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Furan Fused V-Shaped Organic Semiconducting Materials with High Emission and High Mobility Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/

Katsumasa Nakahara,a Chikahiko Mitsui,b Toshihiro Okamoto,*,a,b,c Masakazu Yamagishi,b Hiroyuki Matsui, b Takanari Ueno,d Yuji Tanaka,d Masafumi Yano,d Takeshi Matsushita,e Junshi Soeda,a,b Yuri Hirose,b Hiroyasu Sato,f Akihito Yamanof and Jun Takeya*,a,b

We report the facile synthetic protocol of dinaphtho[2,3b:2',3'-d]furan (DNF–V) derivatives. DNF–V derivatives showed high emissive behaviours in solids. A solutioncrystallized transistor based on alkylated DNF–V derivatives showed an excellent carrier mobility of up to 1.3 cm2/Vs, thereby proving to be a new solution-processable active organic semiconductor with high emission and high mobility.

The development of organic semiconductor materials for organic field-effect transistors (OFETs) is crucial for the creation of ‘printed plastic electronics’ with commercial products such as flexible displays and low-price identification tags. This is because they require a higher charge carrier mobility than the value of approximately 1 cm2/Vs, which corresponds to that of the amorphous silicon used at the moment. Additionally, solutionprocessable and chemically stable semiconductors are desired to create low-cost, easily handled devices. So far, most solutionprocessable high-performance materials reported for OFETs have relied on thiophene-based compounds with a mobility of more than 1 cm2/Vs.1 Recently, furan (instead of thiophene), as an oxygencontaining congener, has been of particular interest because of its densely packed nature, which originates from a smaller element size.2-6 Among them, solution-processable furan-based semiconductors with chemical stability comparable to that of thiophene analogues and an excellent carrier mobility of up to 3.6 cm2/Vs in a single-crystal transistor, were reported. 2 Since such promising compounds were reported, furan-based π-conjugated systems have proven to be potential candidates as organic semiconductors, rivalling thiophene-based semiconductors.

This journal is © The Royal Society of Chemistry 2012

We recently designed and developed dinaphtho[2,3-b:2',3'-

Figure 1. Structural comparisons between DNF–V and DNT– V. d]thiophene (DNT–V)-based materials with high carrier mobilities of 0.7 to 9.5 cm2/Vs, depending on their length and the position of their alkyl chains on the DNT–V core.7 As illustrated in Figure 1, a dinaphtho[2,3-b:2',3'-d]chalcogenophene core could be drastically modulated through its physical and structural properties by simply replacing the central chalcogen atom. In this communication, we report the oxygen-incorporated congeners, dinaphtho[2,3-b:2',3'd]furan derivatives (DNF–Vs), as organic semiconducting materials. DNF–V derivatives bearing soluble substituents of long alkyl chains (C10H21) exhibited a solubility of 0.01–1.58 wt% in commonly used

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organic solvents at room temperature with high chemical and thermal stability. Notably, unlike alkyl-substituted DNT–V derivatives, alkyl-substituted DNF–V derivatives, regardless of the position of their substituents, exhibited a high hole mobility of more than 1 cm2/Vs in the form of solution-crystallized single-crystalline thin film through edge-casting.8 Single crystal analysis proved that all types of DNF–V derivatives form well overlaps between chargetransporting π-electronic cores. These results indicate that an

dichloroethane solution, and 0.40 in a solid obtained from vacuum sublimation. This highly emissive character of DNF–V derivatives provides the opportunity to demonstrate how organic light-emitting transistors (OLETs) show a pure blue emission. The application to OLET using DNF–V derivatives will be reported elsewhere. The thermal behaviours of C10–DNF–VW and C10–DNF–VV were evaluated to understand their chemical stabilities against heated

Scheme 1. Synthetic route for C10–DNF–VW and C10–DNF– VV.

Table 1. Estimation of molecular displacements and distances based on the single-crystal data of DNF–V, C10–DNF–VV and C10–DNF–VW, compared to DNT–V derivatives.

H 21 C10 H 21 C10

OM e

OM e

a) Homo-coupling b) Demethylation

a, b

HO

a, b

HO

C10 H 21 H 21 C10

H 21 C10

C10 H 21 OH

OH c

c) Dehydration

c

H 21 C10 H 21 C10

C10H 21

C10 H 21 O

O

C10 –DN F–VW

C10 –DN F–VV

Reagents and Reaction Conditions: a) n-BuLi (1.1 eq.), 0 °C 2) Fe(acac)3 (1.1 eq.), rt, 2 h, 66–73%, b) BBr3 (2.2 eq.), CH2Cl2, 0 °C, 3 h, 97%, and c) Zeolite, o-dichlorobenzene, 160 °C, 16 h, 94–98%. oxygen-bridged V-shaped π-core is less susceptible to molecular displacement in an aggregated structure than its sulphur-bridge counterparts, mainly due to its smaller element size. We first synthesized decyl group-substituted DNF–V (hereinafter referred to as ‘C10–DNF–VW’ and ‘C10–DNF–VV’) to examine the effect of the position of the alkyl chain on the physicochemical properties. DNF–V itself has already been synthesized in multiple steps in low yields (7 steps, 2% overall yield).9 Thus, according to the facile and cost-effective synthetic protocol for constructing the DNF–V framework that we recently developed,10 alkylated DNF–V derivatives were synthesized as shown in Scheme 1. Starting from either 2-decyl-7-methoxynaphthalene or 2-decyl-6methoxynaphthalene, homo-coupling smoothly proceeded via deprotonation using n-butyl lithium, followed by oxidation using an iron (III) reagent. Subsequently, the demethylation afforded 2,2’binaphthalene binols as the key precursors. Finally, by treatment with a zeolite catalyst, dehydration proceeded to afford DNF–V derivatives in an almost quantitative yield. In this synthetic methodology, the overall yields of C10–DNF–VW and C10–DNF– VV were 67% and 63%, respectively. We estimated the ionization potentials of C10–DNF–VW and C10– DNF–VV in thin-film through photoelectron yield spectroscopy (PYS) (see Figure S1).11 From the PYS results, we found that alkylated DNF–Vs possess smaller ionization potentials of 5.73 eV (C10–DNF–VW) and 5.77 eV (C10–DNF–VV), respectively, compared to DNF–V itself (5.93 eV). The smaller ionization potentials of alkyl DNF–Vs stem from the electron-donating nature of the alkyl chain. In terms of p-type device operation, it would be preferable for a hole injection from the electrodes to occur. Due to the large HOMO–LUMO gap estimated from the absorption spectrum measurements in solution, the DNF–V derivatives showed a deep-blue emission (see Figure S5). C10–DNF– VW showed a high quantum efficiency of 0.88 in a 1,2-

2 | J. Name., 2012, 00, 1-3

Compound

a

Displacement (Å)a

Centre of Mass (Å)b

HH

HT

TT

HH

HT

TT

DNF–V C10–DNF–VW C10–DNF–VV

0.01 0.00 0.00

0.03 0.00 0.00

0.43 0.00 0.00

4.83 4.75 4.74

5.97 5.91 6.06

4.82 4.68 5.07

DNT–V C10–DNT–VW C10–DNT–VV

0.75 0.00 1.06

0.22 0.00 0.24

1.20 0.00 1.27

4.92 4.88 4.90

6.39 6.09 6.75

5.12 5.02 5.27

Displacements between the centres of masses. between the centres of masses.

b

Distances

conditions and phase-transition temperatures, which are important factors in achieving thermally durable organic electronic devices. Thermogravimetric analysis guaranteed their thermal stability. Heating the samples up to 500 °C, no thermal decomposition was observed for any material, which completely evaporated after melting under nitrogen (see Figure S6). Phase-transition temperatures were also measured by differential scanning calorimetry. As depicted in Figure S2, C10–DNF–VW exhibits phase-transition from the crystal-crystal phase transition until 190 °C. In the range from 190 °C to 210 °C, C10–DNF–VW forms Sc liquid crystalline phase. On the other hand, C10–DNF–VV does from crystal to isotropic liquid at 176°C without liquid crystal phase. The solubility of organic semiconductors is crucial to the solution process. Thus, we tested the solubility of these materials at room temperature. C10–DNF–VV exhibited a moderate solubility (1.58 wt% in chloroform and 1.35 wt% in toluene), while unsubstituted DNF–V and C10–DNF–VW dissolved only at c.a. 0.01 wt% under the same condition. At an elevated temperature of 60 °C, the solubility of C10–DNF–VW reached 0.165 wt% in toluene. To unambiguously determine the molecular and packing structures, and to investigate the charge transporting ability, singlecrystal structural analyses of C10–DNF–VW and C10–DNF–VV were carried out. Both of their single crystals were obtained as very

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Journal Name thin platelets and successfully analysed by X-ray diffraction measurements. It is noted that C10–DNF–VW assumes a slightly bent structure with a dihedral angle of 5.91° between two naphthalene units in crystal; this conflicts with the calculated results, in which the optimized geometries were completely planar in structure. This trend is the same as DNF–V.10 Both DNF–V itself and alkylated DNF–V derivatives stack into a herringbone packing manner. In this manner, each molecule interacts with adjacent molecules with either slight or no displacement, while DNT–V and C10–DNT–VV form the assembled structure with non-negligible molecular displacement due to the size of the elements. These results indicate that the oxygen-bridged V-shaped π-core is less susceptible to molecular displacement in an aggregated structure than its sulphur-bridge counterparts. This means that all DNF–V derivatives synthesized in this study exhibit high FET performance, regardless of the position of their substituents.

Finally, we evaluated the intrinsic charge carrier mobility of C10– DNF–VW and C10–DNF–VV in the form of single-crystalline films, which were prepared by solution-crystallizing ‘edge-casting’ methods originally developed by our group.8 Such single-crystal thin-film transistor (TFT) devices are advantageous in that they are capable of exhibiting the intrinsic performances of the materials without parasitic effects from grain boundaries. Indeed, the crystalline films of C10–DNF–VW and C10–DNF–VV were successfully prepared. Thus, a droplet of a 0.1 wt% solution of C10– DNF–VW in o-DCB was placed at the edge of a liquid-sustaining piece on a SiO2 substrate treated with βphenylethyltrimethoxylsilaneβ -PTS). Along with the direction of the solvent-evaporation, a single crystalline domain, which formed on top of the solution’s surface, softly landed on the substrate. The uniformity and molecular orientation of the obtained crystalline thin films were guaranteed by X-ray diffraction (XRD) and AFM image measurements. The out-of-plane XRD data of C10–DNF–VW revealed that that b axis was orientated perpendicular to the substrate, and the a-c plane, the conduction plane, was parallel to the substrate (see Figure S13). The AFM image shows that a step corresponds to the molecular height of C10–DNF–VW, indicating that a molecularly flat area with a micrometre size covers the substrate (see Figure S15). Using the prepared single crystalline film, TFT was fabricated by evaporating F4-TCNQ and the Au electrodes through a patterned shadow mask to construct the bottomgate-top-contact architecture. The use of an electron-accepting F4TCNQ layer helps hole injection to the organic semiconductor active channels with reduced contact resistance at the Au electrodes.12 The measurement was performed in the ambient condition. The TFT performances of C10–DNF–VW and C10–DNF–VV were high with hole mobilities of up to 1.1 and 1.3 cm2/Vs, respectively (see Figure S10–12). Note that despite the position of alkyl chains on the DNF– V π-core, C10–DNF–VW and C10–DNF–VV exhibited the same order of hole mobility. Optimising device configuration could further improve transfer characteristics from the aspect of the threshold voltage. In conclusion, we synthesized alkylated DNF–V derivatives by the facile and efficient synthetic method that we recently developed. By means of the solution crystallised method, C10–DNF– VW and C10–DNF–VV crystallised thin film exhibited high hole mobility over 1 cm2/Vs, which was different from the DNT–V sulphur analogue cases. Besides, from the results of the photophysical property measurements, DNF–Vs showed a deep-blue emission with high quantum efficiency. Thus, DNF–V derivatives are promising candidates for the development of blue-light emitting

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COMMUNICATION DOI: 10.1039/C3CC47577H organic semiconducting devices for OLET devices. Applications using such materials are currently ongoing in our laboratory. T. O. thanks JSPS Grant-in-Aid for Scientific Research (B) (No.25288091). C. M. also thanks JSPS Grant-in-Aid for Young Scientists (B) (No. 25810118). This work was financially supported by JNC Petrochemical Corp. and JNC Corp., JAPAN. The authors also thank the Comprehensive Analysis Center (CAC), ISIR, Osaka University for measurement of elemental analysis.

Notes and references a

The Institute of Scientific and Industrial Research (ISIR), Osaka Univ., 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. b Department of Advanced Materials Science, School of Frontier Sciences, The Univ. of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 2778561, Japan. c JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan. d Faculty of Chemistry, Materials and Bioengineering, Kansai Univ., 3-335 Yamate-cho, Suita, Osaka 564-8680, Japan. e JNC Petrochemical Corp., 5-1 Goikaigan, Ichihara, Chiba 290-8551, Japan. f Rigaku Corp., 3-9-12 Matsubara-cho, Akishima, Tokyo 196-8666, Japan.

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Furan fused V-shaped organic semiconducting materials with high emission and high mobility.

We report a facile synthetic protocol for preparation of dinaphtho[2,3-b:2',3'-d]furan (DNF-V) derivatives. DNF-V derivatives showed high emissive beh...
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