Light-controllable reflection wavelength of blue phase liquid crystals doped with azobenzene-dimers

Article abstract of DOI:10.1039/c3cc46117c

A new series of azobenzene-dimers were synthesized and doped into the blue phase liquid crystals to broaden the temperature range of BPs. It is found that not only can the reflection wavelength of BPI be reversibly controlled but BPI can also be transform

Full text of DOI:10.1039/c3cc46117c

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Light-controllable reflection wavelength of blue phase
liquid crystals doped with azobenzene-dimers†
Cite this: Chem. Commun., 2013,
49, 10097
a
b
Xingwu Chen,z Ling Wang,z Chenyue Li,a Jiumei Xiao,c Hangjun Ding,a Xin Liu,d
Received 10th August 2013,
Accepted 4th September 2013
Xiaoguang Zhang,d Wanli He*a and Huai Yang*ab
DOI: 10.1039/c3cc46117c
www.rsc.org/chemcomm
A new series of azobenzene-dimers were synthesized and doped into units which are linked by a flexible spacer. Due to their special
the blue phase liquid crystals to broaden the temperature range of structures, they have attracted growing attention because their
BPs. It is found that not only can the reflection wavelength of BPI be mesophase behaviour is clearly different from that of the corre-
reversibly controlled but BPI can also be transformed into the cholesteric sponding monomers.10 Especially, the dimers with an odd spacer
phase owing to isomerization of azobenzene induced by light.
have a bent-shaped structure and show a large flexoelectric effect,
which is considered beneficial for the stability of BPs.11
Liquid crystalline blue phases (BPs), which are potentially useful for
Azobenzene compounds have recently given rise to enormous
applications such as fast light modulators or tunable photonic interest owing to their good chemical stability and reversible isomeriza-
crystals,1 have attracted considerable attention in recent years due tion between the trans and cis isomers.12 The trans-isomer, which is
to their intriguing electro-optical characteristics and unique self- linear and thermodynamically stable, is beneficial for the stability of
assembled cubic structures.2 According to the difference in their LCs; whereas the cis-isomer, which is bent and thermodynamically
microstructures, BPs can be divided into BPI, BPII and BPIII. BPIII is metastable, destabilizes the LC phase. Depending on the trans–cis
an amorphous phase with a local cubic lattice structure, while BPI transformation of azobenzene, extensive studies have been carried
and BPII have a body-centred-cubic structure and a simple cubic out to control the orientation of LCs, such as inducing smectic phase
structure, respectively.3 The periodicity of the microstructure, about transition,13 adjusting the reflection wavelength of the N* phase14 or
several hundred nanometres, is on the scale of the wavelength of Bragg reflection of BPs.15 Among them, the tuneable wavelength of BPs
visible light, so the BPs can produce Bragg reflection around visible is attracting since it can be potentially used in three-dimensional
light, which is important to the liquid crystal (LC) lasers and other photonic crystals and optically addressable BP-LC displays, which was
applications.4 Usually, BPs are found in a very narrow temperature first studied by H. Y. Liu et al.15 However, the adjustment range
range about 1 1C between the chiral nematic (N*) and isotropic (Iso) mentioned in their report is limited to several tens of nanometres
phases, which is one of the major obstacles to their practical and the reflected color of the LC-BP platelet only changed from brown
applications. To overcome the challenge, several methods have been to green, so there is an urgent need to overcome their limitations.
proposed, including stabilizing the BPs with a polymer network5 or
Herein, we firstly report two series of azobenzene-dimers by
nanoparticles6 and using low-weight molecules such as T-shaped combining the structural specificity of a dimer with the optical
molecules,7 dimer molecules8 and bent-core molecules.9 Interest- properties of azobenzene, as shown in Fig. 1. Then, the azobenzene-
ingly, H. J. Coles et al.8 have reported that the dimers can stabilize dimers were separately doped into a BP-LC mixture; it was found
the BPs to a large degree, where the dimers contain two mesogenic that the temperature range of BPs could be broadened to some
degree. Importantly, the reflection wavelength of the composite can
a Department of Materials Physics and Chemistry, University of Science and
Technology Beijing, Beijing 100083, P. R. China.
be successfully controlled under UV or visible light irradiation.
As can be seen from the POM picture, the reflected color of the
LC-BPI platelet1,5,7 can be reversibly changed from red to blue,
and light-induced BPI to N* phase transition was observed in the
BP-LCs doped with a high concentration of azobenzene-dimers,
which is potentially used for light-control switches or optically
addressable BP-LC displays.
Table 1 displays the phase transition behaviours of the
azobenzene-dimers, and the details about their synthesis are
shown in the ESI.† It can be found that the azobenzene-dimers
with trifluoromethyl linked by an alkyl chain show no mesophases,
E-mail: yanghuai@mater.ustb.edu.cn, hewanli@mater.ustb.edu.cn;
Fax: +86-010-62333165
b Department of Materials Science and Engineering, College of Engineering,
Peking University, Beijing 100871, P. R. China. E-mail: yanghuai@coe.pku.edu.cn
c Department of Applied Mechanics e, University of Science and Technology Beijing,
Beijing 100083, China
d Science and Technology on Electro-Optical Information Security Control
Laboratory, Hebei Sanhe 065201, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3cc46117c
‡ These authors contributed equally to this work.
c
This journal is The Royal Society of Chemistry 2013
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azobenzene-dimers with an odd spacer have a bent-shaped
structure and show a large flexoelectric effect, which are two
significant factors to the stability of BPs.9,11
In order to investigate the effects of trans–cis isomerization
of azobenzene-dimers on the Bragg reflection of BP-LCs, first,
the azobenzene-dimer 5a, which has the greatest impact on the
temperature range of BPs, was selected to study the absorption
Fig. 1 Structures of the investigated compounds.
Table 1 The phase transition temperature and corresponding transition enthal-
pies of the azobenzene-dimers
Sample no. Phase transition during the cooling cycle/(1C, DH/kJ molꢀ1
)
2a
3a
4a
2b
3b
4b
5a
5b
Iso 149.5(43.8) Cr
Iso 139.1(49.2) Cr
Iso 136.6(55.5) Cr
Iso 221.6(4.0) N 161.4(68.6) Cr
Iso 205.4(2.9) N 147.7(65.7) Cr
Iso 194.1(3.5) N 146.6(73.3) Cr
Iso 114.3(13.7) SmX1 109.2(4.9) SmX2 89.0(12.0) Cr
Iso 113.8(0.5) N 67.1(0.8) SmX3 38.6(8.8) Cr
Iso: isotropic; Cr: crystal; SmX: unknown smectic; N: nematic phase.
while the one with cyano linked by an alkyl chain shows the nematic
(N) phase, and with a longer alkyl chain, the temperature range of the
N phase is broadened and the clearing point is reduced. This can be
attributed to the fact that the trifluoromethyl has a greater lateral
volume, which is bad for the ordered arrangement of the compounds,
and an increase in the length of the alkyl chain also increases the
flexibility of the molecule, which makes it more favourable for the
formation of a mesophase. Importantly, all the azobenzene-dimers
linked by tetraglycol chains show mesophases, two unknown smectic
phases (SmX) for 5a and an N phase and SmX for 5b, as the tetraglycol
chain is more flexible than the alkyl chain that makes it more
favourable for the mesophases and leads to a lower clearing point.
As the azobenzene-dimers 5a and 5b have a relatively low
clearing point and a wide temperature range of the mesophase, they
were chosen to study the influence of azobenzene-dimers on the
temperature range of BPs when they were incorporated in BP-LCs, as
presented in Table 2 (the details of 5b can be seen in the ESI†). With
the increase of the solubility of azobenzene-dimers, the temperature
range of BPs first broadened and then decreased. The widest range
can reach 19.4 1C for the mixture with 9% 5a, which is two times
more than that observed for the mixture without the azobenzene-
dimers. This can be explained due to the fact that the
Table 2 The liquid crystalline composites prepared and the corresponding BP
temperature range when 5a was doped into BPs-LC
Composition
Transition temperature/1C
Sample no.
5a (wt%)
BP-LCa (wt%)
N*-BP
BP-I
DTb
A0
A1
A2
A3
A4
A5
A6
—
100.0
99.0
97.0
95.0
93.0
91.0
89.0
37.5
33.0
26.0
26.3
24.1
23.4
29.3
43.5
43.1
42.5
42.6
42.8
42.8
43.3
6.0
10.1
16.5
16.3
18.7
19.4
14.1
1.0
3.0
5.0
7.0
9.0
11.0
Fig. 2 (a) The transmittance spectral change of the composite A1 when
irradiated by UV at 38.5 1C; (b) the transmittance spectral change of A1 when
irradiated by visible light after irradiation by UV; (c) change in POM pictures of
the composite A1 when irradiated by UV at 38.5 1C; (d) POM picture of A1
irradiated by visible light after irradiation by UV.
a
b
BP-LC: 67% (wt%) SLC-4 and 33% S811. DT: BP temperature range.
c
10098 Chem. Commun., 2013, 49, 10097--10099
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irradiation time is 15 s and the BPI completely disappears at 30 s,
which can be further confirmed by the transmittance spectra, as seen
in the ESI.† This can be attributed to the fact that too many cis-
azobenzenes with a bent-shaped structure cannot be changed to tune
the double twist alignment of BPI, thus resulting in the breaking
of the double-twist cylinders of BPI, thereby destroying the structure of
the BPI, which is consistent with the previous studies.15,17
In summary, a new series of azobenzene-dimers were synthesized
and their mesophases were found to be mainly affected by the spacers
and terminal groups. As they were doped into the BP-LCs, the
temperature range of the BPs could be broadened. More importantly,
the Bragg reflection of BPI could be controlled effectively and
Fig. 3 (a) The possible illustration of azobenzene-dimers doped into the BP-LCs;
(b) partial enlargement of the double twist before irradiation; (c) partial enlarge- reversibly by light upon introduction of azobenzene-dimers, which
ment of the double twist after irradiation.
can be used to tune the color of BPI from red to blue. Furthermore,
the isomerization of azobenzene-dimers can result in the transforma-
tion of BPI to N* when irradiated by UV light. This work suggests a
new direction for designing molecules that can be used to stabilize
blue phases and it is promising for the applications in optically
addressable BP-LC displays and other optical devices.
This work was supported by the National Natural Science
Fund for Distinguished Young Scholar (Grant No. 51025313),
Fig. 4 POM pictures of A3 irradiated by UV at 35 1C.
the National Natural Science Foundation (Grant No. 51333001,
51173003, 51272026, 51273022, 61007016), the Major Program
properties, as shown in the ESI.† Then the transmittance spectra of Chinese Ministry of Education (Grant No. 313002), the Major
and POM pictures of LC-BPI1,5,7 doped with 1% of 5a (sample A1) Project of Beijing Science & Technology Program (Grant No.
were obtained under UV (365 nm) or visible light (450 nm) irradia- Z121100006512002), the Defense industrial technology develop-
tion at 38.5 1C, as shown in Fig. 2. When irradiated by UV light, the ment program (Grant No. B1120110006).
transmission peak shifts from 650 nm to a shorter wavelength.
It shifts to 625 nm in 10 s, 600 nm in 15 s, 570 nm in 20 s and
disappears in 25 s, and then no change is observed as the time
increases, as shown in Fig. 2a. On the other hand, when irradiated
by visible light, it recovers to the original position rapidly (Fig. 2b). As
Notes and references
1 (a) M. J. Gim, S. T. Hur, K. W. Park, M. Lee, S. W. Choi and
H. Takezoe, Chem. Commun., 2012, 48, 9968; (b) Y. Li and
S. T. Wu, Opt. Express, 2011, 19(9), 8045.
observed from the corresponding POM pictures in Fig. 2c, the
transmittance changes from red to yellow when the irradiation time
is 15 s and then changes to green when the time is 20 s and becomes
blue in 25 s, after that no change is observed upon increasing the
irradiation time, which is consistent with the transmittance spectra.
Importantly, the texture of the sample returns to the original color as
it is irradiated by visible light, as seen in Fig. 2d.
Fig. 3 is the possible illustration of azobenzene-dimers doped
into the BP-LCs and partial enlargement of the double twist align-
ment when irradiated by light. According to previous research, it can
be inferred that the isomerization of azobenzene causes the bending
of the rigid part of the azobenzene-dimers,16 which affects both the
2 H. Kikuchi, Struct. Bonding, 2008, 128, 99–117.
3 D. K. Yang and P. P. Crooker, Phys. Rev. A, 1987, 35(10), 4419.
4 (a) W. Cao, A. Munoz, P. Palffy-Muhoray and B. Taheri, Nat. Mater.,
2002, 2, 111; (b) H. Coles and S. Morris, Nat. Photonics, 2010, 4, 676.
5 H. Kikuchi, M. Yokota, Y. Hisakado, H. Yang and T. Kajikawa, Nat.
Mater., 2002, 1, 64–68.
6 (a) L. Wang, W. He, X. Xiao, F. Meng, Y. Zhang, P. Yang, L. Wang,
J. Mei, H. Yang and Y. Lu, Small, 2012, 8(14), 2189; (b) L. Wang,
W. He, X. Xiao, M. Wang, M. Wang, P. Yang, Z. Zhou, H. Yang, H. Yu
and Y. Lu, J. Mater. Chem., 2012, 22, 19629.
7 (a) A. Yoshizawa, M. Sato and J. Rokunohe, J. Mater. Chem., 2005,
15, 3285; (b) B. Li, W. He, L. Wang and H. Yang, Soft Matter, 2013, 9, 1172.
8 H. Coles and M. Pivnenko, Nature, 2005, 436, 997.
9 L. Wang, W. He, X. Xiao, Q. Yang, B. Li, P. Yang and H. Yang,
J. Mater. Chem., 2012, 22, 2383.
10 C. T. Imrie and P. A. Henderson, Chem. Soc. Rev., 2007, 36, 2096.
molecular order and the ‘effective’ helical twisting power (HTP) of 11 F. Castles, S. M. Morris, E. M. Terentjev and H. J. Coles, Phys. Rev.
the mixture17 and results in the azobenzene-dimers surrounding the
Lett., 2010, 104, 157801.
12 T. Ikeda and O. Tsutsumi, Science, 1995, 268, 1873.
13 J. Guan, M. Zhang, B. Li, H. Yang and G. Wang, ChemPhysChem,
rod-like molecules of LC,18 thus making the LC more compact and
resulting in the shrinkage of the double twist cylinder of BPI, which
leads to a decrease of the lattice constant. While irradiated by visible
light, the cis-isomer turns back to the trans configuration, which
results in the restoration of the Bragg reflection. Some similar
phenomena have also been observed in previous publications.19
Furthermore, the isomerization of azobenzene-dimers in BPI
2012, 13, 3812.
14 T. J. White, R. L. Bricke, L. V. Natarajan, N. V. Tabiryan, L. Green,
Q. Li and T. J. Bunning, Adv. Funct. Mater., 2009, 19, 3484.
15 H. Y. Liu, C. T. Wang, C. Y. Hsu, T. H. Lin and J. H. Liu, Appl. Phys.
Lett., 2010, 96, 121103.
16 J. Henzl, M. Mehlhorn, H. Gawronski, K. H. Rieder and K. Morgenstern,
Angew. Chem., Int. Ed., 2006, 45, 603.
17 Y. Wen, Z. Zheng, H. Wang and D. Shen, Liq. Cryst., 2012, 39(4), 509.
results in the phase transition from BPI to N*, as shown in Fig. 4, 18 H. Dube, D. Ajami and J. Rebek, Angew. Chem., Int. Ed., 2010, 122, 3260.
19 (a) T. J. White, R. L. Bricker, L. V. Natarajan, S. V. Serak,
which presents the POM pictures of sample A3 when irradiated by UV
at BPI at 35 1C. Upon observing the textures, it can be seen that the
N. V. Tabiryan and T. J. Bunning, Soft Matter, 2009, 5, 3623–3628;
(b) R. K. Vijayaraghavan, S. Abraham, D. S. S. Rao, S. K. Prasad and
irradiation by UV leads to the transformation of BPI into N* when the
S. Das, Chem. Commun., 2010, 46, 2796.
c
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Chem. Commun., 2013, 49, 10097--10099 10099