π-Conjugated Polymers Composed of BODIPY or Aza-BODIPY

Article pubs.acs.org/Macromolecules

π‑Conjugated Polymers Composed of BODIPY or Aza-BODIPY Derivatives Exhibiting High Electron Mobility and Low Threshold Voltage in Electron-Only Devices Ryousuke Yoshii,† Honami Yamane,† Atsushi Nagai,‡ Kazuo Tanaka,† Hideo Taka,§ Hiroshi Kita,§ and Yoshiki Chujo*,† †

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan Department of Materials Molecular Science, Institutes for Molecular Science, Higashiyama, Okazaki, Aichi 444-8787, Japan § Konica Minolta, Inc., 2970 Ishikawa-machi, Hachiojishi, Tokyo 192-8505, Japan ‡

S Supporting Information *

ABSTRACT: We present efficient electron-transport materials based of polymers. π-Conjugated copolymers composed of boron dipyrromethene (BODIPY) or Aza-BODIPY were synthesized via the efficient Suzuki−Miyaura cross-coupling reaction of (2,5-bis(2(2-(2-(pyridin-2-yloxy)ethoxy)ethoxy)ethoxy)-1,4-phenylene)diboronic acid with each of the diiodo-substituted BODIPY and Aza-BODIPY. Synthesized polymers exhibited high solubility even in polar solvents such as acetic acid. Their electronic and optical properties were studied by cyclic voltammetry, UV−vis absorption, and photoluminescence spectroscopies. The absorption and photoluminescence spectra of the obtained polymers were redshifted in comparison with the corresponding monomers due to the increase in the HOMO level by the formation of donor− acceptor interactions and the expansion of main-chain conjugations, explained by their cyclic voltammograms and theoretical calculations of the model compounds using the densityfunctional theory method. Finally, the electron mobilities of the polymers were determined from the space-charge-limited current with electron-only device structure of ITO/Ca/polymer/BCP/LiF/Al. As a result, owing to their high electron acceptability and strong stacking interaction among the BODIPY or Aza-BODIPY units, it was found that the mobilities for the polymers ((1.5− 3.6) × 10−4 [cm2 V−1 s−1]) were much higher than the value of Alq3 (5.8 × 10−5 [cm2 V−1 s−1]), and their threshold voltages (5− 7 V) were much smaller than that of the Alq3 device (12 V).

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INTRODUCTION π-Conjugated polymers have received a great deal of attention from both academic and industrial researchers. Their high charge-carrier abilities, prominent optical properties, and high processability make them key materials for the developments of advanced opto and/or electronic devices such as organic lightemitting diodes (OLEDs),1 photovoltaic cells (PVCs),2 field effect transistors,3 electrochromic devices,4 electrochemical supercapacitors,5 and switch elements.6 Various conjugated polymers have already been applied to the optoelectronic devices. However, the reports about applications of conjugated polymers to the electron-transporting layer are very limited because of instability and low electron mobilities of electron transport (ET) materials. Organoboron complexes are known as a versatile fluorophore because of relatively high absorption coefficient, fluorescent quantum yield, and high photochemical stability.7 Indeed, many researchers have applied their derivatives for constructing advanced optical materials as molecular probes8 and solar energy harvesters.9 In particular, the conjugated © 2014 American Chemical Society

polymers containing organoboron complexes have been extensively studied because they possess prominent processability and optical properties. For example, Jäkle et al. have presented the synthesis of conjugated polymers containing boron complexes in polymer main chain or side chain. These polymers have high stability, low-lying LUMO, and strong emissive properties.10 We have demonstrated the synthesis of the conjugated polymers incorporated various organoboron complexes.11 Especially, we have presented another possibility that a boron quinolate-based polymer can be used as an ET material.12 The electron mobility is reasonably close to the value of aluminum quinolate (Alq3) which is the most common ET material in OLEDs. This result strongly indicated organoboron complexesbased conjugated polymers have great potentials for application to ET materials with the applicability to a low-cost solution Received: January 27, 2014 Revised: March 16, 2014 Published: March 26, 2014 2316

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process13 and prompted us to explore conjugated polymers incorporated other organoboron complexes as ET materials. We focused on BODIPY (boron dipyrromethene) and AzaBODIPY dyes as the building block of conjugated polymers for development of ET materials. These dyes have various advantages for ET materials because they have high stability, low-lying LUMO, planar molecular structure, and adequate molecular ordering,14 which are favorable properties to develop ET materials with high stability and electron mobilities.15,16a Several research groups have presented that BODIPY-based compounds exhibited n-type semiconducting property.16 For example, Ziessel et al. have reported that a thienyl-BODIPY derivative exhibited comparable hole and electron mobilities in a standard bottom-contact OFET device, and the bulk heterojunction solar cell assembled with the dye and PC61BM showed a high power conversion efficiency 4.7%.16a−c We have previously presented that the conjugated polymers containing BODIPY or Aza-BODIPY derivatives exhibited strong deep-red or near-infrared (NIR) emission and formed various supramolecular self-assembled structures.17 Thus, we expected these dyes also have a large possibility to show high ET ability in the main-chain-type polymers. In this article, we report the synthesis and the electron mobilities of the conjugated polymers based on BODIPY (CP) or Aza-BODIPY (NP). They exhibited high solubility in polar solvents such as acetic acid. We examined their electron mobility using electron-only devices with the configuration of ITO/Ca/polymers/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/LiF/Al. Because of their high electron acceptability and strong stacking interaction among BODIPY or Aza-BODIPY units in the solid states, the polymers exhibited high electron mobilities (CP: 1.5 × 10−4 [cm2 V−1 s−1]; NP: 3.6 × 10−4 [cm2 V−1 s−1]) which were much higher values than the value calculated from the Alq3-fabricated device. Furthermore, the threshold voltages of the polymers (CP: 7 V; NP: 4 V) were much smaller than that of Alq3 (12 V). From the optical measurements, it was proposed that the strong π−π interaction between the BODIPY or Aza-BODIPY cores contributes to presenting high ET ability even in the amorphous phase.

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Current density−voltage (J−V) characteristics were measured by a computer-controlled programmable Keithley 6340 sourcemeter. Powder X-ray diffraction (XRD) patterns were taken by using Cu Kα radiation with a Rigaku Miniflex. Elemental analysis was performed at the Microanalytical Center of Kyoto University. Materials. Diiodo-substituted Aza-BODIPY (NM) and (E)-1-(4iodophenyl)-3-phenylprop-2-en-1-one were prepared according to the literature. 17c (2,5-Bis(2-(2-(2-(pyridin-2-yloxy)ethoxy)ethoxy)ethoxy)-1,4-phenylene)diboronic acid (Ph′) was prepared according to Scheme S1 (see Supporting Information). 2-Dicyclohexylphosphino-2′,6′-dimethoxybiphenyl [S-Phos] (Aldrich Chemical Co., 97%), tris(dibenzylideneacetone)dipalladium(0) [Pd2(dba)3] (Tokyo Kasei Kogyo Co.), boron trifluoride diethyl etherate [BF3·OEt2] (Aldrich Chemical Co.), and cesium carbonate (Wako Chemical Co., 99.8%) were used as received. Tetrahydrofuran (THF) and triethylamine (NEt3) were purified using a two-column solid-state purification system (Glasscoutour System, Joerg Meyer, Irvine, CA). Fabrication of Electron-Only Device. Anode was patterned on the glass substrate (NA45, produced by NH Techno Glass Corp., 100 mm × 100 mm × 1.1 mm) with ITO (100 nm) in a nitrogen-filled glovebox. After that, the substrate was cleaned by ultrasonication with 2-propanol, purged with nitrogen, and washed with UV ozone for 5 min. The Ca (5 nm) was thermally deposited on the substrate to prevent hole injection to the organic layer. The CP, NP, or AzaBODIPY in toluene (20 mg mL−1) was deposited on the substrate as an ET layer, spin-coated for 30 s (1000 rpm), and dried at 100 °C for 1 h, while Alq3 was thermally deposited on the substrate. The cathode of the device, consisting of BCP (70 nm) as an ET layer to increase film thickness, LiF (1 nm) as a buffer layer, and Al (100 nm) as a cathode, was thermally deposited on the top of polymer film at 1.0 × 10−5 Pa. Synthesis of 1. Diethylamine (0.19 mL, 0.13 g, 1.8 mmol) was added to the solution of (E)-1-(4-iodophenyl)-3-phenylprop-2-en-1one (6.0 g, 18 mmol) and ethyl nitroacetate (3.0 mL, 3.6 g, 27 mmol) in ethanol (40 mL). The reaction mixture was stirred at reflux temperature for 12 h and cooled to room temperature. Acetic acid (40 mL) and water (100 mL) were added to the solution to neutralize. The precipitate was collected by filtration and recrystallized from ethanol to give pure 1 as a white solid (6.6 g, 79%). 1H NMR (CDCl3): δ = 7.81 (2H, d, J = 8.31 Hz, Ar−H), 7.59 (2H, d, J = 8.31 Hz, Ar−H), 7.35−7.20 (5H, Ar−H), 5.53 (1H, dd, J = 23.82, 8.92 Hz, −CH−), 4.54−4.40 (1H, m, −CH−), 4.31−4.14 (1H, m, −CH−), 4.13−4.01 (1H, m, −CH−), 3.71−3.37 (2H, m, −CH2−), 1.25 (1.5H, t, J = 7.09 Hz, −CH3), 1.07 (1.5H, t, J = 7.09 Hz, −CH3) ppm. HRMS (APCI): Calcd for [M + H]+, 468.0302; found, m/z 468.0295. Synthesis of 2. NEt3 (2.1 mL, 1.3 g, 15 mmol) was added to the solution of 1 (7.1 g, 15 mmol) and formamidinesulfinic acid (6.6 g, 61 mmol) in 2-propanol (200 mL). The reaction mixture was stirred under a nitrogen atmosphere at reflux temperature for 12 h. The solvent was removed by rotary evapolator, and the residure was dissolved in cyclopentyl methyl ether (CPME). The organic layer was individually washed with water and brine. The organic layer was dried over magnesium sulfate and concentrated by a rotary evaporator. The obtained residure was dissolved in a small amount of THF, and then the product was reprecipitated from methanol. The precipitate collected by filtration was dried in vacuum to give 2 as a pare blue powder (1.3 g, 21%). 1H NMR (CDCl3): δ = 9.26 (1H, br s, −NH−), 7.76 (2H, d, J = 8.53 Hz, Ar−H), 7.58 (2H, dd, J = 8.16, 1.34 Hz, Ar− H), 7.43−7.29 (5H, Ar−H), 6.62 (1H, d, J = 3.17 Hz, Ar−H), 4.28 (2H, q, J = 7.15 Hz, −OCHH2CH3), 1.26 (3H, t, J = 7.07 Hz, −OCHH2CH3) ppm. HRMS (APCI): Calcd for [M + H]+, 418.0298; found, m/z 418.0288. Synthesis of 3. Aqueous solution of NaOH (2 M, 20 mL) was added to the solution of 2 (1.2 g, 2.9 mmol) in ethanol (60 mL). The reaction mixture was refluxed for 2 h and cooled to room temperature. Water and concentrated HCl (35%) were added to the solution to neutralize. The precipitate was collected by filtration and washed with water twice to give 3 as a pare blue solid (1.0 g, 89%). 1H NMR (DMSO-d6): δ = 12.36 (1H, br s, −COOH), 11.91 (1H, s, −NH−), 7.72 (4H, Ar−H), 7.53 (2H, d, J = 6.58 Hz, Ar−H), 7.34 (2H, t, J =

EXPERIMENTAL SECTION

Instrumentation. 1H (400 MHz), 13C (100 MHz), and 11B (128 MHz) NMR spectra were recorded on JEOL JNM-EX400 spectrometers. 1H NMR spectra used tetramethylsilane (TMS) as an internal standard in CDCl3 (11B NMR spectra; BF3 ·OEt2 [sealed capillary], and 13C NMR spectra; TMS as an internal standard in CDCl3). Number-average molecular weight (Mn) and molecular weight distribution [weight-average molecular weight/number-average molecular weight (Mw/Mn)] values of all polymers were estimated by size exclusion chromatography (SEC) at 40 °C. The system was operated at a flow rate of 1.0 mL/min with THF or CHCl3 as an eluent. Polystyrene standards were employed for calibration. TGA was performed on an EXSTAR TG/DTA6220, Seiko Instrument, Inc., with the heating rate of 10 °C/min up from 30 to 800 °C under nitrogen flowing (200 mL/min). Residual water was removed by keeping on the platinum pan at 120 °C for 1 h before the curve profiling. The decomposition temperatures (Td) were determined from the onset of the weight loss. DSC thermograms were carried out on a SII DSC 6220 instrument. The sample on the aluminum pan was cooled to −100 °C at the rate of 10 °C/min under nitrogen flowing (30 mL/min) and then heated from −100 to 250 °C with the same rate. UV−vis spectra were recorded on a SHIMADZU UV-3600 spectrophotometer and fluorescence emission spectra on a HORIBA JOBIN YVON Fluoromax-4P spectrofluorometer. Cyclic voltammetry (CV) was performed with a BAS ALS-600D electrochemical analyzer. 2317

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Scheme 1. Synthetic Route of CM

Scheme 2. Synthetic Route of the Polymers

6.82 Hz, Ar−H), 7.26 (1H, t, J = 6.94 Hz, Ar−H), 6.76 (1H, s, Ar−H) ppm. HRMS (APCI): Calcd for [M−H]−, 387.9840; found, m/z 387.9834. Synthesis of 4. Trifluoroacetic acid (20 mL) was added to 3 (1.0 g, 2.6 mmol), the reaction mixture was stirred under a nitrogen atmosphere at 50 °C for 0.5 h, and then triethyl orthoformate (10 mL) was added to the solution at 50 °C. The solution was poured into CPME (100 mL), and the solution was neutralized by sodium hydrogen carbonate and washed with water three times. The organic layer was dried over magnesium sulfate and concentrated by a rotary evaporator. The product was purified by silica gel column chromatography eluted with hexane/CHCl3 (1/4) to give 4 as a brown solid (0.75 g, 78%). 1H NMR (CDCl3): δ = 9.78 (1H, br s, −NH−), 9.65 (1H, s, −CHO), 7.79 (2H, dd, J = 6.70, 1.83 Hz, Ar− H), 7.58−7.35 (7H, Ar−H), 6.72 (1H, s, Ar−H) ppm. HRMS (APCI): Calcd for [M + H]+, 374.0036; found, m/z 374.0025. Synthesis of CM. Trifluoroacetic acid (62 μL, 92 mg, 0.8 mmol) was added to the solution of 3 (0.31 g, 0.8 mmol) and 4 (0.30 g, 0.8 mmol) in CH2Cl2 (3 mL). The reaction mixture was stirred under a nitrogen atmosphere at 40 °C for 3 h and cooled to −78 °C. NEt3 was added dropwise to the solution at −78 °C, and the solution was stirred at −78 °C for 5 min. After BF3·OEt2 (0.79 mL, 0.91 g, 6.4 mmol) and CH2Cl2 (6 mL) were added to the solution, the mixture was warmed to room temperature. Then the solution was refluxed for 12 h and poured into a large amount of methanol. The precipitate was collected by filtration. The product was purified by silica gel column chromatography eluted with THF. The obtained product was dissolved in a small amount of THF, and then the product was reprecipitated from ethanol. The precipitate collected by filtration was

dried in vacuum to give pure CM as a metallic brown solid (0.42 g, 70%). 1H NMR (CDCl3): δ = 7.79 (4H, d, J = 8.55 Hz, Ar−H), 7.66 (4H, d, J = 8.55 Hz, Ar−H), 7.56−7.38 (11H, Ar−H and −CH=), 6.70 (2H, s, Ar−H) ppm. 13C NMR (CDCl3): δ = 156.71, 146.05, 137.48, 134.71, 133.01, 131.65, 130.89, 129.08, 128.90, 128.71, 127.96, 118.88, 96.73 ppm. 11B NMR (CDCl3): δ = 1.51 (t, J = 31.30 Hz) ppm. HRMS (APCI): Calcd for [M + H]+, 748.9928; found, m/z 748.9900. Anal. Calcd for C33H21BF2I2N2: C, 52.98; H, 2.83; N, 3.74. Found: C, 52.77; H, 2.89; N, 3.67. Synthesis of CP. Water (0.1 mL) was added to the solution of 5 (0.11 g, 0.15 mmol), Ph′ (90 mg, 0.15 mmol), Pd2(dba)3 (1.3 mg, 1.5 μmol), S-Phos (2.3 mg, 5.8 μmol), and cesium carbonate (0.48 g, 1.5 mmol) in THF (1 mL). The reaction mixture was stirred at 60 °C for 24 h under a nitrogen atmosphere. The reaction mixture was poured into a large amount of methanol to collect the polymer by filtration. The precipitate was dissolved in a small amount of THF, and then the product was reprecipitated from ethanol three times. The polymer collected by filtration was dried in vacuum to give CP as a dark blue solid (0.14 g, 91%). Mn = 6200, Mw/Mn = 2.0. 1H NMR (CDCl3): δ = 8.26−6.49 (31H, Ar−H and −CH=), 4.55−4.24 (4H, −CH2−), 4.24− 3.95 (4H, −CH2−), 3.95−3.28 (16H, −CH2−) ppm. 13C NMR (CDCl3): δ = 163.51, 157.53, 150.41, 146.56, 145.44, 141.33, 139.54, 138.30, 134.73, 133.41, 130.94, 130.45, 129.46, 129.14, 128.99, 128.77, 119.35, 116.55, 116.25, 114.83, 111.17, 70.87, 70.70, 69.78, 69.67, 65.02 ppm. 11B NMR (CDCl3): δ = 1.42 (t, J = 31.30 Hz) ppm. Synthesis of NP. NP was prepared from NM (0.30 g, 0.41 mmol) in 76% yield (0.34 g, black solid) according to the same method for CP. Mn = 4300, Mw/Mn = 2.5. 1H NMR (CDCl3): δ = 8.36−6.46 (30H, Ar−H), 4.59−4.23 (4H, −CH2−) 4.23−3.98 (4H, −CH2−) 3.94− 2318

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3.35 (16H, −CH2−). ppm. 13C NMR (CDCl3): δ = 163.42, 159.18, 150.42, 146.55, 145.67, 140.73, 138.31, 133.73, 132.37, 130.28, 129.74, 129.32, 129.06, 128.53, 128.35, 128.15, 126.22, 119.16, 116.59, 111.11, 70.90, 70.73, 69.81, 69.69, 64.98 ppm. 11B NMR (CDCl3): δ = −4.02 ppm.

electronegativity of nitrogen atom. Furthermore, both polymers (CP and NP) showed the red-shifted absorption bands and smaller HOMO−LUMO band gaps than those of the corresponding monomers (CM and NM) and their derivatives (CM′ and NM′) without iodide groups.14h,17c These results suggest that the main-chain conjugations are elongated through the polymer main chains. In the PL spectra (Figure 1b), the peak positions of the PL maxima (λPL) showed good correlation with the order of their Eg values: The PL maxima of CP and NP exhibited obvious bathochromic shifts relative to those of the corresponding monomers (CM and NM). These data indicate efficient extension of π-conjugation along the polymer main chain. For the BODIPY-based compounds (CM and CP), they showed strong red emissions. From the Aza-BODIPY-based compounds (NM and NP), deep-red and NIR emissions were observed, respectively (Figure 1d). Meanwhile, the absolute quantum yields (ΦPL) of NM and NP strongly decreased compared to the BODIPY-based compounds. The decreases in ΦPL of the Aza-BODIPY-based compounds could be due to increasing internal conversion according to decreasing the energy gaps. That is, the nonradiative deactivation probability of S1 → S0 increases as energy gap of S0 → S1 decreases.18 Especially, NP with the smallest HOMO−LUMO energy gap would show the lowest ΦPL in the synthesized compounds. On the other hand, in the solid states, their emissions were strongly quenched by aggregation caused quenching effect (Figure S7).19 These results indicate that the synthesized compounds have strong intermolecular interactions among the BODIPY or Aza-BODIPY units in the solid sates. Cyclic Voltammetries. The electrochemical properties of the synthesized compounds were investigated with cyclic voltammetry (CV) (Table 3 and Figure 2). Oxidation peaks were hardly detected in their cyclic voltammograms. In contrast, NM and NP showed consecutive reversible two and three reduction peaks, respectively. In addition, CM and CP showed a strong irreversible reduction peak. These results indicate that BODIPY and Aza-BODIPY derivatives can easily receive electrons. Moreover, it is suggested that the rate of the electrochemical redox reaction between the Aza-BODIPYbased compounds and the electrode should be faster than those of the BODIPY compounds. It is assumed that strong electronegativity of the nitrogen atom could induce the change of the reaction rate. Their LUMO energy levels were estimated from the onsets of the first reduction waves by the empirical formula.20 Subsequently, their HOMO energy levels were calculated from the LUMO energy levels and optical band gaps (Eg) of the corresponding compounds (Table 3). Owing to the effect of the chelating with a boron atom, all compounds showed lowlying LUMO energy levels from 3.54 to 4.06 eV. Especially, Aza-BODIPY derivatives have more stabilized LUMO than those of BODIPY derivatives. The strong electronegativity of the nitrogen atom in the meso-position could introduce the lowering effect. Comparing to CM and NM, the corresponding polymers exhibited high-lying HOMO. The expansion of the πconjugation and the substitution of dialkoxybenzene derivatives with an electron donative property could be responsible for the increases of the HOMO levels. Despite the high-lying HOMO, their energy levels are still deeper than 5.2 eV.2b Basically, the threshold HOMO energy level of oxidative reaction with oxygen molecule is known as about 5.2 eV. Thus, it is likely that

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic routes toward diiodo-substituted BODIPY (CM) and the conjugated polymers based on BODIPY (CP) or Aza-BODIPY (NP) are outlined in Schemes 1 and 2. Efficient palladium-catalyzed Suzuki−Miyaura cross-coupling polymerization of (2,5-bis(2(2-(2-(pyridin-2-yloxy)ethoxy)ethoxy)ethoxy)-1,4-phenylene)diboronic acid (Ph′) with each of the diiodo-substituted BODIPY (CM) and Aza-BODIPY (NM) was carried out with 2-(2′,6′-dimethoxybiphenyl)dicyclohexylphoshine (S-Phos) in the presence of tris(dibenzylideneacetone) dipalladium [Pd2(dba)3] to afford the corresponding polymers. Since acid solvents are often used as convenient media on the device fabrication using wet process, we introduced the 2-pyridyl units to improve the solubility in acid solvents such as AcOH. The structures of the polymers were determined by 1H, 13C, and 11B NMR spectroscopies (Figures S1−S5). Synthesized polymers can be readily dissolved in common organic solvents such as CHCl3, THF, and toluene. Furthermore, the polymers showed high solubilities even in high polar solvents such as acetic acid due to their hydrophilic side chains, which are commonly used in the low-cost solution process. The number-average molecular weights (Mn) of the synthesized polymers were determined by size-exclusion chromatography (SEC) in CHCl3 using polystyrenes as the standards. The results are shown in Table 1. The thermal properties of the synthesized polymers Table 1. Results of the Polymerization compd

Mna

Mwa

Mw/Mna

yieldc [%]

CP NP

6200 4300

12 400 10 800

2.0 2.5

91 76

a

Estimated by size-exclusion chromatography (SEC) based on the polystyrene standards in CHCl3. bAverage number of repeating units calculated from Mn and molecular weights of repeating units. cIsolated yields after precipitation.

were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The degradation temperatures with 5% weight loss of CP and NP were 292 and 291 °C (Figure S6), respectively. In DSC measurements, significant thermal transition peaks were hardly observed in the measurement temperature range (from −100 to 250 °C). Optical Properties. The optical properties of synthesized polymers and their monomers were investigated by a UV−vis absorption and photoluminescence (PL) spectroscopy in CHCl3 (c = 1 × 10−5 M, Figure 1). The results are listed in Table 2. In the UV−vis absorption spectra (Figure 1a), the synthesized compounds exhibited strong absorption assigned as π−π* transitions at long wavelength region (λabs = 579−692 nm, ε = 53 400−99 800 M−1 cm−1). Comparing to CM, NM showed the red-shifted absorption band and smaller HOMO− LUMO gap, determined from the onset wavelength of the UV− vis spectrum in CHCl3. These data represent the low-lying LUMO of NM. It is known that the introduction of a nitrogen atom at the meso-position on a BODIPY skeleton induces the decrease in the LUMO level resulting from the strong 2319

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Figure 1. (a) UV−vis absorption and (b) photoluminescence spectra of CM, NM, CP, and NP in CHCl3 (c = 1 × 10−5 mol/L) and photographs of (c) the solution of the synthesized compounds in CHCl3 and (d) their photoluminescences under UV-irradiation.

compounds CM′, NM′, CP′, and NP′, corresponding to CM, NM, CP, and NP, respectively, were designed by using the Gaussian 03 suite of programs.21 Their electronic states were examined by theoretical calculations using a density-functional theory (DFT) method at the B3LYP/6-31G(d)//B3LYP/631G(d) level of theory (Figure 3). The trend of the calculation results in the model compounds shows good agreement with the experimental results for the synthesized compounds. In CM′ and NM′ (Figure 3a), it was clearly shown that their frontier orbitals delocalized through the BODIPY cores. In addition, corresponding to the experimental data, NM′ showed low-lying LUMO energy level and smaller HOMO−LUMO gap than those of CM′ (Figure 3 and Table 3). In CP′ and NP′ (Figure 3b), they showed similar trends to those of CM′ and NM′: NP′ has low-lying LUMO energy level and smaller HOMO−LUMO gap in comparison with CP′. In contrast, although CP′ and NP′ showed largely expanded HOMOs through the BODIPY and the dimethoxybenzene units, the LUMOs distinctly localized at the BODIPY core. These results suggest that BODIPY and Aza-BODIPY units can contribute to efficient electron transportation. Moreover, only the HOMOs of the synthesized polymers can be modulated by the substitution to the comonomers. In fact, it was observed that the substitution of the dimethoxybenzene derivatives with electron donative properties led to strong increase in the HOMO energy levels of the corresponding BODIPY or AzaBODIPY (HOMO = −5.30 eV (CM′) → −4.98 eV (CP′); HOMO = −5.35 eV (NM′) → −5.03 eV (NP′)). Thereby, CP′ and NP′ exhibited smaller HOMO−LUMO gaps than those of CM′ and NM′, respectively. These results can support our interpretation on their optical and electronic properties. Charge-Transport Properties. Charge-carrier mobilities of the synthesized polymers Aza-BODIPY and Alq3 as the typical ET materials were evaluated by the current−voltage measurement with the electron-only thin film devices. The devices were fabricated with the configuration of ITO/Ca (5

Table 2. Photoluminescence Properties of CM, NM, CP, and NPa compd

λPLb (nm)

λPL,solidb (nm)

ΦPLc

ΦPL,solidc

CM CP NM NP

613 648 694 738

663 719 −d −d

0.45 0.40 0.12 0.01

0.01 0.01 −d −d

In CHCl3 (c = 1.0 × 10−5 mol/L). bCM, NM, CP, and NP were excited at 550, 650, 603, and 692 nm, respectively. cΦPL is absolute quantum yield (CM, NM, CP, and NP were excited at 550, 650, 603, and 692 nm, respectively). dNot detected. a

Table 3. UV−Vis Absorption and Electrochemical Properties of CM, NM, CP, and NP compd CM CP NM NP

λabsa (nm) 579, 380, 289 603, 411, 325, 273 664, 480, 316 692, 496, 340

optical band gap (Egopt)b (eV)

Eredc,d

HOMOe (eV)

LUMOf (eV)

2.01

−1.20

−5.61

−3.60

1.90

−1.26

−5.44

−3.54

1.77

−0.74

−5.83

−4.06

1.60

−0.86

−5.54

−3.94

a

UV−vis spectra of CM, NM, CP, and NP were measured in CHCl3 (c = 1.0 × 10−5 mol/L). bThe optical band gap estimated from the onset wavelength of the UV−vis spectra in CHCl3. cCV was carried out in THF with 0.1 M Bu4NPF6 as supporting electrolyte. dEred is the onset potential of first reduction wave. fCalculated from the empirical formula, LUMO = −Ered −4.80 (eV).16 eCalculated from LUMO and optical band gap (Egopt) of the synthesized compounds, HOMO = LUMO − Egopt (eV).

these compounds are thermodynamically stable against the oxidative reaction with oxygen molecule in the air. Theoretical Calculations. To explain the optical and electronic properties of the synthesized compounds, model 2320

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Figure 2. Cyclic voltammograms of (a) monomers and (b) polymers in THF (c = 1 × 10−3 M) with 0.1 M Bu4NPF6 as a supporting electrolyte, AgCl/Ag as a reference electrode, Pt as working and counter electrodes, and scan rate at 100 mV/s.

Figure 3. Model compound structures and molecular orbital diagrams for the LUMO and HOMO of (a) CM′ and NM′ and (b) CP′ and NP′ (B3LYP/6-31G(d)//B3LYP/6-31G(d)).

where J is the measured current density, μ is the charge mobility, ε0 (= 8.85 × 10−14 C V−1 cm−1) is the free space permittivity, εr (= 0.3) is the dielectric constant of the material, V is the applied voltage, and L is the thickness of the device. Because the value of J/V2 in the equation is measurable as a slope of a J−V2 curve at the space-charge-limited current (SCLC) region, the electron mobilities for the polymers can be easily obtained by eq 2. At the SCLC region,22 the electron mobilities calculated using eq 2 and the threshold voltages for CP, NP, AzaBODIPY, and Alq3 are shown in Table 4. The mobilities for CP, NP, and Aza-BODIPY were much higher than that of Alq3, while their threshold voltages are much smaller than that of the Alq3 device. These data mean that NP and CP have prominent electron-carrier abilities compared to Alq3. Interestingly, though the LUMO energy level of CP is starkly higher than that of NP,

nm)/synthesized polymers, Aza-BODIPY or Alq3/BCP (70 nm)/LiF (1 nm)/Al (100 nm) (Figure 4a,b), and the Ca was thermally deposited on ITO anode to prevent hole injection to the organic layer. After the polymers in toluene were spincoated on the ITO/Ca, ITO/Ca/polymer was dried at 80 °C for 30 min under a nitrogen atmosphere. The film thickness of CP, NP, Aza-BODIPY, and Alq3 was 30, 30, 30, and 60 nm, respectively. Figure 4c shows the relationship between current density (J) and voltage (V) in the devices. The J−V2 curves were analyzed by the modified Mott−Gurney equation (Figure 4d, eqs 1 and 2).22 J = 9ε0εrμV 2/8L3

(1)

μ = (8/9)ε0εrL3J /V 2

(2) 2321

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Figure 4. (a) Chemical structures of Aza-BODIPY and Alq3, (b) schematic structure of the device, (c) current density−voltage (J−V), and (d) J−V2 characteristics from the device of CP, NP, Aza-BODIPY, and Alq3.

Table 4. ET Properties of CP, NP, Aza-BODIPY, and Alq3 compd CP NP Aza-BODIPY Alq3

μea (cm2 V−1 s−1) 1.5 3.6 1.8 5.8

× × × ×

10−4 10−4 10−4 10−5

Vthb (V) 7 4 5 12

a Estimated from the slope of the J−V2 curve at the SCLC region by eq 2. bVth is the onset potential on the J−V curve.

the mobility of CP is almost similar to that of NP. These results imply that their electron-carrier abilities would be less influenced by their LUMO energy levels. On the fabrication of the multilayered devices, the modulation of the energy levels of each layer is critical to obtain higher device efficiency. These data propose that the BODIPY-based polymers can enable us to regulate the energy levels for optimizing device efficiencies without loss of electron-carrier ability. According to these results, we conclude that the BODIPY and Aza-BODIPY have promising structures as ET materials for highly efficient drive of various electronic devices. Their remarkable electron-carrier abilities would be attributed to the high electron acceptability of BODIPY or Aza-BODIPY units with low-lying LUMO. XRD Measurements. Polymeric materials generally show lower carrier mobility in comparison with small molecular materials. Because of random distribution, the pathways for electron transportation should be limited. In contrast, the mobilities of the synthesized polymers hardly change among the polymers and Aza-BODIPY, indicating moderate molecular ordering among the polymers still remains. To understand the high electron mobilities of the polymers, we investigated the stacking structure of the synthesized compounds in the solid state by a powder X-ray diffraction (XRD). The XRD results of the CM and NM exhibited sharp and intense reflections (Figure 5), indicating the existence of regular structures. On the other hand, in the diffraction pattern of CP and NP, a broad

Figure 5. X-ray powder diffraction pattern of CM, NM, CP, and NP. CP or NP in toluene (20 mg mL−1) was deposited on the substrate and dried at 100 °C for 1 h.

diffraction peak was observed at around 23° (3.8−3.9 Å), which seems to be attributed to the distance of the π−π stacking between the BODIPY or Aza-BODIPY. From the results of XRD and PL measurements, it is suggest that the prominent electron-carrier ability of BODIPY-based polymers would be obtained from their strong π−π interaction between the BODIPY cores.

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CONCLUSION We have demonstrated the synthesis of the conjugated polymers based on BODIPY or Aza-BODIPY derivatives and their superior properties as ET materials. The synthesized polymers showed high solubility even in high polar solvents such as acetic acid due to their hydrophilic side chains. The electron-only devices fabricated with the configuration of ITO/ 2322

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Ca/CP or NP/LiF/Al exhibited a small threshold voltage and high electron mobilities (CP: 1.5 × 10−4 [cm2 V−1 s−1]; NP: 3.6 × 10−4 [cm2 V−1 s−1]) which were much higher than that of the Alq3-fabricated device. It is likely due to their high electron acceptability and the strong π−π interaction among the BODIPY or Aza-BODIPY units. This result gives us prospects for the possibility of using the BODIPY-based polymer as a new class of electron-transfer materials which are applicable in a low-cost solution process for electronic devices.

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ASSOCIATED CONTENT

* Supporting Information S

Text giving synthetic method of Ph′, the copies of 1H, 13C, and 11 B NMR spectra of Ph′, CM, NM, CP, and NP, TGA profiles for CP and NP, PL spectra of the synthesized compounds in the solid state, and J−V2 curves of CP, NP, Aza-BODIPY, and Alq3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by “the Adaptable and Seamless Technology Transfer Program” through target-driven R&D, Japan Science and Technology Agency (JST) and a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on Element-Blocks (No. 2401)” (24102013) of The Ministry of Education, Culture, Sports, Science, and Technology, Japan.

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