Photo-crosslinkable second order nonlinear AB2-type monomers: Convenient synthesis and enhanced NLO thermostability

Article abstract of DOI:10.1039/d0tc00308e

In this article, two AB₂-type second-order nonlinear optical monomers (DN-SH and DS-SH) containing two double bonds and one thiol group, respectively, were synthesized successfully. These two monomers could be in situ photo-crosslinked via a th

Full text of DOI:10.1039/d0tc00308e

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Photo-crosslinkable second order nonlinear
AB2-type monomers: convenient synthesis
Cite this:DOI: 10.1039/d0tc00308e
and enhanced NLO thermostability†
a
Ruifang Wang,a Ziyao Cheng,a Xiaocong Deng,a Wenjing Zhao,a Qianqian Li
ab
and Zhen Li
*
In this article, two AB2-type second-order nonlinear optical monomers (DN-SH and DS-SH) containing
two double bonds and one thiol group, respectively, were synthesized successfully. These two
monomers could be in situ photo-crosslinked via a thiol–ene click reaction under 365 nm UV light
without a photoinitiator after poling. Thanks to the photo-crosslinking process of thin films, the
thermostability of the second-order nonlinear optical property was much enhanced. DS-SH exhibited a
larger NLO effect and better NLO thermostability than DN-SH, because of the introduction of sulfonyl-
based chromophores which can decrease the electrostatic interaction between nitro-based
chromophore moieties. T80% of the photo-crosslinked DS-SH films increased to 112.5 1C, much higher
than that of uncrosslinked ones (75 1C), while T80% of DN-SH films increased from 58 to 95 1C. More
excitedly, the doped thin films of DN-SH and DS-SH at different molar ratios (1 : 1, 2 : 1, and 1 : 2) showed
better NLO properties (123, 122, and 114 pm Vꢀ1, respectively) after photo-crosslinking than single
DN-SH (89 pm Vꢀ1) and DS-SH (108 pm Vꢀ1) respectively.
Received 16th January 2020,
Accepted 17th March 2020
DOI: 10.1039/d0tc00308e
rsc.li/materials-c
the thermostability of molecules and chromophores, which
would be degraded at such a high temperature of poling.
Introduction
Second-order nonlinear optical (NLO) materials have been widely Another method to improve the NLO stability is to create
studied in the past few decades, because of their extensive Diels–Alder thermal-crosslinkable networks, such as trismale-
applications such as in high-speed electrooptic modulation, imides and dienes, which can react partly at a baking tempera-
frequency doubling, data storage, telecommunications and ture of 50 1C according to their highly reactive nature.5
so on.1 Compared to inorganic nonlinear materials having a However, it is hard to control the best poling temperature,
low electrooptical coefficient, high dielectric constant and which strongly relies on the balance between glass transition
difficulties in crystal growth, organic nonlinear materials have temperature, dissociation and crosslinking. The last method is
attracted great attention because of their definite advantages of the design of photocrosslinkable polymers (Chart S1, ESI†).6
ultra-fast response speed, large NLO properties and convenient It has a great advantage because the photo-crosslinking process
processability.2 In order to meet the requirement for practical can be conducted easily during or after poling under UV light
applications, NLO materials should not only have large NLO with/without photoinitiators. As early as 1994, two photo-
properties, but also good thermal and optical stability.3 There- crosslinked acrylic monomers bearing nitroazobenzene chromo-
fore, many efforts have been made to improve the stability of phores showed no relaxation for a week at 80 1C after relatively
NLO materials. One of the methods is to synthesize polymers quick UV-initiated polymerization.7 Another photo-crosslinked
with high glass transition temperatures, such as polyimides, NLO epoxy polymer containing cinnamate-functionalized sub-
especially when chromophore moieties were attached to the stituents was prepared successfully with improved NLO thermo-
polymer backbone.4 However, there are strict requirements for stability after UV curing.8 Recently, a direct writing approach was
conducted to obtain Mach–Zehnder waveguides after photo-
crosslinking.9
a Sauvage Centre for Molecular Science, Department of Chemistry,
Another implementation of an NLO material device is to
promote the second harmonic generation (SHG) effect, charac-
Wuhan University, Wuhan 430072, China. E-mail: lizhen@wh.edu.cn,
lichemlab@163.com
b Institute of Molecular Aggregation Science, Tianjin University, Tianjin 300072,
terized by d33. In 2006, the concept of a ‘‘suitable isolation
group’’ (SIG) was proposed by our group,10 and then a series of
China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc00308e systematic research studies were conducted accompanied by
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rational molecular designs and optimized spatial configuration. click reaction may be a promising method to improve the
In particular, the successful preparation of dendrimers, hyper- thermostability of NLO property. However, the use of a cross-
branched polymers and dendronized hyperbranched polymers linking agent (PETMP) without an NLO effect will reduce the
efficiently translates large microscopic mb to high macroscopic loading density of chromophore moieties, leading to a reduction
NLO properties.11 Thus, high performance of NLO properties of the NLO properties. Therefore, we wonder if simply synthe-
was achieved, up to 299 pm Vꢀ1 (Chart S2, ESI†), which was the sized low-generation azo dendrimers can be utilized to form
best value based on azo chromophores so far. As mentioned functionalized compounds which can be photo-crosslinked by
above, we aimed to further enhance the thermostability of NLO themselves in the absence of a cross-linking agent.
property through a simple and controllable method, and achieve
In this work, we further developed this photo-crosslinking
a relatively good NLO effect at the same time. Generally, the method. As shown in Fig. 1, two AB2-type monomers, DN-SH
thiol–ene click reaction can be carried out efficiently with/ and DS-SH, were synthesized successfully by the azide-alkyne
without photoinitiators under UV light, which attracted much click reaction, esterification of hydroxyl groups and reduction
attention in various applications.12 Hence, our group made of disulfide bonds, in which, two double bonds and one thiol
a preliminary exploration by synthesizing a kind of photo- group were present on the head and tail of the chromophores
crosslinkable second-order NLO linear polymer containing respectively. Firstly, the chromophore moieties with donor–p–
double bonds which can react with pentaerythritol tetra(3- acceptor (D–p–A) structures can achieve noncentrosymmetric
mercaptopropionate) (PETMP) under UV light (Chart S1, ESI†). alignment through poling under a high electric field. However,
As expected, the crosslinked films exhibited better thermo- molecules were easily able to return to their centrosymmetric
stability of NLO property than uncrosslinked ones, demonstrat- alignment because of the strong dipole–dipole interaction
ing the feasibility and operability of the photo-crosslinking between chromophore moieties without the application of an
method by utilizing the thiol–ene click reaction in second- electric field.14 Thus, 365 nm UV light was applied to initiate a
order nonlinear materials.13 Thus, the photo-induced thiol–ene photo-crosslinking process of chromophores under an electric
Fig. 1 Schematic illustration of the photo-crosslinking process and the structures of DN-SH and DS-SH.
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field, so that free-rotating molecules can be locked by cross-
All target monomers and the related intermediates were well
linking. As a result, the thermostability of NLO property was characterized by nuclear magnetic resonance (NMR) (specific
greatly improved after photo-crosslinking in comparison with analysis is shown in the Experimental section and the ESI†).
that of uncrosslinked films. The T80% of DN-SH increased from Besides, high resolution mass spectrometry (HRMS) and
58 to 95 1C, and 75 to 112.5 1C for DS-SH. Herein, we will elementary analysis (EA) were further utilized to characterize
present the synthesis, characterization, photo-crosslinking AB2-type monomers. The calculated mass of DN-SH and DS-SH
process, and NLO properties of these new monomers in detail. is in good accordance with the results of HRMS (Table S1, ESI†).
The thermal stability of monomers was measured by thermo-
gravimetric analysis (TGA). As shown in Table S1 (ESI†), the
degradation temperatures (Td) of DN-SH and DS-SH were about
245.9 and 216.3 1C, respectively. Generally, the temperature of
nonlinear materials is below 200 1C for practical applications.
The glass transition temperatures (Tg) of monomers were also
investigated by differential scanning calorimetry (DSC), with the
results summarized in Table S1 (ESI†). The Tg value of monomer
Results and discussion
Synthesis and characterization
Two photo-crosslinkable second-order nonlinear optical AB2-type
monomers DN-SH and DS-SH were prepared successfully. The
carbon–carbon double bonds and thiol groups of the chromo-
phore moieties can be crosslinked via the thiol–ene click reaction
DS-SH (70.5 1C) was higher than that of DN-SH (60.5 1C), possibly
under UV light after poling. The specific synthetic routes and
due to the introduction of sulfonyl-chromophore moieties.
preparation procedures are shown in Scheme 1 and in the
Experimental section. Taking DN-SH for example, firstly, we
The UV-vis absorption spectra of target monomers in different
solvents are demonstrated in Fig. 2, with the maximum absorp-
obtained an important intermediate of DN-OH through the
tion wavelengths (lmax) for the p–p* transitions derived from azo
Cu(I)-catalyzed azide–alkyne cycloaddition click reaction accom-
moieties summarized in Table 1. The lmax of DS-SH blue-shifted
in the same solvent in comparison with that of DN-SH. As we all
know, the mb value of the sulfonyl-based chromophore is lower
than that of the nitro-based chromophore, thus it is normal that
panied by the formation of triazole rings,15 which can act as
isolation groups to decrease strong dipole–dipole interactions
between chromophores. The target monomer DN-SH was
obtained in a satisfactory yield by another two steps, subsequent
esterification of hydroxyl groups in DN-OH and reduction of
DS-SH showed better optical transparency than DN-SH. And the
two monomers exhibited solvatochromism phenomena, that is to
disulfide bonds in DS-S-S-DS, respectively. The synthetic route
say, the UV-vis spectra and lmax of molecules are different in
of another monomer DS-SH is similar to that of DN-SH (ESI†).
And these two monomers have good solubility in common polar
organic solvents, such as DCM, CHCl3, THF, and DMF.
different solvents, and D was defined as the difference between
the lmax values in different polar solvents of 1,4-dioxane and
DMSO.16 As shown in Table 1, DN-SH exhibited a larger D value
(29 nm) than that of DS-SH (22 nm), indicating that the introduc-
tion of sulfonyl-based chromophores can decrease the strong
dipole–dipole interactions between azo-based chromophores.
Fourier transform infrared (FT-IR) spectroscopy was performed
to identify the functional groups of DN-SH and DS-SH. As shown
Fig. 2 UV-vis spectra of DN-SH and DS-SH in different solvents
(1,4-dioxane, N,N-dimethylformamide, dimethyl sulfoxide, tetramethylene
oxide, chloroform, and dichloromethane; 0.02 mg mLꢀ1).
Table 1 Maximum absorption wavelength (lmax, nm) of monomers in
different solvents (0.02 mg mLꢀ1
)
1,4-Dioxane CHCl3 DCM THF DMF DMSO
D
DN-SH 466
DS-SH 455
475
470
475
472
471
458
487
470
495
477
29
22
D = lmax (DMSO) ꢀ lmax (1,4-dioxane).
Scheme 1 Synthesis of DN-SH.
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were monitored accompanied by the increase of temperature at
the rate of 4 1C minꢀ1. T80% represents the temperature at
which the SHG signal decreased by 80% of the initial value
during the depoling process. Thus, the T80% can be used as a
parameter to judge whether NLO properties are stable or not.
Generally, the higher the T80% is, the better stability SHG has,
which is beneficial for practical applications.
The specific photo-crosslinkable process and preparation of
films are described in the Experimental section. At first, films
DN-SH1 and DS-SH1 were measured to obtain the poling and
depoling curves without UV light. As shown in Fig. S4 and S6
(ESI†), DS-SH1 exhibited higher Te (90 1C) compared to that of
DN-SH1 (64 1C), because of the existence of the sulfonyl-based
chromophore. At the same time, the T80% of DN-SH1 and
DS-SH1 was 58 and 75 1C respectively. Next, we chose other
films to conduct the photo-crosslinking process. And the photo-
crosslinking process was carried out under different conditions
in order to get better results. Plan A: the films (DN-SH5 and
DS-SH3) were heated approximately at the best poling tempera-
ture (64 and 90 1C respectively) under the electric field. After
maintaining for 10 min at Te, the poled films were irradiated for
30 min under 365 nm UV light with the voltage still being
applied. Plan B: the films (DN-SH2 and DS-SH2) were also
heated to the best poling temperature (64 and 90 1C respectively).
After maintaining for 10 min at Te, the poled films were cooled to
the room temperature with the frozen alignment of chromo-
phores. Then, the films were exposed to 365 nm UV light for
30 min. And there was always a high voltage applied during the
process of poling and UV light. The photo-crosslinking process
was repeated three times to ensure complete crosslinking, with
the results summarized in Fig. S8 (ESI†). For films DN-SH2 and
DN-SH5, there was no obvious difference of T80% between
the two test methods, however, the d33 value can be greatly
influenced by the temperature of illumination. The d33 value of
DN-SH2 (89 pm Vꢀ1) is much higher than that of DN-SH5
(75 pm Vꢀ1) after photo cross-linking. For films DS-SH2 and
DS-SH3, although the DS-SH3 film exhibited a little better NLO
stability after photo crosslinking, the d33 value of DS-SH3
(95 pm Vꢀ1) is lower than that of DS-SH2 (108 pm Vꢀ1) after
the crosslinking process, which is almost consistent with the
results of DN-SH films. These results indicated that irradiation
conditions (such as, temperature) can greatly influence the d33
value. It is easy to understand why the d33 value can be
influenced. As we all know, the molecules are easier to move
especially at a temperature approaching glass transition tem-
perature. So, there should be an equilibrium state between the
random motion of molecules and the orderly arrangement of
Fig. 3 FT-IR spectra of the monomers and the corresponding photo-
crosslinking systems. (a) Monomer DN-SH was illuminated for 0 min
(purple curves), and DN-SH-UV was illuminated for 30 min (orange
curves); (b) monomer DS-SH was illuminated for 0 min (red curves), and
DS-SH-UV was illuminated for 30 min (black curves).
in Fig. 3, DN-SH (purple curves) and DS-SH (red curves) both
showed absorption bands associated with the carbonyl groups
(at about 1739 cmꢀ1) and the nitro groups (at about 1515 and
1338 cmꢀ1). Meanwhile, a weak absorption band associated with
carbon–carbon double bonds appeared at about 1640 cmꢀ1
,
further confirming the successful preparation of target
monomers. Unfortunately, another characteristic peak of thiol
groups was not observed in FT-IR spectra. There may be two
main reasons for this: the content of thiol groups in the whole
molecule is too low to be detected17 and besides, the small thiol
groups are wrapped in the interior of much larger monomers so
that weak absorption cannot be observed.
Photo-crosslinking of AB2-monomer films
AB2-type monomers DN-SH and DS-SH can react with them-
selves through the thiol–ene click reaction under UV light. And
this process can be monitored by FT-IR spectroscopy (Fig. 3).
The specific operations are as follows: monomers DN-SH and
DS-SH were dissolved in dichloromethane, respectively, and
then the solution was coated on a KBr pellet. The FT-IR spectra
of monomers with different illumination times (0 min and
30 min) were obtained. As shown in Fig. 3a, the characteristic
peak of DN-SH at about 1640 cmꢀ1, which is ascribed to the
carbon–carbon double bond, became weaker after UV exposure
for 30 min, compared to initial monomers without the UV light.
Similarly, the intensity of carbon–carbon double bonds stemming
from DS-SH decreased after illuminating for 30 min (Fig. 3b).
All these results demonstrated that the thiol–ene addition reaction
of two monomers was conducted successfully and efficiently.
NLO properties of photo-crosslinked films
DN-SH and DS-SH exhibited good film-forming properties. molecules at a high voltage at the best poling temperature.
Their solutions can be easily spin-coated into thin solid films, When the molecules were in a state which is not beneficial to
no matter DN-SH and DS-SH have relatively low molecular produce higher NLO activity, the molecules were locked through
weight (1369 and 1316 respectively). Thus, their NLO properties a thiol–ene reaction under UV light, resulting in a lower d33 value.
can be tested based on the thin films conveniently. The most In contrast, molecular orientation was ‘‘frozen’’ at room
convenient method to study the second-order NLO property of temperature with the application of an electric field and the
materials is to explore the second harmonic generation SHG molecules could not move freely, resulting in a relatively higher
coefficient (d33). The method to calculate d33 of films has been d33 value. Thus, it is better that the photo-crosslinking process
reported before.11 The real-time poling and depoling processes was conducted at room temperature for 30 min.
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without UV irradiation for comparison. The DN-SH1 film is
almost completely soluble in dichloromethane. By contrast, the
DN-SH2 film after being irradiated with UV light is only a little
soluble in dichloromethane. In the same way, the poor solubi-
lity of film DS-SH2 in dichloromethane also indicated thiol–ene
reactions occurring under 365 nm UV light, without photo-
initiators. A free radical-induced thiol–ene click reaction has
been widely applied in protective coatings and films, because
of its enormous advantages, such as high efficiency, no
by-products, insensitivity to oxygen and so on.12 This reaction
involves the process of chain initiation and chain transfer
which can form polymers with larger molecular weight.
Besides, the thickness of the films we prepared is about
200 nm and high-intensity UV light makes the thiol group react
with double bonds quickly, and the chromophore moieties are
easier to move under a high electric field at the same time.
Thus, insoluble polymers were obtained.
Besides, under the optimized conditions, the doping experi-
ments were also carried out. Monomers DN-SH and DS-SH were
mixed at molar ratios of 1 : 1, 1 : 2 and 2 : 1 respectively. The
poling and depoling curves of doped systems are shown in
Fig. S9–S11 (ESI†), with the results summarized in Fig. 5. All the
doped films, which were irradiated for 30 min at room tem-
perature after poling, demonstrated obviously enhanced NLO
thermostability compared to that of uncrosslinked doped ones,
in agreement with that of the single component.
According to the experimental data, the d33 values of photo-
crosslinked films DN-SH, DS-SH and doped systems were
calculated, respectively. As shown in Fig. 6, the d33 value of
the crosslinked film of DS-SH (108 pm Vꢀ1) is higher than that
of DN-SH (89 pm Vꢀ1), owing to the presence of sulfonyl-based
chromophores which can decrease the dipole–dipole inter-
Fig. 4 Depoling curves of films DN-SH1, DN-SH2 (illuminated at room
temperature for 30 min), DS-SH1 and DS-SH2 (illuminated at room
temperature for 30 min) and subsequent solubility tests (insert pictures).
The depoling curves of second-order nonlinear materials actions between azo-based chromophores. Excitedly, all doped
after poling were recorded to explore the optical stability of systems (1 : 1 doped, 2 : 1 doped, and 1 : 2 doped) showed better
films by the real time decays of the SHG signals (Fig. 4). There NLO properties with d33 values up to 123, 122, and 114 pm Vꢀ1
,
was no electric field applied during the depoling process, and respectively, after photo-crosslinking, in comparison with that
then the poled films were heated gradually from room tem- of the single component. For doped systems, the density of the
perature until no obvious signals were observed. As shown in azo-based chromophores increased in comparison with single
Fig. 4(a), T80% of DN-SH2 after first depoling is 62.5 1C, slightly DS-SH, and meanwhile the sulfonyl-based chromophore can
higher than that of DN-SH1 (about 57.5 1C) without UV light also act as an isolation chromophore to decrease the strong
irradiation, while in the second and third depoling, T80% of
DN-SH2 gradually increased to 95 1C, much higher than that of
DN-SH1. These results demonstrated that the thermostability
of NLO property greatly enhanced after the photo-crosslinking
process. Similarly, in Fig. 4(b), T80% of photo-crosslinked
DS-SH2 increased from 77.5 1C to 110 1C after three times of
depoling, much higher than that of uncrosslinked DS-SH1
(75 1C). Thus, the photo-crosslinking process can indeed
improve the thermostability of NLO property, and it is very
convenient to control this process unlike thermo-crosslinking.
Also, the solubility tests of films were conducted to monitor
the crosslinking process. As shown in Fig. 4 (inserted small
pictures), there are four pictures of uncrosslinked and cross-
linked films which have undergone the poling and depoling
Fig. 5 T80% of films DN-SH, DS-SH and 1 : 1 doped, (DN-SH : DS-SH, 1 : 1,
process, immerged in dichloromethane. First DN-SH2 was
exposed to UV light for 30 min, and then DN-SH1 was tested
M : M), 2 : 1 doped (DN-SH : DS-SH, 2 : 1, M : M), and 1 : 2 doped
(DN-SH :DS-SH, 1: 2, M :M) with and without photo-crosslinking processes.
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Thermogravimetric analysis (TGA) was performed on a NETZSCH
STA449C thermal analyzer at a heating rate of 10 1C minꢀ1 in
nitrogen at a flow rate of 50 cm3 minꢀ1. Differential scanning
calorimetry (DSC) was investigated using DSC Q2000 V24.11
Build 124 under nitrogen at a scanning rate of 10 1C minꢀ1
.
Elemental analysis (EA) was performed using a CARLOERBA-1106
micro-elemental analyzer.
Synthesis
DN-OH. Compound 1 (200 mg, 0.40 mmol), 2 (363.58 mg,
0.926 mmol), CuSO4ꢁ5H2O (10% mol), NaHCO3 (20% mol) and
ascorbic acid (20% mol) were added into a Schlenk flask. THF
(9 mL) and water (1.8 mL) were added to react for 3 h under
nitrogen at room temperature. Then the reaction solution was
extracted by water, and the solvent was removed by rotary
evaporation. The crude product was purified by column chro-
matography on silica gel using dichloromethane and ethyl
acetate (1/1.5) as eluents, to yield a red solid (400 mg, 78%).
1H NMR (400 MHz, CDCl3, 298 K), d (TMS, ppm): 1.24 (t, J =
8.0 Hz, 6H, –CH3), 1.62–1.39 (m, 6H, –CH2–), 1.91 (m, 2H,
–CH2–), 2.21 (m, 4H, –CH2–), 2.70 (s, 1H, –OH), 2.95 (t, J =
4.0 Hz, 4H, –CH2–), 3.47 (q, 4H, –CH2–), 3.73–3.65 (m, 6H,
–CH2–), 4.19–4.01 (m, 10H, –CH2–), 4.38 (t, J = 4.0 Hz, 4H,
–CH2–), 5.20–5.14 (m, 4H, –CQCH2), 5.81–5.88 (m, 2H,–CHQC),
6.48 (d, J = 8.0 Hz, 2H, ArH), 6.70 (d, J = 8.0 Hz, 4H, ArH), 7.23
(s, 2H,–CQCH–), 7.95–7.46 (m, 15H, ArH). 13C NMR (100 MHz,
CD2Cl2, 298 K), d (ppm): 155.77, 155.06, 151.64, 149.55, 148.70,
148.00, 147.14, 147.09, 146.34, 145.34, 144.11, 132.99, 126.06,
125.82, 122.40, 117.13, 116.97, 116.43, 116.12, 116.00, 111.70,
111.46, 109.33, 109.15, 70.02, 68.69, 62.37, 52.62, 51.16, 47.31,
45.36, 32.94, 29.11, 28.52, 26.10, 25.64, 21.75, 12.21. HRMS
(ESI, m/z): calcd for C66H77N18O10: 1281.6065 [M + H]+; found:
1281.6060.
DN-S-S-DN. DN-OH (236 mg, 0.18 mmol), 3,30-dithio-
dipropionic acid (19.36 mg, 0.09 mmol), EDC (52.96 mg,
0.27 mmol) and DMAP (30% mol) were added in distilled
dichloromethane (2.3 mL). The resultant mixture was stirred
at 30 1C for 7 h. The organic layer was extracted by a citric acid
aqueous solution and water, and then the solvent was removed
by rotary evaporation. The crude product was purified by
column chromatography on silica gel using dichloromethane
and ethyl acetate eluents (1/1), to yield a red solid (150 mg,
61%). 1H NMR (400 MHz, CDCl3, 298K), d (TMS, ppm): 1.23
(t, J = 8.0 Hz, 12H, –CH3), 1.65–1.46 (m, 12H, –CH2–), 1.92
(m, 4H, –CH2–), 2.23 (m, 8H, –CH2–), 2.67 (t, J = 8.0 Hz, 4H,
–CH2–), 2.85 (t, J = 8.0 Hz, 4H, –CH2–), 2.95 (t, J = 8.0 Hz, 8H,
–CH2–), 3.48 (q, 8H, –CH2–), 3.71 (t, J = 8.0 Hz, 8H, –CH2–),
Fig. 6 d33 values of films DN-SH2, DS-SH2 and 1 : 1 doped (DN-SH :
DS-SH, 1 : 1, M : M), 2 : 1 doped (DN-SH : DS-SH, 2 : 1, M : M), and 1 : 2 doped
(DN-SH : DS-SH, 1 : 2, M : M) after photo-crosslinking.
interactions between azo-based chromophores, leading to
better NLO performance.
Conclusions
In summary, two AB2-type monomers with alkyne and thiol as
terminal groups were synthesized successfully. The films of
DS-SH and DN-SH were respectively irradiated at different
temperatures for 30 min by utilizing the thiol–ene click
reaction. The photo-crosslinking process conducted at room
temperature after poling showed better results, with the T80% of
DS-SH increasing from 75 before poling to 112.5 1C, while T80%
of the DN-SH film increased from 58 to 95 1C after poling. The
doped films of DN-SH and DS-SH at a molar ratio of 1 : 1
showed larger NLO properties (123 pm Vꢀ1) after the photo-
crosslinking process than single DN-SH (89 pm Vꢀ1) and DS-SH
(108 pm Vꢀ1). Thus, the utilization of photo-crosslinking is a
promising method to achieve good NLO stability and relatively
large NLO properties simultaneously by regulating the balance
between electrostatic interactions and molecular structures.
Experimental section
Materials
Dichloromethane was dried with CaH2 and distilled before use.
Tetrahydrofuran (THF) was dried from K–Na alloy and distilled
under an atmosphere of dry nitrogen. The synthetic routes of 1
and 2 are shown in the ESI.† All other reagents were used as
received.
Instrumentation
1H and 13C NMR spectra were recorded on a Bruker AVANCE III 4.00 (t, J = 4.0 Hz, 8H, –CH2–), 4.19–4.06 (m, 16H, –CH2–), 4.37
HD 400 MHz spectrometer using tetramethylsilane (TMS; (t, J = 8.0 Hz, 8H, –CH2–), 5.20–5.13 (m, 8H,–CQCH2), 5.88–5.81
d = 0 ppm) as the internal standard. High resolution mass (m, 4H,–CH QC), 6.58 (d, J = 12.0 Hz, 4H, ArH), 6.69 (d, J =
spectra (HRMS) were measured on an LTQ-Orbitrap Elite high- 8.0 Hz, 8H, ArH), 7.63–7.57 (m, 6H, ArH), 7.84–7.75 (m, 24H,
resolution mass spectrometer equipped with an electrospray ArH). 13C NMR (100 MHz, CDCl3, 298 K), d (ppm): 171.77,
ionization (ESI†) source. UV-visible spectra were obtained using 155.54, 154.90, 151.57, 149.18, 148.60, 147.90, 147.21, 147.18,
a Shimadzu UV-2550 spectrometer. The Fourier transform 146.34, 145.34, 144.20, 132.72, 126.30, 126.11, 122.43, 117.37,
infrared (FTIR) spectra were recorded on a PerkinElmer-2 117.32, 116.64, 116.57, 116.23, 111.72, 111.45, 109.19, 108.96,
spectrometer in the region of 3000–400 cmꢀ1 on KBr pellets. 69.78, 68.54, 64.79, 52.67, 51.30, 47.25, 45.37, 34.05, 33.13, 28.88,
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28.51, 28.40, 25.72, 25.70, 21.87, 12.50. EA: (%, found/calcd): 365 nm UV light for 30 min with the voltage applied, and then
C, 60.59/60.56; H, 5. 62/5.82; N, 18.17/18.42; S, 2.06/2.34.
the photo-crosslinked films were obtained.
DN-SH. Under an atmosphere of nitrogen, DN-S-S-DN
The depoling curves of second-order nonlinear materials
(135 mg, 0.05 mmol), DTT (16.74 mg, 0.11 mmol), Et3N after poling were conducted as follows: poled films were heated
(11.33 mg, 0.11 mmol) and distilled dichloromethane (2 mL) gradually from room temperature until no obvious signals were
were added into a Schlenk flask. The reaction mixture was observed. The heating rate was 4 1C minꢀ1. T80% represents the
stirred at room temperature for 5 h. The organic layer was temperature at which the SHG signals decreased to initial 80%.
washed with HCl (0.1 mol Lꢀ1) and water, and the solvent was
removed by rotary evaporation. The crude product was purified
by column chromatography on silica gel using dichloro-
Conflicts of interest
methane and ethyl acetate as eluents (2/1), to yield a red solid
There are no conflicts to declare.
(60 mg, 44%). 1H NMR (400 MHz, CDCl3, 298 K), d (TMS, ppm):
1.24 (t, J = 4.0 Hz, 6H, –CH3), 1.71–1.48 (m, 7H, –CH2–, SH),
1.95–1.90 (m, 2H, –CH2–), 2.24 (m, 4H, –CH2–), 2.60 (t, J = Acknowledgements
8.0 Hz, 2H, –CH2–), 2.71 (q, 2H, –CH2–), 2.96 (t, J = 8.0 Hz, 4H,
We are grateful to the National Natural Science Foundation of
China (21734007) for financial support.
–CH2–), 3.48 (q, 4H, –CH2–), 3.73 (t, J = 4.0 Hz, 4H, –CH2–), 4.00
(t, J = 4.0 Hz, 4H,–CH2–), 4.20–4.09 (m, 8H, –CH2–), 4.38 (t, J =
8.0 Hz, 4H, –CH2–), 5.18 (m, 4H, –CQCH2), 5.88–5.81 (m, 2H,
–CHQC), 6.59 (d, J = 8.0 Hz, 2H, ArH), 6.70 (d, J = 8.0 Hz, 4H,
Notes and references
ArH), 7.63–7.58 (m, 3H, ArH), 7.85–7.76 (m, 12H, ArH). 13C
NMR (100 MHz, CDCl3), d (TMS, ppm): 171.74, 155.55, 154.90,
151.58, 149.18, 148.62, 147.90, 147.22, 147.19, 146.36, 145.36,
144.20, 132.72, 126.30, 126.12, 122.42, 117.37, 117.33, 116.64,
117.57, 116.23, 111.73, 111.46, 109.20, 108.95, 69.78, 68.55,
64.68, 52.67, 51.30, 47.26, 45.37, 38.47, 28.89, 28.53, 28.41,
25.76, 25.71, 21.87, 19.80, 12.50. HRMS (ESI, m/z): calcd for
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Preparation of films
The monomers DN-SH and DS-SH, and doped systems were
dissolved in dichloromethane respectively with the concentration
of 30 mg mLꢀ1, and the solutions were filtered by syringe filters.
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NLO measurements and photo-crosslinking of monomers
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different monomers.
The thin films were heated to the best poling temperature
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the photo-crosslinking process was conducted immediately
or at room temperature. The poled films were exposed to the
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