Effect of alkyl chain and linking units on mesophase transitions and molecular order of rod-like thiophene mesogens: 13C NMR investigation

Article abstract of DOI:10.1039/c7nj03525j

Thermotropic liquid crystals with π-conjugated cores are increasingly witnessed due to their promising optoelectronic and optophotonic properties. As π-conjugated mesogens are often realized by direct linking of the rings of the core, the mesophase transitions are usually modulated by inserting the required alkyl/alkoxy chains either at a terminal or lateral location. In this work, five mesogens in which thiophene and two phenyl rings are common in the core unit with terminal alkyl/alkoxy chains are investigated to probe the influence of (a) a terminal alkyl chain on the mesophase characteristics, (b) the linking units on the mesophase range and (c) the disparity of thiophene orientational constraints. The mesophase properties reveal that whenever the alkyl chain is located at one end of the molecule, polymesomorphism is observed. The XRD studies reveal layer ordering typical of smectic mesophases with lower temperature and higher order phases, namely SmB and CrE phases. A detailed ¹³C NMR study of the mesogens in the liquid crystalline phase indicates that the order parameters are governed by the nature of the substitution of thiophene and its linkage with two phenyl rings. Importantly, the ¹³C NMR data of the terminal chains in the liquid crystalline phase are compared with the recently reported thiophene mesogen in which the n-hexyl chain is located at the lateral position and completely different orientation characteristics were found for the terminal versus lateral chains.

Full text of DOI:10.1039/c7nj03525j

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Effect of alkyl chain and linking units on
mesophase transitions and molecular order
of rod-like thiophene mesogens: C NMR
Cite this:DOI: 10.1039/c7nj03525j
13
investigation†
a
Y. Santhosh Kumar Reddy,a Nitin P. Lobob and T. Narasimhaswamy
*
Thermotropic liquid crystals with p-conjugated cores are increasingly witnessed due to their promising
optoelectronic and optophotonic properties. As p-conjugated mesogens are often realized by direct
linking of the rings of the core, the mesophase transitions are usually modulated by inserting the
required alkyl/alkoxy chains either at a terminal or lateral location. In this work, five mesogens in which
thiophene and two phenyl rings are common in the core unit with terminal alkyl/alkoxy chains are
investigated to probe the influence of (a) a terminal alkyl chain on the mesophase characteristics, (b) the
linking units on the mesophase range and (c) the disparity of thiophene orientational constraints.
The mesophase properties reveal that whenever the alkyl chain is located at one end of the molecule,
polymesomorphism is observed. The XRD studies reveal layer ordering typical of smectic mesophases
with lower temperature and higher order phases, namely SmB and CrE phases. A detailed 13C NMR
study of the mesogens in the liquid crystalline phase indicates that the order parameters are governed
by the nature of the substitution of thiophene and its linkage with two phenyl rings. Importantly, the
13C NMR data of the terminal chains in the liquid crystalline phase are compared with the recently
reported thiophene mesogen in which the n-hexyl chain is located at the lateral position and
completely different orientation characteristics were found for the terminal versus lateral chains.
Received 19th September 2017,
Accepted 28th November 2017
DOI: 10.1039/c7nj03525j
rsc.li/njc
due to inherent rigidity of the core unit, the melting and clearing
transitions are generally found to be high.15–17 To modulate the
1. Introduction
Thermotropic liquid crystals based on a rod-like core are continu- mesophase transitions of such mesogens, either normal or
ing to lure the attention of investigators because of their structural branched alkyl chains are introduced at different locations of the
simplicity and well-known structure–property correlation.1–4 The molecules.18–24 For instance, in the case of thiophene-based
recent past has witnessed increasing interest in p-conjugated p-conjugated systems, the alkyl chains are often inserted at the
mesogens in which the core is often rod-like in shape.5–8 These 3-position of the thiophene ring so that the molecules show
molecules are generally synthesized by directly linking the carbo- solubility in common solvents at room temperature.25,26 In low
cyclic or heterocyclic aromatic rings with suitable terminal molar mass and oligomeric p-conjugated mesogens, the alkyl
chains.9–11 Because of the direct link of the rings of the core, chains are placed at the terminal position not only to achieve
the p-conjugated mesogens conceivably exhibit interesting opto- solubility without disrupting the p-conjugation but also to
electronic and opto-photonic properties.12–14 Even though a direct enhance the aspect ratio of the molecule.27,28 Recent research
link between the rings of the core favors excellent p-conjugation, results also emphasized the role of alkyl chains in influencing
the property characteristics as well as the morphology of
materials.29,30 In other words, depending on the position of
the alkyl chain, i.e., the terminal versus lateral location, the
a Polymer Science & Technology, CSIR-Central Leather Research Institute, Adyar,
Chennai 600020, India. E-mail: tnswamy99@hotmail.com
b Inorganic & Physical Chemistry, CSIR-Central Leather Research Institute, Adyar,
mesophase properties dramatically differ.23,31 For instance,
placing the alkyl chain at the terminal location of mesogens
may favor layer ordering while its location at the lateral position
would disrupt the layer ordering.32,33 It is generally noticed that
mesogens with two terminal chains which can be indepen-
dently varied in length can exhibit tilted phases if the lengths
are similar.19–22 On the other hand, orthogonal phases such as
Chennai 600020, India
† Electronic supplementary information (ESI) available: It contains synthetic
details, HRMS data, figure of SAMPI-4 pulse sequence, solution 1D and 2D
NMR spectra of mesogens, plots of SAMPI-4 1D dipolar slices, thiophene models
for orientational order parameter computation, figure of PELF pulse sequence,
PELF experimentsl details, data and spectra for aliphatic region of mesogens. See
DOI: 10.1039/c7nj03525j
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SmB, crystal B and crystal E are found if the shorter chains are were purchased from Sigma-Aldrich, USA and used without further
at the terminal location.19–22 In order to understand the influ- purification. Dimethyl formamide, chloroform, potassium
ence of alkyl chains on the mesophase properties as well as hydroxide, sodium chlorite, sodium dihydrogen orthophosphate
orientational constraints of thiophene mesogens, hot-stage dihydrate, anhydrous sodium sulfate, acetonitrile, dichloro-
optical polarizing microscopy (HOPM), differential scanning methane, hexane, ethyl acetate and silica gel (100–200 mesh)
calorimetry (DSC) and 2D solid state 13C NMR investigations were obtained from Merck, India, and 5-octyl thiophene-2-
are carried out respectively.
carbaldehyde was obtained from Avra Synthesis, India, and used
In recent years, solid state 13C NMR studies of liquid crystals as received.
have been increasingly witnessed owing to the accessibility of
Instrumental details
anisotropic information in the form of alignment-induced
1
chemical shifts, as well as 13C–1H dipolar couplings that can Both H and 13C NMR spectra were run on a 400 MHz Bruker
be extracted from 1D and 2D experiments in the mesophase.34–37 Avance-III HD spectrometer at room temperature using CDCl3
Furthermore, the extracted anisotropic information of the as a solvent and tetramethylsilane (TMS) as an internal refer-
different moieties of the mesogens, i.e. the constituent units of ence. The resonance frequencies of 1H and 13C were 400.23 and
the core as well as the terminal/lateral chains, provide order 100.64 MHz respectively. The optical polarizing micrographs
parameter values which are a direct consequence of the were taken using a Carl Zeiss axiocam MRC5 Polarizing micro-
dynamics of the corresponding moieties.38,39 By comparing the scope equipped with a Linkam THMS heating stage with a TMS
orientational order of the constituent rings of the core unit, 94 temperature programmer. The samples were placed between
the molecular shape or topology can be extracted. As a result, a 12 mm glass coverslips and transferred to the heating stage and
wide range of topologically different mesogens have been sub- heated at a programmed heating rate. The photomicrographs
jected to 13C NMR investigation in the mesophase to map the were taken using an imager A2M digital camera. The differential
orientational constraints. It is also noticed that on the replace- scanning calorimetry traces were recorded using a DSC Q 200
ment of the phenyl ring with a thiophene ring in the core, instrument at a heating rate of 10 1C per minute in a nitrogen
profound changes in molecular shape or topology results which atmosphere. The samples were subjected to two heating and two
is well reflected through the order parameter values.40,41 Hence, cooling cycles. The data obtained from the second heating and
in the present work, 13C NMR is employed for understanding the cooling cycle were considered for discussion. High-Resolution
orientational constraints of phenyl as well as thiophene rings of Mass Spectra (HRMS) of the mesogens were recorded using a
rod-like mesogens with terminal alkyl/alkoxy chains. Further- MICROMASS-Q-TOF mass spectrometer (Fig. S1–S3, ESI†).
more, the orientational constraints of terminal alkyl chains
The synthetic details of the intermediates and final meso-
determined from 13C NMR data are compared with the recently gens are given in the ESI.†
reported mesogen in which the alkyl chain is located on the
thiophene at the 3-position i.e. at a lateral location.23 In other
Solid state NMR measurements
words, the influence of orientational constraints on terminal All the solid-state NMR experiments were conducted under
versus lateral location of the alkyl chain is evaluated. For better static conditions on a Bruker Avance III HD 400 WB NMR
1
comprehension, for all the mesogens examined, the core is spectrometer (9.4 T) at a frequency of 400.07 MHz for H and
designed to have two phenyl rings with an additional thiophene 100.61 MHz for 13C. A 5 mm double resonance Bruker VTN
ring as part of the mesogenic core. The main focus of the study is probe with a horizontal solenoid coil was used for the measure-
to address (i) the influence of the terminal alkyl chain on the ments. At room temperature, the powdered samples were
mesophase characteristics, (ii) the effect of the linking unit on packed in a 4 mm Zirconia rotor with a Zirconia cap and placed
the mesophase range and (iii) the variation of thiophene orienta- inside a 5 mm glass tube. In order to achieve the sample
tional constraints with respect to the molecular structure. alignment, all the samples were gradually heated to their corres-
Towards this goal, the synthesized mesogens are investigated ponding isotropic phase and later slowly cooled to the respective
by HOPM, DSC and powder X-ray diffraction (XRD) for phase mesophases for the measurement. The cross-polarization (CP)42
assignment as well as transition temperatures and by 13C NMR sequence along with a linear ramp from 100–50% (1H channel)
to find the orientational constraints.
was employed to get 1D static 13C NMR spectra of the mesogens in
their respective mesophases. Typically, a contact time t of 3 ms,
number of scans 128, recycle delay of 8 s and 62.5 kHz of radio
1
frequency (RF) field strength on both the H and 13C channels
2. Experimental section
Materials
during the period t were used. High-resolution 2D separated local
field (SLF)43,44 spectra were obtained by using the SAMPI-4 pulse
5-Hexylthiophene-2-carbaldehyde, 2-hexyl thiophene, 5-bromo- scheme45 (Fig. S4, ESI†) and its utility for topologically different
thiophene-2-carboxaldehyde, 4-(dodecyloxy) phenol, 4-hydroxybenz- mesogens was highlighted in our earlier works.23,31,32,36–40,46 The
aldehyde, thiophene 2-boronic acid, potassium carbonate, dichloro- SAMPI-4 experimental parameters for the five mesogens were as
methane, tetrakis(triphenylphosphine) palladium (0), tetrahydro- follows KTC/HTC/OTC/OTPC/BTC: CP contact time t = 3 ms, t1
furan, dicyclohexyl carbodiimide, 4-dimethylamino pyridine, phos- increments = 128, number of scans = 24/28/20/42/28, 1H 901
phorous oxychloride, 4’-bromobiphenyl-4-ol, and 1-bromododecane pulse length = 5 ms and recycle delay = 13 s/15 s/14 s/16 s/13 s
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(to avoid RF heating effects). The 2D data sets were zero-filled and the 13C chemical shifts were referenced externally to
in both the t1 and t2 dimensions to form a 256 ꢀ 4096 data adamantane methine resonance at 29.5 ppm.
matrix. Phase-shifted sine bell multiplication was applied
Computational details
in both dimensions prior to Fourier transformation. For all
the experiments, 30 kHz SPINAL-6447 proton decoupling was The gas phase molecular geometries of the representative
employed during carbon acquisition. The Bruker BVTB-3500 mesogens were optimized using the density functional theory
temperature control unit was used for temperature regulation (DFT)48 based Becke’s three-parameter hybrid exchange functional
Scheme 1 Synthetic strategy for mesogens. (a) KTC, HTC, OTC, BTC and (b) OTPC mesogens.
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and Lee–Yang–Parr correlation functional (B3LYP) method followed by a threaded nematic texture. At 73 1C transition bars
employing the 6-31G(d) basis set using the Gaussian 09W suite of (Fig. 2b) are noticed succeeded by a broken fan texture57,58
programs.49
indicating the phase transition to the SmC phase. The crystalli-
zation of the sample, on the other hand, is noticed at 62.6 1C.
For OTC also birefringent threads are witnessed at 125.4 1C on
cooling the isotropic phase. Similar to HTC, the phase change at
102 1C is noticed where transition bars59 are observed (Fig. 2c).
The appearance of a schlieren texture57,58 at 102.9 1C indicates
the SmC phase. On further cooling, the sample crystallizes at
70.8 1C. For BTC, the appearance of the nematic phase is
confirmed by noticing birefringent threads (Fig. 2d) at 188.6 1C
and crystallization is noted at 109 1C. In the case of OTPC, upon
cooling from the isotropic liquid, formation of broken fans
characteristic of the SmC phase (Fig. 3a) is observed and on
further cooling OTPC shows the SmB and CrE phases (Fig. 3b
and c). The crystallization of OTPC is observed at 60 1C.
Powder X-ray measurements
The powder XRD studies of the un-oriented samples in a
Lindemann capillary with a diameter of 1 mm (Hampton
Research, Aliso Viejo, CA, USA) were carried out using a
PANalytical instrument (DY 1042-Empyrean) operating with
a line focused Ni-filtered Cu-Ka (l = 1.54 Å) beam and a linear
detector (PIXcel 3D). The sample temperature was controlled
with a precision of 0.1 1C using a heater and a temperature
controller (Linkam).50
3. Results and discussion
The HOPM assignments are further confirmed by DSC
investigations where multiple peaks associated with the corres-
ponding phase transitions are clearly noted (Fig. 4 and Tables 1
and 2). On cooling, the transition enthalpy values of the
isotropic to nematic phase for the mesogens are in the range
of 0.12 to 0.13 kcal molꢁ1, while that of isotropic to SmC phase
is found to be 3.40 kcal molꢁ1. For OTC the nematic to SmC
Five mesogens in which thiophene is at one end of the core unit
and a dodecyloxy chain at the other end are synthesized as per
the strategy outlined in Schemes 1a and b and in the ESI.†
The two phenyl ring based thiophene containing mesogens are
synthesized from 5-substituted thiophene-2-carboxaldehyde. In
the case of the mesogen with the bithiophene unit (BTC), the
second thiophene is introduced by Suzuki coupling51–54 of the
corresponding bromo mesogen with thiophene-2-boronic acid.
For the case of OTPC, where no connecting units are present in
the core, synthesis was conducted by direct arylation31,55,56 of
2-hexyl thiophene using 4-bromo-40-(dodecyloxy)-1,10-biphenyl
employing palladium acetate as a catalyst. The main focus of
the investigation is to find the influence of terminal chains on
the mesophase transition temperatures as well as on the
molecular order. Furthermore, the 13C NMR data of terminal
chains in the liquid crystalline phase are compared with the
recently reported thiophene mesogen in which the n-hexyl
chain is located at the lateral position. It is anticipated that
by comparing the ordering of the terminal chain on thiophene
versus the chain at the lateral location of the reported mesogen,
the utility of 13C NMR for understating the dynamics and
orientation of the chains in mesogens can be correlated with
the molecular structure (Fig. 1). Among the five mesogens i.e.
KTC, HTC, OTC, OTPC and BTC, KTC with no substituent on
the terminal thiophene serves as a benchmark mesogen for
comparing the mesophase properties. The synthesized meso-
gens are initially characterized by solution 1D 1H and 13C NMR,
and later 2D NMR experiments are performed to establish the
structural integrity (Fig. S5–S50, ESI†).
transition (98.5 1C) showed an enthalpy value of 0.27 kcal molꢁ1
.
For OTPC the SmC–SmB transition is observed at 165.6 1C
showed an enthalpy value of 0.81 kcal molꢁ1. For all the
mesogens, the enthalpy values of mesophase to crystallization
varied in the range of 2–10 kcal molꢁ1. In the case of KTC only
nematic phase is noticed (Fig. 4a). Furthermore, for HTC and
OTC (cooling cycle) multiple peaks are noticed with character-
istic transition enthalpies (Fig. 4b and c). For OTPC, the SmB to
CrE transition which is further confirmed by X-ray diffraction
exhibited an enthalpy of 0.75 kcal molꢁ1. XRD is also used for
confirming the SmC–SmB transition. The HOPM and DSC
investigations provide data which can be correlated with the
molecular structure. The common feature of all the mesogens
is the presence of two phenyl rings along with either one or two
thiophene rings in the core. Since the alkyl chain is located at
one end of the molecule for HTC, OTC and OTPC, polymeso-
morphism is observed. Furthermore, the presence of smectic
phases in HTC, OTC, and OTPC could be attributed to a high
aspect ratio of the molecule due to the presence of the alkoxy
and alkyl chains at both ends of the molecule.19–22 As a result,
the segregation of aliphatic chains from the rigid core units
leads to nano segregation resulting in layer order.39,60–62
Powder XRD studies
Mesophase identification by HOPM and DSC
The existence of smectic mesophases for OTC and OTPC is
The mesophase assignment of the synthesized molecules is ascertained using the powder XRD technique. Fig. 5a shows the
carried out by HOPM, where the formation of texture upon XRD scan of OTC in the smectic C phase in the temperature
cooling the sample from the isotropic phase is observed (Fig. 2 range of 85–95 1C. The XRD profile measured at 95 1C shows an
and 3). In KTC, on cooling from the isotropic phase, birefringent intense sharp reflection at 2y = 2.81 with a layer spacing (d) of
threads (Fig. 2a) are observed at 118 1C. On further cooling, the 31.51 Å and a diffuse and broad peak in the wide angle region
threaded nematic texture57,58 characteristic of the nematic phase (4.54 Å). The intense peak in the small angle region is a
is noticed. The sample experienced crystallization at 85.8 1C. For characteristic of layer ordering commonly observed in smectic
HTC, the formation of birefringent droplets is seen at 116.8 1C, mesophases.63 The wide angle broad diffuse reflection, on the
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Fig. 1 Planar and DFT energy optimized structures of mesogens. (a) KTC, (b) HTC, (c) OTC, (d) BTC and (e) OTPC.
Fig. 2 HOPM textures of mesogens on cooling from the isotropic phase.
(a) Nematic phase at 118 1C for KTC, (b) SmC phase at 73 1C for HTC,
(c) SmC phase at 102 1C for OTC and (d) nematic phase of BTC at 188.6 1C.
Fig. 3 HOPM textures of OTPC on cooling from the isotropic phase.
(a) SmC phase at 169 1C, (b) SmB phase at 164 1C, and (c) CrE phase at 160 1C.
other hand, indicates the liquid-like nature of molecules within nature of the smectic mesophase. To estimate the tilt angle of
the layers.58 These observations further indicate the fluid the molecules in the fluid smectic phase, the d/L ratio is
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Fig. 4 DSC scans of mesogens in heating and cooling cycles. (a) KTC, (b) HTC, (c) OTC, (d) BTC and (e) OTPC.
calculated where L is the energy optimized molecular structure accordingly, the d/L ratio is 0.72 which suggests that the
of OTC in the fully extended conformation. The molecular molecules are tilted in the layers. The estimated tilt angle based
length of OTC is found to be L = 42.90 Å (Fig. 1c) and on the expression d = L cos y is 431, which is the value usually
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Table 1 Transition temperatures and enthalpy values of the synthesized
mesogens from the second heating cycle
the presence of two phenyl rings in the core unit and also the
location of thiophene at one end of the molecule. Thus, the
determination of the order parameters of the synthesized
mesogens would yield information that can be correlated with
the individual molecular structure. Furthermore, the presence
of flexible hexyl/octyl chains at one end of the mesogen
significantly influences the mesophase sequence as well as
the ordering of the thiophene rings. Hence, all the mesogens
are examined by 1D and 2D 13C experiments in the mesophase
to find the alignment-induced chemical shifts (dLC–diso) as well
as the 13C–1H dipolar couplings. For all the molecules, the
solution 13C NMR is run in CDCl3 at room temperature and
then the static 1D 13C NMR experiments in the liquid crystal-
line phase. Furthermore, 2D SAMPI-4 experiments were also
performed to get the 13C–1H dipolar couplings and using them
the orientational order parameters are computed.
Phase transition temperatures and
enthalpy values (DH kcal molꢁ1
)
Code
KTC
Cr–N 96.6 1C
(4.41)
N–I 118.9 1C
(0.12)
HTC
OTC
OTPC
BTC
Cr–N 83.7 1C
(7.13)
Cr–SmC 91.7 1C
(6.94)
Cr–SmB 162.3 1C
(0.71)
Cr–N 126.3 1C
(12.81)
N–I 117.4 1C
(0.20)
SmC–N 99.4 1C
(0.15)
SmB–SmC 165.6 1C
(0.81)
N–I 189.7 1C
(0.20)
N–I 122.5 1C
(0.23)
SmC–I 170.5 1C
(3.49)
Cr: crystallization; N: nematic; I: isotropic; SmC: smectic C.
found for mesogens that show a nematic to smectic C phase.39,64
Thus, the XRD data are in agreement with the HOPM and DSC
observations where the appearance of a smectic C phase is
noted. Furthermore, the d spacing shows a decreasing trend in
OTC with lowering of the temperature suggesting an increase in
tilt angle in the SmC phase. The XRD patterns of OTPC mea-
sured in different smectic mesophases in the temperature range
of 160–175 1C are shown in Fig. 5b. The XRD profile measured at
175 1C shows a sharp and intense signal in the small angle
region with 2y = 2.701 corresponding to a layer spacing (d) of
32.68 Å and a diffuse peak in the wide angle region at 4.58 Å.
At 168 1C, the XRD pattern shows changes both in the small
angle and wide angle regions. The position of the small angle
reflection changed from (2y) 2.70 to 2.51 while the wide angle
peak split into two peaks. At 160 1C, in the wide angle region,
multiple peaks are noted while the reflection in the small-angle
region is unchanged (2y = 2.511). Accordingly, the phases have
been identified as SmC, SmB and CrE31,58,65 respectively. The tilt
angle in the SmC phase is 33.131 and is determined from
d = L cos y where L is the length of the molecule (39.03 Å)
calculated from the energy optimized structure from DFT calcu-
lations (Fig. 1e). Thus, the smectic polymesopmorphism in
OTPC is clearly established by XRD studies.
The solution 13C NMR spectra of the mesogens are shown in
Fig. 6a–e. For KTC, HTC and OTC, the solution 13C NMR
spectra show 14 lines for the core unit which include phenyl
rings, ester carbonyls and a thiophene ring. The chemical shift
assignment of the core unit carbons is attempted by comparing
the spectra generated from chemsketch and also comparing the
literature 13C NMR data.23,31,32,36–40 Table 3 lists the chemical
shift values of 14 carbons of the core unit of KTC, HTC and
OTC. A close look at the chemical shift values indicates that the
chemical shifts of C1–C10 are similar for the three mesogens
while for C11–C14, a slight change is noticed. Furthermore, the
change in the values is attributed to the presence of a hexyl/
octyl chain in the HTC and OTC cases as opposed to KTC. For
these two mesogens, however, the C11–C14 values are similar
since the hexyl/octyl chain is part of the thiophene ring. For
BTC and OTPC, not only the number of lines but also the
chemical shifts of the core unit carbons are different. This is
comprehensible since ester linking units are part of the core for
BTC while in OTPC, the rings are connected directly without
any linking units. Accordingly, BTC shows 18 lines while OTPC
exhibited 12 lines for the core unit carbons. For all the meso-
gens the chemical shift range for the core unit carbons is from
114.0 to 165.0 ppm.
Solid state 13C NMR investigations of the mesogens
In the liquid crystalline phase, static 1D 13C NMR experi-
ments are carried out to find the alignment-induced chemical
shifts of all the mesogens (Fig. 6f–j). For assigning the chemical
shifts of the core unit carbons of the mesogens, the literature
The synthesized mesogens are characterized by 13C NMR in the
liquid crystalline phase to determine the orientational order
parameters. The common structural feature of the mesogens is
Table 2 Transition temperatures and enthalpy values of the synthesized mesogens from the second cooling cycle
Phase transition temperatures and enthalpy values (DH kcal molꢁ1
)
Code
KTC
I–N 116.1 1C
(0.12)
N–Cr 85.8 1C
(1.57)
HTC
OTC
OTPC
BTC
I–N 116.3 1C
(0.24)
I–N 121.8 1C
(0.30)
I–SmC 169.2 1C
(3.40)
I–N 188.6 1C
(0.16)
N–[SmC] 71.2 1C
(0.24)
N–SmC 98.5 1C
(0.27)
SmC–SmB 165.1 1C
(0.81)
N–Cr 110.1 1C
(2.16)
[SmC]–Cr 65.6 1C
(9.40)
SmC–Cr 72.5 1C
(7.20)
[SmB]–CrE 161.6 1C
(0.75)
[CrE]–Cr 68.7 1C
(0.67)
[ ] indicates monotropic transition. Cr: crystallization; N: nematic; I: isotropic; SmC: smectic C. SmB: smectic B; CrE: crystal E.
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mono substituted. For other cases i.e. HTC, OTC, BTC and
OTPC, the thiophene ring is 2,5-disubstituted. This variation in
thiophene substitution is clearly reflected in the 13C NMR data
in the mesophase. In particular, the 13C–1H dipolar couplings
of the thiophene ring show a characteristic trend. In HTC, due
to the presence of three CH carbons in thiophene, the CH
contours in 2D are clearly separated from the rest of the phenyl
ring CH contours owing to the variation in the orientation of
the phenyl rings from the thiophene ring with reference to the
long axis. For other rings, owing to the 2,5-disubstitution
nature of thiophene, only two CH carbons are noticed. The
2D spectra of these mesogens show 13C–1H dipolar couplings
which are almost comparable in magnitude with the phenyl
ring methine carbons. This particular feature, i.e., a large variation
in the 13C–1H dipolar couplings from monosubstituted thiophene
to 2,5-disubstituted thiophene is due to the change in thiophene
ring orientation with respect to the location of the long axis.23,31
As a result, the 13C–1H dipolar couplings of KTC versus other
mesogens dramatically vary as noticed in 2D SAMPI-4. BTC, by
virtue of the presence of two thiophene rings, is different from the
other mesogens. In the 2D spectrum, five CH contours of the
bithiophene unit are expected as seen in the spectrum (Fig. 7).
As stated earlier, for OTPC, the linking units such as ester groups
are absent and all the rings are directly connected.
Generally, in SAMPI-4 experiment when carbons are coupled to
more than one proton, the dipolar coupling information needs to be
extracted from the experimental dipolar oscillation frequency.69,70
Considering the established procedure,23,31,32,36–40,46,66–68,71 the
relationship between the main orientational order parameters
and dipolar couplings for the phenyl or thiophene ring will be
expressed by the following equation.72,73
Fig. 5 Powder X-ray diffraction profile of the mesogens. (a) OTC in the
SmC phase at 95 1C and (b) OTPC at 160 1C, 166 1C, and 175 1C.
DCH = K[Szz(3cos2 yz ꢁ 1)/2 + (Sxx ꢁ Syy) (cos2 yx ꢁ cos2 yy)/2
data of structurally related mesogens are considered.23,31,32,36–40,66
For instance, 13C NMR data from our earlier work on thiophene-
+ Sxz(cos yx cos yz)]
(1)
based mesogens as well as the data of 4-hexyloxy benzoic acid are where K = ꢁhgHgC/4p2rCH3, where h is the Planck constant,
utilized for assigning the core unit carbons.23,31,32,36,37,67,68 gH and gC are the gyromagnetic ratios of 1H and 13C nuclei
Further, 2D SAMPI 4 experiments that provide 13C–1H dipolar respectively and rCH is the C–H bond, y s are the angles formed
couplings are also employed for assigning the thiophene ring by rCH with the respective coordinate axes of the molecular
carbons. It is clear from Table 3 that the mesophase 13C NMR data fragment (phenyl or thiophene ring) under consideration. For a
of KTC, HTC and OTC mesogens are dissimilar from their 1–4 disubstituted phenyl ring having D2 symmetry, the z axis is
respective solution NMR chemical shifts. For example, in solution, taken along the para axis and the remaining perpendicular
the chemical shift values of C1–C10 are exactly the same, while x and y axes are respectively, in and out of the ring plane.
for C11–C14, minor variation is noticed. In the mesophase, on During fitting, the standard bond distances rCH = 1.1 Å for the
the other hand, the differences in the 13C chemical shifts C–H bond and rCC = 1.4 Å for the C–C bond are employed. The
between the core units of KTC, HTC and OTC are due to the CCH bond angles are varied B1201 to arrive at the best fit.74 On
fact that the mesophase transition temperatures are not the other hand, the thiophene ring possessing irregular penta-
similar. Furthermore, OTC exhibits the SmC phase as opposed gonal geometry requires three order parameters, namely, Szz,
to KTC and HTC which show the nematic phase. The calculated (Sxx ꢁ Syy), and Sxz for an arbitrary choice of the axis system.75,76
alignment induced chemical shift values of the mesogens are The procedure and model adopted for computing the order
listed in Table 3.
parameters for the thiophene moiety which are connected by a
The 2D SAMPI-4 experiments for all the mesogens are linking unit or directly linked to the phenyl ring are described
performed in the liquid crystalline phase to find the 13C–1H in our previous work in detail.23,31,32,36,40,76 Initially, the z-axis
dipolar couplings (Fig. 7). The 1D dipolar slices for aromatic is chosen along the C10–C11 bond for the KTC, HTC, OTC and
resonances for each mesogen are shown in Fig. S51–S55, ESI.† BTC mesogens, the C8–C9 bond for the OTPC mesogen and the
From the structure point of view, the thiophene ring in KTC is C14–C15 bond for the lateral thiophene of the BTC mesogen
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Fig. 6 13C NMR spectra of mesogens in different phases. Left column: solution 13C NMR spectra of (a) KTC, (b) HTC, (c) OTC, (d) OTPC and (e) BTC. Right
column: mesophase 13C NMR spectra of (f) KTC in the nematic phase (98 1C), (g) HTC in the SmC phase (85 1C), (h) OTC in the SmC phase (96 1C),
(i) OTPC in the SmC phase (166 1C) and (j) BTC in the nematic phase (130 1C).
(Fig. S56, ESI†). Later this z-axis is slightly varied by an angle b parameters of the phenyl as well as thiophene rings are listed in
in order to improve the quality of the fit. This fit is further Table 4. In all the cases, two order parameters for two phenyl
improved by the marginal variation in CCH bond angles. rings and one order parameter for thiophene are determined
Apparently, the addition of the third order parameter Sxz, did except for BTC where two thiophene rings are present in the
not improve the fit significantly and hence, only two order core unit. For KTC, the order parameter values for both the
parameters were retained, namely, Szz and (Sxx ꢁ Syy). The order phenyl rings are 0.55 and for the thiophene ring, the value is 0.51.
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Table 3 13C NMR data for the core unit of the five mesogens in solution and mesophasesa
KTC
HTC
OTC
BTC
OTPC
166 1C (smectic C)
AIS
DOFb Soln LC DOFb
98 1C (nematic)
85 1C (smectic C)
DOFb Soln LC
96 1C (smectic C)
DOFb Soln LC
130 1C (nematic)
DOFb Soln LC
Soln LC
AIS
AIS
AIS
AIS
C. N. (ppm) (ppm) (ppm) (kHz) (ppm) (ppm) (ppm) (kHz) (ppm) (ppm) (ppm) (kHz) (ppm) (ppm) (ppm) (kHz) (ppm) (ppm) (ppm) (kHz)
1
2
3
4
5
6
7
8
156.9 217.3 60.4 1.29 156.2 229.6 73.4 1.51 156.9 234.0 77.1 1.71 156.9 223.9 67.0 1.41 158.8 229.1 70.3 1.75
115.1 137.5 22.4 2.40 115.1 142.3 27.2 2.83 115.1 143.9 28.8 3.15 115.1 139.9 24.8 2.59 114.9 143.1 28.2 3.33
122.3 146.5 24.2 2.13 122.3 151.8 29.5 2.54 122.3 153.9 31.6 2.85 122.3 148.9 26.6 2.36 127.7 154.0 26.3 3.65
144.1 202.7 58.6 1.25 144.2 211.3 67.1 1.42 144.2 214.3 70.1 1.61 144.1 208.3 64.2 1.38 132.9 208.2 75.3 1.63
164.7 201.5 36.8 0.47 164.8 210.3 45.5 0.70 164.8 214.3 49.5 1.17 164.7 206.2 41.5 0.53 141.4 217.3 75.9 1.71
127.3 186.5 59.2 1.26 127.2 197.4 70.2 1.52 127.1 201.6 74.5 1.75 127.3 192.2 64.9 1.42 125.7 150.3 24.6 3.54
131.8 158.9 27.1 2.20 131.7 164.6 32.9 2.65 131.7 166.7 35.0 2.95 131.8 161.4 29.6 2.47 126.9 152.8 25.9 3.63
121.8 146.5 24.7 2.13 121.9 152.1 30.2 2.58 121.9 153.9 32.0 2.85 121.8 148.9 27.1 2.36 133.1 214.2 81.1 1.70
154.6 208.7 54.1 1.27 156.9 219.0 62.1 1.53 154.8 222.6 67.8 1.74 154.6 213.5 58.9 1.40 139.4 208.6 69.2 1.25
159.9 201.5 41.6 0.47 159.9 210.3 50.4 0.70 159.9 214.3 54.4 1.17 159.7 206.2 46.5 0.53 122.5 157.7 35.2 5.69
132.3 166.4 34.1 0.71 129.1 173.2 44.1 1.10 129.1 176.4 47.3 1.28 129.7 177.3 47.6 0.76 124.9 178.1 53.2 4.25
135.1 171.2 36.1 5.29 135.4 168.1 32.7 2.45 135.4 169.8 34.4 2.47 136.0 172.4 36.4 3.73 145.6 192.6 47.0 1.19
128.1 163.9 35.8 4.38 125.6 177.4 51.8 2.80 125.6 180.9 55.3 2.74 124.3 152.7 28.4 3.00
134.0 182.5 48.5 7.20 154.8 214.3 59.5 1.51 156.2 218.4 62.2 1.61 136.1 172.4 36.3 0.98
146.0 197.2 51.2 0.93
9
10
11
12
13
14
15
16
17
18
125.6 177.3 51.7 2.10
128.1 159.2 31.1 1.85
126.6 177.3 50.7 8.90
a
b
Soln: solution chemical shift, LC: mesophase chemical shift; AIS: alignment induced shift; DOF: dipolar oscillation frequencies. The errors on
the values of DOF are about ꢂ0.040 and 0.055 kHz respectively, for protonated and non protonated carbons.
In the case of HTC, the phenyl ring order parameter values are the location of the long axis depends on the second substituent on
0.638 (ring I) and 0.65 (ring II) and 0.753 for the thiophene ring. In the thiophene ring. In other words, for 2,5-disubstituted thio-
the case of OTC, the values are 0.729 and 0.743 for the phenyl phene, the long axis is more or less collinear to the C12–C13 bond
rings and for thiophene, the value is 0.83. A large variation in the (HTC, OTC) and C10–C11 bond (OTPC). For the case of KTC where
order parameter values of the phenyl as well as thiophene for thiophene is mono substituted, the long axis is not collinear to the
these mesogens is understandable if the phase transition tem- C12–C13 bond. These subtle changes are clearly reflected in the
peratures as well as the nature of the mesophase are taken into 13C NMR data in the mesophase where the chemical shifts as well
consideration. For instance in KTC and HTC, the experiments are as 13C–1H dipolar couplings are completely governed by the
done in the nematic phase whereas for OTC, the measurement is orienting axis with respect to the magnetic field. In solution or
carried out in the smectic phase. Furthermore, the mesophase the isotropic phase, due to tumbling of molecules, the anisotropic
transition temperatures of KTC and HTC are slightly different chemical shifts observed in the mesophase vanish. Thus the order
even though experiments are performed in the nematic phase. For parameter values derived from 13C NMR provide information at
BTC and OTPC, the phenyl ring order parameters are 0.601 and the molecular (local) level which is correlated with the molecular
0.607 and for thiophene rings they are 0.55 and 0.471 whereas in structure of the mesogen.
OTPC the phenyl ring order parameters are 0.748 (ring 1), 0.757
The 2D proton encoded local field (PELF)77–79 scheme
(ring 2) and 0.809 for the thiophene ring. Here again, the (Fig. S57, ESI†) is employed for extracting the 13C–1H dipolar
mesophase transitions of both the mesogens largely differ. Also couplings of the terminal alkyl chains of the KTC, HTC and
for BTC, the experiments are carried out in the nematic phase OTC mesogens as it provides better resolution for aliphatic
while for OTPC the experiments are done in the SmC phase. carbons. The experimental details are provided in the ESI.† The
A close inspection of Table 4 pertaining to the order parameters 2D PELF spectra for the three mesogens are shown in Fig. S58,
indicates some noticeable trend. For the mesogens in which the ESI† and the corresponding extracted 1D dipolar slices are
alkyl chain is present in the terminal thiophene ring, the order plotted in Fig. S59–S61, ESI.† The 13C NMR data of the terminal
parameters of the thiophene ring are higher than those corres- alkyl chains for KTC, HTC and OTC are compared with a
ponding to the phenyl rings of the core unit. However, if an alkyl recently reported mesogen23 where the alkyl chain is located
chain is not present (KTC) or if thiophene is connected to other at the lateral position of the thiophene ring. In particular, the
thiophene rings (BTC) directly, the thiophene order parameter is alignment-induced chemical shift values of carbons that are
lower than those corresponding to the phenyl rings of the core linked to the core unit versus the carbons of the chains located
unit. Thus, the presence of an alkyl chain in thiophene, i.e., when at the lateral position can provide qualitative information about
thiophene is 2,5-disubstituted, the order parameter of the ring the chain ordering. Table S1, ESI† lists the terminal chain
is higher than that of the corresponding phenyl rings of the carbon chemical shift values of the KTC and HTC mesogens
mesogen. If KTC is considered a benchmark molecule, for HTC, in solution. The chemical shift assignment is completed by
OTC and OTPC in which thiophene is 2,5-disubstituted, the comparing the structurally similar compounds in the literature.31
thiophene ring order parameters are high. This suggests that For HTC, the n-hexyl chain carbon chemical shifts are identified by
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Fig. 7 2D SAMPI-4 spectra of mesogens in various phases. (a) KTC in the nematic phase (98 1C), (b) HTC in the SmC phase (85 1C), (c) OTC in the SmC
phase (96 1C), (d) OTPC in the SmC phase (166 1C) and (e) BTC in the nematic phase (130 1C). Thiophene methine carbon contours are indicated in red
color.
considering the HTC aliphatic region. In other words, the dodecy- chain is present in only HTC. For the static 1D 13C NMR, again the
loxy chain is common for both the mesogens whereas the n-hexyl chemical shift values of the terminal chain carbons of KTC are
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Table 4 Orientational order parameter for phenyl and thiophene rings of five mesogensa
CCH bond angles
Calculated dipolar oscillation frequencies (kHz)
Mesogen Ring yb
yc
Szz
Sxx ꢁ Syy
b
c
d
a
RMSD (kHz)
KTC
HTC
OTC
BTC
I
II
I
II
I
II
I
II
I
II
119.8
120.4
119.6
120.5
119.8
120.6
119.8
120.4
119.5
119.1
120.8
120.7
120.5
120.7
120.6
120.9
120.6
120.8
118.6
118.8
0.549 ꢂ 0.022 0.056 ꢂ 0.009 2.39
0.551 ꢂ 0.024 0.050 ꢂ 0.007 2.20
0.638 ꢂ 0.023 0.062 ꢂ 0.009 2.82
0.650 ꢂ 0.019 0.070 ꢂ 0.006 2.65
0.729 ꢂ 0.020 0.068 ꢂ 0.007 3.13
0.743 ꢂ 0.023 0.076 ꢂ 0.008 2.95
0.601 ꢂ 0.024 0.058 ꢂ 0.009 2.59
0.607 ꢂ 0.022 0.060 ꢂ 0.008 2.47
0.748 ꢂ 0.021 0.072 ꢂ 0.008 3.33
0.757 ꢂ 0.019 0.075 ꢂ 0.007 3.54
2.14
2.12
2.56
2.59
2.87
2.85
2.37
2.36
3.65
3.65
1.28
1.27
1.48
1.53
1.69
1.74
1.39
1.42
1.68
1.72
1.26
1.27
1.46
1.52
1.67
1.73
1.37
1.41
1.72
1.72
0.02
0.01
0.04
0.01
0.05
0.01
0.02
0.01
0.03
0.01
OTPC
CCH bond angles
yB yC yD
Calculated dipolar oscillation frequencies (kHz)
Mesogen Ring
b
Szz
Sxx ꢁ Syy
B
C
D
A
RMSD (kHz)
KTC
HTC
OTC
BTC
III
III
III
III
IV
III
0.5 106.9 40.0 32.2 0.510 ꢂ 0.012 0.053 ꢂ 0.004 5.29
4.39
2.79
2.74
2.99
1.84
4.25
7.20
1.45
1.61
0.92
8.86
1.16
0.72
1.20
1.28
0.82
0.74
1.30
0.01
0.06
0.02
0.04
0.05
0.03
17.0 122.5 58.5
17.0 122.7 58.0
7.5 114.6 45.6
—
—
—
0.753 ꢂ 0.024 0.079 ꢂ 0.009 2.42
0.830 ꢂ 0.022 0.065 ꢂ 0.008 2.47
0.550 ꢂ 0.012 0.033 ꢂ 0.006 3.74
7.5 118.4 48.7 33.3 0.471 ꢂ 0.010 0.008 ꢂ 0.003 2.18
OTPC
4.8 114.7 45.5
—
0.809 ꢂ 0.010 0.078 ꢂ 0.005 5.70
a
Ring I (KTC/HTC/OTC/BTC/OTPC): a = C1, b = C2, c = C3, d = C4. Ring II (KTC/HTC/OTC/BTC): a = C6, b = C7, c = C8, d = C9. Ring II (OTPC):
a = C5, b = C6, c = C7, d = C8. Ring III (KTC/HTC/OTC/BTC): A = C11, B = C12, C = C13, D = C14. Ring III (OTPC): A = C9, B = C10, C = C11, D = C12.
used for identifying the n-hexyl chain of HTC. In a recent work,23 dipolar couplings the order parameters of the terminal chains of
we examined a p-conjugated thiophene based mesogen in which HTC, KTC and OTC are calculated. It is clear from Table S1, ESI†
the n-hexyl chain is located at the lateral position. In this work, the that the order parameter values follow an established style in
n-hexyl chain carbons (HTC) in the liquid crystalline phase showed concurrence with trends observed from alignment induced
a different trend compared to the terminal chain carbons. In the chemical shift values of terminal chain carbons.
reported mesogen, the hexyl chain is away from the long axis as it
The mesophase transition temperatures of mesogens
is positioned at the lateral location while the terminal chains are provide scope for understanding the structure–property rela-
along the long axis. Accordingly, the n-hexyl chain carbons of the tionship. For structurally close mesogens i.e. KTC, HTC and
reported mesogen showed positive sign for alignment-induced OTC, the melting transition varies between 83–97 1C while the
chemical shifts whereas the terminal chain carbons of the same clearing temperatures are in the range of 116–123 1C. With the
mesogen exhibited the usual negative sign. In the present work, introduction of an alkyl chain i.e. the hexyl chain for HTC and
the aliphatic carbons of the HTC mesogen exhibited alignment- octyl chain for OTC, an increase in the mesophase range is
induced chemical shifts with a negative sign. This is in sharp noticed. Furthermore, due to increase in the length of the
contrast to the reported p-conjugated mesogen where the n-hexyl molecule, the aspect ratio increases from KTC to OTC, and as
chain carbons in the liquid crystalline phase showed positive sign a result, the tendency for layer ordering increases. In the case of
for the alignment-induced chemical shift values. Interestingly, the KTC, only the nematic phase is noticed whereas for HTC and
solution 13C NMR values of the n-hexyl chain of the reported OTC, the smectic C phase is observed as monotropic and
mesogen as well as the n-hexyl chain of HTC are comparable enantiotropic respectively in addition to the nematic phase.
whereas the 13C chemical shifts in the liquid crystalline phase For the case of BTC, the melting and clearing temperatures are
show a dramatically different trend. In HTC, as the n-hexyl chain is high due to the rigid bithiophene ring. Interestingly, despite
at the terminal location and in the liquid crystalline phase, it increase in the length of the molecule, the smectic phase is not
orients along the long axis. However, for the reported mesogen,23 seen in BTC. For OTPC also, the melting and clearing tempera-
the orientation of the hexyl chain which is at the lateral location is tures are quite high similar to BTC but smectic polymeso-
different. It is also clear from earlier studies that the methylene morphism is noticed. Thus OTPC exhibits enantiotropic SmC,
carbons that predominantly have trans conformation and orient monotropic SmB and crystal E phases. The most interesting
with respect to the long axis show negative alignment-induced feature is that despite the very low mesophase range (8.5 1C),
chemical shift values while those with cis conformation and away the appearance of smectic polymesomorphism could be attributed
from the long axis exhibit a positive trend. Using the 13C–1H to the absence of flexible and polarizable ester units. The high
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melting and clearing temperatures are a clear indication of the Acknowledgements
overall rigidity of the mesogen due to the direct link between the
We thank Dr B. Chandrasekaran, Director, CSIR-CLRI and Prof.
K. V. Ramanathan, NMR Research Centre, Indian Institute of
Science, Bangalore, India for their support and keen interest in
this work. We thank Dr B. V. N. Phani Kumar and Mr R.
Ravikanth Reddy, CSIR-CLRI for the help extended in carrying
out the 2D solution NMR studies. We are grateful to Ms K. N.
Vasudha, Raman Research Institute, Bangalore, India for the
powder X-ray measurements. We also thank Dr R. Srinivasan,
CSIR-CLRI for extending help in HRMS data interpretation.
One of the authors Y. Santhosh Kumar Reddy thanks the
Council of Scientific and Industrial Research (CSIR), New Delhi
for the grant of a Senior Research Fellowship.
phenyl as well as thiophene rings. Thus, insertion of bithiophene
produces the highest mesophase range with an enantiotropic
nematic mesophase.
4. Conclusion
Five mesogens in which thiophene is at one end of the core and
a dodecyloxy chain at the other terminus were synthesized
either by Suzuki coupling or direct arylation. The common
feature of the synthesized mesogens was the presence of two
phenyl rings and a thiophene ring in the core. The HOPM and
DSC investigations revealed that mesogens with a terminal
alkyl chain exhibited polymesomorphism. The appearance of
smectic phases in HTC, OTC and OTPC was due to the high
aspect ratio of the molecules since the alkoxy and alkyl chains
at both ends result in segregation of the aliphatic chains from
the rigid core units as supported by XRD investigation. The
orientational order parameters of the constituent rings as well
as the terminal alkyl chain were determined from 13C–1H
dipolar couplings from 13C NMR studies. For KTC, the order
parameters of both the phenyl rings were 0.55 whereas for the
thiophene ring, the value was 0.51. In the case of HTC, the
phenyl ring order parameter values were 0.638 (ring I) and 0.65
(ring II) and 0.753 for the thiophene ring. For OTC, the values
were 0.729 and 0.743 for the phenyl rings and 0.83 for the
thiophene ring. The notable feature of the 13C NMR study was
about mesogens in which the alkyl chain was present on the
terminal thiophene ring, which showed higher order para-
meters than the corresponding phenyl rings. If the alkyl chain,
on the other hand, was absent or if thiophene was connected to
other thiophene rings directly, the order parameter of the
thiophene was lower than that of the corresponding phenyl
rings. These variations were rationalized by considering the
location of the long axis with respect to the substituent on the
thiophene ring. Importantly, the 13C NMR data of the terminal
chains in the liquid crystalline phase were compared with a
thiophene mesogen in which the n-hexyl chain is located at the
lateral position. The n-hexyl chain carbons of the reported
mesogen showed positive sign for alignment-induced chemical
shifts whereas the aliphatic carbons of the HTC mesogen exhibited
a negative sign. Quite remarkably, the solution 13C NMR values of
the n-hexyl chain of the reported mesogen as well as the n-hexyl
chain of HTC are similar, whereas a distinct difference in the
liquid crystalline phase was noticed. The stark contrasting trend
was attributed to their location leading to different kinds of
orientation with respect to the long axis. Thus, this study delivered
interesting structure–property and orientational information on
the core as well as the terminal chains of thiophene and phenyl
ring based mesogens.
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Conflicts of interest
There are no conflicts to declare.
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