Enhanced electro-optic activity of two novel bichromophores which are synthesized by Cu(I) catalyzed click-reaction

Article abstract of DOI:10.1016/j.dyepig.2017.01.009

Two novel bichromophores YL1 and YL2 based on traditional aniline chromophore YL were synthesized by Cu(I) catalyzed click-reaction and systematically investigated in this paper. The UV–Vis, electrochemical property, thermal stability and EO activity of t

Full text of DOI:10.1016/j.dyepig.2017.01.009

Dyes and Pigments 139 (2017) 756e763
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journal homepage: www.elsevier.com/locate/dyepig
Enhanced electro-optic activity of two novel bichromophores which
are synthesized by Cu(I) catalyzed click-reaction
,
,
,
,
,
Yanling He a b, Lu Chen a b, Fenggang Liu a b, Hua Zhang a b, Fuyang Huo a b, Zhuo Chen a,
Jialei Liu a, Zhen Zhen a, Xinhou Liu a, Shuhui Bo 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:
Two novel bichromophores YL1 and YL2 based on traditional aniline chromophore YL were synthesized
by Cu(I) catalyzed click-reaction and systematically investigated in this paper. The UVeVis, electro-
chemical property, thermal stability and EO activity of these chromophores were systematically studied
and discussed. These two bichromophores showed good thermal stability. Then nonlinear optical
polymer films were fabricated by doping chromophores into polymethylmethacrylate (PMMA). The
doped films containing bichromophores YL1 and YL2 showed r33 values of 20 and 17 p.m./V at con-
centration of 25 wt% at 1310 nm. These values were respectively 2.85 times and 2.43 times of the EO
activity of chromophore YL (7 p.m./V). High r33 values indicated that the new structure of bichromo-
phores YL1 and YL2 can reduce intermolecular electrostatic interactions, thus enhancing macroscopic EO
activity. These properties suggested the potential use of the new bichromophores in nonlinear optical
materials.
Received 6 December 2016
Received in revised form
4 January 2017
Accepted 5 January 2017
Available online 8 January 2017
Keywords:
Nonlinear optical materials
Bichromophore
Second-order
Electro-optic
Click-reaction
© 2017 Elsevier Ltd. All rights reserved.
1. Introduction
Rational structure designs of bichromophores structure are
considered as an effective solution along with much research [17].
Organic electro-optic (EO) materials have attracted considerable
attention over past three decades because of their potential appli-
cations in high-speed and broadband information technology
[1e5]. The most highly studied material type in this field is called
poled guest-host polymeric material in which EO chromophores
are dispersed in a polymer matrix [6,7]. In recent years, many
The reason is that the core or other groups in bichromophores can
act as isolation groups to weak interchromphore dipole-dipole in-
teractions. And bichromophores also exhibit high physical stability,
high chemical stability, and good optical transparency. Generally,
bichromophores and trichromophores were mostly synthesized by
incorporating chromophores with ester groups in past reports
[18e21]. However, this single design method sometimes could not
meet further processing requirements of all kinds of chromophores.
We need some other high efficient methods to prepare bichromo-
phores and trichromophores.
The Cu(I) catalyzed click-reaction mostly was used by synthe-
sizing the azo-type chromophores in organic NLO materials field
and this reaction usually has very high yield (about 90%) [22e24].
The triazole group in chromophores can act as isolation groups for
enhancing macroscopic NLO effect. Moreover, the synthesis con-
dition of click-reaction is moderate, which is an important advan-
tage because most chromophores are unstable in acid or alkali.
Moreover, the chromophores containing triazole groups also
exhibit good chemical and thermal stability [25].
chromophores with large hyperpolarizability (
successfully and studied [8e13]. However, the large microscopic
hyperpolarizability ( ) of chromophores usually could not be
b) were synthesized
b
translated into high macroscopic nonlinear optical (NLO) activity
(electro-optic coefficient r33) of polymer materials effectively due
to very strong interchromphore dipole-dipole interactions. These
interchromphore dipole-dipole interactions could lead to unfav-
ourable anti-parallel packing of the chromophoric units [14e16].
Thus, how to suppress the dipole interactions among chromo-
phores has become the focus of researchers.
* Corresponding author.
E-mail address: boshuhui@mail.ipc.ac.cn (S. Bo).
In this work, we utilized Cu(I) catalyzed click-reaction to
http://dx.doi.org/10.1016/j.dyepig.2017.01.009
0143-7208/© 2017 Elsevier Ltd. All rights reserved.

Y. He et al. / Dyes and Pigments 139 (2017) 756e763
757
produce EO bichromophores YL1 and YL2 with traditional aniline
chromophore YL as branch and phenyl or pentafluorophenyl as core
(Chart 1). The traditional aniline chromophore YL is easily syn-
thesized and can represent aniline-type chromophores well. EO
chromophores containing pentafluorophenyl groups have been
reported in the literature and shown to exhibit enhancements in
solubility and NLO coefficient (r33) [26,27]. At the same time, the
bichromophore YL2 containing phenyl group was designed as
comparative item. In this paper, the synthesis, UVeVis, sol-
vatochromic, electrochemical property, thermal stability and EO
activity of these chromophores were systematically studied and
compared to illustrate architectural influences on rational NLO
bichromophore designs.
tetrabutylammoniumperchlorate as the supporting electrolyte. The
ferrocene/ferrocenium (Fc/Fcþ) couple was used as an internal
reference. The melting points were obtained by TA DSC Q10 under
N2 at a heating rate of 10 ꢀC$minꢁ1
.
2.2. Syntheses
2.2.1. Synthesis of compound 1
A solution of 4-[(2-hydroxyethyl)(methyl)amino]-benzaldehyde
(1 g, 5.6 mmol) and Et3N (1.15 mL, 7.3 mmol) in anhydrous CH2Cl2
(50 mL) was stirred for 10 min at room temperature. para-
Toluenesulfonyl chloride (1.6 g, 8.4 mmol) was added to the reac-
tion mixture, which was stirred for 22 h. The reaction mixture was
extracted with a saturated solution of NaHCO3. The organic phase
was dried over anhydrous MgSO4. After filtration and removal of
solvent under vacuum, the crude product was purified by silica
chromatography, eluting with (AcOEt: Hexane ¼ 1: 1) to give
compound 1 as a white solid in 80% yield (1.49 g, 4.48 mmol).
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), triethylamine (N(C2H5)3), tetrahydrofuran (THF) and
acetone were distilled over calcium hydride and stored over mo-
lecular sieves (pore size 3 Å). 2-Dicyanomethylene-3-cyano-4-
methyl-2,5-dihydrofuran(TCF) acceptor was prepared according
to the literature [28]. Chromophore YL was synthesized according
to the literature [18]. Compound 3, 4 and 5 were synthesized ac-
cording to literature [29]. TLC analyses were carried out on 0.25 mm
thick precoated silica plates and spots were visualized under UV
light. Chromatography on silica gel was carried out on Kieselgel
(200e300 mesh).
1HNMR (400 MHz, Acetone)
d 9.74 (s, 1H), 7.68 (m, 4H), 7.35 (d,
J ¼ 7.8 Hz, 2H), 6.73 (d, J ¼ 8.6 Hz, 2H), 4.29 (t, J ¼ 5.2 Hz, 2H), 3.82 (t,
J ¼ 5.1 Hz, 2H), 3.01 (s, 3H), 2.38 (s, 3H).
MS (EI): m/z calcd for C17H19NO4S: 333.10; found: 333.15.
2.2.2. Synthesis of compound 2
Sodium azide (12 g, 185 mmol) was added into a solution of
compound 1 (15 g, 45 mmol) in 50 mL dry DMF. The reaction
mixture was stirred at room temperature to subside heat by this
exothermic reaction. After stirring for 24 h, the resulting mixture
was poured into 200 mL of cold water with crushed ice and
extracted with 3 ꢂ 50 mL of ethyl acetate. The organic layers were
combined, dried over MgSO4 and concentrated via rotary evapo-
ration. The residual amount of DMF was removed in vacuo. After
removal of solvent under vacuum, the crude product was purified
by silica chromatography, eluting with (AcOEt: Hexane ¼ 1: 6) to
give compound 2 as a white solid in 90% yield (8.26 g, 40.5 mmol).
1HNMR spectra were determined using an Advance Bruker 400
(400 MHz) NMR spectrometer (tetramethylsilane as an internal
reference). The MS spectra were obtained on MALDI-TOF-(Matrix
Assisted Laser Desorption/Ionization of Flight) on a BIFLEXIII (Bro-
ker Inc.) spectrometer. The UVeVis spectra were performed on a
Cary 5000 photo spectrometer. The TGA was determined by
TA5000-2950TGA (TA Co) at a heating rate of 10 ꢀC$minꢁ1 under
the protection of nitrogen. Cyclic voltammetry (CV) experiments
were performed on a CHI660D electrochemical workstation by a
cyclic voltammetry (CV) technique in CH3CN solution, using a Pt
disk electrode and a platinum wire as the working and counter
electrodes, respectively, and a saturated Ag/AgCl electrode as the
1HNMR (400 MHz, Acetone)
d
9.77 (s,1H), 7.75 (d, J ¼ 8.1 Hz, 2H),
6.90 (d, J ¼ 8.3 Hz, 2H), 3.74 (t, J ¼ 5.8 Hz, 2H), 3.62 (t, J ¼ 5.7 Hz, 2H),
3.14 (s, 3H).
MS (EI): m/z calcd for C10H12N4O: 204.10; found: 204.13.
2.2.3. Synthesis of compound 3
To a stirred solution of propargyl bromide (3.26 g, 27.37 mmol)
and methyl 3,5-dihydroxybenzoate (2 g, 11.9 mmol), in acetone
reference electrode in the presence of 0.1
M
n-
Chart 1. Chemical structure for chromophores YL, YL1 and YL2.

758
Y. He et al. / Dyes and Pigments 139 (2017) 756e763
(40 mL) were added potassium carbonate (7 g, 50.72 mmol). The
reaction mixture was heated at reflux under nitrogen for 24 h,
filtered, evaporated to dryness, and partitioned between water and
dichloromethane. The aqueous layer was then extracted with
dichloromethane (2 ꢂ 200 mL). The combined extracts were
washed with water and dried over MgSO4. After filtration and
removal of solvent under vacuum, the crude product was purified
by silica chromatography, eluting with (Acetone: Hexane ¼ 1: 8) to
give compound 3 as a white solid in 90% yield (2.19 g, 10.71 mmol).
J ¼ 7.9 Hz, 2H), 6.92 (t, J ¼ 7.3 Hz, 1H), 6.75 (s, 2H), 6.60 (s, 1H), 5.03
(s, 2H), 4.75 (d, J ¼ 1.8 Hz, 4H), 3.03 (s, 2H).
MS (EI): m/z calcd for C19H11F5O3: 292.11; found: 292.16.
2.2.8. Synthesis of compound 7a
Compound 2 (1.18 g, 5.76 mmol), compound 6a (1 g, 2.62 mmol),
CuSO4$5H2O (10 mol%), NaHCO3 (20 mol%), and ascorbic acid
(20 mol%) were dissolved in tert-butanol/H2O (10 mL/10 mL) under
nitrogen in a Schlenk flask. The mixture was stirred at 25 ꢀC over-
night, then extracted with chloroform, washed with 1N HCl, 1N
NH4OH and water subsequently. The organic layer was dried over
MgSO4. After filtration and removal of solvent under vacuum, the
crude product was purified by silica chromatography, eluting with
(Methanol: Dichloromethane ¼ 1: 1) to give compound 7a as a
yellow solid in 85% yield (1.76 g, 2.23 mmol).
1HNMR (CDCl3)
d
: 7.31 (s, 2H), 6.83 (s, 1H), 4.73 (d, J ¼ 2.4 Hz,
2H), 3.92 (s, 3H), 2.55 (t, J ¼ 2.4 Hz, 2H).
MS (EI): m/z calcd for C14H12O4: 244.07; found: 244.09.
2.2.4. Synthesis of compound 4
To a stirred solution of the compound 3 (6.9 g, 28.3 mmol) in
anhydrous THF (100 mL) was added lithium aluminum hydride (2 g,
52.6 mmol) in small portions, and the reaction mixture was stirred
at room temperature for 2 h. Beckstrom's reagent (7 g) was then
added to quench the remaining lithium aluminum hydride. The
reaction mixture was filtered under vacuum, the solid was rinsed
with dichloromethane, and the filtrate was dried with MgSO4. After
evaporation of the solvents, the crude product was purified by
recrystallization from methanol to give compound 4 as white
crystals in 90% yield (5.5 g, 25.47 mmol).
1HNMR (400 MHz, CDCl3)
d 9.68 (s, 2H), 7.66 (s, 4H), 7.64 (s, 2H),
6.62 (t, J ¼ 4.9 Hz, 4H), 6.60 (s, 2H), 6.54 (s, 1H), 5.11 (s, 4H), 5.07 (s,
2H), 4.61 (t, J ¼ 5.7 Hz, 4H), 3.96 (t, J ¼ 5.7 Hz, 4H), 2.83 (s, 6H).
MS(MALDI-TOF): m/z (Mþ, C39H35F5N8O5): calcd:791.27; found:
791.20.
2.2.9. Synthesis of compound 7b
The procedure for compound 7a was followed to prepare 7b as
white solid in 82% yield (1.51 g, 2.15 mmol).
1HNMR (CDCl3)
d
: 6.56 (s, 2H), 6.46 (s, 1H), 4.61 (d, J ¼ 2.4 Hz,
1HNMR (400 MHz, Acetone)
d 9.72 (s, 2H), 8.04 (s, 2H), 7.67 (d,
4H), 4.45 (s, 2H), 2.46 (t, J ¼ 2.4 Hz, 2H).
J ¼ 8.2 Hz, 4H), 7.28 (t, J ¼ 7.5 Hz, 2H), 7.00 (d, J ¼ 7.8 Hz, 2H), 6.93 (t,
J ¼ 7.2 Hz, 1H), 6.76 (d, J ¼ 8.4 Hz, 4H), 6.73 (s, 2H), 6.65 (s, 1H), 5.15
(s, 4H), 5.05 (s, 2H), 4.72 (t, J ¼ 5.7 Hz, 4H), 4.05 (t, J ¼ 5.7 Hz, 4H),
2.86 (s, 6H).
MS (EI): m/z calcd for C13H12O3: 216.08; found: 216.04.
2.2.5. Synthesis of compound 5
To a stirred solution of the compound 4 (4 g, 18.5 mmol) in
anhydrous THF (100 mL) was added carbon tetrabromide (9.2 g,
27.75 mmol) followed by the portionwise addition of triphenyl-
phosphine (7.27 g, 27.75 mmol). The reaction was stirred at room
temperature for 1 h and the quenched with 50 mL of water.
Tetrahydrofuran was evaporated, and the crude product was
extracted with dichloromethane (2 ꢂ 100 mL). The organic layer
was dried with MgSO4. After filtration and removal of solvent under
vacuum, the crude product was purified by silica chromatography,
eluting with (Dichloromethane: Hexane ¼ 1: 1) to give compound 5
as a pale yellow solid in 85% yield (4.39 g, 15.72 mmol).
MS(MALDI-TOF): m/z (Mþ, C39H35F5N8O5): calcd:701.31; found:
791.35.
2.2.10. Synthesis of chromophore YL1
A solution of 7a (1.57 g, 1.98 mmol) and TCF (0.87 g, 4.36 mmol)
in 50 mL ethanol was refluxed for 2 h, then cooled to room tem-
perature. After the removal of solvent, the residue was purified by
column chromatography (Acetone: Hexane ¼ 1: 2) to give a golden
solid chromophore YL1 (1.37 g, 1.19 mmol) in 60% yield.
1HNMR (400 MHz, DMSO)
d
8.24 (s, 2H), 7.89 (d, J ¼ 15.9 Hz, 2H),
7.72 (d, J ¼ 8.5 Hz, 4H), 6.89 (d, J ¼ 15.9 Hz, 2H), 6.75 (d, J ¼ 8.5 Hz,
4H), 6.68 (s, 3H), 5.14 (s, 2H), 5.09 (s, 4H), 4.63 (t, J ¼ 5.7 Hz, 4H),
3.99 (t, J ¼ 5.7 Hz, 4H), 2.85 (s, 6H), 1.75 (s, 12H).
1HNMR (CDCl3)
d
: 6.57 (s, 2H), 6.46 (s, 1H), 4.58 (d, J ¼ 2.4 Hz,
4H), 4.33 (s, 2H), 2.46 (t, J ¼ 2.4 Hz, 2H).
MS (EI): m/z calcd for C13H11BrO2: 277.99; found: 277.91.
13CNMR (101 MHz, DMSO)
d 177.75, 176.06, 159.59, 153.04,
149.62, 143.15, 138.27, 133.11, 129.81 125.56, 122.95, 119.24, 115.68,
113.81, 112.97, 112.20, 109.45, 107.91, 102.24, 98.81, 93.41, 76.87,
68.97, 61.67, 56.31, 51.93, 47.45, 26.04.
2.2.6. Synthesis of compound 6a
To a stirred solution of 5 (0.46 g, 1.65 mmol) and Penta-
fluorophenol (0.28 g, 1.5 mmol), in acetone (20 mL) were added
potassium carbonate (0.23 g, 1.65 mmol) and 18-crown-6 (0.1 g,
0.4 mmol). The reaction mixture was heated at reflux under ni-
trogen for 24 h, filtered, evaporated to dryness, and partitioned
between water and dichloromethane. The aqueous layer was then
extracted with dichloromethane (2 ꢂ 100 mL). The combined ex-
tracts were washed with water and dried over MgSO4. After
filtration and removal of solvent under vacuum, the crude product
was purified by silica chromatography, eluting with (AcOEt:
Hexane ¼ 1: 1) to give compound 6a as a pale yellow solid in 80%
yield (0.46 g, 1.2 mmol).
MS(MALDI-TOF): m/z (Mþ,
C61H49F5N14O5): calcd:1153.12;
found: 1153.45.
HRMS (ESI) (M þ H)þ: calcd, 1153.4010; found, 1153.4003.
2.2.11. Synthesis of chromophore YL2
The procedure for compound YL1 was followed to prepare YL2
as golden solid chromophore in 55% yield (1.16 g, 1.09 mmol).
1HNMR (400 MHz, DMSO)
d
8.23 (s, 2H), 7.89 (d, J ¼ 15.9 Hz, 2H),
7.72 (d, J ¼ 8.5 Hz, 4H), 7.29 (t, J ¼ 7.5 Hz, 2H), 7.02e6.96 (m, 2H),
6.94 (t, J ¼ 6.8 Hz, 1H), 6.89 (d, J ¼ 15.9 Hz, 2H), 6.75 (d, J ¼ 8.4 Hz,
4H), 6.67 (s, 2H), 6.63 (s, 1H), 5.10 (s, 4H), 5.00 (s, 2H), 4.62 (t,
J ¼ 5.7 Hz, 4H), 3.99 (t, J ¼ 5.7 Hz, 4H), 2.84 (s, 6H), 1.74 (s, 12H).
1HNMR (400 MHz, Acetone)
d 6.77 (s, 2H), 6.67 (s, 1H), 5.24 (s,
2H), 4.80 (s, J ¼ 1.6 Hz, 4H), 3.08 (s, 2H).
13CNMR (101 MHz, DMSO)
d 177.75, 176.04, 159.60, 153.01,
MS (EI): m/z calcd for C19H11F5O3: 382.06; found: 382.12.
149.62, 143.25, 140.00, 133.13, 129.89, 125.60, 122.96, 121.23, 115.21,
113.83, 112.67, 112.30, 109.47, 107.11, 101.27, 98.80, 93.46, 69.31,
68.97, 61.62, 56.31, 51.97, 47.45, 26.06.
2.2.7. Synthesis of compound 6b
The procedure for compound 6a was followed to prepare 6b as
white solid in 79% yield (0.34 g, 1.18 mmol).
MS(MALDI-TOF): m/z (Mþ, C61H54N14O5): calcd:1063.17; found:
1063.44.
1HNMR (400 MHz, Acetone)
d
7.27 (t, J ¼ 7.4 Hz, 2H), 6.99 (d,
HRMS (ESI) (M þ H)þ: calcd, 1063.4481; found, 1063.4473.

Y. He et al. / Dyes and Pigments 139 (2017) 756e763
759
3. Results and discussion
hydroxyethyl)(methyl)amino]-benzaldehyde with azide group
through two step reactions. The para-Toluenesulfonyl group is a
good leaving group, which is beneficial for following SN2 reaction.
On the other hand, starting from 3,5-dihydroxybenzoate, dendritic
compounds 6a-b were synthesized in good overall yields (40%e
50%) through four step reactions. The derivative 3 was obtained by
substituting with propargyl bromide. Then the ester group of 3 was
reduced to the hydroxyl group to give compound 4 with lithium
aluminum hydride. After introduction of carbon tetrabromide, the
compound 5 was prepared with a high yield (about 90%). Treatment
of compound 5 with pentafluorophenol and phenol gave dendritic
compounds 6a-b.
3.1. Synthesis and characterization of chromophores
Chromophores YL, YL1 and YL2 were synthesized following the
general route laid out in Scheme 1. Starting from 4-((2-
hydroxyethyl)(methyl)amino)benzaldehyde, the chromophore YL
was synthesized through one step reaction.
The difference between bichromophores YL1 and YL2 is core
group of the compound structures. At beginning, the synthesis was
carried out in two aspects. On one hand, the derivative 2 was ob-
tained by substituting of the hydroxyl group of 4-[(2-
Scheme 1. Chemical structures and synthetic routes for chromophores YL1 and YL2.

760
Y. He et al. / Dyes and Pigments 139 (2017) 756e763
Table 1
Next, the compounds 7a-b were prepared by click reaction of
the derivative and dendritic compounds 6a-b. The target
Thermal and Optical Properties data of the Chromophores.
2
Dlc (nm)
a
b
Cmpd
Td (ꢀC)
lmax (nm)
lmax (nm)
bichromophores YL1 and YL2 were successfully synthesized by
condensing aldehydes 6a and 6b with acceptor TCF in ethanol.
All the chromophores were characterized by 1HNMR, 13CNMR,
MS, UVeVis spectroscopic analysis and the data obtained were in
full agreement with the proposed formulations. These chromo-
phores possess good solubility in common organic solvents, such as
dichloromethane, trichloromethane and acetone.
YL
YL1
YL2
283
264
309
587
566
567
532
522
523
55
44
44
a
lmax was measured in DMF.
lmax was measured in dioxane.
Dl was the difference between lmax and lmax
b
c
a
b
.
3.2. Thermal stability
wavelengths (lmax) of YL, YL1 and YL2 in different solvents show a
red shift with increase of solvent polarity. In the case of molecules
held together via a terminal ring, the most significant UVeVis ab-
sorption is attributed to monochromophore antenna, as literature
reported [31]. Besides, the absorption spectra was slightly broad-
ened from monomer to bichromophores, which could be resulted
from larger conformational disorder in the bichromophore [32].
The maximum absorption wavelengths (lmax) of YL, YL1 and YL2
are 587 nm, 566 nm and 567 nm in DMF respectively, as Table 1
shows. Therefore, compared with chromophore YL, the lmax of
bichromophores YL1 and YL2 show blue shifts, which may be
attributed to spatial interactions between the two sub-
chromophores in YL1 and YL2 [33,34]. This blue shift behaviour
can be good for enhancement of transparency of EO device because
the light absorptions of the bichromophores YL1 and YL2 are less
than that of chromophore YL. In addition, as the core group of the
bichromophores don't change their conjugate structure, the lmax of
YL1 is similar with that of YL2.
NLO chromophores must be thermally stable enough to with-
stand high temperatures (>200 ꢀC) in electric field poling and
subsequent processing of chromophore/polymer materials. Ther-
mal properties of these two bichromophores were measured by
Thermogravimetric Analysis (TGA) at a heating rate of 10 ꢀC$minꢁ1
under nitrogen. Fig. 1 shows thermogravimetric analysis of
bichromophores YL1 and YL2. The two bichromophores exhibited
good thermal-stabilities and their decomposition temperatures
(Td) were above 250 ꢀC as summarized in Table 1. The triazole group
formed by click reaction is thermally stable and the decomposition
temperatures of compounds usually increase along with their
molecular weight increase. We could conclude that the bichro-
mophore YL2 had better thermal stability than the chromophore YL
by comparing their decomposition temperatures. Furthermore, the
decomposition temperature(Td) of YL1 is lower by about 40 ꢀC than
that of YL2, possibly owing to the presence of the penta-
fluorophenyl group [30]. In a word, the excellent thermal stabilities
of bichromophores YL1 and YL2 make them suitable for practical
device fabrication and EO device preparation.
3.4. Electrochemical properties
In order to determine the redox properties of chromophores YL,
YL1 and YL2, cyclicvoltammetry (CV) measurements were con-
ducted in degassed anhydrous acetonitrile solutions containing
0.1 mol/L tetrabutylammonium hexafluorophosphate (TBAPF) as
supporting electrolyte. The relative data and voltammograms of
1 ꢂ 10ꢁ4 mol/L chromophores YL, YL1 and YL2 were recorded, as
shown in Table 2 and Fig. 3.
The HOMO and LUMO levels of the chromophores can be
calculated from their corresponding oxidation and reduction po-
tentials [35]. The difference between these two values provides the
HOMO-LUMO energy difference △E (CV), which are 1.788, 1.810
and 1.809 eV for chromophores YL, YL1 and YL2 respectively.
Comparing the △E (CV) value of chromophore YL, the values of
bichromophores YL1 and YL2 are a little bigger. This phenomenon
may result from the electron withdrawing group-triazole in donor
parts of YL1 and YL2. The narrower energy gap indicated easier
charge transfer [36]. These results corresponded with the conclu-
sion of UVeVis spectra analysis.
3.3. Optical properties
The UVeVis absorption spectra of three chromophores
(c ¼ 1 ꢂ 10ꢁ5 mol Lꢁ1) were measured in a series of solvents with
different constants as shown in Fig. 2. In order to reveal the effect of
bichromophores on the intramolecular charge-transfer (ICT) of
dipolar chromophores, the chromophore YL was used as reference
spectrum.
As shown in Fig. 2, the synthesized YL, YL1 and YL2 chromo-
phores exhibited a similar
absorption band in visible region. The maximum absorption
pep* intramolecular chargetransfer
3.5. Electro-optic performance
In order to study macroscopic EO response of these chromo-
phores, the polymer films doped with 25 wt% chromophores into
amorphous polymethylmethacrylate (PMMA) were prepared using
cyclopentanone 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 70 ꢀC
in a vacuum oven overnight. 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 transparency in order to minimize the contribution from
Fig. 1. TGA curves of chromophores YL1 and YL2 with a heating rate of 10 ꢀC minꢁ1 in
nitrogen atmosphere.

Y. He et al. / Dyes and Pigments 139 (2017) 756e763
761
Table 2
Summary of electro-chemical data and EO coefficients of chromophores.
Cmpd
D
E(CV)a (eV)
Eoxb(V)
Eredc(V)
r33(pm/V)
YL
YL1
YL2
1.79
1.81
1.81
0.999
1.036
1.043
ꢁ0.789
ꢁ0.774
ꢁ0.766
7
20
17
a
D
E(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
Fig. 3. Cyclic voltammograms of chromophores YL, YL1 and YL2 recorded in CH3CN
solutions containing 0.1 M Bu4NPF6 supporting electrolyte at a scan rate of 100 mV sꢁ1
.
À
Á
3=2
n2 ꢁ sin2
q
3
l
Im
1
sin2
r33
¼
4p
À
Á
n2 ꢁ 2 sin2
q
q
VmIcn2
where r33 is the EO coefficient of the poled polymer,
l
is the
optical wavelength, 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.
If intermolecular electrostatic interactions are neglected, the
electro-optic coefficient (r33) should increase linearly with chro-
mophore density, dipole moment, first hyperpolarizability and the
strength of electric poling field. But chromophores with a large
dipole moment generate intermolecular static electric field dipole-
dipole interaction, which leads to the unfavourable antiparallel
packing of chromophores. So the number of truly oriented chro-
mophore is small. To achieve high EO coefficient, the chromophores
should have large mb, meanwhile the dipole-dipole interactions of
chromophores should be reduced [40]. The rational design of
bichromophores has been considered as an excellent method
which not only can take use of chromophores with high first
hyperpolarizability (b) but also restrain the dipole-dipole in-
teractions of chromophores to some extent [17].
The EO coefficient r33 values of the poled films of YL/PMMA,
YL1/PMMA and YL2/PMMA are 7, 20 and 17 p.m./V, respectively.
The r33 values of YL1/PMMA and YL2/PMMA are 2.85 times and
2.43 times of YL/PMMA, which can be attributed to the effect of
novel structures of these bichromophores. Comparing with regular
chromophore YL, the unfavourable antiparallel packing of bichro-
mophores YL1 and YL2 in EO polymeric material may become
difficult. This should because the triazole groups formed by click
reaction can act as the intramolecular isolation groups of
Fig. 2. UVeVis absorption spectra of chromophores YL, YL1 and YL2 in six kinds of
aprotic solvents with varying dielectric constants.
multiple reflections [37,38].
The r33 values can be calculated by the following equation [39]:

762
Y. He et al. / Dyes and Pigments 139 (2017) 756e763
materials [25]. In addition, the r33 value of YL1/PMMA is a little
bigger than that of YL2/PMMA. The reason may be that the pen-
tafluorophenyl groups can obviously improve the compatibility of
bichromophores in polymer. We have tried to prepare EO films
doped different concentrations of bichromophores YL1 and YL2.
When the doping concentration of YL2 reached 25 wt%, the YL2/
PMMA system was at the edge of the phase separation. However,
the YL1 bichromophores with pentafluorophenyl in structure show
better compatibility and the highest doping concentration can be
30 wt%. Besides, the pentafluorophenyl can act as a better isolation
group in EO materials as the literature [23] reported.
The order parameters (F) of poled EO films were also studied by
recording UVeVis absorption spectra of poled films and depoled
films. After corona poling, the chromophores in the polymer were
aligned, and the absorption intensity decreased due to birefrin-
gence. The order parameters (
following equation:
¼ 1 ꢁ A1/A0, where A0 and A1 are the
absorbance of depoled and poled EO films at normal incidence. So
the order parameters ( ) can indicate proportion of aligned chro-
F) can be calculated according to the
F
F
mophores in poled films, which represent poling efficiency of EO
films to some extent. Depoled films can be obtained by heating
poled films above Tg for 20 min to make chromophores become
disorganized again. The
F values of YL/PMMA, YL1/PMMA and YL2/
PMMA films were about 11%, 20% and 16% respectively. These
values reached the average values (10%e20%), which meant that
the poling process was typically efficient [8]. The
F values of YL1/
PMMA and YL2/PMMA films both were bigger than that of YL/
PMMA films. The result was in accordance with the EO coefficient
r33 values, which can illustrate the advantage of bichromophores in
poling process. We verified the conclusion that triazole groups can
act as excellent isolation groups to suppress possible aggregation.
Besides, the
F values of the YL1/PMMA and YL2/PMMA also had
small difference that might be ascribed to the effect of penta-
fluorophenyl group (see Fig.4).
4. Conclusions
Two novel bichromophores basing on simple aniline chromo-
phore were designed by utilizing Cu(I) catalyzed click-reaction and
synthesized successfully. The yields of bichromophores were rela-
tively high (about 20%) through 8 steps. Moreover, the reaction
condition of Cu(I) catalyzed click-reaction is moderate, which could
be applied to many chromophores. The decomposition tempera-
tures (Td) of bichromophores YL1 and YL2 were both above 250 ꢀC.
The ability of intramolecular charge-transfer (ICT) of bichromo-
phores YL1 and YL2 become a little weaker comparing chromo-
phore YL from the UVeVis spectra and cyclic voltammogram. This
effect can lead to the improvement of transparency of EO device
made by bichromophores. The doped EO films containing bichro-
mophores YL1 and YL2 showed higher r33 values and poling effi-
ciency comparing with chromophore YL. These results indicated
that there were advantages of the novel bichromophores structures
in promoting the translation from microscopic hyperpolarizability
(b) of chromophores to macroscopic nonlinear optical (NLO) ac-
tivity of corresponding polymer materials. We believe that the
novel bichromophores can be used in exploring high-performance
organic EO materials which have high optical nonlinearity, good
thermal stability and low optical loss.
Acknowledgments
Fig. 4. UVeVis absorption spectra of the poled and depoled YL/PMMA, YL1/PMMA and
YL2/PMMA films.
We are grateful to the National Nature Science Foundation of
China (no. 21504099) and the National Key Research and Devel-
opment Program of China (Grant No. 2016YFB0402004) for finan-
cial support.
bichromophores, which are utilized frequently in dendrimer EO

Y. He et al. / Dyes and Pigments 139 (2017) 756e763
763
Theory-guided design and synthesis of multichromophore dendrimers: an
analysis of the electro-optic effect. J Am Chem Soc 2007;129(24):7523e30.
[21] Gao JK, Cui YJ, Yu JC, Lin WX, Wang ZY, Qian GD. Enhancement of nonlinear
optical activity in new six-branched dendritic dipolar chromophore. J Mater
Chem 2011;21(9):3197e203.
[22] Wu W, Li C, Yu G, Liu Y, Ye C, Qin J, et al. High-generation second-order
nonlinear optical (NLO) Dendrimers that contain isolation chromophores:
convenient synthesis by using click chemistry and their increased NLO effects.
Chemistry 2012;18(35):11019e28.
[23] Wu W, Wang C, Li Q, Ye C, Qin J, Li Z. The influence of pentafluorophenyl
groups on the nonlinear optical (NLO) performance of high generation den-
drons and dendrimers. Sci Rep 2014;4:6101.
[24] Tang RL, Zhou SM, Xiang WD, Xie YJ, Chen H, Peng Q, et al. New 'X-type'
second-order nonlinear optical (NLO) dendrimers: fewer chromophore moi-
eties and high NLO effects. J Mater Chem C 2015;3(17):4545e52.
[25] Wu WB, Huang LJ, Song CF, Yu G, Ye C, Liu YQ, et al. Novel global-like second-
order nonlinear optical dendrimers: convenient synthesis through powerful
click chemistry and large NLO effects achieved by using simple azo chromo-
phore. Chem Sci 2012;3(4):1256e61.
References
[1] Sullivan PA, Dalton LR. Theory-inspired development of organic electro-optic
materials. Accounts Chem Res 2010;43(1):10e8.
[2] Dalton LR, Sullivan PA, Bale DH. Electric field poled organic electro-optic
materials: state of the art and future prospects. Chem Rev 2010;110(1):
25e55.
[3] Wu J, Bo S, Liu J, Zhou T, Xiao H, Qiu L, et al. Synthesis of novel nonlinear
optical chromophore to achieve ultrahigh electro-optic activity. Chem Com-
mun (Camb) 2012;48(77):9637e9.
[4] Liu FG, Wang HR, Yang YH, Xu HJ, Yang D, Bo SH, et al. Using phenoxazine and
phenothiazine as electron donors for second-order nonlinear optical chro-
mophore: enhanced electro-optic activity. Dyes Pigments 2015;114:196e203.
[5] Yang YH, Liu FG, Wang HR, Bo SH, Liu JL, Qiu L, et al. Enhanced electro-optic
activity from the triarylaminophenyl-based chromophores by introducing
heteroatoms to the donor. J Mater Chem C 2015;3(20):5297e306.
[6] Hammond SR, Clot O, Firestone KA, Bale DH, Lao D, Haller M, et al. Site-iso-
lated electro-optic chromophores based on substituted 2,2'-bis(3,4-
propylenedioxythiophene) pi-conjugated bridges. Chem Mater 2008;20(10):
3425e34.
[7] He MQ, Leslie TM, Sinicropi JA, Garner SM, Reed LD. Synthesis of chromo-
phores with extremely high electro-optic activities. 2. Isophorone- and com-
bined isophorone-thiophene-based chromophores. Chem Mater 2002;14(11):
4669e75.
[8] Hu CL, Liu FG, Zhang H, Huo FY, Yang YH, Wang HR, et al. Synthesis of novel
nonlinear optical chromophores: achieving excellent electro-optic activity by
introducing benzene derivative isolation groups into the bridge. J Mater Chem
C 2015;3(44):11595e604.
[26] Knorr DB, Zhou XH, Shi ZW, Luo JD, Jang SH, Jen AKY, et al. Molecular mobility
in self-assembled dendritic chromophore glasses.
J
Phys Chem
B
2009;113(43):14180e8.
[27] Kim TD, Kang JW, Luo J, Jang SH, Ka JW, Tucker N, et al. Ultralarge and
thermally stable electro-optic activities from supramolecular self-assembled
molecular glasses. J Am Chem Soc 2007;129(3):488e9.
[28] He MQ, LaJAS TM. r-Hydroxy ketone precursors leading to a novel class of
electro-optic acceptors. Chem Mater 2002;2002(14):2393e400.
[29] Malkoch M, Schleicher K, Drockenmuller E, Hawker CJ, Russell TP, Wu P, et al.
Structurally diverse dendritic libraries: a highly efficient functionalization
approach using Click chemistry. Macromolecules 2005;38(9):3663e78.
[30] Wu W, Ye C, Qin J, Li Z. Dendrimers with large nonlinear optical performance
by introducing isolation chromophore, utilizing the Ar/ArF self-assembly ef-
fect, and modifying the topological structure. ACS Appl Mater interfaces
2013;5(15):7033e41.
[31] Bazan GC. Novel organic materials through control of multichromophore in-
teractions. J Org Chem 2007;72(23):8615e35.
[32] Gao JK, Cui YJ, Yu JC, Wang ZY, Wang MQ, Qiu JR, et al. Molecular design and
synthesis of hetero-trichromophore for enhanced nonlinear optical activity.
Macromolecules 2009;42(6):2198e203.
[33] Wu W, Ye C, Yu G, Liu Y, Qin J, Li Z. New hyperbranched polytriazoles con-
taining isolation chromophore moieties derived from AB4 monomers through
click chemistry under copper(I) catalysis: improved optical transparency and
enhanced NLO effects. Chemistry 2012;18(14):4426e34.
[34] Liao Y, Firestone KA, Bhattacharjee S, Luo JD, Haller M, Hau S, et al. Linear and
nonlinear optical properties of a macrocyclic trichromophore bundle with
parallel-aligned dipole moments. J Phys Chem B 2006;110(11):5434e8.
[35] Mukundan Thelakkat H-WS. Synthesis and properties of novel derivatives of
1,3,5-Tris(diarylamino)benzenes for electroluminescent devices. Adv Mater
1998;10(3):219e23.
[36] Xu HJ, Zhang ML, Zhang AR, Deng GW, Si P, Huang HY, et al. Novel second-
order nonlinear optical chromophores containing multi-heteroatoms in
donor moiety: design, synthesis, DFT studies and electro-optic activities. Dyes
Pigments 2014;102:142e9.
[37] Teng CC, Man HT. Simple reflection technique for measuring the electrooptic
coefficient of poled polymers. Appl Phys Lett 1990;56(18):1734e6.
[38] Park DH, Lee CH, Herman WN. Analysis of multiple reflection effects in
reflective measurements of electro-optic coefficients of poled polymers in
multilayer structures. Opt Express 2006;14(19):8866e84.
[39] Kajzar F, Lee KS, Jen AKY. Polymeric materials and their orientation tech-
niques for second-order nonlinear optics. In: Lee KS, editor. Polymers for
photonics applications ii: nonlinear optical, photorefractive and two-photon
absorption polymers. Berlin: Springer-Verlag Berlin; 2003. p. 1e85.
[40] Wu J, Luo J, Cernetic N, Chen K, Chiang K-S, Jen AKY. PCBM-doped electro-
optic materials: investigation of dielectric, optical and electro-optic proper-
ties for highly efficient poling. J Mater Chem C 2016;4(43):10286e92.
[9] Andreu R, Cerdan MA, Franco S, Garin J, Marco AB, Orduna J, et al. Decreased
optical nonlinearities upon CF3 substitution on tricyanofuran acceptors. Org
Lett 2008;10(21):4963e6.
[10] Liao Y, Eichinger BE, Firestone KA, Haller M, Luo JD, Kaminsky W, et al. Sys-
tematic study of the structure-property relationship of a series of ferrocenyl
nonlinear optical chromophores. J Am Chem Soc 2005;127(8):2758e66.
[11] Luo JD, Haller M, Ma H, Liu S, Kim TD, Tian YQ, et al. Nanoscale architectural
control and macromolecular engineering of nonlinear optical dendrimers and
polymers for electro-optics. J Phys Chem B 2004;108(25):8523e30.
[12] Si P, Liu JL, Zhen Z, Liu XH, Lakshminarayana G, Kityk IV. Synthesis and
characterization of NLO chromophore with benzo [1,2-b:4,5-b ']dithiophene
unit as pi-electron bridge. Tetrahedron Lett 2012;53(26):3393e6.
[13] Sen Liu MAH, Ma Hong, Dalton Larry R, Jang Sei-Hum, Jen Alex K-Y. Focused
microwave-assisted synthesisof 2,5-dihydrofuran derivatives as Electro-
nAcceptors for highly efficient nonlinear opticalchromophore. Adv Mater
2003;2003(15):603e7.
[14] Liakatas I, Cai C, Bosch M, Jager M, Bosshard C, Gunter P, et al. Importance of
intermolecular interactions in the nonlinear optical properties of poled
polymers. Appl Phys Lett 2000;76(11):1368e70.
[15] Wang HR, Yang YH, Liu JL, Liu FG, Qiu L, Liu XH, et al. Great improvement of
performance for NLO chromophore with cyclopentadithiophenone unit as pi-
electron bridge. Mater Lett 2015;161:674e7.
[16] Wu J, Xiao H, Qiu L, Zhen Z, Liu X, Bo S. Comparison of nonlinear optical
chromophores containing different conjugated electron-bridges: the rela-
tionship between molecular structure-properties and macroscopic electro-
optic activities of materials. RSC Adv 2014;4(91):49737e44.
[17] Gao JK, Cui YJ, Yu JC, Lin WX, Wang ZY, Qian GD. Design and synthesis of
bichromophores for nonlinear optical applications in polymer films. React
Funct Polym 2011;71(4):496e501.
[18] Gao JK, Cui YJ, Yu JC, Lin WX, Wang ZY, Qian GD. A 3-phenoxypropane-1, 2-
diol based bichromophore for enhanced nonlinear optical properties. Dyes
Pigments 2010;87(3):204e8.
[19] Sullivan PA, Akelaitis AJP, Lee SK, McGrew G, Lee SK, Choi DH, et al. Novel
dendritic chromophores for electro-optics: influence of binding mode and
attachment flexibility on electro-optic behavior. Chem Mater 2006;18(2):
344e51.
[20] Sullivan PA, Rommel H, Liao Y, Olbricht BC, Akelaitis AJP, Firestone KA, et al.