Thermotropic columnar liquid crystals based on wedge-shaped phenylphosphonic acids

Article abstract of DOI:10.1246/bcsj.20190105

Wedge-shaped phenylphosphonic acids with variation in the peripheral alkoxy chains have been synthesized. These derivatives show a hexagonal columnar liquid-crystalline behavior upon thermal treatment. These materials have potential to be used as efficien

Full text of DOI:10.1246/bcsj.20190105

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Thermotropic Columnar Liquid Crystals Based on Wedge-Shaped Phenylphosphonic Acids
K. R. Sunil Kumar, Monika Gupta, Takeshi Sakamoto, and Takashi Kato*
Advance Publication on the web June 5, 2019
doi:10.1246/bcsj.20190105
© 2019 The Chemical Society of Japan
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Publication manuscripts.

Thermotropic Columnar Liquid Crystals Based on Wedge-Shaped Phenylphosphonic
Acids
K. R. Sunil Kumar, Monika Gupta, Takeshi Sakamoto, and Takashi Kato*
Department of Chemistry and Biotechnology, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656
E-mail: kato@chiral.t.u-tokyo.ac.jp
Abstract
Wedge-shaped phenylphosphonic acids with variation in
nanostructured materials for transport of ions and proton.4a,4c-
e,8,13,14
They exhibited columnar and bicontinuous cubic LC
the peripheral alkoxy chains have been synthesized. These
derivatives show a hexagonal columnar liquid-crystalline
behavior upon thermal treatment. These materials have potential
to be used as efficient anisotropic proton conductors.
phases. In the present molecular design, the phosphonic acid is
directly attached to the phenyl ring while previous reports show
that phosphonic acid group connected through a methylene
spacer to the phenyl ring did not show thermotropic LC
behavior.9,10
Keywords: Liquid crystal, Columnar structure,
Phosphonic acid
2. Experimental
General Procedures.
Phase transition properties were
examined by differential scanning calorimetry (DSC) by using a
NETZCH DSC 204 Pheonix system. Polarizing optical
microscope (POM) observation was conducted with an Olympus
BX-51 polarizing optical microscope equipped with a Linkam
LTS350 hot-stage. X-ray diffraction (XRD) patterns were
obtained by a Rigaku RINT-2500 diffractometer with a heating
stage using a Ni-filtered CuKα radiation. Proton conductivities
were measured by an alternating current impedance method
using a Schlumberger Solartron 1260 impedance analyzer
(frequency range: 100 Hz − 1 MHz, applied voltage: 0.3 V) and
a temperature controller.
Materials. All chemical reagents and solvents of the
highest quality were purchased from Sigma-Aldrich, Kanto
Chemical, TCI, and FUJIFILM Wako Pure Chemical, and were
used as received. Unless otherwise noted, all of the reactions
were carried out under an argon atmosphere in a dry solvent
purchased from Kanto.
1. Introduction
Phosphonic acid is of interest for various areas because it
exhibits coordination or self-assembling properties via hydrogen
bonding.1 It also shows proton conductive properties.2 In
particular, polymers with phosphonic acid groups have been
studied as non-aqueous proton conducting membranes for fuel
cells because they are operated at high temperature.3
Preparation of oriented nanochannels using liquid-
crystalline (LC) materials is a promising approach for efficient
ion transportation.4,5 Proton conduction in liquid crystals and
molecular ordered systems have attracted a great deal of
attention.4–9 However, for LC molecules having phosphonic acid
moieties, only aqueous lyotropic liquid crystals6,10 and
thermotropic smectic liquid crystals7 were reported. In contrast
to phosphonic acids, a variety of benzoic acid8d,e,11 and sulfonic
acid-based liquid crystals5,8a–c,12 were reported.4 If we establish
design of phosphonic acid groups in ordered self-assembling
materials, a new type of functional materials can be obtained.
We have developed ion-conductive LC materials based on
wedge-shaped ammonium,13 phosphonium,13b imidazolium,14
and sulfobetaine compounds.8a These compounds show
thermotropic columnar and/or cubic bicontinuous LC
nanostructures. This structural behavior is due to delicate
balance between hydrophilic and hydrophobic segments of the
molecules.13b In this context, phosphonic acid functionalized
liquid crystals are good candidates for proton conduction.7 This
is because the ordered arrangement of the protogenic
phosphorous moieties in the self-assembled LC state may induce
the formation of stable hydrogen bonded network to achieve
long range proton conduction.
Scheme 1. Synthesis of wedge-shaped phenylphosphonic acids.
Reagent and conditions: (i) BBr3, CH2Cl2, r.t., 15 h; (ii)
CnH2n+1Br, K2CO3, DMF, 80 °C, 12 h; (iii) diisopropylphosphite,
Herein, we show the design and synthesis of phosphonic
acid based thermotropic columnar liquid crystals 1(n) (Scheme
1). We prepared a homologous series of wedge-shaped non-ionic
molecules bearing directly attached phosphonic acid moiety and
benzene ring with peripheral alkoxy chains. We have shown that
wedge-shaped ionic molecules are useful to obtain LC
N,N-diisopropylethylamine,
palladium
acetate,
1,1'-
bis(diphenylphosphino) ferrocene, DMF, 110 °C, 24 h; (iv)
trimethylsilyl bromide, CH2Cl2, r.t., 24 h.

3. Results and Discussion
(102 °C) although the crystallization temperature (30 °C) was
lower compared to 1(12) and 1(16). The DSC measurement also
revealed that enthalpy changes during the crystallization become
larger due to increase of terminal alkyl chain length. It suggests
that assembly of the alkyl chain significantly affect
crystallization of these molecules.
Compounds 1(n) were prepared in four steps starting from
5-bromo-1,2,3-trimethoxy benzene as shown in Scheme 1 (for
details, see Supporting Information). LC properties of
phosphonic acid derivatives [3,4,5-tris(alkoxy)-phenyl-
phosphonic acid] 1(6), 1(12), and 1(16) were examined by POM
observation and DSC measurement (Figure 1, Table 1, and
Figure S1–S4). In the POM observation on cooling, all the
compounds showed focal conic textures suggesting formation of
columnar mesophases (Figure 1a). DSC thermograms of 1(12)
are shown in Figure 1b and the thermal properties of 1(n) are
presented in Table 1. The XRD pattern of compound 1(12) at
75 °C exhibit peaks with the d-spacing values of 2.95, 1.67, and
1.44 nm (Figure 1c) in the small-angle region. The ratio of the
peaks is 1: 1/√3: 1/2. These peaks are attributed to the (100),
(110), and (200) planes of hexagonal columnar (Colh) lattice
(Figure 1d). In addition, a diffuse halo due to correlation of
molten alkyl chains in the LC phase is also observed at 0.45 nm.
Compounds 1(6) and 1(16) also show similar XRD patterns
assigned to Colh phases (Figures S5 and S6).
The intercolumnar distances in the Colh phases of the each
compound are shown in Table 2. Compounds 1(n) showed an
increasing trend in the values of intercolumnar distance due to
increase of terminal alkyl chain length. The number of molecules
of 1(n) per cross-section layer of the columns n was estimated
from the following equation:
n = √3 (NAa2h)/2Mw
Here NA is the Avogadro constant, a is the intercolumnar
distance, h is the thickness of the columnar slice and Mw is the
molecular weight of compounds 1(n). The numbers of molecules
per cross-section layer were calculated to be 3.3, 3.8 and 3.9 for
compounds 1(6), 1(12) and 1(16) respectively.
Figure 2 shows comparison of the thermal properties of
1(12) with those of other non-ionic, ionic, benzyl or phenyl type
wedge-shaped compounds, 3,4,5-tris(dodecyloxy)benzoic acid
5(12),11g 3,4,5-tris(dodecyloxy)-N,N,N-trimethyl-
Compound 1(6), whose alkyl chain length is the shortest in
the three compounds, showed the widest temperature ranges for
the formation of the Colh phases (Table 1). Phase transition from
the isotropic melt of 1(6) occurred at higher temperature
Table 2. Intercolumnar distances of Colh phases of 1(n).
Temperature
/ °C
Intercolumnar
distance / nm
Compound
1(6)
1(12)
1(16)
95
75
85
2.5
3.4
3.8
Figure 1. (a) Polarizing optical microscope (POM) image of
compound 1(12) at 60 °C on cooling at the rate of 10 °C/min
from the isotropic state. (b) Differential scanning calorimetric
(DSC) thermograms of compound 1(12). (c) X-ray diffraction
patterns of compound 1(12) in the LC phase at 75 °C. (d)
Schematic illustration of self-assembly in the Colh phase.
Table 1. Thermal properties of the phenylphosphonic acids 1(n)
Compound
1(6)
Phase transition behavior a
Iso 102 (1.0)
Colh 30 (2.8×101) Cr
1(12)
Iso
Iso
78 (1.5)
95b
Colh 47 (6.1×101) Cr
Colh 47 (7.8×101) Cr
1(16)
a)
Cr: crystalline, Colh: hexagonal columnar, Iso: isotropic;
transition temperatures (°C) and enthalpy changes (kJ mol−1, in
parentheses) were determined by DSC on the first cooling cycle.
b) Determined by POM observation.
Figure 2. Comparison of the thermal properties of seven types
of wedge-shaped compounds having same trialkoxy chains (RO
= C12H25O) and different head groups, 1(12), 5(12),11g 6(12),13c
7(12),9,10 8(12),13b 9(12),13b and 10(12).12c

benzene-ammonium
tetrafluoroborate
6(12),13c
3,4,5,-
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tris(dodecyloxy)-phenylmethyl phosphonic acid 7(12),9,10
trimethyl-[3,4,5-tris(dodecyloxy)benzyl]phosphonium
tetrafluoroborate 8(12),13b trimethyl-[3,4,5-tris(dodecyloxy)-
benzyl]ammonium tetrafluoroborate 9(12),13b and caesium
2,3,4-tris(dodecyloxy)-benzenesulfonate
10(12).12c
These
4.
compounds contain the same trialkoxy (OR = OC12H25) phenyl
moieties but their hydrophilic head groups and spacer units
between the hydrophobic part and head group are different.
Among these wedge-shaped compounds, all ionic
compounds, 6(12), 8(12), 9(12), and 10(12) show LC phases
such as columnar and micellar cubic phases regardless of ionic
species, in the presence or absence of the methylene spacer. In
contrast, LC properties of the non-ionic compounds are
dependent on both of the head groups and their position. For the
non-ionic compounds, only 1(12) shows the LC phase. Although,
previously Gin et al., reported the synthesis of wedge-shaped
compound bearing phosphonic acid 7(12),10 this compound did
not show thermotropic LC properties.9,10 These results suggest
that the direct attachment of phosphonic acid functional moiety
at 1'-position on the phenyl ring without any methylene spacer
played a key role to induce thermotropic LC behavior.
The non-aqueous proton conductivities of compounds 1(6),
1(12), and 1(16) were measured on a cooling process from the
isotropic states. Compound 1(6) showed conductivity of the
order of ca. 10−7 S cm−1 in the isotropic state at around 115 °C,
which gradually decreased upon cooling to Colh phase (Figure
S7). The proton conductivity of 1(6) was lower than that of
polystyrene-based phosphonic acid materials3e and similar to
those of the phosphonic acid derivatives forming smectic
phases.7 The wedge-shaped benzyl-type phosphonic compounds
also showed similar proton conductivities.9 Phase transition
from isotropic melt to assembled structures decreased the
conductivity, which may be due to increase of their viscosities.
These measurements were conducted without orientation control
of the LC materials. If we could achieve alignment, higher
conductivities may be obtained.8i,14
5.
6.
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4. Conclusion
We designed and synthesized thermotropic LC materials based
on phosphonic acid moiety which showed hexagonal columnar
LC properties. These liquid crystals have a potential to be used
as efficient proton conductors. Moreover, the liquid crystals
containing phosphonic acids may be useful in a wide variety of
fields such as supramolecular chemistry, surface chemistry,
biomaterials, and hybrid materials.
This work was partially supported by JST, CREST,
JPMJCR1422.
Supporting Information
Synthesis, characterization, and experimental details. This
material is available on http://XXXXXXXXXX.
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