Abstract
Organic molecules and polymers with extended π-conjugation are appealing as advanced electronic materials, and have already found practical applications in thin-film transistors, light emitting diodes, and chemical sensors. Transition metal (TM)-catalyzed cross-coupling methodologies have evolved over the past four decades into one of the most powerful and versatile methods for C–C bond formation, enabling the construction of a diverse and sophisticated range of π-conjugated oligomers and polymers. In this review, we focus our discussion on recent synthetic developments of several important classes of π-conjugated systems using TM-catalyzed cross-coupling reactions, with a perspective on their utility for organic electronic materials.
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1. Introduction
Advances in conducting and semiconducting organic materials have produced game-changing technologies for electronic devices and optoelectronic displays. The scope of organic electronic materials is extensive and cover multiple areas of technological importance: thin-film transistors (TFTs), radiofrequency identification tags, organic solar cells and light-harvesting devices, organic semiconductor lasers, and chemical sensing applications based on optoelectronic modulation [1–10]. The potential benefits of organic electronics over conventional Si-based electronics have also been stressed, such as its processability in solution, low manufacturing cost, low weight with favorable mechanical properties (e.g. flexibility and stretchability), and promise for reduced environmental impact [11, 12].
Nearly all organic electronic materials consist of π-conjugated backbones. Most of these are comprised of polyaromatic or heteroaromatic units; common building blocks include thiophenes, oligoacenes, fluorenes, and fused heterocyclic rings. A key contributing factor toward the tremendous advances in organic electronics is the availability of efficient synthetic methods for coupling aromatic monomers into extended π-conjugated systems. In particular, transition metal (TM)-catalyzed cross-coupling reactions have yielded a diverse and sophisticated class of π-conjugated oligomers and polymers used in organic electronics research, and TM-based catalysts with high substrate generality and turnover number continue to be developed. In this review, we focus much of our discussion on recent synthetic developments in TM-catalyzed cross-coupling reactions, with a perspective on their utility for organic electronic materials. We pay particular attention to reactions that have had a strong impact on several important classes of π-conjugated systems, namely oligomers and polymers of thiophenes, fluorenes, arenes and heteroarenes, arylethynylenes, and arylvinylenes.
2. Development of synthetic methodologies
Prior to the advent of Pd and other TM catalysts, cross-coupling reactions were limited to a handful of examples, mostly involving Grignard reagents and organoalkali species (M = Li, Na, K). Such strong nucleophiles could react with unhindered alkyl (sp3) electrophiles in a general fashion, but their use in cross-coupling reactions between unsaturated carbon atoms (sp2−sp2 or sp2−sp bonds) was severely limited. This problem was solved in the 1970s by the introduction of TM-catalyzed cross-coupling, which has been evolving at a rapid pace ever since. Cross-coupling methodologies were initially centered on Pd-catalyzed reactions between aryl or alkenyl halides and organometals containing Al, Zn, Zr (Negishi) [13–15], B (Suzuki) [16, 17], Sn (Stille) [18, 19], Si (Hiyama) [20, 21], and terminal alkenes (Heck) [22, 23] or alkynes (Sonogashira) [24]. Ni-catalyzed Grignard cross-couplings (Tamao−Kumada) [25, 26] (Corriu) [27] also proved to be valuable for synthesis; while more limited in scope, their efficiency could be comparable or even superior to some of the Pd-catalyzed variants mentioned above.
The widely accepted mechanism for Pd-catalyzed cross-coupling reactions of organometals R1M with electrophiles R2X is depicted in scheme
The catalytic cycle in scheme
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Standard image High-resolution imageIn addition to these well-established classes of TM-catalyzed cross-coupling, arylation by direct C–H activation has recently emerged as a new type of cross-coupling reaction, and is being increasingly applied toward the synthesis of small molecules [35–37]. In principle, TM-catalyzed C–H activation does not require organic halides or the generation of organometallic intermediates in contrast to classical cross-coupling reactions (scheme
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Standard image High-resolution image3. Synthesis of organic electronic materials
3.1. Polythiophenes and related structures
Thiophene-based polymers and oligomers represent one of the largest classes of π-conjugated organic materials. Polythiophenes have low band gaps and are valued for their high charge carrier mobility at low voltage bias, and also for their excellent thermal and chemical stability [6]. Unsubstituted 2,5-polythiophenes are poorly soluble and difficult to work with, but their processability is greatly improved by introducing pendant groups at the C3 position. The intrinsic asymmetry of 3-substituted thiophenes gives rise to three types of coupling products: 2 → 5' or head–tail (HT), 2 → 2' or head–head (HH), and 5 → 5' or tail–tail (TT) (figure 2). The steric interactions between C3 groups in HH-coupled units promote twisted, non-planar backbone conformations, leading to loss of π-conjugation and carrier mobility. For this reason, synthetic methods have been largely focused on the preparation of regioregular poly(3-alkylthiophene)s (P3ATs) with continuous HT couplings.
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Standard image High-resolution imageRegioregular P3ATs were first synthesized over 30 years ago by McCullough and co-workers [38], and further improved by the use highly reactive (Rieke) Zn to prepare regioregular P3ATs (scheme
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Standard image High-resolution imageP3ATs can also be synthesized using a Grignard metathesis (GRIM) protocol and Ni-catalyzed (Kumada-type) coupling [41, 42]. 2,5-Dibromo-3-alkylthiophene reacted with one equivalent of alkyl or vinyl Grignard reagent to form a mixture of intermediates by metal−halogen exchange, referred to as Grignard metathesis (scheme
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Standard image High-resolution imageIn addition to the Negishi- and Kumada-type reactions, regioregular P3ATs have also been synthesized by other Pd-catalyzed cross-coupling reactions using organotins (Stille coupling) [43] and organoborons (Suzuki coupling) [44]. The ease of isolating and purifying these precursors has enabled numerous thiophene-based compounds to be synthesized from building blocks with widely different characteristics. However, application of Stille or Suzuki coupling toward polythiophene synthesis tends to produce P3ATs of lower molecular weight and/or regioregularity compared with Negishi- and Kumada-type conditions. This is gradually being improved by the development of new coordinative ligands; the use of sterically demanding phosphines [45] or N-heterocyclic carbenes [46, 47] has increased the scope of these couplings to include less activated substrates (e.g., aryl chlorides) in Pd-catalyzed cross-coupling reactions, while maintaining mild reaction conditions. For example, 2-chlorotetracene was efficiently coupled with a 5'-stannylbithiophene via a modified Stille coupling by using the electron-rich and sterically demanding ligand P(tBu)3, to produce 5'-tetracenyl bithiophene for vapor deposition in organic TFT production (scheme
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Standard image High-resolution imageThe synthesis of regioregular P3ATs by direct C–H bond activation was first reported in 1999, using 2-halo-3-alkylthiophenes as monomers and Pd(OAc)2 as the catalyst [52]. Although the molecular weights of these polymers were initially low (Mw ∼ 3 kDa) and with 90% HT regioregularity, the Pd-mediated dehydrohalogenative polymerization of 2-bromo-3-hexylthiophene yielded regioregular P3ATs of higher molecular weight (Mw ∼ 30 kDa) when using the thermally stable bis-palladacycle known as Herrmann's catalyst [53]. The catalytic efficiency could be further improved by adding tris(2-dimethylaminophenyl)phosphine as a stabilizing ligand, enabling the polymerization of 3-hexylthiophene into P3HT in 99% yield with high molecular weight (Mw > 30 kDa) and > 98% HT regioregularity (scheme
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Standard image High-resolution imageDeviations from regioregularity in P3AT synthesis increases the number of head-to-head (HH) isomers, which adopt nonplanar conformations in order to reduce torsional strain. Several strategies have been developed to synthesize P3AT-like polymers with extended planar backbones whose conformations do not depend on regioregularity (figure 3). These include: (i) the incorporation of unsubstituted oligothiophenes as spacers within the P3AT chain, which effectively removes the steric interactions between 3AT units; (ii) locking the polymer backbones into planar conformations by using fused thiophenes, and (iii) introducing electronic stabilizing interactions between the ring sulfur and substituents of neighboring 3ATs to promote planarity between HH subunits [54]. Many of the modified P3ATs in figure 3 were synthesized by Pd-catalyzed cross-coupling using Stille conditions [47, 55, 56].
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Standard image High-resolution imageStille copolymerization has also been used to make P3HT-like polymers containing tail-to-tail bithiophene (TT−BT) units, such as poly(quaterthiophene) (PQT), poly(bithiophene−thieno(3,2-b]thiophene) (PBT−TT), and poly(bithiophene−dithieno(3,2-b:2',3'-d)pyrrole) (PBT−DTP) (scheme
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Standard image High-resolution imageDirect C−H activation can also be used to produce π-conjugated polymers with TT−BT units, as exemplified in the polycondensation of thieno(3,4-c)pyrrole-4,6-dione (TPD) with a 3,3'-dioctyl-BT derivative (scheme
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Standard image High-resolution imageWith regard to oligothiophenes and related molecules, Pd-catalyzed Negishi cross-coupling reactions have been used to prepare a variety of π-conjugated molecules using halogenated heterocycles and zincated oligothiophenes. Novel polyheteroaromatic systems containing two oligothiophene subunits bridged by phenanthroline derivatives have been prepared as metal-chelating units for electronically-based chemical sensors [61], and as intermediates for π-conjugated catenanes and other topologically interesting structures (scheme
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Standard image High-resolution imageA high molecular-weight copolymer of benzothiadiazole (BTZ) and cyclopentadithiophene (CDT) has been prepared by Suzuki polycondensation (Mn = 50 kDa, scheme
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Standard image High-resolution image3.2. Polyfluorenes and related structures
Polyfluorenes (PFs) are conformationally constrained polyarylenes that are comprised of π-conjugated biphenyl units bridged by sp3 carbon atoms (C9 position). PFs are well known for their strong electroluminescence with emissions at blue wavelengths [65]. A variety of aryl and alkyl substituents have been introduced at C9 to solubilize these rigid polymers; these can also modulate the electronic or photophysical properties by applying torsional strain to the π-conjugated backbone. The electrochromic properties of PFs have made them one of most widely used organic electronic materials, with applications in organic light-emitting diods (OLEDs), lasers, TFTs, and sensors [66]. These tunable materials have also given rise to π-conjugated polymers containing spirofluorene units and related polycyclic derivatives, many of which have been described in earlier reviews [67, 68].
Oligo- and polyfluorenes are often synthesized using TM-catalyzed polycondensation, which can be sorted into three categories: (i) homopolymerization of 2,7-dibromofluorenes (AA monomer) via Ni-catalyzed Yamamoto coupling; (ii) copolymerization of AA monomers with their bismetallated congeners (BB monomers) via Pd-catalyzed cross-coupling (Kumada, Negishi, Stille, or Suzuki conditions); and (iii) homopolymerization of monometallated bromofluorenes (AB monomers) via Pd-catalyzed cross-coupling. AB and BB-type monomers are readily prepared from 2,7-dihalofluorenes (AA-type monomers) by metal−halogen exchange using one or two equivalents of n-butyllithium or Grignard reagent, followed by transmetallation with borates, ZnBr2 or Me3SnCl (scheme
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Standard image High-resolution imageYamamoto polymerization of a dihaloaromatic compound (AA-type monomer) gives a polymer. The main advantage of Yamamoto polymerization is that AA-type monomers are straightforward to work with. For example, poly(9,9-bis(2-ethylhexyl)fluorene) was prepared by Yamamoto-type polycondensation of a 2,7-dibromofluorene with bis(COD)nickel and bipyridine [73]. However, this method usually requires stoichiometric amounts of catalyst. Investigations on Ni-catalyzed reductive polymerization of 2,7-dibromofluorenes with excess zinc dust as a reductant led to the synthesis of highly conjugated and processable PFs (scheme
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Standard image High-resolution imagePd-catalyzed heteropolymerization of AA- and BB-type monomers and homopolymerization of AB-type monomers are all used to prepare polyfluorenes. Suzuki conditions have been found to be efficient and have become widely adopted for the synthesis of high molecular weight PFs (50−60 kDa) [75, 76]. Suzuki couplings can be accelerated by over two orders of magnitude by using microwave heating, with increased chain propagation: heteropolymerization of 2,7-dibromo- and 2,7-di(pinacolato)boryl)-9,9-dihexylfluorene under microwave conditions produced PFs in under 15 min at twice the molecular weight (Mw = 40 kDa), compared with that synthesized by conventional heating for 48 h (Mw = 20 kDa) (scheme
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Standard image High-resolution imageChain-growth polycondensations of AB-type monomers are a popular alternative for the preparation of well-defined π-conjugated polymers [78]. Propagation typically involves an intramolecular catalyst-transfer pathway, following formation of a TM-based initiator. Recently there has been much progress in chain-growth polycondensation using TM complexes with bulky and electron-rich ligands. For example, a universal chain-growth polymerization initiated with aryl-Pd(Ruphos)-X species has been developed using a Negishi coupling protocol, which is applicable to the synthesis of PFs, P3HT, and other π-conjugated copolymers [72]. 2-Bromo-7-iodo-9,9-dioctylfluorene (F8) was converted into an AB-type organozinc monomer, then polymerized at room temperature into poly-F8 using a fluorene−Pd complex as an initiator (scheme
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Standard image High-resolution imagePd/PtBu3-catalyzed Negishi chain-growth polycondensation have recently been reported to produce polyfluorenes with molecular weights of up to 120 kDa (scheme
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Standard image High-resolution imageThe Pd-catalyzed oliogomerization of fluorene derivatives can also be controlled to the extent that discrete 'molecular wires' with different end groups can be designed and synthesized. Polymerization could be initiated from a bromoarene (Ar1-Br) by oxidative addition of Pd(PtBu3)2 to generate a Pd(PtBu3)(Ar)Br complex, similar to that featured above, followed by chain growth using Suzuki coupling conditions with AB-type monomer then terminated by addition of a arylborate ester (Ar2-B(OR)2) to produce oligofluorenes (n = 10–23) in high yields with full control over the end groups (scheme
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Standard image High-resolution imageActivated fluorenes can be copolymerized with other π-conjugated systems to create hybrid PFs, allowing further tuning of the electronic band structures and emission wavelengths. This is generally achieved by the Pd-catalyzed cross-coupling of alternating AA- and BB-type monomers, using either component as the bismetallated species. For example, a hybrid copolymer comprised of alternating 4,7-bis-(3-decyloxythiophen-2-yl)benzothiadiazole (DTB) and 9,9-dioctylfluorene (F8) was synthesized by Suzuki polycondensation (scheme
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Standard image High-resolution imageCopolymers of alternating 9,9-dioctylfluorene (F8) and benzothiadiazole (BT) units have been prepared by dispersion polymerization using Suzuki conditions (scheme
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Standard image High-resolution imageTetrathiafulvalene−fluorene (TTFV-F) copolymers with protohelical conformations, which have been prepared by Sonogashira polycondensation, have been shown to undergo large conformational changes upon protonation (scheme
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Standard image High-resolution imageFluorene derivatives have also been copolymerized with isothianaphthene (ITN), a bicyclic compound with attractive electric and optical properties for generating low-bandgap polymers based on its o-quinoidal character [85]. Copolymers with alternating 9,9-dihexylfluorene (F6) and ITN units have been synthesized by Stille polycondensation (scheme
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Standard image High-resolution imageBlock copolymers with extended π-conjugation have also been synthesized, using Negishi-type couplings for the sequential polymerization of AB-type monomers with Pd complexes as polymer initiators [72]. Copolymers comprised of F8 and 3AT segments have been prepared by the stepwise polymerization of the corresponding AB-type monomers, catalyzed either by fluorene−Pd complex (to initiate growth of PF block) or thiophene−Pd complex (to initiate growth of P3AT block) (scheme
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Standard image High-resolution imageTriblock π-conjugated copolymers have also been synthesized by Pd-catalyzed cross-coupling using Suzuki-type protocols, with narrow molecular weight distributions. PF-based copolymers were prepared using AB-type monomers derived from di(6'-bromohexyl)fluorene (F6-Br) and dioctylfluorene (F8) using (t
Bu3P)Pd(Ph)Br as the catalyst [89]. The terminal bromides on F6-Br could be substituted with pyridine (F6-Py+) to introduce polyelectrolyte character at both ends of the triblock copolymer (scheme
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Standard image High-resolution imagePhotovoltaic properties such as charge generation, power conversion efficiency (PCE), and external quantum efficiency (EQE) can be enhanced in π-conjugated polymers by the inclusion of select transition metals. Such an effect has been investigated by the incorporation of Ir into a PF-based backbone, which was prepared by Suzuki condensation of BB monomer (diboryl-F8) with AA-type-monomers generated from F8 and Ir-substituted phenylpyridine (Ir-PPy) (scheme
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Standard image High-resolution image3.3. Polyphenylenes and poly(heteroarylene)s
Poly- and oligoarylenes have been developed as basic model compounds for studying redox properties and understanding of the spectroscopic data [91]. These compounds are frequently synthesized by the Pd-catalyzed polycondensation of AA and BB monomers. The exceptional physical properties of polyphenylenes and related compounds have supported the development of organic electronic materials that are highly processable [80, 92, 93].
Suzuki polycondensation has been applied in the synthesis of poly(3-butoxy-4',5-meta-biphenylene) (PBmP), a novel example of a protohelical π-conjugated polymer (scheme
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Standard image High-resolution imageStille cross-coupling conditions have been used to synthesize poly- and oligoarylenes in the presence of sensitive functional groups, a limitation frequently encountered in the Suzuki reaction. For example, bis(thioacetyl)oligoarenes designed as potential molecular 'alligator clips' were prepared by using triphenylarsine as an ancillary ligand, without adverse effect on the thiol-containing units [96] (scheme
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Standard image High-resolution imageKumada cross-couplings can also be used to make polyphenylenes, especially in the industrial-scale production of organic electronic materials. This reaction has been proposed to proceed through a Ni(0)−arene π-complex after reductive elimination, an intermediate which can participate either in intermolecular chain transfer through a dissociative mechanism, or in intramolecular oxidative addition to support chain-growth polymerization (scheme
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Standard image High-resolution imageDirect C−H bond activation has recently been used to prepare heteroarylene copolymers containing electron-deficient aromatic units. Copolymerization of 1,2,4,5-tetrafluorobenzene (TFB) with brominated dioctylfluorene (F8) or N-alkylcarbazoles has been achieved in the presence of Pd(OAc)2 with PtBu2Me and base (scheme
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Standard image High-resolution imagePd-catalyzed direct arylation has also been used to prepare a variety of diketopyrrolopyrrole (DPP) derivatives [101]. Bisaryl-DPP heterotrimers were synthesized in excellent yields from dithienyl-DPP and bromoarenes, and also from dibromo-DPP and pentafluorobenzene (scheme
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Standard image High-resolution image3.4. Oligo- and poly(arylethynylene)s
Over the last 25 years, poly(arylethynylene)s (PAEs) have received much attention as rigid, π-conjugated interconnects, with applications as polarizing materials in liquid crystal displays [102, 103], as molecular wires [104], and in the chemical detection of explosives and other volatile compounds [105, 106]. A wide variety of PAEs with different functional unit structures have been synthesized and studied [107], some examples of which are shown in figure 4. Despite their structural variety, they all share the same backbone feature, i.e. they are conjugated through ethyne-linked aromatic or heteroaromatic rings.
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Standard image High-resolution imageThe most common approach to preparing PAEs is by Sonogashira coupling, a cross-coupling reaction between aromatic halides and terminal alkynes in the presence of catalytic CuI [24, 108]. This Pd-mediated coupling was in fact first described without Cu(I) by Heck [109] and Cassar [110], but required neat conditions at high temperatures or the use of strong base. In the Sonogashira−Hagihara reaction, the reaction is much milder and thus more compatible with various functional groups, and the addition of CuI as co-catalyst further enables the couplings to proceed at ambient temperatures.
The mechanism of Sonogashira coupling has been a subject of considerable debate, but many agree that the proposed mechanism involves two independent catalytic cycles (scheme
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Standard image High-resolution imageWhile Sonogashira coupling has been successfully used to synthesize a variety of PAEs, there are also several shortcomings of this methodology, including the homocoupling of terminal alkynes, dehalogenation or substitution by phosphine ligands [111], and the relatively low degree of polymerization (DP < 100) which is strongly affected by the electronic structure of the monomers [112]. This has led to substantive efforts to improve Sonogashira coupling conditions to generate PPEs of either low polydispersity or high molecular weight. A catalyst transfer polycondensation (CTP) protocol using PhPd(t-Bu3P)Br has recently been developed as a 'living' chain growth process (as opposed to step growth) to afford poly(p-phenylethynylene)s (PPEs) of well-defined size and polydispersity (scheme
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Standard image High-resolution imagePPEs can also serve as precursors for novel hybrid copolymers, some having highly nonlinear conformations or branched structures [114–116]. For example, PPE-like networks were prepared by polycondensation of 1,3,5-tris(ethynyl)benzene and 2,5-diiodohydroquinone using Sonogashira conditions, then transformed into hyperbranched networks containing benzodifuran units formed by intramolecular cyclization, followed by macromolecular reorganization to produce a microporous organic network (scheme
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Standard image High-resolution imageIn another example featuring multiple Sonogashira couplings, substituted pyridines were coupled to dipropargyl ether then subjected to a Rh-catalyzed [2 + 2 + 2] cycloaddition to produce a 1,4-phenylene-bridged bipyridine (PyPPy, scheme
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Standard image High-resolution imageNegishi and Stille cross couplings have been used to synthesize redox-active, star-shaped oligo(phenylethynylene)s containing metal-coordinated complexes. Multiple Negishi couplings with C6Br6 could be performed in a single pot to produce hexakis(ferrocenylethynyl)benzene and similar π-conjugated molecules, using excess ZnCl2 to prevent formation of homodimers (disubstituted butadiynes) (scheme
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Standard image High-resolution imageOligo(arylethynylenes) are candidate materials for organic TFTs, due to their propensity to adopt planar conformations and form crystalline domains. For example, the molecule 9,10-teranthrylethynylene (D3ANT) was synthesized via Negishi and Sonogashira coupling in good yield (scheme
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Standard image High-resolution image3.5. Oligo- and poly(arylvinylene)s
Polyarylvinylenes (PAVs) represent one of the largest classes of organic semiconductors, and are well known for their high electron affinities and superior electroluminescent properties. The π-conjugated backbones of PAVs can support a variety of functional groups for enhanced processability and adjustable reduction potentials; the parent structure, poly(p-phenylene)vinylene (PPV), has been extensively modified to produce OLEDs with tunable emissions at visible wavelengths [122, 123]. Here we present some recent examples of PAVs prepared by TM-catalyzed cross-coupling reactions.
Many PAVs can be prepared by Stille polycondensation, which can couple vinylstannanes with electron-poor or electron-rich components with similar efficiencies. This method has been used to prepare PPV copolymers containing tetrafluorophenylvinyl and 2,5-dialkoxyphenylvinyl units with unusually high nonlinear optical response (scheme
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Standard image High-resolution imagePd-catalyzed cross-couplings are also compatible with coordinative heteroarenes, and their polymerizations can proceed with similar levels of efficiency as with standard aryl subunits. Both Stille and Heck polycondensations have been used to generate various poly(pyridylvinylene)s (PPyV)s, which can serve as electron-deficient PPV analogs [125]. Stille couplings were used to produce HH bis(pyridyl)ethene, which was further condensed with distannylethylene to produce regioregular HH-PPyVs, whereas the polymerization of 5-bromo-2-vinylpyridine using Heck conditions yielded HT-PPyVs (scheme
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Standard image High-resolution imageStille cross couplings have been used to prepare benzodifurandione-based PPV (BD-PPV) derivatives, electron-deficient analogues designed to address the issue of carrier mobility in solid-state PPVs. Problems associated with low carrier mobility include low crystallinity, cis/trans isomerization of the vinylene subunits in individual chains, and high LUMO levels that discourage efficient electron transport [127]. The BD-PPV backbone is more rigid than typical PPVs and is further stabilized by intramolecular CH···O hydrogen bonds (scheme
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Standard image High-resolution imagePAVs and their precursors have also been synthesized by Pd-catalyzed Suzuki and Hiyama couplings, using arylborinates and vinylsiloxanes, respectively. Carbazolyl−fluorenyl-based PPV copolymers and their polyazomethine analogs were prepared by Suzuki couplings with average molecular weights of 5 kDa (scheme
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Standard image High-resolution imageHiyama coupling conditions are milder than Suzuki or Stille conditions, and the vinyl- or arylsiloxane building blocks are stable and easy to work with. Moreover, Hiyama conditions can circumvent the use of Brönsted bases, relying instead on fluoride ions for desilylative coupling [130]. For example, oligo(4-halostyryl)arenes have been prepared by a one-pot synthesis using Pd2(dba)3 and the corresponding styryldisiloxane, which in turn is readily prepared by Ru-catalyzed olefin cross metathesis (scheme
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Standard image High-resolution imageFinally, olefin metathesis has been used to prepare polymers with alternating cyclohexene−ethylene units, en route to PPVs. Ring-opening metathesis polymerization (ROMP) of a disubstituted dicyclooctadiene was performed using Schrock's catalyst to produce polymers with low polydispersity (PDI = 1.2–1.3, Mn
= 46–63 kDa), which could undergo double elimination to produce PPVs with a mixture of cis and trans alkenes (scheme
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Standard image High-resolution image4. Concluding remarks
Enormous progress has been achieved in the synthesis of organic electronic materials over the past two decades. TM-catalyzed cross-coupling has played a key role in producing numerous types of π-conjugated oligomers and polymers, with many appealing properties for practical applications in organic electronics. The main advantages of the synthetic methodologies discussed herein are their compatibility with many types of functional groups and the ease in preparing various monomers, making it possible to synthesize complex polymers with high molecular weight. Polythiophenes, polyfluorenes, polyarylenes, -arylethynylenes, and -arylvinylenes have been produced using TM-catalyzed cross-couplings, some already used in commercial production.
Despite the many significant discoveries and developments in TM-catalyzed cross-coupling, there remain some unsolved problems. The low reactivity of aryl or alkenyl chlorides represents one limitation. The use of chlorides would present a great advantage, considering these substrates are often less expensive and in greater supply than the corresponding bromides. It is also desirable to develop more powerful catalysts, particularly those featuring sterically demanding phosphine or N-heterocyclic carbene ligands, to improve the cross-coupling efficiency of challenging substrates. High catalyst loadings present another challenge: palladium catalysts are relatively expensive, and the colloidal palladium formed during polymerization could be detrimental to the electronic materials properties of the resulting polymers if not properly removed. Efforts to improve catalyst efficiency and turnover number are now in development, as recently demonstrated by a PdP(tBu)3-catalyzed Negishi polymerization with exceptionally high catalyst turnover numbers (TON > 200 000) [79]. An alternative solution is to recycle and reuse catalysts immobilized on solid support or suspended in separable liquid media (e.g., fluorous or ionic liquid phases).
TM-mediated C–H activation continues to be a promising avenue for polymer synthesis, as it circumvents the use of organic halides and/or the generation of organometallic precursors. This field is still at an early stage of development relative to established TM-catalyzed cross-coupling reactions, but its advancement will likely be influenced by socioeconomical factors and by demonstrations of its practical use on a large scale. In the meantime, it is desirable to identify solutions to issues of chemo- and regioselectivity, which has so far limited the choice of substrates amenable to C–H bond activation. Taking into account the current state of development in TM-catalyzed cross-coupling, we believe that this class of reactions is likely to become increasingly important to the future of organic electronic materials.