Amphotropic azobenzene derivatives with oligooxyethylene and glycerol based polar groups

Article abstract of DOI:10.1039/c5tc02583d

A series of amphiphilic azobenzenes with one to three lipophilic alkyl chains at one end and polar groups with oligooxyethylene (EO) and racemic 3-glyceryl units at the opposite end was synthesized and their thermotropic and lyotropic liquid crystalline self-assemblies were studied by POM, DSC and XRD. Tilted and non-tilted lamellar phases with interdigitated double layer structures (SmC_d and SmA_d, respectively) were found for the compounds with a single alkyl chain, whereas hexagonal columnar phases were formed by the compounds with two or three alkyl chains. The effect of protic solvents, like formamide, ethylene glycol and water, was investigated for representative examples. For the compounds with the single chain, induction and stabilization of SmA phases were observed, though broad regions of lyotropic SmC phases were retained in most cases. Depending on the structure of the polar group, the hexagonal columnar phases were either removed or drastically stabilized by the solvents. Photoisomerisation of an azobenzene chromophore was also studied.

Full text of DOI:10.1039/c5tc02583d

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Materials Chemistry C
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PAPER
Amphotropic azobenzene derivatives with
oligooxyethylene and glycerol based polar groups†
Cite this:DOI: 10.1039/c5tc02583d
Xiaoping Tan,‡a Ruilin Zhang,‡ab Chunxiang Guo,a Xiaohong Cheng,*a
Hongfei Gao,a Feng Liu,*c Johanna R. Bruckner,d Frank Giesselmann,*d
Marko Prehme and Carsten Tschierske*e
A series of amphiphilic azobenzenes with one to three lipophilic alkyl chains at one end and polar
groups with oligooxyethylene (EO) and racemic 3-glyceryl units at the opposite end was synthesized
and their thermotropic and lyotropic liquid crystalline self-assemblies were studied by POM, DSC and
XRD. Tilted and non-tilted lamellar phases with interdigitated double layer structures (SmCd and SmAd,
respectively) were found for the compounds with a single alkyl chain, whereas hexagonal columnar
phases were formed by the compounds with two or three alkyl chains. The effect of protic solvents, like
formamide, ethylene glycol and water, was investigated for representative examples. For the compounds
with the single chain, induction and stabilization of SmA phases were observed, though broad regions of
lyotropic SmC phases were retained in most cases. Depending on the structure of the polar group, the
hexagonal columnar phases were either removed or drastically stabilized by the solvents. Photoisomerisation
of an azobenzene chromophore was also studied.
Received 20th August 2015,
Accepted 28th September 2015
DOI: 10.1039/c5tc02583d
www.rsc.org/MaterialsC
biphenyl,6–8 phenylpyrimidine9,10 and other rigid cores11,12 ter-
minated with alkyl chains at one end and oligooxyethylene (EO)
1. Introduction
Design of molecules which can self-assemble into ordered nano- or glycerol-based groups at the other end.5 Amphotropic LCs
structures is of current interest for the general understanding involving EO chains and glycerol units are potentially useful,
of the processes of molecular self-assembly.1 Amphiphilic and for example, for design of nanostructured 1D, 2D or 3D ion
anisometric compounds have aroused special interest as they conducting materials in combination with ionic liquids.13 On
can organize into ordered liquid crystalline (LC) superstructures the other hand, LC azobenzene derivatives have attracted attention
with surprising complexity and great interest for materials because of their photoresponsive properties caused by the trans–cis
science as well as life science.2 By combining an anisometric photoisomerization of the azobenzene units.14 Therefore, a
molecular shape and an amphiphilic structure, both representing wide diversity of azobenzene derivative LCs have been designed
structural concepts supporting LC self-assembly, new amphotropic to introduce light-modulated functionalities, for example for
molecules can be obtained.3–5 Such molecules can display thermo- application as organic light-driven actuators.15,16 Photoisomerizable
tropic and lyotropic LC phases and form well defined monolayers amphiphiles can find applications in photoswitchable surfactants,17
at interfaces.12 Typical examples are amphotropic LCs involving for drug delivery,18 switchable vesicles19 and controlled gelation.20
Considering the vast variety of useful properties and potential
applications, photoswitchable azobenzene based amphiphiles21–27
a Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry Department,
and especially photoswitchable amphotropic LCs28–30 have
Yunnan University, Kunming, Yunnan 650091, P. R. China.
E-mail: xhcheng@ynu.edu.cn; Fax: +86 871 5032905
received comparatively little attention to date. Herein we report
the design, synthesis and thermotropic as well as lyotropic
liquid crystalline self-assembly of new amphiphilic azobenzenes,
consisting of EO31 or glycerol based polar groups or combina-
tions of both involving one or two hydroxyl groups in the polar
groups, and having one to three lipophilic n-alkyl chains at the
opposite end.
b Forensic Medicine of Kunming Medical University, Kunming 650500, P. R. China
c State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University,
Xi’an, 710049, P. R. China. E-mail: feng.liu@mail.xjtu.edu.cn; Fax: +86 29 82663453
d University of Stuttgart, Institute of Physical Chemistry, Pfaffenwaldring 55, D-075069,
Stuttgart, Germany. E-mail: f.giesselmann@ipc.uni-stuttgart.de;
Fax: +49 711-685-62569
e Institute of Chemistry, Organic Chemistry, Martin Luther University Halle-Wittenberg,
In the notation of the compounds 1–3 (1nEOmOH to 3nEOmOH
and 1nEOm(OH)2 to 3nEOm(OH)2, see Scheme 1), the first number
indicates the number of terminal n-alkyl chains and the superscript
n represents the length of these chains. Note that in compounds of
Kurt-Mothes Str. 2, 06120 Halle/Saale, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 55 27346
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5tc02583d
‡ Both authors contributed equally to this work.
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Scheme 1 Synthesis of compounds 1–3. Reagents and conditions: (i) KOH, H2O, 195–200 1C, 5 h; (ii) TsCl, CH2Cl2, Et3N, 0–25 1C; (iii) K2CO3, DMF,
90 1C, 12 h; (iv) CH3CN, K2CO3, 70 1C, 24 h; (v) allyl bromide, NaH, THF, 70 1C, 12 h; (vi) OsO4, NMMNO, H2O, acetone, 25 1C, 24 h.
type 1 the single alkoxy chain is directly attached to the azobenzene 2.2 Thermotropic self-assembly of the single chain
core whereas for compounds of types 2 and 3 with more than only compounds 1 in SmA and SmC phases
one alkoxy chain these chains are attached via an additional
benzylether unit to the azobenzene core. EO indicates the presence
of an EO chain with a number m of EO units in this chain. At the
end OH denotes the presence of a single terminal hydroxyl group,
The phase transition temperatures and enthalpies of the azo-
benzene derivatives with a single alkyl chain (compounds 1) are
listed in Tables 1 and 2. No LC phases were found for the EO
ethers with only one OH group and comparatively short hexyl
chains (compounds 16EOmOH with m = 3, 4). Elongation of the
whereas (OH)2 indicates a racemic 3-glyceryl unit. Compounds
1n(OH)2–3n(OH)2 have the 3-glyceryl units directly connected to
alkyl chain leads to SmC phases. All compounds with terminal
glycerol groups show smectic phases with enhanced stability
the azobenzene core by an ether linkage.
compared to the compounds with only a single OH group.
Among them, compounds 1n(OH)2 without EO units have
enhanced mesophase stability compared to the related compounds
2. Results and discussion
2.1. Synthesis
1nEOm(OH)2 with additional EO units. SmC phases were found in
all cases and for most compounds with glycerol groups, additional
The synthesis of the compounds under discussion is shown in
Scheme 1. The key intermediate is azobenzene-4,40-diol C, which
¨
was prepared based on the method of Willstatter and Benz from
Table
1
Mesophases, transition temperatures (T/1C) and transitional
4-nitrophenol32 and monoalkylated with appropriate n-bromo- enthalpy values (DH/kJ molꢀ1) of the single chain compounds 1nEOmOH
with a single OH groupa
alkanes or monobenzylated with alkoxysubstituted benzyl chlorides,
leading to the 40-alkoxysubstituted azobenzene-4-ols 1nOH and
the 40-benzyloxy-azobenzene-4-ols 214OH and 3nOH, respec-
tively. Compounds 1nOH and 3nOH were alkylated using
tri- and tetraethylene glycol monotosylates EOmTs yielding
Comp
R1
m
Phase transitions
compounds 1nEOmOH and 3nEOmOH. These compounds and
the azobenzene-4-ols 1nOH–3nOH were etherified with allyl-
bromide, followed by dihydroxylation of the double bonds using
OsO4/N-methylmorpholine-N-oxide (NMMNO).33 The obtained
target compounds, the alcohols 1nEOmOH and 3nEOmOH and
the glycerol ethers 214(OH)2, 3n(OH)2 and 3nEOm(OH)2 were purified
by column chromatography or crystallization from ethyl acetate.
Detailed synthetic procedures and analytical data are given in
the ESI.†
16EO3OH
16EO4OH
112EO3OH
C6H13
C6H13
C12H25
C12H25
C18H37
O
O
O
O
O
3
4
3
4
3
Cr 102 [51.5] Iso
Cr 104 [49.5] Isob
Cr 97 [35.6] SmCd 106 [12.3] Iso
Cr 96 [42.6] SmCd 113 [16.5] Iso
Cr 106 [52.1] SmCd 114 [16.1] Iso
1
1
12EO4OH
18EO3OH
a
Transition temperature and enthalpy changes (in square brackets) were
determined by DSC (peak temperature, first heating scan, 10 1C minꢀ1);
abbreviations: Cr = crystalline solid; SmCd = tilted smectic phase with a
double layer structure; Iso = isotropic liquid state. Supercooling was
possible to 95 1C.
b
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Table 2 Mesophases, transition temperatures (T/1C) and transitional enthalpy values (DH/kJ molꢀ1) of the single chain compounds 1n(OH)2 and
1nEOm(OH)2 with a racemic 3-glyceryl groupa
Comp
R1
m
Phase transitions
112(OH)2
C12H25
C14H29
C12H25
C14H29
O
O
O
O
0
0
3
3
Cr 156 [30.8] SmCd 168 [1.3] SmAd 176 [5.1] Iso
Cr 144 [31.1] SmCd 166 [2.6] Iso
Cr 94 [18.6] SmCd 129 [2.6] SmAd 131 [6.0] Iso
Cr 95 [41.9] SmCd 121 [2.0] SmAd 126 [7.4] Iso
1
1
1
14(OH)2
12EO3(OH)2
14EO3(OH)2
a
SmAd = non-tilted smectic phase with a double layer structure; for conditions and other abbreviations, see Table 1.
SmA phases are formed above the SmC phases. The SmA–SmC
transitions have comparatively large enthalpy values (DH = 1.3–
2.6 kJ molꢀ1) for this kind of transition and thus this phase
transitions appear to be first order. Fig. 1 shows representative
DSC traces for compound 112(OH)2 with a SmC–SmA sequence.
Interestingly, the Sm–Iso transition enthalpy values of the glycerol
derivatives, having two terminal OH groups, are only about one half
of those of the amphiphiles with only one hydroxyl group (compare
Tables 1 and 2), which is discussed later.
By microscopic investigation between non-treated glass plates
through crossed polarizers, the smectic C phases of these
compounds occur with typical broken fan and SmC schlieren
textures34–36 (see Fig. 2a) and the SmA phases are characterized
by focal-conic fan-like textures and pseudoisotropic regions
(Fig. 2b), often separated by oily streaks.
Compounds 112EO3OH and 112(OH)2 were investigated as
representative examples by X-ray diffraction (XRD; see Fig. 3).
Diffuse wide angle scattering with maxima at d = 0.45–0.47 nm,
corresponding to the mean lateral distance between the molecules,
indicate fluid smectic phases without in-plane order for all
investigated compounds. This excludes the presence of other
low temperature smectic mesophases37 and thus assignment to
SmA and SmC was confirmed by XRD. The sharp fundamental
Fig. 2 Textures of 112(OH)2 in the (a) SmCd phase at T = 167 1C and (b) in
the SmAd phase at T = 169 1C as observed upon heating; at the right
homeotropic alignment and at the left predominately planar alignment
with fan texture.
and second-order pseudo-Bragg peaks in the small-angle regime
originate from the long-range lamellar order along a single
direction. For compound 112EO3OH the layer reflection has a
tilt angle of y = 291 with respect to the maxima of the diffuse
wide angle scattering, indicating the presence of a synclinic SmC
phase with an average tilt angle of 291 (see Fig. 3c). The layer
spacing is d = 5.57 nm while the length of the molecule is only
Lmax = 3.7 nm (calculated by the MM2 method38 and assuming a
molecule in the most stretched conformation with all-trans
conformation for the alkyl chains and EO chains39). Thus the
layer thickness is significantly larger than the molecular length
(d/L = 1.5). Considering the tilt, the effective molecular length is
Fig. 1 DSC traces of 112(OH)2 (10 K minꢀ1).
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Table 3 Comparison of X-ray data and molecular dimensions of selected
single chain compounds 1
Comp.
Phase
d/nm (T/1C)
L/nm
1
12(OH)2
SmAd
SmCd
SmCd
5.16 (169)
5.05 (167)
5.57 (105)
3.3
3.3
3.7
1
12EO3OH
Fig. 3 XRD patterns of the smectic phases after subtraction of the
scattering in the isotropic liquid states: (a) 2D-XRD pattern of a surface
aligned sample of compound of 112(OH)2 in the SmCd phase at T = 167 1C
and (b) in the SmAd phase at T = 169 1C and (c) 2D pattern of a surface
aligned sample of the SmC phase of 112EO3OH at T = 105 1C; (d) shows a
model of the molecular organization in one layer of the double layer SmC
phase with partial intercalation of the alkyl chains; complete intercalation
would lead to d B 5.2 nm. For additional crystallographic data, see
Tables S1–S3 (ESI†).
reduced to cos 291 Lmol = 3.24 nm, leading to the effective d/L
ratio of 1.72. This means that the layer distance is quite a bit
smaller than twice the effective molecular length, indicating a
bilayer structure with a partial intercalation of the alkyl chains as
shown in Fig. 3d (the SmCd phase). The degree of alkyl chain
intercalation could become smaller if there is strong disorder of
the alkyl chains and especially of the EO chains.
In the SmC phase of the glycerol derivative 112(OH)2 with the
same alkyl chain length, but without EO units the thickness of
the smectic layers is d = 5.16 nm at 167 1C and the tilt angle is
only 121 (Fig. 3a). The layer thickness decreases to 5.05 nm at
the transition from the SmA phase to the SmC phase at 167 1C,
in line with the emerging uniform tilt angle of only 121. The
ratio d/L = 1.56 (Lmax = 3.3 nm) also indicates an intercalated
Fig. 4 Small angle XRD patterns of compounds (a) 112EO3OH and (b)
112EO3(OH)2 at the indicated temperatures in the smectic phase (black
lines) and at different temperatures in the Iso phase in proximity to the
SmAd–Iso transition (colored lines), showing a significant cybotaxis for
112EO3(OH)2 being much weaker for 112EO3OH.
bilayer structure for the SmC and SmA phases (SmCd and SmAd measured for the related compounds with a single OH group (see
phases) with a degree of intercalation being very similar to that Tables 1 and 2). It appears that for 112EO3(OH)2 significant
found in the SmCd phase of compound 112EO3OH (Table 3).
cybotactic clusters are already formed in the Iso phase which then
XRD investigation of the Iso phases occurring immediately fuse to infinite layers at the Iso–Sm phase transition, whereas for
above the smectic–Iso phase transition of compounds 112EO3(OH)2 112EO3OH the clusters are much smaller and just at the Iso–Sm
and 112EO3OH, as shown in Fig. 4, indicates a higher intensity of transition layer formation starts with much smaller clusters, thus
the diffuse small angle scattering in the Iso phase of the glycerol leading to a larger enthalpy for the phase transition.
derivative 112EO3(OH)2 compared to 112EO3OH having only one OH
2.3 Thermotropic self-assembly of compounds 2 and 3 with
two and three alkyl chains in columnar LC phases
group. This indicates the formation of clusters in the Iso phase of
112EO3(OH)2 which is much weaker for compound 112EO3OH with
only one terminal OH group. This could contribute to the observa- Double and triple chain benzylether 2 and 3 are collated in
tion that the enthalpy values of the Sm–Iso transition of the glycerol Table 4. Except for compound 314EO3OH, other compounds
substituted compounds are only one half or even less than those show exclusively hexagonal columnar phases (Colhex). In general
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the stability of LC phases increases with increasing number of hexagonal lattice with p6mm symmetry (Fig. 5d and Fig. S2c, ESI†).
OH groups. The Colhex–Iso transition enthalpies are in the range The 2D pattern of an aligned sample of compound 312(OH)2 also
between DH = 0.4 and 1.5 kJ molꢀ1, which is about one order of confirms this phase assignment (Fig. 5e and Fig. S2b, ESI†).
magnitude smaller than those for the Sm–Iso transitions of Although SAXS was not carried out on all of the columnar phases,
compounds 1, and there is obviously no dependence of the based on textural similarities (Fig. S1, ESI†), it is very likely that the
Col–Iso transition enthalpy on the number of OH groups.
columnar phases of the other compounds represent the hexagonal
There is a strong effect of the number of alkyl chains, columnar phases too. The number of molecules organized in a slice
reducing the mesophase stability with growing chain number of the columns with a height of h = 0.45 nm (a typical value for the
(compare compounds 214(OH)2 and 314(OH)2 in Table 4). In maximum of the diffuse wide angle scattering) ncell was estimated
contrast, the effects of the EO unit between the azobenzene core using eqn (1) and assuming a density of r = 1 g cmꢀ3 (NA
=
and the glycerol unit on melting temperatures and mesophase Avogadro constant; M = molecular mass, see Table 4). The number
stability are only marginal (compare compounds 3nEO3(OH)2 decreases from ncell = 13 for the two-chain compound 214(OH)2 to
and 3n(OH)2). The textures of the columnar mesophases, as ncell = 10 for the three-chain compound 312(OH)2. Introduction of
observed between crossed polarizers, are characterized by the (EO)3 unit at constant alkyl chain volume leads to a slight
‘spherulitic’ domains (see Fig. 5a, Fig. S1a and c, ESI†), com- increase of the number of molecules in the column cross section
bined with regions appearing completely dark, where the optic from ncell = 10 to 11 for compound 312EO3(OH)2.
axis is normal to the LC film (homeotropic regions), see Fig. S1b
pffiffi
ꢁ
ꢀ ꢂ
ncell ¼ a2
2
3hðNA=MÞr
(1)
(ESI†). These textures indicate an optically uniaxial columnar LC
phase The orientation of the p-conjugated aromatic cores is
deduced from the colour of the fans in the micrograph taken
These two- and triple-alkyl chain amphotropic azobenzene
with a l-retarder plate (Fig. 5b and Fig. S1c, ESI†). The directions derivatives can be either considered as taper shaped molecules,40,41
of yellow and blue fans with respect to the indicatrix of the providing a radial organization of the molecules in circular
retarder confirm the orientation of the high-index axis as radial columnar aggregates, thus, directly leading to a packing of the
to the fans. As the columns are tangential and the high-index axis circular columns on a hexagonal lattice (see the model in Fig. 5f),
is parallel to the p-conjugated aromatic cores. This means that or as polycatenar (three- and tetracatenar, respectively) meso-
in the columns the preferred direction of the intra-molecular gens, which can organize into ribbons of antiparallel organized
p-conjugation pathway (i.e. the long axis of the azobenzene cores) molecules, resembling the organization in the smectic phases of
is on average perpendicular to the column long axis, in line with the single chain compounds (see the model in Fig. 5h).42,43 In
the proposed mode of self-assembly (see below).
the latter case the time and space averaging of the ribbon local
The columnar phases of the double chain glycerol substituted orientation would lead to the globally averaged hexagonal lattice.
compound 214(OH)2, the three chain compound 312(OH)2 and Probably the truth is somewhere in the middle, i.e. the columns
the related compound with additional (EO)3 unit 312EO3(OH)2 have an elliptical shape which retains some ribbon-like organi-
were investigated as representative examples by small-angle zation in the middle and simultaneously the contact between
X-ray scattering (SAXS), and grazing-incidence small-angle X-ray polar groups and aliphatic chains in the periphery is minimized
scattering (GISAXS) on thin surface-aligned films on silicon by fusing the aromatic regions to elliptical shells covering the
using a synchrotron source (see Fig. 5d, e and Fig. S2, ESI†). All polar regions (Fig. 5g). These elliptical columns are then
columnar phases show three or four small angle reflections arranged on a time and space averaged hexagonal lattice. This
with a ratio of their reciprocal spacing 1 : 31/2 : 2 : (71/2) which model is in line with the reconstructed electron density maps
can be indexed to the 10, 11, 20 (and 21) reflections of a (see Fig. 6a and c) where the column centers with high electron
Table 4 Mesophases, transition temperatures (T/1C) and transitional enthalpy values (DH/kJ molꢀ1) of the multi-chain compounds 2 and 3a
Comp
R1
R2, R3
m
Phase transitions
a/nm (T/1C)
ncell
214(OH)2
H
OC14H29
OC12H25
OC14H29
OC12H25
OC14H29
OC12H25
OC14H29
0
0
0
3
3
3
3
Cr 115 [25.1] Colhex 165 [1.5] Iso
Cr 59 [34.4] Colhex 119 [0.7] Iso
Cr 64 [36.7] Colhex 115 [0.4] Iso
Cr 74 [57.9] (Colhex 70 [0.4]) Iso
Cr 78 [55.3] Isob
Cr 66 [49.6] Colhex 115 [0.9] Iso
Cr 71 [39.5] Colhex 116 [0.5] Iso
6.65 (140)
6.20 (90)
13.0
9.6
3
3
3
3
12(OH)2
OC12H25
OC14H29
OC12H25
OC14H29
OC12H25
OC14H29
14(OH)2
12EO3OH
14EO3OH
312EO3(OH)2
7.17 (90)
11.3
3
14EO3(OH)2
a
b
Abbreviations: Colhex = hexagonal columnar phase with p6mm symmetry; for conditions and other explanations see Table 1. No LC phase was
observed upon cooling before crystallization at T = 62 1C.
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Fig. 6 Electron density maps of the Colhex phases of (a) compound
312(OH)2 and (c) compound 312EO4(OH)2 as reconstructed from the
synchrotron powder diffraction patterns (color code: purple/blue = high,
green/yellow = medium and red = low electron density; for details of the
reconstruction, see the ESI†); (b) snapshot of the structure of the Colhex phase
of 312(OH)2 and (d) of 312EO3(OH)2 after MD annealing (polar groups are
colored purple and the aromatic cores green, alkyl chains gray); red and yellow
circles represent the boundaries between high and medium and medium and
low electron density regions in the electron density maps, respectively.
organized in the cross section of the columns (ncell = 10) compared to
the double chain compound 214(OH)2 (ncell = 13). Enlargement of the
polar group by the (EO)3 chain increases the polar volume and this
reduces the interface curvature, thus leading to a slight increase in
the number of molecules organized in the cross section of the
columns (ncell = 11 for 312EO3(OH)2).
Fig. 5 Investigation of compound 312(OH)2 (a) texture of the columnar
phase at T = 90 1C as observed between crossed polarizers; (b) shows a
section of the texture with an additional l-retarder plate and the orienta-
tion of the indicatrix of the l-plate is shown in (c); (d) powder X-ray
diffraction pattern at T = 90 1C and (e) GISAXS pattern of a surface-aligned
sample at T = 90 1C; (f–h) models showing the organization of the
amphiphilic molecules, (f) in circular columns, (g) in elliptical columns
and (h) in ribbons; for more details, see Tables S4–S7 and Fig. S2 (ESI†).
In Fig. 6 the reconstructed electron density maps of the
Colhex phases of compounds 312(OH)2 and 312EO3(OH)2 (left)
are compared with the molecular dynamics (MD) annealed
models of the columnar phases (right). The MD annealed
model of the columnar phase of 312EO3(OH)2 (Fig. 6d) indicates
that the polar chains form the majority of the column cores and
the azobenzenes form a kind of shell separating the polar core
from the aliphatic continuum, in which the alkyl chains are
completely disordered and partly interdigitated. The formation
of this kind of aromatic shell is favored by the possibility of
hydrogen bonding of the OH groups to the azo groups, to the
electron rich aromatic p-systems and to the ether oxygens at the
aromatics. The MD annealed model of the columnar phase of
312(OH)2 (Fig. 6b) without EO units shows better defined shells
of the aromatic cores with a larger degree of parallel alignment
of the cores around the smaller core regions involving the polar
groups (structure corresponding to the model in Fig. 5g).
density (purple) are surrounded by medium electron density shells
(blue/green/yellow). The medium electron density shells might result
from the rotational disorder of the elliptical columns leading to a
space and time averaging of the high electron density polar cores
and the low electron density alkyl chains in the continuum around
the columns. In the column cores only the aromatic/polar cores of
the elliptical columns overlap on the space and time average, leading
to high electron density. Independent of the precise shape of the
columns the rigid aromatics, the polar groups (polyether and
glycerol unit) and the non-polar alky chains segregate into distinct
domains. The polar units form the column centers, which are
2.4 Effects of solvents and lyotropic self-assembly
surrounded by the azobenzene cores and assembled on a hexagonal The effect of protic solvents on the phase behavior of 112EO3OH
lattice within the lipophilic continuum mainly formed by the alkyl as a representative example was investigated by POM and XRD.
chains. The three alkyl chains of 312(OH)2 provide a stronger inter- By adding ethylene glycol, the SmC phase of this compound is
face curvature leading to a 30% decrease of the number of molecules first stabilized, but already at nEO B 1 (nEO = n(EO)/n(112EO3OH))
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the stability of the SmC phase decreases and a SmA phase occurs
(Fig. 7). With further increase in solvent concentration, the SmC–
SmA transition temperature decreases whereas the SmA phase
stability increases. The SmC phase exhibits a typical schlieren texture
as observed for the dry compound and the SmA phase shows oily
streaks and homeotropic areas as characteristic of lyotropic SmA
phases (Fig. 8). At nEO B 4 the maximum of the phase stability is
reached and with a further increase in solvent concentration the
transition temperatures slightly decrease up to nEO B 11 which
represents the maximum uptake of the solvent (Fig. 7). After
reaching this concentration the system remained heterogeneous
with excess solvent coexisting with the lyotropic LC system.
The system was in addition investigated by XRD for the
concentrations nEO B 1 (B10 wt% EO), nEO B 3 (B30 wt% EO)
and nEO B 10 (B50 wt% EO). The XRD patterns showed the typical
diffraction patterns of smectic phases. There is a significant
Fig. 8 Texture of the solvent induced lyotropic SmA phase of 112EO3OH
increase of the d-spacings with increasing solvent content, in line at T = 105 1C in a mixture with ethylene glycol (EO) at nEO = 10.
with a swelling of the polar layers by the added solvent molecules.
Remarkably, the layer shrinkage at the SmA–SmC transition and in
the temperature range of the SmC phase increases with increasing
solvent content. In contrast, the layer spacing d remains nearly
constant in the SmC phase region of the solvent-free sample (see
Fig. 9). This could have different reasons, for example, the solvation
of the head groups could change, i.e. the number of molecules
phase was induced and an SmC–SmA–Iso phase sequence was
found for the solvent enriched samples. For formamide a bit higher
transition temperatures were observed for the solvent saturated
sample (Cr 77 1C SmC 90 1C SmA 135 1C Iso), compared with EO,
whereas for water the observed transition temperatures are lower
(Cr 78 1C SmC 80 1C SmA 97 1C Iso for the water saturated
separating the head groups laterally increases and simultaneously
sample). This is a bit surprising, as usually water provides the
the number of solvent molecules arranged between the layers
most efficient polar protic solvent, leading to the highest lyotropic
decreases with decreasing temperature. This could further lead to
a stronger intercalation of the alkyl chains and an increase of the
uniform (SmC) or average tilt (SmA) of the aromatic at lower
temperature. The combined contribution of these effects is thought
to lead to the observed effect.
A similar behaviour to that with EO was also observed using
water and formamide, which were investigated by contact prepara-
tion. For this purpose the amphiphile and the solvent were placed
side-by-side between two glass plates and the phases developing in
the contact region due to the developing concentration gradient
phase stabilities.5
Additional compounds were investigated using formamide
as the protic solvent. In the case of 112EO3(OH)2 with an
additional glycerol group there is an even stronger stabilization
of the SmA phase, reaching about 180 1C for the formamide
saturated sample. This formamide saturated sample crystal-
lizes at 68 1C without formation of a SmC phase, i.e. there is a
stronger destabilization of the tilted SmC phase. Compound
312(OH)2 with a glycerol unit but without the EO segments
were observed under a polarizing microscope. In all cases a SmA
Fig. 9 Temperature dependence of the layer spacing depending on the
EO content of the system 112EO3OH/ethylene glycol (EO).
Fig. 7 Phase diagram of the system 112EO3OH/ethylene glycol (EO).
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behaves similar, the SmA phase being stabilized to 4200 1C, nearly parallel to the column surface form a relatively thin
but the formamide saturated sample rapidly crystallizes already shell. This might reduce the sensitivity to steric distortions of
at 123 1C before the transition of the SmA to the SmC phase can columnar self assembly and could lead to the very special
take place.
The lyotropic behavior of the three chain compounds is very
behavior of this compound.
Investigation of aqueous systems was more difficult due to
different and for these compounds a strong dependence of easy water evaporation, but in principle the behaviour can be
the phase sequence on the type of polar group is observed. considered as similar to formamide. So the Colhex phase of
Compounds 312EO3(OH)2, 312EO3OH and 312(OH)2 having identical 312EO3(OH)2 is retained in the contact region in the whole
alkyl chain lengths but different types of polar groups were temperature region available for investigation, whereas for
investigated as representative examples using formamide as the 312EO3OH and 312(OH)2 the columnar phases are removed for
polar protic solvent (contact preparations). The columnar phases of the water saturated samples. It appears that for 312(OH)2 an
312(OH)2 and 312EO3OH, having either the glycerol group or the additional lamellar phase is induced in the water saturated
triethyleneglycol chain, were completely removed in the contact sample with a clearing temperature of 71 1C, but this is not
region and only crystallization was observed at T = 50 1C for both completely certain due to the above mentioned difficulties in
compounds. In contrast, for compound 312EO3(OH)2, combining handling these amphiphile water systems.
the EO chain with the glycerol group, a dramatic stabilization of the
Colhex phase was observed (see Fig. 10), it is retained up to the
2.5 Photoisomerization behavior
boiling point of this solvent at 210 1C. It appears that the molecular All compounds 1–3 exhibit the expected reversible photore-
organization in columns with a polar interior restricts the effect of sponsive behavior (see Fig. 11). All azobenzene derivatives in
the solvent on the self assembly. In the case of relatively small polar the E-form show a strongly absorbing band in the UV region
groups the volume added by the solvent molecules to the polar (B360 nm) which is attributed to the p–p* transition, and a
groups distorts the formation of columnar aggregates, but it weakly absorbing band in the visible region (B450 nm) due to
appears to be too small for a transition to a lamellar organization; the n–p* transition. The E-form is generally more stable than
therefore order is lost and an isotropic liquid is formed. For the Z-form, but each isomer can be converted into the other by
compound 312EO3(OH)2 with the largest polar group, involving a light-irradiation caused a decrease of the absorption at around
strong hydrogen bond donor group (the glycerol units) and having 355 nm and an increase of the absorption at around 450 nm.
several additional hydrogen bonding acceptor sites along the For example, dark incubation of a dichloromethane solution
EO chains, a small number of formamide molecules might be (1 ꢁ 10ꢀ5 M) of 112EO3OH have a maximum absorbance at
sufficient to dramatically increase the number of attractive 358 nm corresponding to the E-azobenzene chromophore
hydrogen bonds, thus stabilizing the columnar aggregates with- (p–p* transition). Upon irradiation of this solution with 365 nm
out requiring too much additional space, and thus retaining the light resulted in photoisomerization from the E-azobenzene to
interface curvature. Moreover, as shown in Fig. 6d, the major the Z-azobenzene chromophore, as evidenced by a decrease in
part of the columns is formed by the polar groups, i.e. polar– absorbance at 358 nm and an increase in absorbance at 450 nm
apolar segregation is dominating the behaviour of 312EO3(OH)2. (n–p* transition). Also the absorbance at 313 nm (p–p* transi-
The azobenzenes being strongly tilted and some of them organized tion) increased. The E–Z transition was complete in 35 s (see
Fig. 11a). Under visible light, the reverse E–Z transition process
was achieved within 6 min (see Fig. 11b).
The effect of E-to-Z photoisomerization caused by the UV
irradiation on the thermotropic LC phases was investigated
using compound 112EO3OH as an example. The sample was
placed between two thin quartz glass plates to form a thin film
and placed on a Linkam hot stage that can control the sample
temperature within 0.1 1C. The sample was cooled from the
isotropic liquid state to 98 1C in the range of the SmCd phase
(see Fig. S6a, ESI†). After annealing for 5 minutes the sample
was exposed to UV irradiation at 365 nm (5 mW cmꢀ2) for 60 s.
After B20 s exposure time the schlieren texture of the SmCd
phase has changed to a plain fan texture separated by home-
otropic areas, indicating that the SmCd phase is replaced by a
SmA phase (most probably SmAd; see Fig. S6b and c, ESI†).
After thermal annealing at the same temperature of 98 1C
without irradiation a slow change in the texture back to the
SmCd phase took place, indicating a thermal Z–E isomerization
Fig. 10 Texture of the solvent stabilized lyotropic Colhex phase of
(see Fig. S6d and e, ESI†). So the E–Z photoisomerization allows a
reversible switching between SmC and SmA at a fixed temperature.
This is due to a change in the phase transition temperatures.
312EO3(OH)2 (right) at T = 170 1C in the contact region with formamide
(left) as observed upon cooling, the inset shows the texture with the
additional l-plate.
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group form exclusively the SmCd phase whereas a SmCd/SmAd
dimorphism was observed for most of the related diols. SmA
phases were induced and stabilized by addition of protic solvents.
This means that increased head group size favors SmA phases, but
nevertheless broad regions of lyotropic SmC phases were retained.
Such SmC phases are rarely observed in lyotropic LC phases and
these materials are of interest as lyotropic ferroelectrics after
introduction of chirality.10b The SmC–SmA transition can be con-
sidered as mainly resulting from the randomization of the tilt by
the lateral separation of the molecules by the solvent molecules
located around the polar groups. The increased effective polar
group volume also reduces the tilt correlation between the layers.
Compounds with two or three alkyl chains form exclusively
Colhex phases and there is a strong effect of protic solvents,
stabilizing Colhex phases of compounds combining EO chains
with glycerol units and removing columnar phases for com-
pounds having only one of them, either EO chains or glycerol
units. All amphiphiles show photoresponsive behavior, which
makes these compounds interesting for potential applications
as photoresponsive amphiphiles and LCs.
Acknowledgements
This work was supported by NSFC (No. 21364017, 21274119,
21374086), YNSF (2013FA007), YED (ZD2015001), DFG FOR
1145 (Ts 39/21-2). MP acknowledges support from the Cluster
of Excellence ‘‘Nanostructured Materials’’ of the government of
Saxony-Anhalt and JRB acknowledges financial support from the
¨
‘‘Landesgraduiertenforderung’’ of the state Baden-Wu¨rttemberg.
Fig. 11 UV spectra of 112EO3OH in CH2Cl2 solution (c = 10ꢀ5 mol Lꢀ1),
changing with time: (a) from E- to Z-azobenzene under irradiation with
365 nm light; (b) from Z- to E-azobenzene under irradiation with visible
light; for additional examples, see Fig. S3 and S4 (ESI†).
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