Blue-Greenish Electroluminescent Poly(p-phenylenevinylene) Developed for Organic Light-Emitting Diode Applications

Article abstract of DOI:10.1021/acs.macromol.5b01249

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 criti

Full text of DOI:10.1021/acs.macromol.5b01249

Article
pubs.acs.org/Macromolecules
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-phenylenevi-
nylene) (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 constitu-
tionally 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.
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.
INTRODUCTION
■
Since the first observation of electroluminescence in π-
conjugated 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 chain-
growth 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 so-
called Gilch route.4 Here the treatment of α,α′-dihalogenated p-
xylene 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
EXPERIMENTAL SECTION
■
Materials. Morpholine, paraformaldehyde, 4-bromophenol, 2-
ethylhexyl 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,N-
dimethylformamide (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 accord-
ing 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.
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
Received: June 9, 2015
Revised: February 10, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.macromol.5b01249
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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 chromatog-
raphy (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 Mess-
and 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 spin-
coated 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.
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 stretch-
ing), 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 n-
hexane (3 × 200 mL) to remove the excess of 1,2,3-tribromo-5-
fluorobenzene (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).
13C 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
3,6-Bis(morpholinomethyl)benzene-1,2-diol (1). 1 was syn-
thesized 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
1
received as a colorless liquid (50.61 g, 77%). H 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).
1
176−177 °C). H 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-disubstitu-
tion).
4-(2′-Ethylhexyloxy)benzene Boronic Acid (6). 6 was synthe-
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
sized 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
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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 n-
hexane/ethyl acetate (2:1) to yield a light beige liquid (5.10 g, 58%).
1H 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
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).
(m, 1H, CH), 1.58−1.29 (m, 8H, CH2), 0.99−0.88 (m, 6H, CH3). 13
C
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).
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,
4,4′-(2,3-Bis(1,2,3-tris(4-(2′-ethylhexyloxy)phenyl)-5-phe-
noxy)-1,4-phenylene)bis(methylene)dimorpholine (3). A mix-
ture 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
74%). H 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-FT-
IR (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.
1
(1:1) to yield a yellow liquid (0.72 g, 57%). H 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-disubstitu-
tion).
1,4-Bis(bromomethyl)-2,3-bis(1,2,3-tris(4-(2′-ethylhexyl-
oxy)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) di-
acetate (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).
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 1,2,3-
tribromo-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 2-
ethylhexyloxy 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 side-
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/
1
chloroform (1:1) to yield a colorless liquid (0.12 g, 17%). H 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,
C
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a
Scheme 1. Synthesis of Monomer M1 and Polymer OC48C48 PPV
a
Reagents and conditions: (a) paraformaldehyde, morpholine, isopropyl alcohol, 90 to 70 °C, 53%; (b) cesium carbonate, 1,2,3-tribromo-5-
fluorobenzene, 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%.
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
the electrolyte and seemed to be irreversible. The correspond-
ing 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 Mikroyanni-
dis,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 longest-
wavelength absorption maximum detected for the poly(2,3-
bis(2-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV) ap-
pears 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
1
PPV H 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 CH3-
groups 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 aceto-
nitrile. 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
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Figure 1. 1H NMR spectrum of the OC48C48 PPV recorded in CD2Cl2
(∗) solution.
Figure 3. Normalized ultraviolet−visible (UV−vis), photolumines-
cence (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 longest-
wavelength 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 measure-
ments, was investigated. Corresponding luminance−voltage
curves are displayed in Figure 4 to determine the turn on
voltage and the maximum luminance of the device.
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
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
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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
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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
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0.02 lm W−1.
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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.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.5b01249.
1H NMR and 13C NMR spectra of compounds 1−7,
monomer M1, and OC48C48 PPV, mass spectra, and FT-
IR spectra (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail n.vilbrandt@mc.tu-darmstadt.de (N.V.).
■
Notes
The authors declare no competing financial interest.
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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