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Tuning the charge transport properties of dicyanodistyrylbenzene derivatives by the number of fluorine substituents
Synthetic Metals 216 (2016) 51–58
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Tuning the charge transport properties of dicyanodistyrylbenzene derivatives by the number of fluorine substituents Hyeong-Ju Kim, Jin Hong Kim, Jangwon Seo, Jaehun Jung, Dong Ryeol Whang, Soo Young Park* Center for Supramolecular Optoelectronic Materials, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 August 2015 Received in revised form 29 December 2015 Accepted 31 December 2015 Available online 15 January 2016
This work describes controlling charge transport properties of organic field-effect transistors (OFETs) with dicyanodistyrylbenzene-based organic semiconductors. Four fluorinated dicyanodistyrylbenzene derivatives (hTF1, hTF2, hTF3, and hTF5) with different degree of fluorination at the phenyl peripheries were synthesized and subjected to OFET devices. It was shown that the fluorination distinctly alters the polarity of charge transfer characteristics from p-type (hTF1), ambipolar (hTF2, hTF5), to n-type (hTF3). Systematic studies on photophysical, electrochemical, structural, and electronic properties revealed the fine tuning of the charge transport properties by the degree of fluorination. While p-type mobility of hTF1 was smaller than 0.01 cm2V1 s1, n-type mobility of hTF3 was as large as 0.7 cm2 V1 s1. On the other hand, well-balanced ambipolar mobilities were achieved for both hTF2 and hTF5 (hole and electron mobilities were virtually equal to be 0.07 cm2 V1 s1). ã 2016 Elsevier B.V. All rights reserved.
Keywords: Ambipolar semiconductor Dicyanodistyrylbenzene Organic field-effect transistor Fluorination Mobility
1. Introduction Organic field-effect transistors (OFETs) have attracted increasing interest due to their advantages of lightness, flexibility, facile processing, and low costs [1–7]. However, as has been reported, most of the organic semiconductors have shown unipolar transport characteristics, which are only suitable for application to either hole-transporting (p-type) or electron-transporting (ntype) OFET devices [8,9]. Good performing ambipolar organic semiconductors are relatively difficult to find due to their highly sophisticated requirements of charge transport properties tuning for viable ambipolarity. Recently, a few ambipolar small molecules, especially partially fluorinated acene-based molecules, have been reported suggesting that fluorination is a promising approach for attaining high performance ambipolar OFETs [10–13]. Nevertheless, it was also noted that these fluorinated ambipolar small molecules are not practical for fine tuning of their charge transport properties, since controlling the number of fluorine substituents into acene-based molecules is synthetically challenging and limited. As an alternative fluorinated organic semiconducting molecules, herein we designed and synthesized a novel class of
* Corresponding author. E-mail address: [email protected] (S.Y. Park). http://dx.doi.org/10.1016/j.synthmet.2015.12.028 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
synthetically feasible fluorinated organic molecules comprising the highly soluble p-conjugated dicyanodistyrylbenzene (DCS) backbone instead of the insoluble acenes. Recently, we have reported a high performance n-type semiconducting DCS molecules (1 and 2, see Scheme 1) which feature tight molecular stacking and excellent electron mobility [14]. However, the CF3 unit is relatively bulkier than F atom and thus their multiple substitution to the phenyl ring to fine tune their charge transport properties is synthetically infeasible. Therefore, in this work, we have designed and synthesized new DCS derivatives by changing electron-withdrawing CF3 unit to multiple – F units to study structure–property relationships in the fluorinated DCS based OFET molecules (Scheme 1). Four new hTF series molecules (hTF1, hTF2, hTF3, and hTF5) consisting of the same DCS backbone could be easily synthesized via Suzuki or Stille coupling reactions as described in Scheme 2. 2. Experimental 2.1. General methods Commercially available chemicals were used as received. All glass wares and magnetic stirring bars were thoroughly dried in an oven (60 C). Reactions were monitored using thin layer chromatography (TLC). Commercial TLC plates (silica gel 254, Merck Co.)
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Scheme 1. Chemical structures of reference molecules (1 and 2) and newly synthesized multi-fluorinated DCS molecules (hTF1,hTF2, hTF3, and hTF5).
were developed and the spots were visualized under UV light at 254 or 365 nm. Silica gel column chromatography was performed with silica gel 60G (particle size 5–40 mm, Merck Co.). 2.2. Synthesis (see Scheme 2) 2.2.1. 2-(5-(4-fluorophenyl) thiophen-2-yl) acetonitrile (S1) 2-(5-Bromothiophen-2-yl) acetonitrile [14] (1 g, 4.95 mmol), 4fluorophenylboronic acid (0.83 g, 5.94 mmol) and tetrakis(triphenyl phosphine) palladium(0) (0.28 g, 0.247 mmol) were dissolved in the mixed solvents comprising 30 mL of toluene, 15 mL of 2 N K2CO3 aqueous solution, and 8 mL of ethanol, followed by stirring at 80 C for 24 h. After cooling to room temperature, the
mixture was poured into distilled water and the organic layer was extracted with dichloromethane (DCM) and dried with anhydrous MgSO4. The residual solvent was removed by rotary evaporation. Silica gel column chromatography (EA:n-hexane = 1:8 v/v) followed by reprecipitation with DCM and n-hexane gave a brown powder (0.40 g, yield = 37%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.51 (dd, 2H, Ar–H), 7.11–7.03 (m, 3H, Ar–H), 7.01 (d, 1H, Ar–H) 3.91 (s, 2H, Alkyl). 2.2.2. 2-(5-(3,5-difluorophenyl) thiophen-2-yl) acetonitrile (S2) Same procedures for S1 was used for the synthesis of S2, while 3,5-difluorophenylboronic acid was used instead of 4-fluorophenylboronic acid (yield = 35%). 1H NMR (300 MHz, CDCl3) d[ppm]:
Scheme 2. Synthesis of hTF series.
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7.19 (d, 2H, Ar–H), 7.10–7.04 (m, 3H, Ar–H), 6.75 (m, 1H, Ar–H), 3.93 (d, 2H, Alkyl). 2.2.3. 2-(5-(3,4,5-trifluorophenyl) thiophen-2-yl) acetonitrile (S3) Same procedures for S1 was used for the synthesis of S3, while 3,4,5-trifluorophenylboronic acid was used instead of 4-fluorophenylboronic acid (yield = 40%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.17–7.09 (m, 3H, Ar–H), 7.04 (d, 1H, Ar–H), 3.92 (d, 2H, Alkyl). 2.2.4. 2-(5-(perfluorophenyl) thiophen-2-yl) acetonitrile (S4) 2-(5-(Tributylstannyl) thiophen-2-yl) acetonitrile [15] (0.98 g, 2.37 mmol), 1-bromo-2,3,4,5,6-pentafluorobenzene (0.88 g, 3.56 mmol) and tetrakis(triphenylphosphine) palladium(0) (0.13 g, 0.119 mmol) were dissolved in 12 mL of dimethylformamide (DMF), followed by stirring at 150 C for 24 h. After cooling to room temperature, the mixture was poured into distilled water and the organic layer was extracted with DCM and dried with anhydrous MgSO4. The residual solvent was removed by rotary evaporation. Silica gel column chromatography (EA:n-hexane = 1:3 v/v) gave a brown powder (0.25 g, yield = 36%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.40 (d, 1H, Ar–H), 7.15 (m, 1H, Ar–H), 3.97 (d, 2H, Alkyl). 2.2.5. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(4fluorophenyl) thiophen-2-yl) acrylonitrile) (hTF1) Compound S1 (0.35 g, 1.61 mmol) and 2,5-bis(hexyloxy) terephthalaldehyde [14] (0.26 g, 0.0.77 mmol) were dissolved in t-BuOH (10 mL) and stirred at 50 C. Tetrabutylammonium
53
hydroxide (TBAH, 1 M solution in MeOH) (0.18 mL, 0.16 mmol) was then injected into the mixture followed by stirring for 2 h. After cooling to room temperature, the orange precipitate was collected by filtration and washed with methanol. Flash silica gel column purification in THF, followed by recrystallization with THF, gave red powder (0.33 g, yield = 72%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 2H, Ar-H), 7.91 (s, 2H, Ar-H), 7.73 (d, 2H, Vinyl), 7.70 (d, 2H, Ar-H), 7.40 (s, 4H, Ar-H), 7.17 (t, 4H, Ar-H), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.42 (m, 8H, Alkyl), 0.93 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H42F2N2O2S2: 732, found: 732.27; EA anal. calcd for C44H42F2N2O2S2: C, 72.10; H, 5.78; N, 3.82; S, 8.75; found: C, 71.78; H, 5.75; N, 3.82; S, 8.78. 2.2.6. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(3,5difluorophenyl) thiophen-2-yl) acrylonitrile) (hTF2) Same procedures for hTF1 were used for the synthesis of hTF2, while compound S2 was used instead of compound S1. (yield = 70%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 4H, Ar-H), 7.58 (d, 2H, Ar-H), 7.43 (d, 2H, Vinyl), 7.34 (d, 4H, Ar-H), 6.96 (m, 2H, ArH), 4.17 (t, 4H, Alkyl), 1.85 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.40 (m, 8H, Alkyl), 0.90 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H40F4N2O2S2: 768, found: 768.25; EA anal. calcd for C44H40F4N2O2S2: C, 68.73; H, 5.24; N, 3.64; S, 8.34; found: C, 68.80; H, 5.27; N, 3.67; S, 8.37. 2.2.7. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(3,4,5trifluorophenyl) thiophen-2-yl) acrylonitrile) (hTF3) Same procedures for hTF1 were used for the synthesis of hTF3, while compound S3 was used instead of compound S1.
Fig. 1. UV–vis absorption spectra of (a) hTF1, (b) hTF2, (c) hTF3, and (d) hTF5 in solution (blue line) and aggregated NPs (red line).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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(yield = 72%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 4H, Ar-H), 7.53 (m, 6H, Ar-H), 7.43 (d, 2H, Vinyl), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.41 (m, 8H, Alkyl), 0.91 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H38F6N2O2S2: 804, found: 804.23; EA anal. calcd for C44H38F6N2O2S2: C, 65.66; H, 4.76; N, 3.48; S, 7.97; found: C, 65.62; H, 4.95; N, 3.49; S, 7.90. 2.2.8. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5(perfluorophenyl) thiophen-2-yl) acrylonitrile) (hTF5) Same procedures for hTF1 were used for the synthesis of hTF5, while compound S4 was used instead of compound S1. (yield = 56%). 1H NMR (300 MHz, THF-d8) d[ppm]: 8.02 (s, 2H, Ar-H), 7.97 (s, 2H, Ar-H), 7.61 (d, 2H, Vinyl), 7.53 (d, 2H, Ar-H), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.40 (m, 8H, Alkyl), 0.91 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H34F10N2O2S2: 876, found: 876.19; EA anal. calcd for C44H34F10N2O2S2: C, 60.27; H, 3.91; N, 3.19; S, 7.31; found: C, 60.26; H, 4.13; N, 3.16; S, 7.22. 2.3. Characterization The UV/Visible absorption spectrum was recorded on a Shimadzu UV-1650 PC spectrometer. 1H NMR spectra were recorded on a Bruker avance 300 (300 MHz). All NMR spectra were referenced to the solvent. Mass spectra were measured using a JEOL, JMS-600W mass spectrometer. Elemental analysis was carried out using a CE Instruments EA1110 elemental analyzer. The current–voltage characteristics of the OFETs were measured using a Keithley 4200 semiconductor parameter analyzer connected to a probe station. XRD measurements were performed on X-ray Diffractometer (Bruker, Germany), operating at 3 kW. Differential scanning calorimetry (DSC) was performed on a PerkinElmer DSC7 at a heating rate of 10 C min1. Cyclic voltammetry (CV) was performed on a Princeton applied research potentiostat/galvanostat model 273A. The CV measurements were carried out using a three-electrode system consisting of hTF series/ITO/glass as a working electrode, a platinum (Pt) wire as a counter electrode, and a Ag/Ag+ electrode as a reference electrode in an acetonitrile solution of 0.1 M Bu4NPF6. The scan rate was 100 mV s1 for all measurements. Atomic force microscopy (AFM) was performed using a Multimode with a NanoScope III controller, Bruker, in the tapping mode.
2.4. Device fabrication Prior to device fabrication, SiO2/Si (300 nm thick SiO2) substrates were rinsed by sonication in acetone and isopropyl alcohol. The substrates were then exposed to UV (360 nm) for 30 min. The substrate surfaces were treated with octadecyltrichlorosilane (OTS) in the vapor phase in a vacuum oven (80 C, under 102 Torr). Treating the SiO2 surface with an OTS monolayer has previously been shown to improve the OFET performances [16,17]. The substrates were brought into a nitrogen-filled glove box. The substrates were carried into a vacuum deposition chamber, and active layers (hTF1, hTF2, hTF3, and hTF5) were deposited (30 nm thick) by thermal evaporation under a vacuum of 2 106 Torr at a deposition rate of 0.1–0.2 Å s1. Finally, top-contact gold electrodes (50 nm thick) were thermally deposited under a vacuum of 3 106 Torr at a deposition rate of 0.2–0.3 Å s1. The channel lengths (L) were 30, 40, and 50 mm, and the width (W) was 1 mm. 2.5. Calculations Density functional theory (DFT) calculations were carried out in the gas phase using the Gaussian 09 quantum-chemical package. The geometry optimizations of the ground state of the compounds were performed using the B3LYP functionals with the 6-31G** basis set. In the calculations, all hexyloxy chains were replaced with a methoxy group. 3. Results and discussion The hTF series were synthesized by palladium-catalyzed Suzuki–Miyaura coupling or Stille coupling reactions, followed by Knoevenagel reactions in good yields. Full synthetic details, 1H NMR, mass spectroscopy, and elemental analysis characterization, are described in the Experimental section. Good solubility of all the intermediates and final products rendered facile syntheses, purifications, and characterizations. The intrinsic electronic structure and the influence of intermolecular interactions on the spectra of hTF series in the solid state were investigated by UV–vis absorption spectroscopy. Fig. 1 shows the absorption spectra of hTF series in tetrahydrofuran (THF) solution (2 105 mol/L) and its nanoparticle suspension (NPs)
Fig. 2. Energy minimized structures and visualized frontier molecular orbitals of hTF series (Gaussian envelope used for the representation of MOs).
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Fig. 3. (a) Out-of-plane XRD patterns and (b) second heating DSC curves of hTF1 (black line), hTF2 (red line), hTF3 (blue line), and hTF5 (green line).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
(THF/water (2:98) mixture), respectively. The latter conveniently represents the solid state spectra of a given compound. The absorption spectra of solutions of all hTF series exhibited a similar absorption band to the reference molecules 1 and 2 (Absonset = 550 nm) [14]. This observation indicates that the optical energy band gaps (Eg) of solutions of hTF series depend little on the position and number of the terminal CF3 and —F units (Table S1). DFT calculation results support these similar absorption spectra of
hTF series in solution. To simplify the calculations, the hexyloxy chains were replaced with methoxy groups. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) of hTF series are not quite different and their optimized geometries are almost same (see Fig. 2), which indicates the solution conformations of isolated molecules are not significantly affected by the different number of terminal —F units due to their characteristically small size comparable to that of hydrogen atom.
Fig. 4. AFM topographic images of (a) hTF1, (b) hTF2, (c) hTF3, and (d) hTF5 films deposited on the OTS-treated SiO2/Si substrates held at Tsub = 50 C.
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Table 1 Hole mobilities (mh) and electron mobilities (me) of OFETs based on hTF series at different substrate deposition temperatures (Tsub). Tsub ( C)
mh,avg (mh,max) (cm2 V1 s1)
Ion/Ioff
me,avg (me,max)
Ion/Ioff
hTF1
RT 50 70
– 0.003 0.0007(0.004) 0.007 0.003 (0.012)
– 104–105 105–106
– – –
– – –
hTF2
RT 50 70
0.013 0.008 (0.019) 0.034 0.02 (0.078) 0.026 0.01 (0.042)
105 105–106 105–106
0.020 0.004 (0.024) 0.054 0.01 (0.073) 0.028 0.02 (0.073)
105 106–107 106–107
hTF3
RT 50 70
– – –
– – –
hTF5
RT 50 70
0.008 0.005 (0.017) 0.012 0.005 (0.020) 0.011 0.005 (0.020)
105 104–105 103–104
Compounds
(cm2 V1 s1)
0.20 0.09 (0.36) 0.33 0.2 (0.72) 0.11 0.1 (0.44) 0.018 0.007 (0.026) 0.035 0.01 (0.050) 0.026 0.02 (0.065)
106 106 106 105–106 105–106 105–106
All data obtained from 5 devices at RT and 10 devices at 50 and 70 C.
In the solid state, however, the absorption spectra of hTF series are different from each other. While the similar vibronic structures in the absorption bands of hTF1, hTF2, and hTF3 solids suggest that the similarly well-ordered and tight molecular stacking is made, the different broadness and maximum wavelength of the absorption bands indicate their intermolecular interactions in the solid states are slightly different from each other (Table S1). Interestingly, hTF5 molecule containing the largest number of F substituents exhibited completely different solid state spectrum from those of other hTF compounds. While the absorption spectrum of hTF5 solution was similar to those of other hTF solutions, the spectral position and shape of hTF5 solid were very different from those of other solid samples (hTF1, hTF2, and hTF3). The spectral shape of hTF5 solid is similar to that of hTF5 solution except the presence of bathochromic shift and long-wavelength shoulders. Therefore, it is assumed that the features of molecular stacking and intermolecular interaction in hTF5 solid is remarkably different from those of others, most likely due to the different charge interaction and also the steric congestion originating from the excessive number of F atoms. For more detailed analysis of hTF series in the solid states, we carried out X-ray diffraction (XRD), differential scanning calorimeter (DSC), and atomic force microscopy (AFM) measurements. As shown Fig. 3a and Fig. 4a–c, out-of-plane XRD peaks of hTF1, hTF2, and hTF3 are located at comparable d-spacing position and also the film morphologies of them similarly featured two-dimensional (2D) lamellar-type grains, supporting the similarity of molecular stacking between these hTF series. On the other hand, the results of differential scanning calorimeter (DSC) measurements showed differences in melting temperatures (Tm) of the hTF1, hTF2, and hTF3. The Tm of hTF1, hTF2, and hTF3 increased as the number of fluorine in the molecules increased, indicating that terminal —F units could fine control the intermolecular interactions in the solid states. Therefore, these results provide evidence that a difference in the position and number of the terminal —F units especially plays an important role in affecting intermolecular interactions in the solid states of hTF1, hTF2, and hTF3 molecules. In the case of the hTF5 solid state, however, different features of molecular stacking and intermolecular interaction from those of the other hTF series were observed. Out-of-plane XRD peaks of hTF5 shown in Fig. 3a qualitatively supports the relatively larger intermolecular spacing compared with other hTF series molecules. Furthermore, the AFM results showed that hTF5 thin film dominantly consisted of large 1D needle-type grains (Fig. 4d). Such morphological singularity of hTF5 obviously supports different molecular
stacking features between hTF5 and the other hTF molecules. The Tm of hTF5 shown in Fig. 3b was also found to be the lowest among those of all hTF series compounds despite its highest molecular weight, indicatingthe relatively weak intermolecular interactions for hTF5 solid as we expected from the optical property analysis. To better understand the role of —F units in the hTF series, the HOMO energy levels of solid-state hTF series were measured by cyclic voltammetry (CV) (see Fig. S1) and LUMO energy levels were calculated using optical band gap of hTF series measured for the NP aggregates (shown in Fig. 1). It was found that the number of —F units in the molecules delicately modulated the energy levels (Fig. S2): HOMO/LUMO levels (eV) were 5.77/3.83, 5.91/ 3.88, 5.89/3.92, and 5.92/3.89 for hTF1, hTF2, hTF3, and hTF5, respectively. Finally, we measured the device performances of thermally evaporated thin film transistors. Based on the transfer and output characteristics shown in Figs. S3, S4, and S5, a thin film of hTF1 showed p-type OFET performance (maximum hole mobility mh,max = 0.012 cm2 V1 s1). On the other hand, hTF2 films showed well-balanced ambipolar behavior with high mobilities (maximum hole and electron mobilities, mh and me, were 0.073 and 0.078 cm2 V1 s1), while hTF3 exhibited clear and high n-type performance (me,max = 0.72 cm2 V1 s1). Especially, such well-balanced and high-performance ambipolar semiconductivity in small molecules has rarely been reported, so the results of hTF2 films are meaningful for ambipolar OFET application. The charge transport properties of these hTF series are listed in Table 1 and the maximum mobilities are summarized in Fig. 5. In hTF1, hTF2, and hTF3, the charge transport properties
Fig. 5. The maximum hole and electron mobilities of hTF series.
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Fig. 6. Typical transfer ((a), (b)) and output ((c), (d)) curves of the OFETs based on hTF5 (Tsub = 50 C) films.
were finely tuned by the number of –F units, and the field effect mobilities increased as the number of –F units increased. On the other hand, a thin film of hTF5 showed ambipolar OFET performances even though it has the largest number of F groups among hTF series (Fig. 6). This ambipolarity of hTF5 can be clearly rationalized, however, by its similar HOMO/LUMO energy levels to those of hTF2 (see SI Fig. S2). It is deduced that the increased electronic effect in hTF5 with five F substituents is counter-balanced by the reduced intermolecular stacking tendency to give the solid state electronic states comparable to those of hTF2. It should be noted however that the ambipolar mobility values of hTF5 are also quite balanced and high enough to be useful as small molecule ambipolar semiconductor. In summary, we have successfully synthesized and characterized a series of DCS-based organic semiconductors hTF1, hTF2, hTF3, and hTF5 with different number of fluorine substituents. The —F units played an important role in tuning the charge transport properties of the hTF series and enhancing their molecular interactions in solid states. While hTF1 showed a p-type unipolar mobility, hTF2 showed ambipolar mobility, and hTF3 showed ntype unipolar mobility to be consistent with the increasing number of electron withdrawing F substituents to gradually reduce the LUMO energy levels. On the other hand, hTF5 showed ambipolar mobility since the electronic effect in hTF5 with five F substituents is counter-balanced by the reduced intermolecular interactions
and molecular stacking tendency to render their LUMO level raised again to that of hTF2. It is to be noted that the ambipolarity in hTF2 and hTF5 is remarkable with virtually equal hole and electron mobilities of 0.07 cm2 V1 s1. Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean government (MSIP; No.2009-0081571). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. Wang, H. Dong, W. Hu, Y. Liu, D. Zhu, Chem. Rev. 112 (2012) 2208. Y. Wen, Y. Liu, Y. Guo, G. Yu, W. Hu, Chem. Rev. 111 (2011) 3358. J. Zaumseil, H. Sirringhaus, Chem. Rev. 107 (2007) 1296. V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, J.-L. Bre’das, Chem. Rev. 107 (2007) 926. A.R. Murphy, J.M.J. Fre’chet, Chem. Rev. 107 (2007) 1066. J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, J. Am. Chem. Soc. 135 (2013) 6724. D. Braga, G. Horowitz, Adv. Mater. 21 (2009) 1473. Q. Shuai, H.T. Black, A. Dadavand, D.F. Perepichka, J. Mater. Chem. C 2 (2014) 3972. P. Sonar, J. Chang, Z. Shi, J. Wu, J. Li, Angew. J. Mater. Chem. C 3 (2015) 2080. K. Liu, C.-L. Song, Y.-C. Zhou, X.-Y. Zhou, X.-J. Pan, L.-Y. Cao, C. Zhang, Y. Liu, X. Gong, H.-L. Zhang, J. Mater. Chem. C 3 (2015) 4188. M.L. Tang, A.D. Reichardt, P. Wei, Z. Bao, J. Am. Chem. Soc. 131 (2009) 5264.
58
H.-J. Kim et al. / Synthetic Metals 216 (2016) 51–58
[12] M.L. Tang, A.D. Reichardt, N. Miyaki, R.M. Stoltenberg, Z. Bao, J. Am. Chem. Soc. 130 (2008) 6064. [13] Y.-Y. Liu, C.-L. Song, W.-J. Zeng, K.-G. Zhou, Z.-F. Shi, C.-B. Ma, F. Yang, H.-L. Zhang, X. Gong, J. Am. Chem. Soc. 132 (2010) 16349. [14] S.W. Yun, J.H. Kim, S. Shin, H. Yang, B.-K. An, L. Yang, S.Y. Park, Adv. Mater. 24 (2012) 911.
[15] O.K. Kwon, J.-H. Park, S.K. Park, S.Y. Park, Adv. Energy Mater. 5 (2015) 1400929. [16] C.D. Sheraw, J.A. Nichols, D.J. Gundlach, J.R. Huang, C.C. Kuo, H. Klauk, T.N. Jackson, M.G. Kane, J. Campi, F.P. Cuomo, B.K. Greening, Tech. Dig. - Int. Electron Devices Meet. (2000) 619. [17] Y.Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Electron Device Lett. 18 (1997) 606.
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Tuning the charge transport properties of dicyanodistyrylbenzene derivatives by the number of fluorine substituents Hyeong-Ju Kim, Jin Hong Kim, Jangwon Seo, Jaehun Jung, Dong Ryeol Whang, Soo Young Park* Center for Supramolecular Optoelectronic Materials, Department of Materials Science and Engineering, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 151-744, South Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 August 2015 Received in revised form 29 December 2015 Accepted 31 December 2015 Available online 15 January 2016
This work describes controlling charge transport properties of organic field-effect transistors (OFETs) with dicyanodistyrylbenzene-based organic semiconductors. Four fluorinated dicyanodistyrylbenzene derivatives (hTF1, hTF2, hTF3, and hTF5) with different degree of fluorination at the phenyl peripheries were synthesized and subjected to OFET devices. It was shown that the fluorination distinctly alters the polarity of charge transfer characteristics from p-type (hTF1), ambipolar (hTF2, hTF5), to n-type (hTF3). Systematic studies on photophysical, electrochemical, structural, and electronic properties revealed the fine tuning of the charge transport properties by the degree of fluorination. While p-type mobility of hTF1 was smaller than 0.01 cm2V1 s1, n-type mobility of hTF3 was as large as 0.7 cm2 V1 s1. On the other hand, well-balanced ambipolar mobilities were achieved for both hTF2 and hTF5 (hole and electron mobilities were virtually equal to be 0.07 cm2 V1 s1). ã 2016 Elsevier B.V. All rights reserved.
Keywords: Ambipolar semiconductor Dicyanodistyrylbenzene Organic field-effect transistor Fluorination Mobility
1. Introduction Organic field-effect transistors (OFETs) have attracted increasing interest due to their advantages of lightness, flexibility, facile processing, and low costs [1–7]. However, as has been reported, most of the organic semiconductors have shown unipolar transport characteristics, which are only suitable for application to either hole-transporting (p-type) or electron-transporting (ntype) OFET devices [8,9]. Good performing ambipolar organic semiconductors are relatively difficult to find due to their highly sophisticated requirements of charge transport properties tuning for viable ambipolarity. Recently, a few ambipolar small molecules, especially partially fluorinated acene-based molecules, have been reported suggesting that fluorination is a promising approach for attaining high performance ambipolar OFETs [10–13]. Nevertheless, it was also noted that these fluorinated ambipolar small molecules are not practical for fine tuning of their charge transport properties, since controlling the number of fluorine substituents into acene-based molecules is synthetically challenging and limited. As an alternative fluorinated organic semiconducting molecules, herein we designed and synthesized a novel class of
* Corresponding author. E-mail address: [email protected] (S.Y. Park). http://dx.doi.org/10.1016/j.synthmet.2015.12.028 0379-6779/ ã 2016 Elsevier B.V. All rights reserved.
synthetically feasible fluorinated organic molecules comprising the highly soluble p-conjugated dicyanodistyrylbenzene (DCS) backbone instead of the insoluble acenes. Recently, we have reported a high performance n-type semiconducting DCS molecules (1 and 2, see Scheme 1) which feature tight molecular stacking and excellent electron mobility [14]. However, the CF3 unit is relatively bulkier than F atom and thus their multiple substitution to the phenyl ring to fine tune their charge transport properties is synthetically infeasible. Therefore, in this work, we have designed and synthesized new DCS derivatives by changing electron-withdrawing CF3 unit to multiple – F units to study structure–property relationships in the fluorinated DCS based OFET molecules (Scheme 1). Four new hTF series molecules (hTF1, hTF2, hTF3, and hTF5) consisting of the same DCS backbone could be easily synthesized via Suzuki or Stille coupling reactions as described in Scheme 2. 2. Experimental 2.1. General methods Commercially available chemicals were used as received. All glass wares and magnetic stirring bars were thoroughly dried in an oven (60 C). Reactions were monitored using thin layer chromatography (TLC). Commercial TLC plates (silica gel 254, Merck Co.)
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Scheme 1. Chemical structures of reference molecules (1 and 2) and newly synthesized multi-fluorinated DCS molecules (hTF1,hTF2, hTF3, and hTF5).
were developed and the spots were visualized under UV light at 254 or 365 nm. Silica gel column chromatography was performed with silica gel 60G (particle size 5–40 mm, Merck Co.). 2.2. Synthesis (see Scheme 2) 2.2.1. 2-(5-(4-fluorophenyl) thiophen-2-yl) acetonitrile (S1) 2-(5-Bromothiophen-2-yl) acetonitrile [14] (1 g, 4.95 mmol), 4fluorophenylboronic acid (0.83 g, 5.94 mmol) and tetrakis(triphenyl phosphine) palladium(0) (0.28 g, 0.247 mmol) were dissolved in the mixed solvents comprising 30 mL of toluene, 15 mL of 2 N K2CO3 aqueous solution, and 8 mL of ethanol, followed by stirring at 80 C for 24 h. After cooling to room temperature, the
mixture was poured into distilled water and the organic layer was extracted with dichloromethane (DCM) and dried with anhydrous MgSO4. The residual solvent was removed by rotary evaporation. Silica gel column chromatography (EA:n-hexane = 1:8 v/v) followed by reprecipitation with DCM and n-hexane gave a brown powder (0.40 g, yield = 37%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.51 (dd, 2H, Ar–H), 7.11–7.03 (m, 3H, Ar–H), 7.01 (d, 1H, Ar–H) 3.91 (s, 2H, Alkyl). 2.2.2. 2-(5-(3,5-difluorophenyl) thiophen-2-yl) acetonitrile (S2) Same procedures for S1 was used for the synthesis of S2, while 3,5-difluorophenylboronic acid was used instead of 4-fluorophenylboronic acid (yield = 35%). 1H NMR (300 MHz, CDCl3) d[ppm]:
Scheme 2. Synthesis of hTF series.
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7.19 (d, 2H, Ar–H), 7.10–7.04 (m, 3H, Ar–H), 6.75 (m, 1H, Ar–H), 3.93 (d, 2H, Alkyl). 2.2.3. 2-(5-(3,4,5-trifluorophenyl) thiophen-2-yl) acetonitrile (S3) Same procedures for S1 was used for the synthesis of S3, while 3,4,5-trifluorophenylboronic acid was used instead of 4-fluorophenylboronic acid (yield = 40%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.17–7.09 (m, 3H, Ar–H), 7.04 (d, 1H, Ar–H), 3.92 (d, 2H, Alkyl). 2.2.4. 2-(5-(perfluorophenyl) thiophen-2-yl) acetonitrile (S4) 2-(5-(Tributylstannyl) thiophen-2-yl) acetonitrile [15] (0.98 g, 2.37 mmol), 1-bromo-2,3,4,5,6-pentafluorobenzene (0.88 g, 3.56 mmol) and tetrakis(triphenylphosphine) palladium(0) (0.13 g, 0.119 mmol) were dissolved in 12 mL of dimethylformamide (DMF), followed by stirring at 150 C for 24 h. After cooling to room temperature, the mixture was poured into distilled water and the organic layer was extracted with DCM and dried with anhydrous MgSO4. The residual solvent was removed by rotary evaporation. Silica gel column chromatography (EA:n-hexane = 1:3 v/v) gave a brown powder (0.25 g, yield = 36%). 1H NMR (300 MHz, CDCl3) d[ppm]: 7.40 (d, 1H, Ar–H), 7.15 (m, 1H, Ar–H), 3.97 (d, 2H, Alkyl). 2.2.5. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(4fluorophenyl) thiophen-2-yl) acrylonitrile) (hTF1) Compound S1 (0.35 g, 1.61 mmol) and 2,5-bis(hexyloxy) terephthalaldehyde [14] (0.26 g, 0.0.77 mmol) were dissolved in t-BuOH (10 mL) and stirred at 50 C. Tetrabutylammonium
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hydroxide (TBAH, 1 M solution in MeOH) (0.18 mL, 0.16 mmol) was then injected into the mixture followed by stirring for 2 h. After cooling to room temperature, the orange precipitate was collected by filtration and washed with methanol. Flash silica gel column purification in THF, followed by recrystallization with THF, gave red powder (0.33 g, yield = 72%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 2H, Ar-H), 7.91 (s, 2H, Ar-H), 7.73 (d, 2H, Vinyl), 7.70 (d, 2H, Ar-H), 7.40 (s, 4H, Ar-H), 7.17 (t, 4H, Ar-H), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.42 (m, 8H, Alkyl), 0.93 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H42F2N2O2S2: 732, found: 732.27; EA anal. calcd for C44H42F2N2O2S2: C, 72.10; H, 5.78; N, 3.82; S, 8.75; found: C, 71.78; H, 5.75; N, 3.82; S, 8.78. 2.2.6. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(3,5difluorophenyl) thiophen-2-yl) acrylonitrile) (hTF2) Same procedures for hTF1 were used for the synthesis of hTF2, while compound S2 was used instead of compound S1. (yield = 70%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 4H, Ar-H), 7.58 (d, 2H, Ar-H), 7.43 (d, 2H, Vinyl), 7.34 (d, 4H, Ar-H), 6.96 (m, 2H, ArH), 4.17 (t, 4H, Alkyl), 1.85 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.40 (m, 8H, Alkyl), 0.90 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H40F4N2O2S2: 768, found: 768.25; EA anal. calcd for C44H40F4N2O2S2: C, 68.73; H, 5.24; N, 3.64; S, 8.34; found: C, 68.80; H, 5.27; N, 3.67; S, 8.37. 2.2.7. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5-(3,4,5trifluorophenyl) thiophen-2-yl) acrylonitrile) (hTF3) Same procedures for hTF1 were used for the synthesis of hTF3, while compound S3 was used instead of compound S1.
Fig. 1. UV–vis absorption spectra of (a) hTF1, (b) hTF2, (c) hTF3, and (d) hTF5 in solution (blue line) and aggregated NPs (red line).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
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(yield = 72%). 1H NMR (300 MHz, THF-d8) d[ppm]: 7.96 (s, 4H, Ar-H), 7.53 (m, 6H, Ar-H), 7.43 (d, 2H, Vinyl), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.41 (m, 8H, Alkyl), 0.91 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H38F6N2O2S2: 804, found: 804.23; EA anal. calcd for C44H38F6N2O2S2: C, 65.66; H, 4.76; N, 3.48; S, 7.97; found: C, 65.62; H, 4.95; N, 3.49; S, 7.90. 2.2.8. (2E,20 E)-3,30 -(2,5-bis(hexyloxy)-1,4-phenylene) bis(2-(5(perfluorophenyl) thiophen-2-yl) acrylonitrile) (hTF5) Same procedures for hTF1 were used for the synthesis of hTF5, while compound S4 was used instead of compound S1. (yield = 56%). 1H NMR (300 MHz, THF-d8) d[ppm]: 8.02 (s, 2H, Ar-H), 7.97 (s, 2H, Ar-H), 7.61 (d, 2H, Vinyl), 7.53 (d, 2H, Ar-H), 4.17 (t, 4H, Alkyl), 1.90 (m, 4H, Alkyl), 1.58 (m, 4H, Alkyl), 1.40 (m, 8H, Alkyl), 0.91 (t, 6H, Alkyl); m/z (FAB, positive): calcd for C44H34F10N2O2S2: 876, found: 876.19; EA anal. calcd for C44H34F10N2O2S2: C, 60.27; H, 3.91; N, 3.19; S, 7.31; found: C, 60.26; H, 4.13; N, 3.16; S, 7.22. 2.3. Characterization The UV/Visible absorption spectrum was recorded on a Shimadzu UV-1650 PC spectrometer. 1H NMR spectra were recorded on a Bruker avance 300 (300 MHz). All NMR spectra were referenced to the solvent. Mass spectra were measured using a JEOL, JMS-600W mass spectrometer. Elemental analysis was carried out using a CE Instruments EA1110 elemental analyzer. The current–voltage characteristics of the OFETs were measured using a Keithley 4200 semiconductor parameter analyzer connected to a probe station. XRD measurements were performed on X-ray Diffractometer (Bruker, Germany), operating at 3 kW. Differential scanning calorimetry (DSC) was performed on a PerkinElmer DSC7 at a heating rate of 10 C min1. Cyclic voltammetry (CV) was performed on a Princeton applied research potentiostat/galvanostat model 273A. The CV measurements were carried out using a three-electrode system consisting of hTF series/ITO/glass as a working electrode, a platinum (Pt) wire as a counter electrode, and a Ag/Ag+ electrode as a reference electrode in an acetonitrile solution of 0.1 M Bu4NPF6. The scan rate was 100 mV s1 for all measurements. Atomic force microscopy (AFM) was performed using a Multimode with a NanoScope III controller, Bruker, in the tapping mode.
2.4. Device fabrication Prior to device fabrication, SiO2/Si (300 nm thick SiO2) substrates were rinsed by sonication in acetone and isopropyl alcohol. The substrates were then exposed to UV (360 nm) for 30 min. The substrate surfaces were treated with octadecyltrichlorosilane (OTS) in the vapor phase in a vacuum oven (80 C, under 102 Torr). Treating the SiO2 surface with an OTS monolayer has previously been shown to improve the OFET performances [16,17]. The substrates were brought into a nitrogen-filled glove box. The substrates were carried into a vacuum deposition chamber, and active layers (hTF1, hTF2, hTF3, and hTF5) were deposited (30 nm thick) by thermal evaporation under a vacuum of 2 106 Torr at a deposition rate of 0.1–0.2 Å s1. Finally, top-contact gold electrodes (50 nm thick) were thermally deposited under a vacuum of 3 106 Torr at a deposition rate of 0.2–0.3 Å s1. The channel lengths (L) were 30, 40, and 50 mm, and the width (W) was 1 mm. 2.5. Calculations Density functional theory (DFT) calculations were carried out in the gas phase using the Gaussian 09 quantum-chemical package. The geometry optimizations of the ground state of the compounds were performed using the B3LYP functionals with the 6-31G** basis set. In the calculations, all hexyloxy chains were replaced with a methoxy group. 3. Results and discussion The hTF series were synthesized by palladium-catalyzed Suzuki–Miyaura coupling or Stille coupling reactions, followed by Knoevenagel reactions in good yields. Full synthetic details, 1H NMR, mass spectroscopy, and elemental analysis characterization, are described in the Experimental section. Good solubility of all the intermediates and final products rendered facile syntheses, purifications, and characterizations. The intrinsic electronic structure and the influence of intermolecular interactions on the spectra of hTF series in the solid state were investigated by UV–vis absorption spectroscopy. Fig. 1 shows the absorption spectra of hTF series in tetrahydrofuran (THF) solution (2 105 mol/L) and its nanoparticle suspension (NPs)
Fig. 2. Energy minimized structures and visualized frontier molecular orbitals of hTF series (Gaussian envelope used for the representation of MOs).
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Fig. 3. (a) Out-of-plane XRD patterns and (b) second heating DSC curves of hTF1 (black line), hTF2 (red line), hTF3 (blue line), and hTF5 (green line).(For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
(THF/water (2:98) mixture), respectively. The latter conveniently represents the solid state spectra of a given compound. The absorption spectra of solutions of all hTF series exhibited a similar absorption band to the reference molecules 1 and 2 (Absonset = 550 nm) [14]. This observation indicates that the optical energy band gaps (Eg) of solutions of hTF series depend little on the position and number of the terminal CF3 and —F units (Table S1). DFT calculation results support these similar absorption spectra of
hTF series in solution. To simplify the calculations, the hexyloxy chains were replaced with methoxy groups. The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) of hTF series are not quite different and their optimized geometries are almost same (see Fig. 2), which indicates the solution conformations of isolated molecules are not significantly affected by the different number of terminal —F units due to their characteristically small size comparable to that of hydrogen atom.
Fig. 4. AFM topographic images of (a) hTF1, (b) hTF2, (c) hTF3, and (d) hTF5 films deposited on the OTS-treated SiO2/Si substrates held at Tsub = 50 C.
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Table 1 Hole mobilities (mh) and electron mobilities (me) of OFETs based on hTF series at different substrate deposition temperatures (Tsub). Tsub ( C)
mh,avg (mh,max) (cm2 V1 s1)
Ion/Ioff
me,avg (me,max)
Ion/Ioff
hTF1
RT 50 70
– 0.003 0.0007(0.004) 0.007 0.003 (0.012)
– 104–105 105–106
– – –
– – –
hTF2
RT 50 70
0.013 0.008 (0.019) 0.034 0.02 (0.078) 0.026 0.01 (0.042)
105 105–106 105–106
0.020 0.004 (0.024) 0.054 0.01 (0.073) 0.028 0.02 (0.073)
105 106–107 106–107
hTF3
RT 50 70
– – –
– – –
hTF5
RT 50 70
0.008 0.005 (0.017) 0.012 0.005 (0.020) 0.011 0.005 (0.020)
105 104–105 103–104
Compounds
(cm2 V1 s1)
0.20 0.09 (0.36) 0.33 0.2 (0.72) 0.11 0.1 (0.44) 0.018 0.007 (0.026) 0.035 0.01 (0.050) 0.026 0.02 (0.065)
106 106 106 105–106 105–106 105–106
All data obtained from 5 devices at RT and 10 devices at 50 and 70 C.
In the solid state, however, the absorption spectra of hTF series are different from each other. While the similar vibronic structures in the absorption bands of hTF1, hTF2, and hTF3 solids suggest that the similarly well-ordered and tight molecular stacking is made, the different broadness and maximum wavelength of the absorption bands indicate their intermolecular interactions in the solid states are slightly different from each other (Table S1). Interestingly, hTF5 molecule containing the largest number of F substituents exhibited completely different solid state spectrum from those of other hTF compounds. While the absorption spectrum of hTF5 solution was similar to those of other hTF solutions, the spectral position and shape of hTF5 solid were very different from those of other solid samples (hTF1, hTF2, and hTF3). The spectral shape of hTF5 solid is similar to that of hTF5 solution except the presence of bathochromic shift and long-wavelength shoulders. Therefore, it is assumed that the features of molecular stacking and intermolecular interaction in hTF5 solid is remarkably different from those of others, most likely due to the different charge interaction and also the steric congestion originating from the excessive number of F atoms. For more detailed analysis of hTF series in the solid states, we carried out X-ray diffraction (XRD), differential scanning calorimeter (DSC), and atomic force microscopy (AFM) measurements. As shown Fig. 3a and Fig. 4a–c, out-of-plane XRD peaks of hTF1, hTF2, and hTF3 are located at comparable d-spacing position and also the film morphologies of them similarly featured two-dimensional (2D) lamellar-type grains, supporting the similarity of molecular stacking between these hTF series. On the other hand, the results of differential scanning calorimeter (DSC) measurements showed differences in melting temperatures (Tm) of the hTF1, hTF2, and hTF3. The Tm of hTF1, hTF2, and hTF3 increased as the number of fluorine in the molecules increased, indicating that terminal —F units could fine control the intermolecular interactions in the solid states. Therefore, these results provide evidence that a difference in the position and number of the terminal —F units especially plays an important role in affecting intermolecular interactions in the solid states of hTF1, hTF2, and hTF3 molecules. In the case of the hTF5 solid state, however, different features of molecular stacking and intermolecular interaction from those of the other hTF series were observed. Out-of-plane XRD peaks of hTF5 shown in Fig. 3a qualitatively supports the relatively larger intermolecular spacing compared with other hTF series molecules. Furthermore, the AFM results showed that hTF5 thin film dominantly consisted of large 1D needle-type grains (Fig. 4d). Such morphological singularity of hTF5 obviously supports different molecular
stacking features between hTF5 and the other hTF molecules. The Tm of hTF5 shown in Fig. 3b was also found to be the lowest among those of all hTF series compounds despite its highest molecular weight, indicatingthe relatively weak intermolecular interactions for hTF5 solid as we expected from the optical property analysis. To better understand the role of —F units in the hTF series, the HOMO energy levels of solid-state hTF series were measured by cyclic voltammetry (CV) (see Fig. S1) and LUMO energy levels were calculated using optical band gap of hTF series measured for the NP aggregates (shown in Fig. 1). It was found that the number of —F units in the molecules delicately modulated the energy levels (Fig. S2): HOMO/LUMO levels (eV) were 5.77/3.83, 5.91/ 3.88, 5.89/3.92, and 5.92/3.89 for hTF1, hTF2, hTF3, and hTF5, respectively. Finally, we measured the device performances of thermally evaporated thin film transistors. Based on the transfer and output characteristics shown in Figs. S3, S4, and S5, a thin film of hTF1 showed p-type OFET performance (maximum hole mobility mh,max = 0.012 cm2 V1 s1). On the other hand, hTF2 films showed well-balanced ambipolar behavior with high mobilities (maximum hole and electron mobilities, mh and me, were 0.073 and 0.078 cm2 V1 s1), while hTF3 exhibited clear and high n-type performance (me,max = 0.72 cm2 V1 s1). Especially, such well-balanced and high-performance ambipolar semiconductivity in small molecules has rarely been reported, so the results of hTF2 films are meaningful for ambipolar OFET application. The charge transport properties of these hTF series are listed in Table 1 and the maximum mobilities are summarized in Fig. 5. In hTF1, hTF2, and hTF3, the charge transport properties
Fig. 5. The maximum hole and electron mobilities of hTF series.
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Fig. 6. Typical transfer ((a), (b)) and output ((c), (d)) curves of the OFETs based on hTF5 (Tsub = 50 C) films.
were finely tuned by the number of –F units, and the field effect mobilities increased as the number of –F units increased. On the other hand, a thin film of hTF5 showed ambipolar OFET performances even though it has the largest number of F groups among hTF series (Fig. 6). This ambipolarity of hTF5 can be clearly rationalized, however, by its similar HOMO/LUMO energy levels to those of hTF2 (see SI Fig. S2). It is deduced that the increased electronic effect in hTF5 with five F substituents is counter-balanced by the reduced intermolecular stacking tendency to give the solid state electronic states comparable to those of hTF2. It should be noted however that the ambipolar mobility values of hTF5 are also quite balanced and high enough to be useful as small molecule ambipolar semiconductor. In summary, we have successfully synthesized and characterized a series of DCS-based organic semiconductors hTF1, hTF2, hTF3, and hTF5 with different number of fluorine substituents. The —F units played an important role in tuning the charge transport properties of the hTF series and enhancing their molecular interactions in solid states. While hTF1 showed a p-type unipolar mobility, hTF2 showed ambipolar mobility, and hTF3 showed ntype unipolar mobility to be consistent with the increasing number of electron withdrawing F substituents to gradually reduce the LUMO energy levels. On the other hand, hTF5 showed ambipolar mobility since the electronic effect in hTF5 with five F substituents is counter-balanced by the reduced intermolecular interactions
and molecular stacking tendency to render their LUMO level raised again to that of hTF2. It is to be noted that the ambipolarity in hTF2 and hTF5 is remarkable with virtually equal hole and electron mobilities of 0.07 cm2 V1 s1. Acknowledgement This research was supported by the National Research Foundation of Korea (NRF) through a grant funded by the Korean government (MSIP; No.2009-0081571). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
C. Wang, H. Dong, W. Hu, Y. Liu, D. Zhu, Chem. Rev. 112 (2012) 2208. Y. Wen, Y. Liu, Y. Guo, G. Yu, W. Hu, Chem. Rev. 111 (2011) 3358. J. Zaumseil, H. Sirringhaus, Chem. Rev. 107 (2007) 1296. V. Coropceanu, J. Cornil, D.A. da Silva Filho, Y. Olivier, R. Silbey, J.-L. Bre’das, Chem. Rev. 107 (2007) 926. A.R. Murphy, J.M.J. Fre’chet, Chem. Rev. 107 (2007) 1066. J. Mei, Y. Diao, A.L. Appleton, L. Fang, Z. Bao, J. Am. Chem. Soc. 135 (2013) 6724. D. Braga, G. Horowitz, Adv. Mater. 21 (2009) 1473. Q. Shuai, H.T. Black, A. Dadavand, D.F. Perepichka, J. Mater. Chem. C 2 (2014) 3972. P. Sonar, J. Chang, Z. Shi, J. Wu, J. Li, Angew. J. Mater. Chem. C 3 (2015) 2080. K. Liu, C.-L. Song, Y.-C. Zhou, X.-Y. Zhou, X.-J. Pan, L.-Y. Cao, C. Zhang, Y. Liu, X. Gong, H.-L. Zhang, J. Mater. Chem. C 3 (2015) 4188. M.L. Tang, A.D. Reichardt, P. Wei, Z. Bao, J. Am. Chem. Soc. 131 (2009) 5264.
58
H.-J. Kim et al. / Synthetic Metals 216 (2016) 51–58
[12] M.L. Tang, A.D. Reichardt, N. Miyaki, R.M. Stoltenberg, Z. Bao, J. Am. Chem. Soc. 130 (2008) 6064. [13] Y.-Y. Liu, C.-L. Song, W.-J. Zeng, K.-G. Zhou, Z.-F. Shi, C.-B. Ma, F. Yang, H.-L. Zhang, X. Gong, J. Am. Chem. Soc. 132 (2010) 16349. [14] S.W. Yun, J.H. Kim, S. Shin, H. Yang, B.-K. An, L. Yang, S.Y. Park, Adv. Mater. 24 (2012) 911.
[15] O.K. Kwon, J.-H. Park, S.K. Park, S.Y. Park, Adv. Energy Mater. 5 (2015) 1400929. [16] C.D. Sheraw, J.A. Nichols, D.J. Gundlach, J.R. Huang, C.C. Kuo, H. Klauk, T.N. Jackson, M.G. Kane, J. Campi, F.P. Cuomo, B.K. Greening, Tech. Dig. - Int. Electron Devices Meet. (2000) 619. [17] Y.Y. Lin, D.J. Gundlach, S.F. Nelson, T.N. Jackson, IEEE Electron Device Lett. 18 (1997) 606.