Design, synthesis, photophysical, and electrochemical properties of DCM-based conjugated polymers for light-emitting devices

Article abstract of DOI:10.1002/pola.26169

A series of conjugated hyperbranched polymers, hyperbranched copolymers, and linear polymers containing 2-pyran-4-ylidenemalononitrile (acceptor) and triphenylamine/fluorene (donor) units were synthesized and characterized by FTIR, ¹H NMR, thermogravimetric analyses, differential scanning calorimetry, gel permeation chromatography, UV-visible, photoluminescence, and cyclic voltammetry measurements. All the polymers show red-light emission in the range of 566-656 nm both in solution and in solid state. The quantum efficiency of the polymers was in the range of 56-82%. Among the six polymers synthesized, only polymers containing fluorene units show T_g and polymers based on triphenylamine not exhibit T_g. The band gap of these polymers were found to be reasonably low; hyperbranched copolymer containing fluorene unit shows lowest band gap of 2.18 eV due to the stabilization of LUMO energy level by the electron withdrawing -CN groups. The thermal and solubility behavior of the polymers were found to be good. All the EL spectra of the devices (indium-tin oxide/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/ polymer/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline/tris(8-hydroxyquinoline) aluminum)/LiF/Al) show red-light emission, and the device fabricated with P3 and P4 shows maximum luminance and luminous efficiency of 4104 cd m^-2 and 0.55 cd A^-1 and 3696 cd m^-2 and 0.47 cd A^-1, respectively, indicates that they had the best carrier balance.

Full text of DOI:10.1002/pola.26169

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Design, Synthesis, Photophysical, and Electrochemical Properties of
DCM-Based Conjugated Polymers for Light-Emitting Devices
Mahalingam Vanjinathan,1* Hong-Cheu Lin,1 A. Sultan Nasar2
1Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
2Department of Polymer Science, University of Madras, Guindy Campus, Chennai 600025, India
Correspondence to: A. S. Nasar (E-mail: drasultannasar@unom.ac.in)
Received 30 September 2011; accepted 9 May 2012; published online
DOI: 10.1002/pola.26169
ABSTRACT: A series of conjugated hyperbranched polymers,
hyperbranched copolymers, and linear polymers containing 2-
pyran-4-ylidenemalononitrile (acceptor) and triphenylamine/fluo-
rene (donor) units were synthesized and characterized by FTIR,
1H NMR, thermogravimetric analyses, differential scanning calo-
rimetry, gel permeation chromatography, UV–visible, photolumi-
nescence, and cyclic voltammetry measurements. All the
polymers show red-light emission in the range of 566–656 nm
both in solution and in solid state. The quantum efficiency of
the polymers was in the range of 56–82%. Among the six poly-
mers synthesized, only polymers containing fluorene units show
Tg and polymers based on triphenylamine not exhibit Tg. The
band gap of these polymers were found to be reasonably low;
hyperbranched copolymer containing fluorene unit shows low-
est band gap of 2.18 eV due to the stabilization of LUMO energy
level by the electron withdrawing ACN groups. The thermal and
solubility behavior of the polymers were found to be good. All
the EL spectra of the devices (indium-tin oxide/poly(3,4-ethylene-
dioxythiophene):poly(styrenesulfonate)/polymer/2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline/tris(8-hydroxyquinoline)aluminum)/
LiF/Al) show red-light emission, and the device fabricated with
P3 and P4 shows maximum luminance and luminous efficiency
ꢀ1
of 4104 cd mꢀ2 and 0.55 cd A and 3696 cd mꢀ2 and 0.47 cd
˚
A
ꢀ1, respectively, indicates that they had the best carrier bal-
˚
C
V
ance.
2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym
Chem 000: 000–000, 2012
KEYWORDS: A2 1 B3 approach; conjugated polymers; charge trans-
fer; cyclic voltammetry; electroluminescence; hyperbranched
polymers; Knoevenagel reaction; photophysical property; PLEDs
INTRODUCTION Linear-conjugated polymers have been
investigated extensively owing to their many applications,
such as in electrochromics,1 sensors,2,3 thin-film transistors,4
light-emitting diodes (LED),5 and photovoltaics.6 The poten-
tial of these polymers will be greatly enhanced, if these poly-
mers could be easily processable, because processability has
significant implications in the device fabrication. One of the
problems associated with linear-conjugated polymers is that
polymer chains tend to aggregate due to p–p stacking inter-
actions, which act as energy traps and hence lead to
increased nonradiative decay. Besides, electronic properties
of the conjugated polymers and solubility could not be tuned
individually, which is a disadvantage for comprehensive opti-
mization of materials. Introduction of bulky side chains will
solve the first problem by means of decreasing greatly the
aggregation or excimer formation.7–9 The second problem is
inherently hard to solve for structural reasons. Hyper-
branched polymers (HBP) have been intensively studied in
recent years due to their good solubility, processability, and
tunable physical and chemical properties, which are advanta-
geous for their application in LED. Moreover, they have glob-
ular structure; the highly twisted structure will decrease
intermolecular interactions. The highly branched and globu-
lar features can reduce or eliminate interchain interactions
and p–p stacking interactions between the planar-conjugated
molecular chains.10–12 Moreover, three-dimensional struc-
tures make the materials to form good quality amorphous
films so as to improve stability and efficiency of polymer
light emitting diodes (PLEDs).13 Therefore, HBP have become
of current interest in terms of developing efficient light-emit-
ting and other optoelectronic devices.14–21
Recently, series of novel soluble conjugated copolymers con-
sisting of coplanar donor (bithiophenevinyl)-acceptor [2-py-
ran-4-ylidenemalononitrile (DCM)-based] unit coupled to dif-
ferent moieties with electron-donating ability were
synthesized by Suzuki coupling polymerization.22 We believe
that symmetrical combination of DCM with fluorene or triphe-
nylamine will assure the better conjugated length and copla-
narity. DCM is a strong electron-accepting group, which can
*Present address: Postgraduate and Research, Department of Chemistry, Dwaraka Doss Goverdhan Doss Vaishnav College,
Arumbakkam, Chennai 600106, India.
C
V
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increase electron affinity and reduce the band gap of the con-
jugated system.23,24 Polyfluorenes are shown to be the most
promising materials for LEDs due to their thermal stability
and high efficiencies for both PL and electroluminescence
(EL).25–27 The copolymerization of fluorene with various aryl
partners or tailings can bring various interesting properties to
the polymers.28–30 Triphenylamine is a well-known hole-trans-
porting unit due to the electron-donating capability of nitro-
gen atom.31,32 Incorporation of functional electron-donating
moieties into the copolymers with different electron-donating
ability will bring intermolecular charge transfer (ICT) from do-
nor to acceptor, to the conjugated system and thus provide a
means to tune the energy levels. To the best of our knowledge,
reports on polymer cored with DCM derivative and containing
cyanovinylene linkages at the backbone are meager.
In this article, we report the design, synthesis, and characteri-
zation of a series of soluble novel HBPs, hyperbranched (HB)-
copolymers, and linear polymers in which fluorene or triphe-
nylamine was used as donor chromophores and 2-(2,6-bis-2-
(5-(cyanomethyl)thiophen-2-yl)vinyl)-4H-pyran-4-ylidene)ma-
lononitrile was used as acceptor chromophore. The aim of sys-
tematic structural variations on these molecules is to provide
insight into understanding the relationship between the struc-
tures and properties and to modulate their absorption bands
to cover the whole visible region. Such structural modulation
made our star-shaped D-p-A molecules as good candidates for
solution-processable light-emitting materials.
FIGURE 1 Structure of B3 monomers and B2 monomers.
using polystyrene as a standard and THF as an eluent. UV–visi-
ble absorption and PL spectra were recorded on a HP G1103A
and Hitachi F-4500 spectrophotometer, respectively. THF solu-
tions (10ꢀ5 M) were used for measurements on solution. For
measurements on solid films, the polymers were spin-coated
using THF solutions with a concentration of 10 mg mLꢀ1 on a
quartz substrate. Cyclic voltammetry (CV) measurements
were performed using a BAS 100 electrochemical analyzer
with a standard three-electrode electrochemical cell in a 0.1 M
acetonitrile solution of tetrabutylammonium hexafluorophos-
phate at room temperature in the range of 0–1.8 eV with a
scanning rate of 100 mV sꢀ1. In each experiment, a carbon-
working electrode spin-coated with a thin layer of polymer, a
platinum wire as counter electrode, and a silver wire as quasi-
reference electrode were used. Ag/AgCl electrode was used as
a reference electrode for all potentials quoted herein. During
the CV measurements, the solutions were purged with nitro-
gen for 30 s, and the redox couple ferrocene/ferrocenium ion
EXPERIMENTAL
Materials
All reagent grade chemicals were purchased from Aldrich,
ACROS, TCI, and Lancaster and used as received. 4-Dicyano-
methylene-2,6-dimethyl-4H-pyran (1) was synthesized from
2,6-dimethyl-4-pyrone as described by Woods.33 Tris(4-for-
mylphenyl)amine (B3 monomer) (5), 4,40-bis(formyl)-triphe-
nylamine (B2 monomer) (7) were synthesized by Vilsmeier
formylation of triphenylamine.34 2,4,7-Tris(formyl)-9,9-dieth-
ylfluorene (B3 monomer) (6) and (8) and (9) were prepared
according to literature methods.35 The structure of B3 and
B2 monomers (5, 6 and 7, 12) were shown in Figure 1. Tolu-
ene was distilled to keep anhydrous before use. Tetrahydro-
furan (THF) was distilled over sodium/benzophenone and
acetonitrile was distilled over P2O5 before use.
(Fc/Fcþ) was used as an external standard. The HOMO levels
onset
ox
in polymer films were calculated from E
values of the elec-
trochemical experiments.
EL Device Fabrication and Measurements
A series of EL devices with the configuration of indium-tin
oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene-
sulfonate) (PEDOT:PSS)/polymer/2,9-dimethyl-4,7-diphenyl-
Measurements
1,10-phenanthroline
(BCP)/tris(8-hydroxyquinoline)alumi-
FTIR spectra of samples (dispersed in KBr disks) were
recorded on a Perkin-Elmer Spectrum 100 Series instrument.
1H NMR and 13C NMR spectra were recorded on a Varian Unity
300 MHz spectrometer using CDCl3 and DMSO-d6 as solvents.
The EI mass spectra were recorded using a JEOL DX-303 spec-
trometer. Elemental analyses were performed on a HERAEUS
CHN-OS RAPID elemental analyzer. Glass transition tempera-
tures were determined by differential scanning calorimetry
(DSC; Perkin-Elmer Pyris 7) with a heating and cooling rate of
10 ꢁC minꢀ1. Thermogravimetric analyses (TGA) were coꢁn-
ducted with a TA instrument Q500 at a heating rate of 20 C
minꢀ1 under nitrogen. Gel permeation chromatography (GPC)
analyses were conducted on a Waters 1515 separation module
num) (Alq3)/Lithium fluoride (LiF)/Al were fabricated,
where BCP was used as a hole-blocking layer and Alq3 was
used as an electron-transporting layer. ITO substrates, where
glass substrates were coated with ITO having a sheet resist-
ance of ꢂ20 X/sq and an effective individual device area of
3.14 mm2, were routinely cleaned by ultrasonic treatments
in detergent solutions and distilled water, followed by rins-
ing with acetone and then ethanol. After drying, ITO sub-
strates were kept in oxygen plasma for 4 min before being
loaded into the vacuum chamber. The PEDOT:PSS films were
first deposited on precleaned ITO substrates by spin-coating
at 6000 rpm for 1 min and subsequently cured in an oven at
120 ꢁC for 1 h. Then, light-emitting polymers in THF
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solutions (10 mg mLꢀ1
)
were spin-coated onto the
Synthesis of Compound (4) (A2 Monomer)
PEDOT:PSS layer at 4000–5500 rpm. The thicknesses of
PEDOT:PSS and LED polymers were measured by an Alfa
Step 500 surface profiler (Tencor). BCP and Alq3 were ther-
mally deposited at a rate of 1–2 Å sꢀ1 under a pressure of
ꢂ2 ꢃ 10ꢀ5 Torr in an Ulvac cryogenic deposition system.
Under the same deposition condition, one layer of LiF was
Compound 3 (2.0 g, 3.266 mmol) and 2-(4-bromophenyl)ace-
tonitrile (1.91 g, 9.798 mmol) were dissolved in 50 mL of
dry THF. To this was added 5 mL of a solution of 2 M K2CO3,
and the mixture was purged with nitrogen for 30 min. Then,
2 mol % (75 mg) of Pd(PPh3)4 was added into the reaction
mixture, and the mixture was refluxed for overnight under
nitrogen atmosphere. The reaction mixture was cooled to
room temperature and poured into 2 N HCl. The organic
layer was extracted with ethyl acetate and dried over anhy-
drous magnesium sulfate. The product was purified by col-
umn chromatography using ethyl acetate and hexane (2:3)
as eluent.
thermally deposited as a cathode at a rate of 0.1–0.2 Å sꢀ1
,
which was followed by capping with aluminum. The cur-
rent–voltage–luminescence characteristics were measured in
ambient condition by a Keithley 2400 source meter and a
Newport 1835C optical meter equipped with an 818ST sili-
con photodiode.
Yield: 1.04 g (54%); IR (KBr, cmꢀ1): 3027 (¼¼CH), 2201
(ACN), 1590, 1505, 1401 (aromatic), 1288 (C¼¼C), 2912
Synthesis of Compound (2)
Compound (1) (2.0 g, 11.62 mmol), 5-bromo-2-thiophene
carboxaldehyde (4.5 g, 23.69 mmol), and five drops of piper-
idine were dissolved in 150 mL of dry acetonitrile and
stirred for overnight under reflux temperature. The reaction
mixture was cooled; the solution was filtered through a D3
glass frit to isolate the product. Thin-layer chromatogram
confirmed absence of impurities in the product after washing
it twice with 20 mL of methanol. Vacuum drying followed by
recrystallization in methylene chloride gave the product (2)
as orange yellow crystalline solid.
1
(alkyl), 802 (CAS); H NMR (300 MHz, DMSO-d6, ppm): 7.69
(d, 2H), 7.29 (d,2H), 7.22 (d, 2H), 6.89 (d, 2H), 6.87 (d, 4H),
6.75 (s, 2H), 6.66 (d, 4H), 3.81 (s, 4H); 13C NMR (75 MHz,
CDCl3-d, ppm): 22.9, 84.5, 99.8, 112.5, 117.8, 120.1, 125.5,
128.7, 128.9, 129.3, 130.1, 131.8, 133.3, 137.5, 142.9, 158.1.
164.5. MS (70 eV, EI): m/z: 590 [Mþ]. Anal. Calcd. for
C36H22N4S2O: C, 73.20; H, 3.75; N, 9.48, O, 2.41; S, 10.86.
Found: C, 72.85; H, 3.56; N, 9.30.
Synthesis of Compound (10)
Yield: 5.27 g (88%). IR (KBr, cmꢀ1): 3024 (¼¼CH), 2209
(ACN), 1590, 1506, 1400 (aromatic), 1287 (C¼¼C), 960
Compound 9 (2.0 g, 4.899 mmol), anhydrous NaOAc (2.0 g,
36.74 mmol), acetic anhydride (2.25 g, 22.0 mmol), and ace-
tic acid (10 mL) were mixed thoroughly and heated at 90 C
ꢁ
1
(CABr), 807 (CAS); H NMR (300 MHz, CDCl3-d, ppm): 7.69
for 24 h. Water was added and the mixture was extracted
with methylene chloride (3 ꢃ 50 mL). The crude product
obtained after solvent removal was purified by column chro-
matography using hexane/ethyl acetate (2:1) as eluent. The
product obtained was sticky oil.
(d, 2H, J ¼ 15 Hz), 7.29 (d, 2H, J ¼ 15 Hz), 7.21 (d, 2H),
6.86 (2H, d), 6.75 (s, 2H);13C NMR (75 MHz, CDCl3-d, ppm):
84.5, 99.8, 112.5, 118.9, 120.1, 125.5, 131.4, 136.4, 140.4,
157.9, 164.7. MS (70 eV, EI): m/z: 515 [Mþ], 517 [Mþ þ 2].
Anal. Calcd for C20H10ON2S2Br2: C, 46.35; H, 1.94; N, 5.41; S,
12.37; Br, 30.84; O, 3.09. Found: C, 45.58; H, 1.83; N, 5.32.
Yield: 0.97 g (79%). IR (KBr, cmꢀ1): 3017 (¼¼CH), 1690
(AC¼¼O), 1602, 1506, 1403 (aromatic), 2920 (alkyl); 1H
NMR (300 MHz, CDCl3-d): 7.63 (d, 2H), 7.36 (s, 2H), 7.34 (d,
2H), 5.17 (s, 4H), 2.12 (s, 6H), 2.05 (q, 4H), 0.31 (t, 6H); 13C
NMR (75 MHz, CDCl3-d, ppm): 8.9, 21.4, 33.1, 56.1, 64.9,
66.6, 121.2, 123.4, 125.9, 136.8, 137.3, 140.3. MS (70 eV, EI):
m/z: 366 [Mþ]. Anal. Calcd. for C23H26O4: C, 75.38; H, 7.15;
O, 17.46. Found: C, 75.30; H, 7.04.
Synthesis of Compound (3)
To a solution of compound (2) (5.0 g, 9.69 mmol) in THF
(100 mL) at ꢀ78 ꢁC, n-butyllithium (4.6 mL of 2.5 M solu-
tion in hexane, 11.628 mmol) was added dropwise. After the
mixture was stirred for 1 h, 2-isopropoxy-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolate (5.9 mL, 29.0 mmol) was added,
and the stirring was continued for additional 1 h at the
same temperature. The mixture was stirred overnight after
slowly warming it up to room temperature. Then, the mix-
ture was poured into the water, extracted with ethyl acetate,
and dried over anhydrous MgSO4. After the solvent was
removed, the product was obtained by a flash column with
hexane: ethyl acetate (3:2).
Synthesis of Compound (11)
A mixture of compound 10 (2.0 g, 5.45 mmol) and NaOH
(0.65 g, 16.35 mmol) in EtOH (50 mL) was heated at 40 ꢁC
for 21 h. Then, the mixture was neutralized with 1N HCl and
extracted with methylene chloride (3 ꢃ 20 mL). The crude
product obtained on solvent removal was purified by passing
it through a column with CH2Cl2/MeOH (10:1) as eluent.
Yield: 3.97 g (67%). IR (KBr, cmꢀ1): 3032 (¼¼CH), 2923
(alkyl), 2210 (ACN), 1600, 1506, 1403 (aromatic), 1288
(C¼¼C), 807 (CAS); 1H NMR (300 MHz, CDCl3-d, ppm): 7.69
(d, 2H), 7.36 (d, 2H), 7.29 (d, 2H), 7.19 (d, 2H), 6.75 (s, 2H),
1.35 (s, 24H); 13C NMR (75 MHz, CDCl3-d, ppm): 24.7, 84.3,
88.1, 99.8, 112.5, 120.1, 125.5, 127.1, 128.3, 130.5, 137.8,
157.8, 164.9. MS (70 eV, EI): m/z: 613 [Mþþ 1]; Anal. Calcd.
for C32H34O5N2S2B2: C, 62.76; H, 5.60; N, 4.57; S, 10.47; B,
3.53; O, 13.06. Found: C, 61.95; H, 5.32; N, 4.44.
Yield: 1.12 g (73%). IR (KBr, cmꢀ1): 3420 (AOH), 3032
(¼¼CH), 1601, 1505, 1416 (aromatic), 2910 (alkyl); 1H NMR
(300 MHz, CDCl3-d): 7.70 (d, 2H), 7.34 (d, 2H), 7.23 (d, 2H),
4.85 (s, 4H), 1.95 (q, 4H), 0.166 (t, 6H); 13C NMR (75 MHz,
CDCl3-d, ppm): 9.3, 33.0, 55.6, 62.4, 119.8, 121.2, 123.4,
125.9, 137.3, 140.3. MS (70 eV, EI): m/z: 282 [Mþ]; Anal.
Calcd. for C19H22O2: C, 80.82; H, 7.85; O, 11.33. Found: C,
80.30; H, 7.60.
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Synthesis of Compound (12) (B2 Monomer)
2932 (alkyl), 810 (CAS); 1H NMR (300 MHz, DMSO-d6,
ppm): 10.12 (s, AHC¼¼O), 6.87–8.78 (br, m, aromatic), 1.98
(br, s, A(CH2)A), 0.25 (br, s, A(CH3)).
To a solution of diol 11 (1.0 g, 3.54 mmol) in CH2Cl2 (40
mL) was slowly added pyridinium chlorochromate (2.20 g,
10.62 mmol) under cooling in an ice-water bath. The mixture
was stirred at room temperature for 6 h. Then, 50 mL of
ether was added to the mixture, the stirring was continued
for additional 30 min, and then the reaction mixture was
passed through a short column filled with silica gel to
remove the pyridinium compounds. The residue obtained af-
ter removal of the solvent was purified with column chroma-
tography using hexane/ethyl acetate (2:1) as eluent.
Polymer P5. The polymer P5 was obtained from compound
(4) and compound (7). Yield: 58%. IR (KBr, cmꢀ1): 3034
(¼¼CH), 2745 (AHC¼¼O), 2225 (ACN), 1689 (¼¼C¼¼O), 1601,
1508, 1409 (aromatic), 1287 (CAN), 1298 (C¼¼C), 2921
1
(alkyl), 809 (CAS); H NMR (300 MHz, DMSO-d6, ppm): 9.85
(s, AHC¼¼O), 6.50–7.87 (br, m, aromatic).
Polymer P6. The polymer P6 was obtained from compound
(4) and compound (12). Yield: 54%. IR (KBr, cmꢀ1): 3028
(¼¼CH), 2758 (AHC¼¼O), 2226 (ACN), 1690 (¼¼C¼¼O), 1600,
1506, 1403 (aromatic), 1285 (C¼¼C), 2926 (alkyl), 812
(CAS); 1H NMR (300 MHz, DMSO-d6, ppm): 10.08 (s,
AHC¼¼O), 6.66–8.88 (br, m, aromatic), 2.05 (br, s, A(CH2)A),
0.28 (br, s, A(CH3)).
Yield: 0.61 g (62%). IR (KBr, cmꢀ1): 3064 (¼¼CH), 2732
(AHC¼¼O) 1689 (¼¼C¼¼O), 1588, 1506, 1434 (aromatic),
2935 (alkyl); 1H NMR (300 MHz, CDCl3-d) 10.12 (s, 2H),
8.88 (d, 2H), 8.40 (d, 2H), 7.95 (d, 2H), 2.08 (q, 4H), 0.26 (t,
6H); 13C NMR (75 MHz, CDCl3-d, ppm): 8.7, 33.1, 56.6, 122.3,
127.9, 138.3, 144.2, 146.3, 153.0, 191.0. MS (70 eV, EI): m/z:
278 [Mþ]; Anal. Calcd for C19H18O2: C, 81.99; H, 6.52; O,
11.50. Found: C, 81.30; H, 6.46.
RESULTS AND DISCUSSION
Design and Synthesis of Monomers
Synthesis of Polymers
We designed an A2 monomer (4) with electron accepting
DCM unit at the centre and polymerizable phenyl acetonitrile
groups at the periphery. Knoevenagel condensation between
this A2 monomer and electron-donating triphenylamine or
fluorene-containing aldehyde groups afforded the polymers
with donor–acceptor moieties linked through cyanovinylene
linkage {(CN) AC¼¼C¼¼} at the backbone. This structure ful-
fills the requirement for a conjugated system in which
energy can be funneled from donor to acceptor.
Knoevenagel condensation reaction was used to synthesize
the polymers as shown in Schemes 3–5. In a typical synthe-
sis, stoichiometrically equivalent of monomers (total amount
not exceeded 0.765 g) and catalytic amount of piperidine
were dissolved in 40 mL of 1:1.5 mixture of THF and aceto-
nitrile mixed solvent. The solution was refluxed under nitro-
gen atmosphere for 24 h. The reaction mixture was cooled
to room temperature and then poured into methanol. The
polymer was filtered, washed, and reprecipitated in acetoni-
trile. The product obtained after drying under vacuum was
reddish brown color solid.
The synthetic route for the A2 monomer is outlined in
Scheme 1. The main purpose of introducing the thiophene
ring and ACN group in the monomer structure is to increase
the absorption and emission maxima of the monomer, and to
increase the electron affinity of the monomer. In addition,
these groups improve electron injection and transport prop-
erties, respectively. To incorporate thiophene ring in the
monomer structure, we used thiophene molecule containing
aldehyde and bromo function as the starting material. This
molecule served as the handle to install the DCM molecule
at the centre and phenyl acetonitrile groups at the periphery.
The extended conjugation in the monomer will result red
shift in the absorption spectrum of the polymer obtained
from thereof.
Polymer P1. The polymer P1 was obtained from compound
(4) and compound (5).Yield: 72%. IR (KBr, cmꢀ1): 3038
(¼¼CH), 2739 (AHC¼¼O), 2201 (ACN), 1699 (¼¼C¼¼O), 1600,
1508, 1405 (aromatic), 1283 (CAN), 1293 (C¼¼C), 2922
1
(alkyl), 806 (CAS); H NMR (300 MHz, DMSO-d6, ppm): 9.77
(s, AHC¼¼O), 6.56–7.83 (br, m, aromatic).
Polymer P2. The polymer P2 was obtained from compound
(4) and compound (6). Yield: 76%. IR (KBr, cmꢀ1): 3024
(¼¼CH), 2756 (AHC¼¼O), 2213 (ACN), 1693 (¼¼C¼¼O), 1581,
1505, 1416 (aromatic), 1290 (C¼¼C), 2928 (alkyl), 810
(CAS); 1H NMR (300 MHz, DMSO-d6, ppm): 10.05 (s,
AHC¼¼O), 6.75–8.93 (br, m, aromatic), 2.02 (br, s, A(CH2)A),
0.36 (br, s, A(CH3)).
The high purity of the monomer was proven by FTIR, 1H
NMR, 13C NMR, mass spectra, and elemental analysis. 1H
NMR spectrum of the A2 monomer (4) is shown in Figure 2.
In this spectrum, the coupling constant (J ¼ 15.5 Hz) of ole-
finic protons at d 7.69 and 7.29 ppm indicates that the pres-
ence of trans isomers. Pyrone ring protons appeared as a
singlet at 6.75 ppm and thiophene ring protons show two
distinct doublets at 6.89 and 7.22 ppm. A sharp singlet at
3.81 ppm is due to presence of benzyl nitrile proton and
appearance of two doublets at 6.66 and 6.87 ppm corre-
sponds to aromatic protons.
Polymer P3. The copolymer P3 was obtained from com-
pound (4), compound (5), and compound (7). Yield: 79%. IR
(KBr, cmꢀ1): 3033 (¼¼CH), 2742 (AHC¼¼O), 2219 (ACN),
1689 (¼¼C¼¼O), 1598, 1509, 1407 (aromatic), 1278 (CAN),
1289 (C¼¼C), 2912 (alkyl), 806 (CAS); 1H NMR (300 MHz,
DMSO-d6, ppm): 9.89 (s, AHC¼¼O), 6.66–7.80 (br, m,
aromatic).
Polymer P4. The copolymer P4 was obtained from com-
pound (4), compound (8), and compound (12). Yield: 69%.
IR (KBr, cmꢀ1): 3035 (¼¼CH), 2759 (AHC¼¼O), 2205 (ACN),
1693 (¼¼C¼¼O), 1590, 1505, 1401 (aromatic), 1290 (C¼¼C),
Synthesis of B2 monomer (12) is outlined in Scheme 2. Ace-
toxy substitution of the bromo atoms in compound (9) was
4
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SCHEME 1 Synthetic route for A2 monomer (4).
done with sodium acetate, acetic anhydride, and acetic acid
104 and 1.8–2.3, respectively. The structure of the B3 and B2
monomer also has little effect on molecular weight of poly-
mers. The molecular weights of polymers derived using fluo-
rene monomers are higher than that of derived using triphe-
nylamine based monomers. The values of Mn and Mw of
HBPs and HB copolymers may be underestimated owing to
the topology of the polymers and method of determination.36
When compared to inherent viscosity (ginh) of linear poly-
mers, the same of HBPs and its copolymers are seems to be
low with respect to molecular weight and is a characteristic
of HBP.37
at 90 ꢁC for 24 h gave compound (10). Base hydrolysis of
ꢁ
(10) using NaOH and ethanol at 40 C followed by oxidation
using pyridinium chlorochromate afforded 2,7-bis formyl
9,9-diethyl fluorene (12) in 62% yield. Analytical data of this
compound unambiguously confirm its structure.
Synthesis of Polymers
The synthesis of HBPs (P1 and P2), HB copolymers (P3 and
P4), and linear polymers (P5 and P6) is described in
Schemes 3, 4, and 5, respectively. Polymerization of A2
monomer (4) with B3 or B2 monomers was carried out in
the mixture of THF/CH3CN (1:1.5 in volume) in the presence
of piperidine as a catalyst. The polymers were purified by
repeated reprecipitation of THF solution in methanol to
remove the low molecular weight oligomers.
Thermal and Solubility Properties of Polymers
The thermal properties of the polymers P1–P6 were investi-
gated by TGA and DSC in nitrogen atmosphere. The results
obtained are given in Table 1. All the polymers, except P1
were found to be stable up to 340 ꢁC; the polymer P1 was
found to be stable for still higher temperature (Fig. 5). DSC
was measured up to 280 ꢁC to investigate the glass transi-
tion state of the polymers. It was found that only the poly-
mers containing fluorene units (P2, P4, and P6) shown Tg in
the range of 120–144 ꢁC (Fig. 6), and none of polymers
based on triphenylamine shown Tg. This is due to the pres-
ence of flexible ethyl side groups in fluorene monomers.
The molecular structures of the polymers were verified by
FTIR and 1H NMR spectroscopy and their peak assignments
were consistent with the structure. Typical 1H NMR spec-
trum of polymer containing triphenylamine or fluorene moi-
ety is shown in Figures 3 and 4, respectively. The peak
appeared at 9.77 and 10.05 ppm corresponds to presence of
the aldehyde end groups in polymer P1 and P2, respectively.
The
proton
attached
to
cyanovinylene
linkage
The solubility of polymers is also included in Table 1. Poly-
mers derived from fluorene are found to be more soluble in
(AC(CN)¼¼CHA) formed in the polymer P1 and P2 appears
at 7.63 and 6.92 ppm, respectively; the broadness of the
peak is a characteristic of polymer. Moreover, the peak corre-
sponds to Ph-CH2ACN group in the A2 monomer was disap-
peared at 3.81 ppm in the polymers, which implies that po-
lymerization had taken place. The NMR spectra of other
polymers were also in good agreement with the correspond-
ing structures. FTIR spectra of all the polymers exhibited
characteristic peaks at 2225, 2925, and 1600 cmꢀ1 corre-
sponds to ACN, alkyl and ¼¼C¼¼C¼¼ stretching frequencies,
respectively.
The polymerization results such as yield, molecular weight
and viscosity data are summarized in Table 1. These results
reveal that the reaction time has significant effect on molecu-
lar weight and yield of polymers. The weight-average molec-
ular weight (Mw) and polydispersity index (PDI) of the poly-
mers are found to be in the range of 2.1 ꢃ 104 to 10.5 ꢃ
FIGURE 2 A 300-MHz 1H NMR spectrum of A2 monomer in
DMSO-d6.
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SCHEME 2 Synthetic route for B2 monomer (12).
SCHEME 3 Synthesis of conjugated hyperbranched polymers via A2þB3 approach.
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SCHEME 4 Synthesis of conjugated hyperbranched copolymers via A2þB3þB2 approach.
polar aprotic solvents such as DMF, N,N0-Dimethyl Acetamide,
NMP, DMSO, and THF at room temperature compared to that
derived from triphenylamine. In the solvents like acetone
and ethyl acetate, branched polymers are found to be more
soluble than the linear polymers. All the polymers can be
cast easily on a glass plate to give bright red thin films.
energy transfer from donor chromophore (triphenylamine or
fluorene) to acceptor chromophore (DCM).
UV–vis absorption spectra of conjugated HBPs P1, P3, and
P5 [Fig. 8(a)] based on DCM and triphenylamine show a
band in the region of 485–494 nm. This can be assigned to
p–p* transitions of the conjugated polymer backbone and
ICT between triphenylamine and DCM. Similarly, the absorp-
tion spectra of polymers P2, P4, and P6 [Fig. 8(b)] consist-
ing of DCM and fluorene molecule show bands at 483, 492,
and 460 nm, respectively. When compared with kmax of A2
Photophysical Properties of Polymers
Photophysical properties of polymers were studied using
absorption and emission spectroscopy. All the spectra were
measured in THF solution (10ꢀ5 M), and the main optical
properties are listed in Table 2. The PL spectra of 5 and 6
(donors), and absorption spectrum of 4 (acceptor) were
superimposed and given in Figure 7. From this figure, it is
evident that the emission of 5 and 6 and the absorption of 4
overlaps completely, which enables an efficient Forster
FIGURE 3 A 300-MHz 1H NMR spectrum of hyperbranched
polymer P1 in DMSO-d6.
SCHEME 5 Synthesis of conjugated linear polymers via A2þB2
approach.
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ꢁ
of the polymers subjected to heat treatment at 200 C for 60
min were also recorded in THF solution and found that there
was no change in the fluorescence intensities compared with
the unheated samples. It is noteworthy to mention here that
high Tg, good thermal stability, and fluorescent property
even after heating the sample at high temperature are the
important criteria for a organic luminescent material to be
used for practical applications in PLEDs.38
The quantum efficiency of the polymers in THF solution was
measured using a dilute solution of quinine sulfate in 0.1 M
H2SO4 (Uf ¼ 0.55) as the standard. All the polymers exhib-
ited moderate quantum yield and the results are summar-
ized in Table 2. The quantum efficiency increased from HBP
to HB copolymer and to linear polymers. The low quantum
yield of HBP is due to loss of coplanarity in the three dimen-
sional structure.
FIGURE 4 A 300-MHz 1H NMR spectrum of hyperbranched
polymer P2 in DMSO-d6.
The UV–visible and PL spectra of polymer films of P1, P3,
and P5, and P2, P4, and P6 are shown in Figure 9(a,b),
respectively. It has been noticed that the absorption of poly-
mers in thin films exhibits red shift compared with polymer
in solution. This may be due to extension of p-conjugation
system in solid state,39 which lead to more p–p stacking.40
The PL spectra of the conjugated polymers also show little
difference when changing its state from solution to film.
These results indicate that the polymers P1–P6 behave
almost in a similar way in both solution and solid state.
monomer (4), the kmax of polymers is distinctly shifted to-
ward low energy (Stokes Shift). This is due to the increase
in effective conjugation length along polymer backbone,
which leads to high ICT strength and thus electronic delocali-
zation. Besides, there is a significant increase in the molar
absorptivity (emax) of these materials, concomitant with sig-
nificant broadening of the absorption spectrum, which is an
important attribute in light-harvesting applications.
As shown in Figure 8, the PL spectra of polymers recorded
at the excitation wavelength of 475 nm display characteristic
strong red emission band around 630 nm, which is the
result of energy transfer from the triphenylamine/fluorene
to the DCM-based A2 monomer. The emission from triphenyl-
amine/fluorene was not observed. Indeed, there will be
emission from triphenylamine/fluorene, but the same may
be quenched by the formation of donor–acceptor complex
between triphenylamine/fluorene units and DCM-based units
in the polymers. Among the six polymers, PL spectra of the
polymers P1, P3, and P5 obtained from triphenylamine as a
donor molecule shows high stokes shift (44–66 nm) com-
pared with the polymers containing fluorene units due to
strong electron donating ability of triphenylamine. PL spectra
Electrochemical Properties of Polymers
The electrochemical properties of conjugated polymers (P1–
P6) were investigated by using CV and the CVs are given
in Figure 10. All the polymers showed irreversible process
in an oxidation scan, which indicates moderate electrical
onset
ox
activity. The onset oxidation potentials (E
) of the
polymers were in the range of 0.65–0.82 V. The redox
potential of Fc/Fcþ which has absolute energy of ꢀ4.8 eV
relative to the vacuum level for calibration is found to be
at 0.08 V in 0.1 M TBAF6/acetonitrile solution. Hence, the
evaluation of HOMO was done according to the following
equation:
TABLE 1 Properties of Conjugated Polymers (P1–P6)
Solubilitye
Reaction
a
a
b
c
d
Polymer
Code
Time
(h)
Yield
(%)
Mn
Mw
Td
Tg
ginh
DMF, DMSO
NMP, THF
Acetone,
(g molꢀ1
)
(g molꢀ1
)
PD
1.8
(ꢁC)
(ꢁC)
(dL gꢀ1
)
Ethylacetate
P1
24
72
19,500
35,100
377
NOc
0.46
þ
þ
P2
P3
P4
P5
P6
a
24
48
48
12
12
76
79
79
58
54
22,800
46,800
52,600
11,300
13,600
52,500
84,300
105,100
21,500
28,700
2.3
1.8
2.0
1.9
2.1
338
348
340
343
341
120
NOc
128
NOc
144
0.34
0.69
0.47
0.42
0.44
þþ
þ
þþ
þ
þþ
þ
þ
þ
6
6
d
Determined by GPC method using polystyrene standards in THF.
The 5% weight-loss temperatures under inert atmosphere.
Not observed up to 280 ꢁC.
Measured at a concentration of 0.5 g dLꢀ1 at 30 ꢁC in THF using an
Ubbelhode viscometer.
b
c
e
þþ, highly soluble; þ, soluble; 6, partially soluble.
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FIGURE 6 Second heating DSC traces of polymers P2, P4, and
P6.
FIGURE 5 TGA traces of polymers P1–P6.
of linear polymers (P5 and P6) are lower than that of HBPs
(P1–P4). This may be due to linear structures of polymers,
which contain more conjugation. In conclusion, the low band
gap in combination with good solubility and moderate quan-
tum efficiency of HB copolymers suggest that these materials
may be useful for PLED and photovoltaic applications.
onset
ox
HOMOðeVÞ ¼ ꢀe½E
þ 4:72ꢄðeVÞ:
The optical band gap (Eg.opt) of polymers was evaluated
from UV–vis absorption edge of thin-film spectra, and the
values are in the range of 2.30–2.18 eV. Among the six conju-
gated polymers, HB copolymers P3 and P4 show low optical
band gap in the range of 2.18–2.20 eV, which is slightly
higher than that of poly(3-hexyl thiophene) homopolymer
(ꢂ2.0 eV).41,42 The LUMO levels were calculated according
to the following equation:
Electroluminescence Properties of Polymers
The electrochemical properties as well as the HOMO and
LUMO energy levels of conjugated polymers are crucial pa-
rameters for the device configuration and further confine the
electron-hole recombination zone to the emission layer suffi-
ciently. Therefore, conjugated polymers were fabricated into
ELUMOðeVÞ ¼ EHOMO þ Eg:opt
:
four-layer PLED devices, with
a configuration of ITO/
PEDOT:PSS (50 nm)/polymer (55–70 nm)/BCP (10 nm)/
Alq3 (30 nm)/LiF (1 nm)/Al (150 nm) using standard proce-
dures of spin-coating and vacuum deposition methods, in
which polymer was used as the emission layer and
PEDOT:PSS as the hole transporting layer (anode buffer).
Besides, BCP (HOMO ¼ ꢀ6.70 eV, LUMO ¼ ꢀ3.20 eV), Alq3
(HOMO ¼ ꢀ6.00 eV, LUMO ¼ ꢀ3.30 eV), and LiF/Al were
used as a hole-blocking layer, an electron-transporting layer,
and a bilayer cathode, respectively.
The electrochemical properties such as the onset potentials,
band gaps, HOMO, and LUMO values are summarized in Ta-
ble 2. The LUMO energy levels of P1, P3, and P5 are very
similar to that of P2, P4, and P6. The substitution of donor
with different electron-donating ability has almost no effect
on the reduction potential of the polymers, and all the poly-
mers show electron affinity due to the incorporation of the
electron-deficient DCM moiety. The HOMO and LUMO levels
TABLE 2 Optical and Electrochemical Properties of Conjugated Polymers (P1–P6)
In Solution
In Film
onset
E
ox
c
d
d
a
b
Polymer
Code
kmax(abs)c(nm)
[e (Mꢀ1cmꢀ1) ]
kedge
kmax(flu)
kmax(abs)
kedge
kmax(flu)
(V)/HOMO
(eV)
LUMO
(eV)
Egap.opt
Uflu
(nm)
(nm)
(nm)
(nm)
(nm)
(eV)
(%)
P1
490 (28,890)
576
643
497
556
654
0.82/ꢀ5.54
ꢀ3.31
2.23
56
P2
P3
P4
P5
P6
a
483 (34,020)
494 (26,980)
492 (29,570)
485 (25,900)
460 (23,400)
626
578
642
643
616
593
652
606
622
566
468
510
483
492
458
563
563
570
550
540
c
597
667
608
637
584
0.73/ꢀ5.45
0.77/ꢀ5.49
0.69/ꢀ5.41
0.75/ꢀ5.47
0.65/ꢀ5.31
ꢀ3.25
ꢀ3.29
ꢀ3.23
ꢀ3.21
ꢀ3.01
2.20
2.20
2.18
2.26
2.30
65
62
65
74
82
The optical band gap (Egap.opt) was obtained from absorption edge.
10ꢀ5 M in THF.
b
d
The quantum yields of the compounds in THF were determined using
Spin-coated from a 10 mg mLꢀ1 THF solution.
a solution of quinine sulfate (1 ꢃ 10ꢀ5 M in 0.1 M H2SO4 solution, hav-
ing a quantum yield of 0.55) as a standard.
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FIGURE 7 UV–vis absorption spectrum of A2 monomer (4) and
PL spectra of B3 monomers (5 and 6). Note the overlap
between the PL spectrum of 5 and 6 with the absorption spec-
trum of 4. All the spectra were recorded in THF at a concentra-
tion of 10ꢀ5 M.
FIGURE 9 UV–vis absorption and PL spectra polymer films
based on (a) triphenylamine and (b) fluorene. PL spectra were
recorded by excitation with 475-nm light.
FIGURE 8 UV–vis absorption and PL spectra of polymers
based on (a) triphenylamine and (b) fluorene. All the spectra
were recorded in THF at a concentration of 10ꢀ5 M. PL spectra
were recorded by excitation with 475-nm light.
FIGURE 10 Cyclic voltammograms of conjugated polymers P1–
P6 (thin solid films) at a scan rate of 100 mV sꢀ1
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nylamine units (P1, P3, and P5), has a lower turn-on voltage
than that bearing fluorene units (P2, P4, and P6). From this
result, it can be attributed that the integration of triphenyl-
amine (electron transporting) into the hyperbranched struc-
tures can remarkably improve device performance with a
reduced turn-on voltage. In addition, the PLED device bearing
P3 and P4 has the best EL performance characteristics with a
maximum luminance and luminous efficiency of 4104 and
0.55 cd Åꢀ1, 3696 cd mꢀ2, and 0.47 cd Åꢀ1, respectively, which
are generally better than that of corresponding PLED devices
bearing P1, P2, P5, and P6. Such improvement in EL device
performance with the copolymers is due to the introduction of
hyperbranched disorder in the conjugated polymer chains.43 It
is worth noting that the PLED devices remained quite stable
when the bias voltages was increased from 4 to 25 V, ascribed
to the stability of DCM, triphenylamine, and fluorene units in
the polymer backbone, as well as the moderate Tg of the poly-
mers. Although breakdown voltage of the device is not exam-
ined, the device shows bright red emission. Therefore, the
properties of devices with polymers as the emitting layer indi-
cated that hyperbranched copolymers had a better charge car-
rier compared with hyperbranched and linear polymers.
Thereby, the DCM units/cyano groups and thiophene, triphe-
nylamine, fluorene units present in the copolymers functioned
well as electron-withdrawing and electron-donating moieties,
respectively.
FIGURE 11 Normalized EL spectra of PLED devices with the
configuration of ITO/PEDOT:PSS/polymer /BCP/Alq3/LiF/Al.
As shown in Figure 11, under forward bias voltages, the EL
spectra of conjugated polymers show EL emissions in the
range of 584–667 nm and the Commission International
´
d’Enclairage (CIE) chromaticity coordinates of the devices are
shown in Table 3, which matches with CIE chromaticity coor-
dinates (x ¼ 0:67, y ¼ 0:33) of national television system com-
mittee (NTSC) for red color very well. These EL features are
similar to those observed with corresponding PL emissions of
the respective solid films, which suggest that identical excited
state species is responsible for both PL and EL emissions and
also suggest that there is no aggregation in the devices. The EL
properties of the PLED devices are summarized in Table 3,
and their current density–voltage and luminescence–voltage
characteristics are displayed in Figure 12(a,b), respectively.
The current density Vs voltage plots of the PLEDs reveal the
same trend as the increase in current density with voltage
occurs over a broader voltage range in the device. A compari-
son of their turn-on voltages, the PLED device bearing triphe-
CONCLUSIONS
We have successfully synthesized p-conjugated HBP, hyper-
branched copolymers, and linear polymers, consisting of
DCM-based unit coupled to different electron donating moi-
eties by the Knoevenagel condensation reaction through
A2þB3, A2þB3þB2, and A2þB2 approaches, respectively. The
HBPs and HB copolymers were found to be highly soluble in
common organic solvents compared to their linear counter
parts. These polymers exhibit well-resolved absorption and
emission spectra both in solution and in solid state and
show red-light emission. The HB and linear polymers had
optical band gap in the range of 2.18–2.30 eV. All the
TABLE 3 Electroluminescence (EL) Device Performance Characteristics of Conjugated
Polymersa
Power
Luminance
Efficiencyc
Max
CIE
Polymer
Code
kEL, film
Von
Efficiencyc
Luminance
(cd mꢀ2
(x,y)
Coordinated
b
ꢀ1
(nm)
(V)
(cd A
)
(lm Wꢀ1
)
)
˚
P1
P2
P3
P4
P5
P6
a
651
597
658
602
626
577
4.4
4.7
4.2
4.6
4.5
5.1
0.40
0.36
0.55
0.47
0.25
0.21
0.72
0.66
0.98
0.85
0.54
0.48
3179
2475
4100
3702
2255
1409
0.634, 0.322
0.613, 0.302
0.637, 0.328
0.609, 0.312
0.628, 0.329
0.615, 0.317
Device configuration: ITO/PEDOT:PSS/polymer /BCP/Alq3/LiF/Al.
b
c
V
on: the turn-on voltage of light (defined as the voltage required to give a luminance of 1 cd mꢀ2).
Measured at 100 mA/cm2.
d
´
Commission International d’Enclairage (CIE) chromaticity coordinates.
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FIGURE 12 (a) Current density–voltage (I–V) curves and (b)
luminance–voltage (L–V) curves of PLED devices with the con-
figuration of ITO/PEDOT:PSS/polymer /BCP/Alq3/LiF/Al.
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polymers show good thermal stability and fluorescent prop-
erty even after heating the sample at high temperatures. The
characterization results indicate that the properties like solu-
bility, quantum yield, and band gap can be tuned by chang-
ing the structure (topology) of polymer and found that the
copolymers P3 and P4 possess these properties at optimal
level. EL spectra show a single-peak emission in the region
of red light in the device with the configurations of ITO/
PEDOT:PSS/polymer/BCP/Alq3/LiF/Al. The device with P3
and P4 shows maximum luminance and luminous efficiency,
which could be attributed to a better carrier balance of the
polymer itself and suggests that these polymers had the best
balance with AlQ in the device. Further research on optimiz-
ing the chemical structures to adjust the balance of carriers
and device architectures to improve efficiency is in progress.
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