A study on regulating the conjugate position of NLO chromophores for reducing the dipole moment and enhancing the electro-optic activities of organic materials

Article abstract of DOI:10.1039/c9tc05704h

In order to improve the first-order hyperpolarizability (β) of the chromophore and transform it into a high macroscopic electro-optic activity, a series of novel second-order nonlinear optical chromophores with different push-pull electron groups introduced on the thiophene π-conjugate bridge for tuning the shape and dipole moment (μ) of chromophores were designed and synthesized. These chromophores are based on the same thiophene π-conjugated bridge, where the donor (N,N-diethylaniline) and acceptor (2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile or malononitrile) are linked to positions 2 and 3 of thiophene, respectively, affording a boomerang-like shape instead of a rod-like shape. Besides, an electron-poor group, Br (bromine atom), or an electron-rich group, DEA (N,N-diethylaniline), as an auxiliary acceptor or donor are linked to position 5 of thiophene. In addition, all chromophores showed good thermal stability as per the results from the DSC and TGA analysis. Through UV-vis analysis and DFT calculation, it has been concluded that chromophores with additional electron-rich groups as auxiliary donors display better intermolecular charge-transfer (ICT) absorption and lower HOMO-LUMO energy gaps (ΔE). Furthermore, the boomerang-like chromophore with the same push-pull structure shows a smaller dipole moment (μ) and β value than the traditional FTC. The poling results of guest-host EO polymers FTC/APC, FTC-H/APC, FTC-Br/APC and FTC-DEA/APC with the same number density afford r₃₃ values of 17 pm V^-1, 11 pm V^-1, 10 pm V^-1 and 25 pm V^-1, respectively. Although the β value of FTC-DEA is smaller than that of FTC, the r₃₃ value of FTC-DEA (25 pm V^-1) is 47% greater than that of FTC (17 pm V^-1) under the same number density. Hence, the above-mentioned results indicated that regulating the conjugate position of chromophores can efficiently decrease the dipole moment of the chromophores, weakening the dipole-dipole interactions and thereby enhancing the macroscopic electro-optical activity of poled polymers. These results indicate the potential application of these novel chromophores in electro-optical devices.

Full text of DOI:10.1039/c9tc05704h

Journal of
Materials Chemistry C
View Article Online
View Journal
PAPER
A study on regulating the conjugate position
of NLO chromophores for reducing the dipole
moment and enhancing the electro-optic
activities of organic materials
Cite this:DOI: 10.1039/c9tc05704h
a
a
b
a
a
Hui Zhang,
Yanxin Tian, Shuhui Bo,
Linghan Xiao, Yuhui Ao,
*
a
a
Ji Zhang* and Ming Li
*
In order to improve the first-order hyperpolarizability (b) of the chromophore and transform it into a high
macroscopic electro-optic activity, a series of novel second-order nonlinear optical chromophores with
different push–pull electron groups introduced on the thiophene p-conjugate bridge for tuning the shape
and dipole moment (m) of chromophores were designed and synthesized. These chromophores are based on
the same thiophene p-conjugated bridge, where the donor (N,N-diethylaniline) and acceptor (2-(3-cyano-
4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile or malononitrile) are linked to positions 2 and 3 of thiophene,
respectively, affording a boomerang-like shape instead of a rod-like shape. Besides, an electron-poor group,
Br (bromine atom), or an electron-rich group, DEA (N,N-diethylaniline), as an auxiliary acceptor or donor are
linked to position 5 of thiophene. In addition, all chromophores showed good thermal stability as per the
results from the DSC and TGA analysis. Through UV-vis analysis and DFT calculation, it has been concluded
that chromophores with additional electron-rich groups as auxiliary donors display better intermolecular
charge-transfer (ICT) absorption and lower HOMO–LUMO energy gaps (DE). Furthermore, the boomerang-
like chromophore with the same push–pull structure shows a smaller dipole moment (m) and b value than
the traditional FTC. The poling results of guest–host EO polymers FTC/APC, FTC-H/APC, FTC-Br/APC and
ꢀ
1
ꢀ1
ꢀ1
FTC-DEA/APC with the same number density afford r33 values of 17 pm V , 11 pm V , 10 pm V and
ꢀ
1
2
5 pm V , respectively. Although the b value of FTC-DEA is smaller than that of FTC, the r33 value of
ꢀ
1
ꢀ1
Received 18th October 2019,
Accepted 25th November 2019
FTC-DEA (25 pm V ) is 47% greater than that of FTC (17 pm V ) under the same number density. Hence,
the above-mentioned results indicated that regulating the conjugate position of chromophores can
efficiently decrease the dipole moment of the chromophores, weakening the dipole–dipole interactions
and thereby enhancing the macroscopic electro-optical activity of poled polymers. These results indicate
the potential application of these novel chromophores in electro-optical devices.
DOI: 10.1039/c9tc05704h
rsc.li/materials-c
5–11
devices, etc.
The second-order NLO chromophores are the most
1
. Introduction
important part of the electro-optic (EO) material. Hyperpolarizability
Organic second-order nonlinear optical (NLO) materials have (b) has been used to characterize the microscopic NLO activities
attracted much attention over the past two decades because of of the chromophore. However, the challenges in preparing
their potential advantages such as ultrafast response, broad excellent electro-optic (EO) materials are the development of
1
2–15
bandwidth, low driving voltage, tunable structures and large NLO chromophores with a large b value
and the translation
1–4
electro-optic (EO) coefficients (r33). These materials have potential of the microscopic NLO activities into macroscopic electro-optic
applications in optical switches, optical sensors, telecommunication activities.
Generally, the NLO dipolar chromophores have a dipolar
a
structure, where the electron-donor (D) and electron-acceptor (A)
groups are end-capped in a p-conjugated bridge (D–p–A structure),
College of Chemistry and Life Science, Jilin Province Key Laboratory of Carbon
Fiber Development and Application, Changchun University of Technology,
Changchun 130012, People’s Republic of China. E-mail: liming15@ccut.edu.cn,
zhangj158@ccut.edu.cn, aoyuhui@ccut.edu.cn; Fax: +86-431-85968078;
Tel: +86-431-85968078
with large intramolecular charge transfer (ICT) absorption for
16–18
enhancing the first-order hyperpolarizability (b).
However,
b
strong intermolecular dipole–dipole interactions between the chro-
mophores inevitably existed among dipolar chromophores and
impeded the poling-induced noncentrosymmetric alignment, leading
Key Laboratory of Photochemical Conversion and Optoelectronic Materials,
Technique Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing, 100190, People’s Republic of China
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
to a lower r33 of the material. Moreover, the larger the b value and the tuning and introduction of auxiliary donors or acceptors at
greater the concentration the chromophore had, the stronger the position 5 of thiophene p-conjugated bridge would affect the
1
9–23
dipole–dipole interaction in the material.
achieve higher EO activity and weaken the intermolecular dipole– and EO performance of the materials.
dipole interactions among chromophores, many efforts had been In this work, we designed and synthesized six dipole chromo-
The phores with thiophene as a p-conjugated bridge with different
Therefore, in order to electron transfer of the chromophore and optical properties
24–28
made to design and synthesize novel NLO chromophores.
structure that links chromophores together in a head-to-head auxiliary groups on the thiophene p-conjugated bridge, such as
and tail-to-tail fashion can effectively block the intermolecular DEA (N,N-diethylaniline) groups with the electron-donating ability
3
4
dipole–dipole interactions and realize the preparation of a and Br (bromine atom) with the electron-withdrawing ability,
2
9
single-molecule nonlinear film. The chromophore designed affording FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and MTD-DEA.
by Dalton et al. showed a decrease in the dipole moment (m) and Major push–pull conjugated structures of all the chromophores
enhanced the electro-optic activity of the material. It can be used thiophene as a p-conjugated bridge, N,N-diethylaniline and
seen that reducing intermolecular dipole–dipole interactions by TCF (2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile)
reducing the dipole moment (m) of the chromophore molecule or malononitrile as the donor and acceptor linked to positions
was a very effective method for increasing the electro-optic 2 and 3 of thiophene, respectively, for tuning the shape and dipole
3
0
3
0
activity.
moment. This means that FTC-DEA and MTD-DEA chromophores
Up to now, NLO chromophores conjugated with thiophene have two electron-donating groups. Synthetic chromophores (in
have been widely used, and FTC has been used as a classic Chart 2) showed great solubility in common organic solvents
1
13
high-performance chromophore in many devices. Therefore, and good compatibility with polymers. H NMR, C NMR and
further modification of the FTC has been extensively studied to MS analyses were carried out to demonstrate the successful
3
1,32
achieve better performance.
Xu’s group designed FTC preparation of these chromophores. Thermal stability, photo-
derivatives by introducing sterically hindered groups into the physical properties, DFT calculations and EO activities of these
donor and conjugated bridges of the FTC. The introduction of chromophores were systematically studied.
hindered groups efficiently prevented the dipole–dipole
moment and led to a higher doping content of the chromo-
ꢀ
1
phore (up to 49 wt%), and the r33 value (56 pm V ) was 43%
2
. Results and discussion
ꢀ
1
33
higher than that of the FTC (39 pm V with 25 wt% in APC).
Based on the previous work on the variety of structures of
2.1 Synthesis and characterization of chromophores
chromophores, we found that donors and acceptors of FTC All NLO chromophores were designed with the same electron
derivatives were linked to positions 2 and 5 of the thiophene- acceptor (TCF and malononitrile) and the same electron donor
conjugated bridge, respectively, and this rod-like structure (N,N-diethylaniline group). The synthesis of FTC-H, MTD-H,
facilitated the ICT absorption but exhibited a large dipole FTC-Br, MTD-Br, FTC-DEA and MTD-DEA chromophores is
moment, as shown in Chart 1(a). The FTC molecules that depicted in Schemes 1 and 2. Chromophores FTC-H, MTD-H,
connect the donors and acceptors to positions 2 and 3 of FTC-Br, MTD-Br, FTC-DEA and MTD-DEA were prepared
thiophene, respectively, were rarely reported (Chart 1(b)). We according to different strategies. All reactions were performed
proposed that the dipole moment of this shape might be under nitrogen protection. The TCF acceptor was prepared
3
5
smaller than that of the rod-like FTC molecules, leading to according to the literature. FTC-H and MTD-H were obtained
relatively small dipole–dipole interactions and high EO activity in seven steps starting from the commercially available 3-methyl-
of materials. Therefore, we are eager to know if the shape thiophene. Aldehyde 11 was obtained in moderate yields by free
Chart 1 (a) Molecular structure and its dipole moment diagram of FTC. (b) Molecular structure and dipole moment diagram of non-traditional FTC
molecular FTC-H.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
Chart 2 Structure of the chromophores FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and MTD-DEA and the reference compound FTC.
Scheme 1 Chemical structure and synthetic scheme of chromophores FTC-DEA, MTD-DEA, FTC-Br and MTD-Br.
13
radical and oxidation reactions. Compound 12 was synthesized
C-NMR, and MS. These chromophores possessed good solubility
by the palladium-catalyzed Heck reaction. Finally, the two electron in common organic solvents, such as dichloromethane, chloro-
acceptors (TCF and malononitrile) were respectively attached to methane and N,N-dimethylformamide.
thiophene by the Knoevenagel condensation reaction to finally
2.2 Optical properties
form the chromophores. FTC-Br and MTD-Br and FTC-DEA and
MTD-DEA were obtained in seven steps starting from the It is generally believed that the NLO properties of organic
commercially available 3-methylthiophene. Aldehyde 4 was molecules are greatly influenced by the nature of intramolecular
obtained in moderate yields by free radical and oxidation reactions. charge-transfer (ICT) absorption properties and electron density
Compounds 6 and 7 were synthesized by the palladium-catalyzed distributions of the chromophore. In order to reveal the effect of
Heck reaction after controlling the receptor content. Finally, two boomerang-like chromophores and the introduction of auxiliary
electron acceptors (TCF and malononitrile) were attached to 6 and donor receptors on the ICT absorption properties, UV-vis absorp-
ꢀ
5
ꢀ1
7
by the Knoevenagel condensation reaction, respectively, to finally tion spectra of the six chromophores (c = 1 ꢁ 10 mol L ) were
form chromophores FTC-Br, MTD-Br, FTC-DEA and MTD-DEA. measured in series solvents with different dielectric constants.
All the chromophores were structurally characterized by H-NMR, Therefore, the solvatochromic behavior of these chromophores
1
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
malononitrile. Comparing the maximum absorption of FTC-H
and MTD-H to that of FTC-Br, MTD-Br, FTC-DEA and MTD-DEA,
the introduction of the bromine atom or DEA at position 5 of
thiophene could shift the ICT absorption band of the chromo-
phore to a lower energy level, which indicated that the introduction
of auxiliary donors and acceptors into the chromophores of
the TCF receptor and the malononitrile acceptor have the same
effect. Especially, the maximum absorption wavelength of FTC-DEA
(lmax = 714 nm) showed a bathochromic shift than FTC (lmax
=
676 nm), which illustrated that the introduction of the electron-rich
group DEA as an auxiliary donor could effectively shift the ICT
absorption band of the chromophore to a lower energy level.
Besides, the solvatochromic behavior is also an important
aspect for studying the polarity of chromophores. In solvents of
different polarities including dioxane (DO), toluene, dichloro-
methane (DCM), chloroform (CF) and N,N-dimethylformamide
Scheme 2 Chemical structure and synthetic scheme of chromophores
FTC-H and MTD-H.
(DMF), the series of chromophores FTC-H, MTD-H, FTC-Br,
MTD-Br, FTC-DEA and MTD-DEA showed very obvious optical
features in terms of solvatochromism, as shown in Fig. 2. All
the chromophores displayed a broad ICT absorption band. All
the spectra displayed a red-shift of lmax from a nonpolar solvent to
a polar solvent (DO to CF) and a blue-shift from a polar solvent to
a strong polar solvent (CF to DMF). The solvatochromic behavior
suggested that the dipolar chromophores FTC-H, MTD-H, FTC-Br,
MTD-Br, FTC-DEA and MTD-DEA could enter the zwitterionic
region beyond the limit of the cyanine. In Table 1, dipolar
chromophores FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and
MTD-DEA showed bathochromic shifts of 52 nm, 27 nm, 53 nm,
32 nm, 97 nm and 39 nm from dioxane to chloroform, respec-
tively. This confirmed that chromophores FTC-H, FTC-Br
and FTC-DEA were more easily polarizable than chromophores
MTD-H, MTD-Br and MTD-DEA. Particularly, FTC-DEA (97 nm)
has the largest solvatochromic behavior and is much larger than
FTC (55 nm), which indicated that the introduction of additional
electron donors assisted the enhancement of ICT absorption. It
Fig. 1 UV-vis absorption spectra of chromophores FTC-H, MTD-H, FTC-
ꢀ5
ꢀ1
Br, MTD-Br, FTC-DEA and MTD-DEA in chloroform (c = 1 ꢁ 10 mol L ). also illustrated that FTC-DEA was the most easily polarized, which
generally indicates the large molecular first-order hyperpolariz-
ability. The Dl values of FTC-H and FTC-Br are basically the same.
could be studied to explore the polarity of chromophores in This indicated that the introduction of a bromine atom has little
various dielectric environments as shown in Fig. 1 and 2. The effect on the ICT absorption properties.
spectral data are summarized in Table 1. As shown in Fig. 1, the
2.3 Thermal stability
chromophores FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and
MTD-DEA showed two prominent absorption bands in the Thermal stability is very important for the chromophore
visible range. The bands at B400 nm could be attributed to application during the NLO device preparation. Most of the
the p–p* transition of aromatic groups. The absorption bands processes need high temperatures: the poling temperature
located in the range of 500–800 nm could be attributed to should be higher than the glass transition temperature of
intramolecular charge transfer (ICT) transitions in D–p–A dipole the polymer during the poling process. Therefore, the NLO
molecules. All the chromophores displayed similar p–p* chromophore must be thermally stable enough to withstand
ICT absorption bands in the visible region and FTC-H, MTD-H, the encountered high temperatures (usually need to withstand
FTC-Br, MTD-Br, FTC-DEA and MTD-DEA showed the maximum temperatures above 200 1C). The thermal stability of all the
absorption (lmax) of 659 nm, 531 nm, 676 nm, 545 nm, 741 nm chromophores was investigated by thermal gravimetric analysis
and 582 nm in CHCl , respectively. Compared to the absorption (TGA) and differential scanning calorimetry (DSC) at the same
3
ꢀ1
bands of chromophores MTD-H, MTD-Br and MTD-DEA, the heating rate of 10 1C min under nitrogen, as shown in Fig. 3
absorption bands of chromophores FTC-H, FTC-Br and FTC-DEA and 4, respectively. The TGA curves of all the chromophores
showed a bathochromic shift, which could be attributed to the showed the 5 wt% weight loss temperature (T
d
), which are
TCF acceptor having a higher electron-accepting ability than listed in Table 1. It was found that all the chromophores
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
Fig. 2 UV-vis absorption spectra of chromophores FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and MTD-DEA in different solvents, including dioxane
DO), toluene, chloroform (CF), dichloromethane (DCM) and N,N-dimethylformamide (DMF).
(
exhibited excellent stability at decomposition temperatures (Td)
Table 1 Summary of optical and thermal properties of the chromophores
FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-DEA and MTD-DEA and the refer- higher than 200 1C, which was high enough for the application
3
6
ence compound FTC
in EO device preparation. It is well known that decomposition
usually leads to an exothermic or endothermic event, which
would be reflected on the DSC curve. Considering that the TGA
only reflects the weight loss, the combination of TGA and DSC
analysis under the same heating condition may be a more
reasonable way to characterize the thermal stability. As shown
in Fig. 4, no significant changes were found in the DSC and
TGA curves of chromophores FTC-H, MTD-H, FTC-Br, MTD-Br,
FTC-DEA and MTD-DEA below 175 1C. Therefore, it could be
deducted that chromophores FTC-H, MTD-H, FTC-Br, MTD-Br,
FTC-DEA and MTD-DEA were thermally stable below 175 1C.
a
b
c
d
l
max (nm)
l
max (nm)
Dl (nm)
d
T (1C)
FTC
FTC-H
MTD-H
FTC-DEA
MTD-DEA
FTC-Br
MTD-Br
676
659
531
741
582
676
545
621
607
504
644
543
623
513
55
52
27
97
39
53
32
242
258
273
268
258
205
251
a
b
c
l
max was measured in chloroform.
lmax was measured in dioxane. Dl =
d
l
max (in chloroform) ꢀ lmax (in dioxane). 5% weight loss temperature
ꢀ
1
determined from TGA in nitrogen at a heating rate of 10 1C min
.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
and first hyperpolarizability (b) values of the chromophores have
2
5,37
been calculated. The DFT
hybrid B3LYP
calculations were carried out at the
38,39
level by employing the split valence 6-31G(d,p)
basis set. All data obtained from DFT calculations are summarized
in Table 2.
The frontier molecular orbitals are often used to characterize
the chemical reactivity and kinetic stability of a molecule and to
obtain qualitative information about its optical and electrical
4
0,41
properties.
Besides, the HOMO–LUMO energy gap is also
used to understand the transfer interaction occurring in a
chromophore molecule. Fig. 5 shows the frontier molecular
orbitals of chromophores FTC-H, MTD-H, FTC-Br, MTD-Br,
FTC-DEA and MTD-DEA. It is obvious that the electron density
distribution of the HOMO level is delocalized over thiophene
and the donor, whereas the LUMO level is mainly distributed in
the TCF acceptor part.
ꢀ1
Fig. 3 TGA curves of chromophores at a heating rate of 10 1C min in a
nitrogen atmosphere.
The HOMO–LUMO energy gaps (DE) were also calculated by
DFT calculations, as shown in Table 2. The DE of chromo-
phores FTC-H, FTC-Br and FTC-DEA were 2.057 eV, 2.034 eV,
4
2
and 1.836 eV, respectively. According to reports, the energy
gap is lower and the degree of intramolecular charge transfer
(ICT) absorption is greater, which improves the nonlinearity
of the chromophore. According to the data, chromophore
FTC-DEA showed a lower DE than chromophores FTC-H and
FTC-Br. The lower DE of FTC-DEA indicated better ICT absorp-
tion and NLO properties than chromophores FTC-H and FTC-Br.
It also demonstrated that the introduction of the electron-rich
group DEA as an auxiliary donor narrowed the energy gap, which
is in accordance with the results obtained for optical properties.
The theoretical microscopic zero-frequency molecular first-
order hyperpolarizability (b) was calculated using Gaussian 09
1
9
at the B3LYP/6-31G(d,p) level. The scalar of b can be calcu-
lated from the x, y and z components according to the following
equation:
ꢀ1
Fig. 4 DSC curves of chromophores at a heating rate of 10 1C min in a
nitrogen atmosphere.
2
2
2 1/2
b = (bx + by + b )
z
where
1
All the chromophores FTC-H, MTD-H, FTC-Br, MTD-Br, FTC-
DEA and MTD-DEA exhibited satisfactory thermal stabilities.
Therefore, the integrated DSC and TGA curves show that the
chromophores have high thermal stability and can be widely
used in the preparation of electro-optic devices.
1 X ꢀ
ꢁ
bi ¼ biii
þ
bijj þ biji þ bjji
;
i; j 2 ðx; y; zÞ
3
iaj
The calculated b values of chromophores FTC-H, FTC-Br and
FTC-DEA are summarized in Table 2. The variations in the b
value of chromophores with different push–pull structures can
be clearly observed. The b values of FTC-H, FTC-Br and FTC-
2.4 Theoretical calculations
After optimizing the ground state of their molecular geo- DEA are 323 ꢁ 10ꢀ30 esu, 320 ꢁ 10ꢀ30 esu and 379 ꢁ 10ꢀ30 esu,
metries, the HOMO–LUMO energy gaps (DE), dipole moment (m), respectively. It can be obviously seen that FTC-DEA has the
Table 2 Data from DFT calculations and EO coefficients
a
b
ꢀ30
c
ꢀ1
Chromophores
EHOMO (eV)
ELUMO (eV)
DE (eV)
b (1 ꢁ 10
esu)
m (D)
r33 (pm V
)
FTC-H
FTC-DEA
FTC-Br
FTC
ꢀ5.304
ꢀ4.929
ꢀ5.375
ꢀ5.488
ꢀ3.247
ꢀ3.090
ꢀ3.341
ꢀ3.483
2.057
1.836
2.034
2.005
323
379
320
711
4.70
6.63
4.28
9.11
11
25
10
17
a
b
c
DE = ELUMO ꢀ EHOMO
.
b values were calculated using Gaussian 09 at the B3LYP/6-31G(d,p) level. m is the total dipole moment.
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
ꢀ1
The EO coefficients (r33) of the films were 11, 10, 25 and 17 pm V
for FTC-H/APC, FTC-Br/APC, FTC-DEA/APC and FTC/APC, respec-
tively. As shown in Table 2, r33 of the guest–host materials appeared
to have the same tendency as the calculated b values for FTC-H/
APC, FTC-Br/APC and FTC-DEA/APC. It is obvious that FTC-DEA
with an electron-rich group had the highest EO activity, while the
EO coefficient of FTC-H and FTC-Br were very similar, which
indicated that the introduction of the electron-rich group DEA
could significantly increase the macroscopic EO activity. For the
FTC-based structure containing strong electron-acceptors, the
introduction of bromo atoms may have little effect on r . Most
33
importantly, the b value of FTC-DEA is less than that of FTC, but
the value of r33 is 47% greater than that of FTC. It is probably due to
the smaller dipole moment of FTC-DEA, leading to the smaller
intermolecular dipole–dipole interaction and higher efficiency for
translating microscopic NLO activities into the macroscopic EO
coefficient. Thus, the boomerang-like chromophores lead to the
small dipole moment, which could effectively prevent the dipole–
dipole interaction between chromophores, and the introduction of
an electron-rich group at position 5 of thiophene could enhance the
electro-optic activity. This was favorable for obtaining the organic
electro-optic film materials of high electro-optic coefficient.
Fig. 5 The frontier molecular orbitals of chromophores FTC-H, FTC-Br
and FTC-DEA.
largest b values, and FTC-H and FTC-Br have very similar
values, which indicated that the introduction of the auxiliary
donor can effectively increase the nonlinear properties of the
molecules and the introduction of an auxiliary acceptor has
little effect on the nonlinear properties of the molecules. As
stated above, DFT calculations and the UV-vis spectral analysis
suggested that the incorporation of the electron-rich group
might lead to a higher EO activity. This result corresponds with
the UV-vis characterization results.
3
. Experimental
The DFT calculation results demonstrated that the total
dipole moment (m) of chromophores FTC-H, FTC-Br and FTC-
3.1 Materials and instruments
DEA are smaller than that of chromophore FTC, which indicated All reagents were purchased from Aladdin Co., Ltd, Shanghai,
that the introduction of electron donors and electron acceptors China and used without further purification. The compound
at positions 2 and 3 of thiophene, respectively, was beneficial to 2-(3-cyano-4,5,5-trimethylfuran-2(5H)-ylidene)malononitrile
3
5
1
reduce the dipole moment of the chromophores. The smaller m (TCF) was prepared according to the literature. The H NMR
1
3
may reduce the aggregation of the chromophore FTC-DEA in and C NMR spectra were recorded using a Bruker III HD 400
4
3,44
organic materials.
spectrometer. The MS spectra were recorded using a Thermo
Fisher ITQ1100 mass spectrometer. The UV-vis spectra were
recorded using an Agilent Cary 5000 spectrophotometer. The
decomposition temperature of chromophores was analyzed using
a Mettler Toledo TGA 2 thermogravimetric analyzer at a heating
2
.5 EO activity characterization
For application, the microscopic hyperpolarizability (b) has to
be translated into a large macroscopic EO coefficient (r ). For
studying the macroscopic nonlinearity of the chromophores
FTC-H, FTC-Br and FTC-DEA and the reference chromophore
FTC in poled polymers, a series of electro-optic films with the
same doping content (0.38 mmol g ) were fabricated by for-
mulating the chromophores into amorphous poly(bisphenol
A carbonate) (APC) using dibromomethane as the solvent. The
3
3
ꢀ
1
rate of 10 1C min under the protection of nitrogen. The glass
transition temperatures were obtained using NETZSCH 4 under N
2
ꢀ
1
at a heating rate of 10 1C min
.
ꢀ
1
3.2 Syntheses
3.2.1. Synthesis of 2,5-dibromo-3-methylthiophene (com-
electro-optical coefficient (r ) is proportional to the content of pound 1). 3-Methylthiophene (5 g, 50.9 mmol) was dissolved
3
3
chromophore doping. In order to compare the effect of the in chloroform and acetic acid (50 mL : 50 mL) in a 250 mL
chromophore structure on the electro-optic properties of the one-necked flask equipped for stirring. N-Bromosuccinimide
material, we have selected the same number density to prepare (19.94 g, 112.1 mmol) was added to the solution in one portion,
the doped film. The FTC content is approximately 18 wt%, and and the solution was stirred for 6 h. The reaction mixture was
the content of FTC-H is 18 wt%, FTC-Br is 20 wt% and FTC-DEA then poured into water and extracted with chloroform; the
is 25 wt%.
organic extracts were washed with water three times and with
Contact poling was applied to obtain the noncentrosymmetric NaHCO (aq.) twice. The organic layer was dried over anhydrous
3
alignment. In order to prevent damage to the films, a moderate MgSO , and then the solvent was removed to obtain a crude
4
ꢀ
1
field of about 100 V mm was applied and no breakdown current product, which was then purified by silica gel column chromato-
was observed. graphy (petroleum) to give compound 1 as a transparent liquid
The r33 values of the poled films were measured by the Teng– (11.88 g, 91.11% yield). H NMR (CDCl
1
3
, 400 MHz, ppm): d 6.77
3
9
Man simple reflection technique at a wavelength of 1310 nm.
3
(1H, s, thiophene-H), 2.15 (3H, s, –CH ).
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C

View Article Online
Paper
.2.2. Synthesis of 2,5-dibromo-3-(bromomethyl)thiophene and Bu
Journal of Materials Chemistry C
3
4
NBr (0.24 g, 0.745 mmol) in dry DMF (5 mL), triethyl-
2
(compound 2). Compound 1 (2 g, 7.81 mmol) and N-bromo- amine (0.077 g, 0.757 mmol), Pd(OAc) (0.017 g, 0.076 mmol)
succinimide (1.7 g, 9.55 mmol) were dissolved in carbon tetra- and triphenylphosphine (0.0193 g, 0.074 mmol) were added.
chloride (CCl , 20 mL). A small amount of benzoyl peroxide was The mixture was stirred under a N atmosphere at 100 1C for
4
2
(
10 mg, 0.04 mmol) added as an initiator. The mixture was 24 h and was monitored by TLC. After cooling to room
refluxed for 4 h. The reaction mixture was then poured into water temperature, the mixture was poured into water (20 mL) and
and extracted with dichloromethane. The organic extracts were extracted with CH Cl . The combined organic phases were
washed with water two times. The organic layer was dried over washed with brine, and dried with anhydrous Na SO . Then
anhydrous MgSO , and the solvent was removed to obtain a the solvent was removed to give a crude product, which was
2
2
2
4
4
crude product, which was then purified by silica gel column then purified by silica gel column chromatography (ethyl
chromatography (petroleum) to give compound 2 as a transparent acetate : petroleum = 1 : 6) to give compound 6 as a dark red
1
1
liquid (2.13 g, 81.3% yield). H NMR (CDCl , 400 MHz, ppm): solid (0.07 g, 26.5% yield). H NMR (400 MHz, CDCl ) d 10.11
3
3
d 7.00 (1H, s, thiophene-H), 4.36 (2H, s, –CH –).
(1H, s, –CHO), 7.69 (1H, d, J = 15.8 Hz, –CHQCH–), 7.41 (2H, d,
2
3
.2.3. Synthesis of (2,5-dibromothiophen-3-yl)methanol J = 8.8 Hz, –CHQCH–), 7.34 (2H, d, J = 8.7 Hz, Ar-H), 7.17 (1H, s,
compound 3). Compound 2 (1 g, 2.99 mmol) was dissolved thiophene-H), 7.07 (1H, d, J = 15.8 Hz, –CHQCH–), 6.91 (1H, d,
in 1,4-dioxane (20 mL), and calcium carbonate (0.36 g, J = 15.9 Hz, –CHQCH–), 6.81 (1H, d, J = 16.0 Hz, –CHQCH–),
.6 mmol) and water (10 mL) were added to this. Then the 6.66 (4H, dd, J = 8.8, 3.1 Hz, Ar-H), 3.44–3.35 (8H, m, –CH –),
mixture was stirred and refluxed for 12 h and was monitored by 1.20 (12H, dd, J = 11.9, 6.9 Hz, –CH ).
TLC. When the reaction was completed, 1 M HCl was added to 3.2.7. Synthesis of (E)-5-bromo-2-(4-(diethylamino)styryl)-
(
3
2
3
the mixture to remove excess calcium carbonate and extracted thiophene-3-carbaldehyde (compound 7). To a solution of com-
with dichloromethane. The organic layer was dried over anhydrous pound 4 (0.69 g, 2.56 mmol), compound 5 (0.3 g, 1.71 mmol) and
MgSO
which was then purified by silica gel column chromatography potassium carbonate (0.3 g, 2.17 mmol), Pd(OAc)
petroleum : ethyl acetate = 10:1) to give compound 3 as a white 0.267 mmol) and triphenylphosphine (0.132 g, 0.503 mmol) were
4
, and the solvent was removed to obtain a crude product, Bu
4
NBr (0.66 g, 2.05 mmol) in dry DMF (5 mL), anhydrous
2
(0.06 g,
(
1
solid (0.67 g, 82.7% yield). H NMR (CDCl
1H, s, thiophene-H), 4.54 (2H, d, E = 3.3 Hz, –CH
OH).
.2.4. Synthesis of 2,5-dibromothiophene-3-carbaldehyde extracted with CH Cl . The combined organic phases were
3
, 400 MHz, ppm): d 7.00 added. The mixture was stirred under a N
–), 2.03 (1H, s, for 24 h and was monitored by TLC. After cooling to room
temperature, the mixture was poured into water (20 mL) and
2
atmosphere at 100 1C
(
–
2
3
2
2
(
compound 4). Compound 3 (0.4 g, 1.47 mmol) was dissolved washed with brine, and dried with anhydrous Na SO . The
2 4
in dichloromethane (15 mL) and pyridinium chlorochromate solvent was removed to give a crude product, which was then
0.38 g, 1.76 mmol) was added. Then the mixture was stirred at purified by silica gel column chromatography (ethyl acetate:
room temperature for 12 h and was monitored by TLC. The petroleum = 1 : 6) to give compound 7 as a dark red solid (0.2 g,
(
1
3
solvent was removed, and the residue was purified by column 32% yield). H NMR (400 MHz, CDCl ) d 9.99 (1H, s, –CHO), 7.58
chromatography (petroleum : dichloromethane = 5 : 1) to give (2H d, J = 15.6 Hz, –CHQCH–), 7.37 (2H, d, J = 8.8 Hz, Ar-H), 7.28
compound 4 as a light yellow solid (0.359 g, 91.2% yield). (1H, s, thiophene-H), 6.96 (1H, d, J = 15.9 Hz, –CHQCH–), 6.64
1
H NMR (CDCl , 400 MHz, ppm): d 9.79 (1H, s, –CHO), 7.34 (2H, d, J = 8.4 Hz, Ar-H), 3.39 (4H, q, J = 7.1 Hz, –CH –), 1.19 (6H,
3
2
(
1H, s, thiophene-H).
t, J = 7.1 Hz, –CH3).
3.2.8. Synthesis of 2-bromo-3-methylthiophene (compound 8).
In an ice bath, methyltriphenylphosphonium bromide (3.02 g, 3-Methylthiophene (5 g, 50.9 mmol) was dissolved in chloroform
.47 mmol) and potassium tert-butoxide (1 g, 8.83 mmol) were and acetic acid (50 mL : 50 mL) in a 250 mL one-necked flask
3.2.5. Synthesis of N,N-diethyl-4-vinylaniline (compound 5).
8
mixed in freshly dried tetrahydrofuran (THF, 40 mL). The yellow equipped with stirring. N-Bromosuccinimide (9.07 g, 51 mmol) was
suspension was stirred for 30 min. Then 4-(diethylamino)benz- added to the solution in one portion, and the solution was stirred
aldehyde (1 g, 5.65 mmol) in THF (10 mL) was slowly added for 5 min. The reaction mixture was then poured into water and
dropwise into the mixture and stirred at room temperature for extracted with chloroform; the organic extracts were washed with
5
h and was monitored by TLC. Methanol was added and the water three times and with NaHCO (aq.) twice. The organic layer
3
4
solution was filtered. After the removal of the solvent, the residue was dried over anhydrous MgSO , and the solvent was removed to
was purified by column chromatography (ethyl acetate: petro- obtain a crude product, which was then purified by silica gel
leum = 1 : 30) to give compound 5 as a yellow oil (1.62 g, 61% column chromatography (petroleum) to give compound 8 as a
1
1
yield). H NMR (400 MHz, CDCl
.69 (1H, dd, J = 17.5 Hz, J = 11.2 Hz, –CHQCH
J = 8.4 Hz), 5.49 (1H, d, J = 17.4 Hz, –CHQCH ), 4.96 (1H, d, J = thiophene-H), 2.20 (3H, s, –CH3).
3
): d 7.28 (2H, d, J = 8.5 Hz, Ar-H), transparent liquid (7.41 g, 82% yield). H NMR (400 MHz, CDCl
3
) d
6
1
2
2
), 6.60 (2H, d, 7.17 (1H, d, J = 5.6 Hz, thiophene-H), 6.78 (1H, d, J = 5.6 Hz,
2
1
0.8 Hz, –CHQCH ), 3.37 (4H, q, J = 7.1 Hz, –CH –N–CH –), 1.17
3.2.9. Synthesis of 2-bromo-3-(bromomethyl)thiophene
2
2
2
(6H, t, J = 7.1 Hz, –CH3).
(compound 9). Compound 8 (2 g, 11.3 mmol) and N-bromo-
3
.2.6. Synthesis of 2,5-bis((E)-4-(diethylamino)styryl)thio- succinimide (2.01 g, 11.3 mmol) were dissolved in carbon
phene-3-carbaldehyde (compound 6). To a solution of com- tetrachloride (CCl , 20 mL). A small amount of benzoyl peroxide
pound 4 (0.1 g, 0.37 mmol), compound 5 (0.16 g, 0.913 mmol) (10 mg, 0.04 mmol) was added as an initiator. The mixture was
4
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
4
refluxed for 4 h. The reaction mixture was then poured into water 3.27 mmol), acceptor TCF (0.078 g, 3.92 mmol) and NH OAc
and extracted with dichloromethane; the organic extracts were (0.0025 g, 0.0327 mmol) were dissolved in anhydrous ethanol
washed with water two times. The organic layer was dried over (3 mL). The reaction mixture was stirred and refluxed for 6 h
anhydrous MgSO , and the solvent was removed to obtain a and was monitored by TLC. The solvent was removed, and the
4
crude product, which was then purified by silica gel column residue was purified by column chromatography (petroleum :
chromatography (petroleum) to give compound 9 as a transparent tetrahydrofuran = 2.5 : 1) to give compound FTC-DEA as a blue
1
1
liquid (2.26 g, 78.3% yield). H NMR (400 MHz, CDCl
1H, m, thiophene-H), 7.00 (1H, d, J = 5.7 Hz, thiophene-H), 4.45 d 8.23 (1H, d, J = 15.7 Hz, –CH
2H, s, –CH –). Ar-H), 7.30 (2H, d, J = 8.8 Hz, Ar-H), 7.16 (1H, d, J = 15.5 Hz,
.2.10. Synthesis of (2-bromothiophen-3-yl)methanol (com- –CH QCH –), 7.00–6.90 (2H, m, –CH QCH – and thiophene-
pound 10). Compound 9 (1 g, 3.91 mmol) was dissolved in 1,4- H), 6.83 (1H, d, J = 15.9 Hz, –CH QCH –), 6.76 (1H, d, J =
3
) d 7.27–7.24 black solid (0.087 g, 41.6% yield). H NMR (400 MHz, CDCl
3
)
(
(
2
QCH –), 7.36 (2H, d, J = 8.8 Hz,
2
2
3
2
2
2
2
2
2
dioxane (20 mL), and calcium carbonate (0.47 g, 4.69 mmol) and 15.9 Hz, –CH QCH –), 6.65 (4H, dd, J = 8.6, 6.1 Hz, Ar-H), 6.35
2
2
water (10 mL) were added to this. Then the mixture was stirred (1H, d, J = 15.6 Hz, –CH QCH –), 3.41 (8H, p, J = 7.0 Hz, –CH –),
2
2
2
1
3
and refluxed for 12 h and was monitored by TLC. When the 1.68 (6H, s, –CH
reaction was completed, 1 M HCl was added to the mixture to (101 MHz, CDCl
remove excess calcium carbonate and extracted with dichloro- 137.88, 134.86, 133.78, 131.55, 129.31, 128.20, 125.49, 123.04,
methane. The organic layer was dried over anhydrous MgSO 120.47, 115.50, 112.37, 112.29, 111.83, 111.71, 97.09, 94.39,
3
), 1.21 (12H, q, J = 6.9 Hz, –CH
3
). C NMR
3
) d 176.03, 173.72, 152.08, 148.85, 142.17,
4
,
+
and the solvent was removed to obtain a crude product, which 55.34, 44.57, 34.23, 30.35, 26.21, 12.74, 12.68. MS (EI ) m/z
was then purified by silica gel column chromatography (petro- calcd for C H N OS(M ): 639.30, found: 639.3.
+
4
0
41 5
leum : ethyl acetate = 10: 1) to give compound 10 as a white solid
3.2.14. Synthesis of 2-((2,5-bis((E)-4-(diethylamino)styryl)-
1
(
0.64 g, 84.8% yield). H NMR (400 MHz, CDCl ) d 7.25 (1H, J = thiophen-3-yl)methylene)malononitrile (compound MTD-DEA).
3
5.8 Hz, thiophene-H), 7.01 (1H, d, J = 5.6, 1.3 Hz, thiophene-H), Compound 6 (0.15 g, 3.27 mmol), acceptor malononitrile
2
4.61 (2H, J = 2.2 Hz, –CH –), 2.01–1.65 (1H, m, –OH).
4
(0.03 g, 4.9 mmol) and NH OAc (0.0025 g, 0.0327 mmol) were
3.2.11. Synthesis of 2-bromothiophene-3-carbaldehyde (com- dissolved in anhydrous ethanol (3 mL). The reaction mixture was
pound 11). Compound 3 (0.9 g, 4.66 mmol) was dissolved in stirred and refluxed for 6 h and was monitored by TLC. After the
dichloromethane (20 mL), and pyridinium chlorochromate (1.21 g, removal of the solvent, the residue was purified by column
5
.59 mmol) was added. Then the mixture was stirred at room chromatography (petroleum: ethyl acetate = 10: 1) to give com-
1
temperature for 12 h and was monitored by TLC. Following the pound MTD-DEA as a black solid (0.074 g, 44.5% yield). H NMR
removal of the solvent, the residue was purified by column (400 MHz, CDCl ) d 7.80 (1H, s, –CH QCH–), 7.66 (1H, s,
3
2
chromatography (petroleum : dichloromethane = 5 : 1) to give com- thiophene-H), 7.41 (2H, d, J = 8.5 Hz, Ar-H), 7.34 (2H, d, J =
1
pound 11 as a light yellow solid (0.58 g, 92.14% yield). H NMR 8.1 Hz, Ar-H), 7.08 (2H, s, –CH
400 MHz, CDCl ) d 9.94 (1H, s, –CHO), 7.36 (1H, d, J = 5.8 Hz, –CH QCH –), 6.81 (1H, d, J = 15.9 Hz, –CH
thiophene-H), 7.29 (1H, d, J = 5.8 Hz, thiophene-H). J = 8.1 Hz, Ar-H), 3.56–3.23 (8H, m, –CH
.2.12. Synthesis of (E)-2-(4-(diethylamino)styryl)thiophene- 7.1 Hz, –CH
-carbaldehyde (compound 12). To a solution of compound 11 161.69, 154.18, 148.41, 147.36, 135.51, 131.31, 129.14, 128.77,
0.1 g, 0.523 mmol), compound 5 (0.14 g, 0.785 mmol) and 127.69, 122.26, 120.89, 114.90, 113.66, 111.14, 44.03, 29.17,
2
QCH
2
–), 6.93 (1H, d, J = 15.9 Hz,
QCH –), 6.67 (4H, d,
–), 1.21 (12H, q, J =
NMR (101 MHz, CDCl 166.56,
(
3
2
2
2
2
2
1
3
3
3
).
C
3
) d
3
(
+
+
Bu NBr (0.2 g, 0.628 mmol) in dry DMF (5 mL), anhydrous 19.15, 12.12. MS (EI ) m/z calcd for C H N S(M ): 506.25,
4
32 34 4
potassium carbonate (0.09 g, 0.628 mmol), Pd(OAc)
2
(0.012 g, found: 506.3.
3.2.15. Synthesis of 2-(3-cyano-4-((E)-2-(2-((E)-4-(diethyl-
2
atmosphere at amino)styryl)thiophen-3-yl)vinyl)-5,5-dimethylfuran-2(5H)-ylidene)-
0
.0535 mmol) and triphenylphosphine (0.014 g, 0.0534 mmol)
were added. The mixture was stirred under a N
100 1C for 24 h and was monitored by TLC. After cooling to room malononitrile (compound FTC-H). Compound 12 (0.1 g,
temperature, the mixture was poured into water (20 mL) and 0.35 mmol) and accepter TCF (0.092 g, 0.463 mmol) were
extracted with CH Cl . The combined organic phases were dissolved in anhydrous ethanol (2.5 mL). The reaction mixture
2
2
washed with brine and dried with anhydrous Na SO . After was stirred and refluxed for 6 h and was monitored by TLC.
2
4
removing the solvent, a crude product was obtained, which Following the removal of the solvent, the residue was purified
was then purified by silica gel column chromatography (ethyl by column chromatography (petroleum : dichloromethane =
acetate: petroleum = 1 : 6) to give compound 12 as a dark red 1 : 4.5) to give compound FTC-H as a black green solid (0.07 g,
1
1
solid (0.06 g, 41.2% yield). H NMR (400 MHz, CDCl
3
) d 43.2% yield). H NMR (400 MHz, CDCl
3
) d 8.43 (1H, d, J =
1
0.17 (1H, s, –CHO), 7.43 (2H, d, J = 8.9 Hz, Ar-H), 7.37 (1H, d, 14.6 Hz, –CH
2
QCH –), 7.42 (2H, s, Ar-H), 7.27 (1H, s, thiophene-
2
J = 5.4 Hz, –CHQCH–), 7.13 (1H, s, thiophene-H), 7.09 (1H, s, H), 7.14 (1H, s, thiophene-H), 7.06 (1H, d, J = 15.8 Hz,
thiophene-H), 7.02 (1H, d, J = 5.4 Hz, –CHQCH–), 6.68 (2H, d, J = –CH QCH –), 6.67 (2H, s, Ar-H), 6.51 (1H, d, J = 15.9 Hz,
2
2
8
7
.8 Hz, Ar-H), 3.41 (4H, q, J = 7.1 Hz, –CH –), 1.21 (6H, t, J = –CH QCH –), 3.42 (4H, s, –CH –), 1.73 (6H, s, –CH ), 1.21 (6H,
2 2 2 2 3
1
3
.1 Hz, –CH3).
t, J = 6.7 Hz, –CH3). C NMR (101 MHz, CDCl ) d 175.48, 173.62,
3
3
.2.13. Synthesis of 2-(4-((E)-2-(2,5-bis((E)-4-(diethylamino)- 138.42, 128.57, 123.99, 111.41, 96.78, 44.04, 31.20, 29.16, 28.79,
+
styryl)thiophen-3yl)vinyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)- 26.69, 25.68, 25.02, 22.15, 13.56, 12.02. MS (EI ) m/z calcd for
malononitrile (compound FTC-DEA). Compound 6 (0.15 g,
+
28 26 4
C H N OS(M ): 466.18, found: 466.2.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C

View Article Online
Paper
Journal of Materials Chemistry C
3.2.16. Synthesis of (E)-2-((2-(4-(diethylamino)styryl)thio- (APC) using dibromomethane as the solvent. The solution had a
phen-3-yl)methylene)malononitrile (compound MTD-H). Com- solid content of 5.5 wt% and was filtered through a 0.2 mm PTFE
pound 12 (0.1 g, 0.35 mmol), accepter malononitrile (0.03 g, filter and spin-coated onto an indium tin oxide (ITO) glass sub-
0.42 mmol) and ammonium acetate (0.03 g, 0.035 mmol) were strate. All the films were baked in a vacuum oven at 80 1C overnight
dissolved in anhydrous ethanol (2.5 mL). The reaction mixture to ensure the removal of the residual solvent. A thin layer of aurum
was stirred and refluxed for 6 h and was monitored by TLC. was deposited on the films as the top electrode using the vacuum
Following the removal of the solvent, the residue was purified evaporation method. The thickness of the Au electrode was about
by column chromatography (petroleum : dichloromethane = B300 nm. The films were polarized by contact poling. The poling
1
: 1) to give compound MTD-H as a black green solid conditions are as follows: at a temperature of about 145 1C, a
1
ꢀ1
(0.054 g, 46.5% yield). H NMR (400 MHz, CDCl
3
) d 7.91 moderate field of about 100 V mm was applied. The r33 values
(
2H, d, J = 4.9 Hz, –CH QCH – and thiophene-H), 7.42 (2H, were measured using the Teng–Man simple reflection technique at
2
2
39
d, J = 8.2 Hz, Ar-H), 7.17 (1H, d, J = 5.4 Hz, –CH QCH –), a wavelength of 1310 nm.
2
2
7.10 (2H, s, thiophene-H), 6.67 (2H, d, J = 6.7 Hz, Ar-H), 3.42
(
4H, q, J = 6.9 Hz, –CH
C NMR (101 MHz, CDCl
23.54, 114.55, 113.31, 111.05, 110.95, 44.02, 12.07. MS (EI )
2
–), 1.21 (6H, t, J = 7.1 Hz, –CH
4. Conclusions
) d 148.37, 136.12, 128.69, 125.23,
3
).
1
3
3
+
1
A series of second-order nonlinear optical chromophores FTC-H,
MTD-H, FTC-Br, MTD-Br, FTC-DEA and MTD-DEA were developed
by tuning the shape to adjust the dipole moment of the chromo-
phores, in which the major conjugated push–pull structure was
located at positions 2 and 3 of the thiophene bridge and auxiliary
donors or acceptors were also introduced at position 5 of
the thiophene bridge, for improving the electro-optic activity of
materials. The chromophores can be successfully synthesized by
+
m/z calcd for C20
.2.17. Synthesis of 2-(4-((E)-2-(5-bromo-2-((E)-4-(diethylamino)-
styryl)thiophen-3-yl)vinyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)-
malononitrile (compound FTC-Br). Compound (0.157 g,
.431 mmol), accepter TCF (0.103 g, 0.518 mmol) and NH OAc
19 3
H N S(M ): 333.13, found: 333.1.
3
7
0
4
(0.002 g, 0.0431 mmol) were dissolved in anhydrous ethanol (3 mL).
The reaction mixture was stirred and refluxed for 5 h and was
monitored by TLC. Following the removal of the solvent, the residue
was purified by column chromatography (petroleum : ethyl acetate =
1
13
H NMR, C NMR and MS characterization. From the results of
UV-vis analysis, it could be found that the chromophores with
auxiliary groups (Br and DEA group) at position 5 of thiophene
showed different degrees of bathochromic shift. FTC-DEA showed
greater ICT absorption properties than the reference chromophore
FTC, illustrating that it was a suitable auxiliary donor. Thermal
analysis also indicated that all the chromophores showed excellent
thermal stabilities. As we proposed, the DFT calculation analysis
indicated that the new structure had a smaller dipole moment than
3
.5 : 1) to give compound FTC-Br as a black solid (0.103 g, 46%
1
yield). H NMR (400 MHz, CDCl ) d 8.38 (1H, d, J = 16.0,
–
H), 7.18 (1H, d, J = 15.7, –CH QCH –), 6.95 (1H, d, J = 15.7,
3
CH QCH –), 7.39 (2H, d, J = 8.9, Ar-H), 7.24 (1H, s, thiophene-
2 2
2
2
–
–
(
1
CH
CH
6H, t, J = 7.1, –CH
73.17, 138.46, 136.91, 128.83, 128.60, 126.36, 111.47, 110.84, 96.83,
2
QCH
QCH
2
–), 6.66 (2H, d, J = 8.9, Ar-H), 6.37 (1H, d, J = 15.7,
–), 3.41 (4H, q, J = 7.1, –CH –), 1.70 (6H, s, –CH ), 1.20
). C NMR (101 MHz, CDCl ) d 175.44, 173.63,
2
2
2
3
13
3
3
the rod-like chromophore FTC (mFTC = 9.11, mFTC-H = 4.70, mFTC-Br
=
+
4
4.01, 29.81, 27.62, 25.70, 25.55, 23.87, 12.10. MS (EI ) m/z calcd for
4
.28 and mFTC-DEA = 6.63). The EO coefficients (r33) of the films were
+
C H BrN OS(M ): 544.09, found: 544.1.
ꢀ1
ꢀ1
ꢀ1
ꢀ1
28
25
4
11 pm V , 10 pm V , 25 pm V and 17 pm V for FTC-H/APC,
3.2.18. Synthesis of (E)-2-((5-bromo-2-(4-(diethylamino)-
FTC-Br/APC, FTC-DEA/APC and FTC/APC, respectively. Interest-
ingly, although the b of FTC-DEA was smaller than that of FTC,
styryl)thiophen-3-yl)methylene)malononitrile (compound MTD-Br).
Compound 7 (0.157 g, 0.431 mmol), accepter malononitrile
(
ꢀ1
the r33 of FTC-DEA (25 pm V ) was 47% greater than that of FTC
0.103 g, 0.518 mmol) and NH
4
OAc (0.002 g, 0.0431 mmol) were
ꢀ1
(17 pm V ) at the same number density. This indicated that the
dissolved in anhydrous ethanol (3 mL). The reaction mixture was
stirred and refluxed for 5 h and was monitored by TLC. After the
removal of the solvent, the residue was purified by column
chromatography (petroleum : tetrahydrofuran = 8 : 1) to give
compound MTD-Br as a black solid (0.087 g, 51% yield).
boomerang-like molecular structure containing the auxiliary donor
was beneficial for efficiently transforming the first-order hyper-
polarizability of the chromophore into the electro-optic coefficient
of the material, thereby increasing the macroscopic EO activity. In a
word, adjusting the conjugated structure of chromophores was an
efficient way for tuning the dipole moment and optimizing the EO
activity.
1
H NMR (400 MHz, CDCl ) d 7.85 (2H, t, J = 23.5, –CH QCH –),
3
2
2
7
.40 (2H, s, Ar-H), 7.10 (1 H, s, thiophene-H), 7.01 (1H, s,
CH QCH –), 6.67 (2H, s, Ar-H), 3.42 (4H, d, J = 6.8, –CH –),
.22 (6H, t, J = 7.0, –CH ) d 157.35,
–
1
1
1
2
2
2
13
3
). C NMR (101 MHz, CDCl
3
48.38, 146.76, 136.65, 128.93, 128.68, 127.24, 125.26, 121.24, Conflicts of interest
+
20.60, 114.25, 112.98, 111.15, 44.11, 12.01. MS (EI ) m/z calcd
+
There are no conflicts to declare.
for C H BrN S(M ): 411.04, found: 411.0.
20
18
3
3
.3 Preparation of guest–host films and their poling process
Acknowledgements
In order to study the EO properties, guest–host films were
formulated by doping chromophores FTC-H, FTC-Br, FTC- The authors acknowledge financial support from The National
ꢀ
1
DEA and FTC (0.38 mmol g ) into poly(bisphenol A carbonate) Natural Science Foundation of China [grant numbers 21644003,
J. Mater. Chem. C
This journal is ©The Royal Society of Chemistry 2019

View Article Online
Journal of Materials Chemistry C
Paper
2
1704006], and the Natural Science Foundation of Jilin Province 21 S. R. Hammond, O. Clot, K. A. Firestone, D. H. Bale, D. Lao,
[
2
grant numbers 2018052008JH, 20190201280JC, 20180101193JC,
0190201301JC].
M. Haller, G. D. Phelan, B. Carlson, K. Y. Jen and P. J. Reid,
Chem. Mater., 2008, 20, 3425–3434.
22 S. J. Benight, D. H. Bale, B. C. Olbricht and L. R. Dalton,
J. Mater. Chem., 2009, 19, 7466–7475.
2
3 D. R. Kanis, M. A. Ratner and T. J. Marks, Chem. Rev., 1994,
References
94, 195–242.
1
2
Y. Shi, Science, 2000, 288, 119.
P. A. Sullivan and L. R. Dalton, Acc. Chem. Res., 2010, 43,
24 A. Leclercq, E. Zojer, S.-H. Jang, S. Barlow, V. Geskin,
A. K.-Y. Jen, S. R. Marder and J. L. Bredas, J. Chem. Phys.,
2006, 124, 195.
10–18.
3
4
H. Kang, A. Facchetti, P. Zhu, H. Jiang, Y. Yang, E. Cariati, 25 X. Ma, F. Ma, Z. Zhao, N. Song and J. Zhang, J. Mater. Chem.,
S. Righetto, R. Ugo, C. Zuccaccia and A. Macchioni, Angew.
Chem., Int. Ed., 2005, 44(48), 7922–7925.
2010, 20, 2369–2380.
26 P. R. Varanasi, K. Y. Jen, J. Chandrasekhar, I. N. N. Namboothiri
and A. Rathna, J. Am. Chem. Soc., 1996, 118, 12443–12448.
T.-D. Kim, J.-W. Kang, J. Luo, S.-H. Jang, J.-W. Ka, N. Tucker,
J. B. Benedict, L. R. Dalton, T. Gray and R. M. Overney, J. Am. 27 J. Luo, F. Lin, Z. Li, M. Li, T.-D. Kim, S.-H. Jang and
Chem. Soc., 2007, 129, 488–489. A. K. Y. Jen, J. Mater. Chem. C, 2017, 5, 2230–2234.
L. R. Dalton, P. A. Sullivan and D. H. Bale, Chem. Rev., 2010, 28 H. Sun, Z. A. Li, J. Wu, Z. Jiang, J. Luo and K. Y. Jen, J. Mater.
10, 25–55. Chem. C, 2018, 6, 2840.
L. Dalton, Advances in Polymer Science, Springer, Berlin 29 M. Li, S. Huang, X.-H. Zhou, Y. Zang, J. Wu, Z. Cui, J. Luo
Heidelberg, 2002. and A. K. Y. Jen, J. Mater. Chem. C, 2015, 3, 6737–6744.
J. Luo, Y.-J. Cheng, T.-D. Kim, S. Hau, S.-H. Jang, Z. Shi, 30 L. Yi, B. Sanchali, K. A. Firestone, B. E. Eichinger, P. Rajan,
5
6
7
8
1
X.-H. Zhou and A. K.-Y. Jen, Org. Lett., 2006, 8, 1387–1390.
C. A. Anderson, B. H. Robinson, P. J. Reid and L. R. Dalton,
S.-K. Kim, Y.-C. Hung, B.-J. Seo, K. Geary, W. Yuan, B. Bortnik,
J. Am. Chem. Soc., 2006, 128, 6847–6853.
H. R. Fetterman, C. Wang, W. H. Steier and C. Zhang, Appl. 31 J. Wu, C. Peng, H. Xiao, S. Bo, L. Qiu, Z. Zhen and X. Liu,
Phys. Lett., 2005, 87, 061112. Dyes Pigm., 2014, 104, 15–23.
L. R. Dalton, D. Lao, B. C. Olbricht, S. Benight, D. H. Bale, 32 I. D. Albert, T. J. Marks and M. A. Ratner, ChemInform, 2010,
9
J. A. Davies, T. Ewy, S. R. Hammond and P. A. Sullivan, Opt.
Mater., 2010, 32, 658–6682.
0 H. C. Song, M.-C. Oh, S.-W. Ahn, W. H. Steier, H. R.
28, 6575–6582.
33 H. Xu, W. Nan, X. Zhang, Z. Li and G. Deng., Dyes Pigm.,
2018, 157, 230–237.
1
1
Fetterman and C. Zhang, Appl. Phys. Lett., 2003, 82, 4432–4434. 34 C.-Z. Zhang, C.-Y. Wang, C. Im, G.-Y. Lu and C.-S. Wang,
1 F. Liu, S. Chen, S. Mo, G. Qin, C. Yu, W. Zhang, W.-J. Shi, J. Phys. Chem. B, 2010, 114, 42–48.
P. Chen, H. Xu and M. Fu, J. Mater. Chem. C, 2019, 7, 35 M. He, T. M. Leslie and J. A. Sinicropi, Chem. Mater., 2002,
019–8028. 14, 2393–2400.
2 M. He, T. M. Leslie, J. A. Sinicropi, S. M. Garner and 36 H. Xu, M. Zhang, A. Zhang, G. Deng, S. Peng, H. Huang,
8
1
L. D. Reed, Chem. Mater., 2012, 14, 4669–4675.
C. Peng, M. Fu, J. Liu and Q. Ling, Dyes Pigm., 2014, 102,
13 L. Dalton and S. Benight, Polymers, 2011, 3, 1325–1351.
142–149.
1
4 S. Peng, J. Liu, Z. Zhen, X. Liu, G. Lakshminarayana and 37 R. V. Solomon, P. Veerapandian, S. A. Vedha and
I. V. Kityk, Tetrahedron Lett., 2012, 53, 3393–3396. P. Venuvanalingam, J. Phys. Chem. A, 2012, 116, 4667–4677.
5 F. Liu, H. Zhang, H. Xiao, H. Xu, S. Bo, L. Qiu, Z. Zhen, 38 K. S. Thanthiriwatte and K. M. N. D. Silva, THEOCHEM,
L. Lai, S. Chen and J. Wang, Dyes Pigm., 2018, 157, 55–63. 2002, 617, 169–175.
6 L. R. Dalton, A. W. Harper and B. H. Robinson, Proc. Natl. 39 C. C. Teng and H. T. Man, Appl. Phys. Lett., 1990, 56, 1734–1736.
Acad. Sci. U. S. A., 1997, 94, 4842–4847. 40 Z. Li, Q. Li and J. Qin, Polym. Chem., 2011, 2, 2723–2740.
7 Y. He, C. Lu, Z. Hua, C. Zhuo, F. Huo, L. Bing, Z. Zhen, 41 Z. a. Li, W. Wu, Q. Li, G. Yu, L. Xiao, Y. Liu, C. Ye, J. Qin and
X. Liu and S. Bo, J. Mater. Chem. C, 2018, 6, 1031. Z. Li, Angew. Chem., Int. Ed., 2010, 49, 2763–2767.
8 J. Wu, J. Luo, N. Cernetic, K. Chen, K.-S. Chiang and A. K.-Y. 42 L. I. Zhong’An, L. I. Zhen, Z. Zhu, L. I. Qianqian, K. Zhang,
1
1
1
1
1
2
Jen, J. Mater. Chem. C, 2016, 4, 10286–10292.
D. I. Zhong’An, Y. Liu, Y. E. Cheng and J. Qin, Macromolecules,
2006, 39, 6951–6961.
9 W. Wu, J. Qin and L. Zhen, Polym. Chem., 2011, 2,
2723–2740.
43 J. Gao, Y. Cui, J. Yu, Z. Wang, M. Wang, J. Qiu and G. Qian,
Macromolecules, 2009, 42, 2198–2203.
0 Y. Cheng, J. Luo, S. Hau, D. Bale, T. Kim, Z. Shi, D. Lao,
N. Tucker, Y. Tian and L. Dalton, Chem. Mater., 2007, 19, 44 Y. Yang, H. Xu, F. Liu, H. Wang, G. Deng, P. Si, H. Huang,
1154–1163.
S. Bo, J. Liu and L. Qiu, J. Mater. Chem. C, 2014, 2, 5124.
This journal is © The Royal Society of Chemistry 2019
J. Mater. Chem. C