Synthesis and Optical Properties of π-Conjugated Polymers

Nov 17, 2016 - Synthesis and Optical Properties of π-Conjugated Polymers Containing Fused Imidazole Skeleton. Koji Takagi†, Takuya Miwa†, and Hyu...
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Synthesis and Optical Properties of π‑Conjugated Polymers Containing Fused Imidazole Skeleton Koji Takagi,*,† Takuya Miwa,† and Hyuma Masu‡ †

Life Science and Applied Chemistry, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa, Nagoya 466-8555, Japan ‡ Center for Analytical Instrumentation, Chiba University, 1-33 Yayoi, Inage, Chiba 263-8522, Japan S Supporting Information *

ABSTRACT: Fused imidazole monomers (1b, 1i, and 2b) having the octyl group were synthesized by the microwave-assisted intramolecular direct arylation, which were subjected to the crosscoupling polymerization. For obtaining π-conjugated polymers with better solubility, fused imidazole monomers (si-1b and si-3b) having the bulky trisiloxane-terminated decyl chain were likewise synthesized and polymerized. On the basis of absorption and emission spectra of obtained π-conjugated polymers in conjunction with the X-ray crystal structure of model compounds and theoretical calculation, the fused structure and coupling partner were found to affect the optical properties in CHCl3 solution as a result of steric and electronic factors. The spectroscopic measurements in film revealed that the bulky trisiloxane group helps πconjugated polymers to form the π-stacked structure upon thermal annealing. The proton doping experiment was also carried out to find out that the protonation of imidazole nitrogen results in the different peak shift depending on the comonomer structure. whole solar spectrum and generate more excitons.6,7 One of the most powerful methodologies to regulate the electronic structure of π-conjugated polymers is known as the donor− acceptor (D−A) architecture, in which the electron-rich and electron-deficient building blocks are incorporated in an alternating fashion.8−16 On the other hand, the highly coplanar polymer backbone as well as the strong intermolecular stacking force is necessary for obtaining superior semiconducting properties. Even if the proper FMO energy levels are attained, the poor intermolecular interaction results in the low charge carrier mobility. High molecular weight π-conjugated polymers tend to have a twisted and kinked conformation in solutions, and entangled polymer chains are no longer able to crystallize in solid states. The introduction of fused or ladder monomer structure is one of reliable techniques to synthesize πconjugated polymers with the low degree of torsional twisting.17−22 In contrast, such kinds of π-conjugated molecules and polymers are prone to form excimers leading to the fluorescence quenching and/or undesired longer wavelength emission, which may be the shortcoming for applying them as light-emitting materials. Accordingly, the control and deep understanding of electronic and ordering structures are much important to develop new π-conjugated polymers. In this paper, a series of bifunctional fused and planar imidazole monomers were prepared by utilizing the intramolecular direct arylation,23 and the palladium-catalyzed Suzuki−Miyaura and Sonogashira−Hagihara cross-coupling

1. INTRODUCTION π-Conjugated polymers have been attracting much attention from academic and industrial researchers because of the potential application to optoelectronic devices such as organic field-effect transistors (OFETs), 1 organic photovoltaics (OPVs),2 and organic light-emitting diodes (OLEDs).3 Although polymer materials have the low crystallinity as compared with small molecules and concern about the batchto-batch sample reliability in some occasions, the great advantage in solution processability and film-forming property in large area is the important characteristics for the commercialization of π-conjugated polymers. In the past several decades, a large number of π-conjugated polymers have been synthesized. For applying π-conjugated polymers to aforementioned devices, the careful molecular design of electronic structure and ordering structure is quite important. The geometry of frontier molecular orbitals (FMOs) affects the charge carrier transport property of materials; namely, the widely delocalized molecular orbitals could overlap between πconjugated polymer chains to efficiently deliver cation and anion charges. The energy levels of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) must be precisely tuned to match the work function of electrodes. When π-conjugated polymers are applied as the hole-transporting donor material in bulk heterojunction solar cells using fullerene derivatives as the electron-transporting acceptor material, the deep HOMO energy level of π-conjugated polymer is a key factor to ensure the large open-circuit voltage (Voc) and gain the high power conversion efficiency (PCE).4,5 The narrow energy gap between HOMO and LUMO is also essential to cover the © XXXX American Chemical Society

Received: August 13, 2016 Revised: November 9, 2016

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DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

were prepared as reported previously. All reactions were performed under a dry nitrogen atmosphere unless otherwise noted. 2.2. Instrumentation. Microwave reactions were performed on a Biotage Initiator 8 in the normal absorption level. 1H and 13C nuclear magnetic resonance (1H NMR and 13C NMR) spectra were recorded on a Bruker AvanceIII HD 400 FT-NMR spectrometer in CDCl3. Melting points (mp) were determined on a Yanagimoto micro melting point apparatus MP-500D and were uncorrected. Elemental analyses (EA) were performed on a Elementar vario EL cube in the CHN mode. Polymer characterizations using gel permeation chromatography (GPC) were carried out on a Shodex 104 system using tandem LF-404 columns (tetrahydrofuran (THF) as an eluent, flow rate = 1.0 mL/min, 40 °C) equipped with an ultraviolet−visible (UV−vis) detector (Shimadzu SPP-20A). Number-averaged molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined on the basis of a calibration curve made from standard polystyrene samples and ethylbenzene. UV−vis and fluorescence spectra were recorded on a Shimadzu UV-1650 spectrophotometer and a Shimadzu RF-5300 spectrofluorometer, respectively, using quartz cell or quartz plate. Fluorescence quantum yields (QYs) in solution were determined relative to quinine sulfate in 0.1 M H2SO4 with QY of 0.55. 2.3. Monomer Syntheses. 2.3.1. Monomer 1b. (Step 1) 2Octylimidazole (0.72 g, 4.0 mmol) was added to a THF suspension (12 mL) of NaH (55% oil suspension, 0.35 g, 8.0 mmol). After the evolution of H2 gas ceased, 1,4-bis(bromomethyl)-2,5-dibromobenzene (0.84 g, 2.0 mmol) was added. The reaction mixture was stirred at room temperature for 3 h. After pouring the reaction mixture into H2O (80 mL) and Et2O (80 mL), an aqueous phase was extracted with Et2O. The combined organic phase was dried over MgSO4, and compound 1 was obtained as a dark yellow solid after removing solvents (1.2 g, 94% yield). 1H NMR (CDCl3) δ ppm 0.86 (t, J = 6.20 Hz, 6H), 1.25 (20H), 1.71 (m, 4H), 2.57 (t, J = 7.30 Hz, 4H), 5.06 (s, 4H), 6.76 (s, 2H), 6.81 (d, J = 1.26 Hz, 2H), 7.07 (d, J = 1.26 Hz, 2H). This compound was used without purification in the second step. (Step 2) A mixture of 1 (1.9 g, 3.1 mmol), K2CO3 (0.85 g, 6.1 mmol), PPh3 (0.08 g, 0.31 mmol), and Pd(OAc)2 (34 mg, 0.15 mmol) in dimethyl sulfoxide (20 mL) was heated at 180 °C for 1.5 h under the microwave irradiation. After CH2Cl2 was added, insoluble solids were removed by Celite filtration. Water was added, and an aqueous phase was extracted with CH2Cl2. The combined organic phase was repeatedly washed with water and dried over MgSO4. After removing organic solvents, the residue was washed with EtOH to obtain compound 2 as a pale white solid (0.52 g, 37% yield). 1H NMR (CDCl3) δ ppm 0.77−0.95 (6H), 1.07−1.53 (20H), 1.72−1.92 (4H), 2.75 (t, J = 7.71 Hz, 4H), 4.91 (s, 4H), 7.10 (s, 2H), 7.52 (s, 2H). This compound was used without purification in the third step. (Step 3) NBromosuccinimide (0.08 g, 0.45 mmol) was added to 2 (0.10 g, 0.22 mmol) in CH3CN (1.5 mL) at 0 °C under dark, and the reaction mixture was stirred at room temperature for 4 h. After solvent was removed, water and ethyl acetate were added. An aqueous phase was extracted with ethyl acetate, and the combined organic phase was dried over MgSO4. The crude product was purified by SiO2 column chromatography (CHCl3, Rf = 0.2) and recrystallization from acetone to obtain 1b as a yellow needle (45 mg, 34% yield); mp 181−182 °C. 1 H NMR (CDCl3) δ ppm 0.88 (t, J = 6.80 Hz, 6H), 1.18−1.48 (20H), 1.78 (m, 4H), 2.71 (t, J = 7.80 Hz, 4H), 4.89 (s, 4H), 7.64 (s, 2H). 13C NMR (CDCl3) δ ppm 14.0, 22.6, 26.9, 27.7, 29.1, 29.2, 29.3, 31.8, 48.2, 102.9, 114.3, 127.8, 133.8, 140.4, 146.0. Anal. Calcd for C30H40Br2N4: C, 58.45%; H, 6.54%; N, 9.09%. Found: C, 58.51%; H, 6.54%; N, 8.94%. 2.3.2. Monomer 1i. This compound was synthesized in a similar manner to 1b using N-iodosuccinimide instead of N-bromosuccinimide; mp 210 °C (dec). 1H NMR (CDCl3) δ ppm 0.88 (t, J = 6.50 Hz, 6H), 1.16−1.46 (20H), 1.78 (m, 4H), 2.74 (t, J = 7.83 Hz, 4H), 4.94 (s, 4H), 7.79 (s, 2H). 13C NMR (CDCl3) δ ppm 14.1, 22.6, 27.2, 27.9, 29.2, 29.3, 29.4, 31.8, 48.2, 68.7, 114.3, 128.6, 139.1, 140.6, 148.3. Anal. Calcd for C30H40I2N4: C, 50.72%; H, 5.67%; N, 7.89%. Found: C, 51.54%; H, 6.78%; N, 8.24%.

polymerizations thereof were carried out. The chemical structures of π-conjugated polymers are shown in Figure 1.

Figure 1. Chemical structure of π-conjugated polymers.

The influences of fused structure, coupling partner, and solubilizing side chain were investigated. The absorption and emission spectra in solution and film states were measured to gain information about the structure−property relationship, which was discussed on the basis of single crystal X-ray crystallography of model compounds. In addition, the protonation of imidazole nitrogen was carried out to tune the electronic state of π-conjugated polymers by manipulating the D−A charge transfer interaction.24,25 The density functional theory (DFT) calculation revealed the steric and electronic perturbations induced by the imidazolium cation. There are some π-conjugated polymers composed of imidazole units in the main chain,26−28 while the present approach is unique in the point that both the reactivity and basicity of imidazole are utilized to control the electronic and ordering structures of πconjugated polymers. We believe that the obtained results give valuable information for designing new π-conjugated polymers.

2. EXPERIMENTAL SECTION 2.1. Materials. Dichlorobis(triphenylphosphine)palladium(II) [Pd(PPh3)2Cl2] and 1,1,1,3,5,5,5-heptamethyltrisiloxane were purchased from Tokyo Chemical Industry. Tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4] and platinum(0)−1,3-divinyl-1,1,3,3tetramethyldisiloxane complex solution in xylene (Pt ∼ 2%) were purchased from Aldrich. Palladium(II) acetate [Pd(OAc)2] was purchased from Kishida. Other organic reagents were obtained from commercial suppliers and used without purification. 1,4-Bis(bromomethyl)-2,5-dibromobenzene,29 1,5-bis(bromomethyl)-2,4-dibromobenzene,30 and 2,6-bis(bromomethyl)-1,5-dibromonaphthalene31 B

DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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2.3.5. Monomer si-3b. This compound was synthesized in a similar manner to si-1b using 2,6-bis(bromomethyl)-1,5-dibromonaphthalene instead of 1,4-bis(bromomethyl)-2,5-dibromobenzene. 1H NMR (CDCl3) δ ppm 0.00 (s, 6H), 0.09 (s, 36H), 0.45 (t, J = 7.46 Hz, 4H), 1.26−1.44 (30H), 1.79 (m, 4H), 2.67 (t, J = 7.83 Hz, 4H), 4.85 (s, 4H), 7.45 (d, J = 8.56 Hz, 2H) 8.89 (d, J = 8.56 Hz, 2H). 13C NMR (CDCl3) δ ppm −0.28, 1.85, 17.6, 23.1, 26.8, 27.4, 29.4, 29.5, 29.6, 33.2, 48.0, 103.8, 121.0, 125.9, 126.2, 127.1, 135.4, 137.8, 145.8. Anal. Calcd for C52H90Br2N4O4Si6: C, 53.67%; H, 7.80%; N, 4.81%. Found: C, 53.49%; H, 7.93%; N, 4.82%. 2.4. Syntheses of Model Compounds. 2.4.1. Model m1. A mixture of 1b (0.10 g, 0.16 mmol), phenylboronic acid (50 mg, 0.41 mmol), and Pd(PPh3)4 (9.4 mg, 8.1 μmol) in THF (2 mL) and 2 M aqueous Na2CO3 solution (0.5 mL) was heated to 100 °C for 12 h under the microwave irradiation. After saturated NH4Cl solution was added, an aqueous phase was extracted with CHCl3. The combined organic phase was washed with water and dried over MgSO4. After removing organic solvents, crude products were purified by SiO2 column chromatography (hexane/EtOAc (3/1 and then1/1), Rf = 0.6) to obtain m1 as a yellow solid (0.52 g, 53% yield); mp 206−208 °C. 1H NMR (CDCl3) δ ppm 0.88 (t, J = 6.60 Hz, 6H), 1.25−1.48 (20H), 1.82 (m, 4H), 2.78 (t, J = 7.70 Hz, 4H), 4.95 (s, 4H), 7.30− 7.36 (m, 2H), 7.48 (4H), 7.83 (6H). 13C NMR (CDCl3) δ ppm 14.1, 22.6, 27.3, 27.8, 29.2, 29.3, 29.5, 31.8, 47.6, 114.6, 126.7, 126.8, 128.6, 129.3, 132.3, 132.8, 135.2, 141.3, 145.7, Anal. Calcd for C42H50N4: C, 82.58%; H, 8.25%; N, 9.17%. Found: C, 81.97%; H, 8.61%; N, 9.03%. 2.4.2. Model m2. This compound was synthesized in a similar manner to m1 using 2b instead of 1b; mp 197−198 °C. 1H NMR (CDCl3) δ ppm 0.88 (t, J = 6.79 Hz, 6H), 1.21−1.51 (20H), 1.83 (m, 4 H), 2.79 (t, J = 7.70 Hz, 4H), 4.94 (s, 4H), 7.37 (3H), 7.47 (4 H), 7.85 (4H), 8.31 (s, 1H). 13C NMR (CDCl3) δ ppm 14.1, 22.6, 27.4, 27.9, 29.2, 29.3, 29.5, 31.8, 47.5, 110.4, 119.0, 126.6, 126.7, 128.7, 131.7, 132.6, 132.9, 135.1, 138.6, 145.5. Anal. Calcd for C42H50N4: C, 82.58%; H, 8.25%; N, 9.17%. Found: C, 82.01%; H, 8.52%; N, 8.98%. 2.4.3. Model m3. This compound was synthesized in a similar manner to m1 using si-3b′ (with octyl group at the 2-position; synthetic details are not described here) instead of 1b; mp 93−95 °C. 1 H NMR (CDCl3) δ ppm 0.88 (t, J = 6.72 Hz, 6 H), 1.23−1.51 (20H), 1.86 (m, 4H), 2.84 (t, J = 7.83 Hz, 4H), 5.00 (s, 4H), 7.23 (d, J = 8.56 Hz, 2H), 7.45 (6H) 7.60 (6H). 13C NMR (CDCl3) δ ppm 14.1, 22.6, 27.2, 27.9, 29.2, 29.3, 29.5, 31.8, 47.8, 121.0, 126.1, 126.4, 127.5, 127.8, 129.4, 130.5, 133.2, 134.0, 136.3, 138.2, 145.3. Anal. Calcd for C46H52N4: C, 83.59%; H, 7.93%; N, 8.48%. Found: C, 83.18%; H, 7.88%; N, 7.86%. 2.5. Polymerization. 2.5.1. Typical Procedure of Suzuki− Miyaura Cross-Coupling Polymerization. A microwave vial containing 1b (0.10 g, 0.16 mmol), 9,9-dihexylfluorene-2,7-diboronic acid (68 mg, 0.16 mmol), Pd(PPh3)4 (18 mg, 16 μmol), THF (2.0 mL), and 2 M aqueous Na2CO3 (0.5 mL) was heated under the microwave irradiation condition at 100 °C for 24 h. After saturated NH4Cl solution was added, an organic phase was extracted with CHCl3. The combined organic phase was dried over MgSO4. The solution was concentrated and poured into EtOH followed by washing with acetone to obtain P1-F (88 mg, 69% yield). 1H NMR (CDCl3) δ ppm 0.74− 2.15 (58H), 2.83 (4H), 4.98 (4H), 7.45−8.50 (8H). Mn = 3200, Mw/ Mn = 1.65. Other π-conjugated polymers (P2-F, Si-P1-F, and Si-P3-F) were likewise obtained. P2-F: Mn = 1900, Mw/Mn = 1.29; Si-P1-F: Mn = 7200, Mw/Mn = 1.33; Si-P3-F: Mn = 4800, Mw/Mn = 1.49. 2.5.2. Typical Procedure of Sonogashira−Hagihara CrossCoupling Polymerization. A solution of 1i (0.10 g, 0.14 mmol), 9,9-dihexyl-2,7-diethynylfluorene (54 mg, 0.14 mmol), CuI (1.3 mg, 7.0 μmol), and Pd(PPh3)4 (8.1 mg, 7.0 μmol) in THF (4 mL) and trimethylamine (2 mL) was heated to 50 °C overnight. After solvents were removed, CHCl3 was added and washed with saturated NH4Cl solution. The CHCl3 solution was dried over MgSO4, concentrated, and poured into EtOH followed by washing with acetone to obtain P1-EF (82 mg, 71% yield). 1H NMR (CDCl3) δ ppm 0.64−1.99 (54H), 2.77 (4H), 4.93 (4H), 7.40−7.95 (8H). Mn = 4700, Mw/Mn = 1.74. P1-EN was likewise obtained except that trimethylsilyl-protected acetylene monomer was used.

2.3.3. Monomer 2b. This compound was synthesized in a similar manner to 1b using 1,5-bis(bromomethyl)-2,4-dibromobenzene instead of 1,4-bis(bromomethyl)-2,5-dibromobenzene in three steps; mp 210−212 °C. 1H NMR (CDCl3) δ ppm 0.88 (t, J = 6.80 Hz, 6H) 1.21−1.43 (20H), 1.74 (m, 4H), 2.61 (t, J = 7.80 Hz, 4H), 4.82 (s, 4H), 7.35 (s, 1H), 7.63 (s, 1H). 13C NMR (CDCl3) δ ppm 14.0, 22.6, 26.9, 27.5, 29.1, 29.3, 29.4, 31.8, 48.3, 102.6, 108.9, 118.9, 129.5, 133.5, 138.0, 145.8, Anal. Calcd for C30H40Br2N4: C, 58.45%; H, 6.54%; N, 9.09%. Found: C, 58.35%; H, 6.54%; N, 8.72%. 2.3.4. Monomer si-1b. (Step 1) To a mixture of 10-undecenal (1.0 g, 5.9 mmol), methanol (32 mL), and 40% glyoxal solution (7.6 mL, 5.9 mmol) was added 28% ammonia solution (3.3 mL, 48 mmol) at 0 °C, and the mixture was stirred at room temperature overnight. After concentrating the solution, water and CHCl3 were added, and an aqueous phase was extracted with CHCl3. The combined organic phase was washed with brine and dried over MgSO4. The crude product was purified by SiO2 column chromatography (CHCl3 and then CHCl3/methanol (10/1)) to obtain 5 as a yellow solid (1.1 g, 88% yield). 1H NMR (CDCl3) δ ppm 1.17−1.49 (10H), 1.61−1.88 (2H), 1.91−2.13 (2H), 2.74 (t, J = 7.71 Hz, 2H), 4.85−5.07 (2H), 5.80 (ddt, J = 17.01, 10.22, 6.66 Hz, 1H) 6.95 (s, 2H), 7.13 (brs, 2H). (Step 2) 5 (1.2 g, 5.7 mmol) was added to a THF suspension (18 mL) of NaH (55% oil suspension, 0.88 g, 11 mmol). After the evolution of H2 gas ceased, 1,4-bis(bromomethyl)-2,5-dibromobenzene (1.2 g, 2.8 mmol) was added. The reaction mixture was stirred at room temperature for 3 h. After pouring the reaction mixture into H2O (100 mL) and Et2O (100 mL), an aqueous phase was extracted with CH2Cl2. The combined organic phase was dried over MgSO4, and crude products were purified by SiO2 column chromatography (CHCl3 and then CHCl3/methanol (10/1)) to obtain 6 as a yellow solid (1.2 g, 94% yield). 1H NMR (CDCl3) δ ppm 1.28 (20H), 1.71 (quin, J = 7.45 Hz, 4H), 1.95−2.13 (4H), 2.57 (t, J = 7.71 Hz, 4H), 4.86−5.10 (8H), 5.80 (ddt, J = 17.02, 10.26, 6.57 Hz, 2H), 6.76 (s, 2H), 6.81 (d, J = 1.26 Hz, 2H), 7.07 (d, J = 1.10 Hz, 2H). (Step 3) A mixture of 6 (2.0 g, 3.0 mmol), K2CO3 (0.88 g, 6.4 mmol), PPh3 (0.08 g, 0.32 mmol), and Pd(OAc)2 (19 mg, 85 μmol) in dimethyl sulfoxide (20 mL) was heated at 130 °C for 13 h under the microwave irradiation. After CH2Cl2 was added, insoluble solids were removed by Celite filtration. Water was added, and an aqueous phase was extracted with CH2Cl2. The combined organic phase was repeatedly washed with water and dried over MgSO4. After removing organic solvents, crude products were purified by SiO2 column chromatography (EtOAc/CHCl3 (1/1) and then EtOAc/CHCl3/methanol (5/5/1)) to obtain 7 as a brown solid (0.67 g, 44% yield). 1H NMR (CDCl3) δ ppm 1.14−1.54 (20H), 1.79 (m, 4H), 2.03 (m, 4H), 2.75 (t, J = 7.64 Hz, 4H), 4.69−5.20 (8H), 5.81 (ddt, J = 16.99, 10.23, 6.60 Hz, 2H), 7.09 (s, 2H), 7.52 (s, 2H). (Step 4) N-Bromosuccinimide (36 mg, 0.20 mmol) was added to 7 (50 mg, 98 μmol) in CH3CN (1 mL) and CHCl3 (2 mL) at 0 °C under dark, and the reaction mixture was stirred at room temperature overnight. After solvent was removed, water and ethyl acetate were added. An aqueous phase was extracted with ethyl acetate, and the combined organic phase was dried over MgSO 4 . The crude product was purified by SiO 2 column chromatography (hexane/EtOAc (4/1), Rf = 0.3) obtain 8 as a brown solid (53 mg, 81% yield). 1H NMR (CDCl3) δ ppm 1.22−1.48 (20H), 1.68−1.91 (4H), 2.04 (m, 4H), 2.71 (t, J = 7.64 Hz, 4H), 4.72−5.17 (8H), 5.81 (ddt, J = 16.99, 10.17, 6.63 Hz, 2H), 7.65 (s, 2H). (Step 5) A toluene (3 mL) solution of 8 (0.20 g, 0.30 mmol), 1,1,1,3,5,5,5-heptamethyltrisiloxane (0.20 g, 0.90 mmol), and platinum(0)−1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex solution in xylene (Pt ∼ 2%) (20 μL) was heated to 60 °C overnight. After solvents were removed, crude products were purified by SiO2 column chromatography (hexane/EtOAc (8/1 and then 5/1), Rf = 0.7) obtain si-1b as a pale brown solid (0.19 g, 58% yield). 1H NMR (CDCl3) δ ppm 0.00 (s, 6H), 0.09 (s, 36H), 0.45 (t, J = 7.52 Hz, 4H), 1.19−1.45 (34H), 1.80 (m, 4H) 2.75 (t, J = 7.76 Hz, 4H), 4.95 (s, 4H), 7.69 (s, 2H). 13C NMR (CDCl3) δ ppm −0.28, 1.85, 17.6, 23.0, 27.1, 27.8, 29.3, 29.5, 29.6, 33.2, 48.4, 102.9, 114.7, 128.2, 134.0, 140.5, 146.1. Anal. Calcd for C48H88Br2N4O4Si6: C, 51.77%; H, 7.97%; N, 5.03%. Found: C, 51.61%; H, 8.21%; N, 4.83%. C

DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules 2.6. Theoretical Calculation. Theoretical calculations were performed on the Gaussian 09W (Revision C.01) package of program using the density functional theory (DFT) method at the B3LYP/631G(d) level of theory to optimize the ground state structure and verify the FMO surfaces.

temperature met with no success, while the microwave heating at 100 °C in a sealed vial resulted in the polymerization. Because the steric hindrance, particularly in the case of 2b, might prevent the smooth reaction, the number-averaged molecular weight of π-conjugated polymers remained low (3200 for P1-F and 1900 for P2-F).32 In order to discuss the optical properties of π-conjugated polymers in detail, model compounds (m1 and m2, Figure 2) were prepared by the

3. RESULTS AND DISCUSSION 3.1. Synthesis. Dibromo monomer 1b which has two imidazole rings at the para-position of benzene and carries the octyl group at the 2-position of imidazole was synthesized by the nucleophilic substitution, intramolecular direct arylation,23 and bromination sequences (Scheme 1). For the preparation of Scheme 1. Synthetic Route of Monomers (1b, 1i, and 2b) Bearing Octyl Group

Figure 2. Chemical structure of model compounds.

Suzuki−Miyaura cross-coupling reaction of 1b and 2b with phenylboronic acid. The Sonogashira−Hagihara cross-coupling polymerizations of 1i with 9,9-dihexyl-2,7-diethynylfluorene and 1,5-dioctyloxy-2,6-bis(trimethylsilylethynyl)naphthalene were subsequently carried out to obtain P1-EF and P1-EN, respectively. For the synthesis of P1-EN, trimethylsilylprotected electron-rich acetylene monomer was used because the unprotected derivative was gradually decomposed upon exposing to air in the solid state.33 Thus, the polymerization was conducted in the presence of n-tetrabutylammonium fluoride as the desilylation reagent. The number-averaged molecular weight of THF-soluble part of P1-EF is 4700; however, the considerably low solubility of P1-EN in THF makes the estimation of molecular weight difficult. The structure of all π-conjugated polymers (oligomers) were checked by 1H NMR spectra to confirm that the integral ratio of proton signals agree with the theoretical value. As mentioned above, the cross-coupling polymerization of fused imidazole monomers bearing the octyl group at the 2position successfully proceeded; however, the solubility of products were low to decrease the polymer molecular weight. For the practical use of π-conjugated polymers, not only the chemical structure but also the solubility, molecular weight, and molecular packing are of great importance to affect the optical and electrochemical properties in the solid state. The good solubility and processability of π-conjugated polymers have been acquired by the attachment of flexible long alkyl groups as the side chain in the past decades. On the other hand, simple linear alkyl chains are not enough to furnish the solubility and processability to significantly rigid rod π-conjugated polymers. Since the pioneering work of Bao et al.,34 several research groups have revealed the utility of siloxane-terminated solubilizing side chains on the π-conjugated main chain.35−38 Unlike in the case of branched alkyl chains such as 2-ethylhexyl and 2-hexyldecyl groups, the introduction of bulky siloxane group at the terminal of linear alkyl chains nicely merges the good solubility and tight π−π stacking structure to record the improved charge carrier mobility in π-conjugated polymerbased OFET devices. With these facts in our mind, we have conceived to prepare fused imidazole monomers (si-1b and si3b) bearing the trisiloxane unit at the terminal of decyl group at the 2-position of imidazole. The synthetic routes starting from 2-(9′-decen-1′-yl)imidazole (5) are indicated in Scheme 2. Four steps reaction

an intermediate 2, the palladium-catalyzed intramolecular direct arylation was conducted with heating under the microwave irradiation at 180 °C for 1.5 h. Although the isolated yield was not good due to the purification procedure (just washing with EtOH), this method was actually effective to complete the reaction in a short time. When the same reaction was performed under the conventional oil bath heating, the longer reaction time and purification by SiO2 column chromatography were required to obtain the product. Diiodo monomer 1i was synthesized by the iodination of 2. Dibromo monomer 2b with the meta-linked fused imidazole structure was obtained in a similar manner using 1,5-bis(bromomethyl)-2,4-dibromobenzene instead of 1,4-bis(bromomethyl)-2,5-dibromobenzene. The structure and purity of these monomers were confirmed by NMR (1H and 13C) spectra and elemental analyses. The Suzuki−Miyaura cross-coupling polymerizations of 1b and 2b with 9,9-dihexylfluorene-2,7-diboronic acid were carried out in THF to obtain π-conjugated polymers P1-F and P2-F, respectively. The oil bath heating at the THF refluxing D

DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

conjugated polymers were checked by 1H NMR spectra to confirm that the integral ratio of proton signals agree with the theoretical value. In addition, a model compound (m3, Figure 2) was prepared by the Suzuki−Miyaura cross-coupling reaction of 3b′, carrying linear octyl group instead of trisiloxane-terminated decyl chain, with phenylboronic acid. The structure and purity of m3 were also confirmed by NMR (1H and 13C) spectra and elemental analysis. 3.2. Optical Properties. Initially, UV−vis absorption and fluorescence emission spectra of P1-F and P2-F obtained by the Suzuki−Miyaura cross-coupling polymerization were measured in CHCl3 solution (10 μM) (Figure 3). Both

Scheme 2. Synthetic Route of Monomers (si-1b and si-3b) Bearing Trisiloxane Unit at the Termini of Decyl Group

Figure 3. UV−vis absorption (left) and fluorescence emission (right) spectra of P1-F and P2-F in CHCl3 solution (10 μM).

absorption and emission maximum wavelengths of P1-F (λabs = 398 nm and λem = 444 nm) corresponding to the π−π* transition exhibited red-shifts from those of P2-F (λabs = 355 nm and λem = 440 nm) because of the para-linked fused imidazole structure. The fluorescence quantum yields relative to quinine sulfate as a standard (0.55) were comparable (0.15 for P1-F, 0.12 for P2-F). The single crystals of m1 and m2, which are model compounds of P1-F and P2-F, respectively, were obtained by the vapor diffusion method using acetone (good solvent) and hexane (poor solvent). In order to investigate the optical properties of P1-F and P2-F in detail, the X-ray crystallographic analyses of m1 and m2 were performed (Figure 4). The torsion angle between imidazole and phenyl rings of m1 is 9°, while that of m2 is 17° suggesting the twisted conformation induced by the steric repulsion of

including the nucleophilic substitution with 1,4-bis(bromomethyl)-2,5-dibromobenzene, intramolecular direct arylation, bromination, and hydrosilylation gave si-1b in totally 20% yield. Other reaction sequences were found to be ineffective; namely, the hydrosilylation of 7 unexpectedly resulted in a poor yield (16%). In contrast to the palladium-catalyzed intramolecular direct arylation of 1 bearing the octyl group (vide supra), the reaction of 6 should be carried out at the lower temperature (130 °C) for the longer reaction time (13 h) to suppress undesirable side reactions of the vinyl group. Monomer si-3b bearing the naphthalene core was likewise synthesized from 2,6bis(bromomethyl)-1,5-dibromonaphthalene. The structure and purity of these monomers were confirmed by NMR (1H and 13 C) spectra and elemental analyses. The Suzuki−Miyaura cross-coupling polymerizations of si-1b and si-3b with 9,9-dihexylfluorene-2,7-diboronic acid were carried out in THF under the microwave heating at 100 °C to obtain π-conjugated polymers Si-P1-F and Si-P3-F, respectively. The number-averaged molecular weight of Si-P1-F (7200) was increased as compared with P1-F (3200), and SiP1-F could be completely dissolved in THF thanks to the trisiloxane-terminated decyl chain. On the other hand, the number-averaged molecular weight of Si-P3-F was 4800, reflecting the steric hindrance between naphthalene and fluorene segments (vide inf ra). The structure of these π-

Figure 4. Single crystal X-ray structures of m1 (a: top view; b: side view) and m2 (c: top view; d: side view) in a stick model (alkyl chains are trimmed for clarity). E

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Macromolecules neighboring benzene rings. It can be deduced that notable steric repulsion should be considered when the benzene ring of m2 is substituted by the 9,9-dihexylfluorene segment. Accordingly, the shorter effective conjugation length of P2-F as compared with P1-F can be ascribed to not only the metalinked fused imidazole structure but also the less planar main chain conformation. Also, the smaller molecular weight of P2-F, which is of course originated from the steric repulsion, might contribute to the hypsochromic shift of spectra. Subsequently, UV−vis absorption and fluorescence emission spectra of P1-EF and P1-EN obtained by the Sonogashira− Hagihara cross-coupling polymerization were measured in CHCl3 solution (10 μM) (Figure 5). The absorption spectra

Figure 6. UV−vis absorption (left) and fluorescence emission (right) spectra of Si-P1-F and Si-P3-F in CHCl3 solution (10 μM).

emission spectra of Si-P3-F having the naphthalene core exhibited blue- and red-shifts, respectively, as compared with those of Si-P1-F. The single crystal of m3, which is a model compound of Si-P3-F, obtained by slowly evaporating a toluene solution was subjected to the X-ray crystallographic analysis (Figure 7). In contrast to the X-ray crystal structure of m1 and

Figure 5. UV−vis absorption (left) and fluorescence emission (right) spectra of P1-EF and P1-EN in CHCl3 solution (10 μM).

of P1-EF having the ethynylene spacer between the fused imidazole segment and fluorene exhibited the π−π* transition band at λabs = 411 nm that shows a red-shift from that of P1-F (λabs = 398 nm). The fluorescence quantum yield of P1-EF was 0.28. The ground state structure of model compounds of P1-F and P1-EF were optimized using the DFT method at the B3LYP/6-31G(d) level of theory (Figure S1). Long alkyl chains were replaced by the methyl group to save the computation cost. The torsion angle between imidazole and fluorene rings of the model of P1-F is 29°, while that of P1-EF is almost 0° due to the release of steric hindrance by the ethynylene spacer. Thus, the different steric environment is supposed to bring about the bathochromic shifts of spectra of P1-EF. On the other hand, both absorption and emission spectra of P1-EN (λabs = 415 nm and λem = 448 nm) were observed at the relatively longer wavelength region due to the wide conjugation system of dialkoxynaphthalene and/or the charge transfer from the electron-rich dialkoxynaphthalene unit to the electrondeficient fused imidazole segment.39 The torsion angle between imidazole and fluorene rings of the model of P1-EN is 20°, which is in between those of P1-F (29°) and P1-EF (0°) (Figures S2 and S3). The fluorescence quantum yield of P1-EN was 0.22. UV−vis absorption and fluorescence emission spectra of SiP1-F and Si-P3-F bearing the trisiloxane unit at the terminal of decyl group were likewise measured in CHCl3 solution (10 μM) (Figure 6). As shown in Figure S5, the absorption spectrum of Si-P1-F exhibited a subtle red-shift from that of P1-F, which might be stemmed from the difference in the polymer molecular weight (vide supra). Because the shapes of spectra resemble each other, the trisiloxane unit at the termini has a negligible influence on the effective conjugation length of π-conjugated polymers in the solution state. The fluorescence quantum yield of Si-P1-F was 0.11. The absorption and

Figure 7. Single crystal X-ray structure of m3 (a: top view; b: side view) in a stick model (alkyl chains are trimmed for clarity).

m2, the torsion angle between imidazole and phenyl rings of m3 is significantly large (58°) due to the steric repulsion between naphthalene and benzene rings. The marked deviation from the planar conformation is thus responsible for the hypsochromic shift of the absorption spectrum of Si-P3-F. It can be deduced that in the excited state the polymer chain conformation might change to make Si-P3-F more planar structure. The fluorescence quantum yield of Si-P3-F was low (0.05) owing to the nonradiative decay from the excited state. The CHCl3 solution (5 wt %) of P1-F and Si-P1-F was spincoated on the quartz glass substrate. Because the polymer molecular weights are limited, the microscopic and X-ray diffraction analyses of polymer films were unfortunately impossible. Thus, the ordering structure in the solid state was studied on the basis of spectroscopic measurements. The absorption spectra shown in Figure S6 indicate that both the absorption maximum and absorption onset in film were clearly red-shifted as compared with those observed in CHCl3 solution (see Figure 3). These facts are resulted from the intermolecular interaction of P1-F and Si-P1-F in the solid state, and the effective conjugation lengths are extended owing to the π−π stacking interaction of polymer chains. Figure 8 shows the fluorescence emission spectra before and after thermal annealing at various temperatures. In comparison to the emission spectrum in CHCl3 solution (λem = 444 nm), the as-cast film of P1-F showed a pronounced red-shift (λem = 565 nm) along with the emission bands at 460 and 492 nm. The single crystal X-ray structure analysis of m1 revealed that each molecule is closely packed by the C−H···π and π···π stacking interactions within the distance of ca. 3.56 Å and forms a F

DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Figure 8. Change of fluorescence emission spectra of P1-F (left) and Si-P1-F (right) in film at various annealing temperatures (as-cast, 80 °C, and 120 °C).

columnar structure along the b-axis (space group = P21/c, Z = 4) (Figure S8). Therefore, the large bathochromic shift is likely stemmed from the excimer formation between π-conjugated polymer chains, and the shorter wavelength emissions should be ascribed to the emission from the single polymer chain. The thermal annealing brought about a small spectral change, suggesting that the packing structure is tight to inhibit the morphological transition. On the other hand, the as-cast film of Si-P1-F showed emission bands at 458 and 490 nm in the intermediate intensity as well as the main emission band at 563 nm. The relatively large peak at the shorter wavelength, which originated from the emission from the single polymer chain, might be ascribed to that the bulky trisiloxane terminal group located at the side chain somewhat hinders the π−π stacking interaction of polymer main chain. Upon thermal annealing at 80 °C, however, the shorter wavelength emission bands were notably diminished. By comparing two spectra in Figure 8, it was found that the emission bands at the shorter wavelength of Si-P1-F became much smaller than those of P1-F after the thermal annealing at 120 °C. The emission maximum wavelength of Si-P1-F was also shifted from 563 to 568 nm. These spectral change upon the thermal annealing can be attributable for the higher chain mobility of Si-P1-F endowed by the trisiloxane-terminated side chain.36 Finally, using P1-F, P1-EF, and P1-EN, the protonation of imidazole nitrogen was performed to tune the optical properties of π-conjugated polymers and clarify the influence of comonomer structure. For example, Figure S9 shows the 1H NMR spectra of P1-EF in CDCl3 before and after adding excess amount of trifluoroacetic acid (TFA). Aromatic proton signals at around 7.6 ppm and methylene proton signals included in the fused imidazole segment at 5 ppm were unambiguously shifted to the downfield region, implying the decreased electron density triggered by the protonation. After the proton doping, both the UV−vis absorption (π−π* transition band) and fluorescence emission maxima of P1-F exhibited blue-shifts of about 10 nm (Figure 9a,b), which is due to the increased steric hindrance as Carter et al. previously discussed.24 In contrast, the UV−vis absorption maxima (π−π* transition band) of P1-EN exhibited red-shift from 415 to 446 nm, while the fluorescence emission maxima also red-shifted from 448 to 490 nm (Figure 9c,d). It can be considered that the protonation of imidazole nitrogen facilitates the charge transfer interaction between the electron donor (dialkoxynaphtharene) and electron acceptor (fused π-conjugated imidazolium cation). The steric hindrance occurring in P1-F is negligible in the case of P1-EN for the sake of ethynylene spacer. P1-EF with the

Figure 9. UV−vis absorption (a, c) and fluorescence emission (b, d) spectra of P1-F (a, b) and P1-EN (c, d) before and after adding TFA in CHCl3 solution (10 μM).

weakly electron-donating fluorene unit and ethynylene spacer showed small peak shifts (Figure S7). To gain deeper insight into the proton doping experiment, the DFT calculation was performed to optimize the ground state structure of model compounds of P1-F and P1-EN along with their proton adducts. Upon protonation, the torsion angle between imidazole and fluorene rings of the model of P1-F was increased from 29° to 40° (Figure S2), while the conformation change was unimportant in the case of P1-EN (Figure S3). Furthermore, the pronounced charge transfer D−A interaction was confirmed in the protonated P1-EN; namely, the HOMO and LUMO surfaces were apparently localized on the dialkoxynaphtharene and fused π-conjugated imidazolium cation units, respectively (Figure S4).

4. CONCLUSIONS A series of fused imidazole monomers bearing octyl chain or siloxane-terminated decyl chain as the solubilizing group at the 2-position of imidazole were synthesized by utilizing the microwave-assisted intramolecular direct arylation. The Suzuki−Miyaura and Sonogashira−Hagihara cross-coupling polymerizations were carried out to obtain rigid rod π-conjugated polymers (oligomers). The bulky siloxane group at the side chain termini contributes the improvement of polymer solubility to increase the molecular weight. The UV−vis absorption and fluorescence emission spectra in solution and film figure out that the fused structure and coupling partner affect the optical properties, and the bulky siloxane group endows π-conjugated polymer with better chain mobility in the solid state. The protonation of imidazole nitrogen could handle the donor−acceptor charge transfer interaction in π-conjugated polymers. These results prove the potential utility of imidazole as the constituent of π-conjugated polymers with the tunable electronic state and ordering structure. G

DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01768. Theoretical calculations, spectroscopic data, and X-ray crystal structure (CCDC 1498337-1498339) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (K.T.). ORCID

Koji Takagi: 0000-0002-6107-0761 Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.macromol.6b01768 Macromolecules XXXX, XXX, XXX−XXX