Synthesis of phenyltetraene chromophores-based hybrid materials for large nonlinear optical activity

Article abstract of DOI:10.1016/j.dyepig.2013.03.016

Two new chromophores, CLDS1 and CLDS2, based on a similar D-π-A structure as the phenyltetraene chromophore CLD, but with an alkylthio group perpendicular to the direction of the long molecular axis as isolation spacer were synthesized. Through a urethane

Full text of DOI:10.1016/j.dyepig.2013.03.016

Accepted Manuscript
Synthesis of phenyltetraene chromophores-based hybrid materials for large nonlinear
optical activity
Ting Liang, Yuanjing Cui, Jiancan Yu, Wenxin Lin, Yu Yang, Guodong Qian
PII:
S0143-7208(13)00105-8
10.1016/j.dyepig.2013.03.016
DYPI 3885
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 February 2013
Revised Date: 16 March 2013
Accepted Date: 20 March 2013
Please cite this article as: Liang T, Cui Y, Yu J, Lin W, Yang Y, Qian G, Synthesis of phenyltetraene
chromophores-based hybrid materials for large nonlinear optical activity, Dyes and Pigments (2013), doi:
10.1016/j.dyepig.2013.03.016.
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ACCEPTED MANUSCRIPT
Synthesis of phenyltetraene chromophores-based hybrid materials for
large nonlinear optical activity
Ting Liang, Yuanjing Cui*, Jiancan Yu, Wenxin Lin, Yu Yang, Guodong Qian
State Key Laboratory of Silicon Materials, Cyrus Tang Center for Sensor Materials and
Applications, Department of Materials Science & Engineering, Zhejiang University, Hangzhou,
China
Abstract: Two new chromophores, CLDS1 and CLDS2, based on a similar D-π-A structure as
the phenyltetraene chromophore CLD, but with an alkylthio group perpendicular to the direction
of the long molecular axis as isolation spacer were synthesized. Through a urethane forming
reaction between 3-isocyanatopropyltriethoxysilane (ICTES) and chromophores, the alkoxysilane
dyes were synthesized. Subsequently, a series of transparent and homogeneous inorganic-organic
hybrid films were prepared via a sol-gel process of the alkoxysilane dye with triethoxyvinylsilane.
1
The molecular structures of the resultants were confirmed by H NMR, FTIR and UV-visible
spectra. After electric poling, the nonlinear optical coefficients (d33) of the hybrid films F-CLDS1
and F-CLDS2 were determined to be 104.54 and 98.17 pm V-1, respectively, which are almost 3
times the d33 value of hybrid film containing CLD (37.78 pm V-1), indicating that the
incorporation of alkylthio group can efficiently improve the macroscopic optical nonlinearities of
hybrid materials.
Keywords: nonlinear optics; phenyltetraene chromophore; isolation group; inorganic-organic
hybrid; thin films; sol-gel
Corresponding author. Tel.: +86-571-87952334; Fax: +86-571-87951234.
E-mail address: cuiyj@zju.edu.cn (Y. Cui), gdqian@zju.edu.cn (G. Qian)
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1. Introduction
Second-order nonlinear optical (NLO) materials have received considerable attention due to
their wide applications in all-optical switching, fast optical communications, and electro-optic
modulation [1-2]. Compared with NLO inorganic crystals, organic NLO polymers display many
advantages including large nonlinear optical coefficients, low dielectric constants, fast response
time and excellent processing [1-9]. However, the thermal instability of poling-induced
chromophore dipole alignment of the poled polymers limits their practical applications in EO
devices. Linking the chromophores via covalent bonds onto a highly rigid matrices or
cross-linkable polymers can effectively stabilize the dipole orientation. Recently,
inorganic-organic hybrid NLO materials through sol-gel process have been considered to be one
of the most potential matrix candidates for stabilizing poled dipole orientation [10-19]. By
incorporating the NLO active chromophores into the inorganic silica networks, the resulting
inorganic-organic materials may display high optical transparency, good thermal stability, and
excellent NLO properties. Moreover, the inorganic silica matrix provides an inert environment
for organic chromophores and theoretically will prevent damage induced by corona discharge
poling and thermal decomposition [20-22].
To achieve large NLO activity, the rational molecular designs of dipole chromophores are
crucial and have been demonstrated in a series of conjugated push-pull molecules. Among these
molecular systems, the phenyltetraene conjugated bridges attached with terminal strong acceptors,
such as FTC- and CLD-type chromophores, represents one of the most effective chromophores
that result in large hyperpolarizability (β) [23-25]. However, the polymers doped with FTC- and
CLD-chromophores tend to lower the NLO response at higher doping content because of the
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strong dipole–dipole interactions among the chromophore moieties. According to the
site-isolation principle, the introduction of some isolation groups to the chromophores is a very
promising approach to decrease the dipole-dipole interactions and increase the poling efficiency
[26, 27]. Indeed, Dalton and Jen have demonstrated a series of dendritic polymers with FTC- and
CLD-chromophores for significantly enhanced optical nonlinearities [28, 29]. However, there are
very few reports concerning the phenyltetraene chromophores linked hybrid materials. In addition,
how to efficiently translate the large β values of the organic chromophores into high macroscopic
NLO activities of hybrid materials receives little attention.
In this report, we design and synthesize two new chromophores, CLDS1 and CLDS2, based
on a similar D-π-A structure as the chromophore CLD, but with an alkylthio group perpendicular
to the direction of the long molecular axis as isolation spacer (Fig. 1). The chromophore CLDS1
and CLDS2 have a comparable β value, but differ in the position of the hydroxyl group, enabling
them to be covalently anchored in silica matrix by their donor end or by their isolation spacer.
Herein, we report the preparation and structural characterizations of the chromophore-linked
hybrid materials, and discuss the influence of isolation group and anchoring position on NLO
properties.
2. Experimental Section
2.1 Materials and measurements
All commercially available starting chemicals were purchased and used as received.
Tetrahydrofuran (THF) was dried over molecular sieves (3A). Chromophore CLD was
synthesized as reported [23].
UV-visible absorption spectra were collected with a Hitachi U-4100 spectrometer. Infrared
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specra were recorded by using a vector 22 Fourier transform infrared spectrometer (Bruker) in
1
KBr disks. H NMR spectra of compounds were measured on a Bruker Avance DMX500
spectrometer. Elemental analyses were taken from a Thermo Finnigan Flash EA1112
microelemental analyzer. The thickness of hybrid films was collected via a Tencor alpha-step 200
surface profiler. Thermogravimetric analysis (TGA) was performed with a TA Instruments SDT
o
Q600 at a heating rate of 10 C min-1 and under a nitrogen atmosphere. Differential scanning
o
calorimetry (DSC) was studied using a Netzsch Instruments 200 F3 with a heating rate of 10 C
min-1 in a nitrogen atmosphere. In situ second harmonic generation (SHG) measurements of the
poled films were taken using a Q-switched Nd:YAG laser at 1064 nm and a Y-cut quartz crystal
as the Reference.
2.2 Synthesis of chromophore CLDS1 and CLDS2
2-(butylthio)-3-(4-((2-hydroxyethyl)(methyl)amino)styryl)-5,5-dimethylcyclohex-2-enone
(1a). 1a was synthesized according to a literature procedure [30].
3-(4-(diethylamino)styryl)-2-(2-hydroxyethylthio)-5,5-dimethylcyclohex-2-enone (1b). To a
sodium ethylate (105 mmol) in ethanol (100 mL) was added mercaptoethanol (7.00 mL, g, 100
mmol), and the solution was stirred for 15 min followed by the addition of isophorone oxide. The
solution became dark immediately and was stirred for 30 min to form intermediate
2-((2-hydroxyethyl)thio)-3,5,5-trimethylcyclohex-2-enone,
and
then
4-(diethylamino)-
benzaldehyde (110 mmol) was added and vigorously stirred at 70 oC for 12 h. The mixture was
cooled to room temperature and extracted with dichloromethane, and purified by chromatography
1
using AcOEt/DCM=1/6. H NMR (500 MHz, CDCl3, δ ppm): 7.92 (d, J=16.1 Hz, 1H), 7.47 (d,
J=8.9 Hz, 2H), 7.08 (d, J=16.1 Hz, 1H), 6.66 (d, J=8.9 Hz, 2H), 3.56 (t, 3H), 3.41 (q, 4H), 2.82 (t,
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2H), 2.66 (s, 2H), 2.45 (s, 2H), 1.20 (t, 6H), 1.09 (s, 6H).
3-(4-((2-(tert-butyldimethylsilyloxy)ethyl)(methyl)amino)styryl)-2-(butylthio)-5,5-dimethylc
yclohex-2-enone (2a). 1a (7.90 g), imidazole (2.57 g) and tert-butylchlorodimethylsilane were
mixed in DMF (30 mL), and the solution was stirred for 30 min and poured into saturated NH4Cl
solution. The product was extracted with AcOEt and purified by chromatography
1
(PE/AcOEt=30/1) to yield orange oil. H NMR (500 MHz, CDCl3, δ ppm): 7.93 (d, J=16.2 Hz,
1H), 7.46 (d, J=8.8 Hz, 2H), 7.00 (d, J=16.2 Hz, 1H), 6.69 (d, J=8.8 Hz, 2H), 3.79 (t, 2H), 3.53 (t,
2H), 3.06 (s, 3H), 2.74 (t, 2H), 2.63 (s, 2H), 2.42 (s, 2H), 1.46 (m, 2H), 1.42 (m, 2H), 1.09 (s,
6H), 0.88 (m, 12H), 0.02 (s, 6H).
2-(2-(tert-butyldimethylsilyloxy)ethylthio)-3-(4-(diethylamino)styryl)-5,5-dimethylcyclohex-
1
2-enone (2b). In a similar manner described above, 2b was synthesized as a red oil (68%). H
NMR (500 MHz, CDCl3, δ ppm): 7.90 (d, J=16.2 Hz, 1H), 7.47 (d, J=8.8 Hz, 2H), 7.00 (d,
J=16.2 Hz, 1H), 6.65 (d, J=8.8 Hz, 2H), 3.70 (t, J=7.2 Hz, 2H), 3.40 (q, J=7.1 Hz, 4H), 2.87 (t,
J=7.2 Hz, 2H), 2.61 (s, 2H), 2.41 (s, 2H), 1.19 (t, J=7.0 Hz, 6H), 1.08 (s, 6H), 0.87 (s, 9H), 0.02
(s, 6H).
2-(3-(4-((2-(tert-butyldimethylsilyloxy)ethyl)(methyl)amino)styryl)-2-(butylthio)-5,5-dimeth
ylcyclohex-2-enylidene)acetonitrile (3a). To a mixture of NaH (4.52 g, 60 wt%, 113 mmol) in
THF (100 mL) at 0 oC was added diethyl cyanomethylphosphonate (17 mL) dropwise by syringe.
After the solution clear, 2a (16.46 g, 32.8 mmol) in THF (30 mL) was added to above solution
and the solution was refluxed for 14 h. The product was extracted with AcOEt, evaporated and
purified by chromatography (ethyl acetate/petroleum ether=1/30) to yield yellow oil (3.94 g,
23%). 1H NMR (500 MHz, CDCl3, δ ppm): 7.89 (d, J=16.3 Hz, 1H), 7.43 (d, J=8.9 Hz, 2H), 6.86
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(d, J=16.2 Hz, 1H), 6.70 (d, J=8.9 Hz, 2H), 6.25 (s, 1H), 3.80 (t, J=6.0 Hz, 2H), 3.53 (t, J=6.1 Hz,
2H), 3.06 (s, 3H), 2.59 (s, 2H), 2.58 (t, J=7.0 Hz, 2H), 2.49 (s, 2H), 1.50 (m, 2H), 1.43 (m, 2H),
1.03 (s, 6H), 0.90 (m, 12H), 0.03 (s, 6H). 48 H (48H). 13C NMR (75 MHz, CDCl3, δ ppm): 158.8,
149.9, 149.1, 134.8, 128.9, 126.7, 124.8, 124.4, 119.6, 112.0, 94.6, 60.7, 54.8,43.6, 41.7, 39.4,
35.4, 32.0, 30.4, 28.2, 26.1, 22.1,18.4, 13.9, -5.2.
2-(2-(2-(tert-butyldimethylsilyloxy)ethylthio)-3-(4-(diethylamino)styryl)-5,5-dimethylcycloh
ex-2-enylidene)acetonitrile (3b). In a similar manner described above, 3b was synthesized as an
orange oil (71%). 1H NMR (500 MHz, CDCl3, δ ppm): 7.85 (d, J=1H), 7.42 (d, 2H), 6.86 (d, 1H),
6.66 (d, 2H), 6.26 (s, 1H), 3.69 (t, 2H), 3.41 (q, 4H), 2.70 (t, 2H), 2.58 (s, 2H), 2.48 (s, 2H), 1.20
(t, 6H), 1.03 (s, 6H), 0.89 (s, 9H), 0.04 (s, 6H).
2-(3-(4-((2-(tert-butyldimethylsilyloxy)ethyl)(methyl)amino)styryl)-2-(butylthio)-5,5-dimeth
ylcyclohex-2-enylidene)acetaldehyde (4a). To a 3a (3.94 g, 7.5 mmol) in toluene (50 mL) at -78
oC was added DIBAL-H (20 mL, 1M in hexane, 20 mmol) dropwise. The solution was stirred for
o
1 h at this temperature, warmed at -40 C for 30 min, added with AcOEt (5 mL) and water (20
mL), and vigorously stirred at room temperature for 8h after another AcOEt (50 mL) was added.
The alumina gel was filtered off by filtration and the solution was dried over rotary evaporator,
1
and was purified by chromatography (AcOEt/PE=1/25-2/20) to yield red oil (2.51g, 63%). H
NMR (500 MHz, CDCl3, δ ppm): 10.16 (d, J=8.1 Hz, 1H), 7.99 (d, J=16.3 Hz, 1H), 7.44 (d,
J=8.9 Hz, 2H), 7.03 (d, J=8.0 Hz, 1H), 6.87 (d, J=16.3 Hz, 1H), 6.69 (d, J=8.9 Hz, 2H), 3.80 (t,
J=6.0 Hz, 2H), 3.52 (t, J=6.0 Hz, 2H), 3.05 (s, 3H), 2.75 (s, 2H), 2.56 (t, J=7.2 Hz, 2H), 2.52 (s,
2H), 1.49 (m, 2H), 1.40 (m, 2H), 1.05 (s, 6H), 0.88 (m, 12H), 0.03 (s, 6H).
2-(2-(2-(tert-butyldimethylsilyloxy)ethylthio)-3-(4-(diethylamino)styryl)-5,5-dimethylcycloh
6

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ex-2-enylidene)acetaldehyde (4b). In a similar manner described above, 4b was synthesized as a
red oil (54%). 1H NMR (500 MHz, CDCl3, δ ppm): 10.15 (d, J=8.0 Hz, 1H), 7.95 (d, J=16.2 Hz,
1H), 7.43 (d, J=8.8 Hz, 2H), 7.01 (d, J=8.1 Hz, 1H), 6.87 (d, J=16.2 Hz, 1H), 6.66 (d, J=8.8 Hz,
2H), 3.69 (t, J=7.2 Hz, 2H), 3.41 (q, J=7.1 Hz, 4H), 2.75 (s, 2H), 2.71 (t, J=7.2 Hz, 2H), 2.51 (s,
2H), 1.21 (t, J=7.1 Hz, 6H), 1.05 (s, 6H), 0.87 (s, 9H), 0.02 (s, 6H).
2-(4-((1E,3E)-3-(3-(4-((2-(tert-butyldimethylsilyloxy)ethyl)(methyl)amino)styryl)-2-(butylthi
o)-5,5-dimethylcyclohex-2-enylidene)prop-1-enyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene
)malononitrile (5a). To
a
solution of 4a (2.45 g, 5.57 mmol) and
2-dicyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran (1.33 g, 6.69 mmol) in CH2Cl2
(50 mL) was added AcOH (0.1 mL) and piperidine (0.2 mL) mixture. The solution was stirred at
room temperature for 4 h, dried and purified by chromatography (CH2Cl2) to yield dark blue solid
o
1
(2.30 g, 58%). m.p.: 157 and 206 C. Td (95 wt%): 260 oC. H NMR (500 MHz, CDCl3, δ ppm):
8.11 (t, J=13.5 Hz, 1H), 8.03 (d, J=16.1 Hz, 1H), 7.52 (d, J=12.2 Hz, 1H), 7.45 (d, 8.9 Hz, 2H),
6.95 (d, J=16.1 Hz, 1H), 6.70 (d, J=9.0 Hz, 2H), 6.39 (d, J=14.8 Hz, 1H), 3.80 (t, J=5.9 Hz, 2H),
3.54 (t, J=5.9 Hz, 2H), 3.07 (s, 3H), 2.59 (t, J=7.2 Hz, 2H), 2.55 (s, 2H), 2.50 (s, 2H), 1.70 (s,
6H), 1.53 (m, 2H), 1.43 (m, 2H), 1.03 (s, 6H), 0.88 (m, 12H), 0.02 (s, 6H).
2-(4-((1E,3E)-3-(2-(2-(tert-butyldimethylsilyloxy)ethylthio)-3-(4-(diethylamino)styryl)-5,5-di
methylcyclohex-2-enylidene)prop-1-enyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malono
nitrile (5b). In a similar manner described above, 5b was synthesized as a dark solid (0.55 g,
58%). 1H NMR (500 MHz, CDCl3, δ ppm): 7.15 (t, J=13.5 Hz, 1H), 8.00 (d, J=15.7 Hz, 1H), 7.52
(d, J=12.1 Hz, 1H), 7.45 (d, J=8.1 Hz, 2H), 6.96 (d, J=16.1 Hz, 1H), 6.67 (d, J=7.5 Hz, 2H), 6.37
(d, J=15.0 Hz, 1H), 3.73 (t, J=6.9 Hz, 2H), 3.43 (q, J=6.8 Hz, 4H), 2.74 (t, J=6.9 Hz, 2H), 2.55 (s,
7

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2H), 2.51 (s, 2H), 1.70 (s, 6H), 1.22 (t, J=6.9 Hz, 6H), 1.03 (s, 6H), 0.89 (s, 9H), 0.04 (s, 6H).
Anal. Calcd. for C34H38N4O3S (582.76): C, 70.07; H, 6.57 ; N, 9.61. Found: C, 69.91; H, 6.61; N,
9.50.
2-(4-((1E,3E)-3-(2-(butylthio)-3-(4-((2-hydroxyethyl)(methyl)amino)styryl)-5,5-dimethylcycl
ohex-2-enylidene)prop-1-enyl)-3-cyano-5,5-dimethylfuran-2(5H)-ylidene)malononitrile
(CLDS1). To the 5a (2.05, 2.89 mmol) solution in acetone (50 mL) was added dilute HCl (4.30
mL, 1.2 N, 5.16 mmol). The solution was stirred overnight and diluted with water (100 mL) and
neutralized with potassium carbonate, the precipitation was collected as a dark blue powder (1.68
g, 98%). m.p. (with decomposition): 232 oC. Td (95 wt%): 254 oC. 1H NMR (500 MHz, CDCl3, δ
ppm): 8.08 (t, J=13.5 Hz, 1H), 8.03 (d, J=16.2 Hz, 1H), 7.50 (d, J=12.3 Hz, 1H), 7.47 (d, J=8.8
Hz, 2H), 6.95 (d, J=16.1 Hz, 1H), 6.78 (d, J=8.9 Hz, 2H), 6.39 (d, J=14.8 Hz, 1H), 3.87 (q, J=5.7
Hz, 2H), 3.58 (t, J=5.6 Hz, 2H), 3.08 (s, 3H), 2.58 (t, J=7.2 Hz, 2H), 2.53 (s, 2H), 2.50 (s, 2H),
1.71 (s, 6H), 1.66 (t, J=5.9 Hz, 1H), 1.52 (m, 2H), 1.43 (m, 2H), 1.04 (s, 6H), 0.88 (t, J=7.3 Hz,
3H).
2-(3-cyano-4-((1E,3E)-3-(3-(4-(diethylamino)styryl)-2-(2-hydroxyethylthio)-5,5-dimethylcycl
ohex-2-enylidene)prop-1-enyl)-5,5-dimethylfuran-2(5H)-ylidene)malononitrile (CLDS2). In
a similar manner described above, chromophore CLDS2 was synthesized as a green powder
o
o
1
(86%). m.p.: 246 C. Td (95 wt%): 252 C. H NMR (500 MHz, CDCl3, δ ppm): 8.14 (t, J=13.5
Hz, 1H), 8.00 (d, J=15.8 Hz, 1H), 7.51 (d, J=12.1 Hz, 1H), 7.46 (d, J=7.7 Hz, 2H), 6.98 (d,
J=16.0 Hz, 1H), 6.67 (d, J=8.2 Hz, 2H), 6.39 (d, J=14.8 Hz, 1H), 3.70 (t, J=6.0 Hz, 2H), 3.42 (q,
J=6.8 Hz, 4H), 2.81 (t, J=5.9 Hz, 2H), 2.57 (s, 2H), 2.51 (s, 2H), 1.70 (s, 6H), 1.22 (t, J=7.0 Hz,
6H), 1.04 (s, 6H). Anal. Calcd. for C35H40N4O2S (580.78): C, 72.38; H, 6.94; N, 9.65. Found: C,
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72.23; H, 6.93; N, 9.60.
2.3 Synthesis of alkoxysilane dye ICTES-CLD, ICTES-CLDS1 and ICTES-CLD2
ICTES-CLD. To the solution of CLD (0.40 g, 0.79 mmol) in dry THF (6 mmol) were added Tin
(II) 2-ethylhexanoate (3 drops) and 3-isocyanatopropyltriethoxysilane (ICTES, 0.45 g, 1.82
o
mmol). After the mixture was stirred at 65 C for 8 h, the solution was cooled to room
temperature was added to petroleum ether (150 mL) and a lot of precipitation occurred. The
precipitation was collected by filtration and purified by chromatography using AcOEt/petroleum
1
ether=1/1 as eluents to yield 0.24 g of dark blue solid (0.32 mmol, 41%). H NMR (500 MHz,
CDCl3, δ ppm): 8.01-8.29 (m, 1H), 7.39 (t, 2H), 6.71-6.86 (m, 4H), 6.25-6.38 (m, 3H), 4.92 (s,
1H), 4.23 (t, 2H), 3.82 (q, 6H), 3.63 (t, 2H), 3.18 (q, 2H), 3.05 (s, 3H), 2.42 (t, 4H), 1.69 (d, 6H),
1.26 (m, 9H), 1.05 (d, 6H), 0.62 (t, 2H).
ICTES-CLDS1. In a similar manner described above, ICTES-CLDS1 was synthesized as a dark
1
blue solid (71%). H NMR (500 MHz, CDCl3, δ ppm): 8.06 (m, 2H), 7.51 (d, J=12.2 Hz, 1H),
7.46 (d, J=8.3 Hz, 2H), 6.94 (d, J=16.1 Hz, 1H), 6.74 (d, J=8.0 Hz, 2H), 6.39 (d, J=14.8 Hz, 1H),
4.94 (s, 1H), 4.24 (s, 2H), 3.82 (q, 6H), 3.64 (s, 2H), 3.17 (d, 2H), 3.06 (s, 3H), 2.57 (m, 4H),
2.50 (s, 2H), 1.71 (s, 6H), 1.62 (m, 2H), 1.53 (m, 2H), 1.43 (m, 2H), 1.23 (q, 9H), 1.03 (s, 6H),
0.88 (s, 3H), 0.62 (t, 2H).
ICTES-CLDS2. In a similar manner described above, ICTES-CLDS2 was synthesized as a blue
1
solid (54%). H NMR (500 MHz, CDCl3, δ ppm): 8.11 (d, J=12.3 Hz, 1H), 8.00 (d, J=16.2 Hz,
1H), 7.49 (d, J=12.4 Hz, 1H), 7.46 (d, J=8.6 Hz, 2H), 6.96 (d, J=16.0 Hz, 1H), 6.67 (d, J=8.3 Hz,
2H), 6.40 (d, J=14.8 Hz, 1H), 4.87 (t, J=5.1 Hz, 1H), 4.12 (t, J=6.7 Hz, 2H), 3.81 (q, J=7.0 Hz,
6H), 3.43 (q, J=6.8 Hz, 4H), 3.10 (q, J=6.3 Hz, 2H), 2.82 (t, J=6.6 Hz, 2H), 2.56 (s, 2H), 2.51 (s,
9

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2H), 1.71 (s, 6H), 1.24 (m, 15H), 1.04 (s, 6H), 0.59 (t, J=8.1 Hz, 2H).
2.4 Preparation of Hybrid Films
The alkoxysilane dyes (0.1 mmol) were mixed with triethoxyvinylsilane (VTES, 1.9 mmol) in
THF, and then 8 mmol acidic water (pH=1) was added. The solutions were kept stirring for 12
hours and aged for another 4 days. Then the solutions were filtered through 0.22 ꢀm filter and
spin-coated on clean indium tin oxide (ITO) glass substrate to afford blue organic-inorganic
hybrid films.
3. Results and Discussion
3.1 Synthesis and Characterization
The detailed synthetic procedure of chromophores was presented in Scheme 1. The
chromophore CLDS1 was synthesized start from a nucleophilic substitution of epoxyisophorone
with alkanethiol, giving a thiol-substituted isophorone. Horner-Emmons olefination followed by
DIBAL-H reduction converted dienone 2a into aldehyde 4a. Knoevenagel condensation of the
2-cyanomethylene-3-cyano-4, 5, 5-trimethyl-2, 5-dihydrofuran to aldehyde 4a was performed to
yield 5a, and then the deprotection procedure of 5a produced hydroxyl-functionalized
chromophore CLDS1. In the synthesis of the analogous chromophore CLDS2,
2-mercaptoethanol was selected to react with epoxyisophorone to introduce hydroxyl group at the
side
of
chromophore.
These
chromophores
were
further
reacted
with
3-isocyanatopropyltriethoxysilane to give alkoxysilane dyes via a urethane forming reaction
[31-33]. Following the hydrolysis and copolymerization process of the alkoxysilane dyes and
triethoxyvinylsilane, a series of inorganic-organic hybrid film were prepared.
1
The structures of chromophores and alkoxysilane dyes were confirmed by H NMR and FTIR
10

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spectroscopy. All the spectral assignments clearly support the proposed structure. The peaks
assigned to vinylic protons of alkoxysilane dye ICTES-CLD illustrate the presence of cis- and
trans-isomer mixtures of the dye, while for ICTES-CLDS1 and ICTES-CLDS2, protonic peak
with a coupling constant 14.8 Hz at 6.38 ppm show the all-trans double-bond configuration of
derived tetraene alkoxysilanes which could provide a better conjugation and increase the
hyperpolarizability of the dyes significantly.
The IR spectra of chromophore CLDS1, alkoxysilane dye ICTES-CLDS1 and hybrid film
F-CLDS1 are shown in Fig. 2. In terms of chromophore CLDS1, the absorption of cyano group
in acceptor 2-cyanomethylene-3-cyano-4,5,5-trimethyl-2,5-dihydrofuran appears at 2220 cm-1,
and the characteristic absorption band at 3430 cm-1 is assigned to stretching vibration of the
hydroxyl group. Compared with CLDS1, the presences of the secondary amino group and
carbonyl group indicate that alkoxysilane dye ICTES-CLDS1 was obtained from the coupling
reaction between hydroxyl groups in chromophore and isocyanate group in ICTES. For hybrid
film F-CLDS1, the significantly increased broad absorption band around 1080 cm-1 and
decreased ethyl group absorption around 2900 cm-1 suggests the formation of Si-O-Si network
due to the hydrolysis and condensation of Si-O-R.
UV-vis absorption spectra of chromphores in CHCl3 indicate that chromophore CLD has a
maximum absorption wavelength (λmax) of 661 nm, and CLDS1 almost exhibits the same
absorption band with a λmax at 662 nm, indicating that the butylthio substituent does not affect the
molecular D-π-A structure. Compared with chromophore CLD and CLDS1, the λmax of
chrmophore CLDS2 shifts bathochromicly to 691 nm, which can be explained by the variation of
donor group in CLDS2. As shown in Fig. 3, the shapes of absorption bands the hybrid films
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F-CLDS1 and F-CLDS2 are similar but broader to the corresponding chromophores in solution,
indicating that the chromophores have been successfully incorporated in the silica networks. The
broadening of absorption bands in films can attributed to the fact that chromophores exhibit more
vibrational and rotational in the complex environment of silica matrix than in pure solution.
The thermal stability of the chromophores and hybrid materials are measured by
thermogravimetric analysis. As shown in Fig. 4, the decomposition temperature (Td) of
chromophores CLDS1 and CLDS2 are 254 and 252 oC, respectively. The hybrid films F-CLDS1
o
and F-CLDS2 show the decomposition temperatures at 270 and 257 C, respectively. This
implies that the inorganic silica backbone can indeed improve the thermal stability of
chromophores.
3.2 Nonlinear optical properties
The βCTµg values of chromophores were measured by solvatochromic method [15, 31]. As
shown in Table 1, the chromophore CLD shows a βCTµg value of 6967.9×10-30 esu D, while the
chromophore CLDS1 and CLDS2 exhibit a larger βCTµ
g value of 7207.6 and 7976.8×10-30esu D,
respectively. This indicates that the introduction of alkylthio substituent can enhance the βCTµg
value because the auxiliary π-accepting alkylthio group can make the chromophores have better
conjugation [30]. Except of the enhancement of βCTµg value, it is expected that the alkylthio side
group will affect the poling efficiency and the macroscopic NLO response of hybrid materials. To
investigate the substituent effect on NLO properties of hybrid materials, the films F-CLD,
o
F-CLDS1 and F-CLDS2 were poled at 5.5 kV dc voltage from 120 to 160 C for half an hour
and the second order NLO properties were characterized by in situ SHG measurement. The SHG
coefficients (d33) were calculated by equations according to the references [34] and summarized
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in Table 2. The experimental data were collected at 1064 nm fundamental wavelength, and
considering the severe absorption at the doubling frequency 532 nm, absorption correction was
applied.
The d33 values of hybrid films F-CLD, F-CLDS1 and F-CLDS2 are 37.78, 104.54 and 98.17
pm V-1, respectively. Compared with the film F-CLD, the film F-CLDS1 shows about 3 times
enhancement of d33 value. The significantly enhanced macroscopic optical nonlinearities of
F-CLDS1 could be attributed to the fact that by adding an alkylthio group perpendicular to the
conjugated backbone at the center of the bridge, not only it can improve the βCTµg value, but also
serve as an efficient isolation spacer to reduce the interchromophore electrostatic interactions. It
needs to be mentioned that the film F-CLDS2, in which the chromophore CLDS2 was covalently
linked to silica matrix, also exhibits an excellent NLO property. This clearly shows that the
enhancement of NLO response is mainly attributed to the incorporation of alkylthio group as
isolation spacer, while scarcely be influenced by anchoring position of chromophore. It may
provide an efficient approach to design and synthesize new hybrid materials for large optical
nonlinearity.
For the purpose of determining the temperature at which dipolar relaxation of SHG signal is
halved, a thermal dynamic induced depoling experiment was performed while the half-decay
temperature (Td/2) was used as notation (Fig. 5). As shown in Table 1, the hybrid film F-CLDS2
o
has a highest half-decay temperature of 127 C. This behavior could be rationalized by the more
difficult dipole orientation in hybrid film F-CLDS2, indicating that the side-on chromophore
attachment geometry can enhance the thermal stability of NLO properties in comparison with end
attachment model.
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4. Conclusion
Two new chromophores, CLDS1 and CLDS2, based on a similar D-π-A structure as the
phenyltetraene chromophore CLD, but with an alkylthio group perpendicular to the direction of
the long molecular axis as isolation spacer were synthesized. By anchoring the chromophores in
inorganic silica matrix, a series of hybrid materials were prepared. Due to the incorporation of
alkylthio group, the hybrid films F-CLDS1 and F-CLDS2 exhibit a significantly enhanced d33
value of 104.54 and 98.17 pm V-1, respectively, which are almost 3 times the d33 value of hybrid
film containing CLD. Furthermore, the film F-CLDS2 shows a quite high thermal stability of
optical nonlinearity. This result could provide an efficient approach to design and synthesize new
hybrid materials for large optical nonlinearity.
Acknowledgement
This work was financially supported by the National Natural Science Foundation of China
(Nos. 51010002, 51272229 and 51272231) and Fundamental Research Funds for the Central
Universities (Nos. 2012QNA4006 and 2012XZZX001).
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Table 1 Solvatochromic data and hyperpolarizabilities of the chromophores
a
b
a
λmax
ε
∆
ν1/2
λmax
∆
ν
βCT µg
(10-30esu D)
6967.9
Chromophore
(nm)
669
667
673
(cm-1)
4762
5099
5617
(nm)
626
622
633
(cm-1)
1026
1085
939
nm
1.1
1.1
1.1
(104 mol-1 L cm-1)
3.32
3.08
3.41
CLD
7207.6
CLDS1
CLDS2
7976.8
a measured in DMF b measured in 1,4-dioxane
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Table 2 Nonlinear optical property of hybrid films.
Hybrid film
F-CLD
d33(pm/V)
37.78
Td/2a(oC)
110
λmax(nm䟊
599
635
104.54
98.17
102
F-CLDS1
F-CLDS2
666
112
a Half-decay temperature of d33 value.
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Figure Captions
Scheme 1. Synthetic route of alkoxysilane dyes ICTES-CLDS1 and ICTES-CLDS2
Fig. 1. Structures of the chromophores CLD, CLDS1 and CLDS2
Fig. 2. FT-IR spectra of CLDS1, ICTES-CLDS1 and film F-CLDS1
Fig. 3. UV-vis spectra of chromophores in CHCl3 and hybrid films
Fig. 4. TGA curves of chromophores and hybrid films
Fig. 5. Decay of the normalized d33 values as a function of temperature for film F-CLDS2.
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Scheme 1
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Fig. 1
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CLDS1
OH
CN
ICTES-CLDS1
CH2CH3
NH
Si-O-R
F-CLDS1
C=O
Si-O-Si
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber(cm-1)
Fig. 2
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(a)
(b)1.0
CLDS1
F-CLDS1
CLDS2
F-CLDS2
1.0
0.8
0.6
0.4
0.2
0.0
0.8
0.6
0.4
0.2
0.0
400 500 600 700 800 900 1000
Wavelength(nm)
400 500 600 700 800 900 1000
Wavelength(nm)
Fig. 3
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100
80
CLDS1
CLDS2
F-CLDS1
FCLDS2
60
40
100
200
300
400
500
Temperature (oC)
Fig. 4
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1.0
0.8
0.6
0.4
0.2
0.0
40
60
80
100
120
140
o
Temperature ( C)
Fig. 5
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Highlights
Two new phenyltetraene-based chromophores with alkylthio group were synthesized
Inorganic-organic hybrid films are prepared through sol-gel process
Incorporation of alkylthio group effectively improves the nonlinear optical property