Blue-Greenish Electroluminescent Poly(p-phenylenevinylene

Feb 23, 2016 - A novel electroluminescent poly(p-phenylenevinylene) (PPV) derivative was synthesized via the Gilch route, which emits in the blue-gree...
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Blue-Greenish Electroluminescent Poly(p‑phenylenevinylene) Developed for Organic Light-Emitting Diode Applications Nicole Vilbrandt,*,† Andrea Gassmann,‡ Heinz von Seggern,‡ and Matthias Rehahn† †

Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science and ‡Materials Science and Geoscience Department, Darmstadt University of Technology, 64287 Darmstadt, Germany S Supporting Information *

ABSTRACT: A novel electroluminescent poly(p-phenylenevinylene) (PPV) derivative was synthesized via the Gilch route, which emits in the blue-greenish region. The required monomer synthesis is a multistep process starting from catechol and does not involve any critical step. The polymer synthesis itself proceeds via standard Gilch conditions and results in constitutionally homogeneous and extraordinary high-molecular-weight PPVs. The characterization of these materials was carried out using nuclear magnetic resonance spectroscopy and size exclusion chromatography measurements. The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels were estimated by combining information provided by cyclic voltammetry and UV−vis measurements. Finally, the electroluminescent behavior of the polymer was confirmed in an organic light-emitting diode.

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INTRODUCTION Since the first observation of electroluminescence in πconjugated polymers, specifically in poly(p-phenylenevinylene) (PPV) and their thus recognized value as functional materials, for e.g. organic light-emitting diode (OLEDs) applications, much effort has been spent to improve the property profiles of this class of polymeric semiconductors systematically.1 From the early days on, especially the development of efficient chaingrowth polymerization leading to soluble precursor polymers followed by their final conversion into PPVs paved the way to fabricate promising OLED devices.2−6 Later, the attachment of lateral substituents solved some prohibitive solubility issues additionally.7 To date, chain-growth methods are broadly useddespite of the development of some promising alternative step-growth polymerization protocols.8−14 One of the economically most attractive and synthetically most fascinating synthetic strategies toward PPVs is the socalled Gilch route.4 Here the treatment of α,α′-dihalogenated pxylene derivatives with a base, like potassium tert-butoxide, leads to high-molecular-weight materials in good to excellent yields within seconds only, even at room temperature. Another important benefit of the Gilch route is its high tolerance toward a variety of lateral substituents needed to e.g. improve solubility, electric characteristics, conductivity, and emission color.15 For applications such as solid-state lighting it is important to realize at least the three fundamental colors red, green, and blue to get white-light emission.16,17 PPVs, on the other hand, are known to emit in the orange-red or green region of the visible spectrum, depending on the substitution pattern.15 Alkoxy substituents in the 2,5-position of the aromatic backbone cause a shift to orange-red emission; the use of those alkoxy © XXXX American Chemical Society

substituents in the 2,3-position of the aromatic backbone results in green emission.18−20 However, the number of reports dealing with PPVs that give blue emission is significantly lower.21−24 Mikroyannidis, for example, describes different PPVs bearing a dendritic aromatic system as bulky lateral substituent that are reported to show blue photoluminescence (PL).25,26 On the basis of the findings mentioned above, we are interested in developing new PPV materials that complete the required emission spectrum. In the present contribution we describe the PPV derivative OC48C48 PPV, polymerized via the Gilch route, which shows extremely high molecular weights and emits blue-greenish when applied in OLEDs.

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EXPERIMENTAL SECTION

Materials. Morpholine, paraformaldehyde, 4-bromophenol, 2ethylhexyl bromide, n-butyllithium (n-BuLi), 4-fluoroaniline, bromine, tert-butyl nitrite, sodium sulfite, acetic anhydride, glacial acetic acid, hydrogen bromide in glacial acetic acid, hydrogen chloride, dry N,Ndimethylformamide (DMF), acetonitrile, and potassium tert-butoxide (KOtBu) were purchased from Acros Organics. Cesium carbonate and trimethyl borate were purchased from Sigma-Aldrich, catechol was from Alfa Aesar, and sodium hydroxide, sodium sulfate, and magnesium sulfate were purchased from Gruessing. Tetrakis(triphenylphosphine)palladium (Pd(PPh3)4) was synthesized according to literature procedures.27 All solvents were purchased from Fisher Scientific. All chemicals and solvents were used without further purification except for tetrahydrofuran (THF) which was dried over sodium benzophenone. All reactions were carried out under N2 conditions. Received: June 9, 2015 Revised: February 10, 2016

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DOI: 10.1021/acs.macromol.5b01249 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Analytical Methods. 1H NMR and 13C NMR spectra were recorded on a Bruker DRX 300 or DRX 500 spectrometer. NMR chemical shifts were referenced relative to the corresponding deuterated solvent. Mass spectra were obtained on a Finnigan MAT 95. Fourier transform infrared spectroscopy (FT-IR) was performed using a Paragon 1000, and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FT-IR) was performed using a FT-IR spectrometer from PerkinElmer. Size-exclusion chromatography (SEC) was performed with THF as the mobile phase (flow rate 1 mL min−1) on a SDV column set (SDV 1000, SDV 100000, SDV 1000000) from PSS (Polymer Standards Service GmbH, Mainz). All measurements were carried out at 30 °C using polystyrene standards (from PSS, Mainz). Cyclic voltammetry (CV) measurements were performed as thin-film measurements in a custom-made measurement cell using a multichannel potentiostat VMP2 (Princeton Applied Research). All measurements were performed under an inert atmosphere (N2). To calibrate the detected potentials, ferrocene was used as a reference. Ultraviolet−visible (UV−vis) absorption spectra were recorded using a Tidas II spectrometer (J&M Analytische Messand Regeltechnik GmbH, Aalen) and a quartz cuvette of 10 mm width. Device Preparation. Glass substrates coated with 100 nm indium tin oxide (ITO) (Visiontek Systems Ltd.) were photolithographically structured and ultrasonically cleaned in a detergent, acetone, and isopropanol. Afterward, the substrates were subjected to a UV ozone treatment. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) dispersed in water (Clevios AI4083 by Heraeus) was spin-coated on the substrates in air and annealed at 110 °C for 5 min to form a 30 nm thick film. Subsequently, OC48C48 PPV was spincoated in a glovebox (N2) from toluene solution to yield layers of 50− 70 nm thickness. In a vacuum chamber 20 nm of calcium and 100 nm of aluminum were deposited by physical vapor deposition (PVD). The organic diodes were defined by the overlap of the ITO anode structure and the Ca/Al cathode strip and have an active area of 10 mm2. Device Characterization. All measurements were conducted in a glovebox (N2). Current density−luminance−voltage characteristics of the OLEDs were measured in the dark using a HP 4145B parameter analyzer, employing calibrated silicon photodiodes for measuring the light output. The electroluminescence (EL) spectrum was recorded with a Maya2000 Pro spectrometer from Ocean Optics Inc. The photoluminescence (PL) spectrum was recorded in air using a Varian Cary Eclipse fluorescence spectrophotometer from Varian Inc. and an excitation wavelength of 400 nm. 3,6-Bis(morpholinomethyl)benzene-1,2-diol (1). 1 was synthesized according to the procedure in the literature.28 A mixture of paraformaldehyde (6.0 g, 0.20 mol) and morpholine (17.4 g, 0.20 mol) in isopropyl alcohol (70 mL) was heated to 90 °C for 20 min under N2 conditions until it was homogeneous. After that it was cooled to 70 °C before catechol (11.0 g, 0.10 mol) in isopropyl alcohol (50 mL) was added. The mixture was stirred for 20 h at 70 °C, cooled to room temperature, and filtered. The resulting solid was recrystallized from ethanol to give the product as light beige crystals (16.34 g, 53%) (mp 176−177 °C). 1H NMR (300 MHz, CDCl3): δ 6.42 (s, 2H, ArH), 3.73 (t, 8H, J = 4.6 Hz, CH2O), 3.66 (s, 4H, CH2), 2.55 (s, 8H, CH2N). 13C NMR (75 MHz, CDCl3): δ 145.5, 120.9, 118.9, 66.9, 61.7, 53.0. HR-MS (EI, m/z): [M+] calcd for C16H24N2O4, 308.1731; found, 308.1720. FT-IR (KBr, cm−1): 3400−2854 (ν(−OH)), (ν(=C−H), aromatic stretching), (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), (ν(−N−CH3)), 1578, 1459 (δ(−C−H), aliphatic deformation), (ν(−CC), aromatic stretching), 1386 (δ(−CH3), aliphatic deformation), (δ(−OH)), 1116 (ν(C−O−C), ether stretching), 824 (δ(C−H), aromatic deformation 1,4-disubstitution). 1,2,3-Tribromo-5-fluorobenzene (7). 7 was synthesized in the style of a procedure described in the literature.29 Acetonitrile (50 mL) was heated to 50 °C before the addition of bromine (15 mL, 0.29 mol) followed by the careful and slow addition of 4-fluoroaniline (10 g, 0.09 mol) in acetonitrile (25 mL). After 1 h at 50 °C tert-butyl nitrite (11 mL, 0.09 mol) in acetonitrile (50 mL) was added dropwise to the reaction mixture and stirred for 1 h at 50 °C before cooling to room temperature. Afterward, a solution of prechilled saturated aqueous

sodium sulfite was added to reduce excess bromine until the reaction mixture became light brown. Later on, HCl (2.0 M, 400 mL) and diethyl ether (200 mL) were added. The water phase was extracted with diethyl ether (200 mL) twice, and the combined organic phases were washed with HCl (2.0 M, 2 × 200 mL). The organic phase was dried over MgSO4 before the solvent was evaporated. The resulting solid was recrystallized from n-hexane to give the product as light orange needles (8.02 g, 27%) (mp 98−100 °C). 1H NMR (300 MHz, CDCl3): δ 7.38 (d, 2H, J = 7.7 Hz, ArH). 13C NMR (75 MHz, CDCl3): δ 162.4, 159.1, 126.3, 123.1, 120.6, 120.3. MS (EI, m/z): [M+] calcd for C6H2Br3F, 332; found, 332. FT-IR (KBr, cm−1): 3075 (ν(C−H), aromatic stretching), 1234 (ν(C−F), aromatic stretching), 857 (δ(C−H), aromatic deformation 1,3-disubstitution). 4,4′-((2,3-Bis(3,4,5-tribromophenoxy)-1,4-phenylene)bis(methylene)dimorpholine (2). A mixture of 1 (1.5 g, 0.004 mol) and cesium carbonate (3.75 g, 0.012 mol) in dry DMF (20 mL) was heated under N2 conditions for 1 h to 80 °C before dropwise addition of 1,2,3-tribromo-5-fluorobenzene (3.75 g, 0.009 mol) in dry DMF (20 mL). The reaction mixture was stirred for 72 h at 80 °C. After cooling the mixture to room temperature, it was extracted with nhexane (3 × 200 mL) to remove the excess of 1,2,3-tribromo-5fluorobenzene (7). Afterward the reaction mixture was extracted with methylene chloride (3 × 100 mL). The combined organic phases were washed with water several times (100 mL) and dried over MgSO4 before the solvent was evaporated. The product was received as a light beige crystalline solid (1.01 g, 22%) (mp 155−156 °C). 1H NMR (300 MHz, CDCl3): δ 7.35 (s, 2H, ArH), 6.90 (s, 4H, ArH), 3.56 (t, 8H, J = 4.4 Hz, CH2O), 3.42 (s, 4H, CH2), 2.39 (t, 8H, J = 4.5 Hz, CH2N). 13 C NMR (75 MHz, CDCl3): δ 157.1, 145.2, 132.8, 128.2, 125.8, 120.3, 120.1, 67.0, 57.2, 53.5. HR-MS (EI, m/z): [M+] calcd for C28H26Br6N2O4, 933.6926; found, 933.6961. FT-IR (KBr, cm−1): 3056 (ν(C−H), aromatic stretching), 3000−2806 (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), (ν(−N−CH3)), 1679−1563 (ν(−C C), aromatic stretching), 1410 (δ(−C−H), aliphatic deformation), 1230, 1116 (ν(C−O−C), ether stretching), 866 (δ(C−H), aromatic deformation 1,3-disubstitution). 1-Bromo-4-(2′-ethylhexyloxy)benzene (5). 5 was synthesized according to literature procedure with slight modification.30 4-bromo phenol (40.0 g, 0.23 mol), dried potassium carbonate (91.0 g, 0.66 mol) and dry DMF (100 mL) were mixed under N2 conditions and heated to 60−65 °C. After 1 h 2-ethylhexyl bromide (42 mL, 0.23 mol) was added dropwise, the reaction mixture was stirred for 20 h at 60−65 °C. After cooling the mixture to room temperature, it was quenched by addition of HCl (2.0 M, 125 mL) and n-hexane (200 mL) was added. The water phase was extracted with n-hexane twice, and the combined organic phases were washed with HCl (2.0 M, 3 × 100 mL). Afterward, the organic phase was washed with NaOH (2.0 M, 100 mL) and water (2 × 100 mL). The organic phase was dried over Na2SO4 before the solvent was evaporated. The product was received as a colorless liquid (50.61 g, 77%). 1H NMR (300 MHz, CDCl3): δ 7.37 (d, 2H, J = 9.1 Hz, ArH), 6.79 (d, 2H, J = 9.1 Hz, ArH), 3.81 (d, 2H, J = 5.7 Hz, OCH2), 1.76−1.68 (m, 1H, CH), 1.55− 1.29 (m, 8H, CH2), 0.95−0.89 (m, 6H, CH3). 13C NMR (75 MHz, CDCl3): δ 158.7, 132.3, 116.5, 112.6, 70.9, 39.5, 30.7, 29.2, 24.0, 23.2, 14.2, 11.2. HR-MS (EI, m/z): [M+] calcd for C14H21BrO, 284.0770; found 284.0770. FT-IR (KBr, cm−1): 3010−2925 (ν(C−H), aromatic stretching), (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), 1590, 1487 (ν(−CC), aromatic stretching), 1380 (δ(−CH3), aliphatic deformation), 1243, 1032 (ν(C−O−C), ether stretching), 1072 (ν(C−Br), aromatic stretching), 820 (δ(C−H), aromatic deformation 1,4-disubstitution). 4-(2′-Ethylhexyloxy)benzene Boronic Acid (6). 6 was synthesized according to literature procedure with slight modification.30 A solution of 5 (10.0 g, 0.035 mol) in anhydrous, degassed THF (116 mL) was cooled to −80 °C under N2 before the addition of n-BuLi (21 mL, 0.053 mol, 2.5 M in n-hexane). The reaction mixture was stirred for 1 h at −80 °C before the addition of trimethyl borate (14 mL, 0.123 mol). The mixture was warmed to room temperature and stirred for 18 h before quenching with HCl (2.0 M, 150 mL). The resulting mixture was extracted with chloroform (2 × 150 mL). The resulting B

DOI: 10.1021/acs.macromol.5b01249 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules organic phase was washed with water (2 × 150 mL) and brine (100 mL) and dried over Na2SO4 before evaporating the solvent. The residue was purified with column chromatography on silica gel with nhexane/ethyl acetate (2:1) to yield a light beige liquid (5.10 g, 58%). 1 H NMR (300 MHz, CDCl3): δ 8.16 (d, 2H, J = 8.1 Hz, ArH), 7.02 (d, 2H, J = 8.2 Hz, ArH), 3.94 (d, 2H, J = 5.8 Hz, OCH2), 1.84−1.72 (m, 1H, CH), 1.58−1.29 (m, 8H, CH2), 0.99−0.88 (m, 6H, CH3). 13C NMR (75 MHz, CDCl3): δ 163.2, 137.6, 114.2, 70.6, 39.6, 30.7, 29.3, 24.1, 23.2, 14.2, 11.3. HR-MS (EI, m/z): [M+] calcd for C14H23BO3, 250.1740; found 250.1738. FT-IR (KBr, cm−1): 3437 (ν(−OH)), 3039 (ν(C−H), aromatic stretching), 3000−2921 (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), 1601 (ν(−CC), aromatic stretching), 1385 (δ(−CH3), aliphatic deformation), (δ(−OH)), 1168, 1110, 1034 (ν(C−O−C), ether stretching), 835 (δ(C−H), aromatic deformation 1,4-disubstitution). 4,4′-(2,3-Bis(1,2,3-tris(4-(2′-ethylhexyloxy)phenyl)-5-phenoxy)-1,4-phenylene)bis(methylene)dimorpholine (3). A mixture of 2 (0.7 g, 0.82 mmol), 6 (1.44 g, 5.8 mmol), degassed aqueous sodium carbonate solution (2.0 M, 5 mL), degassed THF (10 mL), and Pd(PPh3)4 (0.057 g, 0.05 mmol) was heated under N2 conditions for 72 h to 80 °C. After cooling the reaction mixture to room temperature, n-hexane (50 mL) was added. The organic phase was washed with water (2 × 50 mL) and brine before drying over MgSO4. Afterward the solvent was evaporated. The residue was purified with column chromatography on silica gel with n-hexane/ethyl acetate (1:1) to yield a yellow liquid (0.72 g, 57%). 1H NMR (300 MHz, CDCl3): δ 7.43−6.51 (m, 30H, ArH), 3.87−3.59 (m, 24H, OCH2, CH2Ar, CH2O), 2.48 (s, 8H, CH2N), 1.71−1.61 (m, 6H, CH), 1.50− 1.27 (m, 48H, CH2), 0.93−0.87 (m, 36H, CH3). 13C NMR (75 MHz, CDCl3): δ 157.9, 157.4, 156.6, 146.3, 142.8, 134.3, 133.0, 131.8, 130.9, 128.3, 127.1, 119.3, 116.2, 113.7, 113.7, 70.6, 67.1, 56.9, 53.7, 39.5, 30.7, 29.3, 24.0, 23.2, 14.2, 11.3. MS (EI, 200−280 °C, m/z): [M+] calcd for C112H152N2O10, 1686; found, 1686. FT-IR (KBr, cm−1): 3050−2928 (ν(C−H), aromatic stretching), 2928, 2859 (ν(−C− H), ν(−CH2), ν(−CH3), aliphatic stretching), (ν(−N−CH3)), 1604 (ν(−CC), aromatic stretching), 1462 (δ(−C−H), aliphatic deformation), 1287, 1245, 1173, 1118, 1034 (ν(C−O−C), ether stretching), 831 (δ(C−H), aromatic deformation 1,4-disubstitution). 1,4-Bis(bromomethyl)-2,3-bis(1,2,3-tris(4-(2′-ethylhexyloxy)phenyl)-5-phenoxy)benzene (M1). A mixture of 3 (0.7 g, 0.42 mmol) and acetic anhydride (10 mL) was heated under N2 conditions for 72 h to 150 °C. After cooling the reaction mixture to room temperature cold water (50 mL) was added, and the mixture stirred for 1 h at room temperature. The aqueous phase was extracted with chloroform (2 × 50 mL) before the combined organic phases were washed with water (4 × 50 mL) before drying over MgSO4. Afterward, the solvent was evaporated to give crude 4,4′-(2,3-bis(1,2,3-tris(4-(2′ethylhexyloxy)phenyl)-5-phenoxy)-1,4-phenylene)bis(methylene) diacetate (4) as a brown liquid (1.10 g), which was used for the next reaction without further purification. MS (EI, 300−350 °C, m/z): [M+] calcd for C108H142O12, 1632; found, 1632. FT-IR (KBr, cm−1): 3392 (ν(−OH)), 3000−2964 (ν(−C-H), ν(−CH2), ν(−CH3), aliphatic stretching), 1745 (ν(−CO), carbonyl stretching), 1609 (ν(−CC), aromatic stretching), 1379 (δ(−CH3 ), aliphatic deformation), (ν(−O−CO−CH3), ester stretching), 1244, 1175, 1118, 1034 (ν(C−O−C), ether stretching), 830 (δ(C−H), aromatic deformation 1,4-disubstitution). A mixture of 4 (1.10 g), HBr in acetic acid (2 mL, 5.7 M), and glacial acetic acid (2 mL) was stirred for 20 h at room temperature. Afterward, cold water (100 mL) and chloroform (50 mL) were added. The aqueous phase was extracted with chloroform (3 × 50 mL). The combined organic phases were washed with water (2 × 50 mL) and saturated sodium hydrogen carbonate solution (100 mL) before drying over MgSO4 and evaporating the solvent. The residue was purified with column chromatography on silica gel with n-hexane/ chloroform (1:1) to yield a colorless liquid (0.12 g, 17%). 1H NMR (300 MHz, CDCl3): δ 7.47−6.51 (m, 30 H, ArH), 4.56 (s, 4H, CH2Br), 3.88−3.71 (m, 12H, OCH2), 1.69−1.62 (m, 6H, CH), 1.53− 1.27 (m, 48H, CH2), 0.97−0.84 (m, 36H, CH3). 13C NMR (75 MHz,

CDCl3): δ 157.9, 157.4, 155.8, 145.7, 142.9, 134.1, 133.9, 133.0, 131.7, 131.1, 127.7, 122.7, 121.8, 117.4, 116.4, 113.7, 70.6, 39.6, 30.7, 29.3, 27.2, 24.0, 23.2, 14.2, 11.3. MS (EI, 250−280 °C, m/z): [M+] calcd for C104H136O8Br2, 1674; found, 1674. FT-IR (KBr, cm−1): 3036 (ν( C−H), aromatic stretching), 2920 (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), 1607 (ν(−CC), aromatic stretching), 1446 (δ(−C−H), aliphatic deformation), (ν(−CC), aromatic stretching), 1246, 1034 (ν(C−O−C), ether stretching), 830 (δ(C−H), aromatic deformation 1,4-disubstitution), 542 (ν(C−Br), aliphatic stretching). Polymerization of M1. Monomer M1 (0.12 g, 0.07 mmol) was dissolved in dry, degassed THF (1.0 mL) and cooled to −10 °C. Afterward a solution of potassium tert-butoxide (0.03 g, 0.3 mmol) in dry, degassed THF (0.5 mL) was added to the vigorously stirred monomer solution in one portion. After 10 min the mixture was warmed to room temperature. After 2 h at room temperature, further THF (3 mL) was added. The mixture was stirred at room temperature for 72 h before the resulting polymer was precipitated in methanol (35 mL), filtered, redissolved in THF, and again precipitated in methanol. The resulting solid was filtered and dried under reduced pressure for 24 h at 40 °C. The product was received as a yellow powder (0.08 g, 74%). 1H NMR (500 MHz, CDCl3): δ 7.47−6.45 (m, br, 32H, ArH, HCCH), 3.77−3.65 (m, br, 12H, OCH2), 1.67−1.56 (m, br, 6H, CH), 1.53−1.26 (m, br, 48H, CH2), 0.86 (s, br, 36H, CH3). ATR-FTIR (cm−1): 3061, 3035 (ν(C−H), aromatic stretching), 2957, 2924, 2871, 2858 (ν(−C−H), ν(−CH2), ν(−CH3), aliphatic stretching), 1607 (ν(−CC), aromatic stretching), 1444, (δ(−C−H), aliphatic deformation), (ν(−CC), aromatic stretching), 1380 (δ(−CH3), aliphatic deformation), 1239, 1173, 1033 (ν(C−O−C), ether stretching), 999 (γ(C−H), vinylic out-of-plane deformation), 826 (δ(C−H), aromatic deformation 1,4-disubstitution). Mn (SEC/IR) 449 000 g mol−1, PDI = 3.4.

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RESULTS AND DISCUSSION Synthesis and Characterization of the Polymer. The synthesis of monomer M1 was achieved following a multistep process described by Martin et al. starting with a double Mannich reaction of catechol (see Scheme 1).18,19,28 The second step, an etherification, was performed using 1,2,3tribromo-5-fluorobenzene (7), which was synthesized in the style of a multiple bromination described by Doyle et al.,29 to yield 2 in 22%. Here, the rather low amount of resulting material was mainly due to steric hindrance and slow reaction rates. The third step consisted of a Suzuki coupling between 2 and the corresponding boronic acid 6, which was synthesized according to literature procedures.30,31 Similar to the procedure described by Martin et al.18,19 compound 3 was acetylated by the addition of acetic anhydride to yield 4 before hydrolysis and nucleophilic substitution yielded monomer M1 in 17%. Subsequent polymerization was performed under standard Gilch conditions for 3 days using an excess of base to significantly lower the amount of remaining halogen content in the polymers main chain. Finally, the OC48C48 PPV was received as a yellow powder in good yield (74%). PPVs molecular weight determined by SEC (calibrated with polystyrene standards) was Mn = 450 000 g mol−1 (∼297 repeating units) and a polydispersity of 3.4. Although the molecular weight determined by SEC is known to be overestimated caused by the rigidity of the conjugated polymer using polystyrene standards for calibration, the OC48C48 PPVs molecular weight was extraordinarily high for a material containing two bulky lateral side groups. In comparison, the molecular weight reported for a PPV derivative containing 2ethylhexyloxy substituents in the 2,3-position of the aromatic backbone (BEH-PPV) is only Mn = 51 000 g mol−1 and for a PPV derivative containing merely one similar bulky lateral sideC

DOI: 10.1021/acs.macromol.5b01249 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of Monomer M1 and Polymer OC48C48 PPVa

Reagents and conditions: (a) paraformaldehyde, morpholine, isopropyl alcohol, 90 to 70 °C, 53%; (b) cesium carbonate, 1,2,3-tribromo-5fluorobenzene, DMF, 80 °C, 22%; (c) 4-(2′-ethylhexyloxy)benzene boronic acid, Na2CO3, THF, H2O, Pd(PPh3)4, 80 °C, 57%; (d) acetic anhydride, 150 °C; (e) HBr, acetic acid, rt, 17%; (f) KOtBu, THF, −10 °C to rt, 74%; (g) K2CO3, 2-ethylhexyl bromide, DMF, 60−65 °C, 77%; (h) n-BuLi, trimethyl borate, THF, −80 °C to rt, 58%; (i) bromine, tert-butyl nitrite, acetonitrile, 50 °C, 27%. a

chain in the 2-position of the aromatic backbone is Mn = 28 000 g mol−1 when synthesized under analogous conditions.18,26 To verify the expected chemical constitution of the OC48C48 PPV 1H NMR spectroscopy was performed in methylene-d2 chloride solution (see Figure 1). Here the intense and broad absorption confirmed SEC results, as this represents the characteristic signature of a high-molecular-weight material. Between δ ≈ 7.65 and 6.17 ppm the signals of the vinylene protons of the regular trans-configured PPV repeating unit, the aromatic protons of the polymeric main chain, and the aromatic protons of the dendritic lateral substituents were observed. At δ ≈ 3.79 and 3.69 ppm the signals of the substituents aliphatic OCH2-groups were detected. The protons of the aliphatic CH groups absorbed between δ ≈ 1.69 and 1.62 ppm, those of the CH2-groups between δ ≈ 1.51 and 1.27 ppm, and the CH3groups at δ ≈ 0.86 ppm. As the signals of the benzylic protons between δ ≈ 4.58 and 4.50 ppm (CH2Br) completely disappeared, the Gilch polymerization was assumed to be successful and complete. Afterward, OC48C48 PPVs oxidation and reduction potentials were identified by cyclic voltammetry (CV). The respective measurement cell consisted of a homemade flask and three electrodes: a working electrode made of glassy carbon, a platinum wire as counter electrode, and an Ag/AgCl electrode as reference electrode. All electrodes were connected to a potentiostat and immersed in an electrolyte that consisted of tetrabutylammonium tetrafluoroborate (Bu4NBF4) and acetonitrile. The polymer thin films were deposited from a THF solution (∼4 mg mL−1) and dried at ambient temperature. The measurements were carried out under inert atmosphere (N2) at a scan rate of 100 mV s−1. The standard potential of ferrocene (Fc/Fc+) acted as a reference to calibrate all measurements. In Figure 2 the oxidation and reduction curves of the polymer are displayed. The onset of oxidation of the OC48C48 PPV occurred at Eox,onset = 1.4 V and the onset of reduction at Ered,onset = −1.5 V. Both events took place near the edge of the measuring range of

the electrolyte and seemed to be irreversible. The corresponding values for the ionization potential (IP) and electron affinity (EA) respective the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energetic levels were calculated using those redox potentials.32 Thus, the HOMO energy level of the OC48C48 PPV was estimated to be −5.8 eV and the LUMO energy level −2.9 eV, resulting in a band gap of Eelec = 2.9 eV. In comparison to the BEH-PPV reported by Martin et al.19 introducing those bulky lateral substituents to the OC48C48 PPV lowers the HOMO energy level by −0.4 eV and the LUMO energy level by −0.1 eV. To analyze the photophysical properties of the OC48C48 PPV ultraviolet−visible absorption (UV−vis), photoluminescence (PL) and electroluminescence (EL) measurements were performed. To provide better comparability, the intensities of all measurements were normalized (see Figure 3). UV−vis absorption measurements were conducted using a solution of OC48C48 PPV in spectroscopy grade THF (∼0.3 mg mL−1). As can be seen in Figure 3 (black curve), the main absorption maximum appeared at λmax = 277 nm. This absorption band originates, in accordance with Mikroyannidis,25,26 from the dendritic aromatic substituents. In addition to the main absorption maximum, a few less intensive absorption maxima were detected at 250 nm, 308 nm, and a broad maximum at 442 nm. The onset of absorption for the broad absorption band was 497 nm, corresponding to an optical band gap of Eopt = 2.49 eV. According to Martin et al.,19 the longestwavelength absorption maximum detected for the poly(2,3bis(2-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV) appears at 446 nm, and by replacing the substituent with a “smaller” e.g. butyloxy group (DB-PPV33), it appears at 454 nm. In both cases the longest-wavelength absorption maximum is assigned to the π−π* interaction of the polymers main chain. The same applies for the OC48C48 PPV; only here the π−π* interaction of the polymers main chain is much weaker and the effective conjugation length much shorter, as the bulky lateral D

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Figure 1. 1H NMR spectrum of the OC48C48 PPV recorded in CD2Cl2 (∗) solution.

Figure 3. Normalized ultraviolet−visible (UV−vis), photoluminescence (PL), and electroluminescence (EL) spectra of the OC48C48 PPV.

Figure 2. CV curve of an OC48C48 PPV thin film measured on a glassy carbon electrode in acetonitrile and Bu4NBF4 (0.1 M) as electrolyte at a scan rate of 100 mV s−1. Figure 4. Luminance−voltage curves of a device based on OC48C48 PPV.

substituents cause a more pronounced twist between the aromatic ring and the double bond along the main chain. This weaker π−π* interaction results in a lower HOMO energy level and therefore a slight hypsochromic shift for the longestwavelength absorption maximum. This explanation also applies for the emission spectra of the OC48C48 PPV. For the PL measurements performed at an excitation wavelength of 400 nm a polymer thin film was spin-coated from toluene solution (∼50 nm). As displayed in Figure 3 (red curve) the PL emission maximum appeared at λem,PL = 495 nm, which equates to bluish light. The overall color impression for the OC48C48 PPV is blue-greenish as additional much weaker emission maxima appeared at 531 and 571 nm. EL measurements for an OLED based on ITO/PEDOT:PSS/OC48C48 PPV (∼50 nm)/ Ca/Al operated at 15 V confirmed this result as well. As can be seen in Figure 3 (blue curve) the EL emission maximum appeared at λem,EL = 495 nm. To characterize the device performance an OLED, based on the device architecture described above for the EL measurements, was investigated. Corresponding luminance−voltage curves are displayed in Figure 4 to determine the turn on voltage and the maximum luminance of the device. While in the literature18 the turn-on voltage for a comparable device based on the BEH-PPV is 6.5 V, the resulting turn-on

Figure 5. Luminance efficiency−voltage curves (red curves) and power efficiency−voltage curves (black curves) of a device based on OC48C48 PPV.

voltage for a OC48C48 PPV-based OLED was unexpectedly low, just about 4.3 V. Basically we assumed the turn-on voltage to be much higher due to the poor conductivity of the OC48C48 PPV E

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Macromolecules evoked by a weaker π−π* interaction along the polymers main chain and the higher injection barrier at the PEDOT:PSS/ OC48C48 PPV interface. At the moment we cannot provide any explanation for this behavior; hopefully further experiments will help to clear this issue. The maximum luminance for an OC48C48 PPV based device was 5.36 cd m−2 at 12 V (see Figure 4). This rather low emission intensity is most probably related to the high injection barrier for holes (∼0.6 eV), causing a relocation of the recombination zone at the PEDOT:PSS/ OC48C48 PPV interface by hole accumulation and therefore acting as quenching site. Here the use of an electron blocking layer between the PEDOT:PSS and the OC48C48 PPV layer might remedy the issue. As was already expected from the low emission intensity of the device, the observed luminance efficiency and the power efficiency were likewise low. With help of the luminance efficiency−voltage curves and the power efficiency−voltage curves in Figure 5 the luminance efficiency was determined 0.08 cd A−1 at 12 V and the power efficiency 0.02 lm W−1.

(2) Wessling, R. A. The Polymerization of Xylylene Bisdialkyl Sulfonium Salts. J. Polym. Sci., Polym. Symp. 1985, 72, 55−66. (3) Roex, H.; Adriaensens, P.; Vanderzande, D.; Gelan, J. Identification and quantification of polymerization defects in C-13labeled sulfinyl and Gilch OC1CO10-PPV by NMR spectroscopy. Macromolecules 2003, 36 (15), 5613−5622. (4) Gilch, H. G.; Wheelwri, W. L. Polymerization of AlphaHalogenated P-Xylenes with Base. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4, 1337−1349. (5) Kesters, E.; Gillissen, S.; Motmans, F.; Lutsen, L.; Vanderzande, D. Polymerization behavior of xanthate-containing monomers toward PPV precursor polymers: Study of the elimination behavior of precursor polymers and oligomers with in-situ FT-IR and UV-Vis analytical techniques. Macromolecules 2002, 35 (21), 7902−7910. (6) Kesters, E.; de Kok, M. M.; Carleer, R. A. A.; Czech, J. H. P. B.; Adriaensens, P. J.; Gelan, J. M.; Vanderzande, D. J. The thermal conversion reaction of sulphonyl substituted poly (para-xylylene): evidence for the formation of PPV structures. Polymer 2002, 43 (21), 5749−5755. (7) Braun, D.; Heeger, A. J. Visible light emission from semiconducting polymer diodes. Appl. Phys. Lett. 1991, 58 (18), 1982− 1984. (8) Bao, Z.; Chen, Y.; Cai, R.; Yu, L. Conjugated liquid-crystalline polymers - soluble and fusible poly(phenylenevinylene) by the Heck coupling reaction. Macromolecules 1993, 26 (20), 5281−5286. (9) Koch, F.; Heitz, W. Soluble poly(1,4-phenylenevinylene)s and poly(1,4-phenyleneethynylene)s via Suzuki coupling. Macromol. Chem. Phys. 1997, 198 (5), 1531−1544. (10) Spring, A. M.; Yu, C. Y.; Horie, M.; Turner, M. L. MEH-PPV by microwave assisted ring-opening metathesis polymerisation. Chem. Commun. 2009, 19, 2676−2678. (11) Mcdonald, R. N.; Campbell, T. W. The wittig reaction as a polymerization method. J. Am. Chem. Soc. 1960, 82 (17), 4669−4671. (12) Rehahn, M.; Schluter, A. D. Soluble Poly(ParaPhenylenevinylene)S from 2,5-Dihexylterephthalaldehyde Using the Improved Mcmurry Reagent. Makromol. Chem., Rapid Commun. 1990, 11 (8), 375−379. (13) Klingelhofer, S.; Schellenberg, C.; Pommerehne, J.; Bassler, H.; Greiner, A.; Heitz, W. Regioselectivity of the Pd-catalyzed synthesis of arylenevinylenes and its impact on polymer properties: Model reaction and polyreactions. Macromol. Chem. Phys. 1997, 198 (5), 1511−1530. (14) Bao, Z. N.; Chan, W. K.; Yu, L. P. Exploration of the Stille coupling reaction for the syntheses of functional polymers. J. Am. Chem. Soc. 1995, 117 (50), 12426−12435. (15) Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B. Synthesis of Light-Emitting Conjugated Polymers for Applications in Electroluminescent Devices. Chem. Rev. 2009, 109 (3), 897−1091. (16) Parthasarathy, G.; Gu, G.; Forrest, S. R. A full-color transparent metal-free stacked organic light emitting device with simplified pixel biasing. Adv. Mater. 1999, 11 (11), 907−910. (17) D’Andrade, B. W.; Forrest, S. R. White organic light-emitting devices for solid-state lighting. Adv. Mater. 2004, 16 (18), 1585−1595. (18) Martin, R. E.; Geneste, F.; Riehn, R.; Chuah, B. S.; Cacialli, F.; Holmes, A. B.; Friend, R. H. Efficient electroluminescent poly(pphenylene vinylene) copolymers for application in LEDs. Synth. Met. 2001, 119 (1−3), 43−44. (19) Martin, R. E.; Geneste, F.; Chuah, B. S.; Fischmeister, C.; Ma, Y. G.; Holmes, A. B.; Riehn, R.; Cacialli, F.; Friend, R. H. Versatile synthesis of various conjugated aromatic homo- and copolymers. Synth. Met. 2001, 122 (1), 1−5. (20) Kraft, A.; Grimsdale, A. C.; Holmes, A. B. Electroluminescent conjugated polymers - Seeing polymers in a new light. Angew. Chem., Int. Ed. 1998, 37 (4), 402−428. (21) Chung, S. J.; Jin, J. I.; Lee, C. H.; Lee, C. E. Improved-efficiency light-emitting diodes prepared from organic-soluble PPV derivatives with phenylanthracene and branched alkoxy pendents. Adv. Mater. 1998, 10 (9), 684−688.

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CONCLUSIONS In this contribution the new monomer M1 was synthesized via a multistep process before subsequent polymerization via the Gilch method. The resulting OC48C48 PPV was obtained in good yield (74%) and high purity. Its molecular weight, Mn = 450 000 g mol−1, was superior compared to similar PPV derivatives described in the literature. PL and EL spectrometry confirmed the OC48C48 PPV to be a blue-greenish emitter with a maximum emission peak at 495 nm. The observed turn-on voltage for a device based on ITO/PEDOT:PSS/OC48C48 PPV/Ca/Al was merely 4.3 V, which is extremely low in due consideration of the materials properties.

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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.5b01249. 1 H NMR and 13C NMR spectra of compounds 1−7, monomer M1, and OC48C48 PPV, mass spectra, and FTIR spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (N.V.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the German Science Foundation (DFG) of the collaborative research center SFB 595 “Electrical fatigue in functional materials”. The authors also thank Marion Trautmann, Katharina Toran, and Alexandra Gresika for synthetic contributions and Gabi Andress for device preparation (all done at TU Darmstadt).

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REFERENCES

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