Monolayer to Interdigitated Partial Bilayer Smectic C Transition in Thiophene-Based Spacer Mesogens: X-ray Diffraction and 13C Nuclear Magnetic Resonance Studies

Article abstract of DOI:10.1021/acs.langmuir.5b02327

Mesophase organization of molecules built with thiophene at the center and linked via flexible spacers to rigid side arm core units and terminal alkoxy chains has been investigated. Thirty homologues realized by varying the span of the spacers as well as the length of the terminal chains have been studied. In addition to the enantiotropic nematic phase observed for all the mesogens, the increase of the spacer as well as the terminal chain lengths resulted in the smectic C phase. The molecular organization in the smectic phase as investigated by temperature dependent X-ray diffraction measurements revealed an interesting behavior that depended on the length of the spacer vis-a-vis the length of the terminal chain. Thus, a tilted interdigitated partial bilayer organization was observed for molecules with a shorter spacer length, while a tilted monolayer arrangement was observed for those with a longer spacer length. High-resolution solid state ¹³C NMR studies carried out for representative mesogens indicated a U-shape for all the molecules, indicating that intermolecular interactions and molecular dynamics rather than molecular shape are responsible for the observed behavior. Models for the mesophase organization have been considered and the results understood in terms of segregation of incompatible parts of the mesogens combined with steric frustration leading to the observed lamellar order.

Full text of DOI:10.1021/acs.langmuir.5b02327

Article
pubs.acs.org/Langmuir
Monolayer to Interdigitated Partial Bilayer Smectic C Transition in
13
Thiophene-Based Spacer Mesogens: X‑ray Diffraction and C
Nuclear Magnetic Resonance Studies
M. Kesava Reddy,† E. Varathan,‡ Nitin P. Lobo,∇,∥ Arun Roy,⊥ T. Narasimhaswamy,§
and K. V. Ramanathan*,#
†Department of Chemistry, S. V. University, Tirupati, 517502, India
§
‡Chemical Laboratory and Polymer Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai 600020, India
∥Department of Physics, Indian Institute of Science, Bangalore, 560012, India
⊥Soft Condensed Matter, Raman Research Institute, Bangalore, 560080, India
#NMR Research Centre, Indian Institute of Science, Bangalore, 560012, India
S
* Supporting Information
ABSTRACT: Mesophase organization of molecules built with thiophene at
the center and linked via flexible spacers to rigid side arm core units and
terminal alkoxy chains has been investigated. Thirty homologues realized by
varying the span of the spacers as well as the length of the terminal chains have
been studied. In addition to the enantiotropic nematic phase observed for all
the mesogens, the increase of the spacer as well as the terminal chain lengths
resulted in the smectic C phase. The molecular organization in the smectic
phase as investigated by temperature dependent X-ray diffraction measure-
ments revealed an interesting behavior that depended on the length of the
spacer vis-a-vis the length of the terminal chain. Thus, a tilted interdigitated
partial bilayer organization was observed for molecules with a shorter spacer
length, while a tilted monolayer arrangement was observed for those with a
longer spacer length. High-resolution solid state 13C NMR studies carried out
for representative mesogens indicated a U-shape for all the molecules,
indicating that intermolecular interactions and molecular dynamics rather than molecular shape are responsible for the observed
behavior. Models for the mesophase organization have been considered and the results understood in terms of segregation of
incompatible parts of the mesogens combined with steric frustration leading to the observed lamellar order.
ring would result in a variety of molecular architectures, namely,
star-like, bent-core, λ, and T-shaped mesogens.12−16 Depending
on the spacer length as well as the length of the rigid core, a
wide range of mesophase morphologies like nematic, smectic,
banana, and columnar phases have been realized.17,18 In this
scenario, it will be interesting to investigate mesogens
containing five-membered heterocyclic systems in the spacer
unit. Even though five-membered heterocyclic rings have made
significant inroads into the armory of calamitic mesogens as
core units,19−22 to the best of our knowledge, investigations on
spacer containing mesogens with five ring moiety at the center
have not been explored so far. The recent observation of biaxial
nematic phases in oxadiazole centered mesogens has fuelled
significant interest in five-membered heterocyclic systems, and,
accordingly, many molecular mesogens have appeared in the
literature.23−26 In this work, we have undertaken a
INTRODUCTION
■
Molecular mesogens containing flexible spacers commonly
known as dimers, trimers, and oligomers have gained
importance in view of their unusual mesophase characteristics
as well as their structural resemblance to mesogenic
polymers.1−4 The construction of dimers/oligomers follows
the principle of connecting two or more rigid cores through
flexible spacers with terminal alkoxy chains at both the ends.5,6
The remarkable feature of these dimers/oligomers in contrast
to conventional low molar mass mesogens (calamitic
mesogens) is the dependency of mesophase characteristics on
the length and the parity of the spacer.1−6 The recent past
witnessed growing interest in symmetric and nonsymmetric
trimers in which the flexible spacer is connected to the central
rigid unit.7−11 These mesogens differ from the classical dimers
in the sense that the center part of the mesogen is made up of
an anisometric rigid ring rather than a flexible methylene unit
often employed for realizing dimers. Among the nonsymmetric
trimers, those based on central phenyl ring draw particular
interest as the variation in the location of spacer on the phenyl
Received: June 25, 2015
Revised: August 19, 2015
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Strategy for the Target Mesogens
comprehensive characterization of a 2,5-thiophene dicarbox-
ylate-based spacer containing mesogens in which the side arm is
built with a core containing three phenyl rings and a terminal
alkoxy chain. By varying the length of the spacer (4−12 even
carbons) as well as the terminal chain length (2−12 even
carbons), 30 molecules have been synthesized. The hot-stage
polarizing microscopy (HOPM) and differential scanning
calorimetry (DSC) studies have been employed to determine
the transition temperatures of the synthesized mesogens. The
molecular packing and organization have been examined by
variable temperature powder X-ray diffraction (XRD). For two
representative mesogens, high-resolution solid state 13C NMR
studies have been carried out, and the ordering and
orientational behavior of different segments have been
obtained. The investigations provide an insight into the liquid
crystalline behavior in terms of the molecular shape and the
influence of the length of the spacer on the molecular
organization such as the monolayer versus the bilayer
arrangement. It is anticipated that such investigations on
conformationally sensitive mesogens could help to establish the
molecular origin of self-assembly in liquid crystalline phases,
which is of great fundamental interest.27,28
EXPERIMENTAL SECTION
■
All the homologues of the series were synthesized by multistep route.
The experimental protocols and the spectral data of the intermediates
as well as final compounds are listed in the Supporting Information.
The molecular structures of mesogens were confirmed by Fourier
transform infrared spectroscopy (FT-IR), and the spectra were
recorded on an ABB BOMEM MB3000 spectrometer using KBr
1
pellets. H and 13C NMR spectra of the compounds were recorded in
CDCl3 either on JEOL ECA 500 MHz or Bruker AV-III 400 MHz
instrument at room temperature using tetramethylsilane as an internal
standard. The resonance frequencies of H and 13C were 500.15 and
1
125.76 MHz for the former and 400.23 and 100.64 MHz for the later,
respectively. The mesophase transitions such as melting and clearing
temperatures were examined using Olympus BX50 HOPM equipped
with a Linkam THMS 600 stage with a TMS 94 temperature
controller. The DSC scans were carried out using a TA Q200
instrument with a heating rate of 10 °C/min in nitrogen atmosphere.
Powder X-ray Diffraction Measurements. Variable-temperature
X-ray diffraction measurements were carried out on unoriented
samples filled in Lindemann capillary of diameter of 1 mm (Hampton
Research, Aliso Viejo, CA) using Cu−Kα (λ = 1.54 Å) radiation from
PANalytical instrument (DY 1042-Empyrean) and a linear detector
(PIXcel 3D). The sample temperature was controlled with a precision
of 0.1 °C using a heater and a temperature controller (Linkam).29
Computational Details. Density functional theory (DFT) has
been used, and the ground state (S0) geometrical optimization of S12−
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C12 mesogen in gas phase was carried out at the level of B3LYP/6-
31G*.30,31 The NMR chemical shifts were calculated at the B3LYP/6-
311+G(2d,p) level of theory using the gauge invariant atomic orbitals
(GIAO) method to circumvent the gauge problem using B3LYP/6-
31G(d) geometry.32 Cheeseman recommended the 6-311+G(2d,p)
basis set for NMR chemical shifts after scrutiny of various basis sets,
and hence the same was selected for the present study.33 The scale
factor determined for B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d)
methodology from the previous study was used for the calculation
of 13C NMR chemical shifts.34 Accordingly, dscalc = 0.95dcalc + 0.30,
where dcalc and dscalc are the calculated and the linearly scaled values
of the 13C chemical shifts, respectively. The chemical shift values of
tetramethylsilane calculated at the same level of theory were used as
the reference (182.5 ppm).34 All calculations were carried out with the
Gaussian 09 package.35
molecular organization of the mesophases is examined in detail
by variable temperature powder X-ray diffraction. High-
resolution solid state 13C NMR studies are carried out in
order to obtain insights into the molecular orientation as well as
ordering. The magnitude and the orientation of dipole
moments are calculated from the energy minimized structures
using the DFT method.
The presence of flexible methylene spacer between the
central thiophene ring and the side arm core can result in
numerous conformations for the molecule. Out of these, three
prominent energy minimized conformations leading to three
different shapes are considered. A rod-like conformation
representing linear dimers where both the arms are fully
stretched is shown in Figure 1A. A U-shaped conformation
Solid-State 13C NMR Measurements. Solid-state 13C NMR
experiments were carried out in the smectic C and nematic
mesophases for S12−C12 and S6−C10mesogens. For S12−C12mesogen,
a Bruker Avance-III 500 WB NMR spectrometer was used, while for
the S6−C10, the experiments were carried out on Bruker Avance-III
1
HD 400 WB NMR spectrometer. The resonance frequencies of H
and 13C were 500.15 and 125.76 MHz for 500 MHz spectrometer and
400.07 and 100.61 MHz for the 400 MHz instrument (Supporting
Information). The experiments in the liquid crystalline phases were
performed using a static double resonance probe equipped with a 5
mm horizontal solenoid coil. 13C NMR spectra were acquired by using
standard Hartmann−Hahn cross-polarization pulse sequence with a
contact time of 2 ms. Typical 90° proton pulse width for all the
experiments was 4 μs. During carbon signal acquisition SPINAL-64,36
decoupling was employed using a proton decoupling power of 30 kHz.
A recycle delay of 8 s was used to avoid sample heating. Each spectrum
was obtained typically with 512 scans. The 2D separated local field
(SLF) NMR spectra under static condition at three different
temperatures were obtained by using the SAMPI-437 pulse sequence
shown in Scheme S1 of the Supporting Information. The earlier
reports highlight the application of this pulse sequence to a variety of
liquid crystalline systems.38,39 The method yields a 2D spectrum that
correlates 13C chemical shifts (F2 dimension) with 13C−1H dipolar
oscillation frequencies (F1 dimension). The SAMPI-4 experimental
parameters were chosen as follows; contact time (τ): 3 ms, number of
data t1 points: 128, number of data t2 points: 768, number of scans: 32,
recycle delay: 12 s. The 2D data matrix is double Fourier transformed
with 2k and 512 points in the F2 and F1 dimensions, respectively. A
shifted sine bell window function was applied to the time domain data,
and the spectrum was processed in the phase sensitive mode.
RESULTS AND DISCUSSION
■
The mesogens are constructed from 2,5-thiophene dicarboxylic
acid, which is linked through a flexible methylene spacer to a
rigid three phenyl ring side arm core and a terminal alkoxy
chain. Scheme 1 shows the strategy adopted for synthesizing
the mesogens. Interestingly, similar mesogens with two-phenyl
ring side arm cores (Scheme S2 of the Supporting Information)
failed to exhibit liquid crystalline property, which suggests that
three-phenyl rings are essential to compensate the flexibility
imparted by the spacer. The lengths of the spacer (4−12) and
the terminal chain (2−12) were varied with an even number of
carbons, and 30 homologues were realized. The mesogens are
denoted as Sm−Cn series in which ‘mʼ signifies the length of the
spacer while ‘nʼ represents that of the terminal alkoxy chain. In
order to establish the relation between the structure of these
mesogens and their liquid crystalline properties, a range of
techniques are employed. The HOPM and DSC measurements
are performed on the mesogens to characterize them in terms
of the transition temperatures and the transition enthalpies.
The HOPM studies also helped in identifying the presence of
the tilted smectic phase in some of the mesogens. The
Figure 1. Molecular structure of mesogen S12−C12. Energy optimized
space-filled models, magnitude, and direction of dipole moment
(DFT) for different conformations: (A) linear shape, (B) U-shape and
(C) V-shape.
(Figure 1B) has the side arm cores parallel to each other and to
the C2v axis, which is the polar axis of the molecule. A V-shaped
conformation (Figure 1C) may also be envisaged where the
side arm cores make an angle with respect to the C2v axis as in
the case of banana-like molecules. It may be expected that the
gross shapes of these three extreme conformations would result
in different molecular packing leading to different mesophase
morphologies. The investigations presented here are aimed at
understanding the observed mesophase properties in terms of
the molecular shape and molecular organization.
C
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Mesophase Transitions and Their Trends. The com-
plete details of HOPM and DSC investigations for all the
compounds are available in the Supporting Information
(Figures S1−S3, Table S1). Only important aspects of these
results are highlighted in this section. The HOPM studies
revealed for the whole series of mesogens, viz., S4−C2 to S12−
C12, the existence of enantiotropic nematic phase, confirmed by
the observation of birefringent droplets on cooling the isotropic
phase.40 Upon further cooling the nematic phase, the smectic C
mesophase is observed for those molecules in which the spacer
length and the length of the terminal chain are optimal. Thus,
the appearance of the smectic C is first noticed in S4−C8 as a
monotropic phase, while for the rest of the mesogens, i.e., C10
and C12, it appeared enantiotropically. The smectic C phase is
realized for all C10 and C12 mesogens. For lower homologues,
the occurrence of smectic C is governed by the spacer length.
The length of the chain versus the length of the spacer also
influences the nature of molecular organization in the smectic
phase, which is discussed elsewhere. The nematic−isotropic
(N−I) transition temperatures of the mesogens showed a
decreasing trend with the increase in the terminal chain length
for a given spacer length (Figure S4 of Supporting
Information). The nematic mesophase range also showed
decreasing trend with increase in terminal chain length. Similar
behavior is noticed across the series, where increase in spacer
length resulted in decrease in nematic phase range. For the N−I
transition, the ΔH values were in the range of 0.4−0.98 k cal/
mol and showed an increasing trend with the increase in spacer
length. Thermogravimetric analysis (TGA) of two mesogens
(S6−C2 and S8−C2) (Figure S5 of Supporting Information) is
carried out to find the initial decomposition temperature since
the clearing temperatures of them are in the range of ∼250−
275 °C. The TGA analysis reveals that the initial decom-
position temperature is above 300 °C, which is higher than the
clearing temperatures.
The smectic mesophase range, on the other hand, revealed
increasing tendency with increase in terminal chain length,
within the series as well as across the series. These trends
suggest the segregation of incompatible parts of the mesogens,
i.e., segregation of flexible aliphatic chains and spacer from rigid
aromatic parts leading to lamellar order typical of smectic
phases.41 These observations are additionally confirmed by
DSC, where each mesogen is subjected to two heating and
cooling cycles (Table S2 of Supporting Information). For
smectogens, the DSC scans showed two types of behavior. For
a majority of mesogens, the transition enthalpy values
associated with nematic to smectic phases are found to be
∼0.14 to 0.49 k cal/mol, whereas for S4−C8, S4−C10, S4−C12,
S6−C10, S6−C12, and S8−C12, the values are found to be about 6
times higher (∼1.0−3 k cal/mol). Such a large difference in
transition enthalpies for nematic−smectic C transition
encouraged us to examine the HOPM textures of smectic C
mesophases more carefully. It was observed that for the
mesogens with low transition enthalpy values, transformation
from nematic to smectic C was preceded by transition bars
resulting in the schlieren texture.42 The phase associated with
such mesogens with low transition enthalpies are categorized as
usual smectic C phases. For those mesogens in which the
transition enthalpies are higher, on the other hand, the HOPM
showed unusual textures (Figure 2), and the associated phase is
considered as unusual smectic C phase. These two phases also
revealed different XRD patterns, which are presented in the
next section.
Figure 2. HOPM photographs of (A) S4−C10: 162.5 °C, (B) S6−C12:
142.2 °C (interdigitated partial bilayer smectic C phases), (C) S12−
C12: 140 °C (monolayer smectic C phase) and (D) S12−C10: 186.2 °C
(nematic phase).
The electric polarization studies of the mesogens exhibiting
usual smectic mesophase as well as unusual smectic mesophase
are carried out in respective mesophases. In both the cases, the
polarization current peaks are not observed, even with the
application of relatively high voltage, indicating the absence of
layer polarization in these smectic phases.
Powder X-ray Diffraction Studies. The variable-temper-
ature powder X-ray diffraction studies are undertaken to
examine the molecular organization in the liquid crystalline
phases and to obtain a correlation to the optical microscopy
and scanning calorimetry results in terms of the molecular
arrangement in the context of the appearance of usual and
unusual smectic mesophases. Sixteen mesogens, some of which
exhibit the usual and the others the unusual smectic
mesophases, are examined by powder X-ray diffraction. XRD
scans in the smectic C phase of two representative mesogens,
namely S6−C8 and S6−C10 are considered (Figure 3). For S6−
C8 at 125 °C, the XRD shows a sharp peak at 30.87 Å in the
small angle region and a broad diffuse peak at 4.44 Å in the
wide angle region. The presence of the sharp reflection in the
small angle region and a broad hump in the wide angle region
suggests fluid lamellar ordering in the smectic mesophase43
with layer spacing (d) equal to 30.87 Å. In contrast, for the
other mesogen, namely, S6−C10, at 150 °C, three reflection
peaks at 62.33, 31.13, and 20.67 Å are seen in the small angle
region in addition to a broad diffuse peak at 4.47 Å in the wide
angle region. The small angle reflections are indexed as (01),
(02), and (03) planes of the lamellar structure with layer
spacing as 62.33 Å. The broad hump in the wide angle region
indicates liquid-like order within the layers. The higher orders
of reflections suggest greater positional order in the smectic
phases.43 It is interesting to note that the layer spacing for S6−
C10 mesogen is approximately 2 times larger in comparison to
that of S6−C8. This remarkable increase in the layer spacing for
a nominal increase in the terminal chain length indicates a
different kind of molecular packing for the S6−C10 homologue.
Following this observation, the XRD data of all other mesogens
are also examined. Typical values of reflections obtained for
each of the 16 mesogens considered are listed in Table 1. The
complete data obtained for all the mesogens at several
temperatures are given in Table S3 of the Supporting
Information. Table 1 also shows the d/L ratio, where L is the
molecular length. The value of L has been obtained by
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Figure 3. High-resolution X-ray powder diffraction patterns for (A) S6−C8 (125 °C), (B) S6−C10 (150 °C), (C) S8−C10 (150 °C), and (D) S8−C10
(110 °C).
Table 1. Powder X-ray Diffraction Data of Mesogens at
Representative Temperatures
clearly noticed. The mesogens that exhibit the usual smectic
phases show only one order of reflection in this region, similar
to S6−C8 discussed earlier. The d/L ratios for these mesogens
are in the range of 0.74−0.76. On the other hand, mesogens
that display the unusual smectic phase show three orders of
reflections as observed for the case of S6−C10.The d/L ratios in
these cases are also different and occur in the range of 1.4−1.6.
These observations suggest that the molecular packing for the
usual and the unusual mesophases are not the same. This
information correlates well with the different values of
transition enthalpies for the two types determined from DSC
measurements as well as with the differences in the HOPM
textures mentioned earlier. In summary, the phases associated
with the usual smectic C textures observed in HOPM have
lower transition enthalpies for the nematic−smectic C (N−S)
transition and have a smaller layer spacing of 34−40 Å as
revealed by XRD. On the other hand, the phases associated
with the unusual smectic C textures have higher transition
enthalpies and have a larger layer spacing on the order of 56−
70 Å.
code
T (°C)
d1 (Å)
d2 (Å)
d3 (Å)
d4 (Å)
d/L
S4−C10
S4−C12
S6−C8
S6−C10
S6−C12
S8−C8
S8−C10
155
165
125
140
145
125
150
110
135
120
130
135
130
125
130
130
140
56.11
60.13
30.87
61.22
63.55
32.35
34.68
66.05
68.04
34.67
35.05
36.18
37.31
37.80
38.24
38.68
40.06
28.48
18.9
4.50
4.54
4.44
4.44
4.53
4.44
4.51
4.46
4.49
4.48
4.44
4.51
4.43
4.48
4.47
4.51
4.52
1.33
1.35
0.73
1.37
1.39
0.74
0.73
1.42
1.42
0.76
0.75
0.73
0.72
0.81
0.77
0.74
0.74
31.13
-
-
-
30.58
20.38
32.35
21.64
-
-
-
-
32.67
21.70
S8−C12
S10-C6
34.31
22.87
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
S10-C8
S10-C10
S10-C12
S12−C6
S12−C8
S12−C10
S12−C12
It is of particular interest to note the XRD pattern of the S8−
C10 mesogen for which two significantly different layer spacings
are noticed at different temperatures (Table 1). At 150 °C, the
XRD scan shows a sharp reflection in the small angle region
and a broad peak in the wide angle region. The corresponding
layer spacing is 34.7 Å. At 110 °C, on the other hand, XRD
shows three orders of reflection in the small angle region and a
broad peak in the wide angle region. The layer spacing for this
lower temperature phase is found to be 66.0 Å. The ratio of the
layer spacing at the lower to the one at the higher temperature
is about 1.9. Thus, lowering the temperature leads to a
remarkable change in the molecular packing in the smectic
phase of this particular mesogen. As discussed elsewhere, for
this mesogen, the lengths of the terminal chain and the spacer
tentatively assuming the U-like shape for the mesogens and
calculating the L value by taking the length of the side arm core,
the terminal chain, and the spacer lengths along with the
thiophene unit, all of which are obtained from DFT
calculations. For example, for the longest mesogen, namely,
S12−C12, L is found to be 54 Å. For other mesogens, L varies in
the range of 41.9−51.5 Å. From Table 1, it is noticed that in all
the cases the wide angle peak occurs at nearly the same place
(∼4.5 Å). As seen in Figure 3 this peak is also a broad hump,
indicating the fluid nature of the smectic phases.44 The
interesting features observed in the small angle region as well as
the examination of data from Table 1, two kinds of behavior are
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lie at the boundary where this series of mesogens make a
transition from the usual to the unusual smectic phase, resulting
in both the behaviors being observed. In this scenario, it would
be interesting to investigate these mesogens using the solid
state NMR methodology. As NMR provides information at the
molecular level, it would be useful to know the molecular
orientation and order as well as how the molecular subgroups
are organized with respect to each other. Further differences in
the molecular structures, if present in the different phases, will
throw light on the origin of the phase behaviors. With this
objective, NMR studies on two representative mesogens have
been carried out, and the results are presented and discussed in
the next section.
isotropic phase, particularly in the aromatic region, due to the
effect of chemical shift anisotropy coming into play in the liquid
crystalline phase. The assignment of the spectra in the LC
phase is carried out by comparing structurally similar fragments
of mesogens, which are known from the literature.44,45
Information from the 2D-SLF spectra is also useful, which is
discussed subsequently. Accordingly, the peaks appearing in the
range 140−229 ppm are assigned to thiophene, linking units,
and side arm phenyl ring carbons. The five peaks appearing in
the region of 140−165 ppm in the smectic C phase have been
assigned to the phenyl ring methine carbons with the intense
peak at 140.7 ppm being assigned to two of them. The weak
peak in this region at 153.4 ppm has assigned to the methine
carbon (C17) of thiophene. The identification of the character-
istic imine carbon (C10) signal at 199.6 ppm is enabled due to
observation of a high 13C−1H dipolar coupling in the 2D
spectrum. The spacer as well as the terminal chain carbon
signals appears in the range 17−60 ppm. Similar observations
for the spectra in the nematic phase (Figure 4C) have been
made. Table 2 lists the assignment of the core as well as the
thiophene carbons at two temperatures in the smectic phase
and also at 175 °C in the nematic phase. The alignment-
induced chemical shifts (AIS) are calculated as δLC − δsolution
and are also listed in Table 2. The AIS suggests a parallel
alignment of the side arm core unit in the magnetic field
(Figure 5A). The observation of only one set of signals for both
arms means that both the arms have identical orientation and
order in the LC phase. For S6−C10 mesogen similar
observations can also be made based on the spectra obtained
in its smectic phase (Figure S6 of the Supporting Information).
Figure 5B shows the expanded 2D SLF spectrum of the side
arm core and thiophene methine carbons of S12−C12 in the
smectic C phase at 140 °C, while the full spectrum is shown in
Figure S7 of Supporting Information. The 2D spectrum shows
well-resolved contours arising from side arm core as well as
spacer and terminal chain carbons. In the region of 140−229
ppm, the imine carbon contours (marked in green) and the
thiophene methine carbon contours (marked in red) are clearly
separated from the contours of the phenyl ring carbons. Table 2
lists the 13C−1H dipolar frequencies measured from 2D
spectrum.
High-Resolution Solid State 13C NMR Investigation.
13C NMR measurements have been carried out on two
representative mesogens, namely, S12−C12 and S6−C10. S12−C12
displays the usual smectic C phase, while the S6−C10 shows the
unusual smectic C mesophase. The 13C NMR spectrum of S12−
C12 is considered for detailed discussion. Figure 4A shows the
Figure 5C shows the expanded part of 2D spectrum
depicting the side arm core and thiophene methine carbons
of S12−C12 in the nematic phase at 175 °C. The trends noticed
in the smectic C phase are also found in the nematic phase.
Decrease in chemical shift as well as 13C−1H dipolar coupling
values noticed in this phase are the result of the reduction in
the orientational order parameters. The 13C−1H dipolar
coupling values (Table 2) of phenyl ring methine carbons of
the side arm core are comparable with those mesogens in which
the core is constructed with three phenyl rings.44,45 The 2D-
SLF experiments were also carried out in the smectic C and the
nematic phases of S6−C10 mesogen, and the 13C−1H dipolar
couplings are obtained, the details of which are available in the
Supporting Information (Figure S8 and Table S4).
Figure 4. 13C NMR spectra of S12−C12 in (A) solution, (B) smectic C
phase (140 °C), and (C) nematic phase (175 °C).
proton decoupled 13C NMR spectrum of the compound in
solution. The assignment of these peaks is carried out by ACD
chemsketch software, which is further refined by computing the
chemical shifts by DFT calculations (Table 2). The appearance
of only half the number of spectral lines in comparison to the
total number of carbon atoms, particularly in the aromatic
region where the chemical shift dispersion is high, suggests that
both arms are chemically equivalent. For the case of the S6−C10
mesogen, the spectral features are similar to S12−C12 mesogen
except for the difference in the number of signals in the
aliphatic region due to the shorter spacer as well as the terminal
chain lengths.
The local orientational order parameters of the phenyl rings
of the mesogen S12−C12 have been calculated in its smectic and
nematic mesophases from the experimental dipolar oscillation
frequencies obtained from the 2D SLF technique by using the
following equation:44−47
The spectrum of a static sample of the liquid crystal S12−C12
obtained in its smectic C and nematic phases are shown
respectively in Figures 4B,C. These two spectra show a large
dispersion in chemical shifts compared to the spectrum in the
F
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Table 2. 13C NMR Data of S12−C12 Mesogen in Solution and Liquid Crystalline Phases
smectic C 140 °C
smectic C 160 °C
nematic 175 °C
C. No solution (ppm) DFT (ppm) CS (ppm) AIS (ppm) DC (kHz) CS (ppm) AIS (ppm) DC (kHz) CS (ppm) AIS (ppm) DC (kHz)
1
161.9
114.2
132.2
121.6
164.9
148.9
121.6
122.2
149.9
159.6
129.0
130.4
114.6
161.6
163.5
139.1
132.7
163.6
114.2
134.0
121.9
164.6
149.4
120.6
124.3
149.2
153.9
131.4
132.7
117.9
161.6
159.8
143.6
131.8
228.0
140.7
160.4
188.8
214.0
224.9
146.2
151.7
228.0
199.6
199.6
163.9
140.7
228.0
214.0
183.8
153.4
66.1
26.5
28.2
67.2
49.1
76.0
24.6
29.5
78.1
40.0
70.6
33.5
26.1
66.4
50.5
44.7
20.7
1.62
2.95
2.97
1.60
-
220.0
137.4
156.5
180.7
206.9
215.6
142.9
147.9
220.0
194.6
190.7
159.8
137.4
220.0
206.9
181.1
151.9
58.1
23.2
24.3
59.1
42.0
66.7
21.3
25.7
70.1
35.0
61.7
29.4
22.8
58.4
43.4
42.0
19.2
1.36
2.56
2.60
1.31
-
211.7
134.1
152.7
172.5
200.1
206.1
139.7
144.1
211.7
189.4
182.0
155.7
134.1
211.7
200.1
177.7
149.4
49.8
19.9
20.5
50.9
35.2
57.2
18.1
21.9
61.8
29.8
53.0
25.3
19.5
50.1
36.6
38.6
16.7
1.13
2.20
2.26
1.11
-
2
3
4
5
6
1.68
3.20
2.97
1.62
5.00
2.18
2.78
2.95
1.62
-
1.42
2.76
2.63
1.36
4.38
1.83
2.48
2.56
1.36
-
1.17
2.39
2.30
1.13
3.78
1.52
2.12
2.20
1.13
-
7
8
9
10
11
12
13
14
15
16
17
0.81
1.67
0.56
1.50
0.51
1.31
CS: Chemical Shift; AIS: Alignment Induced Shift; DC: 13C−1H Dipolar Couplings
Figure 5. (A) Picture shows the orienting axis (C2v) of S12−C12mesogen, which is parallel to the magnetic field. 2D SAMPI-4 spectra of S12−C12
mesogen in (B) smectic C phase at 140 °C and (C) nematic phase at 175 °C. Red and green contours denote thiophene ring methine and imine
carbons.
⎤
⎡
1
2
1
2
(C2 axis) of the ring; the x-axis is in the plane of the ring, while
the y-axis is perpendicular to the plane. Standard bond
distances, namely 1.1 Å for the C−H bond and 1.4 Å for the
C−C bond, are taken for calculating K in eq 1. By following the
established procedure,38,39,44−48 the two order parameters,
namely, S′zz and (S′xx − S′yy) for the phenyl rings, are calculated
by using eq 1. The estimated orientational order parameter
values for phenyl rings in the smectic C and nematic phases are
DCH = K (3 cos2 θ − 1)S′zz
+
(cos2 θ − cos2 θy)(S′xx − S′ )
⎥
⎦
⎢
⎣
z
x
yy
(1)
where K = −hγHγC/4π2r3CH, γH and γC are the gyromagnetic
ratios of H and 13C nuclei, respectively, and θx, θy and θz are
1
angles between the internuclear vector, rCH, and the coordinate
axes. For the phenyl rings, the z-axis is taken as the para-axis
G
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Table 3. Orientational Order Parameter Values of Phenyl Rings and the Thiophene Ring of S12−C12 Mesogen
Article
a
angles
calculated dipolar oscillation frequencies (kHz)
b
ring
T (°C)
θb
θc
S′zz
S′xx − S′yy
b
c
a
d
I
140
120.1
119.7
120.7
119.9
119.5
120.2
119.6
119.2
120.0
120.0
120.3
120.1
119.7
120.0
119.9
119.3
119.5
119.6
0.69
0.71
0.69
0.58
0.60
0.58
0.49
0.50
0.49
0.076
0.080
0.078
0.066
0.071
0.068
0.052
0.060
0.053
2.95
3.18
2.77
2.55
2.77
2.48
2.20
2.39
2.11
2.98
2.98
2.96
2.60
2.63
2.56
2.26
2.32
2.20
1.61
1.65
2.18
1.35
1.39
1.83
1.13
1.16
1.54
1.60
1.66
1.61
1.34
1.40
1.35
1.12
1.16
1.13
II
III
I
160
175
II
III
I
II
III
calculated dipolar oscillation frequencies
(kHz)
thiophene
T (°C)
tilt angle θ
S′zz
S′xx − S′yy
C17
C16
140
160
175
34.0
29.0
30.0
0.41
0.25
0.23
0.158
0.043
0.036
1.67
1.50
1.31
0.73
0.50
0.44
a
b
In the figure above the table, b and c are methine carbons, and a and d are quaternary carbons, respectively, for rings I, II, and III. For carbon a in
ring-III, the contribution of the azomethine proton has also been taken into account.
shown in Table 3. The order parameter of the side arm core is
taken to be the same as that of the phenyl ring-II, which is more
ordered than ring-I and ring-III. Accordingly, the Szz values are
found to respectively be 0.71 and 0.60 at 140 and 160 °C in the
smectic phase and 0.50 at 175 °C in the nematic phase. For the
S6−C10 mesogen, these values are 0.69 at 140 °C in the smectic
C and 0.64 at 160 °C in the nematic phase (Table S5 of the
Supporting Information). These values are consistent with
typical mesogens constructed with three phenyl rings for which
literature data is available.44,45
or higher than the side arm phenyl rings. In such a case, as
obtained with other bent-core systems,38,39 the bent-angle β
can be calculated from S′zz (PheII) = P2 cos (β)·S′zz (Thio),
where S′zz (PheII) and S′zz (Thio) are the order parameters of
the side arm phenyl ring-II and the thiophene ring, respectively.
However, a lower order parameter for the thiophene ring as
well as the local order parameter of the attached O−CH2
carbons indicates that this part of the molecule is relatively
flexible and cannot support a V-shape. Thus, NMR measure-
ments of both types of molecules, namely those which exhibit
the usual smectic C phase as exemplified by S12−C12 and those
which exhibit the unusual smectic C phase similar to S6−C10,
indicate the U-like conformation for both series. It may be
noted that despite a large variation in d/L ratios between the
two series of molecules arising out of the monolayer versus
interdigitated bilayer smectic C organization, they have
comparable orientational order parameter values, which
suggests that the different molecular packing has no significant
impact on the local molecular dynamics.
The order parameters of the center thiophene ring have been
obtained from the experimental 13C−1H dipolar oscillation
frequencies of the thiophene carbons using a model in which
the plane of the thiophene ring is oriented at an angle with
respect to the C2v axis of the molecule, which is aligned with the
magnetic field (Figures 5A and S9 of the Supporting
Information). Values of 0.41, 0.25, and 0.23 for S′zz,
respectively, at 140, 160, and 175 °C are obtained for the
thiophene moiety of the S12−C12 mesogen (Table 3). For S6−
C10 the corresponding values are 0.23 and 0.19, respectively, at
Model of Molecular Organisation. The set of mesogens
investigated here, though they can be classified as symmetric
dimers, are unusual for the reason that the two arms of the
dimer are not joined by a simple aliphatic chain, but with a
flexible spacer and a rigid thiophene unit at the center. For such
a configuration, three major possible shapes as depicted in
Figure 1 are possible. Among these, for the linear conformation
(Figure 1A), the L value is nearly double that of the other two.
The conventional linear symmetrical dimers are in general
observed to exhibit intercalated phases with a typical d/L ratio
of 0.5.1−7 This is, however, not considered as a possibility in
our case, as the central thiophene dicarboxylate unit is expected
to promote for the molecule a bent shape rather than a linear
shape for the reason that the bend angle of 2,5-thiophene
dicarboxylate is typically 145°.49 It was shown previously38 that,
without the flexible spacer and only with a thiophene unit
140 and 160 °C phase (Table S6 of the Supporting
Information). The low S′zz value for thiophene in comparison
to the side arm phenyl rings is due to the presence of the
flexible spacer between the thiophene and the side arm core
resulting in the increased mobility of the thiophene ring. In
Table 3 it is seen that, with the increase in temperature, the
order parameters decrease as the phase changes from smectic C
to nematic. The 13C NMR measurements of the mesogens in
the smectic C as well as in the nematic phases indicate that the
C2v symmetry axis of the molecule is parallel to the magnetic
field (Figure 5A). The results are also taken to support the
hypothesis that the molecule adopts a U-shape rather than a V-
shape. This is based on the argument that for a V conformation,
the central thiophene and the attached spacers will be rigid, and
the thiophene ring will have an order parameter comparable to
H
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connecting the two arms, the molecule can be classified as a
bent core mesogen and has a V shape. The molecule orients in
the magnetic field with the C2v symmetry axis of the molecule
perpendicular to the magnetic field. The thiophene moiety is
rigid and shows a large order parameter. However, in the
present case, we rule out the V-shape conformation shown in
Figure 1C, as the mesophases observed are different from the
smectic phases exhibited by the bent core banana like
molecules.50 The switching current experiments confirmed
the absence of layer polarization in these smectic phases.
Further, the layer ordering as well as layer spacing does not
support the V-shape. With the flexible spacer unit connecting
the thiophene to the side arm core units, the NMR results
support a U shape for the molecule, since it is observed that the
thiophene unit is much less rigid and has a relatively lower
order parameter. The molecule also orients with the C2v
symmetry axis parallel to the magnetic field. Based on these
observations, it is concluded that the molecule most probably
adopts the U-shaped conformation, as shown in Figure 1B.
In this series of mesogens, molecules exhibit a variety of
phases depending on the length of the spacer unit and the
terminal chain length. These are summarized in Figure 6A. As
layer spacing of 34−40 Å, while for the unusual phases larger
layer spacing on the order of 56−70 Å is observed. With the
assumption of a U shape for the molecules, the molecular
length L has been calculated, and this gives rise to a d/L ratio in
the range of 0.74−0.76 for the smectic C phase. This
corresponds to a tilted monolayer organization of the U-
shaped molecules in the layers is proposed. The tilt angle θ
determined from the d/L ratio is found to be about 40°, which
agrees with that estimated from the HOPM studies. On the
other hand, for the unusual smectic mesophases, a tilted
interdigitated partial bilayer organization51 of the U-shaped
molecules is considered, which accounts for the d/L ratio being
greater than 1 but less than 2. The dipole moment values of the
three shapes presented in Figure 1 calculated from DFT are 2.7,
4.8, and 6.6 D for the linear, U-shaped, and V-shaped
molecules, respectively. The large longitudinal dipole moment
of the thiophene unit in the U-shaped configuration tends to
stabilize the interdigitated partial bilayer organization of the
molecules in layers. It is known in the literature that highly
polar group attached to one end of a rigid rod-like molecules
often leads to such interdigitated partial bilayer organization
and double-layer arrangements of the molecules in layers.52 It is
interesting to note that a highly polar group such as the cyano
group, even if it is only attached to one end of the rigid core
through a flexible aliphatic spacer chain, leads to the formation
of a double-layered SmA2 phase.53 The strong dipolar
interaction of the highly polar end groups tends to stabilize
these higher order smectic phases, even when the polar end
group is mechanically decoupled from the rigid core through a
flexible aliphatic chain.
Based on the above observations, a model for the molecular
packing for both types of smectic phases is shown in Figure 6B.
For the usual smectics, the tilted monolayer organization is
suggested (Figure 6B(i)) that takes into account the observed
d/L ratio of 0.74 and the tilt angle of 40° for the molecules
within the layer. Similarly, for the unusual smectics, the tilted
interdigitated partial bilayer structure is shown in Figure 6B(ii),
which corresponds to a larger d/L ratio of 1.4. In order to
understand the driving force responsible for the two dissimilar
molecular packing for the synthesized mesogens, it would be
useful to consider the existing information for dimers/trimers
reported in the literature. For instance, malonic acid-based54
dimers revealed the presence of nematic and smectic A phases
with a typical layer intercalation. The replacement of malonic
acid in these mesogens with thiophene dicarboxylate yields the
present mesogens. From this it is easy to recognize the role of
thiophene in influencing the phase properties. Similarly,
mesogens with a three ring core linked to a hydroxyl alkoxy
chain at one end and an alkoxy chain at the other end44 show
an enantiotropic nematic and the usual smectic C phases similar
to the thiophene mesogens. This observation indicates that the
side arms also play a part in deciding the phase characteristics
of the current set of mesogens. These are further illustrated in
Figure 6A, which shows the various phases exhibited by the
current set of mesogens as a function of the number of carbon
atoms in the spacer unit and the terminal chain. In the figure, it
may be noticed that, for shorter spacer and terminal chains,
only nematic phases are noticed. With increasing length of
either the spacer unit or the terminal chain, the smectic phases
start appearing. It may be noted that increasing the terminal
chain length favors the formation of the unusual smectic phase
(interdigitated partial bilayer configuration), while increasing
the spacer length favors the formation of the usual smectic
Figure 6. (A) The phase characteristics of mesogens as a function of
spacer and terminal chain length. (B) Schematic representation of (i)
usual smectics, tilted monolayer organization, and (ii) unusual
smectics, tilted interdigitated partial bilayer organization.
seen in the figure, molecules with shorter spacer and terminal
chain lengths exhibit only the nematic phase, while those with
more numbers of carbons either in the spacer unit or in the
terminal chain exhibit the smectic phase also. In the smectic
phase, two different types are observed, which have been
named as the usual and the unusual smectic phases. The
distinction between the usual and the unusual smectic phases
has been made based on HOPM, DSC, and XRD results. The
XRD results reveal for the usual smectic C phases a smaller
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phase (monolayer configuration). For example, in the case of
the C-12 mesogens, for shorter spacer lengths, namely, S-4, S-6,
and S-8, the bilayer smectics are observed, while for S-10 and S-
12 the monolayer smectics are seen. Interestingly, the S8−C10
mesogen lies at the crossing point of these two behaviors, and
for this case both phases are observed as a function of
temperature. The transition is found to be weakly first-order
with the bilayer conformation being favored at the lower
temperature. It may be mentioned that a similar behavior has
been postulated in side-chain liquid crystalline polymers where
the mesogens are attached to the main chain through a flexible
spacer, with the short spacer favoring the bilayer formation,
while the longer ones result in monolayer organization.55,56
Another important result comes from the polarization
measurements, which revealed that both the usual and unusual
smectic mesophases are nonswitchable. Despite the polar
nature of the mesogens, the absence of net-polarity signifies an
antiparallel packing of the mesogens. Based on this fact and
considering the effect of the longer terminal chains, one may
conclude that the thiophene−thiophene interaction coupled
with the interaction between the side-chains favors the bilayer
configuration in mesogens with a short spacer. Increasing the
length of the spacer or increasing temperature as in the case of
S8−C10 mesogen gives rise to additional mobility at the
thiophene end of the molecule, leading to collapse of the
bilayer arrangement. Thus, the monolayer configuration is
preferred for mesogens with a longer spacer length. These
trends suggest that the driving force responsible for the
formation of the molecular arrangements is the segregation of
incompatible parts of the mesogens combined with steric
frustration, leading to lamellar order.57 Thus, the findings here
provide an understanding of the properties of the mesogens
investigated, which behave differently than classical dimers, and
also of the role of the polar five membered thiophene ring at
the middle in the mesogens in influencing these properties.
and the side-arm phenyl rings. This arises because of the
flexible spacer that connects the thiophene to the side-arm core
phenyl rings. The results have been interpreted to indicate a U-
shape conformation for the mesogens. The absence of net
polarity as indicated by polarization measurements supports an
antiparallel packing of the mesogens in the smectic C phase. A
systematic analysis of the above results in terms of the length of
the spacer versus the length of the chain has been made, and
the results indicate that the spacer length plays an important
role in influencing the molecular organization in terms of
monolayer versus bilayer arrangement. It is, however, necessary
to recognize the role of the other components of the molecule
such as the thiophene moiety in influencing the molecular
arrangements. The molecular shape, say U or V, may also be
expected to significantly influence the property of the phase.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.lang-
muir.5b02327.
Synthetic details of 30 mesogens as well as experimental
conditions employed for generating the data from
different techniques. Further, tables and figures of
DSC, a table of T/TN‑SC and d/L ratios, and figures of
TN−I versus terminal chain length, TN−I versus spacer
length, TSC‑N versus terminal chain length, HOPM
images of the mesogens, SAMPI-4 pulse sequence, 1D
as well as 2D static liquid crystalline 13C NMR plots and
orientational order data of S6−C10 mesogen, a proposed
model for the thiophene unit, and TGA curves for
mesogens are also included. (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: kvr@sif.iisc.ernet.in.
■
CONCLUSION
■
Present Address
Thirty mesogens built with thiophene at the center and linked
to a side-arm, three-phenyl ring core by flexible spacers as well
as terminal alkoxy chains exhibited enantiotropic mesophases.
The mesogens with short spacer and terminal chains showed
predominantly nematic mesophase, while longer spacer and
terminal chains favored both nematic and smectic C phases.
The HOPM and DSC investigations revealed the following two
kinds of smectic C mesophases: (i) a phase with typical
schlieren textures and with normal enthalpy values and (ii) a
phase with unusual textures associated with high transition
enthalpy. A detailed XRD investigation of the mesogens in the
smectic C phase revealed different features for the above two
cases. The layer spacing and d/L ratio for mesogens exhibiting
the unusual smectic C phase was found to be higher than those
of mesogens exhibiting the usual smectic C phase. By
considering the U-shape as the most preferred conformation,
the molecular organization in the usual smectic C phase was
found to be a monolayer, while for the unusual case an
interdigitated partial bilayer organization has been suggested.
Interestingly, for one of the homologues, S8−C10, both types of
molecular organization are observed with a transition from
monolayer to interdigitated partial bilayer smectic C mesophase
taking place by a decrease of temperature. 13C NMR
measurements carried out in the nematic and the smectic C
mesophases of two of the mesogens showed that there is a large
variation in the order parameters between the thiophene ring
∇Chemical Physics Laboratory, CSIR-Central Leather Research
Institute, Adyar, Chennai, 600020, India.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.K.R. would like to thank Prof. K. Subramanyam Reddy, Sri
Venkateswara University, Tirupati, India, and Dr. A. B. Mandal,
CSIR-CLRI, Chennai, India, for their help, constant support,
and encouragement. The authors are grateful to Prof. V. A.
Raghunathan and Ms. K. N. Vasudha, RRI, Bangalore, India, for
the X-ray measurements. Thanks are also due to Dr. V.
Subramanian, CLRI, for the quantum chemical calculations.
The partial financial support from STRAIT project under XII
five year plan of CSIR is duly acknowledged. The use of the
Bruker AV-III-400 and Bruker AV-III-500 WB NMR
spectrometers funded by the Department of Science and
Technology, New Delhi, at the NMR Research Centre, Indian
Institute of Science, Bangalore, India, is gratefully acknowl-
edged.
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