Branched quaternary ammonium amphiphiles: Nematic ionic liquid crystals near room temperature

Article abstract of DOI:10.1039/b909605a

Branched quaternary ammonium molecules were synthesized and characterized by calorimetric, optical and X-ray diffraction studies; two of the molecules exhibited interesting nematic liquid crystalline behavior close to room temperature.

Full text of DOI:10.1039/b909605a

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Branched quaternary ammonium amphiphiles: nematic ionic liquid
crystals near room temperaturew
Wen Li,a Jing Zhang,a Bao Li,a Mingliang Zhangb and Lixin Wu*ab
Received (in Cambridge, UK) 14th May 2009, Accepted 29th June 2009
First published as an Advance Article on the web 21st July 2009
DOI: 10.1039/b909605a
Branched quaternary ammonium molecules were synthesized
and characterized by calorimetric, optical and X-ray diffraction
studies; two of the molecules exhibited interesting nematic liquid
crystalline behavior close to room temperature.
consisting of rod-like mesogens with an oligo(oxyethylene)
group at the lateral position could disturb the original layer
structure and form new and complex LC structures.12 In
addition, calamitic liquid crystals connected with flexible
lateral alkyl chains can result in the formation of nematic
phases.13 Such a strategy for obtaining a nematic phase
has also been employed in polymer14 and hybrid LCs15
successfully. We believe that an ionic nematic mesophase can
be achieved through a branched modification of charged
molecules with mesogenic groups.
In this communication, we report nematic ionic liquid crystals
(ILCs) formed near room temperature. ILCs that combine
both the solvent properties of ionic liquids and the self-
organization features of liquid crystals, have been of considerable
interest in recent years because they present unique potential
applications lacking in normal liquid crystals.1 These kinds of
liquid crystals have been expected to serve as anisotropic
ion-conductors,2 ordered reaction media3 or templates for
the synthesis of zeolites,4 mesoporous materials5 and nano-
materials.6 Additionally, ILCs also possess potential as
electrolytes in dye-sensitized solar cells.7 The rich properties
observed for these materials have inspired the synthesis of
ILC materials and the hunt for new mesophases. Many
ILCs have been designed, synthesized and characterized,
such as ammonium, phosphonium, imidazolium, pyridinium,
vinamidinium, and dithiolium salts, etc., and have been well
reviewed.1d However, most of the ILCs reported up to date are
smectic polymorphism1d,8 or columnar phases.2,9 The simple
yet more significant nematic phase is still very rarely
reported.10,11 One of the main reasons is that most of the
popular ILCs are linear molecules consisting of a single ionic
head group connecting one or multiple long aliphatic tails.
As can be easily understood, the microsegregation of
incompatible units, the aggregation of compatible units, and
the minimization of volume are the main driving forces that
give rise to a general tendency for lamellar or columnar
structures. From the technical point of view, a simple nematic
ILC is more important, because it has the least order and
possesses the lowest viscosity of all types of mesophase,
making the nematic phase easier to realign under external
stimulation. Therefore, creating nematic ILCs might allow the
improvement of their applications in ion transport, or the
development of new switchable liquid crystalline electrolytes.
In order to realize nematic ILCs, it is necessary to reduce the
interactions existing between ionic molecules moderately.
Recently, some reports demonstrated that T-shaped amphiphiles
Based on this motivation, we designed a series of branched
(close to T-shaped) quaternary ammonium molecules (B-n), by
connecting an azobenzene mesogen with a flexible alkyl chain
to the lateral benzoic group of an ammonium bromide
molecule via esterification, as shown in Fig. 1. We expected
to utilize the branched molecular geometry as well as the alkyl
chain length (n) to adjust the interactions between molecules,
and thus leading to a nematic phase near room temperature.
B-n were synthesized according to modified routes from
the literature.16–18 The detailed preparation procedures,
characterizations and instrumentation are all presented in
the electronic supplementary information (ESI).w
The mesomorphic properties of the B-n series were
characterized using differential scanning calorimetry (DSC),
polarizing optical microscopy (POM) and temperature
dependent X-ray diffraction (XRD), in detail. The phase
sequences and assignments, transition temperatures, associated
enthalpies and entropies are summarized in Table 1.
By taking B-8 as a representative example, its DSC traces
exhibit typical thermal properties in the first cooling and
second heating runs, as shown in Fig. 2. Upon cooling, an
a State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, Changchun 130012, P. R. China.
E-mail: wulx@jlu.edu.cn; Fax: þ86 431 85193421;
Tel: þ86 431 85168481
b College of Chemistry, Jilin University, Changchun 130012,
P. R. China
w Electronic supplementary information (ESI) available: Detailed synthesis
and characterization of amphiphiles. See DOI: 10.1039/b909605a
Fig. 1 (a) Schematic molecular structure of quaternary ammonium
bromides, B-n, and (b) proposed packing model of the nematic phase
of B-6 and B-8.
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Table 1 Summary of assignments of mesophases, transition
temperatures, enthalpies and entropies of B-n by means of DSC
(second heating run)
Amphiphiles Transitionsa T/1C DH/kJ molꢁ1 DS/J Kꢁ1 molꢁ1
B-6
g–N
N–Iso
g–N
N–Iso
g–SmA
SmA–Iso
g–SmA
SmA–Iso
23
36
25
68
32
87
31
92
—
0.38
—
1.17
—
1.90
—
—
1.23
—
3.43
—
5.31
—
B-8
B-10
B-12
2.63
7.23
a
g, N, SmA and Iso denote glass transition, nematic, smectic A and
isotropic phases, respectively.
exothermal change appears at 60 1C, and the estimated
transition enthalpy and entropy are quite low as compared
to the normal phase transition from liquid crystal to the
isotropic state (see Table 1). This result suggests a much
smaller increase in ordering going from the isotropic liquid
to the mesophase. With further decreasing temperature, a glass
transition occurs at ca. 25 1C. Similar transition temperatures
from the glass state to the mesophase and from the mesophase
to the isotropic state are also observed upon second heating
and following cycles, indicating that the transition behavior is
reversible. The DSC curves of other B-n molecules (n ¼ 6, 10, 12)
exhibit similar phase transitions, as provided in the ESI.w The
alkyl chain length of B-n has an influence on the phase
transitions. With the increase in the alkyl chain length from
B-6 to B-12, both the phase transition temperatures and the
enthalpy changes tend to rise accordingly. Interestingly,
the mesophases of all B-n molecules emerge at 23B32 1C,
indicative of suitable candidates for an anisotropic ionic liquid
at near room temperature.
Fig. 3 POM images of (a) nematic droplets of B-8 occurring just
below the isotropic liquid phase, (b) a schlieren texture of the nematic
phase of B-8 (thin), (c) marbled patterns of B-6 at 25 1C, and (d)
orthogonal isogyre texture of B-10 at 60 1C with mechanical press.
into the glass state. B-6 shows marbled patterns even at lower
temperature, which can be regarded as a room-temperature
LC material (Fig. 3c). Notably, B-10 and B-12 show
preferential homeotropic aligned areas in their mesophases,
which are optically isotropic between crossed polarizers. The
orthogonal isogyre texture of B-10 can be clearly viewed when
the top glass slide was pressed, as presented in Fig. 3d, which
implies the formation of smectic A phase.
In agreement with the POM observations, powder XRD
results also support the assignment of the nematic phase, as
seen in the data of B-8 (Fig. 4). When cooling from the
isotropic state, diffuse diffractions at both low and wide angles
were observed and assigned to the intermolecular short-range
interactions which are parallel and perpendicular to the
molecule long axis, respectively. The estimated distance from
the low-angle diffraction is ca. 2.4 nm through Bragg’s
equation, which corresponds to the average length along the
molecular long-axis. Meanwhile, the broad wide-angle
diffraction indicates an average intermolecular separation of
0.46 nm, close to the typical value for liquid crystalline phases.
At 25 1C, the two diffuse diffractions are still maintained,
indicating the near room temperature mesophase, which is
consistent with the data from POM and DSC. The XRD
patterns of B-6 are similar to that of B-8, showing the presence
The mesophases deduced from the DSC results can be
further understood from the optical textures upon heating
and cooling the samples. The isotropic liquid of B-8 placed
between glass slides was examined through a slow cooling rate
of 2 1C minꢁ1. The appearance of distinct nematic droplets
reveals the phase definitely (Fig. 3a). The texture flashed when
the top glass slide was pressed gently. Upon cooling further,
the nematic schlieren texture (Fig. 3b) with four-brush
disclinations (strength S ¼ 1) was persistent at the edges of
the slide. The texture was maintained until it was cooled down
to room temperature, indicating that the mesophase can freeze
Fig. 2 DSC thermograms of B-8 on the first cooling and second
heating processes with a rate of 5 1C minꢁ1
Fig. 4 Variable-temperature X-ray diffractions of B-8 (inset: wide-angle
.
X-ray diffraction of B-8).
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of a nematic phase. For B-10 and B-12, strong low-angle
diffraction peaks are observed, suggesting lamellar LC
structures.w Combining the microscopic observations, DSC
and XRD data, we conclude that B-6 and B-8 with shorter
alkyl chains form nematic phases, and B-10 and B-12
form smectic A phases due to the increasing interactions.
A schematic packing model of the nematic phase of B-6 and
B-8 is proposed, as shown in Fig. 1b.
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In summary, we have designed and synthesized branched
quaternary ammonium amphiphiles. The special chemical
structure can lead to the formation of nematic ILCs. The
results obtained suggest
a direct relationship between
molecular structure and LC properties. Most importantly,
the temperature at which the nematic phase occurs is low
and close to room temperature, which is beneficial for
functionalizations. Detailed research for more of these kinds
of molecule is currently in progress in this laboratory.
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We acknowledge the financial support from National Basic
Research Program (2007CB808003), National Natural Science
Foundation of China (20703019, 20731160002), Open Project
of State Key Laboratory of Polymer Physics and Chemistry of
CAS, and 111 Project (B06009) for supporting the fruitful
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