Synthesis and properties of highly birefringent azo-tolane liquid-crystalline polymers: Effect of the position of the tolane moiety in the side chain

Article abstract of DOI:10.1080/154214091009932

We have synthesized two types of azobenzene liquid-crystalline polymers with a tolane moiety exhibiting high birefringence: one is the copolymer having an azobenzene moiety and a 3-ring tolane group in the side chain (PA-3T) and the other is the polymer c

Full text of DOI:10.1080/154214091009932

This article was downloaded by: [University of Bath]
On: 05 October 2014, At: 21:09
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Synthesis and Properties of
Highly Birefringent Azo-Tolane
Liquid-Crystalline Polymers:
Effect of the Position of the
Tolane Moiety in the Side Chain
Kunihiko Okano a , Atsushi Shishido a , Tomiki Ikeda a
& Takeshi Shiono b
a Chemical Resources Laboratory , Tokyo Institute of
Technology , Yokohama, Japan
b Department of Applied Chemistry , Graduate
School of Engineering, Hiroshima University ,
Hiroshima, Japan
Published online: 17 Oct 2011.
To cite this article: Kunihiko Okano , Atsushi Shishido , Tomiki Ikeda & Takeshi Shiono
(2005) Synthesis and Properties of Highly Birefringent Azo-Tolane Liquid-Crystalline
Polymers: Effect of the Position of the Tolane Moiety in the Side Chain, Molecular
Crystals and Liquid Crystals, 441:1, 275-285, DOI: 10.1080/154214091009932
To link to this article: http://dx.doi.org/10.1080/154214091009932
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and

are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 441, pp. 275–285, 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/154214091009932
Synthesis and Properties of Highly Birefringent
Azo-Tolane Liquid-Crystalline Polymers: Effect of the
Position of the Tolane Moiety in the Side Chain
Kunihiko Okano
Atsushi Shishido
Tomiki Ikeda
Chemical Resources Laboratory, Tokyo Institute of Technology,
Yokohama, Japan
Takeshi Shiono
Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, Hiroshima, Japan
We have synthesized two types of azobenzene liquid-crystalline polymers with a
tolane moiety exhibiting high birefringence: one is the copolymer having an
azobenzene moiety and a 3-ring tolane group in the side chain (PA-3T) and the
other is the polymer containing a long mesogen core, in which the azobenzene
group is directly connected to the tolane moiety (P3AT). The values of 0.35 and
0.39 in birefringence were obtained in PA-3T and P3AT, respectively. When the
film was irradiated with a writing beam at 488 nm, the change in birefringence
was efficiently induced with P3AT compared with PA-3T.
Keywords: azobenzene; change in birefringence; liquid-crystalline polymer; tolane
INTRODUCTION
Azo-dye-containing organic and polymeric compounds are promising
materials for optical applications. They have been extensively stud-
ied because of their applicability for nonlinear optics [1], optical
memory [2,3], photoswitching devices [4], and holography [5–7].
Among diversified azobenzene-containing polymers, liquid-crystalline
polymers (LCPs) with the azobenzene in the side chain have
Address correspondence to Tomiki Ikeda, Chemical Resources Laboratory, Tokyo
Institute of Technology, R1-11, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp
275

276
K. Okano et al.
attracted much attention because a photoinduced change in birefrin-
gence Dn is large when compared with amorphous counterparts
[8–11]. The azobenzene moiety in a trans form, which is rodlike, sta-
bilizes the phase structure of liquid crystals (LCs). On the other
hand, bent cis isomers disorganize the phase structure of the LCs.
Consequently, trans-cis photoisomerization of the azobenzene chro-
mophores leads to a change in large Dn by disorganizing the align-
ment of LCs. It has also been known that polarized light can
induce reorientation of azobenzene groups through photochemically
induced trans-cis-trans isomerization cycles. Previously, we have
revealed that a large change in Dn was induced by photoisomeriza-
tion [12–14] of the azobenzene and the resultant alignment change
[15,16].
Tolane moieties are known as a highly birefringent mesogen
because of large anisotropy of polarizability [18]. Therefore, to obtain
a larger change in Dn in LCPs, we previously prepared a copolymer
having a 2-ring tolane and an azobenzene in the side chain, and found
that the polymer exhibits higher birefringence than that of conven-
tional LCPs [17]. In this study, we have synthesized two types of
azobenzene LCPs with a tolane moiety to induce larger Dn: one is
the copolymer having an azobenzene moiety and a 3-ring tolane group
in the side chain (PA-3T), and the other is the polymer containing a
long mesogen core in which the azobenzene group is directly connec-
ted to the tolane moiety (P3AT). In addition, we investigated the
photoinduced birefringence of the films upon exposure to linearly
polarized light.
EXPERIMENTAL
Characterization
NMR spectra were recorded in CDCl3 with Lambda-300. Molecular
weight of the polymer was determined by gel permeation chromato-
graphy (GPC; JASCO DG-980-50; column, Shodex GPC K802 þ
K803 þ K804 þ K805; eluent, chloroform) calibrated with standard
polystyrenes. LC behavior and phase transition behavior were exam-
ined on an Olympus Model BH-2 polarizing microscope equipped with
Mettler hot-stage models FP-90 and FP-82. Thermotropic properties
of LCPs were determined with a differential scanning calorimeter
(Seiko I&E SSC-5200 and DSC220C) at a heating rate of 10ꢀC=min.
At least three scans were performed for each sample to check repro-
ducibility. Absorption spectra were recorded with a UV-vis absorption
spectrometer (JASCO, V-550).

Highly Birefringent Azobenzene Liquid-Crystalline Polymers
277
Materials
The monomer, 6-[4-(4-ethoxyphenylazo)phenoxy]hexyl methacrylate
(M6AB2), was prepared and purified as previously reported [19,20].
The synthesis of monomer, 6-[4-[3-methyl-4-(4-cyanophenylethynyl)-
phenylethynyl]phenoxy]hexyl methacrylate (3T), is outlined in
Scheme 1. Six compounds from 1 to 6 shown in Scheme 1 were pre-
pared using a procedure similar to the literature [18]. The synthesis
and thermal property of P3AT was previously reported [21].
1-[(6-hydroxyhexyloxyphenyl)ethynyl]-3-methyl-
4-[(4-cyanophenyl)ethynyl]benzene(7)
1-Bromo-3-methyl-4-[(4-cyanophenyl)ethynyl]benzene (3) (2.6 g, 4.5
mmol), 1-(6-hydroxyhexyloxy)-4-ethynylbenzene (6M) (1.8 g, 4.1 mmol),
SCHEME 1 Synthetic route of the polymer used in this study.

278
K. Okano et al.
PdCl2(PPh3)2 (0.28 g, 0.4 mmol), CuI (0.28 g, 0.5 mmol), PPh3 (0.56 g,
3.0 mmol) were dissolved in triethylamine (10 ml) and THF (10 ml).
The mixture was stirred under nitrogen at 60ꢀC for 8 h, and then
cooled to room temperature, followed by extraction with ethyl
acetate. After the organic layer was dried with anhydrous
magnesium sulfate, the solvent was removed by evaporation. The
product was purified by column chromatography on silica gel (eluent:
ethyl acetate:hexane ¼ 1:1) to yield 2.2 g (5.3 mmol, 59%) of yellow
powder.
6-[4-[3-methyl-4-(4-cyanophenylethynyl) phenylethynyl]
phenoxy]hexyl methacrylate 3T
A solution of methacryloyl chloride (0.5 g, 4.8 mmol) in THF (10 ml)
was added dropwise at 0ꢀC to a mixture of 7 (2.0 g, 4.8 mmol), triethy-
lamine (0.5 g, 4.8 mmol), and a trace amount of hydroquinone, and the
reaction mixture was stirred at room temperature for 24 h. The sol-
ution was poured into saturated aqueous sodium hydrogen carbonate,
and the product was extracted with ethyl acetate. After the organic
layer was dried with anhydrous magnesium sulfate, the solvent was
removed by evaporation. The crude solid was purified by column
chromatography on silica gel (eluent: chloroform) and finally recrys-
tallized from methanol to yield 1.5 g (3.0 mmol, 63%) of white powder.
1H-NMR (CDCl3, d, ppm): 1.46–1.54 (m, 8H), 1.58–1.82 (m, 4H), 1.93
(s, 3H), 2.48 (s, 3H), 3.96 (t, J ¼ 6.3 Hz, 2H), 4.14 (t, J ¼ 6.6 Hz, 2H),
5.53 (s, 3H), 6.80 (s, 3H), 6.85 (d, J ¼ 9.0 Hz, 2H), 7.29–7.46 (m, 5H),
7.61 (q, J ¼ 8.7 Hz, 4H). 13C-NMR (CDCl3, d, ppm): 18.28, 20.52,
25.71, 25.77, 28.53, 29.05, 64.60, 67.84, 87.78, 91.60, 92.59, 92.99,
111.46, 114.55, 114.82, 118.51, 121.46, 124.39, 125.23, 128.25,
128.72, 131.93, 131.98, 132.07, 132.40, 133.09, 136.47, 140.44,
159.34, 167.52. Anal. Calcd for C34H31NO3: C, 81.40; H, 6.23; N,
2.79. Found: C, 81.23; H, 6.35; N, 2.91.
Copolymerization
The procedure for copolymerization of P3T-A is shown in Scheme 1.
3T (0.9 g, 2.5 mmol), M6AB2 (0.1 g, 0.28 mmol) and AIBN (0.05 g,
2.8 mmol) were dissolved in dry DMF (5 ml) in a sealed tube and kept
at 60ꢀC for 48 h. The resulting solution was cooled to room tempera-
ture and poured into 400 ml of methanol with vigorously stirring to
precipitate the polymer. The polymer obtained was purified by
repeated reprecipitation from DMF into a large excess of methanol
and dried under vacuum to yield 0.80 g of P3T-A in 80% conversion.

Highly Birefringent Azobenzene Liquid-Crystalline Polymers
279
Preparation of Films
P3AT was dissolved in chloroform, and then a small portion of the
resultant solution was cast on a polyimide-coated glass substrate,
which had been rubbed to align mesogens. Homogeneously aligned
films were obtained after annealing at 200ꢀC for 30 min. Thickness
of the sample films was measured as 400 nm with a surface profiler
(Veeco Instruments Inc., Dektak 3ST). This film was used for the
investigation of the photoinduced alignment change.
Optical Setup and Experimental Conditions
Photoinduced alignment change of LCPs was investigated by the
following procedure. The polymer film was irradiated with linearly
polarized light at 488 nm from an Arþ laser in a glassy state. The
direction of the polarization of light was parallel to the aligned direc-
tion of mesogens, i.e., s-polarization. Intensity of the probe light at
633 nm from a He-Ne laser (NEC, GLC5370, 1 mW) transmitted
through a pair of crossed polarizers, with the sample film between
them, was measured with a photodiode.
RESULTS AND DISCUSSION
Characterization of Polymers
Figure 1 shows the 1H NMR spectrum for the copolymer P3T-A. From
the peak assignments, the composition of the copolymer can easily be
calculated.
The phase transition behavior of P3T-A was investigated by means
of DSC and polarizing optical microscopy. From the third heating
curves of DSC, glass transition and an endothermic peak were
observed at 48ꢀC and 210ꢀC, respectively. Since the LC phase showed
typical Shlieren texture (Fig. 2), we concluded that P3T-A exhibited a
nematic phase between 48ꢀC and 250ꢀC, which is a much wider tem-
perature range than that of conventional LCPs without 3-ring tolane
moiety or an azo-tolane moiety. The structures and thermodynamic
properties of the LCPs used in this study are shown in Figure 3.
Photoisomerization of LCPs
We attempted the trans-cis photoisomerization of the azobenzene
moiety in the side chain of P3AT and P3T-A upon irradiation at
366 nm, which roughly corresponds to the absorption band of normal
trans azobenzene moieties (Fig. 4). As shown in Figure 4(a), P3AT

280
K. Okano et al.
FIGURE 1 1H NMR spectrum for the copolymer P3T-A.
exhibited strong absorption around 400 nm, which may be attributed
to the p-pꢁ transition band of the trans-azobenzene moiety (kmax at
379 nm in DMF). This absorption rapidly decreased upon irradiation
at 366 nm. The isosbestic points were observed at 337 nm and
463 nm. These spectral changes clearly indicate that the azobenzene
moieties underwent the trans-cis isomerization. Furthermore, we
evaluated the ratio of cis-azobenzene moieties at the photostationary
state by the following equation:
S ¼ ðA0 ꢂ AtÞ=A0 ꢃ 100
where A0 and At are absorbance at kmax before and after irradiation,
respectively. It was found that about 60% of cis-azobenzene moieties
were produced at the photostationary state.
In P3T-A, on the other hand, no change in the spectra was recog-
nized under irradiation (Fig. 4(b)). This is probably because the

Highly Birefringent Azobenzene Liquid-Crystalline Polymers
281
FIGURE 2 Polarizing optical micrograph of the texture of P3T-A at 200ꢀC.
FIGURE 3 Chemical structure and properties of P3AT and P3T-A used
in this study. Mn, number-average molecular weight; Mw, weight-average
molecular weight; G, glass; N, nematic; I, isotropic phase.

282
K. Okano et al.
FIGURE 4 Change in the absorption spectrum of P3AT (a) and P3T-A (b) in
DMF at 25ꢀC. Times of UV (366 nm) irradiation are indicated in the figure.
absorption of 3-ring tolane moieties (strong absorption around 340 nm)
is completely overlapped with the p-pꢁ transition band of the trans-
azobenzene moiety.
Change in Alignment of the LCP
As shown in Figure 5, we have estimated the order parameter (S) of
the monodomain film by the following equation:
S ¼ ðA ꢂ A Þ=ðA þ 2A Þ
?
?
k
k
FIGURE 5 Polarized absorption spectra of monodomain films of P3AT (a)
and P3T-A (b).

Highly Birefringent Azobenzene Liquid-Crystalline Polymers
283
FIGURE 6 Change in transmittance as a function of irradiation time of
P3AT (a) and P3T-A (b). Photoirradiation at 488 nm (250 mW=cm2) was
performed at 80ꢀC.
where A and A are absorbance parallel and perpendicular to the
k
?
polarization direction of the irradiation light. The value of S in
P3AT and P3T-A was 0.41 and 0.50, respectively. These monodomain
films transmitted the probe light through a pair of crossed polarizers,
with the sample film between them, because of the birefringence of the
LCPs. From the transmittance (T) of the probe light, we have esti-
mated the values of birefringence (Dn) by using the following equation:
[22,23]
T ¼ sin2ðpdDn=kÞ
where d is the film thickness and k is the wavelength of the probe light
(633 nm). In the initial state, the calculated values of the LCP films
were 0.39 (P3AT), and 0.35 (P3T-A), respectively. Next, from the
change in transmittance, we investigated the photoinduced alignment
change in P3AT and P3T-A (Fig. 6) The transmittance of the probe
light decayed upon irradiation of s-polarized light in P3AT. This result

284
K. Okano et al.
indicates that photoinduced alignment change causes the destruction
of the initial order of homogeneously aligned azo-tolane moieties, and
induces a large change in Dn (ꢄ0.35). On the other hand, in P3T-A,
the reduction of the transmittance was very low. As mentioned above,
this result could be assigned to the overlapping of the absorption of
tolane moieties with that of the p-pꢁ transition bands of the trans-
azobenzene moieties.
CONCLUSION
In this study, we have synthesized two types LCPs containing a tolane
moiety and explored the effect of the position of the tolane moiety in
the side chain. Both polymers showed a nematic phase in a broad tem-
perature range. The homopolymer containing an azo-tolane moiety
showed trans-cis photoisomerization reversibly. On the other hand,
in the copolymer having an azobenzene moiety and a 3-ring tolane
moiety, photoisomerization didn’t induced photoisomerization because
the absorption of 3-ring tolane moieties was overlapped with the p-pꢁ
transition bands of the trans-azobenzene moieties. Despite both LCPs
exhibit the large value of Dn (>0.35), the change in Dn of the homopo-
lymer is much efficiently induced (ꢄ0.35) compared with that of the
copolymer.
REFERENCES
[1] Okada, S., Matsuda, H., Masaki, A., Nakanishi, H., Abe, T., & Ito, H. (1992). Jpn. J.
Appl. Phys., 31, 365.
[2] Natansohn, A., Rochon, P., Barrett, C., & Hay, A. (1995). Chem. Mater., 7, 1612.
[3] Ho, M., Natansohn, A., & Rochon, P. (1996). Macromolecules, 29, 44.
[4] Ikeda, T., Sasaki, T., & Ichimura, K. (1993). Nature, 361, 428.
[5] Todorov, T., Nikolva, L., & Tomova, N. (1984). Appl. Opt., 23, 4588.
[6] Eich, M. & Wendorff, J. H. (1987). Makromol. Chem. Rapid Commun., 8, 59.
[7] Eich, M. & Wendorff, J. H. (1987). Makromol. Chem. Rapid Commun., 8, 467.
¨
[8] Inanov, S., Yakovlev, I., Kostromin, S., Shibaev, V., Lasker, L., Stumpe, J., &
Kreysig, D. (1991). Makromol. Chem. Rapid Commun., 12, 709.
[9] Stumpe, J., Fischer, Th., & Menzel, H. (1996). Macromolecules, 29, 2831.
[10] Geue, Th., Ziegler, A., & Stumpe, J. (1997). Macromolecules, 30, 5729.
[11] Zebger, I., Rutloh, M., Hoffmann, U., Stumpe, J., Siesler, H.W., & Hivilsted, S.
(2002). J. Phys. Chem. A, 106, 3454.
[12] Ikeda, T. & Tsutsumi, O. (1995). Science, 268, 1873.
[13] Shishido, A., Tsutsumi, O., Kanazawa, A., Shiono, T., Ikeda, T., & Tamai, N. (1997).
J. Am. Chem. Soc., 119, 7791.
[14] Ikeda, T. (2003). J. Mater. Chem., 13, 2037.
[15] Wu, Y., Demachi, Y., Tsutsumi, O., Kanazawa, A., Shiono, T., & Ikeda, T. (1998).
Macromolecules, 31, 1104.
[16] Wu, Y., Demachi, Y., Tsutsumi, O., Kanazawa, A., Shiono, T., & Ikeda, T. (1999).
Macromolecules, 3(2), 3951.

Highly Birefringent Azobenzene Liquid-Crystalline Polymers
285
[17] Yoneyama, S., Yamamoto, T., Tsutsumi, O., Kanazawa, A., Shiono, T., & Ikeda, T.
(2002). Macromolecules, 35, 8751.
[18] Sekine, C., Iwakura, K., Konya, N., Minami, M., & Fujisawa, K. (2001). Liq. Cryst.,
28, 1375.
[19] Ringdorf, H. & Schmidt, H. W. (1984). Makromol. Chem., 185, 1327.
[20] Angeloni, A. S., Caretti, D., Carlini, C., Chiellini, G., Galli, G., & Altomare, A.
(1989). Liq. Cryst., 4, 513.
[21] Okano, K., Shishido, A., Tsutsumi, O., Shiono, T., & Ikeda, T. (2003). Proc. SPIE.,
139, 5213.
[22] Zilker, S. J., Beiringer, T., Haarner, D., Stein, R. S., Egmond, J. W., & Kostromine, S. G.
(1998). Adv. Mater., 10, 855.
[23] Stracke, A., Wendorff, J. H., Mahler, J., & Rafler, G. (2000). Macromolecules, 33,
2605.