Synthesis, characterization, and electronic and optical properties of donor-acceptor conjugated polymers based on alternating bis(3-alkylthiophene) and pyridine moieties

Article abstract of DOI:10.1021/ma021082+

A novel series of conjugated polymers comprising alternating π-excessive bis(3-alkylthiophene) and π-deficient 2,5-pyridine (or 2,6- Or 3,5-pyridine) moieties were synthesized using a Stille coupling approach and characterized by FT-IR, NMR, UV-vis, fluorescence spectroscopy, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), electrochemical analysis, and X-ray photoelectron spectroscopy (XPS). The derived polymers were highly soluble in common organic solvents and trifloroacetic acid. The electronic and optical properties of these polymers were closely related to the polymer backbone structures. Polymers containing 2,6-/3,5-pyridine moieties exhibited blue shifts in UV and fluorescent spectra relative to those containing 2,5-pyridine analogues, and polymers containing 3,5-pyridine units depicted the highest thermal stability, UV absorption energy, fluorescence energy, and reduction potentials. The electrochemical behavior of these new polymers showed facile n-doping and p-doping properties. The lower reduction potential of these polymers implied their good electron-transporting and easy electron-injection properties due to the presence of the electron-withdrawing pyridine unit. The optical properties of the polymers in trifloroacetic acid were also investigated. XPS studies performed on both neutral and doped states implied the formation of charge carriers in the polymer backbones when doped.

Full text of DOI:10.1021/ma021082+

Macromolecules 2003, 36, 1543-1552
1543
Synthesis, Characterization, and Electronic and Optical Properties of
Donor-Acceptor Conjugated Polymers Based on Alternating
Bis(3-alkylthiophene) and Pyridine Moieties
Hon g-F a n g Lu , Ha r d y S. O. Ch a n , a n d Siu -Ch oon Ng*
Department of Chemistry, National University of Singapore, Singapore 119260, Singapore
Received J uly 11, 2002; Revised Manuscript Received October 21, 2002
ABSTRACT: A novel series of conjugated polymers comprising alternating π-excessive bis(3-alkyl-
thiophene) and π-deficient 2,5-pyridine (or 2,6- or 3,5-pyridine) moieties were synthesized using a Stille
coupling approach and characterized by FT-IR, NMR, UV-vis, fluorescence spectroscopy, gel permeation
chromatography (GPC), thermogravimetric analysis (TGA), electrochemical analysis, and X-ray photo-
electron spectroscopy (XPS). The derived polymers were highly soluble in common organic solvents and
trifloroacetic acid. The electronic and optical properties of these polymers were closely related to the
polymer backbone structures. Polymers containing 2,6-/3,5-pyridine moieties exhibited blue shifts in UV
and fluorescent spectra relative to those containing 2,5-pyridine analogues, and polymers containing 3,5-
pyridine units depicted the highest thermal stability, UV absorption energy, fluorescence energy, and
reduction potentials. The electrochemical behavior of these new polymers showed facile n-doping and
p-doping properties. The lower reduction potential of these polymers implied their good electron-
transporting and easy electron-injection properties due to the presence of the electron-withdrawing
pyridine unit. The optical properties of the polymers in trifloroacetic acid were also investigated. XPS
studies performed on both neutral and doped states implied the formation of charge carriers in the polymer
backbones when doped.
In tr od u ction
erated systems,21-23 little work has been done on the
syntheses of these polymers using chemically grown
polymers that are soluble and processable. Meanwhile,
their physical properties such as thermal and optical
properties have not been reported.
In recent years, many donor-acceptor conjugated
polymers have been studied showing unique optoelec-
tronic properties with potential applicability in devices.
Thus, CN-PPVs in which poly(phenylvinylene)s were
functionalized with flexible alkyl/alkoxy and CN side
chains depicted high electron affinities and electron-
transporting properties, depicting interesting properties
in optical absorption,1 electronic structure,2 photoin-
duced absorption, excitation spectra,3 luminescence
efficiency,4 characteristics of LEDs,5-7 and photodiodes.8
The presence of donor and acceptor functionalities in
the polythienylene backbone has been reported to result
in the formation of highly localized polarons.9-11 In
addition, conjugated polymers incorporating alternating
π-excessive and π-deficient aromatic moieties depicted
significant decrease in their optical band gaps due to
the introduction of intramolecular charge-transfer (ICT)
structures and showed large third-order nonlinear opti-
cal susceptibility and interesting electrochemical proper-
ties.12-18 These desirable properties afford the necessary
motivation for further work toward the preparation of
novel donor-acceptor materials.
Our research group has previously reported on the
syntheses and characterization of the ABA type poly-
mers poly[bis(3-alkylthienylenephenylene)] (PBTBCx)
based on electron-donating backbone structure.19,20 In
the work reported herein, we introduce an electron-
accepting pyridinyl moiety in place of the phenylene unit
in the polymer backbone with a view to investigate the
properties of these novel donor-acceptor conjugated
polymers. Although the polymers with this backbone
structure have been reported in electrochemically gen-
The reason pyridine is chosen as the electron-deficient
moiety arises from its electron-deficient and acid-
sensitive properties, which has been demonstrated to
afford tunable electronic and optical properties in the
derived polymers reported by others21,26 and our
group.24,25 However, possibly arising from the introduc-
tion of the pyridinyl moiety between the functionalized
thiophene rings, the widely used ferric chloride oxida-
tion approach for polythiophene derivatives27 could not
achieve high molecular polymers. Here, we have used
the Stille coupling approach for polymerization. We have
also incorporated meta-linked pyridinyl moieties into
polymer backbone (Scheme 1) for the purpose of inves-
tigating the structure-property correlation studies.
Exp er im en ta l Section
Ma ter ia ls. The following reagents, 3-bromothiophene (Flu-
ka, >95%), N-bromosuccinimide (Merck, 99%), Ni(dppp)Cl2
(Fluka), 2,5-dibromopyridine (Fluka), 2,6-dibromopyridine
(Fluka), 3,5-dibromopyridine (Fluka), 1-bromoalkane (Fluka),
n-butyllithium (Merck, 1.6 M in hexane) and iodine (Fisher,
99%), were purchased from commercial sources and used
without further purification, except Bu4NBF4 (TCI, for elec-
trochemistry) which was dried under high vacuum before used.
Chloroform (J .T. Baker) and acetonitrile (J .T. Baker) were
reagent grade solvents and freshly distilled over calcium
hydride. Tetrahydrofuran (J .T. Baker) and diethyl ether (J .T.
Baker) were refluxed over metallic sodium wire/benzophenone
for 4 h and distilled under a nitrogen atmosphere before used.
3-n-Hexylthiophene, 3-n-octylthiophene, 3-n-dodecylthiophene,
2-bromo-3-n-hexylthiophene, 2-bromo-3-n-octylthiophene, and
2-bromo-3-n-dodecylthiophene were synthesized according to
the literature.28
* To whom correspondence should be addressed: Tel +65-6874-
2922; Fax +65-6779-1691; e-mail chmngsc@leonis.nus.edu.sg.
10.1021/ma021082+ CCC: $25.00 © 2003 American Chemical Society
Published on Web 02/08/2003

1544 Lu et al.
Macromolecules, Vol. 36, No. 5, 2003
Sch em e 1. Str u ctu r es of Novel Con ju ga ted P olym er s
In str u m en ta tion . Elemental analyses, mass spectra (MS),
FTIR spectra,1H NMR and 13H NMR spectra, UV and fluo-
rescence spectra, scanning electron micrographs (SEM), ther-
mogravimetric analysis (TGA), gel permeation chromatogra-
phy analysis (GPC), and cyclic voltammetry (CV) were
performed as previously reported.25 Conductivity measure-
ments were carried out on polymer pellets of known thickness
using a four-point probe connected to a Keithley constant
current source. XPS measurement of polymer powders was
performed by means of a VG ESCA/SIMLAB MMKII with a
Mg KR radiation source (1253.6 eV). The binding energies were
corrected for surface charging by referencing to the designated
hydrocarbon C(1s) binding energy as 284.6 eV. Spectrum
deconvolutions were carried out using the Gaussian component
with the same full width at half-maximum (fwhm) for each
component in a particular spectrum.
n-dodecylthiophene was used instead of 2-bromo-3-n-hexyl-
thiophene. Yield: 55%; mp: 42-44 °C. 1H NMR (CDCl3,
ppm): δ ) 8.70 (1H, d), 7.73 (1H, dd, J ) 2.0 Hz), 7.57 (1H, d,
J ) 8.0 Hz), 7.30 (2H, t, J ) 5.2 Hz), 7.01 (2H, dd, J ) 5.2
Hz), 2.93 (2H, t, J ) 7.8 Hz), 2.68 (2H, t, J ) 7.8 Hz), 1.64
(4H, m), 1.25 (36H, m), 0.88 (6H, t, J ) 6.4 Hz). 13C NMR: δ
151.90, 149.22, 141.07, 139.94, 136.72, 133.54, 130.67, 129.70,
128.38, 125.78, 124.56, 120.98, 31.80-13.97 (aliphatic Cs). FT-
IR: 3057, 2919, 2850, 1590, 1556, 1466, 1435, 1384, 1354,
1292, 1229, 1084, 1027, 987, 930, 884, 843, 734, 693, 653. HR-
MS:
C37H57NS2, Calcd: 579.3933; Found: 579.3951. Anal.
Calcd: C, 76.64; H, 9.91; N, 2.42; S, 11.06. Found: C, 76.25;
H, 10.24; N, 2.55; S, 12.22.
2,6-Bis(3′-n -d od ecyl-2′-th ien yl)p yr id in e (6). The proce-
dure of synthesis 5a was repeated except that 2-bromo-3-n-
dodecylthiophene and 2,6-dibromopyridine were used instead
of 2-bromo-3-n-hexylthiophene and 2,5-dibromopyridine, re-
spectively. Yield: 65%; mp: 39-40 °C. 1H NMR (CDCl3,
ppm): δ ) 7.70 (1H, t, J ) 7.8 Hz), 7.40 (2H, d, J ) 8.0 Hz),
7.29 (2H, d, J ) 5.2 Hz), 6.99 (2H, d, J ) 4.8 Hz), 3.00 (4H, t,
J ) 7.6 Hz), 1.69 (4H, m), 1.25 (36H, m), 0.88 (6H, t, J ) 6.5
Hz). 13C NMR: δ 152.99, 141.19, 137.63, 136.87, 130.71,
125.53, 118.90, 31.83-14.03 (aliphatic Cs). FT-IR: 2953, 2918,
2851, 1564, 1534, 1465, 1451, 1437, 1376, 1330, 1277, 1226,
1167, 1126, 944, 865, 832, 794, 755, 714, 671, 652, 638. HR-
Syn th esis. 2,5-Bis(3′-n -h exyl-2′-th ien yl)p yr id in e (5a ).
2-Bromo-3-n-hexylthiophene (6.05 g, 30 mmol) was slowly
added to a suspension of magnesium turnings (0.84 g, 36
mmol) in 20 mL of dry THF at room temperature. The mixture
was heated to reflux for 2 h. The produced Grignard reagent
was transferred to a mixture of 2,5-dibromopyridine (3.54 g,
15 mmol) and Ni(dppp)Cl2 (0.162 g, 0.3 mmol) suspended in
20 mL of THF at 0 °C dropwise. After the addition was
finished, the mixture was allowed to warm to room tempera-
ture and heated to reflux for 24 h. The reaction mixture was
quenched by ice water. The aqueous solution was neutralized
by dilute hydrochloric acid and then extracted with diethyl
ether (15 mL × 3). The organic layers were combined and
washed with water and dried over magnesium sulfate. After
filtration and concentration, the product was separated by
column chromatography (silica gel, hexane and dichlorometh-
MS:
C37H57NS2, Calcd: 579.3933; Found: 579.3937. Anal.
Calcd: C, 76.64; H, 9.91; N, 2.42; S, 11.06. Found: C, 76.17;
H, 10.72; N, 3.04; S, 11.38.
3,5-Bis(3′-n -d od ecyl-2′-th ien yl)p yr id in e (7). The proce-
dure of synthesis of 6 was repeated, except that 2-bromo-3-n-
dodecylthiophene and 3,5-dibromopyridine were used instead
of 2-bromo-3-n-hexylthiophene and 2,5-dibromopyridine, re-
1
1
ylene as eluting solvent). The yield of 5a was 50%. H NMR
spectively. Yield: 64%. H NMR (CDCl3, ppm): δ ) 8.64 (2H,
(CDCl3, ppm): δ ) 8.70 (1H, d), 7.73 (1H, dd, J ) 2.0 Hz),
7.57 (1H, d, J ) 8.0 Hz), 7.30 (2H, t, J ) 5.2 Hz), 7.02 (2H, d,
J ) 5.3 Hz), 2.94 (2H, t, J ) 7.8 Hz), 2.70 (2H, t, J ) 7.8 Hz),
1.64 (4H, m), 1.32 (12H, m), 0.87 (6H, t, J ) 6.4 Hz). 13C
NMR: δ 151.94, 149.30, 140.97, 139.91, 136.68, 133.60, 130.70,
129.71, 128.34, 125.75, 124.58, 120.94, 31.76-14.00 (aliphatic
Cs). FT-IR: 3108, 3068, 2959, 2926, 2855, 1590, 1556, 1468,
1435, 1384, 1361, 1292, 1229, 1090, 1028, 987, 964, 925, 889,
839, 768, 724, 693, 654. HR-MS: C25H33NS2, Calcd: 411.2054;
Found: 411.2034. Anal. Calcd: C, 72.96; H, 8.08; N, 3.40; S,
15.58. Found: C, 73.00; H, 8.76; N, 3.20; S, 15.28.
2,5-Bis(3′-n -octyl-2′-th ien yl)p yr id in e (5b). The procedure
of synthesis 5a was repeated except that 2-bromo-3-n-octyl-
thiophene was used instead of 2-bromo-3-n-hexylthiophene.
Yield: 52%. 1H NMR (CDCl3, ppm): δ ) 8.70 (1H, d), 7.73
(1H, dd, J ) 2.4 Hz), 7.57 (1H, d, J ) 8.0 Hz), 7.30 (2H, t, J )
5.2 Hz), 7.01 (2H, dd, J ) 5.2 Hz), 2.92 (2H, t, J ) 7.8 Hz),
2.68 (2H, t, J ) 7.8 Hz), 1.67 (4H, m), 1.26 (20H, m), 0.87 (6H,
t, J ) 6.4 Hz). 13C NMR: δ 151.96, 149.32, 140.97, 139.91,
136.68, 133.59, 130.70, 129.72, 128.35, 125.78, 124.58, 120.94,
31.80-14.00 (aliphatic Cs). FT-IR: 3108, 3068, 2959, 2924,
2854, 1590, 1557, 1476, 1435, 1378, 1366, 1292, 1229, 1153,
1096, 1028, 987, 964, 930, 884, 839, 768, 722, 693, 659. HR-
s), 7.81 (1H, d), 7.34 (2H, d, J ) 4.8 Hz), 7.04 (2H, d, J ) 5.6
Hz), 2.70 (4H, t, J ) 7.6 Hz), 1.60 (4H, m), 1.24 (36H, m), 0.88
(6H, t, J ) 6.4 Hz). HR-MS: C37H57NS2, Calcd: 579.3932;
Found: 579.3941. Anal. Calcd: C, 76.64; H, 9.91; N, 2.42; S,
11.06. Found: C, 76.23; H, 10.21; N, 2.83; S, 11.54.
2,5-Bis(3′-n -h exyl-5′-iod o-2′-th ien yl)p yr id in e (8a ).29 2,5-
Bis(3′-n-hexyl-2′-thienyl)pyridine (0.411 g, 1 mmol) was added
to a two-neck 50 mL flask fitted with a magnetic stirrer and
capped with a rubber septum. The system was vacuumed and
flushed with nitrogen successively. Then, 15 mL of anhydrous
ether was added via a syringe. The solution was cooled to -70
°C, and n-butyllithium (1.6 M in hexane) (1.31 mL, 2.10 mmol)
was added dropwise by a syringe. The mixture was stirred at
-70 °C for half an hour and then warmed to room temperature
for a further hour and then cooled back to -70 °C; iodine
solution (0.56 g, 2.19 mmol of I2 in 20 mL of anhydrous ether)
was added. After addition, the reaction mixture was allowed
to warm to room temperature and kept for half an hour. The
reaction was quenched by ice water. The aqueous solution was
extracted with diethyl ether (15 mL × 3). The combined
organic layers were washed with water and dried over
magnesium sulfate. After filtration and concentration, the
product was separated by column chromatography (silica gel,
hexane as eluting solvent). Pure 8a (0.55 g, 83% yield) is
yellowish crystals; mp 37-38 °C. 1H NMR (CDCl3, ppm): δ )
8.59 (1H, d), 7.66 (1H, dd, J ) 2.4 Hz), 7.46 (1H, d, J ) 8.0
Hz), 7.15 (2H, d), 2.86 (2H, t, J ) 7.8 Hz), 2.62 (2H, t, J ) 7.8
Hz), 1.63 (4H, m), 1.32 (12H, m), 0.88 (6H, t, J ) 6.4 Hz). 13C
MS:
C29H41NS2, Calcd: 467.2680; Found: 467.2684. Anal.
Calcd: C, 74.48; H, 8.84; N, 3.00; S, 13.71. Found: C, 74.49;
H, 8.94; N, 3.01; S, 12.78.
2,5-Bis(3′-n -d od ecyl-2′-th ien yl)p yr id in e (5c). The pro-
cedure of synthesis 5a was repeated except that 2-bromo-3-

Macromolecules, Vol. 36, No. 5, 2003
Donor-Acceptor Conjugated Polymers 1545
NMR: δ 151.21, 149.07, 143.47, 142.70, 141.97, 140.54, 139.47,
136.61, 127.54, 120.41, 75.59, 72.59, 31.52-14.00 (aliphatic
Cs). FT-IR: 3010, 2954, 2920, 2846, 1652, 1583, 1553, 1517,
1479, 1468, 1415, 1377, 1279, 1225, 1162, 1109, 1025, 992, 961,
The separated organic phase was dried over anhydrous
magnesium sulfate. The solvent was evaporated by vacuum
distillation, and a yellow viscous liquid was obtained. The yield
1
was 87%. H NMR (CDCl3, ppm): δ ) 8.71 (1H, d), 7.72 (1H,
916, 827, 750, 723, 701, 618. HR-MS:
C
25H31NS2I2, Calcd:
dd, J ) 2.2 Hz), 7.54 (1H, d, J ) 8.4 Hz), 7.02 (2H, d), 2.93
(2H, t, J ) 7.8 Hz), 2.70 (2H, t, J ) 7.8 Hz), 1.60-0.90 (76H,
m). 13C NMR: δ 151.80, 148.97, 142.85, 141.68, 140.81, 139.28,
138.33, 137.91, 136.27, 128.42, 120.61, 31.81-10.68 (aliphatic
Cs). FT-IR: 3036, 2956, 2924, 2853, 1663, 1589, 1554, 1531,
1459, 1418, 1377, 1340, 1292, 1244, 1178, 1152, 1073, 1023,
960, 930, 875, 839, 767, 692, 663, 598, 504. Anal. Calcd for
662.9987; Found: 662.9984. Anal. Calcd: C, 45.25; H, 4.70;
N, 2.11; S, 9.67; I, 38.25. Found: C, 45.22; H, 4.64; N, 2.18; S,
10.35; I, 38.35.
2,5-Bis(3′-n -octyl-5′-iod o-2′-th ien yl)p yr id in e (8b). The
procedure of synthesis 8a was repeated except that 2,5-bis-
(3′-n-octyl-2′-thienyl)pyridine was used instead of 2,5-bis(3′-
n-hexyl-2′-thienyl)pyridine. Yield: 85%; mp: 47.5-49.0 °C. 1H
NMR (CDCl3, ppm): δ ) 8.60 (1H, d), 7.66 (1H, dd, J ) 2.4
Hz), 7.46 (1H, d, J ) 8.0 Hz), 7.13 (2H, d), 2.86 (2H, t, J ) 7.8
Hz), 2.62 (2H, t, J ) 7.8 Hz), 1.63 (4H, m), 1.32 (20H, m), 0.87
(6H, t, J ) 6.4 Hz). 13C NMR: δ 151.22, 149.07, 143.45, 142.70,
141.97, 140.52, 139.45, 136.61, 127.52, 120.41, 75.60, 72.59,
31.82-14.02 (aliphatic Cs). FT-IR: 3025, 2950, 2919, 2847,
1665, 1588, 1549, 1462, 1422, 1374, 1275, 1221, 1188, 1118,
1082, 1032, 995, 927, 907, 883, 837, 765, 747, 723, 706, 675.
C
49H85NS2Sn2: C, 59.46; H, 8.65; N, 1.42; S, 6.48. Found: C,
60.08; H, 8.76; N, 1.57; S, 6.54.
2,5-Bis(3′-n -oct yl-5′-t r ib u t ylst a n n yl-2′-t h ie n yl)p yr i-
d in e (9b). The procedure of synthesis of 9a was repeated
except that 2,5-bis(3′-n-octyl-2′-thienyl)pyridine was used
instead of 2,5-bis(3′-n-hexyl-2′-thienyl)pyridine. Yield: 88%.
1H NMR (CDCl3, ppm): δ ) 8.70 (1H, d), 7.75 (1H, dd, J ) 2.0
Hz), 7.51 (1H, d, J ) 7.9 Hz), 7.02 (2H, d), 2.92 (2H, t, J ) 7.8
Hz), 2,70 (2H, t, J ) 7.8 Hz), 1.60-0.90 (84H, m). 13C NMR:
δ 151.80, 148.97, 142.83, 141.68, 140.81, 139.28, 138.34,
137.91, 136.29, 128.42, 120.62, 31.81-10.68 (aliphatic Cs). FT-
IR: 3036, 2956, 2924, 2853, 1664, 1589, 1555, 1531, 1459,
1418, 1377, 1340, 1292, 1244, 1178, 1152, 1073, 1023, 960, 930,
875, 839, 767, 692, 663, 598, 504. Anal. Calcd for C53H93NS2-
Sn2: C, 60.87; H, 8.96; N, 1.34; S, 6.13. Found: C, 61.12; H,
9.31; N, 1.56; S, 6.50.
2,5-Bis(3′-n -d od ecyl-5′-tr ibu tylsta n n yl-2′-th ien yl)p yr i-
d in e (9c). The procedure of synthesis of 9a was repeated
except that 2,5-bis(3′-n-dodecyl-2′-thienyl)pyridine was used
instead of 2,5-bis(3′-n-hexyl-2′-thienyl)pyridine. Yield: 90%.
1H NMR (CDCl3, ppm): δ ) 8.70 (1H, d), 7.72 (1H, dd, J ) 2.4
Hz), 7.51 (1H, d, J ) 8.4 Hz), 7.02 (2H, d), 2.92 (2H, t, J ) 7.8
Hz), 2.70 (2H, t, J ) 7.8 Hz), 1.60-0.91 (100H, m).13C NMR:
δ 151.80, 148.95, 142.85, 141.68, 140.80, 139.40, 139.28,
138.34, 136.27, 128.42, 120.61, 63.24, 31.52-10.67 (aliphatic
Cs). FT-IR: 3030, 2956, 2924, 2853, 1663, 1589, 1555, 1531,
1464, 1418, 1377, 1340, 1292, 1244, 1179, 1152, 1073, 1018,
960, 930, 872, 840, 767, 692, 663, 599, 504. Anal. Calcd for
HR-MS:
C29H39NS2I2, Calcd: 719.0613; Found: 719.0620.
Anal. Calcd: C, 48.41; H, 5.46; N, 1.95; S, 8.91; I, 35.27.
Found: C, 48.51; H, 5.31; N, 2.10; S, 9.50; I, 33.18.
2,5-Bis(3′-n -dodecyl-5′-iodo-2′-th ien yl)pyr idin e (8c). The
procedure of synthesis of 8a was repeated except that 2,5-bis-
(3′-n-dodecyl-2′-thienyl)pyridine was used instead of 2,5-bis-
(3′-n-hexyl-2′-thienyl)pyridine. Yield: 86%; mp: 60.0-61.0 °C.
1H NMR (CDCl3, ppm): δ ) 8.60 (1H, d), 7.66 (1H, dd, J ) 2.4
Hz), 7.46 (1H, d, J ) 8.4 Hz), 7.15 (2H, d), 2.86 (2H, t, J ) 7.8
Hz), 2.61 (2H, t, J ) 7.8 Hz), 1.60 (4H, m), 1.25 (36H, m), 0.87
(6H, t, J ) 6.4 Hz). 13C NMR: δ 151.22, 149.08, 143.47, 142.73,
141.98, 140.55, 139.53, 139.48, 136.54, 127.55, 120.40, 75.60,
72.59, 31.82-14.02 (aliphatic Cs). FT-IR: 2920, 2848, 1651,
1557, 1461, 1424, 1373, 1030, 832, 723, 673. HR-MS: C37H55
-
NS2I2, Calcd: 831.1865; Found: 831.1853. Anal. Calcd: C,
53.43; H, 6.67; N, 1.68; S, 7.71; I, 30.51. Found: C, 53.79; H,
6.72; N, 1.56; S, 8.19; I, 30.08.
2,6-Bis(3′-n -dodecyl-5′-iodo-2′-th ien yl)pyr idin e (10). The
procedure of synthesis of 8a was repeated except that 2,6-bis-
(3′-n-dodecyl-2′-thienyl)pyridine was used instead of 2,5-bis-
(3′-n-hexyl-2′-thienyl)pyridine. Yield: 83%; mp: 34.0-36.0 °C.
1H NMR (CDCl3, ppm): δ ) 7.69 (1H, t, J ) 7.8 Hz), 7.28 (2H,
dd, J ) 8.0 Hz), 7.11 (2H, s), 2.88 (4H, t, J ) 7.8 Hz), 1.65
(4H, m), 1.25 (36H, m), 0.88 (6H, t, J ) 6.4 Hz). 13C NMR: δ
151.93, 142.79, 140.57, 137.10, 118.53, 75.42, 31.82-14.01
(aliphatic Cs). FT-IR: 2958, 2922, 2849, 1563, 1532, 1457,
1427, 1268, 1170, 1068, 825, 795, 726, 699, 664, 618. HR-MS:
C
61H109NS2Sn2: C, 63.27; H, 9.49; N, 1.21; S, 5.54. Found: C,
63.85; H, 9.58; N, 1.33; S, 5.43.
Gen er a l P r oced u r e for P olym er iza tion . 2,5-Bis(3′-n-
alkyl-5′-tributylstannyl-2′-thienyl)pyridine (1.5 mmol), 2,5-bis-
(3′-n-alkyl-5′-iodo-2′-thienyl)pyridine (1.5 mmol), and tetrakis-
(triphenylphosphino)palladium(0) (0.03 mmol) were placed in
a 100 mL two-neck, round-bottomed flask and deareated with
nitrogen. Thereafter, THF (60 mL) was syringed into the
reaction flask. This reaction mixture was stirred and refluxed
for 5 days. The reaction mixture was concentrated to 10 mL
by vacuum distillation and poured into 400 mL of methanol.
The precipitated polymer was collected by filtration and
further purified by redissolving in chloroform and precipitating
into methanol. The resulting polymer was washed with
methanol for 24 h and with acetone for 12 h and finally
extracted with chloroform through a Soxhlet apparatus. The
polymer was obtained by concentrating the solution and
subsequent precipitation into methanol.
C
37H55NS2I2, Calcd: 831.1865; Found: 831.1847. Anal. Calcd:
C, 53.43; H, 6.67; N, 1.68; S, 7.71; I, 30.51. Found: C, 53.68;
H, 6.90; N, 1.85; S,8.01; I, 29.89.
3,5-Bis(3′-n -dodecyl-5′-iodo-2′-th ien yl)pyr idin e (11). The
procedure of synthesis of 6a was repeated except that 3,5-bis-
(3′-n-dodecyl-2′-thienyl)pyridine was used instead of 2,5-bis-
(3′-n-hexyl-2′-thienyl)pyridine. Yield: 80%; mp: 43.0-45.5 °C.
1H NMR (CDCl3, ppm): δ ) 8.58 (2H, s), 7.64 (1H, s), 7.16
(2H, s), 2.61 (4H, t, J ) 7.8 Hz), 1.55 (4H, m), 1.25 (36H, m),
0.88 (6H, t, J ) 6.4 Hz). FT-IR: 3040, 2954, 2916, 2849, 1589,
1541, 1468, 1403, 1374, 1312, 1276, 1216, 1154, 1077, 1024,
923, 882, 837, 766, 718, 691, 672, 638. HR-MS: C37H55NS2I2,
Calcd: 831.1865; Found: 831.1847. Anal. Calcd: C, 53.43; H,
6.67; N, 1.68; S, 7.71; I, 30.51. Found: C, 53.67; H, 6.68; N,
1.63; S, 8.36; I, 30.18.
2,5-Bis(3′-n -h exyl-5′-t r ib u t ylst a n n yl-2′-t h ien yl)p yr i-
d in e (9a ). 2,5-Bis(3′-n-hexyl-2′-thienyl)pyridine (0.411 g, 1
mmol) was dissolved in 15 mL of dry anhydrous ether in a 50
mL flask, which was fitted with a magnetic stirrer and rubber
septum under a N2 atmosphere. n-Butyllithium (1.6 M in
hexane) (1.31 mL, 2.10 mmol) was added via a syringe at -70
°C. After the mixture was stirred for 30 min and then warmed
to room temperature for 30 min, the resulting solution gradu-
ally became an orange suspension. The mixture was then
cooled back to -70 °C, and tributyltin chloride (0.683 g, 2.10
mmol) was added. The solution was gradually warmed to room
temperature, stirred for 1 h, and then hydrolyzed by ice water.
P oly[(3′-n -h exylth iop h en e-2′,5′-d iyl)(p yr id in e-2,5-d iyl)-
(3′-n -h exylth iop h en e-2′,5′-d iyl)] (12a ). Yield: 68%. 1H NMR
(CDCl3, ppm): δ 8.73 (br, pyridinyl proton, 1H), 7.76 (br peaks,
pyridinyl proton, 1H), 7.61 (br peaks, pyridinyl proton, 1H),
7.17 (br peaks, thiophenyl proton, 2H), 2.93 (br, -thiophenyl-
CH2-, 2H), 2.69 (br, -thiophenyl-CH2-, 2H), 1.68-0.89 (m,
-(CH2)4CH3). FT-IR: 2922, 2854, 1700, 1651, 1558, 1541, 1459,
1370, 1367, 1270, 1034, 920, 824, 668. Anal. Calcd for (C25H31
-
NS2)n: C, 73.38; H, 7.63; N, 3.42; S, 15.65. Found: C, 69.34;
H, 8.19; N, 3.42; S, 13.37; I, 1.50.
P oly[(3′-n -octylth iop h en e-2′,5′-d iyl)(p yr id in e-2,5-d iyl)-
(3′-n -octylth iop h en e-2′,5′-d iyl)] (12b). Yield: 70%. 1H NMR
(CDCl3, ppm): δ 8.73 (br, pyridinyl proton, 1H), 7.76 (br peaks,
pyridinyl proton, 1H), 7.61 (br peaks, pyridinyl proton, 1H),
7.16 (br peaks, thiophenyl proton, 2H), 2.92 (br, -thiophenyl-
CH2-, 2H), 2.69 (br, -thiophenyl-CH2-, 2H), 1.72-0.87 (m,
-(CH2)6CH3). FT-IR: 2921, 2852, 1700, 1651, 1558, 1548, 1463,

1546 Lu et al.
Macromolecules, Vol. 36, No. 5, 2003
Sch em e 2. Syn th etic Sch em e to Mon om er s 5, 6,
a n d 7a
Sch em e 3. Syn th etic Ap p r oa ch to P olym er s w ith
2,5-P yr id in yl Lin k a gesa
a
Reagents and conditions: (i) RMgBr, Et2O, Ni(dppp)Cl2;
(ii) NBS/AcOH/CHCl3; (iii) Mg, Et2O; (iv) 2,5-dibromopyridine,
Ni(dppp)Cl2; (v) 2,6-dibromopyridine, Ni(dppp)Cl2; (vi) 3,5-
dibromopyridine, Ni(dppp)Cl2.
a
Reagents and conditions: (vii) nBuLi, Et2O, and then I2;
n
(viii) BuLi, Et2O, Bu3SnCl, -70 °C; (ix) Pd(PPh3)4/THF/∆.
1371, 1270, 1023, 921, 824, 716, 663. Anal. Calcd for (C29H39
-
NS2)n: C, 74.78; H, 8.44; N, 3.01; S, 13.77. Found: C, 71.66;
H, 8.32; N, 2.70; S, 12.45; I, 3.37.
9a -c and 8a -c, 10, and 11, respectively (Scheme 3 and
Scheme 4).
P oly[(3′-n -d od ecylt h iop h en e-2′,5′-d iyl)(p yr id in e-2,5-
d iyl)(3′-n -d od ecylth iop h en e-2′,5′-d iyl)] (12c). Yield: 65%.
1H NMR (CDCl3, ppm): δ 8.73 (br, pyridinyl proton, 1H), 7.76
(br peaks, pyridinyl proton, 1H), 7.60 (br peaks, pyridinyl
proton, 1H), 7.16 (br peaks, thiophenyl proton, 2H), 2.91 (br,
-thiophenyl-CH2-, 2H), 2.69 (br, -thiophenyl-CH2-, 2H),
1.68-0.87 (m, -(CH2)10CH3). FT-IR: 2919, 2850, 1650, 1548,
1467, 1371, 1271, 1217, 1071, 1023, 921, 825, 717, 663. Anal.
Calcd for (C37H55NS2)n: C, 76.89; H, 9.59; N, 2.42; S, 11.09.
Found: C, 71.96; H, 8.38; N, 1.78; S, 10.11; I, 3.55.
P oly[(3′-n -d od ecylt h iop h en e-2′,5′-d iyl)(p yr id in e-2,5-
d iyl)(3′-n -d od ecylth iop h en e-2′,5′-d iyl)-co-a lt-(3′-n -d od e-
cylt h iop h en e-2′,5′-d iyl)(p yr id in e-2,6-d iyl)(3′-n -d od ecyl-
th iop h en e-2′,5′-d iyl)] (13). The procedure of synthesis 12c
was repeated except that 2,6-bis(3′-n-dodecyl-5′-iodo-2′-thie-
nyl)pyridine (10) was used instead of 2,5-bis(3′-n-dodecyl-5′-
iodo-2′-thienyl)pyridine (8c). Yield: 70%. 1H NMR (CDCl3,
ppm): δ 8.73, 7.71, 7.60, 7.40, 7.15, 7.10, 2.93, 2.69, 1.68-
0.89. FT-IR: 2923, 2852, 1652, 1559, 1457, 1357, 1158, 829,
716, 663. Anal. Calcd for (C37H55NS2)n: C, 76.89; H, 9.59; N,
2.42; S, 11.09. Found: C, 72.71; H, 9.42; N, 3.23; S, 11.33; I,
2.25.
P oly[(3′-n -d od ecylt h iop h en e-2′,5′-d iyl)(p yr id in e-2,5-
d iyl)(3′-n -d od ecylth iop h en e-2′,5′-d iyl)]-co-a lt-(3′-n -d od e-
cylt h iop h en e-2′,5′-d iyl)(p yr id in e-3,5-d iyl)(3′-n -d od ecyl-
th iop h en e-2′,5′-d iyl)] (14). The procedure of synthesis of 12c
was repeated, except that 3,5-bis(3′-n-dodecyl-5′-iodo-2′-thie-
nyl)pyridine (11) was used instead of 2,5-bis(3′-n-dodecyl-5′-
iodo-2′-thienyl)pyridine (8c). Yield: 75%. 1H NMR (CDCl3,
ppm): δ 8.73, 8.67, 7.84, 7.79, 7.61, 7.20, 7.11, 2.93, 2.69, 1.68-
0.89. FT-IR: 2923, 2852, 1652, 1558, 1541, 1460, 1373, 1021,
828, 716. Anal. Calcd for (C37H55NS2)n: C, 76.89; H, 9.59; N,
2.42; S, 11.09. Found: C, 76.32; H, 9.94; N, 2.35; S, 9.80; I,
0.35.
Compounds 5-11 showed different physical proper-
ties due to the different substituents or backbones. 5a
and 5b were orange viscous liquids with strong fluo-
rescence under UV light. 7 was a brown liquid with
weak fluorescence. The other two compounds 5c and 6
were light yellow solids. Compounds 9a -c were light
yellow viscous liquids, and all diiodo-substituted com-
pounds (8a -c, 10, and 11) were yellow crystals. The
structure and purity of these monomers were confirmed
by NMR (1H and 13C), HR-MS, and elemental analyses
as reported in the Experimental Section.
Polymerization was carried out in THF in the pres-
ence of catalytic amounts (ca. 2 mol %) of Pd(PPh3)4
under a nitrogen atmosphere. When the reaction ended,
the polymer was precipitated into methanol and sub-
jected to Soxhlet extractions to eliminate oligomers.
Purification was carried out by dissolving polymer in
CHCl3 and then reprecipitation in methanol. All poly-
merizations were carried out smoothly with good yields.
The results are summarized in Table 1.
P olym er P h ysica l P r op er ties. The derived poly-
mers with pyridinyl moieties having meta vs para
linkages depicted different physical properties. 12a -c
with p-pyridinyl linkages were red powders, while 13
and 14 with meta-linkages were red resins. All polymers
have good solubility in organic solvents such as tetrahy-
drofuran, dichloromethylene, and chloroform as well as
trifluoroacetic acid and formic acid.
The structures of the derived polymers were charac-
terized by spectroscopic methods. The spectral assign-
ments clearly corroborated the proposed structure. For
polymers 12a -c, similar characteristic chemical shifts
Resu lts a n d Discu ssion
1
1
Syn th esis of th e P olym er s. The synthesis of bis-
(2-thienyl)pyridines (5a -c, 6, and 7) was achieved using
Grignard coupling reactions, as shown in Scheme 2.
2-Bromo-3-n-alkylthiophenes (3) were reacted with
magnesium to afford the corresponding Grignard re-
agents, which were then coupled with dibromopyridine
in the presence of a catalytic amount of Ni(dppp)Cl2 to
afford compounds 5-7. Then, 5-7 were reacted with
n-butyllithium at -70 °C, followed by treatment with
tributyltin chloride or iodination to give compounds
were observed in H NMR spectra. Figure 1 showed H
NMR spectra of the representative polymer 12b in
comparison with monomer 5b. The chemical shifts of
the pyridinyl protons of 12b were manifested at δ 8.73,
7.76, and 7.61 ppm while the Câ-proton of the thiophene
ring appeared at 7.16 ppm. The remaining resonance
at δ 2.92, 2.69, and 1.72-0.87 ppm can be correlated to
the octyl pendant chains on thienylene unit. Polymers
13 and 14 containing para- and meta-linked pyridinyl
moieties depicted more complicate NMR spectra com-

Macromolecules, Vol. 36, No. 5, 2003
Donor-Acceptor Conjugated Polymers 1547
Sch em e 4. Syn th etic Ap p r oa ch to P olym er s w ith m -P yr id in yl Lin k a gesa
a
n
n
Reagents and conditions: (vii) BuLi, Et2O, and then I2; (viii) BuLi, Et2O, Bu3SnCl, -70 °C; (ix) Pd(PPh3)4/THF/∆.
Ta ble 1. Syn th etic Yield a n d P h ysica l P r op er ties of P olym er s
GPC results
conductivity/S cm-1
polymer
yield (%)
Mn × 10-3
Mw × 10-3
PDI
na
I2-doped
FeCl3-doped
12a
12b
12c
13
68
70
65
70
75
5.4
6.7
5.7
10.6
12.7
8.6
12.0
7.8
15.4
21.6
1.59
1.80
1.36
1.46
1.71
39
45
27
54
66
8.75 × 10-5
3.16 × 10-4
2.79 × 10-5
8.26 × 10-5
8.42 × 10-6
b
b
b
b
14
a
b
Number of aromatic rings. The pellets of the polymers are difficult to compress.
F igu r e 2. IR spectra of polymer 12c: (a) the neutral polymer;
(b) FeCl3-doped polymer; (c) I2-doped polymer.
length. This correlated to the observation in the study
20
of PBTBCx and poly(alkylthiophene)s.30
F igu r e 1. 1H NMR spectra of the representative polymer 12b
in comparison to compound 5b.
The doped polymers showed a dramatic change in
their FTIR spectra (Figure 2). The stretching vibrations
in the region of 1000-1500 cm-1 became broad due to
the positively charged state of the polymer backbone.
The relative intensities of the stretchings are all greatly
reduced because of “the rise of the baseline” as a result
of the enhanced absorption in IR region due to the
formation of polarons and bipolarons.23
pared to 12c due to the different chemical environments
of the protons on the thienylene and pyridine units
within the polymer backbone. FT-IR spectra of the
polymers are consistent with the expected structures.
Elemental analyses of the novel polymers are within
range of the expected formula.
Th er m a l P r op er ties of th e P olym er s. The polymer
thermal stability was evaluated using TGA under either
a nitrogen or air atmosphere. Polymers 12a -c showed
similar thermal stability, as minimal weight change
(<10%) was observed on heating to 420 °C in air and a
nitrogen atmosphere. In a nitrogen atmosphere, an
apparently one-step degradation mainly attributable to
the cleavage of pendant group and partial decomposition
of backbone was observed. The amount of residues
decrease with increasing alkyl chain length, which is
consistent with degradation of pendant chains of in-
creasing bulkiness. The degradation of the polymers in
air depicted a two-step decomposition pattern, corre-
sponding to the cleavage of the alkyl chain and the
subsequent degradation of the polymer chain.
The molecular weights of polymers were determined
by GPC using THFas eluant and polystyrene as stan-
dard. The results are summarized in Table 1. The novel
polymers had a number-average weight (Mn) ranging
from 5.4 to 12.7 kDa with polydispersity (PDI) from 1.36
to 1.80, corresponding to the presence of 27-66 aromatic
rings. Meta-linked polymers 13 and 14 showed higher
molecular weights compared to para-linked ones.
When doped with I2 and FeCl3, the derived polymers
were electrically conductive. The conductivity of the
polymers 12a -c in their doped states were of the order
of 10-6-10-4 S cm-1, which is lower than that of ABA
type PBTBCx.19,20 All measurement results are listed
in Table 1. It is apparent that the conductivities of the
polymers decrease with increasing substituted chain

1548 Lu et al.
Macromolecules, Vol. 36, No. 5, 2003
of the bonds connecting the neighboring units due to
accentuated steric repulsion.33 Interestingly, the poly-
mers depicted nearly identical emission peaks at ca. 548
nm (greenish-yellow) in CF3COOH solution although
they have different backbones and different UV absorp-
tion maxima.
The excitation spectra of polymers were obviously
different when using CHCl3 and CF3COOH as solvents
(see Table 2). In CHCl3 solution, the excitation curves
of all polymers are closely to their UV absorption curves,
as often observed in polymers soluble in common organic
solution. However, when emitted at 548 nm in CF3-
COOH solution, the excitation maxima of all polymers
were shifted to longer wavelength. Figure 4 showed the
UV-vis absorption, excitation, and emission curves of
polymers 12c, 13, and 14 in CF3COOH solutions.
F igu r e 3. UV-vis absorption and fluorescence spectra of the
polymers with different linked backbones in CHCl3 solution.
Optical Properties in Film Phase. All novel polymers
are easily spin-coated onto glass substrates to afford
highly transparent, pinhole-free films. The film of
polymers 12a -c showed π-π* absorption maxima at
about 449 nm. Similarly, polymer 13 and 14 depicted
blue-shifted absorption maxima in the film state in
comparison to polymers 12a -c for the same reason
mentioned for polymers in solution. The polymers in the
film phase depicted a red shift of about 20-30 nm from
their solutions, which could be ascribed to conforma-
tional changes increasing the degree of conjugation in
the polymer backbone of the condensed state.31 How-
ever, this bathochromic shift is somewhat less than that
observed for poly(3-alkylthiophene)s in the range 60-
100 nm,31 suggesting that the introduction of pyridinyl
unit imparted a significant amount of rigidity to the
polymer backbone. The band gap energy of the polymers
deduced from the energy absorption edge of the UV-
vis spectrum according to the approach of J ohnson et
al.34 was about 2.34, 2.39, and 2.46 eV for polymers
12a -c, 13, and 14, respectively.
The temperature-dependent absorption of the polymer
was studied in film phases. Figure 5 depicts the
representative spectra for polymers 12c, 13, and 14. For
polymer 12c, the absorption maximum (449 nm) un-
derwent a blue shift with gradual decrease in the
intensity with increasing temperature, while a new
band (288 nm) appeared with increasing intensity. This
indicates a decrease of the conjugation length. This
observation is due to heat-induced disorder in the side
chain leading to accentuated steric interaction and
concomitant twisting of the polymer chain, reducing the
extent of inter-ring conjugation.35 The magnitude of the
blue shift is significantly lesser in comparison to poly-
(alkylthiophene)s,35 indicating that the extent of twist-
ing of the polymer chain is very much reduced. This
might be due to the spacer effect of the pyridinyl unit
on the polymer chain, which helps diminish the repul-
sive intrachain steric interactions exerted by the alkyl
pendant groups. It should also be noted that one
isosbestic point is observed in UV spectra, implying the
coexistence of two different phases. The isosbestic point
would imply the presence of some cooperative effects in
the simultaneous rotation of long sequences of polymer
repeat units.36
Meta-linked polymers 13 and 14 depicted better
thermal stability with both of them exhibiting an onset
of degradation greater than 300 °C in N2 and air, and
polymer 14 containing 3,5-pyridinyl units showed the
highest degradation onset temperature (440 °C) in
those.
Op tica l P r op er ties of P olym er s. Optical Properties
in Solution. The UV-vis absorption and fluorescence
emission spectra of polymers in solution phase were
recorded in dilute chloroform and trifloroacetic acid at
room temperature.
The optical properties of the derived polymers in
CHCl3 solution are mainly dependent on pyridinyl
linked patterns of their backbones. Polymers 12a -c
with 2,5-linked pyridinyl units depicted almost identical
behavior in optical absorption and emission spectra. The
absorption spectra exhibited a single absorption peak
(λmax) at 429-432 nm. The PL spectra were obtained
by excitation at the absorption maxima. Polymers
12a -c afforded a strong green fluorescence with emis-
sion maxima at 511-513 nm, which were roughly at
the onset positions of their absorption band.
As to polymers 13 and 14 containing meta-linked
pyridinyl units, the absorption and emission maxima
were blue-shifted in comparison to 12a -c. This might
be attributed to the lesser extent of π-electron delocal-
ization of the polymer main chain arising from meta-
linkages, causing a reduction of the π-conjugation
lengths. Polymer 14 containing 3,5-linked pyridinyl
units depicted the highest energy transitions in the UV
and fluorescence spectra, implying the least mean
conjugation length. Figure 3 depicted the UV-vis
absorption and fluorescence spectra of polymers 12c, 13,
and 14 in CHCl3 solution.
The fluorescent quantum yield in solution was deter-
mined according to the method described previously25
relative to quinine sulfate in 0.1 N H2SO4. The results
are summarized in Table 2. The fluorescent quantum
yields of the novel polymers were higher than that of
poly(3-alkylthiophene)s31 and PBTBCx.19
For the purpose of comparison, we have measured the
optical properties of the derived polymers in CF3COOH
solution. Like other soluble conjugated polymers con-
taining pyridinyl32,25 or quinoline units,33 the derived
polymers showed UV absorption maxima and emission
peaks at a longer wavelength in CF3COOH solution
than in CHCl3 solution. This indicates that the absorp-
tion and emission of these polymers are strongly affected
by the protonation of nitrogen, which cause distortion
Other polymers exhibited similar behavior in their
UV spectra upon heating. From Figure 5, the three
polymers which have the same substituents but differ-
ent linkages in the backbone, the same new band (288
nm) with increasing intensity in their UV spectra was
observed upon heating. This thermochromic behavior

Macromolecules, Vol. 36, No. 5, 2003
Donor-Acceptor Conjugated Polymers 1549
Ta ble 2. Op tica l P r op er ties of th e Der ived P olym er s in Solu tion a n d F ilm Sta tes
solution state
In CHCl3 solution
in CF3COOH solution
film stated
UV λmax
(nm)
Exa
(nm)
λ
Emb λmax
(nm)
UV λmax
(nm)
Exa
(nm)
λ
Emb λmax
(nm)
UV λmax
(nm)
Em λmax
(nm)
polymer
æc
Ege (eV)
12a
12b
12c
13
432
432
429
414
407
431
432
429
414
407
513
514
511
503
500
43.8
42.3
40.3
42.8
37.2
465
472
474
464
453
479
480
479
492
492
548
549
548
548
548
459
458
449
443
424
540
540
541
534
528
2.34
2.34
2.34
2.46
2.39
14
a
b
Ex ) excitation. Emitted by emission maxima. Em ) emission. Excited by UV maxima. c æ ) quantum yield. Relative to 10-5
M
quinine sulfate in 0.1 N H2SO4 solution. Films were obtained by spin-coating from CHCl3 solution. e Eg ) band gap.
d
F igu r e 4. (a) UV absorption, (b) excitation, and (c) emission
curves of polymers 12c, 13, and 14 in CF3COOH solution.
was reversible, regaining the initial absorption state
upon cooling.
The emission maxima (upon excitation at absorption
F igu r e 5. Variation of absorption maxima in UV-vis spectra
of polymers 12c, 13, and 14 upon heating in film states.
maxima) of the derived polymers occurred in the green
region at ca. 540 nm for 12a -c and 534 and 528 nm for
13 and 14, respectively. Similarly, polymer with meta-
linked backbone exhibited a blue shift from those with
para-linked analogues. Polymer 14 with 3,5-pyridinyl
units depicted the shortest emission wavelength. The
optical properties of polymer films are summarized in

1550 Lu et al.
Macromolecules, Vol. 36, No. 5, 2003
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD, Epc ) -1.97
V vs SCE) and copolymers containing PBD moieties.37
This implies that the polymers may have electron-
transporting properties similar to these typical electron-
transporting materials.
From the cyclic voltamogram and UV-vis absorption
spectra, the electron affinity (EA) of these polymers is
ca. 2.57-2.60 eV, which is higher than previously
reported green-emitting copolymers based on 3-alkyl-
thienylene and phenylene units.20,38 This may be at-
tributed to the presence of electron-withdrawing pyridi-
nyl moieties in the polymer backbone which decreases
the LUMO energy. Consequently, these polymers may
provide for more facile electron injections when used as
active materials in PLEDs. In addition, facile electron
injection from the cathode will make possible the use
of more stable metals as the cathode.
F igu r e 6. UV-vis absorption and photoluminescence spectra
of the polymer films on ITO glass.
The p-doping of 12a depicted an oxidation peak at
1.01 V (Epa) and a reduction peak at ca. 0.89 V, which
is lower than that of poly[(2,5-bis(2′-thienyl)pyridine)]
(P25H) (1.1 V vs SSCE)39 prepared from electrochemical
polymerization. The onset for p-doping is determined
to be 0.86 V. During the oxidation process, the film
changed from orange to brown color, with the polymer
film being stable after multiple scans. Polymer 12b, 12c,
and 13 showed a similar p-doping process. From the
difference of onset potential between the reduction and
oxidation processes, the band gaps of the polymers could
be estimated, which were slightly larger than those
evaluated from the absorption edge in the UV-vis
spectra, as listed in Table 3.
XP S. The electronic environments of neutral and
doped polymers were evaluated from the XPS core level
spectra. The results suggest that both the sulfur and
nitrogen atoms are oxidized in the polymer backbone.
Figure 8 shows the N(1s) signals for neutral and
doped polymer 12a , which is a representative example.
The deconvolution of the N(1s) envelope of 12a revealed
only one component at 398.5 eV (fwhm ) 1.60 eV). After
doping, two nonequivalent nitrogen environments were
revealed in the N(1s) spectra, as shown in Figure 8b.
The low binding energy component at about 398.5 eV
(fwhm ) 1.60 eV) is attributable to neutral nitrogen.
The high BE component (400.9 eV, fwhm ) 1.60 eV),
which has a chemical shift of about +2.4 eV from the
neutral nitrogen, is associated with the oxidized pyri-
dinium nitrogen.40,41
The deconvolution of the S(2p) envelope for neutral
polymer 12a , as shown in Figure 9a, revealed only one
sulfur environment as a set of doublet [S(2p3/2) and
S(2p1/2)] with spin-orbit splitting of 1.2 eV and area
rations of 2:1. The binding energy of the S(2p3/2) peak
is at ca. 163.8 eV (fwhm ) 1.5 eV). These are attribut-
able to neutral sulfur in the thienylene ring.42 For doped
samples, two sets of doublets were deconvoluted from
the S(2p) curve with S(2p3/2) components located at
163.8 eV (fwhm ) 1.5 eV) and 164.8 eV (fwhm ) 1.5
eV), respectively (Figure 9b). The former originates
similarly from neutral sulfur in the neutral polymer,
while the latter suggests that some sulfur atoms are
positively polarized or partially charged, a result of the
oxidation of polymer chain by dopants.42
F igu r e 7. Cyclic voltammograms of polymer 12a in film state
coated on Pt electrodes in CH3CN solution of 0.1 M Bu4NBF4
at a scan rate of 20 mV/s.
Table 2. Figure 6 depicted the UV and fluorescence
spectra of polymer films 12c, 13, and 14 on ITO-glass.
Electr och em ica l P r op er ties of P olym er s. The
electrochemical properties of these conjugated polymers
were investigated using cyclic voltammetry. The poly-
mer films dip-coated on a Pt electrode were scanned
positively and negatively separately in a 0.1 M tetrabu-
tylammonium tetrafloroborate (Bu4NBF4) solution in
anhydrous acetonitrile. Figure 7 depicts the CV curves
of both the p- and n-doping processes of polymer 12a
on a representative example. During the cathodic scan,
the film of 12a exhibited good reversible n-doping and
dedoping processes. A reduction peak appeared at -1.85
V (Epc) with the corresponding reoxidation peak at -1.81
V (Epa). The onset potential of the reduction was -1.70
V. The electrochemical reduction process was very stable
during repeat scanning with no obvious changes in the
features of the CV. The n-doping of polymers was
accompanied by an obvious color change (electro-
chromism) from red in the original films to green in the
n-doped polymer films.
Other polymers depicted similar n-doping processes,
except that the potential value shifted slightly with
different alkyl pendant or backbones. Polymer 14 con-
taining a 3,5-pyridinyl unit showed the highest reduc-
tion potential among those. However, it is noted that
due to the presence of the electron-withdrawing pyridi-
nyl unit, the reduction of all polymers is more facile than
that electron-donating PBTBCx (Epc ) -2.14 to -2.39
V vs SCE)20 and can be comparable to some good
electron-transporting materials such as 2-(4-biphenylyl)-
Con clu sion
A novel series of donor-acceptor conjugated polymers
comprising alternating bis(3-alkylthienylene) and para-
or meta-linked pyridinyl repeating units were synthe-

Macromolecules, Vol. 36, No. 5, 2003
Donor-Acceptor Conjugated Polymers 1551
Ta ble 3. Su m m a r y of On set a n d P ea k P oten tia ls of p - a n d n -Dop in g P r ocesses a n d th e Ba n d ga p Va lu es
p-doping
n-doping
band gap
polymer
Eon (V)
Epa (V)
Epc (V)
Eon (V)
Epc (V)
Epa (V)
Ega (eV)
12a
12b
12c
13
0.80
0.75
0.71
0.90
0.90
1.01
1.05
1.06
1.03
b
0.89
0.96
1.02
0.94
b
-1.65
-1.68
-1.70
-1.68
-1.75
-1.86
-1.88
-1.93
-1.90
-1.97
-1.82
-1.82
-1.84
-1.85
-1.88
2.45
2.43
2.41
2.58
2.65
14
a
b
Electrochemical band gap. Unresolved.
solution. Polymers with meta-linkages exhibited blue
shifts from those with para-linkages in their UV and
fluorescence spectra. The optical behavior of these
polymers in CF3COOH solution depicted bathochromic
shifts from those in CHCl3 solution and interestingly
showed nearly identical emission maxima although they
have different backbone linkage structures. The derived
polymers were electrically conductive when doped with
I2 and FeCl3. FTIR and XPS investigations showed that
doping-induced charge-transfer complexes were formed
in polymers. The electrochemical behavior of the poly-
mers depicted facile p- and n-doping characteristics and
good electron-transporting properties ascribable to the
introduction of electron-withdrawing pyridinyl unit. On
the basis of these results, the novel polymers might be
promising materials for applications in light-emitting
diodes, rechargeable batteries, and light-emitting elec-
trochemical cells.
Refer en ces a n d Notes
(1) Broms, P.; Fahlman, M.; Xing, K. Z.; Salaneck, W. R.;
Dannetun, P.; Cornil, J .; Dossantos, D. A.; Bredas, J . L.;
Moratti, S. C.; Holmes, A. B.; Friend, R. H. Synth. Met. 1994,
67, 93-96.
F igu r e 8. XPS spectra of polymer 12a : (a) N(1s) spectrum
of neutral polymer; (b) N(1s) spectrum of FeCl3-doped polymer.
(2) Fahlman, M.; Broms, P.; Dossantos, D. A.; Moratti, S. C.;
J ohansson, N.; Xing, K.; Friend, R. H.; Holmes, A. B.; Bredas,
J . L.; Salaneck, W. R. J . Chem. Phys. 1995, 102, 8167-8174.
(3) Harrison, N. T.; Baigent, D. R.; Halls, J . J . M.; Pichler, K.;
Friend, R. H. Synth. Met. 1996, 76, 43-45.
(4) Samuel, I. D. W.; Rumbles, G.; Collison, C. J .; Crystall, B.;
Moratti, S. C.; Holmes, A. B. Synth. Met. 1996, 76, 15-18.
(5) Greenham, N. C.; Friend, R. H.; Bradley, D. D. C. Adv. Mater.
1994, 6, 491-494.
(6) Liu, Y. Q.; Li, Q. L.; Xu, Y.; J iang, X. Z.; Zhu, D. B. Synth.
Met. 1997, 85, 1279-1280.
(7) Rasmusson, J . R.; Broms, P.; Birgerson, J .; Erlandsson, R.;
Salaneck, W. R. Synth. Met. 1996, 79, 75-84.
(8) Halls, J . J . M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E.
A.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Nature
(London) 1995, 376, 498-500.
(9) Demanze, F.; Yassar, A.; Garnier, F. Macromolecules 1996,
29, 4267-4273.
(10) Roncali, J . Chem. Rev. 1997, 97, 173-205.
(11) Demanze, F.; Yassar, A.; Garnier, F. Adv. Mater. 1995, 7,
907.
(12) Lee, B. L.; Yamamoto, T. Macromolecules 1999, 32, 1375-
1382.
(13) Saito, H.; Ukai, S.; Iwatsuki, S.; Itoh, T.; Kubo, M. Macro-
molecules 1995, 28, 8363-8367.
(14) Zhang, Q. T.; Tour, J . M. J . Am. Chem. Soc. 1998, 120, 5355-
5362.
F igu r e 9. XPS spectra of polymer 12a : (a) S(2p) spectrum of
(15) Naka, K.; Umeyama, T.; Chujo, Y. Macromolecules 2000, 33,
7467-7470.
neutral polymer; (b) S(2p) spectrum of FeCl3-doped polymer.
(16) Karikomi, M.; Kitamura, C.; Tanaka, S.; Yamashita, Y. J .
Am. Chem. Soc. 1995, 117, 6791-6792.
(17) Yamamoto, T.; Takagi, M.; Kizu, K.; Maruyama, T.; Kubota,
K.; Kanbara, H.; Kurihara, T.; Kaino, T. J . Chem. Soc., Chem.
Commun. 1993, 797-798.
(18) Yamamoto, T.; Zhou, Z. H.; Kanbara, T.; Shimura, M.; Kizu,
K.; Maruyama, T.; Nakamura, Y.; Fukuda, T.; Lee, B. L.;
Ooba, N.; Tomaru, S.; Kurihara, T.; Kaino, T.; Kubota, K.;
Sasaki, S. J . Am. Chem. Soc. 1996, 118, 10389-10399.
(19) Ng, S. C.; Xu, J . M.; Chan, H. S. O. Synth. Met. 1998, 92,
33-37.
sized by a Stille coupling approach. The derived poly-
mers depicted good solubility in common organic sol-
vents such as chloroform, tetrahydrofuran, and trifloro-
acetic acid. Our investigations showed that the polymer
electronic and optical properties were consistent with
their conjugated backbone structure. They showed
strong green fluorescence in their solution and film
states with high photoluminescence quantum yields in

1552 Lu et al.
Macromolecules, Vol. 36, No. 5, 2003
(20) Ng, S. C.; Xu, J . M.; Chan, H. S. O. Macromolecules 2000,
33, 7349-7358.
(32) Fu, D. K.; Xu, B.; Swager, T. M. Tetrahedron 1997, 53,
15487-15494.
(33) Yamamoto, T.; Sugiyama, K.; Kushida, T.; Inoue, T.; Kanbara,
T. J . Am. Chem. Soc. 1996, 118, 3930-3937.
(34) J ohnson, E. G.; Willardson, R.; Beer, A. C. Semiconductors
and Semimetals; Academic Press: New York, 1967; p 153.
(35) Roux, C.; Bergeron, J . Y.; Leclerc, M. Makromol. Chem. 1993,
194, 869-877.
(36) Roux, C.; Leclerc, M. Chem. Mater. 1994, 6, 620-624.
(37) J anietz, S.; Wedel, A. Adv. Mater. 1997, 9, 403.
(38) Ng, S. C.; Xu, J . M.; Chan, H. S. O.; Fujii, A.; Yoshino, K. J .
Mater. Chem. 1999, 9, 381-385.
(39) Tanaka, S.; Sato, M. A.; Kaeriyama, K. J . Macromol. Sci.,
Chem. 1987, A24, 749.
(21) Irvin, D. J .; DuBois, C. J .; Reynolds, J . R. Chem. Commun.
1999, 2121-2122.
(22) J enkins, I. H.; Salzner, U.; Pickup, P. G. Chem. Mater. 1996,
8, 2444-2450.
(23) Kaeriyama K.; Sato, M.; Tanaka, S. Synth. Met. 1987, 18,
233.
(24) Ng, S. C.; Lu, H. F.; Chan, H. S. O.; Fujii, A.; Laga, T.;
Yoshino, K. Adv. Mater. 2000, 12, 1122.
(25) Ng, S. C.; Lu, H. F.; Chan, H. S. O.; Fujii, A.; Laga, T.;
Yoshino, K. Macromolecules 2001, 34, 6895-6903.
(26) Wang, H.; Helgeson, R.; Ma, B.; Wudl, F. J . Org. Chem. 2000,
65, 5862-5867.
(27) Ruiz, J . P.; Dharia, J . R.; Reynolds, J . R.; Buckley, L. J .
Macromolecules 1992, 25, 849-860.
(40) Ng, K. T.; Hercules, D. M. J . Am. Chem. Soc. 1975, 97, 4169.
(41) Tan, K. L.; Tan, B. T. G.; Kang, E. T.; Neoh, K. G. J . Mol.
Electron. 1990, 6, 5-13.
(42) Kang, E. T.; Neoh, K. G.; Tan, K. L. Phys. Rev. B 1991, 44,
10461-10469.
(28) McCullough, R. D.; Lowe, R. D.; J ayaraman, M.; Anderson,
D. L. J . Org. Chem. 1993, 58, 904-912.
(29) Li, J .; Pang, Y. Macromolecules 1997, 30, 7487-7492.
(30) Sato M.; Tanaka S.; Kaeriyama, K. Makromol. Chem. 1987,
188, 1763.
(31) Inganas, O.; Salaneck, W. R.; Osterholm, J . E.; Laakso, J .
Synth. Met. 1988, 22, 395-406.
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