Symmetric bent-core mesogens with m-carborane and adamantane as the central units

Article abstract of DOI:10.1039/b719629f

Several members of two homologous series of symmetric bent-shaped compounds with either m-carborane or adamantane (1[n] or 2[n], n = 9-13) in the central position were synthesized and investigated by optical, calorimetric, X-ray diffraction, and electro-o

Full text of DOI:10.1039/b719629f

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www.rsc.org/materials | Journal of Materials Chemistry
Symmetric bent-core mesogens with m-carborane and adamantane as the
central units†‡
a
b
c
c
Damian Pociecha, Kiminori Ohta, Adam Januszko, Piotr Kaszynski and Yasuyuki Endob
*
Received 20th December 2007, Accepted 11th March 2008
First published as an Advance Article on the web 8th April 2008
DOI: 10.1039/b719629f
Several members of two homologous series of symmetric bent-shaped compounds with either
m-carborane or adamantane (1[n] or 2[n], n ¼ 9–13) in the central position were synthesized and
investigated by optical, calorimetric, X-ray diffraction, and electro-optical methods. Results show that
the two shortest members of the m-carborane series, 1[9] and 1[10], exhibit an intercalated lamellar B6
phase. The adamantane derivatives 2[9]–2[13] display a monotropic weakly birefringent soft crystalline
or crystalline phase with homochiral domains. Neither phase undergoes polarization switching in an
electric field.
The use of large, bulky central groups in bent-core mesogens
Introduction
was avoided because of the belief that molecules with planar
rings are necessary for achieving close packing in the fluid
phase and the formation of banana phases. Also there are only
a few possible candidates for such sterically demanding and
conformationally rigid connectors. Among those are m-carbo-
rane and adamantane, which are approximately spherical and
provide the desired connectivity patterns with an angle between
the substituents of approximately 120ꢀ. The former is an
inorganic boron cluster8,9 with C2v point group symmetry and
a moderate dipole moment of 2.85 D.10 Adamantane, in contrast,
is a non-polar, Td-symmetric hydrocarbon.
Bent-core molecules, in which mesogenic units are attached to
the central ring at an angle of about 120ꢀ, have been intensively
investigated over the past decade.1–3 Prompted by the discovery
of electrooptical switching in one such achiral bent-core
compound,4 subsequent detailed investigations have uncovered
spontaneous formation of homochiral domains, supramolecular
chirality, and polar phases in many achiral bent-core mesogens.
To date, several liquid crystalline phases (banana phases) have
been identified in this new class of materials.1,5
As part of an effort to better understand structure–properties
relationships in bent-core mesogens and to develop a rational
design for molecules with desired polar properties, many series of
compounds were prepared and investigated by diffraction,
optical, and dielectric methods. Typically, planar central units
such as 1,3-disubstituted benzene, 2,6-disubstituted pyridine,
2,7-disubstituted naphthalene, 3,40-disubstituted biphenyl, and
some 5-membered heteroaromatic rings have been used in the
design of such bent-core molecular systems.1,3 Occasionally, the
central ring has an additional small substituent, which modifies
the polar properties and packing of the molecules. The use of
non-aromatic rings as the central unit is very rare. Examples
Here we report the preparation and characterization of the
first bent-shaped compounds in which m-carborane, in series
1[n], and adamantane, in series 2[n], serve as the bulky central
connectors (Fig. 1).
Results and discussion
Synthesis
The two series of compounds were prepared by reaction of
carboxylic acid 3[n] with either 1,7-bis(4-hydroxyphenyl)-m-
carborane (4a) or 1,3-bis(4-hydroxyphenyl)adamantane (4b)
following a general procedure (Scheme 1).11
include tetrahydropyran ring6 and
a
recently reported
pyrazabole,7 which, in spite of its sterically demanding ‘‘roof’’
shape, supports the formation of nematic and soft crystalline
banana phases.7
aDepartment of Chemistry, Warsaw University, Al. Zwirki i Wigury 101,
02-089 Warsaw, Poland
bTohoku Pharmaceutical University, 4-4-1, Komatsushima, Aoba-ku,
Sendai, 981-8558, Japan
cOrganic Materials Research Group, Department of Chemistry, Vanderbilt
University, Nashville, 37235, TN, USA. E-mail: piotr.kaszynski@
vanderbilt.edu; Fax: +1-615-343-1234; Tel: +1-615-322-3458
† This paper is part of a Journal of Materials Chemistry theme issue on
Liquid Crystals Beyond Display Applications. Guest editor: Carsten
Tschierske.
‡ Electronic supplementary information (ESI) available: Synthesis and
analytical details for all compounds, transition temperatures for 3[n]
and 5[n], powder XRD for 2[11] and electrooptical response for 1[9].
See DOI: 10.1039/b719629f
Fig. 1 Two series of compounds derived from m-carborane (1[n]) and
adamantane (2[n]). In m-carborane, each vertex represents a BH fragment
and each sphere is a carbon atom.
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Analysis of the carborane series 1[n] revealed that all melt
about 155 ꢀC and only the two lowest members of the series
exhibit liquid crystalline behavior. The nonyloxy derivative 1[9]
exhibits an enantiotropic smectic phase, which easily supercools
to about 130 ꢀC before it crystallizes. In the next homolog, 1[10],
the phase becomes monotropic and it appears 10 K below the
melting point (Table 1). The clearing transitions for both
compounds have the enthalpy change of nearly 18 kJ molꢁ1
.
The nonyloxy derivative 1[9] exhibits a rich solid-state
polymorphism. A virgin sample of 1[9] shows two high-enthalpy
Cr–Cr transitions at 99 ꢀC and at 140 ꢀC and a melting transition
at 154 ꢀC. In the second heating cycle, the low temperature
Cr–Cr transition is absent and the melting transition appears at
Scheme 1
ꢀ
148 C. Higher homologues exhibit a low energy Cr–Cr transi-
ꢀ
tion at about 120 C.
DSC analysis of the adamantane series 2[n] did not reveal
a mesophase formation. All compounds melt to an isotropic
phase at about 180 ꢀC and crystallize upon cooling with a small
(0.5–5 K) hysteresis of the transition temperature. In addition,
several broad Cr–Cr transitions are observed with the most
Scheme 2
ꢀ
common one at about 105 C.
Optical analysis
Microscopic observations of 1[9] and 1[10] confirmed the
formation of a mesophase. Upon cooling between two glass
slides both compounds form regular fan-shaped textures with
additional thin lines and small areas of different birefringence
(Fig. 2A). Removing the top cover glass by shearing and
applying mechanical stress resulted in the formation of
a schlieren texture as shown in Fig. 2B. No homeotropic region
of the sample was observed in either of the textures. Considering
the bent-core geometry of the molecules, such textures can be
attributed to an intercalated lamellar B6 phase.15,16
Scheme 3
Carboxylic acids12 3[n] were prepared by oxidation of alde-
hydes 5[n], which were obtained according to a general literature
procedure13 (Scheme 2). The bisphenol 4a was obtained by
demethylation of dimethoxy derivative 6a with BBr3 (Scheme 3).
Compound 6a was prepared by arylation of m-carborane with
4-iodoanisole according to a general procedure.14
Although DSC did not show mesophase formation in series
2[n], microscopic observations revealed that upon fast cooling
from the isotropic phase all adamantane derivatives form
a weakly birefringent phase with a dark non-characteristic
texture. The texture exhibits optical activity (OA), and domains
with opposite OA sign always coexist and have comparable areas
(Fig. 3). The texture does not respond to electric field. However,
cooling of the sample in the field leads to a texture with larger
homochiral domains. At temperatures about 15 K below
melting, the sample hardens and forms a glassy, rigid material
Thermal analysis
Phase transition temperatures and enthalpies for series 1[n] and
2[n] are shown in Table 1. Phase structures were assigned on the
basis of microscopic textures observed using a birefractive setup.
Table 1 Transition temperatures (ꢀC) and enthalpies (in parentheses, kJ molꢁ1) obtained for 1[n] and 2[n] in the heating modea
n
1[n]
Cr1
Cr1
Cr1
Cr1
Cr1
2[n]
Cr1
Cr1
Cr1
Cr1
Cr1
9
99
(32.1)
122
(27.2)
118
(22.1)
89
(3.0)
127
(5.3)
Cr2
Cr2
Cr2
Cr2
Cr2
140
(20.2)
158
(47.0)
157
(46.7)
117
(3.5)
154
(44.9)
Crb
(B6
I
154
(21.6)
B6
I
159
(17.6)
I
76
Cr2
Cr2
Cr2
Cr2
Cr2
116
(6.4)
94
Cr3
180
I
(8.2)
82
(4.4)
105
(28.5)
103
(39.9)
108
(23.5)
(41.9)
107
(3.9)
10
11
12
13
148)c
Cr3
Cr4
180
(40.7)
I
(17.7)
(3.1)
176
(35.5)
179
(45.6)
176
(37.6)
I
I
I
Cr3
I
157
(48.4)
I
a
c
Cr ¼ crystal; B ¼ banana phase; I ¼ isotropic. b An additional Cr–Cr transition was observed at 147 ꢀC (1.0 kJ molꢁ1). Monotropic transition.
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Fig. 4 Temperature-dependent small angle reflections in the smectic and
crystalline phases for 1[9] recorded in the cooling mode. The slope is for
the smectic layer spacing (circles) is ꢁ9.5(1) ꢂ 10ꢁ3 A Kꢁ1 in the range of
˚
139–154 ꢀC. The vertical line marks the crystallization temperature.
Fig. 2 Texture photomicrographs of 1[9] obtained on cooling to about
155 ꢀC from the isotropic phase: (A) fan-shaped texture and (B)
a schlieren texture. See text for details.
Fig. 5 UFF optimized molecular structure of 1[9] in the fully extended
bow-like conformation. The molecule’s projection on the horizontal axis
˚
is 47 A.
˚
corresponding to a smectic layer distance d of about 25 A
(Fig. 4). The observed layer spacing d is about half of the
molecular length in the most extended bow-like conformation
(Fig. 5), according to the calculations with the Universal Force
Field (UFF) algorithm. This indicates that molecules are
strongly intercalated between layers, which supports the assign-
ment to the B6 phase (SmAc).1
The layer spacing
d slightly decreases with increasing
Fig. 3 A natural texture of 2[10] obtained upon cooling to 170 ꢀC from
the isotropic phase in the absence of electric field viewed with crossed
(middle) and decrossed polarizers. Arrows indicate the polarizer
configuration. A crystallite is visible in the upper-left corner.
temperature. This suggests increasing disorder of the nonyl
chains and consequent reduction of the effective length of the
alkyl substituent and contraction of the layer spacing.
Despite efforts, XRD analysis of the adamantane derivative
2[11] showed a practically identical pattern for a virgin sample
before melting and for one cooled from the isotropic phase at
about 10 K below melting point (see ESI‡). This indicates that
the weakly birefringent metastable phase shown in Fig. 3 either
quickly crystallizes or is not formed at all under the XRD
experimental conditions. It is also possible that the monotropic
phase is a crystalline modification.
without changing the texture. The texture observed for the
monotropic phase found in 2[n] could be attributed to a B4
phase.17,18 However, this assumption could not be confirmed by
XRD analysis because the metastable phase always co-exists
with crystallites (Fig. 3), and easily crystallizes at temperatures
near the isotropic transition.
X-Ray diffraction
Electrooptical analysis
X-Ray powder diffraction analysis of the mesophase formed
Brief investigation of the electric field effect on the mesophase
texture showed no switching behavior (no change of light
by 1[9] revealed one reflection in the small angle region,
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part, to the five-fold minima for rotation around the carborane–
R bond, which leads to significant conformational mobility of
the substituents and overall molecular flexibility. The mobility
results in difficulties in formation of lamellar phases and the
compact packing arrangements in the solid phase. In contrast,
both bicyclo[2.2.2]octane and adamantane analogs have three-
fold rotational potentials, which results in a greater molecular
rigidity. Consequently, the two carbocycles are expected to
exhibit more organized phases and higher transition tempera-
tures than the carborane analogs. This is indeed observed
experimentally for both rod-like and bent-core compounds.
The present findings open new possibilities for controlling
mesophase properties of bent-shaped molecules by using steri-
cally demanding central groups. Future structural modifications
in series 1[n] and 2[n] by desymmetrization of the substituents,
partial fluorination of the alkyl groups and/or benzene rings may
allow for induction of enantiotropic polar, switchable phases in
m-carborane and adamantane derivatives. For instance, partial
fluorination of the C8 tail in the analogous m-terphenyl series
induced a broad range antiferroelectic B2 phase.11
Fig. 6 Transmission as a function of applied dc electric field obtained
for a sample of 1[9] in a standard planar 3.2 mm cell at ꢃ155 ꢀC.
extinction direction upon application of the field) in either series
of compounds. This lack of interaction with electric field further
supports the assignment of the observed mesophases as non-
polar B6 for carboranes 1[n] and is consistent with the possible
B419 phase for adamantanes 2[n]. It was found, however, that for
1[9] the birefringence varies significantly and reproducibly with
the applied field. The birefringence change is not accompanied by
polarization switching (no current peak), and thus has a dielec-
tric origin.
Experimental
Thermal analysis results were obtained using a TA Instruments
2920 DSC. Transition temperatures (onset) and enthalpies were
obtained using small samples (1–2 mg) and a heating rate of 5 ꢀC
minꢁ1 under a flow of nitrogen gas. Optical microscopy and
phase identification were performed using a PZO ‘‘Biolar’’
polarized microscope equipped with a HCS402 Instec hot stage.
Electrooptical studies were carried out in 3.2 mm thick planar
cells with ITO electrodes using a setup based on a Nikon
Optiphot2-Pol microscope with a P102 photomultiplier. X-Ray
diffraction experiments were performed with a Bruker D8
Discover system equipped with an Anton Paar DCS350 heating
stage.
A similar effect has been observed in another bent-core
mesogen,20 and was explained by molecular rotation to orient
their planes parallel to the applied field. Moreover, a well defined
threshold was observed on the transmission vs. electric field curve
(Fig. 6). The existence of the threshold, of about 2 V mmꢁ1 in
a 3.2 mm thick cell, could be interpreted as partial breaking of
molecular anchoring to the glass. Further experiments in cells
with different thickness and alignment layers are necessary for
a better understanding of this phenomenon.
The preparation of intermediates and compounds in series 1[n]
and 2[n] is described in the ESI.‡
Discussion and conclusions
Experimental results demonstrate that compounds with a bent-
core molecular architecture containing sterically demanding
central groups, m-carborane and adamantane, form non-polar
phases. Carborane derivatives 1[n] exhibit an intercalated
lamellar B6 phase, which is quickly destabilized with increasing
length of the alkyl chain. Compounds in the adamantane series
2[n] form a monotropic phase, whose optical texture and lack of
interaction with electric field are consistent with a soft crystalline
B4 phase. The mesophase in the carborane series appears to be
destabilized relative to that in the benzene analogs. For instance,
the benzene analogs of 1[9] and 1[12] exhibit nearly 60 K and 40 K
wide, respectively, polar B1 phases (Colr) with clearing temper-
atures of over 200 ꢀC.11,21 A higher homolog of the m-terphenyl
derivative shows an antiferroelectric B2 phase (SmCPA).11
Acknowledgements
This work was supported in part by the NSF grant (DMR-
0111657) and by a Grant-in-Aid for High Technology Research
Program from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. XRD experiments were
performed in The Structural Research Laboratory at Warsaw
University, established under the European Regional Develop-
ment Found Program (WKP-1/1.4.3/1/2004/72/72/165/2005) and
funded in part by the ESF/2007/03 grant.
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