Short-conjugated zwitterionic cyanopyridinium chromophores: Synthesis, crystal structure, and linear/nonlinear optical properties

Article abstract of DOI:10.1016/j.dyepig.2014.06.005

A new class of short-conjugated zwitterionic cyanopyridinium chromophores with the absorption maxima less than 550 nm were designed and synthesized. These chromophores display a good solubility in organic solvents and exist in a charge-separated ground state, as confirmed by IR, X-ray crystallography, negative solvatochromism and fluorescence studies. They can be doped up to 10% by weight into poly(ether sulfone) and attached onto a methacrylate polymer by grafting. Without optimization of the poling conditions, a high electro-optic coefficient can be readily achieved using the chromophore-grafted nonlinear optical polymers.

Full text of DOI:10.1016/j.dyepig.2014.06.005

Dyes and Pigments 111 (2014) 145e155
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Short-conjugated zwitterionic cyanopyridinium chromophores:
Synthesis, crystal structure, and linear/nonlinear optical properties
,
, c, *
Wen Hui Hao a b, Pengfei Yan a, Guangming Li a, Zhi Yuan Wang b
a Key Laboratory of Functional Inorganic Material Chemistry (MOE), School of Chemistry and Materials Science, Heilongjiang University, Harbin 150080,
PR China
b Department of Chemistry, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario K1S5B6, Canada
c State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new class of short-conjugated zwitterionic cyanopyridinium chromophores with the absorption
maxima less than 550 nm were designed and synthesized. These chromophores display a good solubility
in organic solvents and exist in a charge-separated ground state, as confirmed by IR, X-ray crystallog-
raphy, negative solvatochromism and fluorescence studies. They can be doped up to 10% by weight into
poly(ether sulfone) and attached onto a methacrylate polymer by grafting. Without optimization of the
poling conditions, a high electro-optic coefficient can be readily achieved using the chromophore-grafted
nonlinear optical polymers.
Received 8 March 2014
Received in revised form
2 June 2014
Accepted 3 June 2014
Available online 14 June 2014
Keywords:
© 2014 Elsevier Ltd. All rights reserved.
Zwitterionic chromophores
Hyperpolarizability
Solvatochromism
Nonlinear optical
Electro-optic
Fluorescence
1. Introduction
Of course, the most widely studied properties are their large
negative first molecular hyperpolarizability (b) and the corre-
Cyanopyridinium-based zwitterionic chromophores are an
important type of the intramolecular charge-transfer (ICT) mole-
cules, in which negatively charged cyano-moiety and pyridium
sponding figure of merit (mb), which is used to characterize their
microscopic nonlinear optical (NLO) efficiencies. Zwitterionic cya-
nopyridinium chromophores have been demonstrated to be one of
the promising classes of second-order NLO materials for electro-
optic (EO) applications [7,8]. To fabricate EO devices, NLO chro-
mophores are usually doped into or covalently bonded onto poly-
mers, followed by poling films under an externally applied electric
field at the glass transition temperature (Tg) in order to achieve a
noncentrosymmetric arrangement with a large macroscopic EO
coefficient [9e15]. More recently, a novel approach has been
cation are separated by a
characteristics, such as highly charge-separated ground state
(Ddþe eAdꢀ; D: donor; A: acceptor;
: degree of CT), large molar
extinction coefficient, large negative solvatochromism, large dipole
moment ( ), and large negative first molecular hyperpolarizability
), arise from this peculiar structure, which leads to a wide variety of
p-conjugated system. Many interesting
p
d
m
(b
applications including functional dyes [1], nonlinear optical chro-
mophores [2,3], photochromic materials [4] and chemosensors [5].
Based on the high absorptive feature, various forms of deep-colored
solutions, as well as films containing zwitterionic cyanopyridinium
chromophore can be obtained [6]. Recently, the cyano moiety of the
zwitterionic chromophores was found to be capable of selectively
and reversibly coordinating with certain metal ions, resulting in
distinct color change due to the switch on/off of CT band [5].
explored in which zwitterionic chromophores with negative
linked to the neutral-ground-state NLO chromophores with posi-
tive [16e18] and showed the interesting application of zwitter-
b were
b
ionic chromophores in EO devices.
The previously studied zwitterionic cyanopyridinium chro-
mophores are typically synthesized through the addition between
tertiary amines and 7,7,8,8-tetracyanoquino-dimethane (TCNQ),
such as DEMI [19], PQDM [20e22] and PeQDM [23] (Fig. 1). Be-
sides complicated procedures and low yield, the chromophores
obtained by this method have their absorption maxima limited in
the red region of the visible spectrum, usually 600 nme750 nm.
* Corresponding author. Department of Chemistry, Carleton University, 1125
Colonel By Drive, Ottawa, Ontario K1S5B6, Canada.
E-mail address: wayne_wang@carleton.ca (Z.Y. Wang).
http://dx.doi.org/10.1016/j.dyepig.2014.06.005
0143-7208/© 2014 Elsevier Ltd. All rights reserved.

146
W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
Fig. 1. Chemical structures of zwitterionic chromophores of DEMI, PQDM, PeQDM and SZC.
As functional dyes and chromic materials, zwitterionic cyano-
pyridinium chromophores with a wide absorptive band are
desirable (e.g. 400e600 nm). In addition, as a material for NLO
application, in order to minimize the optical loss and light dam-
age, the chromophores should ideally be transparent at the
communication wavelengths (1310e1550 nm) and have low ab-
sorption at visible region, especially near the double frequency
wavelengths (650e750 nm). The basic requirement in zwitter-
ionic cyanopyridinium chromophore design calls for the maximal
absorption peak to be significantly blue shifted or ideally less
than 600 nm [24,25].
Furthermore, zwitterionic chromophores often display poor
solubility in normal organic solvents and low compatibility with a
polymer matrix. The large dipole moment and planar structure
cause the chromophores to aggregate aggressively in the matrix,
thereby limiting their loading level in the guestehost polymer
systems. The highly polar zwitterionic nature of the chromophores
also leads to an anti-parallel dipole arrangement, which could
further diminish or even eliminate the macroscopic nonlinearity in
bulk and hamper their EO application [19e23].
acid, hydroxyethyl cyanoacetate and poly(methyl methacrylate)
(PMMA) were purchased from Aldrich Chemicals. Azobisisobutyr-
onitrile (AIBN) was purchased from Aldrich Chemicals and recrys-
tallized from methanol. DMF and DMAc were dried over CaH2 and
distilled under vacuum. Poly(ether sulfone) (PES, commercial trade
name: Ultrason E, Tg ¼ 225 ꢁC) was obtained from BASF. The LiNbO3
wafer was purchased from Thorlabs Inc. (New Jersey, USA) and used
for calibration of electro-optic measurement.
2.2. Instruments
1H and 13C NMR spectra were recorded on a Varian 300 MHz
(300 and 75 MHz for 1H and 13C NMR, respectively) or a Bruker
AMX 400 MHz spectrometer using tetramethylsilane (TMS;
d
¼ 0 ppm) as an internal standard. The Fourier transform infrared
(FTIR) spectra were recorded on a PerkineElmer 1600 or a Bomen
Michelson 120 FTIR spectrometer in the regions of 4000e400 cmꢀ1
.
Mass spectra were measured with a Micromass Quattro LC ESI mass
spectrometer (EI and TOF-ESI). The melting points were deter-
mined using a Fisher-Johns melting point apparatus. Differential
scanning calorimetric analysis was carried out in nitrogen on a TA
DSC Q100 with a heating rate of 10 ꢁC/min. Thermogravimetric
analysis (TGA) was performed under a nitrogen atmosphere on a
Hi-Res TGA 2950 thermogravimetric analyzer with a heating rate of
10 ꢁC/min. The decomposition temperatures (Td) were determined
by 5% weight loss from TGA under nitrogen atmosphere. The
UVeViseNIR spectra were recorded on a PerkineElmer Lambda
900 UVeViseNIR spectrometer at room temperature. Fluorescence
emission spectra were measured on a PTI fluorescence system. The
absorption and fluorescence emission spectra of all the samples
were taken in a quartz cuvette with a path length of 10.0 mm.
Refractive indices were measured using a Metricon 2010 prism
coupler. An alpha-step 200 surface profiler was used for measuring
the thin film thickness. The Mw and Mn of the polymers were
evaluated in DMF with a Waters GPC system (2414 Refractive Index
Detector) with Styragel HR4E and HR5E columns using polystyrene
standards.
Therefore, it is desirable to develop a simple versatile method
for the synthesis of processable dipolar zwitterionic chromophores
with a short transparency cut-off wavelength. To achieve the blue-
shifted zwitterionic chromophores, adjusting the conjugation
length of the p-electron bridge is important; meanwhile, due to the
fact that zwitterionic chromophores often exist as the combination
of two limiting resonance forms (zwitterionic and neutral forms),
maintaining the zwitterionic structure as the predominant ground
state is critical in order to obtain large mb values [26,27].
Here we report the highly efficient and facile synthesis of a new
class of cyanopyridinium chromophores, composed of a pyridium
donor and cyano-moiety acceptor (SZC, in Fig.1) and corresponding
polymers. These chromophores are purposely designed to have a
short conjugation length in order to achieve a blue-shift in ab-
sorption. They also have various bulky substituents in order to
minimize chromophore dipoleedipole interactions and to improve
solubility and compatibility with polymer matrix. The new class of
zwitterionic chromophores and polymers are fully characterized by
spectroscopic methods, X-ray crystallographic analysis, thermal
analysis and nonlinear optical measurement.
2.3. Synthesis
2.3.1. Synthesis of 2-(4-bromophenyl)-3-pyridine-4-yl-acrylonitrile
(1)
2. Experimental section
Under anhydrous and oxygen-free conditions, to a 250-mL
round-bottomed flask, 4-bromophenyl acetonitrile (5.90 g,
30.0 mmol), 4-pyridinecarboxaldehyde (3.0 mL, 30.0 mmol), dry
THF (80 mL) and DBU (1 mL) were added. The mixture was heated
under reflux for 18 h and then cooled to room temperature. The
solvent was removed under reduced pressure; the red residue was
2.1. Materials
4-Bromophenyl acetonitrile, 4-pyridinecarboxaldehyde, 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU), 1-bromohexane, malononi-
trile, fluorophenyl acetonitrile, methyl cyanoacetate, cyanoacetic

W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
147
washed with methanol (30 mL), filtered and dried. The final
product was an off-white crystalline (3.90 g, 95% yield); mp:
138e140 ꢁC; 1H NMR (300 MHz, DMSO-d6):
8.77 (2H, d,
J ¼ 6.0 Hz), 8.15 (1H, s), 7.81 (2H, d, J ¼ 5.9 Hz), 7.76 (4H, m); IR (KBr,
cmꢀ1): 2217 (C^N), 1645 (C]N), 1594, 1588 (C]C); UVeVis:
lmax ¼ 312 nm (DMF); MS (EI, m/z): 284 [Mþ], 286 [Mþ2]þ.
2.3.6. General synthesis of SZC chromophores (with SZC-1 as an
example)
d
Under anhydrous and oxygen-free conditions, to a 50-mL round-
bottomed flask, malononitrile (0.50 g, 7.4 mmol), sodium hydride
(0.30 g, 60%, 7.5mmol), compound 4 (1.0g, 2.6mmol)and THF (20mL)
were added. After reaction for 30 min at 0 ꢁC, the resulting insoluble
inorganic salt was removed by filtration. The filtrate was concentrated
under reduced pressure. The residue was purified with the use of
column chromatography on silica gel (acetone and hexane, 1:1 v/v).
The final product was obtained as orange-red crystal (0.56 g, 63%
2.3.2. Synthesis of 2-(4-bromophenyl)-3-N-(n-hexylpyridinium)-4-
yl-acrylonitrile bromide salt (2)
yield); mp: 167 ꢁC; 1H NMR (400 MHz, DMSO-d6):
d 7.81 (2H, d,
A mixture of compound 1 (0.350 g, 1.23 mmol), 1-bromohexane
(2.8 mL, 20.0 mmol) and benzonitrile (2 mL) was heated under
reflux for 18 h. The reaction mixture was cooled and the precipi-
tate was filtered. The crude product was dissolved in hot aceto-
nitrile (10 mL) and then added into hot ether (50 mL). The
precipitate was filtered and dried in air to give yellow amorphous
J ¼ 7.1 Hz), 7.36 (2H, d, J ¼ 8.7 Hz), 7.32 (2H, d, J ¼ 8.7 Hz), 6.34 (2H, d,
J¼7.1Hz),5.88(1H, s),3.92(2H, t, J¼ 7.4 Hz), 1.63 (2H, t, J¼ 6.8Hz),1.22
(6H, m), 0.83 (3H, t, J ¼ 6.8 Hz); 13C NMR (100 MHz, DMSO-d6): 161.2,
151.4, 139.8, 133.7, 130.0, 129.9, 119.8, 119.2, 118.2, 116.6, 116.4, 101.7,
56.7, 30.4, 29.8, 24.9, 21.8, 13.7; IR (KBr, cmꢀ1): 2192 (yC^N of cyano
groups),1640 (yC]N ofpicoliniummoiety),1571,1450;MS(MALTI-TOF,
DMF/acetonitrile 1:1): calcd for (C22H22FN3): m/z [M]þ: 347.1798;
found: m/z 347.1793; UVeVis: lmax ¼ 486 nm (DMF); Td ¼ 312 ꢁC;
Fluorescence emission: 534 nm (excitation at 486 nm). Anal. Calcd (%)
for C22H22N3F: C, 76.06; H, 6.38; N, 12.09. Found: C, 75.93; H, 6.44; N,
12.12.
powders (0.30 g, 66% yield); 1H NMR (300 MHz, DMSO-d6):
d 9.22
(2H, d, J ¼ 8.4 Hz), 8.48 (2H, d, J ¼ 8.4 Hz), 8.44 (1H, s), 7.86 (4H,
m), 4.64 (2H, t, J ¼ 7.3 Hz), 1.95 (2H, m), 1.32 (6H, m), 0.89 (3H, t,
J ¼ 6.9 Hz); IR (KBr, cmꢀ1): 2219 (C^N), 1637 (C]N), 1604, 1583
(C]C); MS (ESI, acetonitrile, m/z): 369 [Mþ, 100%]; UVeVis:
lmax ¼ 347 nm (DMF).
2.3.7. Synthesis of SZC-2
2.3.3. Synthesis of 2-(4-fluorophenyl)-3-pyridine-4-yl-acrylonitrile
(3)
Methyl cyanoacetate (0.74 g, 7.5 mmol) was used; 0.60 g of
product (61% yield) was obtained as orange-red crystal. mp:
Under anhydrous and oxygen-free conditions, to a 250-mL,
round-bottomed flask, 4-fluorophenyl acetonitrile (4.05 g,
30.0 mmol), 4-pyridinecarboxaldehyde (3.20 g, 30.0 mmol), dry
THF (90 mL) and DBU (1 mL) were added. The mixture was heated
under reflux for 18 h and then cooled to room temperature. The
solvent was removed under reduced pressure and the red residue
was washed with methanol (30 mL), filtered and dried. The final
product was an off-white crystalline (3.40 g, 55% yield). mp:
204e206 ꢁC; 1H NMR (400 MHz, DMSO-d6):
d 7.72 (2H, d,
J ¼ 6.8 Hz), 7.29 (2H, d, J ¼ 8.5 Hz), 7.21 (2H, d, J ¼ 8.5 Hz), 7.20
(1H, s), 6.16 (2H, d, J ¼ 6.8 Hz), 3.88 (2H, t, J ¼ 7.2 Hz), 3.57 (3H, s),
1.63 (2H, t, J ¼ 6.8 Hz), 1.21 (6H, m), 0.83 (3H, t, J ¼ 6.8 Hz); 13C
NMR (100 MHz, DMSO-d6): 163.2, 160.7, 152.2, 139.3, 136.7, 129.9,
122.2, 118.6, 116.3, 116.1, 104.7, 75.1, 56.5, 49.9, 30.4, 29.8, 24.9, 21.7,
13.7; IR (KBr, cmꢀ1): 2180 (C^N), 1750 (C]O), 1639, 1505; MS
(MALTI-TOF, acetonitrile): calcd for (C23H25FN2O2): m/z [M]þ:
380.1900; found: m/z 380.1880. Anal. Calcd (%) for C23H25N2O2F: C,
72.61; H, 6.62; N, 7.36. Found: C, 72.35; H, 6.51; N, 7.35.
166e167 ꢁC; 1H NMR (400 MHz, DMSO-d6):
d 8.75 (2H, d,
J ¼ 6.0 Hz), 8.15 (1H, s), 7.80 (2H, d, J ¼ 5.9 Hz), 7.76 (4H, m); IR (KBr,
cmꢀ1): 2216 (C^N), 1592, 1543 (C]C); MS (EI, m/z): 224 [Mþ,
100%]; UVeVis: lmax ¼ 319 nm (DMF).
2.3.8. Synthesis of SZC-3
2-Hydroxyethyl cyanoacetate (0.99 g, 7.7 mmol) was used;
2.3.4. Synthesis of 2-(4-fluorophenyl)-3-N-(n-hexylpyridinium)-4-
yl-acrylonitrile bromide salt (4)
0.42 g of product (40% yield) was obtained. 1H NMR (400 MHz,
DMSO-d6):
d
9.01 (2H, d, J ¼ 6.8 Hz), 8.12 (2H, d, J ¼ 6.8 Hz), 7.38
A mixture of compound 3 (1.75 g, 7.80 mmol), 1-bromohexane
(20.0 mL, 143.0 mmol) and benzonitrile (3 mL) was heated under
reflux for 18 h. The reaction mixture was cooled to room temper-
ature and the resulting precipitate was filtered. The crude product
was dissolved in hot acetonitrile (40 mL) and then poured into
ether (250 mL). The precipitate was filtered as yellow amorphous
(2H, d, J ¼ 8.8 Hz), 7.17 (2H, d, J ¼ 8.8 Hz), 6.92 (1H, s), 4.62 (2H, t,
J ¼ 7.6 Hz), 4.48 (2H, t, J ¼ 5.6 Hz), 3.75 (2H, t, J ¼ 5.6 Hz), 1.93 (2H, t,
J ¼ 6.8 Hz),1.28 (6H, m), 0.86 (3H, t, J ¼ 6.8 Hz); IR (KBr, cmꢀ1): 3387
(OH), 2934, 2175 (C^N), 1637 (C]O), 1509; MS (EI, m/z): 410 [Mþ,
100%]; UVeVis: lmax ¼ 496 nm (DMF), lmax ¼ 556 nm (CHCl3).
powders (2.40 g, 80% yield). 1H NMR (300 MHz, DMSO-d6):
d 9.23
2.3.9. Synthesis of SZC-4
(2H, d, J ¼ 8.4 Hz), 8.49 (2H, d, J ¼ 8.4 Hz), 8.45 (1H, s), 7.87 (4H, m),
4.64 (2H, t, J ¼ 7.3 Hz), 1.95 (2H, m), 1.32 (6H, m), 0.88 (3H, t,
J ¼ 6.9 Hz); IR (KBr, cmꢀ1): 2224 (C^N), 1637 (C]N), 1597, 1557
(C]C); MS (ESI, acetonitrile, m/z): 309 [Mþ, 100%]; UVeVis:
lmax ¼ 341 nm (DMF).
Under anhydrous and oxygen-free conditions, to a 25-mL
round-bottomed flask, malononitrile (0.027 g, 0.40 mmol), so-
dium hydride (0.024 g, 60%, 0.60 mmol) and compound 2 (0.10 g,
0.2 mmol) were added. After reaction for 30 min at 0 ꢁC, the
resulting insoluble inorganic salt was removed by filtration. The
filtrate was concentrated under reduced pressure. The residue was
purified with the use of column chromatography on silica gel
(acetone and hexane, 1:1 v/v) to give the final product as orange-
red crystal (0.059 g, 65% yield). mp: 196 ꢁC; 1H NMR (400 MHz,
2.3.5. Synthesis of 2-hydroxyethyl cyanoacetate
A mixture of cyanoacetic acid (4.25 g, 50.0 mmol), ethylene
glycol (3.41 g, 55.0 mmol), and TsOH (0.2 g) in benzene (15 mL) in a
round-bottomed flask equipped with a DeaneStark trap was
heated under reflux. After the calculated volume of water (0.9 mL)
was distilled off, the solvent was evaporated and the residue was
vacuum distilled to give the product (2.58 g, 40% yield). 1H NMR
DMSO-d6):
d
7.81 (2H, d, J ¼ 7.1 Hz), 7.36 (2H, d, J ¼ 8.7 Hz), 7.32 (2H,
d, J ¼ 8.7 Hz), 6.34 (2H, d, J ¼ 7.1 Hz), 5.88 (1H, s), 3.92 (2H, t,
J ¼ 7.4 Hz), 1.63 (2H, t, J ¼ 6.8 Hz), 1.22 (6H, m), 0.83 (3H, t,
J ¼ 6.8 Hz); IR (KBr, cmꢀ1): 2190 (yC^N of cyano groups), 1640 (yC]N
of picolinium moiety),1571,1450 (yC]C aromatic ring); MS (EI, m/z):
407 [M]þ, 409 [Mþ2]þ; UVeVis: lmax ¼ 486 nm (DMF); Td ¼ 323 ꢁC
(TGA).
(300 MHz, DMSO-d6): d 3.52 (2H, s), 4.02 (2H, m), 4.40 (1H, s), 4.55
(2H, m); IR (NaCl, cmꢀ1): 3426 (OH), 2268 (C^N), 1743 (C]O),
1194.

148
W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
2.3.10. Synthesis of SZC-5
2.6. Thin film preparation and EO coefficient measurement
A Schlenk flask was charged with THF (23 mL), methacryloyl
chloride (0.53 mL, 5.4 mmol), SZC-3 (1.86 g, 4.0 mmol) and Et3N
(0.76 mL, 5.4 mmol). The reaction mixture was stirred at ambient
temperature for 6 h and then diluted with 200 mL of ether
(200 mL). The organic layer was washed with water
(2 ꢂ 200 mL) and brine (200 mL) and then dried over anhydrous
K2CO3. The solvent was removed under reduced pressure, and
the crude product was purified by chromatography on an
alumina column, eluting with dichloromethane. The red band
was collected to give SZC-5 as red powder (1.52 g, 70% yield). 1H
Doping SZC into PES to form a hosteguest system was done with
5 wt% chromophore content. To cast films, a DMF solution (2 wt%)
containing a chromophore and host polymer or a chromophore-
grafted polymer such as NP polymers was stirred overnight at
room temperature in darkness. The solution was filtered through a
0.2-
about 1e3
m
m syringe filter and casted onto ITO glass to form the films of
m
m thickness and then baked at 45 ꢁC overnight under
nitrogen. The films were further heated in a vacuum oven overnight
at 85 ꢁC. Then a layer of gold with a thickness of 100e150 nm was
sputtered or vacuum deposited on the films as the top electrode for
electric poling. During the thermo-electric poling process, the
sample cells were gradually heated to the final poling temperature,
while the voltage was gradually applied across the polymer films
with an average of 70 V/mm. The EO coefficients of the poled
samples were measured at the laser wavelength of 1550 nm using
the Teng-Man reflection technique setup [30]. The EO measure-
NMR (400 MHz, DMSO-d6):
d
8.99 (2H, d, J ¼ 6.8 Hz), 8.07 (2H, d,
J ¼ 6.8 Hz), 7.37 (2H, d, J ¼ 8.8 Hz), 7.14 (2H, d, J ¼ 8.8 Hz), 6.91
(1H, s), 5.67 (1H, t, J ¼ 1.6 Hz), 5.60 (1H, t, J ¼ 1.6 Hz), 4.61 (2H, t,
J ¼ 7.2 Hz), 4.47 (2H, t, J ¼ 6.2 Hz), 4.07 (2H, t, J ¼ 6.2 Hz), 1.98
(3H, s), 1.92 (2H, t, J ¼ 6.8 Hz), 1.23 (6H, m), 0.83 (3H, t,
J ¼ 6.8 Hz); IR (KBr, cmꢀ1): 2177 (C^N), 1739 (C]O), 1672 (C]
C), 1637 (C]O).
ment was calibrated using
a
commercial LiNbO3 wafer
(r33 ¼ 31 pm/V at 1550 nm).
2.3.11. A typical procedure for the synthesis of polymer NP
A two-neck, round-bottomed flask was charged with SZC-5
(0.105 g, 0.220 mmol), methyl methacrylate (0.450 mL,
4.18 mmol), AIBN (3.0 mg) and of dry benzene (1 mL). The re-
action mixture was heated to 80 ꢁC under nitrogen for 18 h. After
dilution with benzene, the polymer was precipitated into
methanol and collected as light red amorphous powder on filter
3. Results and discussion
3.1. Preparation of the SZC chromophores
The general synthesis of SZC chromophores is shown in Scheme
1. In order to introduce the alkene bridge and functional bulky
groups in the center of the chromophores, 4-fluorophenyl aceto-
(0.11 g, 89% yield). 1H NMR (CDCl3):
d 8.5, 8.2, 7.5, 7.0, 4.3, 3.6,
1.3e1.5, 0.8e1.1; IR (KBr, cmꢀ1): 2177 (C^N), 1756 (C]O), 1731
(C]O), 1637, 1485, 1150; UVeVis: lmax ¼ 502 nm (DMF), 558 nm
(CHCl3). NP-2 (Number-average molecular weight (Mn) of
51,954 Da, polydispersity index (PDI) of 1.25): Tg ¼ 126 ꢁC,
Td ¼ 232 ꢁC.
nitrile
(or
4-bromophenyl
acetonitrile)
and
4-
pyridinecarboxaldehyde were used as starting materials to pre-
pare compound 1. The crucial pyridinium salt 2 was then prepared
by alkylation of compound 1 with 1-bromohexane in benzonitrile.
The alkylated pyridinium is highly reactive towards active methy-
lene compounds, such as malononitrile, cyanoacetates, allowing for
a highly efficient nucleophile-replacement of the cyano group in
the central part of compound 1 and clean access to SZC chromo-
phores under mild conditions without any catalysts. The target
chromophores including a bromo-cyanide containing chromo-
phore (SZC-4) and three fluoro-cyanide containing chromophores
were prepared in good yields (>60%). Amongst SZC chromophores,
hydroxyl-containing chromophore (SZC-3) is considered to be a
potential monomer for the synthesis of grafted NLO polymers.
Halides (e.g., F or Br) on the aromatic rings may alter the polarity of
the chromophores and also allow for further functionalization on
the phenyl rings, for example, by the Suzuki cross-coupling reac-
tion. The long aliphatic chains on the nitrogen of pyridine are
deemed to impart the solubility to the chromophores. To prepare
SZC-3, the required 2-hydroxyethyl cyanoacetate was synthesized
according to a known procedure [31].
2.4. Single crystal X-ray diffraction
All diffraction measurements were carried out on a Bruker
SMART APEX CCD diffractometer with graphite-monochromated
Mo-K
a
(
l
¼ 0.71073 Å) radiation. Data were collected using the
Bruker SMART detector, processed using the SAINT Plus package
from Bruker. The structures were solved by direct methods
(SHELXS-97) and expanded using Fourier techniques (SHELXL-97).
All calculations were performed using the Bruker SHELXTL97
crystallographic software package.
2.5. Quantum yield measurement
The fluorescence emission spectra were measured with a PTI
spectrofluorometer. The fluorescence emission spectra of SZC
(2 ꢂ 10ꢀ5 M) were measured at 25 ꢁC, with excitation at 470 nm.
The fluorescent quantum yield (FF) of SZC were obtained with
reference to Rhodamine 6G (FF ¼ 0.88 in ethanol) [28]. The
The SZC chromophores were fully characterized by 1H NMR, 13
C
NMR, IR and mass spectrometry. In the IR spectrum of SZC-1
(Fig. S5), the cyano group is observed at 2192 cmꢀ1, and the in-
tensity of the peak is strikingly high, which is characteristic of the
cyano group connected to an electron-rich group with a negative
charge. In order to illustrate the extent of ground-state charge
separation, some IR data of compounds 1 and 2, SZC-1, SZC-2 and
other model compounds are listed in Table 1. Since the stretching
frequency of the cyano groups is sensitive to the vicinal electron
density, it is a good probe for the ground-state charge transfer. The
bond strength and therefore the vibrational frequency of CN groups
are reduced by the presence of increased vicinal electron density,
quantum yield (F) of SZC is expressed by eq. (1) [29], where the
subscript “r” stands for the reference and “x” for the sample), A
is the absorbance at the excitation wavelength, I ( ) is the rela-
tive intensity of the exciting light at wavelength , n is the
l
l
refractive index of the solvent for the luminescence, and D is the
area (on an energy scale) of the luminescence spectra. The
samples and the reference were excited at the same wavelength.
The sample absorbance at the excitation wavelength was kept as
low as possible to avoid fluorescence errors (Aexc <0.1).
which interacts with the p* orbital of the CN group [32].
The stretching frequency of nCN was found at 2178 cmꢀ1 for
lithium dicyano-phenylmethanide (Liþ[C(CN)2Ph]ꢀ), which in-
dicates a full formal vicinal negative charge, while the stretching
ꢀ
ꢁꢀ
ꢁꢀ ꢁꢀ
ꢁ
Arðl Þ
Iðl Þ n2x
Dx
Dr
r
r
Qx ¼ Qr
(1)
nr2
Axðl Þ Iðl Þ
x
x

W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
149
Scheme 1. Synthesis of zwitterionic chromophores SZC.
frequency, nCN ¼ 2226 cmꢀ1, for TCNQ serves as a model without
formal vicinal charge [33]. The CN stretches at 2220 cmꢀ1 and
2224 cmꢀ1 for intermediate compounds 1 and 2, respectively, are
from typical neutral conjugated cyano groups. On the other hand,
atoms. By comparing the observed two CeCN bonds (1.411 Å and
1.420 Å) with the one (1.427 Å) in typical TCNQs, there is a sub-
stantial negative charge localization within the eC(CN)2 group; the
elongated CN (1.151 Å and 1.147 Å) bond compared to that in typical
TCNQs (1.144 Å) [35] indicates a charge resonant stabilization via
the two CN groups.
For comparison, the well-known Brooker dye (Scheme 2), which
has a similar conjugated backbone and a length of the central bond
of 1.35 Å in the solid state, is considered as the prototype of a
zwitterionic merocyanine dye, with an estimated degree of the
zwitterionic form of 81.8% in its ground state [36]. The longer
central bond, 1.384 Å, in SZC-1 indicated that less than 80% of the
zwitterionic form in its ground state. Compared to the central bond
the lower values for SZC-1 (2192 cmꢀ1) and SZC-2 (2180 cmꢀ1
)
imply considerable CT character in the ground state. In comparison,
the smaller nC^N frequency of PeQDM (2146 cmꢀ1) with that of
PQDM (2173 cmꢀ1) indicate that PeQDM with one cyano and one
ester group at the acceptor part gain more aromatization stabili-
zation than PQDM, which shows that PeQDM has a higher molec-
ular hyperpolarizability and larger EO coefficient than PQDM [31]. A
similar trend of the smaller nC^N frequency for SZC-2 than SZC-1
was observed.
3.2. Structure by X-ray crystallography
Further evidence for the formation of zwitterionic chromo-
phores is provided by X-ray crystallography of SZC-1 (Fig. 2) [34].
Single crystal of SZC-1 was obtained by slow crystallization from a
hexane/acetone solution at room temperature.
The central bond length between C7 and C11 (1.384 Å) is similar
to the value observed in other zwitterionic compounds [1], which
clearly indicates the double bond
p-bridge separating the pyr-
idinium and dicyanomethanide groups. This conclusion is also
supported by additional selected bond lengths shown in Table 2.
The bond lengths between C7 and C8 (1.408 Å), C7 and C11
(1.384 Å) suggest electron delocalization among the three carbon
Table 1
Selected IR data for 1, 2, SZC-1 and SZC-2.
Fig. 2. ORTEP drawing of the structure of SZC-1 in the crystal with atomic number.
Hydrogen atoms were omitted for clarity. The crystal data for SZC-1 is given as fol-
Compound
nCN (cmꢀ1
)
Compound
nCN (cmꢀ1
)
lowed:
c ¼ 21.313(5) Å,
C
22H22FN3,
M
¼
347.43, monoclinic,
a
¼
10.485(3) Å,
b
¼
8.809(2) Å,
Liþ[C(CN)2Ph]ꢀ
TCNQ
1
2
217833
222633
2220
SZC-1
SZC-2
PQDM
PeQDM
2192
a
¼ 90.00ꢁ,
b
¼ 100.628(4)ꢁ,
g
¼ 90.00ꢁ, V ¼ 1934.7(8) Å3, T ¼ 293
2180
(2) K, Space group P 2(1)/c, Z ¼ 4, 11,920 reflections measured, 4589 independent
reflections. The final R values were 0.0505(I > 2sigma (I)). The final R1 values were
0.1084 (all data). The final wR2 values were 0.0902 (all data). CCDC number: 861785.
217321
214623
2224

150
W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
¼ 48.8ꢁ) than that in PeQDM-Ben (
q
¼ 64.5ꢁ) is observed
Table 2
angle (
q
Selected bond distances for SZC-1 in the crystal.a
which indicated that there is no H-aggregation between two
chromophores. The intermolecular distance (d) is considerably
larger than the sums of van der Waals radii between planar cofacial
Atom1
Atom2
Å
Atom1
Atom2
Å
C4
C7
C7
C11
C12
C12
C13
C15
C7
C11
C8
C12
C13
C15
C14
C16
1.487(2)
1.384(4)
1.408(2)
1.416(2)
1.409(2)
1.413(2)
1.352(2)
1.349(2)
C16
C14
C17
C8
C8
C9
N3
N3
N3
C9
C10
N1
N2
F
1.352(1)
1.349(2)
1.480(1)
1.411(2)
1.420(2)
1.147(2)
1.151(2)
1.362(4)
p-electron systems (~3.50 Å) [37], the dihedral angle between the
pyridinium and benzene rings is 65.65ꢁ, result in the difficulty in
close face-to-face packing of the aromatic rings in these central
bulky molecules. Therefore, incorporation of bulky groups onto the
central moiety is an effective strategy to minimize molecular
interaction. The weak electrostatic interactions between SZC-1
(compared to PQDMs and PeQDMs) make these chromophores
soluble not only in polar solvents such as DMF, acetonitrile (ACN)
and methanol, but also moderately soluble in less polar solvents
such as chloroform, tetrahydrofuran (THF) and benzene.
C10
C1
a
Estimated value in parenthesis refer to the last digit.
length of PeQDM (1.354 Å, double bond character), SZC-1 has a
lower degree of charge transfer in ground state than that of PeQDM.
Although the bond lengths in the conjugated bridge and
3.3. Solvent-dependent optical property of SZC chromophores
acceptor part clearly demonstrated
a zwitterionic molecular
structure of SZC-1, the bond length of the pyridinium ring is qui-
noidal rather than aromatic. Similar phenomena have also been
reported for other zwitterionic molecules such as DEMI [19] and
PeQDM [23]. Therefore, the best description of the ground state
structure of such chromophores is the combination of the two
limiting forms, zwitterionic and neutral forms wherein the zwit-
terionic contribution predominates.
As shown in Fig. 3 (left), two SZC-1 molecules are arranged in an
antiparallel fashion with an intermolecular distance of 4.0645 Å
between the aromatic backbones in the crystal cell. A smaller slip
All of SZC chromophores present a strong CT band in the visible
region of the spectrum, the largest absorption wavelengths of SZC-
1 and SZC-2 in selected solvents are collected in Table 3. SZC-1
shows good transparency, approximately 200 nm blue-shift of the
maximum absorption of SZC-1 in DMF solution was observed
compared with that of PQDM [21]. The CT band of SZC-1 exhibits
negative solvatochromic effect, a hypsochromic shift from THF
(ET ¼ 37.4) to MeOH (ET ¼ 55.5) is about 15 nm toward shorter
wavelengths with increasing solvent polarity. The negative sol-
vatochromism result indicates that the charge-separated resonance
Scheme 2. Resonance structures of Brooker dye and chromophore SZC.
Fig. 3. Crystal packing diagrams of chromophore SZC-1 (the hydrogen atoms are omitted for clarity) (left); Model for a centrosymmetric dimer of two dipolar dyes,
of the dimer, d is the distance between the systems (right).
q is the slip angle
pep

W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
151
Table 3
80
MeOH
ACN
DMSO
DMF
Acetone
DCM
THF
lmax of SZC chromophores in different solvents.
Compound THF 7.6a/ Acetone
Methanol DMF
ACN
DMSO
37.4b
20.7/42.2 32.7/55.4 36.7/43.2 37.5/ 46.7/45.1
45.6
60
40
20
0
SZC-1
SZC-2
495
502
487
495
480
486
485
493
483
490
484
492
1,4-dioxane
a
Solvent Dielectric Constant (ε) [39].
Reichardt E(T30) values [38].
b
form of SZC-1 is more dominant in the ground state than the
excited state (mg me) [38].
>
In comparison, the magnitude of the solvatochromic shift in the
PQDM chromophores (on average 113 nm) [21] is 7.5 times larger
than that in the SZC chromophores (on average 15 nm). This can be
explained by the magnitude of the gain in resonance stabilization
energy upon aromatization of the two donor and acceptor units.
The gain in aromaticity for the SZC chromophores is expected to be
less than that in the case of PQDMs due to the absence of one
benzene unit in the conjugated bridge.
500
550
600
650
Wavelength (nm)
Fig. 5. Fluorescence spectra of SZC-1 solutions with excitation at 460 nm. The suc-
cessive maxima correspond to methanol, ACN, DMSO, DMF, acetone, DCM, THF and 1,4-
dioxane.
As shown in Fig. 4, the strongest emission band of SZC-1 has a
maximum at 530 nm. The fluorescence spectrum of SZC-1 associ-
ated with the lowest excited singlet state displays a mirror image of
the corresponding absorption band (485 nm) with a Stokes shift of
45 nm. As shown in Fig. 5, a set of fluorescence spectra of SZC-1
show a gradual blue shift with increasing solvent polarity, from
539 nm in 1,4-dioxane to 524 nm in methanol. The negative
polarity-dependent results are consistent with the negative sol-
vatochromism of SZC-1, which further confirms the charge-
separated ground state of these chromophores.
In comparison with DEMI and PpQDM [5], the Stokes shift
values are dramatically influenced by an increase in the conjuga-
tion length from SZC to DEMI to PpQDM (Table 4). The large Stokes
shift of PpQDM could be explained by the magnitude of the gain in
resonance stabilization energy upon aromatization of the donor
and acceptor units. The gain in aromaticity for DEMI is expected to
be less than that of PpQDMs due to the absence of one aromatic ring
in the donor. SZC have the shortest conjugation length and
accordingly have the smallest Stokes shift among all.
Table 4
Optical properties of SZC, DEMI and PpQDM in DMF.
Compound
lA (nm)a
lF (nm)b
lF
ꢀ
lA (nm)
FF (%)
SZC-1
SZC-2
DEMI
485
493
690
680
530
533
770
875
45
40
80
0.40c
0.27c
e
0.20d
PpQDM
195
a
Maximal wavelength of absorption and fluorescence measured at a concentra-
tion of 2 ꢂ 10ꢀ5 M in DMF. Excitation wavelength is 470 nm for SZC-1 and SZC-2;
670 nm for DEMI and PpQDM.
b
Maximal wavelength of absorption and fluorescence measured at a concentra-
tion of 2 ꢂ 10ꢀ5 M in DMF. Excitation wavelength is 470 nm for SZC-1 and SZC-2;
670 nm for DEMI and PpQDM.
c
Fluorescence quantum yield, relative to Rhodamin 6G (FF ¼ 0.88 in ethanol).
d
Fluorescence quantum yield measured in DMF, relative to IR-125 (FF ¼ 0.13 in
DMSO) [40].
moments are expected on the basis of the zwitterionic ground state
structure of SZC. Since the ester is considered to be a weaker
electron acceptor than the nitrile, the dipole moment of SZC-2 is
expected lass than SZC-1. A decrease in dipole moment from SZC-1
to SZC-2 has been confirmed experimentally by the most common
method based on dielectric and refractive index measurements
according to Debye's theory. The changes in dielectric constant and
refractive index of solutions of the chromophores in nonpolar sol-
vents are measured to give a dipole moment of 21.6 D for SZC-1 and
11.4 D for SZC-2 (Table 5).
3.4. Dipole moment and molecular hyperpolarizability
The ground-state dipole moment (
m), first hyperpolarizability
(b) and the corresponding figure of merit (mb) are representative
characteristics of zwitterionic chromophores. Large dipole
The first hyperpolarizabilities of SZC chromophores are
measured by hyper-Rayleigh scattering (HRS) method in DMF so-
1.0
0.8
0.6
0.4
0.2
0.0
lution at 1.07
m
m wavelength. The bzzz values are ꢀ320 ꢂ 10ꢀ30 esu
for SZC-1 and ꢀ495 ꢂ 10ꢀ30 esu for SZC-2, assuming all other
components are negligible. More details about the setup, data
collection, and dispersion modeling are given in our previous re-
ports [19]. The structural feature of DEMI, PQDM and PeQDM im-
plies that the aromatic/quinoidal bridging unit is critical in
maintaining a charge-separated state and might be necessary for
their optical nonlinearity. However, this bridging unit is absent in
SZC chromophores, which still exhibit optical nonlinearity. In
addition, either the replacement of the third cyano group on the
central part or substitutions on one of the cyano group of the
acceptor does not significantly change the dipole and optical
nonlinearity of SZC chromophores. Further comparison can be
made between SZC and the adducts of N,N-dimethylaniline/
tetracyanoethylene (TCNE) [41], as the former exhibits a negative
400
450
500
550
600
650
Wavelength (nm)
Fig. 4. The normalized absorption and emission spectra of SZC-1 in DMF (2 ꢂ 10ꢀ5 M).

152
W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
Table 5
Dipole moment and hyperpolarizability of SZC and other chromophores.
100
80
60
40
20
0
SZC-1
SZC-4
Chromophore
m
bc
b0
mb
(10ꢀ30 esu)
(10ꢀ30 esu)
(10ꢀ48 esu)
(a)
(D)
SZC-1
SZC-2
PQDM-Met
PeQDM-Ben
DEMI
21.6a
11.4a
38b
ꢀ320
ꢀ495
ꢀ1865
ꢀ1800
e
e
ꢀ6720
ꢀ5643
e
247e397d
416e546d
350e
ꢀ70,870
ꢀ70,000
ꢀ9450f
40b
27b
a
Experimental value measured in CHCl3.
Theoretical value (Experimental value has not been measured in chloroform due
b
to solubility difficulties) [7,26].
c
The experimental value measured by HRS in DMF at 1.07 mm.
d
Static values derived by means of the single-mode vibronic model (serving as a
lower limit); Static values obtained from the purely homogeneous vibronic-like
model (serving as an upper limit) [26].
e
Measured in chloroform [7].
Measured value of mb0 in chloroform [7].
f
100
150
200
250
300
350
400
450
Temperature (oC)
first molecular hyperpolarizability and the latter in the predomi-
nantly non-charge separated ground state shows the positive
values.
120oC
130oC
(b)
0
3.5. Thermal properties
For EO applications, the chromophore is typically doped in a
host polymer and then subjected to electric poling at elevated
temperatures. Thermal stability of the chromophore is a key
parameter for the selection of host polymer and poling tempera-
ture. Thermal properties of SZC chromophores were evaluated by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). Thermogravimetric analysis of SZC-1 and SZC-2 was
done under nitrogen and the onset temperatures for 5% weight loss
were found at 312 ꢁC and 323 ꢁC, respectively (Fig. 6a), being higher
than those of PQDM (287 ꢁC) [21] and PeQDMs (215e287 ꢁC) [23].
The characterization by DSC revealed typical phase transitions,
namely crystallization at 130 ꢁC and 120 ꢁC and melting process at
167 ꢁC and 196 ꢁC for SZC-1 and SZC-4, respectively (Fig. 6b).
During the cooling process, no crystallization was observed for
these two chromophores.
167oC
SZC-1
SZC-4
196oC
50
100
150
200
250
Temperature(oC)
Fig. 6. TGA thermograms (a) and DSC traces (b) of SZC-1 and SZC-4.
obtained in 85e90% yields as light red powders (Table 6). The IR
spectra of the NP polymers exhibit the characteristic peaks for the
cyano groups at 2177 cmꢀ1. The 1H NMR spectra of two NP poly-
mers displayed the expected peaks corresponding to the structures
of both MMA and SZC chromophore (Fig. S8).
3.6. Synthesis of SZC-containing NLO polymers (NP)
To achieve a large and stable macroscopic EO activity, chromo-
phores are usually doped in a polymer host or covalently bonded
onto a polymer and then oriented under an externally applied
electric field at the glass transition temperature (Tg) of the materials
in order to obtain a noncentrosymmetric order. To choose a suitable
host for SZC, PMMA and PES are typically good candidates, due to
their availability, good processability and suitable compatibility
with zwitterionic chromophores [42e44]. A relatively large amount
of SZC-1 (5e10 wt%) could be blended into PMMA and PES, forming
the films with no phase separation. Considering a higher Tg, PES
was used to make SZC/PES guestehost polymers for EO
measurement.
To incorporate SZC chromophores into polymer by copoly-
merization, SZC-5 monomer was needed and thus synthesized by
the reaction of SZC-3 with methacryloyl chloride. The general
synthesis of SZC-containing NLO polymers (NP) by free radical
polymerization of SZC-5 with methyl methacrylate (MMA) is
shown in Scheme 3.
The contents of SZC chromophore in NP polymers were deter-
mined from the UV absorption that is calibrated using SZC-1 in
DMF and found to be 7 wt% and 14 wt% for NP-1 and NP-2,
respectively. Negative solvatochromism for NP polymers was also
confirmed, as the absorption of NP-1 is at 558 nm in CHCl3 and
502 nm in DMF. NP Polymers have good thermal stability
(Td > 232 ꢁC). The Tg value of NP-2 (126 ꢁC) is higher than that of
NP-1 (95 ꢁC), due to a higher content of polar SZC in the polymer.
NP polymers have good solubility in common organic solvents such
as CHCl3, benzene, ACN and DMF. Flexible and transparent films
without any phase separation can be readily prepared on ITO
coated glass by solution casting and are suitable for electric poling
and EO measurement.
3.7. Poling and EO properties
The IR spectrum of SZC-5 displayed the characteristic bands of
cyano group at 2177 cmꢀ1 and of the esters at 1739 and 1672 cmꢀ1
The maximal absorption of SZC-5 is at 556 nm in chloroform and
496 nm in DMF, which indicates that SZC-5 has the same charge-
separated ground state as SZC-3. Polymers NP-1 and NP-2 were
.
To investigate the potential of SZC chromophores in EO appli-
cations, both SZC/PES doped guestehost polymers and NLO poly-
mers NP were poled and the EO coefficients were assessed. For the
poling studies, SZC/PES films with a thickness of 1e3
mm were

W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
153
Scheme 3. Synthesis of SZC-containing NP polymers.
prepared by casting a solution in DMF (2 wt%/V) onto ITO glass
substrates. NP polymer films with a thickness of 1e3 m on ITO
polymers with 7 and 14% SZC were subjected to poling. In com-
parison with SZC-doped PES films, NP polymer films showed a
m
glass were prepared by solution casting. The films were dried at
60e80 ꢁC under vacuum (1 mmHg) for 24 h before sputtering a thin
layer (100e150 nm) of gold as a top electrode onto the polymer
films. The parallel-electrode poling was carried out under the
protection of nitrogen in dark and the sample cells were gradually
heated from room temperature to the poling temperature, about
5e10 ꢁC below the Tg of the polymer films (e.g., 190 ꢁC for SZC/PES)
at a heating rate of 5 ꢁC/min. The poling field was then applied to
the sample at a ramping rate of 10 V/min. During the poling pro-
cess, the electric current was monitored to optimize poling effi-
ciency and to control local electrical breakdown in the films. After
the films were held at the final poling temperature for 20 min, the
heating was turned off while keeping the poling field. After
reaching room temperature, the poling voltage was taken off and
the EO coefficient, r33 was determined on a Teng-Man reflection
setup immediately at 1550 nm. The EO properties of SZC-doped
polymers and NP polymers were summarized in Table 7.
Although SZC chromophores exhibit moderate mb values but
still ten times less than PQDM (Table 5), SZC (5%) doped PES films
display almost the same EO coefficients as PQDM (1 wt%) doped
PES films (21 pm/V) [23]. SZC can be doped up to 10 wt% into PES, 5
times higher than PQDM (about 2 wt%), which can be attributed to
less dipoleedipole interaction of the short-conjugated SZC mole-
cules. In addition, due to relatively smaller dipole moment, SZC
chromophores (11e20 D) are more compatible with and soluble in
PES than PQDM (38e40 D).
better stability to the electric field above 70 V/mm and displayed the
EO coefficients of 24 and 30 pm/V for NP-1 and NP-2 polymers,
respectively. A slightly higher EO coefficient for NP-2 polymer is
likely due to its higher chromophore content than NP-1. As ex-
pected, the EO coefficient (30 pm/V) of the poled NP-2 (14 wt%
chromophore) is higher than that of the SZC doped PES system
(5 wt%) and is indeed comparable to that of lithium niobate (31 pm/
V). It is known that a trade-off between the chromophore content
and poling efficiency for highly polar chromophore-polymer sys-
tems and a maximum of chromophore incorporation exists for a
maximal poling efficiency, depending on the chromophore polarity
and dipoleedipole interaction. For PQDM, an amount of 11 wt% can
be covalently incorporated in polyimide to achieve a maximum of
EO coefficient [6]. A higher concentration of SZC (14%) in a poled
polymer with the EO coefficient of 30 pm/V can be attributed to the
short conjugation length and low dipoleedipole interaction of SZC.
Further increase of the concentration of SZC in a polymer is highly
possible when using a more polar polymer as a carrier, which
should lead to a higher EO coefficient.
4. Conclusions
Short-conjugated zwitterionic cyanopyridinium chromophores
(SZC) with a rigid non-planar structure and the absorption less than
550 nm were designed and synthesized by the use of nucleophile
substitution reaction between cyano-pyridinium salt and cyanoa-
cetates without any catalysts. The synthetic process developed was
simple and versatile and allows the preparation of a blue-shift SZC
with high yield, the easy introduction of functional groups and the
The poled SZC-doped PES polymers showed a small EO coeffi-
cient (<20 pm/V), presumably due to a low number density of SZC.
Attempt to increase the SZC content to above 10 wt% led to the
formation of brittle films that are not suitable for poling. To achieve
a higher poling efficiency and thus higher EO coefficient, the
polymer with high chromophore content is desirable. Thus, NP
Table 7
EO properties of SZC-doped PES and NP polymers.
NLO polymer
N
(wt%)
Film thickness
(mm)
n
Poling
r33
temperature (oC) (pm/V)
Table 6
Characterizations of NP polymers.
SZC-1/PES
SZC-2/PES
NP-1
5
5
7
2.12
2.06
2.01
1.89
1.6206 190
1.6204 180
20
18
24
30
Polymer
X
Y
lmax
Tg
Td
1.4972
90
(wt%)
(mol%)
(DMF, nm)
(oC)
(oC)
NP-2
14
1.4970 120
NP-1
NP-2
7
14
95
90
502
498
95
126
264
232
N: chromophore content in polymer.
n: refractive index at 1550 nm.

154
W.H. Hao et al. / Dyes and Pigments 111 (2014) 145e155
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alteration of the geometry of SZC. The new zwitterionic cyano-
pyridinium chromophores display excellent solubility in organic
solvents, good film forming ability, higher loading level in polymer
matrix and good thermostability. IR, X-ray crystallography, negative
solvatochromism and fluorescence studies confirm the charge-
separated ground state of SZC. Absorption spectra of SZC chromo-
phores show a blue shift by nearly 200 nm in comparison with the
previously studied PQDM chromophores. Structureefunction rela-
tionship revealed that an olefinic conjugation bridge is critical in
maintaining the charge-separated ground state.
The studies of the linear and nonlinear optical properties of SZC
show moderate dipole moments and first hyperpolarizabilities. SZC
can be doped up to 10% by weight into poly(ether sulfone) to form a
uniform film. A chromophore-grafted polymer containing 14% by
weight of SZC exhibits a promising EO coefficient (30 pm/V at
1550 nm). This versatile preparation method not only opens the
new avenue for the design and synthesis of a variety of blue-shifted
zwitterionic cyanopyridinium chromophores, but also enables easy
further structure modification to achieve desired linear and
nonlinear optical properties for specific applications.
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Notes
The authors declare no competing financial interest.
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properties of picolyltricyanoquinodimethan, the zwitterionic donor-p-
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Acknowledgments
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We thank the Natural Sciences and Engineering Research
Council of Canada and National Natural Science Foundation of
China (21272061, 51272069) for financial support and Dr. W.
Wenseleers of University Antwerp for measurement of the hyper-
polarizability of SZC chromophores.
Appendix A. Supplementary data
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