Synthesis and characterization of color tunable, highly electroluminescent copolymers of polyfluorene by incorporating the N-phenyl-1,8-naphthalimide moiety into the main chain

Article abstract of DOI:10.1039/c5tc01899d

A series of six new organic light emitting copolymers were prepared from 9,9′-dioctylfluorene (DOF) and N-phenyl-1,8-naphthalimide (NPN) using palladium catalysed Suzuki polymerization. The feed ratios of poly[2,7-(9,9′-dioctylfluorene)-co-N-phenyl-1,8-na

Full text of DOI:10.1039/c5tc01899d

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Synthesis and Characterization of Color Tunable, Highly
Electroluminescent Copolymers of Polyfluorene by Incorporating
N-phenyl-1,8-Naphthalimide Moiety into Main Chain
Receiꢅed 00th January 20xx,
Acceꢁted 00th January 20xx
Peddaboodi Gopikrishna,a Dipjyoti Dasa and Parameswar Krishnan Iyera,b
DOIꢇ 10.1039/x0xx00000x
A series of six new organic light emitting copolymers were prepared from 9,9’-dioctylfluorene (DOF) and N-phenyl-1,8-
naphthalimide (NPN) using palladium catalysed Suzuki polymerization. The feed ratios of Poly[2,7-(9,9’-dioctylfluorene)-co-
N-phenyl-1,8-naphthalimide] (PFONPN) copolymers were 50:50, 65:35, 75:25, 90:10, 95:05 and 99:01 respectively. These
copolymers have good thermal stability with onset decomposition temperature (Td) of ~340-405°C and glass transition
temperature (Tg) of 123-134°C. All the copolymers are highly fluorescentandsoluble in common organic solvents such as
chlorobenzene, chloroform, dichloromethane, THF, toluene etc. allowing their processing from desired solvent. The
electroluminescence (EL) properties of the copolymers were also studied by fabricating single layer devices with
ITO/PEDOT: PSS/PFONPN/Ca: Al configuration. The PL and the EL spectra of the copolymers reveals that by changing the
content of NPN moiety in the polyfluorene main chain from 1-50 mol %, the emission color of the polymers can be tuned
from blue to green with Commission International de l’Echairage (CIE) coordinates being (0.17, 0.22) to (0.24, 0.49). This
color tuning can be attributed to the strong energy transfer from the fluorene to NPN unit in the polymer backbone. The
devices made with these copolymers are found to be very bright with PFONPN01 giving the highest brightness of 5236
cd/m2 with a luminous efficiency of 3.52cd/A.
www.rsc.org/
benzothiadiazole,8-10
bithiophene15
and
Introduction
naphthoselenadiazole.13 Recently organic molecules21 and
polymers22-24 containing 1,8-Naphthalimide and its derivatives
have generated lots of research interest as luminescent
materials in organic EL devices due to their superior thermal,
chemical and optical properties and high fluorescent quantum
yields. Generally 1,8-Naphthalimide derivatives have high
electron affinities (because they possess carbonyl groups) and
demonstrate good electron transporting or hole blocking
capabilities that can help balance the charge in PLEDs.25
Introduction of electron donating substituents at 4-position of
Since the discovery of polymer light-emitting diodes (PLEDs)
based on poly(p-phenylenevinylene) in 1990,1 conjugated
polymers (CPs) have attracted significant attention towards
PLEDs due to their potential application in large-area, flat-
panel displays because of their wide structural variety, low
cost, very high flexibility and ease of processing.2-5 In the past
decades, polyfluorenes have been extensively used as active
materials for multicolour PLEDs because of their excellent
thermal and chemical stability, good solubility in common
organic solvents, easy functionalization at the C-9 position of
fluorene and high fluorescence quantum yields in solid films.6
Generally polyfluorenes are p-type (electron donor) materials
with large band gaps7 and emit blue light. However, the
emission color of polyfluorenes can be judiciously tuned over
the entire visible region by introducing the narrow band gap
units into the polyfluorene backbone as mentioned in few
examples earlier.8-20 The commonly used narrow band gap
units are aromatic heterocyclic compounds such as
1,8-Naphthalimide
derivatives
makes
them
highly
fluorescent.26-28 Unlike 1,8-naphthalimide containing side
chains non-conjugated polymers which have the disadvantage
of low light-emitting efficiency,29-31 CPs containing 1,8-
naphthalimide derivatives have emerged very promising as
potential candidates for PLED applications owing to the
possibility of energy transfer from the conjugated backbone to
the low-band-gap 1,8-naphthalimide moieties.22,32,33Indeed,
numerous organic light emitting homo polymers have been
reported by incorporation of N-phenyl-1,8-naphthalimide
derivatives
into
polyfluorene
by
end
capping
method.34,35Recently, few studies have reported that when N-
aryl-1,8-naphthalimidederivatives were incorporated into the
CP the resulting polymer could show large EL and PL
efficiency.33,35
a.Center for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati-
781039, Assam, India
b.Department of Chemistry, Indian Institute of Technology Guwahati,Guwahati-
781039, Assam, India. E-mail: pki@iitg.ernet.in; Fax: +0091 361 258 2349
ꢀElectronic Suꢁꢁlementary Informaꢂon ꢃESIꢄ aꢅailaꢆleꢇ 1H and 13C NMR scanned
sꢁectra of all the newly synthesized comꢁounds and DSC curꢅes are included. See
DOIꢇ 10.1039/x0xx00000x
Herein, the synthesis, characterization and color tuning of a
series of new donor and acceptor based, highly
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electroluminescent conjugated random copolymers derived controlled by adjusting the molar ratio of 1 and 2 while
from 9,9’-dioctylfluorene (DOF) and N-phenyl-1,8- maintaining a 1:1 molar ratio between the dibromo monomers
naphthalimide (NPN) by Suzuki cross coupling reaction and 9,9’-Dioctylfluorene-2,7-diboronic acid bis(1,3-
(scheme 1) is reported. The NPN co-monomer was inserted propanediol)ester. The colors of the PFONPN01, PFONPN05
into polyfluorene main chain as an acceptor to enhance the and PFONPN10 polymer powders were light yellow and that of
electron transporting properties and to improve the PFONPN35, PFONPN25 and PFONPN50 polymers were yellow.
recombination efficiency of holes and electrons. To the best of
our knowledge, this is the first report of block copolymer using
(a)
(a)
NPN unit with polyfluorene by Suzuki coupling reaction. By
controlling the NPN unit in polymer main chain the emission
color of polymer was tuned efficiently from blue to green. The
emission color of copolymers changes when NPN content
increases from 1-50 mol % into the polyfluorene main chain.
We observed that in solid state PL, the polyfluorene
fluorescence completely quenched even for 1 mol% of NPN
content. We believe that the NPN unit allows efficient
intramolecular energy transfer from the fluorene segment to
the NPN unit. The EL spectra of these polymers show excellent
color tuning and except PFONPN01, all copolymers showed
only single peak because of strong energy transfer from the
fluorene backbone to NPN unit. The emission spectra of the
polymers were highly red shifted with gradual increment of
NPN content in the polymer backbone. The devices made with
these copolymers are found to be very bright with PFONPN01
giving the highest brightness of 5236 cd/m2 with a luminous
efficiency of 3.52cd/A.
(b)
Br
Br
O
N
O
i)
Br
Br
NH2
2
O
O
O
O
N
O
O
B
O
O
B
O
Br
Br
Br
Br
C8H17
C8H17
Fig. 1 1H (a) and 13C NMR (b) spectra of polymers in CDCl3.
C8H17
C8H17
1
2
3
55-67 %
yields
ii)
The 1H and 13C NMR spectra (Fig. 1) of the polymers in
chloroform-d, and FT-IR spectra (Fig. 2) confirmed the
O
N
O
1
expected polymer structures. In H NMR spectra we observed
Y
n
x
C8H17 C8H17
peaks at 8.75 and 8.44 ppm due to the NPN moiety protons
along with some other peaks merged with fluorene peaks.
Similarly, in 13C NMR spectra, we observed peaks at 164.77,
132.53, 131.31, 130.31, 128.86, 128.20, 127.95 and 127.11
ppm due to the NPN moiety carbons. The relative intensity of
NPN peaks in both the 1H and 13C NMR spectra gradually
increases as the NPN moiety increases in the polymer chain.
Scheme 1. Synthetic route for monomer and polymers. Feed ratio of Monomers:
99/01 (PFONPN01), 95/05 (PFONPN05), 90/10 (PFONPN10), 75/25 (PFONPN25),
65/35 (PFONPN35), 50/50 (PFONPN50).i) Glacial acetic acid, Reflux, 12 hrs.
ii)Pd(pph3)4, Aliquat 336, 2M K2CO3, THF, 48 hrs, Benzene boronic acid,
Iodobenzene.
Results and discussion
Table 1 NPN content in the polymers.
Synthesis and characterization of the polymers
NPN content in thePolymers (mol %)
The synthetic routes to the preparation of monomers and
polymers are shown in scheme 1. The monomers 2,7-dibromo-
9,9’-dioctylfluorene (donor) (1) was prepared according to
reported procedure36 and 4-bromo-9-N-4-bromophenyl-1,8-
naphthalimide (acceptor) (2) was prepared from 4-bromo-1,8-
napthalicanhydride and 4-bromo aniline. Donor-Acceptor
based random conjugated copolymers were synthesized from
monomers 1, 2 and 3 using palladium (0) catalysed Suzuki
coupling polymerization. The molar ratio of 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide moiety in the copolymers was
Polymer
Feed ratio
Actual contenta
PFONPN01
PFONPN05
PFONPN10
PFONPN25
PFONPN35
PFONPN50
1
0.8
5
5
10
25
35
50
12
28
39
48
aCalculated from the 1H NMR spectra.
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The actual content of NPN in the polymers was estimated from
1
their H NMR spectra by comparing the integral values of NPN
50%
35%
25%
10%
05%
01%
1.0
0.8
0.6
0.4
0.2
0.0
unit with that of the fluorene monomer and are listed in Table
1. It is found that the actual content of NPN is very close to the
feed ratio, suggesting that NPN has been fully incorporated
into the main chain of the polymers during polymerization. In
FT-IR spectra of the PFONPN polymers the five peaks located
at 1710, 1669, 1585, 1357 and 1237 cm-1 correspond to the
NPN unit, and with increasing NPN moiety in the polymer
chain, the relative intensity of these five peaks also show an
increase. All copolymers were soluble in common organic
solvents such as chlorobenzene, toluene, chloroform,
dichloromethane and THF.
100
200
300
400
500
)
600
Temperature ( °C
Fig. 3 TGA curves of polymers.
01
05
10
The number-average molecular weights (Mn) of the
copolymers, determined by gel permeation chromatography
using a PMMA standard in THF were found to be in the range
of 11729–13389 with a polydispersity index (Mw/Mn) of 1.14 to
1.31 (Table 2).The polymerization results of the synthesized
copolymers are summarized in Table 2.Thermal properties of
these copolymers were determined by TGA and DSC (See ESI,
figure S8) under nitrogen flow at a heating rate of 10
°C/min.The TGA curves of the copolymers are shown in Fig
3.All the copolymers were found to exhibit very good thermal
stability with onset decomposition temperature (Td) 342-405°C
(Table 2) and no weight loss observed at lower temperature.
Interestingly all copolymers showed two step degradation due
to the presence of two different units in the polymer main
chain. The glass transition temperature (Tg) of the copolymers
were investigated by DSC (Table 2). The amorphous nature of
all polymers was very useful in solution processing for PLED
fabrication.
25
35
50
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber (cm-1)
Optical and Photoluminescence properties
The normalized UV-visible absorption properties of the
PFONPN conjugated polymers, both in solution (chloroform,
10-5 M) and solid state are shown in Fig.4 and the results are
summarised in Table 3.In solution state PFONPN50 polymer
shows two absorption peaks at 325 and 393 nm and remaining
all polymers showed single peaks viz. PFONPN35 at 365nm,
PFONPN25 at 370nm, PFONPN10 at 363nm and PFONPN05,
PFONPN1 at 380nm.
Fig. 2 FT-IR spectra of polymers.
Table 2 Polymerization results and Thermal data of polymers.
a
Mn
a
c
Polymer
Mw
PDIa
Yield
(%)
Tdb(˚
Tg (˚C)
C)
PFONPN01
PFONPN05
PFONPN10
PFONPN25
PFONPN35
PFONPN50
12463 14622
13389 15213
12057 14032
12135 14146
11729 13663
12568 14747
1.31
1.17
1.16
1.26
1.14
1.17
58
67
55
62
57
61
388
405
344
380
355
342
134.20
123.48
132.23
133.33
125.46
132.85
For solid state measurements, the copolymer films with a
thickness approximately of 80 nm was spin coated on glass
substrate from a chloroform solution. In solid state also
PFONPN50 showed two peaks at 327 and 395nm whereas the
remaining polymers showed single peaks viz. PFONPN35 at
370nm, PFONPN25 at 375nm, PFONPN10 at 365nm,
PFONPN05 at 381nm and PFONPN01 at 380nm. The optical
band gaps (Eg) of all the polymers were estimated from the
solid state of UV-visible absorption edges. Both solution and
solid state absorption maximum of all copolymers showed
slight blue shift with fraction of NPN content increased
because increasing the fraction of NPN content in polymer
main chain induces a decrease in the effective conjugation
length of fluorene units.37,38 The normalized PL spectra of the
PFONPN conjugated polymers, both in solution (chloroform 10-
aMn, Mw and PDI of the polymers were determined by gel permeation
chromatography using PMMA standards. bOnset decomposition temperature
measured by TGA under nitrogen. cGlass transition temperature.
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5
M) and solid state are shown in Fig.5 and the results are fluorescence intensity to that of a thin film sample of PFO
summarised in Table 3.
polymer (ФF = 0.55).39 The quantum yield values of copolymers
decrease with increasing numbers of the NPN unit due to the
concentration quenching effect of the NPN unit due to
aggregation of the chromophores.22,40,41 A very good way to
suppress this is by developing supramolecular compounds,
such as rotaxanes, which suppress aggregation / quenching
very effectively.42
50%
35%
25%
10%
05%
01%
PFO
1.0
0.8
0.6
0.4
0.2
0.0
1.0
a
50%
1.0
35%
25%
10%
a
0.8
05%
01%
PFO
0.6
0.4
0.2
50%
35%
25%
10%
05%
01%
PFO
0.8
0.6
0.4
0.2
b
0.0
1.0
50%
35%
25%
10%
05%
01%
PFO
b
0.8
0.6
0.4
0.2
0.0
0.0
300
400
500
600
Wavelength (nm)
Fig. 4 UV-Vis absorption spectra of polymers in solution state (a) and solid state
(b).
400
500
600
The solution state PL spectra of copolymers were taken at
different excitation. The PL spectrum of PFONPN50 was taken
Wavelength (nm)
under an excitation of 340nm, and single peaked at 508nmwas Fig. 5 Photoluminescence spectra of polymers in solution state (a) and solid state
(b).
obtained. The emission color of the PFO is completely
quenched due to energy transfer from the PFO to NPN unit.
Table 3 Photo physical properties of the polymers.
All the remaining copolymers PFONPN35, PFONPN25,
CHCl3 solution
Solid state
PFONPN10, PFONPN05 and PFONPN01 showed two peaks
when excited at a wavelength of 380nm. The peak in blue
region is due to fluorene unit whereas the peak in green region
is because of the NPN unit. We observed that the intensity of
the green peak increases on increasing the NPN unit in the
polymer chain. All these copolymers showed single peak in
solid phase due to efficient charge transfer and strong energy
transfer from PFO to NPN units. The solid state PL spectra of
the PFONPN50, was recorded at an excitation wavelength of
340nm whereas for the remaining copolymers the solid state
PL spectra were recorded at an excitation wavelength of 380
nm. The emission colors of the polymers are red shifted from
466 nm to 511 nm (Blue to Green) when NPN content
increases from 01 to 50 mol % in the polymer backbone.
c
Polymer
λ max,abs
λ max, PL
[nm]
λ max
,
λ max
,
ФPL
abs
[nm]
[nm]
PL
[nm]
466
477
490
500
505
511
PFONPN01
PFONPN05
PFONPN10
PFONPN25
PFONPN35
PFONPN50
380
380
415, 519
415, 520
415, 520
416, 517
416, 504
508
380
381
0.74
0.69
0.42
0.34
0.27
0.18
363
365
370
375
365
370
325, 393
327, 395
cSolid state PL quantum yields are estimated with polydioctylfluorene standard.
Electrochemical properties
Cyclic voltammetry (CV) (Fig.6) was performed to investigate
The PL quantum yields (ФPL) of the copolymers in solid state the reduction and oxidation peaks of the copolymers and to
were measured with excitation at 380nm except PFONPN50 estimate their highest occupied molecular orbitals (HOMO)
which was excited at 330nm (Table 3).The solid state quantum and lowest unoccupied molecular orbitals (LUMO) energy
yields of the polymers are estimated by comparing their levels.
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0.0002
200
160
120
80
50%
35%
25%
10%
05%
01%
01%
05%
10%
25%
35%
50%
0.0001
0.0000
-0.0001
-0.0002
40
0
-2
-1
0
1
2
0
2
4
6
8
10
12
Voltage ( V )
Voltage (V)
Fig.6 Cyclic Voltagramms of polymers.
Fig. 7 Current Density-Voltage (J-V) characteristics of PLEDs.
The copolymers were drop casted onto carbon working
electrode and silver and platinum wire were used as the
reference electrode and counter electrode respectively. The
electrochemical properties of the copolymers were
investigated in an electrolyte consisting asolution of 0.1M
tetra butyl ammonium hexafluorophosphate (Bu4NPF6) in
acetonitrile (CH3CN) at room temperature under argon
atmosphere.
The Current density-Voltage (J-V) characteristics and the
typical EL spectra of all the PLEDs are shown in Figure 7 and 8
respectively. The onset values for all the fabricated devices
along with their emission maxima and CIE coordinates are
listed in Table 5. From Figure 8 it is clear that the copolymer
PFONPN01 shows three peaks at 434 nm, 464 nm and 486 nm
respectively. The first two peaks in the spectrum can be
attributed to the polyfluorene main chain whereas the peak
centred at 486 nm is due to the partial energy transfer from
the PFO to NPN unit.43-46 The polyfluorene emission, however,
was completely quenched as the NPN content of the
copolymer was increased.The EL spectra of all the other
copolymers show only a single peak centred at 483 nm for
PFONPN05, 493 nm for PFONPN10, 497 nm for PFONPN25,
504 nm for PFONPN35 and 506 nm for PFONPN50 respectively.
The slight difference in wavelength between the solid state
fluorescence emission and the electroluminescence emission
can be attributed to the different emission mechanism
between PL and EL.
Table 4 Electrochemical Potentials and Energy Levels of the PFONPN Polymers
Polymer
Eox/V Ered/V EHOMO/eV ELUMO/eV Eg/eV Optical
band
gap/eV
PFO
1.38
1.21
1.15
1.25
1.17
1.16
1.1
-0.69
-1.2
-5.8
-5.97
-5.85
-5.95
-5.87
-5.86
-5.8
-2.9
-3.5
2.9
2.95
2.71
2.70
2.69
2.67
2.62
2.52
PFONPN01
PFONPN05
PFONPN10
PFONPN25
PFONPN35
PFONPN50
2.47
2.45
2.41
2.37
2.34
2.33
-1.3
-3.4
-1.16
-1.2
-3.52
-3.5
-1.18
-1.23
-3.5
-3.47
The measurements were calibrated using ferrocene as
standard. The PFONPN copolymers show two potential peaks
(Fig. 6). The reduction potential, in negative potential region (-
1.18 to -1.23V) is because of the NPN moiety as it has high
electron affinity (because they possess carbonyl groups).
Similarly, the peak in the positive potential region (1.1 to
1.25V) is oxidation potential and is for fluorene unit.The
HOMO and LUMO energies were calculated by submitting the
PFO
1.0
01%
05%
0.8
0.6
0.4
0.2
0.0
10%
25%
35%
50%
onset reduction and onset oxidation peak values in ELUMO
=
[(Ered – E1/2 (ferrocene)) + 4.8] eV, EHOMO = [(Eox – E1/2
(ferrocene)) + 4.8] eV. The onset reduction and onset oxidation
peak values, energy levels and band gaps of the copolymers
were represented in Table 4.
400
450
500
550
600
650
700
Wavelength (nm)
Electroluminescence properties
Fig. 8 Electroluminescence of PLEDs.
In order to investigate the EL properties of the polymers, we
fabricated single-layer devices with a configuration of ITO/
PEDOT:PSS(40 nm)/Polymer(80 nm)/Ca(10 nm)/Al(100 nm).
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As the concentration of the NPN unit increases, the electron
accepting capacity of the polymer al o increases due to the
ss
excess of electron attracting effect leading to decreasing
device performance.
6000
01%
5000
4000
3000
2000
1000
0
05%
10%
25%
35%
50%
0
25
50
75 100 125 150 175 200
Current Density (mA/cm2)
Fig. 10 Brightness Vs Current Density characteristiccs of PLEDs.
Fig. 9 Chromaticity diagram representing the CIE coordinattes of the fabricated
PLEDs.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
01%
05%
10%
25%
35%
50%
Fig. 9 shows the chromaticity diagram repr
e
senting the CIE
coordinates of the fabricated PLEDs.Th
e
coordinates of the copolymers changed from x=0.17, y=0.22,
for PFONPN01 to x=0.24, y=0.49 for PFONPPN50. The PLED
devices fabricated using these copolymers are ffound to exhibit
very good results and the device performance aare presented in
Table 5.Fig 10 and 11 shows that the brightneess and current
efficiency of the devices are 5236 cd/m2 andd 3.52 cd/A for
PFONPN01, 3120 cd/m2 and 1.96 cd/A for P
FFONPN05, 2064
cd/m2 and 1.5 cd/A for PFONPN10, 1755 cd/m2 and 1.16 cd/A
for PFONPN25, 1756 cd/m2 and 1.1 cd/A for PFONPN35 and
345 cd/m2 and 0.22 cd/A for PFONPN50.The eenhanced device
performance of PFONPN01 as compared to other polymers
results due to the well-balanced electron and hholes in polymer
main chain.From these results, it is clear that with increasing
0
25
50
75 100 125 150 175 200
Current Density (mA/cm2)
Fig. 11 Luminous Efficiency Vs Current density characteristics of PLEDs.
NPN content the device brightness showed a deecreasing trend.
This is due to the high electron affinity of NPN uunit.
Conclusions
Table 5 Device Characteristics of PLEDs based on polymers.
In summary, we have synthesized a n
acceptor based random conjugated ccopolymers PFONPN by
introducing NPN unit as an acceptor i to the polyfluorene by
oovel class of donor and
Polymer
Onset
voltage
(V)
EL
Max.
Brightn
ess
Lum
i
nou
CIEcoordi
emission
λmax (nm)
ss
nate (x,y)a
nn
efficieency
(cd//A)
(cd/m2)
well-known palladium catalysed Suzuki coupling reaction. All
copolymers are highly soluble in common organic solvents
such as chlorobenzene, chloroform, dicchloromethane and THF.
The resulting copolymers exhibit exccellent thermal stability
and high glass transition temperature. The emission color of
the polyfluorene (blue) could easily be tuned to green via
efficient energy transfer to acceptor (NPN) unit. In solid state
photoluminescence, the polyfluorene fluorescence is totally
PFO
6.5
6.3
436, 463,
493
1949
0.
9
9
2
6
0.17, 0.15
0.17, 0.22
PFONPN01
434, 464,
486
5236
3.
55
PFONPN05
PFONPN10
PFONPN25
PFONPN35
PFONPN50
5.4
6.5
5.7
5.3
5.5
483
3120
2064
1755
1756
345
1.
1
9
0.19, 0.32
0.22, 0.45
0.22, 0.37
0.23, 0.42
0.24, 0.49
493
..5
497
1.
1
116
504
..1
quenched at NPN concentrations a
PFONPN copolymers exhibit maxi
511nm. The single layer PLED dev
PFONPN polymers with configur
s
s
low as 1% and the
506
0.
222
mmum emission at 466-
aDetermined from the EL spectrum.
ices made with these
a
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01899D
ARTICLE
PSS/PFONPN/Ca:Al are found to be very bright with PFONPN01 feasibility to oxidative addition with palladium. In case of
giving the highest brightness of 5236 cd/m2 with a luminous PFONPN01 to PFONPN35 we used three monomers viz.
efficiency of 3.52 cd/A. Like PL spectra, the EL spectra also fluorene boronic ester, 2,7-dibromo-9,9’-dioctylfluorene and
show the color tuning property of the copolymers.
4-bromo-9-N-4-bromophenyl-1,8-naphthalimide. In these
copolymers, due to the high reactivity of fluorene bromide,
there is a probability that it will react first with fluorene
boronic ester forming a polyfluorene block. Similarly, the
remaining fluorene boronic ester will react with 4-bromo-9-N-
4-bromophenyl-1,8-naphthalimide forming another block.
When they undergo polymerization, they may form
copolymers containing long oligofluorenes segments.
However, in case of PFONPN50, as we used only two
monomers, viz. fluorene boronic ester and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide, it will form only alternating
copolymers.
Experimental
Synthesis of 4-bromo-9-N-4-bromophenyl-1,8-naphthalimide (2)
4-bromo 1,8-naphthalicanhydride (0.500 g, 1.80 mmol), 4-
bromo aniline(0.4649 g, 2.67 mmol) and 15mL glacial acetic
acid were added into a 50mL round bottom flask at room
temperature and refluxed for 12 hours. The reaction mixture
was then cooled to room temperature and poured into
distilled water (100 mL) the resultant solid was collected by
filtration and recrystallized from acetone. (Yield 0.572 g, 73 %).
1H NMR (400 MHz, DMSO-d6) δ (ppm): 8.62-8.58(m, 2H), 8.36-
8.34 (d, 1H) 8.27-8.25(d, 1H), 8.06-8.04 (t, 1H), 7.74-7.72(d,
1H), 7.39-7.37 (d, 1H); 13C NMR (150 MHz, DMSO-d6) δ (ppm):
163.09, 163.03, 135.16, 133.68, 132.54, 131.71, 131.66,
131.42, 131.37, 131.07, 129.29, 129.16, 122.55, 121.51;
Electrospray ionization mass spectrum (ESI-MS): [M + H]+:
calcd: 432.08; found: 432.2.
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
(PFONPN50)
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 4-bromo-9-N-4-bromophenyl-
1,8-naphthalimide (2) (0.2155g, 0.50 mmol), were used in this
1
polymerization. H NMR (600 MHz, CDCl3) δ (ppm): 8.75 (br,
2H), 8.42 (br,1H), 7.86 (br, 2H), 7.69 (br, 4H), 7.59 (br, 1H),
7.48 (br, 1H), 7.38 (br, 1H), 6.88 (br, 1H), 2.10 (br, 4H), 1.15
(br, 24H), 0.82 (br, 6H); 13C NMR (150 MHz, CDCl3) δ (ppm):
164.64, 151.87, 140.71, 140.23, 131.60, 130.30, 129.30,
128.73, 128.27, 127.11, 124.98, 120.53, 55.88, 40.63, 31.99,
30.25, 29.44, 24.38, 22.82, 14.27; FT-IR (KBr pellets,cm-1):
2953, 2922, 2850, 1710, 1669, 1585, 1466, 1357, 1237, 810.
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
General polymerization procedure
9,9-dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester, dibromo compounds ( 2,7-dibromo-9,9-dioctylfluorene
and 4-bromo-9-N-4-bromophenyl-1,8-naphthalimide), 12 mL
Tetrahydrofuran
(THF)
and
tetrakis(triphenyl
phosphine)palladium(0) (0.015 mmol) were added into a dry
two neck round bottom flask. Subsequently 5 mL 2M aqueous
potassium carbonate and aliquat 336 (0.025 mmol) were
added to the flask. The reaction mixture was degassed thrice
by freeze–thaw cycles to remove trace amounts of oxygen
completely. The reaction mixture was stirred at 80 °C for 2
days, and iodobenzene was added as an end capper. After 4
hours, benzene boronic acid was dissolved in 1 mL THF and
added into the reaction mixture as another end capper and
stirring continued further for 4 hours. The reaction mixture
was then cooled to room temperature, poured into 100 mL
methanol and further stirred at room temperature for 4 hours.
The desired polymer was collected by filtration and
reprecipitated twice from methanol and acetone. The
polymers were further purified by soxhlet filtration with
acetone to remove oligomers and catalyst residues. After
drying the polymers we obtained a yield of 55-67%. The
resulted polymers were soluble in common organic solvents.
The reactivity of the three different types of aryl-bromides
differ under the reaction conditions reported herein. Fluorene
unit has two identical bromides, whereas, the NPN unit has
two different type of bromides, viz. attached to napthyl and
phenyl rings. As compared to the NPN bromides the fluorene
bromides are more reactive due to the absence of electron
withdrawing group as well as its high electron density and
there is a high possibility of oxidative addition predominatly.
On the other hand, among the NPN bromides, the napthyl
bromide is more reactive than the phenyl bromide since the
napthyl moiety has more electron density. Hence, it has more
(PFONPN35)
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 2,7-dibromo-9,9-
dioctylfluorene (1) (0.0822g, 0.15 mmol) and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide (2) (0.1508g, 0.35 mmol),
1
were used in this polymerization. H NMR (600 MHz, CDCl3) δ
(ppm): 8.74 (br, 2H), 8.43 (br, 1H), 7.84 (br, 2H), 7.69 (br, 4H),
7.58 (br, 1H), 7.48 (br, 1H), 7.37 (br, 1H), 6.94 (br, 1H), 2.11
(br, 4H), 1.14 (br, 24H), 0.81 (br, 6H); 13C NMR (150 MHz,
CDCl3) δ (ppm): 164.77, 151.82, 151.70, 139.95, 132.53,
131.63, 131.31, 130.38, 128.86, 128.74, 128.68, 128.20,
127.95, 127.11, 126.85, 126.74, 126.11, 121.39, 120.27,
119.87; FT-IR (KBr pellets,cm-1): 2953, 2922, 2850, 1710, 1669,
1585, 1466, 1357, 1237, 810.
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
(PFONPN25)
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 2,7-dibromo-9,9-
dioctylfluorene (1) (0.1371g, 0.25 mmol) and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide (2) (0.1077g, 0.25 mmol)
1
were used in this polymerization. H NMR (600 MHz, CDCl3) δ
(ppm): 8.74 (br, 2H), 8.43 (br, 1H), 7.84 (br, 2H), 7.68 (br, 4H),
7.58 (br, 1H), 7.49 (br, 1H), 7.37 (br, 1H), 6.94 (br, 1H), 2.11
(br, 4H), 1.14 (br, 24H), 0.81 (br, 6H); 13C NMR (150 MHz,
CDCl3) δ (ppm): 164.57, 152.01, 140.71, 140.23, 132.33,
131.51, 130.67, 129.61, 129.19, 128.71, 128.46, 128.24,
127.13, 126.36, 121.70, 120.56, 120.15, 111.5, 55.54, 40.57,
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C5TC01899D
ARTICLE
31.98, 30.22, 29.40, 24.12, 22.78, 14.24; FT-IR (KBr pellets, cm- Varian
Journal Name
Cary
Eclipse
spectrometer.
Electrochemical
1): 2953, 2922, 2850, 1710, 1669, 1585, 1466, 1357, 1237, 810. measurements were carried out under argon atmosphere on a
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
deoxygenated solution of 0.1M tetra butyl ammonium
perchlorate (TBAP) in acetonitrile by using CH instrument
(Model 700D series). Glassy carbon as a working electrode,
platinum as a counter electrode, and Ag/Ag+ was used as
reference electrode. Thermo Gravimetric Analysis (TGA) was
recorded on Mettler Toledo, model TG/SDTA 851 e,
Differential Scanning Calorimetry (DSC) was measured on a
mettler Toledo, model DSC1, stare system. The gel permeation
chromatography (GPC) measurements were recorded on
waters 515 chromotograph connected to waters 2414
refractive index detectors by using tetrahydrofuran as an
eluent and calibration curve of PMMA standards. FT-IR analysis
was carried out with a Perkin-Elmer-Spectrum with samples
prepared as KBr pellets.
(PFONPN10)
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 2,7-dibromo-9,9-
dioctylfluorene (1) (0.2193g, 0.40 mmol) and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide (2) (0.0431g, 0.10 mmol)
1
were used in this polymerization. H NMR (600 MHz, CDCl3) δ
(ppm): 8.79, (br, 2H), 8.44 (br, 1H), 7.87 (br, 2H), 7.70 (br, 4H),
7.57 (br, 1H), 7.49 (br, 1H), 7.37 (br, 1H), 6.88 (br, 1H), 2.12
(br, 4H), 1.14 (br, 24H), 0.82 (br, 6H); 13C NMR (150 MHz,
CDCl3): δ (ppm) 154.04, 140.73, 140.24, 126.38, 121.72,
120.18, 55.56, 40.60, 32.01, 30.25, 29.43, 24.14, 22.81, 14.26;
FT-IR (KBr pellets,cm-1): 2953, 2922, 2850, 1710, 1669, 1585,
1466, 1357, 1237, 810.
PLED fabrication and characterization
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
(PFONPN05)
In order to study the EL properties, PLEDs were fabricated
using the as synthesized copolymers as active layer. The PLED
device configuration is composed of a pre-cleaned and pre-
patterned indium tin oxide (ITO) as the transparent anode. The
ITO surface was ultrasonically agitated and cleaned in 2%
detergent, acetone and isopropanol, each for five minutes at
60 °C. After cleaning, ~40 nm thin layer of poly (3,4-
ethylendioxythiophene) : poly(styrenesulfonate) (PEDOT:PSS)
as a hole injecting layer was spin-coated at 3000rpm for 60
seconds, then baked for 15 minute in argon environment at
130 °C. The copolymers were dissolved in chlorobenzene (15
mg/mL) and spin-coated above the PEDOT:PSS layer under
ambient atmosphere and thermally treated at 150 °C for 15
minute for the removal of residual solvent. The film thickness
of the active layer was ~80 nm. Finally, calcium and aluminum
was thermally evaporated at a rate of 0.1 Å/s and 10 Å/s
respectively at a base pressure of 10-6 mbar to form the
cathode electrode. The active area of the diodes was 12 mm2.
The current density-voltage (J-V) characteristics of the
fabricated PLEDs were measured using Keithley 2400 source
meter whereas the luminance and the EL spectra were
measured using LCS-100 integrated sphere. All the devices
were fabricated and characterized under Argon atmosphere
inside a glove box.
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 2,7-dibromo-9,9-
dioctylfluorene (1) (0.2467g, 0.45 mmol) and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide (2) (0.0215g, 0.05 mmol)
1
were used in this polymerization. H NMR (600 MHz, CDCl3) δ
(ppm): 8.75 (br, 2H), 8.44 (br, 1H), 7.85 (br, 2H), 7.70 (br, 4H),
7.56 (br, 1H), 7.49 (br, 1H), 7.37 (br, 1H), 6.83 (br, 1H), 2.1 (br,
4H), 1.14 (br, 24H), 0.80 (br, 6H); 13C NMR (150 MHz, CDCl3) δ
(ppm): 152.05, 140.74, 140.28, 126.39, 121.73, 120.19, 55.57,
40.59, 32.01, 30.26, 29.44, 24.14, 22.82, 14.28; FT-IR (KBr
pellets, cm-1) 2953, 2922, 2850, 1669, 1468, 814.
Poly[2,7-(9,9’-dioctylfluorene)-co-N-phenyl-1,8-naphthalimide
(PFONPN01)
9,9-Dioctylfluorene-2,7-diboronic acid bis(1,3-propanediol)
ester (3) (0.2792g, 0.50 mmol), 2,7-dibromo-9,9-
dioctylfluorene (1) (0.2687g, 0.49 mmol) and 4-bromo-9-N-4-
bromophenyl-1,8-naphthalimide (2) (0.0041g, 0.01 mmol)
were used in this polymerization.1H NMR (600 MHz, CDCl3) δ
(ppm): 7.85 (br, 2H), 7.71 (br, 4H), 2.1 (br, 4H), 1.14 (br, 24H),
0.73 (br, 6H); 13C NMR (150 MHz, CDCl3) δ (ppm): 152.04,
140.73, 140.24, 126.38, 121.72, 120.18, 55.56, 40.60, 32.01,
30.25, 29.43, 24.14, 22.81, 14.26; FT-IR (KBr pellets, cm-1)
2953, 2922, 2850, 1468, 814.
Materials and Measurements
2,7-dibromofluorene,
1-bromooctane,
4-bromo
1,8-
Acknowledgements
naphthalicanhydride, 9,9’-dioctylfluorene-2,7-diboronic acid
bis(1,3-propanediol)ester (3), 4-bromoaniline, glacial acetic
The authors are thankful to the Department of Science and
Technology
(DST),
India,
(DST/TSG/PT/2009/11,
acid
(99.85%),
Tetrakis(triphenylphosphine)palladium(0),
DST/TSG/PT/2009/23), (No. DST/SERB/EMR/2014/000034),
and IGSTC/MPG/PG(PKI)/2011A/48 for financial support.
Central Instruments Facility, IIT Guwahati, is acknowledged for
providing various instrumentation facilities.
Aliquat 336, were purchased from Sigma-Aldrich and used
without further purification. Chloroform, dichloromethane and
toluene were distilled over calcium chloride and anhydrous
tetrahydrofuran was dried over sodium-benzophenone. 2, 7-
dibromo-9, 9’-dioctylfluorene (1) was prepared by following
published procedures.36
1H NMR and 13C NMR spectra were recorded on Varian AS 400
MHz and Bruker 600 MHz NMR spectrometers. UV-visible
spectra were measured by PerkinElmer, Model Lambda-25
spectrometer. Photoluminescence spectra were recorded with
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DOI: 10.1039/C5TC01899D
ARTICLE
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Journal of Materials Chemistry C
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DOI: 10.1039/C5TC01899D
Table of contents entry
Synthesis and Characterization of Color Tunable, Highly Electroluminescent Copolymers of
Polyfluorene by Incorporating N-phenyl-1,8-Naphthalimide Moiety into Main Chain
Peddaboodi Gopikrishna, Dipjyoti Das and Parameswar Krishnan Iyer*
50%
35%
25%
1.0
0.8
10%
05%
01%
0.6
0.4
0.2
0.0
PFO
NPN
400 500 600 700
Wavelength (nm)
Block copolymers comprising N-phenyl-1,8-Naphthalimide (NPN) moiety into the main polyfluorene
chain are introduced to tune the emission color in PLED devices. High performance devices are
obtained by changing the NPN content in the polyfluorene main chain from 1-50 mol % thereby
tuning the emission color of the PLEDs from blue to green with Commission International de
l’Echairage (CIE) coordinates being (0.17, 0.22) to (0.24, 0.49).