Energy-level tuning of poly(p-phenylenebutadiynylene) derivatives by click chemistry-type postfunctionalization of side-chain alkynes

Article abstract of DOI:10.1016/j.reactfunctpolym.2016.06.001

A series of poly(p-phenylenebutadiynylene) polymers substituted with electron-rich alkynes as the side chain were synthesized by homocoupling polymerization of asymmetric bifunctional monomers. The electron-rich alkynes underwent "click chemistry" with te

Full text of DOI:10.1016/j.reactfunctpolym.2016.06.001

Reactive and Functional Polymers 105 (2016) 114–121
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Reactive and Functional Polymers
journal homepage: www.elsevier.com/locate/react
Energy-level tuning of poly(p-phenylenebutadiynylene) derivatives by
click chemistry-type postfunctionalization of side-chain alkynes
a,
a
b
a
a
a
a
a
Dong Wang ⁎, Ruirui Zhang , Hong Gao , Xiangke Wang , Huihui Wang , Zhou Yang , Wanli He , Hui Cao ,
c
d
a
Jianming Gu , Huiying Hu , Huai Yang
a
Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, People's Republic of China
China Academy of Space Technology, Beijing 100094, People's Republic of China
Department of Adult Joint Reconstruction, Beijing Jishuitan Hospital, Beijing 100035, People's Republic of China
b
c
d
Department of Obsterics and Gynecology, Peking Union Medical College Hospital, Beijing 100730, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of poly(p-phenylenebutadiynylene) polymers substituted with electron-rich alkynes as the side chain
were synthesized by homocoupling polymerization of asymmetric bifunctional monomers. The electron-rich al-
kynes underwent “click chemistry” with tetracyanoethylene (TCNE) to produce donor–acceptor chromophores.
Optical and electrochemical characterizations clearly indicated that the energy level and band gap of P2 could be
precisely controlled by the addition of acceptor molecules. One of the most important conclusions of this study is
that a linear relationship between the lowest occupied molecular orbital (LOMO) and the amount of TCNE was
observed. From the Z-scan measurement, all the compounds exhibited very special nonlinear optical properties,
which suggested a tendency to transfer from saturable absorption (SA) to reverse saturable absorption (RSA).
© 2016 Published by Elsevier B.V.
Received 28 February 2016
Received in revised form 2 June 2016
Accepted 3 June 2016
Available online 5 June 2016
Keywords:
Energy-level tuning
Poly(p-Phenylenebutadiynylene)
Click chemistry
Postfunctionalization
Nonlinear optical
1
. Introduction
Click chemistry provides efficient, reliable, and selective reactions
for synthesizing new compounds and generating combinatorial librar-
Conjugated organic molecules and polymers play a primary role in
the development of a new generation of optical and electronic materials
1–6]. In particular, conjugated polymers are now of significant com-
mercial importance for the construction of organic light-emitting diodes
OLEDs), thin-film transistors (TFTs), and field effect transistors (FETs),
ies [19]. Click chemistry can be defined as a highly efficient addition re-
action, essentially yielding no by-product [19]. At present, the most
famous and well-studied click reaction is the copper(I)-catalyzed
azide-alkyne cycloaddition reaction (CuAAC), forming a triazole ring,
which has been widely used for the preparation of functional materials
[20–25]. Nevertheless, one drawback of these 1,3-dipolar azide-alkyne
reactions is the little effect on the conjugation and energy level, which
are commonly used for nonlinear optical (NLO) properties [26,27].
This may hinder their general applicability for optical polymers, such
as NLO. In order to solve this problem, other click-type reactions have
simultaneously been developed [28–33]. For introducing strong elec-
tron-withdrawing groups for NLO application, the typically thermal
[2 + 2] cycloadditions, followed by retro-electrocyclization of
tetracyanoethylene (TCNE) and 7,7,8,8-tetracyanoquinodimethaneI
(TCNQ), were linked to “electronically confused” alkynes [34–40]. By
using this reaction, the polymer energy levels and thermal stability
were significantly controlled.
[
(
which are finding applications in computing hardware and electronic
appliances [7–16].
In order to achieve an appreciable device performance, it is impor-
tant to design polymer structures with energy levels suitable for a spe-
cific application. One of the most direct and promising approaches is the
modification of the side-chain groups of the polymer [17,18]. The intro-
duction of electron-donating or electron-accepting groups directly or
via a π-spacer into the polymer side chain dramatically altered the ener-
gy levels. Tuning of the electronic highest occupied molecular orbital
(
HOMO) and lowest occupied molecular orbital (LUMO) levels is crucial
in enhancing the optoelectronic properties of organic materials. Design-
ing new donor–acceptor-type π-conjugated molecules is one solution,
because through-bond interactions between donor and acceptor moie-
ties result in narrow band gaps originating from the elevated HOMO and
lowered LUMO levels.
In this study, novel poly(p-phenylenebutadiynylene)s (PPBs) con-
taining full conjugated main chain and postfunctionalized side groups
were designed and synthesized. Because of the conjugated relationship
between main chain and side groups, the postfunctionalized reaction af-
fects the electronic cloud distribution of not only the side groups but
also the whole main chain. It could be assumed that the energy levels
⁎
Corresponding author.
E-mail address: wangdong@ustb.edu.cn (D. Wang).
http://dx.doi.org/10.1016/j.reactfunctpolym.2016.06.001
381-5148/© 2016 Published by Elsevier B.V.
1

D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
115
of polymers are precisely controlled by the extent of postfunctionalized
reaction. It is interesting to note that the selection of [2 + 2] click chem-
istry method has resulted in the accurate quantification of the amount
of click moieties. It was for precisely tuning energy levels, at the same
time, the NLO properties may be challenged to control the
postfunctionalized reaction.
2925, 2854, 2203, 2156, 1604, 1550, 1532, 1519, 1466, 1405, 1369,
−
1
1249, 1188, 1140, 1097, 1029, 952, 860, 843, 812, 760 cm . MALDI-
1
TOF-MS (dithranol): m/z: calculated for C46
found: 800.3 g•mol
H
73Br
2
N: 799.41 g•mol−
,
−
1
+
[MH] ; elemental analysis calculated (%) for
46 2
C H73Br N (799.41): C 69.11, H 9.48, N 1.70. found: C 69.10, H 9.49, N
1.69.
2
. Experimental
2.3.2.
4-((2,5-bis((trimethylsilyl)ethynyl)phenyl)ethynyl)-N,N-
dihexadecylaniline (2)
2
.1. Materials
Compound (1) (2.00 g, 2.51 mmol) and (triisopropylsilyl)acetylene
(
TMSA) (0.74 g, 7.53 mmol) were dissolved in TEA/THF (VTEA:VTHF
1:1, 40 mL). After the solution was purged with bubbling Ar for
30 min, PdCl (PPh (0.11 g, 0.15 mmol) and CuI (0.06 g, 0.30 mmol)
=
Chemicals purchased from TCI, J&K, Alfa Aesar, and Aldrich were
used as received. 2,5-dibromoiodobenzene [41], N,N-dihexadecyl-4-
iodoaniline [42], 4-ethynyl-N,N-dihexadecylaniline [42], 4-[(2-
bromophenyl)ethynyl]-N,N-dihexadecyl niline (7) [43], 4-((2,5-
dibromophenyl)ethynyl)-N,N-dihexadecylaniline (1) [44], 4-((2,5-
bis((trimethylsilyl)ethynyl)phenyl)ethynyl)-N,N-dihexadecylaniline
2
3 2
)
were added. The reaction mixture was then stirred at 80 °C for 12 h
under Ar atmosphere. The mixture was concentrated, rediluted with
DCM, and filtered through a plug of silica gel. The solvent was removed
in vacuo and the crude product was purified by column chromatography
1
(
(
2) [44], and 4-((2,5-diethynylphenyl)ethynyl)-N,N-dihexadecylaniline
3) [44] were synthesized according to the methods described in the
(SiO
(CDCl
1.58 (m, 4H), 3.29 (m, 4H), 6.58 (d, J = 9.0 Hz, 2H), 7.28 (s, 1H), 7.40
2
, Vhexane/VDCM = 10:1) to produce (2) (1.00 g, 48%). H NMR
3
, 500 MHz): δ = 0.27 (m, 18H), 0.89 (m, 6H), 1.28 (m, 52H),
literature.
(
m, 3H), 7.60 (s, 1H) ppm. FT-IR (KBr): ν = 2916, 2851, 2206, 1604,
−
1
2
.2. General measurements
1515, 1472, 1402, 1369, 1198, 1122, 1078, 1029, 878, 808, 715 cm
.
MALDI-TOF-MS (dithranol): m/z: calculated for
C
56
H
91Br
833.70 g•mol , found: 834.8 g•mol [MH] ; elemental analysis cal-
culated (%) for C56 91Br NSi (833.70): C 80.37, H 11.01, N 1.72.
found: C 80.34, H 11.03, N 1.71.
2 2
NSi :
1H nuclear magnetic resonance (NMR) spectra were measured on a
−1
−1
+
Bruker AV300 NMR spectrometer (300 MHz) at 20 °C. Chemical shifts
are reported in parts per million downfield from SiMe , using the
H
2
2
4
solvent's residual signal as an internal reference. The resonance multi-
plicity is described as s (singlet), d (doublet), and m (multiplet). Infra-
red (IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer.
All matrix-assisted laser desorption/ionization time-of-flight mass spec-
tra (MALDI-TOF-MS) were measured on a Shimadzu AXIMA-CFR mass
spectrometer. The operation was performed at an accelerating potential
of 20 kV by a linear positive-ion mode with dithranol as a matrix. Gel
permeation chromatography (GPC) was measured on a Shodex system
equipped with polystyrene gel columns using tetrahydrofuran (THF)
as an eluent at a flow rate of 1.0 mL/min. Relative molecular weights
were determined by comparison with the calibrated standard poly-
styrenes. Thermogravimetric analysis (TGA) was carried out on a
Seiko SII TG 6220 under nitrogen flow at a scanning rate of 10 °C/min. Ul-
traviolet–visible (UV–Vis) spectra were recorded in a quartz cuvette on a
JASCO V-570 spectrophotometer. Cyclic voltammetric measurements
were carried out in a conventional three-electrode cell using glassy carbon
2.3.3. 4-((2,5-diethynylphenyl)ethynyl)-N,N-dihexadecylaniline (3)
To a 100-mL flask, (2) (1.00 g, 1.20 mmol), K CO (0.50 g,
3.60 mmol), and MeOH (15 mL) were added, and the mixture was
stirred at 20 °C for 3 h. The mixture was diluted with DCM and the or-
2
3
ganic phase was washed thrice with water. After drying over Na
the solution was filtered. Removal of the solvent in vacuo and column
chromatography (SiO , Vhexane/VDCM = 10:1) yielded the desired prod-
2 4
SO ,
2
1
3
uct (3) (0.70 g, 85%). H NMR (CDCl , 500 MHz): δ = 0.86 (m, 6H), 1.26
(m, 52H), 1.55 (m, 4H), 3.13 (s, 1H), 3.25 (m, 4H), 3.39 (s, 1H), 6.54 (d,
J = 8.5 Hz, 2H), 7.28 (d, J = 10.0 Hz, 1H), 7.36 (d, J = 10.5 Hz, 2H), 7.42
(d, J = 5.5 Hz, 1H), 7.59 (s, 1H) ppm; FT-IR (KBr): ν = 3340, 2918, 2853,
2166, 1604, 1515, 1472, 1402, 1369, 1198, 1122, 1078, 1029, 865,
−
1
715 cm . MALDI-TOF-MS (dithranol): m/z: calculated for C50
H
75N:
[MH] ; elemental analysis
75N (689.62): C 87.16, H 10.80, N 2.04.
−
1
−1
+
689.62 g•mol , found: 690.8 g•mol
calculated (%) for C50
H
working electrodes of diameter 2 mm, a platinum wire counterelectrode,
found: C 87.15, H 10.82, N 2.03.
+
and an Ag/Ag /CH
3
CN/Bu
4
NPF
6
reference electrode on a computer-
controlled CHI 660C instrument at room temperature (rt). All potentials
were referenced to the ferricinium/ferrocene (Fc/Fc ) couple used as
an internal standard. The NLO response was measured by Z-scan tech-
nique, using 21-ps laser pulses at 532 nm delivered by a mode-locked
Nd:YAG laser. Elemental analyses were conducted using the Flash EA
2.3.4. 1,4-dibromo-2-((4-pentylphenyl)ethynyl)benzene (4)
In a 250-mL round-bottom flask, 2,5-dibromoiodobenzene (3.60 g,
10.0 mmol) and 1-ethynyl-4-pentylbenzene (1.89 g, 11.0 mmol) were
+
dissolved in Et
with bubbling Ar for 40 min, Pd(PPh )
3
N (TEA)/THF 1:1 (80 mL). After the solution was purged
(347 mg, 0.30 mmol) and CuI
3 4
1
2
2
5
112 instrument.
(114 mg, 0.60 mmol) were added. The reaction mixture was then
stirred at 40 °C for 12 h under Ar atmosphere. The mixture was concen-
.3. Synthesis of monomers
trated, rediluted with CH
The solvent was removed in vacuo, and the crude product was purified
by column chromatography (SiO , hexane/CH Cl 10:1) to produce 4
(3.27 g, 81%) as a yellow liquid. H NMR (CDCl , 300 MHz): δ = 0.92
2 2
Cl , and filtered through a plug of silica gel.
.3.1. 4-((2,5-dibromophenyl)ethynyl)-N,N-dihexadecylaniline (1)
To a degassed solution of 4-ethynyl-N,N-diihexylaniline (3.00 g,
.31 mmol) and 1,4-dibromo-2-iodobenzene (2.30 g, 6.37 mmol) in
2
1
2
2
3
(m, 3H), 1.36 (m, 4H), 1.63 (m, 2H), 2.65 (m, 2H), 7.21 (d, J = 4.5 Hz,
2H), 7.31 (d, J = 4.8 Hz, 1H), 7.48 (d, J = 5.1 Hz, 1H), 7.51 (d, J =
4.8 Hz, 1H), 7.70 (s, 1H) ppm. FT-IR (KBr): ν = 2920, 2848, 2205,
1606, 1553, 1519, 1466, 1405, 1370, 1248, 1136, 1095, 1029, 952, 862,
triethylamine (TEA) (40 mL) and THF (40 mL), bis(triphenylphosphine)
palladium(II) dichloride (PdCl (PPh ) (0.22 g, 0.32 mmol) and cu-
prous iodide (CuI) (0.12 g, 0.64 mmol) were added under Ar atmo-
sphere. The mixture was stirred at 40 °C for 15 h. After removal of the
2
3 2
)
−
1
844, 762 cm . MALDI-TOF-MS (dithranol) m/z: calculated for
−
1
−1
+
precipitated salt, evaporation and column chromatography (SiO
2
,
C
19
H
18Br
2
: 403.98 g•mol , found: 405.1 g•mol
[MH] . Elemental
V
hexane/Vdichloromethane(DCM) = 20:1) afforded the desired product (1)
analysis calculated (%) for: C 56.19, H 4.47; found: C 56.15, H 4.49.
1
(
2.62 g, 62%). H NMR (CDCl
2H), 1.58 (m, 4H), 3.30 (m, 4H), 6.60 (d, J = 8.5 Hz, 2H), 7.24 (d,
J = 8.5 Hz, 1H), 7.42 (d, J = 9.0 Hz, 2H), 7.45 (d, J = 8.5 Hz, 1H), 7.65
s, 1H) ppm. Fourier transform infrared spectra (FT-IR) (KBr): ν =
3
, 500 MHz): δ = 0.90 (m, 6H), 1.29 (s,
5
2.3.5. 1,4-bis((trimethylsilyl)ethynyl)-2-((4-pentylphenyl)ethynyl)benzene (5)
Compound
4
(2.02 g, 5.0 mmol) was cross-coupled with
N/THF
(
(trimethylsilyl)ethyne (1.47 g, 15.0 mmol) and dissolved in Et
3

116
D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
1
:1 (40 mL). After the solution was purged with bubbling Ar for 40 min,
Pd (PPh (231 mg, 0.20 mmol) and CuI (76 mg, 0.40 mmol) were
added. The reaction mixture was then stirred at 40 °C for 12 h under
Ar atmosphere. The mixture was concentrated, rediluted with CH Cl
the product (0.36 g, 87%). FT-IR (KBr): ν = 2922, 2851, 2194, 1606,
1536, 1518, 1466, 1401, 1397, 1363, 1185, 1140, 1029, 888, 811,
3 4
)
−
1
720 cm
.
2
2
,
and filtered through a plug of silica gel. The solvent was removed in
vacuo, and the crude product was purified by column chromatography
2.3.8. P2 (x = 1)
To a solution of P1 (100 mg, 0.145 mmol) in 1,2-dichloroethane
(10 mL), TCNE (18.68 mg, 0.145 mmol) was added, and the mixture
was stirred at 25 °C for 1 h. Removal of the solvent in vacuo yielded a
brown solid P2 (x = 1; 118 mg, 99%). FT-IR (KBr): ν = 2922, 2851,
(
SiO
2
, hexane/CH
2
Cl
2
20:1) to produce 5 (1.28 g, 58%) as a yellow
, 300 MHz): δ = 0.29 (m, 18H), 0.93 (m, 3H),
1
solid. H NMR (CDCl
3
1
.35(s, 4H), 1.65 (m, 2H), 2.64 (m, 2H), 7.19 (d, J = 4.8 Hz, 2H), 7.34
−
1
(
d, J = 5.1 Hz, 1H), 7.45 (d, J = 5.1 Hz, 1H), 7.48 (d, J = 4.8 Hz, 2H),
2214, 1602, 1485, 1415, 1401, 1338, 1182, 805, 720 cm
.
7
1
.64 (s, 1H) ppm. FT-IR (KBr): ν = 2913, 2853, 2202, 1611, 1505, 1468,
−
1
411, 1342, 1178, 1121, 1075, 1030, 867, 841, 710 cm . MALDI-
2.3.9. P3
: 440.24 g•mol−
1
,
To a solution of monomer 6 (177 mg, 0.60 mmol) in toluene (2 mL),
CuCl (30 mg, 0.3 mmol) and N,N,N′,N′-tetramethylethylenediamine
(250 μL, 1.68 mmol) were added. The mixture was stirred at 60 °C for
TOF-MS (dithranol) m/z: calculated for C29
H
36Si
[MH] . Elemental analysis calculated (%) for:
C 78.92, H 8.28; found: C 78.97, H 8.26.
2
−
1
+
found: 441.1 g•mol
2
4 h, and the reaction mixture was poured into an excess amount of
2
.3.6. 1,4-diethynyl-2-((4-pentylphenyl)ethynyl)benzene (6)
methanol. The precipitate was collected by filtration, washed with
Compound 5 (0.88 g, 2.0 mmol) in THF (10 mL) was added to tetra-
methanol twice, and dried under vacuum to yield an orange solid as
n-butylammonium fluoride (TBAF) (4 M in THF 20 mL). The reaction
mixture was stirred at rt for 30 min. Then, the mixture was diluted
the product (150 mg, 85%). FT-IR (KBr): ν = 2923, 2846, 2204, 1605,
1558, 1519, 1403, 1378, 1245, 1136, 1029, 952, 862, 846, 760 cm−
1
.
2 2 2 4
with CH Cl , washed with H O, dried (MgSO ), and concentrated in
vacuo. In general, the resultant terminal alkyne was then used with no
2.3.10. N,N-dihexadecyl-4-((2-((trimethylsilyl)ethynyl)phenyl)ethynyl)aniline
further purification. A yellow liquid (monomer 6) was obtained
(8)
1
(
3
53 mg, 90%) as the product. H NMR (CDCl , 300 MHz): δ = 0.96 (m,
The compound 7 (1.44 g, 2.0 mmol) was cross-coupled with
3
7
1
2
H), 1.39 (s, 4H), 1.67 (m, 2H), 2.66 (m, 2H), 3.23 (s, 1H), 3.50 (s, 1H),
(trimethylsilyl)ethyne (0.246 g, 2.5 mmol) and dissolved in Et
THF1:1 (30 mL). After the solution was purged with bubbling Ar for
40 min, Pd (PPh (69 mg, 0.06 mmol) and CuI (23 mg, 0.12 mmol)
were added. The reaction mixture was then stirred at 80 °C for 12 h
under Ar atmosphere. The mixture was concentrated, rediluted with
3
N/
.22 (d, J = 4.8 Hz, 2H), 7.41 (d, J = 4.8 Hz, 1H), 7.52 (d, J = 5.4 Hz,
H), 7.54 (d, J = 4.8 Hz, 2H), 7.72 (s, 1H) ppm. FT-IR (KBr): ν = 3315,
912, 2850, 2148, 1603, 1512, 1468, 1401, 1369, 1198, 1122, 1078,
3 4
)
−
1
1
029, 878, 808, 714 cm . MALDI-TOF-MS (dithranol) m/z: calculated
20: 296.16 g•mol− , found: 297.3 g•mol
1
−1
[MH] . Elemental
+
CH
moved in vacuo, and the crude product was purified by column chroma-
tography (SiO , hexane/CH Cl 20:1) to produce 8 (1.06 g, 72%) as an
orange solid. H NMR (CDCl , 300 MHz): δ = 0.34 (m, 9H), 0.93 (m,
for C23
H
2 2
Cl , and filtered through a plug of silica gel. The solvent was re-
analysis calculated (%) for: C 93.20, H 6.80; found: C 93.18, H 6.82.
2
1
2
2
2
.3.7. P1
To a solution of monomer 3 (0.41 g, 0.60 mmol) in toluene (2 mL),
CuCl (30 mg, 0.3 mmol) and N,N,N′,N′-tetramethylethylenediamine
250 μL, 1.68 mmol) were added. The mixture was stirred at 60 °C for
4 h, Then, the reaction mixture was poured into an excess amount of
3
6H), 1.33 (s, 52H), 1.62 (m, 4H), 3.31 (m, 4H), 6.61 (d, J = 5.4 Hz,
2H), 7.21 (m, 1H), 7.27 (m, 1H), 7.43 (d, J = 5.1 Hz, 2H), 7.52 (d, J =
5.1 Hz, 1H), 7.61 (d, J = 5.4 Hz, 1H) ppm. FT-IR (KBr): ν = 2916,
2851, 2206, 1604, 1515, 1472, 1402, 1369, 1198, 1122, 1078, 1029,
(
2
−
1
methanol. The precipitate was collected by filtration, washed with
methanol twice, and dried under vacuum to yield an orange solid as
878, 808, 715 cm . MALDI-TOF-MS (dithranol) m/z: calculated for
−
1
−1
+
C
51
H
83NSi: 737.63 g•mol , found: 738.5 g•mol
[MH] . Elemental
2 2 3 4 3 3 4 3
Scheme 1. Synthesis routes of monomers (3 and 6). (a) NaNO , KI, HCl/H O; (b) Pd(PPh ) , CuI, Et N/THF, rt; (c) Pd(PPh ) , CuI, Et N/THF, 80 °C; (d) TBAF, THF, 0 °C.

D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
117
Scheme 2. Synthesis routes of all the polymers and the postfunctionalization by TCNE addition. (a) CuCl, N,N,N′,N′-tetramethylethylenediamine, toluene, 60 °C; (b) TCNE, dichlorobenzene,
00 °C.
1
1
analysis calculated (%) for: C 82.97, H 11.33, N 1.90; found: C 82.92, H
3
(compound 9) was obtained (0.61 g, 92%). H NMR (CDCl , 300 MHz):
1
1.36, N 1.91.
δ = 0.93 (m, 6H), 1.32 (s, 52H), 1.62 (m, 4H), 3.31 (m, 4H), 6.62 (d,
J = 5.4 Hz, 2H), 7.24 (m, 1H), 7.27 (m, 1H), 7.46 (d, J = 5.4 Hz, 2H),
7.56 (d, J = 5.1 Hz, 1H), 7.65 (d, J = 5.1 Hz, 1H) ppm. FT-IR (KBr):
ν = 3290, 2921, 2853, 2196, 1606, 1516, 1470, 1400, 1368, 1190,
2
.3.11. 4-((2-Ethynylphenyl)ethynyl)-N,N-dihexadecylaniline (9)
Compound 8 (0.74 g, 1.0 mmol) in THF (10 mL) was added to TBAF
−
1
(
2 M in THF 10 mL). The reaction mixture was stirred at rt for 30 min.
Then, the mixture was diluted with CH Cl , washed with H O, dried
MgSO ), and concentrated in vacuo. In general, the resultant terminal
alkyne was then used with no further purification. An orange liquid
1078, 878, 808, 713 cm . MALDI-TOF-MS (dithranol) m/z: calculated
−
1
−1
+
2
2
2
for C48H75N: 665.59 g•mol , found: 666.7 g•mol [MH] . Elemental
(
4
analysis calculated (%) for: C 86.55, H 11.35, N 2.10; found: C 86.50, H
11.37, N 2.12.
3 4 3
Scheme 3. Synthesis routes of model compounds. (a) TMSA, Pd(PPh ) , CuI, Et N/THF, 80 °C; (b) TBAF, THF, 0 °C; (c) CuCl, N,N,N′,N′- tetramethylethylenediamine, toluene, 60 °C; (d) TCNE,
dichlorobenzene, rt.

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D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
2
.3.12. 4,4′-((buta-1,3-diyne-1,4-diylbis(2,1-phenylene))bis(ethyne-2,1-
diyl))bis(N,N-dihexadecylaniline) (M)
To a solution of compound 9 (0.40 g, 0.60 mmol) in toluene (2 mL),
CuCl (30 mg, 0.3 mmol) and N,N,N′,N′-tetramethylethylenediamine
(
250 μL, 1.68 mmol) were added, and the mixture was stirred at 60 °C
for 24 h. The mixture was concentrated, rediluted with CH Cl , and fil-
tered through a plug of silica gel. The solvent was removed in vacuo,
and the crude product was purified by column chromatography (SiO
2
2
2
,
1
hexane/CH
NMR (CDCl
2
Cl
, 300 MHz): δ = 0.90 (m, 12H), 1.28 (s, 104H), 1.58 (m,
H), 3.23 (m, 8H), 6.52 (d, J = 5.4 Hz, 4H), 7.22 (m, 1H), 7.33 (m, 1H),
2
20:1) to produce M (1.06 g, 72%) as an orange solid. H
3
8
7
2
8
C
.45 (d, J = 5.4 Hz, 4H), 7.51 (d, J = 4.5 Hz, 2H), 7.57 (d, J = 4.5 Hz,
H) ppm. FT-IR (KBr): ν = 2922, 2853, 2206, 1607, 1520, 1298, 1190,
08, 753 cm . MALDI-TOF-MS (dithranol) m/z: calculated for
96 2
H148N : 1329.6 g•mol , found: 1330.5 g•mol [MH] . Elemental
−
1
−
1
−1
+
analysis calculated (%) for: C 86.68, H 12.21, N 2.11; found: C 86.62, H
1
2.25, N 2.13.
2
.3.13. M-TCNE
To a solution of M (66 mg, 0.05 mmol) in 1,2-dichloroethane (5 mL),
Fig. 2. FT-IR spectra of P1 before “click” and P2 (x = 1) after reaction.
TCNE (12.80 mg, 0.10 mmol) was added, and the mixture was stirred at
5 °C for 1 h. Removal of the solvent in vacuo yielded the brown solid M-
2
1
TCNE (78 mg, 91%). H NMR (CDCl
3
, 300 MHz): δ = 0.92 (m, 12H), 1.28
oxidative coupling polymerization in the presence of catalytic CuCl
and N,N,N′,N′-tetramethylethylenediamine under air at 60 °C, and the
crude materials of PPBs (P1 and P3) were produced (Scheme 2). The
corresponding polymers P1 and P3 were precipitated by slow addition
of the reaction mixture into a large volume of well-stirred methanol.
The precipitates were washed and collected, and the pure polymers
P1 and P3 were obtained. These polymers are soluble in common or-
ganic solvents such as THF, DCM, chloroform, and benzene, because of
the presence of long alkyl chains. The number-average molecular
(
1
(
1
(
1
s, 104H), 1.59 (m, 8H), 3.33 (m, 8H), 6.55 (d, J = 5.1 Hz, 4H), 7.32 (m,
H), 7.39 (m, 1H), 7.48 (d, J = 4.5 Hz, 2H), 7.54 (d, J = 5.4 Hz, 4H), 7.59
d, J = 5.1 Hz, 2H) ppm. FT-IR (KBr): ν = 2924, 2853, 2215, 1733, 1603,
492, 1467, 1416, 1367, 1342, 1184, 823, 758 cm−1. MALDI-TOF-MS
1
dithranol) m/z: calculated for C108
H
148
N
10: 1585.19 g•mol− , found:
−
1
+
586.3 g•mol [MH] . Elemental analysis calculated (%) for: C 81.77,
H 9.40, N 8.83; found: C 81.75, H 9.44, N 8.81.
3
. Results and discussion
n w n
weight (M ) and polydispersity (M /M ) of P1 and P3 were deter-
mined by GPC (eluent: THF). As the polymerization results were repro-
ducible, there were no significant side reactions in this case.
Both asymmetric bifunctional monomers with various alkyne
substituents were synthesized by the sequential Sonogashira cross-
coupling and silyl deprotection protocol. Starting from 2,5-
dibromoiodobenzene, selective introduction of one equivalent of
alkyne groups into the iodo positions resulted in 1 or 4, respectively
The development of a unique class of nonplanar push–pull chromo-
phores by means of [2 + 2] cycloaddition, followed by cycloreversion, of
electron-deficient olefins, such as TCNE, TCNQ, and 2,3,5,6-tetrafluoro-
7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) has been studied [45,
46]. The high-yielding, click chemistry-type transformation always af-
fords electron sinks capable of multiple electron uptake with a narrow
potential range. In this study, the unexpected results are that only P1
can be reacted with click reagents, and the clicked products exhibit
poor solubility. Only P2 with TCNE moieties can be dissolved in general
solvents such as THF, DCM, chloroform, and benzene. In this case, P3
cannot be clicked by TCNE, but P1 can be. It was suggested that TCNE
can only react with electron-rich alkyne groups (see Schemes 2 and 3)
and generated the chromophore group (1,1,4,4-tetracyanobuta-1,3-
diene (TCBD)).
(Scheme 1). Compounds 2 or 5 were synthesized by Sonogashira cou-
pling with (trimethylsilyl)ethyne and compounds 1 or 4. Subsequent
silyl deprotection of compounds 2 or 5 with TBAF afforded the asym-
metric monomers 3 or 6. All the compounds were characterized by
NMR, IR, and MS spectra. Monomers 3 and 6 were used in a acetylenic
The addition of TCNE solution in 1,2-dichloroethane to the precursor
polymer in o-dichlorobenzene at rt resulted in a color change from yel-
low to dark red, together with a gradual increase in a charge transfer
(
CT) band around 570 nm (Fig. 1). The presence of isosbestic points
Table 1
Summary of the polymerization results and thermal properties.
Polymer
Mna (104)
Mwa (104)
(Mw/Mn)a
Td5%(°C)b
Td10%(°C)b
P1
3.66
4.01
4.16
4.31
4.52
0.29
7.72
8.38
7.88
9.10
8.75
0.71
2.11
2.09
1.89
2.11
1.94
2.45
360
352
347
325
261
190
390
377
374
357
316
220
P2 (x = 0.25)
P2 (x = 0.5)
P2 (x = 0.75)
P2 (x = 1)
P3
a
Molecular weights determined by GPC (eluent: THF, calibrated by polystyrene
Fig. 1. UV–Vis NIR spectral change of P1 with TCNE addition reaction to the electron-rich
alkyne at the polymer side chain. Inset: Plots of TCNE addition amount vs. absorbance
increase at 570 nm.
standards).
b
The 5% and 10% weight loss temperatures determined by TGA at the heating rate of 10
°C/min.

D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
119
Fig. 4. Relationship between the amount of TCNE and the LOMO and HOMO levels,
determined by the E on and E on, the solid lines show a fit of the data.
Fig. 3. Normalized UV–Vis spectra of monomer 3, P1, P2 (x
dichloromethane.
= 1) and P3 in
red
ox
(
311 and 416 nm) in the UV–Vis spectral change indicates the absence
transitions and π–π* transition of full conjugated main chain in them.
A comparison of model monomeric compound M and monomer 3
shows that the intensification of conjugation was much larger and the
intramolecular charge transferred over a longer conjugation length
along the molecular backbones. M-TCNE significantly lowered the ener-
gy of the CT bands compared with M, because of the introduction of the
electron-withdrawing group CN. The λend of P1 measured in THF was
575 nm (2.16 eV), while P2 (x = 1) displayed the bathochromically
shifted λend value at 822 nm (1.50 eV). This result suggested that the
formation of donor–acceptor chromophores in the side chains by the
postfunctionalization lowered the polymer band gap. All the com-
pounds showed complicated absorption behaviors in UV–Vis absorp-
tion spectra, and only the λmax values are listed in Table 2. It is evident
from the table that λmax values are not as highly regular as the onset
wavelength of optical absorption.
of undesired side reactions. In order to reveal the quantitative yield of
the polymer reaction, the full TCNE adduct (TCNE: alkyne in polymer =
1
shows the linear relationship between the amount of TCNE added and
absorbance at 570 nm. It was suggested that a quantitative reaction oc-
curred between the alkyne groups of P1 and TCNE. In the IR spectra (Fig.
:1) was prepared and unambiguously characterized. The inset of Fig. 1
−
1
2
), the precursor polymer P1 displayed a peak at 2194 cm , which was
attributed to the alkyne vibration of the side chains. After the addition of
TCNE, the peak (alkyne) completely disappeared and became equivocal
due to overlap with a new and strong cyano vibrational peak. The TCNE-
adducted polymer P2 (x = 1) showed a new and stronger peak attrib-
uted to the cyano vibration at 2214 cm . After TCNE addition, the po-
sition and intensity of these peaks were consistent with previous
reports [47,48]. GPC showed an appreciable increase in molecular
−
1
n
weight (M ), as shown in Table 1.
In contrast to the previous report that the [2 + 2] click reaction
sometimes enhances the thermal stability of polymers, in this study,
3.2. Electrochemistry
5
% and 10% decomposition temperatures (Td5% and Td10%) of polymer
In order to verify the optical band gaps and determine the absolute
energy levels, the cyclic voltammograms (CVs) were measured in
CH CN with 0.1 M Bu NPF at 20 °C for the polymer thin films cast on
3 4 6
P2 series were lower than those of the corresponding precursor poly-
mer P1 (Table 1 and Fig. S1) [42,48–50]. However, both the obtained
polymers were thermally stable without any decomposition at least
up to 260 °C. The result may be derived from reduced individual bond
energy by introducing strong acceptor cyano groups.
a glassy carbon electrode. Several postfunctionalized polymers P2
with different acceptor addition amounts were prepared to elucidate
the relationship between the added acceptor amount (x) and the abso-
lute energy levels. CVs of P1 and P2 (x = 1) are shown in Fig. S3 and the
peak top values are summarized in Table 2. P1 showed the first oxida-
3
.1. UV–Vis spectroscopy
ox
+
tion peak potential (E on) at 0.54 eV (vs. Fc/Fc ), which was attributed
to the N,N-dihexadecylaniline moieties. From the CV curves, P1 did not
exhibit any clear reduction peak in the potential range. A stepwise TCNE
addition to P1 gradually changed reduction potential, but oxidation
The normalized UV–Vis absorption spectra are shown in Figs. 3 and
S2. Compared to the UV spectra of monomer 3, P1 and P3 show double
absorption peaks because of the presence of both intramolecular CT
Table 2
Optical andelectrochemical properties of monomers and polymers.
Eoxona(V)
Eredona(V)
HOMO (eV)
LOMO (eV)
Eg (eV)
b
Eg (eV)
c
Materials
λmax (nm)
P1
364
364
364
395
395
398
329
295
0.54
0.53
0.52
0.53
0.52
0.64
0.51
0.53
−
−5.13
−5.12
−5.11
−5.12
−5.11
−5.23
−5.10
−5.12
−
−
2.16
1.55
1.53
1.52
1.50
2.42
1.86
1.35
P2 (x = 0.25)
P2 (x = 0.5)
P2 (x = 0.75)
P2 (x = 1)
P3
−0.46
−0.42
−0.39
−0.37
−
−4.13
−4.17
−4.20
−4.22
−
0.99
0.94
0.92
0.89
−
M
−1.25
−0.87
−3.34
−3.72
1.76
1.40
M-TCNE
a
Onset potentials determined from cyclic voltammograms in CH
3
CN with 0.1 M Bu
Band gap calculated from the energy level of cyclic voltammograms.
Band gap estimated from the onset wavelength of optical absorption in THF solution.
0
NPF
6
at a scan rate of 100 mV/s.
b
c

120
D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
a good agreement between the electrochemical and optical band
gaps determined by the end absorption.
3.3. Nonlinear optics
The NLO responses of P1 and P2 were measured by the Z-scan tech-
nique, and Fig. 5 displayed the nonlinear absorptive and refractive be-
haviors of P1 and P2. A good fit was achieved between the
experimental data and the theoretical curves, and the NLO data are
shown in Table 3.
In Fig. 5, a profound transmittance valley could be seen around the
focal plane for P1, which was characteristic of reverse saturable absorp-
tion (RSA)-type behavior of P1. Materials with RSA became more
opaque on exposure to high photon fluxes due to the high absorption
from the excited state, and the properties had been exploited in the
field of optical limiting for laser protection. For P2 (x = 0.5), a similar
RSA-type behavior was observed, and the NLO response was reduced
by click reaction. However, Fig. 5 showed typical transmittance peaks
of click reaction products P2 (x = 1.0), which exhibited saturable ab-
sorption (SA)-type behaviors. Compared to P1 without click moieties,
the clicked products exhibited clearly RSA–SA transition, and the transi-
tion was achieved by click-type reactions in organic molecules. Thresh-
old intensity was the reason behind conversion from RSA to SA
phenomenon [38,39,54], which was determined by parameters such
as absorption cross section, level lifetime and decline of saturation in-
tensity. Once the incident light intensity crossed the threshold intensity,
conversion from RSA to SA happened. Comparing with curves in Fig. 5a,
the same phenomenon was observed in Fig. S4(a) for the model com-
pounds. The model compound M did not show the NLO response be-
cause of the short conjugated length. The M-TCNE exhibited SA-type
behaviors because of the introduced clicked moieties. From all the fig-
ures and data in Table 3, it can be proved that the click addition was
the most important factor to affect the NLO properties. The reasons
were dramatically facilitated by the elongation of conjugation lengths
of molecular backbones and enhancement of electron affinities via
introduction of strong electron-withdrawing substitutes [38,39,54].
Fig. 5. Z-scan results of dichloromethane. (a) Data collected under the open-aperture
configuration; (b) data collected under the closed-aperture configuration.
4
. Conclusions
potential remains unchanged. For example, P2 (x = 0.25) and P2 (x =
0
0
.50) displayed N,N-dihexadecylaniline-centered Eoxon at 0.53 and
A series of PPB derivatives were prepared by the [2 + 2] cycloaddi-
.52 V, respectively. In contrast to the oxidation peaks, the origin of
tion of TCNE to donor-substituted alkynes in excellent yields and full
characterization. The data suggested that a careful selection of the spe-
cies and amount of the added acceptor molecules enables good control
of the polymer energy levels. The data clearly indicated that a linear re-
lationship between LOMO and the amount of TCNE was an objectively
existing rule. These results provide a promising prospect for applying
this methodology to other conjugated polymers. Z-scan measurements
to investigate the third-order NLO properties confirmed that the click
reaction was key factor for the typical RSA–SA reverses.
red
the first reduction peak potential (E on) was changed from the poly-
mer main chain to the TCBD moieties when TCNE was added to P1.
Thus, the reduction potential of P1 was facilitated as more TCNE was
red
added. The E on of P1 bathochromically shifted to −0.45, −0.42,
−
0.39, and −0.36 V for P2 with x = 0.25, 0.50, 0.75, and 1, respectively.
The HOMO and LUMO energy levels of all the polymers were calculat-
ox
red
ed from the E on and E on values, respectively, based on the assumption
of Fc/Fc+ [51–53]. The energy levels were plotted against the amount of
TCNE (x; Fig. 4). The data clearly indicated that the HOMO remained un-
changed and LUMO levels decreased as more acceptor molecules were
added (Table 2). The fitting line of LOMO shows a linear relationship
between LOMO and the amount of TCNE. It was suggested that the
energy levels of conjugated polymers be accurately adjusted by the
postfunctionalization method. Accordingly, the electrochemical
band gaps, calculated from the difference between the HOMO and
LUMO levels, considerably decreased after postfunctionalization.
The electrochemical properties have been studied by CVs, indicating
Acknowledgments
This study was supported by the National Key Basic Research Program
of China (2014CB931804), the National Natural Science Foundation of
China (Grant Nos. 51473020, 61370048, 51333001, and 51373024), the
Fundamental Research Funds for the Central Universities (Grant No.
FRF-TP-15-003A3), and Beijing Higher Education Young Elite Teacher
Project (Grant No. YETP0356).
Table 3
Third-order NLO parameters of all the compounds.
Appendix A. Supplementary data
Samples
P1
P2 (x = 0.5)
P2 (x = 1.0)
M
M-TCNE
−
11
(m·w−1)
β × 10
2.6
−
11.0
−2.7
−0.51
−
−
−
−3.6
−2.4
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.reactfunctpolym.2016.06.001.
(
3)
−18
Imχ × 10
(esu)

D. Wang et al. / Reactive and Functional Polymers 105 (2016) 114–121
121
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