36) Zhang, Y.; de Boer, B.; Blom, P. W. M. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085201. (37) Olivier, Y.; Niedzialek, D.; Lemaur, V.; Pisula, W.; Müllen, K.; Koldemir, U.; Reynolds, J. R.; Lazzaroni, R.; Cornil, J.; Beljonne, D. Adv. Mater. 2014, 26, 2119. (38) Dell, E. J.; Capozzi, B.; Xia, J.; Venkataraman, L.; Campos, L. M. Nat. Chem. 2015, 7, 209. (39) Schumacher, R. F.; Rosário, A. R.; Souza, A. C. G.; Menezes, P. H.; Zeni, G. Org. Lett. 2010, 12, 1952. (40) Franchetti, P.; Cappellacci, L.; Abu Sheikha, G.; Jayaram, H. N.; Gurudutt, V. V.; Sint, T.; Schneider, B. P.; Jones, W. D.; Goldstein, B. M.; Perra, G.; De Montis, A.; Loi, A. G.; La Colla, P.; Grifantini, M. J. Med. Chem. 1997, 40, 1731. (41) Yokoyama, A.; Miyakoshi, R.; Yokozawa, T. Macromolecules 2004, 37, 1169. (42) Miyakoshi, R.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2005, 127, 17542. (43) Miyakoshi, R.; Shimono, K.; Yokoyama, A.; Yokozawa, T. J. Am. Chem. Soc. 2006, 128, 16012. (44) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough, R. D. Macromolecules 2004, 37, 3526. (45) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macromolecules 2005, 38, 8649. (46) Yokozawa, T.; Nanashima, Y.; Ohta, Y. ACS Macro Lett. 2012, 1, 862. (47) Bridges, C. R.; McCormick, T. M.; Gibson, G. L.; Hollinger, J.; Seferos, D. S. J. Am. Chem. Soc. 2013, 135, 13212. (48) Bridges, C. R.; Yan, H.; Pollit, A. A.; Seferos, D. S. ACS Macro Lett. 2014, 3, 671. (49) Pollit, A. A.; Bridges, C. R.; Seferos, D. S. Macromol. Rapid Commun. 2015, 36, 65.

Predicted absorbance spectra, cyclic voltammograms, AFM height and phase images, SCLC measurements and differential scanning calorimetry for P-ketal and P-keto. 1 H NMR of the metathesis quenching experiments for the monomer. Representative GPC traces for polymers synthesized at each catalyst loading before and after postpolymerization conversion. Cartesian coordinates and energy levels of the calculated stationary points for the hexamers of P-ketal and P-keto. Complete reference for ref 77(PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail (D.S.S.): [email protected]. *E-mail (Y.L.): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSERC, the CFI, the Ontario Research Fund, DuPont, and the Alfred P. Sloan Foundation. C.R.B is grateful for the Chem Club Graduate Scholarship. M.B.M. is grateful for an Ontario Graduate Scholarship. 1-D XRD measurements were performed by Abdolkarim Danaei at the Walter Curlook Materials Characterization and Processing Lab at the University of Toronto. 2-D XRD measurements were performed by Victora Jarvis at the MAX Diffraction Facility at McMaster University.

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REFERENCES

(1) Nath, N.; Chilkoti, A. Adv. Mater. 2002, 14, 1243. (2) Sundaram, H. S.; Ella-Menye, J.-R.; Brault, N. D.; Shao, Q.; Jiang, S. Chem. Sci. 2014, 5, 200. (3) Elbert, J.; Gallei, M.; Rüttiger, C.; Brunsen, A.; Didzoleit, H.; Stühn, B.; Rehahn, M. Organometallics 2013, 32, 5873. (4) Wiktorowicz, S.; Aseyev, V.; Tenhu, H. Polym. Chem. 2012, 3, 1126. (5) Han, D.; Tong, X.; Boissière, O.; Zhao, Y. ACS Macro Lett. 2012, 1, 57. (6) Alarcón, C. de L. H.; Pennadam, S.; Alexander, C. Chem. Soc. Rev. 2005, 34, 276. (7) Scherowsky, G.; Schliwa, A.; Springer, J.; Kühnpast, K.; Trapp, W. Liq. Cryst. 1989, 5, 1281. (8) Texter, J. Macromol. Rapid Commun. 2012, 33, 1996. (9) Xu, C.; Liu, L.; Legenski, S. E.; Ning, D.; Taya, M. J. Mater. Res. 2004, 19, 2072. (10) Cho, K.; Cho, J. H.; Yoon, S.; Park, C. E.; Lee, J.-C.; Han, S.-H.; Lee, K.-B.; Koo, J. Macromolecules 2003, 36, 2009. (11) Jeong, B.; Gutowska, A. Trends Biotechnol. 2002, 20, 305. (12) Bandara, H. M. D.; Burdette, S. C. Chem. Soc. Rev. 2012, 41, 1809. (13) Zatsepin, T. S.; Abrosimova, L. A.; Monakhova, M. V.; Le Thi Hien; Pingoud, A.; Kubareva, E. A.; Oretskaya, T. S. Russ. Chem. Rev. 2013, 82, 942. (14) Beharry, A. A.; Woolley, G. A. Chem. Soc. Rev. 2011, 40, 4422. (15) Schenderlein, H.; Voss, A.; Stark, R. W.; Biesalski, M. Langmuir 2013, 29, 4525. (16) Bulloch, R. H.; Kerszulis, J. A.; Dyer, A. L.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2015, 7, 1406. (17) Ö sterholm, A. M.; Shen, D. E.; Kerszulis, J. A.; Bulloch, R. H.; Kuepfert, M.; Dyer, A. L.; Reynolds, J. R. ACS Appl. Mater. Interfaces 2015, 7, 1413. (18) Thompson, B. C.; Schottland, P.; Zong, K.; Reynolds, J. R. Chem. Mater. 2000, 12, 1563. (19) Sankaran, B.; Reynolds, J. R. Macromolecules 1997, 30, 2582. H


Article

Macromolecules (50) Brown, P.; Sirringhaus, H.; Harrison, M.; Shkunov, M.; Friend, R. Phys. Rev. B: Condens. Matter Mater. Phys. 2001, 63, 125204. (51) Berson, S.; De Bettignies, R.; Bailly, S.; Guillerez, S. Adv. Funct. Mater. 2007, 17, 1377. (52) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Fréchet, J. M. J.; Toney, M. F. Macromolecules 2005, 38, 3312. (53) Pommerehne, J.; Vestweber, H.; Guss, W.; Mahrt, R. F.; Bässler, H.; Porsch, M.; Daub, J. Adv. Mater. 1995, 7, 551. (54) Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.; Gordeyev, S.; Hamilton, R.; Higgins, S. J.; McCulloch, I.; Skabara, P. J.; Sparrowe, D.; Tierney, S. Chem. Commun. 2007, 5061. (55) Kozycz, L. M.; Gao, D.; Seferos, D. S. Macromolecules 2013, 46, 613. (56) McCormick, T. M.; Bridges, C. R.; Carrera, E. I.; DiCarmine, P. M.; Gibson, G. L.; Hollinger, J.; Kozycz, L. M.; Seferos, D. S. Macromolecules 2013, 46, 3879. (57) Prosa, T. J.; Winokur, M. J.; Moulton, J.; Smith, P.; Heeger, A. J. Macromolecules 1992, 25, 4364. (58) Müller, C.; Zhigadlo, N. D.; Kumar, A.; Baklar, M. A.; Karpinski, J.; Smith, P.; Kreouzis, T.; Stingelin, N. Macromolecules 2011, 44, 1221. (59) Zen, A.; Pflaum, J.; Hirschmann, S.; Zhuang, W.; Jaiser, F.; Asawapirom, U.; Rabe, J. P.; Scherf, U.; Neher, D. Adv. Funct. Mater. 2004, 14, 757. (60) Bolognesi, A.; Botta, C.; Mercogliano, C.; Porzio, W.; Jukes, P. C.; Geoghegan, M.; Grell, M.; Durell, M.; Trolley, D.; Das, A.; Macdonald, J. E. Polymer 2004, 45, 4133. (61) Li, L.; Hollinger, J.; Jahnke, A. A.; Petrov, S.; Seferos, D. S. Chem. Sci. 2011, 2, 2306. (62) Malik, S.; Nandi, A. K. J. Polym. Sci., Part B: Polym. Phys. 2002, 40, 2073. (63) Zen, A.; Saphiannikova, M.; Neher, D.; Grenzer, J.; Grigorian, S.; Pietsch, U.; Asawapirom, U.; Janietz, S.; Scherf, U.; Lieberwirth, I.; Wegner, G. Macromolecules 2006, 39, 2162. (64) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.; Bradley, D. D. C.; Giles, M.; McCulloch, I.; Ha, C.-S.; Ree, M. Nat. Mater. 2006, 5, 197. (65) Huang, J.; Zhao, Y.; He, W.; Jia, H.; Lu, Z.; Jiang, B.; Zhan, C.; Pei, Q.; Liu, Y.; Yao, J. Polym. Chem. 2012, 3, 2832. (66) Kim, D. H.; Park, Y. D.; Jang, Y.; Yang, H.; Kim, Y. H.; Han, J. I.; Moon, D. G.; Park, S.; Chang, T.; Chang, C.; Joo, M.; Ryu, C. Y.; Cho, K. Adv. Funct. Mater. 2005, 15, 77. (67) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L. J.; Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. Adv. Mater. 2007, 19, 833. (68) Tseng, H.-R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Nguyen, T.-Q.; Bazan, G. C.; Heeger, A. J. Adv. Mater. 2014, 26, 2993. (69) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee, M. D.; Toney, M. F. Nat. Mater. 2006, 5, 328. (70) Hollinger, J.; Jahnke, A. A.; Coombs, N.; Seferos, D. S. J. Am. Chem. Soc. 2010, 132, 8546. (71) Yan, H.; Hollinger, J.; Bridges, C. R.; McKeown, G. R.; AlFaouri, T.; Seferos, D. S. Chem. Mater. 2014, 26, 4605. (72) Hollinger, J.; DiCarmine, P. M.; Karl, D.; Seferos, D. S. Macromolecules 2012, 45, 3772. (73) Hollinger, J.; Seferos, D. S. Macromolecules 2014, 47, 5002. (74) Love, B. E.; Jones, E. G. J. Org. Chem. 1999, 64, 3755. (75) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (76) Becke, A. D. J. Chem. Phys. 1996, 104, 1040. (77) Frisch, M. J.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT, 2009.

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