Synthesis and 2nd order nonlinear optical properties of soluble polyimides bearing nitroazobenzene type chromophore pendants attached in side-on mode

Article abstract of DOI:10.1039/b107553e

An organic soluble polyimide was prepared from 3,3′-dihydroxybenzidine and 2,2-bis(phthalic anhydride)hexafluoropropane. The two hydroxy groups in the repeating unit of the polyimide thus prepared were utilized in attaching 4-nitro-4′-diphenylaminoazobenz

Full text of DOI:10.1039/b107553e

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Synthesis and 2nd order nonlinear optical properties of soluble
polyimides bearing nitroazobenzene type chromophore pendants
attached in side-on mode{
Bong-Jin Jeon,a Soon Wook Cha,a Mi-Yun Jeong,a Tong Kun Limb and Jung-Il Jin*a
aDivision of Chemistry and Molecular Engineering, and Center for Electro- and
Photo-Responsive Molecules, Korea University, Seoul 136-701, South Korea.
E-mail: jijin@mail.korea.ac.kr
bDepartment of Physics, Korea University, Seoul 136-701, South Korea
Received 21st August 2001, Accepted 14th November 2001
First published as an Advance Article on the web 31st January 2002
An organic soluble polyimide was prepared from 3,3’-dihydroxybenzidine and 2,2-bis(phthalic
anhydride)hexafluoropropane. The two hydroxy groups in the repeating unit of the polyimide thus prepared
were utilized in attaching 4-nitro-4’-diphenylaminoazobenzene chromophores onto the main chain
perpendicularly (or in an end-on fashion) or laterally (or in a side-on fashion) through either the
oxydimethylene or oxyhexamethylene spacer. All the polymers revealed two strong absorptions: one lmax at
300 nm and the other lmax at 493 nm. The polymers, when electrically poled, showed 2nd order nonlinear
optical properties, r33 ~ 9–23 pm V21 at 1300 nm, r33 ~ 102–194 pm V21 at 633 nm, and d33 ~ 59–109 pm V21
at 1064 nm depending on their structure. The polymers bearing side-on attached chromophores revealed higher
r33 values than those with end-on attached chromophores. Temporal stability of the former was also greater
than that of the latter.
polyoxetane backbones. In particular, we27 observed extra-
Introduction
ordinarily fast photoinduced reorientation of chromophores
and decay behaviour at room temperature, far lower than their
glass transition temperatures. Polymers bearing the laterally
attached chromophores revealed faster writing than polymers
which bear the perpendicularly attached chromophores. More-
over, we28 previously demonstrated that the following poly-
imide (n ~ 6) could control the orientational ordering of the
5CB liquid crystal by changing the incident angle of the pump
beam utilized in the photoalignment of the azo chromophores.
In this report, we would like to describe the synthesis and 2nd
order NLO properties of SP-2 and SP-6 (Fig. 1), where ‘‘SP’
stands for side-on attachment of the chromophores and, 2 and
6 for the numbers of the methylene groups in the spacers. For
the sake of comparison, discussion on the synthesis and
properties of the homologous polymer (EP-2 and EP-6, Fig. 1)
bearing end-on or perpendicularly attached chromophores is
also included in this paper. ‘‘EP’ stands for end-on attachment
of the chromophore pendants.
A wide variety of polymers bearing chromophoric structures
revealing nonlinear optical properties have been reported.1–10
Especially, the second order nonlinear optical properties such
as second harmonic generation and electro-optic properties of
the polymers, have been studied in detail in relation to the
structures of the polymer backbone and chromophores. Since
noncentrosymmetrical orientation of dipolar chromophores is
required in order to bring about 2nd order optical nonlinearity,
electrical poling, either corona or contact poling, is usually
applied to polymer thin films. There are exceptional examples
where poling is not necessary. Representative examples are
octupolar compounds11 which, by symmetry, have zero-
permanent dipole moment. Poled or aligned chromophores,
however, tend to relax back to a disordered state, which has to
be prevented from occurring for long-term utilization of the
materials. Earlier, we12 reported that poly(p-phenyleneviny-
lene) derivatives having oriented chromophores, due to their
very rigid backbones, did not exhibit any relaxation even at
elevated temperatures. Polyamides13,14 and crosslinkable
polymers15 also provide us with thermally stable NLO
compositions. Polyimides16–24 are another example of poly-
mers with rigid backbones. Organic–inorganic composites,25
especially silica composites prepared by sol–gel methods, have
been attracting much attention too.
Experimental
Synthesis of polymers
The polymers were prepared via a three step synthetic route as
shown in Scheme 1. Synthetic details for the preparation of
compound 3 can be found elsewhere.27 Other synthetic details
are available from the Electronic Supplementary Information{.
Preparation of SP-2: PI (0.69 g; 1.1 mmol), compound 3(2)
(1.5 g; 3.3 mmol) and triphenylphosphine (1.4 g; 5.5 mmol)
were dissolved in 30 mL of dry tetrahydrofuran under a dry
nitrogen atmosphere. Diisopropyl azodicarboxylate (1.1 g;
5.4 mmol)29 was added dropwise to the solution. The mixture
was stirred for 2 days. The reaction mixture was added
dropwise with vigorous stirring to a mixture of 500 mL of
methanol and 10 mL of 2 M hydrochloric acid. The precipitate
was collected by filtration. The crude product thus obtained
On the other hand, polymers26 containing azobenzene
chromophores are the subject of numerous reports due to
their unique optical properties resulting from the ability of azo
chromophores to undergo photoinduced orientation involving
cis<trans isomerization. Recently, we27 have been trying to
compare the dependence of photoinduced orientation of
azobenzene type chromophores on their mode of attachment,
i.e. either laterally (side-on) or perpendicularly (end-on), on
{Electronic supplementary information (ESI) available: experimental
details. See http://www.rsc.org/suppdata/jm/b1/b107553e/
546
J. Mater. Chem., 2002, 12, 546–552
This journal is # The Royal Society of Chemistry 2002
DOI: 10.1039/b107553e

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Fig. 1 Structures of polymers (SP-2, SP-6, EP-2 and EP-6).
was subjected to Soxhlet extraction with methanol for 2 days.
The product yield was 1.4 g (85%).
corresponding chromophore, 3(6). Its structure was confirmed
spectroscopically and by elemental analysis. Recovered yield
after Soxhlet purification was 89%.
Preparation of EP-2 and EP-6: the polymers were prepared
from PI and 4(2) or 4(6) in the same manner as described above
for the preparation of SP-2. The structures of the polymers
are confirmed spectroscopically and by elemental analysis.
Recovered yields after Soxhlet purification were 85 and 80%,
respectively.
1H NMR spectrum (CDCl3, d ppm): 4.3–4.6 (br d, 8H,
-OCH2CH2O-), 6.5–8.2 (m, 46H, Ar-H). FT-IR spectrum
(KBr, cm21): 1787, 1732 (stretching of amide), 1504, 1343 (N–
O stretching), 1241, 1102 (C–O stretching). Anal. Calcd. for
C83H54N10O12F6: C 66.58, H 3.63, N 9.35%. Found: C 65.91, H
3.99, N 8.98%.
SP-6 was prepared in the exact same manner from PI and the
Scheme 1
J. Mater. Chem., 2002, 12, 546–552
547

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Characterization of polymers
utilized in this experiment for the wavelengths 633, 1300,
1064 nm, respectively. Refractive indices of polymer films were
determined using an ellipsometer (Rudolps-2000, USA) at the
Korea Basic Science Institute — Seoul branch. The d33 values
were estimated relative to the intensity of the second harmonics
by a Y-cut quartz plate of 1.025 mm thickness. The d11 value of
Y-cut quartz plate was taken to be 0.96 6 1029 esu.38 The
temporal stability of some of the poled polymer films was
studied either by measuring the decrease in the r33 value at
80 uC or by observing the decrease in the intensity (dI2v) of the
second harmonics as the films were stepwise heated from room
temperature to 160 uC. The samples were kept at each
temperature for 1 h.
1H NMR and FT-IR spectra of intermediates and polymers
were recorded on a Varian AM 300 spectrometer and on a
Bomem Michelson instrument, respectively . UV-Vis spectra
were recorded on a Hewlett Packard HP 8452A Diode Array
spectrophotometer. Elemental analysis was performed by the
Center for Organic Reactions, Sogang University, Seoul,
Korea, using an Eager 200 elemental analyzer. Molecular
weights of the polymers were determined by a Waters 745 GPC
instrument with a refractive index detector against a poly-
styrene reference. Tetrahydrofuran was employed as an eluent.
Thermal properties of the polymers were examined under a
nitrogen atmosphere on a Mettler 821e and TGA 50 at a
heating and cooling rate of 10 uC min21
.
Results and discussion
Measurement of 2nd order optical nonlinearity
Synthesis and general properties of SP and EP polymers
The 2nd order nonlinear optical properties of polymer films
(0.60–0.75 mm thick) were measured by the reflection
method30,31 and the Maker fringe method.32,33 The polymer
solutions (5 wt%) in 1,1,2,2-tetrachloroethane were filtered
through 0.2 mm Teflon filters and then the films were prepared
by spin-coating onto ITO coated glasses and onto glasses for
the reflection method and the Maker fringe method. The first
method provides us with electro-optic coefficient (r33)34 and the
second method, the second harmonic coefficient34 (d33; the
values were calculated by using Herman and Hayden’s theory35
where the absorption effect was considered at the second
harmonic generated frequency of 532 nm). Experimental
details of the methods can be found in our previously published
papers.12,13,36,37 The first and the second methods require
contact poling and corona poling, respectively. For contact
poling, an electric field of 1.0–1.2 MV cm21 was applied to the
polymer films between ITO and gold electrodes for 15 min at
the glass transition temperature (Tg) of each sample. For
corona poling, an electric field of 3.5 kV was applied to the
polymer film for 10 min and the distance from a tungsten tip to
the sample was 1.5 cm. Poling was conducted at the Tg
temperature of each sample. In both cases, the electric field was
kept on until the samples were cooled to room temperature
after poling. Three lasers (He-Ne, diode, and Nd : YAG) were
In order to prepare the four polyimides, SP-2, SP-6, EP-2 and
EP-6, bearing chromophores as pendants, we had to synthesize
separately PI (Scheme 1) and the chromophores, [3(2), 3(6),
4(2), and 4(6)], and then coupling between PI and chromo-
phores followed.
As far as the synthesis of the second chromophoric
compounds, 4(2) and 4(6), is concerned, the synthetic route
(Scheme 2) had to be altered from the route27,39 to compounds
3(2) and 3(6) as discussed above in Experimental. 4-(v-
Hydroxyalkoxy)iodobenzenes (5) were reacted with tert-
butyldiphenylsilyl chloride to protect the terminal hydroxy
group in 5. The Ullmann coupling40 between 6 and the azo
compound, Disperse orange 1, produced compound 7, whose
silyl protecting group was then removed by fluoride41 to result
in the final chromophore 4.
The SP and EP polymers could be readily prepared from
PI and chromophoric compounds. Condensation between
phenolic hydroxy compounds and aliphatic alcohols can be
readily performed by using the condensing agent pair,
triphenylphosphine and diisopropyl azodicarboxylate. Such
condensations are called the Mitsunobu reaction.29 PI contains
two phenolic hydroxy groups per repeating unit and the
chromophoric azo compounds contains one aliphatic hydroxy
Scheme 2
548
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Table 1 General properties of polymers
rate of 10 uC min21. The Tg value of PI is the highest, 375 uC,
among the polymers. The polymers bearing the chromophore
pendants exhibit much lower Tg values due to the increase in
their free volume by the presence of the bulky pendants. The
longer, hexamethylene spacer in SP-6 and EP-6 reduces the Tg
value to a greater extent when compared with the shorter,
dimethylene spacer in SP-2 and EP-2. One interesting point to
note is that the two pairs of SP-2 and EP-2, and SP-6 and EP-6
exhibit the same Tg values, 193 and 152 uC, respectively,
regardless of the difference in the mode (side-on vs. end-on) of
binding of the chromophore pendants. The presence of the
same polymethylene spacers between the backbone and the
chromophore pendants must be the major reason for this
observation. Thermal stability of the present polymers is rather
poor because the azobenzene moieties undergo thermal
decomposition to produce nitrogen and other products. The
initial decomposition temperature (265–270 uC) of the SP
polymers appear to be significantly lower than that (305 uC) of
the EP polymers: probably side-on attachment of the
chromophoric unit to the spacer makes the surroundings
around the thermally labile azo group more congested leading
to easier decomposition. However, the initial decomposition
temperatures (Table 1, 265–305 uC) are much higher than the
Tg values. This ensures safe poling of the present polymers at or
near their Tg temperatures. At the same time, high Tg values of
the polymers are expected to bring about improved temporal
stability of molecular orientation of poled polymer films, which
is discussed in the next Section.
Mna 6 103
Mw/Mn
(DP)n
Tg/uCb
Td/uCb
a
a
Polymer
PI
20.8
37.7
43.7
44.4
52.5
2.2
2.1
2.9
2.8
3.0
33
25
27
30
32
375
193
152
193
152
—
265
270
305
305
SP-2
SP-6
EP-2
EP-6
aDetermined against a polystyrene standard by GPC using tetra-
hydrofuran as an eluent. bTg and Td stand for the glass transition
and initial decomposition temperature, respectively: estimated from
the DSC or TGA thermograms, respectively, at the heating rate of
10 uC min21. The temperature where the initial slope change occurred
was taken to be the transition or decomposition temperature.
group. In order to ensure complete reaction, the chromophoric
aliphatic alcohols were used in an excess of 50 mole%.
Structures of the intermediates and the final polymers were
1
confirmed by H-NMR and FT-IR spectroscopy and also by
elemental analysis as detailed in the Experimental section.
All the SP and EP polymers were soluble in common organic
solvents such as THF, chloroform, 1,1,2,2-tetrachloroethane,
and N,N-dimethylformamide. Average molecular weights of
PI, SP and EP polymers were determined by GPC against a
polystyrene reference. THF was employed as an eluent. The
number average molar mass (Mn) and the weight average molar
mass (Mw) of PI were 20800 and 45900, respectively. This
corresponds to the average degree of polymerization of 33 and
polydispersity index of 2.2. Molecular weights of the final SP
and EP polymers determined by GPC were Mn ~ 37700–52500
and Mw ~ 80000–162600. As shown in Table 1, these values
correspond to an average degree of polymerization of 25–32
and polydispersity indices of 2.1–3.0. Discrepancies between
molecular weights and polydispersity indices of PI and those of
SP or EP polymers can be ascribed to the fact that all the data
for molecular weights were obtained by GPC against
polystyrene. Since the chromophores in the SP and EP
polymers are quite bulky and polar, it is highly conceivable
that chain conformations, and thus molecular hydrodynamic
volumes, of the polymers in THF solutions are not only much
different from that of PI but also differ greatly between each of
the SP and EP polymers.
Fig. 3 compares the UV-Vis absorption spectra of SP-6 and
EP-6. Since spectra of SP-6 and EP-6 are exactly the same as
those of shorter spacers, only the spectra of these two
representative polymers are given. The two spectra given in
Fig. 3 are practically the same. They reveal two strong absorp-
tions, one at lmax ~ 300 nm and the other at lmax ~ 500 nm.
The first absorptions arise from the p–p* transitions of
substituted benzenes and the second from p–p* transitions of
the delocalized p-systems all over the chromophore structures.
The 2nd order nonlinear optical properties
Fig. 4 show the dependence of the electro-optic coefficient
(r33)1a values on the applied poling field for the four polymers.
The data for r33 values were obtained for an incidence
wavelength of 633 nm (He-Ne laser, p-polarized wave), which
is absorbed by the polymers to some extent (7–18%, see Table 2
and Fig. 3). Therefore, the data shown in Fig. 4 are all
All the SP and EP polymers are amorphous as shown by the
X-ray diffractograms in Fig. 2. The diffractograms show only a
very broad diffraction in the wide-angle region centered around
˚
2h ~ 20u (4.4 A), which reflects interchain spacing. The starting
polymer, PI, is also amorphous. The glass transition tempera-
tures given in Table 1 were obtained using DSC at a heating
resonance-enhanced values. At a poling field of 1.0–
1.2 MV cm21, they reveal r33 values higher than 100 pm V21
with the value for EP-2 polymer being particularly greater than
that for the SP-2 polymer at a poling field 1.2 MV cm21. When
the wavelength of the incident light was changed to 1300 nm
(diode laser) from 633 nm, significantly lower r33 values were
obtained. Moreover, the polymer (SP-2) bearing the side-on
attached chromophore through the shorter dimethylene spacer
exhibited a significantly higher r33 value of 23 pm V21 at a
Fig. 2 Wide angle X-ray diffractograms of polymers at room
temperature.
Fig. 3 UV-Visible absorption spectra of SP-6 and EP-6.
J. Mater. Chem., 2002, 12, 546–552
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Fig. 4 The electro-optic coefficient r33 values of the contact poled
polymers obtained for an incident wavelength of 633 nm. The values
are relative to that for the Y-cut quartz plate.
Fig. 5 Maker fringe pattern of the corona poled SP-2 film and quartz.
poling field of 1.2 MV cm21, when compared with 16 pm V21
for EP-2, that is, the corresponding polymer bearing the end-
on attached chromophore. In other words, mode of attachment
of the chromophore to the spacer influences NLO properties of
the polymers. Since all the polymers do not absorb any light at
1300 nm, the higher r33 value for SP-2 indicates that side-on
attachment of the NLO chromophore brings about more
efficient orientation than end-on attachment. This effect is not
observed when the spacer becomes much longer, from
dimethylene to hexamethylene. In other words, the maximum
r33 values obtained for SP-6 and EP-6 are about the same. This
must be due to a much higher degree of conformational
freedom possible for the longer polymethylene spacers regard-
less of the mode of attachment of the chromophores.
The second harmonic coefficients, d33,1a of the SP-2 and EP-
2 polymers were obtained by the Maker fringe method32,33
using a Nd : YAG laser beam (1064 nm) with a pulse width of
7 nm. The incident angle was varied from 270u to 170u. Fifty
data points were obtained stepwise for every 0.6u. All the data
points were calibrated against the second harmonic intensity of
Y-cut quartz (1.025 mm thick).
NLO chromophore pendants per repeating group. One of their
polymers exhibited a d33 value of 138 pm V21 at a wavelength
of 1064 nm, which remained unchanged for 360 h even at
160 uC. Shen et al.18 recently prepared a series of polyimides
bearing different types of azobenzene or azothiophene type
NLO chromophoric pendants. Although the d33 values of the
polymers are not very high, they appear to be fairly thermally
stable. It19 is also noted that NLO polyimides with dipole
moments aligned transverse to the imide linkage show very
good temporal stability even at elevated temperatures.
We studied temporal stability of poled polymers by
examining the decrease of the r33 values at 80 uC as a function
of time (Fig. 6) and also by in situ observation of decrease in
SHG intensity with increasing temperature (Fig. 7). In the
latter experiment the polymer films were kept for one hour at
each temperature during which time the SHG intensity was
continuously monitored. From the former experiments the
relaxation rate of the poled NLO chromophores was found to
increase in the order of SP-2 v EP-2 v SP-6 v EP-6. After
24 hours at 80 uC, they retained 89, 80, 75 and 67% of their
original r33 values, respectively (see Fig. 6), and the dependence
of the relaxation on temperature (Fig. 7) clearly demonstrates
a lower thermal stability of chromophore orientation in
EP-2 than in SP-2. In other words, longer spacers and end-
on attachment of NLO chromophores are less efficient in
the stabilization of oriented chromophores. Since the azo
Fig. 5 presents a representative example of a Maker fringe
pattern obtained for a corona poled SP-2 film. The d33 values
(Table 2) of SP-2 and EP-2 were 110 and 86 pm V21
,
respectively, for the polymer films poled for 10 min at a
corona field of 3.5 kV. Here again, the value for SP-2 is greater
than that for EP-2, implying that poling is more efficient for the
side-on attached chromophore.
Although we did not try to optimize the poling conditions for
the present polymer films in order to achieve as high r33 or d33
values as possible, the 2nd order NLO coefficients obtained are
comparable to those reported for polyimides bearing the same
or similar chromophore pendants. For example, a copolymer
of disperse red 1 (DR 1)-substituted methacrylate and methyl
methacrylate, when poled by a corona field, gave d33
43 pm V21 at 1064 nm and r33 ~ 18 pm V21 at 633 nm.42
~
Recently, Davey et al.16 reported the synthesis of NLO
chromophore-embedded polyimides that reveal very high
thermal stability in 2nd-order nonlinear optical properties.
These polymers, after corona poling, exhibited x(2) responses as
high as 82 pm V21 and negligible decay at 100 uC over 1000 h.
Ueda et al.17 have reported various polyimides bearing two
Fig. 6 Thermal stability of the contact poled polymers at 80 uC by the
reflection method.
Table 2 Optical properties of polymers
Transmittance
(%) at 633 nm
r33 (pm V21
at 633 nm
)
r33 (pm V21
)
at 1300 nm
Polymer
d33 (pm V21
c
)
n (532 nm)
n (1064 nm)
SP-2
SP-6
EP-2
EP-6
88
93
82
84
115a
102a
194a
107b
23a
8.6a
16a
9.5b
110
77
86
59
c
1.78
1.55
1.74
1.55
1.71
1.55
1.63
1.55
aContact poling at 1.2 MV cm21 for 15 min. bContact poling at 1.0 MV cm21 for 15 min. Corona poling at 3.5 kV cm21 for 10 min.
550
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performed in the CRM supported by the Korea Science and
Engineering Foundation, which is gratefully acknowledged.
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Conclusion
We have successfully prepared soluble aromatic polyimides
bearing azo NLO chromophores bound to polymethylene
spacers either by a side-on or an end-on attachment fashion.
The so-called Mitsunobu reaction29 between the hydroxy-
substituted polyimides and aliphatic alcohols bearing chromo-
phores were found to be very useful in preparing the title
polymers. All the polyimides were amorphous and exhibited
high glass transition temperatures higher than 150 uC. The Tg
values did not depend on the mode of binding of the
chromophores. The average degree of polymerization was
not very high, DP ~ 25–32, but the polymers were able to form
high quality films by spin-coating the solutions in 1,1,2,2-
tetrachloroethane. Since the chromophores carry an electron-
donating group at one end and an electron-attracting group at
the other, their 2nd order nonlinear optical properties were
studied by measuring electro-optic coefficients (r33) and second
harmonic coefficients (d33) after their films were poled at their
Tg. This study demonstrates that side-on attachment of NLO
chromophores improves 2nd order optical nonlinearity and
temporal stability of their orientation when compared to the
case where they were bound in an end-on fashion. This can be
explained by easier rotational orientation of the chromophores
in the former case. Side-on attachment of chromophores also
appears to stabilize their orientation by stronger interactions
with the polymer backbone due to parallel alignment.
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