The preparation of two-dimensional spindle-type chromophores for second-order nonlinear optical materials

Article abstract of DOI:10.1016/j.dyepig.2011.08.021

A series of two-dimensional spindle-type chromophores has been synthesized. The detailed synthetic procedure was reported. The factors that influence the nonlinear optical properties such as the donors and acceptor character have been investigated. The chemical structures of the target products have been confirmed by IR, UV-Vis and NMR spectroscopy, elemental and thermal analysis. The spindle-type chromophores have been used to prepare host-guest doped films which exhibited very good thermal and temporal stability. The nonlinear electro-optical coefficients of the host-guest doped films were measured by the Teng-Man technique and gave values in the range from 21 to 29 pm/V.

Full text of DOI:10.1016/j.dyepig.2011.08.021

Dyes and Pigments 92 (2012) 982e987
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
The preparation of two-dimensional spindle-type chromophores for second-order
nonlinear optical materials
,
a
b
b
Xiaolong Zhanga, Ming Lia, Zuosen Shia , Lisha Zhao , Rulong Jin , Maobin Yi ,
*
,
Daming Zhangb, Zhanchen Cuia
*
a State Key Lab of Supramolecular Structure and Materials, College of Chemistry, Jilin University, 2699# Qianjin Road, Changchun 130012, PR China
b State Key Laboratory on Integrated Opto-electronics, Jilin University Region, 2699#, Qianjin Road, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of two-dimensional spindle-type chromophores has been synthesized. The detailed synthetic
procedure was reported. The factors that influence the nonlinear optical properties such as the donors
and acceptor character have been investigated. The chemical structures of the target products have been
confirmed by IR, UVeVis and NMR spectroscopy, elemental and thermal analysis. The spindle-type
chromophores have been used to prepare host-guest doped films which exhibited very good thermal
and temporal stability. The nonlinear electro-optical coefficients of the host-guest doped films were
measured by the Teng-Man technique and gave values in the range from 21 to 29 pm/V.
Ó 2011 Elsevier Ltd. All rights reserved.
Received 1 April 2011
Received in revised form
27 August 2011
Accepted 30 August 2011
Available online 8 September 2011
Keywords:
Nonlinear optics
Two-dimensional chromophores
Electro-optic coefficient
Host-guest doped films
Cross-linking
Temporal stability
1. Introduction
heterobridges have been investigated. Experimental and theoret-
ical studies have demonstrated that replacing the benzene ring of
Electro-optical (EO) materials with potential applications in
photonic devices for telecommunications and optical information
processing usually involve a host polymeric matrix containing
second-order nonlinear optical (NLO) chromophores either as guest
molecules or covalently attached to the polymer backbone [1e3].
The structural noncentrosymmetry of the material, a necessary
condition for second-order NLO activity, is generally achieved by
electric field orientation of the chromophore at the temperature
close to the glass transition temperature (Tg) of the polymer.
Designing organic molecules with large molecular nonlinearity
continues to be a challenging subject for materials application in
optoelectronic fields [4]. Besides a high molecular nonlinear optical
(NLO) response, many optical applications also require materials to
possess good thermal stability and transparency in the funda-
mental region of emission [5e7].
a chromophore bridge with easily delocalizable heteroaromatic
rings [8e11], such as thiophene, pyrrole, thiazole and benzimid-
azole, results in an enhanced molecular hyperpolarizability of
donor-acceptor compounds [9,12e20]. These chromophores
exhibit large first-order hypersusceptibility. However, the one-
dimensional compounds provide less phase-matching behavior
owing to their small off-diagonal component [21]. Experiments
have shown that two-dimensional (2-D) chromophores possess
better phase-matching than one-dimensional chromophores
because of their larger off-diagonal components [22]. Such chro-
mophores provide not only a better phase-matching capacity but
also an asymmetric arrangement [23].
Here, we present the synthesis, characterization of two-
dimensional spindle-type chromophores STC-1-4. The chromo-
phores and Poly(GMA-co-MMA) were used to prepare host-guest
doped second-order nonlinear optical films. The nonlinear prop-
erty of the films was investigated. To our best knowledge, no similar
chromophores containing a terphenyl structure conjugated bridge
linked to donor and acceptor groups have been reported in the
literature. Thermogravimetric analysis showed that these two-
dimensional spindle-type chromophores have good thermal
stability (Td > 200 ꢀC) in nitrogen.
Molecular NLO-phores are traditionally constituted by electron
donor and acceptor moieties covalently connected through
a conjugated bridge, and a wide variety of
p-conjugated and
* Corresponding authors. Tel.: þ86 431 85168217; fax: þ86 431 85193423.
E-mail addresses: shizs@jlu.edu.cn (Z. Shi), cuizc@jlu.edu.cn (Z. Cui).
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.08.021

X. Zhang et al. / Dyes and Pigments 92 (2012) 982e987
983
2. Experimental method
mixture stirred at room temperature for 2 h. A yellow solution was
obtained. Solvents were removed under reduced pressure, and then
column chromatography, eluting with petroleumeethyl acetate
(1:1) gave the title compound 0.81 g (80%) as a yellow solid.
2.1. Materials
Tetrahydrofuran (THF) and toluene were purified by fractional
distillation over sodium. Potassium tert-butoxide was purchased
from Acros Organics and piperidine was purchased from Aldrich
and purified by distillation under reduced pressure before use.
1,3,3-Trimethyl-5-dicyanovinyl-1-cyclohexene (TDC) and tricya-
novinyldihydrofuran (TCF) were prepared according to previous
literatures [24,25]. Poly(MMA-co-GMA) was synthesized according
to the literature [26]. All the other solvents and chemical reagents
were used as received, without further purification.
Mp ¼ 101 ꢀC 1H NMR (500 MHz, CDCl3, TMS):
d (ppm) trans:-
cis
¼
2:3; trans 2.05(s, 2H, eOH), 2.98(s, 6H, eCH3),
4.75e4.77(d, 4H, eCH2e), 7.28e7.30(d, 2H, NeAr), 6.54e6.57(d,1H,
CH]CH), 7.14e7.17(d, 1H, CH]CH), 7.66e7.68(d, 2H, NeAr),
7.65e7.67(d, 2H, HOCH2eAr), 7.51e7.53(d, 4H, HOCH2eAr),
7.69(s, 1H, AreAreAr), 8.03(s, 1H, AreAreAr), 9.99(s, 2H, eCHO).
cis: 2.05(s, 2H, eOH), 2.97(s, 6H, eCH3), 4.81e4.83(d, 4H, eCH2e),
7.22e7.24(d, 2H, NeAr), 6.95e6.97(d, 1H, CH]CH), 7.00e7.02(d,
1H, CH]CH), 7.64e7.66(d, 2H, NeAr), 7.38e7.40(d, 4H,
HOCH2eAr), 7.46e7.48(d, 4H, HOCH2eAr), 7.97(s, 1H, AreAreAr),
7.77(s, 1H, AreAreAr), 9.99(s, 2H, eCHO). 13C NMR (125 MHz,
2.2. Measurements
CDCl3, TMS):
d (ppm) 40.3, 65.15, 112.3, 121.7, 122.2, 125.5, 126.9,
Nuclear magnetic resonance (NMR) spectra were measured on
a Bruker AVANCE NMR spectrometer at a resonance frequency of
500 MHz in CDCl3, and tetramethyl chlorosilane (TMS) was used as
internal standard. IR spectra were taken on an AVATAR 360 FTIR
spectrometer. Elemental analysis was carried out with a vario
MICRO elemental analyzer. The melting point data of all solid
compounds were measured on X-4 Melting point apparatus with
microscope. The decomposition temperatures of the chromophore
molecules were analyzed by using Perkin-Elmer TGA 7 thermog-
ravimetric analyzer (TGA) at a heating rate of 10 ꢀC/min in nitrogen.
Thermal degradation temperatures were measured in the range
50 ꢀCe750 ꢀC. Ultravioletevisible (UVevis) absorption spectra
were measured on an SHIMADZUUV-3100 spectrophotometer.
130.2, 132.5, 133.0, 136.8, 140.32, 144.5, 150.6, 191.9. nmax
(KBr)/cmꢁ1: 3385(eOH), 2855(eCH3), 1681(eCHO), 1606(Ar). Anal.
calcd for C31H29NO3: C, 80.32; H, 6.31; N, 3.02. Found: C, 80.46; H,
6.34; N, 3.15.
2.3.4. 2,5-(4-Hydroxymethylphenyl)-4-(4-(diphenylamino)styryl)
benzaldehyde (4)
A similar procedure was followed for compound 3 and for
compound 2 (0.93 g, 2.69 mmol) (4-diphenylaminobenzyl)triphe-
nylphosphonium bromide (1.62 g, 2.69 mmol) and t-BuOK (0.493 g,
4.03 mmol) were dissolved in THF (10 mL). The product was puri-
fied by column chromatography, eluting with petroleumeethyl
acetate (2:1) gave the title compound (0.9 g, 53%) as a dark yellow
solid. Mp ¼ 135 ꢀC 1H NMR (500 MHz, CDCl3, TMS):
d (ppm)
2.3. Synthesis
trans:cis ¼ 1:1; trans 9.96(s, 1H, CHO), 7.99(s, 1H, CHOeAr), 7.77(s,
1H, CHOeAr), 7.45e7.47(m, 4H, HOCH2eAr), 7.42e7.44(m, 4H,
HOCH2eAr), 7.59e7.61(d, 2H, NeAr), 7.10e7.11(m, 6H, NeAr),
7.08e7.09(m, 4H, NeAr), 6.97e6.99(d, 2H, NeAr), 7.18e7.21(d, 1H,
CH]CH), 7.13e7.16(d, 1H, CH]CH), 4.79e4.82(d, 4H, eCH2e); cis:
10.00(s, 1H, CHO), 8.01(s, 1H, CHOeAr), 7.54(s, 1H, CHOeAr),
7.50e7.52(m, 4H, HOCH2eAr), 7.49e7.51(m, 4H, HOCH2eAr),
7.27e7.28(d, 2H, NeAr), 7.23e7.24(m, 6H, NeAr), 7.24e7.25(m, 4H,
NeAr), 6.94e6.96(d, 2H, NeAr), 6.58e6.60(d, 1H, CH]CH),
6.34e7.36(d, 1H, CH]CH), 4.75e4.77(d, 4H, eCH2e). nmax
(KBr)/cmꢁ1: 3390(eOH), 2921, 2856(eCH3，eCH2e), 1681(eCHO),
2.3.1. 4-(Hydroxymethyl)phenylboronic acid (1)
This compound was synthesized according to the literature [27],
and a white solid was obtained. Mp ¼ 265 ꢀC nmax (KBr)/cmꢁ1
3434(eOH). 1H NMR (500 MHz; D2O)
d (ppm) 1.94 (2H, s, eB(OH)2),
4.54 (2H, s, eCH2e), 7.30e7.32 (2H, d, Ar) and 7.65e7.66(2H, d, Ar).
Anal. calcd for C7H9BO3: C, 55.33; H, 5.97. Found: C, 55.50; H, 5.86.
2.3.2. 2,5-(4-Hydroxymethylphenyl)benzene-1,4-dicarbaldehyde
(2)
2,5-Dibromobenzene-1,4-dicarbaldehyde (1.0 g, 3.43 mmol),
4-(hydroxymethyl)phenylboronic acid (1.04 g, 6.85 mmol, 2 equiv)
were dissolved in THF (60 mL) under a nitrogen purge. The solution
was treated with palladium acetate (9.3 mg, 0.041 mmol,
0.006 equiv), triphenylphosphine (32.3 mg, 0.124 mmol,
0.018 equiv), 25.2 mL of 2 M sodium carbonate (Na2CO3) (0.321 mol,
1.2 equiv), deionized water (13.2 mL) and heated under reflux
under a nitrogen atmosphere. The resulting solution was stirred
under reflux at 90 ꢀC for 6 h. The organic layer was concentrated
under reduced pressure and purified by column chromatography
on silica gel eluting with dichloromethane-methanol (50:1) to give
yellow solid 0.9 g (83%). Mp ¼ 179 ꢀC 1H NMR (500 MHz, CDCl3,
1586(Ar). 13C NMR (125 MHz, CDCl3, TMS):
d (ppm) 65.6, 123.5,
123.7, 124.9, 127.2, 127.4, 127.8, 129.4, 129.7, 130.1, 130.4, 130.6,
132.0, 134.9, 139.6, 140.2, 140.4, 140.5, 147.7, 147.9, 168.1. Anal. calcd
for C41H33NO3: C, 83.79; H, 5.66; N, 2.38. Found: C, 83.53; H, 5.74;
N, 2.45.
2.3.5. 2-(3-(2,5-(4-Hydroxymethylphenyl)-4-(4-(dimethylamino)
styryl)styryl)-5, 5-dimethylcyclohex-2-enylidene)malononitrile
(STC-1)
Compound 3 (0.253 g, 0.55 mmol) was dissolved in THF 10 mL,
TDC (0.102 g, 0.55 mmol) and piperidine (0.05 mL) were added, and
the stirred mixture was then heated to reflux for 6 h. A dark solu-
tion was obtained. The product was purified by column chroma-
tography on silica gel eluting with dichloromethaneemethanol
(50:1) to give a red powder (0.26 g, 79%). Mp ¼ 113 ꢀC.1H NMR
TMS):
d (ppm) 10.00(s, 2H, eCHO), 7.96(s, 2H, AreAreAr),
7.65e7.67(m, 4H, HOCH2eAr), 7.58e7.60(m, 4H, HOCH2eAr),
5.38(s, 2H, eOH), 4.61(s, 4H, eCH2eOH). 13C NMR (125 MHz, CDCl3,
TMS):
d
(ppm) 65.2, 127.6, 130.6, 134.6, 136.1, 138.4, 144.6, 192.87.
(500 MHz, CDCl3, TMS): d (ppm) 7.72(s, 1H, AreAreAr), 7.66(s, 1H,
nmax (KBr)/cmꢁ1 3440(eOH), 1652(eCHO). Anal. calcd for C22H18O4:
AreAreAr), 7.50e7.52(d, 4H, HOCH2eAr), 7.46e7.48(d, 2H, NeAr),
7.43e7.45(d, 4H, HOCH2eAr), 7.27e7.30(d, 1H, NeAreCH]CH),
7.11e7.14(d, 1H, CH]CH-TDC), 7.06e7.09(d, 1H, NeAreCH]CH),
7.00e7.02 (d, 2H, NeAr), 6.90e6.93(d, 1H, CH]CH-TDC), 6.77(s, 1H,
CH)， 4.81(s, 4H, HOeCH2), 2.98(s, 6H, CH3eN), 2.55(s, 2H, -CH2-),
2.24(s, 2H, eCH2e), 1.01(s, 6H, CH3eC). 13C NMR (125 MHz, CDCl3,
C, 76.29; H, 5.24; N, 18.48. Found: C, 75.96; H, 5.41; N, 18.25.
2.3.3. 2,5-(4-Hydroxymethylphenyl)-4-(4-(dimethylamino)styryl)
benzaldehyde (3)
Compound 2 (0.75 g, 2.17 mmol) was dissolved in THF 30 mL,
(4-dimethylaminobenzyl)triphenylphosphonium iodide (1.136 g,
2.17 mmol) and t-BuOK (0.37 g, 3.26 mmol) were added, and the
TMS):
d (ppm) 28.3, 30.1, 32.4, 40.9, 43.3, 65.7, 78.0, 112.7, 112.9,
122.2, 124.2, 127.3, 128.0, 128.3, 128.7, 130.6, 131.4, 131.8, 132.3,

984
X. Zhang et al. / Dyes and Pigments 92 (2012) 982e987
136.0, 136.9, 137.9, 140.3, 140.8, 150.4, 169.6. nmax (KBr)/cmꢁ1
3405(eOH), 2923, 2852(eCH3, eCH2e), 2219(CN), 1605(Ar). Anal.
calcd for C43H41N3O2: C, 81.74; H, 6.54; N, 6.65. Found: C, 81.62;
H, 6.63; N, 6.59.
:
4.80e4.81(d, 4H, HOeCH2), 1.49 (s, 6H, CH3eC). 13C NMR (125 MHz,
CDCl3, TMS): (ppm) 26.3, 30.1, 37.8, 65.4, 87.1, 112.0, 115.5, 121.2,
d
123.2, 123.8, 124.4,125.2,127.5,127.7,127.8,128.1,129.0, 129.7,130.1,
130.3, 130.6, 130.9, 131.1, 131.9, 138.9, 139.3, 140.9, 141.6,
144.4, 143.5, 148.5, 176.7. nmax (KBr)/cmꢁ1: 3347(eOH), 2924,
2856(eCH3, eCH2e), 2226(CN), 1580(Ar). Anal. calcd for
C52H40N4O3: C, 81.23; H, 5.24; N, 7.29. Found: C, 80.98; H, 5.45;
N, 7.42.
2.3.6. 2-(3-(2,5-(4-Hydroxymethylphenyl)-4-(4-(diphenylamino)
styryl)styryl)-5, 5-dimethylcyclohex-2-enylidene)malononitrile
(STC-2)
A similar procedure was followed to that for STC-1, compound 4
(0.3 g, 0.511 mmol), TDC (0.095 g, 0.55 mmol) and piperidine
(0.04 mL) were dissolved in THF (10 mL). The product was purified
by column chromatography on silica gel eluting with dichlor-
omethaneemethanol (50:1) to give a red powder(0.26 g, 79%).
2.4. Preparation of guest-host doped thin films
To prepare the doped films, the chromophores STC-1-4 and the
Poly(MMA-co-GMA) containing 0.5wt% 2-methylimidazole were
dissolved in butyl acetate with a weight concentration of 0.01 g/mL
and 0.1 g/mL, respectively. The solutions were filtered through
Mp ¼ 126 ꢀC 1H NMR (500 MHz, CDCl3, TMS):
d (ppm) 7.78(s, 1H,
AreAreAr), 7.70(s, 1H, AreAreAr), 7.50e7.52 (d, 4H, HOCH2eAr),
7.44e7.46(d, 4H, HOCH2eAr), 7.24e7.25(d, 2H, NeAreCH]CH),
7.21e7.23(m, 4H, NeAr), 7.18e7.20(d, 1H, NeAreCH]CH),
7.11e7.14(d, 1H, CH]CH-TDF), 7.08e7.11(m, 6H, NeAr),
7.02e7.04(m, 2H, NeAr), 6.97e6.99(d, 1H, NeAreCH]CH),
6.91e6.94(d, 1H, CH]CH-TDF)，6.78(s, 1H, eCHe), 4.79e4.80(d,
4H, HOeCH2), 2.55(s, 2H, eCH2e), 2.24(s, 2H, eCH2e), 1.01(s, 6H,
a syringe fitted with a 0.22 mm pore size filter, and spin-coated on
indium-tin-oxide (ITO)-coated glass substrates, which had been
previously cleaned by N,N-dimethylformide (DMF), acetone, THF
and distilled water sequentially in an ultrasonic bath. The films
were baked in air for 1 h to remove the majority of any volatiles. The
residual solvent was removed by heating the films in a vacuum
oven at 40 ꢀC for 10 h.
CH3eC). 13C NMR (125 MHz, CDCl3, TMS):
d (ppm) 27.9, 29.6, 40.8,
42.9, 65.1, 80.1, 112.3, 121.6, 122.5, 122.5, 123.0, 123.2, 123.4, 123.9,
124.5,124.7,124.8,126.8,126.9,127.3,127.4,127.7,127.5,129.0,129.1,
129.3, 129.9, 130.0, 130.1, 181.2. nmax (KBr)/cmꢁ1: 3347(eOH), 2926,
2860(eCH3, eCH2e), 2219(CN), 1590(Ar). Anal. calcd for
C53H45N3O2: C, 84.21; H, 6.00; N, 5.56. Found: C, 84.31; H, 6.07; N,
5.59.
3. Results and discussion
3.1. Synthesis
The preparation of the chromophores was accomplished by
three different strategies in Scheme 2. Compound 2 was obtained
2.3.7. 2-(3-Cyano-4(2,5-(4-hydroxymethylphenyl)-4-(4-
(dimethylamino)styryl)styryl)-5, 5-dimethyl-5H-furan-2-ylidene)-
malononitrile (STC-3)
via Suzuki cross-coupling reaction. The compound
3 and
compound 4 were synthesized using Wittig reaction. The chro-
mophores STC-1-4 were obtained via Knoevenagel condensation
reaction.
The synthesis of the target chromophores were outlined in
Scheme 1. Compound 1 was achieved by a one-step synthesis in
a reasonable yield by reduction of the 4-formylphenylboronic acid
with NaBH4 (4-Diphenylaminobenzyl) triphenylphosphonium
bromide and (4-dimethylaminobenzyl) triphenylphosphonium
iodide were easily obtained under routine conditions [28]. 2,5-
A similar procedure was followed for the STC-1, compound 3
(0.32 g, 0.7 mmol), TCF (0.14 g, 0.7 mmol) and piperidine (0.05 mL)
were dissolved in THF (10 mL). The product was purified by column
chromatography on silica gel eluting with petroleumeethyl acetate
(1:1) to give a red powder (0.28 g, 63%). Mp ¼ 109 ꢀC 1H NMR
(500 MHz, CDCl3, TMS):
d (ppm) 7.78(s, 1H, AreAreAr), 7.69(s, 1H,
AreAreAr), 7.66e7.68 (d, 4H, HOCH2eAr), 7.64e7.66(d, 4H,
HOCH2eAr), 7.38e7.40(d, 2H, NeAr), 7.11e7.14(d, 1H, CH]CH-TCF),
7.18e7.21(d, 1H, NeAreCH]CH), 6.91e6.94(d, 1H, CH]CH-TCF),
6.81e6.84(d, 1H, NeAreCH]CH)，6.65e6.67(d, 2H, NeAr),
4.80e4.82(d, 4H, HOeCH2), 2.97(s, 6H, CH3eN), 1.49(s, 6H, CH3eC).
Dibromobenzene-1,4-dicarbaldehyde with excess compound
1
under Suzuki cross-coupling conditions in the presence of palla-
dium acetate as a catalyst and aqueous solution of 2 M Na2CO3 as
13C NMR (125 MHz, CDCl3, TMS):
d (ppm) 27.9, 29.6, 44.4, 46.0,
65.0, 99.8, 100.1, 111.5, 112.5, 114.5, 120.1, 127.0, 127.1, 127.3, 127.4,
128.1, 128.3, 128.5, 128.6, 129.7, 129.9, 131.8, 131.8, 132.0,
132.1, 132.3, 153.5, 160.6, 184.7. nmax (KBr)/cmꢁ1: 3383(eOH),
2928, 2856(eCH3, eCH2-), 2226(CN), 1608(Ar). Anal. calcd for
C42H36N4O3: C, 78.24; H, 5.63; N, 8.69. Found: C, 78.43; H, 6.01;
N, 8.82.
2.3.8. 2-(3-Cyano-4-(2,5-(4-hydroxymethylphenyl)-4-(4-
(diphenylamino)styryl)styryl)-5, 5-dimethyl-5H-furan-2-ylidene)-
malononitrile (STC-4)
A similar procedure was followed for the STC-1, compound 4
(0.16 g, 0.273 mmol), TCF (0.055 g, 0.273 mmol) and piperidine
0.03 mL were dissolved in THF 10 mL. The product was purified by
column chromatography on silica gel eluting with petroleumeethyl
acetate (1:1) to give a red powder(0.117 g, 56%). Mp ¼ 123 ꢀC 1H
NMR (500 MHz, CDCl3, TMS):
d (ppm) 7.78(s, 1H, AreAreAr),
7.70(s, 1H, AreAreAr), 7.50e7.52 (d, 4H, HOCH2eAr), 7.44e7.46(d,
4H, HOCH2eAr),7.38e7.40(d, 2H, NeAreCH]CH), 7.22e7.25(m,
4H, NeAr), 7.07e7.09(m, 6H, NeAr), 7.03e7.05(m, 2H, NeAr),
7.14e7.17(d, 1H, CH]CH-TCF), 7.17e7.20(d, 1H, NeAreCH]CH),
6.82e6.85(d, 1H, CH]CH-TCF), 6.93e6.96(d, 1H, NeAreCH]CH)，
Scheme 1. The structure of the chromophores.

X. Zhang et al. / Dyes and Pigments 92 (2012) 982e987
985
Scheme 2. Synthetic route of the chromophores.
base in THF at reflux afforded the desired compound 3 in 80% yield
as a mixture of isomers (trans:cis ¼ 2:3). Compound 4 was prepared
as a mixture of isomers (trans:cis ¼ 1:1) by NMR spectroscopy from
a Wittig reaction between phosphonium salt and compound 2. The
final compounds STC-1 e 4 were obtained by a Knoevenagel
condensation with the acceptor TDC and TCF to afford chromo-
phores containing 4-(hydroxymethyl) phenyl as side group and
divinyl phenyl as the conjugating bridge conveniently.
displayed high thermal stability with 5% weight loss temperature
(Td) higher than 200 ꢀC. STC-4 had the highest Td about 254 ꢀC.
This result indicated that the different donors and accepters had
a significant impact on the overall thermal stability of the
chromophores.
3.2. Structural characterization
All of the compounds were characterized by spectroscopic
methods and gave satisfactory spectral data (see Fig. 1 for detailed
analysis data). The 1H NMR spectrum of chromophores shows
a singlet for the equivalent methyl groups at 1.01 ppm assigned to
the TDF units for STC-1 and STC-2, and a single signal at 1.50 ppm
was assigned to the equivalent methyl groups of the TCF units for
STC-3 and STC-4. Fig. 1 shows the IR spectra of the chromophores.
The strong band around 3340 cmꢁ1-3410 cmꢁ1 was attributed to
the OeH stretching. The OeH stretching was broad, and this sug-
gested that the triphenyl structure was formed under Suzuki cross-
coupling condition. A strong absorption peak appeared at about
2220 cmꢁ1, which was attributed to the vibration of the cyano
group.
3.3. Thermal properties of the chromophores
The thermal property of chromophores was investigated by TGA
under a nitrogen atmosphere. In Fig. 2, all of the chromophores
Fig. 1. IR spectrum of the chromophores.

986
X. Zhang et al. / Dyes and Pigments 92 (2012) 982e987
Fig. 4. UVevis absorption spectra of the chromophores in THF.
Fig. 2. TGA thermograms of the chromophores in nitrogen atmosphere at a heating
rate of 10 ꢀC/min.
3.5. Nonlinear optical properties
3.4. UVeVis spectra
The films were prepared using Poly(GMA-co-MMA) and
evidence of cross-linking was determined by FTIR spectroscopy
through monitoring the CeOeC stretching band ar ca. 910 cmꢁ1
.
The UVevis absorption spectra of chromophore STC-1 in
different solvents is presented in Fig. 3. The charge transfer band
shows a bathochromic shift with the increase of the solvent
polarity from dioxane to NMP. This positive solvatochromism has
been commonly regarded as an indication of molecular nonline-
Dynamic DSC analysis at 150e190 ꢀC was also used to confirm that
the polymer was fully cross-linked. To exhibit an EO effect the
chromophores in the films must be noncentrosymmetric. High
electric field poling was utilized to orient the chromophores along
the applied electric field and thus produce a noncentrosymmetric
arrangement. Poling conditions were as follows: at a temperature
about 110 ꢀC and applying a high dc voltage 5.0 kV at the tungsten
wire across the films for about 1 h at the gap distance 1.0 cm. The
temperature was then increased to 130 ꢀC for about 30 min to
ensure the epoxy groups were fully cross-linked. Finally, the
temperature was decreased to room temperature with the electric
field still applied.
arity (mb) of NLO-phores [29,30]. The absorption wavelength (lmax
)
of the four chromophores was different in the same solvent in
Fig. 4. In the UVeVis spectra the absorption coefficients were
6.9 Mꢁ1 cmꢁ1, 2.4 Mꢁ1 cmꢁ1, 1.2 Mꢁ1 cmꢁ1 and 1.7 Mꢁ1 cmꢁ1 for
STC-1 e STC-4 in dichloromethane. All of the absorption wave-
lengths (lmax) of chromophores were less than 480 nm, which
indicated that the chromophores possessed good transparency in
the longer wavelength visible region of the spectrum.
The EO coefficients of the guest-host doped films were
measured with the Teng-Man setup at 1310 nm [31]. All of the films
had the same composition and amount of chromophore and their
EO coefficients were determined to be 25, 26, 21 and 29 pm/V for
STC-1-4, respectively. Chromophore STC-4 with stronger donor and
acceptor groups displayed the largest EO coefficient [32].
The long-term stability of the films at elevated temperature is
very important for the practical use of NLO materials. To investigate
the long-term NLO stability of the guest-host doped films, dipole
reorientation of chromophores was observed by measuring the EO
coefficient (r33(t)/r33(t0)) as a function of time at 80 ꢀC. Fig. 5 shows
the temporal stability of the EO coefficient for the host-guest doped
films at 80 ꢀC. The temporal stability of the EO response was found
to be good, as the r33 value of the films maintained more than 85%
of their original value after holding at 80 ꢀC in air over 100 h. There
are two reasons for obtaining these results. The former is that the
structural architecture of the chromophore has a large effect on the
temporal stability.
A two-dimensional chromophore requires
a larger rotational sphere volume than one-dimensional one [23].
Short-range relaxation such as crankshaft motion cannot provide
enough local free volume to randomize the oriented dipoles. The
local free volume can be increased by increasing the temperature
which provides sufficient free volume for reorientation of the
chromophores. The second reason is that the increased tempera-
ture in the poling process ensures that all of the epoxy groups
Fig. 3. UVevis absorption spectra of chromophore STC-1 in different solvents.

X. Zhang et al. / Dyes and Pigments 92 (2012) 982e987
987
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This work was supported by the National Natural Science
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