Azobenzene moiety variation directing self-assembly and photoresponsive behavior of azo-surfactants

Article abstract of DOI:10.1039/c4tc00321g

The effect of varying the position of the azobenzene group within two comparable photoresponsive amphiphiles on their capability to form lyotropic liquid crystals (LLCs) was investigated in detail in this study. Two photoresponsive amphiphiles having comparable structures were designed and synthesized consisting of hydrophilic oligooxyethylene units, a hydrophobic alkyl chain and a light-sensitive azobenzene moiety. When the azobenzene group was located in the middle of the hydrophobic alkyl chain, multiple LLC phases were observed at various water contents in the azo-surfactant-water binary system. In contrast, when the azobenzene group was directly attached to the hydrophilic domain, the azo-surfactant-water binary system exhibited only lamellar phases. The temperature dependence of these self-organised nanostructures was also investigated by the combination of small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and rheology. Under alternating UV and visible light irradiation, reversible trans-cis photoisomerization of the azobenzene group occurred efficiently in dilute solution for both azo-surfactants. However, only photoisomerization of the surfactant possessing the azobenzene group localized in the middle of the alkyl chain induced significant changes in the self-assembled structure and its bulk properties. This study demonstrates that self-assembly and photoresponsive behaviour of photosensitive amphiphiles is extremely sensitive to the position of the photoactive moiety within the surfactant molecular architecture. This journal is

Full text of DOI:10.1039/c4tc00321g

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Materials Chemistry C
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PAPER
Azobenzene moiety variation directing self-
assembly and photoresponsive behavior of azo-
Cite this: J. Mater. Chem. C, 2014, 2,
surfactants†
8303
a
b
b
Shuhua Peng,ab Qipeng Guo, Patrick G. Hartley and Timothy C. Hughes
*
*
*
The effect of varying the position of the azobenzene group within two comparable photoresponsive
amphiphiles on their capability to form lyotropic liquid crystals (LLCs) was investigated in detail in this
study. Two photoresponsive amphiphiles having comparable structures were designed and synthesized
consisting of hydrophilic oligooxyethylene units,
a hydrophobic alkyl chain and a light-sensitive
azobenzene moiety. When the azobenzene group was located in the middle of the hydrophobic alkyl
chain, multiple LLC phases were observed at various water contents in the azo-surfactant–water binary
system. In contrast, when the azobenzene group was directly attached to the hydrophilic domain, the
azo-surfactant–water binary system exhibited only lamellar phases. The temperature dependence of
these self-organised nanostructures was also investigated by the combination of small angle X-ray
scattering (SAXS), differential scanning calorimetry (DSC), and rheology. Under alternating UV and visible
light irradiation, reversible trans–cis photoisomerization of the azobenzene group occurred efficiently in
dilute solution for both azo-surfactants. However, only photoisomerization of the surfactant possessing
the azobenzene group localized in the middle of the alkyl chain induced significant changes in the self-
assembled structure and its bulk properties. This study demonstrates that self-assembly and
photoresponsive behaviour of photosensitive amphiphiles is extremely sensitive to the position of the
photoactive moiety within the surfactant molecular architecture.
Received 18th February 2014
Accepted 20th May 2014
DOI: 10.1039/c4tc00321g
www.rsc.org/MaterialsC
component thermotropic liquid crystal systems, LLC phase
behaviour depends on both concentration in solution and
Introduction
temperature. LLC assemblies have immediate relevance in
biology due to the prevalence of organized lipid structures in
living systems.10 Considerable efforts have been devoted to the
investigation of amphiphile self-assembly in water to generate
LLCs.11–16 As a result, LLCs have been successfully applied
to material synthesis,17–19 drug delivery,20,21 and protein
crystallisation.22,23
Amphiphilic molecules functionalized with stimuli respon-
sive groups which are able to be manipulated upon demand
into desired structures, morphologies, and properties by
external environmental stimuli have great potential for
numerous practical applications such as drug delivery, catal-
ysis, diagnostics, and biosensors.24 The stimuli may include
temperature,25 ultrasound,3 an electric or magnetic eld,26 light
irradiation,27,28 and chemicals (pH, ionic strength),29 depending
on the functionality of the stimuli responsive group employed.
In particular, pH-sensitive amphiphiles are of interest in drug
delivery due to the different pH conditions existing in normal
tissue vs. tumours.30 Kataoka and co-workers reported that pH-
responsive micelles formed by block copolymers containing
citraconic amide as a pH-sensitive charge masking group
selectively released active lysozyme in response to reduced pH.31
Light-sensitive amphiphiles are another attractive alternative
Self-assembly of amphiphilic molecules into a variety of well-
dened molecular architectures with controlled size, orienta-
tion, shape and morphology have been drawing increasing
attention in recent years due to their potential for application in
the areas of drug delivery, imaging, sensing and catalysis.1–3
Driven by the reduction of surface energy, amphiphiles with
different molecular structures can self-organise into various
nanostructures according to their different chemical natures.
For instance, highly ordered nanotubes with notable thermal
and chemical stability have been prepared by self-assembly of
amphiphilic peptides at different pHs.4,5 Apart from nanotubes,
the self-assembly of amphiphilic molecules has also been used
to develop morphologies such as nanobres6,7 and spheres.8,9
Self-assembled lyotropic liquid crystals (LLCs) are another
important class of materials, occupying an intermediate phase
between solids and liquids. Unlike the more well-known single
aPolymers Research Group, Institute for Frontier Materials, Deakin University, Locked
Bag 2000, Geelong, Victoria 3220, Australia. E-mail: qguo@deakin.edu.au
bCSIRO Materials Science and Engineering, Bayview Avenue, Clayton South, Victoria
3169, Australia. E-mail: patrick.hartley@csiro.au; tim.hughes@csiro.au
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4tc00321g
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since they possess a broad range of tunable parameters, e.g.,
wavelength, exposure duration, and intensity to manipulate
their self-assembly behaviours. Several photosensitive units,
such as azobenzene,32,33 stilbene,34,35 dithienylcyclopentene,36
naphthopyran,28,37 and merocyanine,38 show reversible confor-
mational changes through external photo stimuli. Among those
photosensitive groups, azobenzene has attracted much atten-
tion because polymers containing azobenzene have signicant
potential for use in holographic optical data-storage devices,
surface relief graings, sensors and articial muscles.39
Experimental section
Chemicals and materials
Poly(ethylene glycol) methyl ether with number-average molec-
ular weight of about 350 (Aldrich) was dried in vacuum oven at
80 ꢀC overnight before use. Sodium nitrite, 4-butylaniline,
phenol, ethyl 6-bromohexanoate, triethylamine, ethyl p-amino-
benzoate, and 1-bromononane were purchased from Sigma-
Aldrich and used without further purication. No purication
was performed on the solvents except for the tetrahydrofuran
(THF), which was dried using an Innovative Technologies, Inc.,
solvent purication system. Sodium hydrogen carbonate
(NaHCO3), potassium hydroxide (KOH), hydrogen chloride
(HCl, 37%), potassium carbonate (K2CO3), neutral alumina, and
potassium iodide (KI) were from Merck Co. and used as
received. Thionyl chloride was purchased from Scharlau and all
the organic solvents were from Merck.
To date, the majority of reports concerning the self-
assembly of azobenzene based amphiphiles describe simple
colloidal aggregates such as micelles and vesicles while self-
assembled LLC systems remain rarely reported. For instance,
Zhao et al. found that azobenzene-containing amphiphilic
diblock copolymers with poly(acrylic acid) as the hydrophilic
block self-assembled into micelles in water and exhibited
light-sensitive reversible morphological changes through
alternating exposure to UV and visible light.32,40 Yu et al.
prepared an amphiphilic diblock copolymer composed of a
hydrophilic poly(ethylene oxide) block and a hydrophobic
polymethacrylate block with photochromic azopyridine side
groups. The resulting copolymer vesicles showed photoin-
duced fusion, damage and defect formation, disruption,
disintegration and rearrangement under UV irradiation.41
More recently we reported a new class of photorheological
materials with reversible rheological response of unprece-
dented magnitude based on the photo-induced conforma-
tional changes of an LLC forming azo-surfactant in solution.42
While Hatton et al.43,44 established that the change in shape
and size of azobenzene based surfactants upon photo-
isomerization was the driving force behind their photo-
responsiveness, the effect of surrounding molecular structure
on their properties has not been systematically investigated for
the corresponding LLC systems. Moreover, in order to develop
and exploit new functional materials based on the photo-
responsive behaviour of azobenzene-containing amphiphiles,
a deeper understanding of the inuence of molecular struc-
ture on azobenzene based photoreversible LLC is required.
We now present investigations of the effect of structural
variation of azobenzene amphiphiles on their LLC self-assembly
and photoresponsive behaviour, with a view to developing a
fundamental understanding of relationship between molecular
structure and the light-responsive properties of azo-surfactants.
Two comparable photoresponsive amphiphiles containing an
azobenzene moiety were designed and their ability to form LLC
phases was evaluated. The photoresponsive amphiphiles con-
sisted of oligooxyethylene units as the hydrophilic moiety, with
Nuclear magnetic resonance (1H and 13C NMR) spectra
1H and 13C NMR spectra were recorded on a Bruker NMR
spectrometer at 400 MHz using deuterated chloroform as the
solvent. 1H NMR chemical shis (d) in parts per millions (ppm)
were referenced relative to chloroform (d ¼ 7.26 ppm) as an
internal standard.
Mass spectra (MS)
Positive ion EI mass spectra were obtained on a ThermoQuest
MAT95XL mass spectrometer using ionization energy of 70 eV.
Peruorokerosene (PFK) was used as the reference sample.
UV-vis spectroscopy
Photoisomerization of azo-surfactants in ethanol (5 ꢁ 10ꢂ6 mol
Lꢂ1) was measured on a Cary 50 Bio UV-vis spectrophotometer
(Varian) against a background of ethanol in a quartz cuvette.
The isomerization degree at the photostationary state is esti-
mated based on this equation:
A0 ꢂ Ai
Isomerization degree ¼
ꢁ 100%
(1)
A0
where A0 is the absorbance at lmax before the light irradiation
and Ai is the absorbance at the same wavelength measured at
the photostationary state.45
UV-visible light exposures
a simple alkyl chain as the hydrophobic group. An azobenzene Trans to cis isomerisation was induced by an EXFO Acticure
group was inserted as the photoresponsive site, systematically 4000 light source via a liquid light-guide working at 365 nm
either in the middle of the hydrophobic part or between the wavelength. The UV light intensity was about 200 mW cmꢂ2 for
hydrophilic and hydrophobic parts to produce two structurally small angle X-ray scattering and photorheology measurements.
comparable azo-surfactants. The study of the relationship Cis to trans isomerisation was induced by visible light at 38 mW
between molecular structure and both morphological and cmꢂ2. In order to slow down rate of the photoisomerization, a
photoresponsive properties of azo-surfactants may aid the lower intensity UV and visible light was used for UV-vis spec-
design of appropriate photoresponsive amphiphiles for specic troscopy measurements, 3.8 mW cmꢂ2 and 12.1 mW cmꢂ2
,
applications.
respectively. UV-vis data was collected every 30 s.
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Polarized optical microscopy (POM)
Liquid crystal textures were observed with a Nikon Eclipse 80i
cross-polarised optical microscope equipped with a Linkam hot
stage and controller (LTS 120 with PE94 controller, Linkam UK).
Images were captured with a Nikon Ds-Fi1 CCD camera equip-
ped with DS-U2 controller (Nikon Australia Pty. Ltd.; Mel-
bourne, Australia).
Differential scanning calorimetry (DSC)
DSC measurements were performed on a Mettler Toledo DSC
821 system. Neat samples weighing 5–15 mg were sealed in
aluminium pans (40 mL) with pierced lids and heated or cooled
at a scan rate 10 C minꢂ1. Thermograms were recorded in a
ꢀ
Scheme 1 Chemical structures and molecular schematic of azo-
surfactants A (a) and B (b).
nitrogen atmosphere, using empty aluminium pans as the
reference.
Photo-rheometry
Results and discussion
Concentration dependence of self-assembly
In situ monitoring of the phase transitions of binary water–
surfactant systems was conducted using an ARES rheometer (TA
Instruments, USA) as previously reported.42 The sample was
loaded in the centre of two parallel plates of 20 mm in diameter.
The gap between the two plꢂa1tes was set at 0.3 mm. The heating/
As shown in Scheme 1(a), azo-surfactant A has four main units:
(1) oligooxyethylene units as the hydrophilic part; (2) an alkyl
chain (C5H10) between hydrophilic part and the azobenzene
unit; (3) an azobenzene unit as a photoresponsive component;
(4) an alkyl tail group (C4H9) as a hydrophobic part. For azo-
surfactant B, the azobenzene unit is directly attached to its
hydrophilic oligooxyethylene block and it has a longer alkyl tail
group (C9H19) as the hydrophobic part. The overall length of the
hydrophobic alkyl chain has been kept constant in both azo-
surfactants at C9. The effect of the chemical structure variation
of the azo-surfactants on their self-assembly behaviour was rst
investigated as a function of concentration in water at 25 ꢀC.
POM and SAXS measurements were performed to evaluate their
ability to form LLC phases. As for the POM images shown in
Fig. 1(a1), no brightness was observed for the neat azo-surfac-
tant A, suggesting it is in the isotropic state. Aer incorporating
20 wt% water, a streaked texture suggesting a lamellar structure
was observed for azo-surfactant A as shown in Fig. 1(a2).48 At 50
wt% water, a smoke-like texture consistent with a hexagonal
phase was observed as shown in Fig. 1(a3). In contrast, neat azo-
surfactant B exhibits strong birefringence under polarised light
(Fig. 1(b1)). Similar optical textures were also observed for the
azo-surfactant B–water system with 20 wt% (Fig. 1(b2)) and 50
wt% H2O (Fig. 1(b3)), respectively. The observed anisotropic
phases from azo-surfactant–water binary system were further
investigated by SAXS in detail.
cooling rate was 5 C min . Storage shear modulus (G0), the
ꢀ
loss shear modulus (G00), and viscosity (h*) were measured as a
function of temperature at a constant frequency of 10 rad sꢂ1
and a strain of 1.0%.
Small angle X-ray scattering (SAXS)
SAXS experiments were performed at the Australian Synchro-
tron on the small/wide angle X-ray scattering beamline working
at 12.0 KeV with the q-range of 0.2–9 nmꢂ1. The magnitude of
the scattering vector (q) is usually dened by this expansion46
ꢀ ꢁ
4p
q
q ¼
sin
(2)
l
2
where l is the wavelength of the X-ray and q is the scattering
angle. Samples were inserted into 1.0 mm borosilicate glass
capillaries which were then sealed. The background correction
was performed by measuring the scattering of an empty capil-
lary and correcting for sample absorption. Sliver behenate was
used to calibrate the sample to detector distance. The relative
peak positions for a hexagonal phase are 1 : O3 : O4 : O7 and for
a lamellar phase are 1 : 2 : 3 : 4. Data analysis (calibration and
integration) of data collected using a 2D detector was achieved
using AXcess, a custom-written SAXS analysis program written
by Dr Andrew Heron from Imperial College, London.47
As shown in Fig. 2(a), for water concentrations 0–10 wt% azo-
surfactant A, no peaks were observed in the SAXS spectrum with
q-range of 0.5–4 nmꢂ1, indicating an absence of a long-range
ordered structure. With increasing water content to 20 and 30
wt%, the SAXS spectrum with three peaks at q-spacing ratios of
Synthesis of azo-surfactants
The chemical structures of the two comparable photo- 1, 2 and 3 were observed, indicating the presence of a lamellar
responsive amphiphiles are shown in Scheme 1. Details about phase. In addition, it was found that the d-spacing of lamellar
the synthesis and characterization of azo-surfactant A can be phase increased from 6.44 nm to 7.04 nm when the water
found in our previous work.42 Azo-surfactant B was prepared in content was increased from 20 to 30 wt%. Larger d-spacing may
a similar manner and details are included in the ESI.†
be attributed to swelling of the water containing interlayer of
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Fig. 1 POM images for the azo-surfactant–water binary system at room temperature: (a1) neat azo-surfactant A; (a2) azo-surfactant A with 20
wt% H2O; (a3) azo-surfactant A with 50 wt% H2O; (b1) neat azo-surfactant B; (b2) azo-surfactant B with 20 wt% H2O; (b3) azo-surfactant B with 50
wt% H2O. (Magnification ꢁ 100).
the lamellar phase. The formation of hexagonal phases at water
contents of 40 to 60 wt% was conrmed by the observation of
peaks in the SAXS spectrum at the q-spacing ratios of 1, O3, O4
and O7. The d-spacing for the hexagonal phase increased from
7.35 to 7.95 nm, and further to 8.72 nm for water contents of 40,
50 and 60 wt% respectively. The binary system evolved to
isotropic phase when the water content was increased above
65 wt% (data not shown). The variety of LLCs observed in the
azo-surfactant A–water binary system is similar to the results
from commercial surfactants like Brij 97.49
Compared with the azo-surfactant A–water binary system,
azo-surfactant B exhibiꢀts completely different self-assembly
behavior in water at 25 C. As shown in Fig. 2(b), for the neat
azo-surfactant B, three scattering peaks with the ratios of
1 : 2 : 3 in the q-range of 0.5–4 nmꢂ1 were observed, demon-
strating the presence of a long-range ordered lamellar struc-
ture.50 This is probably because the hydrophobic domains
phase separate from the hydrophilic oligooxyethylene units and
Fig. 2 SAXS patterns for azo-surfactant and water binary systems at
this nanophase separation process promotes the bilayer
packing of the molecules. Similar results have been reported for
various concentrations of water at 25 ꢀC: (a) azo-surfactant A–water;
(b) azo-surfactant B–water. Water contents are indicated and inten-
liquid-crystalline polymers.50,51 The d-spacing calculated using
sities offset for clarity.
‘AXcess’ for the neat azo-surfactant B is about 4.87 nm. By
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incorporating water into azo-surfactant B, the SAXS patterns in oxyethylene units and the hydrophobic domains decreases as
Fig. 2(b) show that the azo-surfactant B–water binary system the temperature increases. The phase transition from hexag-
retained lamellar structure upon water dilution, with SAXS onal to lamellar was also observed for the azo-surfactant A–
peaks shiing toward a lower q range as water content water binary system with 45 wt% H2O (ESI Fig. S3†). Meanwhile,
increased. Quantitatively analysis of these SAXS patterns reveals this OOT from hexagonal to lamellar was also veried by the
that the d-spacing gradually increased from 5.03 to 7.38 nm rheology measurements. Fig. 4 shows the temperature depen-
when the water content in the binary system was increased from dence of viscosity for the azo-surfactant A–water binary system
10 to 60 wt%. The increase of d-spacing is likely due to the with 50 wt% H2O. At initial 25 ꢀC, it exhibits very high viscosity
swelling of the hydrophilic layer formed by oligooxyethylene around 2000 Pa s, which can be attributed to the presence of a
units with increasing water content. Comparing azo-surfactants highly ordered hexagonal phase. A sharp decrease of viscosity
A and B, these results clearly indicate that the different position was observed as the temperature increased from 25 to 33 ꢀC.
of the azobenzene group brought about signicantly different This may be due to the melting of hexagonal phase and the
self-assembly behaviors.
initiation of highly ordered packing of the azo-surfactant A
molecules in hexagonal phase was destroyed, resulting in the
decrease of viscosity. Aer that, the viscosity levelled off at
around 10 Pa s in the temperature range of 33–50 ꢀC. These
steady state viscosities were associated with the lamellar phases
according to the SAXS patterns in Fig. 3(a). Compared to
hexagonal phase, the units in lamellar phase were organized in
sheets and that may slide over each other. The relatively
increased mobility of the chains in the lamellar phase sheet
gives rise toꢀlower viscosity.53 With further increase of temper-
ature to 60 C, the azo-surfactant A–water binary system even-
tually transformed to the isotropic phase with extremely low
viscosity. It is worth noting that these phase transitions were
reversible. As shown in Fig. 4, the viscosity nearly returns to the
initial values for each state as the temperature decreases, which
indicates its phase transition as the function of temperature
was reversible. The partial phase diagram derived from the
SAXS measurements for azo-surfactant A with a concentration
accuracy of 5 wt% water content and a temperature accuracy of
5 ꢀC is plotted in Fig. 5. It can be clearly observed that the azo-
surfactant A and water binary systems exhibit rich liquid crystal
phases and an interesting order–order transition (OOT) from
hexagonal to lamellar can be induced by increasing temperature
at high water contents (35–50 wt%).
Temperature dependence of self-assembly
Although LLCs are characterized by the fact that concentration
is the determining factor in their phase transitions, tempera-
ture also plays an important role. Temperature dependence of
the self-assembly behavior of azo-surfactants A and B in water
was also investigated. Fig. 3(a) presents the SAXS spectra for
azo-surfactant A–water binary system with 50 wt% H2O at the
temperature ranging from 25 to 60 ꢀC. From the SAXS spectrum
at 25 and 30 ꢀC, it can be clearly seen that there exists hexagonal
phase in this system due to the presence of SAXS peaks situated
at the positions 1 : O3 : O4 : O7. Comparable lattice spacing ꢀfor
ꢀ
the hexagonal phases, 7.92 nm at 25 C and 8.01 nm at 30 C,
were obtained. With further increasing temperature to 35 ꢀC,
SAXS peaks assignable to hexagonal phase faded away and
characteristic peaks with relative positions 1 : 2 : 3 for a
ꢀ
lamellar phase emerged when the temperature reached 40 C.
Similar results concerning the temperature induced phase
transition from hexagonal to lamellar have been reported for
poly(styrene-b-isoprene) (SI) diblock copolymers in selective
solvents.52 This order–order transition (OOT) is believed to be
related to the fact that the selectivity of water for the hydrophilic
The corresponding temperature dependence of phase
behavior for the azo-surfactant B–water binary system with 50
Fig. 3 SAXS patterns for azo-surfactant and water binary systems at
various temperatures with 50 wt% water: (a) azo-surfactant A–water; Fig. 4 Plot of the viscosity of azo-surfactant A–water system with 50
(b) azo-surfactant B–water. Intensity offset for clarity.
wt% water as a function of temperature.
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the structure. This decreasing tendency of lattice parameter as a
function of temperature is commonly observed in LLCs systems
with long hydrocarbon amphiphile molecules and lipids.49,54–56
With further increasing temperature to 70 ꢀC, no long range
ordered structures could be observed, as evidenced by the
absence of the periodic peaks in the SAXS spectrum. This was
presumably due to the melting of the lamellar phases, which
was further conrmed by the DSC results discussed in the
following text.
The melting points of these lamellar phases were deter-
mined by DSC measurements. Fig. 7 shows the DSC curves from
25 to 90 ꢀC of neat azo-surfactant B and azo-surfactant B–water
binary system with different water contents. For the neat azo-
surfactant B, one endothermic peak at 58 ꢀC was recorded due
to the melting of the surfactant crystals. Upon incorporation of
10 wt% water into azo-surfactant B, the melting pointꢀincreased
ꢀC for the binary system with 20 and 30 wt% H2O respectively.
Aer that, an identical melting point 69 ꢀC was obtained for the
binary system with 40, 50 and 60 wt% H2O respectively. The
increase of the melting point is most likely caused by the
enhanced segregation of hydrophilic and hydrophobic domains
aer adding water into the azo-surfactant B. The highly sepa-
rated bilayer structure results in the improved thermal stability.
The DSC measurements for the azo-surfactant B–water binary
system with 50 wt% H2O were in good agreement with those of
SAXS spectra shown in Fig. 3(b).
Fig. 5 Data points for the partial phase diagram derived from SAXS
patterns of the azo-surfactant A–water binary system. L: lamellar
phase; H: hexagonal phase; O: isotropic phase; ꢁ: lamellar and
hexagonal.
ꢀ
to 63 C. The melting points further increased to 66 C and 68
wt% H2O is shown in Fig. 3(b). The SAXS patterns with relative
peak positions 1 : 2 : 3 were observed for this binary system at
ꢀ
25 C, indicating the presence of a lamellar phase. Moreover,
the lamellar phase remains unchanged with increasing the
ꢀ
temperature up to 65 C. Nevertheless, it was clearly observed
that these SAXS peaks shi towards higher q as the temperature
increases. The d-spacing plotted as a function of temperature is
shown in Fig. 6. It shows that d-spacing decreased gradually
from 7.31 nm to 6.05 nm with increasing temperature from 25
to 65 ꢀC. Compared with azo-surfactant A, as shown in Scheme
Photoresponse of azo-surfactants
As both azo-surfactants A and B have a potentially photosensi-
tive azobenzene unit within them, the photoresponsive behav-
iour of azo-surfactant in dilute solution as well as in LLCs was
investigated. The UV-vis spectra for both azo-surfactants are
1, azo-surfactant B has a longer hydrophobic alkyl chain (C9H19
)
adjacent to the rigid azobenzene group. As the temperature
increases, the exibility of the long hydrophobic alkyl chain of
azo-surfactant B is increased, which results in the shrinking of
Fig. 6 Variation of d-spacing for the azo-surfactant B–water system Fig. 7 DSC curves for the azo-surfactant B–water binary system with
with 50 wt% H2O as a function of temperature.
different water content.
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shown in Fig. 8. As shown in Fig. 8(a), before UV light irradia- phase faded away and a SAXS pattern with two broad peaks was
tion, the trans-form of azo-surfactant A exhibits a maximum recorded. This may be correlated to an isotropic state because
absorption band at around 350 nm. This absorbance peak no brightness was observed when the sample was visualised
decreased aer UV-light irradiation due to trans–cis isomeriza- between two polarized lms under UV irradiation (data not
tion and a photoequilibrium was established within 10 min shown). According to the previous UV-vis spectroscopy
(Fig. 8(a), spectrum 2). Overlapping spectra recorded at different measurements, this order-disorder transition (ODT) is most
time points conrmed that the photostationary state was likely attributed to the local isomerization of azobenzene units
established. A more detailed kinetic study of azo-surfactant A in azo-surfactant A. Under UV irradiation, the rodlike trans
can be found in our previous publication.42 The trans–cis tran- conformation azobenzene units in azo-surfactant A trans-
sition was conrmed by the appearance of a weak absorption formed to their cis ‘bent’ conformation. The disruption of
band around 440 nm based on n–p* transition of cis-isomer.32,57 hexagonal phase may be due either to molecular-geometrical
Isomerization degrees calculated based on eqn (1) for azo- changes or to the difference in hydrophobicity of the two
surfactant A and B were 89% and 86% respectively. Aer UV isomers of azo-surfactant A. Azobenzene based surfactants in
irradiation, the sample was irradiated with visible light for 10 the cis form usually have greater hydrophilicity.58 Aer switch-
min and the UV-vis spectrum was recorded. The maximum ing off UV light and turning on visible light, the well-dened
absorption band at round 350 nm fully recovered and the periodic SAXS peaks for a hexagonal phase returned. Referring
spectrum essentially overlapped the original UV-vis spectrum, to the UV-vis spectroscopic results in Fig. 8(a), under visible
indicating a completely reversible trans–cis isomerization was light exposure, the bent azobenzene units in cis form relaxed
achieved under alternating UV and visible light irradiation. back to its rod-like trans form and hence azo-surfactant A
Meanwhile, Fig. 8(b) shows that a similar reversible behaviour assembled back to hexagonal phase. The detailed photo-
was observed for azo-surfactant B. A comparison of the kinetic responsive behaviour of azo-surfactant A has been reported in
plots of UV spectra of azo-surfactants A and B under UV and our previous work.42
visible light exposure indicated that they had very similar rates
A corresponding photoresponsive study for the azo-surfac-
of photoisomerisations (data not shown). Based upon the effi- tant B and water binary system with 50 wt% H2O was also
cient reversible photoisomerization of these azo-surfactants in conducted and the SAXS proles are shown in Fig. 9(b). Before
dilute solution, a similar approach was used to assess material UV irradiation, it shows periodic SAXS peaks with relative
properties of their self-assembled LLCs aggregations as outline positions 1 : 2 : 3 for lamellar phase. Surprisingly, the SAXS
in the following section.
pattern still exhibits characteristic peaks for a lamellar phase
To determine the effect of photoisomerization of azobenzene aer UV irradiation, which is in contrast to order-disorder
group within azo-surfactants on their LLC phase behaviour, transition for azo-surfactant A and water binary system. It
SAXS studies were carried out for the above two azo-surfactants should be noted that these periodic peaks shied towards
in binary water systems before UV irradiation, aer UV irradi- higher q and the resulting d-spacing decreased from 7.31 to 6.66
ation, and followed by visible light exposure respectively. The nm aer UV irradiation. This presumably results from the
obtained SAXS spectra are given in Fig. 9. As shown in Fig. 9(a), trans–cis isomerization of azobenzene units in azo-surfactant B.
the azo-surfactant A–water system with 50 wt% H2O before UV The effective chain length of azo-surfactant B in the bent cis
light irradiation shows an ordered hexagonal phase. Aer UV form is decreased under UV irradiation, resulting in the
light irradiation, these periodic SAXS peaks for a hexagonal decrease of d-spacing. Although azo-surfactant B showed
Fig. 8 Influence of photo-irradiation upon absorption spectra of azo-surfactant A (a) and azo-surfactant B (b): (1) before UV irradiation; (2) after
UV irradiation (10 min); (3) after UV irradiation followed by visible light exposure (10 min).
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Fig. 9 SAXS patterns for the azo-surfactant–water systems with 50 wt% H2O before UV irradiation (I), after UV irradiation (5 min) (II), and followed
by visible exposure (5 min) (III) at 25 ꢀC: (a) azo-surfactant A–water; (b) azo-surfactant B–water.
Fig. 10 Schematic illustration of different phase behaviours and stimulus-response properties of azo-surfactants A and B at 50 wt% H2O content
respectively.
similar UV-vis spectra as azo-surfactant A in dilute solution light,42 no signicant rheological change was observed for the
during tran–cis isomerization (Fig. 8), the isomerization of azo-surfactant B with 50 wt% water under alternating UV and
azobenzene group did not disrupt the bilayer packing of azo- visible light irradiation due to their highly stable ordered
surfactant B molecules in water. This may be due to that the lamellar phase. A schematic of the different phase behaviours
azobenzene group of surfactant B was directly attached to a and stimulus-response properties of azo-surfactants A and B at
exible PEG chain, the changes in azobenzene conformation 50 wt% H2O content are schematically shown in Fig. 10.
under UV irradiation were dissipated when it was attached to
the exible PEG chain. Aer switching off UV light and turning
on visible light, the bent cis form azo-surfactant B molecules
Conclusions
relaxed back to rod-like trans form and therefore, it was In summary, two structurally similar and directly comparable
observed that the periodic SAXS peaks for lamellar phase shif- photoresponsive azo-surfactants were successfully synthesized
ted back to low q range. It should be noted, however, that the d- and their capability to form LLCs and the photoresponsive
spacing (7.01 nm) did not fully recover aer a long (more than behaviour of these LLCs were investigated. The results show
10 min) visible light exposure. Unlike the mixture of azo- that the self-organised structures of azo-surfactants A and B in
surfactant A–water (50 wt%) which exhibited a large rapid water were signicantly different depending on the position of
reversible change in modulus upon exposure to UV and visible azobenzene unit. Neat azo-surfactant A with azobenzene group
8310 | J. Mater. Chem. C, 2014, 2, 8303–8312
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Journal of Materials Chemistry C
localized in the centre of its hydrophobic alkyl chain did not
show liquid crystal structures at room temperature. The pres-
ence of water, however, increases the segregation between the
hydrophilic and hydrophobic blocks of azo-surfactant A and
induced ordering in the amphiphilic system. This was evident
in the formation of lamellar LLCs in the azo-surfactant A–water
system with 20 wt% water at room temperature, which evolved
to hexagonal phase when the water content reached to 40 wt%.
In contrast, the neat azo-surfactant B with azobenzene unit
directly attached to the hydrophilic block exhibited lamellar
phase which remains the bilayer structures upon water dilution.
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Acknowledgements
SAXS/WAXS research was undertaken at the Australian
Synchrotron, Victoria, Australia and the authors thank Dr Nigel 25 Y. Yin, L. Wang, H. Jin, C. Lv, S. Yu, X. Huang, Q. Luo, J. Xu
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