Synthesis and characterization of room temperature columnar mesogens of cyclotriphosphazene with Schiff base units

Article abstract of DOI:10.1039/c3tc31371a

New supermolecular liquid crystals have been synthesized by attaching polycatenar aromatic units with three terminal alkyl chains through imine linking groups to a central cyclotriphosphazene ring, and these were investigated by polarizing microscopy, DSC

Full text of DOI:10.1039/c3tc31371a

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DOI: 10.1039/C3TC31371A
TOC
Synthesis and characterization of room temperature
columnar mesogens of cyclotriphosphazene with Schiff
base unit
New star-like cyclotriphosphazenes bearing six polycatenar Schiff base units adopt a
staggered organization in columns, forming hexagonal columnar liquid crystalline
phases.

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DOI: 10.1039/C3TC31371A
Synthesis and characterization of room temperature columnar mesogens of
cyclotriphosphazene with Schiff base unit
a
a
a
a
b
c
Qingye He, Heng Dai, Xiaoping Tan, Xiaohong Cheng,* Feng Liu,* and Carsten Tschierske
a
Prof. Dr. X. Cheng, Q. He, H. Dai, X. Tan,
Key Laboratory of Medicinal Chemistry for Natural Resources
Yunnan University
Kunming, Yunnan 650091, P. R. China
Fax: (+86) 871 5032905
E-mail: xhcheng@ynu.edu.cn
b
Prof. Dr. F. Liu
State Key Laboratory for Mechanical Behavior of Materials
Xi’an Jiaotong University
Xi’an, 710049, P. R. China
Fax: (+86) 29 82663453
E-mail: feng.liu@mail.xjtu.edu.cn
c
Prof. Dr. C. Tschierske
Institute of Chemistry
Martin-Luther University Halle-Wittenberg
Kurt-Mothes Str. 2, 06120 Halle/Saale, Germany
1

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Abstract: New supermolecular liquid crystals have been synthesized by attachin
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10
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/Cte3T
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r
aromatic units with three terminal alkyl chains through imine linking groups to a central
cyclotriphosphazene ring, and these have been investigated by polarizing microscopy, DSC and
XRD. Compounds with several different linear terminal alkyl chains are liquid crystals with
columnar phases, and most of them are room temperature columnar phases. A model is proposed
for the organization of the molecules in the columns, which is based on nanosegregation of
aromatic cores and alkyl chains and a staggered organization of the star shaped molecules in
columns. Branching the peripheral alkyl chains leads to an amorphous material.
Key words: liquid crystal; cyclotriphosphazene; Schiff base; self-assembly, organic-inorganic
hybride materials
1
. Introduction
Self assembly of -conjugated compounds into columnar liquid crystalline phases has been
widely investigated because of their potential applications as one-dimensional semiconductors,
1
2
light emitting diodes, photovoltaics and as building blocks for organic ferroelectric etc.
Compounds that exhibit columnar mesomorphism at room temperature or mesomorphic glasses
having a columnar structure are especially interesting for the design of functional self-healing
3
,4
organic semiconducting materials.
Supermolecular liquid crystals (LCs) based on giant and monodisperse liquid crystalline
supermolecules, have been the subject as a novel approach for preparing nano-structured
5
materials by self-assembly. Most of the supermolecular liquid crystals are based on a
6
7 , 8
multivalent
or dendritic central core to which mesogenic units are linked.
Cyclotriphosphazenes are very attractive in this respect, as this inorganic core provides synthetic
versatility, chemical stability and optical transparency for near-infrared to about 210-190 nm in
the ultraviolet spectrum. It is easily functionalized on the phosphorus atoms giving rise to a wide
9
variety of branched molecules. Recently great progress has been made by Serrano et al. with
cyclotriphosphazenes based supermolecular liquid crystals by attaching polycatenar aromatic
mesogens through carboxylate and amide linking groups to the central cyclotriphosphazene
1
0
ring. Different mesophase structures including smectic phases, room temperature columnar
2

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phases and even micellar cubic phases have been realized depending on the n
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terminal alkyl chains in the promesogenic groups. The great versatility of cyclotriphosphazenes as
templates for the design of supermolecular liquid crystals opens new possibilities for the
preparation of assemblies of -conjugated aromatics for optical and electronic applications.
Imines represent one of the earliest and most widely studied linking groups in liquid
1
1
crystals. The advantages of incorporating imine functionalities are their linearity, -conjugation,
versatility, structural variety and the easy preparation and derivatization, combined with
11
comparatively low melting points and wide mesomorphic ranges. Especially, a large variety of
different new LC phases were found by introducing the imine moiety to metallomesogens and
11, 12
bent-core (banana-shaped) molecules.
Salicylaldimines are additionally stabilized by
1
3
intramolecular hydrogen bonding. Other imines without the adjacent OH group are more
sensitive to hydrolysis, and an exchange of aldehyde and amine components can take place under
certain conditions. This could also be an advantage which allows the study of imines as materials
1
4,15
for dynamic polymers, dynamic liquid crystals and dynamic combinatorial LC libraries.
With
this background, it is interesting to know the influence of the imine linking group on the
properties of supermolecular liquid crystals based on cyclotriphosphazenes.
Here, we report the design, synthesis and mesomorphic property of a novel series of
cyclotriphosphazenes based supermolecular liquid crystal that bears polycatenar aromatic imines
mesogens. The purpose of this study is as follows: firstly, to provide a simple approach to
supermolecular liquid crystals with room temperature columnar phases, and secondly, to study the
influence of the type of alkyl terminal chains (linear or branched) on the mesophase properties.
All compounds with linear alkyl chains, reported here, are columnar liquid crystals, most of
which are already at room temperature. However, branching of the alkyl chains leads to the lost of
mesogenic property and the formation of an amorphous material.
2
2
. Results and Discussion
.1 Synthesis
The synthesis of the target molecules I/n is shown in Schemes 1 and S1. The three hydroxyl
group of methyl gallate were alkylated with appropriate alkyl bromides in refluxing DMF in the
3

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presence of K
2
CO
3
resulting in 1/n. Reduction of the ester group with LiAl
D
H
O 4I : 1
l
0
e
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d
039
t
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o
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h
e
3,4,5-trialkoxybenzylalcohols which were then converted to the corresponding benzylchlorides 3
with SOCl
2
in THF at RT. The 3,4,5-trialkoxybenzylchlorides 3/n were then etherified with
CO . The anilines 5/n were obtained by reduction of
the 4-nitrophenylethers 4/n by hydrogenation with H -Pd/C in THF as solvent.
Hexachlorocyclotriphosphazene was reacted with excess 4-hydroxybenzaldehyde in the presence
of K CO in refluxing toluene affording the hexaaldehyde 6. The target compounds I/n were
synthesized by Schiff’s base formation of the hexaaldehyde 6 with excess of the
p-nitrophenol in DMF in the presence of K
2
3
2
2
3
4
-(3,4,5-trialkoxybenzyloxy)anilines 5/n in refluxing ethanol in the presence of a catalytic amount
1
6
of glacial acetic acid.
The final compounds were purified by recrystallization from ethanol/CH
2
Cl
2
and the structure
1
13
and purity of all compounds were confirmed by H NMR, C NMR, elemental analysis and
1
13
gel-permeation chromatography. All peaks in the H NMR and C NMR spectra of each product
I/n are consistent with their chemical structures. The complete reaction of all peripheral aldehyde
groups and the presence of a single completely substituted species were confirmed by GPC,
1
giving single peaks and by H NMR (absence of aldehyde proton around 10 ppm), for more
details, see Figs. S1 and S2.
4

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DOI: 10.1039/C3TC31371A
a
Scheme 1. Chemical structure and synthetic pathway of the cyclotriphosphazenes I/n
Reagents and conditions: a) LiAlH , THF, 40 °C, 12 h; b) SOCl
, 25 °C 14 h; e) K CO , toluene, reflux, 24 h; f) CH
a
4
2
, THF, 25 °C 5 h; c) K
2 3
CO ,
DMF, reflux, 12 h; d) Pd/C, H
2
2
3
3
COOH,
ethanol, 50 °C.
2
.2 Investigations
The liquid crystalline properties of the synthesized compounds were studied by polarizing
optical microscopy (POM, Optiphot 2, Nikon, in conjunction with a FP 82 HT heating stage,
Mettler), differential scanning calorimetry (DSC, NETZSH 200F 3) and different X-ray
diffraction (XRD) techniques, using powder and surface aligned samples. A synchrotron X-ray
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source was used for the investigation of the powder samples by SAXS (beamli
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1
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t
Shanghai Synchrotron Radiation Facility, SSRF) and surface aligned films by grazing-incidence
small-angle X-ray scattering (GISAXS) (beamline BL14B1, SSRF). Based on the powder data,
electron density reconstruction was carried out. The transition temperatures of the synthesized
cyclotriphosphazenes I/n are collected in Table 1.
a
Table 1 Phase transitions of the cyclotriphosphazenes I/n
-
1
Comp.
I/8
T/°C [ΔH/kJmol ]
(cooling: Iso 130 [2.9] Colhex
Cr <20 Colhex 133 [2.6] Iso
Cr <20 Colhex 123 [2.3] Iso
)
)
I/12
(cooling: Iso 117 [2.6] Colhex
I/16
Cr 50 [152] Colhex 107 [2.5] Iso (cooling: Iso 102 [3.0] Colhex 39 [184] Cr)
I/b10
Amorphous
a
-1
Transition temperatures were determined by DSC (5 K min ) and confirmed by POM, peak temperatures from
the second heating/cooling scan are given; abbreviations: Cr = crystalline solid, Iso = isotropic liquid, Colhex
=
hexagonal columnar LC phase with plane group p6mm.
(
a)
(
b)
-1
Fig. 1 DSC traces of compounds I/8, I/12 and I/16 a) on heating and b) on cooling (5 K min ,
second scans).
6

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All compounds, except I/b10, form liquid crystalline phases. DSC scans (see Table 1, Fig. 1)
show that the homologues I/8 and I/12 are LC at room temperature and the LC phases are
maintained up to 133 and 122 °C, respectively, on heating. The LC phases of these two
compounds can be cooled down to ambient temperature without crystallization. By increasing the
chain length, the melting points increase, while the clearing points decrease. Therefore, the
homologue I/16 shows the narrowest LC phase region. Branching the terminal chains, compound
I/b10, led to the formation of an amorphous material without LC properties. Between crossed
polarizers, all compounds except I/b10 show birefringence, in most cases spherulitic (see Figs. 2
and S3) or filament textures occurring together with homeotropically aligned areas which appear
completely dark, indicating that this mesophase is optically uniaxial as typical for hexagonal
columnar mesophases (see Figs. 2 and S3). The XRD patterns have a diffuse wide angle halo
confirming the absence of crystalline order on the atomic scale (see Figs. 3c, S5b and S6b). The
maximum of this diffuse scattering slightly increases with increasing the length of the alkyl
chains from d ≈ 2π/q = 0.41 nm (compounds I/8 and I/12) to 0.42 nm (compound I/16) (Fig. S8).
Shearing easily destroys these textures and further confirms the liquid crystalline nature of these
mesophases. The viscosity of the phases is almost the same as the usual columnar phases of low
molecular weight LC's.
a)
b)
Fig. 2 Optical textures of the Colhex phase (cooling from isotropic state) of compound I/8: a) at T
40°C as observed between crossed polarizers; b) the same texture with with λ-retarder plate, the
=
indicatrix orientation in the compensator is shown at the right bottom.
Investigation of the Colhex phases by polarizing microscopy between crossed polarizers with
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an additional λ-retarder plate (Figs. 2b, S3b and S3d) indicates that the columna
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7
compounds have negative birefringence.
This means that the major intramolecular
π-conjugation pathway, which is along the long axis of the aromatic cores, has a major component
perpendicular to the column long axis.
a
Table 2 Parameters of the Colhex phases of the cyclotriphosphazenes I/n at T = 100 °C.
Comp.
a
hex[nm] ρ[g/mL]
d
diff[nm]
n
I/8
4.68
5.08
5.35
0.99
0.98
0.97
0.41
0.41
0.42
1.09
1.00
0.93
I/12
I/16
a
ahex= lattice parameter at T = 100 °C; ρ = density measured with the floating equilibrium method at T = 25 °C;
o
d
diff= the height of each stratum of the columns measured at 100 C ; n = number of molecules in a stratum of
the columns in the Colhex phase with a height corresponding to ddiff
.

The LC phases were investigated by powder XRD (see Figs. 3, S5-S7 and Tables 2, S1-S3)
and compounds I/8 and I/12 was additionally investigated by GISAXS on aligned thin films (see
Figs. 3b and S5d). The small-angle reflections of the mesophases of compound I/8 have a ratio of
1
/2
their positions corresponding to 1:3 :2, which can be indexed to the 10, 11, 20 reflections of a
hexagonal lattice (Fig. 3a and Table S1). The indexing was facilitated by the 2D-GISAXS pattern
of an aligned sample of compound 1/8 and I/12. The lattice parameter ahex was calculated to be
4
.7 nm for I/8, 5.1 nm for I/12 (Fig. S5 and Table S2) and 5.4 nm for I/16 (Fig. S6 and Table S3)
respectively. The number of molecules, organized in each stratum of the columns with a height h
corresponding to the maximum of the measured diffuse wide angle scattering ddiff (see Figs. 3c,
A
S5b, S6b, S8), was estimated using eq. (1) to be about n = 1 for the investigated compounds (N =
Avogadro constant, M = molecular mass, see Table 2). Note that n is somewhat overestimated as
o
the density at the temperature of the LC (T = 100 C) will be lower than those measured at room
temperature.
2
a
n  (
) 3h(N M )
(
1)
2
A
8

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DOI: 10.1039/C3TC31371A
(
a)
(b)
c)
d)
d
med3.4nm
Fig. 3 Hexagonal columnar phase of compound I/8: a) SAXS diffractogram at T = 100 °C
synchrotron source); b) GISAXS pattern of the p6mm phase recorded at 25 °C, superimposed is
(
the reciprocal lattice for the observed orientation; c) 2D diffraction pattern at T = 100 °C, WAXS
of the diffraction pattern with the d value for the maximum of the diffuse outer scattering is on the
top right corner; d) electron density map, reconstructed from a), dmed is the diameter of the
medium electron density core regions.
In the hexagonal columnar phases, the stacking of the molecules should satisfy the
microsegregation of the polar central moieties from the alkyl chains and simultaneously should
allow a dense packing of the core units. The cyclotriphosphazene core itself provides a diabolo
like double-cone conformation with three of the arms pointing upwards, forming one cone, and
the other arms pointing downwards, forming the other cone. However, the rotation of the around
the RO-P bonds allows the adoption of different conformations, among them a “rod-like” shown
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in Fig. 4a, a “disc-like”shown in Fig. 4b and a more or less star-shaped showDO
n
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n
.10
F
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/C
.
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.
1
8
Formation of simple taper shaped or single-cone-like conformation with a distinct apex appears
to be unlikely, as the limited conformational freedom of the O-P-O units inhibits adaption of such
a conformation. The O-P-O units also provide a significant angle between adjacent rods and thus
it is also unfavorable for a parallel organization of the rod-like units in a rod-like overall
1
9
conformation, as suggested by Moriya et al. for the Nematic phases of cyclotriphosphazenes
with a single alkyl chain at each aromatic core. Also formation of an ideally flat disc-like shape is
difficult as the cylotriphosdphazene core favors a star shaped molecular conformation.
a)
b)
Lcore ≈ 3.6 nm
dcore ≈ 4.1 nm
Fig. 4 Molecular models of two possible molecular conformations of compound I/8: (a) rod-like
conformation, Lcore is the length of the rod-like aromatic core region; (b) nearly disc-like
2
0
conformation, dcore is the diameter of the aromatic core region.
a)
b)
c)
d)
e)
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Fig. 5 Self assembly of I/8: a) side view on the molecule with star conformatio
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between the length of the extended aromatic core region (dcore = 4.1 nm) and the projected length
of the aromatic core dmed assuming a tilt angle with respect to the normal to the column long axis
of θ = 34° (cos θ = dmed / dcore); b) side view on the molecule with star conformation rotate by 0°;
c) presumed staggered organization of the molecules along the columns rotate by 60°; d) top view
and e) side view on the molecule with star conformation rotate by 90°. Added greyish-green
columns have the diameter (dmed) of the medium electron density, representing the volume
occupied by cyclotriphosphazenes and aromatics.
The calculated number n of molecules in each column stratum with a height corresponding to
d
diff (Table 2) was about 1, suggesting that during self assembly the individual molecules could
possibly adopt a star-like averaged shape (Fig. 5a) and then organize with a staggered packing
along the column cores rotated by ~60° (Fig. 5c) with respect to each other. According to our
presumed model for I/8, there are other rotation possibilities as shown in Figs. 5b, 5d and 5e, and
1
0a
the rotation by ~90° is probably the best for space-filling (Figs. 5d and 5e). As we cannot find
reflections indicating a longer repeat distance along the columns or helical features, it is assumed
that there is a more or less alternating stacking along the column long axis. Remaining space at
the periphery can be filled by interdigitation and folding of the alkyl chains. This model is
supported by the electron density map, reconstructed from the small-angle X-ray scattering
(SAXS) intensities of compound I/8 (Fig. 3a and Table S1) and shown in Figs. 3d and S4. The
purple areas in the middle of the columns represent the regions with highest electron density
which can be assigned to the column centers formed predominately by the cyclotriphosphazene
rings. The blue and green medium electron density shells are formed by the aromatic cores and
the red areas (lowest electron density) should represent the continuum of the fluid alkyl chains.
Based on the diameter of the medium electron density regions (blue + green areas, dmed = 3.4 nm)
and the projected distance between the ends of the aromatic cores for the packing of disk-like
molecules (dcore = 4.1 nm), we are able to conclude a tilt angle θ of the aromatic core of
approximately 34° with respect to the normal to the column long axis (Fig. 5a). This tilt would be
in line with the negative birefringence (Figs. 2 and S3) and the relatively low birefringence of the
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textures, appearing gray instead of showing bright birefringence colors (see Figs.
addition, the tilt angle of I/12 (θ ≈ 26°) and I/16 (θ ≈ 18°) can also be estimated using cosθ =
med / dcore, see Figs. S5e and S6d. This suggests that with increasing the terminal chain length, the
D
2
O
a
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a
0
n
.1
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03
S
9/
3
C
)
3
.
TC31371A
I
n
d
individual molecules have the growing tendency of adopting a more flat disc-like averaged shape.
This might lead to a slightly less dense packing in the core region which could reduce the stability
of the self-assembled structures a bit, as indicated by a reduced mesophase stability (reduced
clearing temperatures, see Table 1 and Fig. 1) with increasing length of the terminal chains. This
is also in line with a reduction of the number of molecules in a stratum of the columns with
increasing chain length (see Table 2). It seems that the short chain compounds are more
dominated by core packing (ddiff = 0.41 nm is unusually small) and the importance of chain
packing increases with increasing alkyl chain length (ddiff = 0.42 nm is still small compared to the
usually observed values around 0.45-0.46 nm).
Attempts to change the mesophase structure to micellar cubic by branching of the peripheral
chains were not successful, and compound I/b10 is completely isotropic and relatively viscous,
but no cubic lattice could be found by XRD; there is only one diffuse scattering in the small angle
range at about d = 3.84 nm (Fig. S7). Hence, LC phases were completely lost and also no
crystallization was observed. It appears that the chain branching leads to a peripheral
overcrowding which distorts the core packing of the aromatics and destabilizes the columnar
aggregates which break up into smaller entities. However, it seems that this does not lead to the
formation of monodisperse spherical aggregates with well defined periodic interfaces between
them, so the amorphous material formed by compound I/b10 can be considered as a kind of
granular matter, formed by only short range ordered aggregates with nonspecific shape.
A direct comparison with the previously reported cyclotriphosphazene based supermolecular
1
0
liquid crystals incorporating ester and amide linkages is at this stage not possible, because their
structures are quite different with respect to core length and chain number.
3
. Conclusion
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2

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A series of cyclotriphosphazenes based supermolecular liquid crystal that be
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aromatic imines was successfully synthesized by Schiff’s base formation of the corresponding
hexaaldehyde 6 with appropriately substituted anilines 5/n. For molecules with three linear alkyl
chains at each rod-unit, hexagonal columnar phases were exclusively observed and a packing
model is proposed where the molecules adopt a star-like average conformation and pack into
columns with a staggering of adjacent molecules. The stacking of the molecules with formation of
a hexagonal columnar mesophase should be mainly driven by microsegregation of the polar
central blocks from the nonpolar alkyl chains. The combination of six rod-like aromatic units with
a distorted double cone-like cyclotriphosphazene core is unfavorable for a strictly parallel
alignment of the mesogenic rod-like units, though these units are preorganized by the
cylotripohosphazens. This competition between preorganization and unfavorable geometry and
also the relatively flexible linkage of the bulky trialkoxybenzyl units to the rod-like cores are
unfavorable for crystallization and also provides relatively low transition temperatures. Thus
room temperature columnar phases were realized by compounds with medium terminal chain
length while branching the peripheral alkyl chains led to an amorphous material.
Acknowledgements. This work was supported by the National Natural Science Foundation of

China (No. 21274119, 21074105 and 11205235) and the Yunnan Science Foundation
(2013FA007and 2010CD018). We thank Dr. Xiangbing Zeng and Prof. Goran Ungar of the
University of Sheffield for writing initial programs for electron density calculation. For help with
the synchrotron experiments, we also thank Drs. Xiuhong Li, Feng Tian (beamline BL16B1) and
Wen Wen, Xiaolong Li (beamline BL14B1) at Shanghai Synchrotron Radiation Facility (SSRF),
China.
1
3

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DOI: 10.1039/C3TC31371A
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7 That the aromatic cores are oriented perpendicular to the column axis is shown by the color of the fans in the
optical micrographs (inset in Figure 2b) taken with a λ-retarder plate. The yellow and blue colors define the
orientation of the high-index axis as radial rather than tangential. Since the columns are tangential in the fans, and
the high-index axis is known to be parallel to the intramolecular -conjugation pathway along the aromatic cores,
it follows that the rigid cores have a preferred direction perpendicular to the columns; (n < n = negative
‖
birefringence).
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Journal of Materials Chemistry C
Page 16 of 16
View Article Online
DOI: 10.1039/C3TC31371A
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2
0 The length of the rod-like aromatic core region Lcore and the diameter of the aromatic core region dcore of the
nearly disc-like conformation were determined with space-filling models (also known as CPK models by Corey,
Pauling and Koltun); see for: R. B. Corey and L. Pauling, Rev. Sci. Instr., 1953, 24, 621-627.
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