En Route to Defect-Free Polythiophene Derivatives by Direct

Article pubs.acs.org/Macromolecules

En Route to Defect-Free Polythiophene Derivatives by Direct Heteroarylation Polymerization Thomas Bura,† Pierre-Olivier Morin,† and Mario Leclerc* Département de Chimie, Université Laval, Quebec City, QC Canada, G1V 0A6 S Supporting Information *

ABSTRACT: We report the synthesis of well-defined poly(3,3‴-didodecyl2,2′:5′,2″:5″,2‴-quaterthiophene) (PQT12) from a direct heteroarylation polymerization (DHAP) of 5-bromo-3,3‴-didodecyl-2,2′:5′,2″:5″,2‴-quaterthiophene (monomer A) and 5-bromo-3′,4″-didodecyl-2,2′:5′,2″:5″,2‴-quaterthiophene (monomer B). Experiments with different catalysts, ligands, additives, and solvents have revealed that the utilization of Herrmann−Beller catalyst and P(oNMe2Ph)3 can lead to selective thiophene−thiophene couplings. In this regard, solid-state optical and thermal measurements were particularly useful to detect the presence of β-branching and indicate that minor molecular defects can induce important changes within the supramolecular organization. We also highlight the fact that steric protection around unsubstituted β-positions of αbromothiophene units is needed to obtain a good selectivity of the crosscouplings at the α-positions. This can be achieved by the presence of a substituent at an adjacent β-position or the utilization of a bulky acidic additive (i.e., neodecanoic acid) in the catalytic system. These synthetic procedures applied to both monomers have led to PQT12 samples showing essentially the same optical and thermal properties and are comparable to those observed with their analogues prepared from chemical oxidation or Stille coupling.

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INTRODUCTION

Scheme 1. Accessible Monomers by DHAP

Low-cost building blocks, few and clean synthetic steps, high polymerization yields, and simple purification methods are important factors for the large-scale development of plastic electronics.1,2 Along these lines, recent advances using direct (hetero)arylation polymerization (DHAP) has brought a new tool for the synthesis of efficient organic semiconducting materials.3−11 Namely, this step-growth polymerization method allows for the formation of aromatic C−C bonds using aromatic C−Br bonds combined with aromatic C−H bonds, leading to only acidic byproducts. Several synthetic steps are skipped compared to the traditional Suzuki or Stille polymerization since the use of boronate or stannyl (toxic waste) comonomers is no longer required with DHAP. Another notable advantage of the DHAP method is that it can lead to higher yields and improved molecular weights when compared to traditional polymerization reactions. In this regard, we recently reported that some timecontrolled DHAP conditions can yield well-defined and processable conjugated copolymers based on dibromoarene compounds, such as 2,7-dibromofluorene, 2,7-dibromocarbazole, or 1,4-dibromobenzene derivatives with various unsubstituted thiophene moieties (see Scheme 1a).12 Those conditions involving a palladium source, a tris(o-anisyl)phosphine ligand, cesium carbonate, pivalic acid in toluene or THF were successfully applied to dibromo-6,6′-isoindigo derivatives as well.13 In these cases, the resulting copolymers © XXXX American Chemical Society

showed comparable, if not slightly better, properties when compared to their Suzuki-synthesized analogues. Moreover, those general conditions (sometimes without pivalic acid) can be applied to β-(also called C-3) protected 2bromothiophene derivatives (Scheme 1b), giving an alternative to the standard Stille coupling reaction and thus avoiding formation of toxic tin byproducts.3,12,14−16 However, some problems of selectivity were noticed with nonprotected brominated thiophene monomers.12,14 Indeed, up to now, there are no studies on polymeric materials that unambiguously report defect-free couplings between two nonprotected thiophene derivatives (see Scheme 1c). These problems could be related to the activation of the aromatic C−H bonds adjacent to the C−Br bonds (leading to β-branching) or dehalogenation reactions (leading to homocoupling in the case Received: June 23, 2015 Revised: July 31, 2015

A

DOI: 10.1021/acs.macromol.5b01372 Macromolecules XXXX, XXX, XXX−XXX

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this procedure led to PQT12ref with a Mn of 22 kDa and a molar-mass dispersity (ĐM) of 2.2. The thin-film UV−vis absorption spectrum of this reference polymer (Figure 1a) is representative of poly(alkylthiophene)s.22,24−26 One can observe a first absorption maximum at 545−550 nm for the vibronic A0−1 band and a second one around 580−590 nm corresponding to the vibronic A0−0 band. The intensity of the latter band (A0−0) has often been found to be dependent upon the polymeric fine structure such as Mn, ĐM, regioregularity, branching and crystallinity.26 As expected, this reference polymer exhibits two main thermal transitions at 119 and 132 °C upon heating (Figure S27) related to the formation of a mesophase and an isotropic phase, respectively.21 An additional small endotherm is observed near 140 °C and could be related to the melting of higher molecular weight fractions. 1H NMR analyses revealed a well-defined structure (see Figure 1c and Figure S46 for assignation) in agreement with previous studies.22 The first series of polymerization conditions by DHAP conducted on monomer A was inspired by our latest studies on the subject12 and are listed in Table 1 (entries A1 and A2). These conditions allowed us to obtain polymers with a Mn also around 20−25 kDa. However, comparisons of the spin-coated UV−vis absorption spectra (Figure 1a) seems to indicate the presence of structural defects (most probably β-branching), resulting in a blue shift of 35 nm of the absorption maximum (514−517 nm) and the absence of well-defined absorption peaks at 550 and 587 nm when compared to PQT12ref. DSC analyses confirm this irregular structure with low melting temperatures and no crystallization processes (Figures S29 and S30). Furthermore, comparisons of the 1H NMR spectra between PQT12ref and entry A2 indicated the presence of unwanted and additional signals in the latter case (Figure 1c). Indeed, two weak signals at 7.30 and 7.45 ppm can be observed in the aromatic part of the spectrum that are not present in the PQT12ref spectrum. Two peaks around 7.02 and 7.27 ppm which are visible in both 1H NMR spectra are related to Hterminated end-groups (Figure S1).22 Interestingly, there is no evidence of signals related to brominated end-groups (Figure S1, 6.99 ppm) which could indicate that debromination reactions take place and could be the limiting polymerization factor. Moreover, despite similar apparent molecular weights from steric exclusion chromatography (SEC) measurements, 1 H NMR spectrum of entry A2 clearly shows more important end-group peaks which could be related to the branched structure of this polymer. Additional peaks are also observed in the aliphatic part at 2.57 and 2.47 ppm (Figure S48). These signals could be assigned to a methylene group adjacent to a structural defect linked to an unwanted β-branching. These results show again that these general DHAP conditions cannot be easily applied for the coupling of nonprotected thiophene units.12 It is also interesting to note that solid-state optical and thermal measurements seem more sensitive to the presence and detection of structural defects than 1H NMR analyses in solution. These features could indicate that minor molecular defects can induce important changes within the supramolecular organization. Similar observations were recently raised with poly(3-hexylthiophene) (P3HT) samples27 and could be applied to conjugated polymers prepared by any synthetic method. In parallel, studies by the group of Itami showed that it was possible to influence the selectively of direct heteroarylation

of thiophene-based monomers or chain termination). It is important to recall here that these issues (branching and homocoupling) does not seem to arise with bromoarene-based (i.e., fluorene, carbazole, benzene, and isoindigo derivatives) monomers due to the relative inertness of the protons in those polymerization conditions. In short, from recent results, it seems that the selectivity of the couplings for α-protons of unsubstituted thiophene12,14,17−19 or even furan20 units is better than anticipated whereas problems of selectivity seem to occur with β-unprotected halogenated thiophene units. To investigate the α−β selectivity of couplings with nonprotected bromothiophene units, we have polymerized 5bromo-3,3‴-didodecyl-2,2′:5′,2″:5″,2‴-quaterthiophene (Monomer A) and 5-bromo-3′,4″-didodecyl-2,2′:5′,2″:5″,2‴quaterthiophene (monomer B) by DHAP in various conditions to obtain the well-known poly(3,3‴-didodecyl-2,2′:5′,2″:5″,2‴quaterthiophene) (PQT12).21 We have then compared their structure and properties with those of polymers obtained from standard polymerization methods. The two starting monomers will also allow to study the influence of a substituent at the 4position on the selectivity and reactivity of the couplings. Indeed, monobrominated quaterthiophenes have been chosen since there are perfect model compounds to focus our investigation on β-branching while avoiding any misleading results coming from homocouplings (which in the present case are not going to affect the properties of the resulting symmetric homopolymer) or variations in the stoiechiometry between two comonomers. Finally, with the exception of the end-groups, it is worth noting that a well-defined polymerization of both monomers should lead to the same polymeric structure.

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RESULTS AND DISCUSSION As shown in Scheme 2, 3,3‴-didodecyl-2,2′:5′,2″:5″,2‴quaterthiophene (QT) and both regio-isomers of monobromiScheme 2. Monomers Used in This Study and Structure of PQT12

nated quaterthiophene (monomers A and B) were prepared from relatively simple procedures as described in the literature (see S.I for experimental details and Figures S1−S16 for 1H NMR analyses).21,22 As a reference compound, PQT12 was first synthesized from oxidative polymerization of QT using ferric chloride as the oxidizing agent.21,23 In agreement with those previous reports, B


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Figure 1. (a) UV−visible absorption spectra of spin-coated A1-A3 samples from CHCl3 (60 °C, 1 mg/mL). (b) UV−visible absorption spectra of spin-coated A5 and B5. (c) 1H NMR spectra of PQT12ref, A2, A3 and A5. Assignation of peaks b, c, and d refers to Scheme 2. Ta and Tb are associated with terminal protons for monomer A (see Supporting Information).

Table 1. First Reaction Conditions for the Polymerization of Monomers A and B by Direct Heteroarylation and the Physical Properties of the Resulting Polymers entry

Pd, L

A1 A2 A3 B1 A4 B2 B3 B4 A5 B5 PQT12ref

Pd(PPh3)2Cl2, P(o-OMePh)3 Pd(PPh3)2Cl2, P(o-OMePh)3 PdCl2, 2,2′-Bipy PdCl2, 2,2′-Bipy Pd(OAc)2, PCy3 Pd(OAc)2, PCy3 Pd(OAc)2, Ph2P(CH2)PPh2 Pd(OAc)2, 2,2′-Bipy PdCl2, 2,2′-Bipy PdCl2, 2,2′-Bipy −

base, additive Cs2CO3, Cs2CO3, Cs2CO3, Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3, Cs2CO3, −

PivOH PivOH PivOH

NDA NDA

solvent, concn (M)

Mn (kDa)/ĐM; yield (%)

λmax (nm)

Tmelting (°C)

Tcryst. (°C)

THF, 0.10 toluene, 0.10 toluene, 0.25 THF, 0.5 THF, 0.35 THF, 0.35 THF, 0.35 THF, 0.35 toluene, 0.25 THF, 0.5 −

20/1.8; 70 24/2.8; 50 23/3.1; 30 27/4.7; 65 oligomer oligomer oligomer oligomer 17/1.8 57 9/1.2 57 22/2.2

514 517 533 537 − − − − 543 536 547; 583

99 95 98 104 − − − − 103; 116; 133 96; 105 119; 131; 141

− − − − − − − − 105; 91; 77 81 122; 73

reactions on thiophene-based compounds by varying the structure of the ligand.28−30 In some cases, with the right catalyst/ligand combination, the selectivity of the couplings at the α-positions was over 99%. We therefore decided to apply those conditions as described in Table 1 (entries A3−A4 and

B1−B4). For entry A3, a slightly red-shifted UV−vis absorption spectrum (15 nm) was observed when compared to entries A1 and A2 (Figure 1a). Still, DSC analyses show a melting process around 100 °C, but no crystallization phenomenon can be observed after this first melting (Figure S31). 1H NMR analysis C


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Table 2. Reaction Conditions Using Herrmann−Beller Catalyst for the Polymerization of A and B and the Physical Properties of the Resulting Polymers entry A6 A7 A8 A9 A10 B6 B7 B8

Pd, L Herrmann, Herrmann, Herrmann, Herrmann, Herrmann, Herrmann, Herrmann, Herrmann,

P(o-OMePh)3 P(o-NMe2Ph)3 P(o-NMe2Ph)3 P(o-NMe2Ph)3 P(o-NMe2Ph)3 P(o-OMePh)3 P(o-NMe2Ph)3 P(o-NMe2Ph)3

base

solvent, concn (M)

Mn (kDa)/ĐM; yield (%)

λmax (nm)

Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3

THF, 0.5 THF, 0.5 dioxane, 0.5 dioxane, 0.3 toluene, 0.5 dioxane, 0.5 THF, 0.5 dioxane, 0.35

28/3.4; 42 29/2.0; 92 43/2.0; 93 28/2.1; 70 14/1.5; 50 − 16/1.5 63 26/2.1 88

522 549 ; 586 552; 587 546; 579 542; 578 − 541 546

Tmelting (°C) − 117; 141; 116; 126; − 121; 113;

134 122 129; 140 141 134 137

Tcryst. (°C) − 125; 131; 118; 108; − 85 112;

80 86 82 93

92

ratio A0−0/A0−1 − 0.951 1.00 0.916 0.911 − − −

Figure 2. (a) UV−visible absorption spectra of spin-coated A7, A8, B8, and PQT12ref samples. (b) DSC thermograms of A7, A8, B8, and PQT12ref.

and crystallization processes at 105, 93, and 77 °C. These thermograms show the same profile but at lower temperatures than our reference polymer but could be related to a lower molecular weight (Mn of 17 kDa (A5) vs 22 kDa (PQT12ref)) and/or the presence of some remaining structural defects. Although having clearly different optical and thermal signatures than the reference PQT12ref, 1H NMR analyses of entry A5 show no evidence of the extra peaks previously observed (Figure 1c); demonstrating again the limit of this technique for the detection of small amount of β-branching. When applied to monomer B, those conditions were even less satisfactory (see entry B5 in Table 1 and Figure 1b). We then decided to study the conditions described by Ozawa et al. as early as 2010 for the preparation of regioregular P3HT (31 kDa).33 Although those polymerization conditions were never applied to nonprotected brominated thiophenes (see Scheme 1c), we utilized them with monomers A and B. First, the use of P(o-OMePh)3 ligand did not afford any well-defined polymeric materials (Entries A6 and B6 in Table 2). Interestingly, the use of Herrmann−Beller catalyst with a different ligand (i.e. P(o-NMe2Ph)3 in THF allowed us to obtain a polymer with a Mn of 29 kDa (ĐM = 2) (entry A7). For the first time, DHAP provides a polymer with an UV−vis absorption spectrum that matches that one of the PQT12ref with comparable A0−0/A0−1 ratios (0.951 vs 0.956, see Figure 2a). The DSC results are also very similar but with slightly higher transition temperatures for entry A7 (Figure 2b). Furthermore, the use of dioxane instead of THF allowed us to increase the reactivity of the catalyst system to give a polymer with a molecular weight of 43 kDa and a polymer-

of entry A3 shows again two residual signals at 7.30 and 7.45 ppm (Figure 1c) and around 2.50 ppm (Figure S49). As for entry A2, we assigned these signals to β-branching. From all these results, one may assume that those conditions are slightly more selective than entries A1 and A2 but remain insufficient to obtain a polymer comparable to a simple oxidative polymerization method. Looking at previous studies, it has been demonstrated that the addition of carboxylate derivatives may improve the efficiency of the catalytic cycle.4,31 The efficiency of the carboxylic acid added has often been linked to the size (or bulkiness) of its aliphatic chain. Recently, Rudenko and Thompson have shown that the nature of the aliphatic chain (linear, secondary, tertiary or cyclic) of the acidic moiety could also have a positive influence on the selectivity of the couplings.32 Therefore, we investigated the effects of a carboxylic acid additive in the conditions developed by Itami et al. (entries A5 and B5). Although Rudenko and Thompson used a wide range of carboxylic acids, we focused our investigation on neodecanoic acid (NDA) since this one was reported to lead to P3HT free of β-branching. With our two monomers, we observed for the first time structured UV−vis absorption spectra with peaks centered at 543 and 536 nm for entries A5 and B5, respectively (Figure 1b). Furthermore, one can see the emergence of a shoulder around 580 nm (vibronic A0−0 band). As previously mentioned for the reference polymer, this fine structure reflects an increase of the structural regularity of the polymer. DSC analyzes corroborate those optical results (Figures S33 and S34). Indeed, entry A5 exhibits endotherms at 103, 116, and 133 °C D


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Table 3. Reaction Conditions with an Additive for the Polymerization of A and B and the Physical Properties of the Resulting Polymers entry

Pd, L

A11 A12 A13

Herrmann, P(o-NMe2Ph)3 Herrmann, P(o-NMe2Ph)3 Herrmann, P(o-NMe2Ph)3

A14 A15

Herrmann, P(o-NMe2Ph)3 Pd(PPh3)2Cl2, P(o-NMe2Ph)3 Pd(MeCN)2Cl2, P(o-NMe2Ph)3 Herrmann, P(o-NMe2Ph)3 Herrmann, P(o-NMe2Ph)3 −

A16 B9 B10 PQT12Stille

solvent, concn (M)

Mn (kDa)/ĐM yield (%)

λmax (nm)

Tmelting (°C)

Tcryst. (°C)

ratio A0−0/ A0−1

Cs2CO3, NDA Cs2CO3, NDA Cs2CO3, PivOH Cs2CO3, NDA Cs2CO3, NDA

dioxane, 0.35 THF, 0.5 THF, 0.5

23/1.8 76 10/1.4 39 19/1.7 75

550; 586 542 541

122; 135; 145 119; 131 119; 127; 139

127 89 119; 95; 78

0.979 − −

dioxane, 0.5 THF, 0.5

13/1.5 52 oligomer

543; 580 −

127; 142 −

108; 96 −

0.913 −

Cs2CO3, NDA

THF, 0.5

no polymerization

−

−

−

Cs2CO3, NDA Cs2CO3, NDA −

dioxane, 0.35 THF, 0.5 −

29/1.9 79 17/1.6 63 24/1.8

137

126; 87

136;119

127; 82

0.972 0.929 0.980

base, additive

550; 587 547; 583 551; 589

Figure 3. (a) UV−visible absorption spectra of spin-coated A11, B9 samples with both reference polymers. (b) DSC thermograms of B9, A11, PQT12ref, and PQT12Stille. (c) 1H NMR spectra of B9, A11, PQT12Stille, and PQT12ref. Ta and Tb are associated with terminal protons for monomer A and Ta′ and Tb′ for monomer B.

(Figure S55) reveals no extra signal and is essentially identical to our reference compound. However, similar polymerization reactions in THF or 1,4-dioxane did not provide as good results with monomer B (B7 and B8). Although we obtained a

ization yield of 93% for entry A8. For this polymer, the UV−vis (Figure 2a) absorption spectrum exhibits an A0−0/A0−1 ratio equal to 1 and a sharp melting process at 141 °C and crystallization at 131 °C. 1H NMR spectrum of this polymer E


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on the selectivity of the couplings. For instance, these results show that it is important to add steric hindrance around the brominated thiophene unit to promote selectivity of the αcouplings. The role of steric hindrance on the selectivity has been verified by the addition of a tertiary acid (NDA), giving polymers comparable to that one obtained from Stille crosscoupling. It is however important to mention again that for αbrominated thiophenes with a substituent at one β-position (i.e. monomer A), the addition of carboxylic acid is not necessary. 1 H NMR analyses have also revealed that debromination occurs during the polymerization and no signals related to brominated end-groups could be observed. Finally, these studies clearly bring new simple and low-cost synthetic tools which should pave the way for the preparation of various polythiophene derivatives for applications in organic electronics.

structured UV−vis absorption spectrum, a blue shift of the absorption maximum is observed (Figure S23). As expected, the DSC results corroborate the optical measurements with lower temperatures for both melting and crystallization transitions (Figures S39 and S40). From this last series of experiments, one can observe differences in the reactivity and selectivity between monomers A and B. Indeed, in similar polymerization conditions, a blue shift is always observed with monomer B. Those results could be explained by the presence of an alkyl chain at the 4-position for monomer A, restricting the approach of the catalyst at the βposition of the thiophene unit. An analogy can be made between the presence of this chain and the use of bulky additive NDA in the first set of experiments. From those observations, we decided to incorporate carboxylate additive in the catalytic system. Repeating the experiments with P(o-NMe2Ph)3 as the ligand and a carboxylate additive gave eight new entries (Table 3). Polymeric materials were only obtained with the utilization of the Herrmann−Beller catalyst. With this catalyst/ligand combination and the presence of NDA, we noticed an improvement in the A0−0/A0−1 ratios (entries A11 and B9) which are now slightly better than our reference PQT12ref for comparable molecular weights. These results can even be compared to a new reference synthesized by Stille crosscoupling polymerization of 5,5′-dibromo-3,3‴-didodecyl2,2′:5′,2″:5″,2‴-quaterthiophene with hexamethylditin which gave a polymer (PQT12Stille) with a Mn of 24 kDa (ĐM 1.8) and enhanced optical (A0−0/A0−1 0.980) and thermal properties. This suggests that oxidative polymerization is not without a small amount of β-branching. DSC analyses (Figure 3b) of the two polymers made by DHAP show a very good fit with that obtained with PQT12Stille. 1H NMR analyses of the polymers show no signals associated with β-branching and only Hterminated end-group signals are present at 7.02 and 7.27 ppm for monomer A and at 7.39 and 7.09 ppm for monomer B. No brominated end-groups (near 6.95 ppm) were noticed with monomer B. Moreover, these 1H NMR spectra (Figure 3c) are quite different from that reported for PQT12Stille where many terminal proton signals are present due to different end-groups (H, Br, SnMe3, Me). Finally, as anticipated, with the exception of the end-groups, the direct heteroarylation polymerization of both monomers in those optimized conditions has led to the same polymeric structure with similar optical and thermal properties (Figure 3).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01372. Experimental procedures and polymers characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*(M.L.) E-mail: [email protected]. Author Contributions †

T.B. and P.-O.M. have equally contributed to this article.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by both discovery and strategic grants from NSERC. M.L. thanks the Killam Foundation for a fellowship. We are thankful to Fontaine’s group for helpful discussions.

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REFERENCES

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CONCLUSION We have shown for the first time that direct heteroarylation polymerization (DHAP) can efficiently and selectively polymerize unprotected thiophene units by using appropriate palladium source, ligand, and additive. More precisely, it was found that the use of P(o-NMe2Ph)3 and Herrmann−Beller catalyst with the presence of a bulky acidic additive in dioxane was particularly helpful to obtain well-defined thiophene-thiophene couplings. In this regard, solid-state optical and thermal measurements were much more useful to detect the presence of structural defects than solution 1H NMR analyses. These features can be explained by the fact that minor molecular defects can induce important changes within the supramolecular organization. These solid-state tools could then be utilized for the future characterization of so-called defect-free conjugated polymers. Experiments with monomers A and B have also pointed out the role of a substituent at the 4-position F


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En Route to Defect-Free Polythiophene Derivatives by Direct

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