Phase characterization and study of molecular order of a three-ring mesogen by 13C NMR in smectic C and nematic phases

Article abstract of DOI:10.1021/jp203388v

Molecules exhibiting a thermotropic liquid-crystalline property have acquired significant importance due to their sensitivity to external stimuli such as temperature, mechanical forces, and electric and magnetic fields. As a result, several novel mesogens have been synthesized by the introduction of various functional groups in the vicinity of the aromatic core as well as in the side chains and their properties have been studied. In the present study, we report three-ring mesogens with hydroxyl groups at one terminal. These mesogens were synthesized by a multistep route, and structural characterization was accomplished by spectral techniques. The mesophase properties were studied by hot-stage optical polarizing microscopy, differential scanning calorimetry, and small-angle X-ray scattering. An enantiotropic nematic phase was noticed for lower homologues, while an additional smectic C phase was found for higher homologues. Solid-state high-resolution natural abundance ¹³C NMR studies of a typical mesogen in the solid phase and in the mesophases have been carried out. The ¹³C NMR spectrum of the mesogen in the smectic C and nematic phases indicated spontaneous alignment of the molecule in the magnetic field. By utilizing the two-dimensional separated local field (SLF) NMR experiment known as SAMPI4, ¹³C-¹H dipolar couplings have been obtained, which were utilized to determine the orientational order parameters of the mesogen. ? 2011 American Chemical Society.

Full text of DOI:10.1021/jp203388v

ARTICLE
pubs.acs.org/JPCB
Phase Characterization and Study of Molecular Order of a Three-Ring
Mesogen by 13C NMR in Smectic C and Nematic Phases
S. Kalaivani and T. Narasimhaswamy
Polymer Laboratory, Central Leather Research Institute (Council of Scientific & Industrial Research, CSIR), Adyar, Chennai 600 020,
India
Bibhuti B. Das,† Nitin P. Lobo,† and K.V. Ramanathan*,‡
†Department of Physics and ‡NMR Research Center, Indian Institute of Science, Bangalore 560 012, India
S Supporting Information
b
ABSTRACT:
Molecules exhibiting a thermotropic liquid-crystalline property have acquired significant importance due to their sensitivity to
external stimuli such as temperature, mechanical forces, and electric and magnetic fields. As a result, several novel mesogens have
been synthesized by the introduction of various functional groups in the vicinity of the aromatic core as well as in the side chains and
their properties have been studied. In the present study, we report three-ring mesogens with hydroxyl groups at one terminal. These
mesogens were synthesized by a multistep route, and structural characterization was accomplished by spectral techniques. The
mesophase properties were studied by hot-stage optical polarizing microscopy, differential scanning calorimetry, and small-angle
X-ray scattering. An enantiotropic nematic phase was noticed for lower homologues, while an additional smectic C phase was found
for higher homologues. Solid-state high-resolution natural abundance 13C NMR studies of a typical mesogen in the solid phase and
in the mesophases have been carried out. The 13C NMR spectrum of the mesogen in the smectic C and nematic phases indicated
spontaneous alignment of the molecule in the magnetic field. By utilizing the two-dimensional separated local field (SLF) NMR
1
experiment known as SAMPI4, 13Cꢀ H dipolar couplings have been obtained, which were utilized to determine the orientational
order parameters of the mesogen.
’ INTRODUCTION
emphasizes molecular shape and topology, reduced symmetry,
microphase segregation, self-assembly, and self-organization.1ꢀ3,7ꢀ9
More recently, amphiphilic polyhydroxy compounds and carbohy-
drate derivatives have received special attention as they form
both lyotropic and thermotropic mesophases. It is found that the
morphologies of these thermotropic mesophases can be tailored by
variation of the number and position of the ꢀOH groups as well as
the lipophilic chains.10 As a consequence, the introduction of the
hydroxyl group as a part of thermotropic mesogens and the study of
such mesogens have gained importance.
Thermotropic mesophases represent unique states of matter
where partial order exists on a molecular and supramolecular
level.1ꢀ3 Molecules exhibiting mesophases are easily influenced
by external stimuli and adopt a new configuration of minimum
energy.2 The calamitic mesogens are rod-like in nature and
generally consist of a rigid core with linking units and terminal
groups.4 By a judicious choice of these structural components, a
wide range of mesogens exhibiting both smectic and nematic
phases have been synthesized.5 However, in recent years, the
molecular designing of mesogens has undergone a dramatic
change. While the conventional approach to the design of the
mesogen is based on parameters like geometrical anisotropy,
linearity, and anisotropic polarizability,6 the current view
Received: April 11, 2011
Revised:
August 3, 2011
Published: August 30, 2011
r
2011 American Chemical Society
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Figure 1. Molecular structure of mesogens.
The understanding of mesophase morphologies at the molec-
ular level requires a thorough investigation of intermolecular
interactions and their influence on the orientational order, which
can be determined by many techniques.11 Among them, solid-
state NMR has acquired popularity and has become a standard
technique in recent years since it is able to provide extensive
information on local order and dynamics.12ꢀ15 Deuterium NMR
spectroscopy is a powerful technique for obtaining the molecular
dynamics and order of mesogens.16,17 However, it requires
uniform/selective deuterium labeling of the sample. It has been
shown in recent years that the natural abundance 13C NMR
technique is a useful alternative capable of providing extensive
information about the mesophase.18,19 The application of the
two-dimensional separated local field (SLF) NMR technique
structural characterization of mesogens with a three-ring core
and a terminal hydroxyl by spectral techniques and phase
characterization by hot-stage optical polarizing microscopy
(HOPM), differential scanning calorimetry (DSC), and small-
angle X-ray scattering (SAXS). One of the terminal hydroxyl
mesogens has been studied using natural abundance 13C NMR
spectroscopy in the solid, smectic C, and nematic phases. In
the mesophase, by employing the 2D SLF NMR technique, the
1
13Cꢀ H dipolar couplings have been obtained to determine the
orientational order at various temperatures. Owing to the pre-
sence of terminal hydroxyl, the mesogens may be classified as a
reactive liquid crystal and can be suitably modified to get
mesogenic monomers for side-chain liquid-crystalline polymers.26
1
provides 13Cꢀ H dipolar couplings and, together with the 13C
’ EXPERIMENTAL SECTION
chemical shifts, can be used to obtain the order parameters of
various segments of the molecule, such as the rigid core and the
terminal groups, and to study the molecular topology.20,21
In the present work, the mesogen core is designed to have
three phenyl rings linked at the 1,4-position, connected via ester
and azomethine groups. One of the terminal units has a hydroxy
hexyloxy chain, while the other end has an alkoxy chain with an
even number of carbons between 4 and 12. The commonly used
linking units such as ester and azomethine are preferred as the
ester is relatively stable and imparts a certain degree of polariz-
ability while the azomethine contributes higher thermal stability
and results in polymesomorphism.22 Usually, three-ring-based
mesogens possess a higher aspect ratio, in contrast to the two-
ring homologues, and accordingly exhibit higher mesophase
thermal stability,23 while four- and five-ring-based cores result
in higher melting and clearing temperatures, and their lower
homologues often undergo decomposition.22 Although the
influence of phenolic OH on the mesophase characteristics of
calamitics is known,24,25 the effect of alcoholic OH as a terminal
functionality has not been widely investigated. Here, we report
Three-ring mesogens with a hydroxy hexyloxy chain at one
end and an alkoxy chain (even number of carbons 4ꢀ12) at the
other have been synthesized (Figure 1). The model mesogens
with terminal methyl have also been obtained for comparing the
mesophase characteristics with terminal hydroxyl series. The
synthetic details and spectral data are furnished in the Supporting
Information.
Solid-State NMR Studies. Solid-state NMR experiments were
performed on a Bruker AVIII 500 WB NMR spectrometer. The
proton and carbon resonance frequencies were 500.17 and
125.79 MHz, respectively, corresponding to a magnetic field of
11.75 T. 13C CP/MAS spectra of one of the mesogens was
recorded at room temperature in the crystalline phase using a
4 mm MAS probe. A sample spinning speed of 5 kHz was used,
and the TOSS27 pulse sequence was employed to avoid spinning
sidebands. Typically, 512 scans were used with a relaxation delay
of 3 s to acquire the time domain signal. Exponential apodization
was used with a line broadening constant of 12 Hz before Fourier
transformation to obtain the frequency domain spectrum.
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Scheme 1. Multistep Synthetic Route for Model and Target
Mesogensa
Figure 2. SAMPI4 pulse sequence used for obtaining the 2D-SLF
spectrum in the mesophase.
The spectra in the liquid-crystalline phase of the same sample
were obtained in the range between 120 and 180 °C using a
double-resonance probe equipped with a 5 mm solenoid coil for
static samples. In this temperature range, the sample exhibited
smectic C and nematic phases. The sample was observed to align
spontaneously in the magnetic field and provide sharp well-
resolved lines. Because the anisotropic interactions are only
partially averaged, 13C spectra could be recorded by employing
HartmannꢀHahn cross-polarization. During acquisition of the
signal, heteronuclear dipolar decoupling was employed using the
SPINAL-64 sequence.28 Typically, 512 scans with a delay of 15 s
between scans to avoid sample heating due to rf were used to
obtain a spectrum with a good S/N ratio.
a (A) DMF/K2CO3/90 °C/RBr (C4ꢀC12 even only); (B) EtOH/
KOH; (C) 4-nitrophenol/DCC/DMAP; (D) Pd/C, THF/EtOH;
(E,F) EtOH/AcOH, reflux; (G) RBr or 6-chloro hexanol/ DMF/
K2CO3/90 °C.
1
Partially averaged 13Cꢀ H dipolar couplings of the sample
were obtained using the SLF two-dimensional NMR spectros-
copy. For this purpose, the SAMPI4 pulse sequence29 (Figure 2)
was applied on the oriented sample under static conditions. This
pulse sequence is similar to the PISEMA30 pulse sequence, and
during the t1 period, magnetization exchange between protons
and carbons takes place under HartmannꢀHahn match. How-
ever, it differs from PISEMA in that for homonuclear proton
dipolar decoupling, the LeeꢀGoldburg (LG) sequence used in
PISEMA is replaced with a modified magic sandwich pulse
sequence.31 The purpose of including homonuclear decoupling
during t1 is to minimize magnetization decay through proton spin
diffusion, thereby achieving line narrowing in the F1 dimension.
During the t2 period, the carbon chemical shift spectrum with
proton decoupling is acquired. A two-dimensional Fourier trans-
form yields a 2D spectrum with carbon chemical shifts along the
F2 axis. The cross-peaks observed along the F1 dimension
provide the protonꢀcarbon dipolar couplings. In order to
maximize the polarization transfer and get higher intensities in
the 2D plots, polarization inversion is used,32 in which carbons
are initially polarized using a CP contact time of 2 ms. The
spectra were recorded by using 62.5 kHz of rf for both the proton
and carbon channels during the t1 period. During the t2 period, a
broad-band heteronuclear decoupling pulse scheme SPINAL6428
with 30 kHz of rf was used. The homonuclear dipolar decoupling
sequence used during the t1 period is based on the idea that the
homonuclear dipolar couplinginthe rotatingframe under spin-lock
is half of its value in the laboratory frame and is of opposite sign.33
Therefore, evolution of magnetization alternately under rotating
and laboratory frames produces echoes, indicating removal of the
interaction over one cycle.34 The π/2 pulses that sandwich the τ2
period in Figure 2 ensure cycling of the magnetization through the
rotating frame during the two τ1 periods and the laboratory frame
during the τ2 period. The τ1 and τ2 ideally should be the same.
However, to take care of magnetization evolution during the
nonideal π/2 pulses, τ1 and τ2 are adjusted to be equal to 7π/
4ω1 and 6π/4ω1, where ω1 is the rf field strength.29 In the present
experiment, values of τ1 and τ2 were respectively 14 and 12 μs. The
pulse sequence uses a supercycle with two dwell periods with rf
phases in mirror symmetry, which suppresses odd-order perturba-
tive terms of the dipolar Hamiltonian. In addition, the use of the
supercycle makes the measurement of dipolar coupling less sus-
ceptible to proton chemical shift and offset compared to PISEMA.
Theoretical calculations have been used to derive the scale factor of
the sequence, which is nearly equal to unity.29 In our case, we
recorded the 1D fully proton coupled 13C spectrum of chloroform
oriented in the liquid crystal N-(4-ethoxybenzylidine)-4-n-butylani-
line (EBBA) and also the 2D-SAMPI4 spectrum of the same
sample. The dipolar doublet splitting obtained in the former
experiment was used to estimate the scaling factor for the latter.
The frequency scale in the F1 dimension during data processing has
been adjusted to take care of the scale factor, so that the actual
dipolar frequencies can be read out from the 2D plots and 1D cross
section. Typically, 16 transients were used for each t1 period with a
recycle delay of 15 s between scans and 128 t1 increments were
employed. A shifted sine bell window function was applied to the
time domain data, and the spectra were processed in the phase-
sensitive mode.
’ RESULTS AND DISCUSSION
The synthesis of mesogens is accomplished as depicted in
Scheme 1. The key intermediate 4-alkoxybenzoyloxy aniline is
reacted with 4-[(6-hydroxyhexyl)oxy]benzaldehyde to get target
mesogens and are named as the 6 series. In order to compare and
understand the influence of the hydroxyl group on the meso-
phase characteristics, compounds with a terminal methyl group
at both the ends have also been synthesized, which serve as model
compounds (Figure 1). All of the synthesized mesogens have
been characterized by FT-IR and 1H and 13C NMR spectroscopy.
The spectral data for a representative mesogen is furnished in the
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Supporting Information. Figure 1 shows the molecular structure of
the mesogens.
blank scotch tape. The sample exposure time was 900 s, and the
ramp rate was 6 °C/min. Once the target temperature was
reached, a 300 s delay was used before starting the experiment.
The results of the studies are depicted in Figure 4.
The hot-stage optical polarizing microscopy studies of the
mesogens showed enantiotropic mesophases. Table 1 lists the
melting and the clearing temperatures along the with transition
enthalpy (ΔH) values of all of the synthesized mesogens. Among
the three model mesogens, lower homologue HPMAPB showed
the enantiotropic nematic phase, while the higher homologues
HPMAPD and HPMAPDd exhibited both smectic C and
nematic phases, respectively (Table 1). For those with the
hydroxy terminal, the lower homologues, that is, HyHPMAPB
and HyHPMAPH showed enantiotropic nematic phases similar
to the corresponding model compounds, whereas the higher
homologues HyHPMAPO, HyHPMAPD, and HyHPMAPDd
exhibited both smectic C and nematic phases. For instance, the
OPM textures of the HyHPMAPD (Figure 3) while cooling the
isotropic phase shows nematic droplets that coalesce to give a
marble texture (Figure 3A). Upon further cooling, the mesogen
changes to the smectic C phase, as indicated by the transition
bars (Figure 3B), and exhibits smectic schlieren texture. The
microscopic data is also confirmed by transition enthalpy (ΔH)
values determined from differential scanning calorimetry
(Table 1). The HOPM and DSC studies thus clearly indicate
the appearance of the smectic C phase for higher homologues. It
is also observed that the temperature range of the smectic C
phase for the terminal hydroxyl mesogens is reduced in compar-
ison to that of the corresponding model compounds. This may be
attributed to the terminal interactions involving the hydroxyl
group through hydrogen bonding. While it may be concluded
that the overall molecular anisotropic polarizability is high in
these systems, as suggested by the thermal stability values, a
marginal increase in the melting point also indicates that the
terminal attractions do play a role in influencing the mesophase
characteristics.
The SAXS trace of HyHPMAPD at 118 °C shows two sharp
peaks at q = 0.1166 and 0.234, which are assigned to first- and
second-order Bragg reflections, the latter being of low intensity.
The corresponding layer spacings (d = 2π/q; d = 4π/q) of these
reflections are 53.88 and 53.70 Å. These reflections clearly
suggest a layer ordering typical of smectic phases.35 Further,
with the increase in temperature, an increase in layer spacing is
observed. Thus, the measurements at 127 and 136 °C also show
two sharp peaks but with increased layer spacings of 54.16 and
55.21 Å, respectively, obtained from first-order reflections. This
suggests that the phase under investigation is the smectic C as the
tilt angle decreases with temperature. In the nematic phase at
SAXS Studies. The mesophase sequence for HyHPMAPD, as
mentioned above, was first observed by hot-stage polarizing
microscopy and was later confirmed by DSC. In order to further
establish the layer ordering of the mesogen, SAXS studies were
carried out at various temperatures. The measurements were
carried out using a Bruker Nanostar instrument equipped with a
rotating anode source and a three-pinhole collimation system
with pinhole diameters for high flux (μm), 750, 400, and 1000.
The wavelength of the X-rays was 1.54 Å (Cu Kα radiation)
operated at 45 kV/110 mA. SAXS data were recorded in the q
range of 0.05ꢀ0.72 (1/Å). A distance of 26 cm was maintained
between the sample and detector. For data collection, the sample
was sandwiched between two pieces of scotch tape of known
thickness, and the backgrounds were subtracted from that of
Figure 3. Optical polarizing micrographs of HyHPMAPD. (A) At
201 °C in the cooling cycle showing the marble texture of the nematic
phase. (B) Characteristic transition bars near transition from the nematic
to smetcic C phase at 137 °C.
Table 1. OPM and DSC Phase Transition Data of the C6 Seriesa
S. No
code
m
n
TCꢀSc °C
TCꢀN °C
TScꢀN °C
TNꢀI °C
ΔTSc
ΔTN
1
2
3
4
5
6
7
8
HPMAPB
4
4
4
4
4
4
4
4
2
113.3 (7.83)
242.4 (0.44)
207.3 (0.37)
197.6 (0.41)
238.9 (0.46)
223.5 (0.60)
213.7 (0.73)
202.7 (0.47)
194.7 (0.42)
129.1
64.7
50.4
113.3
96.3
93.2
65.0
45.0
HPMAPD
8
10
2
104.8 (7.64)
98.8 (6.70)
142.6 (0.39)
147.2 (0.47)
37.8
48.4
HPMAPDd
HyHPMAPB
HyHPMAPH
HyHPMAPO
HyHPMAPD
HyHPMAPDd
125.6 (5.09)
127.2 (6.79)
4
6
110.2 (9.39)
108.9 (7.23)
109.5 (8.54)
120.5 (1.10)
137.7 (0.99)
149.7 (1.18)
10.3
28.8
40.2
8
10
a Values in the parentheses indicate ΔH in kcal/mol.
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lines from the ꢀOCH2 groups and one line corresponding to the
ꢀCH2OH carbon are expected. Of them, the chemical shift
values of the two ꢀOCH2 carbons are very similar, and the
corresponding signals appear at a higher frequency due to their
proximity to the phenyl rings. The line appearing at the lower
frequency may be assigned to the ꢀCH2OH carbon. In the
region of 22ꢀ33 ppm, nine lines are observed, which are from the
decyloxy chain and central methylenes of the hydroxyl hexyloxy
chain. The characteristic methyl carbon of the decyloxy chain is
noticed at 14.13 ppm.
13C CP/MAS NMR Study of Crystalline HyHPMAPD. To
study the mesogen in its solid phase, the 13C CP/MAS spectrum
of the compound is obtained at room temperature. The sample is
spun at 5 kHz, and to eliminate the spinning sidebands, the
spectrum is obtained using the TOSS pulse sequence.27 The
spectrum (Figure 5B) shows 24 peaks in the region of 110ꢀ
170 ppm, in contrast to 14 lines in the solution spectrum. The
increase in the number of lines compared to the solution state
spectrum indicates the presence of either more than one
molecule in the unit cell or molecules with different conforma-
tions. A closer examination of the chemical shift values shows
that 6 out of the 10 additional peaks observed fall in the region of
110ꢀ134 ppm and are attributable to the phenyl ring methine
carbons. The remaining four signals occurring in the region of
142ꢀ158 ppm may be assigned to quaternary carbons. For the
terminal chains, additional peaks are also seen in the region of
60ꢀ70 ppm, while the methylene region (20ꢀ35 ppm) showed
a set of broad peaks, in contrast to the solution spectrum, due to
overlapping of methylene carbon chemical shifts. In an earlier
work, a structurally similar nematogen also showed more peaks
in the solid state at room temperature that however disappeared
when the temperature of the sample was raised.36
Figure 4. Variable-temperature SAXS traces of HyHPMAPD in smectic
C and nematic phases.
190 °C, however, the sharp peaks that are noticed at lower
temperatures are replaced by a broad hump, indicating liquid-like
ordering. These features unambiguously confirm the existence of
smectic C and nematic phases in HyHPMAPD.
13C NMR Studies of HyHPMAPD. Spectral Assignment in
Solution. For the study of molecular dynamics and orientational
order in the mesophase by NMR, the first step is the assignment
of the spectrum to the molecular structure of the mesogen in
solution. The proton-decoupled 13C NMR spectrum of HyHP-
MAPD in solution is shown in Figure 5A. The molecule consists
of three phenyl rings joined by two linking units in the core and is
flanked by terminal hydroxyl and decyloxy chains (Figure 1). The
individual assignment of the chemical shifts of all of the carbons
is carried out by comparing structurally similar compounds and
also using the iterated spectrum generated by the ACD Chems-
ketch (version 3.0) software. The assigned chemical shift values
along with the carbon numbers are listed in Table 2. The solution
NMR spectrum showed 14 well-resolved lines in the region of
113ꢀ166 ppm and accounted for all 20 carbons of the core unit.
The highly intense signals in the region of 113ꢀ133 ppm arise
from the protonated carbons of the phenyl rings. The chemical
shift equivalence of the carbons at the ortho and the meta
positions indicate rapid π-flips of the phenyl rings about their
C2 axes. The azomethine carbon showed a sharp line of higher
intensity at 159.8 ppm due to its attached proton giving rise to a
shorter relaxation time in comparison to the quaternary carbons,
which showed well-resolved lines of lower intensity in this region.
Thus, among the 14 lines, 7 have been assigned to the methine
carbons and the remaining to the quaternary carbons. In the
aliphatic region (13ꢀ69 ppm), 13 lines are seen with varying
intensity accounting for 16 carbons arising from the terminal
hydroxy hexyloxy and decyloxy chains. Table 2 shows the
assignment of these lines to various methylenes and a methyl
carbon. Among them, three lines are seen at 63ꢀ69 ppm with
equal intensity. The larger chemical shift values of these lines, in
contrast to other signals in the region, indicate that they are from
ꢀOCH2 carbons. Because the mesogen consists of a hydroxyl
hexyloxy chain at one end and a decyloxy chain at the other, two
13C NMR Spectrum of HyHPMAPD in the Mesophase.
Figure 5C and D shows the 13C NMR spectrum of a static
sample of the mesogen in its smectic C phase at 120 °C and in the
nematic phase at 160 °C. In contrast to CP/MAS and solution
spectra, the peaks in the mesophase are observed to be well-
dispersed. The other notable feature of the spectra in the
mesophase is the reduction in the number of peaks as compared
to the spectrum in the solid state; the number is now akin to that
in the spectrum in the solution phase. There is also a dramatic
change in the chemical shift values, which cover a much larger
range. The peaks that are seen in the solution and in the solid
state in the range of 113ꢀ166 ppm now appear at a higher
frequency in the region of 140ꢀ250 ppm. On the other hand, the
signals that are seen in the range of 13ꢀ69 ppm in the solution
and in the solid state are slightly shifted to the lower frequency
and appear in the region of 8ꢀ60 ppm. Observation of the sharp
peaks and the spread in the chemical shift values clearly indicate
that the sample in the smectic C and the nematic phases are
oriented in the magnetic field. The changes observed in the
chemical shift values of the mesogen upon going to the meso-
phase suggest that the molecules are aligned parallel to the field.
These are confirmed subsequently from the order parameters
obtained from dipolar couplings. It is seen in the spectrum that
the line widths in the smectic C phase are relatively large
compared to those obtained in the nematic phase.
The assignment of the chemical shift values of the aligned
mesogens to individual carbons is difficult. This is due to the lack
of data on the chemical shielding tensors of different carbons
belonging to different groups involved in the construction of the
mesogen and also due to the possibility of a difference in the
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Figure 5. (A) Proton-decoupled 13C NMR spectrum of HyHPMAPD in solution at 25 °C. (B) 13C CP/TOSS NMR spectrum of HyHPMAPD with
5 kHz spinning speed at 25 °C in the solid phase. (C) 13C spectrum of the static oriented sample of HyHPMAPD at 120 °C in the smectic C phase.
(D)13C spectrum of the static oriented sample of HyHPMAPD at 160 °C in the nematic phase.
ordering along the length of the molecule. Gross assignments
may be made from the peak positions and intensities, while finer
assignments can be done by comparison of the spectral pattern
with those of structurally similar compounds and model meso-
gens for which the assignments are already known. The proce-
dure adopted for assigning the spectrum in the smectic C phase is
described below, and assignment in the nematic phase follows as
an extension of the same logic. In the smectic C phase, the ring 1
chemical shifts have been assigned by comparing with the static
13C NMR spectrum of structurally similar mesogens.19,37
As a result of the π flips that phenyl rings undergo about the
para-axis, a single line is observed for the pair of methine carbons
in the ortho and meta positions. Consequently, 6 peaks may
be expected, corresponding to the 12 methine carbons in the
three phenyl rings. However, the spectrum shows only five peaks
in the region of 140ꢀ180 ppm. In view of the ortho carbons in
ring 3 (carbons 13 and 130) having a chemical environment
similar to carbons 2 and 20 in ring 1, the overlap of corresponding
signals is possible. In solution too, these carbons show two
closely spaced lines at 114.76 and 114.37 ppm. In the spectrum in
the mesophase, the peak that appears at 145.3 ppm is more
intense and can account for four carbons. Hence, this peak is
assigned to carbons at positions 2/20 and 13/130. The azo-
methine carbon has been identified by considering the intensity
and chemical shift value and assigned to the peak at 207.4 ppm,
which is further supported by the SLF-2D data presented in the
next section. On the basis of the intensity pattern and by
comparison with the chemical shifts of other structurally similar
mesogens, the other aromatic methine carbons of the static
spectrum have also been assigned, and the chemical shift values
are listed in Table 2. In contrast to the spectrum in solution, the
13C NMR spectrum in the mesophase shows only three signals
for quaternary carbons, whereas the core unit of the mesogens
consists of seven quaternary carbons. However, in the 2D
spectrum, four peaks can be identified for the quaternary carbons.
The signal appearing at 236.4 ppm has a higher intensity
compared to other peaks in the region and may be attributed
to overlapped lines of structurally similar carbons like C1, C6,
and C14. The ester carbonyl carbon C5 is assigned a chemical
shift of 226.6 ppm. Of the other three quaternary carbons, C4 is
assigned to the peak at 197.7 ppm, while C9 and C11 are assigned
to 234.3 and 207.4 ppm, respectively (Table 2). In the case of
aliphatic chains, by close observation of the region of 50ꢀ
60 ppm and also with the help of the 2D-SLF spectrum, two
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Table 2. 13C Chemical Shifts (ppm) of HyHPMAPD in Solution, Solid, and Smectic C and Nematic Phases
carbon
code
solution
solid CP-MAS
smectic C phase
AIS (ppm)
nematic phase
AIS (ppm)
(ppm) 25 °C
(TOSS)- (ppm) 25 °C
static (ppm) 120 °C
(δlc ꢀ δsoln.
)
static (ppm) 160 °C
(δlc ꢀ δsoln.
)
1
163.6
114.3
132.3
121.7
165.1
148.9
121.7
122.3
149.9
159.8
129.0
130.5
114.7
161.9
68.3
163.9
114.3
131.8
120.6
163.9
148.2
122.0
122.0
152.3
158.8
128.5
129.2
115.3
162.7
66.1
236.4
145.3
169.5
197.7
226.6
236.4
151.3
157.1
234.3
207.4
207.4
165.8
145.3
236.4
62.1
72.8
31.0
37.2
76.0
61.5
87.5
29.6
34.8
84.4
47.6
78.4
35.3
30.6
74.5
ꢀ6.2
ꢀ6.0
ꢀ1.3
ꢀ5.9
ꢀ5.1
ꢀ6.5
ꢀ6.3
ꢀ6.1
ꢀ4.5
ꢀ4.4
ꢀ4.9
ꢀ2.1
ꢀ2.0
222.2
139.7
161.6
182.7
208.6
222.2
144.9
139.8
217.7
197.1
192.8
158.6
139.7
222.2
62.9
58.6
25.4
29.3
61.0
43.5
73.3
23.2
17.5
67.8
37.3
63.8
28.1
25.0
60.3
ꢀ5.4
ꢀ5.2
ꢀ1.6
ꢀ4.7
ꢀ4.5
ꢀ5.2
ꢀ5.0
ꢀ4.8
ꢀ2.6
ꢀ2.5
ꢀ4.5
ꢀ1.7
ꢀ2.1
2
3
4
5
6
7
8
9
10
11
12
13
14
A
a
68.1
65.1
62.1
62.9
f
62.8
62.3
61.5
61.2
e
32.7
33.1
26.8
28.0
B
31.9
31.5
26.8
27.4
G, H
E, F
b, D
c
29.5
31.5
23.0
24.3
29.3
31.5
23.0
24.3
29.1
31.5
23.0
24.3
26.0
26.3
21.5
23.4
d
25.9
24.5
21.5
23.4
C
I
25.5
23.8
20.6
21.0
22.7
23.8
20.6
21.0
J
14.1
14.0
12.1
12.0
peaks can be identified. Of them, the two ꢀOCH2 are assigned to
62.1 ppm, and the terminal ꢀCH2OH is given an assignment of
61.5 ppm. All other methylene carbons appear in the region of
16.9 to 24 ppm as broad peaks due to overlap of signals from
carbons with nearly the same chemical shift. The narrow chem-
ical shift span for these carbons does not favor resolution of the
peaks corresponding to the different carbon sites. The signal due
to the methyl carbon of the decyloxy chain appears at 9.4 ppm.
The spectrum in the nematic phase (160 °C) (Figure 5D)
shows similar trends with better resolution. For instance, in the
smectic C phase, the quaternary carbon (C11) and the azo-
methine carbon (C10) appear as a single line due to overlap, but
in the nematic phase, they appear as separate lines. Similarly, in
the smectic C phase, in the aliphatic region, the signals due to
ꢀOCH2 and CH2OH carbons are overlapped. In the nematic
phase, they are clearly separated into two lines. This improve-
ment in resolution in the nematic phase results from smaller line
width due to faster dynamics of the molecules and low molecular
order compared to that of the smectic C phase.
chemical shift tensors. Also, the quaternary carbons show a larger
shift to the higher frequency than methine carbons. For instance,
for the aromatic methine carbons C7 and C8 in the central phenyl
ring, the AIS values are respectively 29.6 and 34.8 ppm, while the
quaternary carbons C6 and C9 of the same ring have values of 87.5
and 84.4 ppm, respectively. These results are in conformity with
behavior expected for mesogens that are aligned parallel to the
field.38
2D-SLF. The SAMPI4 pulse sequence shown in Figure 2 has
been used to obtain a two-dimensional SLF NMR spectrum of
the aligned mesogens both in the smectic C and in the nematic
phases. The experiment provides 13C chemical shifts along the F2
13
axis and 1Hꢀ C dipolar couplings along the F1 axis. Earlier use
of SAMPI4 on a liquid-crystalline system is made on 4-n-pentyl-
40-cyanobiphenyl (5CB) to demonstrate its applicability and
performance.39 In this work, the use of SAMPI4 is presented for
the first time for a novel mesogen. Figure 6 shows a typical 2D
plot of the mesogen HyHPMAPD in its smectic C phase at
120 °C, and Figure 7 shows expanded plots of the aromatic
methine signals of the 2D spectra in the smectic C (120 °C) as
well as in the nematic (160 °C) phases. For the sake of clarity, the
spectrum obtained in the smectic C phase at 120 °C (Figures 6
and 7A) is discussed in detail, while the data obtained at other
temperatures in the smectic and nematic phases are listed in
Table 3. In Figures 6 and 7, well-resolved cross-peaks along the
F1 axis can be seen for several carbons both for the core unit and
for the terminal chains, with line widths in the F1 dimension in
The alignment-induced chemical shift (AIS) for all of the
carbons has been obtained as the difference between the chemical
shifts in the oriented phase and the isotropic phase (δlc ꢀ δsolution
)
and are listed in Table 2, while Table 3 shows the chemical shifts of
carbons of the core unit of the mesogen in both phases at different
temperatures. As seen in Table 2, unlike the aliphatic carbons, the
aromatic, the carbonyl, and the azomethine carbons (core unit
carbons) have large AIS values due to the large span of their
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The Journal of Physical Chemistry B
ARTICLE
some cases being as low as 200 Hz. Interestingly, the line widths are
reduced even further to 100 Hz by the use of maximum entropy
reconstruction methods,40 the details of which will be presented
elsewhere. The additional resolution available in the 2D spectrum is
used for confirmation of assignment of several carbons.
The core unit of HyHPMAPD consists of seven chemically
inequivalent methine carbons. Among them, six carbons belong
to the three phenyl rings, and the remaining one to the
azomethine group. However, in the 2D spectrum (Figure 7),
only six dipolar contours with a large separation along the dipolar
dimension that could correspond to the aromatic methine
carbons are observed. As in the case of the 1D spectrum, a close
examination reveals that the contours at 145.3 ppm (Figure 7A)
are more intense than others and also show a slightly larger
spread in both dimensions. As discussed in the earlier section,
these contours are attributed to the two methine carbons C2 and
C13 belonging to the rings 1 and 3, respectively. The super-
position of the contours of both of these carbons indicates that
not only the chemical shifts but also the dipolar coupling values are
nearly the same for both carbons. The other phenyl ring methine
carbon contours are seen at 151.3, 157.1, 165.8, and 169.5 ppm,
while the characteristic contours corresponding to the azomethine
linking unit appears at 207.4 ppm. The 2D spectral pattern is thus
consistent with the molecular structure. The frequencies of the
contours along the dipolar dimension correspond to the dipolar
oscillation frequencies (fD) and are listed in Table 3. The dipolar
oscillation frequency corresponds directly to the dipolar coupling
only for an isolated CꢀH pair.41 For more than two protons
coupled to a carbon, the dipolar coupling information needs to be
extracted from the oscillation frequency (vide infra). In Table 3
and Figure 7A, it is seen that the dipolar frequencies are not the
same for all carbons in the core unit. Earlier studies of two ring
nematogens show that typical dipolar oscillation frequencies for
phenyl rings are about 2.00 kHz.18 However, in the present case,
these values are found to be around 3.00 kHz. Also the azomethine
dipolar frequency of 5.55 kHz is significantly higher than the value
of 3.5 kHz observed for mesogens like MBBA and EBBA. These
observations indicate overall a high molecular order for HyHP-
MAPD in the smectic phase.
Table 3. Chemical Shifts (in ppm) and Dipolar Oscillation
Frequencies (in kHz) of Carbons in the Aromatic Core Unit of
HyHPMAPD at Different Temperature in the Mesophase
120 °C
130 °C
140 °C
160 °C
180 °C
C. No ppm kHz ppm kHz ppm kHz ppm kHz ppm kHz
1
2
3
4
5
6
7
8
9
236.4 1.84 235.2 1.83 228.4 1.57 222.2 1.46 212.2 1.21
145.3 3.24 144.7 3.18 142.1 2.87 139.7 2.60 135.3 2.15
169.5 3.02 168.7 2.98 165.0 2.69 161.6 2.41 156.3 1.98
197.7 1.71 196.5 1.74 188.8 1.56 182.7 1.44 172.3 1.13
226.6
226.1
213.6
208.6
201.4
236.4 1.84 235.2 1.83 228.4 1.74 222.2 1.46 212.2 1.21
151.3 3.46 150.5 3.37 147.6 3.04 144.9 2.75 140.7 2.24
157.1 3.21 156.4 3.13 152.9 2.82 149.8 2.53 145.0 2.18
234.3 1.86 233.4 1.78 224.7 1.74 217.7 1.54 207.2 1.23
The 2D spectrum of the mesogen in the aliphatic region
(9ꢀ70 ppm) also shows well-resolved signals. The high resolu-
tion observed in the 2D spectrum, compared to the 1D spectrum,
arises out of the difference in the dipolar couplings of carbons
whose signals are overlapped in the 1D spectrum due to nearly
identical chemical shift values. For instance, the ꢀOCH2 and
ꢀCH2OH carbons whose peaks overlap and are just discernible
10 207.4 5.55 206.4 5.53 201.2 4.88 197.1 4.44 190.3 3.68
11 207.4 2.12 206.4 1.98 198.8 1.82 192.8 1.75 182.2 1.40
12 165.8 3.34 165.2 3.30 161.6 2.97 158.6 2.68 153.7 2.21
13 145.3 3.24 144.7 3.18 142.1 2.87 139.7 2.60 135.3 2.15
14 236.4 1.84 235.2 1.83 228.4 1.57 222.2 1.46 212.2 1.21
1
Figure 6. 13Cꢀ H 2D-SLF spectrum of HyHPMAPD at 120 °C in the smectic C phase.
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The Journal of Physical Chemistry B
ARTICLE
is given by12,20
ꢀ
1
DCꢀH ¼ K ð3 cos2 θz ꢀ 1ÞSz0z
2
ꢁ
1
þ ðcos2 θx ꢀ cos2 θyÞðSx0 x ꢀ S0yyÞ
ð1Þ
2
where K = ꢀhγCγH/4π2r3CH, with γC and γH being the gyro-
magnetic ratios of carbon and hydrogen nuclei, respectively, rCH is
the distance between the nuclei C and H, and θx, θy, and θz are the
angles that the CꢀH vector makes with the coordinate axes. For
each of the phenyl rings, we have chosen the z-axis to be the para-
axis of the ring, the x-axis is in the plane of the ring, and the y-axis is
perpendicular to the plane. In the case of the ortho- and the meta-
carbons, the experimentally measured dipolar frequency fD actually
has two components, namely, a coupling of the carbon to the ipso
proton DCꢀHi and also to the proton at the ortho position DCꢀHo
.
The measured frequency is then obtained as41 [(DCꢀHi)2 +
(DCꢀHo)2]1/2. The quaternary carbons have dipolar couplings to
two equivalent protons in the ortho position. The meta protons
being far away, their coupling may be neglected. In this case, the
√
oscillation frequency may be obtained as 2*(DCꢀHo). The
experimentally determined four oscillation frequencies for each
phenyl ring are used to obtain the two order parameters S0zz and
S0xx ꢀ S0yy. In view of the uncertainty in the position of the H atom
as determined by X-rays, it has been foundnecessaryto vary the two
CꢀCꢀH bond angles, which are found to deviate from the value of
120° corresponding to an ideal hexagonal geometry.43,44 Accord-
ingly, in the fitting procedure, the two CꢀCꢀH bond angles have
also been slightly varied around 120°. Table 4 lists the oscillation
frequencies and the local order parameters for each phenyl ring
obtained in the smectic C phase at 120 °C by fitting the experi-
Figure 7. An expanded plot of the 2D-SLF spectrum of HyHPMAPD
corresponding to the aromatic core; (A) smectic C phase (120 °C);
(B) nematic phase (160 °C).
mental data. Figure 8A and B shows the variation of S0zz and S0xx
ꢀ
S0yy as a function of temperature for all three rings. It is clear from
Figure 8 and Table 4 that the order parameters differ by a small
value between the three rings. It is also observed that the order
parameters increase while passing from the nematic to the smectic
C phase. Similar observation was made earlier in other systems
exhibitinga smectic Cphaseathighmagnetic fields.45 The chemical
shift values shown in Table 3 also further confirm this trend; the
aromatic carbon chemical shifts are the largest at 120 °C and
decrease monotonically at higher temperatures. The order param-
eters while entering the smectic phase from the nematic phase do
not show a drastic change, which indicates that the conformer
dynamics is nearly the same in both phases.
In order to estimate the orientation of the long axis of the
molecular core with respect to the para-axes of the phenyl rings,
o0ne may0use an uniaxial approximation and use the relations
S zz(1)/S zz(2) ≈ P2(cos β1)/P2(cos β2) or S0zz(3)/S0zz(2) ≈
P2(cos β3)/P2(cos β2). However, a whole range of values of β1,
β2, and β3 will satisfy the above relation, and it is not possible to
uniquely fix the orientaation of long molecular axis. To circum-
vent this problem, a relationship between the β’s can be obtained
using a method proposed earlier,42,46,47 namely, S0zz(1)/S0zz(2)
≈ P2(cos θ1ꢀ2) and S0zz(3)/S0zz(2) ≈ P2(cos θ2ꢀ3), where θ1ꢀ2
and θ2ꢀ3 are the angles between the para-axes of rings 1 and 2
and 2 and 3, respectively. This is an approximation and is valid
when one of the para-axes can be expected to be nearly parallel to
the long molecular axis from p0hysical considerations. From the
information on the values of S zz of the three rings shown as a
function of temperature in Figure 8, we have obtained average
angles of 9.5 and 12.9° between the para-axes of rings 1 and 2 and
as two lines in the 1D spectrum are now separated into two sets of
contours with dipolar oscillation frequencies of 9.4 and 3.71 kHz,
respectively, for the ꢀOCH2 and ꢀCH2OH carbons. This
difference in the dipolar frequencies arises due to their locations
in the mesogenic molecule. The ꢀOCH2’s are positioned closer
to phenyl rings of the core unit, while the ꢀCH2OH is at one
terminal of the mesogens where the mobility is high. The dipolar
couplings thus provide a valuable indicator of the local dynamics
and the position of the atoms in the molecule. For the large
number of ꢀCH2 carbons, the 1D spectrum shows one broad
signal with few shoulders, whereas several contours are clearly
distinguished in the range of 16.9ꢀ24 ppm in the 2D spectrum.
The characteristic terminal methyl shows a contour at 9.9 ppm with
a dipolar frequency of 1.91 kHz. No attempt is made here to further
identify the chain carbons in view of the complex dynamics;
subsequent discussion being confined to the aromatic core. From
1
the experimentally measured 13Cꢀ H dipolar frequencies, the
orientational order parameters of the aromatic core of the liquid-
crystal molecule have been obtained, which are presented in the
next section.
Orientational Order Parameters. The smectic C phase
exhibits biaxial ordering. However, for an aligned sample, the
phase biaxiality may not be detected, and hence, only the
molecular order parameters may be obtained.42 The measured
dipolar couplings can be used for estimating the local order
parameters of the liquid-crystal fragme0nts. The phenyl rings
require two order parameters, namely, S zz and S0xx ꢀ S0yy, and
the carbonꢀproton dipolar coupling in terms of these parameters
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The Journal of Physical Chemistry B
ARTICLE
1
Table 4. Local Order Parameters of the Phenyl Rings and the Corresponding 13Cꢀ H Dipolar Oscillation Frequencies for
HyHPMAPD at 120°Ca
calculated dipolar oscillation frequencies (kHz)
120 °C
θb
θc
S0zz
S0xx ꢀ S0yy
b
c
a
d
ring 1
ring 2
ring 3
120.3
120.1
120.1
120.9
120.8
120.3
0.75
0.79
0.74
ꢀ0.097
ꢀ0.099
ꢀ0.099
3.23
3.46
3.29
3.03
3.21
3.22
1.78
1.86
2.33b
1.79
1.88
1.76
a In the figure above the table, b and c are methine carbons, and a and d are quaternary carbons. b For carbon a in ring 3, the contribution of the
azomethine proton has also been taken into account.
been reported in other systems.46 The estimation of the dipolar
coupling of the carbon in the azomethine linking unit will require
a knowledge of the complete order matrix, in the absence of any
symmetry. In view of the lack of this information, one may use a
uniaxial approximation and use only the dominant order para-
meter. Assuming the other order parameters to be small and negli-
gible48 and from the relation DCꢀH = K{1/2(3 cos2 θ ꢀ 1)S0zz
the angle θ between the long molecular axis and the azomethine
CH bond vector may be calculated. Utilizing the order parameter
of the long molecular axis that is found to coincide with the para-
axis of the central phenyl ring and from the dipolar coupling of
the azomethine carbons at different temperatures, an angle of
110.7 ( 0.4° has been obtained for the orientation of the CH
bond of the azomethine carbon, which matches well with the
value of 110.5° estimated in an earlier study on MBBA carried out
using deuteriumꢀcarbon dipolar couplings.49
It may be mentioned that the 13C chemical shifts provide one
of the important means of determining the order parameters of
the system.42,46 This requires a knowledge of the CSAs of the
carbon, which may be obtained either by experiments or by
DFT calculations. In the present case, we have the informa-
tion about CSA available from literature and from experi-
ments for a few of the carbons of ring 1, and for these carbons,
we have used the local order parameters obtained from the
dipolar couplings to estimate the chemical shifts, and the
results are presented in the Supporting Information. It is
observed that the calculated values match reasonably well
the experimental values and are also able to reproduce the
trend in the variation of the chemical shifts as a function of
temperature in both phases.
’ CONCLUSIONS
Novel three-ring-based mesogens built with three phenyl ring
cores linked by ester and azomethine groups and hydroxy
Figure 8. The variation of the order parameters S0zz and S0xx ꢀ S0yy as a
function of temperature for HyHPMAPD in the mesophase.
hexyloxy/alkoxy chains were synthesized by a multistep route.
The HOPM and DSC data indicated enantiotropic nematic
phase for all lower homologues and smectic C for higher
homologues. Introduction of terminal hydroxyl has not altered
the phase sequence, though it influenced the phase and thermal
stability values. The 13C NMR study of a representative hydroxyl-
containing mesogen (HyHPMAPD) showed the parallel align-
ment of mesogens with the magnetic field in the smectic C and
rings 2 and 3, respectively. From the above information, an angle
of 1° between the long axis of the molecular core and the para-
axis of ring 2 was obtained. The order parameter values are
typical for calamitic mesogens exhibiting the smectic C phase
and nematic phase.14,42 The negative values for S0xx ꢀ S0yy are not
unusual, being dependent on the choice of coordinates, and have
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The Journal of Physical Chemistry B
ARTICLE
nematic phases. The 2D SAMPI4 experiment provided 13Cꢀ H
dipolar couplings that were used to confirm the molecular
structure of the mesogens and to compute the order parameters
and the orientation of the molecular director. A large value for the
local order parameters for all three rings in the smectic C phase of
the mesogens has been obtained, which indicates that the lateral
interactions of the core are dominant, a common feature of
calamitic smectogens. In the nematic phase, the order parameter
values are found to be in a range commonly observed in many
nematic liquid-crystalline systems.
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Magn. Reson. Spectrosc. 2007, 50, 1.
1
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’ ASSOCIATED CONTENT
S
Supporting Information. The experimental details per-
b
taining to the synthesis of mesogens as well as details regarding
DSC data and plots of chemical shifts. This material is available
free of charge via the Internet at http://pubs.acs.org.
(16) (a) Zhang, J.; Domenici, V.; Veracini, C. A.; Dong, R. Y. J. Phys.
Chem. B 2006, 110, 15193. (b) Dong, R. Y.; Chen, Y. B.; Veracini, C. A.
Chem. Phys. Lett. 2005, 405, 177.
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Ribeiro1, A. C.; Feio, G.; Apreutesei, D.; Mehl, G. H. Mol. Cryst. Liq.
Cryst. 2008, 495, 348.
’ ACKNOWLEDGMENT
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T.N. would like to thank Dr. A. B. Mandal, Director, CLRI, for
his support and encouragement. The partial financial support
from NWP-23 is duly acknowledged. The use of the AV-III-500
Solid State NMR spectrometer funded by the Department of
Science and Technology, New Delhi, at the NMR Research
Centre, Indian Institute of Science, Bangalore, is gratefully
acknowledged.
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