Ionic liquid crystals bearing bipyridinium and pentaphenylene groups

Article abstract of DOI:10.1016/j.molliq.2010.09.002

Molecular and polymeric liquid crystals bearing bipyridinium and pentaphenylene groups were synthesized and characterized. These new ionic liquid crystals develop single layer smectic-type mesophases with molecules tilted or orthogonal with respect to the

Full text of DOI:10.1016/j.molliq.2010.09.002

Journal of Molecular Liquids 157 (2010) 133–141
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Ionic liquid crystals bearing bipyridinium and pentaphenylene groups
José Carlos Díaz-Cuadros a, Leticia Larios-López a, Rosa Julia Rodríguez-González a,
Bertrand Donnio b,1, Daniel Guillon b,1, Dámaso Navarro-Rodríguez a,
⁎
a
Centro de Investigación en Química Aplicada, Blvd. Enrique Reyna 140, CP 25253, Saltillo, Coahuila, Mexico
b
Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 (CNRS-Université Louis Pasteur), 23 rue de Loess, 67037 Strasbourg Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 June 2010
Received in revised form 23 August 2010
Accepted 10 September 2010
Available online 25 September 2010
Molecular and polymeric liquid crystals bearing bipyridinium and pentaphenylene groups were synthesized and
characterized. These new ionic liquid crystals develop single layer smectic-type mesophases with molecules
tilted or orthogonal with respect to the smectic planes as a function of temperature. A high tendency to develop a
homeotropic behavior was observed for the molecular liquid crystals. A molecular arrangement was proposed in
which the different parts of molecules are fully segregated from one another with the ionic groups stabilizing the
smectic order. These molecules show a luminescence in the blue region that combined with the liquid–crystalline
behavior could be interesting for the fabrication of light emitting thin films with ordered molecular emitters.
© 2010 Elsevier B.V. All rights reserved.
Keywords:
Polymers
Bipyridinium
Pentaphenylene
Liquid crystals
Smectic
1. Introduction
smectic type in a wide temperature range (110–210 °C). By X-ray
diffraction analysis it was found that the different parts of molecules
The driving forces leading to a liquid–crystalline behavior are the
mutual repulsive interactions (dispersive forces) between the different
parts of molecules [1] and if one of these parts is an ionic group it will play
a key role in the structure stabilization, particularly in lamellar
mesophases [2–4]. Ionic liquid crystals show unique characteristics that
can be different from those of conventional or neutral liquid crystals. For
instance, a tendency to form a spontaneous homeotropic phase has been
reported to occur particularly in ionic liquid crystals, such as the N,
N-dialkyl-substituted diazoniabicyclooctane [5] and some alkylammo-
nium derivatives [6]. This property is useful for the surface treatment of
substrates that may allow the deposition of functional thin films with
highly ordered molecules [7]. In this work we are interested in liquid
crystals bearing p-phenylene groups, which are usually modified with
alkyl chains to render them soluble, facilitating in this way their
processing [8]. Recently, this kind of π-conjugated groups has aroused a
huge interest because of their luminescent properties in the blue region
[9–12]. In a previous work, dialkyloxy-substituted oligophenylenes were
functionalized at one chain end with bromine to obtain intermediates
[13], which were further used to quaternize the pyridine [14]. The
resulting pyridinium salts developed a mesomorphic behavior of the
(oligophenylene core, alkyloxy chains and pyridinium ring associated
with its counterion) segregate from one another in layered domains
periodically arranged in space, as it normally occurs for amphiphilic
molecules. These dialkyloxy-substituted oligophenylenes can be also
functionalized at both chain ends with bromine to obtain intermediates
that can be reacted with bipyridine (viologen) to produce molecular and
polymeric ionic liquid crystals. These ionic molecules are interesting not
only because they develop both liquid–crystalline and light emission
properties, but also because they show a tendency to develop a
homeotropic phase when they are deposited onto a glass substrate.
There are few reports on liquid crystals bearing a viologen or bipyridyl
group [15], and we thought it of interest to combine this bifunctional
group with a rigid π-conjugated core to investigate the thermotropic
behavior as well as the light emission characteristics of the resulting ionic
molecules. So, in the present work, the mesomorphic behavior and the
light emission characteristics of new ionic liquid crystals bearing both
bipyridinium and pentaphenylene groups are described and discussed.
2. Experimental
2.1. Materials
⁎
Corresponding author. Tel.: +52 844 4389830; fax: +52 844 4389839.
E-mail addresses: cdiazc2006@yahoo.com.mx (J.C. Díaz-Cuadros), llarios@ciqa.mx
1-bromobutane, 1,10-dibromodecane, 1,12-dibromododecane, 4,4′-
bipyridyl, butyllithium 1.6 M in hexanes, 4′-bromo-(1,1′-biphenyl)-
4-ol, triisopropyl borate, bromine (Br2), calcium chloride, potassium
carbonate, sodium carbonate, hydrochloric acid, hydroquinone, sodium
hydroxide, potassium iodide, calcium sulfate di-hydrated, tetrakis
(L. Larios-López), rjrodrig@ciqa.mx (R.J. Rodríguez-González),
Bertrand.Donnio@ipcms.u-strasbg.fr (B. Donnio), directeur.ecpm@unistra.fr
(D. Guillon), damaso@ciqa.mx (D. Navarro-Rodríguez).
1
Tel.: +33 388 10 71 58; fax: +33 388 10 72 46.
0167-7322/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molliq.2010.09.002

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J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
(triphenylphosphine)palladium(0), ethyl acetate, acetone, chloroform,
methylene chloride, dimethylformamide (DMF), ethanol, ethyl ether,
hexane, methanol, carbon tetrachloride, tetrahydrofuran (THF) and
toluene were purchased from Aldrich Co. and/or J. T. Baker, and were
used as received unless otherwise specified. The 4,4′-bipyridyl was
recrystallized from toluene. Calcium sulfate di-hydrated was dehy-
drated at 250 °C for 8 h. DMF was dried with anhydrous calcium sulfate
for 72 h and distilled at 80 °C under reduced pressure. THF was distilled
from a sodium/benzophenone complex before used.
solution was transferred to a 500 mL separation funnel to be washed
with distilled water. The organic phase was transferred to a round
bottom flask to eliminate the solvent by evaporation under reduced
pressure. Finally, the intermediate IV (or V) was purified by chromato-
graphic column using CHCl3 as solvent and elusion medium. Com-
pounds IV and V were recuperated as white powders. Yields around 70%
were obtained for both IV and V. m.p. 132 °C (IV) and 110 °C (V). 1H
NMR (CDCl3) δ (ppm)=7.65 (m, 12H, Ar), 7.05 (d,s, 6H, Ar), 4.0 (m, 8H,
CH2α-O), 3.4 (t, 4H, CH2α-Br), 1.85 (m, 8H, CH2β-O), 1.75 (m, 4H, CH2β-
Br), 1.2-1.6 (m, 28H IV or 36H V, –CH2–) and 0.92 (t, 6H, CH3). Elemental
analysis (%): IV (C58H76Br2O4) calcd C 69.87, H 7.68; found C 70.19, H
7.43 and V (C62H84Br2O4) calcd C 70.71, H 8.04; found C 69.68, H 8.26.
2.2. Synthesis
Ionic molecules were synthesized following the reaction route
outlined in Scheme 1. The 1, 4-bis(4′(ω-bromoalkyloxy)biphenyl)-2,5-
dibutoxybenzene (IV or V) was synthesized through the Suzuki cross-
coupling reaction between the 1,4-dibutoxy-2,5-phenylbisboronic acid
(I) and the4′(ω-bromoalkyloxy)-4-bromobiphenyl (II orIII) usingPd[0]
as catalyst [16,17]. The chemical precursors I–III were synthesized
according to procedures described in detail in previous works [18]. The
dibromo end-functionalized molecules (IV and V) were then reacted
with 4,4′-bipyridineto obtain two molecules having atboth chain ends a
mono-substituted bipyridinium group (M-10 and M-12) and a polymer
bearing both bipyridinium and pentaphenylene groups linked through
flexible spacers (P-10). The procedures for obtaining the intermediates
(IV and V) and final molecules (M-10, M-12 and P-10) are described in
the next paragraphs.
2.2.2. 1,4-bis-[(4′-(ω-(bipyridinium bromide)alquiloxy)biphenyl]-2,5-
butoxybenzene (M-10 and M-12)
In a 50 mL pear-shaped Schlenk flask 0.2 g (0.2 mmol) of IV or 0.2 g
(0.19 mmol) of V, 0.156 g (1.0 mmol) of 4, 4′-bipyridine and 15 mL of
DMF, were introduced. Then, the solution was degassed, stirred and
heated up to 100 °C for 8 days. After cooling to room temperature, the
solution was poured drop wise into 150 mL of ethyl acetate to obtain a
precipitate that was recuperated by filtration and dried in a vacuum
oven at 30 °C for 24 h. The recuperated compound (M-10 or M-12) is a
beige colored solid. Yields for both reactions were higher than 90%. 1H
NMR (CDCl3/MeOD) δ (ppm)=9.2 (d, 4H, bpy), 8.8 (d, 4H, bpy), 8.3 (d,
4H, bpy), 7.7 (d, 4H, bpy), 7.5–7.7 (m, 12H, Ar), 6.9–7.1 (d, s, 6H, Ar), 4.8
(t, 4H, CH2α-N), 3.9–4.1 (m, 8H, CH2α-O), 2.0 (m, 4H, CH2β-N), 1.6–1.8
(m, 8H, –CH2β-O), 1.1–1.5 (m, 28H M-10 or 36H M-12, –CH2–), and 0.9
(t, 6H, CH3). Elemental analysis (%): M-10 (C78H92Br2N4O4·1H2O) calcd
C 70.57, H 7.13, N 4.22; found C 68.91, H 7.48, N 3.9 and M-12
(C82H100Br2N4O4·1H2O) calcd C 71.18, H 7.43, N 4.05; found C 68.66, H
7.91, N 3.70. Half molecule of water was considered per pyridinium–
bromine ion pair, although it seems that more than half molecule of
water was absorbed by each one of them. The 1H NMR spectrum of these
compounds showed a singlet at 2.45 ppm that was attributed to water.
This signal frequently appears in pyridinium salts, even for those dried
2.2.1. 1,4-bis(4′-(ω-bromoalkyloxybiphenyl)-2,5-dibutoxybenzene
(IV and V)
In a 250 mL two-neck round bottom flask equipped with condenser,
magnetic stirring and an inner gas inlet (argon), 1 g (3.2 mmol) of I,
2.98 g (6.4 mmol) of II or 3.2 g (6.4 mmol) of III, 50 mL of toluene, 0.11 g
of [P(C6H5)3]4Pd(0) and 19.4 mL of an aqueous solution of Na2CO3 2 M,
were introduced. The solution was degassed, vigorously stirred and
heated up to 100 °C for 24 h. After cooling to room temperature, the
Scheme 1. Reactions involved in the preparation of ionic liquid crystals bearing bipyridinium and pentaphenylene groups. (i) Toluene, Pd[0] and Na2CO3/H2O; (ii) DMF, 4,4′-bipyridine
and (iii) DMF and IV (or V).

J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
135
at 80 °C under reduced pressure overnight [4,19]. The bound water is
then difficult to remove as deduced from both the 1H NMR spectra and
the elemental analysis results.
nylene group can be appreciated close to this region. For the polymer P-10
the 1H NMR spectrum (Fig. 1b) shows signals of both mono- and di-
substituted bipyridine moieties, along with those corresponding to the
pentaphenylene groups. The signals of protons of the mono-substituted
bipyridines (chain ends) are located at δ=9.2, 8.8, 8.3 and 7.7 ppm, while
those (only two signals) corresponding to the di-substituted bipyridine
appear at δ=9.0 and 9.2 ppm. From these signals it was calculated that
the resulting molecule contains around 5 repeating units, which seems to
be quite small for a polymer; however, the repeating unit by itself is quite
long in such a way that the resulting molecule can be considered as a low
molecular weight polymer (~5800 g mol–1). It should be mentioned that,
during the quaternization reactions (conducted under vacuum), solutions
turned out colored from yellow to deep blue, though such color vanished
progressively after the reaction flasks were opened and the solutions
entered in contact with air. Purified salts (M-10 and M-12) were yellowish
powders and filtrates were rather colorless. The polymer P-10 remained
brown colored even after purification. Colored reaction solutions have
been previously observed in quaternization reactions of polymers bearing
pyridine rings [20]. Such color was attributed to the formation of ion pairs
that give rise to charge–transfer bands, as it was long time ago proposed
by Boucher and Mollet for polymers bearing pyridinium groups [21]. It is
to point out that several experiments were conducted to obtain the
polymer P-12, however in any of them a polymer was obtained, even for
long reaction times.
2.2.3. Polymer bearing bipyridinium and pentaphenylene groups (P-10)
In a 50 mL pear-shaped Schlenk flask 0.059 g (0.059 mmol) of IV,
0.078 g (0.059 mmol) of M-10 and 15 mL of DMF, were introduced.
Then, the solution was degassed, stirred (magnetic bar) and heated up
to 100 °C for 8 days. After cooling to room temperature, the solution was
poured drop wise into 150 mL of ethyl acetate to obtain a precipitate
that was recuperated by filtration and dried in a vacuum oven at 30 °C
for 24 h. The recuperated compound (P-10) is a brown colored solid. 1H
NMR (CDCl3/MeOD) δ (ppm)=9.2 (4H, bpy+2H, end-bpy), 9.0 (4H,
bpy), 8.8 (2H, end-bpy), 8.3 (2H, end-bpy), 7.75 (2H, end-bpy), 7.4–7.65
(12H, Ar), 6.8–7.1 (6H, Ar), 4.6–4.8 (4H, CH2α-N), 3.8–4.0 (8H, CH2α-O),
2 (4H, –CH2β-N), 1.5–1.8 (8H, –CH2β-O), 1.2–1.5 (28H, –CH2–), and 0.92
(6H, CH3).
2.3. Characterization
The chemical structure of molecules was verified by 1H NMR
spectroscopy using a Jeol spectrometer (300 MHz) and CDCl3, THF-d or
CDCl3/CD3OD as solvents. Elemental analysis was performed in a Carbo
Erba (1106) instrument at the Charles Sadron Institute, Strasbourg France.
The thermal stability of compounds was determined on vacuum dried
samples and using a TGA 951 thermal analyzer from DuPont Instruments
connected to a nitrogen vector gas and heating at 10 °C min−1 from room
temperature to 800 °C. The DSC traces were obtained in a differential
scanning calorimeter (MDSC 2920) from TA Instruments at heating and
cooling rates of 5 °C min−1. Optical textures of mesophases were
registered upon cooling (2 °C/min) in an Olympus BH-2 polarizing optical
microscope coupled with a Mettler FP82HT hot stage. The structure of
mesophases was obtained at different temperatures by means of X-ray
diffraction analyses using an INEL CPS120 diffractometer (Kα1 copper
radiation and a home-made electrical oven). The UV–visible absorption
spectra were recorded on a Shimadzu 2401 PC spectrophotometer with a
detection interval of 190–800 nm, while the emission spectra were
obtained using a Perkin Elmer LS 50B spectrofluorimeter illuminating the
samples with a UV light of γ=10 nm higher than the γabs.max. Solutions
for optical analyzes were prepared in spectroscopic grade chloroform at
3.2. Mesomorphic behavior
The intermediates IV and V were first studied by DSC and their
corresponding thermograms (Fig. 2) showed a monotropic transition. By
optical microscopy a Schlieren texture was observed in both compounds
indicating without ambiguity the presence of a nematic phase (Fig. 3).
The molecules bearing mono-quaternized bipyridines (M-10 and
M-12) were analyzed by thermogravimetry exhibiting an initial decom-
position temperature of around 200 °C, which was considered as the
maximum temperature for the thermotropic examination. This relatively
low decomposition temperature is without doubt associated to the poor
thermal stability of the CH2–N+ bond of pyridinium salts [22]. The DSC
traces of these compounds showed one or two endotherms/exotherms as
it can be appreciated in Fig. 4. On heating from room temperature the M-
10 compound showed only one small endotherm centered around 100 °C
and upon cooling it showed a broad exotherm that extends from 185 to
140 °C. On the other hand, optical textures were registered upon cooling
concentrations of 0.04 mg mL−1
.
3. Results and discussion
3.1. Synthesis
Several reaction steps were considered to obtain the intermediates IV
and V as well as the molecules bearing both pyridinium and pentaphe-
nylene groups (M-10, M-12 and P-10). The intermediates were
synthesized through procedures described in previous papers [18], so
here the discussion is focused only on the molecules bearing pyridinium
and pentaphenylene groups. These ionic molecules were synthesized
through a quaternization reaction conducted in a high dielectric constant
solvent (DMF). The mono-quaternization of the bipyridine proceeds
relatively fast, however the quaternization of the second nitrogen atom of
the bipyridine is much more difficult because it becomes less electropos-
itive. This effect, associated with the poor solubility of the growing
molecules, may be the cause of the limitations in molecular weight as
deduced from the 1H NMR of the polymer P-10. The chemical structure of
M-10 and M-12 was confirmed from the 1H NMR spectra, which were
detailed in the experimental section. The 1H NMR signals of the
bipyridnium moiety are located at low fields (from δ=7.7 to 9.2 ppm)
as it is shown in Fig. 1a for the compound M-10. For the molecules bearing
mono-substituted bipyridines four signals (δ=9.2, 8.8, 8.3 and 7.7 ppm)
can be distinguished in the 1H NMR spectrum; additionally, two complex
signals (δ=6.9 and 7.5 ppm) corresponding to protons of the pentaphe-
Fig. 1. Low field of the 1H NMR spectra of M-10 (a) and P-10 (b). Solvent: CDCl3/MeOD.

136
J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
Fig. 2. DSC thermograms of IV (left) and V (right).
Fig. 3. Optical textures of IV (left) and V (right).
and for this compound a homeotropic behavior (Fig. 5) was observed in
the all temperature range. In fact, the reduced number of batonnets
appearing on cooling (around 184 °C) from the isotropic state vanished
progressively as the temperature decreased, and the image became dark
and remained as it is up to room temperature. This phenomenon has been
frequently observed in rodlike molecules bearing quaternary ammonium
end groups, which adhere to the surface of the glass substrate with
molecules pointing out in a perpendicular direction with respect to the
substrate plane [7]. The DCS traces of the M-12 compound exhibited two
endotherms on heating from room temperature, one of them centered at
100 °C and the other localized between 160 and 175 °C; however, on
cooling from the isotropic state only a broad exotherm was appreciated at
high temperature. It is to point out that the optical textures of M-12
showed batonnets upon cooling (around 175 °C), that also disappear
leading to a homeotropic texture, but in this case a transition leading to a
non-specific birefringent texture (Fig. 5) was observed around 100 °C. By
DSC it was difficult to detect something in this region (according to Fig. 4),
even though by polarizing optical microscopy (POM) and X-ray diffraction
a clear transition was observed around 100 °C. For the polymer P-10, a
transition was observed by DSC on heating around 110 °C, although by
POM it was pretty difficult to locate such transition. Due to discrepancies
observed between DSC and POM experiments, a detailed structural
analysis as a function of temperature was performed by X-ray diffraction,
in both heating and cooling modes.
3.3. Structural characterization
The X-ray diffraction patterns of M-10 and M-12 were obtained at
different temperatures (upon heating and cooling) and results are
shown in Figs. 6 and 7. In order to avoid a thermal degradation of
compounds, a maximum temperature of 190 °C was used in these
measurements. It can be noticed that the X-ray diffraction patterns show
in general two equidistant Bragg reflections (d001 and d002), revealing
the layered structure, typical of a smectic-type mesophase [23]. For the
M-10 compound (Fig. 6) such diffraction peaks are suddenly shifted
Fig. 4. Heating (→) and cooling (←) DSC thermograms of M-10, M-12 and P-10.

J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
137
Fig. 5. Optical textures of M-10, M12 and P-10.
Fig. 6. Low angles region of the X-ray diffraction patterns for the compound M-10.
Fig. 7. Low angles region of the X-ray diffraction patterns for compound M-12.
Registered upon heating and cooling at different temperatures.
Registered upon heating and cooling at different temperatures.

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J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
cooling) a broad peak above 100 °C and a single sharp peak below
100 °C. In order to determine the molecular arrangement of M-10 and
M-12 in the mesophase, the measured layer spacing was compared with
the length of the molecule in its most extended conformation (L=54
and 59 Å for M-10 and M-12, respectively), calculated by using
molecular modeling software (Spartan'04). The d001/L radios shown in
Table 1 indicate that the molecular arrangement corresponds to a
single-layered smectic-type mesophase with molecules orthogonal
(high temperature) or tilted (low temperature) with respect to the
smectic planes, as it is schematically represented in Fig. 9. In this model
the different parts of molecules are fully segregated from one another
and periodically arranged in space. Within the ionic sublayers the
cations and anions are facing each other; they may be also facing the
opposite charge ions of the neighboring smectic layers, stabilizing in this
way the mesophase [4,24]. This model is plausible because both the π–π
interactions of the long pentaphenylene groups and the interaction of
the electrical charges of the ionic groups (pyridinium rings and counter
ions) seem to be well satisfied. Similar molecular arrangements were
recently proposed for relatedmolecules possessingbothpyridiniumand
terphenylene groups [14].
Compared to the M-10 and M-12 compounds, the X-ray diffraction
patterns of the polymer P-10 did not exhibit abrupt changes in the
position of the Bragg diffraction peaks, but a rather continuous and
reversible shift as it is observed in Fig. 10. By plotting d001 against
temperature (Fig. 11) it can be seen that the layer spacing of the polymer
P-10 expands only 7 Å. The d001/L radios found for the polymer also
correspond to a tilted (at low temperature) or to an orthogonal (at high
temperature) single-layered smectic-type mesophase, although in this
case the molecules are progressively tilted attaining an angle smaller than
that calculated for the small molecules (Table 1). In the wide angles region
of the X-ray diffraction patterns of the polymer (not shown here) only a
broad diffraction peak was appreciated in the all temperature range,
indicating that there is no order of molecules within the smectic layers. A
molecular arrangement proposed for the polymer P-10 is schematically
represented in Fig. 12. In this molecular arrangement the repeating units
may be covalently connected from one smectic layer to the other.
According to all these observations, the types of mesophases presented by
M-10, M-12 and P-10 can be assigned as described in Table 1.
The models proposed in this work for the molecular and polymeric
liquid crystals are some similar to that proposed for Ohta et al. [5] for ionic
liquid crystals based on N, N-dialkyl-substituted diazoniabicyclooctanes,
which show a spontaneous homeotropic alignment and that seem to be
stabilized by the ionic groups, as it is proposed in the present work.
The phase transition between orthogonal and tilted smectic phases
can be either first or second-order. First-order transitions are those
characterized by a discontinuous change in the tilt angle of molecules with
respect to the smectic plane, while second-order transitions are those
showing a continuous or smooth change in such tilt angle [25]. In a first-
order transition two distinct phases coexist at the transition temperature
[23]. For the M-10 and M-12 compounds, the first-order transition was
determined from the sudden shift in the position of the low angles X-ray
diffraction peaks, occurring around 100 °C in both compounds (Figs. 6
and 7). Moreover, the presence of diffraction peaks of two distinct phases
in the same X-ray diffraction pattern (Fig. 7, 100 °C upon heating)
demonstrates that the phase transition is of first-order; this evidence
Fig. 8. Smectic period (d001) as a function of temperature for compounds M-10 and M-12.
towards low angles (on heating around 100 °C), indicating a change in
the layer spacing (d001) from 37.5 to 54.5 Å. By plotting d001 as a function
of temperature (Fig. 8) it can be clearly seen that, upon heating, a first-
order transition occurs leading to an expansion in the layer spacing of
approximately 17 Å. In contrast, on cooling from 190 to 50 °C the layer
spacing remains relatively constant meaning that the same or almost the
same molecular arrangement is maintained in all this temperature
range. This behavior seems to be consistent with the optical observa-
tions, where the homeotropic texture remained unaffected from 184 °C
to room temperature. On the other hand, the X-ray diffraction patterns of
M-12 revealed a structural transition in both heating and cooling
experiments (Fig. 7). Upon heating d001 suddenly changes (around
100 °C) from 39 to 56.8 Å, indicating an expansion of the layer spacing of
17.2 Å (Fig. 8), which is of the same order to that observed for M-10.
Upon cooling the layer spacing decreases abruptly (around 100 °C) and
attains in a reversible way a value of 39 Å. This reversible change
coincides well with the optical observations, where it was detected that,
upon cooling, the homeotropic texture turned out birefringent near
100 °C.
The smectic phases were definitely characterized by the shape of the
scattering signal in the wide angles region of the X-ray patterns (not
shown here). Upon heating, the M-10 and M-12 compounds showed,
below 100 °C, a single sharp peak, indicating a hexatic order, and above
100 °C, a broad peak typical of a liquid-like order [23]. Upon cooling, the
M-10 compound showed only a broad peak in the all temperature range
indicating that there is no order of the molecules within the smectic
layers. In a different way, the M-12 compound developed (upon
Table 1
Structural data for M-10, M-12 and P-10. Minimum (*) and maximum (**) attained values.
Compound
M-10
L, Å
d001
Å
Mesophase
Tilt angle, °
53.4
54.5**
37.5*
56.8**
39.0*
46.9**
40.1*
HexF/I→(100 °C) SmA→(185 °C) I
Remains homeotropic←SmA (185 °C)←I
HexF/I→(100 °C) SmA→(175 °C) I
HexF/I (100 °C)←SmA (175 °C)←I
SmC (80°↔120 °C) SmA→(190 °C) I
SmC (80°↔120 °C) SmA←(190 °C) I
SmA~0
HexF/I=45.4
SmA=13.8
HexF/I=48.0
SmA=3.7
SmC=31.4
M-12
P-10
58.5
47.0

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139
Fig. 9. Schematic representation of the molecular arrangement (one layer) for compounds M-10 and M-12 in the mesophase.
indicates that both the orthogonal and the tilted mesophases coexist at the
transition region. Similar results were reported by Ratna et al. [26] for the
coexistence of an orthogonal and a tilted mesophase observed in a very
narrow temperature region (b1 °C) for the 4-(3-methyl-chloropentanoy-
loxy)-4′-heptyloxybiphenyl, although in such a case it corresponds to a
first-order SmA–SmC* phase transition. In the case of the polymer P-10,
the reversible second-order phase transition was attested by the con-
tinuous shift of diffraction peaks as a function of temperature.
The driving forces leading to a smectic order are the mutual
interactions between the different chemical groups of molecules. From
the models (Figs. 9 and 12) it is clear that the smectic layer formation is
primarily due to the strong interactions between the cations and anions,
which are stronger than the π–π and the van der Waals interactions of
pentaphenylenes and alkyl chains, respectively. Nevertheless, the
pentaphenylenes and alky chains are long enough to segregate in
separate sublayers. In rodlike liquid crystals it is quite common to
observe a thermotropic sequence involving orthogonal and tilted
mesophases in which the phase transition is first-order, as for instance
for the reversible first-order HexB–HexF transition observed in an
enaminoketone derivative series [25]. Our experimental data is quite
limited to make a full discussion about the possible driving forces that
induce the first-order transition in the M-10 and M-12 compounds,
however in a first approximation we believe that it could be associated
to the melting process of the alkoxy chains, particularly to the melting of
the butoxy groups substituted in the central ring of pentaphenylenes.
On a heating run, these groups melt in a short temperature interval,
becoming flexible enough to reduce in some extent the packing
constrains; in this case the rodlike molecules can adopt an orthogonal
arrangement. Inversely, in a cooling run the flexible groups become stiff
and probably more unfolded; in this case the packing constrains will
induce the tilt of molecules. The molecular origin of tilt in smectic liquid
crystals has been extensively reviewed in theoretical and experimental
studies and it was frequently attributed to the structural constrains and
specific interactions produced by the lateral chemical groups substitut-
ed in rodlike molecules [27]. No matter what the constrains are, the
observed significant expansion of the layer spacing (around 17 Å) gives
a clear idea of the importance of the packing constrains that are relieved
at the phase transition observed in the M-10 and M-12 compounds. For
the long polymeric molecules (P-10), which are located in several
contiguous smectic layers, the second-order phase transition should be
related to this and some other structural constrains.
3.4. Absorption and emission properties
The absorption and emission spectra were obtained from spectro-
scopic grade chloroform solutions of M-10, M-12 and P-10 (Fig. 13). The
absorption spectra showed two peaks centered at 260 and 345 nm for
the compounds M-10 and M-12, and 285 and 345 nm for the Polymer P-
10. The emission spectra showed only one maximum centered
approximately at 400 nm for the three compounds. It can be noticed
that both absorption and emission peaks are well separated from one
another (Stokes shift) in a way they overlap only in a quite narrow
region of the spectrum. The large Stokes shift indicates that the ground
and excited states are different and probably associated to the torsion of
Fig. 10. Low angles region of the X-ray diffraction patterns for compound P-10
registered upon heating and cooling at different temperatures.
Fig. 11. Smectic period (d001) as a function of temperature for the polymer P-10.

140
J.C. Díaz-Cuadros et al. / Journal of Molecular Liquids 157 (2010) 133–141
Fig. 12. Schematic representation of the molecular arrangement (one layer) of the polymer P-10 in the mesophase.
adjacent phenylene rings due to excitation [12]. The fact that there is
almost no difference in the wavelengths obtained for compounds M-10
and M-12 confirms that the alky spacers have no effect on the conju-
gation [18]. Finally, no differences were found between the character-
istics of the emitted light between small molecules (M-10 and M-12)
and the polymer (P-10); the emission process is associated solely to the
electronic excited states occurring in the pentaphenylene core, which in
all molecules is well separated from the bipyridinium groups through
flexible spacers.
4. Conclusion
New molecular and polymeric liquid crystals bearing bipyridinium
and pentaphenylene groups were synthesized and characterized. They
develop single-layered smectic-type mesophases with molecules tilted
(low temperature) or orthogonal (high temperature) with respect to
the smectic planes. A high tendency to develop a homeotropic phase
was observed for the molecular liquid crystals. A molecular stacking
model was proposed in which the different parts of molecules are fully
segregated from one another and periodically arranged in space; the
ionic sublayers facing each other in neighboring smectic layers
contribute to stabilize the smectic order. This model is plausible because
the π–π interactions of the pentaphenylene groups and the interactions
of the electrical charges of ionic groups are satisfied. These molecules
showed a light emission centered at around 400 nm that combined with
a liquid–crystalline behavior may be interesting for the preparation of
organic active layers having molecular light emitters aligned in a
preferential direction.
Acknowledgments
The authors would like to thank the CONACYT of México for the grant
given to J. C. Díaz-Cuadros during his Master studies (grant number:
205927).
This research work was sponsored by the National Council of Science
and Technology (CONACYT) of Mexico (grant number: 43741-Y).
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