A new crosslinkable system based on thermal Huisgen reaction to enhance the stability of electro-optic polymers

Article abstract of DOI:10.1039/b900669a

The thermal Huisgen cycloaddition reaction is a new and efficient process to crosslink electro-optic polymers, since it leads to an effective increase of the thermal alignment stability of the NLO chromophores.

Full text of DOI:10.1039/b900669a

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A new crosslinkable system based on thermal Huisgen reaction
to enhance the stability of electro-optic polymersw
a
a
b
b
´
Annabelle Scarpaci, Errol Blart, Veronique Montembault, Laurent Fontaine,*
Vincent Rodriguez*c and Fabrice Odobel*a
Received (in Cambridge, UK) 13th January 2009, Accepted 18th February 2009
First published as an Advance Article on the web 6th March 2009
DOI: 10.1039/b900669a
The thermal Huisgen cycloaddition reaction is a new and
efficient process to crosslink electro-optic polymers, since it
leads to an effective increase of the thermal alignment stability
of the NLO chromophores.
to limit the impact of side reactions, such as chromophore
degradation.4
In previous work,7 we have extensively studied a methacrylate-
based crosslinkable polymer named PIII reported by Boutevin
et al.,6 composed of a Disperse Red 1 (DR1) derivative func-
tionalized with a carboxylic acid function, which crosslinked with
a pendant epoxy group as side chain (Fig. 1). Although this
polymer displays significantly appealing features such as easy,
large-scale synthesis and relatively satisfying temporal stability
(up to 100 1C), several deficiencies such as a limited storage
period at room temperature, because of premature crosslinking,
hampered the use of this material. Besides, the largest achievable
thickness of a monolayer film with PIII was only 1.3 mm by spin
coating. Moreover, the appearance of hydroxyl groups upon
crosslinking raises the absorbance at 1.5 mm, since the second
harmonic stretching of the latter is located at this wavelength.8
In this contribution, we report a new crosslinking system based
on the implementation of the thermal Huisgen reaction, which
proved particularly efficient to lock the chromophore orientation
after the poling step. Copper-catalyzed Huisgen 1,3-dipolar
cycloaddition is a very popular reaction, referred to as a click
reaction, and has been used extensively to graft molecular units
on polymers, on surfaces or simply to synthesize new com-
pounds.9 Recently, it has been successfully used to attach site
isolation groups on NLO chromophores to limit their propensity
to form intermolecular ferroelectric associations,10 but to the best
of our knowledge the simple thermal version of the Huisgen
reaction has never been used as a crosslinking system, particularly
with organic electro-optic materials. The absence of any metal-
based compound to catalyse the crosslinking process was a
requirement for such an application, since it would first decrease
the crosslinking temperature much below Tg, second it would
enhance the electric conductivity when the film is submitted to the
One of the major obstacles limiting the practical applications of
polymer-based nonlinear optical (NLO) materials for optical
telecommunications is the relatively low temporal and thermal
stability of the non-centrosymmetric alignment of the chromo-
phores. This limitation has been somewhat solved by the advent
of high glass transition temperature (Tg) polymers such as
polyimides, especially when the NLO chromophores are chemi-
cally bonded to the polymer backbones.1 However, the chromo-
phores can be prone to decomposition or sublimation when the
materials are subjected to the high temperatures endured by the
polymer during the preparation or the poling. An alternative
strategy to enhance the stability of NLO activity is to focus on
retarding the mobility of the polymer segments via crosslinking.
Crosslinking reactions have been performed either thermally or
photochemically, and stabilization of dipole alignment has been
achieved in both approaches.2 In this context, the group of Jen
and coworkers has reported the use of Diels–Alder reaction to
thermally crosslink several electro-optic organic materials, and a
marked improvement of the stability of the chromophore
orientation after poling was observed.3,4 However, only a rela-
tively small number of thermal crosslinking reactions have
been reported in the field of organic second order nonlinear
materials,3–6 because the suitable system should fulfil three
distinct properties which are difficult to bring together. First of
all, the crosslinkable groups must remain inert during the
polymerization step and the storage in order to avoid the
premature crosslinking of the material. Second, the crosslinking
temperature (Tc) has to be above or close to the glass transition
temperature (Tg) to ensure an efficient poling of the chromophores
before the crosslinking reaction starts; and finally the
crosslinking reaction has to be relatively fast at Tc in order
a
´
CEISAM, Chimie Et Interdisciplinarite, Synthese, Analyse,
Modelisation CNRS, UMR CNRS 6230, UFR des Sciences et des
`
´
`
Techniques 2, rue de la Houssiniere – BP 92208, 44322 Nantes
Cedex 3, France. E-mail: Fabrice.Odobel@univ-nantes.fr
b
c
`
LCOM-Chimie des Polymeres, UCO2M-UMR CNRS 6011,
Universite du Maine, Avenue O. Messiaen, 72085 Le Mans Cedex 9,
´
France. E-mail: laurent.fontaine@univ-lemans.fr
Department of Chemistry, Groupe Spectroscopie Moleculaire –
´
Institut des Sciences Moleculaires, UMR 5255 CNRS, University of
´
Bordeaux Bat. A12, 351 cours de la Liberation, 33405 Talence
´
Cedex, France. E-mail: v.rodriguez@ism.u-bordeaux1.fr
w Electronic supplementary information (ESI) available: Synthetic pro-
cedures and characterization of the materials. See DOI: 10.1039/b900669a
Fig. 1 Structures of the NLO polymers described in this study.
Chem. Commun., 2009, 1825–1827 | 1825
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high poling voltage and finally it could induce phase segregation.
Copper-free crosslinking of difluorinated cyclooctyne derivatives
with azides was reported for in vivo imaging or photodegradable
polymers, but this activated alkyne cannot be used to crosslink
NLO materials because it would react far below the Tg of the
polymer thus preventing the chromophore orientation.11
measured by size exclusion chromatography calibrated with
polystyrene standards and the data are collected in Table 1.
The chromophore loading levels are, respectively, 47 and
42 wt% in PAS1 and PAS2. From differential scanning
calorimetry (DSC) analysis, the glass transition temperatures
(Tg) of PAS1 and PAS2 were measured at 88 and 89 1C,
respectively (see Fig. 1 SIw). Moreover, the DSC trace exhibits
a first broad exothermic peak assigned to Huisgen crosslinking
reaction at a temperature around 160 1C, followed by a second
higher exothermic peak around 290 1C attributed to the
classical decomposition of triazoles into nitrenes (Table 1).15
Besides, thermal gravimetric analyses showed that PAS1 and
PAS2 polymers display a good thermal stability since the
decomposition temperatures (Td) are 295 and 285 1C, respec-
tively. These observations indicate that the presence of MMA
monomer does not influence to a great extent the thermal
properties of these new materials and therefore MMA can be
used as a co-monomer to decrease the amount of TMSMA.
A second advantage of this crosslinking reaction is the
possibility to follow and to quantify the extent of crosslinking
by monitoring the decrease in intensity of the azide absorption
band around 2100 cmÀ1 by FTIR spectroscopy. As shown in
Fig. 2, the crosslinking of a PAS1 film is complete in less than
one hour at 150 1C. Finally, films with good optical quality and
satisfying thicknesses (up to 1.8 mm) were prepared by spin
coating solutions of the polymers PAS1 and PAS2 in o-dichloro-
benzene, evidencing the good filmability of these new polymers.
Wire poling under high electric field (3.9 kV) was carried out at
90 1C for one hour for each material. Then, the films were heated
at 150 1C for an additional hour to perform the Huisgen cross-
linking reaction and were rapidly cooled to room temperature in
order to freeze the orientation of the chromophores.
To investigate the potential utility of the thermal Huisgen
reaction as a crosslinking process for electro-optic polymers, we
prepared two new crosslinkable materials (PAS1 and PAS2),
analogous to the known PIII polymer (Fig. 1).6 These polymers
were synthesized by radical copolymerization of three methacrylate
monomers: a monomer bearing an alkyne group (TMSMA), the
monomer 5 incorporating the DR1-type chromophore at the same
concentration as that in PIII (30 mol%), but functionalized by an
azido group, and methyl methacrylate (MMA) (Scheme 1). The
alkyne monomer was protected by a trimethylsilyl (TMS) group to
avoid the anticipated side reactions of terminal alkynes with the
free radicals formed during the polymerization reaction.12 To
minimize the production cost, we also studied how the presence
of MMA as co-monomer impacts the properties of the final
polymer. Therefore, PAS1 and PAS2 were prepared in the presence
and in the absence of MMA, respectively (Scheme 1).
The synthesis of the new polymers is relatively straight-
forward and can be performed on a 10 g scale or even more in
a few days owing to its high efficiency and its technical
simplicity. Firstly, chromophore 3 was prepared by a diazo-
coupling reaction of 4-amino-3-nitrobenzoic acid 1 with com-
pound 2 under acidic conditions in 67% yield.13 Then the
amidification of the carboxylic acid function borne by com-
pound 3 was carried out in the presence of 4-azidoaniline
hydrochloride 4, N-methylmorpholine (NMM) and dimethoxy-
triazine-N-methylmorpholine (DMTMM)14 as condensing
agent to lead to the NLO chromophore 5 in quantitative yield.
Finally, the PAS1 and PAS2 polymers were synthesized in
about 75% yield by free-radical copolymerization of the above
monomers using AIBN as an initiator (Scheme 1). The poly-
mers PAS1 and PAS2 exhibit high solubility in common polar
organic solvents, such as CH2Cl2, CHCl3, THF, DMF and
DMSO, and non saturating concentration of 250 g LÀ1 could
be obtained in o-dichlorobenzene. The number average mole-
cular weight (Mn) and the polydispersity index (PDI) were
The obtained poled/cured polymer films were very tough
and gained good solvent resistance after crosslinking. The
quantitative crosslinking of the films was confirmed by the
disappearance of azide absorption bands (around 2100 cmÀ1
)
in FTIR spectra (Fig. 2).
The d33 coefficients of these films were measured at 1064 nm
by second harmonic generation (SHG). Values of 56 and
50 pm VÀ1 were determined for PAS1 and PAS2, respectively.
Unsurprisingly, these values are similar to the d33 value of PIII
polymer (60 pm VÀ1) since a similar chromophore was used at
the same concentration (30 mol%). Thermal stability of the
electro-optic activity of the polymers was investigated by
depoling experiments, in which the real time decay of the
SHG signal was monitored for non crosslinked PAS1, cross-
linked PAS1 and PAS2 and the reference polymer PIII as a
function of the temperature (Fig. 3).
Table 1 Specific properties of the polymers: glass transition tempera-
ture (Tg), crosslinking temperature (Tc), decomposition temperature
(Td), film thickness (d), and d33 measured at 1064 nm
a
a
Mn
/
Mw
/
d33/pm
g molÀ1 g molÀ1 PDIa Tg/1C Tc/1C Tdb/1C d/mm VÀ1
PAS1 7500
PAS2 7100
PIII 5700
13 500 1.8
13 500 1.9
9600 1.69 65
88
89
160
160
150
295
285
270
1.8
1.6
1.3
56
50
60
a
Determined by size exclusion chromatography (SEC) calibrated with
b
polystyrene standards. Measured at 5 wt.% decomposition.
Scheme 1 Synthetic route for the polymers PAS1 and PAS2.
1826 | Chem. Commun., 2009, 1825–1827
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exists a large body of information, was used to give a proof of
concept for this new crosslinking process, but certainly without
any claim to compete with the best known electro-optic materials
in term of d33. The extension of this concept to a chromophore
exhibiting a larger quadratic hyperpolarizability than DR1 could
lead to polymers which combine high r33 coefficients along with
good electro-optic stability. Furthermore, this version of the
Huisgen reaction will certainly also be useful for other applications
where crosslinking is required, for example light emitting diode
technology or organic field effect transistors.17
Region Pays de la Loire is gratefully acknowledged for the
´
financial support of this research through the CPER and the
MILES-MATTADOR programs.
Fig. 2 Evolution of the intensity of the stretching band of azide in
PAS1 film as a function of the heating time at 150 1C.
Notes and references
1 (a) M. H. Davey, V. Y. Lee, L. M. Wu, C. R. Moylan, W. Volksen,
A. Knoesen, R. D. Miller and T. J. Marks, Chem. Mater., 2000, 12,
1679–1693; (b) H. Saadeh, D. Yu, L. M. Wang and L. P. Yu,
J. Mater. Chem., 1999, 9, 1865–1873.
2 D. M. Burland, R. D. Miller and C. A. Walsh, Chem. Rev., 1994,
94, 31–75.
3 Y. Enami, C. T. Derose, D. Mathine, C. Loychik, C. Greenlee,
R. A. Norwood, T. D. Kim, J. Luo, Y. Tian, A. K. Y. Jen and
N. Peyghambarian, Nat. Photonics, 2007, 1, 180–185.
4 J. Luo, S. Huang, Y.-J. Cheng, T.-D. Kim, Z. Shi, X.-H. Zhou and
A. K. Y. Jen, Org. Lett., 2007, 9, 4471–4474.
5 (a) Y. Bai, N. Song, J. P. Gao, X. Sun, X. Wang, G. Yu and
Z. Y. Wang, J. Am. Chem. Soc., 2005, 127, 2060–2061; (b) J. Luo,
M. Haller, H. Ma, S. Liu, T.-D. Kim, Y. Tian, B. Chen,
S.-H. Jang, L. R. Dalton and A. K. Y. Jen, J. Phys. Chem. B,
2004, 108, 8523–8530.
Fig. 3 Thermal stability of electro-optic activity of polymers PAS1 and
PAS2 upon heating a film, initially poled at a rate of 2 1C minÀ1 in the air.
6 D. Bosc, F. Foll, B. Boutevin and A. Rousseau, J. Appl. Polym.
Sci., 1999, 74, 974–982.
7 (a) C. Monnereau, E. Blart, B. Illien, M. Paris and F. Odobel,
J. Phys. Org. Chem., 2005, 18, 1050–1058; (b) A. Apostoluk,
J. M. Nunzi, V. Boucher, A. Essahlaoui, R. Seveno,
H. W. Gundel, C. Monnereau, E. Blart and F. Odobel, Opt.
Commun., 2006, 260, 708–711.
These results clearly show that this new crosslinking process
increases the stability of the electro-optic activity of the polymers.
When the polymer is simply poled but not crosslinked, the
chromophores start to relax at 80 1C (circles), while NLO proper-
ties are maintained up to 137 1C after crosslinking (squares and
triangles). In other words, the present crosslinking system leads to
a gain of EO stability of ca. 60 1C for the PAS1 and PAS2
materials and represents a real improvement compared to the
previously known polymer PIII (diamonds), in which the NLO
stability was kept only until 105 1C. The temporal stability of PIII
was studied and proved to be satisfactory for several days at
85 1C, therefore it is reasonable to anticipate that PAS1 and PAS2
will be as stable and probably even more so.16 Finally, it should
be pointed out that no significant stability difference is noted for
the polymer incorporating MMA monomer (PAS1 vs. PAS2).
The production cost of these new materials can thus be substan-
tially lowered, without impairing their NLO properties.
8 H. Ma, A. K. Y. Jen and L. R. Dalton, Adv. Mater., 2002, 14,
1339–1365.
9 (a) J. F. Lutz, Angew. Chem., Int. Ed., 2007, 46, 1018–1025;
(b) W. H. Binder and R. Sachsenhofer, Macromol. Rapid
Commun., 2007, 28, 15–54; (c) V. Bock, D. H. Hiemstra and
J. H. van Maarseveen, Eur. J. Org. Chem., 2006, 51–68.
10 (a) Z. Li, Q. Zeng, G. Yu, Z. Li, C. Ye, Y. Liu and J. Qin, Macromol.
Rapid Commun., 2008, 29, 136–141; (b) Q. Zeng, Z. Li, Z. Li, C. Ye,
J. Qin and B. Z. Tang, Macromolecules, 2007, 40, 5634–5637.
11 (a) J. A. Johnson, J. M. Baskin, C. R. Bertozzi, J. T. Koberstein
and N. J. Turro, Chem. Commun., 2008, 3064–3066;
(b) J. M. Baskin, J. A. Prescher, S. T. Laughlin, N. J. Agard,
P. V. Chang, I. A. Miller, A. Lo, J. A. Codelli and C. R. Bertozzi,
Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 16793–16797.
12 (a) A. Hayashi, Y. Goto, M. Nakayama, H. Sato, T. Watanabe and
S. Miyata, Macromolecules, 1992, 25, 5094–5098; (b) V. Ladmiral,
G. Mantovani, G. J. Clarkson, S. Cauet, J. L. Irwin and
D. M. Haddleton, J. Am. Chem. Soc., 2006, 128, 4823–4830.
13 C.-D. Keum, T. Ikawa, M. Tsuchimori and O. Watanabe,
Macromolecules, 2003, 36, 4916–4923.
In conclusion, we present the first example of crosslinkable EO
polymers based on the thermal Huisgen reaction. The polymers
can be prepared on a large scale (410 g) due to a facile synthetic
procedure and they exhibit good film-forming properties. After the
poling process, a smart control of the absorption band of azide
groups, monitored by FTIR, can be used to follow the crosslinking
process. The resulting materials exhibit very satisfying thermal
alignment stability, the d33 coefficient remaining constant up to
about 140 1C. These results demonstrate that the thermal Huisgen
1,3-dipolar azide–alkyne ‘‘click’’ reaction can be used successfully
to ensure the efficient crosslinking of NLO polymers. In this work,
a derivative of the well-known DR1 chromophore, for which there
14 M. Kunishima, C. Kawachi, J. Monta, K. Terao, F. Iwasaki and
S. Tani, Tetrahedron, 1999, 55, 13159.
15 M. Ergin, B. Kiskan, B. Gacal and Y. Yagci, Macromolecules,
2007, 40, 4724–4727; G. L’Abbe, Chem. Rev., 1969, 69, 345–363.
16 R. Levenson, J. Liang, C. Rossier, R. Hierle, E. Toussaere,
N. Bouadma and J. Zyss, Advances in organic polymer-based
optoelectronics, American Chemical Society, Washington DC, 1995.
17 (a) C. Kim, Z. Wang, H.-J. Choi, Y.-G. Ha, A. Facchetti and
T. J. Marks, J. Am. Chem. Soc., 2008, 130, 6867–6878; (b) B. Ma,
F. Lauterwasser, L. Deng, C. S. Zonte, B. J. Kim, J. M. J. Frechet,
C. Borek and M. E. Thompson, Chem. Mater., 2007, 19,
4827–4832.
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