Chromophores with side isolate groups and applications in improving the poling efficiency of second non-linear optical (NLO) materials

Article abstract of DOI:10.1016/j.dyepig.2018.11.001

In this paper, alkyl groups with different lengths are introduced to the two-dimensional spindle-type chromophores as the steric isolators to improve the poling efficiency and long term stability of the poled polymers for second-order nonlinear optical materials. The structure and thermal properties of alkyl functionalized chromophores are characterized by the ¹H NMR, FT-IR, UV–Vis, DSC, TGA et al. The chromophores with different alkyl groups present extremely similar maximum absorbance wavelength (480 nm), which means they have very similar first-order hyperpolarizability. Guest-host polymer doped with chromophores has been prepared. Those materials display different glass transition temperatures and good thermal stability. According to the electro-optic coefficients studies and poling efficiency results, chromophores with longer alkyl side chain groups exhibit better poling efficiency and larger electro-optic coefficients. Especially for the chromophores with octyl group, the poling efficiency increases up to 3 times compared to the reference chromophore 2-(3-{2,5-Phenylmethanol-4-[4-(dimethylamino)styryl]-styryl-5,5-dimethylcyclohex-2-enylidene)malononitrile (STC), while the corresponding electro-optic coefficient of the poled polymers increases up to 4 times, indicating a significant effect of the isolate groups on improving the poling efficiency.

Full text of DOI:10.1016/j.dyepig.2018.11.001

Accepted Manuscript
Chromophores with side isolate groups and applications in improving the poling
efficiency of second non-linear optical (NLO) materials
Qiuli Cheng, Xiaoyu Shi, Chuangyang Li, Yuhang Jiang, Zuosen Shi, Jiawei Zou,
Xuesong Wang, Xibin Wang, Zhanchen Cui
PII:
S0143-7208(18)31526-2
https://doi.org/10.1016/j.dyepig.2018.11.001
DYPI 7145
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 10 July 2018
Revised Date: 15 October 2018
Accepted Date: 1 November 2018
Please cite this article as: Cheng Q, Shi X, Li C, Jiang Y, Shi Z, Zou J, Wang X, Wang X, Cui
Z, Chromophores with side isolate groups and applications in improving the poling efficiency of
second non-linear optical (NLO) materials, Dyes and Pigments (2018), doi: https://doi.org/10.1016/
j.dyepig.2018.11.001.
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ACCEPTED MANUSCRIPT
Chromophores with side isolate groups and applications in improving the
poling efficiency of Second Non-linear Optical (NLO) materials
Qiuli Cheng,a Xiaoyu Shi,a Chuangyang Li,a Yuhang Jiang,a Zuosen Shi,*,a Jiawei Zou,a
Xuesong Wang,a Xibin Wang,c Zhanchen Cui*,a,b
a
State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin
University, Changchun 130012, P. R. China.
b Department of Chemistry and Pharmacy, Zhuhai College of Jilin University, Zhuhai 519041,
People’s Republic of China.
c
State Key Laboratory on Integrated Opto-Electronics, Jilin University Region, Changchun
130012, P. R. China.
Abstract
In this paper, alkyl groups with different lengths are introduced to the two-dimensional
spindle-type chromophores as the steric isolators to improve the poling efficiency and long term
stability of the poled polymers for second-order nonlinear optical materials. The structure and
1
thermal properties of alkyl functionalized chromophores are characterized by the H-NMR, FT-
IR, UV-Vis, DSC, TGA, et al. The chromophores with different alkyl groups present extremely
similar maximum absorbance wavelength (480nm), which means they have very similar first-
order hyperpolarizability. Guest-host polymer doped with chromophores has been prepared.
Those materials display different glass transition temperatures and good thermal stability.
According to the electro-optic coefficients studies and poling efficiency results, chromophores
with longer alkyl side chain groups exhibit better poling efficiency and larger electro-optic
coefficients. Especially for the chromophores with octyl group, the poling efficiency increases up
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to 3 times compared to the reference chromophore 2-(3-{2,5-Phenylmethanol-4-[4-
(dimethylamino)styryl]-styryl-5,5-dimethylcyclohex-2-enylidene)malononitrile (STC), while the
corresponding electro-optic coefficient of the poled polymers increases up to 4 times, indicating
a significant effect of the isolate groups on improving the poling efficiency.
Keywords: Nonlinear optics; Chromophores; Poling efficiency; Electro-optical (EO)
1. Introduction
Over the past two decades, second-order nonlinear optical materials attracted much and deep
research due to its enormous and potential applications in high speed electro-optical (EO)
modulators [1-10]. Inorganic NLO materials, such as LiNbO3, K3PO4, GaAs, etc., were
developed and used much more widely because of their excellent stabilities [11-13]. However,
poled polymers for second-order nonlinear optical (NLO) materials were considered to be the
ideal material for the next generation high-speed electro-optical (EO) modulating devices, due to
the advantages of molecule variable, low micro-wave dielectric constant, high modulating
frequency (>100G), ultra-low drive voltage (<1V), ease of processing molding and integration
into optical electronic devices [9, 14-23]. In general, the first-order hyperpolarizability of the
chromophores, poling efficiency and alignment stability greatly affected their non-linear optical
properties [24-30]. Up to now, people have made great efforts and obtained great progress on the
preparation and application of poled polymer NLO materials. Some of them were even
commercialized and displayed excellent performances. However, one of the main barriers which
limited their widely use was the bad long-term stability of the chromophores in the materials [27,
31-36].
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In our previous works [37-39], we synthesized a series of polymers containing spindle-type
chromophores and studied their structure-properties relationship. We found that these poled
polymer NLO materials exhibited excellent orientation stabilities, since these chromophores had
larger freedom of rotation volume and stereo-hindrance effect, which could effectively decrease
the intermolecular aggregations. Theoretically, decreasing the interaction of the molecules and
enhancing the poling efficiency represent a key point to improve the second-order NLO property
for poled polymers. Therefore, chromophores with “isolation groups” structures [40, 41],
spherical-like hyperbranched polymers or dendrimers [42-44] and many other types of
chromophores [45] were designed and synthesized to reduce the dipole-dipole interactions.
Based on the previous research, in this study, by introducing alkyl chains with different
structures and length, we synthesized alkyl chain modified “spindle-type” chromophores STC,
STC-tBu, STC-C4 and STC-C8 (Fig. 1). The alkyl chains increased the steric hindrance and
improved the poling efficiency [46-50]; we further studied their effect on the NLO properties
before and after poling by the UV absorption measurements, and it was found that chromophores
with octyl chains had better poling efficiency. Finally, the electro-optical (EO) properties of the
poled polymers were tested by simple reflection method [51], the results indicated that the r33
values increased with the prolong of the side chains, the film containing STC-C8 displayed
largest output modulation signal (Im33 =220 mV), which was up to 4 times than STC (50 mV).
2. Experimental
2.1. Materials.
4-(hydroxymethyl) phenyl boronic Acid (99%) was obtained from Sun Chemical Technology
Co., Ltd. Potassium tert-butoxide and tetrakis(triphenylphosphine) palladium (0) (99.8%) were
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supplied from Aladdin. Toluene and sodium carbonate were from Beijing Chemical Co. (Beijing,
China). Ethyl acetate and other reagents were purchased from Tianjin Tiantai Refine Chemicals
Co., Ltd (Tianjin, China). All available reagents were used without further purification. The STC
was prepared according to our previous literatures [38].
Fig. 1. Structures of spindle-like chromophores STC, STC-tBu, STC-C4 and STC-C8.
2.2. Synthesis
Synthesis of the chromophores. 2,5-dibromobenzene-1,4-dicarbaldehyde and 1,3,3-
trimethyl-5-dicyanovinyl-1-cyclohexene (TDC) were synthesized using the literature procedures
[52]. The specific synthesis of the chromophores is showed in Scheme 1. Details of the synthesis
is described in the supporting information.
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Scheme 1. Synthetic scheme of the chromophores.
Preparation of host-guest doped NLO thin films. To prepare the polymer films, the alkyl
group modified chromophores (STC-tBu, STC-C4 and STC-C8) were dissolved in
cyclopentanone with a 0.15 g mL-1 weight concentration. The solutions were filtered through a
0.2 mm pore size filter and spin coated onto indium tin oxide (ITO) glass substrates at 1000 rpm.
The residual solvent was removed by heating the films in a vacuum oven at 70°C overnight.
2.3. Characterization
1
The H NMR spectra were taken on a Bruker AVANCE NMR spectrometer at a 500 MHz
1
resonance frequency for H, and tetramethylsilane (TMS) was used as the internal standard. IR
absorption was recorded on an AVATAR 360 FT-IR spectroscope. Ultraviolet-visible absorption
spectra were obtained by spectrophotometer (SHIMADZU) UV-3100 spectrophotometer. The
glass transition temperature was determined by using differential scanning calorimetry (DSC)
measurement taken on a NETZSCH 4 instrument at a scan rate of 10 K min-1 in the temperature
range of 50-300 °C under nitrogen. The decomposition temperatures were measured using
Perkin-Elmer TGA 7 analyzer from 50 °C to 800 °C at a heating rate of 10 K min-1 under
nitrogen atmosphere. The electro-optical (EO) coefficient was determined by simple reflection
method according to our previous report [53].
3. Results and discussion
3.1. Synthesis and structural characterization
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Spindle-type chromophores STC-tBu, STC-C4 and STC-C8 were prepared from three
different alkyl isocyanates and STC respectively by an addition reaction (Scheme 1). The
structure of chromophores was characterized by 1H NMR and FT-IR. (Fig. 2-3).
Fig. 2. The 1H-NMR spectrum of chromophores.
1
The H-NMR spectra of compounds are displayed in Fig. 2 and showed quite considerable
spectral date. Compared to the chromophore STC, new chemical shifts, originated to the proton
of alkyl group, had been found in the spectra of STC-tBu, STC-C4 and STC-C8. In addition,
the FT-IR spectra of the chromophores shown in Fig. 3 revealed a peak at about 2220 cm-1
attributed to cyano group. The stretching vibration of the N-H displayed around 3340 cm-1 and
the C=O appeared at approximately 1710 cm-1 except the chromophore STC. The absorbance
peak of phenyl group appeared at about 1550 cm-1. The two peaks at 2930 cm-1 and 2846 cm-1
were attributed to the absorption of methyl and methylene group respectively.
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Fig. 3. FT-IR spectra of chromophores.
3.2. Thermal properties of the chromophores
Thermal stability of chromophores was investigated by thermogravimetric analysis (TGA)
under nitrogen atmosphere. As shown in Fig. 4, the temperatures of 5% weight loss (Td) for all
the chromophores were about 200 °C; STC-C4 and STC-C8 exposed slightly higher thermal
property than STC, however, the stability of STC-tBu was weakened, which were likely due to
the introduction of tertiary butyl structure. Fig. 5 showed the DSC thermograms of
chromophores. Chromophores STC-C4 and STC-C8 emerged melting peak at 195 °C and
86 °C, respectively. This appearance could be contributed to the introduction of an alkyl group
that formed barrier effect, leading to a reduction in inter-molecular forces and an increase of the
molecular flexibility simultaneously, thus the chromophores melted before decomposition. There
were two endothermic peaks for STC-tBu, combining with the TGA curve, which related to two
decomposition processes of the chromophore.
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Fig. 4. TGA thermograms of the chromophores in nitrogen at the heating rate of 10 K min-1.
Fig. 5. DSC thermograms of the chromophores in nitrogen at the heating rate of 10 K min-1.
3.3. UV-Vis spectra
The UV-Vis absorption spectra of four chromophores measured in a series of different polar
solvents are presented in Fig. 6. The charge transfer band demonstrated that the four
chromophores had similar absorption peak shape and at the same time when the same solvent is
used they exhibit quite similar absorption peaks, including close wavelengths of maximum
absorption and extinction coefficient. This phenomenon is attributed to the barrier of methylene
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which completely affected electro-optic activity of chromophores due to the barrier effect and
flexible structure. Chromophore molecules presented bathochromic shift effect with increasing
polarity of the solvent (from CH2Cl2 to CHCl3), with the polarity of solvent further increased
(from CHCl3 to CH3CN), the wavelengths of maximum absorption showed hypsochromic shift.
This phenomenon was associated with the molecules interaction with solvent which showed
different charge separation.
Fig. 6. UV-Vis absorption spectra of chromophores in different solvents: a) STC, b) STC-tBu,
c) STC-C4, d) STC-C8.
3.4. Structural simulation
Fig. 7 depicted a MM-minimized structure which could intuitively represent the effect of the
alkyl group on the molecular structure of chromophores. Owing to the introduction of alkyl
groups, the free volume of the chromophore molecules increased (Fig. 7a, front view). Moreover,
due to the bond angle of carbamate linkage between methylene and alkyl groups (calculated
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results), the alkyl groups extended beyond the dipole plane, which hindered molecular
interactions more effectively (Fig. 7b, lateral view). The branched-chain structure of tert-butyl
group and the length of the n-octyl group were more conducive to weaken the intermolecular
interaction.
Fig. 7. Structure Simulation of Chromophores (MM2 method): a) front view of chromophores,
b) lateral view of chromophores.
3.5. Nonlinear optical properties
For exhibiting an electro-optic effect, the chromophore units in the films must be
noncentrosymmetric. We utilized high electric field poling to orient the chromophores along the
applied electric field and thus producing a noncentrosymmetric alignment. Poling conditions
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were as follows: at 140 °C and applying a high dc voltage 5.5 kV at the tungsten wire across the
films for about 1 h at the gap distance 1.0 cm. Finally, the temperature was lowered to ambient
temperature with the electric field still applied.
Fig. 8. UV-Vis absorption spectra of the STC/PMMA doped film before and after poling.
Fig. 9. UV-Vis absorption spectra of the STC-C8/PMMA doped film before and after poling.
To explore the noncentrosymmetric alignment of the chromophores in the films, the
absorption spectra of hybrid films STC and STC-C8 before and after poling were compared.
Fig. 8 and 9 showed the UV-Vis absorption spectra of the guest-host doped film before and after
poling. It can be seen that polarization leaded to a reduction in absorption intensity, and the
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intensity of absorption peaks of STC-C8 changed more obviously than STC. According to the
absorption change, the order parameter (Ф values) for the films could be measured as following
equation:
Ф=1-A1/A0
where A0 and A1 were the absorbance of the unpoled and poled films, respectively. The
calculation of Ф values were summarized in Table 1. The STC-C8/PMMA doped film exhibited
quite high Ф values, up to 0.17, comparing with STC/PMMA doped film (Ф=0.06). The Ф
values tested were well in accordance with their r33 values.
Table 1. Characterization data of chromophores.
c
λamax (nm)
471.0
λbmax (nm) Im33 (mV)
Ф
Td ( )
205
r33 (pm/V)
0.54
485.0
485.5
485.0
487.0
50
55
0.06
0.08
0.10
0.17
STC
471.5
190
0.58
STC-tBu
STC-C4
STC-C8
469.5
70
220
0.71
472.0
220
230
2.13
a The maximum UV-Vis absorption wavelength of chromophores in acetone
b The maximum UV-Vis absorption wavelength of chromophores in trichloromethane
c The 5% weight loss temperature of chromophores measured by the TGA analyses at nitrogen
condition at the heating rate of 10 K min-1
The EO coefficients of the poled guest-host doped films were measured by the Teng-Man
technique at 1310 nm[54, 55]. According to these literatures, due to the complication of the
testing system, which would result in errors, we did the test by referring to a standard sample
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(GaAs crystal, r41=1 pm/V) under the same testing conditions. r33 was calculated according to
the following equation reported in literature[56].
⁄
ꢈ
ꢈ
ꢈ
ꢆ2ꢇꢁꢂ − ꢉꢊꢇꢈ ꢋꢌ
ꢆ
ꢌꢂ ꢈ
rꢀꢀ 3 ꢃꢄꢀꢀꢅꢄꢁꢂ ꢇꢀꢀ − ꢉꢊꢇ ꢋ
.
.
=
ꢆ1ꢌ
ꢈ
ꢈ
⁄
ꢇꢀꢀꢆꢇꢁꢂ − ꢉꢊꢇꢈ ꢋꢌꢂ ꢈ
rꢁꢂ 2 ꢅꢄꢀꢀ ꢄꢁꢂ
ꢃ
where n is the refractive index at 1,310 nm, Im is the intensity of output modulation signal, and
Vm is the alternating current voltage applied to the sample. θ is the incident angle, Im41 and Vm41 is
the parameters of GaAs. Therefore, the equation (1) could also be displayed as follows:
⁄
ꢈ
ꢆ
ꢌꢂ ꢈ
ꢃ
ꢄꢀꢀꢅꢄꢁꢂ
ꢇ
ꢀꢀ − 0.5
.
rꢀꢀ = 16.835 ×
ꢆ2ꢌ
ꢇꢀꢈꢀ
ꢅꢄꢀꢀ ꢄꢁꢂ
ꢃ
According to equation (2), because all of the films were tested under the same conditions, the
value of r33 could be compared by using Im33 (Vm33=20.4V). The testing results (Table 1) showed
that the values of Im33 and Ф of the chromophore molecules with alkyl structure were higher than
that of STC molecule (50 mV), which indicated that alkyl modification made the poling
efficiency more effectively. The Im33 and Ф values of STC-tBu (55 mV) was similar to the value
of STC, which might be due to the large volume and relative rigidity of the t-butyl group. The
STC-C8 showed higher Im33 (220 mV) than the STC-C4 (70 mV). The reason was that the STC-
C8 possesses longer length alkyl and greater proportion of flexible components.
More noteworthy was that different alkyl groups in the lateral group of the chromophores could
impact the poling efficiency and the electro-optic activity. In Fig. 10, temporal stability of the
chromophores was measured by being exposed in air at room temperature for several months.
Although the order parameter Ф values of chromophores declined slightly, the poling efficiency
trend had not changed. Overall, all of the temporal stability were satisfactory.
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Fig. 10. Temporal stability of chromophores.
4. Conclusion
In summary, a series of two-dimensional spindle-like chromophores with different length of
alkyl groups on the conjugate bridge were designed and synthesized. By comparing the electro-
optic (EO) coefficients (r33) of the poled polymers by doping chromophores in PMMA, the
results showed that the chromophores with longer alkyl groups and linear alkyl group had better
poling efficiency and larger electro-optic coefficients. Moreover, two-dimensional spindle-like
chromophores had good temporal stability. Therefore, designing and optimizing chromophore
molecules through modifying the blocking groups might be a new way to improve the poling
efficiency of the chromophores and enhance the second-order nonlinear optical properties of
electro-optic polymers.
Acknowledgments
This article was financially supported by the National Natural Science Foundation of China
(grant no. 21374039, 21204028, 51673081). The authors would like to acknowledge those who
make contributions to this article.
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Highlights
• We synthesized a series of two-dimensional spindle-like chromophores with
different length of alkyl groups.
• Two-dimensional spindle-like chromophores had good thermal stabilities and met
the requirements of the poling process.
• Chromophores with longer alkyl groups and linear alkyl group had better
long-term stability, poling efficiency and larger electro-optic coefficient.
• The poling efficiency of chromophores with octyl group increased up to 3 times
and the electro-optic coefficient of poled polymers increased up to 4 times
compared to STC.