New copolymers with thieno[3,2-b]thiophene or dithieno[3,2-b:2′,3′-d]thiophene units possessing electron-withdrawing 4-cyanophenyl groups: Synthesis and photophysical, electrochemical, and electroluminescent properties

Article abstract of DOI:10.1002/pola.28657

New monomers containing 4-cyanophenyl (–PhCN) groups attached to a thieno[3,2-b]thiophene (TT) or dithieno[3,2-b:2′,3′-d]thiophene (DTT) structure were synthesized and characterized as 4-(2,5-dibromothieno[3,2-b]thiophen-3-yl)benzonitrile (Br–TT–PhCN) or 4,4′-(2,6-dibromodithieno[3,2-b:2′,3′-d]thiophene-3,5-diyl)dibenzonitrile (Br–DTT–PhCN). The Suzuki coupling of 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester and the Br–TT–PhCN or Br–DTT–PhCN monomer was utilized for the syntheses of novel copolymers poly{9,9-dioctylfluorene-2,7-diyl-alt-3-(4′-cyanophenyl)thieno[3,2-b]thiophene-2,5-diyl} (PFTT–PhCN) and poly{9,9-dioctylfluorene-2,7-diyl-alt-3,5-bis(4′-cyanophenyl)dithieno[3,2-b:2′,3′-d]thiophene-2,6-diyl} (PFDTT–PhCN), respectively. The photophysical, electrochemical, and electroluminescent (EL) properties of these novel copolymers were studied. Their photoluminescence (PL) exhibited the same emission maximum for both copolymers in solution. Red-shifted PL emissions were observed in the thin films. The PL emission maximum of PFTT–PhCN was more significantly redshifted than that of PFDTT–PhCN, indicating more pronounced excimer or aggregate formation in PFTT–PhCN. The ionization potential (HOMO level) and electron affinity (LUMO level) values were 5.54 and 2.81 eV, respectively, for PFTT–PhCN and were 5.57 and 2.92 eV, respectively, for PFDTT–PhCN. Polymer light-emitting diodes (LEDs) with copolymer active layers were fabricated and studied. Anomalous behavior and memory effects were observed from the current–voltage characteristics of the LEDs for both copolymers.

Full text of DOI:10.1002/pola.28657

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New Copolymers with Thieno[3,2-b]thiophene or Dithieno[3,2-
b:20,30-d]thiophene Units Possessing Electron-Withdrawing 4-Cyanophenyl
Groups: Synthesis and Photophysical, Electrochemical, and
Electroluminescent Properties
1
2,3
1
1
ꢀ
ꢀ
ꢀ
ꢀ
Drahomır Vyprachticky, Ilknur Demirtas, Vagif Dzhabarov, Veronika Pokorna,
Erdal Ertas,2 Turan Ozturk
,
2,4
1
ꢁ ꢀ
Vera Cimrova
1
ꢀ
Institute of Macromolecular Chemistry, The Czech Academy of Sciences, Heyrovsky Sq. 2, 16206 Prague 6, Czech Republic
2Department of Chemistry, Science Faculty, Istanbul Technical University, Maslak, Istanbul 34469, Turkey
3TUBITAK Marmara Research Center, Gebze, Kocaeli 2141470, Turkey
4Chemistry Group, Organic Chemistry Laboratory, TUBITAK UME, Gebze, Kocaeli 5441470, Turkey
ꢀ
Correspondence to: V. Cimrova (E-mail: cimrova@imc.cas.cz) or T. Ozturk (E-mail: ozturktur@itu.edu.tr)
Received 5 April 2017; accepted 6 May 2017; published online 00 Month 2017
DOI: 10.1002/pola.28657
ABSTRACT: New monomers containing 4-cyanophenyl (–PhCN)
groups attached to a thieno[3,2-b]thiophene (TT) or dithieno
[3,2-b:20,30-d]thiophene (DTT) structure were synthesized and char-
acterized as 4-(2,5-dibromothieno[3,2-b]thiophen-3-yl)benzonitrile
(Br–TT–PhCN) or 4,40-(2,6-dibromodithieno[3,2-b:20,30-d]thiophene-
3,5-diyl)dibenzonitrile (Br–DTT–PhCN). The Suzuki coupling of 9,9-
dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)ester and
the Br–TT–PhCN or Br–DTT–PhCN monomer was utilized for the
syntheses of novel copolymers poly{9,9-dioctylfluorene-2,7-diyl-
alt-3-(40-cyanophenyl)thieno[3,2-b]thiophene-2,5-diyl} (PFTT–PhCN)
and poly{9,9-dioctylfluorene-2,7-diyl-alt-3,5-bis(40-cyanophenyl)di-
thieno[3,2-b:20,30-d]thiophene-2,6-diyl} (PFDTT–PhCN), respectively.
The photophysical, electrochemical, and electroluminescent (EL)
properties of these novel copolymers were studied. Their photolu-
minescence (PL) exhibited the same emission maximum for both
copolymers in solution. Red-shifted PL emissions were observed
in the thin films. The PL emission maximum of PFTT–PhCN was
more significantly redshifted than that of PFDTT–PhCN, indicating
more pronounced excimer or aggregate formation in PFTT–PhCN.
The ionization potential (HOMO level) and electron affinity (LUMO
level) values were 5.54 and 2.81 eV, respectively, for PFTT–PhCN
and were 5.57 and 2.92 eV, respectively, for PFDTT–PhCN. Poly-
mer light-emitting diodes (LEDs) with copolymer active layers
were fabricated and studied. Anomalous behavior and memory
effects were observed from the current–voltage characteristics of
C
V
the LEDs for both copolymers.
2017 Wiley Periodicals, Inc. J.
Polym. Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: conjugated polymers; electrochemistry; electrolu-
minescence; photophysics; synthesis
of interest due to the electron-deficient CN group for opto-
electronic and sensing applications.8–13 The introduction of
cyano groups into the polymer backbone usually lowers its
LUMO level, leading to an increased electron affinity and
decreased band gap energy. A higher electron affinity can
facilitate electron injection when using these materials in
LEDs.
INTRODUCTION Fused thiophenes, such as thieno[3,2-b]thio-
phenes (TT) and dithieno[3,2-b:20,30-d]thiophenes (DTT),
have been widely accepted as important building blocks of
various organic materials (from organic light-emitting devi-
ces to solar cells).1–6 This acceptance is mainly due to their
electron-rich structures, possessing two (TT) and three
(DTT) sulfur atoms, and their flat and rigid geometries, pro-
viding them with good electron delocalization. The unique
features of TT, DTT, and their polymers make them promis-
ing for use in organic optoelectronics. These materials are
used in the design and construction of magnetic and photo-
sensitive receptors, sensors, electrochromic devices (ECD),
organic light-emitting diodes (OLED), field effect transistors
(FET), and solar cells.6,7 Cyano-substituted polymers are also
In this article, we report the syntheses of new two monomers,
4-(2,5-dibromothieno[3,2-b]thiophen-3-yl)benzonitrile (Br–TT–
PhCN) and 4,40-(2,6-dibromodithieno[3,2-b:20,30-d]thiophene-3,5-
diyl)dibenzonitrile (Br–DTT–PhCN), containing 4-cyanophenyl
(–PhCN) electron-withdrawing groups and their utilization for
the syntheses of two new copolymers. The Suzuki coupling of
the Br–TT–PhCN monomer or the Br–DTT–PhCN monomer with
Additional Supporting Information may be found in the online version of this article.
C
V
2017 Wiley Periodicals, Inc.
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9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester
(monomer F) yielded new copolymers poly{9,9-dioctylfluorene-
2,7-diyl-alt-3-(40-cyanophenyl)thieno[3,2-b]thiophene-2,5-diyl}
(PFTT–PhCN) and poly{9,9-dioctylfluorene-2,7-diyl-alt-3,5-
bis(40-cyanophenyl)dithieno[3,2-b:20,30-d]thiophene-2,6-diyl}
(PFDTT–PhCN). Their photophysical, electrochemical, and
electroluminescent properties were investigated.
Sample Preparation
Thin films were prepared by spin-coating from 1,2-dichloro-
benzene solutions. Polymer concentration in the solvent was
approximately 1.1 wt. %. Thin films were spin-coated onto
fused silica substrates for optical studies or coated on a Pt
wire electrode by dipping for electrochemical measurements.
For polymer light-emitting devices (LED), polymer layers
were prepared on indium tin oxide (ITO) substrates covered
with a thin layer of poly[3,4-(ethylenedioxy)thiophene]/poly-
(styrenesulfonate) (PEDOT:PSS). All polymer films were dried
in a vacuum (1023 Pa) at 353 K for 2 h. The ITO glass sub-
strates were purchased from Merck (Germany). PEDOT:PSS
(CLEVIOSTM P VP AI 4083) from Heraeus Clevios GmbH (Ger-
many) was used for LED preparation. The 50 nm thick
PEDOT:PSS layers were prepared by spin-coating and dried in
air at 396 K for 15 min. The calcium (20 nm) and, subse-
quently, 60–80 nm thick aluminum electrodes were vacuum-
evaporated on top of the polymer films to form LEDs. Typical
active areas of the LEDs were 4 mm2. Layer thicknesses were
measured using a KLA-Tencor P-10 profilometer. All thin film
preparations and the device fabrication were carried out in a
glove box under a nitrogen atmosphere.
EXPERIMENTAL
Materials and Methods
3-Bromothiophene and 4-(2-bromoacetyl)benzonitrile were
obtained from abcr Gute Chemie (Germany). The 9,9-dioctyl-
fluorene-2,7-diboronic acid bis(1,3-propanediol) ester (F
monomer), catalyst tetrakis(triphenylphosphine)palladium(0)
[99.99%, Pd(PPh3)4], trioctylmethylammonium chloride (Ali-
R
V
quat 336), calcined Celite 577 fine filter aid, n-butyllithium
(n-BuLi), tert-butyllithium (t-BuLi), polyphosphoric acid (PPA),
N-bromosuccinimide (NBS), zinc, glacial acetic acid, chloroben-
zene, dichloromethane (DCM), and methyl alcohol (MeOH)
were purchased from Sigma–Aldrich. Xylene (mixture of iso-
mers, multisolvent grade XI0059) and Silica gel 60 (70–230
mesh ASTM) were supplied by Scharlau Chemie. Aluminum
oxide (neutral, Brockmann I, 50–200 ml), thiophene and bro-
mine were purchased from Acros Organics. Diethyl ether
(Et2O), sulfur (S8), sodium bicarbonate (NaHCO3), sodium
sulfate (Na2SO4), and sodium thiosulfate (Na2S2O3) were
obtained from Merck. Tetrahydrofuran p.a. (THF, Lachner,
Czech Republic) was dried with LiAlH4 and distilled under
argon. Synthesized compounds were purified by column chro-
matography using a Merck Silica Gel (0.015–0.040 mesh) as
the adsorbent (stationary phase) packed into a glass col-
umn. Commercial TLC (Merck TLC Silica Gel 60 F254) was
used to follow the reaction course and/or find out the elu-
ent (mobile phase) for the column chromatography. 1H and
13C NMR spectra of low-molecular weight compounds were
recorded with a Varian model NMR 500 or 600 MHz in deu-
terated chloroform (CDCl3) or dimethyl sulfoxide (DMSO-
d6) using tetramethylsilane as an internal standard. 1H
NMR and 13C NMR spectra of polymers were measured in
deuterated tetrahydrofuran (THF-d8), chloroform or tri-
fluoroacetic acid (CF3COOD) with an upgraded Bruker
Avance DPX-300 spectrometer at 300.13 and 75.45 MHz,
respectively, using hexamethyldisiloxane as an internal stan-
dard. FT IR spectra were recorded with a Thermo Nicolet
6700 FT-IR or Perkin-Elmer Paragon 1000 PC Fourier trans-
form infrared spectrometer by means of diamond attenu-
ated total reflection (ATR). Size exclusion chromatography
(SEC) measurements were performed using a Pump Del-
tachrom (Watrex Comp.) with a Midas autosampler and two
columns of MIXED-B LS PL gel, particle size 10 lm. An
evaporative light scattering detector (PL-ELS-1000 from
Polymer Laboratories) was used. THF was the mobile
phase. Polystyrene standards were used for calibration.
Mass spectra were recorded on Bruker MICROTOFQ and
Thermo LCQ-Deca ion trap mass instruments. Melting
points were determined by a Buchi Melting Point Analyzer
(B-540).
Cyclic Voltammetric Measurements
Cyclic voltammetry (CV) was performed with a PA4 polaro-
graphic analyzer (Laboratory Instruments, Prague, CZ) with
a three-electrode cell. Platinum (Pt) wire electrodes were
used as both working and counter electrodes. A nonaqueous
Ag/Ag1 electrode (Ag in 0.1 M AgNO3 solution) was used as
the reference electrode. CV measurements were made in an
electrolyte solution of 0.1 M tetrabutylammonium hexafluor-
ophosphate (TBAPF6) in anhydrous acetonitrile under a
nitrogen atmosphere in a glove box. Typical scan rates were
20, 50, and 100 mV s21
.
Photophysical Measurements
UV–vis spectra were measured on a Perkin-Elmer Lambda 35
UV/VIS spectrometer. Solvents of spectroscopic grade were
used. The absorption spectra of thin films were also measured
in the glove box using fiber optics connected to the spectropho-
tometer. Steady-state PL spectra were recorded using a Perkin-
Elmer LS55 fluorescence spectrophotometer. The PL quantum
yield of the polymer in solution was calculated relative to the
quinine sulfate in 0.1 M H2SO4, which was used as a standard
(PL quantum yield 0.577).14
Electroluminescence Measurements
EL spectra were recorded using an Acton Research Spectro-
graph with single photon-counting detection (SPEX, RCA
C31034 photomultiplier). LEDs were supplied from a Key-
sight Technologies B2901A Precision Source Measure Unit,
which served to simultaneously record the current flowing
through the sample. Current–voltage and luminance–voltage
characteristics were recorded simultaneously using the
source measure unit and a Minolta LS110 Luminance Meter
or a silicon photodiode with integrated amplifier (EG&G
HUV-4000B) for the detection of total light output. A voltage
signal from the photodiode was recorded with a Hewlett
2
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(frit S3) and dried to a constant weight. Yield: 0.3581 g
(81%). SEC (THF): Mw 5 6800, Mn 5 3300, Ð 5 2.06, P 5 10–
11 (Fig. 1). FI IR (ATR): m 5 2922, 2850, 2228, 1607, 1460,
1350, 1270, 1167, 1009, 808, 740, 558 cm21 (Supporting
1
Information Fig. S18). H-NMR (300.13 MHz, CDCl3, d): 7.95–
7.10 (m, 10H, aromatic H), 6.88–6.78 (m, 1H, heterocyclic
H), 2.15–1.45 (br d, 4H, 2 3 fluorene–CH2), 1.25–0.95 (m,
20 H, 10 3 CH2), 0.81 (t, 6H, 2 3 CH3), 0.70–0.45 (br s
shoulder, 4H, 2 3 CH2–Me) (Supporting Information Fig.
S19). 13C-NMR (75.45 MHz, CDCl3, d): 153.4, 152.0, 151.8,
151.6, 151.2, 146.1, 142.4, 141.5, 140.5, 139.9, 138.1, 132.7,
132.4, 129.8, 128.8, 128.5, 128.1, 125.0, 124.0, 120.1 (20C
aromatic), 118.6 (CꢀN), 115.6, 111.2, 111.0, 110.2 (4C aro-
matic), 55.2 (5C(octyl)2), 40.4 (2C), 31.8 (2C), 30.0 (2C),
29.8 (2C), 29.3 (2C), 23.8 (2C), 22.7 (2C), 14.1 (2C), all ali-
phatic (Supporting Information Fig. S20).
FIGURE 1 SEC curves of the PFTT–PhCN (dashed), OFDTT–
PhCN (dashed and dotted), and PFDTT–PhCN (solid) copoly-
mers measured in THF as the mobile phase (polystyrene stand-
ards were used for calibration).
Poly/Oligo{9,9-dioctylfluorene-2,7-diyl-alt-3,5-bis(40-
cyanophenyl)dithieno[3,2-b:20,30-d]thiophene-2,6-diyl}
(PFDTT–PhCN or OFDTT–PhCN)
(a) OFDTT–PhCN: High Temperature (130 8C) Procedure in
Xylene/NaHCO3
Packard 34401A multimeter. All measurements were con-
ducted in a glove box under a nitrogen atmosphere.
Comonomers F (0.3909 g, 0.7 mmol) and Br–DTT–PhCN (10)
(0.3894 g, 0.7 mmol) were weighted into a dry preheated
three-necked reactor equipped with a condenser, magnetic stir-
rer, argon inlet, and septum and the reaction vessel was
flushed with argon (30 min). Then degassed xylene (10 mL),
degassed aqueous (15 wt. %) NaHCO3 (10 mL), and Aliquat
336 (40 mg) were added by a syringe via septum and the mix-
ture was flushed with argon for 30 min at 50 8C. The tempera-
ture was increased to 130 8C and the catalyst Pd(PPh3)4
(48 mg, 3.0 mol % per total monomer) in degassed xylene
(5 mL) was added via septum. Reaction mixture was heated at
130 8C for 120 min under argon. Then end-capping was per-
formed: (a) with phenylboronic acid pinacol ester (0.05 g, 0.25
mmol, 15 min, 130 8C) and (b) with 2-ethylhexyl bromide
(0.2 mL, 1.12 mmol, 15 min, 130 8C). After cooling, the reac-
tion mixture was separated into water layer (aq. NaHCO3, bot-
tom, wasted), organic layer (xylene, upper), and insoluble
material (on the walls and magnetic stirrer bar). The insoluble
material was dissolved in THF, mixed with xylene layer, and
precipitated into an excess of methanol. The precipitate was
filtered off (S3), washed with water and methanol, and dried.
Yield: 0.2698 g (49%). The raw material was dissolved in THF,
Synthesis
Organic Synthesis
Organic syntheses of new monomers, 4-(2,5-dibromo-
thieno[3,2-b]thiophen-3-yl)benzonitrile (Br–TT–PhCN) and
4,40-(2,6-dibromodithieno-[3,2-b:20,30-d]thiophene-3,5-diyl)di-
benzonitrile (Br–DTT–PhCN), and their characterization are
described in Supporting Information.
Polymer Synthesis
Poly{9,9-dioctylfluorene-2,7-diyl-alt-3-(40-cyanophenyl)
thieno[3,2-b]thiophene-2,5-diyl} (PFTT–PhCN)
Comonomers F (0.3909 g, 0.7 mmol) and Br–TT–PhCN (5)
(0.2794 g, 0.7 mmol) were weighted into a dry preheated
three-necked reactor equipped with a condenser, magnetic
stirrer, argon inlet and septum and the reaction vessel was
flushed with argon (30 min). Then degassed xylene (10 mL),
degassed aqueous (15 wt. %) NaHCO3 (10 mL), and Aliquat
336 (40 mg) were added by a syringe via septum and the
mixture was flushed with argon for 30 min at 50 8C. The
temperature was increased to 130 8C and the catalyst
Pd(PPh3)4 (32.3 mg, 2.0 mol % per total monomer) in
degassed xylene (5 mL) was added via septum. Reaction
mixture was heated at 130 8C for 90 min under argon. Then
end-capping was performed: (a) with phenylboronic acid
pinacol ester (0.05 g, 0.25 mmol, 15 min, 130 8C) and (b)
with 2-ethylhexyl bromide (0.2 mL, 1.12 mmol, 15 min, 130
8C). After cooling, the upper organic layer of reaction mixture
was poured into an excess of methanol and the precipitated
raw material was filtered off (frit S3), washed with water
and methanol, and dried. Yield: 0.3905 g (89%). The raw
polymer was dissolved in THF, filtered through a short col-
R
filtered through a short column of CeliteV 577/aluminum
oxide/silica gel 60 (ca. 1 4 3 3/3/3 cm), and the volume
was reduced to ca. 15–20 mL by a rotary evaporator. The solu-
tion was precipitated into methanol (700 mL), the khaki-green
oligomer OFDTT–PhCN was filtered off (S3) and dried to a
constant weight. Yield: 0.2387
g
(43%). SEC (THF):
Mw 5 1550, Mn 5 1300, Ð 5 1.19, P 5 2–3 (Fig. 1). FI IR (ATR):
m 5 2922, 2850, 2324, 2228, 1605, 1524, 1460, 1340, 1270,
1170, 1020, 840, 812, 740, 550 cm21 (Supporting Information
1
Fig. S21). H-NMR (300.13 MHz, CDCl3, d): 8.05–6.60 (m, aro-
R
umn of CeliteV 577/aluminum oxide/silica gel 60 (ca. 1 4
3 3/3/3 cm) and the volume was reduced to ca. 15 mL by a
rotary evaporator. The solution was precipitated into metha-
nol (700 mL), the ochre polymer PFTT–PhCN was filtered off
matic H), 2.35–1.45 (m, fluorene–CH2), 1.45–0.90 (m, CH2),
0.81 (s, CH3), 0.54 (br s, CH2–Me) (ratio aliphatic to aromatic
protons is 3.46) (Supporting Information Fig. S22). 13C-NMR
(75.45 MHz, CDCl3, d): 156.1, 153.4, 151.7, 150.8, 141.7, 141.4,
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(Fig. 1). FI IR (ATR): m 5 2922, 2850, 2321, 2227, 1607, 1522,
1460, 1364, 1255, 1060, 1020, 850, 805, 735, 550 cm21
(Supporting Information Fig. S24). 1H-NMR (300.13 MHz,
CDCl3:CF3COOD5 100:1 v/v, d): 8.02–7.16 (m, 14H, aromatic
H), 1.64 (br s, 4H, 2 3 fluorene–CH2), 1.40–0.90 (m, 20 H, 10
3 CH2), 0.80 (t, 6H, J 5 6.7 Hz, 2 3 CH3), 0.51 (br s, 4H, 2 3
CH2–Me) (Supporting Information Fig. S25). 13C-NMR (75.45
MHz, CDCl3:CF3COOD5 100:1 v/v, d): 159.8 (1C), 159.1 (1C),
151.7 (2C), 142.2 (2C), 141.7 (2C), 140.7 (2C), 140.1 (2C),
132.9 (2C), 132.7 (2C), 129.9 (2C), 129.5 (2C), 128.6 (2C),
124.0 (1C), 120.7 (1C), 120.1 (1C), (all aromatic1 heterocy-
clic) 118.2 (2C, CꢀN), 116.3 (2C), 112.6 (2C), 111.1 (2C),
108.8 (1C) (all aromatic1 heterocyclic), 55.3 (1C, 5C(octyl)2),
40.2, 39.0, 32.0, 30.1, 29.3, 23.9, 22.7, 14.2 (8 3 2C, all ali-
phatic) (Supporting Information Fig. S26).
SCHEME 1 Synthesis of the Br–TT–PhCN monomer, 4-(2,5-
dibromothieno[3,2-b]thiophen-3-yl)benzonitrile (5); (a): (i) n-
BuLi/Et2O, 278 8C, 1 h, 74%, (ii) sulfur S8, 1 h, (iii) 4-(2-bromoa-
cetyl)benzonitrile (2), r.t., overnight, 74%; (b): PPA, chloroben-
zene, 130 8C, 4 h, 81%; (c): NBS, 210 8C, in dark, 4 h, 90%.
RESULTS AND DISCUSSION
Synthesis of Monomers
Synthesis of thienothiophene (4) having 4-cyanophenyl (–PhCN)
functional group at C-3 was started with lithiation of 3-bromo-
thiophene (1) using n-BuLi at 278 8C, which was followed by
addition of elemental sulfur and then a-bromoketone 4-(2-bro-
moacetyl)benzonitrile (2, Br-CH2COC6H4-4CN) to produce
a-thioketone 4-(2-(thiophen-3-ylthio)acetyl)benzonitrile (3) in
74%. Ring closure reaction of the ketone in refluxing chloroben-
zene in the presence of polyphosphoric acid (PPA) gave the
corresponding thienothiophene, 4-(thieno[3,2-b]thiophen-3-yl)
benzonitrile (4), in 81% yield, having electron withdrawing 4-
cyanophenyl group. Its bromination with NBS at 210 8C pro-
duced the Br–TT–PhCN monomer (5) in 90% (Scheme 1).
140.0, 133.3, 132.7, 130.8, 129.8, 129.3, 128.5, 128.3, 124.0,
123.8, 121.1, 120.7, 120.3, 119.3 (all aromatic 1 heterocyclic),
118.5 (CꢀN), 114.4, 111.4, 110.2 (all aromatic1 heterocyclic),
55.1 (5C(octyl)2), 40.4, 39.6, 31.9, 30.1, 29.3, 23.8, 22.7, 14.2
(Supporting Information Fig. S23).
(b) PFDTT–PhCN: Low Temperature (85 8C) Procedure in
THF/Na2CO3
Comonomers F (0.3766 g, 0.6744 mmol) and Br–DTT–PhCN
(10) (0.3752 g, 0.6744 mmol) were weighted into a dry pre-
heated three-necked reactor equipped with a condenser, mag-
netic stirrer, and argon inlet and the reaction vessel was
flushed with argon (30 min). Then degassed THF (35 mL)
and degassed aqueous 2M Na2CO3 (20 mL) were added (Ar)
and the reaction vessel was purged-bubbled with argon for
30 min at 50 8C. The catalyst Pd(PPh3)4 (31.2 mg, 2.0 mol %
per total monomer) was added under argon and reaction mix-
ture was refluxed at 85 8C for 67 h under argon. Then end-
capping was performed: (a) with phenylboronic acid pinacol
ester (0.05 g, 0.25 mmol, 1 h reflux at 85 8C) and (b) with 2-
ethylhexyl bromide (0.2 mL, 1.12 mmol, 2 h reflux at 85 8C).
After cooling, the reaction mixture was separated into color-
less water layer (aq. Na2CO3, bottom, wasted) and ochre-
yellow organic layer (THF, upper). The THF layer was precipi-
tated into methanol (600 mL), where an ochre-yellow precipi-
tate was formed. The raw polymer material was filtered off
(frit S4), washed with water and methanol, and dried. Yield:
0.5210 g (98%). Using a Soxhlet apparatus, the raw polymer
was dissolved in chloroform and purified by filtration through
A well-established literature15 method was applied for the
synthesis of S,S’-thiophene-3,4-diyl bis(4-cyanobenzothioate)
(8). Treatment of 3,4-dibromothiophene (7) with t-BuLi was
followed by the addition of elemental sulfur and then a-
bromoketone (2) to obtain 1,8-diketone (8). A ring-closure
reaction of the ketone in refluxing chlorobenzene in the
presence of PPA produced the dithienothiophene 4,40-
(dithieno[3,2-b:20,30-d]thiophene-3,5-diyl)dibenzonitrile (9),
with a yield of 65%, having 4-cyanophenyl electron-with-
drawing groups on both peripheral thiophenes. Next, the
dithienothiophene was reacted with an excess of bromine to
obtain the Br-DTT-PhCN monomer (10), with a yield of 89%
(Scheme 2). The synthesized intermediate materials (3, 4, 8,
9) and the monomers (5, 10) were characterized by using
FTIR and 1H and 13C NMR spectroscopy, as shown in the
Supporting Information Figures S1–S17.
Synthesis of Polymers
The copolymer PFTT–PhCN was prepared by the Suzuki cou-
pling of the 9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-pro-
panediol) ester (monomer F) and 4-(2,5-dibromothieno[3,2-
b]thiophen-3-yl)benzonitrile (5, Br–TT–PhCN) in a high-boiling
xylene/aq. NaHCO3 solvent system under argon (Scheme 3).
R
a short column of CeliteV 577/aluminum oxide/silica gel 60
(ca. 1 4 3 3/3/3 cm). After volume reduction to ca. 50–
70 mL by a rotary evaporator, the solution was precipitated
into methanol (1500 mL), where a bright-yellow precipitate
was formed. The yellow polymer PFDTT–PhCN was filtered
off (frit S4) and dried to a constant weight. Yield: 0.4980 g
(94%). SEC (THF): Mw 5 7640, Mn 5 6180, Ð 5 1.24, P 5 10
R
Pd(PPh3)4 was used as a catalyst, and AliquatV 336 was
used as a phase-transfer agent. The synthesis yielded copoly-
mer PFTT–PhCN soluble in organic solvents (xylene, THF,
4
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SCHEME 4 Synthesis of poly/oligo{9,9-dioctylfluorene-2,7-diyl-
alt-3,5-bis(40-cyanophenyl)dithieno[3,2-b:20,30-d]thiophene-2,6-diyl};
(a) OFDTT–PhCN: high-temperature (130 8C) procedure in xylene/
NaHCO3, reaction time 2 h; (b) PFDTT–PhCN: low-temperature (85
8C) procedure in THF/Na2CO3, reaction time 67 h.
SCHEME 2 Synthesis of the Br–DTT–PhCN monomer, 4,40-(2,6-
dibromodithieno[3,2-b:20,30-d]thiophene-3,5-diyl)dibenzonitrile (10);
(a): Zn/acetic acid, 120 8C; (b): (i) t-BuLi/Et2O, 278 8C, 45 min, (ii) sul-
fur S8, 45 min, (iii) 4-(2-bromoacetyl)benzonitrile (2), r.t., overnight,
68%; (c): PPA, chlorobenzene, 130 8C, 1.5 h, 65%; (d): Br2, chloro-
form, 50 8C, 3 h, 89%.
plane deformation), 808 and 740 cm21 (assigned to CH out-
of-plane deformation), and 558 cm21 (assigned to Ar–CN
bending vibrations). From the 1H NMR spectrum, the peaks
of the aromatic and heterocyclic protons were found at
7.95–6.78 ppm, and the peaks of the aliphatic protons were
found at 2.15–0.45 ppm, with distinguished broad peaks of
the fluorene–CH2, CH2, and CH3 protons. The ratio of the ali-
phatic to aromatic integrals confirmed the repeating struc-
tural –[polymer unit]n–, as shown for PFTT–PhCN in Scheme
3. From the 13C-NMR spectrum, the peaks of the aromatic
(153.4–110.2) and aliphatic (40.4–14.1) carbons were clearly
separated, and the signals of the CꢀN carbons at 118.6 and
the 5C(octyl)2 carbon at 55.2 ppm were assigned.
CHCl3) in a short reaction time (90 min), similar to our pre-
vious polymerizations,16,17 when compared with the Suzuki
reactions using THF, for which several days were
required.18,19 The copolymer was characterized by SEC (Fig.
1), FTIR (Supporting Information Fig. S18), and 1H and 13C
NMR (Supporting Information Figs. S19 and S20) spectros-
copy. The average molecular weight (Mw and Mn) and disper-
-
sity (D) of PFTT–PhCN were determined by SEC to be
-
Mw 5 6800, Mn 5 3300, and D 5 2.06. The FTIR spectrum
exhibited vibration bands at 2922 cm21 (assigned to aro-
matic CH stretching), 2850 cm21 (assigned to the aliphatic
CH stretching), 2228 cm21 (assigned to the CꢀN stretching
in the aromatic nitriles), 1607 cm21 (assigned to aromatic
ring stretching (sharp peak)), 1460 cm21 (assigned to the
aliphatic CH2 scissor vibrations and the CH3 antisymmetric
deformation), 1350 cm21 (assigned to CH3 symmetric defor-
mation), 1270, 1167, and 1009 cm21 (assigned to CH in-
The Suzuki coupling of the monomer F and 4,40-(2,6-dibro-
modithieno[3,2-b:20,30-d]thiophene-3,5-diyl)dibenzonitrile (10,
Br–DTT–PhCN) in high-boiling xylene/aq. NaHCO3 (130 8C)
R
under argon using Pd(PPh3)4 as a catalyst and AliquatV 336 as
a phase-transfer agent only yielded oligomers (OFDTT–PhCN)
with limited solubility in xylene but soluble in THF. The SEC in
THF showed very low average values of Mw 5 1550 and
Mn 5 1300 (Fig. 1). Consequently, in the 1H NMR spectrum, the
ratio of the aliphatic to aromatic protons was measured to be
higher than the value of 2.43 for the repeating structural –
[polymer unit]n–, as shown in Scheme 4. The signals in the
1
FTIR spectrum (Supporting Information Fig. S21) and H and
13C NMR spectra (Supporting Information Figs. S22 and S23)
could only be assigned qualitatively. Because of found solubility
in THF, the Suzuki coupling reaction of the monomers F and
Br–DTT–PhCN (10) was additionally performed with a lower-
temperature (85 8C) procedure in THF/Na2CO3 using Pd(PPh3)4
as a catalyst, which yielded polymer PFDTT–PhCN (Scheme 4).
The copolymer was characterized by SEC (Fig. 1), FTIR (Sup-
porting Information Fig. S24), and 1H and 13C NMR (Sup-
porting Information Figs. S25 and S26) spectroscopy. The
SCHEME 3 Synthesis of poly{9,9-dioctylfluorene-2,7-diyl-alt-3-
(40-cyanophenyl)thieno[3,2-b]thiophene-2,5-diyl} (PFTT–PhCN)
from the high-temperature (130 8C) procedure in xylene/NaHCO3.,
reaction time 90 min.
-
average molecular weight (Mw and Mn) and dispersity (D) of
PFDTT–PhCN were determined by SEC to be Mw 5 7640,
-
Mn 5 6180, and D 5 1.24. The FTIR spectrum exhibited
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TABLE 1 Photophysical Properties of the Polymers Measured
in Dilute DCB Solutions (kabsmax: Absorption Maximum; kPLmax
Emission Maximum; kPLexcmax: Maximum of the Excitation
Spectrum; gPL: Photoluminescence Quantum Yield). The Pri-
mary Maxima Are Printed in Bold
:
a
b
kabsmax
kPLmax
kPLexcmax
Material
(nm)
(nm)
(nm)
gPL
PFTT–PhCN
OFDTT–PhCN
PFDTT–PhCN
321, 423
313, 396
331, 430
494
494
494
332, 428
0.13
0.18
0.34
330, 380, 402
332, 437, 441
a
Excitation wavelength at kabsmax
.
b
Emission wavelength at kPLmax
.
FIGURE 2 UV–vis absorption of the PFTT–PhCN (dashed),
OFDTT–PhCN (dashed and dotted), and PFDTT–PhCN (solid)
copolymers measured in dilute DCB solution.
higher average molecular weight of PFDTT–PhCN in compari-
son with that of OFDTT–PhCN is caused mainly by a better
solubility of reaction components in THF than in xylene. The
heterogeneity of xylene reaction mixture in OFDTT–PhCN
synthesis decreases the progress of the Suzuki polycondensa-
tion, which is generally believed to be of the step-growth
type.20
vibration bands at 2922 cm21 assigned to the aromatic CH
stretch, 2850 cm21 assigned to the aliphatic CH stretch,
2227 cm21 assigned to the CꢀN stretch in the aromatic
nitriles, 1607 cm21 assigned to the aromatic ring stretch
(sharp peak), 1460 cm21 assigned to the aliphatic CH2 scis-
sors vibration and CH3 antisymmetric deformation,
1364 cm21 assigned to the CH3 symmetric deformation,
1255, 1060, and 1020 cm21 assigned to the CH in-plane
deformation, 850, 805, and 735 cm21 assigned to the CH
out-of-plane deformation, and 550 cm21 assigned to the Ar–
Photophysical Properties
The photophysical properties of the new polymers were
studied in solutions and as thin films. The absorption spectra
measured in DCB solutions and for the thin films are dis-
played in Figures 2 and 3, respectively, and their characteris-
tic data are summarized in Tables 1 and 2. The absorption
spectra of the polymers exhibited broad absorption bands in
both solutions and as thin films. In the DCB solution, two
bands are evident in the absorption spectra. The long-
wavelength band can be assigned to the p–p* transition of
the conjugated polymer backbones. The maxima are located
at 423, 396, and 430 nm for PFTT–PhCN, OFDTT–PhCN, and
PFDTT–PhCN, respectively. PFDTT–PhCN, with the highest
Mw, exhibited an absorption maximum at the longest wave-
length of 430 nm, which is redshifted by 34 nm compared to
the wavelength of the absorption maximum of the OFDTT–
PhCN oligomer. The lower extinction coefficients for OFDTT–
PhCN and PFTT–PhCN than for PFDTT–PhCN are in good
agreement with the copolymer Mw values.
1
CN bending vibration. From the H-NMR spectrum, the peaks
of the aromatic protons were found at 8.02–7.16 ppm, and
the peaks of the aliphatic protons were found at 1.64–0.51
ppm, with distinguished broad peaks of the fluorene–CH2,
CH2, and CH3 protons. The ratio of the aliphatic to aromatic
integrals confirmed the structure of the repeating –[polymer
unit]n–, as shown for PFDTT–PhCN in Scheme 4. From the
13C-NMR spectrum, the peaks of the aromatic (159.8–108.8)
and aliphatic (40.2–14.2) carbons were separated, and the
signals of the CꢀN carbons at 118.2 and the 5C(octyl)2 car-
bon at 55.3 ppm were clearly assigned. We assume that a
The absorption spectra of the thin films are displayed in Fig-
ure 3. The quality of the OFDTT–PhCN oligomer thin film
was not good enough, and therefore, the properties of the
TABLE 2 Photophysical Properties of the Polymers Measured
as Thin Films (kabsmax: Absorption Maximum; kPLmax: Emission
Maximum; kPLexcmax: Maximum of the Excitation Spectrum).
The Primary Maxima Are Printed in Bold
a
b
Material
kabsmax (nm)
kPLmax (nm)
kPLexcmax (nm)
PFTT–PhCN
322, 422
331, 427
550
509
305, 423, 443
332, 422, 443
PFDTT–PhCN
a
FIGURE 3 UV–vis absorption of the PFTT–PhCN (dashed) and
Excitation wavelength at kabsmax
Emission wavelength at kPLmax.
.
b
PFDTT–PhCN (solid) copolymers measured as thin films.
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aggregate emission. The PL efficiency also differed. PFDTT–
PhCN showed a value that was higher than that of PFTT–
PhCN by a factor of 2.6, which was well-correlated with the
excimer or aggregate formation ability. The excitation spectra
corresponded well to the absorption spectra.
The PL spectra measured for the thin films are displayed in
Figure 5, and the characteristic data are summarized in
Table 2. For the thin films, the emission maxima are red-
shifted compared to the solutions. PFTT–PhCN showed a
larger redshift of 56 nm than that of PFDTT–PhCN (15 nm).
This indicates that PFTT–PhCN has more pronounced exci-
mer and/or aggregate formation in the solid state, which
also showed a large Stokes shift of 128 nm. The relative PL
efficiency of a thin film, UPLrel, was introduced to compare
the PL efficiencies of the polymers. The PL efficiency of
PFDTT–PhCN is more than five times higher for than that of
PFTT–PhCN. The shapes of the excitation spectra match well
with the absorption spectra shapes.
Electrochemical Properties
Cyclic voltammetry (CV) measurements were performed to
obtain information on the electronic structures of the new pol-
ymers. An example of a representative CV curve of the polymer
thin film coated on a Pt wire is displayed in Figure 6.
FIGURE 4 UV–vis absorption and PL spectra of the (a) PFTT–
PhCN, (b) OFDTT–PhCN, and (c) PFDTT–PhCN polymers measured
in dilute DCB solutions. Normalized absorption spectra are shown
in dashed lines.
PFTT–PhCN and PFDTT–PhCN thin films were only studied.
The long-wavelength absorption maxima are located at 422
and 427 nm and are slightly blueshifted compared with the
corresponding maxima in the solution spectra.
The new polymers exhibited greenish photoluminescence in
solution and as thin films. The PL excitation and emission
spectra measured in the DCB solutions are displayed in Fig-
ure 4, and the characteristic data are summarized in Table 1.
In solution, the PL emission spectra exhibited maxima at
494 nm for all PFTT–PhCN, PFDTT–PhCN, and OPFDTT–PhCN
polymers but had different spectral shapes at their long-
wavelength tails. This tail was more pronounced in the spec-
trum of PFTT–PhCN and could indicate excimer and/or
FIGURE 5 UV–vis absorption and PL spectra of 82-nm thick
PFTT–PhCN (kexc 5 421 nm, kem 5 552 nm) and 73-nm thick
PFDTT–PhCN (kexc 5 423 nm, kem 5 509 nm) films. Normalized
absorption spectra are shown in dashed lines.
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FIGURE 7 Normalized EL spectra (solid) of the ITO/PEDOT:PSS/
PFTT–PhCN/Ca/Al LED (black) and ITO/PEDOT:PSS/PFDTT–
PhCN/Ca/Al LED (blue). PL emission spectra of the correspond-
ing thin films (dashed) are displayed for comparison. [Color
figure can be viewed at wileyonlinelibrary.com].
Electroluminescent Properties
FIGURE 6 Representative cyclic voltammograms of the thin
The new PFTT–PhCN and PFDTT–PhCN polymers were used
to create active layers in samples of light-emitting diodes
(LEDs). LEDs with a hole-injecting electrode formed by ITO
covered with a PEDOT:PSS layer and an electron-injecting
electrode of calcium covered with aluminum (Ca/Al) were
prepared and studied. The electroluminescent (EL) spectra
of the LEDs are shown in Figure 7. The PFDTT–PhCN LED
exhibited a greenish EL emission with a maximum at
509 nm, which coincided with the thin-film PL maximum,
and a shoulder at 535 nm, which was similar to the PL. The
EL maximum of the PFTT–PhCN LED (at 630 nm) was signif-
icantly redshifted compared to the EL of the PFDTT–PhCN
LED and compared with the PFTT–PhCN thin-film PL maxi-
mum. The redshift in the PFTT–PhCN LED indicates that
charge trapping plays a significant role in the PFTT–PhCN EL
process and that different centers are dominant in the EL
and PL processes. In contrast, PFDTT–PhCN exhibited similar
EL and PL thin-film spectra, indicating that similar centers
occur in the EL and PL emission processes.
films coated on a Pt wire recorded at a scan rate of 50 mV s21
.
Quasi-reversibility in the oxidation and reduction was
observed. The ionization potential (HOMO level), EIP, and
electron affinity (LUMO level), EA, were estimated from the
onset potentials, Eonset, of the oxidation and reduction peaks
based on the reference energy level of ferrocene (4.8 eV
below the vacuum level) using the equation EIP (EA) 5 | –
(Eonset – Eferr) – 4.8| eV, where Eferr is the potential for ferro-
cene versus the Ag/Ag1 electrode. The EIP (HOMO) and EA
(LUMO) values of 5.47 and 2.92 eV, respectively, for PFDTT–
PhCN are slightly higher than those of 5.39 and 2.81 eV for
PFTT–PhCN, which correspond well with the content of the
cyano groups in the copolymers. Electrochemical bandgap val-
ues of 2.58 and 2.55 eV were determined for PFTT–PhCN and
PFDTT–PhCN, respectively. These results are in very good
agreement with the optical bandgap values determined from
the thin-film absorption spectra (see Table 3). In our case, the
electron affinity is not very high for the cyano-substituted pol-
ymers with substitution close to the backbone.9–13
The PFDTT–PhCN LED exhibited better performance than the
PFTT–PhCN LED, which is in good agreement with the rela-
tive PL efficiencies of the thin films. An example of the typi-
cal dependences of the current and luminance on the
applied voltage measured on the ITO/PEDOT:PSS/copolymer/
Ca/Al LED is shown in Figure 8. The PFDTT–PhCN LED
showed higher luminance values up to 400 cd m22, where sat-
uration occurred, and a lower EL onset voltage than the PFTT–
PhCN LED. A steep increase in EL occurred at 5 V (i.e., at an
electric field intensity < 7 3 107 V m21), which was associated
with the steep current increase. At higher voltages, when EL
emission begins, the current increases steeply with increasing
applied voltage U, j ꢁ Um, with an m of approximately 10. The
TABLE 3 Electronic Properties of the Copolymers Under Study
(EIP: Ionization Potential; EA: Electron Affinity; Eeglc: Electro-
chemical Bandgap; Eg8pt: Optical Bandgap)
EIP(eV)
EA(eV)
Egelc
Eg8pt
Copolymer
(–EHOMO
)
(–ELUMO
)
(eV)
(eV)
PFTT–PhCN
5.39
5.47
2.81a/3.10
2.92a/3.28
2.58
2.55
2.58
2.55
PFDTT–PhCN
a
Evaluated from the reduction prepeak onset.
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dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester (F
monomer), yielding new copolymers PFTT–PhCN and PFDTT–
PhCN. In solution, both copolymers exhibited greenish PL emis-
sion with the same position of their emission maximum. Red-
shifted PL emission was observed for the thin films. The PL
emission maximum of PFTT–PhCN was more significantly red-
shifted than PFDTT–PhCN, indicating more pronounced excimer
or aggregate formation in PFTT–PhCN. The PL efficiency was
higher by a factor 2 for PFDTT–PhCN than for PFTT–PhCN in
solution and was even higher by a factor of more than 5 in the
thin films. The ionization potential (HOMO level) and electron
affinity (LUMO level) values of 5.54 and 2.81 eV, respectively,
were determined for PFTT–PhCN and 5.57 and 2.92 eV, respec-
tively, for PFDTT–PhCN. The new copolymers were tested as
active layers in polymer LEDs. The performance of the LED
with PFDTT–PhCN as an active layer was better than that of the
PFTT–PhCN LED. The EL spectrum of PFTT–PhCN was red-
shifted compared to its PL thin-film spectrum, whereas the EL
spectrum of the LED made from PFDTT–PhCN was similar to
its PL thin-film spectrum. Anomalous behavior with memory
effects was observed in the current–voltage characteristics of
both copolymer LEDs, which could be of interest for future elec-
tronic applications.
ACKNOWLEDGMENTS
FIGURE 8 Dependence of current density (a) and luminance (b)
on the applied voltage measured on the ITO/PEDOT:PSS/PFTT–
PhCN/Ca/Al (circles) and ITO/PEDOT:PSS/PFDTT–PhCN/Ca/Al
(squares) LEDs. [Color figure can be viewed at wileyonlineli-
brary.com].
We thank the Czech Science Foundation (grant 13-26542S) and
The Scientific and Technological Research Council of Turkey
(TUBITAK, grant 214Z297) for financial support and Istanbul
Technical University for supporting ID (PhD).
REFERENCES AND NOTES
EL intensity, IEL, increased even more steeply with the applied
voltage, IEL ꢁ Un, with an n of approximately 13–15. Anoma-
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reversible memory switching phenomenon.21,22 Depending on
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