TTF-based bent-core liquid crystals

Article abstract of DOI:10.1039/b801196f

The synthesis and characterization of bent-core liquid crystals which incorporate TTF groups is reported; different bent-core mesophases are induced depending on the molecular structure and properties derived from their compact packing have been studied.

Full text of DOI:10.1039/b801196f

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TTF-based bent-core liquid crystalsw
a
a
a
b
´
Inmaculada C. Pintre, Jose Luıs Serrano, M. Blanca Ros,* Josu Ortega,
Ibon Alonso, Josu Martınez-Perdiguero, Cesar L. Folcia, Jesus Etxebarria,
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c
c
c
c
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Faustyna Goc,z David B. Amabilino,d Josep Puigmartı
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-Luisd and Elba Gomar-Nadald
b
Received (in Cambridge, UK) 22nd January 2008, Accepted 27th February 2008
First published as an Advance Article on the web 31st March 2008
DOI: 10.1039/b801196f
The synthesis and characterization of bent-core liquid crystals
which incorporate TTF groups is reported; different bent-core
mesophases are induced depending on the molecular structure
and properties derived from their compact packing have been
studied.
In 1996, a new type of mesogenic material appeared, the
so-called bent-core liquid crystals.4 Bent-core molecules are of
prime interest in liquid crystal research because of their special
properties, which arise from their unique molecular structure
and their compact packing that restricts rotational freedom.
Features such as the occurrence of polar order within layers or
columns, as well as the appearance of some supramolecular
chiral structures from non-chiral molecules are of fundamental
scientific interest. Sometimes the chirality can be switched
under external electric fields. Furthermore, these mesomorphic
materials give rise to antiferroelectric, ferroelectric4 or non-
linear responses,5 often with exceptional values for the
relevant parameters. Mesomorphic properties result from a
suitable combination of the shape of a molecule and the
magnitude and direction of the molecular interactions. The
few examples reported show that the high tendency of TTF
moieties to stack allows classical mesophases, through a
proper molecular structure.1c,6 Here, we report the synthesis
and characterization of three bent-core liquid crystals which
incorporate TTF (Fig. 1). Additionally, these new compounds
add to the scarce polycatenar bent-core materials reported7
which interestingly induce axially polar columnar mesophases.
We have found that different bent-core mesophases are
induced depending on the molecular structure and that they
show properties resulting from their compact packing.
Of the many p-functional units that are employed in molecular
materials, tetrathiafulvalene (TTF) and its derivatives are
heterocyclic systems which are particularly interesting from a
number of viewpoints, especially because they can be oxidized
to produce conducting materials through their p-stacked
columnar arrangement. Intensive studies have focused on
TTF for the preparation of conductive materials in bulk and
thin film states.1 Furthermore, TTFs are also very useful for
sensors, molecular switches, NLO responses, OFETs1c,2a or in
the nanoscience area,2b especially with regard to molecular
electronics. It has been stated that TTF-based materials offer
widespread potential and versatility through incorporation of
the p-rich redox-active building block into materials.1c The
control of the supramolecular arrangement in shape and size is
the key to success in this area. Thus different self-assembling
approaches, using non-covalent interactions, gelification or
liquid crystal phases, have been used to organize this moiety.1,2
Liquid crystals are of great interest not only for their well-
known applications in devices but also, because they are
drawing the attention of fields such as biology, medicine,
physics, chemistry and materials science. Mesophases offer
soft phases with lamellar, columnar or cubic arrangements3
and a current goal is to use the fourth state of matter to
achieve ordered functional systems.
These compounds were prepared following synthetic routes
reported8 or adapted from the literature (see ESIw). The phase
sequences of the compounds were determined by polarizing
optical microscopy and differential scanning calorimetry. The
different mesophases of I, II and III were assigned on the basis
of the textures observed by optical microscopy (Fig. 2), as well
a
´
Quımica Organica, Facultad de Ciencias, Instituto de Ciencia de
Materiales de Aragon, Universidad de Zaragoza-CSIC, 50009-
´
´
Zaragoza, Spain. E-mail: bros@unizar.es; Fax: +34 976 761209;
Tel: +34 976 762277
b
c
´
Departamento de Fısica Aplicada II, Facultad de Ciencias y
Tecnologıa, Universidad del Paıs Vasco, Apdo 644, 48080-Bilbao,
´
´
Spain
Departamento de Fısica de la Materia Condensada Facultad de
´
Ciencias y Tecnologıa, Universidad del Paıs Vasco, Apdo 644,
´
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48080-Bilbao, Spain
d Institut de Ciencia de Materials de Barcelona (CSIC), Campus
Universitari, 08193-Bellaterra, Spain
w Electronic supplementary information (ESI) available: Synthesis
and H-NMR spectra of I, II and III, cyclic voltammograms, DSC
thermograms, proposed molecular packing, conductivity and photo-
response of II. See DOI: 10.1039/b801196f
z Current address: Institute of Molecular Physics, Polish Academy of
Sciences, Poznan, Poland.
Fig. 1 Chemical structure of the compounds I, II and III.
Chem. Commun., 2008, 2523–2525 | 2523
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Fig. 3 Polarization switching current under a triangular-wave electric
field and mechanisms proposed for the polar switching of I (a) and II (b).
directions of the columns cannot be determined with non-
resonant X-ray diffraction. Compounds II and III are no
longer fluid below some temperature but the diffuse peaks
remain. This fact confirms the vitrification of the mesophases.
An attractive feature of some bent-core liquid crystalline
phases is their switching response under electric field. This
behavior was investigated for I, II and III within 5 mm ITO-
coated cells. Upon application of a triangular-wave field an
antiferroelectric-like switching current response (Fig. 3a), but
without texture changes, was detected for I. This implies that
the switching occurs around the long molecular axis. A ferro-
electric-like switching current response was observed for II
(Fig. 3b), but second harmonic generation (SHG) activity
disappears when the field was off, suggesting an antiferro-
electric ground state. Spontaneous polarizations around
400 nC cmꢁ2 were measured for I and II. No appreciable
switching was observed for III.
Fig. 2 Textures (top) and possible molecular packing within the
mesophases (bottom) for: (a) I, Colob; (b) II, SmCP under electric
field and (c) III, USmCP.
Table 1 Phase sequences for compounds I, II and III
Compound
Phase transitionab/1C [kJ molꢁ1
]
I
C 101.0 [25.5] Colob 117.6 [17.9] Is
C 96.5 [26.7] SmCP 158.1 [23.4] Is
USmCP (g) B 90 USmCP 172.6 [4.3] Is
II
III
a
Data determined by DSC from first scans at scanning rate of 10 1C
minꢁ1 on heating. C: crystalline solid, Colob: oblique columnar
b
mesophase, SmCP: SmC polar mesophase, USmCP: undulated SmCP
mesophase, g: glassy mesophase, Is: isotropic liquid phase.
as from X-ray patterns taken in the mesophase. The results of
these studies are collected in Tables 1 and 2.
SHG signals in unaligned cells of I and II under electric
fields of 30 V mmꢁ1 were about 5 times larger than SHG
intensities in classical bent-core liquid crystals in the same
conditions.10 This corresponds to second-order dielectric sus-
ceptibility coefficients d E 20 pm Vꢁ1. Compound III was
SHG inactive, at least under electric fields below 20 V mmꢁ1
(damage threshold).
These compounds exhibit bent-core liquid crystalline phases
within a significant temperature range. X-Ray diffraction
patterns showing a diffuse peak at wide angles were obtained
for the three compounds, suggesting the mesophase order.
Small-angle X-ray diffraction diagrams from I revealed an
oblique lattice consistent with a Colob arrangement, where
layer fragments form the columns (Fig. 2). For II, diffraction
patterns show two peaks, which correspond to the 1st and 2nd
orders of a given periodicity; the difference between the
measured layer thickness (51 A) and the estimated molecular
length (70 A) indicates molecular tilting in the layers, thus a
SmCP phase is proposed. Small-angle X-ray diffraction
experiments for III show peaks that were indexed on the basis
of a rectangular lattice. The existence of a strong (01)
reflection indicates a clear smectic character. An anharmonic
undulated smectic phase is proposed. Fig. 2c shows one of the
possible structures compatible with Fourier analysis of the
intensities.9 Other possibilities for the molecular packing of I
and III can be obtained by changing the distribution of the
polarization directions of the blocks, being compatible with an
antiferroelectric ordering (see below). The precise polarization
The 1st and 2nd one-electron oxidation potentials of these
TTF derivatives in solution are at approximately 0.66 and
1.06 V, respectively (see ESIw).11 Both redox processes are
reversible. The presence of carboxy groups reduces the donor
ability of the TTF core of the molecules in comparison with
others of the TTF family.8b,12
The conductivity of one of the compounds (II) has been
studied (Fig. S5w). The material shows a clear photoconduc-
tivity upon illumination with UV light. In the mesophase the
ratio between photo and dark currents is larger than in the
other phases (Fig. S6w), suggesting electronic conduction.9a
In summary, we have succeeded in the preparation of
innovative polycatenar p-functional bent-core liquid crystals.
Depending on the molecular structure, close-packed lamellar
and columnar arrangements can be induced, which could
provide a significant orbital overlap of the p-rich redox-active
building blocks within these structures. Furthermore, the di-
electric and optical response of these materials, as well as the
redox properties of these compounds, confirm this new type of
TTF-based liquid crystals as a potential strategy for new
functional materials.
Table 2 X-Ray data for compounds I, II and III
Compound
Measured spacing/A
(01) 46.81, (10) 37.76,
ꢀ
(11) 32.99, (11) 26.75, (02) 23.40
Parameters
I
a = 39 A,
c = 48 A
b = 78
I.C.P, I.A and J.M.P thank the Aragon, Basque and
´
II
III
(01) 51.80, (02) 25.26
(20) 72.72, (01) 52.45,
(11) 49.35, (31) 35.51, (02) 26.15
c = 51 A
a = 141 A,
c = 52 A
Spanish Governments for fellowship grants. Research sup-
ported by CICYT-FEDER of Spain-UE MAT2006-13571-
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