Physical gels made of liquid crystalline B4 phase

Article abstract of DOI:10.1039/c3cc41225c

The achiral liquid crystalline materials showing two B₄ (HN) phases have been found to exhibit strong gelation ability for various organic solvents with reversible sol-gel phase transition. The gel is formed by helical tubules, which build entangled 3D network, encapsulating the solvent. The equilibrium of left- and right-handed tubules is preserved in the gel, even if the chiral solvent is used.

Full text of DOI:10.1039/c3cc41225c

ChemComm
View Article Online
View Journal
COMMUNICATION
Physical gels made of liquid crystalline B4 phase†
a
a
Anna Zep, Miroslaw Salamonczyk, Natasa Vaupotic, Damian Pociechaa and
ˇ
ˇ bc
Cite this: DOI: 10.1039/c3cc41225c
Ewa Gorecka*a
Received 15th February 2013,
Accepted 27th February 2013
DOI: 10.1039/c3cc41225c
www.rsc.org/chemcomm
The achiral liquid crystalline materials showing two B4 (HN) phases
have been found to exhibit strong gelation ability for various
organic solvents with reversible sol–gel phase transition. The gel is
formed by helical tubules, which build entangled 3D network,
encapsulating the solvent. The equilibrium of left- and right-handed
tubules is preserved in the gel, even if the chiral solvent is used.
Fig. 1 General molecular structure of the studied dimers; phase transition
temperatures (1C) and enthalpy changes (J gꢀ1) are also given.
The chemical and physical organogels are formed by the self-
assembly of organic molecules into a 3D fiber network that
entraps different types of organic solvents. The fibers of chemical having a well-defined diameter.14 Recently it has been shown
gels are built of covalently bonded polymers. In the case of the that the filament morphology is preserved even if the B4
physical gels the fibrous aggregates are formed by low mass material is mixed with mesogens forming nematic, smectic or
molecules connected through the non-covalent interactions such columnar phases.15–17
as hydrogen bonding,1–5 p–p,6 lipophilic,7 dipole–dipole inter-
Formation of the gel seems to be a general feature of the B4
actions,8 donor–acceptor exchange,9 etc. In recent years growing materials, independently of their molecular architecture. Here
interest in gelators with liquid crystalline properties has been we describe the gelation ability of symmetric mesogenic dimers
observed. There are several examples of mesogenic materials that (Fig. 1) made of 4-decyloxy-40-hydroxybiphenyl units connected
exhibit gelation abilities in organic solvents. Mamiya et al. reported by flexible alkyl spacers with an odd number of carbon atoms
that chiral azobenzene derivatives, which exhibit smectic phases, (synthesis described in ESI†).18 These materials are thermotropic
are capable of forming fibrous aggregates with smectic order.8 liquid crystals, exhibiting two B4-type phases.
Hashimoto et al.,10 Isoda et al.11 and Yoshio et al.12 described the
The LC properties for all the studied homologues are presented
liquid-crystalline disc-like compounds that form physical gels in in Fig. 1. The formation of the B4-type phases was confirmed by
organic solvents with fibers exhibiting columnar structure. Here we characteristic optical textures, with low birefringence and large
report a new type of mesogenic gelators, which give the B4 phase in optically active domains observed when the sample is cooled
the bulk sample. The B4 phase is one of the most unusual liquid from the isotropic phase. The optical texture was not affected by
crystalline (LC) phases; it draws a lot of attention because of its the B4–B40 phase transition. There were no noticeable changes either
chiral nature and strong optical activity, despite the fact that it is in the optical activity or in the domain shape or size. In both phases
built of achiral molecules.13 The structure of the phase was the X-ray diffraction measurements revealed a robust lamellar
attributed to the local saddle-splay curvature of membranes, made structure evidenced by up to 7 harmonics of the main signal, related
of a few molecular layers. Such elastic deformation leads to helical to the layer thickness (Fig. 2). Significant broadening of these signals
nanofilaments (HN), i.e. twisted ribbons, with single filament indicates a finite size of a lamellar membrane, from which filaments
of the B4 phase are formed (line broadening due to a small size of
crystallites19); the correlation length along the layer normal, deter-
a
˙
Department of Chemistry, University of Warsaw, Zwirki i Wigury 101,
02-089 Warsaw, Poland. E-mail: gorecka@chem.uw.edu.pl
mined from diffraction signals width, is only 3–4 layers. The
reflections observed at high angles confirm that molecules exhibit
long range order within layers (Fig. 2).
Signals in the diffraction pattern for compound n = 9 in the
B4 phase can be fitted to the monoclinic crystallographic lattice
with parameters a = 9.30 Å, b = 5.35 Å, c = 56.7 Å and g = 1101.
b Department of Physics, Faculty of Natural Sciences and Mathematics, University of
ˇ
Maribor, Koroska 160, 2000 Maribor, Slovenia
c
ˇ
Jozef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia
† Electronic supplementary information (ESI) available: Experimental details,
synthesis of materials and their spectral characterization. See DOI: 10.1039/
c3cc41225c
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.

View Article Online
Communication
ChemComm
Table 1 Sol–gel phase transition temperatures for dimer n = 9 (B5%) in
different organic solvents. In the table only phase transition temperatures are
given (taken as peak position in heating runs), as the enthalpies (B80 J gꢀ1) are
measured with large error due to some evaporation of the solvent at elevated
temperatures, despite the sealing of the samples
Sol–gel phase transition
temperature (1C)
Solvent
Toluene
Nitrobenzene
(R)-(+)-3-Methylcyclohexanone
(+)-Menthone
(ꢀ)-Menthone
75.0
85.0
81.0
86.0
86.0
0
Fig. 2 X-ray pattern of the B4 and B4 phase, xerogel and gel of compound with
n = 9 with toluene.
Since a/b = 1.73, one can deduce that molecular mass centres form
nearly ideal hexagons in the smectic planes; the molecules are tilted
to the next nearest neighbour. The observed hkl signals with l = 1
and 2 indicate the presence of some interlayer correlations. The
0
structure of the B4 phase is similar to the B4 phase; the same layer
thickness is observed. The changes in the high angle signals show
that the phases differ in their in-plane order of molecules.
All the studied compounds were excellent gelators for various
organic fluids: toluene, nitrobenzene, 3-methylcyclohexanone,
(ꢀ)-menthone, (+)-menthone, chloroform and dichloromethane,
already at concentrations ca. 5 wt%. No gelation was observed for
polar organic solvents (DMF, ethanol, methanol). The gels were
formed when the mixture of LC and solvent is heated and subse-
quently cooled to room temperature; it should be stressed that
gelation occurred in the whole volume of the sample, no precipita-
tion of crystallites from solution was observed (Fig. 3). The gel and
sol states are thermo-reversible with the phase transition tempera-
ture dependent on the organic solvent (Table 1). The sol–gel phase
transition temperatures monitored optically coincide with those
detected by the DSC method (Fig. 4) and are much lower than the
B4-Iso phase transition in pure materials.
Fig. 4 DSC scans (two consecutive runs, ꢁ10 deg minꢀ1) showing reversible
sol–gel transition for dimer n = 9 in (R)-(+)-3-methylcyclohexanone.
helical tubules. The gel is formed by encapsulating the solvent
within the network of percolated gelator and inside the tubes.
Both right- and left-handed helical tubules are present; the
equilibrium between the helices of opposite sense is not broken
even if an optically pure solvent (menthone) is used. The average
diameter, d, of the nanofilament is 50 nm, and the helical pitch,
L, B40–50 nm, which gives the aspect ratio d/L B 1; the values
were not dependent on the homologue used for gel preparation.
The X-ray pattern of the gel (as well as dried xero-gel) (Fig. 2) is
0
similar to the X-ray pattern of the B4 phase. It shows signals
The morphology of gels was examined by the scanning
electron microscopy (SEM). The SEM picture (Fig. 3) revealed
formation of a three-dimensional network composed of long,
entangled fibres; the filaments are made of tightly winded,
coming from the lamellar structure and a few reflections at high
angles. The X-ray signals from the crystal structure disappear at the
gel–sol phase transition, so above Tc only a diffused signal from the
solvent is observed. On subsequent cooling below the sol–gel phase
0
transition temperature the X-ray pattern of the B4 phase is
recovered. Therefore, we can conclude that the fibres forming gels
0
have the structure of the bulk B4 phase. This also shows that the
material which forms tubules is not diluted by the solvent.
The morphology of the gel was compared with that observed
0
for the bulk material. The AFM picture of the B4 phase (Fig. 5)
shows strongly distorted twisted ribbons, made of membranes
5–6 layers thick, with the orientation of the layer normal
perpendicular to the local twist axis. The layer thickness
estimated from the AFM picture is consistent with the spacing
measured by the X-ray method. The AFM picture seems to confirm
the saddle-splay helical structure of the B4 phase, proposed by the
Boulder group.14
Fig. 3 SEM image of xero-gel formed from compound n = 7 in (ꢀ) menthone,
after evaporation of solvent; enlarged picture shows helical tubular filaments.
Photos of material in vial indicate that sol and gel states have similar volumes.
The formation of the gel was also checked for a few other bent
core materials (see ESI†). Gel was formed for the compounds
c
Chem. Commun.
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
would be expected. Alternatively, left- or right-handed tubules can
be obtained from the membranes when they try to accommodate
the spontaneous splay of polarization, which is an inherent
property of the bent-core liquid crystals and is promoted by the
surfaces (thus by a finite width of the ribbon). A similar effect,
i.e. the variation in the tilt direction in smectic C phase was shown
to promote the formation of cylindrical tubules and helical
ribbons of chiral lipid membranes.21,23
ˇ
ˇ
The authors wish to thank Prof. M. Cepic for all the discussions
and acknowledge the FNP Project TEAM/2010-5/4, Self-assembly
of functionalized inorganic–organic liquid crystalline hybrids for
multifunctional nanomaterials, for financial support. Special
thanks to Dr E. Bialecka-Florianczyk for drawing our attention
9 at room to this class of dimeric mesogens.
0
Fig. 5 AFM picture of the B4 phase of compound with n =
temperature.
Notes and references
1 K. Hanabusa, M. Yamada, M. Kimura and H. Shirai, Angew. Chem.,
Int. Ed. Engl., 1996, 35, 1949.
exhibiting the B4 (HN) phase, while it was not observed for
materials showing the dark conglomerate (DC) phase. The DC
phase exhibits non-birefringent but optically active texture
similar to the B4 phase; however its X-ray pattern is typical
for smectics with liquid like order inside the smectic layers; for
this phase a sponge like structure with saddle-splay curvature
was assumed.20 Thus it seems that for this class of compounds
the crystalline character of the layers is an important factor for
the formation of filaments with gelation ability.
2 J. van Esch, F. Schoonbeek, M. de Loos, H. Kooijman, A. L. Spek,
R. M. Kellogg and B. L. Feringa, Chem.–Eur. J., 1999, 5, 937.
3 K. Yoza, N. Amanokura, Y. Ono, T. Akao, H. Shinmori, M. Takeuchi,
S. Shinkai and D. N. Reinhoudt, Chem.–Eur. J., 1999, 5, 2722.
4 A. Aggeli, M. Bell, N. Boden, J. N. Keen, P. F. Knowles, T. C. B.
McLeish, M. Pitkeathly and S. E. Radford, Nature, 1997, 386, 259.
5 R. J. H. Hafkamp, M. C. Feiters and R. J. M. Nolte, J. Org. Chem.,
1999, 64, 412.
6 F. Placin, M. Colomes and J.-P. Desvergne, Tetrahedron Lett., 1997,
38, 2665.
7 K. Murata, M. Aoki, T. Suzuki, T. Harada, H. Kawabata, T. Komori,
F. Ohseto, K. Ueda and S. Shinkai, J. Am. Chem. Soc., 1994, 116, 6664.
8 J. Mamiya, K. Kanie, T. Hiyama, T. Ikeda and T. Kato, Chem.
Commun., 2002, 1870.
9 D. Rizkov, J. Gun, O. Lev, R. Sicsic and A. Melman, Langmuir, 2005,
21, 12130.
In summary, there are two basic types of a helical filament
structures that can accommodate the local twist of smectic
layers: twisted ribbons and helical tubules.21 The theoretical
model, proposed for amphiphilic systems, predicts that the
helical tubular vs. twisted ribbon morphology should be biased by
10 M. Hashimoto, S. Ujiie and A. Mori, Adv. Mater., 2003, 15, 797.
the in-plane crystallinity of the membrane, from which filaments 11 K. Isoda, T. Yasuda and T. Kato, J. Mater. Chem., 2008, 18, 4522.
are made.22 This is apparently not the case for the system studied
here, as both types of filaments, tubules and ribbons are formed
12 M. Yoshio, R. Konishi, T. Sakamoto and T. Kato, New J. Chem., 2013,
37, 143.
13 T. Sekine, T. Niori, J. Watanabe, T. Furukawa, S. W. Choi and
from smectic with crystal-like in-plane order. In the studied system
twisted ribbons are formed in bulk (B4 phase) while in solvent
helical tubules are favored. This shows that the surrounding is of
primary importance for the morphology of the objects. The result
can be justified if one notices that the formation of tubules in bulk
material would require molecules in the center of the tube being in
a molten state (in the nematic or isotropic phase); that would cost
the system some additional energy; while in the liquid solvent the
inner part of a tube can be filled with the molecules of the solvent.
Additionally, tubes are stabilized in solutions, since the membrane
edge exposed to the solvent is much smaller for tubes than for
twisted ribbons. This mechanism should be especially effective in
H. Takezoe, J. Mater. Chem., 1997, 7, 1307.
14 L. E. Hough, H. T. Jung, D. Kruerke, M. S. Heberling, M. Nakata,
C. D. Jones, D. Chen, D. R. Link, J. Zasadzinski, G. Heppke,
J. P. Rabe, W. Stocker, E. Korblova, D. M. Walba, M. A. Glaser and
N. A. Clark, Science, 2009, 325, 456.
15 T. Otani, F. Araoka, K. Ishikawa and H. Takezoe, J. Am. Chem. Soc.,
2009, 131, 12368.
16 D. Chen, M. S. Heberling, M. Nakata, L. E. Hough, J. E. Maclennan,
M. A. Glaser, E. Korblova, D. M. Walba and N. A. Clark, ChemPhysChem,
2012, 13, 155.
17 D. Chen, C. Zhu, H. Wang, J. E. Maclennan, M. A. Glaser,
E. Korblova, D. M. Walba, J. A. Rego, E. A. Soto-Bustamante and
N. A. Clark, Soft Matter, 2013, 9, 462.
18 E. Bialecka-Florianczyk, I. Sledzinska, E. Gorecka and J. Przedmojski,
Liq. Cryst., 2008, 35, 401.
19 P. Scherrer, Goettingen Nachr. Ges., 1918, 2, 98.
systems in which the edges are polar. In isotropic fluid tubular 20 D. Chen, M. S. Heberling, M. Nakata, L. E. Hough, J. E. Maclennan,
M. A. Glaser, E. Korblova, D. M. Walba and N. A. Clark, Chem-
PhysChem, 2012, 13, 155.
21 J. V. Selinger, M. S. Spector and J. M. Schnur, J. Phys. Chem. B, 2001,
filaments easily percolate trapping the solvent, forming the gel state.
It was suggested that the helix sense of ribbons is related to the
symmetry breaking at the molecular level – formation of chiral
conformers of achiral bent-core molecules.14 However, helices of
both handedness are observed in the chiral solvent, although in
such an environment stabilization of one of the chiral conformers
105, 7157.
22 R. Oda, I. Huc, M. Schmutz, S. J. Candau and F. C. MacKintosh,
Nature, 1999, 399, 566.
23 J. V. Selinger, F. C. MacKintosh and J. M. Schnur, Phys. Rev. E, 1996,
53, 3804.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun.