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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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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