Hydrogen-bonded banana liquid crystals

Article abstract of DOI:10.1002/anie.200460549

Top banana! Hydrogen-bonded bent complexes of benzoic acids (H donor) and nonmesomorphic V-shaped 4′-stilbazoles (H acceptor) give rise to polar smectic C (SmCP, see graphic) mesophases. The multifunctional character of these noncovalent materials is conf

Full text of DOI:10.1002/anie.200460549

Angewandte
Chemie
Liquid Crystals
interactions? On the basis of the following results, the answer
to this question is a resounding yes!
Hydrogen-Bonded Banana Liquid Crystals**
Having taken into account the structural molecular
requirements that 1) govern these new macroscopic liquid-
crystal phase arrangements[5] and 2) allow appropriate
H bonded complexation, we focused our interests on the V-
shaped H acceptor structures 1 and 2 shown in Scheme 1 and
NØlida Gimeno, Maria Blanca Ros,* JosØ Luis Serrano,*
and Maria Rosario de la Fuente
There are numerous examples in nature that demonstrate the
crucial role of hydrogen bonds. Through imitation of this
phenomenon, hydrogen bonds have been used to good effect
by researchers to achieve specific synthetic targets.[1] Fields
such as supramolecular chemistry and materials science are
indebted to this strategy,[2] and important breakthroughs have
been achieved by exploiting these inter- or intramolecular
“strong and directional, and… lovely”[3] interactions. Liquid
crystals, which are related to both of the aforementioned
fields, are another example of the attractive possibilities of
this interaction.[4] Mesomorphic properties result from a
suitable combination of the shape of a molecule and the
magnitude and direction of interactions between the mole-
cules. Hydrogen bonding, through self-assembly, has been
used to order thermotropic and lyotropic liquid crystals in
which the magnitude and the direction of the interactions
have been appropriate to maintain order within the fluid
state. Calamitic and columnar mesophases of low or high
molecular-weight materials have been stabilized with this
approach. Such systems have also been obtained by the use of
either mesogenic or non-liquid-crystalline moieties.
In 1996, a new type of mesogenic material appeared in the
field of liquid crystals: the so-called “banana-compounds”.[5]
These mesomorphic materials are of interest from both an
academic and a practical point of view.[6] These systems form a
distinct class of liquid-crystalline compounds as they give rise
to new types of mesophases that do not have analogues
among classical calamitic phases. Interestingly, some of these
compounds exhibit the unique feature of forming polar
ordered mesophases with achiral molecules to provide anti-
ferroelectric, ferroelectric or nonlinear optical responses—
often with exceptional values for the relevant parameters.[7]
Herein, we report a study that addresses a pertinent question
that has not successfully been answered to date: is it possible
to stabilize this type of mesophase through hydrogen bonding
Scheme 1. General structures of the H acceptors (1 and 2) and H do-
nors (3 and 4).
two benzoic acid derivatives of varying lengths as the H donor
moieties 3 and 4. The V-shaped 4’-stilbazoles (1 and 2) were
prepared according to the synthetic route outlined in
Scheme 2 (see also Supporting Information), and the acids
(3 and 4) were prepared according to literature methods.[8]
The synthesis of the desired bent complexes required the two
components (H donor and H acceptor) to be mixed in precise
equimolecular proportions in a common solvent (THF)
followed by removal of the solvent. The formation of the
complex was easily confirmed through polarizing optical
microscopy measurements; the solid samples melted cleanly
without the appearance of biphasic regions, which would
otherwise have indicated the presence of nonstoichiometric
complexes.
Despite the nonmesomorphic nature of both of the
H acceptor compounds studied, all of the complexes were
liquid crystalline over temperature ranges that were different
to the ranges at which the carboxylic acids displayed calamitic
phases (Table 1). More interestingly, the complexes exhibit
textural features which are identical to those reported[8,9] and
observed by us for SmCP (smectic C polar) mesophases. A
schlieren texture and highly birefringent domains were
observed upon cooling the sample from the isotropic liquid
(Figure 1a). From a structure–activity point of view, the larger
the number of aromatic rings, the broader and more stable the
mesophases are. Furthermore, hysteresis of the solidification
processes of around 20 degrees was also observed.
[*] N. Gimeno, Dr. M. B. Ros, Prof. J. L. Serrano
Dpto. Química Orgµnica, Facultad de Ciencias–ICMA
Universidad de Zaragoza–CSIC, 50009-Zaragoza (Spain)
Fax: (+34)976-761209
E-mail: bros@unizar.es
joseluis@unizar.es
Prof. M. R. de la Fuente
Dpto. Física Aplicada II, Facultad de Ciencias
Universidad del País Vasco, 48080-Bilbao (Spain)
Fax: (+34)94-664-8500
[**] This work was supported by the Spanish Government (project
CICYT MAT2003-07806-C02), the European Union (FEDER), and the
Government of Aragón.
The stability of the hydrogen bonding that leads to these
banana complexes was also examined. The IR spectra of these
materials (KBr pellets) show features that are characteristic
of pyridine–carboxylic acid complexes.[4b,10] Thus, the forma-
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200460549
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tion of the H bond is detected through the
À
modification of the O H stretching band
of the acid. The complexes exhibit two
absorptions at ꢀ 2500 and 1950 cmÀ1
À
which correspond to the O H···N(Py)
interaction, whereas carboxylic acid
À
dimers exhibit
a
band (O H) at
ꢀ 2650 cmÀ1. On the other hand, an
absorption band at 1683 cmÀ1 which also
corresponds to the acid dimers was not
detected for the bent complexes. Further-
more, variable-temperature IR spectra
were recorded for all of the complexes
and these reveal that a carbonyl band,
which arises from the dissociation of the
hydrogen bonding, is not observed in
either the crystalline phase or the meso-
phase.
The X-ray diffraction patterns of the
three complexes in their mesophases are
very similar. In the wide-angle region there
is diffuse scattering, whereas in the small-
angle regions there are sharp layer reflec-
tions up to third order (Table 2 and
Figure 2). This indicates well-defined
layer structures for the liquid-crystalline
phase in all cases; that is, the complexes
arrange themselves in smectic phases with-
out in-plane order. The estimated length of
the “molecules” (67–76 Š) is larger than
the interlayer distances measured and
indicates tilted arrangements of the com-
plexes in the liquid-crystalline phase with
angles of around 39–458.
An attractive feature of the SmCP
mesophase is its switching response under
electric fields.[5b,6,7a,9] This behavior was
investigated for our H bonded materials:
5-mm ITO (indium tin oxide)-coated cells
were filled with the samples and their
switching behavior was studied. On slow
cooling from the isotropic liquid, a texture
with focal-conics containing fringe pat-
terns and some circular domains was
perfectly visible. The direction of the
extinction brushes coincides with the
polarizer and the analyzer (see Figure 1b).
Scheme 2. Synthetic routes to the H acceptors 1 and 2. a) DCC, DMAP, CH2Cl2; b) (CH3CO)2O;
c) Pd[P(Ph)3]4, NaHCO3, DME; d) Pd(OH)2/C, cyclohexene, EtOH. DCC=Dicyclohexyl carbodiimide,
DMAP=dimethylaminopyridine, DME=1,2-dimethoxyethane.
Table 1: Thermal properties of the H donors, H acceptors and the
Application of direct current (d.c.) fields causes the brushes to
rotate (see Figures 1c and d). The higher the field, the larger
the tilt of the extinction brushes (with a saturation value
around 458). All these facts are consistent with a SmCP
mesophase in which molecules are organized in layers and
tilted. The tilt alternates from layer to layer to give rise to an
anticlinic and antiferroelectric SmCAPA structure. Under a
high enough d.c. field, the structure becomes a synclinic
ferroelectric SmCSPF structure.[6,9] Moreover, upon applica-
tion of triangular-wave fields, all these complexes show a tri-
stable switching process typical of an antiferroelectric
response. Figure 3 shows a typical plot of the inversion of
complexes studied.
Compounds
Phase transition[a,b] [8C] (DH [kJmolÀ1])
1
2
3
4
1·3
1·4
2·3
K 108.2 (56.2) I
K 168.9 (37.0) I
K 104.6 (53.5) SmC 125.0 (7.9) I
K 101.6 (30.7) SmC 206.5 (13.8) I
K 98.2 (58.6) SmCP118.8 (29.6) I
K 82.9 (59.3) SmCP142.6 (26.1) I
K 117.9 (19.0) SmCP173.8 (28.7) I
[a] Data determined by DSC at a scanning rate of 108CminÀ1. [b] K:
crystalline; SmC: smectic C mesophase; SmCP: smectic C polar mes-
ophase; I: isotropic liquid phase.
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Figure 3. Polarization switching current of complex 1·3 under a trian-
gular-wave electric field: 100 Hz, 36 VppmmÀ1 (5 mm-Linkam cell).
novel systems. As an initial study, the one-photon-excited
fluorescence emissions of these complexes were determined
in solution (10À6 m in THF); representative results from these
Figure 1. Textures of the mesophase SmCP: a) Complex 2·3 at 1448C,
uncoated cell; b) complex 1·3 at 1008C, V=0, coated cell; c) complex
1·3 at 1008C, V=70 (d.c.), coated cell; d) complex 1·3 at 1008C,
V=100 (d.c.), coated cell. The orientation of the extinction brushes is
seen clearly in the circles indicated.
Table 2: X-ray data for the complexes studied.
Complex Measured spacings [Š] Miller index Layer thickness [Š]
1·3
1·4
2·3
52.2 (vs)[a]
26.3 (w)
17.25 (m)
50.5 (vs)
25.7 (m)
17.1 (m)
61.6 (vs)
30.8 (m)
001
002
003
001
002
003
001
002
d: 52.05
d: 51.04
d: 61.6
Figure 4. Fluorescence spectra of compound 1 and complexes 1·3 and
1·4 at room temperature in THF. Excitation wavelength 310 nm, emis-
sion wavelength 355 nm.
[a] Intensity of the reflection: vs: very strong; w: weak; m: medium.
studies are shown in Figure 4. The H donor compounds or the
H bonded complexes absorb at 310 nm (1 and its complexes)
or 320 nm (2 and its complex) and they exhibit fluorescence
emissions around 355 nm and 365 nm, respectively.
A
decrease in the absorptivity of the H donors is observed
upon complexation. To determine the potential of the
complexes as fluorescent materials, more in-depth studies of
their luminescent behavior will be undertaken in a future
project—particularly with respect to the novel SmCP meso-
phase as, to our knowledge, there is no precedence in the
literature for this property in ꢀbananaꢁ liquid crystals.
Although we were unable to find an example of a
compound in the literature that could be considered as a
covalent analogue of our bent complexes, we compared the
mesomorphic properties of 1·3 with the longest related
Figure 2. X-ray diffraction pattern of complex 1·4 at 1208C in the
SmCPmesophase.
the polarization current. Two peaks for each half-period are
clearly visible. The integration of these peaks[11] gives a value
around 200 nCcmÀ2 for the dielectric polarization for all of
the complexes.
The presence of stilbazole structures in these materials
also led us to assess their luminescent response, which, if
present, would increase the multifunctional character of these
Scheme 3. ‘Banana’ ester compound for comparison with 1·3.
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ꢀbananaꢁ ester reported by Pelzlꢁs group[12] (Scheme 3). From
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b) J. Etxebarria, C. L. Folcia, J. Ortega, M. B. Ros, Phys. Rev. E
2003, 67, 042702; c) J. Ortega, J. A. Gallastegui, C. L. Folcia, J.
Etxebarria, N. Gimeno, M. B. Ros, Liq. Cryst. 2004, 31, 579.
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the data obtained, we tentatively conclude that despite the
slight difference in length in the two materials, the similarity
in their thermal behavior is sufficiently close to acknowledge
the formation of H bonded complexes as a useful strategy to
design and, more interestingly, to identify new ꢀbananaꢁ liquid
crystals. In an attempt to obtain new mesophases, as well as to
approach different V-shaped structures by H bonding, the
complexation of some pyridine structures with different
carboxylic acids is currently in progress.
[9] D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark, E.
Körblova, D. M. Walba, Science 1997, 278, 1924.
In summary, further evidence of the versatility of hydro-
gen bonding for the anisotropic self-organization of func-
tional materials has been obtained. A successful method to
obtain novel mesophases, namely the most studied SmCP
liquid-crystalline phase, through H bonding interactions has
been developed. More interestingly, the attractive electro-
optical and dielectric responses of this polar phase were also
determined for these noncovalent systems. Furthermore,
these results indicate that these V-shaped H bonded struc-
tures are an easily obtained and attractive “testing bank”, not
only for the induction of different mesophases but also as a
potential way to obtain different multifunctional ꢀbananaꢁ
liquid crystals.
[10] a) S. L. Johnson, K. A. Rumon, J. Phys. Chem. 1965, 69, 74; b) T.
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[12] M. W. Schröder, S. Diele, G. Pelzl, W. Weissflog, ChemPhys-
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Received: May 4, 2004
Keywords: fluorescence · hydrogen bonds · liquid crystals ·
.
mesophases · supramolecular chemistry
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