Halogen Substitution Effects on the Molecular Packing and Thin Film

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Halogen Substitution Effects on the Molecular Packing and Thin Film Transistor Performances of Carbazoledioxazine Derivatives Hiroki Tatsumi, Yang Wang, Yosuke Aizawa, Masatoshi Tokita, Takehiko Mori, and Tsuyoshi Michinobu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09888 • Publication Date (Web): 08 Nov 2016 Downloaded from http://pubs.acs.org on November 13, 2016

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The Journal of Physical Chemistry

Halogen Substitution Effects on the Molecular Packing and Thin Film Transistor Performances of Carbazoledioxazine Derivatives Hiroki Tatsumi,† Yang Wang,‡ Yosuke Aizawa,‡ Masatoshi Tokita,† Takehiko Mori,† and Tsuyoshi Michinobu*,† †

Department of Materials Science and Engineering, Tokyo Institute of Technology, 2-12-1

Ookayama, Meguro-ku, Tokyo 152-8552, Japan. ‡

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1

Ookayama, Meguro-ku, Tokyo 152-8552, Japan

ABSTRACT

Solution-processable carbazoledioxazine derivatives with different halogen substituents (F, Cl, and Br) were newly synthesized by condensation and subsequent cyclization reactions. The chemical structures were confirmed by 1H NMR and IR spectroscopies as well as MALDI-TOF mass spectrometry. All three carbazoledioxazines possessed a high thermal stability with decomposition temperatures exceeding 270 oC and exhibited thermal transitions upon heating.

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The phases were characterized by their wide-angle X-ray diffraction patterns at various temperatures. In addition, the energy levels of the carbazoledioxazines were estimated from the optical absorption spectra and electrochemical redox potentials of the thin films. All three derivatives displayed more or less the same energy levels; highest occupied molecular orbitals (HOMOs) of -5.3 eV and lowest unoccupied molecular orbitals (LUMOs) of -3.5~-3.6 eV. Despite this fact, the Br derivative showed higher hole mobilities with the maximum mobility of 4.9×10−3 cm2 V−1 s−1 in the thin film transistors as compared to those of the counter F and Cl derivatives. This was attributed to the bimodal carrier pathways formed through the monoclinic molecular orientation of the Br derivative, revealed by grazing-incidence X-ray diffraction (GIXRD) measurements.

INTRODUCTION In the past decade, organic dyes and pigments have been intensively investigated as semiconductors used in organic thin film transistors (TFTs) and organic photovoltaics (OPVs). Such examples are indigo,1-10 isoindigo,11-20 diketopyrrolopyrrole (DPP),21,22 perylene,23-31 and quinacridone.32-43 One of the authors recently revealed that the halogen substitution on the indigo derivatives improves the ambipolar transistor properties due to the supramolecular halogen interactions in solid thin films.44 It is also noteworthy that most of these chromophores were successfully integrated into polymer main chains, producing high-performance semiconducting polymers.45-47 Carbazoledioxazine, also called violet 23, is a well-known violet pigment, which has been employed as a coloring material for plastics and printing inks.48-52 This pigment can usually


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be synthesized by the condensation of 3-amino-9-ethylcarbazole with p-chloranil followed by two-fold cyclization reactions. Substitution of long alkyl chains at the nitrogen atoms of the carbazole unit allowed for the detailed synthetic studies of isomer structures in terms of spectroscopic characterization and X-ray crystal structure analysis.53 Two isomer structures, i.e., linear and angular-type structures, were initially proposed, but it was shown that simple heating of the precursor selectively afforded the angular-type structure. In order to synthesize the linear structure, the methoxy-substituent must be introduced into the 2-position of the carbazole unit.54,55 The optical and electrochemical properties of these alkylated carbazoledioxazines were investigated in the 1990s.56,57 However, there have been, to the best of our knowledge, no reports about their organic electronic device applications. Thus, we became interested in the thin film transistor performances of these violet chromophores. We now describe the synthesis of three solution-processable, novel carbazoledioxazine derivatives with different halogen substituents (Br, Cl, and F). We show that different thermal transition behaviors and molecular packing orientations result in different transistor performances. The results reported here indicate the high potential of the novel π-chromophore used in organic electronic devices.

EXPERIMENTAL SECTION General Measurements Nuclear magnetic resonance (NMR) spectra were recorded using a JEOL model AL300 (300 MHz) at room temperature. Deuterated chloroform was used as the solvent. Chemical shifts of NMR were reported in ppm (parts per million) relative to the residual solvent peak at 7.26 ppm for 1H NMR spectroscopy and 77.6 ppm for 13C NMR spectroscopy. Coupling constants (J) were


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given in Hz. The resonance multiplicity was described as s (singlet), d (doublet), t (triplet), and m (multiplet). Fourier transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR4100 spectrometer in the range from 4000 to 600 cm−1. MALDI−TOF mass spectra were measured on a Shimadzu/Kratos AXIMACFR mass spectrometer equipped with a nitrogen laser (λ= 337 nm) and pulsed ion extraction, which was operated in a linear-positive ion mode at an accelerating potential of 20 kV. Tetrahydrofuran (THF) solutions containing 1 g L-1 of a sample, 20 g L-1 of dithranol, and 1 g L-1 of sodium trifluoroacetate were mixed at a ratio of 1:1:1; and then 1 µL aliquot of this mixture was deposited onto a sample target plate. UV-vis-NIR spectra were recorded on a JASCO V-670 spectrophotometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out on a Rigaku TG8120 and a Rigaku DSC8230, respectively, under nitrogen flow at the scan rate of 10 °C min−1. Electrochemistry measurements were carried out on a BAS electrochemical analyzer model 612C at 20 °C in a classical three-electrode cell. The working, reference, and auxiliary electrodes were a glassy carbon electrode, Ag/AgCl/CH3CN/(nC4H9)4NPF6, and a Pt wire, respectively. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured. It was assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.80 eV to vacuum. The HOMO and LUMO energy levels were then calculated according to the following equations: EHOMO = −(φox + φFc/Fc+ + 4.80) (eV)

(Eq. 1)

ELUMO = −(φre + φFc/Fc+ + 4.80) (eV)

(Eq. 2)

where φox is the onset oxidation potential vs Ag/AgCl, φre is the onset reduction potential vs Ag/AgCl, and φFc/Fc+ is the redox potential of ferrocene/ferrocenium vs Ag/AgCl. Fabrication and Characterization of Thin Film Transistors


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Top-contact/bottom-gate TFTs devices were fabricated using n+-Si/SiO2 substrates where n+-Si and SiO2 were used as the gate electrode and gate dielectric, respectively. The substrates were subjected to cleaning with deionized water, acetone, and 2-propanol. Then, the substrates were cleaned with O3 for 20 min using a Technovision model 208 UV-O3 cleaning system. The cleaned substrates were modified with octadecylsilanes (octadecyltrimethylsilane (OTMS)) to form a self-assembled monolayer (SAM).58 Thin films of the carbazoledioxazines were deposited on the treated substrate by spin-coating a chloroform solution (8 g L−1) in a glove box, optionally followed by thermal annealing at 70, 110, 140, 150, 160, 200 or 220 °C under nitrogen. After the thin film deposition, ~50 nm thick gold was deposited as source and drain contacts using a shadow mask. The TFT devices had a channel length (L) of 100 µm and a channel width (W) of 1 mm. The TFT performances were measured under ambient conditions using a Keithley 4200 parameter analyzer on a probe stage. The carrier mobilities, µ, were calculated from the data in the saturated regime according to the following equation: ISD = (W/2L)Ciµ(VG – VT)2

(Eq. 3)

where ISD is the drain current in the saturated regime, W and L are the semiconductor channel width and length, respectively, Ci (Ci = 13.3 nF cm-2) is the capacitance per unit area of the gate dielectric layer, and VG and VT are the gate voltage and threshold voltage, respectively. VG-VT of the devices was determined from the square root values of ISD at the saturated regime. Current on/off ratios (Ion/Ioff) were determined from the minimum current at around VGS = 0 to −40 V (Ioff) and the current at VGS = −100 V (Ion). The transfer characteristics were obtained by sweeping the VGS from 20 to −100 V. Atomic Force Microscopy (AFM) Measurements


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AFM samples were prepared by spin-coating the chloroform solutions on OTMS-treated Si/SiO2 substrates. Both pristine and thermally-treated films were examined by a Seiko Instruments SPA400 with a stiff cantilever of Seiko Instruments DF-20. X-ray Diffraction Measurements X-ray diffraction (XRD) and grazing-incidence (GI) XRD patterns were obtained using a Bulker D8 DISCOVER equipped with a Vantec-500 detector and Cu Kα radiation. The bulk samples for XRD measurements were prepared by slow evaporation of the carbazoledioxazine solutions in chloroform over 24 h under ambient conditions followed by thermal annealing at 200 °C for 30 min under vacuum. For the GIXRD measurements, thin film samples were prepared by casting the carbazoledioxazine derivatives on cleaned glass substrates. Materials and Synthetic Details All chemicals were purchased from Tokyo Chemical Industry (TCI), Kanto, and Sigma Aldrich and used as received unless otherwise noted. Synthesis of 9-(2-decyltetradecyl)-9H-carbazole (2) To a solution of carbazole (2.00 g, 12.0 mmol) in DMF (20 mL), NaH (1.50 g, 37.5 mmol) was added and stirred for 30 min. 2-Decyltetradecylbromide (5.00 g, 12.0 mmol) was added and stirred at 20 oC for 24 h. After methanol and water were added, the organic phase was extracted with CH2Cl2. Evaporation followed by column chromatography (SiO2, hexane/CH2Cl2 5:1) afforded the desired compound as transparent oil (4.65 g, 77%). 1H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.2 Hz, 6H), 1.20 (m, 40H), 2.12 (br, 1H), 4.15 (d, J = 7.8 Hz, 2H), 7.22 (t, J = 7.2 Hz, 2H), 7.38 (d, J = 8.7 Hz, 2H), 7.45 (t, J = 7.5 Hz, 2H), 8.10 ppm (d, J = 7.2 Hz, 2H). 13C


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NMR (75 MHz, CDCl3): δ = 14.30, 22.91, 23.59, 26.79, 29.50, 29.60, 29.83, 29.89, 30.13, 30.37, 32.16, 32.78, 38.10, 47.89, 109.03, 118.80, 120.35, 122.97, 125.63, 141.08 ppm. IR (neat): ν = 2924, 2853, 1599, 1485, 1464, 1377, 1229, 1153, 911, 742, 651 cm−1. MALDI-TOF MS (Calcd. for C36H57N, Mw = 503.45): m/z = 503.06 ([M+]). Synthesis of 9-(2-decyltetradecyl)-3-nitro-9H-carbazole (3) To a solution of 2 (4.65 g, 9.24 mmol) in acetic acid (50 mL), a mixture of fuming nitric acid (0.395 mL, 9.52 mmol) and acetic acid (5 mL) was added dropwise under reflux over 2 h. After cooling to 20 oC, water was added and the organic phase was extracted with CH2Cl2. Evaporation followed by column chromatography (SiO2, hexane/CH2Cl2 5:1) afforded the desired compound as yellow oil (2.50 g, 49%). 1H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 7.5 Hz, 6H), 1.20 (m, 40H), 2.11 (br, 1H), 4.19 (d, J = 7.2 Hz, 2H), 7.34 (t, J = 7.5 Hz, 1H), 7.36 (d, J = 9.0 Hz, 1H), 7.44 (d, J = 8.4 Hz, 1H), 7.56 (t, J = 7.5 Hz, 1H), 8.13 (d, J = 7.8 Hz, 1H), 8.36 (d, J = 9.0 Hz, 1H), 8.99 ppm (s, 1H).

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C NMR (75 MHz, CDCl3): δ = 14.10, 22.40, 22.67,

23.62, 23.80, 26.50, 29.29, 29.34, 29.46, 29.53, 29.55, 29.60, 29.62, 29.63, 29.82, 29.97, 31.82, 31.88, 31.90, 37.89, 38.23, 48.20, 84.98, 108.43, 109.97, 117.18, 120.64, 120.89, 121.45, 122.40, 122.75, 127.23, 140.46, 142.00, 143.93 ppm. IR (neat): ν = 2922, 2852, 1595, 1517 (νN=O), 1495, 1475, 1325 (νN=O), 1282, 1228, 1140, 1123, 1090, 1023, 811, 788, 752, 721, 635, 623 cm−1. MALDI-TOF MS (Calcd. for C36H56N2O2 Mw = 548.43): m/z = 548.47 ([M+]). Synthesis of 2,5-bis((9-(2-decyltetradecyl)-9H-carbazol-3-yl)amino)-3,6-difluorocyclohexa2,5-diene-1,4-dione (4F)


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Iron (2.35 g, 42.0 mmol) was added to water (20 mL) and aq. HCl (5 mL), and the mixture was stirred at 40 oC for 30 min under nitrogen. After a solution of 3 (2.3 g, 4.2 mmol) in ethanol (50 mL) was added, the mixture was refluxed for 3 h. After cooling to 20 oC, water was added and the organic phase was extracted with CH2Cl2. Evaporation yielded 9-(2-decyltetradecyl)-9Hcarbazol-3-amine, and this compound was subjected to the next reaction without further purification. 3-Amino-9-(2-decyltetradecyl)carbazole, p-fluoranil (0.378 g, 2.10 mmol), and sodium acetate (3.44 g, 42.0 mmol) were mixed with ethanol (30 mL), and the mixture was refluxed under nitrogen for 6 h. After cooling to 20 oC, water was added and the organic phase was extracted with CH2Cl2. Evaporation followed by column chromatography (SiO2, hexane/CH2Cl2 9:5) afforded the desired compound as a reddish-purple solid (0.469 g, 19%). 1H NMR (300 MHz, CDCl3): δ = 0.89 (t, J = 6.6 Hz, 12H), 1.22 (m, 80H), 2.13 (br, 2H), 4.14 (d, J = 6.9 Hz, 4H), 7.25 (t, J = 6.9 Hz, 2H), 7.29 (d, J = 9.3 Hz, 2H), 7.35 (d, J = 8.4 Hz, 2H), 7.40 (d, J = 8.4 Hz, 2H), 7.49 (t, J = 7.8 Hz, 2H), 7.91 (s, 2H), 7.95 (s, 2H), 8.08 ppm (d, J = 7.8 Hz, 2H). 13

C NMR (75 MHz, CDCl3): δ = 14.13, 26.56, 29.32, 29.36, 29.54, 29.58, 29.63, 29.66, 29.91,

31.87, 31.90, 31.92, 37.98, 47.90, 108.86, 109.27, 115.18, 119.10, 120.55, 121.51, 122.27, 122.80, 126.22, 128.59, 129.48, 133.86, 137.08, 139.14, 141.53 ppm. IR (neat): ν = 3230 (νN-H), 2921, 2852, 1667 (νC=O), 1572, 1498, 1464, 1431, 1407, 1352, 1324, 1310, 1195, 1152, 1076, 993, 883cm−1. MALDI-TOF MS (Calcd. for C78H114F2N4O2, Mw = 1176.89): m/z = 1178.75 ([M+]). Synthesis of 2,5-dichloro-3,6-bis((9-(2-decyltetradecyl)-9H-carbazol-3-yl)amino)cyclohexa2,5-diene-1,4-dione (4Cl)


8

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Starting from 3 (3.60 g, 6.57 mmol) 9-(2-decyltetradecyl)-9H-carbazol-3-amine was prepared according to the procedure of 4F. 3-Amino-9-(2-decyltetradecyl)carbazole, p-chloranil (0.81 g, 3.29 mmol), and sodium acetate (4.73 g, 65.7 mmol) were mixed with ethanol (30 mL), and the mixture was refluxed under nitrogen for 6 h. After cooling to 20 oC, water was added and the organic phase was extracted with CH2Cl2. Evaporation followed by column chromatography (SiO2, hexane/CH2Cl2 2:1) afforded the desired compound as a reddish-purple solid (1.73 g, 43%). 1H NMR (300 MHz, CDCl3): δ = 0.88 (t, J = 6.8 Hz, 12H), 1.21 (m, 80H), 2.12 (br, 2H), 4.19 (d, J = 7.3 Hz, 4H), 7.23-7.29 (m, 4H), 7.36 (d, J = 8.4 Hz, 2H), 7.41 (d, J = 8.4 Hz, 2H), 7.49 (t, J = 7.3 Hz, 2H), 7.88 (s, 2H), 8.07 (d, J = 7.0 Hz, 2H), 8.71 ppm (s, 2H). 13C NMR (75 MHz, CDCl3): δ = 14.16, 22.69, 26.56, 29.32, 29.36, 29.53, 29.58, 29.59, 29.65, 29.67, 29.92, 31.90, 31.92, 37.99, 41.03, 47.92, 102.43, 108.43, 109.37, 117.50, 119.23, 120.56, 122.39, 123.50, 126.25, 127.99, 139.63, 141.54, 143.09, 173.86 ppm. IR (neat): ν = 3262 (νN-H), 2923, 2853, 1652 (νC=O), 1574, 1491, 1296, 1207, 1030, 918, 829, 800, 769, 720, 695, 664, 651, 641, 628, 614, 601 cm−1. MALDI-TOF MS (Calcd. for C78H114Cl2N4O2, Mw = 1208.83): m/z = 1209.32 ([M+]). Synthesis of 2,5-dibromo-3,6-bis((9-(2-decyltetradecyl)-9H-carbazol-3-yl)amino)cyclohexa2,5-diene-1,4-dione (4Br) Starting from 3 (1.21 g, 2.21 mmol) 9-(2-decyltetradecyl)-9H-carbazol-3-amine was prepared according to the procedure of 4F. 3-Amino-9-(2-decyltetradecyl)carbazole, p-bromanil (0.468 g, 1.11 mmol), and sodium acetate (1.59 g, 19.4 mmol) were mixed with ethanol (20 mL), and the mixture was refluxed under nitrogen for 6 h. After cooling to 20 oC, water was added and the organic phase was extracted with CH2Cl2. Evaporation followed by column chromatography


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(SiO2, hexane/CH2Cl2 2:1) afforded the desired compound as a reddish-purple solid (0.480 g, 33%). 1H NMR (300 MHz, CDCl3): δ = 0.89 (t, J = 6.8 Hz, 12H), 1.23 (m, 80H), 2.14 (br, 2H), 4.19 (d, J = 7.3 Hz, 4H), 7.24-7.31 (m, 4H), 7.37 (d, J = 9.0 Hz, 2H), 7.42 (d, J = 7.7 Hz, 2H), 7.51 (t, J = 7.1 Hz, 2H), 7.90 (s, 2H), 8.08 (d, J = 7.0 Hz, 2H), 8.87 ppm (s, 2H). 13C NMR (75 MHz, CDCl3): δ = 14.23, 22.69, 26.59, 29.31, 29.36, 29.53, 29.58, 29.64, 29.66, 29.92, 32.12, 38.03, 42.11, 48.03, 52,24, 91.57, 102.22, 108.535, 109.46, 118.14, 119.32, 120.64, 122.35, 123.92, 126.29, 127.74, 140.10, 141.55, 145.23, 173.77 ppm. IR (neat): ν = 3257 (νN-H), 2920, 2851, 1647 (νC=O), 1550, 1478, 1304, 1199, 1017, 915, 881, 854, 809, 795, 764, 722, 711, 649, 632, 619, 600, 586, 576, 566, 548, 530, 516 cm−1. MALDI-TOF MS (Calcd. for C78H114Br2N4O2, Mw = 1298.73): m/z = 1299.09 ([M+]). Synthesis

of

5,15-bis(2-decyltetradecyl)-9,19-difluoro-5,15-

dihydrocarbazolo[3',4':5,6][1,4]oxazino[2,3-b]indolo[3,2-h]phenoxazine (5F) A solution of 4F (0.240 g, 0.400 mmol) and p-toluenesulfonic acid (60.0 mg, 0.315 mmol) in DMSO (4 mL) was heated to 150 oC for 5 h under nitrogen. After cooling to 20 oC, chloroform was added to dissolve the resulting compound and the homogeneous solution was poured into methanol (50 mL). The precipitate was collected by filtration, and column chromatography (SiO2, CH2Cl2) afforded the desired compound as a bluish-purple solid (65.0 mg, 27%). 1H NMR (CDCl3, 300 MHz): δ = 0.87 (m, 12H), 1.24 (m, 80H), 2.04 (br, 2H), 4.13 (d, J = 7.3 Hz, 4H), 7.18 (d, J = 8.8 Hz, 2H), 7.32 (d, J = 8.4 Hz, 2H), 7.35 (t, J = 7.3 Hz, 2H), 7.46 (t, J = 7.1 Hz, 2H), 7.61 (d, J = 8.8 Hz, 2H), 8.37 ppm (d, J = 7.7 Hz, 2H).

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C NMR (75 MHz, CDCl3): δ =

14.10, 22.67, 25.50, 26.48, 29.33, 29.36, 29.53, 29.61, 29.63, 29.66, 29.69, 29.95, 31.83, 31.92, 37.93, 47.84, 101.67, 102.62, 109.97, 120.93, 126.92, 128.64, 129.91, 130.82, 131.15, 132.95,


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139.36, 140.99, 142.35, 165.87, 166.98 ppm. IR (neat): ν = 2924, 2853, 2359, 1636 (νC=N), 1591 (νC=C), 1558 (νC=N), 1540, 1507, 1456, 1396, 1321, 1260, 1084, 1074, 805, 764, 750, 722, 712, 693, 678, 659, 647, 617, 602cm−1. MALDI-TOF MS (Calcd. for C78H110F2N4O2, Mw = 1172.86): m/z = 1172.86 ([M+H]+). Synthesis

of

9,19-dichloro-5,15-bis(2-decyltetradecyl)-5,15-

dihydrocarbazolo[3',4':5,6][1,4]oxazino[2,3-b]indolo[3,2-h]phenoxazine (5Cl) A solution of 4Cl (0.484 g, 0.400 mmol) and p-toluenesulfonic acid (0.100 g, 0.525 mmol) in DMSO (6 mL) was heated to 150 oC for 5 h under nitrogen. After cooling to 20 oC, chloroform was added to dissolve the resulting compound and the homogeneous solution was poured into methanol (50 mL). The precipitate was collected by filtration, and column chromatography (SiO2, CH2Cl2) afforded the desired compound as a bluish-purple solid (0.108 g, 23%). 1H NMR (CDCl3, 300 MHz): δ = 0.87 (m,12H), 1.24 (m, 80H), 2.06 (br, 2H), 4.07 (d, J = 7.7 Hz, 4H), 7.15 (d, J = 8.4 Hz, 2H), 7.22 (d, J = 8.4 Hz, 2H), 7.32 (t, J = 6.2 Hz, 2H), 7.37 (t, J = 6.4 Hz, 2H), 7.59 (d, J = 8.4 Hz, 2H), 8.45 ppm (d, J = 7.7 Hz, 2H).

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14.35, 22.88, 26.70, 29.54, 29.57, 29.73, 29.81, 29.83, 29.87, 29.89, 30.15, 32.03, 32.10, 38.13, 48.09, 106.63, 109.16, 109.80, 109.95, 120.76, 121.00, 123.29, 125.82, 126.45, 128.10, 139.63, 140.89, 142.23, 142.56, 142.87 ppm. IR (neat): ν = 2920, 2851, 1635 (νC=N), 1605 (νC=C), 1570, 1550 (νC=N), 1489, 1457, 1388, 1339 1304, 1275, 1214, 1184, 1143, 1109, 1030, 932, 792, 747, 731, 721, 645, 634, 622, 612 cm−1. MALDI-TOF MS (Calcd. for C78H110Cl2N4O2, Mw = 1204.80): m/z = 1205.54 ([M+H]+). Synthesis

of

9,19-dibromo-5,15-bis(2-decyltetradecyl)-5,15-

dihydrocarbazolo[3',4':5,6][1,4]oxazino[2,3-b]indolo[3,2-h]phenoxazine (5Br)


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A solution of 4Br (0.450 g, 0.400 mmol) and p-toluenesulfonic acid (90.0 mg, 0.472 mmol) in DMSO (5 mL) was heated to 150 oC for 5 h under nitrogen. After cooling to 20 oC, chloroform was added to dissolve the resulting compound and the homogeneous solution was poured into methanol (50 mL). The precipitate was collected by filtration, and column chromatography (SiO2, CH2Cl2) afforded the desired compound as a bluish-purple solid (0.221 g, 49%). 1H NMR (CDCl3, 300 MHz): δ = 0.86 (m, 12H), 1.21 (m, 80H), 2.04 (br, 2H), 4.05 (d, J = 7.7 Hz, 4H), 7.13 (d, J = 8.4 Hz, 2H), 7.20 (d, J = 7.7 Hz, 2H), 7.31 (t, J = 6.6 Hz, 2H), 7.35 (t, J = 6.8 Hz, 2H), 7.57 (d, J = 9.2 Hz, 2H), 8.54 ppm (d, J = 7.0 Hz, 2H).

13


14.10, 22.67, 25.50, 26.48, 29.33, 29.36, 29.53, 29.61, 29.63, 29.66, 29.69, 29.95, 31.83, 31.92, 37.93, 47.84, 101.79, 106.24, 108.75, 109.43, 120.36, 120.75, 123.34, 125.58, 126.05, 128.09, 139.42, 140.62, 141.98, 142.79, 144.12 ppm. IR (neat): ν = 2922, 2853, 1636 (νC=N), 1605 (νC=C), 1548 (νC=N), 1489, 1457, 1389, 1340, 1109, 914, 794, 775, 732, 675, 659, 648, 638, 618, 608cm−1. MALDI-TOF MS (Calcd. for C78H110Br2N4O2, Mw = 1294.70): m/z = 1295.92 ([M+H]+).

RESULTS AND DISCUSSION Synthesis and Characterization Starting from carbazole (1), alkylation at the 9-position followed by mononitration yielded 9-(2decyltetradecyl)-3-nitrocarbazole (3) in an isolated two-step yield of 67% (Scheme 1). The reduction of 3 afforded the corresponding 3-aminocarbazole derivative, and this compound was directly used for the next condensation reaction with tetrahalogenated p-quinones due to its


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limited chemical stability. The resulting products (4F, 4Cl, and 4Br) were further subjected to thermal ring closure reactions in the presence of p-toluenesulfonyl chloride (TsCl), eventually furnishing the desired carbazoledioxazine derivatives (5F, 5Cl, and 5Br) in 22.6-49.2% yields. All the derivatives showed good solubilities in the common organic solvents, such as chloroform, tetrahydrofuran, and toluene. The chemical structures were unambiguously characterized by 1H-, 13

C-NMR, and IR spectroscopies and MALDI-TOF mass spectrometry. Particular attention was

paid to the isomeric structures of the final products. A set of aromatic proton peaks clearly suggested the formation of angular-type structures, which are consistent with the reported spectral pattern (Figure S1).54-57 In addition, after ring closure, the IR peaks ascribed to the C=O and NH vibrations completely disappeared. Furthermore, MALDI-TOF mass spectrometry revealed the exact molecular ion peaks of each compound.

Scheme 1 Synthesis of carbazoledioxazine derivatives.


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Thermal Properties The decomposition temperatures (Td) of the carbazoledioxazine derivatives were determined by thermogravimetric analysis (TGA) under a nitrogen atmosphere at the heating rate of 10 °C min-1 (Figure S2). All the derivatives exhibited a good thermal stability with the 5% weight loss temperatures exceeding 270 oC. The thermal transition properties were also investigated by differential scanning calorimetry (DSC) in the temperature range without thermal decomposition. 5F and 5Cl showed well-defined endothermic and exothermic transitions during the heating and cooling processes, respectively, while the transitions of 5Br were ambiguous under the measurement conditions (Figure 1a). We attempted to identify the possible (meso)phases by polarized optical microscopy (POM); however, no characteristic texture could be observed mainly due to the highly dark purple colors of these molecules, although the birefringence that was maintained over the temperature range was investigated. Thus, in order to obtain more information about the phases of these molecules, XRD measurements were performed at various temperatures. 5F exhibited a profile including a sharp reflection with a d-spacing comparable to the molecular length and an outer broad halo with a d-spacing of 0.46 nm at 50 oC, while the sharp reflection disappeared upon heating to >152 oC (namely, 160 and 200 oC) (Figure 1b). This was consistent with the DSC chart and suggested that the endothermic peak at 152 oC upon heating is ascribed to a phase transition from smectic (Sm) A or C to nematic (N). On the other hand, 5Cl formed three highlyordered smectic phases with orthorhombic, monoclinic, and orthorhombic lattices in the order of increasing temperature and displayed two endothermic peaks at 75 and 222 oC in the DSC thermogram. While the phase behaviors of 5F and 5Cl were reversible during the heating and cooling processes, 5Br showed a unique irreversible phase change from the SmX (monoclinic) to


14

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SmA or C upon heating to 240 oC. The formed SmA or C phase was retained even after cooling to room temperature.

(a) 5F

123 SmA or C

N

SmA or C

5Cl

Heat flow

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N

152

64

SmX 75 (orthorhombic) SmX(monoclinic) SmA or C

5Br

207

222 SmX (orthorhombic)

SmX

240

0

50

100

150

200

250

o

Temperature ( C) (b)

50oC

160oC

200oC

50oC

90oC

230oC

50oC

250oC

50oC (cooling)

5F

5Cl

5Br


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Figure 1 (a) DSC thermograms and (b) WAXD images of 5F, 5Cl, and 5Br.

Optical and Electrochemical Properties The absorption spectra of the carbazoledioxazine derivatives were measured in CHCl3. All the derivatives showed an intense sharp peak at the lowest energy, which is characteristic of fused aromatic compounds (Figure 2a).59-63 The longest wavelength absorption maxima (λmax) bathochromically shifted in the following order: 5F (609 nm) < 5Cl (614 nm) < 5Br (616 nm) (Table 1). Also, the absorption coefficients (ε) at λmax increased in this order (49000, 60000, and 65000 M-1 cm-1 for 5F, 5Cl, and 5Br, respectively). These behaviors were probably due to the electronic effects of the halogen substituents. There was a slight difference in the onset absorption wavelengths (λonset). However, the calculated optical band gaps (Egopt) were more or less the same (1.87~1.89 eV).


16

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(a) 7x104 4

6x10

ε (M-1 cm-1)

5F 5Cl 5Br

4

5x10

4

4x10

4

3x10

4

2x10

4

1x10

0 300

400

500

600

700

800

Wavelength (nm) (b) 5F 5Cl

Current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5Br

-2

-1.5

-1

-0.5

0

0.5

1

1.5

+

Potential (V vs. Fc/Fc ) Figure 2 (a) UV-Vis absorption spectra and (b) cyclic voltammograms (0.1 V s-1) of the carbazoledioxazine derivatives.

Table 1 Optical properties of the carbazoledioxazine derivativesa

5F

λmax (nm)

ε (M-1 cm-1)

λonset (nm)

Egopt (eV)b

609

49000

657

1.89


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5Cl

614

60000

662

1.87

5Br

616

65000

660

1.88

a

Measured in chloroform solution. b Band gap estimated from the λonset.

In order to estimate the energy levels, cyclic voltammograms (CVs) were measured. The thin films of the carbazoledioxazine derivatives were prepared on a glassy carbon electrode, and the CVs were measured at the scan rate of 0.1 V s-1 in CH3CN with 0.1 M (nC4H9)4NClO4 at room temperature. Both 5Cl and 5Br displayed well-defined reversible two-step oxidation peaks and a single reduction peak, whereas the redox responses of 5F were weak and its reduction peak became quasi-irreversible (Figure 2b). Despite the poor redox activity of 5F, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels were estimated from the onset oxidation (Eox) and onset reduction (Ered) potentials, respectively. All the data are summarized in Table 2. The thin film of 5F showed the Eox of 0.48 V (vs. Fc/Fc+) and Ered of -1.23 V, corresponding to the HOMO level of -5.28 eV and the LUMO level of -3.57 eV. The energy levels of 5Cl and 5Br were similarly determined. Although both the HOMO and LUMO levels of 5Cl were slightly lower than those of 5F and 5Br, the differences were mostly within experimental error (merely 0.04 V for HOMO and 0.06 V for LUMO). The electrochemical band gaps (Egec) of the three derivatives, calculated from the Eox and Ered values, were also very close at around 1.7 V, which roughly correlated with the optical band gaps (vide supra). All these results suggested that the energy levels of the carbazoledioxazine derivatives are almost independent of the halogen substituents.


18

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Table 2 Electrochemical properties of the carbazoledioxazine derivatives Eox (V)a

Ered (V)a

HOMO (eV)

LUMO (eV)

Egec (eV)

5F

0.48

–1.23

–5.28

–3.57

1.71

5Cl

0.52

–1.20

–5.32

–3.60

1.72

5Br

0.48

–1.26

–5.28

–3.54

1.74

a

Potentials vs Fc/Fc+.

Fabrication and Measurements of Thin Film Transistors In order to elucidate the semiconducting properties of the carbazoledioxazine derivatives, thin film transistors (TFTs) were fabricated. Bottom-gate/top-contact devices were fabricated by spin-coating the CHCl3 solutions onto an octadecylsilane (octadecyltrimethylsilane (OTMS))treated SiO2 (300 nm)/n++-Si substrate. After thermal annealing at some selected temperatures (70, 110, 140, 150, 160, 200, or 220 °C), ~50 nm thick gold was deposited as the source and drain contacts using a shadow mask. As anticipated from the HOMO and LUMO levels, all the carbazoledioxazine derivatives showed p-type transistor performances. For example, the as-cast films of 5F, 5Cl, and 5Br exhibited an average hole mobility (µh) of 9.7×10−7, 1.2×10−5, and 1.4×10−4 cm2 V−1 s−1, respectively (Table 3). Considering the liquid crystalline behavior of these carbazoledioxazine derivatives, we tried to optimize the TFT performances via thermal annealing. Similar to some liquid crystalline small molecules64,65 and many conjugated polymers,66,67 it was found that thermal annealing is effective for the enhancement of the mobilities. The µh value of 5Cl almost doubled upon thermal annealing to 150 oC. The values of 5F and 5Br more significantly increased by 30~40 times. However, a further increase in the annealing temperature resulted in the decrease of the mobilities, suggesting that there was an optimum annealing


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Page 20 of 36

temperature in each case. Thus, the TFT of 5Br displayed the highest mobility among the three carbazoledioxazine derivatives with the µh of 4.0×10−3 cm2 V−1 s−1 (µh_max of 4.9×10−3 cm2 V−1 s−1) at the optimized annealing temperature of 140 oC. It should be noted that this value is relatively good, considering the noncrystalline small molecules with branched long alkyl chains. A similar trend was also observed for the on-off ratio (Ion/Ioff) upon thermal annealing. The annealed devices, in particular, those of 5Br showed much higher Ion/Ioff values as compared to others (Table 3). The transfer characteristics of the TFTs fabricated under the optimized annealing conditions are depicted in Figure 3. Note that no n-type TFT performances were observed even after the thermal annealing.

Table 3 Summary of TFT device performances sample / annealing temp.

µh [µh_max] (cm2 V−1 s−1)a

Vth (V)

Ion/Ioff

5F (N/A)

9.7×10−7 [9.7×10−7]

−47

1.6×102

70 oC

2.3×10−6 [2.9×10−7]

−32

2.2×102

110 oC

3.1×10−5 [4.4×10−5]

−60

8.4×102

150 oC

b

b

b

5Cl (N/A)

1.2×10−5 [2.6×10−5]

−78

8.9×101

150 oC

2.7×10−5 [7.0×10−5]

−100

5.4×101

200 oC

1.2×10−5 [1.2×10−5]

−50

3.3×103


20

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5Br (N/A)

1.4×10−4 [3.9×10−4]

−52

2.7×103

140 oC

4.0×10−3 [4.9×10−3]

−70

1.1×104

160 oC

3.6×10−3 [4.1×10−3]

−40

1.6×105

220 oC

3.2×10−3 [3.8×10−3]

−60

1.9×102

a

The average values calculated from 5 to 10 devices. Maximum values measured under vacuum

(10−4-10−5 mbar). b Below the detection limit.

(a)

(b)

(c)

Figure 3 Transfer characteristics of TFTs based on the thin films of (a) 5F, (b) 5Cl, and (c) 5Br annealed at optimized temperatures. The blue dash lines describe the gate leakage currents with the same axis scale as the source-drain currents (L = 100 µm and W = 1 mm, all the measurements were done under vacuum (10−4-10−5 mbar)).

Characterization of Molecular Organization and Film Morphologies


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It is important to investigate the molecular organization in the thin films because the differences in the TFT performances between the three carbazoledioxazine derivatives are not strongly correlated with the energy levels (vide supra). The GIXRD patterns of the thin films were measured, and the molecular packing structures of each derivative were revealed (Figure 4). The thin film of 5F showed a diffraction pattern including a series of sharp peaks along the film normal direction (the meridian), characteristic of layer stacking along the substrate normal at a spacing of c = 32.60Å. In contrast, those of 5Cl and 5Br exhibited more sharp reflections off the meridian. The reflections for the thin film of 5Cl are ascribed to an orthorhombic lattice with a = 9.94Å, b = 4.91Å, and c = 35.06Å, whereas that of 5Br to a monoclinic lattice with a = 9.94Å, b = 4.91Å, c = 37.23Å, and β = 109o. A packing motif similar to 5Br was previously predicted for the XRD patterns of a carbazoledioxazine derivative with ethyl substituents (violet 23).68 It was thought that these different molecular orientations resulted in the different TFT mobilities. Based on the GIXRD patterns, 5F and 5Cl packed in orthorhombic lattices had a carrier pathway only in the π−π stacking direction. In contrast, 5Br packed in a monoclinic lattice had multiple carrier pathways in both the π−π stacking direction and adjacent molecular columns. The 2D and 3D conduction models via multiple carrier transport are generally desired for high mobility TFTs.6972


22

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(a)

(b)

(c)

Figure 4 GIXRD patterns of the as-cast thin films of (a) 5F, (b) 5Cl, and (c) 5Br.

To further elucidate the effects of the thermal annealing on the film quality, the surface morphology of the thin films was investigated using tapping-mode atomic force microscopy (AFM). The as-cast thin film of 5F showed a smooth surface with a root-mean-square (rms) roughness of 0.59 nm, whereas those of 5Cl and 5Br were more crystalline with a higher rms roughness of 1.06 and 2.25 nm, respectively (Figure 5). This result was supported by the GIXRD patterns (vide supra). When the films were annealed at selected temperatures, the domain size increased as did the rms roughness, presumably due to the enhanced intermolecular π−π interactions and crystallinity. For example, the rms roughness of 5F increased from 0.59 to 4.27 nm upon annealing to 110 oC. Similar to 5F, 5Cl and 5Br also showed a well-defined increase in


23


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Page 24 of 36

the rms roughness from 1.06 to 2.16 nm upon annealing to 150 oC (for 5Cl) and from 2.25 to 3.24 nm upon annealing to 140 oC (for 5Br). These results are consistent with the observed increase in µh upon thermal annealing.

as-cast

70oC

110oC

as-cast

150oC

200oC

as-cast

140oC

220oC

(a)

(b)

(c)

Fig. 5 Tapping-mode AFM topography images of the selected thin films prepared by spincoating of a chloroform solution of (a) 5F, (b) 5Cl, and (c) 5Br. Annealing temperatures (for 10 min) are depicted in each image. AFM size: 2× 2 µm2.

CONCLUSIONS


24

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In summary, solution-processable carbazoledioxazine derivatives with three different halogen substituents were newly synthesized. Despite the different halogen substituents, the energy levels of the three carbazoledioxazine derivatives, estimated from the UV-Vis absorption and electrochemical redox potentials, were almost the same. However, there were clear differences in the thermal properties and molecular orientations, leading to different TFT performances. 5Br with the bromo substituents showed the best TFT performance with the highest hole mobility due to the multiple carrier pathways. This is the first report of the TFT performances of the carbazoledioxazine derivatives. It should be highlighted that the carbazoledioxazine is the only unexplored pigment in the field of organic semiconducting devices. In order to improve carrier transport properties, further derivatization and copolymerization are currently underway in our laboratory.

ASSOCIATED CONTENT Supporting Information. Isomer structures of carbazoledioxazine, TGA curves, and 1H NMR and 13C NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENT


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Page 26 of 36

This work was supported by the Tokuyama Science Foundation and the Support for Tokyotech Advanced Researchers (STAR). Y.W. is grateful to the Japanese government (MEXT: Monbukagakusho) scholarship. REFERENCES (1) Irimia-Vladu, M.; Troshin, P. A.; Reisinger, M.; Shmygleva, L.; Kanbur, Y.; Schwabegger, G.; Bodea, M.; Schwödiauer, R.; Mumyatov, A.; Fergus, J. W. et al. Biocompatible and Biodegradable Materials for Organic Field-Effect Transistors. Adv. Funct. Mater. 2010, 20, 4069-4076. (2) Irimia-Vladu, M.; Głowacki, E. D.; Troshin, P. A.; Schwabegger, G.; Leonat, L.; Susarova, D. K.; Krystal, O.; Ullah, M.; Kanbur, Y.; Bodea, M. A. et al. Indigo - A Natural Pigment for High Performance Ambipolar Organic Field Effect Transistors and Circuits. Adv. Mater. 2012, 24, 375-380. (3) Głowacki, E. D.; Voss, G.; Leonat, L.; Irimia-Vladu, M.; Bauer, S.; Sariciftci, N. S. Indigo and Tyrian Purple – From Ancient Natural Dyes to Modern Organic Semiconductors. Isr. J. Chem. 2012, 52, 540-551. (4) Kojima H.; Mori, T. Estimated Mobility of Ambipolar Organic Semiconductors, Indigo and Diketopyrrolopyrrole. Chem. Lett. 2013, 42, 68-70. (5) Głowacki, E. D.; Voss, G.; Sariciftci, N. S. 25th Anniversary Article: Progress in Chemistry and Applications of Functional Indigos for Organic Electronics. Adv. Mater. 2013, 25, 67836800. (6) Klimovich, I. V.; Leshanskaya, L. I.; Troyanov, S. I.; Anokhin, D. V.; Novikov, D. V.; Piryazev, A. A.; Ivanov, D. A.; Dremova, N. N.; Troshin, P. A. Design of Indigo


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Table of Contents Graphic


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Halogen Substitution Effects on the Molecular Packing and Thin Film

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