Effect of cyano substituents on electron affinity and electron-transporting properties of conjugated polymers

Article abstract of DOI:10.1021/ma011790f

A series of cyano-containing distyrylbenzenes were synthesized as the model compounds to systematically study the effect of cyano substituents on the redox behaviors of conjugated molecules. By introducing the electron-withdrawing functional groups (cyano and dicyanovinyl) onto the phenylene ring, both electron affinity and electrochemical stability of the resulting distyrylbenzenes are greatly enhanced. The results enabled us to design and synthesize a new class of highly electron affinitive, fluorene-based copolymers with these cyano-containing chromophores as comonomers. The effects of acceptor strength and side chain on electron-transporting properties of these polymers were also investigated. By properly adjusting copolymer compositions, a combined high electron affinity and transport was achieved in a statistic copolymer, poly(fluorenebenzothiadiazole-cyanophenylenevinylene) (PFB-CNPV). An external quantum efficiency up to0.88% and brightness as high as 4730 cd/m² were achieved in a double-layer light-emitting diode (LED) using PFB-CNPV as the emitting layer.

Full text of DOI:10.1021/ma011790f

3532
Macromolecules 2002, 35, 3532-3538
Effect of Cyano Substituents on Electron Affinity and
Electron-Transporting Properties of Conjugated Polymers
Mich elle S. Liu , Xu ezh on g J ia n g, Sen Liu , P etr a Her gu th , a n d Alex K.-Y. J en *
Department of Materials Science and Engineering, Box 352120, University of Washington,
Seattle, Washington 98195-2120
Received October 15, 2001; Revised Manuscript Received February 1, 2002
ABSTRACT: A series of cyano-containing distyrylbenzenes were synthesized as the model compounds
to systematically study the effect of cyano substituents on the redox behaviors of conjugated molecules.
By introducing the electron-withdrawing functional groups (cyano and dicyanovinyl) onto the phenylene
ring, both electron affinity and electrochemical stability of the resulting distyrylbenzenes are greatly
enhanced. The results enabled us to design and synthesize a new class of highly electron affinitive,
fluorene-based copolymers with these cyano-containing chromophores as comonomers. The effects of
acceptor strength and side chain on electron-transporting properties of these polymers were also
investigated. By properly adjusting copolymer compositions, a combined high electron affinity and
transport was achieved in a statistic copolymer, poly(fluorenebenzothiadiazole-cyanophenylenevinylene)
(PFB-CNPV). An external quantum efficiency up to 0.88% and brightness as high as 4730 cd/m2 were
achieved in a double-layer light-emitting diode (LED) using PFB-CNPV as the emitting layer.
In tr od u ction
possible to fine-tune the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) energy levels and alter its electronic
properties. These properties will eventually affect the
charge transport characteristics of the materials.8-10
Previously, the results from a processable poly(cyano-
terephthalylidene) (CN-PPV), a dialkoxy-substituted
PPV derivative with cyano groups on the vinylene units,
have demonstrated a high internal quantum efficiency
up to 4% in a double-layer device (ITO/PPV/CN-PPV/
Al).11 This improved device efficiency was attributed to
better electron injection facilitated by the electron-
withdrawing cyano groups. However, the CN-PPV was
synthesized by using the Knoevenagel condensation
polymerization. A careful control over reaction condi-
tions is required to avoid the Michael addition side
reactions that will cause cross-linking and lead to
polymers with inferior EL properties. In this paper,
a series of cyano-containing distyrylbenzenes (3-6)
with electron-withdrawing groups functionalized on the
phenylene ring were synthesized initially as the model
compounds to study the effect of substituents on the
electrochemical properties of these oligomers. This was
achieved through a systematic changing of acceptor
strength on the aromatic ring (Chart 1). The data
obtained on these molecular models were used to predict
the ultimate electronic and electrochemical properties
of an ideal defect-free polymer chain. Based on this
model study, a series of light-emitting copolymers with
these cyano-containing chromophores as building blocks
were synthesized, and their electron affinity and charge-
transporting properties were investigated (Chart 2).
Conjugated polymers with excellent luminescent prop-
erties are very attractive for indicator and flat-panel
display applications due to their low cost, convenient
processability, and ease to fine-tune the emission colors.
A wide range of polymers, poly(p-phenylenevinylene)
(PPV), polythiophene (PT), poly(p-phenylene) (PPP),
polyfluorene (PF), and their derivatives, have been
extensively investigated as emissive materials.1-3 Elec-
troluminescence (EL) in these materials is usually
achieved by injecting electrons from a cathode into the
conduction band and holes from an indium tin oxide
(ITO) anode into the valence band of the polymers.
These injected holes and electrons capture each other
to form excitons, whose radiative decay emits light. A
balanced charge injection from both electrodes and
comparable mobility of electrons and holes within the
polymers are crucial to achieve high device efficiency.
However, most of the conjugated polymers tend to
possess low electron affinity.1 In addition, electron
transport is often reduced by the presence of defect- and
impurity-related traps in polymer films, while these
defects and impurities show little effect on the hole
transport. For these reasons, the majority of the charge
carriers in these emissive polymers are usually holes,
owing to their smaller injection barriers and higher
mobilities.4,5 Although electron injection can be facili-
tated by using low work function metals as cathode
(such as calcium or magnesium), the low electron
mobility in polymer still results in unbalanced charge
transport.6,7 As a result, the improvement of EL ef-
ficiency is quite limited. Therefore, it is a major chal-
lenge to find suitable conjugated polymers that possess
enhanced electron injection and transport simulta-
neously.
Exp er im en ta l Section
Ma ter ia ls. Commercially available chemicals (Aldrich) were
used without further purification. 2,5-Dicyano-p-xylyl-bis-
(diethylphosphonate),12,13 9,9-dihexylfluorene-2,7-di(ethylenyl
boronate),14 2,5-dibromoterephthalaldehyde,15 4-bromo-2,5-
dioctylbenzaldehyde,16 4,7-dibromo-2,1,3-benzothiadiazole,17
and 1,4-bis(4-bromostyryl)-2,5-dicyanobenzene18 were prepared
according to the procedures from the literature. All reactions
were performed under a dry nitrogen atmosphere.
The optoelectronic properties of a conjugated polymer
are primarily governed by the chemical structure of the
polymer backbone. By choosing suitable functional
groups attached onto the polymer main chain, it is
* To whom correspondence should be addressed.
10.1021/ma011790f CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/27/2002

Macromolecules, Vol. 35, No. 9, 2002
Conjugated Polymers 3533
Ch a r t 1. Ch em ica l Str u ctu r e of Cya n o-Con ta in in g Mod el Com p ou n d s
Ch a r t 2. F lu or en e Bor on ic Ester , Ar yl Dibr om id e Mon om er s, a n d Cop olym er s F or m ed by th e Su zu k i Cou p lin g
Rea ction
Gen er a l P r oced u r e for Wittig-Hor n er Rea ction . To a
solution of 1:2.1 equiv of a 2,5-dicyano-p-xylyl-bis(diethyl-
phosphonate) and the derivatives of benzaldehyde in THF was
added potassium tert-butoxide (2.1 equiv) at 0 °C. After stirring
at room temperature for 2 h, the reaction was quenched with
water. After the solvent was evaporated under vacuum, the

3534 Liu et al.
Macromolecules, Vol. 35, No. 9, 2002
crude product was extracted from water with CH2Cl2 and then
purified by column chromatography and crystallization.
1,4-Bis(4-ter t-bu tylstyr yl)ben zen e (1). The general pro-
cedure described above was followed using p-xylyl-bis(diethyl-
phosphonate) (378 mg, 1 mmol) and 4-tert-butylbenzaldehyde
malononitrile (88 mg, 1.3 mmol) in chloroform (20 mL) was
added pyridine (3-4 drops). The mixture was refluxed under
nitrogen for 24 h. The product was extracted from water with
CH2Cl2 and purified by a silica gel column using hexane/CH2-
Cl2 (1:1) as eluent. The crude product was recrystallized from
acetone/ethanol to afford an orange solid. Yield: 58 mg (90%).
1H NMR (300 MHz, CDCl3): δ 1.35 (s, 18H), 6.97 (d, J ) 15.6
Hz, 2H), 7.08 (d, J ) 15.6 Hz, 2H), 7.43 (dd, J ) 8.7 Hz, 3.9
Hz, 4H), 7.54 (dd, J ) 8.4 Hz, 4.2 Hz, 4H), 7.92 (s, 1H), 8.19
(s, 1H), 8.35 (s, 1H). 13C NMR (CDCl3): δ 158.20, 153.96,
140.26, 138.22, 136.10, 133.55, 132.66, 128.03, 127.83, 126.66,
126.03, 122.26, 117.54, 115.71, 112.61, 35.59, 31.95. Anal.
Calcd for C35H33N3: C, 84.81; H, 6.71; N, 8.48. Found: C, 84.92;
H, 6.76; N, 8.66.
t
(0.35 mL, 2.1 mmol) in the presence of BuOK (235 mg, 2.1
mmol). After column separation, the product was recrystallized
from acetone/ethanol to afford a pale yellow solid. Yield: 240
1
mg (60%). H NMR (300 MHz, CDCl3): δ 1.34 (s, 18H), 7.06
(d, J ) 15.6 Hz, 2H), 7.13 (d, J ) 15.9 Hz, 2H), 7.39 (dd, J )
6.6 Hz, 1.9 Hz, 4H), 7.47 (dd, J ) 6.6 Hz, 1.8 Hz, 4H), 7.50 (s,
4H). 13C NMR (CDCl3): δ 151.55, 137.47, 135.34, 129.02,
128.31, 127.45, 126.98, 126.37, 35.38, 32.03. Anal. Calcd for
C30H34: C, 91.32; H, 8.68. Found: C, 91.48; H, 8.70.
1,4-Bis(4-ter t-bu tylstyr yl)-2,5-d icya n oben zen e (3). The
general procedure described above was followed using 2,5-
dicyano-p-xylyl-bis(diethylphosphonate) (428 mg, 1 mmol) and
4-tert-butylbenzaldehyde (0.35 mL, 2.1 mmol) in the presence
1,4-Di(4-ter t-bu tylstyr yl)-2,5-d ica r bon ylben zen e (3-d i-
CHO). Compound 3-d iCHO was synthesized following the
same procedure for 3-CHO using 3 (222 mg, 0.5 mmol) and
DIBAL (0.3 mL, 1.7 mmol). Yield: 113 mg (50%). 1H NMR (300
MHz, CDCl3): δ 1.35 (s, 18H), 7.19 (d, J ) 16.2 Hz, 2H), 7.43
(dd, J ) 8.4 Hz, 1.8 Hz, 4H), 7.54 (d, J ) 8.4 Hz, 4H), 7.97 (d,
J ) 16.2 Hz, 2H), 8.18 (s, 2H), 10.45 (s, 2H). Anal. Calcd for
t
of BuOK (236 mg, 2.1 mmol). After column separation, the
product was then recrystallized from acetone/ethanol to afford
a yellow solid. Yield: 380 mg (85%). 1H NMR (300 MHz,
CDCl3): δ 1.35 (s, 18H), 7.36 (d, J ) 15.6 Hz, 4H), 7.44 (d, J
) 8.7 Hz, 4H), 7.54 (dd, J ) 8.4 Hz, 1.8 Hz, 4H), 8.04 (s, 2H).
13C NMR (CDCl3): δ 153.75, 139.71, 135.68, 133.52, 130.37,
127.95, 126.71, 121.76, 117.38, 115.71, 35.59, 31.95. Anal.
Calcd for C32H32N2: C, 86.45; H, 7.25; N, 6.30. Found: C, 86.43;
H, 7.24; N, 6.53.
C
32H34O2: C, 85.29; H, 7.60. Found: C, 85.41; H, 7.44.
1,4-Di(4-ter t-bu tylstyr yl)-2,5-bis(d icya n ovin yl)ben zen e
(6). Compound 6 was synthesized following the same proce-
dure for 5 using 3-d iCHO (80 mg, 0.18 mmol) and malono-
1
nitrile (117 mg, 1.8 mmol). Yield: 81 mg (84%). H NMR (300
MHz, CDCl3): δ 1.35 (s, 18H), 7.00 (d, J ) 16.2 Hz, 2H), 7.15
(d, J ) 15.6 Hz, 2H), 7.46 (dd, J ) 8.4 Hz, 3.9 Hz, 4H), 7.52
(dd, J ) 8.7 Hz, 3.9 Hz, 4H), 8.20 (s, 2H), 8.24 (s, 2H). 13C
NMR (CDCl3): δ 157.49, 150.74, 138.72, 138.30, 133.11,
132.51, 128.13, 127.41, 126.25, 121.52, 113.17, 112.24, 35.11,
31.44. Anal. Calcd for C38H34N4: C, 83.48; H, 6.27; N, 10.25.
Found: C, 83.37; H, 6.20; N, 10.16.
1,4-Bis(4-cya n ostyr yl)-2,5-d icya n oben zen e (4). The gen-
eral procedure described above was followed using 2,5-dicyano-
p-xylyl-bis(diethylphosphonate) (428 mg, 1 mmol) and 4-cyano-
t
benzaldehyde (262.3 mg, 2 mmol) in the presence of BuOK
(236 mg, 2.1 mmol). The yellow solid was precipitated out from
THF during reaction. The solid was filtered and then rinsed
with water, methanol, methylene chloride, and acetone se-
quentially to get rid of all the starting materials. Yield: 333
mg (87%). Anal. Calcd for C26H14N4: C, 81.66; H, 3.69; N,
14.65. Found: C, 81.44; H, 3.89; N, 14.37.
Br 2C8CNP V. The general procedure for the Wittig-Horner
reaction was followed using 2,5-dicyano-p-xylyl-bis(diethyl-
phosphonate) (428 mg, 1 mmol) and 4-bromo-2,5-di-n-octyl-
t
benzaldehyde (819 mg, 2 mmol) in the presence of BuOK
4,4′-Di-ter t-bu tyl-1,4-bis(b-cya n ostyr yl)ben zen e (2). 1,4-
Phenylenediacetonitrile (156 mg, 1 mmol) and 4-tert-butyl-
benzaldehyde (0.35 mL, 2.1 mmol) were dissolved in a mixture
(235.7 mg, 2.1 mmol). Yield: 486 mg (52%). 1H NMR (300 MHz,
CDCl3): δ 0.86 (t, J ) 6.2 Hz, 12H), 1.285 (br, 48H), 2.70
(q, J ) 7.8 Hz, 8H), 7.24 (d, J ) 16.2 Hz, 2H), 7.39 (s, 2H),
7.45 (s, 2H), 7.52 (d, J ) 16.2 Hz, 2H), 8.00 (s, 2H). Anal. Calcd
for C56H78Br2N2: C, 71.63; H, 8.37; Br, 7.02; N, 2.98. Found:
C, 71.46; H, 8.55; Br, 6.89; N, 3.04.
Gen er a l P r oced u r e for th e Su zu k i Cou p lin g P olym -
er iza t ion . To a stirred mixture of the 9,9-dihexylfluorene-
2,7-di(ethylenyl boronate) (1 equiv), aryl dibromide (1.05
equiv), tetrakis(triphenylphosphine)palladium (1 wt %), and
Aliquat 336 (tricaprylylmethylammonium chloride 10 wt %,
purchased from Aldrich Chemical) in toluene (10 mL) under
nitrogen was added a 2 M aqueous potassium carbonate
solution (3.3 equiv). The mixture was heated to reflux for
3 days. The polymerization was end-capped with phenyl-
boronic acid for 6 h, followed by bromobenzene for another
6 h. The reaction mixture was cooled and added dropwise
into a methanol/water (2:1 v/v) solution. The precipitated
polymer fibers were collected by filtration. The crude polymer
was further purified by redissolving the polymer into THF
and reprecipitating in methanol several times.
t
of tert-butyl alcohol (9 mL) and THF (3 mL). BuOK (11 mg,
0.1 mmol) and tetra-n-butylammonium hydroxide (1 mL, 1
mmol) were added quickly. The temperature was increased to
50 °C. After stirring 15 min at this temperature, the reaction
mixture was poured into acidified methanol. The yellow
precipitate was filtered and washed with methanol three
times. The product was then recrystallized from ethanol to
obtain a pale yellow solid. Yield: 111 mg (25%). 1H NMR (300
MHz, CDCl3): δ 1.36 (s, 18H), 7.51 (dd, J ) 6.9 Hz, 2.0 Hz,
4H), 7.59 (s, 2H), 7.75 (s, 4H), 7.88 (dd, J ) 6.3 Hz, 2.1 Hz,
4H). 13C NMR (CDCl3): δ 155.38, 143.35, 135.92, 131.51,
130.20, 127.50, 127.18, 126.78, 118.69, 35.80, 31.86. Anal.
Calcd for C32H32N2: C, 86.45; H, 7.25; N, 6.30. Found: C, 86.21;
H, 7.08; N, 6.05.
1,4-Di(4-ter t-bu t ylst yr yl)-2-ca r b on yl-5-cya n oben zen e
(3-CHO). To an ice-cooled suspension of 3 (155 mg, 0.35 mmol)
in ether (20 mL) was added diisobutylaluminum hydride
(DIBAL) (0.07 mL, 0.39 mmol) gradually. The mixture was
stirred at room temperature overnight. The ice-cooled reaction
mixture was then quenched carefully with methanol (0.5 mL),
followed by water (1 mL) and 2 M hydrochloric acid (5 mL).
Finally, concentrated HCl was added until the white solid was
completely dissolved. The aqueous layer was separated and
extracted twice with ether. The combined organic layers were
washed successively with water, saturated NaHCO3 solution
and water, and then dried over Na2SO4. After removal of the
solvent, the crude product was purified by a silica gel column
P F -C8CNP V. The general procedure described above was
followed using 9,9-dihexylfluorene-2,7-di(ethylenyl boronate)
(190 mg, 0.4 mmol) and Br2C8CNPV (394 mg, 0.42 mmol) in
the presence of tetrakis(triphenylphosphine)palladium (1.9
mg), Aliquat 336 (19 mg), and 2 M K2CO3 (0.6 mL). Yield: 355
1
mg (80%). H NMR (300 MHz, CDCl3): δ 0.77 (t, J ) 6.6 Hz,
6H), 0.87 (t, J ) 7.2 Hz, 12H), 1.08-1.28 (m, 64H), 2.68
(br, 4H), 2.82 (br, 8H), 7.18 (br, 4H), 7.33-7.51 (m, 6H),
7.61-7.81 (m, 4H), 8.08 (s, 2 H). Anal. Calcd for (C81H110N2)n:
C, 87.51; H, 9.97; N, 2.52. Found: C, 88.31; H, 9.52; N, 2.14.
P F -CNP V. The general procedure described above was
followed using 9,9-dihexylfluorene-2,7-di(ethylenyl boronate)
(181 mg, 0.38 mmol), 9,9-dihexylfluorene-2,7-dibromide (98 mg,
0.2 mmol), and 1,4-bis(4-bromostyryl)-2,5-dicyanobenzene (98
mg, 0.2 mmol) in the presence of tetrakis(triphenylphosphine)-
palladium (2 mg), Aliquat 336 (18 mg), and 2 M K2CO3 (0.6
mL). Yield: 257 mg (51%). 1H NMR (300 MHz, CDCl3): δ 0.80
1
using hexane/CH2Cl2 (7:3) as eluent. Yield: 47 mg (30%). H
NMR (300 MHz, CDCl3): δ 1.35 (s, 18H), 7.19 (d, J ) 16.2
Hz, 2H), 7.43 (dd, J ) 8.7 Hz, 3.9 Hz, 4H), 7.54 (dd, J ) 8.4
Hz, 4.2 Hz, 4H), 7.97 (d, J ) 16.2 Hz, 2H), 8.19 (s, 1H), 8.24
(s, 1H), 10.46 (s, 1H). Anal. Calcd for C32H33NO: C, 85.87; H,
7.43; N, 3.13. Found: C, 86.11; H, 7.28; N, 3.05.
1,4-Di(4-ter t-bu tylstyr yl)-2-cya n o-5-d icya n ovin ylben -
zen e (5). To a solution of 3-CHO (58 mg, 0.13 mmol) and

Macromolecules, Vol. 35, No. 9, 2002
Conjugated Polymers 3535
(br, 18H), 1.15 (br, 48H), 2.11 (br, 12H), 7.45 (br, 4H), 7.69-
7.85 (m, 26H), 8.14 (br, 2 H). Anal. Calcd for (C99H110N2)n: C,
89.54; H, 8.35; N, 2.11. Found: C, 90.15; H, 8.02; N, 1.68.
P F B-CNP V. The general procedure described above was
followed using 9,9-dihexylfluorene-2,7-di(ethylenyl boronate)
(181 mg, 0.38 mmol), 9,9-dihexylfluorene-2,7-dibromide (39 mg,
0.08 mmol), 4,7-dibromo-2,1,3-benzothiadiazole (35 mg, 0.12
mmol), and 1,4-bis(4-bromostyryl)-2,5-dicyanobenzene (98 mg,
0.2 mmol) in the presence of tetrakis(triphenylphosphine)-
palladium (2 mg), Aliquat 336 (18 mg), and 2 M K2CO3 (0.6
mL). Yield: 163 mg (86%). 1H NMR (300 MHz, CDCl3): δ 0.79
(br, 24H), 1.15 (br, 64H), 2.19 (br, 16H), 7.45 (br, 6.67H), 7.68-
8.13 (m, 42.66H). Anal. Calcd for (C146H153.33N5.33S1)n: C, 87.03;
H, 7.67; N, 3.71. Found: C, 87.90; H, 7.25; N, 4.05.
P F -TCNE. The general procedure described above was
followed using 9,9-dihexylfluorene-2,7-di(ethylenyl boronate)
(237 mg, 0.5 mmol) and 2,5-dibromoterephthalaldehyde (153
mg, 0.525 mmol) in the presence of tetrakis(triphenylphos-
phine)palladium (2.4 mg), Aliquat 336 (24 mg), and 2 M K2-
CO3 (0.8 mL). Yield: 193 mg (83%). 1H NMR (300 MHz,
CDCl3): δ 0.79 (t, J ) 6.6 Hz, 6H), 1.12 (br, 16H), 2.07 (t, J )
6.3 Hz, 4H), 7.41-7.96 (m, 6H), 8.08 (s, 1H), 8.26 (s, 1H), 10.11
(s, 1 H), 10.16 (s, 1H). P F -TCNE was formed by the further
condensation of above prepolymer (100 mg, 0.2 mmol) with
malononitrile (132 mg, 2 mmol). Yield: 36 mg (33%). 1H NMR
(300 MHz, CDCl3): δ 0.79 (t, J ) 6.3 Hz, 6H), 1.13 (br, 16H),
2.08 (t, J ) 6.6 Hz, 4H), 7.44-7.98 (m, 6H), 8.09 (s, 1H), 8.22
(s, 1H), 8.44 (s, 2 H). Anal. Calcd for (C39H36N4)n: C, 83.53; H,
6.47; N, 10.00. Found: C, 83.02; H, 7.02; N, 8.94.
vacuum-deposited on the top as a protecting layer for Ca. The
active emissive area defined by the cathode was about 8 mm2.
The “electron-only” devices were prepared as a sandwich
structure between Al and Ca electrodes.
Resu lts a n d Discu ssion
Syn th esis. Chart 1 presents a series of cyano-
containing compounds 3-6, which are arranged accord-
ing to the increasing strength of electron-withdrawing
substituents. Compounds 3 and 4 were synthesized
by the Wittig-Horner condensation reaction between
1,4-bis(2,5-dicyanobenzalphosphonate) and 4-tert-butyl-
benzaldehyde and 4-cyanobenzaldehyde, respectively
(Scheme 1). Further chemical reduction of the cyano
groups on compound 3 by diisobutylaluminum hydride
(DIBAL) afforded a monoaldehyde and a dialdehyde
depending on the stoichiometric ratios between 3 and
DIBAL. Condensation of the resulting aldehydes with
malononitrile gave compounds 5 and 6 in almost
quantitative yield. The unsubstituted compound 1 and
the cyanoterephthalylidene 2 were also synthesized for
comparison (Chart 1). Compound 4 is insoluble in
common organic solvent, such as THF, chloroform, and
DCE, due to its symmetrical and rigid structure.
However, the result obtained from elemental analysis
verified the composition of the desired compound.
Compounds 1-3, 5, and 6 could be purified by recrys-
tallization from acetone/ethanol after brief purification
through column chromatography. The stereochemistry
of the expected trans olefinic linkage was verified by
Gen er a l Ch a r a cter iza tion Meth od s. 1H and 13C NMR
spectra were obtained on a Bruker AF300 (300 MHz) spec-
trometer and recorded in ppm relative to tetramethylsilane
(TMS) (δ ) 0 ppm) as an internal standard. Molecular weights
of the polymers were determined by gel permeation chroma-
tography (GPC) on a Waters Styragel (HR 4E, 7.8 × 300 mm)
column with polystyrenes as the standards and THF as the
solvent. Thermal properties of polymers were analyzed with
a TA Instruments thermal analysis and rheology system (TGA
2950, DSC 2010) under nitrogen at a heating rate of 10 °C/
min. UV-vis spectra were recorded on a Perkin-Elmer spec-
trophotometer (Lambda 9 UV/vis/NIR). The film thickness was
measured with a Dektak surface profilometer (model 3030).
Photoluminescence (PL) efficiencies of the neat films (cast on
quartz glass) were recorded on an Oriel Instaspec IV charge
coupled device (CCD) camera and a Newport integrating
sphere.19 EL spectra of LEDs were obtained with the same
CCD system. Current-voltage (I-V) and luminance-voltage
(L-V) characteristics were measured on a Hewlett-Packard
4155B semiconductor parameter analyzer together with a
Newport 2835-C multifunctional optical meter. Photometric
units (cd/m2) were calculated using the forward output power
and the EL spectra of the devices, assuming Lambertian
distribution of the EL emission.20
Cyclic voltammetry (CV) was conducted at room tempera-
ture in a typical three-electrode cell with a working electrode
(ITO glass), a reference electrode (Ag/Ag+, referenced against
ferrocene/ferrocenium (FOC), 0.12 V), and a counter electrode
(Pt gauze) under a nitrogen atmosphere at a sweeping rate
of 100 mV/s (CV-50W voltammetric analyzer, BAS). CV
measurements for oligomers were performed in an electrolyte
solution of 0.1 M tetrabutylammonium hexafluorophosphate
(Bu4NPF6) in 1,2-dichloroethane (DCE), while a solution of 0.1
M tetrabutylammonium perchlorate (TBAP) in acetonitrile was
used as the electrolyte for polymer films.
1
the coupling constant of the vinylene protons in H NMR
(J ) 15-17 Hz).
Suzuki coupling methodology was employed in the
polymerization since it is tolerant of a large variety of
functional groups, insensitive to the presence of water,
and easy to make high molecular weight polymers.23
Polymers I-III were synthesized by coupling fluorene
diboronic esters with various cyano-containing distyryl-
benzene dibromides and benzothiadiazole dibromide in
the presence of Pd complex catalyst (Chart 2). While
the polycondensation of fluorene diboronic ester with
2,5-dibromoterephthalic dialdehyde gave polyaldehyde
initially, PF-TCNE (IV) was obtained by a further
condensation reaction of the resulting prepolymer with
malononitrile. The structures of the monomers and
polymers were confirmed by 1H NMR and elemental
analysis. The polymer compositions were estimated from
the feeding ratio. All the polymers could be very easily
dissolved in common organic solvents, such as chloro-
form, THF, and DCE. The weight-average molecular
weights (Mw) of these polymers determined by gel
permeation chromatography (GPC) range from 27 700
to 148 500 with polydispersity ranging from 1.7 to 3.3
using polystyrenes as the standards (Table 1). Thermal
characterization of polymers was accomplished by ther-
mogravimetry analysis (TGA) and differential scanning
calorimetry (DSC). The corresponding data are sum-
marized in Table 1. All the polymers except PF-TCNE
show excellent thermal stability with decomposition
temperature (onset of weight loss, measured by TGA
analysis, 10 °C/min) in excess of 380 °C. PF-TCNE
undergoes a two-step weight loss: at 224 °C, it starts
to lose dicyanovinyl substituents, while decomposition
temperature of polymer main chain is at 382 °C. These
polymers also exhibit relatively high Tg’s, ranging from
141 to 174 °C.
LEDs F a br ica tion . The EL devices were constructed using
a double-layer configuration with an in-situ polymerized bis-
(tetraphenyldiamino)biphenyl-perfluorocyclobutane (BTPD-
PFCB) as the hole-transporting layer21,22 and the cyano-
containing polymers as the emitting layer. The BTPD-PFCB
layer was first spin-coated from its DCE solution (1 wt %) onto
ITO glass substrates and then cured at 225 °C for 1 h. Then
a thin layer of light-emitting polymers (1 wt % in DCE) was
spin-coated onto the BTPD-PFCB layer. A layer of 30 nm
thick Ca was vacuum-deposited at pressure below 10-6 Torr
through a mask, and another layer of 120 nm thick Ag was
Electr och em ica l a n d Ch a r ge Tr a n sp or t P r op er -
ties. Table 2 lists the values of onset redox potentials

3536 Liu et al.
Macromolecules, Vol. 35, No. 9, 2002
Sch em e 1. Syn th etic Rou te for th e Cya n o-Con ta in in g Distyr ylben zen esa
a
t
Reagents and conditions: (i) BuOK, THF, 2 h; (ii) DIBAL, ether, 0 °C; (iii) malononitrile, pyridine, CHCl3, reflux, 24 h.
Ta ble 1. Ch a r a cter istics of th e Cya n o-Con ta in in g
Cop olym er s
copolymer
Mw
Mw/Mn
Tg (°C)
Td (°C)
389
379
406
PF-C8CNPV
PF-CNPV
PFB-CNPV
PF-TCNE
51 100
64 000
148 000
27 700
2.83
3.12
3.29
1.68
174
141
149
163
224, 382
Ta ble 2. Red ox P r op er ties of th e Distyr ylben zen es
model
compd
Eox (V)a
Ered (V)a
HOMO (eV)24
LUMO (eV)24
1
2
3
4
5
6
1.04
-5.72
-1.52
-1.46
-1.40b
-0.87
-0.67
-3.16
-3.22
-3.28
-3.81
-4.01
a
b
Onset potentials vs Ag/Ag+. Thin film evaporated onto ITO.
F igu r e 1. Cyclic voltammograms of the electrochemical
oxidation of compound 1 and reduction of compounds 2 and 3
in 0.1 M TBAPF6 in DCE at a sweep rate of 100 mV/s.
of the model compounds from electrochemical study. The
unsubstituted parent compound 1 exhibits two revers-
ible oxidative waves (Figure 1), corresponding to the
successive generation of the cation radical and dication.
No reduction is observed for this molecule under the
experimental conditions. However, the addition of the
cyano or dicyanovinyl functional groups enhances the
electron affinity of compounds significantly; only reduc-
tion processes could be observed for the cyano-contain-
ing compounds. Their electron affinities increase with
the increasing electron-withdrawing strength of the
acceptors. Moreover, the position of electron-withdraw-
ing functional groups also affects the stability of the
anion radical. Figure 1 shows the cyclic voltammograms
of compounds 2 and 3. The onset and reduction peak
potentials of 3 are nearly identical to those of 2,
indicating that different positions of the cyano substitu-
tion have a similar influence on electron affinity of the
carbon framework. They possess comparable electron
affinity. However, compound 3 shows a distinct, revers-
ible wave, while only an irreversible wave is observed
for compound 2. The reversible reduction wave of 3 can
be cycled repetitively without any appreciable change.
This can be explained that the vinylenic nitrile (an R,â-
unsaturated system) is very susceptible to attack from
nucleophiles. As a result, it breaks the conjugation and
electroactivity of the molecule. The improved stability
of radical anions through these cyano groups on the
aromatic ring may eventually contribute to the durabil-
ity of the LED devices.
As was expected, by attaching a stronger electron-
accepting dicyanovinyl group onto the phenylene, it can
further improve the electron affinity of the parent
compound. Compound 6 demonstrates a much lower

Macromolecules, Vol. 35, No. 9, 2002
Conjugated Polymers 3537
Ta ble 3. P h ysica l P r op er ties of th e Cya n o-Con ta in in g
Cop olym er s
PL
λUV
HOMO LUMO
λPL
efficiency
(%)
copolymer
(nm)a
(eV)b
(eV)c
(nm)d
PF-C8CNPV 408
-5.75
-5.93
-5.76
-6.33
-3.12
-3.43
-3.22
-4.00
504
513
541
616
17
15
10
4
PF-CNPV
PFB-CNPV
PF-TCNE
380, 432
345, 447
326
a
b
Maximum wavelength of absorption (film). From absorption
spectra. c From cyclic voltammery data. Maximum wavelength
d
of emission (film), excited at absorption maximum.
F igu r e 2. Current density-electric field characteristics of
“electron-only” devices.
F igu r e 3. Optical absorption (a) and photoluminescence (b)
spectra of thin films of cyano-containing polymers.
reduction onset at -0.67 V, which indicates easier
accessible radical anions (Table 2).
Our model study demonstrates that the incorporation
of cyano functional groups onto the phenylene ring not
only enhances electron affinity but also increases anion
stability. Thus, the monomers bearing cyano-containing
distyrylbenzenes were used to copolymerize with fluo-
rene to improve electron-transporting properties of
polyfluorenes.
As we compare the data of the LUMO level obtained
from cyclic voltammetry (CV),24 it shows that the LUMO
levels of the polymers are similar to those of their oligo-
meric model compounds, indicating improved electron
affinity of these cyano-containing polymers (Table 3).
The electron-transporting properties were investi-
gated using an “electron-only” devices consisting of a
polymer layer sandwiched between Al and Ca elec-
trodes.25 By replacing the high work function ITO (4.7
eV) with a lower work function metal Al (4.3 eV), it
increases the energy barrier for hole injection. This will
reduce the number of injected holes to the HOMO level
of the polymers. In addition, there is no energy barrier
for electron injection due to low LUMO levels of these
cyano-containing polymers. Therefore, the current den-
sity-electric field (J -E) characteristics are only related
to the bulk electron conduction of polymer films. A
single-carrier device using fluorene homopolymer, poly-
(9,9-dihexylfluorene) (PHF), as the active layer was also
fabricated for comparison. The J -E characteristics of
these testing devices demonstrate that the best electron
conduction is achieved in the PFB-CNPV system
(Figure 2). At the same electric field, the electron
current density of the PFB-CNPV device is the highest
among all devices. This may be due to the better
planarity of benzothiadiazole that provides a better
electron-hopping manifold. The presence of alkyl chains
on PF-C8CNPV improves the solubility and decreases
chain packing of the polymer. However, it significantly
hinders the electron conduction. Thus, the polymer
suffers from the slowest electron motion. On the other
hand, PF-TCNE possesses the highest electron affinity,
but its electron motion is relatively slow. This is
understandable because the dicyanovinyl group is a very
strong electron acceptor, and it may act as a deep trap
site for electrons in the bulk film. From these studies,
the PFB-CNPV shows a combined high electron affinity
and conductivity that makes it an ideal candidate as a
good electron- transporting material.
P h otop h ysica l P r op er ties. The UV-vis absorption
and emission spectra of the copolymers are shown in
Figure 3, and the results are summarized in Table 3.
The longer absorption wavelength of PFB-CNPV com-
pared with that of other copolymers further confirms
its more planar structure. When the strong electron-
withdrawing dicyanovinyl groups were introduced into
the polymer main chain, a higher energy absorption
peak at 326 nm with a broad shoulder at 410 nm was
observed.
All the polymers possess broad, featureless emission
bands. The emission maximum of the PF-C8CNPV with
octyl substitutions on the distyrylbenzene is at 504 nm,
which is blue-shifted compared to those polymers with-
out octyl chains (PF-CNPV and PFB-CNPV) because
of backbone distortion induced by the alkyl chains. PF-

3538 Liu et al.
Macromolecules, Vol. 35, No. 9, 2002
Ta ble 4. P er for m a n ce of LEDs Ba sed on
Cya n o-Con ta in in g Cop olym er s
the effect of cyano substituents on the redox behaviors
of conjugated molecules. The results derived from the
structure/property relationships among these com-
pounds enabled us to design and synthesize a new class
of fluorene-based copolymers with these cyano-contain-
ing chromophores as comonomers to improve the elec-
tron affinities and transport of these light-emitting
materials.
d
Von
Bmax
ηmax
device configuration
(nm)a (V)b (cd/m2)c (%)d
ITO/BTPD-PFCB/PF-C8CNPV/Ag
ITO/BTPD-PFCB/PF-CNPV/Ag
ITO/BTPD-PFCB/PFB-CNPV/Ag
ITO/BTPD-PFCB/PF-TCNE/Ag
ITO/BTPD-PFCB/PFB-CNPV/Ca
55
50
55
40
60
6.0
3.6
4.4
13.2
2.6
1220
837
2050
36
0.53
0.07
0.32
0.02
0.88
4730
Ack n ow led gm en t. The authors thank the Air Force
Office of Scientific Research (AFOSR) for support through
the MURI (Polymeric Smart Skins) and the DURIP
program. A. J en thanks the Boeing-J ohnson Foundation
for financial support. M. Liu thanks the Nanotechnology
Center at the University of Washington for the nano-
technology fellowship.
a
b
Thickness of the emitting layer. Turn-on voltage, defined as
the voltage required to give a luminance of 1 cd/m2. c Maximum
d
brightness. Maximum external quantum efficiency.
C8CNPV also possesses higher photoluminescence (PL)
efficiency. This may be due to the presence of the octyl
substitutions decreasing intermolecular packing and
thus reducing luminescence quenching. The emission
of PF-TCNE has a significant red shift relative to the
rest of polymers, in which a very low PL quantum yield,
4%, was obtained. This may be caused by the intramo-
lecular charge transfer (ICT) between the fluorene and
the bis(dicyanovinyl)benzene ring. ICT is much stronger
in the PF-TCNE than the other cyano-containing
polymers (I-III) since the dicyanovinyl group is a much
stronger electron acceptor.
Refer en ces a n d Notes
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(24) According to the redox onset potentials of the CV measure-
ments, the HOMO and LUMO energy levels for the oligomers
and polymers are estimated on the basis of the reference
energy level of ferrocene (4.8 eV below the vacuum level):
HOMO/LUMO ) -(Eonset - 0.12 V) - 4.8 eV, where the value
0.12 V for FOC vs Ag/Ag+.
Electr olu m in escen ce. EL of single-layer devices
made from the cyano-containing polymers using the
ITO/polymer/Ag configuration (φAg ∼ 4.3 eV) showed
very poor performance with very high turn-on voltage
(>13 V) and low brightness due to large energy barriers
for both electron and hole injection. By empolying a hole-
transporting layer (BTPD-PFCB), good EL efficiency
and luminance were achieved in the devices with the
configuration of ITO/HTL/polymer/Ag (Table 4). Al-
though the PF-C8CNPV-based device has the highest
efficiency, the brightness is very low due to much slower
electron motion. The ITO/BTPD-PFCB/PFB-CNPV/Ag
device has the best overall performance, with a turn-
on voltage of 4.4 V and a luminance of 2050 cd/m2 at
0.68 A/cm2. This could be attributed to both optimized
carrier injection and electron conduction in this config-
uration. For the polymers with similar structure (I-
III), the turn-on voltage increases with the increasing
energy barrier for the electron injection, which indicates
that the device performance is still limited by electron
injection from the silver electrode, since there is still a
quite large energy barrier (∼1 eV) between Ag and the
polymers. This is confirmed by a device fabricated using
another bilayer configuration (ITO/BTPD-PFCB/PFB-
CNPV/Ca) with a low work function calcium metal as
the cathode. The ohmic contact between PFB-CNPV
(LUMO ∼ 3.3 eV) and calcium (φCa ∼ 2.9 eV) greatly
facilitates the electron injection. The device can be
turned on at 2.6 V, and it reached a high luminance of
4730 cd/m2 at a current density of 1.62 A/cm2. The
external quantum efficiency of the device also improves
by approximately a factor of 3 (ηEL ) 0.88%).
Su m m a r y
(25) Parker, I. D. J . Appl. Phys. 1994, 75, 1656.
A series of cyano-containing distyrylbenzenes were
synthesized as model compounds to systematically study
MA011790F