One-Pot Synthesis and Characterization of All-Conjugated Poly(3

Oct 3, 2014 - Laboratory for Chemistry of Novel Materials, Center of Innovation and Research in Materials & Polymers (CIRMAP), University of Mons − ...
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One-Pot Synthesis and Characterization of All-Conjugated Poly(3alkylthiophene)-block-poly(dialkylthieno[3,4‑b]pyrazine) Pieter Willot,† David Moerman,§ Philippe Leclère,§ Roberto Lazzaroni,§ Yannick Baeten,‡ Mark Van der Auweraer,‡ and Guy Koeckelberghs*,† †

Laboratory for Polymer Synthesis, Department of Chemistry and ‡Laboratory for Photochemistry & Spectroscopy, Department of Chemistry, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium § Laboratory for Chemistry of Novel Materials, Center of Innovation and Research in Materials & Polymers (CIRMAP), University of Mons − UMONS/Materia Nova, Place du Parc 20, B7000 Mons, Belgium S Supporting Information *

ABSTRACT: The Kumada catalyst transfer polymerization (KCTP) was applied on dialkyl-substituted thieno[3,4-b]pyrazine monomer systemsa low-bandgap system consisting of a thiophene ring with a fused pyrazine ring which displays promising properties for use in electronic applications. The poly(thieno[3,4-b]pyrazine)s were synthesized through a chain-growth mechanism with best results using an external otolyl initiator. A block copolymer with poly(3-hexylthiophene) was successfully formed without impurities. It is shown by UV−vis, fluorescence, DSC, and AFM measurements that the electronically fundamentally different blocks influence each other substantially in terms of the self-assembly and the optical properties, generating materials with significantly different properties than the constituting homopolymers.

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INTRODUCTION Research toward catalyst transfer polymerizations (CTP) has up to now mainly focused on the synthesis of electron-rich polymer systems, which can be used, for instance, as donor materials in organic solar cells.1 This can be explained by the benchmarked use of PCBM derivatives as electron accepting entities2−4 as well as by the synthetic difficulties which are encountered while investigating CTP for n-type conjugated polymers.5 The amount of electron-accepting and low-bandgap monomer systems that can be polymerized in a chain-growth mechanism with (some) control over the resulting polymer properties is at the moment still very limited; most of these polymers are synthesized using step-growth couplings.6−9 The general issues are mostly the low solubility of n-type monomer systems, Grignard compatibility, and the limited association between the catalyst entity and the growing polymer chain that is observed for a series of electron-donating polymers, which is the key to success in most controlled chain-growth protocols.5 Concerning the few n-type conjugated polymers prepared via a chain-growth mechanism, the group of Yokozawa investigated the successful synthesis of polypyridine derivatives with the Ni(dppp)Cl2 catalyst (dppp = 1,3-bis(diphenylphosphino)propane) that is also widely used for most electron-rich monomer systems.10,11 The group of Kiriy reported a strategy toward controlled synthesis of another electron-deficient monomer system based upon naphthalene diimide, using a Ni(dppe)Br2 catalyst (dppe = 1,3-bis(diphenylphosphino)ethane).12 However, this approach is only applicable for monomer systems that form stable radical anions. Elmalem et al. also investigated the polymerization of fluorene−benzothiadiazole copolymers using a Pd catalyst.13 Finally, the group of © XXXX American Chemical Society

Seferos implemented new Ni-diimine catalysts to obtain significant control over the polymerization of polybenzotriazoles.14,15 The research area of all-conjugated donor−acceptor (D−A) block polymers is attracting an increasing amount of attention because of their possibilities toward application in all-polymer solar cells, an alternative to the widely investigated polymer/ fullerene solar cells.16−22 Notwithstanding the currently lower power conversion efficiencies, advantages over the traditional polymer/fullerene systems such as better coverage of the solar spectrum and a higher stability of the active layer induce a lot of interest in these materials. However, because of the abovedescribed synthetic limitations, the research area of allconjugated D−A block copolymers is still in its infancy. An interesting monomer toward all-conjugated D−A block copolymers is thieno[3,4-b]pyrazine (TP). TP monomers have been extensively studied, and they combine a very low bandgap for the resulting polymers with a substantial amount of functionalization possibilities, including the implementation of solubilizing side chains, which is a crucial parameter toward processability.23−34 Furthermore, Wen et al. already showed that a CTP polymerization with Ni(dppp)Cl2 of a dihexylsubstituted TP-monomer resulted in the formation of polymeric material, although, because of the limited solubility, a complete insight into the nature of this polymerization was not obtained.31 Received: August 26, 2014 Revised: September 25, 2014

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In this work, first a more extensive study is performed to analyze the nature of the KCTP applied on the TP monomer system. Afterward, the TP monomer system is used in combination with 3-hexylthiophene to obtain an all-conjugated block copolymer in a one-pot synthesis, and the resulting material is analyzed structurally. A further study is performed toward the optical and physical properties of these polymers.

polymerization and low lifetime of the polymerization as for Ni(dppp)Cl2 were also observed for the Ni(dppe)Cl2 catalyst, leading to the same conclusion of a not-controlled polymerization mechanism. The high polydispersity of the polymers made with Ni(dppp)Cl2 and Ni(dppe)Cl2 indicates the presence of different polymerization circumstances, leading to polymers of different molecular weights. A possible explanation is the relative insolubility of the catalysts that are used. This leads to polymer chains starting growth at different times during the polymerization, resulting in broad GPC spectra. This explanation is quite reasonable in this case because of the observed fast polymerization in combination with the poor solubility of the catalyst systems. In order to tackle this problem, we switched to soluble o-tolyl-Ni catalyst systems (Scheme 1).35 This ensures that all initiating particles can start reacting at the beginning of the polymerization, and therefore this should lead to a significant decrease in polydispersity. The polymerizations were investigated both with dppp and dppe ligands under the same circumstances as the earlier polymerizations. The dissolution of the o-tolyl-Ni initiator results in the simultaneous and faster initiation of the polymer chains, leading to significantly lower Đ values as well as a far better monomodality of the GPC curves (Figure 1). While the

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RESULTS AND DISCUSSION Monomer Synthesis. Because initial attempts to polymerize a dihexyl-substituted TP monomer resulted in a partially insoluble material, we opted for longer dodecyl side chains in order to obtain a higher solubility. The formation of the dibromo-substituted dialkylthienopyrazine (1) was performed as described earlier.24 GRIM tests with i-PrMgCl.LiCl at room temperature for 30 min (c = 0.1 M) (Scheme 1) resulted in conversion to 2 of 96%, as confirmed by 1H NMR analysis of H2O-quenched aliquots. These conditions are therefore used for all polymerizations. Scheme 1. Monomer Formation and Polymerization Experiments

Figure 1. GPC curves of the o-tolyl-initiated thienopyrazine polymers PTP3 and PTP4.

Homopolymer Synthesis. Inspired by the work of Wen et al., we started polymerization tests with Ni(dppp)Cl2 (4 mol %; c = 0.03 M).31 After optimization of the reaction conditions, completely soluble polymeric material was formed at room temperature with M̅ n values of up to 9.1 kg/mol (PTP1). However, this catalyst system consistently resulted in multimodal GPC spectra with high polydispersities (Đ), discouraging the idea to obtain control over the polymerization (Figure S1, Supporting Information). By withdrawing small fractions during the polymerization and analyzing the conversion and molar mass, it is observed that the polymerization happens very fast (within 5−10 min), and the highest molar mass is already obtained within this short period of time. Furthermore, after the initial, short polymerization time, the molar mass and the conversion do not increase anymore, while there is still monomer present and no polymer precipitation is observed. From these properties, one can conclude that the polymerization is certainly not controlled. Hoping to obtain more control over the polymerization, we switched to a different, related catalyst, Ni(dppe)Cl2. This resulted in polymeric material (PTP2) with a higher M̅ n value of 12.8 kg/mol. However, the GPC spectrum still showed the presence of different shoulders and a high Đ. The same fast

M̅ n value of the o-tolyl-Ni(dppe) (PTP4) initiator (5.8 kg/mol) is slightly higher than that of the o-tolyl-Ni(dppp) initiator (5.1 kg/mol) (PTP3), the latter has a significantly lower Đ value (1.3). The ratio of the integration value of the 1H NMR signals of PTP3 from the o-tolyl entity and the integration values of the repeating unit implies a chain length of 8 units (Figure S6, Supporting Information). Taking into account the typical overestimation of M̅ n values by GPC, this value is in good correlation with the obtained GPC values (470 g/mol per TP unit), which can be considered as an indication that every chain is initiated with the o-tolyl entity. This indication could be further confirmed by using MALDI-Tof analysis of PTP3, but unfortunately we were unable to produce a representative MALDI-Tof spectrum of this polymer due to a poor ionization of the chains. However, the calculated degree of polymerization of 8 is significantly lower than what is theoretically expected in a chain-growth mechanism if 4 mol % of initiator is used. This suggests that the polymerization mechanism is not controlled, which motivates further analysis. Therefore, a series of polymerizations with different mol % of o-tolyl-Ni(dppp) initiator (between 2 and 20 mol %) were used in combination with the same amount of monomer. This resulted in roughly B

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the same M̅ n values for all synthesized polymers, indicating that the amount of initiator has no effect on the molecular weight (Figure 2). This unfortunately implies that the amount of

generated, which can initiate new polymer chains (which do not contain an o-tolyl group). Conversely, if the dissociation from a growing polymer chain is the dominant termination reaction, the degree of polymerization is governed by the ratio of the rate of oxidative addition and the rate of dissociation. For both possibilities, this leads to disproportionation dominant:

X̅ n =

k TM[In]0 [M]0 kdispr[In]0 [In]0 (1)

dissociation dominant:

X̅ n =

k OA[In]0 kdiss[In]0

(2)

in which kTM, kdispr, kOA, and kdiss are the rate constants for transmetalation, disproportionation, oxidative addition, and dissociation, respectively. These equations reveal that if disproportionation is the dominant termination reaction, the degree of polymerization (and molar mass) depends on [M]0/ [In]0, while in the case of dissociation the degree of polymerization is independent of [M]0/[In]0. Our experiments in which we vary this ratio (Figure 2) clearly show that the molar mass is independent of [M]0/[In]0, revealing that the dominant termination reaction is the dissociation of the catalyst. Moreover, experiments in which the [In]0 was kept constant at 0.67 × 10−3 M and [M]0 was varied between 0.016 and 0.33 M resulted in similar M̅ n (6.0 kg/mol). This result is perfectly in line with a weaker interaction between the Ni(dppp) catalyst and the conjugated polymer for electrondeficient polymers, resulting in a shorter lifetime during which the catalyst must react and insert in the terminal C−Br bond. The use of more electron-donating ligands might therefore be helpful.15,36 Note that we also observed that the presence of competing catalyst dissociation in the electron-rich poly(3alkoxythiophene) limited the degree of polymerization in that system, which can in that case be attributed to a difficult oxidative addition.37 Block Copolymer Synthesis. Because the polymerization of TP starting from an external, soluble initiator resulted in the formation of relatively monodisperse and well-defined polymers, it can be hypothesized that these monomers can be used in a block copolymerization. In fact, at the start of the growth of the second block, the first block can be considered as an external, soluble “macroinitiator” for the polymerization of the second block. Therefore, we considered a block copolymerization of TP starting from an in situ prepared poly(3hexylthiophene) (P3HT) macroinitiator, which should result in a P3HT-b-PTP block copolymer (Scheme 3). For the polymerization of the first block, we used a Ni(dppp)Cl2 catalyst (6 mol %), and after a polymerization time of 1 h, half of the polymerization mixture is quenched with acidified THF, while 1 equiv of the thienopyrazine monomer is added to the other half. After a further polymerization time of 1 h, the block copolymerization is quenched with acidified THF. The block copolymer is then precipitated in methanol, filtered off and washed with hexane in a Soxhlet apparatus, dissolved in CHCl3, and again precipitated in methanol. All of the polymeric material was soluble in CHCl3. The resulting GPC traces of the homo- and block copolymer are given in Figure 3. This graph shows a clear shift toward lower elution time, while maintaining a monodisperse peak, indicating the absence of remaining P3HT homopolymer. The P3HT homopolymer showed a M̅ n value of 6.3 kg/mol with a

Figure 2. Influence of the mol % initiator used on the resulting molar mass ([M]0 = 0.03 M).

control over the polymerization reaction is only very limited. Clearly, the polymerization continues up to a M̅ n value of around 5 kg/mol, after which the chains are terminated. It can be excluded that this is due to solubility issues, as higher, still soluble molecular weight material is readily formed under the same polymerization conditions if another catalyst is used, albeit with multimodal molecular weight distribution. This independence of the molar mass on the amount of initiator used is inconsistent with the hypothesis that the polymers would be formed through a step-growth polymerization mechanism because the o-tolyl initiating entity would act as a chain-stopper, hereby lowering the resulting molar masses, which is not observed. Also considering the low Đ value, the obtained results point at a not-controlled chain-growth mechanism in which the growing chains terminate after the addition of 8 units on average. Although it is clear that the polymerization proceeds via a chain-growth polymerization and that the molar mass is not governed by solubility, the question arises what type of termination reaction is dominant. In KCTP the key to success is the fact that after the reductive elimination the catalyst does not diffuse from the polymer chain, but instead remains complexed to the π-conjugated backbone. It then inserts in a terminal C−Br bond, followed by transmetalation. In this mechanism, two possible termination steps can occur: either disproportionation or dissociation (Scheme 2). If disproportionation is the dominant termination step, it competes with the transmetalation, and the degree of polymerization is determined by the ratio of the rate of transmetalation and the rate of disproportionation. Note that also Ni(dppp)Br2 is Scheme 2. Mechanistic Pathway with Possible Termination Reactionsa

a

OA = oxidative addition, TM = transmetalation, and RE = reductive elimination. C

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Scheme 3. Synthesis of the P3HT-b-PTP Block Copolymer

growth after the addition of the TP monomers. Also, the fact that there is still one triplet present from a terminal αmethylene unit demonstrates that the block copolymer underwent a unidirectional growth resulting in only A−B block copolymers, without B−A−B contamination.39 This result confirms previous findings by the group of Yokozawa which emphasize the importance of the order of addition of the different blocks in a block copolymerization with Ni(dppp)Cl2 if both blocks have significantly different electronic properties.40 This implies that if a well-defined A−B block copolymer can be synthesized, the formation of the B−A block copolymers will be less favorable. This rule also excludes the formation of B−A−B triblock copolymers by bidirectional growth for these monomers because of the unfavorable B−A transition. Therefore, only unidirectional growth will take place in a block copolymerization with Ni(dppp)Cl2 of blocks with significantly different electronic properties, forming A−B block copolymers without the need for external initiation.39 If the intensity of the α-methyl signal of the starting 3hexylthiophene (3HT) unit is then calibrated to 2, it is clear that there are 16 internal 3HT units, which still corresponds with the chain length of the P3HT homopolymer. Furthermore, it can be deduced that there are on average seven thienopyrazine units present in the block copolymer, since one TP unit accounts for four α-methyl protons. This amount is lower than the envisioned 16 units but in line with the degree of polymerization of the TP homopolymers. To further test whether it is possible to make a longer TP block, we repeated the same block copolymerization experiment, but instead of adding 1 equiv of the thienopyrazine block, we added 2 equiv, theoretically leading to a thienopyrazine block length of 32. 1H NMR analysis (Figure S8, Supporting Information) showed that this block copolymer had a thienopyrazine block length of 8, which is only marginally higher than what was obtained for the first block copolymer. From these results, it can be concluded that the same

Figure 3. GPC curves of P3HT and the P3HT-b-PTP block copolymer.

Đ value of 1.2, which shifts to a M̅ n value of 14.8 kg/mol with a Đ value of 1.6 for P3HT-b-PTP. The determination of the lengths of both blocks can be performed by analysis of the 1H NMR spectrum of the P3HT homopolymer and the P3HT-bPTP block copolymer (Figure 4).38 If the α-methylene region of the P3HT homopolymer is considered, the ratio of the two signals can be correlated to the chain length. The small signal centered at 2.60 ppm accounts for the α-methylene protons of the end-groups and integrates for 4 H atoms, whereas the larger signal at 2.80 ppm accounts for all internal units of the P3HT.39 From these integration values, it can be deduced that the P3HT homopolymer, and therefore the first block of the block copolymer, has a length of 17 units, which is in nice correlation with the amount of initiator that is used. If we then consider the 1 H NMR spectrum of P3HT-b-PTP, the first feature that is clear is that the triplet signal of the α-methyl protons of the end-group next to the H-terminus (c) has disappeared, and only the triplet corresponding to the α-methyl protons of the end-group next to the Br-terminus (a) remains. This is a further strong indication that all P3HT chains underwent further

Figure 4. 1H NMR spectra of the α-methylene region of (I) P3HT and (II) P3HT-b-PTP. D

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structure can be observed for lower solvent qualities, indicating aggregation in the polymer. One should note that in contrast to in many other conjugated polymers as e.g. P3HT, where aggregation is accompanied by a bathochromic shift (due to planarization), the absorption maximum is hypsochromically shifted upon addition of a nonsolvent. The observed blue-shift can therefore be attributed to the formation of H-aggregates of the polymers, without significant planarization, as the polymers are already strongly conjugated (and planar) in a good solvent. The UV−vis spectrum in good solvent (THF) of the block copolymer P3HT-b-PTP is compared with the UV−vis spectra of the homopolymers and a blend which was prepared taking the contribution (weight percent) of both blocks to the block copolymer into account (m%P3HT = 42.8; m%PTP = 57.2%) (Figure 5b). This contribution was determined from the block lengths determined from the 1H NMR spectrum. It is clear that the signals of both homopolymers at 450 and 900 nm are also present in the spectrum of the block copolymer and the blend. However, if the UV−vis spectrum of the blend of the two homopolymers is compared with that of the block copolymer, an additional peak with an absorption maximum at 560 nm is observed in the latter. This peak cannot be attributed to one of the two constituting homopolymers, and it clearly demonstrates the occurrence of electronic interaction, possibly of charge transfer type, between the blocks, leading to a new material with distinct properties which differ from those of the polymer blend. When the P3HT-b-PTP polymer is subjected to a solvatochromism experiment (Figure 5c), the signals related to both the constituting blocks change. The signal ascribed to the P3HT block at 450 nm undergoes a red-shift, until it overlaps with the additional band of the block copolymer at 560 nm. This induces a change in color of the block copolymer solution from dark green-brown to purple. This red-shift in the absorption spectrum upon addition of nonsolvent is also present in homopolymers of regioregular P3HT, where it is accompanied by the appearance of a fine structure, correlated with the aggregation (HJ aggregates)41 and long-range order of these polymers. This fine structure is however not found in the spectrum of the block copolymer. This shows that the P3HT block does undergo a planarization if a nonsolvent is added (red-shift) but that there is no or poor aggregation of the P3HT blocks (no fine structure). The signal of the PTP block, on the other hand, decreases in intensity upon addition of nonsolvent, as is also the case for PTP3. In contrast to PTP3 a red-shift (up to 925 nm) rather than a blue-shift and a shoulder around 1100 nm are observed upon addition of methanol. These observations are similar to the spectral changes observed upon aggregation of P3HT. Therefore, it can be concluded that the PTP block in the block copolymer does aggregate, presumably also in HJ aggregates. In summary, the solvatochromism experiments show that the aggregation of the P3HT block in the block copolymer is compromised, while that of the PTP block is enhanced. This aggregation behavior in block copolymers, in which one block influence the aggregation of the other, has already been found in a number of other block copolymers as well.38,42−50 UV−vis measurements were also performed on films of P3HT-b-PTP, which lead to comparable results as in a nonsolvent solution. Note that the block copolymer covers a large part of the solar spectrum, making it interesting for possible e.g. photovoltaic applications. Annealing of the films at 90 °C did not induce any significant change (Figure S9, Supporting Information).

polymerization limits that hold for the homopolymerization also apply for the block copolymers. However, the successful block copolymer synthesis also further strengthens the hypothesis that the PTP homopolymers are indeed formed through a chain-growth mechanism with termination. Finally, it was also tried to make a block copolymer starting from the PTP block, followed by polymerization of 3HT. While the first block was indeed formed, the block copolymerization was unsuccessful. Also, no P3HT homopolymer was formed. This confirms not only the presence of termination reactions in the PTP but also that dissociation of the Ni(dppp) catalyst is the dominant termination reaction. Indeed, disproportionation is accompanied by the formation of Ni(dppp)Br2, which would act as an initiator for the P3HT polymerization. Optical and Electrochemical Properties of the Polymers. To obtain good insight into the optical properties and behavior of the homo- and block copolymers, these materials were studied using UV−vis absorption and fluorescence spectroscopy. The UV−vis spectrum of PTP3 in solution shows two transitions: a shoulder at 580 nm and one larger signal centered at 900 nm (Figure 5a). The latter, structureless band is very

Figure 5. UV−vis absorption data of (a) solvatochromism of PTP3, (b) P3HT-b-PTP, P3HT, and PTP3 and a blend of the constituting blocks, based on their weight percent in the synthesized block copolymer (m%P3HT = 42.8; m%PTP = 57.2%), and (c) solvatochromism of P3HT-b-PTP.

broad and extends up to 1200 nm. A solvatochromism analysis was performed by lowering the solvent quality by systematic addition of a nonsolvent, i.e., methanol. This resulted in a drop in intensity of the signal at 900 nm, combined with a significant hypsochromic shift (blue-shift) by more or less 100 nm while the signal at 580 nm shows no change. Moreover, some fine E

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Further optical studies were performed using fluorescence spectroscopy of solutions of P3HT, PTP3, P3HT-b-PTP, and a blend of P3HT and PTP3 in chloroform while fluorescence decays were recorded upon excitation at 430 nm. Upon excitation at 430 nm the fluorescence spectrum of P3HT consists of a band with maximum at 570 nm and a shoulder, attributed to a vibrational progression at 620 nm. The fluorescence decay is nonexponential and could be fitted to a sum of two exponentials with decay times of 130 and 553 ps (Table 1). As the amplitude of the fast decaying component Table 1. Fluorescence Lifetime Measurements Results with Excitation at 430 nm (Emission Results Measured at 575 nm Are Shown) P3HT P3HT-b-PTP P3HT−PTP blend

t1 (ps)

A1 (%)

t2 (ps)

A2 (%)

t3 (ps)

A3 (%)

130 33 115

19.5 10.4 3

553 130 529

80.5 27.2 97

499

62.4

Figure 6. Fluorescence spectra of P3HT, PTP3, and P3HT-b-PTP with excitation at 430 nm.

complex decay. However, it is clear that the fast decaying components of the decay become shorter and more important. When the average decay time, defined as ⟨τ⟩ = ∑Aiti/∑Ai, is calculated, one obtains 445 and 513 ps for P3HT and the blend of P3HT and PTP3 and 357 ps for P3HT-b-PTP. This indicates that the P3HT block is quenched by the PTP block. The quenching is however less extensive than suggested by the fluorescence spectra. This could suggest the presence of shorter components in the decay which are beyond the time resolution of our single photon timing setup (30−50 ps). Considering the overlap of the P3HT emission with the PTP3 absorption and the expected position of the HOMO and LUMO of P3HT51 and PTP3,31 quenching can occur as well by energy as by electron transfer. However, as no emission of PTP3 can be observed, it is for the moment not possible to discriminate between both mechanisms. Nevertheless, also from the fluorescence measurements it can be concluded that the properties of the block copolymer significantly differ from those of the blend. Finally, the electrochemical properties were evaluated by cyclic voltammetry (CV). A film of P3HT-b-PTP was dropcasted on a Pt electrode, and the film was scanned at speed of 100 mV/s. An irreversible oxidation wave was observed at 0.89 V, but surprisingly, a reduction was not found (Figure S30). This both contrasts with P3HT, in which reduction is also not found, but which oxidizes at much lower voltages (∼0.4 V) and also with PTP, which shows a clear reduction peak.31,51 From the obtained oxidation peak, the EHOMO can be calculated to be −5.69 eV. Using the onset values of the UV−vis spectrum of P3HT-b-PTP, the bandgap of the block copolymer is found to be 1.12 eV, which corresponds with a ELUMO of −4.57 eV. These results again show the difference in behavior between the block polymer and the homopolymers. Thermal Properties. To obtain an insight into the structural behavior and crystallinity of the synthesized polymers, the polymer powders were analyzed using DSC measurements (Figure 7). P3HT showed a melting peak at a temperature of 208 °C with a melting enthalpy of 5.7 J/g. PTP3 showed a melting signal at a temperature of 141 °C with a melting enthalpy of 1.2 J/g. This is a clear sign that TP polymers are up to some extent semicrystalline, although these values are significantly lower in comparison with P3HT. The block copolymer P3HT-b-PTP shows only one, very broad, melting peak with a maximum at around 164 °C with a melting enthalpy of 2.0 J/g, together with a crystallization peak in the cooling cycle (Figure S29, Supporting Information). The absence of a clear melting peak that can be ascribed to the

decreases at longer wavelengths (Supporting Information), this component can be attributed to a relaxation process involving planarization and/or energy transfer to segments with a longer conjugation length. The long decay time corresponds in this framework to decay of the singlet excited state to the ground state and the triplet state. The fluorescence measurements in chloroform show no significant fluorescence for PTP3 (see also Supporting Information for λ ≥ 800 nm), indicating that excitation at both the shoulder at 540 nm and the maximum at 800 nm induce relaxation through faster, nonradiative mechanisms. Upon excitation at 430 nm the features and maximum of the emission spectrum of a solution of the blend resemble that of a solution of P3HT although there is a minor red-shift of the maximum. One should note however that even when correcting for the fraction of light absorbed by PTP3, the emission intensity is slightly smaller than that of a solution of P3HT. In analogy with a solution of P3HT the fluorescence decay of a solution of the blend could be fitted to a sum of two exponentials with decay times of 115 and 529 ps (Table 1) while the amplitude of the fast decaying component decreases at longer wavelengths (Supporting Information). In analogy to the solution of P3HT this component can be attributed to excited state relaxation. It should be noted however that the contribution of this component is significantly smaller than that observed for the solution of P3HT. Together with the reduced intensity this could indicate some interaction between both chains in the blend. The fluorescence of the P3HT-b-PTP solution was measured after excitation at 430 nm, targeting the P3HT absorption maximum (Figure 6), at 540 nm, targeting the small absorption shoulder of PTP3, and at 800 nm, targeting the absorption band of PTP (spectra in Supporting Information). Upon excitation at 430 nm the emission spectrum resembles that of P3HT, but the intensity is strongly reduced, suggesting quenching of the fluorescence of the P3HT block by the PTP block. Upon excitation at 580 or 800 nm no fluorescence was observed in the range that is accessible with our setup. In contrast to the solution of P3HT and that of the blend of P3HT and PTP3, the fluorescence decay of the solution of P3HT-b-PTP has to be analyzed as sum of three exponentials. This should not be considered as a suggestion for the presence of three excited species as it is very likely that this tripleexponential decay is just a phenomenological way to fit a more F

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the UV−vis analysis, we believe that the latter hypothesis is more probable.

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CONCLUSION In the first part of this work, we focused on the Kumada catalyst transfer polymerization behavior of didodecyl-substituted thieno[3,4-b]pyrazines (TP). The Ni-catalyzed polymerization of this monomer system follows a not-controlled chain-growth mechanism. An external, soluble o-tolyl-Ni(dppp)Br initiator delivered best results, with fast formation of monomodal polymers with chain lengths of on average 8 units, after which termination takes place by dissociation of the catalyst. A successful block copolymerization with a Ni(dppp)Cl2 catalyst system was achieved starting from a poly(3-hexylthiophene) (P3HT) block, continuing growth with a poly(thieno[3,4b]pyrazine) (PTP) block (P3HT-b-PTP), proving the chaingrowth nature of the polymerization. A complete further growth of the PTP block was obtained, without the presence of homopolymer impurities. 1H NMR studies resulted in observed block lengths of 17 3-hexylthiophene units and 7 TP units, independent of the equivalents of TP monomers used. In the second part of this work, the optical and structural properties of the PTP and P3HT-b-PTP were studied using UV−vis and fluorescence spectroscopy as well as CV, DSC, and AFM measurements. The PTP homopolymer showed some semicrystallinity. A solvatochromism study of this polymer with UV−vis spectroscopy showed a significant hypsochromic shift with decreasing solvent quality together with a limited amount of fine structure. UV−vis analysis of the P3HT-b-PTP block copolymer showed absorption signals corresponding to the constituting blocks (450 nm of P3HT; 900 nm of PTP), plus an additional absorption band at intermediate wavelengths (560 nm), indicating a strong influence between the constituting blocks. This strong influence is also showed through investigation of the emission properties of the polymers. A solvatochromism study of the P3HT-b-PTP block copolymer demonstrates that the aggregation of the P3HT block is comprisedplanarization still occurs, but aggregation is suppressed, while the aggregation of the PTP-block is enhanced. These findings are also confirmed with the DSC and AFM experiments. From these results it can be concluded that the properties of the P3HT-bPTP block copolymer are significantly different from the constituting homopolymers and a blend thereof. The combination of different polymers in block copolymer materials can result in new and promising materials with properties that are not achievable in homopolymers. These results state that block copolymers are very interesting and promising materials toward the development of organic electronics.

Figure 7. DSC spectra of PTP3, P3HT, and P3HT-b-PTP.

P3HT block is a further indication that the P3HT block of P3HT-b-PTP does not show the organization behavior as the P3HT homopolymer. On the other hand, the broad melting peak has a higher melting temperature and melting enthalpy value compared to PTP3, suggesting a higher semicrystallinity for the block copolymer in comparison with the PTP homopolymer. These results are in line with the findings from the UV−vis measurements in nonsolvent. Microscopic Morphology. Atomic force microscopy measurements were carried out to determine the microscopic morphology of thin deposits of the P3HT and PTP homopolymers and the P3HT-b-PTP block copolymer. Figure 8, left, shows a tapping-mode AFM phase image of P3HT; it

Figure 8. 2 × 2 μm2 tapping-mode phase images of thin deposits of P3HT (left), PTP (center), and P3HT-b-PTP (right).

displays the typical fibrillar morphology observed for that polymer.52−57 Those fibers are thought to be built from assemblies of π-stacked polymer chains, oriented edge-on with respect to the substrate and perpendicular to the fiber axis. In that structural model, the fiber width thus corresponds to the chain length; the width value estimated from those images, i.e., 15 nm, is consistent with the molecular weight of the P3HT considered here. The PTP homopolymer also forms fibers (Figure 8, center). They are less regular and significantly narrower than the P3HT fibers (around 6 nm vs 15 nm), consistent with the low molecular weight of the PTP polymer. The deposits of the P3HT-b-PTP copolymer also show a fibrillar morphology (Figure 8, right). The distribution of the width values is rather broad and centered around 10 nm. This indicates that full π-stacking of the block copolymer chains does not take place (if that were the case, the fiber width would be around 20 nm). The fibers observed for the block copolymer are thus made either of P3HT blocks that assemble only partially, because of the disturbance induced by the PTP blocks, thus leading to narrower fibers, or of the assembly of the PTP blocks, with nonassembling P3HT chains. AFM data provide no information on the chemical nature of the fibers, but given

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ASSOCIATED CONTENT

S Supporting Information *

Used instrumentation and experimental details as well as 1H NMR, GPC, fluorescence, DSC, and CV spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (G.K.). Notes

The authors declare no competing financial interest. G

dx.doi.org/10.1021/ma501757e | Macromolecules XXXX, XXX, XXX−XXX

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ACKNOWLEDGMENTS We are grateful to the Onderzoeksfonds KU Leuven/Research Fund KU Leuven and the Fund for Scientific Research (FWOVlaanderen). P.W. is grateful to the agency for Innovation by Science and Technology (IWT) for a doctoral fellowship. We also thank Tine Hardeman for her help with the DSC measurements. Research in Mons is supported by the Science Policy Office of the Belgian Federal Government (PAI 7/05), Région Wallonne (OPTI2MAT excellence programme), and FNRS-FRFC. Ph.L is senior research associate of FRS-FNRS. D.M. is grateful to FRIA for a doctoral fellowship.

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dx.doi.org/10.1021/ma501757e | Macromolecules XXXX, XXX, XXX−XXX