Synthesis and characterization of new red-emitting polyfluorene derivatives containing electron-deficient 2-pyran-4-ylidene-malononitrile moieties

Article abstract of DOI:10.1021/ma0355397

A novel series of red-light-emitting copolymers derived from fluorene and 2-pyran-4-ylidene-malononitrile (PM) have been synthesized through a palladium-catalyzed Suzuki coupling reaction. The polymers were characterized by FT-IR, NMR, and elemental analysis. All these polymers are completely soluble in common organic solvents, such as THF, CH₂Cl₂, CHCl₃, and toluene, and they have good thermal stability with onset decomposition temperature (T_d) of 406-407°C and glass-transition temperature (T_g) of 73-186°C. Cyclic voltammetry studies reveal that these copolymers have low-lying LUMO energy levels ranging from -3.53 to -3.57 eV and HOMO energy levels ranging from -5.77 to -5.79 eV, which indicated that they may be promising candidates for electron-transporting or hole-blocking materials in light-emitting diodes. These polymers in thin films can emit strong red photoluminescence (PL) around 641-662 nm with the corresponding additional peaks in the range 704-712 nm upon photoexcitation. Double-layer LEDs fabricated with the configuration of ITO/PEDOT/polymer/Ba/Al can emit red light with external quantum efficiencies of 0.21-0.38%. Preliminary electroluminescent (EL) results show that these polymers are novel promising candidates for red emissive materials in polymer light-emitting diodes.

Full text of DOI:10.1021/ma0355397

260
Macromolecules 2004, 37, 260-266
Articles
Synthesis and Characterization of New Red-Emitting Polyfluorene
Derivatives Containing Electron-Deficient
2-Pyran-4-ylidene-Malononitrile Moieties
Qia n g P en g,† Zh i-Yu n Lu ,† Ya n Hu a n g,† Min g-Gu i Xie,*,† Sh a o-Hu Ha n ,‡
J u n -Bia o P en g,‡ a n d Yon g Ca o‡
Department of Chemistry, Sichuan University, Chengdu 610064, China PR; Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640,
China PR; and Institute of Materials Research and Engineering, National University of Singapore,
3 Research Link, Singapore 117602, Republic of Singapore
Received October 13, 2003; Revised Manuscript Received November 6, 2003
ABSTRACT: A novel series of red-light-emitting copolymers derived from fluorene and 2-pyran-4-ylidene-
malononitrile (PM) have been synthesized through a palladium-catalyzed Suzuki coupling reaction. The
polymers were characterized by FT-IR, NMR, and elemental analysis. All these polymers are completely
soluble in common organic solvents, such as THF, CH2Cl2, CHCl3, and toluene, and they have good thermal
stability with onset decomposition temperature (Td) of 406-407 °C and glass-transition temperature (Tg)
of 73-186 °C. Cyclic voltanmetry studies reveal that these copolymers have low-lying LUMO energy
levels ranging from -3.53 to -3.57 eV and HOMO energy levels ranging from -5.77 to -5.79 eV, which
indicated that they may be promising candidates for electron-transporting or hole-blocking materials in
light-emitting diodes. These polymers in thin films can emit strong red photoluminescence (PL) around
641-662 nm with the corresponding additional peaks in the range 704-712 nm upon photoexcitation.
Double-layer LEDs fabricated with the configuration of ITO/PEDOT/polymer/Ba/Al can emit red light
with external quantum efficiencies of 0.21-0.38%. Preliminary electroluminescent (EL) results show that
these polymers are novel promising candidates for red emissive materials in polymer light-emitting diodes.
In tr od u ction
stability.18-29 Indeed, many blue- and green-light-emit-
ting polyfluorene derivatives have been reported, and
recently, red-light-emitting diodes, presumably made
from random copolymers based on fluorene and 4,7-bis-
(2-thienyl)-2,1,3-benzothiadiazole, have been briefly
described.30,31 Other red-emitting materials have also
been reported from energy transfer between emissive
polymers and several dyes, such as tetraphenylporphy-
rin or europium complexes.28,32-34 However, most of
these red-emitting polymers synthesized to date have
low electron affinity. It is established that high quantum
efficiency PLEDs can be obtained by achieving both
efficient charge injection and balanced mobility of both
charge carriers inside the emissive materials. In gen-
eral, many light-emitting polymers inject and transport
holes more efficiently than electrons due to their inher-
ent richness of π-electrons. So it is a very challenging
task to develop new red-light-emitting polymers with
high electron affinitiy, especially for polyfluorene de-
rivatives.
Since the initial report of polymer light-emitting
diodes (PLEDs) based on poly(p-phenylenevinylene)
(PPV) by the Cambridge group, conjugated polymers
have received considerable attention due to their po-
tential application in patterned light sources and flat
panel displays.1-5 In the past decade, many conjugated
polymers, such as poly(p-phenylene)s, poly(phenylenevi-
nylene)s, poly(oxadiazole)s, poly[2,7-(9,9-dialkylfluo-
rene)]s, poly(3,6-carbazole)s, and poly(2,7-carbazole)s,
have been synthesized in order to obtain three primary
(RGB) colors.6-16 Among these polymers, polyfluorene
derivatives show interesting and unique chemical and
physical properties because they contain a rigid planar
biphenyl unit and the facile substitution at the remote
C-9 position can improve the solubility and processabi-
lilty of polymers without significantly increasing the
steric interactions in the polymer backbone.17 As a
result, polyfluorene and its derivatives have emerged
as the most promising light-emitting materials due to
their emission at wavelength spanning the entire visible
spectrum, high fluorescence efficiency, and good thermal
Eastman Kodak’s dyes (DCM class) are well-known
as low molecular weight red-emitting materials, which
can be synthesized by a relatively simple procedure.35-38
All the DCM class red dyes contain 2-pyran-4-ylidene-
malononitrile (PM) derivatives as electron acceptor. In
most of the studies, these dyes were usually doped in a
layer of Alq3, which was used as the electron-transport-
ing layer (ETL) as well as the host. But the Alq3-doped
† Sichuan University.
‡ South China University of Technology.
* To whom correspondence should be addressed: telephone
(86-28) 85410059; Fax (86-28) 85413601; e-mail xiemingg@
mail.sc.cninfo.net.
10.1021/ma0355397 CCC: $27.50 © 2004 American Chemical Society
Published on Web 12/03/2003

Macromolecules, Vol. 37, No. 2, 2004
Red-Emitting Polyfluorene Derivatives 261
2-Hexyloxyl-5-b r om ob en za ld eh yd e (2a ). A mixture of
2-hydroxyl-5-bromobenzaldehyde (7.9 g, 0.0393 mol), 1-bro-
mohexane (6.5 g, 0.0393 mol), K2CO3 (16.4 g, 119 mol), and
DMF (35 mL) was heated at 100 °C overnight under argon.
The mixture was filtered, and DMF was removed under a
reduced pressure. The crude product was purified by a silica
gel column chromatography. Compound 2a was obtained as
light yellow oil with a yield of 90%. 1H NMR (CDCl3, 400 MHz,
δ/ppm): 10.41 (s, 1H), 7.92 (d, 1H), 7.62-7.59 (m, 1H), 6.95-
6.87 (d, 1H), 4.08-4.04 (t, 2H), 1.88-0.89 (m, 11H). Anal. Calcd
for C13H17O2Br: C, 54.75; H, 6.01. Found: C, 54.71; H, 6.03.
2-Dod ecyloxyl-5-br om oben za ld eh yd e (2b). Compound
2b was obtained as a white solid with a yield of 92% from the
reaction of 2-hydroxyl-5-bromobenzaldehyde with 1-bromo-
dodecane according to the procedure described for 2a ; mp:
39.5-40.5 °C. 1H NMR (CDCl3, 400 MHz, δ/ppm): 10.42 (s,
1H), 7.92 (d, 1H), 7.62-7.59 (m, 1H), 6.89-6.87 (d, 1H), 4.07-
4.04 (t, 2H), 1.88-0.87 (m, 23H). Anal. Calcd for C19H29O2Br:
C, 61.79; H, 7.91. Found: C, 61.72; H, 7.93.
2-{2,6-Bis[2-(2-h exyloxyl-5-br om op h en yl)vin yl]p yr a n -
4-ylid en e}-Ma lon on itr ile (3a ). Compound 3a was obtained
as a yellow solid with a yield of 70% from reaction of 2a with
2-(2,6-dimethylpyran-4-ylidene)-malononitrile according to
the procedure described for 1; mp: 166-167 °C. 1H NMR
(CDCl3, 400 MHz, δ/ppm): 7.75-7.62 (m, 4H), 7.46-7.43 (m,
2H), 6.98-6.68 (m, 6H), 4.09-4.06 (t, 4H), 1.91-0.81 (m, 22H).
Anal. Calcd for C36H38Br2N2O3: C, 61.20; H, 5.42; N, 3.96.
Found: C, 61.15; H, 5.40; N, 3.98.
devices are either low in efficiency (at high dopant
concentrations) or poor in color purity (at low dopant
concentrations). The low efficiency at high dopant
concentrations is due to the effect of aggregation and
concentration quenching, which suppress, broaden, and
red shift the EL spectra. However, at low dopant
concentrations, the energy transfer from Alq3 to dopant
is not complete, and the red emission from dopant is
usually mixed with the green emission from Alq3 leading
to an orange emission.
In this contribution, we designed and synthesized a
novel series of fluorene-based red-light-emitting poly-
mers containing a 2-pyran-4-ylidene-malononitrile (PM)
unit in the main chain. These polymers possess high
molecular weight, good solubility, excellent thermal
stability, low-lying lowest unoccupied molecular orbital
(LUMO) energy level, and good red-emitting property
with improved electron affinity. The chemical doping
method by introducing a PM moiety into polyfluorene
main chain might enable these polymer materials to be
used as red-emitting materials with improved device
performance for full color (RGB) displays.
Exp er im en ta l Section
In str u m en ta tion . 1H NMR and 13C NMR spectra were
performed on a Varian Unity INOVA-400. FT-IR spectra were
recorded on a Perkin-Elmer 2000 spectrometer with KBr
pellets. Elemental analysis studies were carried out with a
Carlo Erba 116 elemental analyzer. UV-vis spectra in solu-
tions and thin films were taken on a Shimadzu UV2100 UV-
vis recording spectrophotometer. Photoluminescence (PL)
spectra of the polymers in solutions and thin films were
measured on a Hitachi 850 fluorescence spectrophotometer.
The PL quantum yields in neat films were measured in an
integrating sphere at room temperature. Thermal gravimetric
analysis (TGA) measurements were performed on Perkin-
Elmer series 7 thermal analysis system under N2 at a heating
rate of 10 °C/min. Differential scanning calorimetry (DSC)
measurements were performed on Perkin-Elmer DSC 7 under
2-{2,6-Bis[2-(2-dodecyloxyl-5-br om oph en yl)vin yl]pyr an -
4-ylid en e}-Ma lon on itr ile (3b). Compound 3b was obtained
as a yellow solid with a yield of 65% from reaction of 2b with
2-(2,6-dimethylpyran-4-ylidene)-malononitrile according to
the procedure described for 1; mp 112-113 °C. 1H NMR
(CDCl3, 400 MHz, δ/ppm): 7.74-7.60 (m, 4H), 7.44-7.41 (m,
2H), 6.95-6.65 (m, 6H), 4.07-4.03 (t, 4H), 1.88-0.85 (m, 46H).
Anal. Calcd for C48H62Br2N2O3: C, 65.90; H, 7.14; N, 3.20.
Found: C, 65.86; H, 7.12; N, 3.23.
P oly{(9,9-d ih exyl-9H -flu or en e-2,7-ylen e)-a lt-(2-{2,6-
b is[2-(4-p h en ylen e)vin yl]p yr a n -4-ylid en e }-m a lon on i-
tr ile)} (P 1). To a mixture of 4 (502.3 mg, 1.0 mmol), 1 (506.2
mg, 1.0 mmol), and Pd(PPh3)4 (12 mg, 1.0 mol %) was added
a mixture of toluene (5 mL) and aqueous 2 M potassium
carbonate (5 mL). The mixture was vigorously stirred at 85-
90 °C for 48 h. After the mixture was cooled to room
temperature, it was poured into 200 mL of methanol and
deionized water (10:1). A fibrous solid was obtained by
filtration; the solid was washed with methanol, water, and
then methanol. After washing for 24 h in a Soxhlet apparatus
with acetone, the resulting polymer P1 was obtained as a red
solid with a yield of 80% after drying under vacuum. 1H NMR
(CDCl3, 400 MHz, δ/ppm): 7.85-7.47 (m, 14H), 7.38-7.36 (m,
N2 at
a heating rate of 10 °C/min. The weight-average
molecular weights (Mw) and polydispersity indices (Mw/Mn) of
the polymers were measured on a PL-GPC model 210 chro-
matograph at 25 °C, using THF as the eluent and standard
polystyrene as the reference. The cyclic voltammograms were
recorded on a computer-controlled EG&G potential/galvanostat
model 283. The thickness of films was measured by a Dektak
surface profilometer.
2H), 6.89-6.77 (m, 4H), 2.07 (br, 4H), 1.26-0.75 (m, 22H). 13
C
Ma ter ia ls. All the chemicals were purchased from Aldrich
and Acros Chemical Co. and were used without any further
purification. All the solvents such as toluene, piperidine, and
THF were dried with appropriate drying agents, then distilled
under reduced pressure, and stored over 4 Å molecular sieve.
The catalyst was tetrakis(triphenylphosphine)palladium Pd-
(PPh3)4,39 (2,6-dimethyl-4H-pyran-4-ylidine)propanedinitrile
was synthesized from 2,6-dimethyl-4-pyrone as a starting
material by the method of Woods,40 and 9,9-dihexylfluorene-
2,7-bis(trimethylene boronates) (4)23 were synthesized using
2,7-dibromofluorene according to the previous literature.
NMR (CDCl3, 400 MHz, δ/ppm): 157.01, 150.73, 148.67,
148.41, 148.13, 141.82, 139.35, 137.85, 136.09, 134.57, 134.31,
134.03, 132.53, 130.66, 127.49, 127.31, 126.31, 125.79, 124.95,
122.48, 122.24, 121.95, 120.01, 119.20, 117.29, 113.92, 105.86,
54.19, 38.93, 30.00, 28.23, 22.56, 21.10, 12.54. FTIR (KBr
pellet, cm-1): 3057, 3027, 2926, 2853, 2209, 1643, 1605, 1548,
1500, 1465, 1416, 1330, 1260, 1203, 1160, 1179, 1097, 1019,
943, 809, 756, 698, 536. Anal. Calcd for (C49H46N2O)n: C, 86.69;
H, 6.83; N, 4.13. Found: C, 85.94; H, 6.95; N, 4.19.
P oly{(9,9-d ih elxyl-9H -flu or en e-2,7-ylen e)-a lt-(2-{2,6-
bis[2-(2-h exyloxyl-5-p h en ylen e)vin yl]p yr a n -4-ylid en e}-
Ma lon on itr ile)} (P 2). Polymer P2 was obtained as a red solid
with a yield of 84% from the reaction of 3a with 4 according
to the procedure described for the synthesis of polymer P1.
1H NMR (CDCl3, 400 MHz, δ/ppm): 8.01-7.46 (m, 12H), 7.38-
7.34 (m, 2H), 7.11-6.71 (m, 4H), 4.14-4.10 (m, 4H), 2.05 (br,
4H), 1.90-0.67 (m, 44H). 13C NMR (CDCl3, 400 MHz, δ/ppm):
158.90, 157.33, 156.07, 151.80, 151.56, 139.90, 138.96, 134.51,
132.04, 131.95, 130.28, 128.76, 128.66, 128.54, 127.41, 127.15,
125.68, 124.14, 121.02, 120.11, 119.64, 115.37, 112.70, 106.99,
68.96, 55.38, 40.41, 31.46, 31.43, 29.63, 29.17, 29.08, 28.93,
25.89, 23.81, 22.81, 13.98, 13.92. FTIR (KBr pellet, cm-1):
3507, 3028, 2925, 2853, 2209, 1640, 1609, 1544, 1506, 1463,
2-{2,6-Bis[2-(4-br om op h en yl)vin yl]p yr a n -4-ylid en e}-
Ma lon on itr ile (1). A mixture of 4-bromobenzaldehyde (1.85
g, 10.0 mmol), 2-(2,6-dimethylpyran-4-ylidene)-malononitrile
(0.861 g, 5.00 mmol), piperidine (10 drops), and freshly distilled
acetonitrile (10 mL) were refluxed under argon for 24 h. The
reaction mixture was cooled to room temperature. The yellow
precipitate was filtered and washed with 50 mL of acetonitrile.
The crude product was purified by recrystallization from
methanol to afford compound 1 as a yellow solid with a yield
1
of 75%; mp: 290-291 °C. H NMR (CDCl3, 400 MHz, δ/ppm):
7.60-7.58 (d, 4H), 7.47-7.43 (m, 6H), 6.79-6.73 (t, 4H). Anal.
Calcd for C24H14Br2N2O: C, 56.95; H, 2.79; N, 5.53. Found: C,
56.91; H, 2.77; N, 5.54.

262 Peng et al.
Macromolecules, Vol. 37, No. 2, 2004
Sch em e 1. Syn th etic Rou te to th e Mon om er s a n d P olym er s
1417, 1321, 1247, 1175, 1147, 1120, 1018, 967, 944, 861, 809,
696, 536. Anal. Calcd for (C61H70N2O3)n: C, 83.33; H, 8.03; N,
3.19. Found: C, 82.57; H, 8.11; N, 3.23.
2852, 2209, 1640, 1611, 1545, 1505, 1462, 1418, 1249, 1179,
1120, 1017, 968, 943, 861, 811, 752, 699, 536. Anal. Calcd for
(C73H94N2O3)n: C, 83.70; H, 9.04; N, 2.67. Found: C, 82.94;
H, 9.13; N, 2.72.
P oly{(9,9-d ih exyl-9H -flu or en e-2,7-ylen e)-a lt-(2-{2,6-
bis[2-(2-dodecyloxyl-5-ph en ylen e)vin yl]pyr an -4-yliden e}-
Ma lon on itr ile)} (P 3). Polymer P3 was obtained as a red solid
with a yield of 85% from the reaction of 3b with 4 according
to the procedure described for the synthesis of polymer P1.
1H NMR (CDCl3, 400 MHz, δ/ppm): 8.02-7.46 (m, 12H), 7.38-
7.36 (m, 2H), 7.12-6.71 (m, 4H), 4.14-4.10 (m, 4H), 2.04 (br,
4H), 1.89-0.67 (m, 68H). 13C NMR (CDCl3, 400 MHz, δ/ppm):
158.86, 158.36, 157.29, 156.09, 151.85, 151.75, 151.52, 141.59,
140.06, 139.87, 138.90, 134.42, 134.13, 132.93, 132.09, 132.02,
131.51, 130.25, 128.75, 128.69, 128.54, 127.13, 126.04, 125.65,
124.05, 121.51, 120.93, 120.07, 119.59, 115.41, 112.62, 107.03,
68.90, 59.21, 55.35, 55.28, 40.41, 31.90, 31.43, 29.63, 29.60,
29.56, 29.33, 29.17, 29.05, 28.95, 26.21, 26.10, 23.79, 22.66,
22.52, 14.10, 14.00. FTIR (KBr pellet, cm-1): 3059, 3029, 2925,
E L Device F a br ica t ion s. For the fabrication of the
devices, glass substrates coated with indium-tin oxide (ITO)
with a sheet resistance of 100 Ω/0 (CSG Co. Ltd.) were cleaned
subsequently in ultrasonic baths of ionic detergent water,
acetone, and anhydrous molecules. A thin film layer of PEDOT
(Batron-P 4083) (90 nm) [(PEDOT is poly(ethylenediox-
ythiophene) doped with poly(styrenesulfonic acid)] and the
polymers (80 nm) (from a 10 mg mL-1 of the polymers in
toluene solution) were spin-coated on ITO at 1500 rpm for 30
s in turn; after that a thin layer of Ba (4 nm)/Al (170 mn) was
deposited on the polymer film by thermal evaporation under
a vacuum of 10-6 Torr. The active area of the device was about
0.15 cm2. The applied dc bias voltages for EL devices were in
a
forward direction (ITO, positive; Ba/Al, negative). The

Macromolecules, Vol. 37, No. 2, 2004
Red-Emitting Polyfluorene Derivatives 263
F igu r e 1. FT-IR spectra of the polymers.
F igu r e 2. Thermal gravimetric analysis (TGA) curves of the
polymers (under a nitrogen atmosphere at a heating rate of
10 °C min-1). The representative Td of P3 is given in the figure
to be 406 °C.
Ta ble 1. Molecu la r Weigh ts a n d Th er m a l P r op er ties of
th e P olym er s
a
a
b
polymers
yield (%)
Mw
Mw/Mn
Td (°C)
Tgc (°C)
Ta ble 2. Absor p tion a n d Em ission Da ta for th e P olym er s
P1
P2
P3
80
84
85
35 700
38 200
33 400
1.52
1.58
1.61
407
406
406
186
119
73
absorption
band
PL (film) fwhma ηPL
polymer (film) λmax (nm) gap (eV)
λ
max (nm)
(nm)
(%)
P1
P2
P3
352, 457
345, 435
347, 438
2.22
2.32
2.30
662, 712
641, 704
641, 705
139
148
150
7
5
4
a
Molecular weights and polydispersity indices determined by
b
GPC in THF using polystyrene as the standard. Onset decom-
position temperature measured by TGA under N2. c Glass transi-
tion temperature measured by DSC under N2.
a
Full width at half-maximum of the film PL spectra.
current-voltage characteristic was measured on a voltmeter
and an amperometer. The EL efficiency and brightness
measurements were carried out with a calibrated silicon
photodiode. All the measurements of the EL devices were
carried out in air at room temperature.
(Mw) of 33 400-38 200 with polydispersity indices (Mw/
Mn) of 1.52-1.61. The thermal properties of the poly-
mers were determined by TGA (heating at 10 °C min-1
in nitrogen) and DSC measurements. Figure 2 gives the
thermogravimetric curves of the polymers. All these
polymers show good thermal stability with the onset
decomposition temperature (Td) of 406-407 °C (shown
in Figure 2) under nitrogen, and no weight loss was
observed at lower temperatures. After the temperature
increasing above 400 °C, the curves fall rapidly, indicat-
ing the decomposition of the polymer backbones. Above
600 °C, there are only 33-40 wt % residues, which are
produced by charring during heating. The glass-transi-
tion temperatures (Tg) of the polymers range from 73
to 186 °C. Compared with P1 and P2, polymer P3 has a
lower Tg (°C) because of two long length dodecyoxyl
linked to PM units. However, their Tg values are higher
than those of poly(9,9-dihexylfluorene) (PHF) (∼55 °C)43
and poly(9,9-dioctylfluorene) (POF) (∼51 °C),44 in which
each repeating fluorene unit contained two flexible
n-hexyl or n-octyl chains at C-9. It is obvious that the
incorporation of a PM unit into the main chain can
increase Tg of polyfluorenes. This is very important for
such types of polymers used as emissive materials in
PLEDs.45
P h otop h ysica l P r op er ties. The photophysical char-
acteristics of the polymers were investigated in the solid
films. The absorption and emission spectral data for the
polymers in the films casting from solution in toluene
are summarized in Table 2. As shown in Figure 3, the
absorption spectra of the polymers obtained from the
thin films, P2 and P3, possess very similar absorption
spectra maximum peaks at 345 and 347 nm with the
corresponding additional absorption peaks at 435 and
438 nm, attributed to π-π* transitions of the polymers.
However, P1 shows the different absorption spectrum
maximum peak at 457 nm with an additional peak at
352 nm. Compared with the spectra of P2 and P3, the
difference shape and red-shifted maximum peak may
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The method for
preparing the monomers and the polymers is outlined
in Scheme 1. The monomers 1 and 3 were prepared
through Knoevenagel condensation between 2-(2,6-
dimethypyran-4-ylidene)-malomonitrile and the corre-
sponding bromoarylaldehyde in a yield of over 65%. The
monomer 4, 9,9-dihexylfluorene-2,7-bis(trimethylene bo-
ronate), was prepared using 2,7-dibromofluorene fol-
lowing a literature procedure.23 The polymerization
reaction conducted by the well-known palladium-
catalyzed Suzuki coupling reaction between compounds
1 (or 3) and 4 in toluene yielded a dark red solid of above
polymers in a yield of over 80%. The chemical structures
of the polymers were confirmed by NMR, FTIR, and
elemental analysis. All these polymers readily dissolve
in solvents, such as THF, CH2Cl2, CHCl3, and toluene.
Uniform and transparent films on substrates can be
obtained by spin-casting the solutions in toluene (1%)
at a spin rate of 1500 rpm. The good solubility of these
polymers may result from the hexyl side chain attached
to the fluorene moiety and n-alkyl side chain linked to
the PM unit. Figure 1 shows the FT-IR spectra. The
absorption peak at about 2925 cm-1 corresponds to C-H
stretching vibration of saturated hydrocarbon, the
absorption peak at about 2209 cm-1 corresponds to -CN
stretching vibration, and the weak absorption peaks at
about 3028 cm-1 and relatively strong absorption at
around 967 cm-1 correspond to C-H stretching and out-
of-plane bending motions of trans-vinylene, respec-
tively.41,42 Their molecular weights were determined by
gel permeation chromatography (GPC) using polysty-
rene as the standard. The results are listed in Table 1.
These copolymers have weight-average molecular weights

264 Peng et al.
Macromolecules, Vol. 37, No. 2, 2004
F igu r e 5. Cyclic voltammograms of the polymers recorded
from thin films coated onto platinum plate electrodes in an
electrolyte solution of Bu4NClO4 (0.10 M) in acetonitrile with
a reference electrode of Ag/AgNO3 (0.10 M) at room temper-
F igu r e 3. UV-vis absorption spectra of the polymer thin
films and PL spectrum of poly(9,9-dihexylfluorene) (PHF).
ature. Scan rate ) 50 mV s-1
.
Ta ble 3. Electr och em ica l P oten tia ls a n d En er gy Levels of
th e P olym er s
red
ox
[Ea
]
[Eb
]
LUMOc HOMOd
onset
onset
polymer
(V)
(V)
(eV)
(eV)
Ege (eV)
P1
P2
P3
-0.83
-0.86
-0.87
1.38
1.39
1.37
-3.57
-3.54
-3.53
-5.78
-5.79
-5.77
2.21
2.25
2.24
a
Onset reduction potentials measured by cyclic voltammetry.
b
Onset oxidation potential measured by cyclic voltammetry.
c Calculated from the reduction potentials. Calculated from the
oxidation potentials. e Calculated from the LUMO and HOMO
energy levels.
d
0.10 M tetrabutylammonium perchlorate (Bu4NClO4) in
anhydrous acetonitrile by using platinum wire as a
counter electrode and Ag/AgNO3 (0.10 M) as reference
electrode. As shown in Figure 5, the onset potentials
for oxidation were observed to be 1.38, 1.39, and 1.37 V
for P1, P2, and P3, respectively. On the other hand, the
onset potentials for reduction were -0.83, -0.86, and
-0.87 V, respectively. From the onset potentials of the
oxidation and reduction processes, the band gaps of the
polymers were estimated to be 2.21, 2.25, and 2.24 eV
for polymer P1, P2, and P3. The values are quite close
F igu r e 4. Photoluminescence (PL) spectra of the polymer thin
films.
be caused by the different structure with extended
length of π-conjugation. The absorption onset wave-
lengths of the polymers were 534-558 nm, which
correspond to band gaps of 2.22-2.32 eV.
The PL spectra of these polymers in thin films are
shown in Figure 4. The polymer films with the broad
band red PL emission exhibit similar vibronic features
with maximum emission peaks around 641-662 and
704-712 nm. No typical emission from fluorene (at
about 450 nm) was observed, which implied that the
emission of fluorene was quenched in these polymers.
As shown in Figure 3, there is a entire or almost entire
overlap between the emission spectrum of poly(9,9-
dihexylfluorene) (PHF) and the absorption spectra of
these polymers. So the results indicate that there is a
possibility of either the charge transfer between a
fluorene segment and electron-deficient PM segment of
the polymers or the efficient Fo¨ster energy transfer
between different polymer chains. Compared with P2
and P3, P1 also exhibits the red-shifted PL spectrum
due to the longer length of π-conjugation. The absolute
PL quantum yields of the neat polymer films were
measured to be about 4-7% in an integrating sphere
at room temperature in air using a HeCd laser line of
325 nm as the excitation source according to the
procedure described by Greenham et al.46
to those obtained by the optical method. According to
ox
the equations47,48 IP ) -([Eonset
]
+ 4.4) eV and EA )
red
red
-([Eonset
]
+ 4.4) eV, where [Eonset
]
ox and [Eonset
]
are
the onset potentials for oxidation and reduction of the
polymer vs the reference electrode, the LUMO and
HOMO of the polymers were estimated to be -3.57,
-3.54, and -3.53 eV and -5.78, -5.79, and -5.77 eV
for P1, P2, and P3, respectively. The electrochemical
data of the polymers are summarized in Table 3. From
Table 3, we find that the HOMO and LUMO energy
levels of these polymers are quite similar, and all the
polymers show much high electron affinity due to the
incorporation of the electron-deficient PM moiety.
E lect r olu m in escen ce P r op er t ies of LE D De-
vices. Double-layer LED devices based on the three
polymers with the configuration of ITO/PEDOT/polymer/
Ba/Al were fabricated. Poly(3,4-ethylenedionythiophene)
(PEDOT) doped with poly(styrenesulfonic acid) (PSS)
(Batron-P 4083) with a thickness of 90 nm was used as
the hole injection/transporting layer. A thin layer of Ba
(4 nm) was employed as the cathode because it normally
results in devices with longer lifetime due to its larger
atomic size and mass as compared with Ca or Mg.49 The
Ba layer was coated with a 170 nm layer of Al. The
Electr och em ica l P r op er ties. Cyclic voltammetry
(CV) was employed to investigate the electrochemical
behavior of the polymers as well as estimate the HOMO
and LUMO energy levels of the materials. The polymer
films deposited on a platinum plate electrode were
scanned both positively and negatively separately in

Macromolecules, Vol. 37, No. 2, 2004
Red-Emitting Polyfluorene Derivatives 265
emitting diodes. So further improvements could be
expected after optimization.
Con clu sion
A novel class of conjugated fluorene-based polymers
containing a 2-pyran-4-ylidene-malononitrile (PM) moi-
ety in the main chain were designed and synthesized
through the palladium-catalyzed Suzuki coupling reac-
tion. The resulting polymers show excellent thermal
stability and good red emission properties. The intro-
duction of a PM moiety into the polyfluorene main chain
can improve the electron affinity of this type of polymer.
Double-layer LEDs were fabricated using these poly-
mers as emitting layers. These devices show red emis-
sion with maximum external quantum efficiencies of
0.21-0.38%. The preliminary EL results show that
these polymers are novel candidates for red emissive
materials in polymer LEDS. Intensive studies on the
electroluminescent and electron-transporting/hole-block-
ing properties of these polymers in LED devices are in
progress.
F igu r e 6. EL spectra of the devices with the configuration of
ITO/PEDOT/polymer/Ba/Al.
Ack n ow led gm en t. This work was financially sup-
ported by the National Science Foundation of China
(Project No. 20102004). We thank Professor Wei Huang
for useful discussion when Dr. Zhi-Yun Lu was in
Singapore.
Note Ad d ed a fter ASAP P ostin g
This article was released ASAP on 12/3/2003. Wei
Huang was moved from the author byline to the
Acknowledgment. The manuscript was reposted to the
web on 12/16/2003.
F igu r e 7. Current-voltage (I-V) and luminance-voltage (L-
V) curves of an ITO/PEDOT/polymer/Ba/Al. The curves of a
and d were recorded from the device based on P1, b and e were
from the device based on P2, and c and f were from the device
based on P3.
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