Defect Analysis of High Electron Mobility ... - ACS Publications

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Defect Analysis of High Electron Mobility Diketopyrrolopyrrole Copolymers Made by Direct Arylation Polycondensation Sebastian Broll,† Fritz Nübling,†,‡ Alessandro Luzio,§ Dimitros Lentzas,§ Hartmut Komber,∥ Mario Caironi,§ and Michael Sommer*,†,‡,⊥ †

Makromolekulare Chemie, Universität Freiburg, Stefan-Meier-Straße 31, 79104 Freiburg, Germany Freiburger Materialforschungszentrum, Stefan-Meier-Str. 21, 79104 Freiburg, Germany § Center for Nano Science and Technology @PoliMi, Istituto Italiano di Tecnologia, Via Pascoli 70/3, 20133 Milano, Italy ∥ Leibniz Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany ⊥ Freiburger Institut für interaktive Materialien und bioinspirierte Technologien, Georges-Koehler-Allee 105, 79110 Freiburg, Germany ‡

S Supporting Information *

ABSTRACT: Defect structures in high-performance conjugated polymers are generally known but still challenging to characterize on a quantitative basis. Here, we present a detailed analysis of backbone topology of a series of copolymers PDPPTh2F4 having alternating dithienyldiketopyrrolopyrrole (DPPTh2) and tetrafluorobenzene (F4) units made by direct arylation polycondensation (DAP). In contrast to early expectations of unselective C−H activation during the DAP of monomers with multiple C−H bonds, detailed structure analysis by high-temperature 1H NMR spectroscopy reveals welldefined and alternating backbones with a quantifiable amount of 0−12% DPPTh2 homocouplings as the only defect structure in the main chain. Homocoupled −DPPTh2−DPPTh2− structural units are additionally characterized by UV−vis spectroscopy. While −DPPTh2−H end groups are inert to other side reactions, −F4−Br end groups are weakly susceptible to both dehalogenation and reaction with toluene. However, despite the presence of DPPTh2 homocouplings, high field-effect transistor electron mobilities up to ∼0.6 cm2/(V s) are achieved. This study highlights both that DPPTh2 homocouplings pose a prevalent structural defect in DPPTh2-based conjugated polymers made by DAP and that a very simple four-step DAP protocol can yield materials with varying molar mass and excellent n-type transistor performance.

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INTRODUCTION The diketopyrrollopyrrole (DPP) unit is one of the most popular and important chromophores for use in light absorbing and optoelectronically active materials. Since its original usage as color pigment in paint formulations,1 manifold applications from organic photovoltaics2−4 to organic field-effect transistors (OFET),5,6 sensors7,8 or near-IR probes for biomedical purposes9 have been pursued, whereby the dithienyl-flanked DPP chromophore, hereafter referred to as DPPTh2, is mostly used. When incorporated into alternating copolymers, DPPTh2-based materials exhibit small bandgaps, excellent charge carrier mobility and photovoltaic performance. Another advantage is the low-lying HOMO level, which renders DPPTh2-based copolymers stable against unintentional p-type doping. While many of the presented DPP copolymers exhibit p-type or ambipolar transport characteristics in field-effect transistors,6,10 the observation of predominant n-type performance is less often observed.11−15 N-type performance can be achieved by copolymerization of DPPTh2 with electrondeficient comonomers13,16 or by replacing the commonly used thienyl with electron-deficient thiazole12 or pyridyl14,16 groups. Such materials are also of interest for all-polymer © XXXX American Chemical Society

photovoltaic devices in which the DPP component can be used in both the electron and hole transport material.12,17 Given the great interest and success of the DPP unit in the field of organic electronics and the diverse applicability of DPP-based materials, further improvements toward yet simplified, faster and less expensive synthesis schemes appear worthy of being investigated. To this end, direct arylation polycondensation (DAP) is a particularly useful technique.18−21 Several examples in which DPPTh2 was copolymerized with dihalides by DAP have been reported.22−24 While the electronic properties of DPP copolymers made by DAP are steadily improving, the best results are still being obtained from analogues made by conventional polycondensation techniques that require an increased number of synthetic steps and increased costs.25 Moreover, DPP-based copolymers with excellent n-type performance have to the best of our knowledge not yet been prepared by DAP. Another recently emerging discussion is the formation of main chain defects in copolymers made by DAP Received: August 20, 2015 Revised: September 30, 2015

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DOI: 10.1021/acs.macromol.5b01843 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Direct Arylation Polycondensations Performed in This Study: (a) Synthesis of the Target Copolymer PDPPTh2F4 Using DPPTh2 and F4Br2, (b) Synthesis of the Control Homopolymer PDPPTh2, and (c) Synthesis of the Control Copolymer PDPPTh2F8 Using DPPTh2 and F8Br2

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RESULTS AND DISCUSSION This study aims at shining light on the scope and limitations on the feasibility of using strongly simplified protocols to prepare highly efficient n-type copolymers based on DPPTh2 via DAP. To this end, PDPPTh2F4 was chosen owing to its promising electron transport properties and the suitability of DPP monomers in DAP reactions.13,22 However, the synthesis of PDPPTh2F4 via Stille polycondensation only yielded moderate yields and molecular weights (63% and Mn = 16 kDa, respectively).13 Thus, maximizing yield and controlling molecular weight of this interesting material within a broader range appeared to be of additional interest. This work is organized into the sections (i) optimization of the DAP conditions of DPPTh2 and F4Br2, (ii) detailed structure analyses of PDPPTh2F4 as a function of reaction conditions by high-temperature NMR spectroscopy, and (iii) investigation of the molecular-weight-dependent optical and electronic properties. Reaction Conditions and Optimization of DPPTh2F4 Synthesis via DAP. We elucidated the best choice of monomer functionalities on the core units dithienyldiketopyrrolopyrrole and tetrafluorobenzene. Brominated DPPTh2Br2 and tetrafluorobenzene F4H2 were reacted using Pd2dba3 as catalyst, P(o-anisyl)3 as ligand, Cs2CO3 as base, pivalic acid (PivOH) as proton shuttle and toluene as solvent. These conditions were reported to be suitable for either of the two monomers in combination with another one, hence we envisioned that the same parameters could provide an efficient catalytic system for the combination DPPTh2Br2 and F4H2.24,30 However, these conditions did not lead to polymeric products, and thus we changed the monomer system by using DPPTh2 and F4Br2. Favorably, the use of commercially available and solid F4Br2 eliminates the bromination step of DPPTh2 and additionally facilitates adjustment of monomer stoichiometry. DPPTh2 and F4Br2 were subjected to DAP conditions with Pd(OAc)2, K2CO3, PivOH, and a 1:1 DMAc/solvent mixture

arising from either unselective C−H activation or from homocouplings.21,26,27 However, for DPP-based copolymers made by DAP, detailed backbone sequence analyses have not yet been reported on a spectroscopic basis. Given the major importance of DPP-related copolymers and the general rise of DAP in polymer science, such information is required as a base onto which structure−function relationships of DPP copolymers in electronic devices can be built. These questions have gained additional importance by very recent observations of DPPTh2 homocouplings using traditional cross-coupling schemes.28,29 Here we investigate the DAP of DPPTh2 and dibromotetrafluorobenzene (F4Br2) as a simple four-step route to high electron mobility copolymers PDPPTh2F4. Upon screening a broad range of reaction parameters, various well-defined copolymers with varying molar mass are obtained with the exception that a small and quantifiable amount of DPPTh2 homocoupled units forms. While such homocouplings have recently been reported to occur under DAP26 as well as under Suzuki28 and Stille29 polycondensation reactions, their spectroscopic signatures have not yet been reported, and it remains largely unclear to what extend these are formed as a function of distinct reaction conditions. Moreover, given the steadily increasing number of publications on DPPTh2-based conjugated copolymers in recent years, this article provides a reference for identifying and quantifying homocoupled units of DPPTh2-based copolymers using NMR spectroscopy and hence aims at encouraging other researchers to investigate related materials with respect to such main-chain defects and the corresponding optoelectronic properties. We also shine light on the ongoing debate as to whether or not unsubstituted thiophene-based monomers undergo unselective C−H arylations leading to kinked or branched copolymers. Gratifyingly, defect incorporation still allows high electron mobilities up to 0.6 cm2/(V s) to be measured, whereby the sample with highest molecular weight also exhibits the highest electron mobility. B


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Macromolecules Table 1. Compilation of DPPTh2 Copolymer Synthesis Using DAPa no.

type

1 2 3 4 5 6 7 8 9 10 11 12 13 14

PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F4 PDPPTh2F8 PDPPTh2

phosphine

PCy3 PCy3 P(o-anisyl)3 P(o-anisyl)3 P(o-anisyl)3

base/carb

solvent

c [DPPTh2]/Mb

Mn/kDac

Đc

yield/%d

hc/%e

K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH K2CO3/PivOH KOPiv KOPiv Cs2CO3/PivOH Cs2CO3/PivOH K2CO3/PivOH Cs2CO3/PivOH

Tol:DMAc CF3tol:DMAc CB:DMAc THF:DMAc Tol:DMAc Tol:DMAc Tol:DMAc Tol:DMAc Tol:DMAc Tol:DMAc Tol Tol CB:DMAc Tol

0.2 0.2 0.2 0.2 0.1 0.3 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.1

10.9 3.1 25.3 10.0 5.6 22.0 30.0 5.4 14.2 12.9 13.5 16.5 8.0 14.0

1.5 1.9 1.8 1.5 1.5 2.5 2.4 1.4 1.4 1.5 2.0 1.9 1.3 1.4

76 2 87 65 12 65 75 23 92 93 81 86 80 20

7 n.m. n.q. 10 n.m. 6 n.q. 9 12 11 7 0 12 100

a

Pd(OAc)2 was used as catalyst in all cases except 11 and 12, where Pd2db3 was used. The molar ratio of palladium to phosphine was always 1:1 except entry 12, where the doubled amount of phosphine was used. The reaction temperature was 90 °C and the time 72 h. bConcentration of DPPTh2 in the solvent (mixture). cFrom SEC in CHCl3. dYield of chloroform fraction after Soxhlet extraction with methanol, ethyl acetate, and chloroform. emol % of DPPTh2 homocouplings from 1H NMR. n.q. = not quanitified due to Mn > 22 kDa. n.m. = not measured.

in the absence of PCy3 (entry 7). Below we will show that for all these permutations DPPTh2 homocouplings form. End Group and Defect Analyses of PDPPTh2F4. In order to obtain information on end group (EG) stability and thus on possible chain termination mechanisms, and to identify possible defect structures in the main chain, high temperature 1 H NMR spectroscopy was performed in deuterated tetrachloroethane at 120 °C. Typical 1H NMR spectra of PDPPTh2F4 with Mn= 22 kDa and 5.4 kDa (entries 6 and 8, respectively) are shown with assignments in Figures 1a and 1c, respectively. It is noteworthy that the nonsymmetric line shape of the signal of thiophene proton H 2 next to the diketopyrrolopyrrole unit in Figure 1a indicates chain aggregation typically for samples with Mn ≥ 20 kDa. This value can be regarded as upper limit for which proton spectra can no longer be fully resolved even at 120 °C. EG signals of PDPPTh2F4 are numbered with a−i, PDPPTh2F4 copolymer backbone signals with 1 and 2, and PDPPTh2 homopolymer backbone signals with 1′ and 2′. Th and Th−Br EGs of homopolymer PDPPTh2 are marked by open and filled circles, respectively. Signals of the thiophene end group (He−Hi in Figure 1c) are assigned by comparison of low molar mass PDPPTh2F4 with the monomer DPPTh2. The TOCSY correlations confirm the assignments made (Figure 1d, blue lines). The EGs arising from DPPTh termination dominate the 1 H NMR spectra but also low-intensity signals due to dehalogenation of −F4−Br chain ends leading to −F4−H (a) and solvent end-capping through C−H activation of toluene (b) were seen in some cases. Generally, solvent C−H activation of Tol, CB, and CF3Tol occurred to minor extend and is therefore not treated in detail. The 19F NMR spectrum of PDPPTh2F4 (entry 6) additionally revealed −F4−Br termination (filled squares in Figure 2a). This unit is not directly seen in the 1H NMR spectrum, but the neighboring thiophene protons c and d can be observed (Hc and Hd in Figure 1a and green line in Figure 1d). Thus, the only signal that could not be assigned to a typical EG is a composite signal at 7.5 ppm (Figure 1a,c). As homocoupling reactions occur in DAP to a nonneglectable extent,26,27,33 the PDPPTh2 homopolymer was prepared as a model compound by DAP (Scheme 1b; Table 1,

(Scheme 1a). These conditions have previously been used for the successful copolymerization of F4Br2 and naphthalene diimide-based monomers which are structurally similar to DPPTh2.31 Parameter screening was generally accompanied by isolated polymer yields after Soxhlet extraction, size exclusion chromatography (SEC), and high-temperature 1H NMR spectroscopy. All entries of DAP reactions are compiled in Table 1. As the role of the solvent mixture is important for DAP, we determined the best cosolvent with DMAc (Table 1, entries 1− 4).26 The use of pure DMAc was not attempted as it is unlikely to lead to good molar mass for solubility reasons. According to molar mass and yield, chlorobenzene (CB) and toluene (Tol) worked well, while THF gave moderate yields. The DAP in CF3tol/DMAc failed under these conditions. CB had already proven useful for the synthesis of related NDI-based copolymers PNDITh2F4 with reduced solubility.31 However, while we note that chlorobenzene enabled higher molar masses than toluene, we further optimized the reaction conditions using toluene as co-solvent for toxicity reasons arising from chlorinated aromatic solvents. Recently, we observed monomer concentration to be of pivotal relevance for accelerating and also controlling molar mass of copolymers made via DAP.32 Thus, in order to increase molar mass, DPPTh2 concentration was varied from 0.1 to 0.5 M in Tol/DMAc, whereby a steady increase in molar mass was found from low molar masses around Mn = 6 kDa to satisfying values Mn = 30 kDa (entries 1, 5−7). Thus, monomer concentration appears to be effective to control molecular weight in the indicated range. It is also noteworthy that fine-tuning the herein presented conditions is likely to produce yet higher values upon balancing solvent mixture, monomer concentration, temperature, and stoichiometry. Next, we investigated the influence of the base and the ligand on molar mass and yield of the DAP of DPPTh2 and F4Br2 (entries 7−10). To this end, the influence of the presence or absence of PCy3 and the use of K2CO3/PivOH versus PivOK was investigated. Lombeck et al. observed increased C− H/C−H homocouplings in the absence of PCy3 and when using PivOK instead of K2CO3/PivOH.26 However, this dependence did not hold for PDPPTh2F4 for which the highest molar mass of Mn = 30 kDa was obtained using K2CO3/PivOH C


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Figure 2. 19F NMR spectra of (a) DPPTh2F4 (Mn = 22 kDa) and (b) model polymer DPPTh2F8. ■ marks signals of (a) F4-Br and (b) F8-Br end groups. Signal a results from the F4-H end group and signals b from −C6F4-Tol end groups. Solvent: C2D2Cl4 at 120 °C.

the effect observed for proton 2′. Thus, also the 13C NMR data confirm the signal assignment to the DPPTh2−DPPTh2 homocoupling unit. On returning to the 1H NMR spectra, a more detailed inspection allows the assignment of low intensity signals in the PDPPTh2F4 spectra to thiophene end groups occurring in PDPPTh2 (dotted lines in Figure 1). Obviously, the homocoupled sequence is located not only somewhere within the chain but also at the chain end. Mechanistically, this indicates that homocoupling must occur at a stage at which DPPTh2 monomer is present, which for a polycondensation reaction is typically at low conversion and at the beginning of the reaction. The asymmetry of DPPTh2 and F4-X end groups seen in Figure 1 also suggests unintentional stoichiometry mismatch. Thus, assuming more DPPTh2 than F4Br2 monomer to be initially present (Figure 1c, entry 8) would explain why the probability of the reaction of DPPTh2 monomer with −DPPTh2 chain ends is increased. Mechanistically, DPPTh2 homocoupling could be explained by the reduction of Pd(II) to Pd(0), either as the initial step of the catalytic cycle or during the course of polymerization.34 Finally, the 19F NMR spectra were inspected with respect to the occurrence of an −F4−F4− linkage, corresponding to a halide−halide homocoupling also observed by Lombeck et al.26 A reference polymer for this structural motif is the copolymer PDPPTh2F8 (Scheme 1c, Table 1, entry 13, and Figure 2b). The characteristic signal pattern around −138 ppm could not be observed in the PDPPTh2F4 spectra. Thus, this homocoupling reaction does not occur here in accordance with the results reported by Luzio et al. using F4Br2 under similar conditions.31 At this point it is interesting to note that halide−halide homocouplings were detected for a DAP with dibromocarbazole as dihalide also under similar reaction conditions.26 A deeper understanding of how monomer structure translates into homocoupled sequences under relevant reaction conditions remains to be established in future studies. The comparison of the 1H and 19F NMR spectra of PDPPTh2F4, PDPPTh2, and PDPPTh2F8 shows that the strategy of using homopolymers as controls is simple and efficient to assign low intensity signals to defect structures.26 It is also clear that after having assigned all signals indirectly

Figure 1. Chemical structures of end groups and homocouplings in PDPPTh2F4 (top) and 1H NMR spectra in C2D2Cl4 at 120 °C (aromatic region): (a) entry 6 (Mn = 22 kDa), (b) control PDPPTh2 (entry 14), (c) entry 8 (Mn = 5.4 kDa), and (d) TOCSY spectrum of entry 8.

entry 14).24 Comparing its 1H NMR spectrum (Figure 1b) with those of the PDPPTh2F4 samples (Figure 1a,c) revealed that the composite signal arose from DPPTh2 homocoupling. This is unequivocally confirmed by the TOCSY correlation between the 7.50 ppm signal and the second expected signal at 8.90 ppm (Figure 1d, red line). This signal is overlapped by end-group signals, but the chemical shift fits well with that of the second thiophene signal of PDPPTh2 taking into account that in PDPPTh2F4 the DPPTh2−DPPTh2 diad is between two F4 units. In addition, HSQC spectra give the 13C chemical shifts of carbons 1′ and 2′ for both PDPPTh2 and PDPPTh2F4 (Figure SI-1). The same value was found for 1′ (125.5 ppm) and a small deviation for 2′ (135.5 vs 135.8 ppm) corresponding to D


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Macromolecules indicates that unselective C−H arylation of the DPPTh2 monomer did not occur. This result is again promising, showing that the much debated unselective C−H arylation of monomers having unsubstituted thienyl units is not a major obstacle for DPPTh2. However, as branching reactions have been shown to occur for e.g. poly(3-hexylthiophene) made by DAP (“β-arylation”),35,36 a comprehensive understanding of how C−H selectivity depends on monomer structure and reaction conditions is the subject of future experiments. While a detailed spectroscopic analysis of this side reaction is yet to be related to a broad variety of reactions conditions, it is clear from our studies that typical side reactions during DAP are rather related to homocouplings and chain end degradation (either through dehalogenation or through reaction via C−H activation of the solvent) than to unselective C−H activation.26,31,32 Next, we turned to the most interesting question as to what extent the used reaction conditions were responsible for undesired DPPTh2 homocouplings. In a previous study, we observed that the presence of PCy3 as ligand and PivOH/ K 2 CO 3 instead of KOPiv was important to suppress homocouplings of the C−H monomer.26 Hence, under the optimized conditions of entry 7, we added PCy3 and additionally replaced PivOH/K2CO3 by KOPiv (entries 8− 10) and inspected the resulting copolymers with respect to signals at 7.5 ppm. Interestingly, on comparing the 1H NMR spectra of all PDPPTh2F4 batches, it appears that DPPTh2 homocouplings form in all cases almost independently of the reaction conditions chosen (Figures SI-2 and SI-3). We also subjected monomers DPPTh2 and F4Br2 to DAP using the meanwhile popular catalytic system Pd2dba3, P(o-anisyl)3, Cs2CO3, PivOH in pure toluene, giving PDPPTh2F4 with Mn = 13.5 kDa in 81% yield (entry 11). Also in this case DPPTh2 homocouplings were observed albeit at lower content (Figure SI-4a). However, when increasing the amount of phosphine to a molar ratio palladium to phosphine of 1:2, DPPTh 2 homocouplings could be eliminated (entry 12, Figure SI-4b). Thus, while the fact that a broad variety of reactions conditions lead to DPPTh2 homocoupling sequences gives a hint that the DPPTh2 structure might be unusually sensitive to this type of side reaction, obviously the amount of phosphine is important to suppress C−H/C−H homocouplings. It is interesting to note that polycondensations based on other cross-coupling variants have been reported to produce DPPTh2−DPPTh2 homocouplings.28,29 Thus, this study provides a reference by which such structural defects can be identified, quantified, and eliminated. Conveniently, as signals 1′ and 2′ of −DPPTh2− DPPTh2− exhibit baseline resolution in the 1H NMR spectrum of PDPPTh2F4, which is not the case for e.g. carbazole-based copolymers,26 good quantification is possible as given in Table 1. We also suggest that the chemical shifts of protons 1′ and 2′ of the defect structure −DPPTh2−DPPTh2− are not strongly dependent on comonomer structure and can thus be used for the defect analysis of other DPPTh2-based conjugated polymers as well. This hypothesis is corroborated by the chemical shift of homocoupled DPPTh2 units which occur in PDPPTh2F8 as well at 7.52 ppm (entry 13, Figure SI-5). Figure 3 displays the observed defective backbone sequences of PDPPTh 2 F 4 schematically. Molecular-Weight-Dependent Optical, Thermal, and Electronic Properties. DPPTh2 homocouplings cause an additional red-shifted absorption band in UV−vis spectroscopy as demonstrated by Hendriks et al. for the analogous phenylene

Figure 3. Chemical and schematic structures of DPPTh2 homocoupled units (a) at the chain end and (b) within the chain.

copolymer.28 Thus, any homocoupled units present in PDPPTh2F4 are expected to display this behavior as well with intensities according to their occurrence. To corroborate our findings from NMR spectroscopy, we compared the optical properties of PDPPTh2F4 (Mn = 10.9 kDa, entry 1) and PDPPTh2 (Mn = 14 kDa, entry 14). However, pronounced aggregation of PDPPTh2F4 in chlorobenzene occurred already for low concentration and for low molar mass, burying any potential shoulders from homocouplings. In order to reveal the absorption spectra of molecularly dissolved chains, UV−vis spectra were recorded at high temperatures (Figure 4). Figure 4a shows the temperature-dependent series of UV−vis spectra of entry 1, which is a low molar mass PDPPTh2F4 sample, from 30 to 135 °C. At 30 °C, a pronounced vibronic fine structure between 500 and 800 nm is visible, indicating significant aggregation. Upon heating, these aggregates continuously dissolve, and the intensity of the A00 band at 736 nm decreases accordingly. The remaining intensity between ∼700 and 750 nm at 135 °C despite further heating is ascribed to the presence of DPPTh2 homocouplings (see also Figure 4c). When the same experiment is performed with the homopolymer PDPPTh2, which also displays a strong aggregation behavior, the intensity of the A00 at 890 nm vanishes completely (Figure 4b). The remaining charge-transfer band of the nonaggregated homopolymer at 736 nm matches well with the remaining shoulder observed for the PDPPTh2F4 sample. As the vibronic structure of aggregated PDPPTh2 is similar to PDPPTh2F4 but its synthesis inherently does not allow for structural defects in the polymer backbone such as homocouplings, its temperaturedependent UV−vis spectra display the behavior of a defect-free model polymer. Accordingly, the temperature-dependent intensities of the A00 bands of these two samples are different with respect to residual intensity at ∼736 nm at high temperature. From Figure 4c it becomes clear the residual intensity of PDPPTh2F4 at high temperature indeed arises from homocouplings and not from aggregation. PDPPTh2F4 having 22 kDa (entry 6) exhibits a residual shoulder as well (Figure SI6). For the highest molar mass sample PDPPTh2F4 (30 kDa, entry 7) thermal heating in chlorobenzene does not dissolve aggregates (Figure SI-7). The high-temperature UV−vis spectra of entries 1, 6, and 13 are finally shown in Figure 4d, also E


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Figure 4. Temperature-dependent UV−vis spectroscopy in chlorobenzene: (a) PDPPTh2F4 (entry 1), (b) PDPPTh2 (entry 14), (c) normalized optical density of (a) and (b) at 736 and 890 nm, respectively, and (d) comparison of entries 1, 6, and 13 at 135, 150, and 130 °C, respectively.

Figure 5. (a) n-Channel transfer characteristics of FETs using entry 1 (green), entry 6 (red), and entry 7 (blue). (b−d) n-Channel output curves of FETs using (b) entry 7, (c) entry 6, and (d) entry 1.

not yet been investigated.13 Moreover, we were interested in whether the presence of the DPPTh2 homocoupling defects would be detrimental to electron transport. We fabricated topgate, bottom-contact FETs using entries 1, 6, and 7 with almost equal amount of defects but varying molecular weight (Mn = 10.9, 22, and 30 kDa, respectively) as semiconductors and 550 nm thick PMMA (ε = 3.6) as the dielectric layer. All devices exhibited typical ambipolar behavior, with strong n-channel and

confirming that the wavelength of the shoulder of PDPPTh2F4 matches with the charge transfer band of PDPPTh2. Thus, the optical spectra support the identification of DPPTh2 homocouplings by NMR spectroscopy. Clearly, reliable quantification is not possible from UV−vis spectroscopy; here hightemperature NMR spectroscopy is to be used. PDPPTh2F4 has been reported to exhibit excellent n-type performance in OFETs, but the effect of molecular weight has F


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weak p-channel characteristics (hole mobilities are more than 1 order of magnitude inferior than electron mobilities). In Figure 5, the n-type transfer and output characteristics of all FETs after thermal postannealing at 250 °C are shown. The p-type transfer characteristics are reported and commented on in Figure SI-8. An enhancement of currents with Mn can be observed in the transfer curves reported in Figure 5a. Consequently, the saturation electron mobility values (μe) were strongly dependent on molecular weight. The average electron mobilities μe,ave of entries 1, 6, and 7 were 0.04, 0.38, and 0.49 cm2/(Vs), respectively, and the maximum electron mobilities μe,max 0.05, 0.55, and 0.60 cm2/(Vs), respectively (Table 2). Such a trend is

Mn (kDa)

μe,ave (cm2/(V s))

μe,max (cm2/(V s))

VTh (V)

1 6 7

10.9 22 30

0.04 ± 0.01 0.38 ± 0.17 0.49 ± 0.11

0.05 0.55 0.60

19.1 ± 0.5 36.4 ± 2.1 24.5 ± 2.3

a

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01843. Details of synthesis, measurements, and experimental procedures; additional NMR and UV−vis spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.S.). Funding

Table 2. Summary of Saturation Electron Mobilities (μe) and Threshold Voltages (VTh) of FETs Prepared from PDPPTh2F4 with Different Molecular Weightsa entry

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The Innovationsfonds Forschung of the University of Freiburg, the DFG, the Fonds der chemischen Industrie and the BadenWürttemberg Stiftung, and the IRTG Soft Matter Science 1642 is gratefully acknowledged for funding. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank M. Hagios for SEC, A. Warmbold for DSC, and V. P. Torres for additional NMR measurements.

All values were extracted at VGS = VDS = 60 V.

common for linear conjugated polymers and in agreement with charge transport being mainly dominated by the film interconnectivity, hence strongly favored in the presence of longer chains providing potential tie molecules between crystallites of the film.25,32,37,38 Overall, our results indicate that good charge transport properties are compatible with the presence of homocoupling defects in PDPPTh2F4 and that further improvements in FET mobility might be feasible by exploiting synergetic effects of defect-free samples with yet higher molar masses combined with fine-tuned device structure and processing conditions.

REFERENCES

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CONCLUSION In conclusion, we have identified dithienyldiketopyrrolopyrrole (DPPTh2) homocouplings to occur in copolymers PDPPTh2F4 with dibromotetrafluorobenzene (F4Br2) made by direct arylation polycondensation (DAP) at a content of 0−12 mol % over a broad range of experimental parameters and molecular weights. These homocouplings are conveniently quantified from high-temperature 1H NMR spectra for molar masses up to Mn = 20 kDa. Because the chemical shift of this structural defect signal at ∼7.5 ppm can be considered weakly dependent on comonomer structure, this study serves as a reference to uncover structural defects of any related DPPTh2-based copolymers made by whichever synthetic method, given a similar aggregation behavior compared to PDPPTh2F4 is present. From these results it might be suspected that the structure of DPPTh2 is sensitive to homocouplings, possibly to a greater extent compared to other C−H building blocks. A comprehensive understanding of the reactivity of a distinct C− H monomer toward undesired side reactions is yet to be established, whereby uncovering the underlying mechanisms will guide rational design of improved reaction conditions. Regarding performance of the herein prepared materials in ntype transistors, promising and molecular-weight-dependent electron mobilities up to 0.6 cm2/(V s) have been achieved despite the presence of structural defects. Studies directed to defect−function relationships of DPPTh2-based copolymers of choice can now be tackled on a comprehensive basis. G


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