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Design and Functions of Semiconducting Fused Polycyclic Furans for Optoelectronic Applications Hayato Tsuji*,†,‡ and Eiichi Nakamura*,† †

Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan

‡

CONSPECTUS: The fused polycyclic furan structure is a ubiquitous motif in naturally occurring organic compounds. However, they had been rarely seen in the literature of organic electronic research until very recently, probably because of the lack of stability of simple furans under conditions that the compounds experience in the active layer of the device. Nonetheless, from the viewpoint of molecular structure, furans look to have potential merits as organic semiconductors such as thiophenes, which are more popular in the organic electronic area. For example, the small atomic radius and large electronegativity of oxygen will increase intermolecular molecular orbital (MO) overlap and hence facilitate charge transporting ability in the solid state. In this Account, we describe the molecular design and optoelectronic applications of fused polycyclic furans, such as benzodifurans (BDFs), naphthodifurans (NDFs), and anthradifurans (ADFs). The molecular design that was exploited in this study crucially depends on the synthetic flexibility of a “modular” synthetic strategy that we purposely developed and reviewed in a separate report. Our synthetic strategy comprises two steps carried out in situ: cyclization of an o-alkynylphenol into a zincio benzofuran and its electrophilic Negishi-type trapping to obtain a range of multisubstituted fused furan compounds. These compounds are found to possess electronic structures resembling those of fused polyaromatic hydrocarbons, such as acenes or phenacenes, rather than oxygen-bridged phenylenevinylene, along with unique characteristics: a wide HOMO−LUMO gap originating from the weak aromaticity of the furan rings, an intense photoluminescent character, and mechanofluorochromism. Semiconducting properties of fused furans are also excellent among organic materials: some BDF derivatives show high hole mobility on the order of 10−3 cm2/(V s) in the amorphous state using time-of-flight (TOF) technique. The p-type BDFs exhibit high performance as hole-transporting material in heterojunction organic light-emitting diodes (OLEDs), while carbazolesubstituted BDFs (CZBDFs) are ambipolar with well-balanced high carrier mobility for both hole and electron and serve as host materials for full-color electroluminescence in both hetero- and homojunction architectures. More π-expanded NDFs showed good crystallinity and are effective active materials for organic field-effect transistors (OFETs) with a high hole mobility of up to 3.6 cm2/(V s) using a solution process. These studies have illustrated the high potential of fused polycyclic furans in organic electronics research, which thus far have attracted much less attention than their thiophene congeners. compounds.12 One of the reasons may be that furan is less aromatic and hence less stable than thiophene, as illustrated by the much smaller resonance energies: 29.1 kcal/mol for thiophene and 16.2 kcal/mol for furan, estimated from the heat of combustion.13 Another reason may be synthetic difficulty in obtaining varieties of furan derivatives at will. For example, it was only in 2010 that selective synthesis of unsubstituted α-oligofurans longer than the trimer was achieved by Bendicov and co-workers.14,15 Despite a much smaller number of the examples of optoelectronically useful furans than thiophenes before 2006 when we started to work in this field, we conjectured that, if made stable by appropriate molecular designs, furan-based materials may show semiconductivity

1. INTRODUCTION Large-scale commercialization of organic electronics is just around the corner for applications in display, lighting, photovoltaics, and sensing, by taking advantage of the low weight, flexibility, and potentially low fabrication cost of organic materials. It has been recognized that the development of this field has depended crucially on the invention of new materials. A thiophene polymer, PEDOT (poly(3,4-ethylenedioxythiophene)), may be a good example: it is a stable surrogate of unstable polyacetylene and shows high conductivity upon positive doping. Some fused thiophenes are state-of-the-art high-performance semiconductors and show carrier mobility of greater than 10 cm2/(V s) (Figure 1a).1−6 Furan, the oxygen analogue of thiophene, however, has drawn much less attention7−11 in this field (Figure 1b), although it is a ubiquitous motif in naturally occurring organic © 2017 American Chemical Society

Received: November 28, 2016 Published: February 6, 2017 396

DOI: 10.1021/acs.accounts.6b00595 Acc. Chem. Res. 2017, 50, 396−406

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Accounts of Chemical Research

recently reviewed.16 Several of the fused furan compounds thus synthesized were found to be electrochemically and thermally stable and useful for organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs), which will be discussed in this Account.

2. MODULAR APPROACH: SYSTEMATIC SYNTHESIS OF FUSED FURANS A feature of our synthesis of fused polycyclic furans is its “modular” or divergent-oriented nature.16 The synthesis comprises two steps (Scheme 1): the first step is the quantitative cyclization of an o-alkynylphenol into a thermally stable β-zinciofuran, which serves as a “synthetic module” for further synthetic elaborations.17−19 The second step includes a Negishi-coupling of the zincio intermediate producing a range of multisubstituted fused polycyclic furan compounds. This protocol has a great advantage in material screening over traditional synthetic approaches for obtaining a library of a wide range of fused furans, including BDFs, such as benzo[1,2-b:4,5b′]difuran (p-BDF),20−24 benzo[1,2-b:5,4-b′]difuran (mBDF),25 and benzo[1,2-b:6,5-b′]difuran (o-BDF),26 as well as NDFs27,28 and ADFs.29 3. STRUCTURES AND PHOTOPHYSICAL PROPERTIES 3.1. Electronic Structures of Fused Furans

Figure 1. State-of-the-art semiconducting thiophene derivatives (a) versus furan derivatives (b).

We here describe some structural features and properties of the fused furan compounds. Photophysical property measurements indicated that p- and m-BDFs are more similar to acene than to oxygen-bridged phenylenevinylene in terms of their electronic structures, despite the low aromaticity of furans (Figure 3a,b).25 Likewise, an o-BDF molecule resembles phenanthrene in its chemical and electronic structures. These characteristics have profound effects in the application to organic electronic devices to be described in section 5. By the same token, angular-shaped NDF1−2 and linear NDF3 are analogous to chrysene and tetracene, respectively, and linear ADF1 and angular-shaped ADF2−3 to pentacene and dibenzoanthracence, respectively.

comparable to that of thiophenes. This is because the smaller atomic radius and larger electronegativity of oxygen than sulfur may increase intermolecular molecular orbital (MO) overlap, which facilitates carrier transport in the solid state. One bonus that we can expect for furan-based semiconductors is their potential for effective light emission, which may open new possibilities for certain kinds of optoelectronic applications. To address the lability issues of furans, we envisioned that fusion of furan ring(s) to an all-carbon aromatic system and introduction of substituents (Figure 2) would be a viable strategy for obtaining stable and functional furan compounds, that is, fused polycyclic furans, such as multisubstituted benzo[b]furan, benzodifurans (BDFs), naphthodifurans (NDFs), and anthradifurans (ADFs), and focused first on developing effective synthetic methods based on zinc-mediated cyclization of o-alkynyl phenols. This synthetic effort has been

3.2. Photoluminescent Properties

The fused furans are photoluminescent with a moderate to high quantum yield (PLQY, Table 1). Among the p-BDFs studied here, 2,6-diphenyl-p-BDF and 2,6-diphenyl-3,7-carbazolylphenyl-p-BDF (p-CZBDF) showed PLQYs as high as 0.98 and 0.97 (entries 1 and 4), respectively, in solution, while 3,7diphenylaminophenyl-p-BDF showed a much lower PLQY of 0.19 (entry 3) because of intramolecular charge transfer involving the amino groups. The m-BDFs also exhibited high PLQY with different substituent effects from the p-BDFs (entries 6−9). The PLQYs of expanded NDFs were almost up to unity (entries 10−14) in solution, while those of ADF were lower (entries 15−17). The reasons for the effects of the parent scaffold and the substituents on PLQYs are unclear at present and may be an interesting subject for theoretical studies. Solid-state luminescence of some compounds is also noteworthy. PLQYs of p-CZBDF and β-unsubstituted NDFs (entries 4, 10, and 11) in the solid state are rather high, whereas tetraphenyl-substituted p-BDF and β-phenyl-substituted NDF exhibited significant quenching (entries 2 and 12). Considering the high carrier mobilities of these solid-state luminescent compounds (see sections 4.1 and 5.5), we expect that these fused furans may be useful for application to organic lightemitting transistors (OLETs).30

Figure 2. Strategy for designing stable furan derivatives. 397


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Accounts of Chemical Research Scheme 1. Synthesis of Fused Polycyclic Furans via Zinc-Mediated Cyclization

Figure 3. Three isomeric benzodifuran core structures: (a) p-BDF, (b) m-BDF, and (c) o-BDF. Single-crystal X-ray structures of representative compounds are shown in the top row. (Bond lengths are given in Å unit. Only the BDF core and α-carbon atoms are shown for clarity.) Schematic representations of their structure, along with isoelectronic polycyclic hydrocarbons, are also shown in the following rows.

3.3. Mechanofluorochromism of 2-PyBDF

concentrated solution showed a pale-yellow color (Figure 4a). This concentrated solution showed broad absorption and emission bands in a longer wavelength region than the diluted sample, suggesting the formation of aggregates. Indeed, a strong intermolecular interaction is suggested by X-ray crystallography: 2-PyBDF has slip−stack packing in single crystals

22

Interestingly, 2-pyridyl-substituted p-BDF (2-PyBDF) showed mechanofluorochromism and concentration-dependent photoluminescence.31 A dilute solution was transparent under ambient light and deep-blue emissive upon photoirradiation, similar to the other BDFs, whereas a 10-times more

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Accounts of Chemical Research Table 1. Photophysical Properties of Fused Furans in CH2Cl2 at Room Temperature

a

Absorption maximum wavelengths. bEmission maximum wavelengths. cPLQY measured using the absolute method. dThe data in parentheses are those in the solid state. eNot available.

(Figure 4b in a box), most probably because of its flat molecular structure and charge distribution within the molecules. In the solid state, a sample prepared by sublimation (Sample S) was pale yellow with blue photoluminescence, similar to the color in dilute solution, while a sample prepared by recrystallization from 1,1,2,2-tetrachloroethane (Sample R) showed a yellow color and yellow emission, similar to the concentrated solution (Figure 4b). The latter sample loses its yellow color by mechanical grinding, and the emission color also changed from yellow to bluish (Sample G) similar to that of Sample S. This blue shift of the emission band upon mechanical stimuli is a rather rare case for mechanofluor-

ochromism and must have caused a significant change in molecular ordering from crystalline to amorphous. The molecular origin of the phenomenon still needs to be probed.

4. ELECTRONIC AND THERMAL PROPERTIES OF BDF 4.1. Charge Carrier Mobility Measurements of BDFs

Studies on the carrier drift mobility using the time-of-flight (TOF) technique revealed the utility of BDFs as semiconducting materials (Figure 5). An amorphous sample of the structurally simplest tetraphenyl-p-BDF (p-TPBDF) showed a hole mobility of 6.4 × 10−4 cm2/(V s),20 which 399


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This conjecture is supported by an OFET study using single crystals of NDF derivatives (see section 5.5). p-DPABDF and p-AntBDF possessing extended π-conjugated substituents, such as diphenylamino and anthryl groups, show higher carrier mobilities on the order of 10−3 cm2/(V s), close to the upper limit of amorphous materials. The data indicate two interesting aspects of the carrier mobility of BDFs: first, the carrier mobility improves by a synergetic effect with the π-extended substituents; second, an arylamine is not mandatory to achieve high carrier mobility. For example, pTPBDF and p-AntBDF showed high carrier mobility even without arylamino groups. It is noteworthy that the hole mobility of cross-conjugated m-BDFs was found to be 1 order of magnitude lower than that of the corresponding p-BDF isomers. This is attributable to their dipole moment derived from the molecular shape of m-BDFs and to ineffective intermolecular orbital overlap due to the lower symmetry structures. Interestingly, installation of carbazole groups provides BDFs with an ambipolar character with well-balanced and high carrier mobilities for both the hole and the electron. For example, pCZBDF was proved to be an ambipolar material with among the highest mobilities (hole 3.7 × 10−3 and electron 4.4 × 10−3 cm2/(V s)).21 Its meta- and ortho-isomers also possess high hole and electron mobilities of over 10−3 cm2/(V s). These ambipolar BDF materials are useful as host materials in heterojunction and homojunction OLEDs, as described in sections 5.2−5.4.

Figure 4. Unique photophysical properties of 2-PyBDF. (a) Concentration dependence of the solution color. (b) Fluorescent mechanochromic behavior. Inset: Packing structure in the single crystals drawn with a space-filling model (gray, carbon; white, hydrogen; red, oxygen; blue, nitrogen).

4.2. Ionization Potential and Glass Transition Temperature of the BDFs

Solid-state properties other than those mentioned above are also suitable for applications, for example, in OLEDs. Ionization potentials (IPs) of the BDFs in the thin-film state are similar to that of amorphous hole-transporting materials commonly used in OLEDs (e.g., α-NPD 5.43 eV), as determined by photoemission yield spectroscopy (PYS). Thus, the IP values of α-phenyl-substituted p- and m-BDF are in a range of 5.47−

rivals the hitherto-reported amorphous hole-transporting materials, such as α-NPD, TPD,32 and other furan materials.8 These data indicate that the substituted BDF has an intrinsic ability for carrier transport, probably because of the flat and delocalized π-electron system of the BDF, which facilitates intermolecular MO interactions suitable for carrier hopping.

Figure 5. Carrier mobility of the BDFs measured using TOF technique of amorphous film (h+, hole; e− electron), along with ionization potential (IP) and glass transition temperature (Tg). aN.A. indicates not available. 400


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OLED based on p-TPBDF (Figure 6). In fact, the performance is among the highest for nonarylamine amorphous HTMs. Whereas arylamines have long served as a standard of HTM in OLEDs, since Tang et al. introduced the concept of multilayer architecture,10 it is evident now that the arylamino group is not the only structural motif required for HTMs. Nonetheless, the favorable contribution of the arylamino substituents to the HTM property of BDFs indicates synergetic effects of the BDF core and the arylamino groups, probably because of the increase of hole mobility and decrease of the IP, as was discussed in section 4.

5.81 eV, which is suitable for p-type semiconducting materials, and those of α-methyl-BDFs are slightly larger than the αphenyl counterparts. The stability of the amorphous state of these BDFs was high: the glass transition temperatures (Tg) of the BDFs rival hitherto-reported amorphous materials, and some compounds are higher than 120 °C, which is required for practical applications.

5. APPLICATION TO ORGANIC DEVICES 5.1. BDF as a Hole-Transporting Material in an OLED

Based on the solid-state properties described above, we expected that the p-type and ambipolar BDFs would serve as hole-transporting and host materials for electronic devices, respectively, which was indeed the case in OLED applications. Note that there have been only a few reports on the application of furan-containing materials, including a furan-containing oligophenylene analogue for hole-transporting materials7 and a benzo[b]furan trimer as an emitting material.9 Thus, we employed a simple and standard heterojunction configuration using the p-type BDF as the hole-transporting material (HTM) as shown in Figure 5a. Most interestingly, pTPBDF (R = H) afforded improved performance (i.e., lower driving voltage and higher current efficiency) compared with αNPD, indicating that BDF itself serves as a good platform for constructing HTMs. Further improvement was observed for the OLEDs using BDFs having arylamino substituents (R = Ph2N and p-tol2N). This study showed us that the high performance is primarily derived from the BDF core itself, not from the arylamine groups, as demonstrated by the good performance of the

5.2. BDF as a Host Material

The wide-gap and ambipolar characteristics of BDFs make them useful for application to host materials33 in heterojunction OLEDs (Figure 7). The wide-gap character originates from the

Figure 7. Application of p-CZBDF as the emission layer of an OLED. (a) Configuration of the device. Note that a BCP layer was inserted between the emissive layer and the Alq3 layer as the hole blocking for the Ir(piq)3-doped device. (b) Chemical structures of the dopants used for the p-CZBDF host, along with the external quantum efficiency of the OLEDs. (c) Orbital energy levels of a host and dopants. (d) Pictures of driving OLEDs (A) without dopant, (B) TBPdoped, (C) C545T-doped, (D) rubrene-doped, (E) Ir(piq)3-doped, and (F) TBP/rubrene-doped.

weak aromaticity of furan. Thus, p-CZBDF has a HOMO− LUMO gap of >3.3 eV and functions by itself as a deep-blueemitting dye, albeit with low efficiency (EQE = 1.2%). Doping with a blue fluorescent TBP, a green fluorescent C545T, a yellow fluorescent rubrene, and a red phosphorescent Ir(piq)3 (tris[1-phenylisoquinoline-C2,N]iridium(III)) into the pCZBDF layer achieved electroluminescence over a full color

Figure 6. Application of p-type BDFs to HTMs of OLEDs. (a) Configuration of the device and chemical structures of HTLs. (b) Summary of the device performance. 401


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degradation of p-CH3CZBDF. We tested this idea through a study of the hydrogen/deuterium isotope effect24 and found that the PHOLED device using p-CD3CZBDF (X = D, Figure 9) as a host for Ir(ppy)3 is five times longer lived than that

range, where electroluminescence occurred only from the dopant and not from the p-CZBDF host. Thus, the energy level matching between the host and dopants allows the latter to function effectively as a recombination site. We also succeeded in obtaining a white-luminescent OLED by stacking TBP- and rubrene-doped layers, with CIE coordinates of (0.304; 0.325), which is close to the ideal white (0.333; 0.333). This efficiency, as compared with CBP, a commonly used ambipolar host material, reflects improvement for the EQE and driving voltage. In addition, the half-lifetime of the phosphorescent OLED (PHOLED) using Ir(piq)3 improved significantly: 4250 h vs 260 h for p-CZBDF and CBP, respectively, under constant current mode. p-CZBDF, on the other hand, failed to drive green and blue PHOLED using Ir(ppy)3 and FIrpic, respectively, because of low triplet excited-state energy (ET, Figure 8). To increase the

Figure 9. Isotope and substituent effects on the PHOLED device lifetime using alkyl-CZBDF as a host material for Ir(ppy)3.

using p-CH3CZBDF (X = H). The maximum luminance of PHOLED using deuterated host material also improved, while the electroluminescence spectra and the device efficiency remained unaffected. The data strongly suggest that cleavage of the C−H bond on the methyl group is a critical factor for the degradation of the host material. In agreement with this finding, the device lifetime was further increased by replacing the two methyl groups with two tertbutyl groups. Thus, PHOLED using t-BuCZBDF (X = CH3) was 22.5 times longer lived than that using X = H with higher maximum luminance and a slight decrease in the performance. It is likely that the bulky tert-butyl groups affected the intermolecular interaction of the molecules in the active layer. 5.4. RGB Full-Color p−i−n Homojunction OLED Using Ambipolar BDF

State-of-the-art OLEDs are made of multilayers of different organic materials (heterojunctions), each of which has a different function, such as hole injection and hole transport. However, there is an obvious practical advantage in the use of fewer materials. Homojunction architecture,34−36 comprising a single organic matrix that has a seamlessly layered structure involving different dopants, offers a solution to this issue. To achieve such a homojunction OLED, one needs a host material that can be doped either positively or negatively to inject hole and electron effectively and to confine carrier and exciton within light-emitting dopants. The carbazole-substituted pCZBDF was found to fulfill this function and afforded an RGB full-color p−i−n homojunction OLED for the first time. Thus, as shown in Figure 10a, the homojunction architecture consisted of three layers in the p-CZBDF matrix: the bottom and top layers doped with vanadium(V) oxide (V2O5) and cesium (Cs) as p- and n-dopant, respectively, for carrier injection and transportation, and an interlayer doped with an emitting dye, such as TBP, C545T, or Ir(piq)3 by codeposition under vacuum. Remarkably, the performance of the homojunction devices was comparable to or higher than the corresponding heterojunction devices using the same host and the same emitting dyes. For example, the green fluorescent C545T-doped homojunction device exhibited a high EQE value of up to 4.2% (Figure 10b), which is close to the theoretical

Figure 8. Comparison of triplet energy levels (ET) of a series of CZBDFs (hosts) and phosphorescent Ir complexes (dopants).

ET value, we next examined less π-conjugated 2,6-dimethyl analogues of p- and o-CH3CZBDFs as a host material for Ir(ppy)3 and FIrpic, respectively.26 As expected, these devices showed electroluminescence from each phosphorescent dye with a lower driving voltage than commonly used host materials, such as CBP and m-CP, with higher or comparable EQE. However, the lifetimes of the devices using the α-methylsubstituted BDF hosts were much shorter than those using pCZBDF, which has phenyl substituents at the α-positions, as will be discussed in the next section. 5.3. Deuterium Effect on the OLED Device Lifetime

The lifetime of organic electronic devices has become the subject of considerable interest, as the commercialization of these devices has become a viable issue. During our studies on the application of p-CH3CZBDF to green PHOLED described above, we noticed a mechanistic possibility that upon oxidation of BDF molecules, the loss of a proton or hydrogen atom from the methyl groups attached to the BDF core triggers 402


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Figure 11. Single-crystal OFET of DPNDFs. Structure of compound and pictures of the representative crystals and devices. Figure 10. Homojunction OLED using p-CZBDF: (a) device structure; (b) plot of EQE against applied voltage for the dye-doped homojunction OLEDs.

limit of a fluorescent OLED (ca. 5%),37 indicating that homojunction architecture can be feasible if one can develop a suitable material. p-CZBDF is now commercially available from Tokyo Chemical Industry (TCI). As overviewed in this section, the BDF derivatives serve as light-emitting organic semiconductors by themselves or used as a host matrix for doping. In particular, the ambipolar CZBDFs are among the most promising class of materials in terms of device efficiency and lifetime as well as suitability for homojunction OLEDs. Broad applications beyond OLEDs will be expected as active materials in ambipolar OFETs, OLETs, and photovoltaic devices. 5.5. High Hole Mobility of Naphthodifurans in a Single-Crystal OFET

Rather small, BDF derivatives tend to form an amorphous film suitable for OLED applications. For other uses, such as OFET applications, one may need crystalline materials. To this end, we synthesized NDFs that possess a more extended aromatic system (Figure 11).27 The high crystallinity of the NDFs was suitable for fabricating single-crystal OFETs: platelet crystals for DPNDF1 and DPNDF3 and needle crystals for DPNDF2 were obtained using the physical vapor transport (PVT) method, and they showed hole mobilities of 1.3, 0.6, and 0.2 cm2/(V s), respectively, with an Ion/Ioff ratio of up to 105. Solution-processed OFETs using DPNDF1 also showed high hole mobility. The bottom-gate−top-contact OFETs in which the organic active layer in the form of a single crystal was fabricated via an edge-cast technique38,39 exhibited hole mobilities of 0.6−1.0 cm2/(V s) in the saturation regime with an Ion/Ioff ratio of 104−105 (Figure 12). Use of the octylsubstituted analogue C8-DPNDF1 achieved much higher mobility of 1.5−3.6 cm2/(V s), which is comparable to the mobility of amorphous silicon and comparable to or higher than those for nonfused40 and other fused41 furan materials measured in single crystal. Such high mobility of C8-DPNDF1 can be attributed to several factors that facilitate carrier

Figure 12. Solution-processed single-crystal OFET of DPNDF1 and C8-DPNDF1: (a) Chemical structures. (b) Packing structure of C8DPNDF1 and the calculated transfer integrals. (c) Schematic illustration of the packing diagram of C8-DPNDF1 along the b-axis. The parallelogram represents a unit cell. (d) The cross-sectional profile along the white dotted line of the inset top view of a picture of the AFM image.

transportation. First, an effective carrier transportation path is formed in a single crystal: the π-conjugation in NDFs extends over the whole molecule. C8-DPNDF1 molecules in crystals stack in a herringbone-packing manner with large transfer integrals along the stacking direction (Figure 12b), which can be ascribed to dense molecular packing, as illustrated by a 2.80 Å intermolecular shortest contact in the crystal of C8DPNDF1, a distance much shorter than that reported for a 403


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Accounts of Chemical Research thiophene analogue (DPNDT, 3.31 Å).42 The smaller atomic radius of oxygen (1.52 Å) than sulfur (1.80 Å) contributes to the shortening of the distance. The second critical factor is the small reorganization energy (λ): a value of λ = 0.18 eV was estimated for the hole, and this value is comparable to the thiophene analogue (0.19 eV). The dense crystal packing and the small λ values facilitate carrier transporting according to the Marcus formalism.43 Third, homogeneity of the solution-grown film contributes to the high performance. Atomic force microscopy (AFM) images revealed that there are only a few step defects of a single molecular height, over a micrometer distance (Figure 12c,d).

Chemistry and, since April 2016, Molecular Technology Innovation Chair Professor in the Office of the President and the Department of Chemistry. His research interests span synthetic, physical, and materials chemistry, including transmission electron microscopy for imaging the motions and reactions of single organic molecules and molecular assemblies. He received the Chemical Society of Japan Award in 2003, the Medal of Honor with Purple Ribbon in 2009 and the Arthur C. Cope Scholar Award of the American Chemical Society in 2010, the 55th Fujiwara Award in 2014, and the Centenary Prize of the Royal Society of Chemistry in 2014. He has been a Fellow of the American Association for the Advancement of Science since 1998, an Honorary Foreign Member of the American Academy of Arts and Sciences since 2008, and a Member of the Science Council of Japan.

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6. CONCLUSIONS We have shown in this Account that thermally and electrochemically stable fused furan derivatives serve as a new class of semiconducting materials as demonstrated by OLED and OFET applications. Their high charge carrier mobility coincides with a small reorganization energy and dense molecular packing because of the small atomic radius of oxygen. The furan skeleton allows us to prepare their derivatives with unique photo and electronic characteristics, such as a wide gap attributed to the low level of the aromatic character of furan and efficient luminescent properties. Further extension of the molecular design reported here to polyfuran compounds will provide an interesting opportunity for materials design, as has already been shown in part for benzotrifuran compounds.44 Considering the abundance of fused furan motifs in biologically active compounds, we expect that these furan compounds may find use in biomedical and biosensing applications as well.

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ACKNOWLEDGMENTS We thank our co-workers, Dr. Chikahiko Mitsui, Dr. Laurean Ilies, and others whose names are shown in the references, for their intellectual and experimental contribution. We also appreciate the fruitful collaboration with device specialists Dr. Yoshiharu Sato and Prof. Jun Takeya, as well as their laboratory members. The authors also thank MEXT and JST (Strategic Promotion of Innovative Research to E.N.) for financial support.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

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

Eiichi Nakamura: 0000-0002-4192-1741 Notes

The authors declare no competing financial interest. Biographies Hayato Tsuji graduated from Kyoto University and obtained his Master of Science degree in 1998 and Doctor of Engineering degree in 2001 at Kyoto University. During his doctoral program, he studied at the University of Colorado in Boulder, USA, as a visiting student. He then worked as a JSPS postdoctoral fellow and visiting researcher at Uppsala University, Sweden. He became an assistant professor in 2002 at the Institute for Chemical Research, Kyoto University, and an associate professor at the University of Tokyo in 2006. He also served as a visiting professor at the University of Strasbourg, France, in 2012. In 2016, he was appointed as a full professor at Kanagawa University. His current research interests include organic synthesis, main group chemistry, and physical organic chemistry. He has received several awards, including a Young Scientist Award from the Ministry of Education, Culture, Sports, Science and Technology, Japan, in 2010 and the Thieme Journal Award in 2011. Eiichi Nakamura obtained his Ph.D. in chemistry at the Tokyo Institute of Technology. After postdoctoral work at Columbia University, he returned in 1980 to his Alma Mater as an assistant professor and was eventually promoted to full professor. In 1995, he moved to the University of Tokyo as Professor in the Department of 404


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