Recent Advances in Conjugated Furans

DOI: 10.1002/chem.201703355

Concept

& Conjugated Materials

Recent Advances in Conjugated Furans Hongda Cao and Paul A. Rupar*[a]

Chem. Eur. J. 2017, 23, 14670 – 14675

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T 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Concept Abstract: Thiophene is one of the most ubiquitous moieties in organic conjugated materials; however, furan, its oxygen congener, and furan derivatives have received comparatively less attention. This is primarily due to the intrinsic instability of furan and its tendency to decompose in the presence of oxygen and light. Incorporating furan into conjugated systems can confer many benefits, including increases in conjugation, improved solubility, and better transport properties. In this Concept Article, advances in furan-containing conjugated materials are presented. The impact of furan on the properties of conjugated materials is discussed, recent advances in synthetic methods are overviewed, and strategies for improving the stability of conjugated furans are detailed.

Introduction Organic electronics have developed rapidly over the last couple of decades.[1] This has been driven, in part, by advances in synthetic chemistry which have allowed for the creation of novel conjugated materials with tailored properties. Uses of conjugated materials cover a wide range of applications, including as sensors,[2] dyes, organic photovoltaics (OPVs),[3] organic field effect transistors (OFETs),[4] and organic light emitting diodes (OLEDs).[5] Although the number of novel conjugated systems continues to grow exponentially, the structure of conjugated materials, especially in application driven research, is dominated by a small number of structural motifs, with thiophene being among the most common.[6] The popularity of thiophene is due to several factors, chief among these are its well-developed synthetic chemistry, reasonable stability, and good electronic properties.[6] Surprisingly, furan, the oxygen congener of thiophene, is relatively underrepresented in the literature, even though the properties of furan make it desirable for use in conjugated systems.[7] Furancontaining materials have been suggested to have improved conjugation, and improved solubility. Perhaps unexpectedly, furan has reduced aromaticity compared to thiophene.[8] This impacts how furan moieties behave within larger conjugated systems. For example, the reduced aromaticity of furan leads to a larger contribution from the quinoidal resonance structure in polyfurans (Figure 1).[7] This makes polyfurans, Figure 1. The aromatic and and other furan-containing conjuquinoidal resonance structures of polyfuran. gated materials, much more rigid, likely to adopt a planar conforma[a] H. Cao, Prof. P. A. Rupar Department of Chemistry, The University of Alabama Tuscaloosa, AL 35487-0336 (USA) E-mail: [email protected] Homepage: http://www.as.ua.edu/rupargroup/ The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/chem.201703355. Chem. Eur. J. 2017, 23, 14670 – 14675

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tion, and to have improved conjugation compared to the related thiophene congeners. Unfortunately, the reduced aromaticity of the furan moiety also makes it much more susceptible to undesired reactions. For instance, furan reacts rapidly with the singlet oxygen generated upon exposure to oxygen and light.[9] As such, many furan-containing molecules can only be manipulated in the dark or under an inert atmosphere. It is this susceptibility to decomposition that is responsible for the scarcity of furan-containing conjugated materials. The objective of this Concept Paper is to highlight key papers concerning conjugated furans with a focus on recent progress in this area. In the first section, we discuss the impact of the incorporation of furan into conjugated systems, looking at both potential benefits and drawbacks. Next, we review strategies for synthesizing furan-containing conjugated materials. Finally, we highlight approaches being taken to make furan-containing materials more robust towards decomposition. We hope that this Concept Paper is helpful to those interested in furan-containing conjugated materials and encourages future research in this exciting area. The interested reader is also encouraged to consult prior reviews on oligofurans, furan conjugated polymers, and fused furan ring systems.[7, 10]

The Impact of Furan on Conjugated Materials Observations of the impact of furans on conjugated systems are best illustrated by the works of Bendikov and co-workers. In a series of papers,[11] Bendikov detailed the syntheses and properties of a-oligofurans and found several important differences compared to a-oligothiophenes. The solubility of a-oligofurans is significantly better than aoligothiophenes. For example, sexifuran (Figure 2) was reported to have a solubility of 0.7 mg mL@1 in chloroform, whereas

Figure 2. 2,2’:5’,2’’:5’’,2’’’:5’’’,2’’’’:5’’’’,2’’’’’-Sexifuran (Sexifuran).[11d]

the solubility of sexithiophene was reported to be < 0.05 mg mL@1.[11d] There are two proposed hypotheses to explain the improved solubility of furans relative to thiophenes. First, the increased dipole moment of the furans caused by the larger electronegativity of oxygen is thought to improve solubility in polar solvents.[7b] Second, solid-state interactions between furan moieties are thought to be weaker due to the less diffused oxygen atom, which makes furans easier to solubilize.[11e] Detailed single-crystal X-ray diffraction studies by Bendikov highlighted important structural features of the a-oligofurans. The inter-ring C@C bond lengths in a-oligofurans are significantly shorter compared to a-oligothiophenes due to an increased contribution from the quinoidal resonance structure and the smaller atomic size of the furan oxygen atom.[11d] The contribution of the quinoidal resonance structure also results

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Concept enable better film processability and less aggregates in the film compared to the thiophene-rich systems.[13c] Researchers have taken advantage of the smaller size of furan to improve planarity and to decrease p–p stacking distances in furan-containing conjugated materials. The Sun group designed a novel benzodifuran (BDF) containing copolymer, PBDF-T1, which has furan moieties in place of the thiophene moieties of the well-studied benzodithiophene (BDT) group (Figure 5, left).[14] DFT calculations showed that the BDF

Figure 3. Twisting between head-to-head defects in oligofurans and oligothiophenes.[11a]

in the a-oligofurans being exceptionally rigid. a-Oligofurans with engineered 3-hexylfuran head-to-head defects remained essentially planar (Figure 3).[11a] For comparison, in related thiophene systems, head-to-head sites have dihedral angles of around 268.[12] Another important consequence resulting from the increased rigidity of furans, along with the smaller size of the oxygen atom, is that the spacing between furan oligomers in the solid state is smaller than in thiophene oligomers.[11d] Optically, the onset of absorption for oligofurans is blueshifted compared to thiophenes.[11d] In general, oligofurans have more intense fluorescence and smaller Stokes shifts compared to oligothiophenes. The more intense fluorescence of the oligofurans is thought to be due to a decrease in intersystem crossing due to the lack of a heavier atom (e.g., sulfur).[11a] The smaller Stokes shift is again a reflection of the increased rigidity of the oligofuran backbone. Finally, the last major difference between a-oligofurans and a-oligothiophenes is their frontier molecular orbitals. a-Oligofurans are more electron rich, with higher lying HOMO and LUMO compared to oligothiophenes. Furthermore, the HOMO– LUMO gaps found in oligofurans are slightly wider (ca. 0.3– 0.4 eV greater) compared to oligiothiophenes.[11b] Many of the favorable properties of oligofurans, such as improved solubility, can be transferred to other conjugated systems through the inclusion of furan moieties.[13] Fr8chet found that only linear alkyl side chains are needed to solubilize furancontaining 2,5-dihydropyrrolo[3,4,c]pyrrole-1,4-dione (DPP) copolymers, whereas branched chains were needed for the allthiophene versions (Figure 4, left).[13a] Recently, Yang reported a

Figure 4. Furan–DPP copolymers that show improved solubility compared to thiophene analogues.[13a,c]

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copolymer had a more planar structure compared to the thiophene analogue. Moreover, organic solar cells devices utilizing PBDF-T1 in the active layer exhibited a higher power conversion efficiency (9.4 %) compared to the thiophene version (8.1 %).[14] Li and co-workers. reported a furan–thiophene-based small molecule TFT-CN (Figure 5, right), which showed good planarity and closer p–p stacking distance in the solid state compared to the all thiophene version (TTT-CN).[15] OFET devices fabricated with TFT-CN had an excellent electron mobility of 7.7 cm2 V@1 s@1, which is two orders of magnitude higher than TTT-CN. The increased rigidity found in oligofurans is also found in other furan-containing systems. Tang recently designed and synthesized a furan-cored aggregation-induced emission luminogen (AIEgen).[16] They investigated the solid-state fluorescence properties relative to its corresponding thiophene counterpart and TPE-F had a high quantum yield of 50 %, while TPE-T was 18 % (Figure 6). OLED devices of TPE-F also exhibited higher luminescence compared to TPE-T.

Figure 6. Structure of furan and thiophene containing AIEgens.[16]

furan-containing DPP polymer that can be processed by environmentally benign halogen-free solvents.[13b] The Tovar group developed several methanoannulene-based DPP copolymers with thiophene and/or furan as p-bridge moieties (Figure 4, right). The improved solubility of furan-based polymers can

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Figure 5. Furan-containing conjugated materials that show improved planarity and reduced p–p stacking.[14, 15]

Synthetic Approaches to Furan-Containing Conjugated Materials There are parallels between the chemistry of furans and thiophenes. In many cases, thiophene-based chemistry can often be adopted for use with furans with only slight modifications.

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Concept However, from a practical standpoint, exposure of unstabilized furans to light in the presence of oxygen must be minimized, as this results in reactions with generated singlet oxygen. Furthermore, furans should not be exposed to acidic conditions, since this rapidly causes degradation to occur. Tactics to improve the stability of furans are discussed in the last section of this Concept article. As early as 2001, Curtis reported the synthesis of both regioirregular and regioregular poly(3-octylfuran) (P3OF) using strategies adapted from thiophene polymerizations.[17] First, 2,5-bromofuran was activated with Rieke Zinc to form 2bromo-5-bromozincio-3-octylfuran. As with the analogous thiophene systems, subsequent addition of Pd catalysts produced regio-irregular polymers, whereas the use of a Ni catalyst produced the regioregular form. In 2004, the catalyst-transfer polycondensation (CTP) polymerization of poly(3-alkylthiophenes) (P3AT) was reported and found to be a chain-growth mechanism.[18] This has had a profound impact on the development of polythiophenes, and conjugated polymers in general, as CTP allows control over polymer molecular weight, molecular weight distribution, and polymer chain ends. However, for furans, CTP has only recently been demonstrated. In 2016, Noonan and co-workers reported the CTP of 3-alkylfurans with the synthesis of regioregular poly(3-hexylfuran) (rrP3HF) (Scheme 1).[19] Although the report-

Scheme 2. Synthesis of a furan-containing copolymer by means of Stille coupling.[21]

Scheme 3. Synthetic method for the formation of 3-bromofuran through Diels–Alder and retro-Diels–Alder reactions.[17]

Scheme 1. Synthesis of regioregular poly(3-hexylfuran) by CTP.[19]

ed molecular weights of the rrP3HF were modest, believed to be a consequence of aggregation during the polymerization, the dispersities of the rrP3HF were low and consistent with CTP polymerization. Noonan has also expanded this methodology to comonomers containing mixtures of furans, thiophenes, and selenophenes.[20] The syntheses of many furan-containing copolymers mimic those of related thiophene containing systems.[13] The Li group prepared a bi(alkylthio-thienyl)benzodithiophene copolymer with bridging furans by menas of a Stille coupling (Scheme 2); this procedure closely follows that of the analogous thiophene-bridged systems.[21] Despite the similarities between the synthetic chemistry of furans and thiophenes, there are also important differences. For instance, 3-bromothiophene, which is a key precursor for 3-substituted alkyl thiophenes, can be synthesized by the tribromination of thiophene to form 2,3,5-tribromothiophene, followed by removal of the bromine atoms at the 2- and 5-positions. In the case of furan, this approach is not effective as the resulting tribromofuran is reportedly unstable.[17] Instead, the most effective route to 3-bromofuran involves brominating the Diels–Alder adduct of maleic anhydride and furan, followed by a dehydrobromination/retro-Diels–Alder reaction in the Chem. Eur. J. 2017, 23, 14670 – 14675

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presence of quinoline (Scheme 3). A route to 3-bromofuran starting from 2,3-dibromo-2-buten-1,4-diol has also been reported.[22] Furan can also be synthesized via synthetic strategies that are not available for thiophenes. For example, the Yoshikai group used palladium catalyzed three-component condensation of alkynylbenziodoxoles, carboxylic acids, and enolizable ketimines to form furans (Scheme 4).[23] Through changing vari-

Scheme 4. Synthesis of furan from imines and alkynyliodine(III) by use of a PdII catalyst (PMP = p-MeOC6H4).[23]

ous substituents, a series of furan derivatives can be prepared in high yield and mild conditions. Furthermore, Wei et al. reported copper-catalyzed cyclization reactions of silyl enol ethers to produce furans,[24] and the Wu group developed a one-pot synthesis of polyfurans via a radical addition method.[25] Unlike polythiophenes, polyfurans cannot be electropolymerized from furan as the required high oxidation potential

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Concept results in irreversible oxidation of the forming polymer.[26] On the other hand, the oxidative potential for the electropolymerization of oligifurans is lower than that of furan, and polyfuran can be formed from the electropolymerization of various oligomers, such as terfuran, tetrafuran, and pentafuran.[26, 27] The electropolymerization of 2-(thiophen-2-yl)furan to form a thiophene–furan copolymer has also been reported recently.[28] Finally, it is important to point out that furans are among the few conjugated moieties that are easily derived from biorenewable resources.[11c] Specifically, furan can be synthesized by the decarbonylation of furfural, which in turn is derived from a variety of agricultural byproducts.[29]

Strategies to Stabilize Furan-Containing Materials The primary reason as to why furan-containing materials have received little attention in the literature, is due to their tendency to decompose in the presence of light and oxygen.[7c, 9, 11d] Continuing efforts to increase the stability of furan-containing conjugated molecules have achieved significant progress in the past decade and two general approaches have been developed: appending electron-withdrawing groups to furans and annulation of furan with other p-conjugated systems (Figure 7).

Figure 9. Furan-based polymers stabilized by p-block elements.[30, 35]

showed excellent stability (Figure 9, left).[35] In this case, the stability of the furan is induced by the electron-deficient, three-coordinate boron atom. We have shown that bridging phosphorus, germanium, and silicon atoms can help stabilize bifuran by lowering the frontier molecular orbitals of the conjugated system through s*–p* hyperconjugation.[30] In the case of phosphorus (as a phosphine oxide), the bridged difuran was incorporated into a conjugated copolymer and found to be stable under light and air (Figure 9, right). A further strategy to prepare stable furan-based materials is to fuse furan with other aromatic rings, such as benzene, naphthalene, thiophene, pyrrole, and so forth.[14, 36] The stability of benzofuran is largely improved by the presence of the fused benzene rings, and its germanium and silicon-bridged derivatives showed excellent stabilities (Figure 10, left).[36a] Following

Figure 10. Furans stabilized through annulation.[36a–c]

Figure 7. Illustration of two strategies to stabilize furans.

Several stable, conjugated furan-containing small molecules have been developed using electron-withdrawing functional groups, such as cyano and amide groups.[15, 30] In addition, furans incorporated into conjugated polymers containing strong acceptor units generally show good stability. Examples of strong acceptor moieties copolymerized with furan include thieno[3,4-c]pyrrole-4,6-dione (TPD),[31] naphthalene diimide (NDI) (Figure 8),[32] 2,5-dihydropyrrolo[3,4,c]pyrrole-1,4-dione (DPP),[13a, 33] and benzo-[c][1,2,5]thiadiazole (BT).[21, 34] In addition to stabilizing furans with traditional electronwithdrawing groups, examples of furans stabilized by adjacent p-block inorganic elements have recently been reported. Helten recently published a furan–borane copolymer, which

Figure 8. A furan copolymer containing electron-withdrawing naphthalene diimide and tetrafluorophenylene moieties.[32] Chem. Eur. J. 2017, 23, 14670 – 14675

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this strategy, the Nakamura group, who have extensively studied fused furans,[10] reported a naphthalene-fused furan small molecule, which can be used as p-type organic transistors under an ambient atmospheric environment (Figure 10, middle).[36b] Moreover, thienofuran was synthesized by the Matzger group. Further functionalization of thienofuran was also performed successfully, including selective mono-bromination and metal coupling reactions (Figure 10, right).[36c]

Conclusion and Outlook Furan and its derivatives have received little attention relative to thiophenes because of poor stabilities. However, furan’s intrinsically chemical properties (reduced aromaticity, smaller size, increased electron richness) have motivated researchers to begin exploring the use of furan in conjugated materials. Synthetically, it is often possible to adapt a direct thiopheneto-furan substitution within an existing conjugated material. Reports have shown this to be a powerful technique to improve material properties, such as increased planarity and p–p stacking, improved solubility and processability, enhanced charge transfer, and better power conversion efficiencies. This, combined with recognition that furans can be made stable, has created a tremendous opportunity, as many previously well-known thiophene-containing conjugated systems can now be revisited from a furan perspective. As discussed in this

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Concept Concept Paper, some exciting results have been reported recently with the replacement of thiophene by furan. We believe that the tendency for furans to show improved solubility has the potential to have great impact. As has been shown, the presence of furan can reduce the size of the alkyl side chains necessary for solubilizing conjugated materials. This, in turn, can provide researchers with increased flexibility in designing conjugated materials by allowing reductions in both the size and branching of solubilizing alkyl chains. The improved solubility of furans may also assist in the incorporation of poorly soluble conjugated moieties into extended conjugated systems. The use of furan-based materials in organic electronic applications is very promising and more exciting research results will occur in the future.

Acknowledgements We are grateful for support from the National Science Foundation (Grant CHE1507566) and the University of Alabama.

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