Novel alternating fluorene-based conjugated polymers containing oxadiazole pendants with various terminal groups

Article abstract of DOI:10.1021/ma0489927

A series of soluble alternating fluorene-based copolymers containing symmetrical and asymmetrical 1,3,4-oxadiazole (OXD) pendants with various terminal groups are synthesized by the palladium-catalyzed Suzuki coupling reaction. These polymers possess higher glass transition temperatures than that of the analogous dialkoxy-substituted polymer (PFPOC₆) consisting of the same backbone without OXD pendants. The photophysical and electrochemical properties of these polymers are affected by the polar effect (electron-withdrawing group, -CN, and electron-donating group, -R or -OR) and the size effect (the size of the grafted side chain) of the OXD pendants. Owing to the large steric hindrance of OXD pendants, the aggregation of these polymers in solids is reduced, which results in almost identical PL emissions in both solution and solid states. The bulky OXD pendants on the polymer side chains can provide the polymer films with lower HOMO and LUMO energy levels and better electron injection property. Since only one emission peak is observed in both PL and EL spectra of these polymers, it is evidenced that effective energy transfer from the OXD pendants to the conjugated polymer backbones has occurred, thus eliminating the light emission from the OXD pendants. These asymmetrical OXD-substituted polymers have higher quantum yields and less aggregation in the solid state than the symmetrical OXD-substituted polymers. The symmetrical OXD-substituted polymer (P1) has a longer PL emission wavelength than the asymmetrical OXD-substituted polymers (P2-P8), which may be due to the improvement of the coplanarity between the polymer backbone and the symmetrical OXD pendants and/or the introduction of two electron-withdrawing OXD pendants.

Full text of DOI:10.1021/ma0489927

Macromolecules 2004, 37, 7945-7954
7945
Novel Alternating Fluorene-Based Conjugated Polymers Containing
Oxadiazole Pendants with Various Terminal Groups
Hsia o-Hsien Su n g a n d Hon g-Ch eu Lin *
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu,
Taiwan, ROC
Received May 21, 2004; Revised Manuscript Received August 9, 2004
ABSTRACT: A series of soluble alternating fluorene-based copolymers containing symmetrical and
asymmetrical 1,3,4-oxadiazole (OXD) pendants with various terminal groups are synthesized by the
palladium-catalyzed Suzuki coupling reaction. These polymers possess higher glass transition temper-
atures than that of the analogous dialkoxy-substituted polymer (PFPOC6) consisting of the same backbone
without OXD pendants. The photophysical and electrochemical properties of these polymers are affected
by the polar effect (electron-withdrawing group, -CN, and electron-donating group, -R or -OR) and the
size effect (the size of the grafted side chain) of the OXD pendants. Owing to the large steric hindrance
of OXD pendants, the aggregation of these polymers in solids is reduced, which results in almost identical
PL emissions in both solution and solid states. The bulky OXD pendants on the polymer side chains can
provide the polymer films with lower HOMO and LUMO energy levels and better electron injection
property. Since only one emission peak is observed in both PL and EL spectra of these polymers, it is
evidenced that effective energy transfer from the OXD pendants to the conjugated polymer backbones
has occurred, thus eliminating the light emission from the OXD pendants. These asymmetrical OXD-
substituted polymers have higher quantum yields and less aggregation in the solid state than the
symmetrical OXD-substituted polymers. The symmetrical OXD-substituted polymer (P 1) has a longer
PL emission wavelength than the asymmetrical OXD-substituted polymers (P 2-P 8), which may be due
to the improvement of the coplanarity between the polymer backbone and the symmetrical OXD pendants
and/or the introduction of two electron-withdrawing OXD pendants.
In tr od u ction
widely studied classes of electron-injection and/or hole-
blocking materials12 by virtue of many merits owned by
1,3,4-oxadiazole (OXD) moieties, such as prominent
electron affinity, high photoluminescence (PL) quantum
yield, and good thermal stability.13 On the other hand,
to achieve highly efficient and luminescent PLED
devices, it is important to suppress the self-quenching
process, which originates from the formation of excimers
due to the intermolecular interactions in the solid state.
Since large steric hindrance of lateral substitution will
cause the decrease of crystallinity in the resulting
polymers, the bulky substitution with symmetrical and
asymmetrical structure on the side chains of the poly-
mer can provide the polymer film with an amorphous
property.14 In addition, the pseudo-orthogonal arrange-
ment of the rigid pendant groups can reduce the
π-stacking and thus increase the radiative decay quan-
tum efficiency and photostability of the polymers.12c,15
In this paper, we report a series of alternating fluorene-
based copolymers containing symmetrical and asym-
metrical substituted OXD pendants with different ter-
minal groups (on the side chains of the polymers). The
electrochemical, spectroscopic, and thermal properties
of these polymers with respect to various terminal OXD
groups are also investigated in this study.
Polymer light-emitting diodes (PLEDs) have been
studied intensively during the past decade because of
their potential applications in a new generation of flat
display and lighting technologies.1,2 Compared with
inorganic electroluminescent (EL) materials, π-conju-
gated polymers have many virtues such as ease of
forming large area, mechanical flexibility, low fabrica-
tion cost, and easy color tuning through molecular
engineering.3 Therefore, it is generally expected that
PLEDs will be a possible candidate of the next genera-
tion display technology.
To obtain highly efficient PLED devices, a balance in
the injection and transportation of both holes and
electrons into the polymer emissive layer is necessary.4-8
However, most EL polymers inject and transport holes
more efficiently than electrons due to the inherent
richness of π-electrons in the EL polymers, which results
in an unbalance of opposite carrier currents and a shift
of the recombination zone toward the region near the
interface of the polymer and the cathode. Thus, the
synthesis of efficient electron-transporting polymers is
needed to improve EL device performance. Polymers
containing electron-withdrawing units in the main
chains or side chains usually have large electron affini-
ties. In recent years, polyquinolines, polyquinoxalines,
polybenzobisazoles, and polypyridines have been dem-
onstrated as electron-transporting (ET) polymers due
to the good film quality and the excellent thermal and
oxidative stability.9-11 Molecular and polymeric 2,5-
diphenyl-1,3,4-oxadiazolediyl derivatives are the most
Exp er im en ta l Section
Mea su r em en ts. 1H NMR spectra were recorded on a
Varian Unity 300 MHz spectrometer using CDCl3 sol-
vent. Elemental analyses were performed on a HERAEUS
CHN-OS RAPID elemental analyzer. Transition temperatures
were determined by differential scanning calorimetry (Perkin-
Elmer Pyris 7) with a heating and cooling rate of 10 °C/min.
Thermogravimetric analysis (TGA) was conducted on a Du
Pont Thermal Analyst 2100 system with a TGA 2950 thermo-
gravimetric analyzer under a heating rate of 20 °C/min. Gel
* Author for correspondence: Tel 8863-5712121 ext 55305; Fax
8863-5724727; e-mail linhc@cc.nctu.edu.tw.
10.1021/ma0489927 CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/23/2004

7946 Sung and Lin
Macromolecules, Vol. 37, No. 21, 2004
Sch em e 1. Syn th etic Rou tes of Mon om er s
permeation chromatography (GPC) analysis was conducted on
a Waters 1515 separation module using polystyrene as a
standard and THF as an eluant. UV-vis absorption spectra
were recorded in dilute chloroform solutions (10-5 M) on a HP
G1103A spectrophotometer, and fluorescence spectra were
obtained on a Hitachi F-4500 spectrophotometer. Fluorescence
quantum yields were determined by comparing the integrated
PL density of a reference 9,10-diphenylanthracene in toluene
with a known quantum yield (ca. 5 × 10-6 M, quantum yield
) 1.0). Cyclic voltammetry (CV) was performed at a scanning
rate of 100 mV/s on a BAS 100 B/W electrochemical analyzer,
which was equipped with a three-electrode cell. Pt wire was
used as a counter electrode, and an Ag/AgCl was used as a
reference electrode in the CV measurement. The polymer thin
film was cast onto a Pt disk as a working electrode with
ferrocene as a standard in actonitrile, and 0.1 M tetrabutyl-
ammonium hexafluorophosphate (TBAPF6) was used as a
supporting electrolyte. Polymer thin films were spin-coated on
a quartz substrate from chloroform solution with a concentra-
tion of 10 mg/mL (this condition consists with the electrolu-
minescent device).
The synthetic routes of monomers M1-M8 are shown in
Scheme 1. First, 2,5-dibromo-p-xylene was oxidized with
potassium permanganate in pyridine to afford 2,5-dibromo-
terephthalic acid and 2,5-dibromo-4-methylbenzoic acid. After
then, the acid group was further esterified to give 2,5-
dibromoterephthalic acid dimethyl ester (1) and 2,5-dibromo-
4-methylbenzoic acid methyl ester (2), which were synthesized
by the modified procedure of ref 16. After the ester groups had
been converted to benzoyl hydrazides (3 and 4), they were
reacted with various aryl chloride to form compounds 5 and
6. Finally, monomers M1-M8 were produced by the dehydra-
tive cyclization of hydrazides in POCl3.
2,5-Dibr om oter ep h th a loyld ih yd r a zid es (3). A mixture
of 3.19 g (9.07 mmol) of methyl-2,5-dibromoterephthalate and
4.53 g (90.70 mmol) of hydrazine monohydrate was dissolved
in 35 mL of methanol and then refluxed for 24 h. After cooling,
the product was poured into water and recrystallized from
methanol. Yield: 83%. 1H NMR (ppm, DMSO-d6): 4.51 (s, 4H),
7.58 (s, 2H), 9.69 (s, 2H).
2,5-Dib r om o-4-m et h ylb en zoic Acid H yd r a zid e (4).
1
Yield: 78%. H NMR (ppm, DMSO-d6): 2.33 (s, 3H), 7.52 (s,
Ma ter ia ls. Chemicals and solvents were reagent grades and
purchased from Aldrich, ARCROS, TCI, and Lancaster Chemi-
cal Co. Dichloromathane and THF were distilled to keep
anhydrous before use. The other chemicals were used without
further purification.
1H), 7.68 (s, 1H), 9.57 (s, 1H).
Com p ou n d 5. 0.81 g (3.02 mmol) of 4-octyloxybenzoyl
chloride was added into a solution containing 0.53 g (1.51
mmol) of 1 and 0.25 mL (3.02 mmol) of pyridine in 10 mL of
NMP. The reaction mixture was stirred for 12 h and then

Macromolecules, Vol. 37, No. 21, 2004
Fluorene-Based Conjugated Polymers 7947
Sch em e 2. Syn th etic Rou te of P olym er s
poured into water. The product was filtered and crystallized
from methanol. Yield: 73%. H NMR (ppm, DMSO-d6): 0.83
(t, 6H), 1.21-1.80 (m, 24H), 4.04 (t, 4H), 7.02 (d, J ) 9 Hz,
4H), 7.78 (s, 2H), 7.89 (d, J ) 8.7 Hz, 4H), 10.55 (s, 2H), 10.58
(s, 2H).
) 8.7 Hz, 4H), 8.51 (s, 2H). Anal. Calcd for C38H44Br2N24O4:
C, 58.47; H, 5.68; N, 7.18. Found: C, 58.20; H, 5.79; N, 7.07.
2-(2,5-Dib r om o-4-m et h ylp h en yl)-5-p h en yl-[1,3,4]oxa -
d ia zole (M2). Yield: 56%. 1H NMR (ppm, CDCl3): 2.46 (s,
3H), 7.52-7.65 (m, 3H), 8.14-8.17 (m, 2H), 8.24 (s, 1H). Anal.
Calcd for C15H10Br2N2O: C, 45.72; H, 2.56; N, 7.11. Found:
C, 46.06; H, 2.87; N, 6.87.
1
Ben zoic Acid N-(2,5-Dib r om o-4-m et h yl-b en zoyl)h y-
1
d r a zid e (6-2). Yield: 81%. H NMR (ppm, DMSO-d6): 2.38
(s, 3H), 7.49-7.62 (m, 3H), 7.68 (s, 1H), 7.77 (s, 1H), 7.91 (d,
J ) 8.4 Hz, 2H), 10.38 (s, 1H), 10.44 (s, 1H).
2-(2,5-Dibr om o-4-m eth ylp h en yl)-5-p-tolyl-[1,3,4]oxa d i-
a zole (M3). Yield: 68%. 1H NMR (ppm, CDCl3): 2.45 (s, 3H),
2.46 (s, 3H), 7.33 (d, J ) 7.8 Hz, 2H), 7.64 (s, 1H), 8.02 (d, J
) 8.4 Hz, 2H), 8.22 (s, 1H). Anal. Calcd for C16H12Br2N2O: C,
47.09; H, 2.96; N, 6.86. Found: C, 47.07; H, 3.23; N, 6.80.
2-(2,5-Dibr om o-4-m eth ylp h en yl)-5-(4-m eth oxyp h en yl)-
4-Meth ylben zoic Acid N′-(2,5-d ibr om o-4-m eth ylben -
zoyl)h yd r a zid e (6-3). Yield: 66%. 1H NMR (ppm, DMSO-
d6): 2.37 (s, 3H), 2.38 (s, 3H), 7.30 (d, J ) 8.1 Hz, 2H), 7.68 (s,
1H), 7.76 (s, 1H), 7.84 (d, J ) 7.5 Hz, 2H), 10.40 (s, 1H), 10.58
(s, 1H).
1
[1,3,4]oxa d ia zole (M4). Yield: 44%. H NMR (ppm, CDCl3):
4-Meth oxyben zoic Acid N′-(2,5-d ibr om o-4-m eth ylben -
zoyl)h yd r a zid e (6-4). Yield: 88%. 1H NMR (ppm, DMSO-
d6): 2.38 (s, 3H), 3.82 (s, 3H), 7.03 (d, J ) 8.7 Hz, 2H), 7.68 (s,
1H), 7.76 (s, 1H), 7.90 (d, J ) 8.7 Hz, 2H), 10.36 (s, 1H), 10.51
(s, 1H).
4-Octyloxyben zoic Acid N′-(2,5-d ibr om o-4-m eth ylben -
zoyl)h yd r a zid e (6-5). Yield: 79%. 1H NMR (ppm, DMSO-
d6): 0.83 (t, 3H), 1.25-1.79 (m, 12H), 2.38 (s, 3H), 4.03 (t, 2H),
7.00 (d, J ) 8.7 Hz, 2H), 7.68 (s, 1H), 7.76 (s, 1H), 7.87 (d, J
) 9 Hz, 2H), 10.37 (s, 1H), 10.50 (s, 1H).
4-Cya n oben zoic Acid N′-(2,5-Dibr om o-4-m eth ylben -
zoyl)h yd r a zid e (6-6). Yield: 82%. 1H NMR (ppm, DMSO-
d6): 2.38 (s, 3H), 7.67 (s, 1H), 7.77 (s, 1H), 8.00 (d, J ) 8.4 Hz,
2H), 8.05 (d, J ) 8.1 Hz, 2H), 10.59 (s, 1H), 10.97 (s, 1H).
Th iop h en e-2-ca r boxylic Acid N′-(2,5-d ibr om o-4-m eth -
ylben zoyl)h yd r a zid e (6-7). Yield: 63%. 1H NMR (ppm,
DMSO-d6): 2.38 (s, 3H), 7.20 (t, J ) 5.1 Hz, 1H), 7.65 (s, 1H),
7.77 (s, 1H), 7.86 (d, J ) 9.3 Hz, 1H), 7.88 (d, J ) 8.4 Hz, 1H),
10.47 (s, 1H), 10.71 (s, 1H).
Naph th alen e-1-car boxylic Acid N′-(2,5-dibr om o-4-m eth -
ylben zoyl)h yd r a zid e (6-8). Yield: 71%. 1H NMR (ppm,
DMSO-d6): 2.39 (s, 3H), 7.56-7.78 (m, 6H), 8.00 (d, J ) 7.2
Hz, 1H), 8.06 (d, J ) 7.2 Hz, 1H), 8.36 (d, J ) 8.7 Hz, 1H),
10.58 (s, 1H), 10.64 (s, 1H).
2.46 (s, 3H), 3.90 (s, 3H), 7.02 (d, J ) 9 Hz, 2H), 7.64 (s, 1H),
8.07 (d, J ) 8.7 Hz, 2H), 8.22 (s, 1H). Anal. Calcd for C16H12
-
Br2N2O2: C, 45.31; H, 2.85; N, 6.61. Found: C, 45.09; H, 3.09;
N, 6.51.
2-(2,5-Dibr om o-4-m eth ylp h en yl)-5-(4-octyloxyp h en yl)-
1
[1,3,4]oxa d ia zole (M5). Yield: 81%. H NMR (ppm, CDCl3):
0.90 (t, 3H), 1.20-1.51 (m, 10H), 1.77 (m, 2H), 2.46 (s, 3H),
4.04 (t, 2H), 7.00 (d, J ) 8.7 Hz, 2H), 7.64 (s, 1H), 8.05 (d, J )
8.7 Hz, 2H), 8.22 (s, 1H). Anal. Calcd for C23H26Br2N2O2: C,
52.89; H, 5.02; N, 5.36. Found: C, 53.13; H, 5.34; N, 5.30.
4-[5-(2,5-Dibr om o-4-m eth ylp h en yl)-[1,3,4]oxa d ia zol-2-
yl]ben zon itr ile (M6). Yield: 88%. 1H NMR (ppm, CDCl3):
2.48 (s, 3H), 7.67 (s, 1H), 7.84 (d, J ) 8.4 Hz, 2H), 8.25 (s,
1H), 8.26 (d, J ) 8.1 Hz, 2H). Anal. Calcd for C16H9Br2N3O:
C, 45.86; H, 2.16; N, 10.03. Found: C, 45.82; H, 2.49; N, 10.07.
2-(2,5-D ib r o m o -4-m e t h y lp h e n y l)-5-t h io p h e n -2-y l-
1
[1,3,4]oxa d ia zole (M7). Yield: 88%. H NMR (ppm, CDCl3):
2.45 (s, 3H), 7.20 (t, J ) 5.1 Hz, 1H), 7.59 (d, J ) 5.1 Hz, 1H),
7.63 (s, 1H), 7.85 (d, J ) 5.1 Hz, 1H), 8.19 (s, 1H). Anal. Calcd
for C13H8Br2N2OS: C, 39.03; H, 2.02; N, 7.00. Found: C, 39.37;
H, 2.34; N, 6.92.
2-(2,5-Dib r om o-4-m et h ylp h en yl)-5-n a p h t h a len -1-yl-
1
[1,3,4]oxa d ia zole (M8). Yield: 68%. H NMR (ppm, CDCl3):
2.41 (s, 3H), 7.56-7.68 (m, 4H), 7.88-8.23 (m, 4H), 9.30 (d, J
) 8.1 Hz, 1H). Anal. Calcd for C19H12Br2N2O: C, 51.38; H, 2.72;
N, 6.31. Found: C, 51.32; H, 3.01; N, 6.28.
P olym er iza tion . The synthetic routes of polymers P 1-P 8
are shown in Scheme 2. The general procedure of polymeri-
zation is proceeded through the Suzuki coupling reaction. A
mixture of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
9,9-dioctylfluorene (1 equiv), dibromo compounds M1-M8 (1
equiv), and tetrakis(triphenylphosphine)palladium (1.0 mol %)
Mon om er 1 (M1). 0.8 g (1.02 mmol) of 4a was dissolved in
100 mL of POCl3. The reaction mixture was heated to 130°C
overnight and then cooled to room temperature. Most POCl3
in reaction mixture was removed at reduced pressure and
poured into water. The product was filtered and crystallized
from methanol and chloroform several times to get a yellow
1
solid. Yield: 74%. H NMR (ppm, CDCl3): 0.88 (t, 6H), 1.30-
1.86 (m, 24H), 4.05 (t, 4H), 7.03 (d, J ) 9 Hz, 4H), 8.08 (d, J

7948 Sung and Lin
Macromolecules, Vol. 37, No. 21, 2004
Ta ble 1. Molecu la r Weigh ts a n d Th er m a l P r op er ties of
P olym er s
4900 and 25 200 g/mol, and the polydispersity indexes
(PDI) are found between 1.2 and 2.8. The thermal
stability of the polymers in nitrogen was evaluated by
thermogravimetric analysis (TGA), and their corre-
sponding data are also summarized in Table 1. The TGA
analysis indicates that the degradation temperatures
(Td) of 5% weight loss in nitrogen are between 398 and
446 °C. The results of TGA reveal that the polymers
are quite stable in nitrogen, and compared with the
other polymers (P 2-P 8) containing asymmetrical OXD
pendants, P 1 containing symmetrical OXD pendants
has the lowest Td. The glass transition temperatures
(Tg) and melting temperatures of these polymers were
determined by differential scanning calorimetry (DSC)
in a nitrogen atmosphere at a scanning rate of 10 °C/
min. Their glass transition temperatures are shown
between 85 and 148 °C, whereas their melting temper-
atures are not observed even up to 280 °C. The long
flexible alkoxy-substituted OXD polymer, P 5, has the
lowest Tg, and the polar cyano-substituted OXD poly-
mer, P 6, demonstrates the highest Tg. In contrast to
P 1 containing symmetrical OXD pendants, analogous
P 5 containing asymmetrical OXD pendants possesses
a lower Tg (34 °C lower). Comparing P 8 with P 2, when
the phenyl group is replaced with bulky a naphthyl
group in the terminal group of OXD pendants, an
increase of 23 °C in Tg is observed in P 8. In comparison
with the analogous structure without oxadiazole pen-
dants, i.e., poly(fluorene phenlyene) (PFP),17 the deg-
radation temperatures of these novel polymers are
higher than that of PFP (Td is 398 °C), and the glass
transition temperatures of these polymers are also
higher than that of PFPOC6 (Tg is 72 °C).
b
polymer
yield (%)
Mn
PDI
Tda (°C)
Tg (°C)
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
51
23
40
35
78
54
88
63
4 900
6 700
1.2
1.7
1.5
2.8
2.7
2.6
1.9
1.7
398
418
421
414
410
422
407
446
119
109
125
128
85
148
120
132
14 800
26 600
24 500
25 200
12 200
6 500
a
Temperature of 5% weight loss was measured by TGA in
b
nitrogen. The glass transition temperature (Tg) was measured
on the first cooling cycle after the sample had been heated to 280
°C.
was added in a degassed mixture of toluene ([monomer] ) 0.2
M) and aqueous 2 M potassium carbonate (3:2 in volume). The
mixture was vigorously stirred and reacted at 87 °C for 72 h.
After the mixture was cooled to room temperature, it was
poured into 200 mL of methanol. A fibrous solid was obtained
by filtration, and the solid was sequentially washed with
methanol, water, and methanol.
P 1. Yield: 51%. 1H NMR (ppm, CDCl3): 0.61-1.41 (m, 46H),
1.69-2.00 (m, 10H), 3.88 (br, 4H), 6.73 (d, J ) 8.4 Hz, 4H),
7.37-7.55 (m, 8H), 7.81 (d, J ) 7.2 Hz, 2H), 8.37 (s, 2H). Anal.
Calcd for C63H76N4O4: C, 79.37; H, 8.04; N, 5.88. Found: C,
78.10; H, 7.97; N, 5.60.
P 2. Yield: 23%. 1H NMR (ppm, CDCl3): 0.66-1.20 (m, 22H),
1.91 (br, 4H), 2.46 (s, 3H), 7.22-7.59 (m, 10H), 7.76-7.81 (m,
2H), 8.14 (s, 1H). Anal. Calcd for C40H42N2O: C, 84.77; H, 7.47;
N, 4.94. Found: C, 83.26; H, 7.45; N, 4.83.
P 3. Yield: 40%. 1H NMR (ppm, CDCl3): 0.66-1.23 (m, 22H),
1.92 (br, 4H), 2.28 (s, 3H), 2.46 (s, 3H), 7.03 (d, J ) 7.8 Hz,
2H), 7.36-7.50 (m, 7H), 7.74-7.82 (m, 2H), 8.12 (s, 1H). Anal.
Calcd for C41H44N2O: C, 84.79; H, 7.64; N, 4.82. Found: C,
83.44; H, 7.55; N, 4.62.
P 4. Yield: 35%. 1H NMR (ppm, CDCl3): 0.74-1.26 (m, 22H),
2.1 (br, 4H), 2.48 (s, 3H), 3.77 (s, 3H), 6.75 (br, 2H), 7.41-
7.84 (m, 9H), 8.14 (s, 1H). Anal. Calcd for C41H44N2O2: C,
82.51; H, 7.43; N, 4.69. Found: C, 81.42; H, 7.50; N, 4.48.
P 5. Yield: 78%. 1H NMR (ppm, CDCl3): 0.68-1.41 (m, 35H),
1.74-2.07 (m, 6H), 2.47 (s, 3H), 3.90 (br, 2H), 6.77 (d, J ) 8.4
Hz, 2H), 7.41-7.83 (m, 9H), 8.15 (s, 1H). Anal. Calcd for
C
48H58N2O2: C, 82.95; H, 8.41; N, 4.03. Found: C, 82.03; H,
8.27; N, 3.71.
P 6. Yield: 54%. 1H NMR (ppm, CDCl3): 0.68-1.26 (m, 22H),
1.95 (br, 4H), 2.50 (s, 3H), 7.35-7.86 (m, 11H), 8.21 (s, 1H).
Anal. Calcd for C41H41N3O: C, 83.21; H, 6.98; N, 7.10. Found:
C, 82.26; H, 6.91; N, 6.73.
P 7. Yield: 88%. 1H NMR (ppm, CDCl3): 0.75-1.20 (m, 22H),
2.04 (br, 4H), 2.48 (s, 3H), 6.95 (br, 1H), 7.37-7.49 (m, 7H),
7.76-7.85 (m, 2H), 8.11 (s, 1H). Anal. Calcd for C38H40N2OS:
C, 79.68; H, 7.04; N, 4.89. Found: C, 78.47; H, 7.14; N, 4.69.
P 8. Yield: 63%. 1H NMR (ppm, CDCl3): 0.48-1.20 (m, 22H),
1.92 (br, 4H), 2.48 (s, 3H), 7.12 (br, 1H), 7.39-7.55 (m, 8H),
7.80-7.87 (m, 4H), 8.20 (s, 1H), 9.04 (br, 1H). Anal. Calcd for
C
44H44N2O: C, 85.67; H, 7.19; N, 4.54. Found: C, 84.20; H,
7.31; N, 4.08.
Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion . The synthetic
routes of monomers M1-M8 and polymers P 1-P 8 are
shown in Schemes 1 and 2. All these polymers were
easily dissolved in common organic solvents such as
chloroform, THF, dichloromethane, and toluene to form
good transparent films on glass substrates. The average
molecular weights were measured by gel permeation
chromatography (GPC) with polystyrene as a standard
and THF as an eluting solvent. The related data
obtained from GPC are given in Table 1. The number-
average molecular weights (Mn) of polymers are between
Op tica l P r op er ties. The photophysical characteris-
tics of the monomers and polymers were studied by PL
and UV-vis absorption in both dilute chloroform solu-
tions and thin films. The optical properties of the
monomers and polymers are summarized in Tables 2
and 3 and Figures 1-5 serially. It is noted that similar
absorption patterns between 250 and 400 nm are
observed in various substituted monomers, except M1.

Macromolecules, Vol. 37, No. 21, 2004
Fluorene-Based Conjugated Polymers 7949
Ta ble 2. Absor p tion a n d P L Em ission Sp ectr a l Da ta of
Mon om er s in Ch lor ofor m
a
b
a
b
λabs
λPL,sol
λabs
λPL,sol
monomer
(nm)
(nm)
monomer
(nm)
(nm)
M1
M2
M3
M4
337
283
286
297
418
347
352
370
M5
M6
M7
M8
299
290
302
320
372
361
367
379
a
b
Wavelength of the maximum absorbance. Wavelength of the
maximum PL emission.
Ta ble 3. Absor p tion a n d P L Em ission Sp ectr a l Da ta of
P olym er s in Ch lor ofor m a n d in Th in Solid F ilm s
λabs,sol λabs,film band gapb λPL,sol λPL, film
fwhmsol
(nm) (nm) ΦPL,sol (nm)
a
a
a
c
polymer (nm)
(nm)
(eV)
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
321
327
329
325
322
323
323
328
322
327
326
323
321
322
324
328
3.14
3.33
3.32
3.32
3.33
3.32
3.33
3.34
445
405
402
405
404
430
409
410
447
411
407
412
409
430
415
410
0.15
0.30
0.39
0.36
0.31
0.17
0.26
0.38
70
60
58
58
58
68
61
62
a
Wavelength of the maximum absorbance or PL emission.
b
Band gaps were calculated from the onsets of UV-vis absorption
spectra of polymer solutions. c PL quantum yield in CHCl3.
F igu r e 2. (a) UV-vis spectra of M5 and P 5. (b) UV-vis and
PL emission spectra of M1 and P 1.
F igu r e 1. UV-vis spectra of polymer solutions in dilute
chloroform.
As expected, both of the absorption and PL spectra are
affected by the substituted terminal groups. Compared
with those of M2, the maximum absorption and PL
wavelengths of M3-M8 increase as the polar substit-
uents and π-excessive thiophene (naphthalene) substit-
uents are attached to the OXD rings. Interestingly,
compared with that of M4 (λPL,sol ) 370 nm) containing
a OMe pendant terminal group, the emission wave-
length of M6 (λPL,sol ) 361 nm), in which the polar cyano
group is attached to the OXD segment, is not efficiently
enlarged. This phenomenon is contrary to our previous
study, where the cyano group attached to the chro-
mophoric core will elongate the conjugation and result
in a longer emission wavelength.18 Whereas, the PL
emission spectra of the polymers have shown that the
cyano-substituted polymer P 6 has a longer emission
wavelength (λPL,sol ) 430 nm) than that of the alkoxy-
substituted polymer P 4 (λPL,sol ) 405 nm).
F igu r e 3. UV-vis spectra of polymer films.
It is noticed that the polymers with asymmetric OXD
pendants exhibit two absorption peaks, except P 8. The
absorption spectrum of P 8 shows only one peak, which
may be due to the overlap of absorption peaks in both
OXD pendants and polymer main chains. The shorter
absorption ranging at 290-305 nm is originated from
the absorption of the conjugated OXD segments, which
is confirmed by the same maximum absorption of
The absorption spectra of synthesized polymers in
dilute solutions (chloroform) are depicted in Figure 1.

7950 Sung and Lin
Macromolecules, Vol. 37, No. 21, 2004
OXD segments in P 1 (containing symmetrical OXD
pendants) does not appear as a peak but only shows a
shoulder around 337 nm (see Figure 2b), which corre-
sponds to the absorption peak of M1. This phenomenon
illustrates that the fluorene and both symmetrical OXD
segments of P 1 do not twist away from the conjugated
plane of the polymer backbone and induce an elongated
conjugation. In addition, the absorption strength of P 1
does not show any absorption peak in the region of 270-
320 nm (which corresponds to M5 absorption). It implies
the conjugated OXD units of P 1 keep better coplanarity
and do not twist away from the conjugated plane. We
also observe the solid-state absorption spectrum of P 1
(as shown in Figure 2b) has a tail in the low-energy
region around 370-600 nm; it also proves that P 1 has
a better coplanarity between the symmetrical OXD
pendants and the polymer backbone.
Compared with analogous PFP (λabs ) 370 nm), our
polymers exhibit blue-shifted absorption maxima (41-
49 nm). The blue shifts of the absorption maxima may
be ascribed to two reasons. One is that the coplanarity
of the polymer backbone is lost due to the asymmetrical
bulky substituents, which cause the steric hindrance
and induce the distortion of the backbone. Therefore,
the conjugation of polymer backbone is interrupted and
induces a blue shift of the absorption maximum. The
other is that as mentioned previously the OXD pendants
act as electron-withdrawing substituents on the side of
the polymer backbone. The electron-withdrawing sub-
stituted groups decrease the electron density of the
polymer backbone and reduce the absorption maxi-
mum.17
F igu r e 4. PL spectra of polymer solutions in dilute chloro-
form.
The optical band gap energies obtained from UV-vis
absorption in solutions are also given in Table 3. The
band gap energies of polymers were determined by the
intersection of tangents through the turning point of the
lower energy side of the spectrum and the lengthened
baseline for each polymer. The optical band gaps of
asymmetrical OXD-substituted polymers from the edge
absorption of solution samples are almost identical
(3.32-3.34 eV), regardless of terminal functional groups
(P 2-P 8) of OXD pendants. Nevertheless, the sym-
metrical OXD-substituted polymer (P 1) has a red-
shifted absorption curve and a smaller band gap (3.14
eV). The smaller band gap may be due to the introduc-
tion of two electron-withdrawing OXD moieties and/or
a better coplanarity between the polymer backbone and
the symmetrical substituents in P 1. In comparison with
PFP, all band gap energies of these polymers are larger
than that of PFP (2.92 eV).
F igu r e 5. PL spectra of polymer films.
polymer P 5 and monomer M5 (Figure 2a). The identical
absorption maximum of the OXD segment in dilute
solutions before and after the polymerization suggests
that the electronic interactions between the conjugated
oxadiazole segments and the polymer backbones are
quite limited.19 The result also implies that part of the
conjugated OXD units (the phenyl-oxadiazole-phenyl
conjugated unit) in the asymmetrical side-chain struc-
tures twist away from the plane of the conjugated
polymer backbone. The longer wavelength peak in the
region of 310-370 nm is attributed to the electron
transition of π-π* along the conjugated backbone of the
polymer. Comparing Figures 1-3, the polymer main-
chain absorption peaks of the solution and film samples
(Figure 2a) are almost identical. This result indicates
that the conformation of the polymer chains is very
similar in both solution and solid states; in addition,
the polymer assembly is in a rather disordered state and
refrains from aggregation by this molecular design even
in the solid state.
The fluorescence quantum yield (ΦPL), the PL emis-
sion maxima (λPL), and the full width at half-maximum
(fwhm) values are also presented in Table 3. All emis-
sion data given here were obtained by the excitation at
the maximum absorption peaks. In dilute solutions, all
these polymers only show one emission peak ranging
from 402 to 445 nm (Figure 4). Emissions from the OXD
pendants are not observed, even when polymers are
excited at the absorption peak of the OXD pendants.
This indicates the existence of efficient energy transfer
from the OXD pendants to the polymer backbone.
Except P 6, all asymmetrical OXD-substituted polymer
solutions and films have PL emission in the purple
region (Figures 4 and 5). Among these purple-emitting
polymers, the electron-donating substituted groups
(CH3, OCH3, and OC8H17) in P 3-P 5 do not seem to
affect the emission spectra significantly as compared
The maximum absorption value of P 1 (321 nm) comes
from the fluorene backbone, which can be proved by
similar maximum absorption values of all polymers P 1-
P 8 (321-329 nm), regardless of various oxadiazole
pendant groups in P 1-P 8. The absorption strength of

Macromolecules, Vol. 37, No. 21, 2004
Fluorene-Based Conjugated Polymers 7951
with that of P 2. However, in contrast to P 3-P 5, when
the electron-withdrawing cyano group is introduced into
the terminal group of OXD pendants in P 6, an obvious
red shift (about 25 nm) can be observed in the PL
spectrum. The symmetrical OXD-substituted polymer
(P 1), apparently, has a longer PL emission wavelength
than the other asymmetrical OXD-substituted polymers
(P 2-P 8). This result may be due to the improvement
of the coplanarity between the polymer backbone and
the symmetrical OXD pendants and/or the introduction
of two electron-withdrawing OXD pendants.
the larger fwhm value of the short side-chain polymers
is a contribution of isolated main-chain fluorophores and
a certain degree of aggregation. In our synthetic poly-
mers, the fwhm values of the emission curves of the
polymers in the solution and solid states are almost
independent of the terminal groups (P 2-P 5 and P 7-
P 8) of the OXD pendants. Only about 10 nm of increase
in the fwhm value was observed in symmetrical P 1 and
asymmetrical P 6 (with a strong electron-withdrawing
cyano substituent attached to the end of the OXD
pendant). The strong polar groups of the cyano termi-
nals in the pendants of P 6 may induce more or less
intramolecular and/or intermolecular interactions and
consequently broaden the fwhm value. Otherwise, com-
paring P 5 with P 2-P 4 of our polymers containing OXD
pendants, the fwhm value of the long terminal side-
chain polymer (P 5) is almost identical with those of
short terminal side-chain polymers (P 2-P 4). From this
result, we can generally conclude that the asymmetrical
OXD pendants can provide good steric hindrance and
reduce aggregation, except OXD pendants containing
strong polar cyano groups (such as P 6). In general, the
lowest quantum yields of P 1 (due to the symmetrical
pendant structure) and P 6 (due to the strong polar
cyano substituent) are also correlated to their largest
fwhm values.
The PL spectra of polymer thin films are shown in
Figure 5. These asymmetrical and symmetrical OXD-
substituted polymers emit purple and blue color light,
respectively. Compared with the spectra of asymmetri-
cal OXD-substituted polymers (P 2-P 8) in solutions, the
PL emission peaks of solid films only show a small red
shift (the maximum red-shifted value is 7 nm). In
contrast to the asymmetrical OXD-substituted poly-
mers, the thin film PL peak of P 1 is just red-shifted for
2 nm, but the emission spectrum shows slight tails in
the low-energy ends, which may be due to the aggrega-
tion of polymer chains in the solid state. The PL
quantum yield will be decreaed when pendant groups
(side chains) are conjugated with the polymer main
chains.12c,19,20 It may be due to the multifarious conju-
gated forms which cannot confine the excitons ef-
ficiently; hence, the PL quantum yield decreases in the
OXD-substituted polymer. It is interesting to note that
the PL quantum yield of the symmetrical OXD-substi-
tuted polymer (P 1) is lower than those of asymmetrical
OXD-substituted polymers (P 2-P 8), which may be due
to the different degree of conjugation between the OXD
pendants and the backbone in the symmetrical and
asymmetrical structures, respectively.
In summary of the absorption and PL results, we can
conclude the photophysical properties are strongly af-
fected by the OXD pendants. In asymmetrical substi-
tuted polymers, the absorption maximum peaks and
optical band gaps are almost independent of the elec-
tronic effect (electron-withdrawing group, -CN, electron-
donating group, -R or -OR) and size effect (the size of
the terminal functional group) of the substituted OXD
pendants. However, the PL emission peaks and the
quantum yields are affected by the electronic effect.
When the strong electron-withdrawing cyano group is
attached to the terminal phenyl group of the OXD
pendant, the PL emission peak is red-shifted and the
quantum yield is reduced. Except P 6, these polymers
have similar PL emission peaks as that of PFP (λabs,sol
) 408 nm, λabs,film ) 422 nm), but they show larger
Stokes shifts than that of PFP. As a symmetrical OXD-
substituted polymer, P 1 has an almost identical absorp-
tion maximum value as those of asymmetrical substi-
tuted polymers, but relatively it possesses a longer PL
emission peak and a lower quantum yield.
Compared with PFP, side-chain conjugated PPV
(OPO-PPV), and cross-conjugated PPE (PPE-PPV),14c,20b
these OXD-substituted polymers (P 1-P 8) exhibit al-
most identical spectra in the solution and thin film
samples. This result is distinct from PFP, OPO-PPV,
and PPE-PPV, which have significant red-shifted spec-
tra in the thin film samples as compared with the
solution samples. The results demonstrate that the
configuration of the OXD-substituted polymers in our
study is not affected by the variation of the state in these
polymers. It can be understood by the enhanced steric
effect between the OXD pendant groups and the poly-
mer backbone, which consequently increases interchain
distances and suppresses the formation of aggregation
efficiently. In comparison with PFP, the PL spectra of
all our polymers (no matter in solution or solid states)
exhibit structureless emission spectra; it is different
from PFP, which possesses a well-resolved vibronic band
in the emission spectrum. This phenomenon suggests
that the OXD pendants can efficiently enhance the
amorphism of the polymers and reduce the vibronic
structures in the solid state.
Electr och em ica l P r op er ties. To fit the energy band
structural scheme of PLED device, it is necessary to
determine the energy levels of the highest occupied
molecular orbital (HOMO) and the lowest unoccupied
molecular orbital (LUMO) of each component, which
was carried out by CV to investigate the redox behavior
of the polymer thin films. The potential estimated here
was based on the reference energy level of ferrocene (4.8
eV below the vacuum level) according to the following
equation:21 EHOMO/ELUMO ) [-(Eonset - 0.45) - 4.8] eV.
The onset potentials were determined from the intersec-
tion of two tangents drawn at the rising and background
currents of the cyclic voltammogram. A crude estimation
of the LUMO levels of reduction polymers were deduced
from the HOMO values and the optical band gaps.
Klemm and co-workers reported that the full width
at half-maximum (fwhm) values of alkoxy-substituted
PPV-PPE hybrid polymers strongly depend on the
length of the attached side chains.21 The fwhm value
increases from 36 nm in PPV-PPE hybrid polymers
containing longer side chains (-OC18) to 67 nm in PPV-
PPE hybrid polymers containing shorter side chains
(-OC8). They explain that the smaller fwhm value of
the long side-chain polymers is due to the emission
coming from isolated main-chain fluorophores; however,
These polymers have the same backbone structure of
poly(fluorene-co-alt-phenylene) with different substit-
uents on the OXD pendants which are attached to the
phenylene groups. It is interesting to evaluate that how
do these OXD pendants modulate the electrochemical

7952 Sung and Lin
Macromolecules, Vol. 37, No. 21, 2004
F igu r e 6. Cyclic voltammetry of polymers during the oxidation process.
Ta ble 4. HOMO a n d LUMO En er gies a n d Electr och em ica l
P r op er ties of P olym er s
Ered/onset Ered/peak Eox/onset Eox/peak EHOMO ELUMO
polymer
(V)
(V)
(V)
(V)
(eV)
(eV)
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
-1.65
-1.79
1.55
1.62
1.57
1.58
1.61
1.39
1.69
1.60
1.74
1.90
1.72
1.98
1.73
1.50
1.87
1.77
-5.90 -2.70a
-5.97 -2.64b
-5.92 -2.60b
-5.93 -2.61b
-5.96 -2.63b
-5.74 -2.84a
-6.04 -2.71b
-5.95 -2.61b
-1.51
-1.72
a
LUMO Energies were obtained from the cyclic voltammetry.
LUMO Energies were deduced from HOMO values and optical
b
band gaps.
potential is -1.51 V. It is obvious that P 6 is much easier
to be reduced than the other polymers; this is probably
due to the strong electron-withdrawing cyano group
which increases the overall electron-withdrawing effect
of OXD pendants. Compared with P 2, the HOMO
energy level of P 6 can be easily adjusted for 0.4 eV by
changing the terminal functional groups of OXD pen-
dants with CN terminals. As for the symmetrical OXD-
substituted polymer (i.e., P 1), the oxidation onset
potential is 1.74 V and reduction onset potential is
-1.65 V. The band gap obtained from the difference of
onset potentials between the reduction and oxidation
processes is estimated to be 3.20 V, which is close to
that obtained from the absorption edge in the UV-vis
spectrum (3.14 V).
The OXD pendants act as electron-withdrawing sub-
stituents on the backbones of poly(fluorine-co-alt-phen-
ylene)s and thus decrease the LUMO levels (-2.60 to
-2.84) of all polymers as compared to that of PFP
(HOMO ) -5.36, LUMO ) -2.47). The lower LUMO
potentials of these polymers indicates that the incorpo-
ration of electron-withdrawing OXD pendants increases
the electron affinities and reduces the LUMO levels of
wide band gap polymers (such as PFP) to match the
F igu r e 7. Cyclic voltammograms of P 1 and P 6.
behavior of these polymers. As shown in Figures 6 and
7, most of the polymers possess two anodic peaks, which
can be attributed to two different blocks in the OXD
pendants and polymer backbones of the copolymers,
respectively. Since the reduction potentials are difficult
to be measured, only P 1 and P 6 can be determined,
which may be due to their strong electron-withdrawing
property. The asymmetrical substituted polymers (P 2-
P 5) show oxidized peaks irreversibly at potentials
ranging from 1.72 to 1.98 V (Table 4) and the onset
potentials of oxidation between 1.57 and 1.62 V in the
anodic scan. Comparing P 3-P 5 with P 2, it seems that
the electron-donating groups slightly reduce the overall
electron-withdrawing effect of the OXD pendants. Com-
pared with P 2, the oxidized onset potentials of P 7 and
P 8 are not remarkably changed by the replacement of
the phenyl group with the π-excessive thiophene and
naphthalene groups.
As for P 6 with cyano-substituted OXD pendants, the
oxidation onset potential is 1.39 V and reduction onset

Macromolecules, Vol. 37, No. 21, 2004
Fluorene-Based Conjugated Polymers 7953
Ta ble 5. EL Da ta of P LED Devicesa
λmax,EL
(nm)
max brightness
polymer
Vonb (V)
Vonc (V)
(cd/m2)
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
452
408
409
423
406
431
416
417
7.5
7.5
7.5
8.5
8.5
6.5
7.0
6.5
7.5
7.5
7.5
8.5
9.0
7.0
7.0
7.0
126
29
51
104
376
306
82
462
a
b
Device structure: ITO/PEDOT:PSS/polymer/Ca/Al. Von is the
c
turn-on voltage of current. Von is the turn-on voltage of light.
Figu r e 9. Normalized EL spectra of ITO/PEDOT:PSS/Polymer/
Ca/Al devices.
F igu r e 8. Current-voltage and luminescence-voltage char-
acteristics of ITO/PEDOT:PSS/P3/Ca/Al device.
work function of the cathode. From these results, we
can conclude that both HOMO and LUMO energy levels
can be adjusted not only by the OXD pendants of the
polymer backbone but also by the attached functional
groups of the OXD pendants.
E lect r olu m in escen t (E L) P r op er t ies of P LE D
Devices. The CV and UV-vis results show that both
HOMO and LUMO energy levels of our new polymers
do not match the work functions of indium-tin oxide
(ITO) anode and Al cathode. Therefore, we choose
PEDOT and Ca as the anode and cathode, respectively,
to overcome these large energy barriers. A series of
double-layer EL devices with the configuration of ITO/
PEDOT:PSS/Polymer/Ca/Al were made by spin-coating
the polymer solutions (with a concentration of 10 mg/
mL) onto PEDOT-coated glass substrates, and their EL
data are demonstrated in Table 5.
The current-voltage and luminescence-voltage, i.e.,
EL response, curves of one typical PLED device (ITO/
PEDOT:PSS/P3/Ca/Al) are displayed in Figure 8. All
these devices show turn-on voltages for current from 6.5
to 8.5 V and turn-on voltages for light from 7 to 9 V
(shown in Table 5). Compared with P 2 without flexible
ends on OXD pendants, P 5 with OC8 terminals on OXD
pendants has a higher turn-on voltage. This result may
be due to the intrinsic insulation property of the long
alkoxy side chains in P 5 which consequently increases
the turn-on voltage. The attainable maximum lumi-
nance is 462 cd/m2 at 15 V for P 8 as an emitter. These
similar turn-on voltages for current and light illustrate
that a matched balance of injection and transportation
in charge is achieved. The balanced injection and
transportation of both charge carriers may be due to
the limited electronic interactions between the cross-
F igu r e 10. Normalized EL spectra of ITO/PEDOT:PSS/P1/
Ca/Al device at different voltages.
conjugated OXD pendants and the conjugated polymer
backbone.15c
As shown in Figure 9, these devices of asymmetrical
OXD-substituted polymers give purple to blue emission
colors, and the EL emission peaks are between 406 and
452 nm (Table 5). The EL emission peaks, general
speaking, are well matched with the corresponding PL
emission peaks of the thin films (Table 4). This indicates
that the similar radiative excited states are involved in
both EL and PL processes. In contrast to all asym-
metrical OXD-substituted polymers, the emission spec-
tra (Figure 10) of P 1 at different voltages are slightly
voltage-dependent. Since the absorption and fluores-
cence data reveal that P 1 forms excimers due to the
aggregation in the solid film, the EL spectra of P 1 are
red-shifted and become broader by increasing voltage.
Con clu sion
A series of soluble alternating fluorene-based copoly-
mers with OXD pendants were synthesized by the
palladium-catalyzed Suzuki coupling reaction. These
polymers exhibit good thermal stability up to 400 °C
and possess higher glass transition temperatures than

7954 Sung and Lin
Macromolecules, Vol. 37, No. 21, 2004
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(PFPOC6) consisting of the same backbone without OXD
pendants. Their photophysical and electrochemical prop-
erties are not only affected by the OXD pendants but
also affected by the terminal functional groups. Owing
to the large steric hindrance of OXD pendants, the
aggreagation of these polymers in solid state is reduced,
which results in almost identical PL emissions in both
solution and solid states. The incorporation of OXD
pendants into polymer backbone leads to mitigating the
interchain interaction and improving the electron af-
finity and charge-injection properties effectively. Since
only one emission peak is observed in both PL and EL
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energy transfer from the OXD pendants to the conju-
gated polymer backbones has occurred, thus eliminating
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substituted polymers possess better properties (higher
quantum yield and lower aggregation in the solid state)
than the symmetrical OXD-substituted polymer. The
symmetrical OXD-substituted polymer (P 1) has a longer
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improvement of the coplanarity between the polymer
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the introduction of two electron-withdrawing OXD
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