Syntheses of dibenzo[d,d']benzo[2,1-b:3,4-b']difuran derivatives and their application to organic field-effect transistors.

Ladder-type π-conjugated compounds containing a benzo[2,1-b:3,4-b']difuran skeleton, such as dibenzo[d,d']benzo[2,1-b:3,4-b']difuran (syn-DBBDF) and dinaphtho[2,3-d:2',3'-d']benzo[2,1-b:3,4-b']difuran (syn-DNBDF) were synthesized. Their photophysical and electrochemical properties were revealed by UV-vis absorption and photoluminescence spectroscopy and cyclic voltammetry. Organic field-effect transistors (OFETs) were fabricated with these compounds as organic semiconductors, and their semiconducting properties were evaluated. OFETs with syn-DBBDF and syn-DNBDF showed typical p-type characteristics with hole mobilities of <1.5 × 10(-3) cm(2)·V(-1)·s(-1) and <1.0 × 10(-1) cm(2)·V(-1)·s(-1), respectively.

Introduction

Organic semiconductors have significantly been developed in the past two decades by virtue of their advantages, such as low weight, flexibility, large-area processability, which are different features from conventional silicon-based semiconductors. Organic semiconducting materials can be used as active layers in organic field-effect transistors (OFETs) [1-7], organic light-emitting diodes (OLEDs) [8-10], and organic photovoltaics (OPVs) [11-12]. Among many organic semiconducting materials so far reported, thiophene-fused π-conjugated compounds have been widely studied as organic semiconducting materials and found to exhibit high semiconducting performances [5,13-16].

Furan-containing π-conjugated compounds have attracted less attention until recently [17-27]. The oxygen atom possesses a smaller van der Waals radius than a sulfur atom. Accordingly, furan-containing π-conjugated compounds should be expected to form a denser packing structure in the solid state, which is one of the main requirements for high semiconducting properties [28-31]. In 2007, Nakamura and co-workers reported the synthesis of furan-fused ladder-type π-conjugated compounds, benzo[1,2-b:4,5-b']difurans (BDFs) 1 and their application to OLEDs as hole-transporting materials (Figure 1) [32]. They also synthesized a series of isomeric BDFs (benzo[1,2-b:5,4-b']difurans and benzo[1,2-b:6,5-b']difurans) and studied their structure–property relationship [33-34]. Furthermore, naphthodifurans with a fused-naphthalene between two furan rings have been developed as organic semiconductors for OFETs [19-20]. In particular, the naphtho[2,1-b:6,5-b']difuran derivative 2 has been reported to demonstrate an excellent OFET mobility of 3.6 cm2·V−1·s−1 [19]. Previously, we have reported the synthesis of dibenzo[d,d']benzo[1,2-b:4,5-b']difurans (anti-DBBDFs), which is also a π-extended homologue of BDF [35]. The OFET devices with an anti-DBBDF skeleton exhibited p-type semiconducting properties [36-37]. For example, dialkyl-substituted anti-DBBDF 3 showed a hole mobility of 0.042 cm2·V−1·s−1 [38]. Recently, we have also found that dinaphtho[2,3-d:2',3'-d']benzo[1,2-b:4,5-b']difuran (anti-DNBDF 4) with a more extended π-conjugation afforded higher hole mobility of 0.33 cm2·V−1·s−1 [39-41]. These studies clearly demonstrate that furan-fused π-conjugated compounds are promising candidates as organic semiconducting materials, and it is highly desirable to investigate the structure−property relationship thoroughly for further development of furan-containing semiconducting materials.

Figure 1

Structures of furan-fused ladder-type π-conjugated compounds.

Herein we report the synthesis of ladder-type π-conjugated compounds containing a benzo[2,1-b:3,4-b']difuran skeleton, such as dibenzo[d,d']benzo[2,1-b:3,4-b']difuran (syn-DBBDF 5) and dinaphtho[2,3-d:2',3'-d']benzo[2,1-b:3,4-b']difuran (syn-DNBDF 6, Figure 1) [42-46]. The physical and electrochemical properties of the synthesized compounds are also discussed. OFETs with these compounds as semiconducting layers were found to exhibit relatively high hole mobility of <1.0 × 10−1 cm2·V−1·s−1.

Results and Discussion

Synthesis

The synthetic routes to syn-DBBDF 5 and syn-DNBDF 6 are described in Scheme 1 and Scheme 2. 3-Decylanisole was first synthesized from commercially available 3-bromoanisole via iron-catalyzed cross-coupling reaction with decylmagnesium bromide in 71% yield [47]. Lithiation of the obtained 3-decylanisole with s-BuLi and the following treatment with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (iPrO-Bpin) gave boronate ester 7 in 57% yield. Then, terphenyl 9 was synthesized via palladium-catalyzed Suzuki–Miyaura cross coupling of boronate ester 7 with 2,3-difluoro-1,4-diiodobenzene (96% yield) and subsequent demethylation (95% yield). Finally, the desired syn-DBBDF 5 was successfully synthesized via the double intramolecular cyclization under basic conditions at high temperature (92% yield) [37,43]. The same synthetic strategy was applied to the synthesis of syn-DNBDF (Scheme 2). 2-Decyl-7-methoxynaphthalene was prepared from 7-methoxynaphthalen-2-ol in two steps according to the literature [23,48], and used for the synthesis of boronate ester 10 (45% yield). The following cross coupling (80% yield), demethylation (85% yield), and the double cyclization (87% yield) gave the target syn-DNBDF 6. The obtained syn-DBBDF 5 is soluble in common organic solvents and can be purified by column chromatography. In contrast, because of low solubility in common organic solvents, the crude product of syn-DNBDF 6 was purified by washing several times with water and the subsequent sublimation.

Scheme 1

Synthesis of syn-DBBDF 5.

Scheme 2

Synthesis of syn-DNBDF 6.

Thermal properties

The phase-transition properties and thermal stability of syn-DBBDF 5 and syn-DNBDF 6 were evaluated by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA), respectively. The DSC scans of syn-DBBDF 5 and syn-DNBDF 6 showed some transition peaks with the first phase-transition temperature at 20 °C and 45 °C, respectively, in the heating process (Figure 2a). Such phase-transition temperatures are >50 °C lower than those of their anti-isomers 3 and 4 [38-39]. These results indicate that syn-DBBDF 5 and syn-DNBDF 6 form weaker intermolecular interactions in the solid state than their corresponding anti-isomers. The mesophase of syn-DBBDF 5 was converted to the isotropic phase at 115 °C, while syn-DNBDF 6 did not melt below 250 °C. From the TG measurement, the temperatures of 5% weight loss (Td5) of syn-DBBDF 5 and syn-DNBDF 6 were estimated to be 272 °C and 423 °C, respectively (Figure 2b).

Figure 2

(a) DSC and (b) TG curves of syn-DBBDF 5 and syn-DNBDF 6.

Photophysical properties

The UV–vis spectrum of syn-DBBDF 5 in chloroform showed the strongest absorption maximum at 324 nm, while syn-DNBDF 6 showed a red-shifted absorption spectrum with the strongest absorption maximum at 365 nm (Figure 3a and Table 1). Since syn-DNBDF 6 contains one more benzene ring at each terminal of the π-conjugated skeleton than syn-DBBDF 5, it should possess an extended π-conjugation length, resulting in a red-shifted absorption spectrum. The HOMO–LUMO energy gaps estimated from the absorption edges were 3.72 eV and 3.32 eV for syn-DBBDF 5 and syn-DNBDF 6, respectively. Their photoluminescence spectra as shown in Figure 3b exhibited mirror images of their absorption spectra with small Stokes shifts (376 cm–1 for syn-DBBDF 5; 370 cm–1 for syn-DNBDF 6), which reflect their high rigidity. Similar to its absorption spectra, syn-DNBDF 6 showed a red-shifted emission band with a relatively high quantum yield (Φ = 61% in CHCl3 solution).

Figure 3

(a) UV–vis absorption spectra of syn-DBBDF 5 (blue line) and syn-DNBDF 6 (red line) in CHCl3 (1.0 × 10−5 M) and (b) normalized photoluminescence spectra of syn-DBBDF 5 (blue line) and syn-DNBDF 6 (red line) in CHCl3 (1.0 × 10−7 M).

Table 1

Photophysical and electrochemical properties of syn/anti-DBBDFs and DNBDFs.

Compound	λabs (nm)a	λem (nm)b	Φ (%)c	Stokes shift (cm–1)	Eg (eV)d	Eoxonset (V)e	EHOMO (eV)f
syn-DBBDF 5	324	328	18	376	3.72	0.84	−5.64
syn-DNBDF 6	365	370	61	370	3.32	0.56	−5.36
anti-DBBDF 3	342	–	–	–	3.51	–	–
anti-DNBDF 4	394	–	–	–	3.15	–	–

aIn CHCl3 (1.0 × 10−5 M). bIn CHCl3 (1.0 × 10−7 M). Excitation at 310 nm. cAbsolute quantum yield determined by a calibrated integrating sphere system. Excitation at 275 nm for syn-DBBDF 5 and syn-DNBDF 6. dOptical band gaps estimated from the onset position of the UV–vis absorption spectra in solution. eOnset potentials (vs Fc/Fc+) of the first oxidation wave determined by cyclic voltammetry: 1.0 mM solution in CH2Cl2 (syn-DBBDF 5) or Cl2CHCHCl2 (syn-DNBDF 6) with 0.1 M Bu4NClO4, Pt as working and counter electrodes, scan rate = 50 mV·s−1. fCalculated according to EHOMO = −(Eox + 4.80) eV (Fc/Fc+ redox couple: 4.8 eV below the vacuum level).

To investigate the structure–property relationship of DBBDFs and DNBDFs, the optical properties of syn-DBBDF 5 and syn-DNBDF 6 were compared with those of anti-DBBDF 3 and anti-DNBDF 4. The UV–vis spectra of anti-DBBDF 3 and anti-DNBDF 4 were reported to show absorption maxima (342 nm for anti-DBBDF 3; 394 nm for anti-DNBDF 4) and absorption edges [353 nm (3.51 eV) for anti-DBBDF 3; 410 nm (3.15 eV) for anti-DNBDF 4] at longer wavelengths than syn-DBBDF 5 and syn-DNBDF 6, respectively [38-39]. Accordingly, syn-isomers are indicated to possess shorter π-conjugation lengths than anti-isomers.

Electrochemical properties

Cyclic voltammograms of syn-DBBDF 5 and syn-DNBDF 6 are shown in Figure 4 [1.0 mM solution in CH2Cl2 (syn-DBBDF 5) or Cl2CHCHCl2 (syn-DNBDF 6) with 0.10 M Bu4NClO4], and the electrochemical properties were summarized in Table 1. syn-DBBDF 5 exhibited two oxidation waves, and an onset potential of the first oxidation wave was determined to be 0.84 V (vs Fc/Fc+). Accordingly, the HOMO energy level was estimated to be −5.64 eV under the premise that the energy level of Fc/Fc+ is 4.8 eV below the vacuum level [49-51]. In contrast, syn-DNBDF 6 showed one oxidation wave with an onset potential of 0.56 eV (vs Fc/Fc+, HOMO = −5.36 eV). The lower oxidation potential and higher HOMO energy level of syn-DNBDF 6 should reflect its longer π-conjugation length than syn-DBBDF 5. Based on their HOMO energy levels and HOMO−LUMO energy gaps, syn-DBBDF 5 and syn-DNBDF 6 are expected to work as stable semiconducting materials under ambient conditions.

Figure 4

Cyclic voltammograms of syn-DBBDF 5 and syn-DNBDF 6 (measurement conditions: 1.0 mM in CH2Cl2 for syn-DBBDF 5 or Cl2CHCHCl2 for syn-DNBDF 6 with 0.1 M Bu4NClO4; Pt as working and counter electrodes; scan rate = 50 mV·s−1).

Fabrication of OFETs with syn-DBBDF- and syn-DNBDF-based thin films and evaluation of semiconducting properties

To study the semiconducting properties of syn-DBBDF 5 and syn-DNBDF 6, bottom-gate/top-contact OTFTs were utilized as a device structure. Thin films of syn-DBBDF 5 and syn-DNBDF 6 were deposited by sublimation under high vacuum (p < 10−5 Pa) at a rate of ca. 1 Å·s−1 for syn-DBBDF and ca. 0.4 Å·s−1 for syn-DNBDF onto the Si/SiO2 substrates. The substrate temperature (Tsub) during deposition has been known to have a great impact on the OTFT performance by affecting the nucleation and growth of the organic molecules [52-53]. Accordingly, the thin films were fabricated at different substrate temperatures. In addition to the bare Si/SiO2 substrates, the HMDS (hexamethyldisilazane)-treated substrates were used to evaluate the effect of the substrate structure on the device performance. The gold source/drain electrodes were deposited on the thin films. The channel width and length were 500 μm and 50 μm, respectively.

Both syn-DBBDF- and syn-DNBDF-based OFETs demonstrated typical p-type semiconducting characteristics. The extracted FET parameters and the transfer/output characteristics are summarized in Table 2, Figure 5, and Figure S21 (Supporting Information File 1). The syn-DBBDF-based OFETs fabricated on bare Si/SiO2 substrates at Tsub = 30 °C showed a field-effect mobility μFET of 5.0 × 10−5 cm2·V−1·s−1 and an Ion/Ioff ratio of 101, while those with HMDS-treated substrates demonstrated higher mobility of 1.5 × 10−3 cm2·V−1·s−1 with an Ion/Ioff ratio of 103. The deposition of syn-DBBDF 5 at Tsub = 60 °C did not give a thin film, which should be caused by re-sublimation of syn-DBBDF 5 from the surface. The more π-extended syn-DNBDF 6 afforded higher performances than syn-DBBDF 5. OFETs fabricated on the bare and HMDS-treated Si/SiO2 substrates at Tsub = 30 °C showed a field-effect mobility of 2.3 × 10−2 cm2·V−1·s−1 (Ion/Ioff = 103) and 2.0 × 10−2 cm2·V−1·s−1 (Ion/Ioff = 103), respectively. The FET performance also depends on the substrate temperature during thin-film fabrication. Thus, the highest hole mobility of 1.0 × 10−1 cm2·V−1·s−1 was obtained for the syn-DNBDF-based device fabricated on the HMDS-treated substrate at Tsub = 90 °C, while it was lower than that fabricated with anti-DNBDF derivatives [39].

Table 2

FET characteristics.

Compound	Surfactant	Tsub (°C)	μFET (cm2·V−1·s−1)	Vth (V)	Ion/Ioff
syn-DBBDF 5	–	30	5.0 × 10−5	−26	101
	HMDS	30	1.5 × 10−3	−25	103
syn-DNBDF 6	–	30	2.3 × 10−2	−24	103
	–	90	6.5 × 10−2	−25	104
	HMDS	30	2.0 × 10−2	−22	103
	HMDS	90	1.0 × 10−1	−28	105

Figure 5

Output and transfer characteristics of the representative OFETs with a thin film of (a) syn-DBBDF 5 (Tsub = 30 °C) and (b) syn-DNBDF 6 (Tsub = 90 °C) on HMDS-treated Si/SiO2 substrates.

Analysis of thin films

The vapor-deposited thin films of syn-DBBDF 5 and syn-DNBDF 6 were analyzed by X-ray diffraction (XRD) and atomic force microscopy (AFM). Figure 6 shows the out-of-plane XRD pattern and an AFM image of the thin film of syn-DNBDF 6 on the HMDS-treated Si/SiO2 substrate (Tsub = 90 °C), which demonstrated the highest mobility in this study. The layer structure was confirmed with a monolayer thickness (d-spacing) of 3.94 nm (2θ = 2.24°). Molecular lengths with extended linear alkyl chains are expected to be ca. 4.2 nm. Accordingly, syn-DNBDF 6 should be arranged on the substrate with its molecular long axis almost perpendicular to the substrate. Such a layer structure was also confirmed by AFM. As shown in Figure 6b,c, the thin film of syn-DNBDF 6 forms relatively large grains (ca. 0.5 μm in size) with a layer structure (step heights ca. 4.0 nm) along with heterogeneous protrusions. The molecular arrangement indicated by these observations is advantageous for the in-plane charge transfer of OFETs. Based on XRD patterns and AFM images, the substrate treatment and the substrate temperature seem to have a limited impact on the molecular arrangement (Figures S22 and S23, Supporting Information File 1). The similar layer structure was also confirmed for syn-DBBDF 5 (Figures S22 and S23, Supporting Information File 1).

Figure 6

(a) XRD pattern, (b) AFM image (2 × 2 μm), and (c) cross-section height of a thin film of syn-DNBDF 6 on HMDS-treated Si/SiO2 substrates (Tsub = 90 °C).

Conclusion

In summary, we investigated the synthesis and properties of ladder-type π-conjugated compounds, dibenzo[d,d']benzo[2,1-b:3,4-b']difuran (syn-DBBDF 5) and dinaphtho[2,3-d:2',3'-d']benzo[2,1-b:3,4-b']difuran (syn-DNBDF 6). Based on the photophysical and electrochemical data, both compounds are expected to possess good air stability as organic semiconducting materials. The comparison with their anti-isomers revealed that the π-conjugation in syn-DBBDF 5 and syn-DNBDF 6 is less effective than those of their anti-isomers. OFETs based on these compounds were fabricated as bottom-gate top-contact devices, and their semiconducting properties were evaluated. All devices showed typical p-type transistor characteristics. The highest hole mobility of 1.0 × 10−1 cm2·V−1·s−1 was achieved when using syn-DNBDF-based OFET device.

General experimental procedures, synthetic procedures/characterization data of compounds 5–12, device fabrication/evaluation procedures, OFET characteristics, XRD patterns, and AFM images.