13C NMR Studies, Molecular Order, and Mesophase Properties of Thiophene Mesogens

Article abstract of DOI:10.1021/acs.jpcb.5b09859

Three-ring mesogens with a core comprising thiophene linked to one phenyl ring directly and to the other via flexible ester are synthesized with terminal alkoxy chains to probe the mesophase properties and find the molecular order. The phenyl thiophene link in the core offers a comparison of the mesophase features with the molecular shape of the mesogen. The synthesized mesogens display enantiotropic polymesomorphism and accordingly nematic, smectic A, smectic C and smectic B mesophases are perceived depending upon the terminal chain length. For some of the homologues, monotropic higher order smectic phases such as smectic F and crystal E are also witnessed. The existence of polymesomorphism are originally observed by HOPM and DSC and further confirmed by powder X-ray diffraction studies. For the C8 homologue, high resolution solid state ¹³C NMR spectroscopy is employed to find the molecular structure in the liquid crystalline phase and using the 2D SLF technique, the ¹³C-¹H dipolar couplings are extracted to calculate the order parameter. By comparing the ratio of local order of thiophene as well as phenyl rings, we establish the bent-core shape of the mesogen. Importantly, for assigning the carbon chemical shifts of the core unit of aligned C8 mesogen, the ¹³C NMR measured in mesophase of the synthetic intermediate is employed. Thus, the proposed approach addresses the key step in the spectral assignment of target mesogens with the use of ¹³C NMR data of mesomorphic intermediate.

Full text of DOI:10.1021/acs.jpcb.5b09859

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Article
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C NMR studies, Molecular Order and
Mesophase Properties of Thiophene Mesogens
Bathini Veeraprakash, Nitin Prakash Lobo, and Tanneru Narasimhaswamy
J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b09859 • Publication Date (Web): 09 Nov 2015
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13C NMR studies, Molecular Order and Mesophase Properties of Thiophene
Mesogens
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B. Veeraprakash†, Nitin P. Lobo‡ and T. Narasimhaswamy*†
†Polymer Laboratory and ‡Chemical Physics Laboratory,
CSIRꢀCentral Leather Research Institute, Adyar, Chennai 600020, India.
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ABSTRACT
Three ring mesogens with a core comprising thiophene linked to one phenyl ring
directly and to the other via flexible ester are synthesised with terminal alkoxy chains
to probe the mesophase properties and find the molecular order. The phenyl thiophene
link in the core offers to compare the mesophase features with the molecular shape of
the mesogen. The synthesised mesogens display enantiotropic polymesomorphism
and accordingly nematic, smectic A, smectic C and smectic B mesophases are
perceived depending upon the terminal chain length. For some of the homologs,
monotropic higher order smectic phases such as smectic F and crystal E are also
witnessed. The existence of polymesomorphism are originally observed by HOPM,
DSC further confirmed by powder Xꢀray diffraction studies. For C8 homolog, high
resolution solid state 13C NMR spectroscopy is employed to find the molecular
structure in liquid crystalline phase and using the 2D SLF technique, the 13Cꢀ1H
dipolar couplings are extracted to calculate the order parameter. By comparing the
ratio of local order of thiophene as well as phenyl rings, the bentꢀcore shape of the
mesogen is established. Importantly, for assigning the carbon chemical shifts of the
core unit of aligned C8 mesogen, the 13C NMR measured in mesophase of the
synthetic intermediate is employed. Thus the proposed approach addresses the key
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step in the spectral assignment of target mesogens with the use of C NMR data of
mesomorphic intermediate.
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KEYWORDS: smectic B, smectic C, XRD, 2D SLF, 13Cꢀ1H dipolar couplings, order
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parameter,
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INTRODUCTION
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Thermotropic liquid crystals comprising of thiophene as a constituent of the core unit
have emerged as important class of molecular mesogens.1ꢀ3 In contrast to wholly
phenyl ring based mesogens, molecules consisting of thiophene contributes for
pronounced change in properties like increase in optical anisotropy, decrease in
melting point, encourages negative dielectric anisotropy and notably reduces the
viscosity.4ꢀ6 Additionally, the recent work unambiguously demonstrated that the
replacement of phenyl rings with thiophene moieties favour interesting optoꢀ
electronic and optoꢀphotonic properties leading to plethora of applications in organic
light emitting diodes, thin film transistors and organic photovoltaics etc.7ꢀ10 Among
them, those based on thiophene as a part of the three ring core have attained
significant importance due to their low melting transition as well as excellent
polymesomorphism.11ꢀ13 In this class of mesogens, depending on the location of
thiophene in a three ring core, either rodꢀlike or bent core molecules are realized.14,15
For instance, if the thiophene is located at the end of the core, rodꢀlike mesogens are
obtained while its presence in the centre of the core yields bentꢀcore molecules. Thus
linking the 2ꢀsubstituted thiophene with the rest of the core favour mostly nematic
phase whereas thiophene at a central location with 2, 5ꢀsubstitution promotes
polymesomorphism.16,17 Further, the direct link between thiophene and phenyl ring
i.e. phenyl thiophene based calamitic mesogens are found to be interesting
molecules.18ꢀ20 Many studies on phenyl thiophene based mesogens revealed
polymesomorphism exhibiting nematic as well as wide range of smectic mesophases.
The direct link between phenyl ring and thiophene also favour πꢀconjugation leading
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to photoluminescence properties which are not generally observed if a linking unit
such as ester is present.21,22 In a recent work, we explored the polymesomorphism in
three ring mesogens in which thiophene is placed at the end of the core and reported
the orientational order of phenyl as well as thiophene rings.23 In the present work, we
examine yet another three ring series where thiophene is located at the centre of the
core.
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In recent years, the combined approach of Density Functional Theory (DFT),
powder Xꢀray diffraction (XRD) and solid state 13C NMR has become popular for
investigating the molecular packing and order in liquid crystalline molecules.24ꢀ26
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Thus the DFT provides the energy optimised structure as well as C chemical shifts
in isotropic phase and XRD offers information about molecular packing while 13C
NMR provides site specific chemical shift information across the molecule in
mesophase. This work also addresses one of the important issues related to chemical
shift assignment of molecules in mesophase by making use of a synthetic intermediate
that exhibits liquid crystalline phase. It is anticipated that the proposed method of
chemical shift assignments of core unit carbons in liquid crystalline phase with the aid
of structurally similar intermediate would be a viable approach. Thus the overall
emphasis of the investigation is to characterize the synthesised mesogens by Hotꢀ
stage optical polarising microscopy (HOPM), differential scanning calorimetry (DSC)
and XRD and also find the local order of phenyl as well as thiophene rings by high
resolution solid state 13C NMR spectroscopy.
EXPERIMENTAL SECTION
The experimental details of intermediates and final mesogens (4ꢀalkoxyphenyl 5ꢀ(4ꢀ
(dodecyloxy) phenyl) thiopheneꢀ2ꢀcarboxylates) and spectral data of all the
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intermediates are furnished in Supporting Information (SI). The spectral data of final
mesogens are furnished below.
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Materials
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Thiopheneꢀ2ꢀcarboxylic acid, 4ꢀbromophenol, sodium hydroxide, palladium acetate,
1ꢀbromo dodecane, pivalic acid, PCy3.HBF4, 10% Pd/C, ammonium formate, 4ꢀ
benzyloxy phenol were purchased from Aldrich, USA. Tetrahydrofuran (THF), nꢀ
heptane, ethanol from (SD Fine, India) were used as received. Ethyl acetate, diethyl
ether, nꢀhexane, isopropanol, anhydrous potassium carbonate, anhydrous sodium
sulfate, Celiteꢀ540, and silica gel (100ꢀ200 mesh) were used as received from Merck,
India.
Ethyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate (EDTC)
ꢀ1
°
Yield: 78.3%, m.pꢀ60 C, FTꢀIR (KBr, cm ): 3112(aromatic CꢀHstr), 3077, 2962, (Cꢀ
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Hstr), 1715(C=Ostrester carbonyl), 1623, 1446(C=Cstraromatic). H NMR (400 MHz,
CDCl3); δ 7.73 (d, J = 3.9 Hz, 1H), 7.55 (d, J = 7.8 Hz, 2H), 7.17 (d, J = 3.9 Hz, 1H),
6.92 (d, J = 7.9 Hz, 2H), 4.35 (q, J = 6.9 Hz, 2H), 3.98 (t, J = 6.2 Hz, 2H), 1.87ꢀ1.72
(m, 2H), 1.45 (d, J = 6.6 Hz, 2H), 1.38 (t, 3H), 1.27ꢀ1.15 (m, 16H), 0.88 (t, J = 5.9
Hz, 3H). 13C NMR (100 MHz, CDCl3); δ 162.41, 159.79, 151.37, 134.32, 131.34,
127.49, 126.05, 122.43, 115.01, 68.20, 61.06, 31.93, 29.67, 29.65, 29.60, 29.58,
29.39, 29.36, 29.21, 26.02, 22.70, 14.39 and 14.13. MS (HRMS): m/z calculated for
C25 H36 O3 S [M + H ]+ 417.2457; found, 417.2458.
4-ethoxyphenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate (EDPTC)
Yield (72%), m.pꢀ99.8 C, FTꢀIR (KBr, cmꢀ1): 3102(aromatic CꢀH ), 3073, 3038,
°
str
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2965(CꢀHstr), 1722(C=Ostr), 1605, 1508, 1420(C=Cstr aromatic), 1237(CꢀOꢀCstr); H
NMR (400 MHz, CDCl3); δ 7.81 (d, J = 3.9 Hz, 1H), 7.51 (d, J = 8.5 Hz, 2H), 7.16
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(d, J = 3.9 Hz, 1H), 7.05 (d, J = 8.8 Hz, 2H), 6.87ꢀ6.83 (t, J = 8.1 Hz, 4H), 3.98ꢀ3.89
(t, J = 6.7 Hz, 4H), 1.76ꢀ1.67 (m, 6H), 1.34 (m, 5H), 1.24 (m, 12H), 0.81 (t, J = 6.6
Hz, 3H). 13C NMR (100 MHz, CDCl3); δ 161.12, 159.98, 156.72, 152.65, 144.04,
135.67, 130.09, 127.60, 125.84, 122.69, 122.46, 115.06, 68.22, 63.85, 31.96, 29.71,
29.68, 29.64, 29.62, 29.43, 29.40, 29.22, 26.05, 22.74, 14.88 and 14.18.
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4-butoxyphenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate (BDPTC)
Yield (78.0%), m.pꢀ91.4 °C, FTꢀIR (KBr, cmꢀ1 ): 3032, 3028(CꢀHstr aromatic),
2948(CꢀHstr), 1728 (C=Ostrester), 1610, 1578(C=Cstraromatic), and 1268, 1077(CꢀOꢀ
Casym&symstr); 1H NMR (400 MHz, CDCl3); δ 7.89 (d, J = 3.9 Hz, 1H), 7.6 (d, J = 8.7
Hz, 2H), 7.23 (d, J = 3.9 Hz, 1H), 7.12 (d, J = 9.0 Hz, 2H), 6.94ꢀ6.90 (t, J = 7.5 Hz,
4H), 4.01ꢀ3.94 (t, 6.5 Hz, 4H), 1.84ꢀ1.73 (m, 4H), 1.53ꢀ1.42 (m, 4H), 1.17 (m, 16H),
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0.98 (t, J = 6.5 Hz, 3H), 0.88 (t, J = 6.6 Hz, 3H). C NMR (100 MHz, CDCl3); δ
161.06, 160.01, 156.95, 152.63, 144.06, 135.58, 130.19, 127.60, 125.89, 122.67,
122.38, 115.10, 68.24, 68.15, 31.92, 31.34, 29.64, 29.59, 29.38, 29.35, 29.21, 26.02,
22.68, 19.24, 14.10 and 13.83. MS (HRMS): m/z calculated for C33 H44 O4 S [M + H]+
537.3032; found, 537.3033.
4-(hexyloxy) phenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate
(HDPTC)
Yield (78.0%), m.pꢀ92.5 °C, FTꢀIR (KBr, cmꢀ1 ): 3067, 3048(CꢀHstr aromatic), 2953,
2870(CꢀHstr), 1722(C=Ostr), 1605, 1508(C=Cstraromatic), and 1237, 1037(CꢀOꢀ
Casym&symstr); 1H NMR (400 MHz, CDCl3) δ 7.88 (d, J = 3.9 Hz, 1H), 7.58 (d, J = 7.2
Hz, 2H), 7.23 (d, J = 3.9 Hz, 1H), 7.12 (d, J = 9.0 Hz, 2H), 6.94ꢀ6.90 (t, J = 6.5 Hz,
4H), 4.02ꢀ3.86 (t, J = 6.6 Hz, 4H), 1.87ꢀ1.70 (m, 4H), 1.53ꢀ1.40 (m, 4H), 1.38ꢀ1.32
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(m, 6H), 1.27 (m, 14H), 0.93ꢀ0.86 (m, 6H); C NMR (100 MHz, CDCl3); δ 161.09,
159.99, 156.94, 152.63, 144.03, 135.62, 130.15, 127.59, 125.87, 122.67, 122.40,
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115.08, 68.45, 68.22, 31.94, 31.61, 29.68, 29.66, 29.62, 29.60, 29.41, 29.38, 29.26,
29.22, 26.04, 25.74, 22.71, 22.63, 14.14 and 14.06. MS (HRMS): m/z calculated for
C35 H48 O4 S [M + H]+ 565.3345; found, 565.3346.
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4-(octyloxy) phenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate (ODPTC)
Yield (78.0%), m.pꢀ90.6 °C, FTꢀIR (KBr, cmꢀ1 ): 3070(CꢀHstr aromatic), 2956,
2922(CꢀHstr), 1727(C=Ostr), 1608, 1513(C=Cstraromatic); 1H NMR (400 MHz,
CDCl3); δ 7.89 (d, J = 3.9 Hz, 1H), 7.59 (d, J = 8.5 Hz, 2H), 7.24 (d, J = 3.9 Hz, 1H),
7.12 (d, J = 8.9 Hz, 2H), 6.95ꢀ6.9 (t, J = 8.4 Hz, 4H), 4.01ꢀ3.94 (t, J = 6.5 Hz, 4H),
1.84ꢀ1.75 (m, 4H), 1.45 (m, 4H), 1.37ꢀ1.26 (m, 24H), 0.90ꢀ0.87 (t, J = 6.5 Hz, 6H);
13C NMR (100 MHz, CDCl3); δ 161.13, 159.97, 156.93, 152.63, 143.99, 135.65,
130.11, 127.60, 125.86, 122.68, 122.41, 115.06, 68.44, 68.22, 31.94, 31.84, 29.68,
29.66, 29.62, 29.59, 29.40, 29.38, 29.27, 29.20, 26.06, 26.03, 22.72, 22.69, 14.15 and
14.14. MS (HRMS): m/z calculated for C37 H52 O4 S [M + H]+ 593.3658; found,
593.3659.
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4-(decyloxy) phenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate (DDPTC)
Yield (72.6%), m.pꢀ97.9°C, FTꢀIR (KBr, cmꢀ1 ): 3022(CꢀHstraromatic), 2955, 2933,
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CDCl3); δ 7.89 (d, J = 3.8 Hz, 1H), 7.59 (d, J = 8.5 Hz, 2H), 7.24 (d, J = 3.9 Hz, 1H),
7.12 (d, J = 8.8 Hz, 2H), 6.95ꢀ6.91 (t, J = 6.5 Hz, 4H), 4.01ꢀ3.94 (t, 6.5 Hz, 4H), 1.84ꢀ
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1.75 (m, 4H), 1.50ꢀ1.42 (m, 4H), 1.29 (m, 28H), 0.90ꢀ0.87 (t, J = 6.8 Hz, 6H); C
NMR (100 MHz, CDCl3); δ 161.13, 159.97, 156.93, 152.63, 143.98, 135.65, 130.11,
127.60, 125.85, 122.68, 122.41, 115.06, 68.44, 68.22, 31.94, 31.92, 29.68, 29.66,
29.59, 29.42, 29.38, 29.35, 29.29, 29.20, 26.06, 26.03, 22.71 and 14.15. MS (HRMS):
m/z calculated for C39 H56 O4 S [M + H]+ 621.3971; found, 621.4073.
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4-(dodecyloxy) phenyl 5-(4-(dodecyloxy) phenyl) thiophene-2-carboxylate
(DDDPTC) Yield: (68.7%) m.pꢀ97.9 °C, FTꢀIR (KBr, cmꢀ1): 3097, 3112(Cꢀ
Hstraromatic), 2953(CꢀHstr) 1727(C=Ostr), 1606, 1449(C=Cstraromatic), 1064(CꢀOꢀ
Casym&symstr ether); 1H NMR (400 MHz, CDCl3); δ 7.81 (d, J = 3.7 Hz, 1H), 7.51 (d, J
= 8.3 Hz, 2H), 7.16 (d, J = 3.8 Hz, 1H), 7.05 (d, J = 8.6 Hz, 2H), 6.95ꢀ6.91 (t, J = 8.3
Hz, 4H), 4.01ꢀ3.94 (t, J = 6.5 Hz, 4H), 1.84ꢀ1.75 (m, 4H), 1.37ꢀ1.26 (m, 4H), 1.22 (m,
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32H), 0.90ꢀ0.87 (t, J = 6.5 Hz, 6H); C NMR (100 MHz, CDCl3); δ 161.12, 159.98,
156.72, 152.65, 144.04, 135.67, 130.08, 127.60, 125.84, 122.69, 122.46, 115.06,
115.05, 68.21, 63.85, 31.96, 29.70, 29.68, 29.64, 29.61, 29.42, 29.40, 29.22, 26.05,
22.74, 14.88 and 14.1. MS (HRMS): m/z calculated for C41 H60 O4 S [M + H]+
649.4284; found, 649.4285.
Instrumental Details
FTꢀIR spectra of samples were recorded on ABB BOMEM MB3000 spectrometer
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using KBr pellets. 1H and C NMR spectra of the samples were run on a 400 MHz
BrukerAvanceꢀIII spectrometer at room temperature using CDCl3 as a solvent and
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tetramethylsilane (TMS) as an internal reference. The resonance frequencies of H
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and C nuclei were 400.23 and 100.64 MHz respectively. LCMSꢀESI spectra were
recorded on a Bruker ImpactꢀHD spectrometer. Optical polarizing microphotographs
were taken using Carl Zeiss Axiocam MRC5 polarizing microscope equipped with
Linkam THMS heating stage with TMS 94 temperature programmer. The
photomicrographs were taken using Imager A2M digital camera. The samples were
placed between 12 mm glass cover slips and transferred to heating stage and were
heated with predetermined heating rate. Differential scanning calorimetry (DSC)
traces were recorded using a DSC Q200 instrument with a heating rate of 10
°C/minute in nitrogen atmosphere. The samples were subjected to two heating and
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two cooling cycles and the data obtained from second cycle is considered for
discussion. UVꢀVisible absorption spectra were recorded on Varian Cary 50 Bio UVꢀ
Visible spectrophotometer in chloroform solution and the Photoluminescence (PL)
spectra were measured in chloroform on Varian Cary eclipse fluorescence
spectrophotometer.
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Variable temperature powder Xꢀray diffraction (VTꢀXRD) measurements
were carried out on unoriented samples filled in Lindemann capillary of diameter of 1
mm (Hampton Research, Aliso Viejo, CA, USA) using CuꢀKα (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).27 The gas phase molecular geometries of EDTC
BDPTC and ODPTC mesogens were optimized by using DFTꢀbased Becke’s three
parameter hybrid exchange functional and LeeꢀYangꢀParr correlation functional
(B3LYP) method employing 6ꢀ31G (d) basis set Gaussian 09W suite of programs.28ꢀ30
Solid-state 13C NMR experiments
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The solid state C NMR measurements were performed on a Bruker Avance III HD
400 WB NMR spectrometer operating at resonance frequencies of 400.07 and 100.61
MHz for 1H and 13C nuclei respectively. A double resonance 5 mm static probe with a
horizontal solenoid coil was used to record the spectra in the liquid crystalline phases.
The sample alignment was attained by heating the sample to the isotropic phase and
then slowly cooling to the respective mesophases. The 1D 13C NMR spectra in
various mesophases were obtained by standard crossꢀpolarization (CP) scheme with a
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contact time of 3 ms, number of scans 128 and recycle delay of 8 s. High resolution
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2DꢀSLF spectra were acquired by using the SAMPIꢀ4 pulse sequence (Figure S1 of
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Supporting Information).31 The efficiency of SAMPIꢀ4 for getting the Cꢀ1H dipolar
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couplings in liquid crystalline phase for wide variety of mesogens is reported in our
earlier work.23ꢀ26,32ꢀ35 The SAMPIꢀ4 experimental conditions for EDTC and ODPTC
mesogens were as follows: CP contact time τ = 3 ms/ 3 ms, number of t1 increments=
128/148, number of scans= 20/22 and recycle delay= 16 s/16 s to reduce RF heating
effects. 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
data and the spectrum was processed in the phase sensitive mode. For both 1D and 2D
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measurements, H 90˚ pulse length was 5ꢁs and SPINALꢀ64 decoupling strength of
30 kHz was used during the carbon signal acquisition.36 The variable temperature
measurements were performed by using a Bruker BVTBꢀ3500 temperature control
unit and 207Pb NMR chemical shift of Lead Nitrate sample was used for temperature
calibration.37 The 13C chemical shift was externally referenced by considering the
solid adamantane methine resonance at 29.5 ppm.
RESULTS AND DISCUSSION
The synthetic route for the preparation of 4ꢀalkoxyphenyl 5ꢀ(4ꢀ(dodecyloxy)
phenyl) thiopheneꢀ2ꢀcarboxylates is sketched in (Scheme 1 of Supporting
Information). Accordingly, 1ꢀbromoꢀ4ꢀ(dodecyloxy)benzene is synthesized by
Williamson’s etherification from 4ꢀbromo phenol while ethyl thiopheneꢀ2ꢀcarboxylate
is made from thiopheneꢀ2ꢀcarboxylic acid and ethanol under reflux with an acid
catalyst.38 The synthesis of ethyl 5ꢀ(4ꢀ(dodecyloxy) phenyl) thiopheneꢀ2ꢀcarboxylate
is then achieved via direct arylation of 1ꢀbromoꢀ4ꢀ(dodecyloxy) benzene with ethyl
thiopheneꢀ2ꢀcarboxylate using palladium acetate.39,40 The hydrolysis of the resultant
ester with sodium hydroxide led to acid which is condensed with 4ꢀalkoxy phenols41
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to get 4ꢀalkoxyphenyl 5ꢀ(4ꢀ(dodecyloxy) phenyl) thiopheneꢀ2ꢀcarboxylates (Scheme 1
of Supporting Information). By keeping the dodecyloxy chain at one end constant,
the alkoxy chain at the other side is varied from C2 to C12 with even carbons to get
the homologs of target mesogens. Figure 1 shows the planar structure along with
carbon numbers and the energyꢀoptimized model of EDTC and ODPTC mesogens
computed from the DFT method of quantum chemical calculations.
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Mesophase Transitions and Molecular Organization
The synthesised mesogens are examined by HOPM, DSC and VTꢀXRD to
establish the nature of the mesophases, transition temperatures as well as the
molecular organization respectively. The HOPM and DSC studies revealed
enantiotropic mesophases with polymesomorphism for entire range of mesogens
(Table 1). Thus, the EDPTC homolog in HOPM (Figure S2 AꢀC of Supporting
Information), on cooling the isotropic phase, showed birefringent droplets. These
droplets on further cooling coalesced to give threaded texture characteristic of
nematic phase (Figure S2 A of Supporting Information).42 At 144.4 °C, the texture
changes to focal conic fan with homeotropic domains indicating the formation of
smectic A phase phase (Figure S2 B of Supporting Information).43 The texture is
retained till the sample temperature is lowered to 86.6 °C. Interestingly, the DSC
(Figure S3 A of Supporting Information) cooling cycle of the EDPTC homolog shows
a monotropic transition at 86.6 °C with an enthalpy of 0.67 k.cal/mole. In HOPM,
however, at this temperature, no clear change in texture is noticed. Since the transition
enthalpy associated with the change is high and homeotropic domains persisted in
HOPM even at 86.6 °C, the phase is assigned to be smectic B phase (Figure S2 C of
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Supporting Information). The crystallization of the sample is noticed at 55.5 °C.
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Table 1 lists the transition temperatures and their enthalpy values.
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For BDPTC homolog, upon cooling the isotropic phase, formation of
batonnets are observed which on further cooling transformed to focal conic fan
texture characteristic of smecticA phase (Figure S2 D of Supporting Information).43
At 143.7 °C, a textural change is observed where in the focal conic fans convert to
sanded brokenꢀfan texture (Figure S2 E of Supporting Information) indicating the
formation of smectic C phase. At 99.8 °C, yet another variation is observed where the
sanded brokenꢀfan texture showed arcs for a short duration followed by
transformation to smooth fans signifying the phase change (Figure S2 F of Supporting
Information).44 The sample at 58.3 °C showed concentric lines which persisted till
32.5 °C and the phase is recognized as crystal E phase (Figure S2 G of Supporting
Information).45 The DSC (Figure S3 B of Supporting Information) cooling curve of
BDPTC homolog supports the phase changes noted in HOPM examination (Figure S2
DꢀG of Supporting Information) indicating the existence of polymesomorphism. The
ꢀH values (Table 1) associated with the phase transitions from isotropic to smectic A
and smectic C to smectic B are 1.71 and 0.75 k.cal/mole respectively while the
smectic A to smectic C transition is found to be very low (0.012 k.cal/mole). These
values are very much in agreement with the literature data of calamitic mesogens.46
Further, the occurrence of polymesomorphism in BDPTC homolog is confirmed by
VTꢀXRD investigation (Figure S4 AꢀD of Supporting Information). In XRD, the
existence of smectic A mesophase is established by observing a sharp reflection in
small angle region and broad hump in wide angle region. The broad hump in the wide
angle region indicates the absence of in plane order and liquid like nature of
molecules within the layer.47 The XRD measurements at 150 °C and 147 °C showed a
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sharp peak at small angle region with a layer spacing (d) of 34.369 Å (Figure S4 A of
Supporting Information). The molecular length of the BDPTC mesogen as calculated
from energy optimised molecular structure by DFT is 37.28 Å. The layer spacing (d)
measured in the range 142ꢀ120°C is 34.02 Åꢀ33.68 Å (Table S1 of Supporting
Information). Although variation in layer spacing is noticed in the range 142ꢀ120 °C,
the broad hump observed in the wide angle region is unaltered. As the layer spacing
decreased with lowering of temperature, the phase is assigned as smectic C in
concurrence with the HOPM and DSC results. Further, the XRD profile in the
temperature range 100ꢀ90 °C reveal increase in layer spacing (35.46 Åꢀ35.83 Å) and
also the broad hump seen at wide angle region for both smectic A and smectic C is
replaced by a sharp peak indicating layer in plane order (Figure S4 BꢀC of Supporting
Information).47,48 These features unambiguously support the presence of smectic B
phase and confirm the DSC data.
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For C6ꢀC12 homologs, on cooling the isotropic phase, either broken fan
texture alone (Figure 2B) or broken fans with schlieren texture (Figure S2 H, K and
M of Supporting Information) is noticed. The transition enthalpy of the phase change
for these homologs is 1.84ꢀ2.48 k.cal/mole as per DSC (Table 1). Thus based on the
high enthalpy values as well as the texture in HOPM, the phase is assigned as smectic
C. On further cooling of HDPTC and ODTPC samples, phase change is noticed at
98.1°C and 98.8°C respectively (Figure S2 I of Supporting Information and Figure
2CꢀD). The corresponding enthalpies of the transitions (Figure S3 C of Supporting
Information and Figure 3B) are found to be 0.69 and 0.75 k.cal/mole. Since the ꢀH
values are high for the transition, the XRD investigation is undertaken to confirm the
nature of the mesophase. The XRD profile of HDPTC homolog (Figure S5 AꢀD of
Supporting Information) in the temperature range 148ꢀ110 °C, shows sharp and
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intense reflection in the small angle region with a layer spacing of 34.36 Å and a
broad hump in the wide angle region (Figure S5 A of Supporting Information).
Interestingly, the d values remain constant in the temperature range 148ꢀ110°C
despite the phase being smectic C. This kind of behaviour i.e. constant layer spacing
with lowering of temperature is common feature for mesogens that exhibit isotropic to
direct smectic C transition (Table S2 of Supporting Information).49 At 98 °C, for
HDPTC homolog, the d value showed sudden increase (37.01 Å) and the wide angel
broad hump is replaced by sharp reflection. Thus increase in layer spacing as well as
layer in plane order establishes that the phase under observation is smectic B (Figure
S5 BꢀC of Supporting Information). For ODPTC homolog also, similar XRD profile
(Figure 4B) is noticed where the layer spacing (35.08 Å) is constant in the
temperature range 145ꢀ110°C (Table S3 of Supporting Information). At 98 °C, similar
to HDPTC homolog, the wide angle profile exhibited sharp peak confirming the
presence of smectic B phase (Figure S6 AꢀB of Supporting Information). On further
cooling, in HOPM, the HDPTC homolog exhibited a phase change at 47.7 °C for
which the enthalpy values is 2.15 k.cal/mole (Table 1). The XRD pattern showed
three orders of reflection in the small angle region which are assigned 001, 002, 003
and wide angle region showed multiple reflections with varying intensity (Figure S5
D of Supporting Information). The phase has been assigned as Crystal E (Figure S2 J
of Supporting Information) by considering the wide angle pattern of XRD.45,50ꢀ52 In
the case of ODPTC homolog, on cooling the smectic B phase, crystallisation is
observed with an enthalpy value of 7.68 k.cal/mole (Table 1). Thus it is concluded
based on the HOPM, DSC and XRD studies that the HDPTC and ODPTC homologs
exhibits enantiotropic smectic C and smectic B phases.
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In the case of DDPTC and DDDPTC mesogens (Figure S2 KꢀN of Supporting
Information), upon cooling the isotropic phase, broken fan or schlieren (Figure S2 K
of Supporting Information) or schlierenꢀmosaic texture of smectic C phase is noticed
(Figure S2 M of Supporting Information). On further cooling, smectic C to higher
order smectic phase change is observed (Figure S2 L and N of Supporting
Information) for DDPTC and DDDPTC respectively with transition enthalpies of 0.38
and 0.43 k.cal/mole (Figure S3 DꢀE of Supporting Information). In order to confirm
the low temperature mesophases of DDDPTC homolog, the XRD investigation is
undertaken (Figure S7 AꢀC of Supporting Information). The XRD profile of the
mesogen in the temperature range 145ꢀ110°C shows sharp reflection in the small
angle region and a broad hump in the wide angle region (Figure S7 A of Supporting
Information). The layer spacing in the measured temperature range is 37.80 Å (Table
S4 of Supporting Information). At 105°C, the layer spacing increases (38.23 Å) and
the wide angle region shows sharpened reflection in contrast to smectic C phase (145ꢀ
120°C). Interestingly, for both DDPTC and DDDPTC mesogens, the smectic C to
lower temperature mesophase is found to be monotropic from DSC studies. Based on
the transition enthalpy value and XRD profile in the wide angle region (Figure S7 Bꢀ
C of Supporting Information), the phase is assigned as smectic F for DDDPTC. Since
the transition enthalpy of low temperature smectic phase of DDPTC is comparable
with that of DDDPTC, smectic F is also assigned for the later (Table 1).48 It is to be
noted that for HDPTC and ODPTC homologs, the transition enthalpy values of
smectic C to higher order phase (smectic B) is high and also the wide angle reflection
is much sharper than the one observed for DDDPTC homolog.
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The EDTC (Figure 1A) which is the synthetic intermediate for the target
mesogens is also subjected to HOPM, DSC and XRD investigation to find mesophase
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properties as well as molecular organization. The sample on cooling the isotropic
phase (HOPM), shows focal conic fan texture with homeotropic domains (Figure
2A).43 The DSC measurement (Figure 3A) confirms enantiotropic nature of the
mesophase and the transition temperatures from crystal to smectic A is 56.9°C and
smectic A to isotropic temperature is 71.3°C. The transitions enthalpies for these
pahse changes are 8.23 and 1.65 kcal/mole. The XRD profile of the mesogen
measured at 70°C shows sharp reflection in small angle region and a broad hump at
wide angle region (Figure 4A). These features are characteristic of layer ordering
observed for smectic phases.47,52 In order to find the nature of the smectic phase, the
d/L values are determined where L represents the molecular length calculated from
energy optimised structure from DFT method. Table S5 of Supporting Information
lists the d/L values in the temperature range 50ꢀ70 °C which is ~1 and is constant
across the temperature range suggesting the monolayer smectic A phase (Table S5 of
Supporting Information).
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13C NMR Investigations
A detailed investigation of 13C NMR of EDTC and ODPTC in liquid
crystalline phases are undertaken to find the orientational order of phenyl and
thiophene rings. Additionally, the chemical shift assignment of aligned spectrum of
ODPTC is carried out by making use of EDTC NMR data which exhibits smectic A
phase. Prior to the the spectral assignment of aligned molecules, the solution 13C
NMR spectra of EDTC and ODPTC is first recorded. The proton decoupled 13C NMR
of EDTC is shown in Figure 5A. The assignments of the lines are carried out by
utilizing the spectrum generated from Chemsketch (version 3.0) software. Table 2
lists the assigned carbons of core region of EDTC. The spectrum in the range 110ꢀ165
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ppm shows 9 sharp lines accounting for thiophene ring, phenyl ring, and ester
carbonyl carbons. The identification of phenyl ring methine carbons is relatively easy
owing to higher intensity in contrast to other carbons of the core. The thiophene
carbons showed lower intensity than phenyl ring methine carbons while the
quaternary carbons of both the rings and ester carbonyl carbon showed lowest
intensity. For the terminal chain carbons, the lines appeared in the range 60ꢀ70 ppm
are assigned to OCH2 carbons of alkoxy chains while the terminal two methyl carbons
showed lines at 14 ppm. The other methylene carbons of the terminal chains are
noticed in the range 20ꢀ35 ppm. Thus the well resolved spectral lines of EDTC are
very much in agreement with the molecular structure (Figure 1A). Figure 5B shows
the proton decoupled 13C NMR of ODPTC. For clarity the spectrum of EDTC and
ODPTC are shown in Figure 5 with line assignment of core unit carbons. The ODPTC
spectrum shows 13 lines in the range 110ꢀ165 ppm accounting for two phenyl rings,
thiophene ring and ester carbonyl carbon of core unit. The terminal chains showed
lines in the range 14ꢀ70 ppm. The OCH2 carbons show peaks at 65 ppm while methyl
showed at 14 ppm. The rest of methylene carbons are seen in the range 20ꢀ35 ppm.
Table 3 list the chemical shift values of core unit carbons. Further it is clear from
Figure 5 that the carbon lines of core unit of EDTC and ODPTC are very much
similar since they have structural resemblance (Figure 1A, 1C).
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The static C NMR spectra of EDTC and ODPTC in liquid crystalline phase
are shown in Figure 6 and Figure S8 A of Supporting Information. For EDTC, the
spectrum is recorded in smectic A phase at 66 °C while for ODPTC, the measurement
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is carried out in smectic C phase at 130 °C. The static aligned C NMR spectrum of
EDTC (Figure 6A) shows very well resolved peaks both for the core unit as well as
terminal chains. The core unit carbons exhibited peaks in the range 140ꢀ230 ppm
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while for terminal chain carbons the signals are noticed in the range 15ꢀ65 ppm. It is
relatively easy to identify peaks arising from phenyl as well as thiophene methine
carbons based on intensity. Thus two peaks with high intensity are attributed to
phenyl ring methine carbons while the identification of thiophene methine carbons is
attempted using 2DꢀSLF spectrum which is discussed in subsequent section. The rest
of the quaternary carbons are identified by making use of structurally similar
mesogens reported recently.23 Figure 6A shows the spectrum with line assignments of
carbons of the core unit while Table 2 lists the chemical shifts as well as alignment
induced chemical shifts (AIS). Figure 6B shows the static 13C NMR spectrum of
ODPTC measured in smectic C phase at 130 °C. The spectrum shows resolved peaks
in the range 140ꢀ230 ppm mainly from core unit carbons. The terminal chain carbons
showed peaks in the range 10ꢀ60 ppm. Among the core unit carbons, four peaks are
more intense and are attributed to phenyl rings while the intensity of other carbons is
comparable. As a consequence, the identification of thiophene methine carbons is
found to be difficult. Hence, the 2D SAMPIꢀ4 spectrum is utilized for the location of
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thiophene carbons. Since EDTC is a synthetic intermediate of ODPTC, the static C
NMR spectrum of EDTC is also used for the assignment of core unit carbons. It is
unambiguous from Figure 6B that identification of core unit carbons can be easily
achieved by comparing the EDTC spectrum (Figure 6A). It is to be noted that even
though EDTC and ODPTC show different liquid crystalline phases with dissimilar
transition temperatures, the static 13C NMR data of EDTC is very useful for the
assignment of ODPTC carbons (Table 3). Despite variation in chemical shift values of
core unit carbons of both the mesogens, the spectral assignment of ODPTC could be
carried out since the sequence of appearance of peaks is similar for both the
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molecules (Figure 6). Further, the 2D data provided the additional source of
confirmation for the core unit carbons of ODPTC.
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In addition to static 13C NMR experiments, the 2D SAMPIꢀ4 is also used both
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for EDTC and ODPTC mesogens for determining the Cꢀ1H dipolar couplings which
are used for confirmation of structural assignment attempted by 1D data and find the
orientational order parameter. The 2D SAMPIꢀ4 spectrum of EDTC is shown in
Figure 7A. Similar to 1D static spectrum, 9 contours are seen in the range 145ꢀ230
ppm for the core unit carbons. The remarkable feature of the 2D spectrum is clear
separation of thiophene methine carbons from rest of the core unit carbons.
Accordingly, the Cꢀ1H dipolar couplings of methine carbons of thiophene are noted
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in the range ~4ꢀ5 kHz whereas the phenyl methine dipolar couplings are observed ~3
kHz. The dipolar couplings of quaternary carbons of the core unit are seen in the
range ~0.75ꢀ2 kHz. The clear separation of thiophene methine carbons from rest of
the carbons is mainly due to the high 13Cꢀ1H dipolar couplings which suggest that the
phenyl as well as thiophene rings are not experiencing similar orientation with respect
to long axis. For the terminal chain carbons also, better separation of contours is seen
in contrast to 1D static spectrum due to variation in 13Cꢀ1H dipolar couplings of
metheylene carbons across the chain. The 2D SAMPIꢀ4 spectrum of ODPTC is shown
in Figure 7B and Figure S8 B of Supporting Information. Similar to 1D static feature,
the 2D spectrum also shows 13 contours for all the core unit carbons. Quite
interestingly, the thiophene carbons are not showing large variation in contrast to
phenyl rings as noted for EDTC. Nevertheless, the identification of thiophene methine
carbons in contrast to 1D static 13C spectrum is easy since the 2D data for structurally
similar mesogen is available in literature.23 Table 3 shows the 13Cꢀ1H dipolar
couplings of all the resolved carbons which are used for computing the orientational
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order parameter. For phenyl rings, the Cꢀ1H dipolar oscillation frequencies can be
extracted from the Cꢀ1H dipolar coupling obtained from 2D SAMPIꢀ4 experiment
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by following the established approach.23ꢀ26,32ꢀ35,53ꢀ55 A similar strategy extended for
thiophene ring and the final dipolar frequencies for C6 and C7 carbons can be
expressed as[(DC6-H6)2+ (DC6-H7)2]1/2and [(DC7-H7)2+(DC7-H6)2]1/2, respectively.
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Similarly, for C5 and C8 carbons the dipolar frequencies would be [(DC5-H6)2+2(DC5-
2 1/2
) ] and (DC8-H7), respectively. Now, these dipolar frequencies are used for
H3
computing the orientational order parameter by using following equation 1,56ꢀ59
DCH =K [½ (3 cos2θzꢀ1) Szz+ ½ (cos2θx ꢀ cos2θy) (SxxꢀSyy)]
(1)
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where K=ꢀhγHγC/4π2r3CH, with γH and γC are the gyromagnetic ratios of H and 13C
nuclei respectively, rCH is the interꢀnuclear vector, θx, θy and θz are angles between rCH
vector and the corresponding molecular axes. For the phenyl rings, the molecular
frame is defined by considering paraꢀaxis (C2 axis) as the zꢀaxis, the xꢀaxis is in the
plane of the ring while the yꢀaxis is perpendicular to the plane. Standard bond
distances, rCH= 1.1 Å for the C–H bond and rCC= 1.4 Å for the C–C bond, are taken.
The two C–C–H bond angles are also slightly varied around 120˚ to get the best fit.60
In arriving at the order parameters of thiophene ring, a model suggested earlier is used
(Figure S9 of Supporting Information).23,61 In view of irregular pentagonal geometry
of thiophene moiety, three order parameters namely, Szz, (Sxx–Syy) and Sxz are
required for an arbitrary choice of the axis system.62 Here the zꢀaxis is chosen along
the C4ꢀC5 bond by which thiophene is directly connected to the phenyl ring I (Figure
1C).23 The bond angles and bond distances computed from the energy minimized
structure of thiophene for EDTC and ODPTC mesogens are utilized (Figure 1B, 1D).
Initially, only two order parameter namely, Szz and (Sxx–Syy) taken for fitting with an
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assumption that the other order parameter is likely to be small. The quality of the fit is
improved by introducing an angle β between the zꢀaxis and the C4ꢀC5 bond. Moreover
the efficiency of the fit is enhanced by varying the two CCH bond angles by ±2
degree. Further the addition of third order parameter namely Sxz did not improve the
fit appreciably. Hence, the Szz and (SxxꢀSyy) order parameters for thiophene as well as
phenyl rings of EDTC and ODPTC are considered by employing the equation 1 and
are listed in Tables 4ꢀ7.
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In both the cases i.e. EDTC and ODPTC, the order parameters of thiophene
and phenyl rings are dissimilar. This feature indicates that with respect to the long
axis, the phenyl and thiophene rings are experiencing different orientation. This is
mainly due to the change in the geometry of phenyl and thiophene rings as already
discussed in the earlier work.24 An examination of order parameters of phenyl ring II
and thiophene provides information about the tilt angle between the long axis versus
local axis of the side phenyl ring. Accordingly, the values are found to be 19.4°, 18.8°
at 95 °C (smectic B) and 130 °C (smectic C) for ODPTC. It is well known that 2, 5ꢀ
substitution on thiophene will results in bentꢀcore shape with typical bond angle of ~
147° which is in close agreement with the tilt angles noticed for ODPTC.17
A plot of temperature versus AIS of EDTC is shown in Figure S10 A of
Supporting Information. With increase in temperature, the AIS values showed
decreasing trend. Since all the measurements are carried out in smectic A phase, a
smooth decrease is noticed. The similar plot for ODPTC is also shown in Figure S10
B of Supporting Information, where like EDTC with increase in temperature decrease
in AIS is observed. However, an abrupt change in AIS values is noticed at 98 °C for
ODPTC indicating phase transition from smectic B to smectic C phase. The DSC data
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confirms the first order nature of the transition and accordingly clear change in AIS
values in NMR data is also observed.
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The photophysical studies of all the mesogens in chloroform (2×10ꢀ6 M) are
carried out to find the absorption and photoluminescence properties (Figure S11 of
Supporting Information). Accordingly, the absorption band is noticed at 335 nm while
the fluorescence emission band is observed at 405 nm. Thus the fluorescence
emission of synthesised mesogens is an interesting feature which is mainly attributed
to the πꢀconjugation of the phenyl thiophene link of the core unit. It is to be noted that
the mesogens in which phenyl and thiophenes are separated by linking units; the
fluorescence emission is not commonly noticed. Also thiophene molecular materials
which are used for organic light emitting diodes are often constructed with phenyl and
thiophene rings directly to facilitate π overlap to enable to have photoluminescence in
solid state for the device fabrication.7
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CONCLUSIONS
Six homologs of (4ꢀalkoxyphenyl 5ꢀ(4ꢀ(dodecyloxy) phenyl) thiopheneꢀ2ꢀ
carboxylates) were synthesised from thiophene ꢀ2ꢀ carboxylic acid with direct
arylation using palladium acetate followed by esterification with 4ꢀalkoxy phenols.
The HOPM and DSC measurements indicated enantiotropic mesophases consisting of
nematic, smectic A, smectic C and smectic B phases depending on the terminal chain
length. Additionally, for some homologs, smectic F as well as crystal E phases were
noticed in monotropic fashion. The HOPM and DSC observations were confirmed by
powder Xꢀray diffraction where layer ordering typical of smectic mesophases were
clearly seen. For smectic A and C phases, the wide angle region showed broad hump
whereas for smectic B and F, the broad hump was replaced by sharp reflection
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indicating in plane order within the layers. The high resolution solid state 13C NMR of
ODPTC was carried out in smectic C phase and the 13Cꢀ1H dipolar couplings
measured from 2DꢀSLF experiments were employed for determining the order
parameter of phenyl and thiophene rings. The higher order for thiophene ring (0.85)
over phenyl (0.71) was attributed to its location at the centre and using the order
parameter ratios of phenyl and thiophene rings, the bentꢀcore nature of the mesogen
was established. Further, the 13C chemical shift assignments of octyl homolog was
carried out by making use of the 13C NMR data of synthetic intermediate (EDTC)
which exhibited smectic A phase. This approach facilitated the assignments of 13C
NMR peaks of the core unit of ODPTC relatively easy. Thus the investigation
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demonstrates the utility of C NMR data of mesomorphic synthetic intermediate for
the final assignment of target mesogens.
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AUTHOR INFORMATION
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Corresponding Author
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*Eꢀmail:tnswamy99@hotmail.com
ACKNOWLEDGEMENTS
The authors would like to thank Dr. A. B. Mandal, Outstanding Scientist, CLRI and
Prof. K. V. Ramanathan, NMR Research Centre, Indian Institute of Science,
Bangalore, India for their support and keen interest in this project. We are grateful to
Prof. V. A. Raghunathan and Ms. K. N. Vasudha, Raman Research Institute,
Bangalore, India for the powder Xꢀray measurements. We acknowledge the partial
financial support from NWPꢀ55 while B. Veeraprakash 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 of intermediates, powder XRD data of
EDTC, BDPTC, HDPTC, ODPTC, DDDPTC mesogens, Figure of SAMPIꢀ4 pulse
sequence, HOPM photographs, DSC plots, VTꢀXRD plots of BDPTC, HDPTC,
ODPTC, DDDPTC mesogens, plot of AIS as a function of temperature for EDTC and
ODPTC mesogens, 1D 13C NMR and 2D SAMPIꢀ4 spectra of ODPTC mesogen at 95
°C in smectic B phase, thiophene model, UVꢀvisible spectra and Fluorescence
emission plots. This material is available free of charge via the Internet at
http://pubs.acs.org
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