Synthesis of Conjugated Polymers Containing Octafluorobiphenylene

Letter Cite This: ACS Macro Lett. 2018, 7, 90−94

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Synthesis of Conjugated Polymers Containing Octafluorobiphenylene Unit via Pd-Catalyzed CrossDehydrogenative-Coupling Reaction Hideaki Aoki,† Hitoshi Saito,† Yuto Shimoyama,† Junpei Kuwabara,† Takeshi Yasuda,‡ and Takaki Kanbara*,† †

Tsukuba Research Center for Energy Materials Science (TREMS), Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan ‡ Research Center for Functional Materials, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan S Supporting Information *

ABSTRACT: Polycondensation via Pd-catalyzed cross-dehydrogenativecoupling reaction of 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl with thiophene analogues was studied. The synthetic protocol, in which employment of prefunctionalized starting monomers was fully avoided, allowed straightforward access to an alternating π-conjugated polymer. The addition of K2CO3 to the catalytic system promotes the cross-coupling reaction and suppresses the undesired homocoupling reaction, producing the corresponding donor− acceptor type π-conjugated polymers with minor homocoupling defects. The reaction also proceeded using O2 as the terminal oxidant, resulting in lower loading of the Ag oxidant. The obtained polymer was evaluated as an emitting material for an organic light-emitting diode. functionalization strategies toward the synthesis of πconjugated polymers, we envisioned the development of a polycondensation reaction via the cross-dehydrogenativecoupling reaction of arenes (Scheme 1b). While there have been several recent reports on oxidative dehydrogenative polymerization,18−27 this work is based on the direct C−H/ C−H cross coupling reaction between two kinds of aromatic monomers,28,29 which allows straightforward access to alternating π-conjugated polymers, without the use of prefunctionalized starting monomers. However, this protocol presents a considerable challenge compared to direct arylation polycondensation. To implement the polycondensation reaction efficiently, it is necessary to control the selectivity of successive C−H metalation steps and suppress undesired homocoupling reactions. Consequently, we focused on the Pdcatalyzed cross-dehydrogenative-coupling reaction of polyfluoro arenes with thiophene derivatives.30−33 The acidic nature of the C−H bond owing to the electron-withdrawing fluorine substituents enables preferential C−H palladation of polyfluoro aromatic compounds over thiophene derivatives,30−32 and the presence of fluoro groups also suppresses the homocoupling reaction derived from a reductive elimination of bis(polyfluorophenyl)−Pd(II) complex,33 resulting in the formation of cross-coupling products predominantly. Thiophene is

P

olycondensation via a dehydrohalogenative cross-coupling reaction, so-called direct arylation, has recently been recognized as an effective method for the practical synthesis of π-conjugated polymers (Scheme 1a).1−11 The sp2 C−H

Scheme 1. Synthesis of π-Conjugated Polymers via sp2 C−H Functionalization

functionalization of aromatic monomers eliminates the prior preparation of organometallic reagents and the treatment of metal containing byproducts, thereby decreasing the number of synthesis and purification steps required for the preparation of the monomers and polymers, respectively. Several groups have actively attempted to utilize this method for the synthesis of πconjugated polymers to serve as semiconducting materials for optoelectronic devices.8−11 However, despite numerous efficient protocols having been reported for the direct arylation polycondensation,1−17 the reaction uses dibrominated aromatic monomers as coupling partners; therefore, this method still requires synthesis steps for their prior preparation and purification. In an ongoing study of direct sp2 C−H © XXXX American Chemical Society

Received: November 10, 2017 Accepted: December 17, 2017

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DOI: 10.1021/acsmacrolett.7b00887 ACS Macro Lett. 2018, 7, 90−94

Letter

ACS Macro Letters

Cu(OAc)2 was less effective (entry 6). The Ag salts do not only serve as oxidants, but also promote C−H bond cleavage of 1.34−37 To examine homocoupling reactions, two parallel control reactions were conducted under the standard reaction conditions (entry 2). The homocoupling product of 1,2,4,5tetrafluoro-3-(trifluoromethyl)benzene was not observed in the absence of 2-butylthiophene, whereas dimerization of 2butylthiophene occurred in the absence of 1, giving 4 in about 80% yield (Scheme S1). Other control experiments on the effect of reaction conditions are summarized in Table S1 in the Supporting Information. Subsequently, the polycondensation reaction of 1 with 3,3′dihexyl-2,2′-bithiophene was preformed referring to the reaction conditions of entry 2 in Table 1. The reaction gave the corresponding polymer (P1) with a molecular weight of 53400 g mol−1 in 61% yield after washing with methanol and ethyl acetate (Scheme 2a). The monomer conversion was very

a logical choice as a coupling partner because it also possesses good reactivity toward C−H functionalization, and the Pdcatalyzed direct arylation polycondensation of thiophene analogues is well-precedented, giving various donor−acceptor π-conjugated polymers.1−17 We herein report polycondensation of 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl (1) with 3,3′-dihexyl2,2′-bithiophene to produce the corresponding π-conjugated polymer (P1). Polycondensation of 1 with 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene was also carried out. The obtained polymer (P1) was evaluated as an emitting material for an organic light-emitting diode (OLED) to prove the applicability of this synthetic method for the construction of organic semiconducting materials. To assess the cross-coupling reaction, the investigation began with the model reaction of 1 with 2 equiv of 2-butylthiophene under various conditions; Table 1 summarizes the results. The Table 1. Results of Model Reactiona

Scheme 2. Pd-Catalyzed Cross-Dehydrogenative-Coupling Polycondensation

entry

oxidant (equiv)

base (equiv)

temp (°C)

Time (h)

yield 2/3/4b (%)

1 2 3c 4c 5c 6c

Ag2CO3 (3) Ag2CO3 (3) Ag2CO3 (3) Ag2CO3 (0.5) Ag2O (0.5) Cu(OAc)2 (1)

K2CO3 K2CO3 K2CO3 K2CO3 K2CO3

120 100 100 100 100 100

24 24 24 48 48 48

64/29/10 94(89)/4/1 96/1/1 72/23/0 90/8/1 28/50/36

(1) (1) (1) (0.5) (0.5)

high but low-molecular-weight material was extracted in the ethyl acetate fraction. Other control experiments to investigate the effects of base and reaction time were performed, and the results are summarized in Table S2 in the Supporting Information. As expected, the polycondensation reaction also proceeded in air referring to the reaction conditions of entry 5 in Table 1, resulting in lower Ag oxidant loading (Table S2, entry 7). The reaction was promoted under O2 and gave P1 with a molecular weight of 23200 g mol−1 in 66% yield (Scheme 2b). Alternatively, the polycondensation of 3,3′dihexyl-2,2′-bithiophene using the same reaction protocol as in Scheme 2a in the absence of 1 gave the corresponding homopolymer with low molecular weight (72% yield, Mn = 5000 g mol−1, Scheme S2). The chemical structure of P1 (Scheme 2a) was elucidated using NMR spectroscopy. Figure 1a shows the 1H NMR spectrum of P1, which has been compared with that of the model product, 2 (Figure S1). The main signals were assigned to the repeat unit, and the integral ratios of the signals essentially agreed with the assignments and were consistent with the alternating structure of P1. The minor signals at δ 7.63, 7.39, and 7.05 ppm are assignable to a terminal 3,3′dihexyl-2,2′-bithiophene unit (Figure S3a).38,39 The 19F NMR

a

Reactions were conducted using Pd(OAc)2 (5 mol %) and PivOH (2 equiv) in DMF/DMSO (20:1, 0.125 M). bThe yield was determined by 19F (for 2 and 3) and 1H (for 4) NMR analyses of a crude product with tetrafluoro-p-xylene as an internal standard. Number in parentheses is the isolated yield. cThe reaction was carried out in air.

cross-coupling reaction proceeded with Pd(OAc)2 (5 mol %), pivalic acid (PivOH, 2 equiv), and Ag2CO3 (3 equiv) in a mixture of N,N-dimethylformamide (DMF)/dimethyl sulfoxide (DMSO; 20/1)30,31 and gave the diarylated compound (2) as a main product, while homocoupling of 2-butylthiophene was also observed (entry 1). Alternatively, the addition of K2CO3 was effective for the cross-coupling reaction and allowed the reaction to proceed at lower temperature (100 °C, entry 2), resulting in considerable reduction of the homocoupling product (4); the base promotes deprotonation of 1. The reaction could proceed even when air was introduced into the reaction mixture (entry 3). The aerobic conditions resulted in lower loading of Ag oxidant (0.5 equiv, entry 4),32 while the catalytic activity was somewhat reduced. Under aerobic conditions, the use of Ag2O was preferable (entry 5), while 91

DOI: 10.1021/acsmacrolett.7b00887 ACS Macro Lett. 2018, 7, 90−94

Letter

ACS Macro Letters Scheme 3. Synthesis of P2 and P3

compared with P1 (Figure S7a). The difference in the reactivity of 4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b′]dithiophene probably caused the homocoupling. In contrast, use of 4,4′-dihexyl-2,2′-bithiophene as a coupling partner did not give the polymer (P3) as an ethyl acetate insoluble fraction due to steric hindrance of the alkyl side chain.38,43 The results of a model reaction showed reasonable consistency (Scheme S3). The absorption and emission spectra of P1 and P2 in the film state were measured (Figure S8). The optical bandgaps (Egopt) of P1 and P2 were determined to be 2.96 and 2.19 eV, respectively. The large Egopt is associated with limited πconjugation owing to steric hindrance of fluorine atoms in the octafluorobiphenylene unit (Figure S9). The HOMO energy levels of P1 and P2 were determined to be −6.07 and −5.86 eV, respectively, by photoelectron yield spectroscopy. Since P1 exhibited light green photoluminescence (PL, λmax = 503 nm, quantum yield (φ) = 9%) when excited at 370 nm (Figure 2), Figure 1. (a)1H (600 MHz, C2D2Cl4, 373 K) and (b)19F (376 MHz, CDCl3, 298 K) NMR spectra of P1 (Scheme 2a).

spectrum supports the alternating structure of P1 (Figure 1b); the minor signals at δ −134.1, −134.4, −135.1, and −135.8 ppm are assignable to a terminal octafluoro-4,4′-biphenyl unit (Figure S3b), which has been compared with that of the model product, 3 (Figure S2). The 13C{1H} NMR signals were also assigned to the carbons in the recurring unit (Figure S4). On the other hand, the minor signals at δ 7.65 and 7.15 ppm in the 1 H NMR spectrum were assigned to structural defects in the polymer chain (Figure S3a),38−40 which were caused by a homocoupling reaction between bithiophene units. The integral ratio between the minor defect and the recurring unit in the 1H NMR spectrum indicated that P1 included 4% of the homocoupling defect. This ratio is somewhat larger than the results of the model reaction (Table 1, entry 2). This can be attributed to the difference in reactivity of 2-butylthiophene and 3,3′-dihexyl-2,2′-bithiophene. The extending π-conjugation of the polymer chain may also change the reactivity of the terminal aromatic unit.41 The NMR spectra of P1 synthesized under aerobic conditions (Scheme 2b) were essentially consistent with Figure 1 (Figure S5), showing 2% of the homocoupling defect. The same reaction protocol as in Scheme 2a was used for polycondensation of 1 with 4,4-bis(2-ethylhexyl)-4Hcyclopenta[2,1-b:3,4-b′]dithiophene. Upon changing the solvent from DMF to N,N-diethylpropanamide (DEPA) due to the solubility of the products,42 the reaction afforded the corresponding polymers (P2, Scheme 3). P2 was also characterized by NMR spectroscopy (Figure S7); the results were consistent with the trend observed in the synthesis of P1. The 1H NMR data also indicated that homocoupling of the thiophene moiety occurred more frequently in P2 (7%)

Figure 2. PL spectrum of thin film of P1 and EL spectrum of OLED using P1 at 10 V.

while P2 showed lower emission properties (φ = 1%), the electroluminescent (EL) properties of P1 were evaluated in an OLED device. The EL spectrum is shown in Figure 2 and the details of OLED fabrication and results of the device are shown in the Supporting Information. The EL spectrum was consistent with the PL of P1, and the coordinates of the CIE chromaticity diagram were x = 0.30, y = 0.53 at 0.29 mA cm−2. The electroluninance reached 417 cd m−2 at a current of 223 mA cm−2, and the external quantum efficiency (EQE) of the OLED was 0.12% at 8.5 mA cm−2. While the reported OLED performance is quite moderate, these results indicate that P1 serves as the emitting material of the OLED device. In summary, the synthesis of π-conjugated polymers via Pdcatalyzed cross-dehydrogenative-coupling polycondensation of 1 with thiophene analogues was demonstrated. The addition of 92

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(6) Kuwabara, J.; Kanbara, T. Development of Synthetic Method for π-Conjugated Polymers via Direct Arylation Polycondensation. Yuki Gosei Kagaku Kyokaishi 2014, 72, 1271−1278. (7) Bohra, H.; Wang, M. Direct C−H Arylation: A “Greener” Approach towards Facile Synthesis of Organic Semiconducting Molecules and Polymers. J. Mater. Chem. A 2017, 5, 11550−11571. (8) Marrocchi, A.; Facchetti, A.; Lanari, D.; Petrucci, C.; Vaccaro, L. Current Methodologies for a Sustainable Approach to π-Conjugated Organic Semiconductors. Energy Environ. Sci. 2016, 9, 763−786. (9) Yu, S.; Liu, F.; Yu, J.; Zhang, S.; Cabanetos, C.; Gao, Y.; Huang, W. Eco-Friendly Direct (Hetero)-Arylation Polymerization: Scope and Limitation. J. Mater. Chem. C 2017, 5, 29−40. (10) Wu, W.; Xin, H.; Ge, C.; Gao, X. Application of Direct (Hetero)arylation in Constructing Conjugated Small Molecules and Polymers for Organic Optoelectronic Devices. Tetrahedron Lett. 2017, 58, 175−184. (11) Dudnik, A. S.; Aldrich, T. J.; Eastham, N. D.; Chang, R. P. H.; Facchetti, A.; Marks, T. J. Tin-Free Direct C-H Arylation Polymerization for High Photovoltaic Efficiency Conjugated Copolymers. J. Am. Chem. Soc. 2016, 138, 15699−15709. (12) Wakioka, M.; Takahashi, R.; Ichihara, N.; Ozawa, F. MixedLigand Approach to Palladium-Catalyzed Direct Arylation Polymerization: Highly Selective Synthesis of π-Conjugated Polymers with Diketopyrrolopyrrole Units. Macromolecules 2017, 50, 927−934. (13) Grenier, F.; Goudreau, K.; Leclerc, M. Robust Direct (Hetero)arylation Polymerization in Biphasic Conditions. J. Am. Chem. Soc. 2017, 139, 2816−2824. (14) Matsidik, R.; Komber, H.; Sommer, M. Rational Use of Aromatic Solvents for Direct Arylation Polycondensation: C−H Reactivity versus Solvent Quality. ACS Macro Lett. 2015, 4, 1346− 1350. (15) Kuwabara, J.; Fujie, Y.; Maruyama, K.; Yasuda, T.; Kanbara, T. Suppression of Homocoupling Side Reactions in Direct Arylation Polycondensation for Producing High Performance OPV Materials. Macromolecules 2016, 49, 9388−9395. (16) Hayashi, S.; Takigami, A.; Koizumi, T. Palladium Immobilized on Thiol-Modified Silica Gel for Effective Direct C−H Arylation. ChemPlusChem 2016, 81, 930−934. (17) Matsidik, R.; Komber, H.; Luzio, A.; Caironi, M.; Sommer, M. Defect-Free Naphthalene Diimide Bithiophene Copolymers with Controlled Molar Mass and High Performance via Direct Arylation Polycondensation. J. Am. Chem. Soc. 2015, 137, 6705−6711. (18) Tsuchiya, K.; Ogino, K. Catalytic Oxidative Polymerization of Thiophene Derivatives. Polym. J. 2013, 45, 281−286. (19) Ricciotti, L.; Borbone, F.; Carella, A.; Centore, R.; Roviello, A.; Barra, M.; Roviello, G.; Ferone, C.; Minarini, C.; Morvillo, P. Synthesis of Highly Regioregular Poly[3-(4-Alkoxyphenyl)-thiophene]s by Oxidative Catalysis Using Copper Complexes. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 4351−4360. (20) Gobalasingham, N. S.; Noh, S.; Thompson, B. C. PalladiumCatalyzed Oxidative Direct Arylation Polymerization (Oxi-DArP) of an Ester-Functionalized Thiophene. Polym. Chem. 2016, 7, 1623− 1631. (21) Gobalasingham, N. S.; Pankow, R. M.; Thompson, B. C. Synthesis of Random Poly(hexyl Thiophene-3-Carboxylate) Copolymers via Oxidative Direct Arylation Polymerization (Oxi-DArP). Polym. Chem. 2017, 8, 1963−1971. (22) Zhang, Q.; Wan, X.; Lu, Y.; Li, Y.; Li, Y.; Li, C.; Wu, H.; Chen, Y. The Synthesis of 5-Alkyl[3,4-c]thienopyrrole-4,6-dione-based Polymers Using a Pd-Catalyzed Oxidative C−H/C−H Homopolymerization Reaction. Chem. Commun. 2014, 50, 12497−12499. (23) Huang, Q.; Qin, X.; Li, B.; Lan, J.; Guo, Q.; You, J. CuCatalysed Oxidative C−H/C−H Coupling Polymerisation of Benzodiimidazoles: An Efficient Approach to Regioregular Polybenzodiimidazoles for Blue-Emitting Materials. Chem. Commun. 2014, 50, 13739−13741. (24) Zhang, Q.; Li, Y.; Lu, Y.; Zhang, H.; Li, M.; Yang, Y.; Wang, J.; Chen, Y.; Li, C. Pd-Catalysed Oxidative C−H/C−H Coupling

K2CO3 into the catalytic system induced the desired C−H/C− H cross-coupling reaction and efficiently suppressed the homocoupling reaction, giving rise to the corresponding donor−acceptor (D−A) type π-conjugated polymers. The synthetic protocol was also successful under aerobic conditions, and a reduction of oxidant loading was achieved in this case. Currently, the synthetic examples are very specific, and a small amount of homocoupling defects remains in the polymers. However, the present work enables access to D−A type πconjugated polymers without the need for prior preparation and purification of dibrominated aromatic monomers or organometallic monomers. An OLED device was also fabricated using P1, and the role of the polymer as an emitting material in the device was demonstrated. Therefore, this synthetic methodology provides a promising tool for the synthesis of π-conjugated semiconducting polymers from the atom- and step-economical points of view. Further efforts to minimize the homocoupling defects, reduce oxidant loading, and expand the availability of the protocol for other aromatic monomers are currently underway.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00887. Characterization data for polymers and OLED device characteristics (PDF).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Junpei Kuwabara: 0000-0002-9032-5655 Takaki Kanbara: 0000-0002-6034-1582 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Chemical Analysis Center of University of Tsukuba for the measurements of NMR spectra and elemental analysis. This work was supported by Grant-in-Aid for Scientific Research (B) (17H03063) from JSPS.

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REFERENCES

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