New soluble rigid rod copolymers comprising alternating 2-amino-pyrimidine and phenylene repeat units: Syntheses, characterization, optical and electrochemical properties

Article abstract of DOI:10.1016/j.polymer.2009.11.010

A new type - π-conjugated copolymers of 2-amino-pyrimidine was prepared between 2-amino-4,6-diiodidepyrimidine and 1,4-dibromo-2,5-dialkoxybenzene by Sonogashira polycondensation. The structures of the copolymers were elucidated by FT-IR, ¹H NMR and ¹³C NMR, fluorescence spectroscopy, gel permeation chromatography, thermal analysis and element analysis. The derived polymers were soluble in common organic solvents and trifloroacetic acid and exhibited good thermal stability. They emitted green light under UV irradiation in solid state and blue or green light in solution phase, respectively. Electrochemical behavior of these new polymers depicted facile p-doping and good electron-transporting properties. These polymers displayed bathochromic shift when protonated with CH₃SO₃H acid in chloroform solutions or m-cresol solutions and the red-shifted peaks were observed from 490 nm to 652 nm. XRD patterns of copolymers showed that the intensity of peaks was enhanced with increasing alkyloxy chain length.

Full text of DOI:10.1016/j.polymer.2009.11.010

Polymer 51 (2010) 437–446
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
New soluble rigid rod copolymers comprising alternating 2-amino-pyrimidine
and phenylene repeat units: Syntheses, characterization, optical and
electrochemical properties
*
Xirali Mamtimin, Rukiya Matsidik, Ismayil Nurulla
Key Laboratory of Oil and Gas Fine Chemicals, Educational Ministry of China, School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi, 830046, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type –
p-conjugated copolymers of 2-amino-pyrimidine was prepared between 2-amino-4,6-
Received 27 March 2009
Received in revised form
4 November 2009
Accepted 9 November 2009
Available online 15 November 2009
diiodidepyrimidine and 1,4-dibromo-2,5-dialkoxybenzene by Sonogashira polycondensation. The
structures of the copolymers were elucidated by FT-IR, 1H NMR and 13C NMR, fluorescence spectroscopy,
gel permeation chromatography, thermal analysis and element analysis. The derived polymers were
soluble in common organic solvents and trifloroacetic acid and exhibited good thermal stability. They
emitted green light under UV irradiation in solid state and blue or green light in solution phase,
respectively. Electrochemical behavior of these new polymers depicted facile p-doping and good elec-
tron-transporting properties. These polymers displayed bathochromic shift when protonated with
CH3SO3H acid in chloroform solutions or m-cresol solutions and the red-shifted peaks were observed
from 490 nm to 652 nm. XRD patterns of copolymers showed that the intensity of peaks was enhanced
with increasing alkyloxy chain length.
Keywords:
Conjugated polymer
Acidochromism
Electrochemical redox reaction
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
applications [1,2,4]. Subsequently, blue electroluminescent mate-
rials containing pyridine ring were reported [5]. In comparison
During the past decade,
p
-conjugated copolymers have been an
with the pyridine, 2-amino-pyrimidine seemed to be showing
more electron-deficient nature, and the amino group was very
active [11,18,20].
interesting class of organic materials for optical and electrical
applications and a large number of papers have been published on
conjugated chains with the linear and rigid structures [1–6].
Although several papers have recently been published on the
preparation and properties of the rigid rod conjugated copolymers
[7–10], more detailed studies seem to be required for revealing
their basic properties. In our work on conjugated-systems, we
became interested in copolymers containing electron-deficient
In this work, we chose 2-amino-pyrimidine to act on the elec-
tron density of the conjugated chain by means of an electroactive
functional group, electron acceptor attached on the phenyl moiety.
We also chose to attach an alkoxy chain on the aryl in order to
increase the solubility and the molecular weight. The copolymers
reported in this paper are considered to possess linear molecular
structures [4,21] without branching since a more convenient
method, using a palladium catalyzed polycondensation, has been
applied to the synthesis of rigid rod conjugated chains [22,23],
more generally known in organic synthesis as a Sonogashira
coupling [24–26]. The copolymers which contains 2-amino-
pyrimidine ring are expected to show interesting chemical and
physical properties.
heterocycles like 2-amino-pyrimidine [11]. However, p-conjugated
copolymers having a 2-amino pyrimidine ring have received less
attention, in spite of the presence of a certain number of reports
about copolymers of containing pyridine rings [5,12–17]. Because of
its electron-deficient nature, pyrimidine could establish charge-
transfer (CT) structure with five-membered ring heterocycles such
as thiophene and furan which have a p-excessive nature [18,19] and
their copolymers had been successfully synthesized. Meanwhile,
the linear and rigid rod conjugated chains of copolymers containing
pyridine ring have been more attracted for nonlinear optical
2. Experimental
2.1. Materials
All the chemicals used were of analytical grade. 2-amino-4,6-
dicholoro pyrimidine, (PPh3)4Pd, CuI, bromine, 1-bromododecane,
* Corresponding author. Tel.: þ86 0991 8583575; fax: þ86 0991 8582809.
E-mail address: ismayilnu@sohu.com (I. Nurulla).
0032-3861/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2009.11.010

438
X. Mamtimin et al. / Polymer 51 (2010) 437–446
OH
C12H25Br
R
R
R
R
Br2
TMS
KOH
Br
Br
(H3C)3Si
Si(CH3)3
H
H
R
HO
R
R
R
1a ~b
2a ~c
3a ~c
4a ~c
NH2
N
NH2
R
R
NH2
Pd 0
N
N N
N
N
HI
KI
5
+
4
I
I
Cl
Cl
n
6
5
a: PI R=OC12H25
b: PII R=OC7H15
c: PIII R=H
Scheme 1. The procedure for preparation of the copolymers.
Tetrahydrofuran (THF), trimethylsilylacetylene (TMSA), hydroqui-
none, triphenylphosphine (TPP), dimethylformamide (DMF) were
purchased from ACROS(China) Chemical Co. and used as received.
1,4-Dibromo-2,5-bis (dodecanoxy)-benzene [28], 1,4-bis[(trimethyl-
silyl)ethynyl]-2,6-bis(dodecyloxy) benzene [3], and 1,4-diethynyl-
2,5-bis(dialkoxy)benzene [3] were synthesized according to the
literature. The acetonitrile and diisopropylamine were distilled from
KOH and toluene was dried and distilled from metal sodium powder
under a N2 atmosphere. Both solvents were sparged with N2 for 2 h
and stored under N2. Other reagents such as methanol and chloro-
form were commercially available and used as received unless
otherwise stated.
source (
l
¼ 0.15418 nm). The scanning rate was 2ꢁ/min over a range
of 2
q
¼ 5–60ꢁ. Molecular weight determinations were made by
Waters Alliance GPC V 2000 at a flow rate of 0.94 ml/min in DMF
Waters styragel columns (HT3, HT4, HT6), T ¼ 65 ꢁC. The thermal
stability of the dry sample was determined using Netzsch STA 449C
instrument with a temperature range of 26–900 ꢁC at a heating rate
of 10 ꢁC/min, using a nitrogen purge at a flow rate of 30 ml/min. The
cyclic voltammetry (CV) was performed with a CHI 660A Electro-
chemical Workstation (CH Instruments, China) in a conventional
three electrode cell with a counter electrode (Pt) and a reference
electrode (Ag/AgCl), T ¼ 20 ꢁC. The melting point was measured
with a TECH XT-5 melting point apparatus.
2.3. Synthesis of 2-amino-4,6-diiodopyrimidine
2.2. Characterization
2-amino-4,6-diiodopyrimidine was prepared by modifying the
literature method [27]. A solution of 2-amino-4,6-dichloropyr-
imidine (4 g, 24 mmol) in 40% HI (100 mL) was treated with KI
(19.2 g, 115 mmol) and stirred for overnight at ambient tempera-
ture. The precipitate was filtered, the precipitate was washed with
aqueous NaHCO3 and dried to give the desired product (8 g, 94%).
1H NMR and 13C NMR spectra were taken with a VARIAN INOVA-
400 spectrometer (400 MHz). Photoluminescence and excitation
spectra were recorded on a HITACHI F-4500 spectrophotometer.
Elemental analyses were made on an FLASH EA series 1112 NCHS-O
analyzer. FT-IR spectra of the copolymers were obtained by using
a BRUKEREUINOX-55 Fourier transform infrared spectrometer
(frequency range 4000–500 cmꢀ1). UV–vis spectra of the copoly-
mers were recorded by using Shimadzu UV-2450 spectrophotom-
eter. UV–vis absorption spectra in powder state were measured at
room temperature on a UV-3010 spectrometer equipped with an
integrating sphere. The X-ray diffraction studies were performed
on a D/Max 2400 X-ray diffractometer by using Cu-Ka radiation
1H NMR (CDCl3)
d: 7.523 (s, 1H), 5.327 (broad s, 2H).
2.4. Synthesis of 1,4-bis (dodecanoxy) benzene
1,4-Bis (dodecanoxy) benzene was prepared by modifying the
literature method [4]. To a solution of hydroquinone (7.7 g, 70 mmol)
NH2
OC12H25
1
1
5
6
4
5
N
N
2
NH2
N
OCH2(C10H20)CH3
4 5 6
8
7
3
3
N
n
5
OC12H25
n
8
2
OCH2(C10H20)CH3
2
7
6
3
6
4
1
2 3
4
160
135
110
ppm
1
140
80
40
0
7.5
5.5
3.5
1.5
ppm
ppm
Fig. 1. 1H NMR spectrum of PI.
Fig. 2. 13C NMR spectrum of PI.

X. Mamtimin et al. / Polymer 51 (2010) 437–446
439
1.0
PI cast film
PI solution
0.8
0.6
0.4
0.2
0.0
PII cast film
PII solution
PIII cast film
PIII solution
Fig. 3. 1H NMR spectrum of PIII in CF3COOD.
400
500
600
Wavelength (nm)
in 140 mL of DMF/toluene mixture (1:1) were added potassium
hydroxide (9.52 g, 170 mmol) and potassium carbonate (23.46 g,
170 mmol). The mixture was stirred at room temperature for 30 min.
Then 1-bromododecane (38.83 g, 156 mmol) was added dropwise.
The mixture was stirred and refluxed under N2 for 14 h, cooled and
the toluene was removed under reduced pressure and the remaining
mixture was poured into cold water (210 ml). The brown precipitate
was collected by filtration and was recrystallized twice from ethanol
to yield a fluffy white material in 96% yield, mp 77–78 ꢁC.
Fig. 5. UV–Vis spectra of PI, PII and PIII in solution or cast film.
was little fluorescent, the emission in the early stages of copolymer-
ization being green and in the later stages more reddish yellow. After
cooling the reaction mixture to room temperature it was added
dropwise to 100 mL of rapidly stirred acetone. The yellow precipitate
was collected and washed repeatedly with acetone, hot EtOH, hot
CH3OH, and recrystallized twice from EtOH/CHCl3. After drying
overnight at 40 ꢁC, copolymer was isolated in 92% yield. GPC analysis
(soluble part in DMF) gave Mn of 5951 and Mw of 6119. 1H NMR
2.5. Synthesis of the copolymers
(CDC13) d: 7.06–7.088 (m, 2H), 6.82–6.99 (br, 1H), 5.18–5.24 (br, d, 2H),
The poly(2,5-didodecyloxy-1,4-diethynyl-phenylene-alt-2-amino-4,
6-pyrimidine) (PI), poly(2,5-diheptyloxy-1,4-diethynyl-phenyl-
ene-alt-2-amino-4,6-pyrimidine) (PII) and poly(1,4-diethynyl-
phenylene-alt-2-amino-4,6-pyrimidine) (PIII) were prepared by
modifying the literature method [3] and the procedure for the
preparation of PI was given as a typical example:
3.98–4.01 (br, m, 4H), 1.20–1.85 (m, 40H), 0.86 (t, 6H). Anal. Calcd for
(C38H55N3O2)n (585.86)n: C, 77.90; H, 9.46; N, 7.17. Found: C, 77.78; H,
9.65; N, 7.24. UV–vis (CHC13) lmax: 415 nm.
PI: 0.165 mmol of 1,4-Diethynyl-2,5-dialkoxybenzene, 0.165 mmol
of 2-amino-4,6-diiodopyrimidine, 0.008 mmol of (PPh3)4Pd, and
0.008 mmol of CuI were combined in 3 mL of diisopropylamine and
7 mL of toluene under N2. The mixture was heated at 80 ꢁC for 10 h.
Ammonium iodide salts were formed immediately and the mixture
Monomer 5
Monomer 4a
302
PI in CHCl3
338
(a) Monomer 5
0.5
PI cast film
438
1631
-NH2
415
(b) Monomer 4a
C
C
H
(c) polymer PIII
(d) polymer PII
(e) polymer PI
C C
C C
1631
3000 2500 2000 1500 1000
Wave number (cm -1
-NH2
3500
0.0
400
Wavelength (nm)
600
4000
500
)
Fig. 4. FT-IR spectra of copolymers and monomers.
Fig. 6. UV–Vis spectra of the monomers and copolymer.

440
X. Mamtimin et al. / Polymer 51 (2010) 437–446
3. Results and discussion
powder state
in solution
3.1. Synthesis and characterization of the copolymers
PI PII PIII
PIII
PII
The synthetic procedure of the monomers and copolymers is
described in Scheme 1. The precursor of 1,4-bis (dodecanoxy)
benzene was prepared via a somewhat different route from the
reported route [3,4,28] and the new method gave the precursor in
a stabilized higher yield (95%) than the previously reported
methods (80–91%).
The chemical structure of the monomers and the copolymers
was characterized by FT-IR, 1H NMR and 13C NMR spectroscopy and
elemental analyses. Fig. 1 presents the 1H NMR spectrum of PI with
the assignments of all the protons. The peaks corresponding to the
aromatic protons were centered at the interval of 6.82–7.52 ppm.
The amide NH2 protons appear in the region of 5.18–5.24 ppm. The
alkyl side-chains are dominating in the region of 0.84–1.86 ppm.
The methylene groups adjacent to oxygen are shifted downfield to
4.00 ppm. Fig. 2 represents the 13C NMR spectra of PI. It shows
a signal at 93.5 and 88.11 ppm, assigned to the acetylenic carbons
[1]. The aromatic carbons of the pyrimidine ring are detected at
162, 154 and 111 ppm. The aromatic carbons of the benzene ring are
detected at 151,117 and 114 ppm. The carbons belonging to the
alkoxy side-chains appear as very strong signals at 70 and from 38
to 14 ppm. The peak at 77 ppm is due to CDCl3.
PI
450
500
550
Wavelength (nm)
600
650
Fig. 7. Fluorescence spectra of copolymers in solution and in powder state.
PII: isolated in 81% yield. GPC analysis (soluble part in DMF)
gave Mn of 4798 and Mw of 4969. 1H NMR (CDC13)
d: 6.954–7.058
(m, 2H), 6.802–6.949 (br, 1H), 5.113–5.173 (br, d, 2H), 3.957–3.973
(br, m, 4H), 1.20–1.85 (m, 20H), 0.86 (t, 6H). Anal. Calcd for
(C28H35N3O2)n (445.6)n: C, 75.60; H, 7.71; N, 9.27. Found: C, 75.47;
H, 7.92; N, 9.43. UV–vis (CHC13) lmax: 423 nm.
Fig. 3 presents the 1H NMR spectrum of PIII. The peaks corre-
sponding to the aromatic protons were centered at the interval of
7.15–7.96 ppm. The amide –NH2 protons are not founded in
CF3COOD.
PIII: isolated in 75% yield. GPC analysis (soluble part in DMF)
gave Mn of 3554 and Mw of 4089. 1H NMR (CF3COOH)
d: 7.548–7.969
Fig. 4 presents the comparison of FT-IR spectra of monomers and
copolymers. For copolymers, the strong peak at 3285 cmꢀ1 corre-
sponding to HC^C–stretching vibration and the weak peak at
2107 cmꢀ1 corresponding to C^C in curve (b) was absent in curve
(c), (d) and (e), but a new strong peak at 2212 cm–1 which is
associated to C^C in copolymers structure was appeared in curve
(c), (d) and (e) [29]. According to the IR data, the acetylene group is
considered to be affected by the electron-deficient nature of
pyrimidine ring and electron-donor nature of phenyl ring. This
indicated that the dipoles were instantaneously changed greatly
and the stretching vibration of acetylene group was very efficient.
The peak at 3278 cmꢀ1 and 3481 cmꢀ1 corresponding to –NH2 in
curve (a) was appeared at 3300 cmꢀ1 and 3497 cmꢀ1 in curve (c),
(d) and ꢀ(1e). The typical stretching vibration of alkyl group at
2919 cm and 2846 cmꢀ1 was also detected at 2922 cmꢀ1 and
2851 cmꢀ1 in curve (d) and (e). The characteristic peak of C]N at
the pyrimidine ring at 1625 cmꢀ1 in curve (a) was detected at
almost the same position in curve (c), (d) and (e).
(m, 4H), 7.156–7.275 (m, 1H). Anal. Calcd for (C14H7N3O2)n
(249.22)n: C, 67.80; H, 2.75; N, 16.72. Found: C, 67.47; H, 2.83; N,
16.86. UV–vis (DMF) lmax: 379 nm.
389
521
1.0
0.8
0.6
0.4
0.2
3.2. Optical properties
The chloroform or DMF solutions of copolymers were light
yellow and fluoresced blue or green under the irradiation with UV
light (365 nm). The films of the polymers were prepared by spin-
casting the polymers from their solutions in chloroform or DMF.
The films of polymers are light yellow. The UV–vis absorption
spectra and photoluminescence spectra of the polymers measured
from the solution, films and powder state were shown in Figs. 5–7.
The peak wavelengths of the spectra and the p–
p* band gaps of the
polymers estimated from the absorption onset wavelengths are
summarized in Table 1. The PI and PII showed almost identical
electronic properties in solution or cast film while PIII showed
shorter absorbance both in solution and cast film (Fig. 5).
300
400
500
600
Although all of the three polymers have the same backbone
structure, their optical properties are quite different with the
variation of substituent. Generally, a substituted group affects the
Wavelength (nm)
Fig. 8. UV–vis absorption and fluorescence spectra of PI in powder state.

X. Mamtimin et al. / Polymer 51 (2010) 437–446
441
Table 1
Optical properties of the polymers.
Polymer
Yield
Color
Mn
UV and PL
l
(nm)a
Optical band
gap (eV)
b
Solution (nm)
l
Stokes’ shift
(F
)
Powder
UV
l
(nm)
Stokes’ shift
Film (nm)c
UV
EX
EMd
EX
EMd
abslmax
abslonset
PI
PI
PIII
92%
81%
75%
Yellow
Yellow
Yellow
5951
4969
4089
415
423
379
429
458
436
465
478
495
50
55
116
0.12
0.11
0.07
389
376
348
–
–
–
521
549
554
132
173
206
438
439
385
501
497
445
2.48
2.49
2.78
a
10ꢀ5 M in CHCl3 or DMF solution.
b
c
Relative to 9,10-diphenylanthracence (10ꢀ5 M in cyclohexane).
Spin-coating from CHCl3 or DMF solution.
d
The main emission peak, operated at 300 K.
solution and powder state. The PI, PII and PIII show different PL
peaks in solution and powder state that are summarized in Table 1.
For instance, the photoluminescence peak of PI in powder state is
observed at 521 nm and shifted by 56 nm to a longer wavelength
than chloroform solution. Interestingly, the PL excitation peaks of
the all polymers in powder state are not found at the long interval
of 300–500 nm and the emission peaks were very easy to find at
the interval of 521–554 nm in different intensity. The PL peaks of PI,
PII and PIII in solution appear near the onset position of the UV–vis
absorption band. The PL spectra, which are attributed to fluores-
cence on account of the short lifetime of the excited states [5], were
obtained by irradiative excitation at the wavelength of the
absorption maxima. For polymers, the blue or green fluorescence
resulted with emission maxima from ca. 465–495 nm, which
correlate approximately with the onset of the absorption band. This
is indicative that photoluminescence took place by electrons in
Table 2
Solvents influence on the PL and UV of PI.
Solvents
UV
PL
lmax
Stokes’
shift (nm)
PL (F)
lmax
EX
EM
Chloroform
THF
Tetrachloromethane
415
412
426
429
418
373
465
457
436
50
45
10
0.12
0.098
0.081
electronic and optical properties of a conjugated polymer by steric
hindrance arising from the substituted group. For polymers, the
introduction of dialkoxy groups into the phenylene ring at the 2-
and 5-positions resulted in a remarkable spectral red or blue shift.
In comparison with PIII (379 nm), the red shifts of PI (415 nm) are
36 nm in absorption spectra and in photoluminescence spectra, PI
(464 nm) has the blue shifts (31 nm) compared to PIII (395 nm).
The PIII is a green light-emitting material, whereas polymer PI
emits blue light. Fig. 6 depicts the UV–vis absorption of the
monomers and PI in chloroform solution and cast film. The PI
a conduction band (p* level) to a valence band (
p level).
Fig. 8 depicts the UV–vis absorption and fluorescence spectra of
the PI in powder state. The absorption spectra exhibited an
absorption maximum (lmax) at ca. 389 and green fluorescence
resulted with emission maxima at ca.521 nm.
shows lmax at a longer wavelength (415 nm, log
based on the repeating unit) than those of the corresponding
e
¼ 2.35; molarity is
The quantum yield for emission in solution was determined
according to the method described by Davey et al. [5] relative to
monomers (4a: lmax ¼ 338 nm and 5: lmax ¼ 302 nm), indicating
that the PI has an expanded
p-conjugated system. Films of the
9,10-diphenylanthracence (10ꢀ5
M in cyclohexane). Due to strong
copolymers showed the UV–vis peak at a somewhat longer wave-
length, suggesting the presence of certain intermolecular electronic
interaction. However, the degree of the red shift is considerably
smaller than those observed with copolymers that take face-to-face
stacking [28,30]. Fig. 7 depicts the PL spectrum of copolymers in
self-absorption and excimer formation of the polymers, the deter-
mined quantum yields were highly dependent on the concentra-
tion, solvents and excitation wavelength upon irradiation. The
results are summarized in Tables 1 and 2. All polymers were found
0.8
a
b
415
466
1
12
490
0.6
0.4
0.2
0.0
12
1
400
500
Wavelength (nm)
600
500
600
Wavelength (nm)
Fig. 9. Changes in the absorption and emission spectra of copolymer (3.25 ꢂ10ꢀ3 mol Lꢀ1) in CHCl3–CH3SO3H mixtures at different concentrations of CH3SO3H (mol Lꢀ1): (1) 0, (2)
9.1536 ꢂ 10ꢀ5, (3) 10.6792 ꢂ 10ꢀ5, (4) 12.2048 ꢂ 10ꢀ5, (5) 15.256 ꢂ 10ꢀ5, (6) 18.3072 ꢂ 10ꢀ5, (7) 22.884 ꢂ10ꢀ5, (8) 48.8192 ꢂ 10ꢀ5, (9) 140.3552 ꢂ 10ꢀ5, (10) 292.9152 ꢂ 10ꢀ5, (11)
1360.8352 ꢂ 10ꢀ5, (12)15256 ꢂ 10ꢀ5; (a) UV spectra; (b) Emission spectra (excitation at absorption maxima).

442
X. Mamtimin et al. / Polymer 51 (2010) 437–446
to be emissive with certain quantum yields. This could be attrib-
utable to the rigid structure of the copolymers with the effect that
relaxation from the excited state through nonradiative (e.g.,
thermal) processes will be reduced with consequently certain
fluorescence quantum yield [5]. Their optical properties are
currently in progress in our laboratory.
With the aim of investigating the effect of protonation on the
copolymers, we measured UV–vis and fluorescence spectra of the
copolymers in CHCl3–CH3SO3H mixtures. The copolymers showed
similar protonation properties in their solution.
Fig. 9 depicts the representative changes in the absorption
spectra and fluorescence spectra of PI with increasing concentra-
tion of CH3SO3H. From Fig. 9a, addition of CH3SO3H (0–0.15256
mol Lꢀ1) to a CHCl3 solution of PI (3.25 ꢂ10ꢀ3 mol Lꢀ1) led to
a quick decrease in the intensity of the absorption at 415 nm and, at
the same time, an appearance of a new band at 490 nm. From
Fig. 9b, in chloroform solution, the emission spectra appeared to be
more strongly affected by the addition of CH3SO3H than did the
UV–vis spectra. The emission maxima were not red-shifted unlike
the copolymers based on pyridine [6] when the CH3SO3H concen-
tration was increased from 0 to 0.15256 mol Lꢀ1. Meanwhile, the
emission intensity was disappeared when the CH3SO3H concen-
tration was increased from 0 to 9.1536 ꢂ 10ꢀ5 mol Lꢀ1. For the
purpose of further investigating the effect of protonation on the
copolymers, the representative acidochromism of PI in m-cresol is
studied. Interestingly, the m-cresol solution of PI shows different
acidochromism with wide range of CH3SO3H concentration than
chloroform solution.
NH2
N
N
NH2
N H+
0.2
0.1
0.0
NH2
+H N N H+
N
440
499
7
652
14
7
14
400
600
Wavelength (nm)
800
Fig. 10. Changes in the absorption and emission spectra of copolymer
(0.82 ꢂ 10ꢀ3 mol Lꢀ1) in m-cresol-CH3SO3H mixtures at different concentrations of
CH3SO3H (mol Lꢀ1): (1) 0, (2) 1.186 ꢂ 10ꢀ4
,
(3) 1.723 ꢂ 10ꢀ4
,
(4) 2.974 ꢂ10ꢀ4
, (5)
1.614 ꢂ10ꢀ3, (6) 1.876 ꢂ 10ꢀ2, (7) 1.751 ꢂ10ꢀ1, (8) 2.262 ꢂ 10ꢀ1, (9) 1.131, (10) 3.394,
(11) 5.085, (12) 8.788, (13) 10.182, (14) 15.274.
Fig. 10 depicts the changes in the absorption spectra of PI with
increasing concentration of CH3SO3H in m-cresol. From Fig. 10,
addition of CH3SO3H (0–0.1751 mol Lꢀ1) to an m-cresol solution of
PI (3.25 ꢂ10ꢀ3 mol Lꢀ1) led to a quick decrease in the intensity of
the absorption at 440 nm and, at the same time, an appearance of a
new band at 499 nm.
(d)
(c)
With increasing concentration of CH3SO3H (0.1751–15.274
mol Lꢀ1), the absorption at 499 nm was decreased and, in the
meantime, it is possible to observe an appearance of a new band at
652 nm. These results suggested that absorption at a longer
wavelength in chloroform or m-cresol solution originates from
a
p– p–
p* transition along the main chain. Moreover, this p* tran-
sition is strongly affected by the protonation of nitrogen in copoly-
mer, which causes distortion of the inter-ring bonds ascribable to an
increase in steric repulsion [20,31]. The difference of acidochromism
in the UV–Vis spectral between in chloroform and m-cresol might be
caused by the acidic and polar properties of the m-cresol. The
acidochromism of PI is carried out in two steps in m-cresol solution,
the first step is to accept a proton, and the second step is to accept
two protons, the first step is relatively easy, while the second reac-
tion was more difficult, because of the existence of the space resis-
tance (space effects). In chloroform solution, PI only accept a proton
while in m-cresol solution, the m-cresol solution can offer an acidic
and polar environment and these moiety will lead to further
protonation of PI and the acidochromism was carried out thoroughly
and the PI accept two protons. In the interest of further under-
standing the protonation of nitrogen in copolymers, the represen-
tative FT-IR spectrum of residue was obtained after fully dried the PI
in CH3SO3H (1.751 ꢂ10ꢀ1 mol Lꢀ1 or 15.274 mol Lꢀ1)/CHCl3 solution
(b)
(a)
3000
2000
Wave number (cm )
Fig. 11. FT-IR spectra of copolymer film on glass. (a) pure copolymer film (CHCl3 as
solvent); (b) copolymer film exposed under CH3SO3H (1.751 ꢂ10ꢀ1 mol Lꢀ1)/CHCl3 as
solvent; (c) copolymer film exposed under CH3SO3H(15.274 mol Lꢀ1)/CHCl3 as solvent;
(d) pure CH3SO3H.
NH2
+H N N H+
OC12H25
OC12H25
NH2
N H+
OC12H25
NH2
N
OC12H25
OC12H25
H+
H+
N
N
n
n
n
OC12H25
Scheme 2. Mechanism for the acidochromism.

X. Mamtimin et al. / Polymer 51 (2010) 437–446
443
a
1.5
1.0
0.5
0.0
b
466
a
a
b
269
483
b
c
485
d
415
c
492
d
312
422
450
500
550
200
300
400
500
Fig. 12. Changes in the absorption and emission spectra of copolymer (2.52 ꢂ 10ꢀ3 mol Lꢀ1) in CHCl3–CH3OH mixtures at different volume fractions of methanol: (a) 0, (b) 20%,
(c) 40%, (d) 60%; (a0) UV spectrum; (b0) emission spectrum (excitation at absorption maxima).
(Fig. 11). The peak at 3300 cmꢀ1 corresponding to –NH2 in curve (a)
was appeared at 3184 cmꢀ1 in curve (b) or (c). The characterization
peaks of C]N at the pyrimidine ring and benzenoid units at
1625 cmꢀ1 and 1553 cmꢀ1 in curve (a) was detected at 1591 cmꢀ1
and 1438 cmꢀ1 in curve (b) or (c). The characterization acetylene
group was detected at 2212 cmꢀ1 in curve (b) or (c). According to the
IR data, the characterization peaks of amino group, pyrimidine ring
and benzene ring were shifted to lower wave numbers but the
characterization peaks of acetylene group was detected at almost the
same position in curve (b) or (c). Since the bands ranging from
3000 cmꢀ1 to 2200 cmꢀ1 which are assigned to vibrations associated
with the –NHþ3 group was not observed, the –NH2 group was not
protonated but affected by the intermolecular hydrogen bond
[32,33]. The representative general chemical structure and proteo-
lytic equilibria of PI are shown in Scheme 2 [5,6,34,35].
The representative intermolecular staking properties of the PI in
solution were investigated in CHCl3–CH3OH mixtures. In CHCl3–
CH3OH mixtures, unlike some substituted polythiophenes which
showed strong solvatochromism [36,37], the position of UV–vis
maximum absorption slightly shifted to a longer wavelength (ca.
7 nm) with different volume fractions of CHCl3 and CH3OH (Fig. 12
(a)). The emission spectrum was nearly dependent on addition of
CH3OH, shifted to a longer wavelength (ca. 19 nm) with the emis-
sion intensity diminished rapidly with clear differences at emission
energies (Fig. 12(b)).
3.3. The structure of the copolymers in the solid state
XRD patterns of polymers are shown in Fig. 13. As shown in
Fig. 13, from PIII to PI, the intensity of peaks was increased. This
indicates tends of crystallinity and the side-to-side distance
N
N
TG
n
100
80
Exo
Decomposed product [5].
60
DTA
Endo
40
200
400
600
800
Temperature (°C)
Fig. 14. TG and DTA curves for copolymer with the heating rate of 10 minꢀ1 under dry
Fig. 13. XRD patterns of copolymers in the powder state.
nitrogen.

444
X. Mamtimin et al. / Polymer 51 (2010) 437–446
Table 3
an overall weight loss of 57.4% which was consistent with gradually
Electrochemical properties of polymers.
thermal breakdown of the alkyl side-chains adjacent to oxygen [5].
A very strong exothermic stage observed in the temperature range
of 751–866 ꢁC was due to phase transition of PI.
Polymers
p-doping(V)
n-doping(V)
Epa
Epc
Epc
Epa
PI
PII
PIII
0.65
1.36
1.16
ꢀ0.06
ꢀ0.42
ꢀ0.49
ꢀ0.65
ꢀ0.74
ꢀ0.70
ꢀ1.41
ꢀ1.13
ꢀ1.17
3.5. Electrochemical redox properties
The electrochemical properties of all three polymers were
investigated in an electrolyte consisting of a 0.10 M tetraethy-
lammoniumtetrafluoroborate [Et4N] [BF4] in acetonitrile by cyclic
voltammetry. The derived polymers were electroactive in both
anodic and cathodic region. The electrochemical data of the poly-
mers obtained from their cyclic voltammograms are summarized in
Table 3. The p-doping process of the polymers is sensitive to the
substitution on the phenylene ring. The attachment of dialkoxy
groups at the 2- and 5-positions of the phenylene ring resulted in
changes of p- and n-doping potential. In comparison with the p-
doping process, the n-doping process of the polymers is less
sensitive to the variation of substituent. Table 3 indicates that all
polymers showed similar electrochemical properties.
A representative CV curve of cast film of PI on a platinum plate is
depicted in Fig. 15. The electrochemical behavior of PI depicted
facile p-doping and good charge transporting properties attribut-
able to the presence of the easily protonated pyrimidinyl [5,34,35]
unit. It is seen that electro- chemical oxidation (or p-doping) starts
at about ꢀ0.2 V vs. Ag/AgCl and gives p-doping peak at 0.058 V vs.
Ag/AgCl. However, the anodic peak for PI in the ꢀ0.2 to 0.5 V region
is similar to those previously reported for polyacetylene [40–47].
This indicated that PI may possibly be the same p-doping state as
ethylene group or phenyl group in some extent and correlate with
the acetylenic carbons or phenyl carbons induced by electron-
deficient nature of pyrimidine ring (Scheme 3). At the same time
the p-doping of PI was accompanied by obvious color changes
(electrochromism) from brown in the neutral film to reddish in the
p-doped polymer film. The corresponding p-undoping peak
appears at ꢀ0.18 V vs. Ag/AgCl. The color of the film changed from
red in the p-doped polymer film to brown in the p-undoped
polymer film. The film did not reveal stability upon repeat scanning
of CV and did not give same CV curves. Sweeping to higher
potential in the interval of 1.7–3.0 V vs. Ag/AgCl gave no peak,
revealing high stability of the PI in this oxidative region. PI gives
a reducing (or n-doping) at ꢀ0.62 V, in contrast to other copoly-
mers comprising electron-deficient heterocycles like pyridine (n-
doping peak, ꢀ2.49 V; n-undoping peak, ꢀ2.21 V) [18], and its
corresponding n-undoping peak is observed at ꢀ0.56 V vs. Ag/AgCl.
This indicated that PI was more easily reduced or n-dopable than
other copolymers comprising electron-deficient heterocycles like
pyridine.
Epa and Epc stand for anodic peak potential, and cathodic peak potential,
respectively.
between the interlayer are increased when the dialkoxy groups
were introduced into the phenylene ring at the 2- and 5-positions.
The
p-conjugated copolymers with long side-chains often give an
XRD peak in a low angle region below 2
d spacing calculated from the peak often corresponds to the
q
¼ 10ꢁ (Cu-Ka), and the
distance between the
p-conjugated main chains separated by
the long side-chains [28,30,38]. When flat plane of the
p-conju-
gated copolymers form a face-to-face stacked assembly, it usually
gives another XRD peak corresponding to an interplane distance at
about d ¼ 3.8 Å [25,26,30]. All-aromatic polyesters with long side-
chains also form similar layered structure [21]. Thus, the XRD peak
in Fig. 13, For instance, the broad peak of PIII at about 24ꢁ should be
assigned to a side-to-side distance between the interlayer which
has a packing-dimension of about 3.7 Å [39].
3.4. Thermal properties
The thermal stability of copolymers was determined using
thermogravimetric-differential thermal analysis (TG-DTA) under
nitrogen.
The copolymers depicted very good thermal stability with
exhibiting an onset of degradation greater than 240 ꢁC in nitrogen
atmospheres. A representative TGA and DTA curves of PI in
nitrogen atmospheres are depicted in Fig. 14 and discussed.
Fig. 14 shows the thermograms for the neutral PI, and exhibited
an onset of degradation greater than 247 ꢁC under nitrogen with no
weight loss at lower temperature. The remaining weight at 900 ꢁC
was about 42.6% and it indicates that the char yield of PI in nitrogen
atmosphere was high even at 900 ꢁC, due to the stability of
conjugated backbone [5].
The DTA analysis of the PI showed a broad endothermic peak in
the temperature range of 247.8–685.1 ꢁC that was connected with
0.057
0.001
Optoelectrochemical data of the PI in a 0.10 M [Et4N] BF4
dichloromethane solution are shown in Fig. 16. On application of
a low potential of 0.04 V, the absorption band of the solution is
somewhat changed with a shift of UV–vis peak from 410 nm to
495 nm. The yellow color of the solution turned to red as shown in
Fig. 16. After adding Et3N to this red solution, the red color of the
solution turned back to yellow as show in Fig.16 and the absorption
band of the solution is somewhat changed with a shift of UV–vis
peak from 495 nm to 401 nm. It is considered that the absorption at
a longer wavelength in dichloromethane solution originates from
oxidation (or p-doping) of the PI on anode. Many papers reported
0.000
-0.54
-0.001
-0.63
-0.18
that oxidized state of
p-conjugated copolymers gave new absorp-
tion peak (s) in the wavelength range of 500–1500 nm [28,38]. The
absorption peak was as same as UV–vis spectra of the PI in CHCl3–
CH3SO3H mixtures (Fig. 9, (490 nm)). These results suggested that
oxidized state of PI has same
CH3SO3H mixtures (Fig. 9, (490 nm)), it is considered that oxidized
-1.0
-0.5
0.0
0.5
E(V)
p–
p* transition as PI in CHCl3–
Fig. 15. CV chart for copolymer cast on a Pt plate in a CH3CN solution of [Et4N] [BF4]
(0.10 M) at 20 mVsꢀ1
.

X. Mamtimin et al. / Polymer 51 (2010) 437–446
445
p-doping
0.057V
OC12H25
OC12H25
NH2
N
OC12H25
NH2
N
-
N
N
N
N
+ BF4
-
BF4
p-undoping
-0.18V
n
n
n
n
OC12H25
n-doping
-0.63V
OC12H25
OC12H25
NH2
N
OC12H25
OC12H25
NH2
N
+ Et4N+
Et4N+
n-undoping
-0.54V
Scheme 3. p-doping and n-doping of pyrimidine.
state of PI has coequal electronic energy state with a
tion along the main polymer chain, moreover, this
is strongly affected by the protonation of nitrogen in polymer.
Detailed studies on the relationship between acidochromism of
copolymers and their electrochemical properties are currently in
progress in our laboratory.
Fig. 17 gives CV of cast film of PI in different masses on a plat-
inum plate. The working electrode was a copolymer film electrode
prepared by casting the CHCI3 solution of copolymer on platinum
electrode.
The detailed procedure of the film preparation procedure was as
following: A certain mass of copolymer will be dissolved in
a certain volume of chloroform solution (its concentration could be
calculated), and then the chloroform solution is casted carefully
with a syringe on Pt and the solution on the Pt was dried very
carefully, after that the film electrode was formed. The mass of the
copolymer on the Pt was determined by the volume and concen-
tration of chloroform solution (the copolymer was almost insoluble
in acetonitrile).
p
–
p* transi-
–
p* transition
It has been seen that electrochemical oxidation and deoxidation
peak are sensitive to the mass of the PI on the Pt plate. With the
increasing mass of the PI on the Pt plate, the oxidation and deox-
idation peaks were moved to higher potential. For instance, the p-
doping peak was shifted from 0.05 V to 0.65 V and the p-undoping
peak was shifted from ꢀ0.18 V to ꢀ0.069 V, while the n-doping
peak was shifted from ꢀ0.62 V to ꢀ1.35 V and the n-undoping peak
was shifted from ꢀ0.54 V to ꢀ0.75 V.
p
4. Conclusion
In summary, novel conjugated copolymers comprising of alter-
nating electron donating phenylene units and electron accepting
pyrimidinyl units were synthesized by sonogashira coupling reac-
tion. The copolymers depicted good solubility in common organic
solvents, such as chloroform, THF and trifloroacetic acid. Our
investigation showed that the electronic and optical properties of
the copolymers were consistent with a rigid-rod conjugated back-
bone structure. They showed blue emission in their solution and
green emission in powder state. The copolymers in CHCl3–CH3SO3H
mixture depicted bathochromic shift. The electrochemical behavior
of the copolymers depicted facile p-doping and certain electron-
transporting properties.
4
3
2
1
0
After adding Et3N At 0.04V potential
411
495
0.006
0.004
0.002
0.000
-0.002
-0.004
-0.006
(a)
401
0.65
0.057
(b)
Normal
-0.069
-1.35
-3
-2
-1
0
1
2
3
400
600
E(V)
Wavelength (nm)
Fig. 17. CV chart for copolymer cast on a Pt plate in a CH3CN solution of [Et4N] [BF4]
(0.10 M) at 100 mVsꢀ1: (a) Mass of the copolymer was 6 mg; (b) Mass of the copolymer
was 0.03 mg.
Fig. 16. UV–vis spectra of the copolymer in a dichloromethane solution of 0.10 M
[Et4N] BF4 at 0.04 V potential.

446
X. Mamtimin et al. / Polymer 51 (2010) 437–446
[18] Yamamoto T, Zhou ZH, Kanbara T, Shimura M, Kizu K, Maruyama T, et al.
Acknowledgements
Journal of the American Chemical Society 1996;118:10389–99.
[19] Yasuda T, Sakai Y, Aramaki S, Yamamoto T. Chemistry of Materials
2005;17:6060–8.
[20] Yamamoto T, Sugiyama K, Kushida T, Inoue, Kanbara T. Journal of the American
Chemical Society 1996;118:3930–7.
[21] Watanabe J, Harkness BR, Sone M, Ichimura H. Macromolecules 1994;27:507–12.
[22] Yamamoto T, Koizumi TA. Polymer 2007;48:5449–72.
[23] Kim JY, Fukuda T. Reactive and Functional Polymers 2007;67:693–9.
[24] Tamura K, Fuji T, Shiotsuki M, Sanda F, Masuda T. Polymer 2008;49:4494–501.
[25] Li WW, Tang HY, Chen XF, Fan XH, Shen ZH, Zhou QF. Polymer 2008;49:4080–6.
[26] Michinobu T, Okoshi K, Osako H, Kumazawa H, Shigehara K. Polymer
2008;49:192–9.
The authors gratefully acknowledge support from the National
Natural Science Foundation of China (No. 20974092, 20674066). We
are also indebted to Mr. Wan Fu Sun for developing for the analysis
of spectra.
Appendix. Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.polymer.2009.11.010
[27] Gomtsyan A, Didomenico S, Lee CH, Matulenko MA, Kim K, Kowaluk EA, et al.
Journal of Medicinal Chemistry 2002;45:3639–48.
[28] Fang Q, Yamamoto T. Polymer 2003;44:2947–56.
[29] Yamamoto T, Nurulla I, Hayashi H, Koinuma H. Synthetic Metals
1999;107:137–41.
References
[30] McCullough RD, Nagle ST, Williams SP, Lowe RD, Jayaraman M. Journal of the
American Chemical Society 1993;115:4910–1.
[31] Nurulla I, Sugiyama K, Lee BL, Yamamoto T. Reactive and Functional Polymers
2000;46:49–53.
[32] Furukawa Y, Ueda F, Hyodo Y, Harada I, Nakajima T, Kawagoe T. Macromole-
cules 1988;21:1297–305.
[33] Cataldo F, Maltese P. European Polymer Journal 2002;38:1791–803.
[1] Moroni M, Le Moigne J, Luzzati S. Macromolecules 1994;27:562–71.
[2] Moroni M, Le Moigne J, Pham TA, Bigot JY. Macromolecules 1997;30:1964–72.
[3] Swager TM, Gil CJ, Wrighton MS. The Journal of Physical Chemistry
1995;99:4886–93.
[4] Wautelet P, Moroni M, Oswald L, Le Moigne J, Pham A, Bigot JY, et al.
Macromolecules 1996;29:446–55.
[5] Ng SC, Lu HF, Chan Hardy SO, Fuji A, Laga T, Yoshino K. Macromolecules
2001;34:6895–903.
[6] Heiner D, Ulf S, Erli S. Synthetic Metals 2004;147:227–31.
[7] Dong H, Li H, Wang E, Nakashima H, Torimitsu K, Wenping H. The Journal of
Physical Chemistry C 2008;112:19690–3.
[8] Hennebicq E, Pourtois G, Scholes GD, Herz LM, Russell DM, Silva C, et al.
Journal of the American Chemical Society 2005;127:4744–62.
[9] Tanase T, Goto E, Begum RA, Hamaguchi M, Zhan S, Iida M, et al. Organome-
tallics 2004;23:5975–88.
[10] Leoni P, Marchetti L, Mohapatra SK, Ruggeri G, Ricci L. Organometallics
2006;25:4226–30.
[11] Xieraili Maimaitiyiming, Ismayil Nurulla. Journal of Functional Polymers
2008;21:64–170 [Chinese].
[12] Berestetsky N, Vaganova E, Wachtel E, Leitus G, Goldberg A, Yitzchaik S. The
Journal of Physical Chemistry B 2008;112:3662–7.
[34] Lehmann F, Mohr GJ, Grummt UW. Sensors and Actuators
1997;39:229–34.
B Chemical
[35] Czerney P, Grummt UW. Sensors and Actuators B Chemical 1997;39:395–400.
[36] Bertinelli F, Delta CC. Polymer 1996;37:5469–75.
[37] Lanzi M, Bizzarri PC, Della CC. Synthetic Metals 1997;89:181–6.
[38] Yamamoto T. Macromolecular Rapid Communications 2002;23:583–606.
[39] Swan PR. Journal of Polymer Science 1962;56:403–7.
[40] Park LY, Ofer D, Gardner TJ, Schrock RR, Wrighton MS. Chemistry of Materials
1992;4:1388–95.
[41] Ofer D, Park LY, Schrock RR, Wrighton MS. Chemistry of Materials 1991;3:
573–5.
[42] Bredas JL, Silbey R, Boudreaux DS, Chance RR. Journal of the American
Chemical Society 1983;105:6555–9.
[43] Lonergan MC, Cheng CH, Langsdorf BL, Zhou X. Journal of the American
Chemical Society 2002;124:690–701.
[44] Qu J, Katsumata T, Satoh M, Wada J, Masuda T. Macromolecules
2007;40:3136–44.
[45] Gao L, Johnston D, Lonergan MC. Macromolecules 2008;41:4071–80.
[46] Guay J, Kasai P, Diaz A, Wu R, Tour JM, Dao LH. Chemistry of Materials
1992;4:1097–105.
[47] Harima Y, Patil R, Liu HF, Ooyama Y, Takimiya K, Otsubo T. The Journal of
Physical Chemistry B 2006;110:1529–35.
[13] Liu FT, Eisenberg A. Journal of the American Chemical Society
2003;125:15059–64.
[14] Malynych S, Luzinov I, Chumanov G. The Journal of Physical Chemistry B
2002;106:1280–5.
[15] Gillissen S, Jonforsen M, Kesters E, Johansson T, Theander M, Andersson MR,
et al. Macromolecules 2001;34:7294–9.
[16] Aoyama Y, Novak BM. Macromolecules 2001;34:6842–4.
[17] Kang NG, Changez M, Lee JS. Macromolecules 2007;40:8553–9.