All-optical switching effect in novel chiral biazobenzene polymer films

Article abstract of DOI:10.1021/ma0354445

Chiral biazobenzene polymers were synthesized. Large birefringence and all-optical switching effect were realized in the polymer films at low optical driving power.

Full text of DOI:10.1021/ma0354445

9292
Macromolecules 2003, 36, 9292-9294
Exp er im en ta l Section . a . Syn th esis. p-Amino-
azobenzene, methacryloyl chloride, and N,N-dimethyl-
formamide (DMF) were received from Aldrich Chem-
icals Co. D-(+)-R-Methylbenzylamine was received
from Acros Organics. Tetrahydrofuran (THF) was dried
over sodium/benzophenone and distilled just before
use.
The biazobenzene chromophore was synthesized by
diazotization reaction and coupling with D-(+)-R-meth-
ylbenzylamine; biazobenzene monomer was obtained
through reaction of biazobenzene chromophore with
methacryloyl chloride (Scheme 1).
All-Op tica l Sw itch in g Effect in Novel Ch ir a l
Bia zoben zen e P olym er F ilm s
Sh u izh u Wu ,*,† F a n g Zen g,† Sh en gla n Ya o,†
Zh en Ton g,† Weilon g Sh e,‡ a n d Du a n bin Lu o‡
Department of Polymer Science & Engineering, South China
University of Technology, Guangzhou 510640, China,
and State Key Laboratory of Optoelectronic Materials
and Technologies, Zhongshan University,
Guangzhou 510275, China
Received September 25, 2003
Revised Manuscript Received October 27, 2003
Sodium nitrite (3.1 g) dissolved in water (50 mL) was
added into a solution of p-aminoazobenzene (7.9 g)
dissolved in a mixture of water (50 mL) and 36%
aqueous hydrogen chloride (7.2 mL). Then the solution
was kept at 0-5 °C for 2 h. D-(+)-R-Methylbenzylamine
(4.85 g) dissolved in a mixture of water (125 mL) and
36% aqueous hydrogen chloride (7.5 mL) was added to
the above solution over 30 min, and then the solution
was stirred for 1 h at 0-5 °C and 4 h at 20 °C. The
precipitate was obtained by filtration, washed with a
large amount of water, dried, and recrystallized from
ethanol to yield the biazobenzene chromophore. Biaz-
obenzene chromophore (2.3 g) and methacryloyl chloride
(1.1 g) were dissolved in 100 mL of dry THF. Triethy-
lamine (1.4 g) was added dropwise to the solution. After
stirring for 15 h at room temperature, the solution was
filtered. Then THF in the filtrate was removed, and the
reaction product was washed with 2 N sodium hydrox-
ide, 0.1 N HCl, and distilled water successively. The
dark red product (biazobenzene monomer) was dried
under vacuum at 60 °C for 72 h. UV-vis in THF
(chromophore): λmax ) 410 nm. 1H NMR (monomer): δ
(ppm), 8.1 (NH), 7.2-7.9 (aromatic H), 5.2-5.9 (Cd
CH2), 2.9-3.0 (CH2), 1.9 (CH3). FT-IR (monomer): 3400
In tr od u ction . Nonlinear optics has been recognized
as a field of research with important potential applica-
tions in optoelectronic devices. For nonlinear optics, the
material must be noncentrosymmetric on the molecular
and macroscopic scales. Noncentrosymmetry on the
molecular scale is easy to achieve, for example, by
connecting an electron donor and acceptor by a one-
dimensional conjugated bridge. On the other hand,
macroscopic noncentrosymmetry is more difficult. Meth-
ods to achieve macroscopic noncentrosymmetry include
electric poling, crystal growth, and self-assembly. An
alternative approach is to use chiral materials, which
are inherently noncentrosymmetric. The use of chirality
in nonlinear optics has been theoretically investigated.1,2
These studies showed that chiral contributions can
increase nonlinear optical response.
Fast all-optical switches are crucial components for
high-bit-rate time-division-multiplexing optical com-
munication systems or free-space optical-digital com-
puting systems. So far, many types of all-optical switches
have been studied; among them, the azobenzene-type
optical switches have received great attention recently.3
The azobenzene molecules are well-known to show
reversible photoisomerization between the trans and cis
forms as well as the birefringent properties upon
illumination by linearly polarized light.4,5 Ideal materi-
als for optical switching should possess a large photo-
induced birefringence and long-term stability. By in-
troducing chirality into azobenzene polymer, it will
improve the system’s macroscopic noncentrosymmetry
and nonlinearity, which in turn will increase the sys-
tem’s optical activity including birefringence. In addi-
tion, traditional all-optical switching usually requires
high driving power (e.g., 1 MW/cm2), thus limiting its
applications.6 To make practical optical switching de-
vices, it is necessary to reduce the optical power require-
ments and achieve large modulation depth. To increase
the magnitude and sensitivity of photoinduced birefrin-
gence, new azo polymers including chiral azo polymers
are constantly being designed and synthesized.7-9 In
this study, we synthesized a novel chiral biazobenzene
polymer and studied its photoinduced birefringence and
all-optical switching effect. The optical switching devices
made herein can be operated with a relatively low
driving power, a rising time and a falling time of about
0.4 ms, and a modulation depth more than 90%.
cm-1 (νNH), 3075 cm-1 (νCdCH ), 2950 and 2870 cm-1 (νCH
2
aliph), 1690 cm-1 (νCdO), 1640 cm-1 (νCdC, aliph), 1600
cm-1 (νCdC, arom), 1500 cm-1 (νC-N-H), 770 cm-1 (νCH
arom).
,
The synthesized biazobenzene monomer (2 g for
sample A and 1 g for sample B) was dissolved in dry
DMF; 0.9 g of methyl methacrylate and 0.1 g of butyl
methacrylate were added to the solution. The polymer-
ization was carried out in the presence of azobis-
(isobutyronitrile) (AIBN, 2 wt %) as an initiator. The
polymerization medium was outgassed twice before
heating and stirring at 70 °C for 240 h under nitrogen.
Then the polymerization mixture was poured into cold
methanol. The isolated ternary copolymer was redis-
solved in THF and precipitated in cold methanol,
filtered, and finally dried under vacuum at 60 °C for 72
h: Mw ) 3.1 × 104, polydispersity ) 1.63 (sample A);
Mw ) 4.2 × 104, polydispersity ) 1.66 (sample B).
Biazobenzene chromophore content obtained through
elemental analysis: 63.0 wt % for sample A and 31.5
1
wt % for sample B. H NMR: δ (ppm), 8.1 (NH), 7.2-
7.9 (aromatic H), 4.2 (OdCCH2), 3.6-3.8 (O-CH3), 1.2-
2.2 (CH2), 0.9 (CH3). FT-IR (KBr): 3200 cm-1 (νNH), 2950
and 2870 cm-1 (νCH aliph), 1730 cm-1 (νCdO), 1600 cm-1
(νCdC, arom), 1500 cm-1 (νC-N-H), 1270 and 1150 cm-1
† South China University of Technology.
‡ Zhongshan University.
* To whom correspondence should be addressed. E-mail:
shzhwu@scut.edu.cn.
(νC-O-C), 770 cm-1 (νCH, arom). Optical rotation: [R]D
25
) +10.5° (CHCl3, C ) 1 g/dL, sample B), [R]D25 ) +16.5°
(CHCl3, C ) 1 g/dL, sample A).
10.1021/ma0354445 CCC: $25.00 © 2003 American Chemical Society
Published on Web 11/12/2003

Macromolecules, Vol. 36, No. 25, 2003
Communications to the Editor 9293
Sch em e 1
F igu r e 1. Experimental setup for measurement of optical
switching effect and birefringence: P ) polarizer; B.S. ) beam
splitter.
b. F ilm P r ep a r a tion a n d Mea su r em en t. The bi-
azobenzene polymer was dissolved in THF and filtered
through a 0.50 µm Teflon filter. Before casting polymer
solution on the glass slide, the glass slide was cleaned
by ultrasonic cleaning and then dried and wiped with
lens tissue. The polymer film was prepared by casting
the filtered solution on glass slide (film thickness: 41
µm for sample A and 45 µm for sample B). Then the
dried film was placed in a vacuum oven at 60 °C for 4
days. FT-IR spectra were measured by using Nicolet
F igu r e 2. Intensity of the transmitted probe beam as a
function of time (modulation frequency ) 1250 Hz, control
beam power ) 12 mW).
1
Magna 760 FT-IR spectrometer. H NMR spectra were
recorded on a Varian Unity Inova 500 MHz NMR
spectrometer in CDCl3. The UV-vis spectrum was
recorded on a Hitachi U-3010 UV-vis spectrophotom-
eter in THF at room temperature. Optical rotation of
the polymer films was measured by using WZZ-2A
polarimeter. Molecular weight and molecular weight
distribution were measured by a Waters gel permeation
chromatograph with 2410 RI detector. Elemental analy-
ses were conducted by using the CHN-O fast speed
analyzer (Heraeus Co., Germany). Film thickness was
measured on an Alpha-Step 500 profiler.
The all-optical switching effects of the samples were
measured by using the experimental setup as shown in
Figure 1. An argon laser beam (514 nm) was used as
the control beam. The sample was placed between two
orthogonal polarizers (P1 and P2) in the path of a He-
Ne laser beam (632.8 nm, probe beam). The angle
between the polarization direction of polarizers P1 and
P3 is 45°.
Resu lts a n d Discu ssion . Figure 2 shows the all-
optical switching effect of the samples at room temper-
ature at the modulation frequency of 1250 Hz. Figure 3
shows the photoinduced birefringence of the samples at
room temperature. It is clear that when the control
beam is turned on, the intensity of the probe beam
detected increases with a rising time of 0.4 ms, while
when the control beam is off, the transmitted intensity
of probe beam decreases with a falling time of about 0.4
ms and the modulation depth is close to 100%. Without
the control beam, the samples are optically isotropic,
and no birefringence can be observed. Under this
F igu r e 3. Photoinduced birefringence of the polymer films
(on/off control beam). Control beam 1: 12 mW; 2: 9 mW.
condition, the He-Ne laser beam does not undergo any
polarization modification when passing through the
sample and so is completely stopped by the polarizer
(P2). When the control beam is switched on, biazoben-
zene moieties begin to orientate by photoisomerization
and the segments of polymer main chain might orient
as well, and then the sample exhibits birefringence.
Then the polarization of the probe beam transmitted
through the sample is modified, and the probe beam can
reach the detector.
From Figure 3, it can be seen that birefringence
increases as the biazobenzene content in the sample
increases, while birefringence decreases as the control

9294 Communications to the Editor
Macromolecules, Vol. 36, No. 25, 2003
beam power increases. When the control beam is turned
off, birefringence quickly decreases; this indicates that
it is reversible photoinduced birefringence, which is
ideal for good all-optical switching effect. There are two
mechanisms in the polymer films which influence the
photoinduced birefringence, which in turn determines
the optical switching effect. One is the photoisomeriza-
tion reorientation of biazobenzene moieties, and the
other is the rotation of biazobenzene moieties due to
thermal agitation. The competition between these two
mechanisms can be used to explain the experimental
results. Thermal agitation comes from the thermal effect
of the control beam illumination owing to the absorption
of the sample. As the control beam power increases, the
temperature of the sample increases; biazobenzene
chromophore moieties which are oriented due to pho-
toisomerization are easier to return to their initial
random state. Therefore, with higher control beam
power, thermal disturbance which enhances the rotation
of biazobenzene moieties and disorders the alignment
of the sample becomes stronger, which in turn weakens
the photoinduced birefringence.
Ack n ow led gm en t. This work is supported by Na-
tional Natural Science Foundation of China (Project No.
50173007 and 59803003), Guangdong Provincial Scien-
tific & Technological Plan of China (Grant No. A1060201
and 980505), Guangzhou Municipal Government (Project
No. 2002J 1-C0111 and 2002J 1-C0051), and the Visiting
Scholar Foundation of Key Lab. in University of China.
Su p p or tin g In for m a tion Ava ila ble: FTIR spectra, NMR
1H spectra, and UV-vis spectrum. This material is available
free of charge via the Internet at http://pubs.acs.org.
Refer en ces a n d Notes
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In summary, chiral biazobenzene polymers have been
synthesized. Large birefringence and all-optical switch-
ing effect have been realized in the polymer films at low
optical driving power. The ability to induce large optical
birefringence in the polymer films is expected to find
applications in high-speed photonic devices, such as spa-
tial light modulators, filters, polarizers, and beam split-
ters for optical communication and image processing.
MA0354445