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International Journal of Polymer Science
Volume 2010 (2010), Article ID 908128, 9 pages
http://dx.doi.org/10.1155/2010/908128
Research Article

New Type of Donor-Acceptor Through-Space Conjugated Polymer

Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan

Received 2 November 2009; Accepted 21 February 2010

Academic Editor: Jinying Yuan

Copyright © 2010 Lin Lin et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We report the synthesis and properties of a novel through-space conjugated polymer with a [2.2]paracyclophane skeleton. The obtained polymer possessed donor (fluorene) and acceptor (2,1,3-benzothiadiazole) segments that were alternately -stacked in proximity via the [2.2]paracyclophane moieties. The good overlap between the emission peak of the donor unit (fluorene) and the CT band of the acceptor unit (2,1,3-benzothiadiazole) caused fluorescence resonance energy transfer, and the visible green light emission from the acceptor unit was observed.

1. Introduction

Since [2.2]paracyclophane was first prepared by Brown and Farthing in 1949 [1], [ ]paracyclophane consisting of two benzene rings closely linked (distance of approximately 2.8–3.1 Å) by two ethylene bridges at para positions, it has been attracting attention [24]. However, even though cyclophane compounds have been receiving considerable attention in the field of organic chemistry, only several reports [523] on the synthesis of [ ]paracyclophane-containing polymers have been published in the field of polymer chemistry. In particular, only a few reports on conjugated polymers containing [ ]paracyclophane in the polymer main chain have been published [57, 2023]. Recently, we successfully synthesized novel through-space conjugated polymers possessing [ ]paracyclophanes as a repeating unit into the main chain [57, 2438]. We elucidated their properties and found that the conjugation lengths of these polymers extend via the stacked -electron systems. In addition, we synthesized [2.2]paracyclophane-layered polymers by treating pseudo-p-diethynyl[ ]paracyclophane with diiodoxanthene as a scaffold [3942].

A one-dimensional -stacked structure can be readily obtained by incorporating a [ ]paracyclophane unit into a conjugated polymer backbone. As shown in Scheme 1, for example, copolymerization of pseudo-p-diethynyl[ ]paracyclophane with 2,5-dialkoxy-1,4-diiodobenzene yields a through-space conjugated polymer comprising a stacked -electron system, that is, a stacked xylyl-phenylene-xylyl unit. Therefore, the properties of through-space conjugated polymers synthesized by using a [2.2]paracyclophane monomer depend on this stacked -electron system (Scheme 1). We elucidated that the polymer emitted efficiently ( = 82% in diluted CHCl3 solution) [35] not from the excimer of the stacked xylyl-phenylene-xylyl segments but from the xylyl-phenylene-xylyl segment itself, irrespective of the -stacked structure of the polymer chain [6, 7, 4345].

908128.sch.001
Scheme 1: -Stacked structure of a through-space conjugated polymer based on [2.2]paracyclophane.

The use of [ ]paracyclophane as a monomer for the synthesis of a conjugated polymer enables the development of -stacked structures of various -electron systems. Such through-space conjugated polymers can be expected to transfer charge and/or energy effectively via the through-space interaction. Here, we report the synthesis of a new type of donor-acceptor through-space conjugated polymer with fluorene as a donor component and 2,1,3-benzothiadiazole as an acceptor component. The obtained polymer comprises donor and acceptor -electron systems that are alternately -stacked in proximity and held by covalent bonds, while the construction of -stacked donor-acceptor systems has been achieved by a supramolecular approach [4650].

2. Experimental Section

2.1. General

1H and 13C NMR spectra were recorded on a JEOL JNM-EX400 instrument at 400 and 100 MHz, respectively. The chemical shift values were expressed relative to Me4Si as an internal standard. FTIR spectra were obtained on a Perkin-Elmer 1600 spectrometer. High-resolution mass spectra (HRMS) were obtained on a JEOL JMS-SX102A spectrometer. Analytical thin-layer chromatography (TLC) was performed with silica gel 60 Merck F254 plates. Column chromatography was performed with Wakogel C-300 silica gel. Gel permeation chromatography (GPC) was carried out on a TOSOH 8020 (TSKgel α-3000 column) instrument using CHCl3 as an eluent after calibration with standard polystyrene samples. Recyclable preparative high-performance liquid chromatography (HPLC) was performed in Japan Analytical Industry Co. Ltd., Model 918R (JAIGEL-2.5H and 3H columns) using CHCl3 as an eluent. UV-Vis absorption spectra were obtained on a Shimadzu UV3600 spectrometer. Photoluminescence spectra were obtained on a HORIBA Jobin Yvon FluoroMax-4 luminescence spectrometer. For cyclic voltammetry, a polymer thin film was obtained by spin-coating from a toluene solution on an indium-tin-oxide (ITO) coated-glass electrode. Cyclic voltammetry (CV) was carried out on a BAS CV-50W electrochemical analyzer in CH3CN containing 0.1 M Et4NBF4 with a glassy carbon working electrode, a Pt counter electrode, an Ag/Ag+ reference electrode, and ferrocene (Fc/Fc+) as an external standard at a scan rate of 100 mV/s. Thermogravimetric analysis (TGA) was made on a Seiko EXSTAR 6000 instrument (10°C/min). Elemental analyses were performed with an Elementar Analysensysteme varioMICRO V1.5.8 system using the CHN mode or performed at the Microanalytical Center of Kyoto University.

2.2. Materials

Dehydrated toluene was purchased and used without further purification. THF was purchased and purified by passage through purification column under Ar pressure [51]. Pd(PPh3)4, Pd(OAc)2, 2-dicyclohexylphosphino- , -dimethoxybiphenyl (S-Phos), K2CO3, and K3PO4 were used as received. 9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester (2), 1-bromo-2,5-dimethylbenzene (5), and 2,5-dimethylphenylboronic acid (6) were purchased and used without further purification. Pseudo-p-dibromo[2.2]paracyclophane (1) [52, 53], 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole (4) [54], and 4,7-dibromo-2,1,3-benzothiadiazole (7) [55] were prepared according to the literature. All reactions were performed under Ar atmosphere.

2.3. 9,9-Dioctyl-2,7-bis(pseudo-p-bromo[2.2]paracyclophanyl)fluorene (3)

A mixture of pseudo-p-dibromo[2.2]paracyclophane (1) (366 mg, 1.0 mmol), 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester (2) (150 mg, 0.30 mmol), Pd(OAc)2 (6.7 mg, 0.030 mmol), and S-Phos (24 mg, 0.060 mmol) was placed in a Schlenk tube equipped with a magnetic stirring bar. THF (9.0 mL) and aqueous 2.0 M K3PO4 (0.45 mL) were added, and the reaction was carried out for 4 days at 70°C. After the reaction, the reaction mixture was poured into H2O, and extraction with CHCl3 was carried out. The organic layer was dried over MgSO4. After MgSO4 was filtered off, solvent was removed in vacuo. The crude solid was purified by column chromatography on silica gel (eluent: CHCl3/hexane, v/v = 1/3). Further purification by HPLC yielded monomer 3 as a white solid (30 mg, 0.030 mmol, 10%).

= 0.40 (hexane/CHCl3, v/v = 1/3); 1H NMR (CD2Cl2, 400 MHz) 0.61 (t, J = 6.8 Hz, 3H), 0.7 (br, 8H), 0.73 (t, J = 6.8 Hz, 6H), 0.79, J = 7.0 Hz, 3H), 1.0–1.3 (m, 40H), 2.0 (m, 4H), 2.1 (m, 4H), 2.54 (m, 4H), 2.68 (m, 4H), 2.90 (m, 8H), 3.01 (m, 4H), 3.19 (m, 4H), 3.45 (m, 8H), 6.54 (m, 16H), 6.61 (s, 4H), 7.08 (d, J = 7.8 Hz, 4H), 7.36 (s, 4H), 7.44 (d, J = 8.0 Hz, 4H), 7.80 (d, J = 7.8 Hz, 4H) ppm; 13C NMR (CD2Cl2, 100 MHz) 14.2, 22.9, 24.5, 29.6, 29.8, 30.4, 30.5, 32.0, 32.1, 32.3, 33.6, 34.4, 34.5, 35.8, 41.0, 55.6, 120.2, 125.1, 126.8, 128.6, 128.8, 129.3, 132.4, 134.2, 135.5, 137.3, 137.6, 139.3, 139.9, 140.2, 140.4, 142.3, 143.0, 147.2, 151.6 ppm. HRMS (EI): calcd. for C61H68Br2 [M]+: 958.3688, found 958.3668. Anal. calcd. for C61H68Br2: C 76.24; H 7.13; Br 16.63, found: C 76.08; H 7.12; Br 16.82.

2.4. Polymerization

A typical procedure is as follows. A mixture of 3 (30 mg, 0.030 mmol), 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole (4) (12 mg, 0.030 mmol), Pd(OAc)2 (1.4 mg, 0.006 mmol), and S-Phos (5.0 mg, 0.012 mmol) was placed in a Schlenk tube equipped with a magnetic stirring bar. THF (3.0 mL) and aqueous 2.0 M K3PO4 (0.10 mL) were added, and the reaction was carried out for 24 hours at reflux temperature (bath temp.: 75°C). After the reaction mixture was cooled to room temperature, the mixture was filtered to remove precipitated salts and washed with CHCl3. The filtrate was washed with 28% aqueous NH3 solution, and the organic layer was poured into a large amount of MeOH to obtain polymer P1 (19 mg, 0.080 mmol, 68%) as a yellow powder.

1H NMR (CD2Cl2, 400 MHz) 0.8–1.0 (br m), 1.0–1.4 (br m), 1.7 (br), 2.2 (br), 2.8–3.2 (br m), 3.7 (br), 6.7–7.1 (m), 7.6–7.7 (br), 8.0 (m) ppm; 13C NMR (CD2Cl2, 100 MHz) 13.5, 22.2, 23.8, 29 (m), 31 (m), 34 (m), 40.3, 55.0, 119.6, 124.3, 129.4, 131-134 (m), 139 (m), 142.1, 151.0, 153.8 ppm.

2.5. 9,9-Dioctyl-2,7-dixylylfluorene (M1)

A mixture of 2 (420 mg, 0.75 mmol), 1-bromo-2,5-dimethylbenzene (5) (276 mg, 1.5 mmol), Pd(OAc)2 (1.12 mg, 0.005 mmol), and S-Phos (4.1 mg, 0.010 mmol) were placed in a Schlenk tube equipped with a magnetic stirring bar. THF (1.0 mL) and aqueous 2.0 M K3PO4 (0.10 mL) was added, and the reaction was carried out for 48 hours at 70°C. After the reaction mixture was cooled to room temperature, the mixture was filtered to remove precipitated salts and washed with CHCl3. The organic layer was dried over MgSO4. MgSO4 was removed, and the solvent was dried in vacuo. The solid was purified by column chromatography on silica gel (eluent: CHCl3/hexane, v/v = 3/1) to yield model compound M1 as a white solid (217 mg, 0.36 mmol, 48%).

= 0.85 (CHCl3/hexane, v/v = 3/1); 1H NMR (CDCl3, 400 MHz) 0.74 (br, 4H), 0.81 (t, J = 7.1 Hz, 6H), 1.06 (m, 16H), 1.19 (t, J = 6.8 Hz, 4H), 1.98 (m, 4H), 2.27 (s, 6H), 2.38 (s, 6H), 7.09 (d, J = 7.6 Hz, 2H), 7.15 (s, 2H), 7.18 (d, J = 7.8 Hz, 2H), 7.28 (d, J = 7.8 Hz, 2H), 7.29 (s, 2H), 7.74 (d, J = 7.6 Hz, 2H) ppm; 13C NMR (CDCl3, 100 MHz) 14.1, 20.1, 20.9, 22.6, 23.8, 29.1, 29.2, 30.0, 31.8, 40.4, 55.1, 119.2, 123.8, 127.79, 127.84, 130.3, 130.5, 132.3, 135.1, 139.5, 140.7, 142.3, 150.6 ppm. HRMS (EI): calcd. for C45H58 [M]+: 598.4539, found 598.4541. Anal. calcd. for C45H58: C 90.24; H 9.76, found: C 90.18; H 9.73.

2.6. 4,7-Dixylyl-2,1,3-benzothiadiazole (M2)

A mixture of 2,5-dimethylphenylboronic acid (6) (375 mg, 2.5 mmol), 4,7-dibromo-2,1,3-benzothiadiazole (7) (293 mg, 1.0 mmol), Pd(OAc)2 (8.98 mg, 0.040 mmol), and S-Phos (32.8 mg, 0.080 mmol) were placed in a Schlenk tube equipped with a magnetic stirring bar. THF (2.0 mL) and aqueous 2.0 M K3PO4 (0.20 mL) was added, and the reaction was carried out for 48 hours at 70°C. After the reaction mixture was cooled to room temperature, the mixture was filtered to remove precipitated salts and washed with CHCl3. The organic layer was dried over MgSO4. MgSO4 was removed, and the solvent was dried in vacuo. The solid was purified by column chromatography on silica gel (eluent: CHCl3/hexane, v/v = 1/1) to yield model compound M2 as a white solid (240 mg, 0.70 mmol, 70%).

= 0.43 (hexane/CHCl3, v/v = 1/1); 1H NMR (CD2Cl2, 400 MHz) 2.14 (s, 6H), 2.39 (s, 6H), 7.21 (s, 4H), 7.26 (d, J = 8.3 Hz, 2H), 7.54 (s, 2H), ppm; 13C NMR (CDCl3, 100 MHz) 19.2, 20.2, 128.5, 128.7, 129.7, 130.4, 133.1, 134.0, 134.8, 137.1, 153.7 ppm. HRMS (EI): calcd. for C22H20N2S [M]+: 344.1347, found 344.1344. Anal. calcd. for C22H20N2S: C 76.71; H 5.85; N 8.13; S 9.31, found: C 76.52; H 5.86; N 7.82; S 9.07.

1H and 13C NMR spectra of all compounds described above are shown in Supplementary Material avalible online doi:10.115/2010/908128 Information.

3. Results and Discussion

The synthesis of monomer 3 is outlined in Scheme 2. The treatment of the excess amount (>2.5 equivalent) of pseudo-p-dibromo[2.2]paracyclophane 1 with fluorene diboronic acid ester 2 in the presence of a catalytic amount of Pd(OAc)2 and 2-dicyclohexylphosphino- , -dimethoxybiphenyl (S-Phos) in THF with aqueous K3PO4 [56, 57] afforded the corresponding bis(pseudo-p-bromo[2.2]paracyclophanyl)fluorene 3 in 10% isolated yield. The purification of monomer 3 by column chromatography using SiO2 and recyclable HPLC resulted in this low isolated yield (10%). The 1H NMR spectrum of monomer 3 exhibited two peaks at around 2.1 ppm (approximately 1:1), as shown in Figure ; these peaks were assigned to the methylenes of octyl groups at the 9-position of fluorene. This result suggests that the existence of two isomers was attributed to the two diastereomers (racemi and meso) derived from two planar chiral [2.2]paracyclophane units in monomer 3.

908128.sch.002
Scheme 2: Synthesis of monomer 3.

Polymer P1 was synthesized by the palladium-catalyzed polymerization of 3 and 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-2,1,3-benzothiadiazole 4, as shown in Scheme 3. The palladium-catalyzed coupling reaction of monomers 3 and 4 was carried out to obtain the corresponding through-space conjugated polymer P1; the polymerization results are listed in Table 1. An appropriate catalytic system was critically important for achieving successful polymerization. The standard Suzuki-Miyaura coupling reaction [56] with a Pd(PPh3)4 catalyst and aqueous K2CO3 in toluene at 80°C for 96 hours was ineffective for polymerization. Polymer P1 was obtained in 14% isolated yield with a number-average molecular weight ( ) of 3300 and a weight-average molecular weight ( ) of 3700 by GPC analysis (CHCl3, polystyrene standards, Run 1 in Table 1). Pd(OAc)2 with S-Phos catalytic system considerably increased the catalytic activity for the polymer synthesis as well as the monomer synthesis to afford polymer P1 in 68% isolated yield, and the and were estimated to be 17000 and 49000, respectively (Run 2 in Table 1).

tab1
Table 1: Polymerization results.
908128.sch.003
Scheme 3: Synthesis of polymer P1.

The obtained polymer P1 is a new type of donor-acceptor conjugated polymer in which a donor -electron system and an acceptor -electron system are linked alternately via the through-space interaction in the single polymer main chain (Scheme 3). The structure of P1 was confirmed by 1H and 13C NMR spectra (Figures S3 and S4 in Supplementary Material). Polymer P1 was highly soluble in common organic solvents such as THF, CHCl3, CH2Cl2, toluene, and DMF. In addition, it could be processed into a thin film by casting or spin-coating from toluene solution, and it was found to be air stable in solution and in the solid state. The thermal stability of P1 was evaluated by carrying out thermogravimetric analysis (TGA) under air (Figure in Supplementary Material). The TGA results showed that P1 exhibited good thermal stability with a 10% weight loss and temperature at 408°C.

In order to elucidate the optical properties of polymer P1, we designed and prepared model compounds M1 and M2. These compounds M1 and M2 represent the donor and acceptor unit layers of P1, respectively, as shown in Scheme 4. Figure 1 shows UV-Vis absorption spectra and photoluminescence spectra of M1, M2, and P1 in diluted CHCl3 (1.0  M for UV and 1.0  M for photoluminescence). As shown in Figure 1(a), the typical transition band of a fluorene compound was observed at around 300 nm, and blue emission was observed at 364 nm with a vibrational structure.

908128.sch.004
Scheme 4: Synthesis of model compounds M1 and M2.
908128.fig.001
Figure 1: UV-Vis absorption spectra and photoluminescence spectra of (a) model compounds M1, (b) M2, and (c) polymer P1 in CHCl3 (1.0 ×  M for UV and 1.0 ×  M for photoluminescence). Scattered light was deleted from the spectra.

As shown in Figure 1(b), M2 exhibited a broad absorption band at around 360 nm, which was attributed to a charge-transfer (CT) band from the benzene to the thiadiazole moieties, in addition to a transition band of xylyl-phenylene-xylyl backbone at around 310 nm. When the CT band at 360 nm was excited, the photoluminescence spectrum of M2 showed a maximum peak at 467 nm. The shape and the peak top of the photoluminescence spectrum of M2 were independent of the excitation wavelength (308 nm and 360 nm), indicating that the spectrum is characteristic of the benzothiadiazole moiety.

As shown in Figure 1(c), polymer P1 exhibited a UV spectrum with two absorption bands at around 330 nm and 400 nm. According to the UV-Vis absorption spectra of M1 and M2 (Figures 1(b) and 1(c), resp.), it was observed that the spectrum of P1 comprises the bands of M1 and M2 segments and the CT band of the benzothiadiazole moiety. In contrast, the absorption spectrum of P1 exhibited a red shift of approximately 50 nm in comparison with the absorption spectra of M1 and M2, because of the through-space conjugation. As in the case of M2, the photoluminescence spectrum of P1 exhibited a broad peak at around 505 nm. The photoluminescence spectra obtained at excitation wavelengths of 290 nm and 410 nm were identical, as shown in Figure 1(c). In other words, even if the fluorene segments in P1 were excited, only the benzothiadiazole segments emitted. In addition, from the excitation spectrum of polymer P1, it was confirmed that the benzothiadiazole segments were the emitting species (Figure 2). We confirmed that this concentration (1.0 ×  M) was sufficiently diluted to avoid intermolecular interactions according to the concentration effect of the photoluminescence spectra (Figures ) due to peak shift saturation. These results and the good overlap between the emission peak of M1 and the CT band of M2 suggest the occurrence of fluorescence resonance energy transfer (FRET) [58] from the donor-fluorene segments to the acceptor-benzothiadiazole segments.

908128.fig.002
Figure 2: Excitation spectrum of polymer P1 in CHCl3 (1.0 ×  M), monitoring wavelength at 520 nm.

Figure 3 shows the photoluminescence spectrum of a mixture of compounds M1 and M2 (concentration of each compound: 1.0 ×  M)in a diluted CHCl3 solution excited at 300 nm. Emissions from both M1 and M2 were observed at 361 nm and 467 nm, respectively. Increasing the concentration from 1.0 ×  M to 1.0 ×  M resulted in an increase in the intensity of emission from M2 due to the intermolecular interaction. The closely -stacked structure of alternate donor-fluorene and acceptor-benzothiadiazole segments in the polymer main chain caused FRET. The absolute photoluminescence quantum efficiency ( ) of P1 was calculated to be 0.49, which was lower than that of M2 ( = 0.75). The solvent effect on the photoluminescence of P1 was examined, and the in more polar solvents such as DMF was 0.42 (Figure ). This result implies that photo-excited electron transfer as well as energy transfer causes a decrease in the photoluminescence quantum efficiency. Incidentally, the Commission Internationale de L’Eclairage (CIE 1931) coordinates (x,y) of P1 were (0.2827, 0.5192) in solution and in the thin film, indicating visible green light emission.

908128.fig.003
Figure 3: Photoluminescence spectrum of the 1:1 mixture of M1 and M2 in CHCl3 (1.0 ×  M and 1.0 ×  M) excited at 300 nm. Scattered light (600 nm) was deleted from the spectrum.

The HOMO and LUMO energy levels of polymer P1 were estimated from the cyclic voltammogram as well as UV-Vis absorption spectrum. The cyclic voltammogram was obtained by fabricating thin films on an ITO glass electrode in CH3CN solution of 0.1 M Et4NBF4 using a three-electrode cell with a Pt counter electrode, an Ag/Ag+ reference electrode, and ferrocene (Fc/Fc+) as an external standard. Figure 4 shows the cyclic voltammogram of P1 at a scan rate of 100 mV/s. The oxidation process of P1 resulted in an onset peak at approximately 0.7 V; in the cathodic scan, the onset reduction potential was observed at approximately –1.3 V (versus Fc/Fc+). The HOMO and LUMO energy levels of 5 were roughly estimated to be –5.5 eV and –3.5 eV, respectively. The bandgap energy was approximately 2.0 eV, which was in agreement with the optical band gap energy (approximately 2.2 eV) estimated from the absorption spectrum of a thin film made of P1 (onset = 575 nm, as shown in Figure in Supplementary Material). On the other hand, density functional theory (DFT) calculations at the B3LYP/6-31G* level were carried out for model compounds. As can be seen in Figure in Supplementary Material, the HOMO and LUMO of the polymer comprise the fluorene unit and the benzothiadiazole unit, respectively. Thus, it can be reasonably concluded that both of the electron transfer and the energy transfer from the fluorene unit to the benzothiadiazole unit occurs in the polymer chain (vide supra).

908128.fig.004
Figure 4: Cyclic voltammogram of polymer P1 on an ITO electrode in CH3CN containing 0.1 M Et4NBF4 (versus Fc/Fc+) at a scan rate of 100 mV/s.

4. Conclusion

In summary, we successfully synthesized a novel through-space conjugated polymer with a [2.2]paracyclophane skeleton. The obtained polymer possessed donor and acceptor segments that were alternately -stacked in proximity via the [2.2]paracyclophane moieties. The polymer was soluble in common organic solvents, and a homogeneous thin film was readily obtained by casting or spin-coating techniques. The conjugation length of the polymer was extended by the through-space interaction. The polymer exhibited green emission with of 0.49 and CIE coordinates of (0.2827, 0.5192) in a diluted solution. This emission was attributed to the benzothiadiazole moieties; in other words, the benzothiadiazole moieties emitted due to FRET even when the fluorene moieties were excited. The polymer exhibited oxidation and reduction potentials at 0.7 V and –1.3 V (versus Fc/Fc+), respectively. Finally, it should be emphasized that the polymer is a novel donor-acceptor conjugated polymer that combines the donor and acceptor units alternately through stacking and not through a bond. Further studies on the synthesis of through-space conjugated polymers containing the donor and acceptor units at each polymer chain end are currently in progress.

Acknowledgments

This work was supported by Grant-in-Aid for Young Scientists (A) (no. 21685012) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Financial support from the Mazda Foundation is also acknowledged.

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