Novel chromophores with excellent electro-optic activity based on double-donor chromophores by optimizing thiophene bridges

Article abstract of DOI:10.1016/j.dyepig.2015.06.012

Three novel nonlinear optical chromophores based on the same bis(N,N-diethyl)aniline donor have been synthesized and systematically characterized. These chromophores showed good thermal stability and chromophore y1 showed the best thermal stability of 249 °C. Besides, compared with the chromophore (y1) without the substituted group, chromophores y2 and y3 show better intramolecular charge-transfer absorption. Most importantly, the high molecular hyperpolarizability of these chromophores can be effectively translated into large electro-optic coefficients. The electro-optic coefficient of poled films containing 25% wt of chromophores y1-y3 doped in amorphous polycarbonate afforded values of 149, 138 and 157 pm/V at 1310 nm for chromophores y1-y3 respectively. These results indicated that the double donors of bis(N,N-diethyl)aniline unit can efficiently improve the electron-donating ability and the special structure can reduce intermolecular electrostatic interactions, thus enhancing the macroscopic electro-optic activity. These properties, together with good solubility, suggest the potential use of these chromophores as advanced materials.

Full text of DOI:10.1016/j.dyepig.2015.06.012

Dyes and Pigments 122 (2015) 139e146
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Novel chromophores with excellent electro-optic activity based on
double-donor chromophores by optimizing thiophene bridges
,
,
,
,
Yuhui Yang a b, Shuhui Bo a **, Haoran Wang a b, Fenggang Liu a b, Jialei Liu a, Ling Qiu a,
Zhen Zhen a, Xinhou Liu a
,
*
a Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, PR China
b University of Chinese Academy of Sciences, Beijing 100043, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Three novel nonlinear optical chromophores based on the same bis(N,N-diethyl)aniline donor have been
synthesized and systematically characterized. These chromophores showed good thermal stability and
chromophore y1 showed the best thermal stability of 249 ꢀC. Besides, compared with the chromophore
(y1) without the substituted group, chromophores y2 and y3 show better intramolecular charge-transfer
absorption. Most importantly, the high molecular hyperpolarizability of these chromophores can be
effectively translated into large electro-optic coefficients. The electro-optic coefficient of poled films
containing 25% wt of chromophores y1ey3 doped in amorphous polycarbonate afforded values of 149,
138 and 157 pm/V at 1310 nm for chromophores y1ey3 respectively. These results indicated that the
double donors of bis(N,N-diethyl)aniline unit can efficiently improve the electron-donating ability and
the special structure can reduce intermolecular electrostatic interactions, thus enhancing the macro-
scopic electro-optic activity. These properties, together with good solubility, suggest the potential use of
these chromophores as advanced materials.
Received 31 May 2015
Received in revised form
10 June 2015
Accepted 13 June 2015
Available online 23 June 2015
Keywords:
Doubleedonor chromophores
Electro-optic
Isolation group
Poled film
NLO
DFT
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
thermally and chemically stable materials with macroscopic EO
responses can be achieved by careful modification of the molecular
Polymeric electro-optic (EO) materials with nonlinear optical
(NLO) chromophores have shown commercial potential as active
media in high-speed broadband waveguides for optical switches,
optical sensors, and information processors [1e5]. In the past two
decades, the NLO materials have continuously drawn great interest
and have stimulated a research boom for materials with large EO
structure of chromophores and host polymers under mild electric-
field poling [10e12]. Thus, chromophores with large and weak
inter-molecular electrostatic interaction, as the decisive factor,
must be designed and prepared.
In general, dipolar NLO chromophores used in polymeric EO
materials consist of electron-donor and electron-acceptor groups
interacting through a conjugated bridge. In such molecules, the
donor and acceptor substituents provide the requisite ground-state
b
activities, both at molecular level (b) and as processed materials
(r33). Considerable progress has been made on developing organic
and polymeric electro-optic (EO) materials for applications in high-
speed and broadband information technology [2,5e7]. However,
one major obstacle that hinders the rapid development of this
technology is the lack of an effective mechanism to translate high-
charge asymmetry, whereas the
p-conjugation bridge provides a
pathway for the ultrafast redistribution of electric charges under an
applied external electric field [4,13,14]. The traditional chromo-
phore moieties have a rod-like structure, which leads to strong
inter-molecular dipoleedipole interactions in the polymeric matrix
and makes the poling-induced noncentrosymmetric alignment of
the chromophores a daunting task. For this reason, many efforts
have been carried out to design and synthesize novel NLO chro-
mohores, seeking to engineer NLO molecules both microscopically
molecular nonlinearities (
(r33) [8,9]. Efficient translation of large
b
) into large macroscopic EO activities
chromophores into
b
* Corresponding author. Tel.: þ86 01 82543528; fax: þ86 01 62554670.
** Corresponding author. Tel.: þ86 01 82543528; fax: þ86 01 62554670.
E-mail addresses: boshuhui@mail.ipc.ac.cn (S. Bo), xinhouliu@foxmail.com
(X. Liu).
(b
) and macroscopically (r33). On one hand, it has been well
established that large values of the chromophores can be
b
http://dx.doi.org/10.1016/j.dyepig.2015.06.012
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

140
Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
achieved by careful modification of the strength of donor and
acceptor moieties, as well as the nature of the -conjugated spacer
2950TGA (TA Co) with a heating rate of 10 ꢀC/min under the pro-
tection of nitrogen. Cyclic voltammetry (CV) experiments were
performed on a CHI660D electrochemical workstation by a cyclic
voltammetry (CV) technique in CH3CN solution, using Pt disk
electrode and a Pt wire as the working and counter electrodes,
respectively, and a saturated Ag/AgCl electrode as the reference
electrode in the presence of 0.1 M n-tetrabutylammoniumper-
chlorate as the supporting electrolyte. The ferrocene/ferrocenium
(Fc/Fcþ) couple was used as an internal reference. All chemicals,
commercially available, are used without further purification un-
less stated. The DMF, POCl3 and THF were freshly distilled prior to
p
[15,16]. Until now, NLO chromophores containing thiophene based
bridges have been widely used [17,18]. Further modification was
also widely studied to achieve excellent and better performance
[19e22]. On the other hand, controlling the shape of the chromo-
phore was proven to be an efficient approach for minimizing inter-
molecular dipoleedipole interaction and enhancing the poling ef-
ficiency [23]. So modifying the thiophene bridge may be one of a
reasonable way to achieve excellent and better performance.
Based on above point, as well as our previous work on the
“double-donor chromophore” [5,9,24], we designed and synthe-
sized three new double donor chromophores y1, y2, and y3. In our
previous work, it has been shown from both theoretical and
experimental analysis that the linear and nonlinear optical prop-
erties of double-donor chromophores are excellent. It also can
its
use.
The
2-dicyanomethylene-3-cyano-4-methyl-2,5-
dihydrofuran (TCF) acceptor was prepared according to the litera-
ture [25].
2.2. Synthesis
translate the high
b values of the chromophores to large macro-
scopic optical nonlinearities (r33) values. The double donors with
two N, N-diethylaniline can improve the electron-donating ability
thus enhance the optical nonlinearity. The one of the N, N-dieth-
ylaniline unit can act as both the additional donor and the isolation
group (IG). On the other hand, the double benzene rings with an
appropriate angle make the special structure different from the
general NLO chromophores and can decrease the intermolecular
electrostatic interactions so enhance the macroscopic EO activity.
However, to our regret, the solubility of double-donor chromo-
phore is not so excellent and limited its application, which
encourage us to optimize double-donor chromophore. In this pa-
per, a new synthetic methodology of attaching two different sub-
2.2.1. Synthesis of chromophore y1
Chromophore y1 was synthesized according to the literature
[24].
2.2.2. Synthesis of compound 2b
Dry DMF (3 mL) was dropwise added to 3 mL of phosphorus
oxychloride (0.0327 mol) maintained at 0 ꢀC. After the solution was
stirred for 30 min, compound 1b (4.60 g, 0.0274 mol) was added to
the above Vilsmeier reagent, and the mixture was heated to 90 ꢀC
for an additional 2 h. After cooling, the clear red solution was added
dropwise to NaHCO3 (aq) with stirring, and the resulting mixture
was allowed to stand for 2 h and extracted with CH2Cl2 (20 mL ꢁ 3).
The combined extracts were washed with water and dried over
MgSO4. After filtration and removal of the solvent under vacuum,
the crude product was purified by column chromatography to give
a red liquid product (87%).
stituents onto the
p-conjugation bridge of NLO chromophore was
developed to obtain two new NLO chromophores. The motivation is
to improve the solubility as well as the EO activities of guest-host
EO materials by a simple bridge-modifying method for NLO chro-
mophores. These modified chromophores showed great solubility
in common organic solvents, better thermal stability (with their
onset decomposition temperatures all above 220 ꢀC), good
compatibility with polymers, and large EO activity in the poled
films. 1H NMR and 13C NMR analysis were carried out to demon-
strate the preparation of these chromophores. Thermal stability,
photophysical properties and DFT calculations of these chromo-
phores were systematically studied. Furthermore, the macroscopic
EO activity (r33) of these chromophores in amorphous poly-
carbonate were also analysed Chart 1.
1H NMR (400 MHz, CDCl3)
d
9.99 (s, 1H), 7.58 (d, J ¼ 5.0 Hz, 1H),
6.96 (d, J ¼ 5.0 Hz, 1H), 2.98e2.62 (m, 2H), 1.73e1.53 (m, 2H),
1.37e1.12 (m, 6H), 0.83 (m, 3H).
13C NMR (100 MHz, CDCl3)
d 182.18, 152.90, 137.71, 134.42,
130.80, 31.65, 31.61, 31.45, 30.42, 30.18, 29.00, 28.53, 22.63, 14.09.
MS (EI): m/z calcd for C11H16OS: 196.09; found: 196.31.
2.2.3. Synthesis of compound 3b
To a suspension of diethyl bis(4-(diethylamino) phenyl) meth-
ylphosphonate (1.5 g, 3.36 mmol), and 2b (588 mg, 3 mmol) in
20 mL dry THF and NaH (0.36 g, 15 mmol) were added and the
mixture turned yellow. The mixture was stirred at room tempera-
ture for 24 h. Saturated NH4Cl was added and the resulting mixture
was extracted with EtOAc (20 mL ꢁ 3). The combined extracts were
washed with water and dried over MgSO4. After filtration and
removal of the solvent under vacuum, the crude product was pu-
rified by column chromatography to give a yellow product (0.66 g,
45%).
2. Experiments
2.1. Materials and instrumentation
1H NMR spectra were determined by an Advance Bruker 400
(400 MHZ) NMR spectrometer (tetramethylsilane as internal
reference). The MS spectra were obtained on MALDI-TOF-(Matrix
Assisted Laser Desorption/Ionization of Flight) on BIFLEXIII(Broker
Inc.) spectrometer. The UVeVis spectra were performed on Cary
5000 photo spectrometer. The TGA was determined by TA5000-
1H NMR (400 MHz, Acetone)
d 7.07e7.01 (m, 2H), 6.90 (s, 1H),
6.87e6.82 (m, 3H, overlap), 6.63 (d, J ¼ 3.8 Hz, 1H), 6.62 (d,
J ¼ 8.8 Hz, 2H), 6.50 (d, J ¼ 8.8, 2H), 3.29 (m, 8H), 2.63e2.54 (m, 2H),
1.49 (m, 2H),1.31e1.20 (m, 6H), 1.04 (m, 12H), 0.75 (m, 3H).
MALDI-TOF: m/z calcd for C32H44N2S: 488.32 [M]þ; found:
488.09.
2.2.4. Synthesis of compound 4b
To a solution of 3b (0.98 g, 2.0 mmol) in dry THF (20 mL) was
added a 2.4 M solution of n-BuLi in hexane (1.3 mL, 3.0 mmol)
dropwise at ꢂ78 ꢀC under N2. After this mixture was stirred at this
temperature for 1 h, and the dry DMF (0.20 mL, 2.4 mmol) was
introduced. The resulting solution was stirred for another 1 h
at ꢂ78 ꢀC and then allowed to warm up to room temperature. The
Chart 1. Structure of chromophore y1ey3.

Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
141
reaction was quenched by water. THF was removed by evaporation.
The residue was extracted with CH2Cl2 (3 ꢁ 30 mL). The organic
layer was dried (MgSO4) and concentrated in vacuo. The residue
was purified by column chromatography on silica gel (hexane/
acetone, v/v, 10/1) to obtain a red liquid (0.85 g, 82%).
2.2.9. Synthesis of compound 4c
In a similar manner described above, 4c was synthesized from
3c as a red liquid (78%).
1H NMR (400 MHz, Acetone)
d 9.55 (s, 1H), 7.49 (s, 1H), 7.14 (s,
1H), 7.08 (d, J ¼ 8.9 Hz, 2H), 6.91 (d, J ¼ 8.6 Hz, 2H), 6.73 (d,
J ¼ 8.6 Hz, 2H), 6.56 (d, J ¼ 8.9 Hz, 2H), 4.07 (q, J ¼ 6.7 Hz, 2H),
3.43e3.26 (m, 8H),1.96 (m, 2H),1.74e1.23 (m, 6H),1.12 (m, 6H),1.05
(m, 6H), 0.88e0.71 (m, 3H).
1H NMR (400 MHz, CDCl3)
d 9.69 (s, 1H), 7.44 (s, 1H), 7.31e7.25
(m, 3H, overlap), 7.07 (d, J ¼ 8.6 Hz, 2H), 7.01 (s, 1H), 6.74 (d,
J ¼ 8.6 Hz, 2H), 6.64 (d, J ¼ 8.7 Hz, 2H), 3.43 (m, 8H), 2.72 (m, 2H),
1.67 (m, 2H), 1.39 (m, 6H), 1.23 (m, 12H), 0.97e0.91 (m, 3H).
13C NMR (100 MHz, Acetone)
d 181.76, 155.51, 148.12, 147.71,
13C NMR (100 MHz, CDCl3)
d
182.24, 148.26, 148.04, 147.79,
143.97, 136.99, 132.33, 131.00, 129.25, 128.52, 125.45, 123.66, 112.32,
111.41, 111.16, 71.56, 44.03, 31.47, 25.65, 22.48, 13.50, 12.11, 12.03.
MALDI-TOF: m/z calcd for
found:532.78.
146.37, 141.88, 139.03, 137.59, 131.41, 129.60, 129.12, 125.10, 44.43,
44.36, 31.73, 30.55, 29.14, 28.91, 22.67, 14.14, 12.67, 12.65.
C
33H44N2O2S: 532.31 [M]þ;
MALDI-TOF: m/z calcd for
found:516.73.
C
33H44N2OS: 516.32 [M]þ;
2.2.10. Synthesis of chromophore y3
In a similar manner described above, chromophore y3 was
synthesized from 4c as a dark solid (38%).
2.2.5. Synthesis of chromophore y2
1H NMR (400 MHz, Acetone)
d
7.73 (d, J ¼ 15.6 Hz, 1H), 7.37 (s,
A mixture of aldehydic bridge 4b (0.516 g, 1 mmol) and acceptor
5 (0.24 g, 1.2 mmol) in ethanol (20 mL) with a catalytic amount of
piperidine and stirred at 78 ꢀC for 3 h. After removal of the solvent,
the residue was purified by column chromatography on silica gel
(hexane/ethyl acetate, v/v, 5:1), a dark solid was obtained (0.24 g,
35%).
1H), 7.18 (s, 1H), 7.12 (d, J ¼ 8.9 Hz, 2H), 6.88 (d, J ¼ 8.6 Hz, 2H), 6.74
(d, J ¼ 8.6 Hz, 2H), 6.56 (d, J ¼ 8.9 Hz, 2H), 6.34 (d, J ¼ 15.6 Hz, 1H),
4.04 (t, J ¼ 6.4 Hz, 2H), 3.41e3.25 (m, 8H), 1.76e1.67 (m, 2H), 1.65 (s,
6H), 1.41 (m, 2H), 1.26 (m, 2H), 1.14 (m, 2H), 1.14e1.07 (m, 6H), 1.04
(m, 6H), 0.83e0.75 (m, 3H).
1H NMR (400 MHz, Acetone)
d
7.94 (d, J ¼ 15.7 Hz, 1H), 7.50 (s,
13C NMR (100 MHz, Acetone)
d 176.47, 172.97, 157.46, 148.66,
1H), 7.29 (d, J ¼ 9.0 Hz, 2H), 7.19 (s, 1H), 7.01 (d, J ¼ 8.8 Hz, 2H), 6.86
(d, J ¼ 8.8 Hz, 2H), 6.70 (d, J ¼ 9.0 Hz, 2H), 6.41 (d, J ¼ 15.7 Hz, 1H),
3.47 (m, 8H), 2.80e2.79 (m, 2H),1.78 (s, 6H), 1.73e1.67 (m, 2H),
1.48e1.29 (m, 6H), 1.21 (m, 12H), 0.90 (m, 3H).
148.29, 146.32, 139.56, 135.20, 135.09, 131.02, 128.98, 128.77, 124.91,
121.72, 112.69, 112.47, 111.97, 111.62, 111.32, 111.01, 110.01, 101.41,
97.29, 94.64, 71.66, 44.20, 44.06, 31.36, 25.53, 22.36, 13.33, 12.04.
MALDI-TOF: m/z calcd for
found:713.02.
C
44H51N5O2S: 713.38 [M]þ;
13C NMR (100 MHz, Acetone)
d 176.58, 173.83, 148.64, 148.49,
HRMS (ESI) (M þ H)þ: calcd, 714.3842; found, 714.3835.
148.32, 147.83, 143.98, 140.17, 138.19, 137.30, 131.26, 129.24, 128.78,
124.61, 113.15, 112.64, 112.26, 111.96, 111.19, 110.85, 110.68, 97.59,
95.52, 44.20, 44.07, 31.51, 30.38, 25.52, 22.51, 22.40, 13.46, 12.05,
12.04.
3. Results and discussion
MALDI-TOF: m/z calcd for
found:697.93.
C
44H51N5OS: 697.38 [M]þ;
3.1. Synthesis and characterization of chromophores
HRMS (ESI) (M þ H)þ: calcd, 698.3893; found, 698.3874.
The synthesis of chromophores y1, y2, and y3 are depicted in
Scheme 1. These NLO chromophores were designed having same
strong electron acceptor (TCF) and the same electron donor
bis(N,N-diethyl)aniline group, but have different substituent
2.2.6. Synthesis of compound 1c
Compound 1c was synthesis according to the report [26].
groups on the thiophene p-conjugation. Chromophores y1, y2, and
1H NMR (400 MHz, Acetone)
(dd, J ¼ 5.2, 1.5 Hz, 1H), 6.19 (dd, J ¼ 3.1, 1.5 Hz, 1H), 3.91 (m, 2H),
1.79e1.69 (m, 2H), 1.42 (m, 2H), 1.36e1.28 (m, 4H), 0.91e0.84 (m,
3H).
d
7.13 (dd, J ¼ 5.2, 3.1 Hz, 1H), 6.72
y3 were prepared starting from the phosphonate which was pre-
pared using a procedure described in the literature [24,27]. React-
ing phosphonate with different aldehyde (2ae2c) using sodium
hydride as base under a HornereEmmons reaction condition
afforded alkene 3ae3c exclusively. Lithiation the alkene 3 with n-
butyl lithium followed by the addition of DMF yielded the corre-
sponding aldehyde 4ae4c. The target chromophores y1 (with no
substituent), y2 (with hexyl group) and y3 (with hexyloxy group)
on the bridge thiophene ring were successfully obtained by
condensing aldehydes 4ae4c with acceptor 5 (TCF) in ethanol with
the presence of a catalytic amount of piperidine.
MS (EI): m/z calcd for C10H16OS: 184.09; found: 184.51.
2.2.7. Synthesis of compound 2c
In a similar manner described above, 2c was synthesized from
1c as a red liquid (72%).
1H NMR (400 MHz, CDCl3)
d
10.02 (s, 1H), 7.63 (d, J ¼ 5.4 Hz, 1H),
6.85 (d, J ¼ 5.4 Hz, 1H), 4.16 (t, J ¼ 6.5 Hz, 2H), 1.93e1.74 (m, 4H),
1.40e1.20 (m, 4H), 0.95e0.85 (m, 3H).
3.2. Thermal analysis
MS (EI): m/z calcd for C11H16O2S: 212.09; found: 212.43.
The NLO chromophores must be thermally stable enough to
withstand encountered high temperatures (>200 ꢀC) in electric
field poling and subsequent processing of chromophore/polymer
materials. Thermal properties of these chromophores were
measured by Thermogravimetric Analysis (TGA) with a heating rate
of 10 ꢀC minꢂ1 under nitrogen. Fig. 1 shows the thermogravimetric
analysis of chromophores y1, y2, and y3. All of the chromophores
exhibited good thermal-stabilities and their decomposition tem-
peratures (Td) were above 220 ꢀC as summarized in Table 1. The
excellent thermal stability of these chromophores makes them
suitable for practical device fabrication and EO device preparation.
2.2.8. Synthesis of compound 3c
In a similar manner described above, 3c was synthesized from
2c as a yellow liquid (40%).
1H NMR (400 MHz, CDCl3)
d
7.22 (d, J ¼ 8.8 Hz, 2H), 7.08 (d,
J ¼ 8.5 Hz, 2H), 6.80 (d, J ¼ 5.5 Hz, 1H), 6.74 (d, J ¼ 8.5 Hz, 2H), 6.67
(d, J ¼ 5.5 Hz, 1H), 6.62e6.56 (d, J ¼ 8.8 Hz, 2H), 4.01 (m, 2H),
3.45e3.30 (m, 8H), 1.96 (m, 2H), 1.76e1.27 (m, 6H), 1.22 (m, 12H),
0.89e0.84 (m, 3H).
MALDI-TOF: m/z calcd for
found:504.01.
C
32H44N2OS: 504.32 [M]þ;

142
Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
Scheme 1. Synthesis of chromophore y1ey3.
3.3. Optical properties
(ICT) absorption properties, UVeVis absorption spectra of three
chromophores (c ¼ 1 ꢁ 10ꢂ5 mol/L) were recorded in a series of
solvents with different dielectric constants (Fig. 2). The spectrum
data are summarized in Table 1. The synthesized chromophores
In order to explore the effect of different substituted groups of
the
p-conjugation bridge on the intramolecular charge transfer
exhibited a similar
pep* intramolecular charge-transfer (ICT) ab-
sorption band in the visible region. As shown in Fig. 2, in both non-
polar and polar solvents such as 1,4-dioxane and CHCl3, these
chromophores exhibited a similar
pep* intramolecular charge-
transfer (ICT) absorption band with a continuous red-shift of the
maximum absorption (lmax) from y1 to y3. Chromophore y3
showed the maximum absorption (lmax) of 803 nm in CHCl3. This
value is exceptionally large compared to the chromophore y1 (lmax
of 725 nm in CHCl3) with same donor and the same acceptor but no
substituent on thiophene bridge. With a hexyloxy on the thiophene
bridge, the lmax of y3 red shifted by 78 nm. However, with a hex-
ylxy on the thiophene bridge, the chromophore y2 is only red
shifted by 11 nm to 736 nm compared to chromophore y1, which
indicated that the hexyloxy group on the thiophene bridge can act
as an additional donor and shifted the ICT absorption band of the
chromophore to the much lower energy. Besides, the sol-
vatochromic behaviour was also an important aspect to investigate
the polarity of chromophores. As shown in Fig. 2, the peak wave-
length of chromophores y3 showed a bathochromic shift of 101 nm
from dioxane to chloroform, displaying
a much larger sol-
vatochromism compared with the chromophore y2 (66 nm) and
chromophore y1 (71 nm in Table 1). The resulting spectrum data
Fig. 1. TGA curves of chromophores y1ey3 with a heating rate of 10 ꢀC minꢂ1 in a
nitrogen atmosphere.

Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
143
Table 1
Summary of thermal and optical properties and EO coefficients of chromophores
y1ey3.
Chromophore Td (ꢀC) lmax (nm) lmaxc (nm) Dld (nm) r33 (pm/V)
a
b
e
y1
y2
y3
249
224
246
725
736
803
654
670
702
71
66
101
149
138
157
a
Td was determined by an onset point, and measured by TGA under nitrogen at a
heating rate of 10 ꢀC/min.
b
lmax was measured in CHCl3.
c
lmax was measured in dioxane.
d
Dl
¼
lbmaxꢂlc
.
max
e
r33 values were measured at the wavelength of 1310 nm.
indicated that the chromophores y3 were more easily polarizable
than chromophore y1 and y2.
3.4. Theoretical calculations
For further comparison, we have performed the quantum
chemical calculations within a framework of GAUSSIAN 03 using
the split valence B3LYP functional with 6-311 G** (d, p) basis set to
understand the ground-state polarization of these chromophores
with different bridges [28e30]. The data obtained from DFT cal-
culations 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 the optical and electrical properties
of molecules. [30e32], Besides, the HOMO-LUMO energy gap is also
used to understand the charge transfer interaction occurring in a
chromophore molecule [32e34]. In the case of these chromo-
phores, Fig. 3 represents the frontier molecular orbitals of chro-
mophores y1ey3. According to Fig. 3, it is clear that the electronic
distribution of the HOMO is delocalized over the thiophene linkage
and benzene ring, whereas the LUMO is mainly constituted by the
acceptor moieties [32].
The HOMO and LUMO energy were calculated by DFT calcula-
tions as shown in Table 2. The energy gap between the HOMO and
LUMO energy for chromophores y1ey3 were 2.05 eV, 1.95 eV and
1.81 eV respectively. The HOMO-LUMO gap was the lowest for
chromophore y3 (1.81 eV) and highest for chromophore y1
(2.05 eV). As reported [32], the optical gap is lower and the charge-
transfer (ICT) ability is greater, thus improving the nonlinearity. As
a result, chromophore y2 and y3 showed the lower optical gap than
chromophore y1, so it may indicate that chromophore y2 and y3
can exhibit better ICT and NLO properties than chromophore y1.
This result corresponds with the UVeVis results.
Further, the theoretical microscopic Zero-frequency (static)
molecular first hyperpolarizability (
03. As the reference reported earlier, the
the 6-311 g** (d, p) level in vacuum [35]. From this, the scalar
quantity of can be computed from the x, y, and z components
b) was calculated by Gaussian
b
has been calculated at
b
according to the following equation:
Fig. 2. UVeVis absorption spectra in different solvents of chromophore y1ey3
(c ¼ 1 ꢁ 10ꢂ5 mol Lꢂ1).
ꢀ
1=2
b ¼ ðb2x þ b2y þ b2z
(1)
3.5. Electrochemical properties
P
1
where bi ¼ biii
þ
ðbijj þ bjij þ bjjiÞ; i; j2ðx; y; zÞ.
3
isj
To determine the redox properties of these chromophores, cyclic
voltammetry (CV) experiments were performed on a CHI660D
electrochemical workstation by a cyclic voltammetry (CV) tech-
nique. As shown in Fig. 4, chromophores y1, y2 and y3 all exhibited
one quasi reversible oxidative wave with a half-wave potential, E1/
The data obtained from DFT calculations were summarized in
Table 2. There was no obvious change of the
b values of these
chromophores y1ey3 (684e713 ꢁ 10ꢂ30 esu), which illustrated
that the substituent group on the thiophene bridge had little in-
fluence on their main conjugated structures under the simulated
condition.
¼ 0.5 (Eox þ Ered), at about 0.55, 0.51 and 0.43 V respectively.
2

144
Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
Table 2
Table 3
Data from DFT calculations.
Summary of EO coefficients of chromophores y1ey3 at different densities.
Chromophores
E
HOMO/eV ELUMO/eV △Ea/eV △Eb/eV bmaxc/10ꢂ30 esu
Wt%
ar33 (pm/V)
y1
y1
y2
y3
ꢂ5.04
ꢂ5.13
ꢂ4.98
ꢂ2.99
ꢂ3.18
ꢂ3.17
2.05
1.95
1.81
1.12
1.04
0.95
713
684
686
y2
34
68
117
138
112
y3
41
79
128
157
145
10
15
20
25
30
32
61
120
149
e
D
E ¼ ELUMO ꢂ EHOMO
.
a
Results was calculated by DFT.
b
c
Results was from cyclic voltammetry experiment.
values were calculated using gussian03 at B3LYP/6-311 G** (d, p) level and the
b
a
r33 values were measured at the wavelength of 1310 nm.
direction of the maximum value is directed along the charge transfer axis of the
chromophores.
derived from these chromophores, a series of guest-host polymers
were prepared by formulating the chromophores into amorphous
polycarbonate (APC) using dibromomethane as solvent. The
resulting solutions were filtered through a 0.2 mm PTFE filter and
spin-coated onto indium tin oxide (ITO) glass substrates. Films of
doped polymers were baked in a vacuum oven at 80 ꢀC overnight to
ensure the removal of the residual solvent. The corona poling
process was carried out at a temperature of 10 ꢀC above the glass
transition temperature (Tg) of the polymer. The r33 values were
measured using the Teng-Man simple reflection technique at the
wavelength of 1310 nm using a carefully selected thin ITO electrode
with low reflectivity and good transparency in order to minimize
the contribution from the multiple reflections [36].
The r33 values of films containing chromophores y1 (fillm-A), y2
(film-B), y3 (film-C) were measured in different loading densities,
as shown in Table 3 and Fig. 5, For chromophore y1, the r33 values
were gradually improved from 32 pm Vꢂ1 (10 wt%) to 149 pm Vꢂ1
(25 wt%), as the concentration of chromophores increased, the
similar trend of enhancement was also observed for chromophore
y2, whose r33 values increased from 34 pm Vꢂ1 (10 wt%) to
138 pm Vꢂ1 (25 wt%). Chromophoe y3 gained the r33 value from
41 pm Vꢂ1 (10 wt%) to 157 pm Vꢂ1 (25 wt%).
As reported earlier [37e41], the introduction of some isolation
groups into the chromophore moieties to further control the shape
of the chromophore could be an efficient approach to minimize
interactions between the chromophores, but the isolation group
Fig. 3. The frontier molecular orbitals of chromophores y1ey3.
Meanwhile, these chromophores had an irreversible reduction
wave corresponding to the acceptor moieties at Ered ¼ ꢂ0.58, ꢂ0.53
and ꢂ0.52 V (vs. Ag/AgCl). It showed an energy gap (
DE) value of
1.12, 1.04 and 0.95 eV for chromophores y1, y2 and y3 respectively.
The HOMO and LUMO levels of these new chromophores were
calculated from their corresponding oxidation and reduction po-
tentials. The HOMO levels of y1, y2 and y3 were estimated to
be ꢂ4.95, ꢂ4.91 and ꢂ4.83 eV respectively. In the meantime, the
corresponding LUMO level of y1ey3 only showed slight changes
at ꢂ3.82, ꢂ3.87 and ꢂ3.88 eV, respectively. This result corre-
sponded with the conclusion of UVeVis spectra analysis and the
DFT results.
should suitable. So by introducing the isolation groups into the p-
bridge, it controls the shape of the chromophore and decreased the
interaction between the chromophore thus can increase the r33
value. As Fig. 6 (the optimized structure) shows, the one of the
donor benzene rings and the substituted group are on both sides of
3.6. Electric field poling and EO property measurements
the conjugated plane, which effectively protected the p-conjugated
For application, the large hyperpolarizability (b) has to be
translated into a large EO coefficient (r33). To study EO property
Fig. 5. EO coefficients of NLO thin films as
a function of chromophore loading
Fig. 4. Cyclic voltammograms of chromophore y1ey3.
densities.

Y. Yang et al. / Dyes and Pigments 122 (2015) 139e146
145
Table 4
Representative r33 values of the chromophores.
Chromophores
y1
y2
y3
r33 (pm/V)a
Nb
r
149
2.455
6.07
138
2.160
6.39
157
2.11
7.44
33/Nc
a
b
c
Experimental value from simple reflection at 1310 nm.
Chromophore number density; in units of ꢁ 1020 molecules/cc.
r33 normalized by chromophore number density; in units of ꢁ 10ꢂ19 pm cc/(V
molecules).
obvious phase separation, the film-B and film-C showed a down-
ward trend. The enhancement of the EO activity further demon-
strated that the substituted group on thiophene ring effectively
suppressed the dipoleedipole interaction and assisted the orien-
tation of chromophores during poling process also improved the
compatibility with the polymer. These large r33 values and high
stability together with our previous work of double donors chro-
mophores can effectively prove that the double donor structure can
really decrease the interactions between the chromophores thus
can increase the r33 values.
Fig. 6. Optimized structures of chromophores y1ey3.
bridge from an intermolecular DielseAlder (DA) cycloaddition re-
action [42e44]. This arrangement acts to site-isolate the chromo-
phore within the internal free volume created by the substituted
group on the thiophene ring, which is favourable for dipole
orientation in the poling process. The molecular conformation,
including the hindrance of donor and
p-bridge (Fig. 6) effectively
isolates the chromophores and weakens the dipoleedipole
interactions.
4. Conclusions
The EO coefficient (r33), defining the efficiency of translating
molecular microscopic hyperpolarizability into macroscopic EO
activities, was described as follows:
In this research, two novel chromophores y2 and y3 were
designed and synthesized through modification of y1 which we
reported previously. These chromophores were systematically
characterized by MS, NMR and UVeVis absorption spectra. The
energy gap between ground state and excited state together with
molecular nonlinearity were studied by UVevis absorption spec-
troscopy, DFT calculations and CV measurements. Theoretical and
experimental investigations suggest that the isolation group play a
critical role in affecting the linear and nonlinear properties of
dipolar chromophores. EO polymers were prepared by the chro-
mophores being doped into APC and their r33 values were
measured. The poling results of guest-host EO polymers with 25 wt
% of these chromophores showed that polymers with chromo-
phores y1ey3 afforded the large r33 values of 149, 138 and 157 pm/
V respectively. The r33 values of EO polymers based on y3 were
larger than y1 and y2 due to the alkxoy chains on the thiophene
bridge can not only act as the additional donor group but also an
isolation group. Those consequences indicate that these chromo-
phores with double N,N-diethylaniline donors and isolation group
on the thiophene bridge could efficiently reduce the inter-
chromophore electrostatic interactions and enhance the macro-
scopic optical nonlinearity. All these might be useful in designing
other new NLO chromophores with the modified thiophene bridge
to optimize the molecular structure for achieving excellent prop-
erties. We believe that these novel chromophores can be used in
exploring high-performance organic EO and photorefractive ma-
terials where both thermal stability and optical nonlinearity are of
equal importance.
.
r33 ¼ 2N fðuÞb < cos3q > n4
where r33 is the EO coefficient of the poled polymer, N represents
the aligned chromophore number density and f(
u
) denotes the
> is the
Lorentz-Onsager local field factors. The term <cos3
q
orientationally averaged acentric order parameter characterizing
the degree of noncentrosymmetric alignment of the chromophore
in the material and n represents the refractive index [5]. Before
poling, there is no EO activity in the EO material and the chromo-
phores in material are thermal randomization. Electrical field
induced poling was proceeded to induce the acentric ordering of
chromophores. Realization of large electro-optic activity for dipolar
organic chromophore-containing materials requires the simulta-
neous optimization of chromophore first hyperpolarizability (
b),
acentric order <cos3
q>, and number density (N). But chromophores
with large mb generate intermolecular static electric field dipo-
leedipole interaction, which leads to the unfavourable antiparallel
packing of chromophores, so that the number of truly oriented
chromophore (N) is small. As a result, the isolation groups should
be appropriate.
Based on the above view, it is not hard to explain the r33 results.
When the concentration of chromophore in APC is low, the inter-
molecular dipolar interactions are relatively weak and thus the
influence of different isolation group on r33 value is small. As the
chromophore loading increased, the effect of inter-chromophore
dipoleedipole is becoming stronger. As shown in Table 4,
although the resulting r33 value of chromophore y2 was 138 pm/V,
which was a little smaller than chromophore y1. When effectively
normalizes the r33 value for the relative chromophore content,
dividing the observed r33 by N yields a value of 6.39 ꢁ 10ꢂ19 pm cc/
(V molecules), which is larger than chromophore y1
(6.07 ꢁ 10ꢂ19 pm cc/(V molecules)). Besides, chromophre y3 pre-
sented the largest r33 values. Hence, film C containing chromophoe
y2 showed much higher efficiency of translating molecular
microscopic hyperpolarizability into macroscopic EO activities.
Besides, to our regret, as the loading density increasing from 25 wt%
to 30 wt%, the film-A with the loading 30 wt% of y1 showed an
Acknowledgements
We are grateful to the National Nature Science Foundation of
China (no. 61101054) for financial support.
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