Core-to-core dimers forming switchable mesophase

Article abstract of DOI:10.1039/c6cc09983a

We report structurally new type of dimers composed of bent-core molecules connected through their central cores by an alkylene spacer. Various self-assembling structures are discovered when prolonging the terminal alkyl chains in arms. Among diverse mesop

Full text of DOI:10.1039/c6cc09983a

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
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Core-to-core dimers forming switchable mesophase
M. Horčic,a J. Svoboda*a, V. Novotná,b D. Pociechac and E. Goreckac
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
models to predict liquid crystalline properties of polymers.4
Calamitic5, discotic6 and bent-core dimers7 of versatile topology
have been investigated. The molecular structure of a LC dimer
usually consists of two monomeric units linked by a flexible spacer.
Of the previously designed bent-core dimers,7 the vast majority of
materials are based on end-to-end arrangement of cores (Fig. 1a).7a-
We report structurally new type of dimers composed of bent-core
molecules connected through their central cores by an alkylene
spacer. Various self-assembling structures are discovered when
prolonging the terminal alkyl chains in arms. Among diverse
mesophases, the antiferroelectric type of switching under applied
electric field is observed.
e
Just a few dimers with other geometries were prepared, among
them side-to-end type dimers (Fig. 1b).7f It was shown that the
character of the linking group strongly influences the type of the
phases formed.7a Bent-core dimers having W-shaped geometry with
an additional inner bend unit (Fig. 1c) have been synthesized and
studied.8 BC-dimers based on core-to-core interconnection (Fig.1d)
have been rarely reported.7e
Liquid crystals (LCs) represent unique state of matter, which
links anisotropy of the solid state and fluidity of the liquids. Bent-
core mesogens are fascinating part of LCs due to their unusual
physical properties. Until the discovery of polar switching for bent-
core (BC) compounds in 1996,1 polar properties: ferro- or
antiferroelectric were exclusively reported for chiral mesogenic
molecules (rod- or disc-like). The unique nature of polar order of BC
mesogens is due to the strong steric hindering of close-packed
molecules within smectic layers.
Most of the polar BC mesophases are tilted smectic phases
(denoted as SmCP) with four distinct substructures, classified
according to the tilt and/or the polarity in consecutive layers.
Notation CS means synclinic, CA anticlinic orientation of the
molecules, PF stands for ferroelectric and PA for antiferroelectric (A)
state in consecutive layers.2 Another distinctive feature of bent-
core compounds is that they easily form modulated structures. For
example, there are columnar phases (denoted as B1) made of layer
polar fragments with 2D density modulation parallel to the
polarization vector.3 Contrary to polar smectics, the B1 phases
usually do not reveal optical response and polar switching under
application of electric field.
Fig. 1 Representation of different types of connections in bent-core dimers:
a) end-to-end, b) side-to-end, c) W-shaped, d) and e) core-to-core
Herein, we present a new type of bent-core dimers (Fig. 1e) in
which the central units are connected top-to-bottom. The
molecular structure of the synthesized dimers denoted as D3-n (Fig
2) is derived from substituted resorcinol cores joined with an
propylene-1,3-dioxy linking unit (3 stands for the number of
carbons in the spacer and n corresponds to the length of terminal
alkyl chains). The terminal chains are the same in all arms and vary
from C2H5 (n=2) to C12H25 (n=12). All synthesised compounds
exhibited mesomorphic properties. Phase transition temperatures
and corresponding phase transition enthalpies are collected in
Table 1. DSC thermographs are presented in ESI, see Fig.S1.
Liquid crystalline dimers and oligomers are of contemporary
interest, because they frequently exhibit rich mesomorphic
properties, different from those of monomers and can also serve as
a.Department of Organic Chemistry, University of Chemistry and Technology, CZ-
166 28 Prague 6, Czech Republic. E-mail: Jiri.Svoboda@vscht.cz, Fax:
+420220444182; Tel: +420220444288
b.Institute of Physics, Academy of Science of the Czech Republic, Na Slovance 2, CZ-
182 21 Prague 9, Czech Republic. E-mail: novotna@fzu.cz, Fax: +420286890527;
Tel: +420266052134
c. Laboratory of Dielectrics and Magnetics, Chemistry Department, Warsaw
University, Al. Zwirki i Wigury 101, 02-089 Warsaw, Poland. E-mail:
pociu@chem.uw.edu.pl; Fax: +48228221075; Tel: +48228221075
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available:
See DOI: 10.1039/x0xx00000x
Fig. 2 Structure of the studied dimers (denoted as D3-n).
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For all compounds only a single enantiotropic liquid crystalline
phase has been observed. Nevertheless, character of the phase
depends strongly on the length of alkyl terminal chains (on n
number). For homologues with the shortest chain D3-2 and D3-4, a
fan-shaped texture is observed under polarizing microscope for
planar samples (Fig. 3a), as well as a homeotropic texture for one-
surface-free samples. No electro-optical response was obtained
under applied electric field up to 20 V/m. Type of the optical
textures and lack of electric response suggested an orthogonal SmA
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DOI: 10.1039/C6CC09983A
phase (SmA). The x-ray measurements confirmed
a layered
structure; in the small-angle region a sharp peak was detected,
while in wide-angle region a broad peak was found corresponding
to the mean in-plane distance between molecules about 4.5Å.
(Fig.4a). The observed broad peak is interpreted as related to the
Fig. 3 Textures a) for D3-2 in the SmA phase at temperature T=160ºC and b)
mean distance between terminal alkyl chains. Similarly as for rod- for D3-8 in the columnar B1 phase at T=140ºC.
like molecules, we do not see separate signals from the distance
Homologues with the longest terminal alkyl chain, D3-10 and
between cores. The layer thickness, d, was established from the
small-angle peak position. We found d=16.5 Å and d=18 Å for D3-2
and D3-4, resp. The d value corresponds to the half of calculated
length of BC dimer, l (l33.6Å and l37.4Å for D3-2 and D3-4,
respectively). Therefore, we conclude that short homologues D3-2
and D3-4 formed an intercalated type of the SmA phase (model is
presented in ESI, Fig. S2).
D3-12, exhibit a switchable lamellar phase and the characteristic
electro-optical behaviour allowed to identify the observed
mesophase as the SmCAPA. In the ground state (SmCAPA) an average
optical axis results from alternation of the tilt direction in
neighbouring layers, being parallel to polariser and analyser at zero
voltage (Fig.5a). Under the applied electric field the optical
birefringence increased significantly (Fig. 5b) and the extinction
brushes rotated from the position along the polarizers by the angle
of ~45 degrees, which proves that the SmCAPA phase transforms
into SmCSPF structure. There are three distinct states, which can be
distinguished by optical investigations, confirming the AF switching.
The polarization current measured under triangular electric field
exhibits two sharp peaks per half-period (Fig. 6), which is
characteristic for antiferroelectric phase. The similar value of
electric polarization, P=850 nC/cm2, has been measured for both
D3-10 and D3-12 compounds, by integration of the switching
current peaks.
Table 1
Melting point, m.p., the phase transition temperatures, Ttr, crystallization
temperature, Tcr, in °C and corresponding enthalpy changes, H, in kJ mol1
(in brackets), detected on the second temperature run at a rate of 5 Kmin1.
n
2
4
6
M
m.p. (H)
160 (+41.9)
146 (+21.4)
133 (+30.4)
Tcr (H)
129 (30.1)
98 (12.3)
91 (19.6)
Ttr (H)
D3-2
D3-4
D3-6
SmA
SmA
B1
201 (25.0)
183 (25.3)
157 (31.3)
D3-8
D3-10
D3-12
8
136 (+38.9)
138 (+77.6)
129 (+37.3)
B1
93 (22.8)
112 (31.7)
99 (25.3)
140 (32.7)
133 (35.6)
139 (39.4)
10
12
SmCAPA
SmCAPA
Homologues with the intermediate length of terminal alkyl
chains, D3-6 and D3-8, revealed a columnar phase. Textures with
coloured domains were observed under polarizing microscope (Fig.
3b), and similarly to shorter homologues no electro-optical
response was detected. For both compounds the x-ray diffraction
studies unambiguously evidenced the B1 type of mesophase, with
the columnar phase built of smectic layer fragments. In the x-ray
pattern there are few incommensurate signals in the small-angle
region and a broad maximum in large-angle region (Fig. 4b). The
small angle x-ray patterns could be indexed assuming centred
orthorhombic crystallographic unit cell, one of the unit cell
dimensions was found to be comparable to the calculated
molecular length (for the cell parameters see ESI).
Fig. 4 The x-ray intensity profile versus the scattering angle for a) D3-2 at T=
170°C and b) D3-6 at T= 110°C. At insets the x-ray pattern for partially
oriented sample is shown at corresponding temperature.
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The dielectric spectroscopy revealed one distinct mode in the
SmCAPA phase. The mode disappears in the isotropic and crystalline
phases and originates from polar packing in the SmCAPA phase
rather than from reorientation along molecular axes. Imaginary part
of dielectric permittivity plotted versus frequency and temperature
(Fig. 7 for D3-12) shows that mode strength does not change with
temperature and the dielectric strength, is rather small
(for details see ESI and Fig. S5. We can conclude that the
observed mode probably corresponds to interlayer antiferroelectric
interactions, which are relatively strong for studied system.
The x-ray diffraction experiments performed for D3-10 and D3-
12 confirmed a layered structure of the mesophase (see ESI for
details). The measured layer spacing, d=35.3 Å for D3-10 and
d=37.7Å for D3-12 (only weakly dependent on temperature) was
significantly smaller than l (l51 Å and l56 Å for D3-10 and D3-12,
respectively), which is consistent with strong tilting of molecules
from the layer normal evidenced by the electro-optical observation
(Fig. 5) i.e. rotation of the extinction position under the electric
field.
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DOI: 10.1039/C6CC09983A
Fig. 8 Schematic organization of dimeric molecules in mesophases.
Self-organization of the studied dimers is schematically depicted
in Fig. 8. In Fig. 8a the intercalated SmA phase is presented, in
Fig.8b the smectic layer fragments are shown and the cell unit
depicted for the B1 phase. In Fig. 8c the orientation of dimeric
molecules in neighbouring layers is described for the SmCAPA phase
and effect of the applied electric field, ±E, is scatched.
The mesomorphic behaviour can be compared with previously
reported monomeric BCs.9 In spite of the fact we have no relevant
data for the shortest terminal chains, we can conclude that studied
dimers revealed the same type of mesophases as corresponding
monomeric BCs. Nevertheless, for monomeric BCs the reported
mesophases are only monotropic and existing in a very narrow
temperature range.9 For studied dimers we observed slight increase
of the transition temperatures in comparison with monomeric BCs.
We can conclude that stabilization of mesophases was confirmed in
the studied dimers.
Fig. 5 Planar texture of D3-12 at temperature T=130°C a) without the electric
field and b) under the applied electric field E=+25 V/m. The width of the
microphotograph is 250 m.
In summary, we have synthesized and characterized
mesomorphic behaviour of a new type of bent-core dimers in which
bent-core molecules are connected core-to-core via the propylene-
1,3-dioxy chain. Interestingly, despite short linkage connecting the
mesogenic cores, which should stiffen the molecular structure, the
phase sequence for the dimeric homologue series is similar to thus
observed for many monomeric bent-core homologue series. Dimers
show different type of mesophases depending on the length of the
terminal chains. The shortest studied homologues formed the
intercalated smectic A phase, the longest – switchable SmCAPA
phase, analogously as can be often observed for BCs.2 For the
dimers with intermediate terminal chains the broken-layer
columnar B1 phase appears, built of small layer fragments (blocks)
arranged into body centred 2D crystallographic lattice.
Fig. 6 The switching current at a triangular field, E, frequency of 10 Hz, for
D3-12 at T=150°C in the SmCAPA phase.
Funding: This work was supported by the Czech Science Foundation
(15-02843S).
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Fig. 7 Imaginary part of the permittivity’’, versus temperature and
frequency for D3-12.
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