Second-order nonlinear optical (NLO) polymers containing perfluoroaromatic rings as isolation groups with Ar/ArF self-assembly effect: Enhanced NLO coefficient and stability

Article abstract of DOI:10.1016/j.polymer.2013.07.073

Four new polyaryleneethynylenes P1-P4, which contained different aromatic or perfluoroaromatic isolation groups, were successfully prepared, via the palladium-catalyzed Sonogashira coupling reaction. Among them, P2, due to the presence of the self-assembl

Full text of DOI:10.1016/j.polymer.2013.07.073

Polymer 54 (2013) 5655e5664
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Second-order nonlinear optical (NLO) polymers containing
perfluoroaromatic rings as isolation groups with Ar/ArF self-assembly
effect: Enhanced NLO coefficient and stability
,
Wenbo Wu a, Qi Huang a, Cheng Zhong a, Cheng Ye b, Jingui Qin a, Zhen Li a
*
a Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
b Organic Solids Laboratories, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100080, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new polyaryleneethynylenes P1eP4, which contained different aromatic or perfluoroaromatic
isolation groups, were successfully prepared, via the palladium-catalyzed Sonogashira coupling reaction.
Among them, P2, due to the presence of the self-assembly effect between the perfluoroaromatic and
chromophore moieties, demonstrated much larger NLO effect (d33 value of 162.3 pm/V) and better
stability with onset temperature for decay (111 ꢀC). The research on the polymer conformation suggested
that perfluorophenyl groups in different positions would result in different effects on the NLO coefficient,
providing some useful information for the design of NLO polymers with better performance.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 26 March 2013
Received in revised form
26 May 2013
Accepted 27 July 2013
Available online 6 August 2013
Keywords:
Nonlinear optics (NLO)
Isolation groups
Ar/ArF self-assembly effect
1. Introduction
in the NLO field and according to the site isolation principle [4],
from 2006, our group have designed many different types of NLO
Organic second-order nonlinear optical (NLO) materials have
attracted much attention among scientists for their potential ap-
plications in photonics, due to their large NLO coefficients, ultrafast
response time and ease of integration, in comparison with con-
ventional inorganic NLO crystals (such as LiNbO3 and so on) [1].
One of the major problems encountered in optimizing organic NLO
materials is how to efficiently translate the high mb values of the
organic chromophores into large macroscopic NLO activities of
polymers [2]. To solve this problem, it is badly needed to overcome
or minimize the strong intermolecular dipoleedipole interactions
among the NLO chromophore moieties to achieve their orderly
alignment upon poling. Just this interaction accounts for the fact
that the NLO properties of the polymers are only enhanced several
times even if the mb values of chromophores have been improved
by up to 250-fold. Fortunately, it has been proved to be an efficient
approach for minimizing this interaction and enhancing the poling
efficiency to introduce isolation groups into chromophores to
further control the shape of the chromophores, with spherical
shape, proposed by Dalton and Jen et al., being the most ideal
conformation [3]. Based on the excellent work of pioneer scientists
dendronized polymers containing different isolation groups with
different size in the chromophore moieties (Charts S1eS6 in the
Supporting information). Based on the obtained experimental re-
sults, we proposed the concept of “suitable isolation group”, for the
design of NLO polymers with good performance [5].
However, nearly all the isolation groups in the previous cases
were only designed to adjust the shape of the chromophore and
decrease the strong interactions between the chromophore moi-
eties, and few reports were concerned on the interactions between
chromophore and isolation group or different isolation groups.
Different from normal aromatic rings, perfluoroaromatic rings were
electropositive, and this activity could lead to the reversible self-
assembly of the two types of aromatic rings, perfluoroaromatic
and normal aromatic ones [6]. For example, it has been determined
that the structure of the benzene/hexafluorobenzene mixture con-
sists of alternating stacks of these two molecules, while pure ben-
zene or pure hexafluorobenzene adopts an edge-to-face structure in
the solid-state (Chart S7). Recently, this AreArF self-assembly effect
has been successfully applied to NLO materials, by Jen and co-
workers in 2007: they developed a new class of molecular glasses
(Chart S8), by utilizing aromatic/perfluoroaromatic dendron-
substituted NLO chromophores through the presence of comple-
mentary Ar/ArF interactions, which exhibited the improved poling
efficiency and much enhanced macroscopic NLO effects [7]. Two
* Corresponding author. Fax: þ86 27 68756757.
E-mail addresses: lizhen@whu.edu.cn, lichemlab@163.com (Z. Li).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.07.073

5656
W. Wu et al. / Polymer 54 (2013) 5655e5664
years later, they observed the same phenomenon again in another
series of dendritic NLO chromophores containing perfluoroaromatic
moieties (Chart S9) [8]. This method was also applied into NLO
polymers and dendrimers by us, and the highest NLO effect (d33
value up to 166.7 pm/V) so far for linear polymers was achieved by
usingsimple azochromophore(Chart S10) [9]. However, till now, the
successful example was still very scarce, possibly due to the unclear
self-assembly behavior in the NLO polymers.
hydroxide. Dichloromethane (CH2Cl2, DCM) was dried over from
CaH2 and distilled under normal pressure before use. Diazonium
fluoroborate (3) [10] and N,N-di(4-pentynyl)benzenamine (5) [5f]
were synthesized according to the literature method. 4-
Iodobenzoic acid and pentafluorobenzoic acid were purchased
from Alfa-Aesar. N-Phenyldiethanol (1) was bought from Fluka. All
other reagents were used as received.
In this paper, we attempted to investigate the interactions be-
tween perfluoroaromatic moieties and other normal aromatic
moieties in NLO polymers, and their influence on the macroscopic
NLO effect in detail. Therefore, four polyaryleneethynylenes (P1e
P4) were synthesized (Scheme 1), in which the isolation groups
were elaborately designed as either aromatic or perfluoroaromatic
ones. The tested NLO properties demonstrated that the position of
the perfluoroaromatic groups affected the NLO effects of the
polymers in a large degree. The theoretical calculation research also
supported this point. Furthermore, thanks to the Ar/ArF self-
assembly effect, the stability of NLO effects was improved. Herein,
we would like to present the syntheses, characterization and
properties of these polymers in detail.
2.2. Instrumentation
1H and 13C NMR spectra were measured on a Varian Mercury300
or Bruker ARX 400 spectrometer using tetramethylsilane (TMS;
d
¼ 0 ppm) as internal standard. 19F NMR spectra were measured on
a Varian Mercury600 spectrometer. The Fourier transform infrared
(FT-IR) spectra were recorded on a PerkinElmer-2 spectrometer in
the region of 3000e400 cmꢁ1. UVevisible spectra were obtained
using a Shimadzu UV-2550 spectrometer. Elemental analyses (EA)
were performed by a CARLOERBA-1106 microelemental analyzer.
Gel permeation chromatography (GPC) was used to determine the
molecular weights of polymers. GPC analysis was performed on a
Waters HPLC system equipped with a 2690D separation module
and a 2410 refractive index detector. Polystyrene standards were
used as calibration standards for GPC. THF was used as an eluent,
and the flow rate was 1.0 mL/min. Thermal analysis was performed
on NETZSCH STA449C thermal analyzer at a heating rate of 10 ꢀC/
min in nitrogen at a flow rate of 50 cm3/min for thermogravimetric
analysis (TGA) and the thermal transitions of the polymers. The
thickness of the films was measured with an Ambios Technology
XP-2 profilometer.
2. Experimental section
2.1. Materials
Tetrahydrofuran (THF) was dried over and distilled from KeNa
alloy under an atmosphere of dry nitrogen. Triethylamine (Et3N)
was distilled under normal pressure and kept over potassium
I
I
N
O
O
N
N
O
O
CuI
PPh3
+
N
Pd[PPh3]4
THF/Et3N
N
O
O
N
O
O
Ar2
O
O
Ar1
NO2
N
NO2
n
O
O
N
N
O
O
N
P1-P4
N
O
O
O
Ar2
N
O
O
O
NO2
Ar1
NO2
F
F
F
F
F
F
F
F
Ar =
1
P1:
Ar =
1
Ar =
P3:
1
P4:Ar1=
P2:
F
F
F
F
F
F
F
Ar2=
Ar2=
F
Ar2=
Ar2=
F
F
F
F
Scheme 1. The synthesis of NLO polymers.

W. Wu et al. / Polymer 54 (2013) 5655e5664
5657
2.3. Synthesis of compound 2
2.6.1. Chromophore C1
Compound 4 (425.2 mg, 0.50 mmol), benzoic acid (91.6 mg,
0.75 mmol). The crude product was purified by column chroma-
tography on silica gel using ethyl acetate/chloroform (1/15, v/v) as
N-Phenyldiethanolamin (1) (1.08 g, 6.0 mmol), 4-iodobenzoic
acid (4.46 g, 18.0 mmol), 1-(3-dimethylaminopropyl)-3-ethyl
carbodiimide hydrochloride (EDC) (4.60 g, 24.0 mmol), and 4-
(N,N-dimethyl)aminopyridine (DMAP) (288 mg, 2.40 mmol)
were dissolved in dry CH2Cl2 (120 mL) and stirred at room
temperature for 3 days, and then treated with saturated solution
of citric acid and extracted with CH2Cl2, washed with brine and
saturated solution of citric acid. After the removal of all the
solvent, the crude product was recrystallized from a lot of
acetone to afford white solid (3.80 g, 98.8%). 1H NMR (300 MHz,
eluent to afford deep red solid (459.6 mg, 96.2%). IR (KBr),
y
(cmꢁ1):
1719 (C]O), 1517, 1338 (eNO2). 1H NMR (400 MHz, CDCl3, 298 K),
d
(TMS, ppm): 3.91 (t, J ¼ 8.0 Hz, 4H, eNCH2e), 4.55 (t, J ¼ 8.0 Hz,
4H, eCOOCH2e), 4.63 (t, J ¼ 4.0 Hz, 2H, eOCH2e), 4.81 (t, J ¼ 4.0 Hz,
2H, eCOOCH2e), 6.89 (d, J ¼ 8.0 Hz, 2H, ArH), 7.39 (d, J ¼ 8.0 Hz, 2H,
ArH), 7.53 (d, J ¼ 8.0 Hz, 1H, ArH), 7.69 (m, 5H, ArH), 7.79 (d,
J ¼ 8.0 Hz, 4H, ArH), 7.90 (m, 3H, ArH), 8.04 (m, 3H, ArH). 13C NMR
(75 MHz, CDCl3, 298 K),
d (ppm): 49.5, 61.6, 62.8, 68.4, 101.1, 110.3,
DMSO-d6, 298 K),
d
(TMS, ppm): 3.79 (t, J ¼ 5.1 Hz, 4H, eNCH2e),
111.6, 117.0, 117.4, 126.0, 128.1, 128.6, 129.5, 130.7, 132.9, 137.6, 144.8,
147.0, 148.0, 150.5, 154.8, 165.7. C39H32N4O9I2 (EA) (%, found/calcd):
C, 48.82/49.07; H, 3.82/3.38; N, 5.80/5.87. UVevis (THF,
1 ꢂ 10ꢁ5 mmol/mL): lmax (nm): 465.
4.41 (t, J ¼ 5.1 Hz, 4H, eOCH2e), 6.63 (t, J ¼ 7.2 Hz, 1H, ArH), 6.88
(d, J ¼ 8.4 Hz, 2H, ArH), 7.17 (t, J ¼ 7.8 Hz, 2H, ArH), 7.63 (d,
J ¼ 8.1 Hz, 4H, ArH), 7.87 (d, J ¼ 8.1 Hz, 4H, ArH). 13C NMR
(75 MHz, DMSO-d6, 298 K),
d (ppm): 50.4, 64.1, 103.6, 113.7, 117.9,
130.6, 130.8, 132.5, 139.3, 149.0, 167.1.
2.6.2. Chromophore C2
Compound 4 (425.2 mg, 0.50 mmol), pentafluorobenzoic acid
(159.1 mg, 0.75 mmol). The crude product was purified by column
chromatography on silica gel using ethyl acetate/chloroform (1/15,
2.4. Synthesis of compound 4
Compound 2 (962 mg, 1.50 mmol) and diazonium salt (3)
v/v) as eluent to afford red solid (439.0 mg, 84.1%). IR (KBr),
(cmꢁ1): 1721 (C]O), 1518, 1337 (eNO2). 1H NMR (400 MHz, CDCl3,
298 K),
y
(668 mg, 2.25 mmol) were dissolved in DMF/THF (15 mL/15 mL) at
0
ꢀC. The reaction mixture was stirred for 36 h at 0 ꢀC, and then
d
(TMS, ppm): 3.91 (t, J ¼ 8.0 Hz, 4H, eNCH2e), 4.57 (m, 6H,
treated with H2O and extracted with a lot of CHCl3, washed with
brine. The organic layer was dried over anhydrous sodium sulfate.
After the removal of the organic solvent, the crude product was
purified by column chromatography on silica gel using ethyl ace-
tate/petroleum ether (2/3, v/v) as eluent and then recrystallized
from ethyl acetate to afford red solid (920 mg, 72.2%). 1H NMR
eCOOCH2e and eOCH2e), 4.88 (t, J ¼ 4.0 Hz, 2H, eCOOCH2e), 6.92
(d, J ¼ 8.0 Hz, 2H, ArH), 7.69 (m, 5H, ArH), 7.79 (d, J ¼ 8.0 Hz, 4H,
ArH), 7.88 (d, J ¼ 8.0 Hz, 2H, ArH), 7.95 (m, 2H, ArH). 13C NMR
(75 MHz, CDCl3, 298 K),
d (ppm): 15.6, 25.4, 49.6, 62.8, 68.3, 69.2,
82.7, 110.3, 111.1, 117.1, 117.2, 126.1, 128.0, 129.4, 132.8, 144.0, 147.6,
150.8, 154.5. 19F NMR (564 MHz, CDCl3, 298 K),
d
(ppm): e137.77, e
(400 MHz, CDCl3, 298 K),
d
(TMS, ppm): 3.83 (s, br, 2H, eOCH2e),
148.27, e160.61. C39H27N4O9F5I2 (EA) (%, found/calcd): C, 44.42/
44.85; H, 2.98/2.61; N, 5.56/5.36. UVevis (THF, 1 ꢂ10ꢁ5 mmol/mL):
lmax (nm): 463.
3.98 (s, br, 4H, eNCH2e), 4.32 (s, br, 4H, eOCH2e), 5.0 (s, 1H, eOH),
7.09 (d, J ¼ 9.0 Hz, 2H, ArH), 7.64 (m, 5H, ArH), 7.85 (m, 7H, ArH),
8.01 (s, 1H, ArH). 13C NMR (75 MHz, CDCl3, 298 K),
d (ppm): 49.6,
60.2, 63.1, 72.3, 102.8, 110.5, 112.9, 117.9, 126.4, 129.5, 131.5, 138.4,
144.6, 147.0, 148.5, 152.1, 155.7, 166.1.
2.6.3. Chromophore C3
Compound 6 (499.7 mg, 1.15 mmol), benzoic acid (211.3 mg,
1.73 mmol). The crude product was purified by column chroma-
tography on silica gel using ethyl acetate/chloroform (1/20, v/v) as
2.5. Synthesis of compound 6
eluent to afford deep red solid (614.3 mg, 99.2%). IR (KBr),
3267 (C^CeH), 1711 (C]O), 1519, 1335 (eNO2). 1H NMR (400 MHz,
CDCl3, 298 K), (TMS, ppm): 1.87 (m, 4H, eCH2e), 2.06 (s, 2H, e
y
(cmꢁ1):
N,N-Di(4-pentynyl)benzenamine (5) (901.3 mg, 4.0 mmol)
and diazonium salt (3) (1.19 g, 4.0 mmol) were dissolved in DMF
at 0 ꢀC. The reaction mixture was stirred for 12 h at 0 ꢀC, and
then treated with H2O and extracted with CH2Cl2, washed with
brine. The organic layer was dried over anhydrous sodium sul-
fate. After the removal of the organic solvent, the crude product
was purified by column chromatography on silica gel using ethyl
acetate/petroleum ether (2/1, v/v) as eluent to afford deep
red solid (1.1 g, 63.3%). 1H NMR (400 MHz, CDCl3, 298 K),
d
C^CeH), 2.30 (m, 4H, eCH2e), 3.58 (t, J ¼ 8.0 Hz, 4H, eNCH2e),
4.62 (t, J ¼ 4.0 Hz, 2H, eOCH2e), 4.80 (t, J ¼ 4.0 Hz, 2H, eCOOCH2e),
6.74 (d, J ¼ 8.0 Hz, 2H, ArH), 7.40 (t, J ¼ 8.0 Hz, 2H, ArH), 7.55 (t,
J ¼ 8.0 Hz, 1H, ArH), 7.69 (d, J ¼ 8.0 Hz, 1H, ArH), 7.86 (d, J ¼ 8.0 Hz,
1H, ArH), 7.92 (m, 1H, ArH), 8.05 (m, 3H, ArH), 8.17 (d, J ¼ 8.0 Hz,1H,
ArH). 13C NMR (75 MHz, CDCl3, 298 K),
d (ppm): 15.8, 25.6, 49.8,
63.0, 68.4, 69.5, 83.0, 110.4, 111.2, 117.3, 126.3, 128.3, 129.6, 133.0,
144.1, 147.3, 147.8, 151.0, 154.7, 166.3. C31H30N4O5 (EA) (%, found/
calcd): C, 68.77/69.13; H, 6.06/5.61; N, 10.06/10.40. UVevis (THF,
1 ꢂ 10ꢁ5 mmol/mL): lmax (nm): 489.
d
(TMS, ppm): 1.88 (m, 4H, eCH2e), 2.05 (s, 2H, eC^CeH), 2.30
(m, 4H, eCH2e), 3.59 (t,
J
¼ 6.0 Hz, 4H, eNCH2e), 4.00
(t, J ¼ 4.0 Hz, 2H, eOCH2e), 4.37 (t, J ¼ 4.0 Hz, 2H, eOCH2e), 6.80
(d, J ¼ 8.0 Hz, 2H, ArH), 7.72 (d, J ¼ 8.0 Hz, 1H, ArH), 7.86
(d, J ¼ 8.0 Hz, 2H, ArH) 7.95 (m, 2H, ArH).
2.6.4. Chromophore C4
Compound 6 (499.7 mg, 1.15 mmol), pentafluorobenzoic acid
(365.8 mg, 1.73 mmol). The crude product was purified by column
chromatography on silica gel using ethyl acetate/chloroform (1/20,
2.6. General procedure for the synthesis of chromophores C1eC4
Compound 4 or 6 (1.00 equiv.), carboxyl-containing compound
(1.50 equiv.), 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hy-
drochloride (EDC) (2.00 equiv.), and 4-(N,N-dimethyl)amino-
pyridine (DMAP) (0.20 equiv.) were dissolved in dry CH2Cl2
(0.1 mmol/mL of compound 4 or 6) and stirred at room temperature
for 3 h, and then treated with saturated solution of citric acid and
extracted with CH2Cl2, washed with brine and saturated solution of
citric acid. After the removal of the solvent, the crude product was
purified by column chromatography on silica gel.
v/v) as eluent to afford deep red solid (686.2 mg, 94.9%). IR (KBr),
(cmꢁ1): 3299 (C^CeH), 1743 (C]O), 1516, 1336 (-NO2). 1H NMR
(400 MHz, CDCl3, 298 K), (TMS, ppm): 1.87 (m, 4H, eCH2e), 2.05
y
d
(s, 2H, eC^CeH), 2.30 (m, 4H, eCH2e), 3.58 (t, J ¼ 8.0 Hz, 4H, e
NCH2e), 4.56 (t, J ¼ 4.0 Hz, 2H, eOCH2e), 4.88 (t, J ¼ 4.0 Hz, 2H, e
COOCH2e), 6.76 (d, J ¼ 8.0 Hz, 2H, ArH), 7.69 (d, J ¼ 8.0 Hz, 1H, ArH),
7.83 (d, J ¼ 8.0 Hz, 2H, ArH), 7.94 (m, 2H, ArH). 13C NMR (75 MHz,
CDCl3, 298 K),
d (ppm): 15.9, 25.7, 49.9, 64.2, 67.9, 69.4, 83.0, 110.2,
111.2, 117.5, 126.3, 144.1, 147.5, 147.8, 151.1, 154.4, 158.9. 19F NMR

5658
W. Wu et al. / Polymer 54 (2013) 5655e5664
(564 MHz, CDCl3, 298 K),
d
(ppm): ꢁ137.83, ꢁ148.42 (d,
(eOCH2e), 6.6e6.9 (ArH), 7.2e7.5 (ArH), 7.5e7.6 (ArH), 7.7e8.0
(ArH). 13C NMR (75 MHz, CDCl3, 298 K),
(ppm): 16.1, 25.1, 48.6,
49.3, 60.7, 62.1, 63.3, 67.0, 67.8, 80.0, 91.4, 106.7, 109.6, 110.8, 116.5,
125.2, 127.3, 127.5, 128.5, 128.7, 130.0, 130.5, 131.1, 132.0, 136.8,
143.6, 145.3, 147.2, 149.9, 153.7, 157.8, 164.8. 19F NMR (564 MHz,
J ¼ 1.7 Hz), ꢁ160.67. C31H25N4O5F5 (EA) (%, found/calcd): C, 59.24/
59.66; H, 3.95/4.01; N, 8.91/8.91. UVevis (THF, 1 ꢂ 10ꢁ5 mmol/mL):
lmax (nm): 489.
d
CDCl3, 298 K),
0.02 mg/mL): lmax (nm): 476.
d
(ppm): ꢁ137.79, ꢁ148.28, ꢁ160.63. UVevis (THF,
2.7. General procedure for the synthesis of P1eP4
A mixture of chromophore C1 or C2 (1.00 equiv.), chromophore
C3 or C4 (1.00 equiv.), copper iodide (CuI) (5 mol-%), tetrakis(-
triphenylphosphine)palladium (Pd(PPh3)4) (3 mol-%) and triphe-
nylphosphine (PPh3) (5 mol-%) was carefully degassed and charged
with argon. THF (9 mL)/Et3N (1 mL) was then added. The reaction
was stirred for 3 days at room temperature. The mixture was passed
through a cotton filter and dropped into a lot of methanol. The
precipitate was collected, further purified by several precipitations
of its THF solution into acetone, and dried in a vacuum at 40 ꢀC to a
constant weight.
2.7.3. P3
Chromophore C2 (73.1 mg, 0.07 mmol), chromophore C3
(37.7 mg, 0.07 mmol). P3 was obtained as deep red powder
(86.3 mg, 92.4%). Mw ¼ 7200, Mw/Mn ¼ 1.43 (GPC, polystyrene
calibration). IR (KBr),
NMR (300 MHz, CDCl3 298 K),
y
(cmꢁ1): 1720 (C]O), 1518, 1338 (eNO2). 1H
d
(TMS, ppm): 1.8e2.0 (eCH2e), 2.4e
2.6 (eCH2e), 3.4e3.8 (eNCH2e), 3.8e4.0 (eNCH2e), 4.4e4.6 (e
OCH2e), 4.6e4.9 (eOCH2e), 6.6e6.9 (ArH), 7.2e7.5 (ArH), 7.5e8.0
(ArH). 13C NMR (75 MHz, CDCl3, 298 K),
d (ppm): 16.8, 21.9, 25.7,
27.5, 29.4, 49.5, 61.5, 62.9, 64.0, 67.7, 68.4, 80.8, 92.0, 110.0, 111.5,
117.4, 126.0, 128.1, 128.3, 129.3, 129.5, 130.7, 131.3, 131.8, 132.9, 137.6,
144.7, 146.9, 148.0, 150.2, 154.4, 158.6, 166.0. 19F NMR (564 MHz,
CDCl3, 298 K),
0.02 mg/mL): lmax (nm): 477.
2.7.1. P1
Chromophore C1 (114.5 mg, 0.12 mmol), chromophore C3
(64.6 mg, 0.12 mmol). P1 was obtained as deep red powder
(107.2 mg, 72.3%). Mw ¼ 17 200, Mw/Mn ¼ 1.99 (GPC, polystyrene
d
(ppm): ꢁ137.81, ꢁ148.46, ꢁ160.70. UVevis (THF,
calibration). IR (KBr),
NMR (300 MHz, CDCl3 298 K),
y
(cmꢁ1): 1719 (C]O), 1517, 1338 (eNO2). 1H
2.7.4. P4
d
(TMS, ppm): 1.8e2.1 (eCH2e), 2.4e
Chromophore C2 (114.8 mg, 0.11 mmol), chromophore C4
(69.1 mg, 0.11 mmol). P4 was obtained as deep red powder (126 mg,
80.7%). Mw ¼ 6 600, Mw/Mn ¼ 1.57 (GPC, polystyrene calibration). IR
2.6 (eCH2e), 3.5e4.0 (eNCH2e), 4.4e5.0 (eOCH2e), 6.7e7.0 (ArH),
7.2e7.6 (ArH), 7.6e8.1 (ArH). 13C NMR (75 MHz, CDCl3, 298 K),
d
(ppm): 17.5, 26.4, 50.1, 50.7, 62.2, 63.5, 69.0, 81.5, 92.9, 101.7, 111.0,
(KBr),
CDCl3 298 K),
y
(cmꢁ1): 1724 (C]O), 1518, 1337 (eNO2). 1H NMR (300 MHz,
112.3, 118.0, 126.8, 128.8, 130.2, 132.0, 133.6, 138.3, 145.5, 147.6,
155.5, 166.3, 166.8. UVevis (THF, 0.02 mg/mL): lmax (nm): 475.
d
(ppm): 1.8e2.1 (eCH2e), 2.3e2.4 (eCH2e), 2.4e2.6
(eCH2e), 3.4e3.8 (eNCH2e), 3.8e4.0 (eNCH2e), 4.4e4.7 (eOCH2e
), 4.8e5.0 (eOCH2e), 6.7e6.9 (ArH), 6.9e7.0 (ArH), 7.3e7.5 (ArH),
2.7.2. P2
7.6e8.0 (ArH). 13C NMR (75 MHz, CDCl3, 298 K),
d (TMS, ppm): 17.2,
Chromophore C1 (104.9 mg, 0.11 mmol), chromophore C4
(69.1 mg, 0.11 mmol). P2 was obtained as deep red powder
(128.4 mg, 87.7%). Mw ¼ 8200, Mw/Mn ¼ 1.54 (GPC, polystyrene
26.2, 50.0, 61.9, 63.2, 64.4, 68.3, 81.2, 92.5, 111.6, 112.0, 117.7, 126.3,
128.4,128.7,129.6,129.8,131.0, 131.6, 133.1, 136.8,137.9,144.7, 146.5,
151.0, 154.6, 158.9, 165.9. 19F NMR (564 MHz, CDCl3, 298 K),
calibration). IR (KBr),
1H NMR (300 MHz, CDCl3 298 K),
y
(cmꢁ1): 1718 (C]O), 1517, 1338 (eNO2).
d
(ppm): ꢁ137.81, ꢁ148.27, ꢁ160.65. UVevis (THF, 0.02 mg/mL):
d
(TMS, ppm): 1.7e2.0 (eCH2e),
lmax (nm): 475.
2.4e2.6 (eCH2e), 3.3e3.9 (eNCH2e), 4.3e4.6 (eOCH2e), 4.6e4.8
-
N2+BF4
O
OH
I
I
3
HO
OH
I
COOH
O
O
N
O
NO2
THF,DMF
N
O
O
EDC,DMAP,DCM
2
1
I
I
I
I
O
O
O
O
N
N
N
C1: Ar=
O
O
O
F
F
F
F
Ar COOH
EDC,DMAP,DCM
C1-C2
4
F
C2: Ar=
N
N
N
O
O
O
OH
O
Ar
NO2
NO2
N
N
C3: Ar=
F
F
F
F
N
3
6
Ar COOH
EDC,DMAP,DCM
C3-C4
F
C4: Ar=
N
N
5
N
O
N
THF,DMF
O
O
O
Ar
OH
NO2
NO2
Scheme 2. The synthesis of monomers C1eC4.

W. Wu et al. / Polymer 54 (2013) 5655e5664
5659
the incident beam and poled inside the oven, and the SHG intensity
was monitored simultaneously. Poling conditions were as follows:
temperature, different for each polymer (Table 3); voltage, 7.8 kV at
the needle point; gap distance, 0.8 cm. The SHG measurements
were carried out with a Nd:YAG laser operating at a 10 Hz repeti-
tion rate and an 8 ns pulse width at 1064 nm. A Y-cut quartz crystal
served as the reference.
P1
P2
P3
P4
3. Results and discussion
3.1. Synthesis and characterization
C CH
≡
The overall pathway of the monomer and polymer synthesis
was presented in Schemes 1 and 2. The whole synthetic route
utilized some simple reactions, such as azo-coupling reaction,
esterification reaction and Sonogashira coupling reaction. The ob-
tained polymers could be readily soluble in common polar organic
solvents, including CH2Cl2, CHCl3, THF, DMF and DMSO, making it
convenient to test their NLO and other properties.
C=O
-NO
2
4000
3000
2000
1600
1200
800
400
-1
Wavenumber (cm )
Fig. 1. The FT-IR spectra of polymer P1eP4.
All the prepared polymers are new compounds, which were
characterized by spectroscopic methods, and all gave satisfactory
spectral data (see Experimental section and Supporting
information for detailed analysis data). Figs. 1 and S1 showed the
IR spectra of polymers P1eP4 and chromophores C1eC4, respec-
tively, in which the absorption bands associated with the nitro
groups and carbonyl groups were at about 1338, 1517 and
1720 cmꢁ1, respectively, showing that the chromophore and
isolation groups were stable during the Sonogashira polymeriza-
tions. Meanwhile, there was an absorption band derived from the
C^CeH stretching vibrations at about 3277 cmꢁ1 in the FT-IR
spectra of chromophores C3 and C4, while it disappeared in the
spectra of its corresponding polymers P1eP4, indicating the suc-
cessful polymerizations.
2.8. Preparation of polymer thin films
The polymers were dissolved in THF (concentration w3 wt %),
and the solutions were filtered through syringe filters. Polymer
films were spin coated onto indium-tin-oxide (ITO)-coated glass
substrates, which were cleaned by DMF, acetone, distilled water,
and THF sequentially in an ultrasonic bath before use. Residual
solvent was removed by heating the films in a vacuum oven
at 40 ꢀC.
2.9. NLO measurement of poled films
The second-order optical nonlinearity of the polymers was
determined by in situ second harmonic generation (SHG) experi-
ment, using a closed temperature-controlled oven with optical
windows and three needle electrodes. The films were kept at 45ꢀ to
Fig. 2 showed the 1H NMR spectra of C1, C3 and P1, as well as
their chemical structures, as an example to illustrate the successful
synthesis, while the NMR spectra of other compounds shown in
Fig. 2. 1H NMR spectra of polymer P1 and its corresponding monomers C1 and C3.

5660
W. Wu et al. / Polymer 54 (2013) 5655e5664
Table 1
1.4
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Characterization data of polymers.
B
A
P1
P2
P3
P4
C1
C2
C3
C4
1.2
a
a
b
c
No.
Yield (%)
Mw
Mw/Mn
Tg (ꢀC)
Td (ꢀC)
P1
P2
P3
P4
72.3
87.7
92.4
80.7
17 200
8200
7200
6600
1.99
1.54
1.43
1.57
105
135
111
100
258
224
226
217
1.0
0.8
0.6
0.4
0.2
0.0
a
Determined by GPC in THF on the basis of a polystyrene calibration.
Glass transition temperature (Tg) of polymers detected by the DSC analyses
b
under argon at a heating rate of 10 ꢀC/min.
c
The 5% weight loss temperature of polymers detected by the TGA analyses under
nitrogen at a heating rate of 10 ꢀC/min.
Figs. S2eS26 in the Supporting information. In the 1H NMR spectra
of monomers C1 and C3, no unexpected resonance peaks were
observed, and the chemical shifts were consistent with the pro-
posed structure (the corresponding peaks were marked in Fig. 1, ae
n for C1 while 1e14 for C3). After polymerization, all the peaks of P1
showed an apparent inclination of signal broadening, and the
disappearance of the single peaks associated with the protons of
C^CH (marked 2 in C3) at 2.06 ppm, confirming the successful
polymerization again. The structure of other polymers could be also
confirmed similarly. On the other hand, 19F NMR spectrum was
another way to confirm the structures of the monomers and
polymers due to the presence of perfluoroaromatic moieties. As
shown in Figs. S11 and S16, there were three peaks in the 19F NMR
spectra of chromophores C2 and C4, while no peak in the 19F NMR
spectra of chromophores 4 and 6, indicating the esterifications
were successful. In the same way, the structures of P2eP4 with
pentafluorophenyl as isolation group were confirmed.
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
C
D
P1
C1
C3
P1
the value of C1+C3
C1 and C3 mixed
400
500
600
400
500
600
Wavelength (nm)
Wavelength (nm)
The molecular weights of polymers were determined by gel
permeation chromatography (GPC) with THF as an eluent and
polystyrene standards as calibration standards, with the results
summarized in Table 1. Interestingly, under the same polymeriza-
tion conditions, P2eP4, with pentafluorophenyl as isolation group,
exhibited much lower molecular weights than P1, and the more
pentafluorophenyl groups, the lower molecular weights. Consid-
ering that under the same polymerization conditions, the only
factor to affect the molecular weights was the reaction rate, we
could surmise that the introduction of perfluoroaromatic rings
would make the reactive activity of Sonogashira reaction a little
lower. On the other hand, the low reaction rate usually leads to low
yield. However, the yields of P2eP4 were even higher than that of
Fig. 4. UVevis spectra in THF solutions: A: polymers P1eP4 (0.02 mg/mL); B: chro-
mophores C1eC4 (1.0 ꢂ 10ꢁ5 mg/mL); C: normalized absorptions of P1 and its cor-
responding monomers; D: normalized absorptions of polymer P1, its corresponding
monomers C1 and C3 (1.0 ꢂ 10ꢁ5 mg/mL) mixed with equivalent and the sum value of
C1 and C3 (1.0 ꢂ 10ꢁ5 mg/mL).
P1, and it seemed that there was a contradiction to our surmise. But
it was not true. In order to improve the performance of the obtained
polymers, the precipitation procedure was used to remove the
monomers and oligomers. Meanwhile, the chemical structures of
these four polymers were not the same exactly, which would result
in their different solubility in acetone, the poor solvent used in
precipitation. Thus, the final yields of these polymers did not only
depend on the reaction rate, but also their solubility, and the effect
of solubility should be even more important. Furthermore, the
lower polymerization rate of P2eP4 also indicated that there
should be some interactions between the perfluoroaromatic rings
and normal aromatic rings in these polymers, which was desirable
as expected. Luckily, thanks to their branched structure, the thin
films of P2eP4, with low molecular weights, could be still easily
100
95
90
85
80
P1
75
P2
Table 2
70
The maximum absorption of polymers and chromophores (lmax, nm).a
P3
65
P4
THF
1,4-Dioxane
Chloroform
Dichloromethane
DMF
DMSO
60
55
50
P1
P2
P3
P4
C1
C2
C3
C4
475
476
477
475
465
463
489
489
466
466
467
468
456
458
478
479
475
477
476
477
461
459
493
493
479
478
479
477
463
464
494
495
491
489
490
490
481
480
500
500
498
497
496
497
487
486
507
507
50 100 150 200 250 300 350 400 450 500 550
o
Temperature ( C)
a
Fig. 3. TGA thermograms of P1eP4, measured in nitrogen at a heating rate of 10 ꢀC/
min.
The maximum absorption wavelength of polymers (chromophore molecules)
solutions with the concentrations fixed at 0.02 mg/mL (2.5 ꢂ 10ꢁ5 mol/mL).

W. Wu et al. / Polymer 54 (2013) 5655e5664
5661
Table 3
shown in Fig. 4D, it became obvious that the maximum absorption
wavelengths of the chromophore moieties in the polymer of P1 was
nearly the same as the maximum absorption wavelength of the
mixture of chromophore C1 and chromophore C3 with equivalent.
Meanwhile, the sum value of the UVevis spectra of THF solutions of
C1 and C3 was also shown as the normalized spectrum in Fig. 4D for
reference. The spectrum was nearly the same as the UVevis ab-
sorption spectra of P1, indicating the surrounding environment of
chromophore moieties did not change much during the polymer-
ization process. Similar phenomena were observed in other poly-
mers (Figs. S37eS39). The almost same lmaxs of these four
polymers indicated that the presence of the different isolation
spacers nearly did not influence the electronic properties of the
chormophore moieties, which would ensure the comparison of
their NLO properties on the same level.
Characterization data of polymers.
lb
(mm)
d33 (pm/V)
d33(N) (pm/V)
Fe
Nf
a
c
d
No.
Te (ꢀC)
P1
P2
P3
P4
110
135
110
95
0.38
0.32
0.31
0.37
100.1
162.3
94.4
17.1
27.7
16.1
25.6
0.21
0.33
0.15
0.29
0.48
0.45
0.45
0.42
150.1
a
The best poling temperature.
Film thickness.
Second harmonic generation (SHG) coefficient.
b
c
d
The nonresonant d33 values calculated by using the approximate two-level
model.
e
Order parameter
F
¼ 1 ꢁ A1/A0, A1 and A0 are the absorbance of the polymer film
after and before corona poling, respectively.
f
The loading density of the effective chromophore moieties.
prepared by spin-coating, making it convenient to test their NLO
properties.
3.2. NLO properties
The TGA thermograms were shown in Fig. 3, and the 5% weight
loss temperature (Td) of polymers was listed in Table 1. All the
polymers were thermally stable with the Td values higher than
217 ꢀC. P2eP4 exhibited a little worse thermal stability than P1,
possibly caused by two reasons: first, the pentafluorophenyl group
was not so stable as normal phenyl moieties; secondly, the mo-
lecular weights of P1 were much higher than those of P2eP4. P4,
with the lowest molecular weights and containing more penta-
fluorophenyl groups, thus demonstrated the worst thermal stabil-
ity. But it should be already enough for NLO materials, because the
temperature for its real application was generally lower than
200 ꢀC. The glass transition temperatures (Tg) of the polymers were
also investigated by using a differential scanning calorimeter (DSC),
with the results summarized in Table 1, and all of them showed
moderate Tg in the range of 100e135 ꢀC.
To evaluate the NLO activity of these polymers with per-
fluoroaromatic moieties in different positions, their poled thin
films were prepared. The most convenient technique to study the
second-order NLO activity was to investigate the second harmonic
generation (SHG) processes characterized by d33, an SHG coeffi-
cient. To check the reproducibility, we repeated the measurements
at least three times for each sample. Calculation of the SHG co-
efficients (d33) for the poled films was based on the following
equation: [11]
sffiffiffiffiffiffiffiffiffiffiffiffiffi
cðs2Þ
cðq2Þ
Is lc;q
Iq ls
d33;s
d11;q
¼
¼
F
where d11,q is d11 of the quartz crystals, which is equal to 0.45 pm/V.
Is and Iq are the SHG intensities of the sample and the quartz,
respectively, lc,q is the coherent length of the quartz, ls is the
thickness of the polymer film, and F is the correction factor of the
apparatus and is equal to 1.2 when lc is much greater than ls. From
the experimental data, the d33 values of P1eP4 were calculated at a
fundamental wavelength of 1064 nm (Table 3). Generally, the d33
value of the same NLO polymer can be different when measured by
different methods or different testing systems at different times. To
avoid the above-mentioned possible deviations, the NLO properties
of all the polymers were tested at the same time.
As shown in Table 3, the d33 values of P1eP4 were 100.1, 162.3,
94.4 and 150.1 pm/V, respectively. These results were encouraging,
and it meant that the perfluoroaromatic isolation groups could lead
to higher d33 values than the normal ones, and the d33 value of P2
was even up to 162.3 pm/V. This was a quite amazing value, since its
chromophore moieties were simple azo chromophores [5,9,12].
All the polymers were soluble in common polar organic solvents
such as CHCl3, CH2Cl2, THF, DMF, and DMSO. The UVevis absorption
spectra of the chromophores and polymers in different solvents
were demonstrated in Figs. 4 and S27eS36, and the maximum
absorption wavelengths (lmax) for the
pep* transitions of the azo
moieties were listed in Table 2. Chromophores C1, C2 and C3, C4
consisted of the same nitro acceptor, but different donors, which
led to the different lmax values, as shown in Fig. 4B. It was easily
seen that the lmax of the chromophore moieties in these polymers
were nearly the same (Fig. 4A), no matter there were some differ-
ences among the four chromophores (Fig. 4C). To investigate the
phenomena deeply, the UVevis absorption spectra of the two
corresponding chromophores mixed with equivalent were tested
as control experiments, since these polymers consisted of these
two types of chromophores equivalently. P1 and its corresponding
chromophores C1 and C3 were taken as the example again. As
Fig. 5. The conformation of dimer that contain perfluoro aromatic isolated spacers.

5662
W. Wu et al. / Polymer 54 (2013) 5655e5664
Fig. 6. The abridged general views of the conformation of: (a) polymers with perfluoro aromatic isolation spacers; (b) polymers with normal isolation spacers.
I
HO
O
N
N
I
COOH
3
O
8
7
THF,DMF
EDC,DMAP,DCM
I
I
O
O
10: Ar=
N
N
N
N
O
O
F
F
F
F
Ar COOH
10 and 11
9
11: Ar=
F
EDC,DMAP,DCM
N
O
N
O
O
O
Ar
OH
NO2
N
NO2
N
N
14: Ar=
F
F
F
F
N
3
Ar COOH
EDC,DMAP,DCM
14 and 15
13
12
15: Ar=
F
N
N
O
N
THF,DMF
O
O
O
Ar
OH
NO2
NO2
N
N
O
N
O
10
or
11
14
N
O
M1-M4
+ or
15
O
O
N
Ar2
N
O
O
O
NO2
Ar
1
NO2
F
F
F
F
F
M4: Ar1 =
F
F
F
M3: Ar1 =
M2: Ar1 =
M1: Ar1 =
F
F
F
F
F
F
F
F
F
Ar2=
Ar2=
Ar2=
F
Ar2=
F
F
Scheme 3. The synthesis of model molecules M1eM4.

W. Wu et al. / Polymer 54 (2013) 5655e5664
5663
However, not all the perfluoroaromatic isolation spacers could
1.0
0.8
0.6
0.4
0.2
0.0
result in higher d33 values: the perfluoroaromatic isolation spacers
in chromophore C2 cause a little lower d33 values, just like P3
and P4.
This abnormal phenomenon should be caused by different in-
teractions between the perfluorophenyl groups and normal phenyl
ones at different positions. The different conformations of these
polymers caused by AreArF self-assembly effects were therefore
considered, which was performed on dimer using Moloc software
with MAB forcefield, and the resultant stable conformer was then
used repeatedly to construct oligomer (containing 10 monomers). A
subsequent molecule dynamics run with opls force field was per-
formed to verify the stability of the oligomers, with the results shown
in Figs. 5 and 6. Due to the different self-assembled effects, the con-
formations of polymerswere different. Theperfluoroaromatic ringsin
chromophore C2, which still had another normal phenyl groups as
isolation groups, could self-assemble with the normal isolation group
by itself. This should be therefore useless for the alignment of chro-
mophores, however, making the loading density of the effective
chromophore moieties lower, since the relative atomic mass of Fatom
was much larger than H atom (Table 3). And it was well-knownthat in
theory, under identical experimental conditions, the d33 value was
proportionaltothedensityof thechromophoremoieties.Thus,thed33
value of P4 was a little lower than that of P2, and the d33 value of P3
was a little lower than that of P1. On the contrary, in chromophore C4,
the self-assembly effect could occur between the perfluorophenyl
group in C4 and another chromophore moiety, which could make the
alignment of chromophore moieties much easier (Fig. 5A. Meanwhile,
Fig. 5B showed the conformation of polymers with normal isolation
spacers for comparison), thus, further increasing the NLO coefficient
in a large degree.
P1
P2
P3
P4
50
70
90
110
130
o
Temperature ( C)
Fig. 7. Decay curves of the SHG coefficients of P1eP4 as a function of the temperature.
one, it still has the highest onset temperature for decay in the d33
value as 111 ꢀC, thanks to the Ar/ArF interactions. However, it was
not so clear how these Ar/ArF interactions affect the Tg values of
NLO polymers, for example, the Tg values of P3 and P4 did not
demonstrate advantages in comparison with that of P1, and the
onset temperatures for decay were also lower than that of P1. Thus,
further researches were still needed, to make the effect of these
interactions to Tg values of polymers much clear. That was to say,
the self-assembly effect in the NLO polymers in P2 could lead to the
better stability. Anyhow, in P2, which demonstrated the highest
NLO coefficient in these polymers, the self-assembly effect also
brought out the best stability, making it good candidate for the
practical applications.
To aid the investigation of these different AreArF self-assembly
effect in depth, four model molecules M1eM4 (Scheme 3), with the
similar structure to P1eP4 (see Supporting information for their
synthetic procedure), were also synthesized. However, it was a pity
that no useful information was obtained (including NMR, FT-IR
spectra and the melting point, etc. see Experimental section and
Supporting information for detail). Thus, further researches were
still needed, to find more evidence to check the different AreArF
self-assembly effect, and to apply this effect to other NLO chro-
mophores with higher mb values, with the aim for larger NLO co-
efficients. These researches were in progress in our laboratory.
As there might be some resonant enhancement due to the ab-
sorption of the chromophore moieties at 532 nm, the NLO properties
of P1eP4 should be much smaller (Table 3, (d33(N)), which were
calculated by using the approximate two-level model. Since all the
polymersexhibitednearlythesameUVevisabsorptionbehavior, their
d33(N) values demonstrated the same phenomena as their d33 values.
To further study the alignment behavior of the chromophore
4. Conclusion
In this work, four new polyaryleneethynylenes P1eP4 were
successfully prepared via the simple palladium-catalyzed Sono-
gashira coupling reaction, in which the AreArF interaction was
used to improve the poling efficiency of these NLO polymers. The
preliminary results demonstrated that the concentration and
position of perfluoroaromatic isolation groups in the chromo-
phore could affect the NLO effect of the corresponding polymers
largely. P2 demonstrated the largest NLO effect of 162.3 pm/V.
Coupled with its better stability, P2 would be a good candidate
for the practical applications. Thus, our study might provide
some new useful information on the design of new NLO poly-
mers with better NLO properties by using the higher mb value
chromophores.
moieties in P1eP4, their order parameter (F) (Table 3) was also
measured and calculated from the change of the UVevis spectra of
their films before and after poling under electric field (Figs. S40e
S43), according to the equation described in Table 3 (footnote e).
Acknowledgments
The tested
values: P2 and P4 exhibited higher
F
values were in accordance with their d33 and d33(N)
values (0.33 and 0.29
We are grateful to the National Science Foundation of China (no.
21034006) for financial support.
F
respectively) than those of P1 and P3 (0.21 and 0.15 respectively),
indicating that much effective oriented alignment of the dipole
moments was achieved in P2 and P4, further confirming the above
discussions.
The dynamic thermal stabilities of the NLO activities of the
polymers were investigated by depoling experiments, in which the
real time decays of their SHG signals were monitored as the poled
films were heated from 35 to 140 ꢀC in air at a rate of 4 ꢀC/min. Fig. 7
showed the decay of SHG coefficient of all the polymers as a
function of temperature. Since the Tg value of P2 was the highest
Appendix A. Supplementary material
Supplementary material related to this article can be found at
http://dx.doi.org/10.1016/j.polymer.2013.07.073.
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