Synthesis, characterization, and electroluminescence properties of poly(fluorenevinylene benzobisthiazoles)

Article abstract of DOI:10.1002/pola.26449

A series of vinylene-linked copolymers based on electron-deficient benzobisthiazole and electron-rich fluorene moieties were synthesized via Horner-Wadsworth-Emmons polymerization. Three different polymers P1, P2, and P3, were prepared bearing octyl, 3,7-dimethyloctyl, and 2-(2-ethoxy)ethoxyethyl side chains, respectively. The polymers all possessed moderate molecular weights, good solubility in aprotic organic solvents, and high fluorescence quantum efficiencies in dilute solutions. P2, which bore branched 3,7-dimethyloctyl side chains, exhibited better solubility than the other polymers, but also exhibited the lowest thermal decomposition temperature of all polymers. Overall, the impact of the side chains on the polymers optical properties in solution was negligible as all three polymers gave similar absorption and emission spectra in both solution and film. Guest-host light-emitting diodes using dilute blends of the polymers in a poly(N-vinylcarbazole) host gave blue-green emission with P2 exhibiting the highest luminous efficiency, 0.61 Cd/A at ～500 nm.

Full text of DOI:10.1002/pola.26449

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Synthesis, Characterization, and Electroluminescence Properties
of Poly(fluorenevinylene benzobisthiazoles)
1
1
2
1
2
Jeremy J. Intemann, Jared F. Mike, Min Cai, Charles A. Barnes, Teng Xiao,
1
2
3
1
Robert A. Roggers, Joseph Shinar, Ruth Shinar, Malika Jeffries-EL
1
Department of Chemistry, Iowa State University, Ames, Iowa 50011
2
Ames Laboratory-USDOE and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011
Microelectronics Research Center and Department of Electrical and Computer Engineering, Iowa State University,
3
Ames, Iowa 50011
Correspondence to: M. Jeffries-EL (E-mail: malikaj@iastate.edu)
Received 6 August 2012; accepted 21 October 2012; published online 26 November 2012
DOI: 10.1002/pola.26449
ABSTRACT: A series of vinylene-linked copolymers based on
electron-deficient benzobisthiazole and electron-rich fluorene
moieties were synthesized via Horner–Wadsworth–Emmons
polymerization. Three different polymers P1, P2, and P3, were
prepared bearing octyl, 3,7-dimethyloctyl, and 2-(2-ethoxy)ethox-
yethyl side chains, respectively. The polymers all possessed
moderate molecular weights, good solubility in aprotic organic
solvents, and high fluorescence quantum efficiencies in dilute
solutions. P2, which bore branched 3,7-dimethyloctyl side chains,
exhibited better solubility than the other polymers, but also
exhibited the lowest thermal decomposition temperature of all
polymers. Overall, the impact of the side chains on the polymers
optical properties in solution was negligible as all three poly-
mers gave similar absorption and emission spectra in both solu-
tion and film. Guest-host light-emitting diodes using dilute
blends of the polymers in a poly(N-vinylcarbazole) host gave
blue-green emission with P2 exhibiting the highest luminous ef-
ficiency, 0.61 Cd/A at ꢀ500 nm. VC 2012 Wiley Periodicals, Inc.
J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 916–923
KEYWORDS: conjugated polymers; heteroatom-containing poly-
mers; light-emitting diodes; synthesis
INTRODUCTION Conjugated polymers are currently of inter-
the development of emissive materials with better charge
injection and transport properties.
6(d),7
est due to their potential impact on a diverse range of tech-
1
nologies, including polymer light-emitting diodes (PLED)s,
Among electron-deficient heterocycles, benzobisthiazole is
unique as it contains two sulfur atoms and two nitrogen
atoms within its fused three-ring system. Benzobisthiazole
polymers and small molecules have been used as organic
semiconductors in a variety of applications, including non-
2
organic photovoltaic cells and field-effect transistors
FETs). Research in this area is motivated by the opportu-
3
(
nity to fabricate large area displays via low-cost solution
processing techniques and tuning of the optical properties of
the polymers through chemical synthesis. Since the discovery
of electroluminescence in poly(phenylene vinylene), poly
8
9
linear optical materials, FETs, light-emitting diodes
10
9(a–c),11
(
LEDs), and photovoltaic cells.
When used as elec-
(
arylene vinylene)s (PAVs) have been widely studied for the
tron-transport layers, polybenzobisthiazoles improved the
development of emissive polymers. The incorporation of
vinylene linkages into the polymer backbone is advantageous
as they reduce torsional interactions between adjacent rings,
which increases planarization and effective conjugation
length, as well as reduces ionization potentials (IPs) and
10(b)
performance of PAV based LEDs.
When used as emis-
sive materials, thin films of poly(benzobisthiazo1e-1,4-phe-
nylenebisvinylene) emitted red light with an emission peak
1
0(a)
at 640 nm and an estimated quantum yield of 4–5%.
In
spite of their many beneficial qualities, the development of
benzobisthiazole-containing materials has been hindered by
two factors: the poor solubility of the resulting polymer,
requiring processing from strongly acidic solutions; and
harsh reaction conditions for their synthesis, which limit
the types of substituents that can be incorporated into the
polymer backbone. However, the recent development of
monomers based on functional benzobisthiazoles has
4
hole injection barriers. Vinylene groups also increase the
rotational flexibility of the polymer chain, which can enhance
5
polymer solubility. The properties of PAVs can be further
modified by the incorporation of electron-deficient hetero-
cycles into the polymer backbone to increase the polymers’
6
electron affinity. Accordingly, the synthesis of PAVs compris-
ing electron-deficient heterocycles has been investigated for
Additional Supporting Information may be found in the online version of this article.
2012 Wiley Periodicals, Inc.
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ꢁ
created the opportunity to develop new PAVs based on this
moiety.
were made within the temperature interval of 25–850 C,
1
2
ꢁ
with a heating rate of 20 C/min, under ambient atmosphere.
Differential scanning calorimetry (DSC) was performed with a
first scan at a heating rate of 15 C/min to erase thermal his-
In this article, we report the synthesis, characterization, and
electroluminescent properties of three new PAVs composed
of fluorene and benzobisthiazole. We used 9,9-dialkylfluor-
enes as comonomers as they have been widely utilized
in the synthesis of electroluminescent polymers. In general
fluorene-based polymers have excellent solubility, high solid-
state photoluminescence (PL) quantum yield and good
ꢁ
tory and a second scan to measure transitions from 0 to 200
ꢁ
C under nitrogen. Electrochemical properties were measured
on an eDaq e-corder 410 potentiostat using a three-electrode
cell (electrolyte: 0.1 mol/L TBAPF6 in acetonitrile) with an
Ag/AgNO reference electrode, a platinum auxiliary electrode,
3
and a platinum button electrode as the working electrode.
Polymer films were drop-cast from a chloroform solution on
1
3
charge carrier mobility.
Unfortunately, many fluorene-
containing polymers have low electron affinities and high
IPs, which makes the charge carrier injection diffi-
ꢁ
to the working electrode. All films were annealed at 160 C
for 1 h prior to use. All cyclic voltammetry experiments were
performed under argon atmosphere and were recorded at a
scan rate of 100 mV/s and referenced to ferrocene/ferroce-
nium. Powder diffraction patterns of thin films were obtained
using a Riguku Ultima IV X-Ray diffractometer equipped with
an MPA-U4 multipurpose Eulerian cradle and 1.76 kW Cu Ka
7
(a),13,14
cult.
Previously, we synthesized poly(fluorene vinyl-
ene-alt-benzobisoxazole)s, which exhibited reversible reduc-
tion processes and stable blue emission with luminous
efficiencies of up to 1 Cd/A at ꢀ470 nm in guest-host PLEDs
using a poly(N-vinyl carbazole) matrix. As benzobisthiazole
is a stronger electron-accepting moiety than benzobisoxazole,
we anticipated that the resulting polymers would have lower
lowest occupied molecular orbitals (LUMO) than their oxy-
gen-containing analogs.
ꢁ
radiation. Samples were first scanned from 0.05 to 0.501 2h
using parallel-beam optics to determine the critical angle of
20
the films. The incident angles of the X-ray beam for the dif-
fraction studies were chosen to be at a value slightly above
the external critical reflection for the polymer films. Fluores-
cence lifetime measurements were performed using the time-
correlated–single-photon counting method. Pulses tunable
from ꢀ780 to 880 nm were produced from a homebuilt 82-
MHz mode-locked Ti:sapphire oscillator pumped by a 5 -W
EXPERIMENTAL
Materials
All chemicals were purchased from commercial sources and
used without further purification unless otherwise noted. Tetra-
hydrofuran (THF) was dried using an Innovative Technologies
solvent purification system. 9,9-Dioctylfluorene-2,7-dicarbalde-
Nd:VO laser (Millennia, Spectra Physics). The resulting funda-
4
mental wavelength at ꢀ814 nm was modulated by a Pockels
cell (Model 350–160, Conoptics) to reduce the repetition rate
to ꢀ8.8 MHz. The frequency-doubling of this laser source by a
harmonic generator (Model TP-2000B, U-Oplaz Technologies)
provided the excitation wavelength at ꢀ407 nm. A half-wave
plate and polarizer placed before the sample chamber
ensured vertically polarized excitation. Emission (ꢃ500 nm)
was collected in a perpendicular geometry and passed
1
5
15
hyde, 9,9-bis(3,7-dimethyloctyl)fluorene-2,7-dicarbaldehyde,
16
17
2
,7-dibromofluorene,
and 2b,
phosphonate ester, and 2-(2-ethoxy)ethoxyethyl bromide
2,7-dibromo-9,9-dialkylfluorenes 2a
18
0
2,6-dimethylbenzo[1,2-d;4,5-d ]bisthiazolediethyl-
12
19
were synthesized according to literature procedures. Chromato-
graphic separations were performed using silica gel 60, using
eluents as indicated.
ꢁ
through a polarizer set at the magic angle (54.7 ) for solu-
Instrumentation
ꢁ
tions and 90 for solid films, with respect to the excitation
Nuclear magnetic resonance spectra were obtained on a 400
polarization. For polymer solution lifetime measurements,
dilute solutions of each were prepared in chloroform and
placed in a 1 cm path-length cuvette. Notably, a front-faced
geometry was also used for solid films. The placement of
appropriate filters before the microchannel plate, MCP (Hama-
matsu, R3809U-50) eliminated the excitation light and
allowed selection of emission from the sample. The full width
at half-maximum of the instrument response function was
1
13
1
MHz spectrometer ( H at 400 MHz and C at 100 MHz). H
NMR samples were referenced internally to residual proto-
1
3
nated solvent. C NMR spectra were referenced to the middle
carbon peak of CDCl . In both instances, chemical shifts are
3
given in d relative to solvent. High-resolution mass spectra
were recorded on a double focusing magnetic sector mass
spectrometer using EI at 70 eV. Melting points were obtained
using a melting point apparatus, upper temperature limit
ꢀ
37–40 ps. All measurements were made in a 3–5 ns time
ꢁ
2
60 C. Gel permeation chromatography (GPC) measurements
window with a total of 4096 channels. A total of 65,536
counts were collected at the peak channel for all lifetime
measurements. The fluorescence quantum yield measure-
were performed on a Viscotek GPC Max 280 separation mod-
ule equipped with three 5 lm I-gel columns connected in a
series (guard, HMW, MMW, and LMW) with a refractive index
þ
ments were conducted by using an Ar laser as excitation
ꢁ
detector. Analyses were performed at 35 C using THF as the
source and a homemade integrating sphere connected to a
calibrated Ocean Optics spectrometer through an optical fiber,
eluent with the flow rate at 1.0 mL/min. Calibration was
based on polystyrene standards. Fluorescence spectroscopy
and UV-visible spectroscopy were obtained using polymer sol-
utions in chloroform and as thin films. The films were made
21
as described elsewhere.
Monomer Synthesis
3
by spin-coating 25 ꢂ 25 ꢂ 1 mm glass slides, using a solu-
2,7-Dibromo-9,9-bis(2-(2-ethoxy)ethoxyethyl)fluorene
A round-bottom flask was charged with 2,7-dibromofluorene
(27.05 g, 83.5 mmol) and potassium iodide (1.39 g, 8.35
tion of 10 mg of polymer per 1 mL chloroform at a spin rate
of 2000 rpm. Thermal gravimetric analysis measurements
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mmol) in dimethyl sulfoxide (70 mL). The mixture was
cooled in a cold-water bath and stirred as potassium hydrox-
ide power (23.45 g, 418 mmol) was added in a single por-
tion. 2-(2-Ethoxy)ethoxyethyl bromide (36.0 g, 184 mmol)
was added dropwise while keeping the pot temperature
benzene as the eluent. The solvent was evaporated yielding
a yellow oil (4.56 g, 99% yield).
1
3
H NMR (CDCl ) d: 10.08 (s, 2H), 7.99 (s, 2H), 7.92 (m, 4H),
3
4
1
.35 (q, 4), 3.24 (t, 4H), 3.12 (t, 4H), 2.78 (t, 4H), 2.49 (t,
13
H), 1.09 (t, 6H); C NMR (CDCl ) d: 192.1, 151.6, 145.1,
3
ꢁ
between 0 and 20 C. The reaction was stirred at room tem-
36.7, 130.3, 124.7, 121.6, 70.3, 69.8, 67.1, 66.8, 52.1, 39.5,
perature for 24 h and then poured into water. The mixture
was extracted with chloroform (3 ꢂ 125 mL) and the or-
ganic extracts were combined, washed with brine, dried over
þ
and 15.3; HRMS (EI) Calcd. for C H O (MH ) 455.2428,
27 35 6
found 455.2434, deviation ꢄ1.29 ppm.
MgSO , and the solvent evaporated. The excess alkyl bromide
General Procedure for Polymerization
4
was removed by distillation under reduced pressure and the
remaining residue was purified by column chromatography
using silica gel and a 5:1 mixture of chloroform: ethyl ace-
tate as the eluent to yield a white solid (28.80 g, 62% yield).
A flame-dried Schlenk flask was placed under an argon
atmosphere and charged with compounds 1 (1 mmol) and
2(a–c) (1 mmol) followed by addition of THF (30 mL). The
mixture was stirred while potassium tert-butoxide (1.0 M in
THF, 2.2 mL) was added dropwise over 20 min. The reaction
was then stirred for 24 h followed by precipitation of the
polymer in methanol (ꢀ100 mL). The solid was filtered and
washed in a Soxhlet extractor with methanol, followed by
hexanes, and then extracted with THF. The THF fraction was
evaporated to yield the solid polymer.
ꢁ
1
Mp ¼ 88–89 C; H NMR (CDCl ) d: 7.53 (s, 2H), 7.51 (d,
3
2
2
1
6
H), 7.46 (d, 2H), 3.42 (q, 4H), 3.33 (t, 4H), 3.20 (t, 4H),
.78 (t, 4H), 2.35 (t, 4H), 1.15 (t, 6 H); C NMR (CDCl ) d:
51.0, 138.5, 130.8, 126.8, 121.8, 121.3, 70.3, 69.8, 66.9,
6.8, 52.0, 39.6, and 15.2; HRMS (EI) Calcd. for C H Br O
2 4
MH ) 555.0740, found 555.0744, and deviation ꢄ0.7 ppm.
,7-Bis(hydroxymethyl)-9,9-bis(2-(2-ethoxy)
1
3
3
2
5
33
þ
(
Poly[(9,9-dioctylfluorene-2,7-vinylene)-alt-benzo
2
[
1,2-d;4,5-d’]bisthiazole-2,6-diyl] (P1)
ethoxyethyl)fluorene
The polymer was obtained as an orange solid (510 mg, 81%
yield).
A flame-dried three-neck round-bottom flask was placed
under argon and charged with 2,7-dibromo-9,9-bis(2-(2-
ethoxy)ethoxyethyl)fluorene (15.75 g, 28.3 mmol) and dry
1
H NMR (CDCl ) d: 8.44 (s, ArAH), 7.38–7.86 (br m, ArAH
3
ꢁ
and vinylic ¼¼ CAH), 2.05 (s, ACH A), 1.02–1.28 (br m,
THF (270 mL). The mixture was cooled to ꢄ78 C and
2
ACH
2
A), 0.59–0.92 (br m, ACH
2
A and CH
3
); GPC: M
n
¼
stirred as tert-butyllithium (1.7 M in pentane, 100 mL) was
11,500, Mw ¼ 32,900, and PDI ¼ 2.9.
added dropwise via an addition funnel. It was then stirred
ꢁ
an additional 1 h at ꢄ78 C before adding paraformaldehyde
Poly[(9,9-bis(3,7-dimethyloctyl)fluorene-2,7-vinylene)-
(
2.55 g, 85 mmol) in small portions over 15 min. The reac-
0
alt-benzo[1,2-d;4,5-d ]bisthiazole-2,6-diyl] (P2)
tion was then stirred for another 2 h after which it was
warmed to room temperature and stirred overnight. The
reaction was then quenched by the addition of a saturated
aqueous ammonium chloride solution and the mixture was
extracted with diethyl ether (3 ꢂ 100 mL). The organic
extracts were combined, washed with water and brine, and
The polymer was obtained as an orange solid (427 mg, 62%
yield).
1
H NMR (CDCl ) d: 8.37 (s, ArAH), 7.38–7.74 (br m, ArAH
3
and vinylic ¼¼ CAH), 1.98 (br m, ACH A), 0.38–1.40 (br m,
2
ACH A and ACH ); GPC: M ¼ 13,000, M ¼ 38,000, and
2
3
n
w
PDI ¼ 2.9.
dried over MgSO . After evaporation of the solvent, the crude
4
oil was purified by column chromatography on a silica gel
column using a gradient of 10:1–2:1 of diethyl ether: isopro-
pyl alcohol to yield a yellow oil (8.22 g, 63% yield).
1
Poly[(9,9-bis(2-(2-ethoxy)ethoxyethyl))fluorene-2,7-vinyl-
ene)-alt-benzo[1,2-d;4,5-d ]bisthiazole-2,6-diyl] (P3)
The polymer was obtained as an orange solid (386 mg, 60%
yield).
0
H NMR (CDCl ) d: 7.63 (d, 2H), 7.42 (s, 2H), 7.32 (d, 2H),
3
1
4
4
.72 (s, 4H), 3.39 (q, 4H), 3.30 (t, 4H), 3.15 (t, 4H), 2.75 (t,
H), 2.38 (t, 4H), 2.32 (s, 2H), 1.12 (t, 6H); C NMR (CDCl3)
H NMR (CDCl ) d: 8.47 (s, ArAH), 7.41–7.82 (br m, ArAH
3
1
3
and vinylic ¼¼ CAH), 3.16–3.45 (br m, AOACH A), 2.92 (br s,
2
d: 149.5, 140.3, 139.7, 126.3, 122.0, 119.8, 69.8, 69.7, 67.1,
AOACH A), 2.49 (br s, AOACH A), 1.27 (s, ACH A), 0.89
2
2
2
6
6.5, 65.5, 51.2, 39.6, and 15.0; HRMS (EI) Calcd. for
(
s, ACH
3
); GPC: M
n
¼ 7 349, M ¼ 15,300, PDI ¼ 2.1.
w
þ
C H O Na (MNa ) 481.2561, found 481.2558, deviation
2
7 38 6
PLED Fabrication and Characterization. PLEDs were fabri-
cated on nominally 20 X/sq, 140-nm thick ITO-coated glass
substrates (Colorado Conecpt Coatings). The substrates were
first cleaned with a detergent and organic solvents and then
treated in a UV/ozone oven to increase the work function of
the ITO and hence facilitate hole injection, as described else-
0.57 ppm.
9,9-Bis(2-(2-ethoxy)ethoxyethyl)fluorene-2,7-
dicarboxaldehyde (2 c)
A round-bottom flask was charged with 2,7-bis(hydroxy-
methyl)-9,9-bis(2-(2-ethoxy)ethoxyethyl)fluorene (4.62 g,
22
10.1 mmol) and activated manganese dioxide (6.96 g, 80
where. A 60 nm PEDOT:PSS layer was spin-coated onto the
ꢁ
mmol) in benzene (70 mL). A Dean-Stark trap with a con-
denser was attached to the flask and the reaction was
refluxed overnight. The mixture was then cooled to room
temperature and passed through a short silica gel plug using
ITO and then baked in air at 120 C for 1 h, then in an argon
ꢁ
filled glovebox at 120 C for another 30 min. Blends of PVK
and benzobisthiazole copolymers in chlorobenzene solutions
were spin-coated on top of the PEDOT:PSS layer in the argon
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SCHEME 1 Synthesis of poly(fluorene vinylene-alt-benzobisthiazole)s.
filled glovebox. The combined concentration of the PVK and
guest material was kept constant at 9 mg mL . The solution
Figure 1 and the data is summarized in Table 1. All of the
polymers were found to be thermally stable with weight loss
onsets occurring above 250 C. As expected, the introduction
ꢄ1
ꢁ
was spin-coated at 4000 rpm for 60 seconds. The fabricated
ꢁ
structure was then annealed at 60 C for 30 min. Following
of flexible side chains onto the polymer backbone results in
significantly lower decomposition temperatures in compari-
son to related poly(phenylenevinylene benzobisthiazole),
which does not possess side chains and exhibits 5% weight
this annealing step, the samples were transfered to a thermal
evaporator within the glovebox and the Bphen, LiF, and Al
layers were deposited sequentially by thermal evaporation at
ꢄ6
ꢁ
10(a)
a base pressure of ꢀ1 ꢂ 10 Torr. The PLEDs were charac-
loss at 560 C.
However, the decomposition temperatures
terized by monitoring their EL spectra, brightness as a func-
tion of the applied voltage, and luminous and power
efficiencies.
of these polymers are all high enough to be useful in semi-
conducting applications. Additionally, DSC indicated that
none of the polymers had glass transition temperatures,
25
which is desirable for good color purity in PLEDs.
RESULTS AND DISCUSSION
Optical and Electrochemical Properties
Synthesis and Characterization
The UV–vis absorption and PL spectra of the polymers, both
as dilute solutions in chloroform and as thin solid films,
were evaluated. The normalized absorption and PL spectra
for all of the polymers in solution is shown in Figure 2, the
film spectra are shown in Figure 3, and the data is summar-
ized in Table 2. It is apparent that the alkyl chains have no
impact on the optical properties of the polymers as the
absorption spectra for all of the polymers are virtually iden-
tical and possess two absorption bands, due to vibronic cou-
pling. In comparison to the solution spectra, in the thin film
spectra there is a broadening of the absorption band and the
intensities of the transitions have switched. As a result the
higher energy peak is now more intense than the lower
energy peak. There is also a slight red-shift in the absorption
of the polymers (6–11 nm), due to conformational changes
in going from solution phase to solid and/or pi-stacking. In
either case this effect is small for all of the polymers, an in-
dication that the side chains do not affect the solid-state
packing. This is supported by X-ray analysis, which shows
that thin films of all polymers are completely amorphous
with no observable periodicity (see Supporting Information
9
2
,9-Bis(2-(2-ethoxy)ethoxyethylfluorene-2,7-dicarboxaldehyde
c was synthesized in three steps starting from 2,7-dibromo-
fluorene using methods analogous to those for the synthesis
of 2a and 2b. All fluorene monomers were carefully purified
via column chromatography to remove any monoalkylated
23
side products that could lead to defects in the polymer.
The base-catalyzed Horner–Wadsworth–Emmons (HWE) pol-
ycondensation reaction between the benzobisthiazole mono-
mer 1 and fluorene monomers 2a, 2b, and 2c in anhydrous
THF afforded the polymers P1, P2, and P3 respectively
(
Scheme 1). The HWE reaction is preferable because it is
known to produce polymers with all trans-double bonds
while preventing cross-linking, incomplete double bond for-
2
4
mation, and other undesirable structural defects. All poly-
mers were obtained in 60–81% yields after purification by
Soxhlet extraction with methanol to remove residual salts,
followed by hexanes to remove the lower molecular weight
material, and finally THF to recover the soluble polymer. The
THF was evaporated to yield solid polymer. All of the poly-
mers were soluble in common organic solvents, such as m-
1
cresol, chloroform, o-dichlorobenzene, and THF. The H NMR
opt
Fig. S2). The optical band gap (Eg ) was determined from
spectra for polymers P1, P2, and P3 were in agreement with
the proposed polymer structures (see Supporting Informa-
tion Figs. S10–S12). The number-average molecular weights
TABLE 1 Physical Properties of Poly(fluorene vinylene-alt-
(
M s) of these polymers as determined by GPC against poly-
n
benzobisthiazole)s
styrene standards in THF range from 7500 to 13,700 g/mol
with polydispersity indices [PDIs, PDI ¼ weight-average mo-
lecular weight (M )/M ] of 2.1–2.9, as shown in Table 1.
Based on these results number-averaged degree of polymer-
izations between 11 and 18 were calculated. The thermal
stability of the benzobisthiazole polymers were evaluated
using thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC). The TGA curves are shown in
a
a
b
ꢁ
Polymer
M
n
M
w
PDI
T ( C)
d
w
n
P1
P2
P3
a
11,500
13,000
7,350
32,900
38,200
15,280
2.9
2.9
2.1
361
258
276
Determined by GPC in THF using polystyrene standards.
5% weight loss temperature by TGA.
b
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red-shift in their film PL spectra into the orange region of
the visible spectrum relative to their emissions in solution
which indicates that the polymer chains adopt a more planar
26
conformation in solid state. The wavelength of emitted
light is similar for all of the polymer films (558–560 nm)
and is also independent of alkyl chain substitution. P3 exhib-
its a broader emission band than P1 and P2 a result of
increased aggregation in the P3 film relative to P1 and P2.
This is due to the inability of the 2-(2-ethoxy)ethoxyethyl
chains of P3 to effectively disrupting p-stacking compared to
the alkyl chains on P1 and P2. This is evident from the
reduced solubility and molecular weight of P3.
The redox properties of the polymers were evaluated using
cyclic voltammetry (see Supporting Information Fig. S13 for
voltammograms). The calculated electron affinities (EA) and
IPs are listed in Table 2. All of the polymers had partially re-
versible reduction waves with reduction onsets at ꢄ1.9,
ꢄ1.8, and ꢄ1.8 V for P1, P2, and P3 respectively. These
reduction potentials translate to EAs of 2.9, 3.0, and 3.0 eV
which are larger than the homopolymer poly(9,9-dihexylfluor-
2
7
ene-2,7-vinylene), which has an EA of 2.7 eV, a consequence
of incorporating the benzobisthiazole moiety into the conju-
gated backbone. The EAs for these new polymers are within
range of previously reported poly(arylene vinylene-co-fluo-
FIGURE 1 TGA
plot
of
poly(fluorene
vinylene-alt-
benzobisthiazole)s.
the absorption onset of the polymer films and was 2.4 eV for
all of the polymers, further evidence that alkyl chain substi-
tution has no effect on the electronic properties of these
materials.
rene)s with electron withdrawing moieties in the backbone
7(c),28
(
2.83–3.17 eV).
This indicates that the electron-deficient
benzobisthiazole moiety is beneficial for increasing the EA of
conjugated polymers. The polymers did not show oxidative re-
versibility but gave onsets of 0.7, 0.8, and 0.7 V for P1, P2,
and P3, which corresponds to IPs of 5.5, 5.6, and 5.5 eV,
respectively. Both the EAs and IPs of these polymers are
The PL of the polymers in solution shows a narrow blue-
green emission at ꢀ490 nm which is independent of alkyl
chain substitution as the emission spectra are essentially the
same for all of them. All of the polymers show a significant
higher
than
the
structurally analogous
poly[(9,9-
FIGURE 2 UV-vis absorption (solid lines) and PL (dashed lines)
of P1, P2, and P3 in chloroform solutions.
FIGURE 3 UV–vis (solid lines) and fluorescence (dashed lines)
of thin films of P1, P2, and P3.
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TABLE 2 Electronic and Optical Properties of Benzobisthiazole Polymers
Solution
Thin Film
max
opt
a
EC
b
c
Polymer
k
(nm)
kem (nm)
kmax (nm)
k
em (nm)
E
g
(eV)
E
g
(eV)
EA (eV)
IP (eV)
d
d
d
P1
P2
P3
a
451, 480
492 , 527
486, 455
485, 454
486, 456
559
558
560
2.4
2.4
2.4
2.6
2.6
2.5
2.9
3.0
3.0
5.5
5.6
5.5
d
d
d
d
449, 478
491 , 526
d
d
446, 475
488 , 522
c
Optical band gap measured from the onset of absorption in films.
Ionization potential calculated from oxidation onsent (Eox) in CV trace
b
Electron affinity calculated from reduction onset (Ered) in CV trace
using EA ¼ ꢄ(Ered þ 4.8).
using IP ¼ ꢄ(Eox þ 4.8).
d
Denotes highest intensity peak.
0
dioctylfluorene-2,7-vinylene)-alt-benzo[1,2-d;4,5-d ]bisoxazole-
,6-diyl] (PFVBBO) (EA ¼ 2.87 eV, IP ¼ 5.36 eV) which has
Electroluminescent Devices
2
We first examined the use of the polymers as neat emitting
layers in PLEDs with the structure ITO/PEDOT:PSS (60 nm)/
P1-P3/BPhen (40 nm)/LiF (1 nm)/Al (100 nm), where ITO
is indium tin oxide, PEDOT:PSS is poly(3,4-ethylenedioxythio-
phene):poly(4-styrenesulfonate), and BPhen is 4,7-diphenyl-
1,10-phenanthroline. However, due to low solid-state
quantum yields, electroluminescence was not observed.
Because of their high quantum yield in solution, these poly-
mers are good candidates for guest-host PLEDs. We therefore
fabricated PLEDs where the polymers were used as low level
dopants in poly(N-vinylcarbazole) (PVK) matrix. The device
architecture was ITO/PEDOT:PSS (60 nm)/PVK:P1-P3/
BPhen (40 nm)/LiF (1 nm)/Al (100 nm), and the PVK had
an average molecular weight 50,000–100,000 g/mol). The
devices were optimized by varying the weight percentage
(wt %) of P1, P2, or P3 in PVK and the properties of the
resulting devices are shown in Table 4. As the concentration
of guest polymer in PVK was increased, overall performance
decreased with the most efficient devices made from 1 wt %
polymer guest in PVK. Concentration quenching was particu-
larly evident in the devices containing P3, which did not
show significant emission from the guest at any concentra-
tion. The device containing 1 wt % of P2 in PVK exhibited
the best efficiency at 0.61 Cd/A with a brightness of 478
15
oxygen atoms in place of sulfur in the backbone. The
electron density is more equally shared between sulfur and
carbon in benzobisthiazole than between oxygen and car-
bon in benzobisoxazoles, since sulfur is less electronegative
than the oxygen atom. Therefore, the p-orbitals will be
more delocalized, making benzobisthiazole a better elec-
tron-accepting moiety. Additionally the empty d-orbitals of
the sulfur atom can contribute to the molecular p-orbitals
8
(b),29
decreasing the energy of the p–p* transition.
Collec-
tively this causes benzobisthiazole polymers to have lower
EAs. The higher IPs are a consequence of the increased
aromaticity of the thiazole ring over the oxazole, which sta-
1
1(b)
bilizes the HOMOs of P1-3.
The resulting electrochemi-
cal band gaps are in close agreement with the optical band
gaps and these results are further evidence that the alkyl
chain substitution has no impact on the electronic
properties of these polymers.
The PL decay lifetimes of the polymers were measured both in
dilute solutions and thin films and the results summarized in
Table 3. In solution, the polymers show relatively short life-
times of 0.46–0.60 ns but are comparable to the previously
mentioned PFVBBO polymer which had a PL decay lifetime of
15
0
.51 ns. In films, the lifetimes for PL decay were even shorter
2
Cd/m . This efficiency is higher than the 0.25 Cd/A obtained
(0.07–0.15 ns) due to solid-state interchain interactions leading
15
from a similar device containing PFVBBO. The improved ef-
ficiency over PFVBBO can be attributed to the higher EA of
P2, which may improve the electron injection and transport,
to various non-radiative decay pathways, thus quenching film
fluorescence. P3 showed a much larger drop off in decay life-
times than P1 and P2, which can be attributed to the greater
extent of aggregation in the P3 film. The decay rate for P1 in
film is identical to that of the structurally analogous PFVBBO
(
also 0.12 ns), which suggests replacing the oxygen atoms in
TABLE 3 Photoluminescence Decay Lifetimes (s) and Quantum
Yields (U)
PFVBBO with sulfur atoms does not affect PL decay.
PL quantum yields were measured for all three polymers
and it was found that in solution all of the polymers were
efficient fluorophores with yields between 0.54 and 0.62. In
solution, the alkyl chain substitution did not have a signifi-
cant impact on quantum yield and all three polymers had
yields similar to that of PFVBBO (U ¼ 0.68). In thin films,
as seen with the PL decay lifetimes, the quantum yield fell
to near zero. This is an effect of concentration quenching
of the polymers in film and has also been reported for
PFVBBO.
a
Solution
Thin Film
b
b
Polymer
s (ns)
U
s (ns)
U
P1
P2
P3
a
0.47
0.46
0.60
0.54
0.62
0.56
0.12
0.15
0.07
<0.01
<0.01
<0.01
Taken in dilute chloroform solutions.
Photoluminescence decay lifetimes were obtained using a triexponen-
b
tial decay fit.
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TABLE 4 Device Characteristics of PLEDs Made From Benzobisthiazole Polymers
a
Device
Drive
Current Density
Brightness
Efficiency
b
2
2
c
EL
max [nm]
Polymer
wt %
V
on [V]
Voltage [V]
[J, mA/cm ]
[Cd/m ]
[Cd/A, %EQE ]
k
CIE 1931 [x,y]
PVK
P1
0.0
1.0
5.8
7.3
8.3
9.4
7.4
8.7
7.8
6.6
6.4
7.0
11.2
10.8
13.0
17.6
11.2
13.4
12.8
9.8
306
753
770
543
392
377
377
358
240
311
250
478
289
195
478
452
323
199
208
213
0.31, 0.30
0.30, 0.25
0.24, 0.17
0.14, 0.08
0.61, 0.45
0.35, 0.20
0.21, 0.12
0.25, 0.35
0.22, 0.29
0.19, 0.22
423
(0.17, 0.07)
(0.18, 0.15)
(0.19, 0.20)
(0.20, 0.25)
(0.19, 0.18)
(0.22, 0.27)
(0.24, 0.31)
(0.17, 0.08)
(0.17, 0.09)
(0.17, 0.11)
499, 428
499, 425
499, 427
502, 429
504, 428
505, 429
ꢄ, 429
2
4
.0
.0
P2
P3
a
1.0
2
4
.0
.0
1.0
2
.0
.0
10.2
10.8
ꢄ, 429
495, 427
4
b
c
Device structure: ITO/PEDOT:PSS/PVK:(P1, P2, or P3)/BPhen/LiF/Al. wt
Turn-on voltage (at which EL is visible to the eyes).
%
is weight percent of polymer in PVK.
EQE ¼ external quantum efficiency.
leading to a higher population of electron charge carriers in
the device.
Still, the overall efficiencies of these devices are low and the
electroluminescence spectra of the polymers (Fig. 4) suggest
a possible reason for this. In all of the devices, PVK emission
at ꢀ425 nm dominates the spectrum, overshadowing the
guest emission at ꢀ500 nm, which implies inefficient F o€ rster
resonance energy transfer between the host and guest. The
degree of energy transfer depends on the overlap of the
30
host’s emission spectrum with the guest’s absorption band.
Because these polymers have absorptions much more red-
shifted (ꢀ480 nm) than PVK emission (ꢀ425 nm), there is
poor spectral overlap between the two. This results in a
decrease in guest emission and a reduction in overall device
performance. These materials could benefit from using a
host with a narrower band gap that would give better spec-
tral overlap, improving the energy transfer.
CONCLUSION
In summary, three new solution-processible benzobisthiazole-
fluorene-containing polymers were synthesized via HWE
condensation polymerization. Changing the alkyl chains on the
fluorene units did not lead to a change in the electronic prop-
erties of the materials but did improve the solubility of the
polymers in organic solvents while maintaining high thermal
stabilities. The optical and electrochemical properties of the
polymers showed that incorporation of the benzobisthiazole
moiety increased their EAs relative to fluorene homopolymers.
Preliminary electroluminescent devices made from these poly-
mers showed promising brightness with modest efficiencies.
These results demonstrate that the benzobisthiazoles are not
as good as benzobisoxazoles for the development of electron-
deficient emissive materials for PLEDs. However, these materi-
als may find use as electron-transporting materials, this work
is currently ongoing and will be reported in due course.
FIGURE 4 Electroluminescent spectra of the benzobisthiazole
polymers in guest-host PLEDs.
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