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Photoresponsive behaviors of smectic liquid crystals tuned by an azobenzenechromophore{
Guojie Wang,* Mingzhi Zhang, Tingting Zhang, Jingjing Guan and Huai Yang*
Received 19th August 2011, Accepted 25th September 2011
DOI: 10.1039/c1ra00615k
The photoresponsive behaviors of a smectic liquid crystal, 8CB, tuned by an azobenzene
chromophore have been systematically investigated. For the smectic 8CB doped with the azobenzene
chromophore, 4-n-hexyl-49-(1-bromopropyloxy)azobenzene (AB), the smectic phase could be
switched to nematic and then to isotropic phase induced by the trans-to-cis photoisomerization of AB
upon UV irradiation. For the smectic 8CB doped with AB and the chiral molecule, (S)-(2)-1,
19-binaphthyl-2, 29-diol (BD), the smectic phase could be switched to the cholesteric phase and then to
the isotropic phase. The initial phase could be recovered when the cis isomer changed to the trans
isomer upon visible irradiation. The switching of the position of reflection band of the liquid crystal
mixtures could be also realized by photoisomerization. The photoresponsive behaviors are dependent
on the composition ratios and the temperature performed in the study.
Introduction
Liquid crystals (LCs) are unique materials, which possess not
only the ordering of crystals but also the molecular mobility of
liquid.1 Yet the ordering degree of LCs is not as high as that of
solid crystals and the LCs are classified as nematic, smectic and
cholesteric LCs according to their special molecular arrange-
ment.2 Just because of their fluidity and long-range order, LC
molecules are easier to arrange in a new way when they are
stimulated by electric field,3 temperature,4–10 and light.11–16
It’s well-known that azobenzene chromophores are photo-
responsive molecules that can undergo reversible isomerization
when irradiated with UV and Vis light.17–22 A small amount of
azobenzene chromophores could be used as guest molecules to
affect the structures and properties of host LCs, due to their
trans-to-cis photoisomerization. The trans form, with a rodlike
shape, stabilizes the phase structure of LCs, while the cis form,
with a bent form, tends to destabilize the phase structure of the
mixture because of its bent shape.23–25
As early as 1971, Sackman prepared a LC mixture composed
of azobenzene compounds and cholesteric LCs and found that
the pitch of the cholesteric LCs could be changed by the
photoisomerization of the azobenzene molecules.26 The photo-
isomerization of azobenzene could be also applied to regulate the
alignment of nematic LCs.27 In addition, The photoisomeriza-
tion of chiral azobenzenes had been reported to control the
phase transition between a nematic and a cholesteric phase.28,29
Ikeda reviewed the photomodulation of LC orientations based
on the order–disorder phase transitions and the order–order
alignment changes induced by the azobenzene photoisomeriza-
tion.30 Not long ago, Bunning et al. demonstrated the reflection
bandwidth of cholesteric LCs consisting of a high-helical-
twisting-power chiral azobenzene molecule could be broadened
from 100 nm to as much as 1700 nm.31 The phototuning of more
than 2000 nm could be achieved for the cholesteric liquid crystals
composed of nematic LCs and the chiral azobenzene com-
pound.32 Dozens of other studies on controlling the structures
and properties of nematic and cholesteric LCs by photochemical
reactions of photochromic molecules have been reported.33–48
Although the LC mixtures composed of nematic or cholesteric
LCs and azobenzene compounds have been comprehensively
studied, the LC mixtures composed of smectic LC49 and the
photoresponsive molecule are rarely reported, where the smectic
layer spacing could be changed by the photoisomerization of
azobenzene.50,51 In a preliminary communication,52 we have
reported briefly the photoinduced phase transitions in smectic
LCs doped with a chiral compound and a photochromic
azobenzene. In this paper, we report more details on the
photoresponsive behaviors of the smectic liquid crystal, 8CB,
tuned by an azobenzene chromophore in the absence and
presence of chiral molecules. The effect of azobenzene photo-
isomerization with different concentrations of the chiral mole-
cule and the azobenzene chromophore on the phase transitions
of the smectic LC is systematically explored. By adjusting the
concentration of the composites and the temperature performed,
the phase transitions from smectic to nematic or to cholesteric
and then to isotropic can be reversibly controlled by the
azobenzene isomerization through the UV and Vis irradiations.
This work not only enriches the study of the phase transitions of
School of Materials Science and Engineering, University of Science andTechnology Beijing, Beijing, 100083, China.E-mail: guojie.wang@mater.ustb.edu.cn; yanghuai@mater.ustb.edu.cn.;Fax: 86-10-62333759; Tel: 86-10-62333759{ Electronic Supplementary Information (ESI) available. See DOI:10.1039/c1ra00615k/
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This journal is � The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 487–493 | 487
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liquid crystals by photo-switching but also opens up a new way
to prepare cholesteric liquid crystals with selective reflection
properties using a smectic liquid crystal.
Experimental section
Materials: The chemical structures of 4-octyl-49cyanobiphenyl
(8CB), 4-n-hexyl-49-(1-bromopropyloxy)azobenzene (AB) and
(S)-(2)-1, 19- binaphthyl-2, 29-diol (BD) are given in Scheme 1.
8CB, a smectic LC at around room temperature, was purchased
from Shijiazhuang Chengzhi Yonghua Display Materials Co.
Ltd. AB is a homemade azobenzene compound.52 The chiral
molecule BD was purchased from the Beijing Chunfu Chemical
Company.
Preparation of LC cells: The mixtures examined herein were
formed by mixing AB and BD with 8CB host homogeneously.
The mixtures were heated to clear point and then were
introduced into LC cells with a gap of 5 mm by capillary action.
Planar-oriented samples were obtained with the substrates
having uniaxially rubbed polyimide layers on their inner
surfaces. The composition of the mixtures in the cells is shown
in Table 1.
Photo irradiation: UV irradiation for the mixtures was carried
out with a high-pressure mercury lamp (365 nm, 500 W nominal
power) and UV light intensity was controlled at 10 mw cm22 by
keeping the distance between the lamp and LC cells. The
irradiation was pursued until no changes were observed in the
absorption spectrum of the sample on further irradiation. Vis
irradiation of recovery experiments was afforded by a fluorescent
light and the light intensity was controlled at 2 mw cm22. The
experiments were performed at 21 uC unless specified.
Instruments and optical measurement: UV-Vis absorption
spectra and transmittance spectra were taken with a UV-Vis
spectrometer (JASCO, V-570). Polarized optical microscopy
(POM) was carried out using an OLYMPUS (BX51) polarizing
microscope. The experiments were performed at 21 uC unless
specified.
Results and discussion
Phase transitions in smectic LCs doped with the azobenzene AB
8CB is a smectic LC at around room temperature and the
textures can be observed by POM, shown in the ESI.{ To
investigate the effect of the photoresponsive azobenzene on the
smectic LC, we prepared four samples with different amount of
the azobenzene AB: LC–A1, LC–A2, LC–A3, and LC–A4, the
mass percentages of AB in which are 3%, 5%, 7%, and 10%,
respectively.
Fig. 1 shows the UV absorption spectra of the sample LC–A1
containing 3% AB before and after UV irradiation at 21 uC. The
mixtures doped with AB exhibit their absorption maxima at
about 354 nm and weak bands at about 441 nm which are related
to p–p* and n-p* transition bands of the trans azobenzene,
respectively. Upon UV irradiation, the intensity of the p–p*
transition band at 354 nm decreased and the intensity of the n-p*
transition band at 441 nm increased gradually until the
photostationary states were obtained. The cis isomer fraction
Y53 was determined from the absorbance by
Y = 1.05 6 (12A/A0)
where A0 and A are the maximum absorbance at the p–p* transition
band before and after UV irradiation, respectively. For the sample
LC–A1, a photostationary state was almost reached upon UV
irradiation for 120 s and the cis isomer fractions increased to 10%,
28%, 75%, and 79% upon UV irradiation for 30 s, 60 s, 120 s, and
180 s, respectively. For the other LC cells such as LC–A2, LC–A3,
and LC–A4, a trans-to-cis photostationary state could be reached
upon the UV irradiation for 180 s.
The phase transitions of the LC cells doped with azobenzene
AB upon the UV irradiation were observed by POM, shown in
Fig. 2. Before UV irradiation, the initial phases of the cells were
all smectic, where the azobenzene was in the trans form; after UV
irradiation, disordered phases could be observed, where the
azobenzene became the cis form. For the LC cell of LC–A1 (3%
Scheme 1 Chemical structures of the compounds used in this study.
Table 1 Mass percentage of each component in the LC mixtures
Samples 8CB AB BD
LC–A1 97.0% 3.0% 0.0%LC–A2 95.0% 5.0% 0.0%LC–A3 93.0% 7.0% 0.0%LC–A4 90.0% 10.0% 0.0%LC–A–B1 96.0% 3.0% 1.0%LC–A–B2 89.0% 10.0% 1.0%LC–A–B3 92.5% 5.0% 2.5%LC–A–B4 91.0% 5.0% 4.0%LC–A–B5 90.0% 5.0% 5.0%
Fig. 1 UV-Vis absorption spectra of LC–A1 (97% of 8CB, 3% of AB)
under UV irradiation 365 nm for 0, 30, 60, 120, and 180 s, at 21 uC.
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of AB), the parabolic focal conic texture of the smectic changed
to a nematic schlieren texture upon the UV irradiation, shown in
Fig. 2a, but no transition from nematic to isotropic was found
even after the trans-to-cis photostationary state. With the
increase of the amount of the azobenzene, the smectic states
could be changed to nematic and then to more disordered states.
For the LC cells of LC–A2 and LC–A3 (containing 5% and 7% of
AB, respectively), the bubble-like textures could be observed at
the trans-to-cis photostationary states, shown in Fig. 2b and 2c.
When increasing the amount of azobenzene AB to 10%, an
isotropic state could be obtained at the photostationary state,
which can be seen from the texture changes of LC–A4, shown in
Fig. 2d. The smectic textures of the cells could be recovered after
the samples were kept in dark for 12 h or irradiated by visible
light for 3 h. The photoinduced phase transition from smectic to
nematic and then to isotropic in the LC mixtures is brought out
by the photoisomerization of azobenzene. It has been proved
that the photoinduced phase instabilities in smectic LCs was due
to the photoisomerization and a subsequent increase in the
smectic layer spacing was observed.50,51 The trans form of the
azobenzene, which possesses a rodlike shape, stabilizes the
smectic LC phase. After UV irradiation, the trans form is
converted to the cis form, which possesses a bent shape and
decreases the order of the LC. The cis form induces the phase
transition from smectic to nematic and then to isotropic when
the amount of azobenzene is enough in the 8CB mixtures.
Phase transitions in smectic LCs doped with a chiral molecule and
the azobenzene AB
It is well known that nematic LCs mixed with chiral molecules
can exhibit cholesteric structures. Since the smectic 8CB doped
with the azobenzene AB can be switched to nematic upon UV
irradiation, it may exhibit a cholesteric structure when mixing
with a chiral molecule. To investigate the phase transitions in
smectic LCs doped with azobenzene and chiral molecules, the
sample of LC–A–B1 with 3% of AB and 1% of BD were prepared
and the texture changes upon UV irradiation were characterized
by POM at 28 uC. Fig. 3a shows POM images of the LC cell LC–
Fig. 2 Polarized optical micrographs of 8CB doped with the azoben-
zene chromophore AB before and after UV irradiation for 480 s at 21 uC.
(a1) and (a2) are graphs of the sample LC–A1 doped with 3% of AB
before and after UV irradiation, respectively; (b1) and (b2) are graphs of
the sample LC–A2 doped with 5% of AB before and after UV irradiation,
respectively; (c1) and (c2) are graphs of the sample LC–A3 doped with 7%
of AB before and after UV irradiation, respectively; (d1) and (d2) are
graphs of the sample LC–A4 doped with 10% of AB before and after UV
irradiation, respectively. Magnification: 400.
Fig. 3 Polarized optical micrographs (a), magnification: 400, and
transmission spectra (b) of the sample LC–A–B1 (96% of 8CB, 1% of
BD, 3% of AB) under UV irradiation for 0, 10, 30, and 60 s, at 28 uC.
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A–B1 upon UV irradiation. It can be seen that the structure of
LC–A–B1 was smectic phase (fan-like texture) before UV
irradiation and the texture changed to a cholesteric phase
(oily-steak texture) gradually upon the irradiation. The texture
change was also verified by the increase of transmission, shown
in Fig. 3b. The transmission increased from 60% before UV
irradiation to 75%, 85% and then to 95% in the range of visible
wavelength upon UV irradiation for 10 s, 30 s, and 60 s,
respectively. The oily-steak cholesteric phase possessed a higher
transmission than that of the fan-like smectic phase. For the LC
cell LC–A–B1 with only 1% of the azobenzene AB, no
cholesteric-to-isotropic phase transition was observed under
the prolonged irradiation, as could be ascribed to the low
concentration of the azobenzene AB.
When the concentration of AB was increased to 10%, the
phase transition of the LC cell LC–A–B2 (89% of 8CB, 10% of
AB and 1% of BD) from smectic to cholesteric and then to
isotropic could be observed upon UV irradiation and the
isotropic phase could be recovered to cholesteric and then to
smectic phases upon visible light irradiation, which was revealed
by POM, as shown in Fig. 4. The texture changes from the
smectic to cholesteric and then to isotropic under UV irradiation
for 0 s, 60 s, and 480 s are shown in Fig. 4a, 4b and 4c,
respectively. Fig. 4d and 4e reveal the cholesteric and smectic
textures under Vis irradiation for 0.5 h and 3 h, respectively.
Fig. 5 exhibits the absorbance changes upon the UV and Vis
irradiation, where the trans-to-cis and cis-to-trans transitions
of the azobenzene AB were observed. The absorption band at
356 nm decreased and the band at 440 nm increased upon
UV irradiation and the bands recovered upon visible irradiation
for 3 h.
Although the cholesteric structure could be obtained in the
above LC cells upon UV irradiation, the reflection band was not
observed in the transmission spectra (out of measurement),
which could result from the low concentration of the chiral
dopant. The spectral position of the reflective cholesteric LC
mixtures is given in the following equation.54–56
lb~�np , �n~nozne
2
where lb is the center wavelength of the reflection notch, �n is the
average of the ordinary (no) and extraordinary (ne) refractive indices
for the LC , and P is the helical pitch of the cholesteric LC. The
position of lb is directly related to the concentration of the chiral
dopant. The higher the concentration of chiral dopant, the smaller
the helical pitch, and then the shorter the reflection wavelength.
When the concentration of the chiral BD increased to 2.5%
and 5%, the reflection notch of the LC cells was observed and the
wavelength of lb was 2340 nm and 1274 nm, respectively, before
UV irradiation at 28 uC, shown in Fig. 6a and 6b. The helical
pitch of the cholesteric LCs decreased from 1466 nm to 796 nm
when the concentration of BD increased from 2.5% to 5% (�n =
1.6). Both of the reflection notches of the samples red-shifted and
became weak upon UV irradiation.
For the LC cell LC–A–B3 (96.0% of 8CB, 5.0% of AB, 2.5% of
BD), the lb increased from 2340 nm to 2342 nm and then to
2346 nm under UV irradiation for 10 s and 60 s, respectively, and
then the band disappeared under irradiation for 120 s, as shown
in Fig. 6a. The transmission of the mixture upon UV irradiation
for 60 s is lower than others in the wavelength region of
500y2000 nm, as could be ascribed to the formed cholesteric
focal conic texture in the mixture. The texture changes from
cholesteric planar structure to fan-shaped focal conic structure
and then to isotropic upon UV irradiation were confirmed by the
polarized optical micrographs, shown in the ESI.{ Cholesteric
structures include planar and focal conic structures. Planar
cholesteric structure (oily streaks texture), where the helical axis
Fig. 4 Polarized optical micrographs of the sample LC–A–B2 (89% of
8CB, 1% of BD, 10% of AB) under UV and Vis irradiation at 21 uC. (a),
(b), (c), (d) and (e) corresponding to UV irradiation for 0 s, 60 s, 480 s
and then to Vis irradiation for 0.5 h and 3 h, respectively, at 21 uC.
Magnification: 100.
Fig. 5 UV–Vis absorption spectra of the sample LC–A–B2 (89% of
8CB, 1% of BD, 10% of AB) under UV and Vis irradiation at 21 uC.
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is normal to the substrates (the director of the molecules is oriented
parallel to the substrates), shows a high transmittance in the visible
and infra-red wavelength range except the reflection range. A fan-
shaped focal conic structure, where the helical axes are randomly
distributed (within individual fans the helix axis, equivalent to the
optic axis, is oriented uniformly), scatters light of all frequencies in
all directions. The lower transmission of the mixture upon
irradiation for 60 s in the wavelength region 500y2000 nm might
be ascribed to the transition from planar to focal conic structures
disturbed by the trans-to-cis isomerization of azobenzene.
For the LC cell LC–A–B5 (96.0% of 8CB, 5.0% of AB, 5% of
BD), the lb increased from 1274 nm to 1436 nm and then to
1672 nm under irradiation for 60 s and 120 s, respectively, and
then the band disappeared under UV irradiation for 180 s, as
shown in Fig. 6b.
The red-shift of the reflection bands, the decrease of the
intensity and the disappearance of the bands upon UV
irradiation resulted from the perturbation effect of the cis
azobenzene molecules. By adjusting the cis isomer fraction in the
mixtures, both the position and the intensity of the reflection
bands could be tuned. Thus the optical properties of the smectic
LC hybrid could be controlled by photo-switching the trans-to-
cis photoisomerization of the azobenzene.
Effect of temperature on the photo-switching
Since the phase transition of LCs and the photo-isomerization
process of azobenzene are greatly dependent on temperature, a
sample of LC–A–B4 doped with 5% of the azobenzene AB and
4% of the chiral BD were investigated at different temperatures.
Fig. 7a and 7b show the POM photographs of LC–A–B4 upon
UV irradiation at 21 uC and 28 uC, respectively. At 21 uC the
texture of LC–A–B4 before UV irradiation was smectic (Fig. 7a1)
and then changed to cholesteric under irradiation for 120 s
(Fig. 7a2). With increasing irradiation time further, mixed phases
of the cholesteric and the isotropic were observed. Isotropic
regions (bubble-like) in the sample increased and enlarged when
the irradiation time increased from 300 s to 480 s, shown in
Fig. 7a3 and 7a4, respectively. At 28 uC the higher temperature
resulted in the decrease of the order of the smectic 8CB and then
the cholesteric phase formed before UV irradiation, shown in
Fig. 7b1 where the azobenzene is in the trans form. Under
this temperature, the cholesteric phase gradually changed to
the mixed phase of the cholesteric phase and isotropic phase
upon UV irradiation, shown in Fig. 7b2 and 7b3. At last, the
Fig. 6 Transmission spectra of the sample LC–A–B3 (92.5% of 8CB,
2.5% of BD, 5.0% of AB) (a) and the sample LC–A–B5 (90% of 8CB, 5%
of BD, 5% of AB) (b) under UV irradiation at 28 uC.
Fig. 7 Polarized optical micrographs of the sample LC–A–B4 (91% of
8CB, 4% of BD, 5% of AB) under UV irradiation: a1, a2, a3 and a4
corresponding to UV irradiation for 0 s, 120 s, 300 s and 480 s,
respectively, at 21 uC, magnification: 100; b1, b2, b3 and b4 corresponding
to UV irradiation for 0 s, 60 s, 120 s and 300 s, respectively, at 28 uC,
magnification: 400.
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cholesteric phase disappeared and changed to the isotropic phase
completely upon UV irradiation for 300 s (Fig. 7b4), where the
cis form of the azobenzene was dominant.
The transmission spectra of the sample LC–A–B4 upon UV
irradiation at 21 uC and 28 uC are shown in Fig. 8a and 8b,
respectively. At the lower temperature of 21 uC, the transmission
of the sample before UV irradiation was low, shown in Fig. 8a,
yet after irradiation for 120 s, the transmission increased a lot
and a reflection band appeared, as was consistent with the
appearance of the cholesteric texture shown by the POM. The
position of the reflection band centered at 1616 nm red-shifed to
1806 nm and then to 2032 nm and the intensity decreased
gradually when the sample was irradiated for 300 s and 480 s,
respectively. At the higher temperature of 28 uC, the sample
exhibited cholesteric phase and the reflection band centered at
1580 nm appeared before UV irradiation, shown in Fig. 8b. The
reflection band red-shifted to 1700 nm under irradiation for 60 s
and the band almost disappeared under irradiation for 120 s,
whereas the reflection band at 21 uC still existed under
irradiation for 480 s shown in Fig. 8a. The azobenzene
isomerization reaction is known to be faster at higher
temperature;57,58 the trans isomers could be transformed into
cis isomers more quickly at 28 uC under UV irradiation
compared with the lower temperature 21 uC. Thus it is
understandable that the phase transition of the mixture from
order to disorder disturbed by the isomerization is faster at
28 uC, combining the knowledge that the fluidity of LCs can be
enhanced and it is easier to change its molecular arrangement at
higher temperature.
Conclusion
Photoresponsive behaviors of a smectic liquid crystal, 8CB,
tuned by an azobenzene chromophore were investigated. For the
LC cells of 8CB doped with the azobenzene compound AB, the
nematic phase could be switched to nematic and then to isotropic
phase induced by the trans-to-cis photoisomerization of AB
upon the UV irradiation. For the LC cells of 8CB doped with
the azobenzene compound AB and a chiral compound BD, the
smectic phase could be switched to cholesteric and then to the
isotropic phase induced by the trans-to-cis photoisomerization of
AB upon the UV irradiation. The initial phase could be
recovered when the cis isomer changed to trans form upon
visible irradiation or thermally driving. The switching of the
position of reflection band of the LC mixtures could be also
realized by the photoisomerization. By adjusting the concentra-
tion of the azobenzene compound AB and chiral compound BD
and the temperature performed, the phase transitions from
smectic to nematic or to cholesteric and then to isotropic can be
reversibly controlled by the UV and Vis irradiations. This work
not only enriches the study of the phase transitions of smectic
liquid crystals by photo-switching but also opens up a new way
to prepare cholesteric liquid crystals with selective reflection
properties.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant No. 21074010 and 51025313),
Beijing Municipal Natural Science Foundation (Grant No.
2112029), Beijing Research Foundation for Excellent Talents
(Grant No. 2010D009006000002), Scientific Research
Foundation for the Returned Overseas Chinese Scholars, State
Education Ministry of China.
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