Our group first focused on tetrafluorobenzene as a target aromatic monomer for direct arylation polycondensation because fluorinated benzenes have highly active C-H bonds in Pd-catalyzed direct arylation (Scheme 2).²⁵ The reaction conditions were optimized in the reactions of 1,2,4,5-tetrafluorobenzene with 2,7-dibromo-9,9-dioctylfluorene. The reaction under the optimized conditions afforded the corresponding polymer with high molecular weight (M_n = 31,500) in good yield (81%). The molecular weight is higher than that prepared by conventional polycondensation using the Suzuki–Miyaura coupling.²⁶ Structural analysis via NMR and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) revealed that the polymer has no irregular structure in the main chain. In addition, elemental analysis confirmed the high purity of the polymer. The quantification of polymer quality is important to prove the reliability of new synthetic methods. Owing to its high quality, the polymer could be used as a hole-blocking material in OLEDs, and a device prepared with this polymer in the hole-blocking layer showed higher luminance than that without the hole-blocking layer over the entire current density region.²⁷

Scheme 2. Direct arylation polycondensation of tetrafluorobenzene.

We next focused on the direct arylation polycondensation of thiophene-based monomers because thiophene derivatives are important building blocks of semiconducting materials for optoelectronic devices.^1–5 In the first trial, 3,3′,4,4′-tetramethylbithiophene was selected as a monomer to circumvent the side reactions of the C-H bonds at the 3 and 4 positions of the thiophene unit (Scheme 3).²⁸ The optimization of the reaction conditions in the reaction of 3,3′,4,4′-tetramethylbithiophene with 2,7-dibromo-9,9-dioctyfluorene revealed that Pd(OAc)₂ (2 mol %) with pivalic acid (30 mol %) in the presence of K₂CO₃ in dimethylacetamide (DMAc) is the optimal condition for affording a high-molecular-weight polymer. The developed method does not require the addition of a phosphine ligand, which is necessary in a general Pd-catalyzed coupling reaction. This phosphine-free system showed high catalytic activity, producing a high-molecular-weight polymer in a short reaction time (3 h). This system is available for the direct arylation polycondensation of bithiazole, thienothiophene, electron-rich monothiophene, and naphthalenediimide-based monomers.^29–37 In addition, this simple reaction system has been used by other groups for the preparation of a variety of conjugated polymers.^38–41 Although the developed catalytic system possesses high reactivity and usability, the site selectivity of the C-H bond remains a challenge. Specifically, the reaction of bithiophene instead of 3,3′,4,4′-tetramethylbithiophene afforded a cross-linked polymer because of the side reactions at the 3 and 4 positions of the thiophene unit.²⁸ However, lowering the reaction temperature resulted in a reduction in the amount of the cross-linked polymer.³⁷ In addition, shortening the reaction time was also an effective solution for preventing the undesired side reactions of the C-H bonds.²⁹ Because the number of the expected reactive points at the α-position decreased with increasing degree of polymerization, the undesired reactions at the β-position occurred with relatively high frequency in the late stages of polymerization.

Scheme 3. Direct arylation polycondensation of bithiophene derivatives.

As described above, direct arylation polycondensation occasionally induces side reactions that yield branched and cross-linked polymers. To suppress these undesired side reactions, we carried out the direct arylation of a pyrrole derivative bearing a directing group.⁴² The presence of a directing group, such as a 2-pyrimidinyl substituent, induces ortho-metalation at the α-position of the pyrrole moiety and accelerates the direct arylation reaction smoothly, even without the protection of the β-position.^43,44 In addition, the steric hindrance of the bulky directing group prevents the homo-coupling of the pyrrole moiety. The Ru-catalyzed direct arylation polycondensation of 1-(2-pyrimidinyl)pyrrole with 2,7-dibromo-9,9-dioctylfluorene gave the corresponding polymer in good yield (Scheme 4).⁴² The NMR and MALDI-TOF-MS measurements revealed that the site-selective polycondensation proceeded smoothly at the α-position of the pyrrole moiety, and β-branching was not observed. After polymerization, the directing group could be removed from the pyrrole moiety efficiently, and the steric hindrance arising from the bulky directing group could be released.

Scheme 4. Site-selective direct arylation polycondensation via ortho-metalation.

To identify key points for improving the efficiency of direct arylation polycondensation, the ideal reaction conditions were investigated using a model reaction with 3,4-ethylenedioxythiophene (EDOT) as a C-H monomer (Scheme 5)^31,45,46 because EDOT is known to possess high reactivity in its C-H bonds.⁴⁷ Initially, carboxylic acid additives were tested because these additives act as carboxylato ligands, assisting the deprotonation step in direct arylation reactions (Figure 1).⁴⁸ This is known as the concerted metallation–deprotonation (CMD) pathway. For an assessment of the effects of the added carboxylic acid, several carboxylic acids were examined for the polycondensation reaction. Trifluoroacetic acid was a less effective additive than pivalic acid, the original choice. In contrast, the reaction with 1-adamantanecarboxylic acid gave better results than that with pivalic acid in terms of the high molecular weight of the polymer and good yield. These results show that the choice of carboxylic acid is a key factor for the polycondensation reaction. The weak acidity of 1-adamantanecarboxylic acid is advantageous because it can act as a strong conjugate base in the coordination sphere of the transition state. In addition, the steric bulk of 1-adamantanecarboxylic acid might improve the catalytic efficiency, probably because it prevents the aggregation of the Pd catalyst.⁴⁹ The advantage of the use of bulky carboxylic acid had also been reported by other groups.^50,51 Owing to the high efficiency of the catalytic system with 1-adamantanecarboxylic acid, only 1 mol % of Pd(OAc)₂ was required to give a high-molecular-weight polymer (39,400) in 89% yield (Scheme 5).³¹

Scheme 5. Direct arylation polycondensation of EDOT using 1-adamantanecarboxylic acid as an additive.

Figure 1. A proposed transition state of the CMD pathway.

Direct arylation polycondensation via microwave heating produced the corresponding polymer with a high molecular weight. After optimization of the reaction conditions for a microwave-assisted reaction, a polymer with extremely high molecular weight (M_n = 147,000) was obtained in a short reaction time (30 min) in good yield (Scheme 6a).⁴⁵ The uniform heating under microwave irradiation results in efficient C-C bond formation. The same polymer can be prepared by a conventional method using the Suzuki–Miyaura cross-coupling reaction. However, even after the optimization of this reaction, the molecular weight was only 17,100 (Scheme 6b).⁴⁶ These results demonstrate that the optimized direct arylation polycondensation reaction can produce higher-molecular-weight polymers than those obtained using conventional methods. One reason for the high molecular weights produced by direct arylation polycondensation is the higher tolerance of the C-H bonds to the reaction conditions than the boronate ester moiety used in the conventional method.⁵²

Scheme 6. Synthesis of an EDOT-based polymer by a direct arylation method via microwave heating and synthesis via a conventional method.

We have concentrated on the development of a phosphine-free Pd catalyst in DMAc for the direct arylation reaction.⁵³ Although this system allows the use of many aromatic compounds as C-H monomers, thiophenes with electron-withdrawing groups cannot be used as monomers because of the low reactivity of the C-H bond under the DMAc-based reaction conditions (Scheme 7). To expand the range of the direct arylation method, a new reaction system for electron-poor thiophenes has been developed. As a representative electron-poor thiophene, a thienopyrroledione (TPD) derivative^54,55 was selected and used to identify suitable reaction conditions. An investigation of the reaction conditions for the TPD monomers revealed that the addition of a PCy₃ ligand and the selection of a low polar solvent (toluene) were effective in achieving the smooth polycondensation of the TPD derivative (Scheme 7).⁵⁶ Interestingly, the newly developed toluene-based reaction conditions are not applicable to electron-rich thiophenes such as EDOT. On the basis of these findings, we can select suitable reaction conditions depending on the electrical properties of the monomers.

Scheme 7. Direct arylation polycondensation of an electron-deficient thiophene.

Direct arylation via microwave heating afforded a high-molecular-weight EDOT-based polymer, as shown in Scheme 6a (H-PEDOTF).⁴⁵ The purity and physical properties were compared by the same polymer prepared via the Suzuki–Miyaura coupling reaction (S-PEDOTF, Scheme 6b).⁴⁶ The purity and semiconducting properties of these polymers were investigated to confirm the advantages of the direct arylation method (Table 1). Elemental analysis and microanalysis revealed the high purity of H-PEDOTF compared to that of S-PEDOTF. H-PEDOTF exhibited better material properties in OPVs and OFETs than S-PEDOTF. Because the power conversion efficiency (PCE) correlates with the hole mobility in the polymers, the higher hole mobility of H-PEDOTF is attributed to the better performance in the OPV.⁴⁶ The high hole mobility of H-PEDOTF is considered to be correlated with its high purity owing to a small number of the hole-trapping sites. The subsequent studies showed that the terminal Br moieties in these polymers acted as hole-trapping sites.⁵⁷

Table 1. Results of polycondensation reactions and device performances.

Subsequently, the synthesis of high-performance polymers was conducted. In OPV applications, the donor–acceptor (D–A) structure is a rational design for high-performances devices.¹ For the synthesis of D–A polymers, two strategies can be adopted: direct C-H arylation of a donor monomer or that of an acceptor monomer (Scheme 8). As described in Section 3.4, the DMAc-based conditions are suitable for the direct arylation of a donor monomer; in the other case, toluene-based conditions are required. From these choices, an appropriate monomer combination and reaction conditions should be selected to obtain a high-molecular-weight and non-defective polymer. Based on these requirements, we found that the toluene-based reaction was preferable to the DMAc-based reaction; that is, the reaction in toluene afforded polymers with fewer structural defects, such as branched and homo-coupled structures.⁵⁸ In addition, the toluene-based conditions are superior to the DMAc-based conditions in terms of the molecular weight of the D–A polymers because the D–A polymers have low solubility in highly polar DMAc and precipitate out of the reaction media during polymerization.^36,55 Scheme 9 shows the optimized conditions for synthesis of the D–A polymer. The reaction produced the corresponding D–A polymer with higher molecular weight than previously reported values for the Migita–Kosugi–Stille coupling polycondensation. In this reaction, a Pd(0) catalyst (Pd(PCy₃)₂) was used instead of the Pd(II) precatalyst in the original toluene-based conditions because the Pd(0) catalyst was found to be suitable by mechanistic studies of direct arylation.^36,59 Because these conditions can avoid the unwanted side reactions such as the homo-coupling reaction, structural defects were not identified by NMR and MALDI-TOF-MS analysis. Moreover, the elemental analysis demonstrated the high purity of the polymer and no Br terminus, which might be due to a minor debromination reaction in the late stages of the polycondensation reaction. A bulk heterojunction (BHJ) solar cell with this D–A polymer and PC₇₀BM was found to have a PCE of 5.1%, which is comparable to that of the polymer obtained from the Migita–Kosugi–Stille coupling polycondensation. After optimization of the device structure, the maximum PCE reached 6.8%.⁵⁸

Scheme 8. Two strategies for synthesis of D–A polymers via direct arylation.

Scheme 9. Synthesis of the donor–acceptor polymer under the modified toluene-based conditions.

Regarding transition-metal-catalyzed cross-coupling polycondensation, the highly catalytic active species are often sensitive to the presence of oxygen and moisture. To avoid catalytic deactivation, the polycondensation must be carried out in an oxygen and moisture-free environment, and thus solvent distillation and storage in an inert atmosphere are required. Alternatively, direct arylation polycondensation using biphasic water/toluene conditions has been recently reported,⁶⁰ and some reports concerning the molecular design and development of robust Pd catalysts for direct arylation in air and water have been published.^61,62 These observations prompted our interest in the exploration of the tolerance of Pd-catalyzed direct arylation polycondensation to aerobic conditions. The polycondensation of TPD monomer with 2,7-dibromo-9,9-dioctylfluorene was attempted repeatedly, and we found that refluxing the solvents provided a tolerant polymerization protocol in air without the need for a special catalyst or additional catalyst loading (Scheme 10).⁶³ Although the oxygen solubility increases with increasing temperature because the solubilization of oxygen in toluene is an endothermic process,⁶⁴ refluxing can efficiently degas the dissolved oxygen from the reaction mixture.⁶⁵ Furthermore, the high vapor density of toluene (3.2, relative to air) could also help prevent the re-solubilization of oxygen. This simple modification successfully produced the corresponding π-conjugated polymers with molecular weights and yields comparable to those obtained using conventional anhydrous and oxygen-free conditions. The protocol also allows the use of commercially available reagent-grade solvents without the need for prior purification or storage under an inert atmosphere. The facile reaction protocol makes it possible to synthesize OLED and OPV materials under aerobic conditions.

Scheme 10. Direct arylation polycondensation under aerobic conditions.

Although direct arylation polycondensation has been recognized as a practical tool for the construction of π-conjugated polymers, the reaction still uses brominated aromatic monomers as coupling partners and thus, requires extra steps for preparation and purification. The consecutive one-pot synthetic strategy is a useful solution because it can reduce solvent use, time, and the number of workup steps compared to individual multi-step syntheses.^10,11 Therefore, we have attempted to develop a facile synthetic method for π-conjugated polymers via sequential bromination/direct arylation polycondensation (Scheme 11).^66,67 The protocol could provide a step-economical route to π-conjugated polymers in a one-pot manner starting from unfunctionalized aromatic compounds. Alternatively, to achieve the sequential protocol, the bromination step should yield the corresponding dibrominated compounds regioselectively and quantitatively, and the bromination residues and byproducts should have a negligible impact on the subsequent polymerization step. Benzyltrimethylammonium tribromide is preferable for this protocol, and the bromination of electron-sufficient heteroaromatic compounds such as fluorene, phenothiazine, and bithiophene proceeded successfully under mild conditions. The control experiments showed that benzyltrimethylammonium bromide, a byproduct of the bromination, did not inhibit the polycondensation step. The obtained dibrominated aromatic monomers were used for the subsequent direct arylation polycondensation without isolation and purification. The polycondensation reactions yielded the corresponding D–A type π-conjugated polymers in moderate to good yields (Scheme 11a). In addition, the sequential protocol could be extended to the polycondensation of unsymmetrical AB-type monomer, and one-pot access to P3HT from 3-hexylthiophene was achieved (Scheme 11b).⁶⁷ In this case, the use of tetra-n-butylammonium tribromide served as an efficient brominating agent.

Scheme 11. Sequential bromination and direct arylation polycondensation.

A polycondensation reaction using an aerobic oxidative C-H/C-H coupling reaction is a low-cost and environmentally friendly process because of the naturally abundant feedstock of dioxygen and harmless byproducts such as water. To achieve this, thiazole-based compounds were selected as a target monomer because the C-H bond at the 2-position in thiazole is highly acidic, and thus highly reactive for Cu-catalyzed aerobic oxidative homo-coupling reactions.⁷⁰ Alternatively, the C-H bond at the 5-position is highly reactive for Pd-catalyzed direct arylation. By taking advantage of the unique reactivity of thiazole, the overall number of synthetic steps can be reduced. Thiazole-based π-conjugated polymers were prepared through direct arylation in monomer synthesis and Cu-catalyzed aerobic oxidative homo-coupling polycondensation (Scheme 12).⁷¹ Monomer synthesis via Pd-catalyzed direct arylation afforded the pure monomer in moderate yield. The aerobic oxidative polycondensation reaction with 10 mol % of bench-stable Cu(OAc)₂ afforded the corresponding polymer using air as the oxidant under neutral conditions. The polycondensation reaction afforded the homo-polymer with a molecular weight of 19,800 in quantitative yield. Some synthesized polymers were used as the semiconducting materials in OFETs and OLEDs.

Scheme 12. Aerobic oxidative coupling polycondensation of a thiazole-based monomer.

In our ongoing study of direct sp² C-H functionalization strategies for the synthesis of π-conjugated polymers, the development of Pd-catalyzed cross-dehydrogenative-coupling polycondensation of 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl with 3,3′-dihexyl-2,2′-bithiophene has been demonstrated (Scheme 13).⁷² The acidic nature of the C-H bond owing to the electron-withdrawing fluorine substituents enables the preferential C-H metalation of the polyfluoroarene unit over the thiophene unit.^73,74 The presence of the electron-accepting tetrafluorophenyl unit also suppresses the homo-coupling reaction of polyfluoroarene units derived from a reductive elimination of the bis(polyfluoroarenyl)-Pd(II) complex,⁷⁵ resulting in the formation of a cross-coupling product. In addition, the addition of K₂CO₃ promoted the cross-coupling reaction and suppressed the undesired homo-coupling reaction, producing the corresponding D–A type π-conjugated polymer efficiently (Scheme 13a). ¹H, ¹⁹F, and ¹³C{¹H} NMR spectra of the polymer indicate a highly alternating structure with minor homo-coupling defects (about 4%). The reaction also proceeded in air, and the use of O₂ as a terminal oxidant resulted in lower Ag oxidant loading (Scheme 13b). The NMR spectra of the polymer synthesized under aerobic conditions indicated only 2% homo-coupling defects. Recently, there have also been some reports of the related direct sp² C-H coupling polycondensation of thiophene derivatives.^76,77 The introduction of efficient directing groups such as sulfonyl and ester groups to the 3-position of thiophene monomers yields regioregular polythiophene derivatives. Thus, this is another promising approach for cross-dehydrogenative-coupling polycondensation.

Scheme 13. Cross-dehydrogenative-coupling polycondensation of polyfluoroarene with bithiophene derivatives.

Although the synthetic examples given are very specific, the present methodology is based on the direct C-H/C-H cross-coupling reaction between two kinds of sp² C-H bonds, thereby allowing straightforward access to alternating or regioregular π-conjugated polymers without the need for the prior introduction of reactive functional groups into the starting monomers. Because of the absence of organometallic or dibrominated monomers, this protocol also enables the elimination of the end-capping treatment of terminal reactive functional groups of the polymers, which is known to reduce the semiconducting performance.^78–80 Therefore, this synthetic methodology is a promising tool for the synthesis of π-conjugated polymers with respect to atom economy and the number of synthetic steps.

As described above, direct arylation polycondensation has become a promising sustainable alternative to the conventional Suzuki–Miyaura and Migita–Kosugi–Stille cross-coupling polycondensation reactions. Moreover, the various strategies for direct sp² C-H functionalization for the synthesis of π-conjugated polymers have been extended to the synthesis of π-conjugated poly(arylenevinylene)s via the dehydrogenative alkenylation of arenes, so-called direct alkenylation (i.e., the oxidative Mizoroki–Heck reaction or Fujiwara–Moritani reaction) (Scheme 14).^81,82 This protocol is a promising alternative to the Mizoroki–Heck type polycondensation^83,84 because the direct oxidative C-H/C-H cross-coupling reaction of arenes with alkenes allows straightforward access to poly(arylenevinylene)s without bromo groups in the starting substrates. The molecular design of a pyrrole monomer bearing a directing group⁴² was applied as the target aromatic monomer for direct alkenylation polycondensation.⁸⁵ The presence of the directing group induced ortho-metalation at the α-position of the pyrrole moiety and accelerated the direct alkenylation reaction smoothly. The bulky directing group also prevented the homo-coupling of the pyrrole moiety. The Rh-catalyzed direct alkenylation polycondensation of 1-(2-pyrimidinyl)pyrrole with 2,7-diethenyl-9,9-bis(2-ethylhexyl)fluorene afforded the corresponding poly(arylenevinylene) in good yield (Scheme 14a).⁸⁶ The NMR and MALDI-TOS-MS spectroscopic data show that the polymer has an alternating structure with high regioselectivity and a trans configuration. The same reaction protocol allowed the polycondensation of other target aromatic monomers bearing directing groups and diethenyl monomers. The obtained polymers were used as p-type semiconducting materials in OFET devices. On the other hand, because of the electron-withdrawing fluorine substituents, the acidic C-H bond of polyfluoroarenes enables effective C-H functionalization, leading to the efficient direct alkenylation;^87,88 this reaction strategy could circumvent the need to introduce a directing group in the target aromatic monomers. The Pd-catalyzed direct alkenylation polycondensation of 2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl with 2,7-diethenyl-9,9-bis(2-ethylhexyl)fluorene gave the corresponding poly(arylenevinylene) with a well-defined trans configuration (Scheme 14b),⁸⁹ and the polymer contained less than 3% homo-coupling defects. The obtained polymer was used as an emitting material in an OLED device.

Scheme 14. Direct alkenylation polycondensation.