Synthesis and properties of a ROMP backbone polymer with efficient, laterally appended nonlinear optical chromophores

Article abstract of DOI:10.1039/B314789D

The synthesis and properties of a new ROMP backbone polymer bearing lateral donor-acceptor-substituted (E)-diethynylethene ((E)-hex-3-ene-1,5-diyne) chromophores are described. The polymer with an average of 40 monomeric repeat units is readily soluble in common organic solvents such as tetrahydrofuran, dichloromethane and chloroform. Good optical-quality films were obtained by spin coating. The third-order susceptibility χ⁽³⁾ at a wavelength of 1907 nm was determined to be 100 times greater than that of fused silica. After electrical poling, the polymer also showed a significant second-order nonlinear optical coefficient (d₃₃) value, which decays by a factor of two over a period of one month at room temperature.

Full text of DOI:10.1039/B314789D

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C O M M U N I C AT I O N
Synthesis and properties of a ROMP backbone polymer with
efficient, laterally appended nonlinear optical chromophores{
Jean-Manuel Raimundo,a Steve Lecomte,b Michael J. Edelmann,c Simona Concilio,d
Ivan Biaggio,e Christian Bosshard,f Peter Gu¨nterb and Franc¸ois Diederich*c
aLaboratoire de Chimie des Mate´riaux Organiques et Me´talliques (CMOM),
Universite´ de Nice – Sophia Antipolis,, Parc Valrose, 28 Avenue Valrose,
06108 Nice Cedex 2, France
bInstitute of Quantum Electronics, ETH-Ho¨nggerberg, 8093 Zu¨rich, Switzerland
cLaboratorium fu¨r Organische Chemie, ETH-Ho¨nggerberg, CH-8093 Zu¨rich,
Switzerland. E-mail: diederich@org.chem.ethz.ch; Fax: (141) 1-632-1109;
Tel: (141) 1-632-2992
dUniversity of Salerno, Department of Chemical Engineering, Via Ponte Don Melillo,
84084 Fisciano (SA), Italy
eDept. of Physics, Lehigh University, Bethlehem, PA 18015, USA
fCSEM Alpnach, 6055 Alpnach, Switzerland
Received 15th November 2003, Accepted 18th December 2003
First published as an Advance Article on the web 12th January 2004
The synthesis and properties of a new ROMP backbone
polymer bearing lateral donor–acceptor-substituted
(E)-diethynylethene ((E)-hex-3-ene-1,5-diyne) chromo-
phores are described. The polymer with an average of 40
monomeric repeat units is readily soluble in common
organic solvents such as tetrahydrofuran, dichloro-
methane and chloroform. Good optical-quality films
were obtained by spin coating. The third-order
Here we describe the synthesis by ring-opening metathesis
polymerisation (ROMP)5 and physical investigation of the novel
polymer
1 (Scheme 1) bearing donor–acceptor-substituted
DEEs as NLO-active side-chain chromophores. The prepara-
tion of linear polymers of low polydispersity, featuring precisely
positioned side-chain chromophores and possessing desirable
properties such as high solubility, ease of processability and low
viscosity has become an attractive target, in particular for the
development of optoelectronic devices.6 ROMP has been shown
in the last several years to be a useful technique for achieving low
polydispersities across a wide array of molecular weight,7 but so
far only a few reports have appeared in which this polymerisa-
tion technique has been adopted as a useful synthetic route to
polymers bearing NLO-active chromophores as side-chains.8
We also opted for ROMP polymers since they offer good
solubility, required for using spin coating techniques, and
transparency of the polymer backbone.
The norbornenyl derivative 2, with the NLO-chromophore
appended via an exo-ether linker to the bicyclic framework, was
chosen as the ROMP monomer.9 This monomer was selected
because it is known to add irreversibly to a living ROMP
polymer chain,10 permitting access to homopolymers and block
copolymers of low polydispersity. It is well documented that
exo-norbornenyl derivatives polymerise more readily than
endo-isomers.10a
susceptibility x(3) at
a wavelength of 1907 nm was
determined to be 100 times greater than that of fused
silica. After electrical poling, the polymer also showed a
significant second-order nonlinear optical coefficient (d33
)
value, which decays by a factor of two over a period of
one month at room temperature.
Efficient nonlinear optical (NLO) materials attract much
attention in view of their potential applications in areas such
as optical signal processing, optical computing and telecom-
munications, and a wide range of compounds extending from
inorganic crystals to organic p-chromophores have been
explored for application in NLO devices.1–6 Among them,
organic polymers have advantages with regards to ease
of fabrication and integration into macroscopic devices,
tailorability—allowing the chemical structure and properties
to be fine-tuned for a given nonlinear optical process-, low
dielectric constants, fast nonlinear optical response times and
large off-resonance nonlinear optical susceptibilities.2
During the past decade, we targeted the understanding of
structure–property relationships for second- and third-order
nonlinear optical susceptibilities by studying comprehensive
series of powerful new one- and two-dimensional NLO
chromophores. These compounds are donor– (e.g. 4-dimethyl-
aminophenyl) acceptor- (e.g. 4-nitrophenyl) substituted deri-
vatives of 1,2-diethynylethene (DEE, (E)-hex-3-ene-1,5-diyne)3
and tetraethynylethene (TEE, 3,4-diethynylhex-3-ene-1,5-
diyne)4 featuring fully planar p-chromophores. They are
synthetically accessible via short and high-yielding protocols
and have high thermal stability in air.
For the preparation of 2 (Scheme 1; for full protocols, see
ESI{) commercially available N-methylaniline was iodinated
to give 3. N-Alkylation with 6-bromohexanol followed by
esterification with p-toluenesulfonyl chloride afforded 4.
exo-Norbornenylcarboxylic acid
5 was prepared by the
Diels–Alder reaction between freshly distilled cyclopentadiene
and methyl acrylate, followed by exo–endo equilibration and
saponification. The endo-isomer was subsequently removed by
iodolactonization9 to afford 5 in 18% overall yield. Reduction
with LiAlH4 gave alcohol 6 that was etherified with 4 to
provide 7. Sonogashira cross-coupling11 of aryl iodide 7 with
p-nitrophenyl-substituted DEE 83a afforded the red-colored
monomer 2, featuring a donor–acceptor-substituted chromo-
phore. The metathesis reaction of 2 was carried out with the
commercial catalyst [RuCl2(PCy3)2CHPh] in THF under Ar.
No change in color or viscosity was observed during the
{ Electronic Supplementary Material (ESI): Full details of the synthesis
of polymer 1 and comparison of the 1H-NMR spectra of 1 and
monomer 2. See http://www.rsc.org/suppdata/jm/b3/b314789d/
2 9 2
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 2 – 2 9 5
T h i s j o u r n a l i s ß T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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Scheme 1 Reagents and conditions: i, I2, NaHCO3, H2O, rt, 45 min, 40%; ii, 6-bromohexanol, K2CO3, n-BuOH, D, 72 h, 60%; then
p-toluenesulfonyl chloride, pyridine, CH2Cl2, rt, 18 h, 84%; iii, CH2Cl2, D, 24 h; then NaOMe, D 4 h; then aq. NaOH, D; then I2, KI, NaHCO3, H2O;
18%; iv, LiAlH4, Et2O, rt (9 h) and D (45 min), 90%; v, NaH, THF, 0 uC (30 min) and rt (2 h), 62%; vi, [PdCl2(PPh3)2], CuI, NEt3, rt, 24 h, 60%; vii,
[RuCl2(PCy3)2CHPh], THF, rt, 2 h, then benzaldehyde, rt, 1 h; quantitative yield. Cy ~ cyclohexyl.
transformation. After all of
2
had reacted (thin layer
obtain an absolute value of the refractive index, we scaled the
refractive index curve using the absolute refractive index values
determined at the two telecom wavelengths of 1.313 mm and
1.55 mm. The absolute value of the refractive index was
measured by the waveguide grating–coupling technique12 using
s-polarized light. The results are marked on the refractive index
curve as two crosses (Fig. 1).
chromatography (TLC) control), the metathesis reaction was
irreversibly terminated by addition of an excess of benzalde-
hyde, yielding polymer 1 in nearly quantitative yield.
Analytical gel permeation chromatography (GPC, calibrated
by polystyrene and poly(triacetylene) standards) analysis of 1
revealed the formation of a polymer of low polydispersity with
an average of 40 monomeric repeat units. The polymer was
highly soluble in common organic solvents such as tetrahy-
drofuran, dichloromethane and chloroform. The 1H-NMR
(CDCl3, 300 MHz) of 1 is well defined featuring broadened
peaks compared to the spectrum of monomer 2 (see ESI{). The
resonances of the appended DEE chromophores in 1 appear
nearly unchanged. The resonance for the olefinic protons of the
norbornene ring in 2 (6.05–6.13 ppm) have disappeared and a
new broad peak around 5.1 ppm is observed, belonging to the
protons of the ethene-1,2-diyl moieties connecting the mono-
meric repeat units in 1.
High-quality thin films of polymer 1 were made by spin
coating of a solution of 1 (concentration 2–10 wt%) in CH2Br2
at 40 uC using a spinning rate of 300 min21 for 4 s and 1500–
3000 min21 for 40 s. The film was subsequently dried at 70 uC
for 15 h. This procedure afforded films with good optical
quality having a thickness range of 0.2 to 3 mm. Differential
scanning calorimetry (DSC) analysis of polymer 1 was carried
out under nitrogen at a heating rate of 20 uC min21 and
revealed the glass transition temperature (Tg) at 40.0 uC,
indicating that polymer 1 is amorphous and is in a glassy stable
state at room temperature.
We first assessed the second-order nonlinear optical response
of electrically poled spin-coated thin films of polymer 1, having
thicknesses ranging from 200 to 300 nm, by second-harmonic
generation (SHG) experiments using a fundamental wave-
length of 1064 nm. To pole the thin films we used the standard
technique of corona poling in a needle to plane configuration13
applying voltages between 6.5 to 9 kV. The polar orientation at
room temperature is achieved by electric field poling close to
the glass transition temperature of the sample and subsequent
cooling. Because of the symmetry of the poling field, the
resulting second-order susceptibility tensor has only the two
independent elements d33 and d31 (~ d15 ~ d32 ~ d24 if
Kleinman symmetry can be applied) with the 3-axis along the
poling field. After poling, the nonlinear coefficients were
measured by the Maker fringe method.14,15 The laser used was
a Nd-YAG laser emitting 100 ps pulses at a wavelength of
1064 nm. Quartz serves as reference with a d11 coefficient of
0.3 pm V21 16
The 532 nm second-harmonic wavelength in
.
this experiment is slightly inside the absorption band of the
polymer film.
In the Maker fringe experiment, we measured second-
harmonic generation as a function of the incidence angle. One
experiment was performed with the polarization of both beams
(the fundamental and the second harmonic) in the incidence
plane (p A p). In another experiment, the polarization of the
fundamental wave was set to be perpendicular to the incidence
plane (s A p). The resulting curves of second-harmonic
The linear and nonlinear optical properties of spin-coated
thin films of polymer 1 were measured. A Perkin-Elmer
Lambda 9 spectrometer was used to measure the absorption.
The Kramers–Kronig relations were used to calculate the
corresponding dispersion of the refractive index (Fig. 1). To
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 2 – 2 9 5
2 9 3

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the SHG signal increases with increasing temperature, and that
(ii) the electric-field-induced nonlinearity reaches a plateau at
a value of about d33 ~ 80 pm V21 (with the electric field
applied). The initial rapid decrease (taking place at a time of
4000–5200 s) of the nonlinearity upon cooling can be ascribed
to a first fast reorientation of the chains at temperatures close
to the Tg value of polymer 1. The later abrupt decrease (taking
place at the later time of 5200–6000 s, and at a lower
temperature) is due to the removal of the applied electric field.
In order to estimate the time-dependence of the orientational
stability of the poled polymer films at room temperature, SHG
measurements were repeated on the same films after one
month. In spite of the low Tg value, the nonlinear optical
coefficients were found to be as high as half of the initially
measured values. Of course, a higher Tg would improve
material performance over time by increasing its orientational
stability at a temperature higher than room temperature. An
increase of the Tg value could be achieved, for example, by
decreasing the length of the alkyl chain between the NLO
chromophores and the polymer backbone.
Third-harmonic generation (THG) was performed both in
solution and in thin films at a fundamental wavelength of
1.907 mm. Such a long fundamental wavelength was chosen to
avoid any resonance effect. The fundamental and third-
harmonic wavelengths were not in absorption bands of the
investigated polymer. As a reference for the calibration we
used x(3) ~ 1.62 6 10222 m2 V22 (1.16 6 10214 esu) for
fs
fused silica.16,17 In a chloroform solution, we measured the
third-order nonlinearity of a single chromophore (2) to be
c ~ 355 ¡ 28 6 10236 esu. For the low-polydispersity polymer
1 with an average of 40 side-chain chromophores, we obtained
c ~ 14500 ¡ 600 6 10236 esu, which is about 40 times the
value of a single chromophore. We conclude that there are no
interactions between the side-chain chromophores in solution.
In thin films, the third-order nonlinearity x(3) was measured
to be about 100 times that of fused silica. An extrapolation of
x(3) from c gives a x(3) value of about 170 times that of fused
silica. This reduction in the expected x(3) has already been
observed in poly(triacetylene) thin films.18 Intermolecular
interactions in the solid-state reduce the third-order non-
linearity x(3) of thin films.
In summary, we have synthesized via high-yielding pro-
cedures the new ROMP polymer 1 which features good
processability and promising optical and nonlinear optical
properties. A high off-resonance third-order nonlinearity x(3)
was measured in a thin film of 1, with an average of 40
chromophore-containing repeat units, which was about 100
times the nonlinearity of fused silica. Polymer 1 has a
significant d33 value, comparable to those of other polymers
containing highly conjugated donor–acceptor chromo-
phores,19–21 and it is easily oriented under corona poling
conditions. Although the relatively low glass transition
temperature does not allow the orientation to be completely
frozen during corona poling, being detrimental to its long-term
stability, this might be very interesting for applications in
photorefractivity and dynamic holography, where the dipolar
chromophores must react to small local changes in the electric
field. The general strategy applied to the synthesis of 1 should
readily allow further adjustment and optimisation of pro-
perties, such as viscosity, Tg, SHG and THG, in future work by
varying the time of polymerisation, the linker length between
the polymer backbone and the photoactive chromophore and
the NLO-chromophore itself.
Fig. 1 Absorption (top) and refractive index (below) of polymer 1 as
function of the wavelength.
intensity vs. incidence angle were compared with the theoretical
prediction14 and with the results obtained using a quartz crystal
as a reference to obtain the values of the nonlinear optical
susceptibilities of the polymer film. From the s A p experiment
we directly obtained d31 ~ 10 ¡ 2 pm V21. From the p A p
experiment we obtained d33 ~ 30 ¡ 10 pm V21, using a fixed
ratio d33/d31 ~ 3 while fitting the experimental data (a ratio of 3
between these two coefficients is expected whenever the dipole
energy, leading to the molecular orientation during poling, is
much smaller than the thermal energy kT). Thus, both the s A
p and the p A p experiments consistently deliver a value of
d33 ~ 30 ¡ 5 pm V21
.
Fig. 2 describes the poling process. It is a plot of the SHG
signal and temperature vs. time. This plot clearly shows that (i)
Acknowledgements
We thank the ETH Research Council and the NCCR
‘‘Nanoscale Science’’ for support of this work.
Fig. 2 Poling process: plot of the SHG-signal and temperature vs. time.
J . M a t e r . C h e m . , 2 0 0 4 , 1 4 , 2 9 2 – 2 9 5
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