Reversible Photoresponsive Molecular Alignment of Liquid Crystals at Fluid Interfaces with Persistent Stability

Article abstract of DOI:10.1002/chem.201600095

This work demonstrates a noninvasive approach to control alignment of liquid crystals persistently and reversibly at fluid interfaces by using a photoresponsive azobenzene-based surfactant dissolved in an ionic liquid (IL), ethylammonium nitrate (EAN). As the first report on the orientational behavior of LCs at the IL/LC interface, our study also expands current understanding of alignment control of LCs at the aqueous/LC interface by adding electrolytes into aqueous solutions. The threshold concentration for switching the optical responses of LCs can be changed just by simply manipulating the ratio of EAN to H₂O. This work will inspire fundamental studies and novel applications of using the LC-based imaging technique to investigate various chemical and biological events in ILs.

Full text of DOI:10.1002/chem.201600095

DOI: 10.1002/chem.201600095
Full Paper
&
Liquid Crystals
Reversible Photoresponsive Molecular Alignment of Liquid
Crystals at Fluid Interfaces with Persistent Stability
Tongtong Tian+,[a, b] Qiongzheng Hu+,[c] Yi Wang,[a] Yanan Gao,[d] and Li Yu*[a, b]
Abstract: This work demonstrates a noninvasive approach
to control alignment of liquid crystals persistently and rever-
sibly at fluid interfaces by using a photoresponsive azoben-
zene-based surfactant dissolved in an ionic liquid (IL), ethyl-
ammonium nitrate (EAN). As the first report on the orienta-
tional behavior of LCs at the IL/LC interface, our study also
expands current understanding of alignment control of LCs
at the aqueous/LC interface by adding electrolytes into
aqueous solutions. The threshold concentration for switch-
ing the optical responses of LCs can be changed just by
simply manipulating the ratio of EAN to H2O. This work will
inspire fundamental studies and novel applications of using
the LC-based imaging technique to investigate various
chemical and biological events in ILs.
Introduction
However, aqueous solutions have to be exchanged or ad-
justed alternatively in order to achieve reverse alignment of
LCs. In addition, the substrates have to be chosen carefully to
avoid water accumulation.[22,23]An extra device, designed to
avoid water evaporation, is also indispensable, especially when
only a small amount of water is used. These disadvantages
limit potential novel applications of LCs at fluid interfaces such
as constructing LC switching devices with long-time stability.
Therefore, it is highly desirable to develop a noninvasive
approach that can control the ordering transitions of LCs
reversibly and persistently at fluid interfaces.
Liquid crystals (LCs) are a type of materials that exhibits fea-
tures between the liquid and the solid state. They have been
widely employed in applications such as display devices,[1] op-
toelectronics,[2] and biomedicine devices,[3] owing to their
unique properties of optical and electrical anisotropy and
long-range orientational order. For decades, reversible control
of LC alignment has drawn particular attention as it plays
a key role in the development of active switching components.
It can be achieved by using an electrical field,[4] a magnetic
field,[5] or light.[6–15] Generally, LCs are always supported on
functionalized solid surfaces[6–10,16] or doped with functional
materials[17] to assist in realizing responses of LCs to external
stimuli. Recently, reversible control of the alignment of LCs has
also been reported at fluid interfaces.[18–21] For example,
Kinsinger et al. reported the control of ordering transitions of
LCs by using pH-responsive amphiphilic polymers at the
aqueous/LC interface.[19]
In addition, although numerous literatures have reported
the study of various chemical and biological events at fluid LCs
interfaces in aqueous solution,[24–32] to the best of our knowl-
edge, there is still no report regarding fluid LCs interfaces in
nonaqueous solution because LCs are soluble in most of the
common nonaqueous solvents, such as ethanol, methanol, and
methylene chloride. Ionic liquids (ILs) are functional isotropic
liquid salts with unique characteristics of negligible vapor pres-
sure, nonflammability, high electrical conductivity, controllable
melting point, high thermal and chemical stability, etc.[33,34] As
a novel nonaqueous media, ILs have currently attracted much
attention in various fields, such as solvents or reaction media
in separation techniques,[35] carbohydrate chemistry,[36] organic
synthesis and catalysis,[37] enzymatic reactions,[38] and electro-
chemistry.[39]
[a] T. Tian,+ Y. Wang, Prof. L. Yu
Key Laboratory of Colloid and Interface Chemistry
Shandong University, Ministry of Education
No.27 Shanda Nanlu, Jinan 250100 (PR China).
E-mail: ylmlt@sdu.edu.cn
[b] T. Tian,+ Prof. L. Yu
School of Chemistry and Chemical Engineering
Herein, we report a promising noninvasive approach to
achieve persistent and reversible photoresponsive alignment
of LCs at fluid interfaces by using an azobenzene-based surfac-
tant dissolved in an IL. Ethylammonium nitrate (EAN),
a common protic IL, was selected as the nonaqueous medium.
The orientational transitions of LCs coupled to adsorption of
a photoswitchable azobenzene-based ionic surfactant (4-ethyl-
azobenzene-4’-(oxy-hexyl) trimethyl ammonium bromide
(azoTAB)) were investigated at both the EAN/LC and aqueous/
LC interfaces, respectively.
Qufu Normal University, Qufu, 273165 (PR China)
[c] Q. Hu+
Department of Chemistry, University of Houston
Houston, Texas 77204 (United States)
[d] Y. Gao
China Ionic Liquid Laboratory
Dalian Institute of Chemical Physics
Chinese Academy of Sciences, Dalian 116023 (PR China)
[+] These authors contributed equally to this work
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201600095.
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5CB at the EAN/LC interface or aqueous/LC interface) when
LCs were viewed with white light through crossed polarizers. A
Michel–Levy chart was employed to estimate the retardance
(Dr) of LCs and the tilt angles by using Equation (1), as de-
scribed in the Experimental Section.[42] It was observed that the
optical appearance turned completely dark when the concen-
tration of azoTAB in EAN was up to 0.1 mm (Figure 2a). In con-
trast, under aqueous solutions, the dark images of LCs were
not obtained until the concentration of azoTAB reached
0.005 mm (Figure 2b). The difference can be attributed to the
different solvent structures of EAN and H2O, that is, a tetra-
hedral H-bond network structure in H2O and only a three-di-
mensional network structure in EAN.[43] Therefore, the azoTAB
molecules exhibit the stronger solvophobic effect in water
than in EAN.
Results and Discussion
Figure 1 shows the chemical structures of azoTAB. It has three
main units: a) a photoresponsive azobenzene moiety, b) a hy-
drophilic trimethylammonium head group, and c) a hydropho-
bic alkyl tail group. The azoTAB surfactant generally exists in
trans form but the less stable cis form is obtained upon UV
Figure 1. Chemical structures of azoTAB with the trans and cis configuration
of the azo groups upon light irradiation.
light irradiation due to trans–cis isomerization.[40] Schematic il-
lustration of the experimental system is shown in Figure S1 in
the Supporting Information. A typical nematic LC, 4-cyano-4’-
pentylbiphenyl (5CB), is used in this study. LCs are confined
into gold TEM grids supported on the OTS-coated surface that
can provide stable homeotropic alignment of LCs at the solid/
LC interface. Thus the optical responses are only coupled to
alignment of LCs at fluid interfaces after addition of aqueous
or nonaqueous media.
First, the effect of ILs on the phase behavior of 5CB was ex-
plored. It was observed that the nematic to isotropic phase
transition occurred at 35.0Æ0.18C either for 5CB or 5CB in
contact with EAN. Thus, EAN did not affect the phase behavior
of 5CB. This is similar to the phase behavior of LCs with aque-
ous solutions that contain molar concentrations of salts report-
ed in the previous study.[41] In addition, we quantified the con-
centration of mesogens in the ionic liquid phase by UV/Vis
spectroscopy (Figure S2 in the Supporting Information). The
concentration of mesogens in EAN was measured to be
3.0 mgmLÀ1, approximately. It suggests only a very small
amount of 5CB partitioning into EAN.
Figure 2. a,b) Polarized light microscopy images of LCs in contact with
azoTAB solutions in EAN and H2O at various concentrations, respectively. All
images were captured after incubation of 5CB with the azoTAB solutions for
5 min to obtain a stable self-assembled surfactant monolayer at fluid interfa-
ces. c,d) Illustration of the LC alignment from planar to homeotropic state at
the interface, owing to addition of azoTAB solutions, which corresponds to
change of the optical response of LCs from bright to dark appearance.
It has been well characterized that the bright optical appear-
ance of LCs is caused by the planar orientation of 5CB at fluid
interfaces, whereas the dark appearance is caused by the
homeotropic orientation.[18,44] For aqueous solutions without
azoTAB or with azoTAB in a low concentration range, optical
responses of LCs appear bright, indicating a strong in-plane bi-
refringence at fluid interfaces. As the concentration of azoTAB
surfactant increases, adsorption of the long-chain amphiphilic
surfactant results in formation of a self-assembled monolayer
(SAMs) at fluid interfaces, inducing ordering transition of LCs
from the planar (Figure 2c) to homeotropic orientation
(Figure 2d). Therefore, the optical response of LCs changes
from bright to dark appearance.
Next, we examined whether the concentrations of azoTAB
and solvent types could affect ordering transitions of 5CB mol-
ecules at fluid interfaces. It was observed that the optical re-
sponses of LCs turned from bright to dark appearance when
gradually increasing the concentrations of azoTAB both in EAN
and H2O, respectively. However, the ordering transitions of LCs
were more sensitive to the adsorption of the surfactant at the
aqueous/LC interface than the EAN/LC interface. In our study,
conoscopic examination confirmed homeotropic alignment of
LCs displaying dark appearance (Figure S3 in the Supporting
Information). Quantification of the tilt angle of 5CB at the EAN/
LC interface and the aqueous/LC interface were illustrated in
Figure S4 in the Supporting Information. The transition of an-
choring of 5CB from planar (qtop ꢀ908) to homeotropic (qtop
ꢀ08) was coupled with a continuous variation in the interfer-
ence colors of 5CB (related to a change in the tilt angle of the
Based on the orientational behaviors of LCs coupled to ad-
sorption of different concentrations of azoTAB in EAN and H2O,
we supposed that the optical responses of LCs could be ad-
justed by varying the ratio of EAN to H2O without changing
the concentration of azoTAB. It was observed that LCs incubat-
ed with various concentrations of azoTAB in the mixture of
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Figure 3. Polarized light microscopy images of LCs exposed to various con-
centrations of azoTAB dissolved in the mixture of H2O and EAN with
a volume ratio of a) 1:1 and b) 1:2, respectively.
EAN and H2O with ratios of 1:1 (Figure 3a) and 2:1 (Figure 3b)
displayed dark appearance when the concentration of azoTAB
reached 0.01 and 0.05 mm, respectively. Both of the above
threshold concentrations of azoTAB are between the threshold
concentration in EAN (0.1 mm) and H2O (0.005 mm) for change
of the optical responses of LCs from bright to dark appearance.
It indicates that LC alignment could be adjusted by changing
the ratio of EAN to H2O instead of varying concentrations of
the surfactant in aqueous solutions.
Figure 4. Polarized light microscopy images of LCs in contact with different
concentrations of azoTAB in EAN and H2O upon UV or Vis light irradiation,
respectively.
According to previous studies,[18,41,45] addition of electrolytes
such as NaCl into aqueous solutions can promote adsorption
of ionic surfactants at the LC interface due to screening of
electrostatic interactions, which increases the areal density of
surfactants at the interface, thereby resulting in change of LC
responses from bright to dark appearance. In our work, we
also observed addition of NaCl increases adsorption of surfac-
tants at the aqueous/LC interface. In comparison to the bright
appearance of LCs in the aqueous solution of 0.001 mm
azoTAB, the optical responses changed from bright to dark ap-
pearance by addition of 0.1m NaCl into the azoTAB solution
(Figure S5a in the Supporting Information). Thus, the threshold
concentration was lowered for change of the optical response
from bright to dark appearance due to addition of NaCl. It is
worth noting that decrease of the threshold concentration for
the bright-to-dark shift of the LC response was also observed
by adding 0.1m EAN into the aqueous solution of azoTAB (Fig-
ure S5b in the Supporting Information). Therefore, based on
the above results, the threshold concentration for the optical
responses of LCs can be increased or decreased just by simply
adding different amounts of EAN into the aqueous solution,
which shows the unique characteristics of ILs. Moreover, addi-
tion of electrolytes could decrease the threshold concentration
for the bright-to-dark shift of the optical responses of LCs.
After that, we studied orientational behaviors of LCs when
azoTAB at fluid interfaces underwent reversible UV and visible
light irradiation. The light irradiation time was 5 min. As shown
in Figure 4, the optical responses of LCs remained bright when
5CB was incubated with solutions without or with azoTAB in
a low concentration range. However, the dark-to-bright shift of
optical responses was observed upon UV light irradiation for
LCs incubated with azoTAB solutions under a concentration
sufficient to form a stable self-assembled surfactant monolayer
at the LC interface, indicating the orientation of LCs changes
from homeotropic to planar alignment. When the aqueous or
nonaqueous solutions were irradiated by visible light again,
the optical responses of LCs turned back from bright to dark
appearance. It is caused by photoisomerization of the azo-
benzene group in azoTAB, which changes the structure of the
surfactant from trans to cis configuration, thereby resulting in
the ordering transition of LCs from homeotropic to planar
state at the interface.
As EAN is a nonaqueous medium with negligible vapor pres-
sure, we hypothesize that LCs might exhibit long-time stability
in EAN. It is found that the 5CB films confined within the TEM
grids were stable under EAN during 30 days of observance
(Figure 5a). In contrast, LCs in water became unstable after
a couple of days in our experimental condition (Figure S6 in
the Supporting Information). This is probably attributed to
water accumulation on the substrate-supporting LCs, accord-
ing to previous studies.[22,23] It was also observed that the pho-
toresponsive ordering transitions of LCs were reversible for
many times upon alternative UV and visible light irradiation
and maintained excellent performance even after 30 days (Fig-
ure 5b). These results indicate EAN is an ideal medium to
maintain the long-time stability of LCs for reversible LC
alignment using the surfactant azoTAB.
Figure 5. a) Cross-polarized optical images of LCs after incubation of 5CB
with EAN for up to 30 days; b) polarized light microscopy images of LCs
after reversible light irradiation for more than 10 times. The concentration of
azoTAB in EAN is 0.5 mm.
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treme caution; do not store the solution in closed containers.) The
slides were then rinsed thoroughly with water, ethanol, and metha-
nol, and then dried under a steam of N2, followed by heating to
1208C overnight prior to OTS deposition. The “piranha-cleaned”
glass slides were immersed in the OTS/heptane solution for
30 min. Then, they were rinsed with methylene chloride and dried
under a stream of N2.
Conclusion
In summary, we have demonstrated a noninvasive approach
that enables persistent and reversible photoresponsive align-
ment of LCs by forming a stable azoTAB SAMs at the IL/LCs
interface. Owing to the unique characteristics of ILs, 5CB in ILs
retains its long-term stability. In addition, we also expand
current understanding of controlling orientational behaviors of
LCs by adding electrolytes into aqueous solutions. Future
study will be focused on fundamental studies and novel
applications of investigating chemical and biological events in
ILs using the LCs-based imaging technique.
Preparation of the optical cells
Gold specimen grids were first put onto the OTS-treated glass
slides. Then, ꢀ1 mL of 5CB was heated to the isotropic phase
(>358C) and subsequently dispensed onto each grid. The excess
5CB confined in the grid was removed through a 20 mL capillary
tube. Then, 110 mL of aqueous or nonaqueous solutions were intro-
duced into the optical cell at room temperature. All the results
were repeated at least three times.
Experimental Section
Materials
Examination of polarized light microscopy images
Gold specimen grids (75 mesh, pitch=340 mm, bar=55 mm, hole=
285 mm) were obtained from Beijing Zhongjingkeyi Technology
Co., Ltd. (China). Nematic liquid crystal 4-cyano-4’-pentylbiphenyl
(5CB), Octadecyltrichlorosilane (OTS), and heptane, were bought
from J&K Scientific Co., Ltd (Hebei, China). Sulfuric acid was pur-
chased from Beijing Chemical works (China). Nitric acid (65–
68 wt%) was obtained from Kangde Chemical Reagent Factory
(Laiyang, China). HCl, NaOH, ethanol, phenol, hydrogen peroxide
(30% w/v), and ethylamine aqueous solution (65–70 wt%) were all
obtained from Sinopharm Chemical Reagent Co. Ltd. (Shanghai,
China). 1, 6-dibromohexane, 4-ethylaniline and NaNO2 were ob-
tained from Aladdin Chemistry Co., Ltd. (China). All the above re-
agents were analytical grade and used without further purification.
A polarized light microscope (XPF-800C, Tianxing, Shanghai, China)
was used to acquire the optical image of 5CB. The images were all
obtained by a 2.5” objective lens and a digital camera (TK-9301EC,
JVC, Japan) at room temperature. Conoscopic characterization was
conducted for the dark optical images.
Photoisomerization of the azoTAB surfactant
For light-triggered trans–cis isomerization, the samples were irradi-
ated with a CHF-XM35–500W ultrahigh-pressure short arc mercury
lamp with optical filters (365 and 440 nm). To avoid overheating,
the temperature was kept at 258C using a thermostat water bath,
and the distance between the glass and light source was fixed at
10 cm.
Synthesis and characterization of the azoTAB surfactant
The azoTAB surfactant was synthesized by azo-coupling reaction of
4-ethylaniline with phenol, followed by alkylation and quaterniza-
tion with dibromohexane and trimethylamine, respectively, as de-
scribed by T. Hayashita et al.[40] 1H NMR spectroscopy for AzoTAB
surfactant: 1HNMR(300 MHz, DMSO): d=1.2 (t, 3H, ÀCH3), 1.3–1.8
(m, 8H, ÀCH2), 2.6–2.7 (m, 2H, ÀCH2), 3.0 (s, 9H, ÀNCH3), 3.2–3.3
(m, 2H, ÀCH2), 4.0–4.1 (t, 2H, ÀOCH2), 7.1–7.4 (d, 4H, ArÀH), 7.7–
7.8 ppm (d, 4H, ArÀH).
Measuring the concentration of mesogens in EAN
A UV/Vis near-infrared spectrophotometer (Hitachi, Japan, U-4100)
was used to measure the concentration of mesogens in the ionic
liquid phase. Methanol was chosen as the reference solution and
a 1 cm quartz cuvette was used for the measurement. UV/Vis
absorption spectra were obtained in the wavelength range of
200–700 nm. The specific absorption peak of 5CB was detected
about 278 nm. 10 mg sample was precisely obtained from the EAN
phase on top of the LC layer and dissolved in methyl alcohol to
prepare 1 mgmLÀ1 EAN solution in 10 mL volumetric flask.
Synthesis and characterization of EAN
The synthesis of EAN was performed as described by Evans et al.[43]
50 mL nitric acid was slowly added to ethylamine solution (83 mL)
dropwise within around 2 h while stirring in an ice bath. Then the
water was removed by evaporation under reduced pressure for
more than 2 h. Finally, the product was dried in a vacuum for 48 h
Determination of the tilt angle of 5CB at fluid interfaces
Tilt angles at fluid interfaces were determined by Equation (1):[42]
1
1
Z
at 608C. The final product was characterized by H NMR: H NMR
(300 MHz, D2O): d=1.12–1.16 (t, 3H, ÀCH3), 2.88–2.96 (m, 2H,
ÀCH2CH3). The water content of EAN was 0.95%, measured by
elemental analysis (Vario EL CUBE, Elementar) and found (%):
d
n0ne
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
Dr ꢀ
ð
À n0Þdz
ð1Þ
z
z
n20 sin2ðd qtopÞ þ ne2 cos2ðd qtop
Þ
0
m
total =3.5030 mg, C 25.68, N 22.01, H 7.442.
where retardance (Dr) is obtained from at least three random grid
squares in each optical image. ne indicates the extraordinary indice
of refraction, which is parallel to the long axis of the LC, and no is
the ordinary indice of refraction, which is perpendicular to the
long axis, ne and no of 5CB were taken as constants, reported as
ne =1.711 and no =1.5296 (l=632 nm, at 258C);[46] d is the thick-
ness of the film of 5CB (ꢀ20 mm); z is the position within the 5CB
film in which z=0 represents the OTS-5CB interface; qtop is the tilt
angle at the top interface measured from the surface normal.
Treatment of glass microscope slides
The OTS-coated glass slides were prepared as described by our
previous reports.[28,29] Briefly, the glass microscope slides were first
immersed in “piranha solution” for 30 min at 808C. (“piranha solu-
tion” is 70% H2SO4/30% H2O2. Caution: “piranha solution” reacts
violently with organic materials and should be operated with ex-
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Keywords: amphiphile · fluid interface · ionic liquid · liquid
crystal · persistent and photoreversible alignment
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