Rational Use of Aromatic Solvents for Direct Arylation

Letter pubs.acs.org/macroletters

Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C−H Reactivity versus Solvent Quality Rukiya Matsidik,†,‡ Hartmut Komber,*,§ and Michael Sommer*,†,‡,∥ †

Universität Freiburg, Makromolekulare Chemie, Stefan-Meier-Str. 31, 79104 Freiburg, Germany Universität Freiburg, Freiburger Materialforschungszentrum, Stefan-Meier-Str. 21, 79104 Freiburg, Germany § Leibniz Institut für Polymerforschung Dresden e.V., Hohe Straße 6, 01069 Dresden, Germany ∥ Freiburger Institut für Interaktive Materialien und Bioinspirierte Technologien, Georges-Köhler-Allee 105, 79110 Freiburg, Germany ‡

S Supporting Information *

ABSTRACT: The solvent for direct arylation polycondensation (DAP) is of crucial importance. For conjugated polymers exhibiting reduced solubility, the choice of solvent decides on the maximum molecular weight that can be achieved, hence, good aromatic solvents are generally desirable. However, unintentional activation of C−H bonds present in aromatic solvents under DAP conditions leads to in situ solvent termination which competes with step growth. Here we evaluate relative C−H reactivity and solvent quality of seven aromatic solvents for the DAP of defect-free naphthalene diimide (NDI)-based copolymers of different solubility. C−H reactivity is strongly reduced with increasing degree of substitution for both chlorine and methyl substituents. Mesitylene is largely C−H unreactive and, thus, albeit being a moderate solvent, enables very high molecular weights at elevated temperature for NDI copolymers with limited solubility.

D

high boiling points.14−16 What is largely unknown is the ability of an aromatic solvent to undergo C−H activation under DAP conditions, in which case chain termination competes with step growth. We have recently observed in situ end-capping of a naphthalene diimide (NDI) bithiophene (T2) copolymer (PNDIT2) with toluene during DAP, and that this process may not necessarily be detrimental, as it can be exploited to control MW.11 As such a mechanism opens up entirely new possibilities in terms of end-functionalization, characterizing aromatic solvents with respect to their C−H reactivity and solvent quality for a given DAP reaction appears central to the further development of DAP. Previously, we also reported that NDI copolymers with enlarged comonomers such as PNDITF4T, composed of naphthalene diimide (NDI), thiophene (T), and tetrafluorobenzene (F4), exhibited drastically reduced solubility. Synthesizing PNDITF4T via a so-called double C−H activation approach in a mixture of DMAc and chlorobenzene (CB) gave low molecular weights Mn,NMR = 6.8 kDa only, which was caused by limited solubility in DMAc/CB. However, surprisingly high field-effect transistor electron mobilities up to 1.3 cm2/(V s) were observed.12 In an effort to increase MW of moderately soluble PNDITF4T, we elucidate a redesigned synthesis route here. Taking into account C−H reactivity and solubilizing ability of seven aromatic solvents, we investigate

irect C−H activation polycondensation (DAP) is an emerging alternative to conventional transition-metalcatalyzed cross-coupling techniques such as, for example, Stille, Suzuki, Kumada, or Negishi couplings. By directly activating C−H bonds, DAP offers many advantages such as straightforward synthesis routes, reduced costs, better atom-economy, and enables pathways free of toxic monomers, as used in Stille polycondensations.1−3 Moreover, DAP is capable of producing materials with higher purity due to the absence of additional metals such as magnesium, tin, zinc or copper.4,5 Significant progress has been made by applying DAP to construct conjugated polymers that exhibit high molecular weights (MWs) and properties comparable or even enhanced compared to analogs made by classical cross-coupling techniques.6−11 While high MW copolymers have been reported, the ability to make high MW defect-free chains is a challenge especially for copolymers with reduced solubility.12 Here, next to the catalytic system the solvent is most crucial. Most often used are dimethylacetamide (DMAc), tetrahydrofuran (THF), or toluene (Tol).1−3 Although DMAc often is preferentially used and exhibits a high boiling point enabling high temperatures during polymerization that in turn may lead to higher MWs, it is still a poor solvent for many copolymers and exhibits high toxicity.13 THF and Tol have better dissolving abilities and are less toxic; however, their relatively low boiling points may lead to irreproducible results due to changes in concentration. Moreover, several high performance copolymers exhibit limited solubility in Tol or THF which prohibits the production of high MW material. Notably, only a few studies have elucidated the effect of solvents for DAP, including aromatic ones exhibiting © XXXX American Chemical Society

Received: November 3, 2015 Accepted: November 16, 2015

1346

DOI: 10.1021/acsmacrolett.5b00783 ACS Macro Lett. 2015, 4, 1346−1350

Letter

ACS Macro Letters

Scheme 1. (a) Synthesis of PNDIT2 and PNDITF4T via DAP Starting from NDIBr2; (b) Model Reaction to Probe C−H Reactivity of Different Aromatic Solvents Towards the Reaction with NDIBr

effects of solvent, monomer concentration, and temperature for the DAP of both PNDITF4T and PNDIT2 and show how these parameters can be rationally used to achieve high yet controllable MWs even for copolymers with limited solubility. We have chosen toluene (Tol), p-xylene (PX), mesitylene (Mes), chlorobenzene (CB), o-dichlorobenzene (DCB), 1,2,4trichlorobenzene (TCB), and 1-chloronaphthalene (CN) as solvents for the synthesis of PNDIT2 and PNDITF4T (Scheme 1a). These solvents pose two series of good and commonly used aromatic solvents with increasing number of methyl or chlorine substituents. CN was additionally chosen as it is the only solvent in which PNDIT2 is molecularly dissolved at room temperature.17 A model reaction was used to probe C−H reactivity of the solvent in the absence of T2. Monofunctional NDIBr and Pd2dba3, PivOH, and K2CO3 as the catalytic system were reacted in different solvents, and the entire product distribution was analyzed using NMR spectroscopy. Additionally, the NDIsolvent adducts, which are mixtures of positional isomers for Tol, CB, DCB, TCB, and CN, were synthesized, isolated, and characterized by 1H and 13C NMR spectroscopy to ensure unambiguous signal assignment (Supporting Information, Figure S1). As discovered earlier, next to NDI solvent adducts, other side products resulting from debromination (NDI-H) and nucleophilic substitution (NDI-OPiv and NDI−OH) were found (Scheme 1b and Figure S2). 11,18 The relative contributions of these products, as shown in Table 1, clearly mirror the relative C−H reactivities of the seven solvents investigated. CN exhibits the highest reactivity giving 88 mol % of NDI-CN followed by Tol, PX, CB, DCB, TCB, and Mes. Generally, the C−H reactivity of chlorinated solvents was lower compared to methyl-substituted benzenes. Interestingly, C−H reactivity strongly decreased with increasing number of substituents. This result is likely caused by steric factors which reduce reactivity of sterically hindered C−H bonds

Table 1. Product Distribution of the Model Reactions Shown in Scheme 1b, Determined by 1H NMR Analysisa mol % solvent

NDIsolvent

NDI-H

NDIBr

Σ (NDI-OPiv + NDI-OH)

CN CB DCB TCB Tol PX Mes

88 43 29 21 69 62 5

8 2 3 7 0 2 4

0 0 0 0 0 0 0

4 (1 + 3) 55 (52 + 3) 68 (64 + 4) 72 (56 + 16) 31 (2 + 29) 36 (5 + 31) 91 (23 + 68)

a

For each solvent, approximately 3−6 mol % of the reaction products could not be identified and were not considered in the product distribution.

under the catalytic conditions employed here. Gorelsky reported a decreasing C−H bond energy with increasing number of substituents for chlorinated benzenes. This behavior is not found here, which may be caused by the different halide substrate and catalytic system used.19 Clearly, synthesizing PNDIT2 or PNDITF4T in Mes and TCB will show the least solvent end-capping, potentially leading to high MW. On the contrary, using C−H reactive CN, Tol, and PX as solvent will limit MW. In a C−H reactive solvent, molar mass can be controlled by monomer concentration as reported recently, and hence, C−H reactive solvents are not necessarily detrimental depending on what molecular weight is targeted.11 What can also be inferred from the model reactions is that with reduced C−H reactivity of the solvent NDIBr mainly reacts to NDI-OPiv and NDI-OH by nucleophilic substitution. As NDI-OH forms from saponification of NDI-OPiv, it is the sum of both these end groups, which increases with decreasing C−H reactivity (Table 1). From these results the relative C−H reactivities of the different solvents can be ranked as shown in Scheme 2. 1347


Letter

ACS Macro Letters

and aromatic end groups influence, for example, charge transport is a subject of further investigations. Next, we applied these results to the synthesis of PNDITF4T using the same reaction scheme and NDIBr2/ TF4T as monomers, with the final aim of making controlled and higher MWs than reported recently (Scheme 1a).12 Due to the πextended comonomer TF4T, PNDITF4T is much less soluble, and hence, solubility effects are expected to show up in addition to C−H reactivities of the solvent. Therefore, in order to test the solubility of PNDITF4T in these solvents, UV−vis measurements were performed at room temperature (Figure 2a). To rate solvent quality of the seven solvents toward PNDITF4T, the maximum wavelength of the charge-transfer band in the UV−vis spectrum was determined, resulting in the order CN > DCB > TCB > CB > Mes > PX ∼ Tol, which is a similar trend compared to PNDIT2.17 However, as intensity and position of the charge-transfer (C-T) band can also depend on the type of aggregates,17,20 we measured UV−vis spectra additionally temperature-dependent until no further change was observed in the spectra. Figure 2b exemplarily shows UV−vis spectra of PNDITF4T in Mes (for all other solvents, see Figures S9−S13). A plot of the maximum wavelength of the CT band (λmax) versus temperature is shown in Figure 2c. The temperature at which λmax remained constant was termed dissolution temperature (Tdis). Ordering Tdis of the seven solvents resulted in a slightly altered solvent quality ranking compared to the spectra taken at room temperature (Scheme 2 and Figure 2d). Clearly, chlorinated solvents are generally better solvents than methylated ones, and CN is by far the best solvent also for PNDITF4T. Interestingly, Mes exhibits the lowest solvent quality and the lowest C−H reactivity. However, its higher boiling point compared to toluene (165 °C vs 110 °C) entails the possibility to polymerize at higher temperatures without inducing changes in monomer concentration, making Mes a highly interesting candidate as shown below. After examining C−H reactivity of the aromatic solvent and Tdis of PNDITF4T, we elucidated the possibility to make defectfree high molar mass PNDITF4T in quantitative yield. Starting with the two extreme cases, CN was use as the best solvent with the highest C−H reactivity, and Mes was chosen as the least good solvent with very low C−H reactivity. Additionally CB was used due to its intermediate C−H reactivity and solvent quality. Quite surprisingly, using CB and CN as solvents yielded PNDITF4T in quantitative yield and with Mn,NMR between 25 and 29 kDa (90 °C, [NDIBr 2] = 0.1−0.4 M). The determination of isolated yields includes the sum of chloroform and chlorobenzene fractions (see Table S2). Compared to the original synthesis scheme of PNDITF4T, this corresponds to an increase in molar mass by a factor of ∼5.12 In Mes at 90 °C, however, oligomeric material with Mn,NMR = 11 kDa was obtained, which was strongly limited by solubility. Obviously, at 90 °C, solubility is far more important than solvent C−H reactivity to maximize MW for PNDITF4T exhibiting a much reduced solubility compared to PNDIT2. With respect to the monomer concentration dependence of achievable MW of PNDITF4T in CB and CN, Mn,NMR of ∼30 kDa were achieved for concentrations between 0.2 and 0.4 M. Higher concentrations led to early gelation due to limited solubility and hence lower molecular weight, while lower concentrations caused chain end termination either through solvent end-capping or nucleophilic substitution. Fully in line with the lower C−H reactivity of CB compared to CN, the low monomer

Scheme 2. (Top) Relative C−H Reactivities of Aromatic Solvents Inferred from Model Reactions (Scheme 1b, Table 1); (Bottom) Solvent Quality Towards PNDITF4T

In order to correlate C−H reactivity of the solvent with copolymerization behavior and MW, we synthesized PNDIT2 in all solvents and characterized them by SEC and 1H NMR spectroscopy (Scheme 1a, Figure 1, and Table S1). Using a

Figure 1. Mn,SEC and Mn,NMR of PNDIT2 synthesized in different aromatic solvents, as shown in Scheme 1a; T = 100 °C and c = 0.05 M.

concentration of NDIBr2 of 0.5 M, solvent C−H reactivity is superimposed by solubility and gelation effects, and as a result molecular weights do not correlate with C−H reactivity of the solvent (see Table S1). Upon decreasing NDIBr2 concentration to 0.05 M, chain-end termination by the aromatic solvent is much more efficient and thus is expected to be the predominant factor that governs MW. In fact, besides T2 end groups, only NDI-OPiv(-OH) and NDI-solvent end groups have to be considered for estimating Mn,NMR (Figure S3, plot of all PNDIT2 samples with end groups). The chemical shifts of NDI-solvent end group signals of the polymer were calculated from the corresponding values of the NDI-solvent monomer following a procedure proven in a previous study (for details, see SI, Figures S4−S8).11 Figure 1 shows how both relative and absolute values of Mn correlate with the solvent used in the same order as C−H reactivity obtained from the model reactions. Thus, at 0.05 M, solvent C−H reactivity directly affects molecular weight, with decreasing C−H reactivity leading to higher molecular weight. Mesitylene turns out to give exceptionally high values due to its sterically hindered aromatic C−H bonds. In Mes, the dependence of MW of PNDIT2 on NDIBr2 concentration is also much less distinct and polymer end groups are mainly from nucleophilic substitution (Table S1). This is clearly in contrast to toluene, for which a strong MW dependence on monomer concentration was found.11 The other extreme is the C−H reactive CN giving 8.1 kDa only. Thus, a broad range of MWs of PNDIT2 is accessible by appropriately selecting C−H reactivity of the solvent as well as monomer concentration. To what extent oxygen functionalities 1348


Letter

ACS Macro Letters

Figure 2. (a) UV−vis spectra of PNDITF4T with Mn = 30 kDa in different aromatic solvents at room temperature; (b) temperature-dependent UV− vis spectra of PNDITF4T in mesitylene (0.01g/L); (c) plot of the low energy band λmax of PNDITF4T vs temperature to determine Tdis; (d) Tdis of PNDITF4T in different aromatic solvents.

concentration onset that can be used to get sufficient MW is lower for CB than for CN (Table S2). At this point, the question arises whether temperatures higher than 90 °C change the relative contributions of solubility and C−H reactivity considerably. Considering Tdis= 105 °C of PNDITF4T in Mes, we chose this solvent at 120 °C, where solubility can be expected to be much higher. We envisioned that using a low concentration enabled by a low C−H reactivity would give additional room for sufficient solubility of high MW material. Figure 3 shows MW and yield of chlorobenzene fractions as a function of [NDIBr2]. Clearly, much higher absolute MWs up to Mn,NMR = 50 kDa are possible under these conditions, and for concentrations higher than 0.1 M, residual material was left in the Soxhlet thimble, suggesting that MW even exceeded the solubility limit in hot CB. The absolute values of Mn,NMR were determined from NMR end group analysis, which also revealed the defect-free nature of PNDITF4T chains made at 120 °C (Figure S14a). Homocoupling reactions of both NDI-Br as well as TF4T-H chain ends were not observed here, which is fully consistent with the detailed study on PNDIT2 and further demonstrates the stability and selectivity of the catalytic system.11 This is in contrast to other monomer systems and catalytic conditions reported earlier, which have highlighted homocouplings as major side reactions.21−23 The only difference in terms of

Figure 3. Reaction yield of the isolated chlorobenzene fraction and Mn,NMR of PNDITF4T synthesized in mesitylene at T = 120 °C using different monomer concentration.

additional side reactions at elevated temperature was an end group resulting from the C−H activation of the methyl groups of mesitylene for T > 120 °C (Figure S14b). In conclusion, we have evaluated solubility effects and C−H reactivity of seven aromatic solvents used for the synthesis of NDI-based copolymers made by direct C−H activation polycondensations. The substitution pattern strongly influences 1349


Letter

ACS Macro Letters

(13) Fischmeister, C.; Doucet, H. Green Chem. 2011, 13 (4), 741− 753. (14) Wang, X.; Wang, M. Polym. Chem. 2014, 5 (19), 5784−5792. (15) Kuwabara, J.; Yamazaki, K.; Yamagata, T.; Tsuchida, W.; Kanbara, T. Polym. Chem. 2015, 6 (6), 891−895. (16) Rudenko, A. E.; Thompson, B. C. J. Polym. Sci., Part A: Polym. Chem. 2015, 53 (21), 2494−2500. (17) Steyrleuthner, R.; Schubert, M.; Howard, I.; Klaumünzer, B.; Schilling, K.; Chen, Z.; Saalfrank, P.; Laquai, F.; Facchetti, A.; Neher, D. J. Am. Chem. Soc. 2012, 134 (44), 18303−18317. (18) Matsidik, R.; Martin, J.; Schmidt, S.; Obermayer, J.; Lombeck, F.; Nübling, F.; Komber, H.; Fazzi, D.; Sommer, M. J. Org. Chem. 2015, 80 (2), 980−987. (19) Gorelsky, S. I. Coord. Chem. Rev. 2013, 257 (1), 153−164. (20) Brinkmann, M.; Gonthier, E.; Bogen, S.; Tremel, K.; Ludwigs, S.; Hufnagel, M.; Sommer, M. ACS Nano 2012, 6, 10319−10326. (21) Lombeck, F.; Gorelsky, S.; Komber, H.; Sommer, M. ACS Macro Lett. 2014, 3 (8), 819−823. (22) Broll, S.; Nübling, F.; Luzio, A.; Lentzas, D.; Komber, H.; Caironi, M.; Sommer, M. Macromolecules 2015, 48 (20), 7481−7488. (23) Kowalski, S.; Allard, S.; Scherf, U. Macromol. Rapid Commun. 2015, 36 (11), 1061−1068. (24) Sommer, M. J. Mater. Chem. C 2014, 2 (17), 3088−3098.

C−H reactivity, whereby an increasing number of substituents lowered C−H reactivity of the solvent pointing to steric effects. Due to its unreactive C−H bonds, mesitylene is a very promising solvent that enables very high molecular weight also at low monomer concentration. The low solubilizing ability of mesitylene can additionally be overcome by using higher temperature, at which the catalytic system used is fully stable and additional side reactions are absent. This study opens the door to a multitude of NDI copolymers24 with further varied comonomers that are defect-free in nature and exhibit high and controlled molecular weight.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.5b00783. Materials, methods, and additional characterization, including model compounds, assignments of chemical shifts, full NMR spectra, and additional UV−vis spectra (PDF).

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The DFG (IRTG Soft Matter Science 1645, Project SO-1213/ 8−1) and the Innovationsfond Forschung Universität Freiburg are gratefully acknowledged for funding. R.M. and M.S. thank M. Hagios for SEC measurements and V. Pacheco-Torres for additional NMR measurements.

■

REFERENCES

(1) Blouin, N.; Leclerc, M. Acc. Chem. Res. 2008, 41 (9), 1110−1119. (2) Okamoto, K.; Zhang, J.; Housekeeper, J. B.; Marder, S. R.; Luscombe, C. K. Macromolecules 2013, 46 (20), 8059−8078. (3) Kowalski, S.; Allard, S.; Zilberberg, K.; Riedl, T.; Scherf, U. Prog. Polym. Sci. 2013, 38 (12), 1805−1814. (4) Estrada, L. A.; Deininger, J. J.; Kamenov, G. D.; Reynolds, J. R. ACS Macro Lett. 2013, 2 (10), 869−873. (5) Kuwabara, J.; Yasuda, T.; Choi, S. J.; Lu, W.; Yamazaki, K.; Kagaya, S.; Han, L.; Kanbara, T. Adv. Funct. Mater. 2014, 24 (21), 3226−3233. (6) Wang, Q.; Takita, R.; Kikuzaki, Y.; Ozawa, F. J. Am. Chem. Soc. 2010, 132 (33), 11420−11421. (7) Berrouard, P.; Najari, A.; Pron, A.; Gendron, D.; Morin, P.-O.; Pouliot, J.-R.; Veilleux, J.; Leclerc, M. Angew. Chem., Int. Ed. 2012, 51 (9), 2068−2071. (8) Kowalski, S.; Allard, S.; Scherf, U. ACS Macro Lett. 2012, 1 (4), 465−468. (9) Lu, W.; Kuwabara, J.; Iiyima, T.; Higashimura, H.; Hayashi, H.; Kanbara, T. Macromolecules 2012, 45 (10), 4128−4133. (10) Gao, Y.; Zhang, X.; Tian, H.; Zhang, J.; Yan, D.; Geng, Y.; Wang, F. Adv. Mater. 2015, 27, 6753. (11) Matsidik, R.; Komber, H.; Luzio, A.; Caironi, M.; Sommer, M. J. Am. Chem. Soc. 2015, 137 (20), 6705−6711. (12) Luzio, A.; Fazzi, D.; Nübling, F.; Matsidik, R.; Straub, A.; Komber, H.; Giussani, E.; Watkins, S. E.; Barbatti, M.; Thiel, W.; Gann, E.; Thomsen, L.; McNeill, C. R.; Caironi, M.; Sommer, M. Chem. Mater. 2014, 26 (21), 6233−6240. 1350


Rational Use of Aromatic Solvents for Direct Arylation

Recommend Documents