Molecular order and mesophase investigation of thiophene-based forked mesogens

Article abstract of DOI:10.1021/acs.jpcb.6b05827

Thiophene-based rodlike molecules constructed from a three phenyl ring core and terminal dialkoxy chains recognized as forked mesogens are synthesized, and their mesophase properties as well as the molecular order are investigated. The synthesized forked

Full text of DOI:10.1021/acs.jpcb.6b05827

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Article
Molecular Order and Mesophase Investigation
of Thiophene based Forked Mesogens
Rajasekhar Reddy Katha, Nitin Prakash Lobo, and Tanneru Narasimhaswamy
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.6b05827 • Publication Date (Web): 13 Jun 2016
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Molecular Order and Mesophase Investigation of Thiophene based Forked Mesogens
K. Rajasekhar Reddy, † Nitin P. Lobo, ‡and T. Narasimhaswamy,*†
†Polymer Laboratory, ‡Chemical Physics Laboratory,
CSIRꢀCentral Leather Research Institute, Adyar, Chennai 600020, India.
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ABSTRACT
Thiophene based rodꢀlike molecules constructed from a three phenyl ring core and terminal
dialkoxy chains recognized as forked mesogens are synthesised and their mesophase
properties as well as the molecular order are investigated. The synthesized forked mesogens
would serve as model compounds for tetracatenar or biforked mesogens. Based on the
position of the thiophene link with rest of the core, 2ꢀsubstituted and 3ꢀsubstituted mesogens
are realised in which the length of the terminal alkoxy chains are varied. The mesophase
properties are evaluated with a hotꢀstage polarising microscope and differential scanning
calorimetry. For both the homologs, the appearance of either nematic phase alone or in
conjunction with smectic C phase is noticed depending on the length of the terminal alkoxy
chains. The existence of layer ordering characteristic of smectic C phase is confirmed for a
representative mesogen using the variable temperature powder Xꢀray diffraction. The highꢀ
resolution solidꢀstate13C NMR measurements of C12 homologs of the two series revealed
orientational order parameters of all the rings of the core as well as terminal chains in liquid
crystalline phase. For both the homologs, due to the asymmetry of the ring I, the order
parameter value is higher in contrast to ring II, ring III as well as thiophene ring. The
chemical shifts and 13Cꢀ1H dipolar couplings of OCH2 carbons of the terminal dodecyloxy
chains provided contrasting conformations reflecting the orientational constraints. Further,
the investigations also revealed that the mesophase range as well as the tendency for layer
ordering is higher for 3ꢀsbustituted mesogens as against 2ꢀsubstituted homologs.
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Introduction
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Thermotropic liquid crystals with rodꢀlike core are known as calamitic mesogens while the
discotics are built with the discꢀlike core.1ꢀ4 The poly catenar mesogens are a distinct class of
molecules which are considered as an intermediate shape to rodꢀlike as well as discotic
systems.5ꢀ7 These mesogens are designed to have a lengthy rodꢀlike core with multiple alkoxy
chains at both the terminal positions and are recognized for rich polymesomorphism covering
nematic, smectic, columnar and cubic phases.5ꢀ7 The remarkable feature of these mesogens is
the influence of (i) number and length of the alkoxy chains and (ii) the ratio of phenyl rings
versus alkoxy chains on mesophase sequence, as well as the phase transition temperatures.7
The biforked mesogens are also known as tetracatenar mesogens possess two alkoxy chains
at both ends of the core unit.8,9 If the molecule, on the other hand, has two alkoxy chains at
one end of the core, they are termed as forked mesogens. For instance, Nguyen and
coworkers10ꢀ11 examined the mesophase characteristics of three ring based forked mesogens
with a terminal cyano group and found smectic A phase. Our group has been involved in the
development of thiophene mesogens with an aim of investigating the orientational constraints
of thiophene moiety in liquid crystalline phases employing the highꢀresolution solidꢀstate13C
NMR spectroscopy.12ꢀ14 Thiophene has emerged as an important structural moiety for
functional organic materials and there is an increasing research activity on thiophene based
thermotropic liquid crystals owing to their use in opto electronics as well as photonic
applications. 15ꢀ17The current work deals with the forked mesogens in which thiophene ring is
positioned at one end of the core unit. Two series of mesogens are synthesized from 2 or 3ꢀ
thiophene carboxylic acid with additional three phenyl rings in the core besides dialkoxy
chains at one terminal. By varying the length of the terminal chains, six mesogens are
synthesized and the mesophase characteristics are accomplished by hotꢀstage polarizing
microscope (HOPM) and differential scanning calorimetry (DSC). The existence of smecticC
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mesophase for one of the homologue is confirmed by variable temperature Xꢀray diffraction
(VTꢀXRD) where a change in layer spacing with lowering of temperature is observed.
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The highꢀresolution solidꢀstate13C NMR has emerged as a powerful tool for structural
characterization of topologically different thermotropic liquid crystals.12ꢀ14,18ꢀ20 The utility of
2D separated local field (SLF) based experiments for finding the molecular topology as well
as the molecular order of thermotropic liquid crystals is highlighted in a recent review.21 In
this work, the 1D and 2D solid state 13C NMR technique is employed for examining the
orientational order of thiophene based forked mesogens. The investigations reveal higher
mesophase range as well as the tendency for layer ordering for 3ꢀsubstituted mesogens as
against 2ꢀsubstituted series. Further, dramatic lowering of nematic to isotropic transition
temperature (~110ꢀ130 °C) is noticed for the synthesized molecules in contrast to structurally
similar four ring mesogen with one alkoxy chain. Additionally, the examination of order
parameter values determined from 13C NMR investigations of thiophene as well as phenyl
rings in mesophase supports the asymmetry of the ring I as reflected in the value of local
order parameter.
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EXPERIMENTAL SECTION
The experimental details of intermediates and final mesogens (E)ꢀ4ꢀ(((4ꢀ((3,4ꢀ
bis(alkoxy)benzoyl)oxy)phenyl)imino) methyl)phenylthiopheneꢀ2ꢀcarboxylates (7aꢀ7d) and
(E)ꢀ4ꢀ(((4ꢀ((3,4ꢀbis(alkoxy)benzoyl)oxy)phenyl)imino)
methyl)phenylthiopheneꢀ3ꢀ
carboxylates (8aꢀ8d) along with spectral data is furnished in Supporting Information (SI).
Materials
4ꢀhydroxy benzaldehyde, 3, 4ꢀdihydroxy benzaldehyde, 2ꢀ thiophene carboxylic acid, 3ꢀ
thiophene carboxylic acid, 4ꢀnitro phenol, nꢀbromohexane, nꢀbromooctane, nꢀbromo decane
and nꢀbromododecane, N, N’ꢀdicyclohexylcarbodiimide (DCC), N, Nꢀ dimethylamino
pyridine (DMAP) and palladium on charcoal (Pd/C, 10%), were purchased from Aldrich
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(USA) and used without further purification. N, N’ꢀdimethyl formamide (DMF),
tetrahydrofuran (THF), ethanol, methanol, dichloromethane, ethyl acetate, chloroform,
diethyl ether, nꢀhexane, nꢀheptane, acetone, acetonitrile, ethyl methyl ketone (EMK),
isopropanol, anhydrous potassium carbonate, anhydrous sodium sulphate, sodium chloride,
celiteꢀ540, hydrochloric acid were obtained from Merck (India) and used as received. Sodium
phosphate monobasic dihydrate (NaH2PO4.2H2O) and sodium chlorite (NaClO2) from Loba
Chemie (Mumbai) and used as such.
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Instrumental Details
FTꢀIR spectra of all of the compounds were recorded by an ABB BOMEM MB3000
spectrometer using KBr pellets. 1D13C NMR spectra of the compounds were recorded on
Bruker AvanceꢀIII HD 400 MHz at room temperature using tetramethylsilane as an internal
standard in CDCl3. The nature of the mesophase and the temperature of occurrence of
different phases were examined by using Carl Zeiss Axiocam MRC5 optical polarizing
microscope equipped with a Linkam THMS 600 stage with a TMS 94 temperature controller.
The photographs were taken using an Imager A2M digital camera. Differential scanning
calorimetry traces were recorded by a DSC Q200 instrument with a heating rate of 10 °C per
minute in a nitrogen atmosphere and the samples were subjected to two heating and two
cooling cycles.
Variable temperature powder Xꢀray diffraction studies were carried out on unꢀ
oriented samples filled in Lindemann capillary of diameter of 1mm (Hampton Research,
Aliso Viejo, CA, USA) using Cu K α ( λ= 1.54 Å) radiation from PANalytical instrument
(DY 1042ꢀEmpyrean) and a linear detector (PIXcel 3D). The sample temperature was
controlled with a precision of 0.1 °C using a heater and a temperature controller
(Linkam).22The gas phase molecular geometry of 8d mesogen was optimized by DFTꢀbased
Becke’s threeꢀparameter hybrid exchange functional and the LeeꢀYangꢀParr correlation
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functional (B3YLP) method employing 6ꢀ31G* basis set Gaussian 09W suite of programs.23ꢀ
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Solid state NMR measurements
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The solidꢀstate NMR experiments were performed on a Bruker Avance III HD 400 WB
NMR spectrometer (9.4 T) equipped with a Bruker double resonance 4 mm WVT MAS
probe. The 1H and 13C resonance frequencies were 400.07 and 100.61 MHz, respectively. All
the measurements in the liquid crystalline phases of the samples were recorded under static
condition. The fine powder samples were packed in a 4 mm Zirconia rotor with Teflon
spacer and Zirconia cap. Initially the samples were heated from room temperature to the
isotropic phase and then slowly cooled to the respective mesophases to achieve sample
alignment. The 1D 13C NMR spectra in mesophases were obtained by crossꢀpolarization (CP)
scheme with a contact time of 3 ms and 62.5 kHz of radio frequency (RF) field strength on
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both the H and 13C channels. The other parameters were: number of scans 512 and 1024
respectively, for 7d and 8d mesogens and recycle delay 8 s. SAMPIꢀ4 pulse sequence26
employed to get high resolution 2D SLF spectrum under static condition (Figure S1 of
Supporting Information). Our earlier work demonstrated the utility of SAMPIꢀ4 pulse scheme
for variety of liquid crystalline samples.12ꢀ14,27,28 2D SAMPIꢀ4 spectra were obtained for 7d
and 8d mesogens using the following parameters: CP contact time τ =3 ms/3ms, t1 increments
=100/82, number of scans=28/100. The recycle delay of 15s/11s was employed to minimize
the RF heating effect. The data were zero filled in both the t2 and t1 dimensions, yielding a
4096 × 256 real matrix. A shifted sine bell window function was applied to the time domain
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data and the spectrum was processed in the phase sensitive mode. In all the experiments, H
90º pulses were of duration 4 ꢁs and SPINALꢀ6429heteronuclear decoupling was applied
during 13C acquisition with a proton nutation frequency of 30 kHz. Sample heating was
performed by running the bearing air through a heating coil and the sample temperature was
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calibrated from 207Pb NMR chemical shift of Pb(NO3)2.30The 13C chemical shift was
referenced externally relative to the adamantine methine resonance at 29.5 ppm.
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Results and Discussion
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The molecular structures of synthesized mesogens are shown in Scheme 1. The mesogenic
series are constructed by condensing either 4ꢀformylphenyl thiopheneꢀ2ꢀcarboxylate or 4ꢀ
formylphenyl thiopheneꢀ3ꢀcarboxylate and dialkoxy two ring amine in ethanol under
microwave condition.31To realize the forked mesogens, at one end of the core, two alkoxy
chains are placed while the other terminus has thiophene ring. Further, the aromatic rings of
the core are connected through ester and azomethine groups. By varying the length of the
terminal alkoxy chains, four mesogens for each series are synthesized. The main objective of
the investigation is to find the influence of second alkoxy chain on the mesophase transition
temperatures as well as the phase sequence. Further, the role of second alkoxy chain on the
local order parameters of the phenyl rings are investigated using 13C NMR spectroscopy. For
the ring I which has two alkoxy chains, the order parameter is directly influenced by the
asymmetry of the ring. The forked mesogens are structurally simple and serve as model
compounds for tetracatenar (biforked) mesogens.5ꢀ7Thus investigation of the structureꢀ
property relation of forked mesogens would simplify the course of understanding the
mesophase properties as well as the molecular order of polycatenar mesogens which are
known for their excellent polymesomorphism.5ꢀ9 The synthesized mesogens are structurally
characterized by FTꢀIR, solution 1H and 13C NMR spectroscopy (Figure S2ꢀS7 of Supporting
Information).
MesophaseCharacterization
The mesophase properties are evaluated by HOPM (Figure1) and further confirmed by
DSC (Figure 2). In each series, four mesogens are synthesized in which the terminal chain
has C6, C8, C10 and C12 carbons(7 aꢀd and 8 aꢀd). In the case of 2ꢀthiophene series (7 aꢀd), all
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the mesogens showed nematic phase on heating and cooling cycles. Additionally, a
monotropic smectic C phase is noted for C10 and C12 homologs. In other words, enantiotropic
nematic phase for all the homologs, and monotropic smectic C phase for higher homologs
(C10 and C12) is observed. For the 3ꢀthiophene series too (8 aꢀd), all homologs showed
nematic phase whereas enantiotropic smectic C phase is additionally seen for C10 and C12
homologs. In HOPM, on cooling the sample from isotropic phase, typically either marble or
birefringent threaded texture characteristic of the nematic phase is noticed.32 For those
homologs where the transition from nematic to smectic C mesophase is observed, transition
bars are seen followed by smectic schlieren texture (Figure 1).33 Table 1 lists the melting
(crystalꢀmesophase) and clearing temperatures (nematicꢀisotropic) of the synthesized
mesogens. It is apparent from the Table 1 that the crystal to mesophase transitions are in the
range of ~ 110ꢀ134 °C while the clearing transitions are found to be ~ 135ꢀ165 °C across the
series. With increase in terminal chain length, a decrease in clearing temperatures is
observed. A comparison of mesophase range of both the series indicates higher values for 3ꢀ
substituted homologs.
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To understand the influence of second alkoxy chain on mesophase transition
temperatures, a recently reported structurally similar mesogenwith one terminal alkoxy chain
is considered.34In view of the reported mesogens have a dodecyloxy chain at the terminal
position, the discussion is limited to C12 homologs of both the series. For 2ꢀthiophene series,
remarkably, the clearing temperature showed a reduction of ~ 138 °C whereas mere ~6 °C
decrease in the crystal to mesophase transition is observed. For the case of 3ꢀthiophene series,
lowering of nematic to isotropic temperature is found to be ~113 °C while a ~30 °C decrease
in the crystal to mesophase transition is noticed. Further, the reported mesogens exhibited
only enantiotropic nematic phase whereas for forked mesogens, either monotropic or
enantiotropic smectic C phase is observed for higher homologs. Thus, the additional
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curvature imparted by the presence of second alkoxy chainfavour layer ordering. Also the
increase in aliphatic chains favour segregation of incompatible parts of the forked mesogens,
i.e. separation of flexible aliphatic chains from rigid aromatic units result in layer ordering
characteristic of smectic C phase.35
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The mesophase properties obtained from HOPM are further confirmed by DSC
heating and cooling cycles (Figure 2 and S8ꢀS11 of Supporting Information). Table 1 lists the
transition enthalpy values which are in concurrence with HOPM results. The NꢀI transition
enthalpy values are in the range 0.14ꢀ0.24 (kcal/mole) whereas crystal to smectic C phase
data is typically higher than the NꢀI transition values.36 Despite the presence of two alkoxy
chains at the terminal position, the appearance of enantiotropic mesophases in the forked
mesogens is attributed to the lengthy rodꢀlike core with four rings i.e. three phenyl and one
thiophene ring. The presence of smectic C phase is further proved by variable temperature Xꢀ
ray diffraction experiments. Since the C10 and C12 homologs of 3ꢀthiophene series only
exhibit enantiotropic smectic C phase, the XRD studies are limited toC12 homologue (8d)
only where (Figure 3) an intense sharp reflection in the small angle region and broad hump at
the wide angle region (125°C) is detected. These reflections are characteristics of layer order
arising from smectic phases exhibited by rodꢀlike molecules.37In order to realize the length of
the molecule (L=39.87Å ), the energy optimized structure by DFT of the C12 homologue (8d)
is used (Figure 4). Table 2 lists the layer spacing (d) as well as wideꢀangle reflection for the
8d in the temperature range 105 to 125 °C (Figure S12 of Supporting Information). To find
the nature of the smectic phase, the d/L ratio measured at different temperatures is enlisted in
Table 2. As the d/L values decrease with lowering of temperature, the phase is assigned as
tilted smectic phase. Further, the broad hump in the wide angle region (Figure 3) indicates the
absence of inꢀplane order and liquidlike nature of the molecules within the layer.38 These
features strongly suggest that the mesophase under observation is tilted fluid phase i.e.
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smectic C mesophase. The tilt angle measured from the d/L values in the temperature span of
105ꢀ125 °C is in the range of ~ 18ꢀ25°.39
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13C NMR studies in liquid crystalline phase
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As stated earlier, the main focus of the investigation is to find the orientational order of all
the rings of the core unit as well as terminal chains. To fulfill the objective, 1D as well as 2D
13C NMR spectroscopy experiments of two representative mesogens (7d and 8d) belonging to
each series are carried out in liquid crystalline phase. By making use of 13Cꢀ1H dipolar
couplings from the 2D experiments, the order parameters of all the rings of the core unit are
determined. Since the ring I has three substituents, the order parameter of it reflects the
asymmetry of the ring.40 Further, the 13C NMR experiments would also throw light on the
orientational constraints of thiophene ring with reference to substitution pattern since the
thiophene is linked to rest of the core either at 2ꢀ or 3ꢀposition.
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To accomplish the structural characterization of 13C NMR spectra of mesogens in
liquid crystalline phase, the complete assignment of 13C NMR spectra of them in solution is
essential. Thus the solution 13C NMR spectra of 7d and 8d mesogens recorded in CDCl3at
room temperature are shown in Figure 5A and 5B. The spectrum shows wellꢀresolved lines
with varying chemical shifts as well as intensities in the range 14ꢀ166 ppm. Since the
mesogen has four rings in the core unit and two dodecyloxy chains at the terminal, many
lines in the span of 14ꢀ166 ppm are noticed. The 13C chemical shift assignment of all the
carbons is attempted first by iterating the spectrum with ChemDraw software followed by
verifying them with the values arrived by group contribution approach.41The assignment is
further refined by making use of DEPT,1Hꢀ1H COSY, 1Hꢀ13C HSQC and 1Hꢀ13C HMBC data
(Figures S13ꢀS24 of Supporting Information). Table 3 shows the chemical shift values of all
the carbons of 7d and 8d mesogens. Due to the asymmetry of the ring I, the spectrum has
morenumber of quaternary carbons.
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The static 13C NMR spectrum of 2ꢀthiophene mesogen (7d) in nematic phase
measured at 124 °C is shown in Figure 6A. Molecules exhibiting liquid crystalline property
in the mesophase align with respect to the magnetic field. The alignment is governed by the
sign of the anisotropic magnetic susceptibility tensor. Molecules with positive sign would
align parallel to the magnetic field while those with negative sign favour perpendicular
orientation.42,43The alignment of the molecules in the magnetic field results in sharp lines
with well resolved peaks as indicated by the spectrum. The peaks arising from core unit
carbons are noticed from 134ꢀ216 ppm in nematic phase as against 14ꢀ166 ppm in solution.
The increase in the chemical shift values in nematic phase is due to parallel alignment of the
molecules in the magnetic field.42,43For the case of terminal dodecyloxy chains, in solution
NMR, the lines are noted in the range 14ꢀ70 ppm. However, in the nematic phase, the 13C
spectrum of the terminal chain carbons is different owing to the anisotropic environment in
the mesophase. In other words, the variation in chemical shifts of the dodecyloxy chains
positioned at C1 and C2 of ring I arise due to change in their orientations with respect to
ordering axis. Accordingly, one of the OCH2 carbon of dodecyloxy chains (C1 position)
shows a marginal decrease in chemical shift values while the OCH2 carbon of other chain (C2
position) exhibits a significant increase. The variation in 13C chemical shifts of OCH2 and
CH2 carbons of both the chains in solution as well as in nematic phase are listed in Table4.
Thus the change in chemical shift values of terminal dodecyloxy chains provides further
insight into the location of long axis in the nematic phase. In solution phase, due to tumbling
of molecules, the anisotropic effects which are seen in nematic phase vanish. As a result, the
difference in the chemical shift values of OCH2 and other carbons of dodecyloxy chains in
nematic phase emerge as observed in the spectrum. For the core unit carbons, the peaks in the
range 134ꢀ216 ppm show achange in intensity pattern. Similar to solution 13C NMR
spectrum, the C3 and C6 of core unit show distinct values in nematic phase. For the rest of
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the phenyl rings of the core, intense peaks in the range 140ꢀ160 ppm are noticed. Among
them, the peak appeared at 146.9 ppm is more intense indicating the overlapping of other
carbons of the core unit. The high resolution of the spectrum (7d) in mesophase enables the
identification of almost all the carbons and accordingly, 21 carbons of the core unit are more
or less clearly seen (Figure 6A). Further, with the use of the 2D spectrum which is discussed
in later part of the section, all the assignment of the carbons is accomplished. Despite many
carbons in the core unit, the high resolution of the spectrum aided by well spread chemical
shifts due to the structural heterogeneity of the constituent moieties, the assignment could be
achieved. In other words, the presence of thiophene, 1ꢀ4ꢀdisubstitued phenyl rings,1,2,4ꢀ
trisubstituted phenyl ring as well as linking units in the mesogenic core, a wide span of the
chemical shifts (134ꢀ216 ppm) in the spectrum is noticed.
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For 3ꢀthiophene homolog (8d), the 13C NMR is acquired in thesmectic C phase
(Figure 6B) at 116 °C. Similar to the 2ꢀthiophene homolog, in this case also the assignment
of all the core unit carbons is achieved with the help of 2D spectrum. Table3 lists the
chemical shift values of the solution as well as static 13C NMR data of 8d. The major
difference between 2 and 3ꢀ substituted thiophenes (7d vs 8d) is the linking of the III phenyl
ring of the core to thiophene position. This variation resulted in a great change in the contour
pattern in 2D spectrum (Figure 7). For the terminal dodecyloxy chains, similar to 2ꢀthiophene
isomer (7d), the difference in chemical shift values of OCH2 is noted in smectic C phase.
Orientational order parameters
To determine the order parameter values of all the rings of the core unit of both the
mesogens (7d and 8d), the static 2DSLF measurements are carried out (Figure 7) in
respective mesophases. In the case of 3ꢀthiophene homologue (8d), the spectrum measured
in smectic C phase at 116 °C (Figure 7B) shows well resolved contours which not only differ
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in chemical shifts but also the 13Cꢀ1H dipolar couplings. Based on the 13Cꢀ1H dipolar
coupling values, the 2D spectrum of the core unit can be classified into three regions. For
example, 13Cꢀ1H dipolar couplings in the range 1.0ꢀ1.52kHz appear from quaternary carbons
of ester carbonyl and thiophene ring and the phenyl ring I carbons. The other phenyl ring
quaternary carbons i.e. ring II and III, showed values in the range 2.25ꢀ2.55kHz. The methine
carbons of the rings II and III are found to be ~ 3.0 kHz whereas the ring I carbons are seen~
3.65 kHz. The thiophene13Cꢀ1H dipolar couplings, however, appeared in the range 4.8ꢀ6.4
kHz while the imine carbon gave a contour at 203.0 ppm with a dipolar coupling value of
5.28 kHz. From these values, it clear that for the ring I, the quaternary 13Cꢀ1H dipolar
coupling values are 1.11ꢀ1.25 kHz, whereas the CH dipolar coupling value is 3.68 kHz. For
the ring II and III, on the other hand, the quaternary carbon dipolar coupling values are 2.28
to 2.54 kHz whereas the methine dipolar couplings are 2.99ꢀ3.20 kHz. In other words, the
dipolar couplings of quaternary carbons are higher for ring II and III while the methine
carbon dipolar couplings are high for ring I. These changes are direct consequence of the
influence of long axis on the phenyl rings (IꢀIII). Yet another variation is about the dipolar
coupling values of the thiophene ring. As stated earlier, the 13Cꢀ1H dipolar coupling values of
methine carbons of thiophene ring are highest (4.8ꢀ6.4 kHz) in contrast to other carbons
including imine linking unit. These large variations within the thiophene suggest that the long
axis is passing through the thiophene ring in such a way that among the three methine
carbons, two are in similar orientation while the third one experiences different orientation. It
is enriching to note that the 2D SLF spectrum could provide a convincing quantum of
information about the variation in the orientation of different moieties of the core unit in the
liquid crystalline phase.
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Yet another remarkable feature that can be acknowledged on inspection of 2D spectra
of 7d and 8d is the orientation of methine carbons of thiophene ring reflected on the contour
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pattern. For instance, in solution 13C NMR, the chemical shift values of C20 and C21
methine carbons of 7d and 8d are more or less similar while a ~ 9 ppm difference is noted for
C19 methine carbon. In liquid crystalline phase, on the other hand, the three methine carbons
not only show variation in chemical shift values but also as in 13Cꢀ1H dipolar couplings. This
suggests that the location of thiophene ring in mesogens (7d & 8d) is very sensitive to long
axis resulting in strikingly different contours with large variation in C19ꢀC21 carbons of
thiophene ring. In the spectrum, for 7d, the methine carbons are seen in the range 164ꢀ182
ppm while the dipolar couplings appeared in the span of 4.3ꢀ7.35 kHz. For the case of 8d, the
chemical shifts of thiophene methine carbons are observed in the range 143ꢀ161 ppm whereas
the dipolar couplings are found to be 4.80 to 6.40 kHz. Also in contrast to phenyl ring
methine carbons of the respective mesogens (7d and 8d), the thiophene carbons show higher
dipolar couplings which enable their identification straight forward. These changes suggest
that the location of carboxylic ester on thiophene has profound influence on dipolar couplings
(Table 3). Further, the variation in chemical shifts as well as dipolar couplings of the
thiophene is also influenced by the mesophase in which the measurements are carried out.
For 7d, the spectrum is recorded in nematic phase while for 8d the experiment is performed
in smectic C phase.
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In general, the 2D SLF experiments provide 13Cꢀ1H dipolar oscillation frequencies
from which the 13Cꢀ1H dipolar couplings can be extracted for a given molecular geometry
(phenyl or thiophene ring) as per the reported procedure.14,27,28,44,45 These extracted 13Cꢀ1H
dipolar couplings can be related to the order parameters of particular ring by considering the
local symmetry. For, 1ꢀ4 disubstituted phenyl ring, the D2 symmetry enables two order
parameters namely Szz and SxxꢀSyy whereas for 1,2,4ꢀtrisubstituted phenyl ring, the Cs
symmetry demands at least three order parameters viz. Szz, SxxꢀSyy and Sxz. Thus the
experimental 13Cꢀ1H dipolar couplings are related the order parameters of the core unit of
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forked mesogensby the following equation46,47
DCH =K [Szz (3 cos2θz-1)/2 + (Sxx – Syy) (cos2θx - cos2θy)/2+ Sxz (cos
θ
x cos
θz )]
(1)
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where K = -hγHγC/4π2 3
r
CH, with γH and γC are the gyromagnetic ratios of H and 13C nuclei
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respectively and rCH is the inter nuclear distance between them, θx, θy and θz are angles that
rCH forms with the respective coordinate axes of the system which are the local axes defined
on the molecular fragment under consideration. The standard bond distances rCH=1.1 Å for
the C–H bond and rCC=1.4 Å for the C–C bond are considered during the fitting. Also, the Cꢀ
CꢀH bond angles are found to be necessary to slightly vary around 120° to arrive at the best
fit.48 For ringꢀI, the zꢀaxis is considered along the C1ꢀC4 bond (para axis) and xꢀaxis is the inꢀ
plane axis perpendicular to z. The three local order parameters namely, Szz, SxxꢀSyy, and Sxz
describe the orientation of this rigid fragment of the core unit for both the mesogens. The data
fitting revealed a good agreement between calculated and experimental values for methine
carbons only and attempts to incorporate the quaternary carbons in the fitting procedure
showed large deviation which could be due to nonꢀplanar geometry of ring I. The calculated
order parametersof the ring I for both the mesogensare listed in Table5. For rings II and III
which are 1,4ꢀdisubstituted phenyl rings and possess effective D2 symmetry, the Sxz=0 in the
equation 1. By using the equation 1, the order parameter values of ring II and ring III are
calculated to be 0.86 and 0.82 respectively and for thiophene ring, on the other hand, the
value is 0.69 (Table 6). For the thiophene ring due to irregular pentagonal geometry, like
phenyl ring I, three order parameters are required for an arbitrary choice of the axis
system.28,49 The zꢀaxis is considered along the C17ꢀC18 bond for 7d and 8d mesogens by
which the thiophene is directly connected to the rest of the mesogen. The bond angles and
bond distances computed from the energy minimized structure of thiophene for 7d and 8d
mesogens are utilized (Figure 4 and Figures S25, S26of Supporting Information). Here, the fit
is initiated using only three methine carbons to find two order parameter namely, Szz and Sxx–
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Syy with an expectation that the other order parameter is likely to be small. Further, the
efficiency of the fit is enhanced by introducing an angle β between the zꢀaxis and the C17ꢀ
C18 bond. A plot of temperature versus alignment induced shifts(AIS) (Figures S27 and S28
of Supporting Information) shows abrupt change at nematic to smectic C indicating first
order nature of the transition in concurrence with DSC data.
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Figures S29 and S30 of Supporting Information shows the 2D Proton Encoded Local
Field (PELF) pulse sequence and spectra for 7d and 8d mesogens respectively in the region
14ꢀ80 ppm covering the terminal dodecyloxy chains. Since PELF provides better resolution
for aliphatic carbons this method is adopted for screening the 13Cꢀ1H dipolar couplings of
terminal dodecyloxy chains which are tabulated in Table 4. This table also lists the chemical
shift values of terminal chain carbons in solution as well as in mesophase for both 7d and 8d
mesogens. As observed earlier, the remarkable feature of the 13C NMR of OCH2 carbons in
mesophase is clear distinction of orientation of dodecyloxy chains attached to C1 and C2
carbons of the ring I. The chemical shift differences in solution NMR for OCH2 carbons of
dodecyloxy chains is 0.29 ppm. In mesophase on the other hand, the OCH2 carbons shows a
difference of 8.6 ppm. Further, the OCH2 arising from dodecyloxy chain attached to phenyl
ring at para position shows lower value whereas the OCH2 of the dodecyloxy chain attached
to meta position show increase in value (Table 4). The rest of the methylene carbons of both
chains show decrease in chemical shift value in contrast to solution spectra. Similarly, the
terminal methyl carbons of both chains gave single contour whose value in contrast to
solution is lower. The 2D spectrum shows 13 contours accounting for 24 carbons of both the
chains. The assignment of chemical shift values of all the dodecyloxy chain carbons is carried
out by following the earlier reports of mesogens that have alkoxy chains in the phenyl
ring.50,51 For instance, the variation in OCH2 carbons of the dodecyloxy chains of 7d indicates
that both the chains are experiencing different orientation with respect to long axis. Fung et
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al.52ꢀ56 earlier carried out 13C NMR studies of biforked mesogens and mesogens with four
ring core unit with lateral alkoxy chains, where the difference in orientation for OCH2
carbons chains attached to phenyl ring are noticed. This is explained based on the fact that the
OCH2 attached to meta position is in mostly in cis conformation, where as the one placed at
para position is predominantly in trans conformation. The contrasting profile of OCH2
carbons as against rest of the methylene carbons further indicates that methylene carbons of
both the dodecyloxy chains are in trans conformation. These chemical shift changes are also
supported by the negative sign of the order parameters except for meta OCH2 carbon. For
example, the typical values of order parameters of 7d mesogen arein the range of ꢀ0.019 to ꢀ
0.177 except for meta OCH2 carbon for which the order parameter is 0.026. Similar trends are
also noticed for 8d mesogen (Table4). It is clear from the 2D NMR studies that the
introduction of second alkoxy chain in the meta position of phenyl ring I, the location of long
axis changes (Figure 8). Further based on the HOPM and DSC data, it is clear that the
mesophase transitions experience drastic reduction in view of the flexibility imparted to the
overall molecule by the introduction of second alkoxychain at the meta position of the ring I.
It is important to note that Fung et al. 52ꢀ55could not determine the order parameter of the core
unit of ring to which alkoxy chains are attached due to the overlapping of signals. However,
in the present case, due to the high resolution achieved from the 2D spectrum, the order
parameter of ring I as well as other rings of the core unit could be carried out for 7d as well as
8d mesogens.
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It is clear from Tables 5 and 6 that the order parameter values are not same for phenyl
as well as thiophene rings. Further, relatively lower values observed for 7d than 8d are due to
the fact that the measurements of 8d are carried out in smectic C phase while for 7d the
experiments are performed in nematic phase. In all the cases, the order parameter of ringꢀII
of the core is found to be higher than other rings which suggest that the longꢀaxis of
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molecules is more or less collinear to C2 axis of ringꢀII. Also the thiophene ring shows lower
value than respective phenyl rings order parameters in both the cases (7d and 8d) as they are
located at one end of the mesogen.
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Conclusions
Two series of forked mesogens with a thiophene ring in the core were synthesized
from 2/3ꢀthiophene carboxylic acid. Depending on the position of the carboxylic acid in the
thiophene, 2 and 3 homolog series were realized in which the terminal dialkoxy chain length
was varied between 6ꢀ12 even carbons. The HOPM and DSC investigations indicated the
presence of enantiotropic nematic as well as smectic C phases for higher homologs. The VTꢀ
XRD studies further confirm the tilted fluid nature of the smectic phase. A detailed 13C NMR
investigation of two representative homologs belonging to each series is carried out. For 2ꢀ
thiophene homolog, measurements were performed in nematic phase while for 3ꢀthiophene
mesogen, the NMR spectra were recorded in smectic C phase. The 2D SLF measurements of
the mesogens in mesophase provided well resolved contours with 13Cꢀ1H dipolar couplings.
The important observations of the 13C NMR investigation include (i) asymmetry of the
phenyl ring I leading to three order parameters (ii) the dependence of chemical shifts as well
as 13Cꢀ1H dipolar couplings of thiophene methine carbons on the substitution i.e. 2ꢀ versus 3ꢀ
substitution and (iii) contrasting conformational profile of OCH2 carbons of dialkoxy
terminal chains. Thus the investigation provided complete profile of order parameter of core
unit as well as terminal chain carbons. The orientational constraints mapped from the Cꢀ1H
13
dipolar couplings of the mesogens in liquid crystalline phase demonstrate the greater utility
of 13C NMR for probing the molecular structure as well as the order for forked mesogens.
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AUTHOR INFORMATION
Corresponding Author
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*Eꢀmail:tnswamy99@hotmail.com; phoneꢀ91ꢀ44ꢀ24422059
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ACKNOWLEDGEMENTS
The authors would like to thank to Dr. B. Chandrasekaran, Director, CSIRꢀCLRI and Prof. K.
V. Ramanathan, NMR Research Centre, Indian Institute of Science, Bangalore, India, for
their support and keen interest in this work. We are grateful to Ms. K. N. Vasudha, Raman
Research Institute, Bangalore, India for the powder Xꢀray measurements and Dr. B. V. N.
Phani Kumar and Mr. R. Ravikanth Reddy, CLRI for 2D solution NMR measurements.We
acknowledge the partial financial support from STRAIT CSC0201ꢀWP2 and K. Rajasekhar
Reddy acknowledges the financial support from Council of Scientific and Industrial Research
(CSIR), New Delhi in the form of Senior Research Fellowship.
SUPPORTING INFORMATION
It contains synthetic details, spectral data (FTꢀIR, NMR) of intermediates and final mesogens,
figures of SAMPIꢀ4 pulse sequence, DSC scans of final mesogens,VTꢀXRD plot of 8d
mesogen, 1D and 2D solution NMR spectra for 7d and 8d mesogens, planar and energy
optimized structures of 7d mesogen, thiophene models, plot of AIS as a function of
temperature for 7d and 8d mesogens and PELF experimental details. This material is
available free of charge via the Internet at http://pubs.acs.org
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157, 303ꢀ318.
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(56) Perez, F.; Judeinstein, P.; Bayle, J. P.; Fung, B. M. The Effect of a Lateral Aromatic
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Table 1: Transition Temperatures and Enthalpy values of Synthesized Mesogens (7aꢀ7d) and
(8aꢀ8d) in the second heating cycle.
6
7
8
9
Transition temperatures (°C)
and enthalpy values in
Mesophase
range
Mesogen
Code
parenthesis (△H kcal/mol)
CrꢀN
133.87
(10.05)
CrꢀN
130.62
(11.17)
NꢀI
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
7a
7b
ꢀ
ꢀ
154.41
(0.24)
NꢀI
20.54 (N)
18.66 (N)
149.28
(0.15)
CrꢀN
125.27
(11.97)
NꢀI
142.19
(0.14)
7c
ꢀ
16.92 (N)
CrꢀN
118.25
(11.89)
CrꢀN
123.82
(11.86)
CrꢀN
121.30
(8.99)
CrꢀSmC SmCꢀN
117.09
(8.18)
CrꢀSmC SmCꢀN
111.88
(12.00)
NꢀI
135.66
(0.15)
NꢀI
165.62
(0.18)
NꢀI
156.35
(0.20)
NꢀI
144.21
(0.20)
NꢀI
141.19
(0.17)
7d
8a
8b
8c
8d
ꢀ
ꢀ
ꢀ
17.41 (N)
44.32 (N)
32.53 (N)
6.16 (SmC)
20.96 (N)
123.25
(0.10)
17.83 (SmC)
11.48 (N)
129.71
(0.47)
Cr: crystallisation;SmC: smectic C;N: nematic; I: isotropic
Table 2: Powder Xꢀray Diffraction Data of Mesogen 8d.
T (°C) d1 (Å) d2(Å) d1/L
125
115
105
36.98 4.55 0.92
36.61 4.52 0.92
36.16 4.48 0.91
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Table 3:13C NMR Data of core unit for 7d and 8d mesogens in solution and liquid crystalline
phases
6
7
8
9
7d (124 ºC)
Carbon Solution CS
8d (116 ºC)
CS AIS
(ppm) (ppm)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
AIS 13Cꢀ1H Solution
13Cꢀ1H
DOF
(kHz)
1.44
1.11
3.68
1.25
3.68
3.68
1.09
2.28
3.20
2.99
2.28
5.28
2.54
3.06
2.99
2.29
1.09
1.51
6.34
5.02
4.83
No.
(ppm)
(ppm) (ppm) DOF
(kHz)
(ppm)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
153.9
148.7
112.0
121.6
124.4
114.7
165.2
149.4
121.8
122.1
149.4
159.1
134.0
130.1
122.5
153.0
160.2
132.6
135.0
128.1
133.8
176.2
165.7
135.2
158.6
133.9
135.2
206.3
211.5
142.9
146.9
215.8
194.3
194.3
156.6
146.9
194.3
204.1
167.5
170.9
164.2
181.8
22.3
17.0
23.2
37.0
9.5
1.24
1.19
3.51
1.41
4.17
3.51
0.61
1.59
2.53
2.30
1.51
4.26
1.70
2.42
2.30
1.18
0.67
0.90
5.10
4.38
7.33
153.9
148.7
114.6
121.5
124.4
111.9
165.2
149.4
121.9
122.2
149.3
159.2
133.9
130.1
122.5
153.1
160.7
132.6
126.6
128.2
134.4
187.8
202.7
138.9
174.5
138.9
138.9
215.4
226.1
148.5
153.4
226.1
203.0
208.2
163.1
153.4
231.4
215.4
167.2
150.5
143.3
160.9
33.9
54.0
24.3
53.0
14.5
27.0
50.2
76.7
26.6
31.2
76.8
43.8
74.3
33.0
30.9
78.3
54.7
34.6
23.9
15.1
26.5
20.5
41.1
62.1
21.1
24.8
66.4
35.2
60.3
26.5
24.4
41.3
43.9
34.9
35.9
36.1
48.0
CS: chemical shift; AIS: alignment induced shift; DOF: dipolar oscillation frequencies
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Table 4:13C NMR data of dodecyloxy chain for 7d and 8d mesogens in solution and liquid
crystalline phases
6
7
8
9
7d (124 ºC)
Carbon Solution CS AIS
8d (116 ºC)
CS AIS
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
DCH SCH Carbon Solution
DCH SCH
(ppm) (ppm) (ppm)
(kHz)
No
(ppm) (ppm) (ppm)
No
(kHz)
a
69.09 77.30 8.21 ꢀ0.59 0.026
a
69.08
69.35
29.21
26.01
29.21
29.41
29.45
29.67
29.75
29.08
31.96
22.73
14.18
78.80 9.72 ꢀ0.61 0.027
67.30 ꢀ2.05 4.84 ꢀ0.213
24.70 ꢀ4.51 4.63 ꢀ0.204
22.10 ꢀ3.91 2.89 ꢀ0.127
25.60 ꢀ3.61 2.29 ꢀ0.101
26.10 ꢀ3.31 3.32 ꢀ0.146
26.10 ꢀ3.35 3.32 ꢀ0.146
27.40 ꢀ2.27 1.84 ꢀ0.080
27.70 ꢀ2.05 1.47 ꢀ0.065
23.40 ꢀ5.68 2.04 ꢀ0.090
30.40 ꢀ1.56 1.27 ꢀ0.056
22.10 ꢀ0.63 0.88 ꢀ0.039
13.50 ꢀ0.68 0.42 ꢀ0.019
a’
b
69.38 68.70 ꢀ0.68 3.69 ꢀ0.163
a’
29.21 26.90 ꢀ2.31 1.72 ꢀ0.076 b,b’
29.21 25.10 ꢀ4.11 4.01 ꢀ0.177 c,c’
26.02 22.00 ꢀ4.02 2.59 ꢀ0.114 d,d’
29.23 25.40 ꢀ3.83 2.25 ꢀ0.099 e,e’
29.45 25.70 ꢀ3.75 2.97 ꢀ0.131 f,f’
29.67 26.30 ꢀ3.37 2.36 ꢀ0.104 g,g’
29.71 26.70 ꢀ3.01 1.93 ꢀ0.085 h,h’
b’
c,c’
d,d’
e,e’
f,f’
g,g’
h,h’
i,i’
29.75 28.40 ꢀ1.35 2.11 ꢀ0.093
29.10 23.20 ꢀ5.9 2.05 ꢀ0.090
i,i’
j,j’
j,j’
31.97 29.80 ꢀ2.17 1.49 ꢀ0.066 k,k’
k,k’
l,l’
22.73 21.80 ꢀ0.93 1.11 ꢀ0.049
14.16 13.30 ꢀ0.86 0.44 ꢀ0.019
l,l’
CS: chemical shift; AIS: alignment induced shift; DCH: Dipolar coupling; SCH:Bond order
parameter
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Table 5: Orientational order parameters of phenyl ringꢀI for the mesogens 7d and 8d at 124
and 116 ºC respectively.a
8
9
Calculated dipolar
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Bond Angles
θ3 θ5 θ6
oscillation
frequencies (kHz)
Cꢀ3 Cꢀ5 Cꢀ6
RMSD
(kHz)
Szz SxxꢀSyy Sxz
Mesogens
7d
8d
121.0 120.0 121.8 0.76 ꢀ0.102 ꢀ0.090 3.51 4.17 3.51 0.002
121.0 121.8 121.8 0.80 ꢀ0.122 ꢀ0.081 3.68 3.68 3.68 0.001
Table 6: Orientational order parameters of phenyl (ringꢀII and ringꢀIII) and thiophene rings
for the mesogens 7d and 8d at 124 and 116 ºC respectively.a
b
c
a
d
Calculated dipolar
oscillation
Bond Angles
RMSD
(kHz)
Mesogens Phenyl
Szz SxxꢀSyy
frequencies (kHz)
θb
θc
b
c
ab
d
II
119.9 120.8 0.59 ꢀ0.058 2.53 2.28 1.36 1.38
118.8 119.2 0.54 ꢀ0.028 2.40 2.30 1.64 1.17
121.1 121.6 0.86 ꢀ0.082 3.19 3.00 2.01 2.03
121.1 121.4 0.82 ꢀ0.084 3.08 2.97 2.59 1.94
0.04
0.07
0.01
0.06
7d
III
II
8d
III
RMSD
(kHz)
0.03
Mesogens Thiophene
β
Szz SxxꢀSyy Cꢀ18 Cꢀ19 Cꢀ20 Cꢀ21
7d
8d
5.9
2.3
0.48 ꢀ0.053
0.69 ꢀ0.073
ꢀ
ꢀ
5.10 4.42 7.36
5.03 6.32 4.85
0.02
a
In the figure above the table, b and c are methine carbons, a and d are quaternary carbons
respectively for ringꢀI and II .
bFor carbon a in ring III, the contribution of azomethine proton has also been taken in to
account.
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