The important role of the isolation group (TBDPS) in designing efficient organic nonlinear optical FTC type chromophores

Article abstract of DOI:10.1016/j.dyepig.2016.12.003

Highly efficient and thermally stable nonlinear optical chromophores based on the same thiophene π-conjugation and tricyanofuran acceptor (TCF) possessing different isolation groups attached to electron-donating moiety have been synthesized and systematic

Full text of DOI:10.1016/j.dyepig.2016.12.003

Dyes and Pigments 139 (2017) 239e246
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The important role of the isolation group (TBDPS) in designing
efficient organic nonlinear optical FTC type chromophores
a
,
b
,
c
,
*
d
d
d
d, **
Yuhui Yang
, Jialei Liu , Hongyan Xiao , Zhen Zhen , Shuhui Bo
a
Department of Materials Engineering, Zhejiang Sci-Tech University, Hangzhou 310018, PR China
b
c
National Engineering Laboratory for Textile Fiber Materials and Processing Technology (Zhejiang), Hangzhou 310018, PR China
Key Laboratory of Advanced Textile Materials and Manufacturing Technology (ATMT), Ministry of Education, Hangzhou 310018, PR China
Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
d
Beijing 100190, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 November 2016
Received in revised form
Highly efficient and thermally stable nonlinear optical chromophores based on the same thiophene p-
conjugation and tricyanofuran acceptor (TCF) possessing different isolation groups attached to electron-
donating moiety have been synthesized and systematically investigated in this paper. Density functional
23 November 2016
theory (DFT) was used to calculate the HOMOeLUMO energy gaps and first-order hyperpolarizability (b)
Accepted 1 December 2016
Available online 5 December 2016
of these chromophores. These new chromophores showed excellent thermal stability with their
ꢀ
decomposition temperatures all above 250 C. Most importantly, the molecular hyperpolarizability of
these chromophores can be effectively translated into large electro-optic (EO) coefficients (r33) in poled
polymers. The doped films-B containing 25 wt % chromophore F1 displayed an r33 value of 52 pm/V at
Keywords:
Chromophores
Electro-optic
Isolation group
Poled film
1310 nm, and the doped films-C containing F2 showed a value of 91 pm/V at the concentration of 35 wt
%. These values are all much higher than the traditional FTC chromophore (39 pm/V). These results
revealed that the introduced isolation group can reduce intermolecular electrostatic interactions thus
enhance the macroscopic EO activity. Besides, the position of the isolation group plays an equally
important role in achieving the ultra large EO coefficients of materials. These properties, together with
the good solubility, suggest the potential use of these new chromophores as advanced materials devices.
©
2016 Elsevier Ltd. All rights reserved.
1. Introduction
macroscopical second order susceptibilities for the organic com-
pounds are very crucially dependent on the relation between the
p
-
Integrated circuits and multifunctional nano-devices based on
organic nonlinear optical (NLO) polymers have emerged as one of
the enabling elements for a new generation of optical telecom-
munications, which needs fast modulation and switching of optical
signals at the speed of 100 Gbit/s or above to accommodate antic-
ipated growth in data traffic [1e4]. Over the past two decades,
considerable progress has been made in the development of
organic and polymeric electro-optic (EO) materials for applications
in ultrafast optical modulators, optical sensors, and information
processors, which have stimulated a research boom for materials
conjugated charge transfer and the dipole moments of the matrices
[8,9]. In recent years, a large number of push-pull organic mole-
cules have been synthesized and used for the preparation of ma-
terials with electronic and optical applications [10e12].
Unfortunately, in most of the organic nonlinear optical (NLO) ma-
terials, the chromophore moieties have a rod-like structure and the
strong dipoleedipole interactions between chromophores have led
to unfavorable antiparallel packing of the chromophores, making
the conversion from the high
b values of the chromophores to large
macroscopic optical nonlinearities (r33) a continuous challenge
with large EO activities, both at molecular level (
b
) and as processed
[13e15]. Thus, the major problem in NLO research is how to effi-
materials (r33) [2,5e7]. We should point out that the output
ciently translate the large
b values of the organic chromophores
into high macroscopic NLO activities of the corresponding poly-
mers. Fortunately, controlling the shape of the chromophore was
proven to be an efficient approach for minimizing the dipoleedi-
pole interactions and enhancing the poling efficiency, with a
spherical shape, as proposed by Dalton and Jen et al., being the
*
Corresponding author. Department of Materials Engineering, Zhejiang Sci-Tech
University, Hangzhou 310018, PR China.
** Corresponding author.
E-mail address: yhyang@zstu.edu.cn (Y. Yang).
http://dx.doi.org/10.1016/j.dyepig.2016.12.003
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

240
Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
most ideal conformation [4,5,16e19]. The most effective and facile
way to improve the r33 values of the guest-host EO materials is the
optimization of the push-pull chromophores.
protection of nitrogen. All chemicals, commercially available, are
used without further purification unless stated. The DMF, ethanol
and THF were freshly distilled prior to its use. The 2-
In an attempt to partially solve the above mentioned challenge,
since 2006, based on the isolation principle proposed by Dalton
et al. many researchers have been investigating the preparation of
different kinds of NLO polymers using some bulky isolation groups
linked to chromophores exhibited large NLO coefficient [20,21].
Besides, it has been shown that an ensemble of interacting spher-
ically shaped structures will have greater mobility in the poling
field than a similar system composed of typical prolate ellipsoid-
shaped molecules [22e24]. Hence, besides the steric hindrance,
chromophores' free mobility also plays an equally important role in
achieving the ultra large EO coefficients of materials. The selection
of molecular optimization is an art of tune, because the steric
hindrance and molecular mobility required the equal balance
dicyanomethylene-3-cyano-4-methyl-2,5-dihydrofuran
acceptor was prepared according to the literature [26].
(TCF)
2.2. Synthesis
2.2.1. Synthesis of compound 1b
Under N
ylamino)-2-hydroxybenzaldehyde (3.86 g, 20 mmol) in anhydrous
DMF was added 6-Chlorohexanol (3.55 g, 26 mmol) and K CO
2
atmosphere to a solution of compound 1a (4-(dieth-
2
3
(3.59 g, 26 mmol). The mixture was stirred at the temperature of
ꢀ
120 C for 12 h and then poured into ice water. The organic layers
were extracted with ethyl acetate (3 ꢁ 50 mL). The combined
4
organic extracts were dried over MgSO . After the removal of the
[16,25]. As a result, the optimization of chromophores should meet
solvent, the residue was purified by column chromatography using
two functions: the steric hindrance effectively attenuates the
dipole-dipole interactions and the chromophores in materials have
the fine free mobility for orientation in the electrical field induced
poling. There are many reports about the influence of the isolation
group's size. However, to our best of knowledge, there is only small
number of reports on influence of the position of the isolation
group.
hexane and acetone as eluent to give a yellow liquid (5.04 g, 86%).
1H NMR (400 MHz, CDCl
3
)
d
10.14 (s, 1H), 7.69 (d, J ¼ 9.0 Hz, 1H),
6.26 (dd, J ¼ 9.0, 2.1 Hz, 1H), 6.01 (d, J ¼ 2.1 Hz, 1H), 4.03 (t,
J ¼ 6.3 Hz, 2H), 3.65 (t, J ¼ 6.5 Hz, 2H), 3.41 (q, J ¼ 7.1 Hz, 6H), 2.40 (s,
1H), 1.90e1.78 (m, 2H), 1.66e1.40 (m, 6H), 1.21 (t, J ¼ 7.1 Hz, 6H).
þ
MS (EI): m/z calcd for C17
H
27NO
3
: 293.20 [M] ; found:293.73.
þ
3
HRMS (ESI) (M , C17H27NO ): calcd, 293.1991; found, 293.1979.
In this paper, we had designed and synthesized two new chro-
mophores containing an identical TCF acceptor, but the introduced
isolation group's position is different. The chromophore denoted as
FTC, having the 4-(diethylamino)aryl group as the donor, the
traditional FTC chromophore was chosen as a benchmark chro-
mophore for comparison. These new chromophores (in Chart 1)
showed great solubility in common organic solvents, good
compatibility with polymers, and large EO activity in the poled
films. H-NMR and C-NMR analysis were carried out to demon-
strate the preparation of these chromophores. In order to deter-
mine the electro-optic property of the chromophores, electric-
poled polymer films containing synthesized chromophores were
prepared and the EO coefficients (r33) were measured at 1310 nm
using reflection technique. In addition to relevant linear optical
spectra and thermal properties are presented and discussed herein.
2.2.2. Synthesis of compound 2a
To a solution of 4-(diethylamino)-2-hydroxybenzaldehyde 1a
(4.05 g, 21 mmol) and imidazol (4.08 g, 50.0 mmol) in DMF (40 mL)
was added dropwise tert-butylchlorodiphenylsilane (6.28 g,
23 mmol). The mixture was allowed to stir at room temperature for
3.5 h and then poured into water. The organic phase was extracted
4
by AcOEt, washed with brine and dried over MgSO . After removal
of the solvent under reduced pressure, the crude product was pu-
1
13
rified by silica chromatography, eluting with (AcOEt: Hexane ¼ 1:4)
to give 2a as a white solid (7.3 g, 81%).
1
3
H NMR (400 MHz, CDCl ) d 10.48 (s, 1H), 7.79e7.69 (m, 5H),
7.45e7.36 (m, 6H), 6.29e6.20 (m, 1H), 5.60 (s, 1H), 2.93 (q,
J ¼ 7.1 Hz, 4H), 1.12 (s, 9H), 0.77 (t, J ¼ 7.1 Hz, 8H).
þ
MALDI-TOF: m/z calcd for C27
H33NO
2
Si: 431.23 [M] ; found:
4
4
2
1
31.06.
HRMS (ESI) (M ,
31.2295.
þ
27 2
C H33NO Si): calcd, 431.2281; found,
.2.3. Synthesis of compound 2b
In a similar way, compound 2b was obtained from compound
b.
1H NMR (400 MHz, Acetone)
d 10.19 (s, 1H), 7.74e7.67 (m, 5H),
7
.63e7.57 (m, 1H), 7.46e7.39 (m, 5H), 6.35 (dd, J ¼ 8.9, 2.1 Hz, 1H),
6
.22 (d, J ¼ 2.1 Hz, 1H), 4.11 (t, J ¼ 6.3 Hz, 2H), 3.73 (t, J ¼ 6.3 Hz, 2H),
3.48 (q, J ¼ 7.1 Hz, 4H), 1.82 (p, J ¼ 6.6 Hz, 2H), 1.68e1.56 (m, 2H),
1.51 (dd, J ¼ 12.0, 8.6 Hz, 4H), 1.24e1.14 (m, 6H), 1.04 (s, 9H).
13
C NMR (100 MHz, Acetone)
d 205.09, 184.98, 163.72, 153.55,
Chart 1. Structure of chromophore FTC, F1 and F2.
135.46, 134.83, 133.94, 129.73, 129.49, 127.79, 127.53, 114.36, 104.24,
9
3.56, 67.82, 63.57, 44.46, 32.44, 26.58, 25.79, 25.49, 19.00, 12.22.
þ
2
. Experiments
MALDI-TOF: m/z calcd for C33H45NO Si: 531.32 [M] ; found:
3
5
31.09.
HRMS (ESI) (M , C33
þ
2.1. Materials and instrumentation
3
H45NO Si): calcd, 531.3169; found, 531.
3191.
1
H NMR spectra were determined by an Advance Bruker 400
(
400 MHZ) NMR spectrometer (tetramethylsilane as internal
2.2.4. Synthesis of compound 3a
Under N , to a mixture of compound 2a (4.31 g, 10 mmol) and 2-
thienyl triphenylphosphonate bromide (5.27 g, 12 mmol) in dry
THF (20 mL) at room temperature, NaH (2.4 g, 0.1 mol) was added.
The mixture turned yellow and was stirred at room temperature for
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
5
2
2
000 photo spectrometer. The TGA was determined by TA5000-
ꢀ
950TGA (TA Co) with a heating rate of 10 C/min under the
4
24 h. Saturated NH Cl was added and the resulting mixture was

Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
241
extracted with EtOAc (20 ꢁ 3 mL). The combined extracts were
washed with water and dried over MgSO . After filtration and
removal of the solvent under vacuum, the residue was purified by
2.2.8. Synthesis of compound chromophore F1
4
A mixture of aldehydic bridge 4a (0.54 g, 1.00 mmol) and
acceptor 5 (0.22 g, 1.10 mmol) in ethanol (30 mL) was stirred at
ꢀ
column chromatography on silica gel (hexane/acetone, v/v, 10/1) to
75 C for 4 h in the presence of a catalytic amount of triethylamine
obtain a yellow liquid (2.96 g, 58%).
After removal of the solvent, the residue was purified by column
chromatography on silica gel (hexane/ethyl acetate, v/v, 4: 1). A
þ
MALDI-TOF: m/z calcd for C32
11.49.
HRMS (ESI) (M ,
11.2382.
H
37NOSSi: 511.24 [M] ; found:
5
dark solid was obtained (0.27 g, 38%).
þ
1
C
32
H
37NOSSi): calcd, 511.2365; found,
H NMR (400 MHz, CDCl
3
)
d
7.78 (dd, J ¼ 20.6, 10.5 Hz, 6H), 7.42
5
2
2
(dt, J ¼ 21.6, 7.1 Hz, 7H), 7.19e7.02 (d, J ¼ 15.9 Hz, 1H), 7.01 (s, 1H),
.53 (d, J ¼ 15.2 Hz, 1H), 6.24 (d, J ¼ 8.8 Hz, 1H), 5.73 (s, 1H), 2.93 (s,
4H), 1.74 (s, 6H), 1.20 (s, 9H), 0.79 (t, J ¼ 6.4 Hz, 6H).
6
.2.5. Synthesis of compound 3b
13
In a similar way, compound 3b was obtained from compound
b.
3
C NMR (100 MHz, CDCl ) d 175.94, 172.59, 155.74, 155.65,
149.69, 139.23, 137.99, 136.93, 135.46, 132.76, 130.83, 130.20, 128.08,
127.65, 126.34, 114, 75, 113.90, 112.48, 111.72, 111.21, 111.12, 105.61,
1H NMR (400 MHz, Acetone)
d
7.69 (d, J ¼ 3.9 Hz, 1H), 7.47e7.36
(
(
m, 5H), 7.19 (m, 8H), 7.13 (d, J ¼ 3.9 Hz,1H), 6.50 (m, 2H), 6.33e6.28
102.53, 94.82, 96.35, 44.38, 29.71, 26.80, 26.61, 19.71, 12.37.
þ
s, 1H), 4.03 (m, 2H), 3.73 (m, 2H), 3.39 (m, 4H), 1.89e1.77 (m, 2H),
MALDI-TOF: m/z calcd for C44
720.42.
HRMS (ESI) (M , C44
720.2972.
H
44
N
4
O
2
SSi: 720.30 [M] ; found:
1
9
.75e1.61 (m, 2H), 1.60e1.45 (m, 4H), 1.15 (t, J ¼ 7.0 Hz, 6H), 1.04 (s,
H).
MALDI-TOF: m/z calcd for C38
11.59.
38 2
HRMS (ESI) (M , C H49NO SSi): calcd, 611.3253; found,
þ
44 4 2
H N O SSi): calcd, 720.2954; found,
þ
H
2
49NO SSi: 611.33 [M] ; found:
6
þ
2.2.9. Synthesis of compound chromophore F2
611.3229.
In a similar way, chromophore F2 was obtained from compound
4
b.
2
.2.6. Synthesis of compound 4a
To a solution of compound 3a (1.59 g, 3.00 mmol) in dry THF
1H NMR (400 MHz, Acetone)
d
8.11 (dd, J ¼ 15.7, 6.0 Hz, 1H), 7.68
(
3
6
m, 5H), 7.63 (d, J ¼ 3.8 Hz, 1H), 7.52e7.35 (m, 7H), 7.31 (dd, J ¼ 16.0,
(
20 mL) a 2.4 M solution of n-BuLi in hexane (1.9 mL, 4.50 mmol)
.5 Hz, 1H), 7.12 (d, J ¼ 3.8 Hz, 1H), 6.75 (dd, J ¼ 15.7, 6.0 Hz, 1H),
.37 (d, J ¼ 8.8 Hz, 1H), 6.30 (s, 1H), 4.12 (t, J ¼ 6.2 Hz, 2H), 3.76 (dd,
ꢀ
was added dropwise at ꢂ78 C under N
2
. After this mixture was
stirred at this temperature for 1 h, dry DMF (0.28 mL, 3.60 mmol)
J ¼ 11.5, 5.2 Hz, 2H), 3.47 (m, 4H), 1.88 (m, 2H), 1.85 (s, 6H),
was introduced. The resulting solution was stirred for another
1
1
.74e1.63 (m, 2H), 1.57 (m, 4H), 1.23e1.15 (m, 6H), 1.03 (s, 9H).
ꢀ
h at ꢂ78 C and then allowed to warm up to room temperature.
13
C NMR (100 MHz, Acetone)
50.31, 139.86, 138.31, 137.19, 135.39, 133.91, 130.33, 129.59, 127.71,
26.56, 115.98, 112.66, 112.56, 111.89, 111.59, 111.03, 104.82, 97.88,
6.19, 95.29, 67.82, 63.58, 53.78, 44.26, 32.40, 26.41, 25.90, 25.44,
8.89, 12.21.
MALDI-TOF: m/z calcd for C50
d 176.08, 173.79, 159.06, 155.34,
The reaction was quenched by water. THF was removed by evapo-
ration. The residue was extracted using CH Cl
(3 ꢁ 30 mL). The
organic layer was dried by MgSO and concentrated in vacuo. The
residue was purified by column chromatography on silica gel
hexane/acetone, v/v, 8/1) to obtain a red solid (1.16 g, 72%).
1
1
9
1
2
2
4
(
þ
56 4 3
H N O SSi: 820.38 [M] ; found:
1
H NMR (400 MHz, Acetone)
d 9.86 (s, 1H), 7.88e7.76 (m, 5H),
820.51.
7
.53 (t, J ¼ 6.6 Hz, 1H), 7.52e7.42 (m, 7H), 7.25 (d, J ¼ 16.2 Hz, 1H),
þ
HRMS (ESI) (M , C50
H
56
N
4
O
3
SSi): calcd, 820.3842; found,
7
.17 (d, J ¼ 3.9 Hz, 1H), 6.31 (dd, J ¼ 8.9, 2.4 Hz, 1H), 5.78 (d,
820.3864.
J ¼ 2.4 Hz, 1H), 2.97 (q, J ¼ 7.0 Hz, 4H), 1.20 (s, 9H), 0.80e0.71 (m,
6
H).
13C NMR (100 MHz, Acetone)
38.27, 135.44, 132.47, 130.18, 128.21, 128.03, 127.89, 127.39, 125.02,
15.31, 105.38, 102.00, 44.12, 26.29, 26.01, 19.34, 11.86.
MALDI-TOF: m/z calcd for C33 SSi: 539.23 [M] ; found:
d
182.08, 155.01, 149.09, 140.06,
3. Results and discussion
1
1
5
5
2
3
1
3.1. Synthesis and characterization of chromophores
þ
H
37NO
2
39.19.
The traditional chromophore FTC was synthesized according to
the literature [12]. Synthesized F1 and F2 chromophores were
depicted in Scheme 1. The difference between the chromophores F1
and F2 is the position of the big isolation group (TBDPS) on the
donor part. The Modified 4-(diethylamino)ary compounds 1b was
synthesized from 4-(diethylamino)salicylaldehyde 1a with 1-
þ
HRMS (ESI) (M , C33
H37NO
2
SSi): calcd, 539.2314; found,
39.2301.
.2.7. Synthesis of compound 4b
In a similar way, compound 4b was obtained from compound
b.
2
Chloro-6-hydroxyhexane under N . To modify the shape of the
1H NMR (400 MHz, Acetone)
H), 7.72e7.65 (m, 5H), 7.47e7.36 (m, 7H), 7.29e7.20 (m, 1H), 7.11
d, J ¼ 3.9 Hz, 1H), 6.33 (m, 1H), 6.27 (d, J ¼ 8.5 Hz, 1H), 4.08 (t,
J ¼ 6.3 Hz, 2H), 3.73 (q, J ¼ 5.9 Hz, 2H), 3.48e3.38 (m, 4H), 1.93e1.81
m, 2H), 1.72e1.62 (m, 2H), 1.61e1.48 (m, 4H), 1.17 (t, J ¼ 7.0 Hz, 6H),
.03 (s, 9H).
13C NMR (100 MHz, Acetone)
39.71, 138.19, 135.57, 133.59, 129.67, 129.20, 128.99, 127.73, 124.73,
15.73,112.44,104.63, 95.25, 68.00, 63.72, 44.22, 32.51, 26.37, 25.93,
d
9.82 (s, 1H), 7.75 (d, J ¼ 3.9 Hz,
chromophore to avoid close packing, the hydroxy group on the
termination of compound 1a and 1b was protected by the tert-
Butyldiphenylchlorosilane. The aldehyde 2a and 2b were
condensed with 2-thienyltriphenylphosphonate bromide by Wittig
condensation to gain 3a and 3b. As expected, after introduction of
the thiophene bridge by Wittig condensation, treatment of com-
pound 4a and 4b with n-BuLi and DMF gave an aldehyde 5a and 5b.
The target chromophore F1 and F2 were obtained via Knoevenagel
condensation reaction of the aldehyde 5a and 5b with acceptor TCF
in the presence of a catalytic amount of triethylamine.
(
(
1
d
181.99, 158.66, 155.40, 149.94,
1
1
2
6
6
5.49, 18.87, 12.26.
MALDI-TOF: m/z calcd for C39
þ
1
H
49NO
3
SSi: 639.32 [M] ; found:
All of the chromophores were fully characterized by H-NMR,
13
39.30.
HRMS (ESI) (M , C39
C-NMR, and mass spectroscopy. These new chromophores
þ
H49NO
3
SSi): calcd, 639.3202; found,
possess good solubility in common organic solvents, such as
dichloromethane, chloroform and acetone.
39.3227.

242
Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
Scheme 1. Chemical structures and synthetic scheme for chromophore F1 and F2.
3.2. Thermal analysis
Table 1
Summary of thermal and optical properties and EO coefficients of chromophores
FTC, F1 and F2.
NLO chromophores must be thermally stable enough to with-
ꢀ
a
ꢀ
lbmax (nm)
lcmax (nm)
Dld (nm)
re33 (pm/)
stand encountered high temperatures (>200 C) in electric field
poling and subsequent processing of chromophore/polymer ma-
terials. Thermal properties of these chromophores were measured
Chromophore
T ( C)
d
FTC
F1
F2
244
258
275
676
713
708
621
643
643
55
70
65
39
52
91
by Thermogravimetric Analysis (TGA) under N
2
at the rate of
ꢀ
ꢂ1
a
1
0
C.min . As shown in Fig. 1 and Table 1, the decomposition
T
d
was determined by an onset point, and measured by TGA under nitrogen at a
C/min.
max was measured in CHCl
ꢀ ꢀ
heating rate of 10
ꢀ
temperature (T ) of chromophore F1 was 258 C and F2 was 275 C,
d
b
l
3
.
which are much higher than the traditional FTC chromophore
c
d
e
L
max was measured in dioxane.
ꢀ
(
244 C). As the literature report, introducing the iosolation of
b
c
Dl
¼
l
max max
ꢂl .
r
33 values were measured at the wavelength of 1310 nm.
benzyloxy group to the similar structure of donor-
p
-acceptor
ꢀ
molecules, the T
d
is only 253 C [27]. These data indicate that the
introduced isolation group (bulky siloxane) can effectively increase
the thermal stability of the chromophores. The T of these novel
d
chromophores was higher enough for the EO device preparation.
The excellent thermal stability of the two chromophores makes
them suitable for practical device fabrication and the EO device
preparation.
3.3. Optical properties
In order to reveal the effect of the location of bulky siloxane on
the intramolecular charge transfer (ICT) of dipolar chromophores,
UVeVis absorption spectra of the two chromophores
ꢂ5
ꢂ1
(
c ¼ 1 ꢁ 10 mol L ) were measured in a series of aprotic solvents
with different polarity. The solvatochromic behavior of the two
chromophores could be investigated to explore the polarizability of
chromophores in a wide range of dielectric environments (Fig. 2).
The spectrum data are summarized in Table 1. The synthesized
ꢀ
ꢂ1
Fig. 1. TGA curves of chromophores FTC, F1 and F2 with a heating rate of 10 C min
in a nitrogen atmosphere.

Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
243
part of F2,
lmax was shifted 32 nm to the longer wavelength of
708 nm. Larger shifts (37 nm) of
lmax were measured for F1
(
l
max ¼ 713 nm), indicating the strong electron-donating effect of
the TBDPS units. On the basis of simple solvent studies using polar
chloroform and less polar 1, 4-dioxane, it was found that chromo-
phores F1 and F2 showed a larger energy shift of the charge-
transfer band (△
l
¼ 70, 65 nm) than FTC (△ ¼ 55 nm). The
l
resulting spectrum data confirm the concept that chromophores F1
and F2 were more easily polarizable than the traditional FTC
chromophore.
3.4. Theoretical calculations
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
[
e31 G* (d, p) basis set using the Gaussian 09 program package
28e30]. The first hyperpolarizability ( ) values were calculated at
CAM-B3LYP level by employing 6-31G* basis set using the Gaussian
9 and the direction of the maximum value is directed along the
b
0
charge transfer axis of the chromophores. The data obtained from
DFT calculations are summarized in Table 2.
The frontier molecular orbitals are often used to characterize the
chemical reactivity and kinetic stability of a molecule and to obtain
qualitative information about the optical and electrical properties
of molecules [30,31]. [32] Besides, the HOMO-LUMO energy gap is
also used to understand the charge transfer interaction occurring in
the chromophore molecule [32e34]. In the case of these chromo-
phores, Fig. 3 represents the frontier molecular orbitals of chro-
mophores F1, F2 and FTC. According to Fig. 3, it is clear that the
electronic distribution of the HOMO is delocalized over the donor
and p-bridge part, 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 Fig. 3 and Table 2. For the chromophore F1, the
new donor structure narrows the energy gap between the HOMO
and LUMO energy with
narrows the energy gap between the HOMO and LUMO energy with
E values of 2.03 eV. By contrast, the E value of FTC is 2.07 eV. This
DE values of 2.00 eV chromophore F2
D
D
result indicates that chromophore F1 and F2 may exhibit better ICT
and NLO property than FTC. As reported [32], the optical gap is
lower, the charge-transfer (ICT) ability is greater and thus improve
the nonlinearity. Chromophores F1 and F2 showed a lower optical
gap, so it can indicate that chromophore F1 and F2 may exhibit
better ICT and NLO property than the traditional FTC chromophore.
This result corresponded with the conclusion of UVeVis spectra
analysis.
Further, the theoretical microscopic
Gaussian 09. The value is related to the substituent, molecular
configuration, and intramolecular charge-transfer. As the reference
reported earlier, the has been calculated at CAM-B3LYP/6-31G*
b was calculated by
b
b
level [35]. From this, the scalar quantity of
b can be computed
Fig. 2. UVeVis absorption spectra in different solvents of chromophore FTC, F1 and F2
ꢂ5
ꢂ1
(
c ¼ 1 ꢁ 10 mol L ).
Table 2
Data from DFT calculations.
chromophores exhibited a similar
pep
* intramolecular charge
Chromophores
EHOMO (eV)
ELUMO(eV)
DE
a
(eV)
b
max
b
(10ꢂ30 esu)
transfer (ICT) absorption band in the visible region. As shown in
Fig. 2, in both non-polar and polar solvents such as 1, 4-dioxane and
FTC
F1
F2
ꢂ5.23
ꢂ5.05
ꢂ5.09
ꢂ3.16
ꢂ3.05
ꢂ3.06
2.07
2.00
2.03
549.13
565.28
535.52
CHCl
* intramolecular charge-transfer (ICT) absorption band. The
effect of the electron donor was characterized by shifting the ab-
sorption maximum ( max) compared with max of the benchmark
FTC. When an additional hexyloxyl group was added to the donor
3
, chromophores F1, F2 and FTC exhibited a similar broad
pep
D
E ¼ ELUMOꢂEHOMO
.
a
Results was calculated by DFT.
b values were calculated using gussian 09 at CAM-B3LYP/6-31G* level and the
b
l
l
direction of the maximum value is directed along the charge transfer axis of the
chromophores.

244
Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
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 multiple
reflections [39]. As reported earlier [15,19,20,40], 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 position of the isolation group should be suitable. So these three
chromophores may have an obvious difference on the macroscopic
EO activity. The EO coefficient (r33), defining the efficiency of
translating molecular microscopic hyperpolarizability into macro-
scopic EO activities, was described as follows:
r33 ¼ 2Nf ð
u
Þ
b
Dcos3 E.n4
q
(2)
Fig. 3. The frontier molecular orbitals of chromophores FTC, F1 and F2.
where r33 is the EO coefficient of the poled polymer, N represents
the aligned chromophore number density and f( ) denotes the
Lorentz-Onsager local field factors. The term <cos
u
3
from the x, y, and z components according following equation.
q> is the
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
the chromophore. Realization of large electro-optic activity for the
dipolar organic chromophores containing materials requires the
ꢀ
ꢁ
1
=2
2
x
2
2
b
¼
b
þ
b
þ
b
(1)
y
z
P
1
where
b
¼
b
þ
ðb
þ
b
þ
b
jji
Þ; i; j2ðx; y; zÞ when used
i
iii
3
ijj
jij
isj
carefully and consistently, this method of DFT has been shown to
give relatively consistent descriptions of first-order hyper-
polarizability for a number of similar chromophores [36e38].The
data obtained from DFT calculations were summarized in Table 2.
simultaneous optimization of first hyperpolarizability (
b
), acentric
There was no obvious change of the
b
values of these chromophores
esu), which illustrated that the
3
order <cos
q>, and number density (N) [26,40]. When at low
ꢂ30
J1eJ3 (535.52e565.28 ꢁ 10
concentration, the electro-optic activity increased with chromo-
phore density, dipole moment and the strength of the electric
isolationed group had little influence on their main conjugated
structures under the simulated condition. To our surprising, chro-
mophore F2 showed smaller hyperpolarizability than chromophore
poling field. However, when the concentrations of chromophores
3
increased to a certain extent, the N and <cos
q
> are no longer in-
FTC. The
b value is related to the substituent, molecular configu-
dependent factors. Then
ration, and intramolecular charge-transfer. May be the introduced
big isolation group decreased the hyperpolarizability (see Fig. 4).
Dcos3
E
h
i
2
q
¼ ð
m
F=5kTÞ 1 ꢂ L ðW=kTÞ
(3)
3
.5. Electric field poling and EO property measurements
where k is the Boltzmann constant and T is the Kelvin (poling
temperature). F ¼ [f (0)E ] where E is the electric poling field. L is
P
p
To testify the effect of donor modification on the EO activities, a
the Langevin function, which is a function of W/kT, the ratio of the
intermolecular electrostatic energy (W) to the thermal energy (kT).
L is related to electrostatic interactions between molecules. So
when the intermolecular electrostatic interactions are neglected,
the electro-optic coefficient (r33) should increase linearly with
chromophore density, dipole moment, first hyperpolarizability and
the strength of the electric poling field.
series of guest-host polymers were generated by formulating the
chromophores into amorphous polycarbonate (APC) using dibro-
momethane 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 re-
sidual solvent. The corona poling process was carried out at a
temperature of 10 C above the glass transition temperature (T
ꢀ
The r33 values of films containing chromophores FTC (fillm-A),
F1 (film-B) and F2 (film-C) were measured in different loading
densities, as shown in Table 3 (the error bar is about 10%) and Fig. 5,
For the traditional FTC chromophore, the r33 values were gradually
ꢀ
g
) of
the polymer. The r33 values were measured using the Teng-Man
ꢂ1
ꢂ1
improved from 12 p.m. V (10 wt%) to 39 p.m. V (25 wt%), as the
Table 3
Summary of EO Coefficients of Chromophores FTC, F1 and F2 at different densities.
Wt%
ar33 (pm/V)
F1
FTC
F2
10
15
20
25
30
35
40
12 ± 2
20 ± 2
33 ± 3
39 ± 4
e
15 ± 2
26 ± 3
43 ± 4
52 ± 5
e
17 ± 2
28 ± 3
44 ± 4
62 ± 6
78 ± 7
91 ± 8
64 ± 6
a
Fig. 4. Optimized structures of chromophores FTC, F1 and F2.
r33 values were measured at the wavelength of 1310nm.

Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
245
translating the microscopic hyper-polarizability into macroscopic
EO coefficients was the balance of molecular steric hindrance and
molecular free mobility. The r33 value for F2-APC showed that
chromophore F2 in APC tuned the best balance of steric hindrance
and molecular free mobility. The r33 value of chromophore F1-APC
was much lower than chromophore F2-APC, which might be
attributed that chromophore F1 with less effective steric hindrance
and excessive free mobility. The excessive free mobility led to the
chromophore easy to form the antiparallel dimmer. In film-C con-
taining the chromophore F2, steric hindrance and molecular free
mobility was fine-tuned and the high efficiency of translating the
microscopic nonlinearity into the macroscopic EO coefficients was
achieved.
As results showed, all of these r33 values are higher than the
ꢂ1
traditional FTC chromophores (r33 ¼ 39 p.m. V ). These results
proved that introducing big isolation group into the donor part
have an obvious advantage in reducing intermolecular electrostatic
interactions and enhancing the macroscopic EO activity. Besides,
the position of the isolation plays an equally important role in
achieving the ultra large EO coefficients of materials.
Fig. 5. EO coefficients of NLO thin films as
a function of chromophore loading
densities.
4. Conclusions
concentration of chromophores increased, the similar trend of
In this research, two new NLO chromophores with big isolation
enhancement was also observed for chromophore F1, whose r33
ꢂ1
ꢂ1
group in different positon of the donor part have been synthesized
and systematically investigated by NMR, MS and UVevis absorp-
tion spectra. The energy gap between ground state and excited
state together with molecular nonlinearity were studied by UVevis
absorption spectroscopy, DFT calculations and EO measurements.
Theoretical and experimental investigations suggest that the
isolation group play a critical role in affecting the linear and
nonlinear properties of dipolar chromophores. In general, the en-
ergy gap of F1 and F2 chromophore is 2.00 and 2.03 eV respectively,
which is much lower than FTC chromophore without isolation
group on the donor part. These results suggest that by introducing
isolation group into suitable position of the chromophore can
dramatically improve the nonlinear optical properties. These novel
chromophores with good thermal stability and large molecular
hyperpolarizabilities, which can be effectively translated into large
EO coefficients in poled polymers. The key point of translating the
microscopic hyper-polarizability into macroscopic EO coefficients
was the balance of molecular steric hindrance and molecular free
mobility. The doped film-C containing chromophore F2 displayed a
maximum r33 value of 91pm/V at the doping concentration of 35 wt
values increased from 15 p.m. V (10 wt%) to 52 p.m. V (25 wt%).
Film-C containing chromophoe F2 gained the r33 values from 17
ꢂ1
ꢂ1
p.m. V (10 wt%) to 92 p.m. V (35 wt%), and when the chro-
mophore loading density was increased to 40 wt%, the film-C
presented a downtrend to 64 pm/V. However, when the chromo-
phore loading density was increased to 30 wt%, both the film-A and
film-B showed an obvious phase separation.
Through the outcome above, we can obviously see that the Film-
C containing chromophore F2 have nearly 3 times higher r33 value
than the film-B containing chromophore FTC and nearly 2 times
higher r33 value than the film-B containing chromophore F1,
illustrating that the isolation group and the position of the isolation
group can significantly increase their macroscopic EO activity. Be-
sides, from Fig. 5, we can see that all the films presented the similar
r33 values when the chromophore loading density is low, but as the
chromophore loading densities is becoming higher, these films
presented an obvious difference. These results can be explained as
follows: When the concentration of chromophore in APC is low, the
intermolecular dipolar interactions are relatively weak. So the dif-
ference between these films is not obvious. As the chromophore
loading densities increased, the effect of inter-chromophore dipo-
leedipole interaction is becoming stronger. As results showed,
these r33 values are all higher than the traditional FTC chromo-
phores (r33 ¼ 39 pm/V) [12]. As reported in the literature, the
similar chromophore without isolation group showed maximum
%
, while film-B containing chromophore F1 showed a maximum r33
value of 52 pm/V at 25 wt %. The chromophore F2 showed the
lowerest value of but the largest r33 value. So chromophore F2 has
an advantage in converting the values of the chromophores into
b
b
large macroscopic optical nonlinearities (r33). The r33 value for F2-
APC proved that chromophore F2 in APC tuned the best balance of
steric hindrance and molecular free mobility. We believe that these
new chromophores can be used in exploring high-performance
organic EO and photorefractive materials where both thermal sta-
bility and optical nonlinearity are of equal importance.
r33 values of 43 pm/V [27].This result proved that by introducing
isolation group into the donor part has an obvious advantage in
reducing intermolecular electrostatic interactions thus enhance the
macroscopic EO activity.
The different r33 values between the chromophre F1 and F2 can
be explained as follows: in one hand, huge steric hindrance might
effectively prevent the aggregation of chromophores and attenuate
the dipole-dipole interactions; in the other hand, chromophores
might be fettered by the huge steric hindrance and be twined
around the neighbour polymer chains or other hindrance group of
chromophores, which had an unwanted influence on the acentric
ordering of chromophores in poling. When the steric effect was
excessive, it would restrain chromophores from moving or rotating
for the alignment in the poling process and the number of chro-
mophore truly oriented was small. Herein, the key point of
Acknowledgements
This work was financially supported by the Science Foundatio-
nof Zhejiang Sci-Tech University (ZSTU) under Grant No. 16012174-
Y. Besides, All calculations were performed on the cluster of Key
Laboratory of Theoretical and Computational Photochemistry,
Ministry of Education, China. We are also grateful to the National
Nature Science Foundation of China (no. 61101054, no. 21003143
and no. 21473227) for financial support.

2
46
Y. Yang et al. / Dyes and Pigments 139 (2017) 239e246
Macromolecules 2006;39:6951e61.
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