The design of nonlinear optical chromophores exhibiting large electro-optic activity and high thermal stability: The role of donor groups

Article abstract of DOI:10.1016/j.dyepig.2016.03.003

A novel chromophore based on N-(4-dibutylaminophenyl)tetrahydroquinolinyl donor group was synthesized to compare with traditional chromophores based on either diethylaminophenyl or triarylaminophenyl donor groups. Cyclic voltammetry measurements showed th

Full text of DOI:10.1016/j.dyepig.2016.03.003

Dyes and Pigments 130 (2016) 138e147
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The design of nonlinear optical chromophores exhibiting large
electro-optic activity and high thermal stability: The role of donor
groups
,
,
,
, b
Fenggang Liu a b, Hongyan Xiao a, Yuhui Yang a b, Haoran Wang a b, Hua Zhang a
,
Jialei Liu a, Shuhui Bo a **, Zhen Zhen a, Xinhou Liu a, Ling Qiu 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:
A novel chromophore based on N-(4-dibutylaminophenyl)tetrahydroquinolinyl donor group was syn-
thesized to compare with traditional chromophores based on either diethylaminophenyl or triar-
ylaminophenyl donor groups. Cyclic voltammetry measurements showed that the new chromophore had
a much smaller energy gap than the structurally related chromophores. Density functional theory cal-
culations suggested that the molecular quadratic hyperpolarizability (mb) value of tetrahydroquinolinyl
chromophore is 65% and 110% larger than that of either the diethylaminophenyl or triarylaminophenyl
chromophores, respectively. The doped film containing the tetrahydroquinolinyl chromophore showed
an r33 value of 126 pm/V at the concentration of 25 wt% which is much higher than the electro-optic
activity of the diethylaminophenyl derived chromophore (39 pm/V) and six times higher than that of
the traditional triarylaminophenyl chromophore (19 pm/V). The tetrahydroquinolinyl chromophore
possesses much higher thermal stability (with its onset decomposition temperature above 280 ꢀC) than
those of commonly used alkyl chromophores.
Received 18 January 2016
Received in revised form
27 February 2016
Accepted 3 March 2016
Available online 4 March 2016
Keywords:
Nonlinear optical materials
Chromophore
Second-order
Electro-optic
Donor
Thermal stability
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
interactions in the polymer matrix are needed to effectively
translate these large mb values into bulk EO activities [10].
Functional materials displaying nonlinear optical (NLO) activity
have been extensively studied are extensively studied in recent
years because of their potential applications in the field of optical
switches, high-speed and broadband information technology, and
telecommunications [1e4]. Compared with inorganic materials,
organic materials have the potential advantages in faster response
time, ease of processing, higher nonlinear optical activity [5e7]. To
meet the stringent requirements for device applications, one of the
often critical challenges in making highly efficient electro-optic
(EO) materials is to develop nonlinear optical (NLO) chromo-
phores with large molecular quadratic hyperpolarizability (mb),
good optical transparency, and excellent thermal and chemical
stabilities [8,9]. Moreover, weak molecular electrostatic
Molecular NLO chromophores are traditionally constituted by
electron donor and acceptor moieties covalently connected
through a conjugated bridge, called the De
When considering the design of second-order nonlinear optical
materials, optimization of the -conjugated bridge, electron-donor
peA system [11,12].
p
and electron-acceptor characteristics of the substituents are
needed to obtain maximum nonlinearity at the molecular level
[13e17]. Alkyl and aryl amines type donors [18,19], the
tricyanofuran-based (TCF) family of acceptors [20], and ring-locked
tetraene [21] or 2,5-divinylthienyl [22] bridges have all proven to
afford highly active and stable dipolar EO chromophores. Among all
of the materials studied, the NLO chromophores bearing 4-(dia-
lkylamino)phenyl groups or triarylamine groups as a donating
group were the most promising due to their strong electron-
donating ability and ease of synthesis [1,23].
The NLO chromophores with a triarylamine (TAA) donor and a
thiophene-bridge exhibit the highest thermal stability of all re-
ported chromophores, which seems to be the best candidate for the
* Corresponding author.
** Corresponding author.
E-mail addresses: boshuhui@mail.ipc.ac.cn (S. Bo), tipcqiuling@163.com (L. Qiu).
http://dx.doi.org/10.1016/j.dyepig.2016.03.003
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
139
active EO materials in
a
practical device [24,25]. However,
chromophore NT can increase the thermal stability and mb as well
as reduce dipoleedipole interactions so as to translate their mb
values into bulk EO performance more effectively. The synthesis,
UVeVis, solvatochromic behaviour, redox properties, DFT quan-
tum mechanical calculations, thermal stabilities and EO activities
of these chromophores were systematically studied and
compared to illustrate the influence of electron-donating groups
on rational NLO chromophore designs.
compared to their alkyl counterparts, TAA based chromophores
usually exhibit reduced hyperpolarizabilities due to delocalization
of the nitrogen lone pair into the two phenyl rings which do not
connect with the bridge [26]. The E-O coefficients (r33) values
derived from aryl De
20 pm Vꢁ1 (at 1310 nm), which are much lower than those obtained
from their alkyl De eA counterparts [27,28]. These may be due to
stacking interactions and charge transport between chromo-
peA chromophores remained between 10 and
p
pep
phores [13,29]. However, the dipolar chromophores with 4-(dia-
lkylamino)phenyl donor have demonstrated significantly weaker
thermo- and photostabilities compared to their 4-(diarylamino)
phenyl analogues although they show much higher r33 values
[25,30].
These encouraged us to design and synthesis a novel chro-
mophore combining the advantages of both alkyl and aryl amines
type donors where both thermal stability and optical nonline-
arity are very high. So, in this paper, we have designed and
synthesized a series of chromophores containing an identical
thiophene bridge and TCF acceptor, but with three different
electron donors, including diethylaminophenyl (FTC), triar-
ylaminophenyl (TPA), and N-(4-dibutylaminophenyl) tetrahy-
droquinolinyl groups (NT) (Chart 1). The diethylaminophenyl
donor-based chromophore FTC and triarylaminophenyl donor-
based chromophore TPA were chosen as a reference compound
2. Experimental
2.1. Materials and instrument
All chemicals are commercially available and are used without
further purification unless otherwise stated. N, N-dimethylforma-
mide (DMF), Phosphorusoxychloride (POCl3), tetrahydrofuran
(THF) and ether were distilled over calcium hydride and stored over
molecular sieves (pore size 3 Å). 2-Dicyanomethylene-3-cyano-4-
methyl-2,5-dihydrofuran (TCF) acceptor was prepared according
to the literature [33]. Compound 2a was synthesized according to
literature [34]. Compound 2c was synthesized according to litera-
ture [35]. TLC analyses were carried out on 0.25 mm thick pre-
coated silica plates and spots were visualized under UV light.
Chromatography on silica gel was carried out on Kieselgel
(200e300 mesh).
1H NMR spectra were determined on an Advance Bruker 400 M
(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-
2950TGA (TA co) with a heating rate of 10 ꢀC minꢁ1 under the
protection of nitrogen. Cyclic voltammetric data were measured on
a BAS CV-50W voltammetric analyser using a conventional three-
electrode cell with Pt metal as the working electrode, Pt gauze as
the counter-electrode, and Ag/AgNO3 as the reference electrode at a
scan rate of 100 mV sꢁ1. The 0.1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF) in acetonitrile is the electrolyte. The
melting points were obtained by TA DSC Q10 under N2 at a heating
for comparison. We developed
a novel donor modification
approach by attaching an N-(4-dibutylaminophenyl) substituent
on the donor part of the chromophore. By addition of another
phenyl ring on the aromatic donor, the chromophore could ex-
hibits a higher Td [29]. In order to offset the reduced hyper-
polarizability due to delocalization of the nitrogen lone pair into
the phenyl rings which do not connect with the bridge, an
additional electron-rich nitrogen atom was introduced onto the
benzene ring of the donor. In the meantime, the ring-fused
amino-phenyl structures in the tetrahydroquinolinyl donor
facilitate the overlap of the p-orbital of the amino atom with the
phenyl ring, thus providing a good mechanism to gradually in-
crease the electron-donating strength [31,32]. In a properly
designed system, significant enhancement in the mb of the NLO
chromophores can be achieved. All these modifications of
rate of 10 ꢀC minꢁ1
.
Chart 1. Chemical structure for chromophores FTC, TPA and NT.

140
F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
2.2. Syntheses
3.16e2.98 (m, 1H), 2.17 (s, 3H),1.71e1.55 (m, 5H),1.49e1.41 (m, 4H),
1.41e1.35 (m, 4H), 1.28 (s, 3H), 1.07 (s, 3H), 0.99 (t, J ¼ 7.3 Hz, 6H).
13C NMR (101 MHz, CDCl3)
182.05, 154.95, 147.44, 147.02, 139.65,
137.54, 135.35, 132.80, 131.43, 131.27, 129.66, 124.54, 124.07, 123.73,
121.62, 115.93, 115.65, 112.08, 111.73, 54.66, 50.74, 46.33, 30.63,
29.34, 26.99, 25.74, 20.47, 20.28, 19.63, 13.94. Anal. Calcd (%) for
2.2.1. Synthesis of 1-(4-(dibutylamino)phenyl)-2,2,4,7-tetramethyl-
1,2,3,4-tetrahydroquinoline-6-carbaldehyde (compound 3a)
d
DMF (1.71 g, 23.40 mmol) was added to freshly distilled POCl3
(3.59 g, 23.40 mmol) under an atmosphere of N2 nitrogen at 0 ꢀC,
and the resultant solution was stirred until its complete conversion
into a glassy solid. After the addition of 2a (4.60 g, 11.70 mmol) in
DMF (20 mL) dropwise, the mixture was stirred at room temper-
ature overnight, then poured into a saturation aqueous solution of
sodium bicarbonate (300 mL). After 2 h stirring at room tempera-
ture, the mixture extracted with chloroform (5 ꢂ 30 mL), and the
organic fractions were collected and dried over anhydrous MgSO4.
The crude product was purified through a silica gel chromatog-
raphy eluting with (Acetone: Hexane ¼ 1:5) to afford a yellow oil in
77.6% yield (3.82 g, 9.08 mmol). MS, m/z: 420.43 (Mþ). 1H NMR
C34H44N2OS: C, 77.23; H, 8.39; N, 5.30; found: C, 77.25; H, 8.41; N,
5.26.
2.2.4. Synthesis of 2-(3-cyano-4-((E)-2-(5-((E)-2-(1-(4-
(dibutylamino)phenyl)-2,2,4,7-tetramethyl-1,2,3,4-
tetrahydroquinolin-6-yl)vinyl)thiophen-2-yl)vinyl)-5,5-
dimethylfuran-2(5H)-ylidene)malononitrile (chromophore NT)
To a solution of 5a (0.26 g, 0.50 mmol) and the TCF acceptor
(0.12 g, 0.60 mmol) in MeOH (60 mL) was added several drops of
triethyl amine. The reaction was allowed to stir at 78 ꢀC for 5 h. The
reaction mixture was cooled and green crystal precipitation was
facilitated. After removal of the solvent under reduced pressure, the
crude product was purified by silica chromatography, eluting with
(AcOEt: Hexane ¼ 1:5) to give chromophore NT as a green solid in
45.1% yield (0.16 g, 0.23 mmol). M.p.: 244.2 ꢀC. MS, m/z: 710.3874
(400 MHz, acetone-d6) d 9.80 (s, 1H), 7.50 (s, 1H), 6.82 (m, 2H), 6.66
(m, 2H), 5.72 (s, 1H), 3.27e3.23 (m, 4H), 2.96 (m, 1H), 2.18 (s, 3H),
1.65e1.44 (m, 5H), 1.37e1.22 (m, 8H), 1.19 (s, 3H), 0.97 (s, 3H), 0.87
(t, J ¼ 7.4 Hz, 6H). 13C NMR (101 MHz, acetone-d6)
d 190.49, 151.92,
148.72, 140.21, 133.68, 132.68, 132.26, 130.11, 124.48, 117.22, 113.48,
113.20, 56.36, 51.70, 47.05, 31.26, 27.93, 26.80, 21.27, 20.96, 20.26,
14.67. Anal. Calcd (%) for C28H40N2O: C, 79.95; H, 9.59; N, 6.66;
found: C, 80.04; H, 9.52; N, 6.73.
(Mþ). 1H NMR (400 MHz, acetone-d6)
d
8.14 (d, J ¼ 15.8 Hz,1H), 7.66
(d, J ¼ 4.0 Hz, 1H), 7.59 (s, 1H), 7.40 (d, J ¼ 15.8 Hz, 1H), 7.25 (m, 1H),
7.23 (d, J ¼ 4.0 Hz, 1H), 6.94 (dd, J ¼ 8.9, 7.0 Hz, 2H), 6.86e6.71 (m,
3H, CH), 5.83 (s, 1H), 3.43e3.28 (m, 4H), 3.18e2.99 (m, 1H), 2.14 (s,
3H), 1.87 (s, 6H) 1.72e1.56 (m, 5H), 1.47e1.34 (m, 8H), 1.20 (s, 3H),
1.07 (s, 3H), 0.97 (t, J ¼ 7.3 Hz, 6H). 13C NMR (101 MHz, CDCl3)
2.2.2. Synthesis of (E)-N,N-dibutyl-4-(2,2,4,7-tetramethyl-6-(2-
(thiophen-2-yl)vinyl)-3,4-dihydroquinolin-1(2H)-yl)aniline
(compound 4a)
d
183.34, 177.85, 157.40, 155.60,149.22, 148.60, 141.10, 139.40,
NaH (0.68 g, 23.80 mmol) was added to a stirred solution of
compound 3a (1.00 g, 2.38 mmol) and 3d (2.09 g, 4.76 mmol) in THF
(100 mL) under nitrogen. The solution was stirred for 24 h and then
poured into water. The organic phase was extracted by AcOEt,
washed with brine and dried over MgSO4. After removal of the
solvent under reduced pressure, the crude product was purified by
silica chromatography, eluting with (Acetone: Hexane ¼ 1:20) to
give compound 4a as yellow oil in 86.1% yield (1.03 g, 2.05 mmol).
138.83, 136.66, 132.95, 132.34, 130.92, 130.00, 128.35, 125.68,
125.40, 123.20, 119.73, 117.48, 117.09, 113.72, 113.48, 113.14, 112.20,
99.27, 66.26, 55.92, 51.75, 47.58, 31.72, 28.29, 26.59, 26.43, 23.72,
21.29, 20.38, 20.22, 14.65, 14.35. Anal. Calcd (%) for C45H51N5OS: C,
76.13; H, 7.24; N, 9.86; found: C, 76.09; H, 7.21; N, 9.87.
2.2.5. Synthesis of (E)-N,N-diethyl-4-(2-(thiophen-2-yl)vinyl)
aniline (compound 3b)
MS, m/z: 500.36 (Mþ). 1H NMR (400 MHz, CDCl3)
d
7.43e7.32 (m,
The procedure for compound 4a was followed to prepare 3b
from 2b to 3d as yellow oil in 79.3% yield (0.67 g, 2.59 mmol). MS,
1H), 7.14 (d, J ¼ 15.2 Hz, 1H), 7.01 (d, J ¼ 5.1 Hz, 1H), 6.98 (d,
J ¼ 3.2 Hz, 1H), 6.93e6.86 (m, 3H), 6.85 (m, 1H), 6.58 (d, J ¼ 8.3 Hz,
2H), 5.84 (d, J ¼ 15.2 Hz, 1H), 3.25e3.19 (m, 4H), 3.03e2.96 (m, 1H),
2.08 (s, 3H), 1.64e1.46 (m, 5H), 1.40e1.27 (m, 8H), 1.20 (s, 3H), 0.99
m/z: 257.43 (Mþ). 1H NMR (400 MHz, CDCl3)
d
7.33 (d, J ¼ 8.9 Hz,
2H),7.11e7.07 (m, 1H), 7.01 (d, J ¼ 16.1 Hz, 1H), 6.96 (d, J ¼ 3.4 Hz),
6.85 (d, J ¼ 16.1 Hz, 1H), 6.64 (d, J ¼ 8.9 Hz, 2H), 6.47 (d, J ¼ 3.4 Hz,
1H), 3.41e3.31 (m, 4H),1.17 (t, J ¼ 7.1 Hz, 6H).13C NMR (100 MHz,
(s, 3H), 0.92 (t, J ¼ 7.2 Hz, 6H). 13C NMR (101 MHz, CDCl3)
d 147.00,
146.41, 144.60, 134.25, 133.15, 131.23, 128.63, 128.51, 127.39, 126.87,
126.00, 124.10, 123.39, 122.63, 117.82, 115.64, 112.19, 111.83, 54.32,
50.75, 46.71, 30.88, 29.43, 27.13, 25.63, 20.62, 20.35, 19.79, 13.97.
Anal. Calcd (%) for C33H44N2S: C, 79.15; H, 8.86; N, 5.59; found: C,
79.08; H, 8.91; N, 5.62.
CDCl3) d 148.10, 144.72, 130.66,129.48, 128.22, 127.95, 127.03,
125.05, 124.67, 123.34, 117.69,112.45, 112.06, 44.88, 13.21. Anal.
Calcd (%) for C16H19NS: C, 74.66; H, 7.44; N, 5.44; found: C, 74.71; H,
7.46; N, 5.41.
2.2.6. Synthesis of (E)-5-(4-(diethylamino)styryl)thiophene-2-
carbaldehyde (compound 4b)
2.2.3. Synthesis of (E)-5-(2-(1-(4-(dibutylamino)phenyl)-2,2,4,7-
tetramethyl-1,2,3,4-tetrahydroquinolin-6-yl)vinyl)thiophene-2-
carbaldehyde (compound 5a)
The procedure for compound 5a was followed to prepare 4b
from 3b as a red solid in 85.9% yield (0.63 g, 2.22 mmol). MS, m/z:
Under a N2 atmosphere, 4a (0.59 g, 2.00 mmol) was dissolved in
150 mL freshly distilled THF and cooled to ꢁ78 ꢀC. Approximately 2
equivalents of n-BuLi in hexanes (16 mL, 4 mmol) was added drop
wise over 20 min. Reaction continued at ꢁ78 ꢀC for 1 h at which
time DMF (0.29 g, 4.00 mmol) was added over 1 min. The reaction
was allowed to reach RT while the solution stirred for 1 h. The
organic phase was extracted by AcOEt, washed with brine and dried
over MgSO4. After removal of the solvent under reduced pressure,
the crude product was purified by silica chromatography, eluting
with (Acetone: Hexane ¼ 1:5) to give compound 5a as a red oil with
76.9% yield (0.50 g, 1.54 mmol). MS, m/z: 528.20 (Mþ). 1H NMR
285.47 (Mþ). 1H NMR (400 MHz, CDCl3)
d 9.84 (s, 1H), 7.66 (d,
J ¼ 3.9 Hz, 1H), 7.40 (d, J ¼ 8.6 Hz, 2H), 7.12 (d, J ¼ 15.9 Hz, 1H), 7.07
(d, J ¼ 3.9 Hz, 1H), 7.00 (d, J ¼ 15.9 Hz, 1H), 6.68 (d, J ¼ 8.6 Hz, 2H),
3.43 (m, 4H), 1.22 (t, J ¼ 7.0 Hz, 6H). 13C NMR (101 MHz, CDCl3)
d
181.12, 153.45, 147.45, 139.24, 136.25, 132.73, 129.24, 127.63,
123.73, 122.21, 114.71, 110.74, 43.46, 11.64. Anal. Calcd (%) for
17H19NOS: C, 71.54; H, 6.71; N, 4.91; found: C, 71.58; H, 6.74; N,
C
4.87.
2.2.7. Synthesis of 2-(3-cyano-4-((E)-2-(5-((E)-4-(diethylamino)
styryl)thiophen-2-yl)vinyl)-5,5-dimethylfuran-2(5H)-ylidene)
malononitrile (chromophore FTC)
The procedure for chromophore NT was followed to prepare
chromophore FTC from 4b as a green solid in 40.1% yield (0.41 g,
(400 MHz, CDCl3)
d
9.80 (s, 1H), 7.62 (d, J ¼ 3.9 Hz, 1H), 7.46 (s, 1H),
7.33 (d, J ¼ 15.8 Hz, 1H), 7.04 (d, J ¼ 3.9 Hz, 1H), 6.98e6.86 (m, 3H,
CH), 6.65 (d, J ¼ 8.6 Hz, 2H), 5.87 (s, 1H), 3.33e3.25 (m, 4H),

F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
141
0.89 mmol). M.p.: 191.4 ꢀC. MS, m/z: 467.1923 (Mþ). 1H NMR
(400 MHz, CDCl3)
7.37 (d, J ¼ 3.9 Hz, 1H), 7.09 (d, J ¼ 15.7 Hz, 1H), 7.01 (d, J ¼ 3.9 Hz,
1H), 6.68 (d, J ¼ 8.6 Hz, 2H), 6.57 (d, J ¼ 15.2 Hz, 1H), 3.52e3.29 (m,
4H), 1.73 (s, 6H), 1.28e1.14 (m, 6H).13C NMR (100 MHz, CDCl3)
and 13C NMR spectroscopy and HRMS. These chromophores possess
good solubility in common organic solvents, such as dichloro-
methane, chloromethane and acetone.
d
7.77 (d, J ¼ 15.7 Hz, 1H),7.38 (d, J ¼ 8.6 Hz, 2H),
3.2. Thermal stability
d
182.03, 172.86, 153.58d148.02, 142.13, 140.08, 139.33, 137.39,
129.07, 111.61, 110.91,110.21, 108.85, 104.96, 99.53, 96.90, 44.81,
44.55, 24.42, 12.58. Anal. Calcd (%) for C28H26N4OS: C, 72.07; H,
5.62; N, 12.01; found: C, 72.09; H, 5.65; N, 11.99.
The NLO chromophores must be thermally stable in order to
withstand the poling process and subsequent processing of chro-
mophore/polymer materials. The thermal stability of the chromo-
phores was investigated using thermo gravimetric analysis (TGA) as
shown in Fig. 1 and Table 1. All the chromophores exhibited good
thermal stabilities with the decomposition temperatures (Td)
higher than 200 ꢀC (240 ꢀCe298 ꢀC). The chromophore TPA had the
maximum decomposition temperature (Td ¼ 298 ꢀC) which is easy
to understand because the triaryl amine donors display much
greater thermal stability than the alkyl analogues [26]. More
interestingly, by addition of another phenyl ring on the aromatic
donor, chromophore NT also exhibits a much higher (Td ¼ 282 ꢀC)
than that of dialkylamino analogue chromophore FTC (Td ¼ 240 ꢀC),
a commonly employed chromophore that has a 4-(dialkylamino)
phenyl donor. The enhanced thermal stability of chromophore NT
might be ascribed to the rigidity of the phenyl group [29]. The new
chromophore NT exhibits the similar high thermal stability as the
triarylamine-based chromophore, which seems to be the promising
candidate for the active EO materials in a practical device.
2.2.8. Synthesis of (E)-N,N-diphenyl-4-(2-(thiophen-2-yl)vinyl)
aniline (compound 3c)
The procedure for compound 4a was followed to prepare 3c
from 2c and 3d as yellow oil in 69.2% yield (0.81 g, 2.29 mmol). MS,
m/z: 353.57 (Mþ). 1H NMR (400 MHz, CDCl3)
d
7.32 (d, J ¼ 8.5 Hz,
2H), 7.25e7.23 (m, 4H), 7.16e7.23 (m, 5H), 7.13e7.08 (m, 5H),
7.02e7.08 (m, 2H), 6.87 (d, J ¼ 16.2 Hz, 1H).13C NMR (100 MHz,
CDCl3)
d 120.18, 123.06, 123.52, 123.84,124.52, 125.48, 127.14,
127.55, 127.92, 129.25, 131.06, 143.27,147.20, 147.52. Anal. Calcd (%)
for C24H19NS: C, 81.55; H, 5.42; N, 3.96; found: C, 81.59; H, 5.39; N,
3.93.
2.2.9. Synthesis of (E)-5-(4-(diphenylamino)styryl)thiophene-2-
carbaldehyde (compound 4c)
The procedure for compound 5a was followed to prepare 4c
from 3c as red solid in 80.8% yield (0.71 g, 1.85 mmol). MS, m/z:
381.47 (Mþ). 1H NMR (400 MHz, CDCl3)
d
9.82 (s, 1H), 7.63 (d,
3.3. Optical properties
J ¼ 3.9 Hz, 1H), 7.34 (d, J ¼ 8.5 Hz, 2H), 7.30e7.24 (m, 4H), 7.11 (d,
J ¼ 7.9 Hz, 4H), 7.10e7.04 (m, 5H), 7.02 (d, J ¼ 8.6 Hz, 2H). 13C NMR
To explore the different charge-transfer (CT) absorption prop-
erties of each chromophore, UVeVis absorption spectra of the three
chromophores were measured in a series of aprotic solvents with
different polarity, as shown in Figs. 2 and 3. The spectra data are
summarized in Table 1. It may be seen from Fig. 2 that chromo-
phores TPA, FTC and NT show the maximum absorption (lmax) from
620 nm to 730 nm in chloroform which shows a big difference. The
chromophore TPA was blue-shifted (Dl ¼ 55 nm), in its absorption
spectra, compared to the alkyl chromophore FTC, probably due to
delocalization of the nitrogen lone pair into the other phenyl rings
of the donor. However, chromophore NT showed a maximum ab-
sorption (lmax) of 730 nm in CHCl3. This value is much larger
compared to the traditional FTC (lmax of 675 nm in CHCl3) with an
(100 MHz, CDCl3)
d 177.93, 148.86, 144.33, 142.98, 136.86, 132.86,
128.31, 125.02, 123.66, 121.56, 120.74, 119.35, 118.44, 114.58, 114.55.
Anal. Calcd (%) for C25H19NOS: C, 78.71; H, 5.02; N, 3.67; found: C,
78.76; H, 5.06; N, 3.61.
2.2.10. Synthesis of 2-(3-cyano-4-((E)-2-(5-((E)-4-(diphenylamino)
styryl)thiophen-2-yl)vinyl)-5,5-dimethylfuran-2(5H)-ylidene)
malononitrile (chromophore TPA)
The procedure for chromophore NT was followed to prepare
chromophore TPA from 4c as a purple solid in 37.9% yield (0.39 g,
0.70 mmol). M.p.: 248.3 ꢀC. MS, m/z: 563.1897 (Mþ). 1H NMR
(400 MHz, CDCl3)
d
7.78 (d, J ¼ 15.7 Hz, 1H), 7.41e7.33 (m, 3H), 7.29
(dd, J ¼ 14.6, 6.8 Hz, 5H), 7.10 (m, 8H), 7.03 (d, J ¼ 8.0 Hz, 2H), 6.61
aniline donor and the same p-conjugated bridge and acceptor.
(d, J ¼ 15.7 Hz, 1H),1.76 (s, 6H).13C NMR (100 MHz, CDCl3)
d
171.24,
Except for the stronger donor tetrahydroquinolinyl group, the
additional electron-rich nitrogen atom provides extra donating
strength to the donor part, shifting the CT absorption band of the
chromophore to lower energy.
Besides, the solvatochromic behaviour was also an important
aspect to investigate the polarity of chromophores. It was found
that chromophore NT showed very large bathochromic shifts of
69 nm, on moving from dioxane to chloroform. Chromophores TPA
and FTC showed a slightly smaller bathochromic shift of 43 nm and
54 nm respectively, from dioxane to chloroform. This confirms that
chromophore NT is more easily polarizable than chromophores TPA
and FTC [36].
168.56, 148.52, 144.68,142.78, 134.87, 133.74, 132.62, 129.32, 125.17,
123.63, 123.29,120.95, 119.46, 117.91, 114.11, 108.13, 107.58, 106.83,
106.40, 92.72, 25.28, 22.19. Anal. Calcd (%) for C36H26N4OS: C, 76.84;
H, 4.66; N, 9.96; found: C, 76.87; H, 4.69; N, 9.91.
3. Results and discussion
3.1. Synthesis and characterization of chromophores
Schemes 1 and 2 show the synthetic approach to the chromo-
phores FTC, TPA and NT. Starting from the amine donor in-
termediates compound 1a, 1c or aldehydes 2b, chromophores NT,
TPA and FTC were synthesized in good overall yields through
simple four or five step reactions: The free N-H group on the donors
was N-arylated by the aryl group to improve stability. Treatment of
compound 2a or 1c with POCl3 and DMF gave the aldehydes 3a or
2c. After introduction of the thiophene bridge by witting reaction,
compounds 4a or 3bec were prepared in a high yield. Treatment of
compound 4a or 3bec with n-BuLi and DMF gave aldehyde 5a or
4bec, the final condensations with the TCF acceptor give chromo-
phore TPA as a purple solid and chromophores FTC, NT as green
solids. All of the chromophores were fully characterized by 1H NMR
3.4. Redox properties and theoretical calculations
In order to determine the electrochemical properties of the
chromophores, cyclicvoltammetry (CV) measurements were con-
ducted in degassed anhydrous acetonitrile solutions containing
0.1 mol/L tetrabutylammonium hexafluorophosphate (TBAPF) as
the supporting electrolyte. The relative data of 1 ꢂ 10ꢁ4 mol/L
chromophores were recorded, as shown in Table 2 and Fig. 4. The
HOMO and LUMO levels of the three chromophores can be calcu-
lated from their corresponding oxidation and reduction potentials

142
F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
Scheme 1. Chemical structures and synthetic routes for chromophores NT.
Scheme 2. Chemical structures and synthetic routes for chromophores FTC and TPA.
[37]. The difference between these two values provides the HOMO-
LUMO energy difference E (CV). The calculation results are sum-
marized in Table 2. The energy gaps between the HOMO and LUMO
energy for chromophores FTC, TPA and NT were 1.43, 1.72 and
respectively. The gradual decrease of the oxidation potentials were
dictated by the progressively increased strength of the donor
group. The HOMO energy levels are systematically lowered with an
increase in the donor strength which influences the HOMO to a
greater extent than the LUMO level. From the lmax, the cut-off
wavelength (lcut-off) and the energy gap between the HOMO and
LUMO energy of the three chromophores, we could assign the order
of electron donor strength under the same accepting strength from
TCF as follows: TPA < FTC < NT.
To understand the ground-state polarization and microscopic
NLO properties of the designed chromophores, the DFT calculations
were carried out at the B3LYP level by employing the split valence
6-31G basis set using the Gaussian 09 program package [39,40]. The
molecular quadratic hyperpolarizability (mb) were carried out at the
cam-B3LYP level by employing the split valence 6-31G basis set
D
1.08 eV, respectively. As DE is reduced, resulting in a bathochromic
shift of lmax within the series of compounds. These results corre-
spond with the conclusion of UVeVis spectra analysis. The HOMO-
LUMO gap of chromophore NT is lower than those of chromophores
FTC and TPA, indicate that chromophore NT should exhibit better
charge-transfer (ICT) ability and NLO properties than FTC and TPA
[38].
As shown in Fig. 4, it can be concluded that the LUMO energy
levels are approximately the same for the three chromophores.
Conversely, chromophores FTC, TPA and NT showed oxidation po-
tential versus ferrocene/ferrocenium at 0.59 V, 0.32 V and 0.07 V,

F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
143
Fig. 1. TGA curves of chromophores FTC, TPA and NT with a heating rate of 10 ꢀC minꢁ1
in nitrogen atmosphere.
Table 1
Thermal and optical properties data of the chromophores.
Cmpd
Td (ꢀC)
lamax (nm)
lbmax (nm)
Dlc (nm)
FTC
TPA
NT
240
298
282
675
620
730
621
577
661
54
43
69
almax was measured in chloroform.
blmax was measured in dioxane.
a
b
cDl was the difference between lmax and lmax
.
Fig. 2. UVeVis absorption spectra of chromophores FTC, TPA and NT in chloroform.
Fig. 3. UVeVis absorption spectra of chromophores FTC, TPA and NT in five kinds of
aprotic solvents with varying dielectric constants.
using the Gaussian 09 program package [40]. All molecules were
assumed to be in trans-configurations [41,42]. The geometrically
optimized structures of the molecules were investigated by calcu-
lating the dihedral angles of the donor moieties as shown in Figs. 5
and 6. The geometries of the three chromophores are totally
different. The three benzene rings of the chromophore TPA are not
in the same plane, the dihedral angle between the other two
benzene rings on the N atom of the TPA moiety and the average
plane of the average plane of the chromophores was ca. 66.40ꢀ and
67.39ꢀ, respectively, as shown in picture 5. The twisted structure
may somehow attenuate the delocalization of the nitrogen lone
pair into two phenyl rings. As for chromophore NT, the N-(4-
Table 2
Summary of DFT, electro-chemical data and EO coefficients of chromophores.
E (CV)a (eV)
Eox (V)
Ered (V)
mb (10ꢁ48 esu)
r33 (pm/V)
b
c
Cmpd
D
FTC
TPA
NT
1.43
1.72
1.08
0.321
0.590
0.068
ꢁ1.109
ꢁ1.136
ꢁ1.015
13876.2
10884.9
22939.8
39
19
126
a
DE (CV) was calculated from their corresponding oxidation and reduction
potentials.
b
Oxidation potentials referenced to ferrocene/ferrocenium standard.
Reduction potentials referenced to ferrocene/ferrocenium standard.
c

144
F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
The molecular quadratic hyperpolarizability (mb) of the chro-
mophores obtained from DFT calculations are calculated as shown
in Table 2. When used carefully and consistently, this method of
DFT has been shown to give relatively consistent descriptions of
molecular quadratic hyperpolarizability for a number of similar
chromophores [37,44,45]. The more powerful the donor, the easier
the charge separation, leading to larger mb values in these chro-
mophores. The influence of donor groups on the absorption charge-
transfer band (lmax: NT > FTC > TPA) totally reflects the result of mb
values (mb: NT > FTC > TPA). The trend of increasing lmax is in good
agreement with the trend of increasing mb value. The mb value of
chromophore TPA was smaller than that of chromophore FTC, due
to delocalization of the nitrogen lone pair into the other two phenyl
rings. The mb value of chromophore NT was much larger than that of
chromophores TPA and FTC due to a narrower energy gap between
HOMO and LUMO. Although the delocalization of the nitrogen lone
pair into the other phenyl rings of chromophore NT may result in
reduced mb, with the additional electron-rich nitrogen heteroatoms
into the benzene ring moiety of the donor, the chromophore NT
showed a much larger mb. In the meantime, the ring-fused ami-
nophenyl structures in tetrahydroquinolinyl donor facilitate the
overlap of the p-orbital of the amino atom with the phenyl ring,
thus providing a good mechanism to gradually increase the
electron-donating strength which also explains for the larger mb
.
3.5. Electro-optic performance
In order to investigate the translation of the microscopic
hyperpolarizability into macroscopic EO response, the polymer
films doped with 25 wt% chromophores into amorphous poly-
carbonate (APC) were prepared 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 at 80 ꢀC in a vacuum oven over-
night. 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 of poled films were measured by TengeMan simple
reflection method at a wavelength of 1310 nm using carefully
selected thin ITO electrode with low reflectivity and good trans-
parency in order to minimize the contribution from multiple re-
flections [39,46].
The r33 values were calculated by the following equation [47]:
À
Á
3=2
n2 ꢁ sin2 q
3lIm
1
r33
¼
4pVmIcn2
À
Á
n2ꢁ2 sin2 q sin2 q
where r33 is the EO coefficient of the poled polymer,
l is the optical
wavelength, h is the incidence angle, Ic is the output beam intensity,
Im is the amplitude of the modulation, Vm is the modulating
voltage, and n is the refractive indices of the polymer films.
When the intermolecular electrostatic interactions are neglec-
ted, the electro-optic coefficient (r33) should increase linearly with
chromophore density, dipole moment, first hyperpolarizability and
the strength of electric poling field. But chromophores with a large
dipole moment generate intermolecular static electric field dipo-
leedipole interaction, which leads to the unfavourable antiparallel
packing of chromophores. So the number of truly oriented chro-
mophore is small. So, to achieve high EO coefficient, the chromo-
Fig. 4. Cyclic voltammograms of chromophores FTC, TPA and NT recorded in CH3CN
solutions containing 0.1
M Bu4NPF6 supporting electrolyte at a scan rate of
100 mV sꢁ1
.
dibutylaminophenyl) substituent located on the nitrogen atom
made a large dihedral angle of about 90ꢀ with average plane of the
chromophore. This large dihedral angle caused large steric hin-
drance that suppressed aggregations among molecules.
The frontier molecular orbitals are often used to obtain infor-
mation about the optical and electrical properties of molecules [43].
Fig. 5 depicts the electron density distribution of the HOMO and
LUMO structures which indicated the density of the ground and
excited state electron is asymmetrical along the dipolar axis of the
chromophores.
phore should have large mb
, meanwhile the dipoleedipole
interactions of chromophores should be reduced. In molecular
optimization, introducing the huge steric hindrance group to
isolate chromophores is the most popular and easy way to atten-
uate the dipoleedipole interactions of chromophores [48].
The poled films of TPA/APC, FTC/APC and NT/APC afforded r33

F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
145
Fig. 5. Optimized structures and frontier molecular orbitals HOMO and LUMO of chromophores FTC, TPA and NT calculated by DFT calculations at the hybrid B3LYP level by
employing the split valence 6e31 g basis set.
Fig. 6. The optimized structures of chromophores FTC, TPA and NT.
values of 19, 39 and 126 pmVꢁ1 at 1310 nm, respectively. In these
chromophores, the r33 values were gradually improved from
19 pm/V to 126 pm/V, illustrating that the increases donor strength
of the chromophores significantly increase their macroscopic EO
activities. The stronger the electron-donating power of the donor
dibutylaminophenyl) substituent located on the nitrogen atom at
the donor which almost perpendicular to the average plane of the
chromophore was intended to prevent the strong electrostatic in-
teractions between chromophores. The undesired antiparallel
packing between chromophores is expected to decrease during the
poling process [49]. The large r33value of chromophore NT may be
attributed to the strong donor ability and the isolated group in the
donor moiety. Although the mb value of chromophore NT is 65%
larger than that of chromophore FTC, the r33 value is three times
higher due to a better isolation effect. Moreover, the chromophore
NT showed two times higher mb and more than six times higher r33
value than that of chromophore TPA. So the chromophore NT has
not only a larger mb but also advantages in translating mb into the
macroscopic EO activity due to additional donor nitrogen hetero-
atom and isolation effect of the N-(4-dibutylaminophenyl)
substituent.
leads to higher lmax, mb, and r33 value [49]. Besides, solvatochromic
behaviour indicated that chromophore NT is more easily polariz-
able than chromophores FTC and TPA. Therefore, in corona poling,
chromophore NT may obtain orientation more easily in contrast
with chromophores FTC and TPA. Except for larger mb, the much
larger r33 value of film NT/APC over films TPA/APC and FTC/APC may
also be explained by favourable poling efficiency which related to
chromophore property, chromophore-host compatibility and so on.
The long alkyl chain in chromophores NTcan somehow provide site
isolation to decrease the strong electrostatic interactions among
chromophores. Besides, as shown in Figs. 5 and 6, the N-(4-

146
F. Liu et al. / Dyes and Pigments 130 (2016) 138e147
phenothiazine as electron donors for second-order nonlinear optical chro-
4. Conclusion
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poled polymers. The doped film containing the chromophore NT
showed an r33 value of 126 pm/V at the concentration of 25 wt%
which is much higher than the EO activity of the chromophore FTC
(39 pm/V) and six times higher than that of the traditional chro-
mophore TPA (19 pm/V). Besides, the chromophore NT showed
excellent thermal stability with an onset decomposition tempera-
ture higher than 280 ꢀC. We believe that the new chromophore can
be used in exploring high-performance organic E-O and photore-
fractive materials where both thermal stability and optical
nonlinearity are of equal importance.
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