13C-1H dipolar couplings for probing rod-like hydrogen bonded mesogens

Article abstract of DOI:10.1039/c3nj00271c

Hydrogen bonding is the most important non-covalent interaction utilised in building supramolecular assemblies and is preferred often as a means of construction of molecular, oligomeric as well as polymeric materials that show liquid crystalline properties. In this work, a pyridine based nematogenic acceptor has been synthesized and mixed with non-mesogenic 4-methoxy benzoic acid to get a hydrogen bonded mesogen. The existence of hydrogen bonding between the pyridyl unit and the carboxylic acid was established using FT-IR spectroscopy from the observation of characteristic stretching vibrations of unionized type at 2425 and 1927 cm^-1. The mesogenic acceptor and the complex have been investigated using ¹³C NMR in solution, solid and liquid crystalline states. Together with the 2D separated local field NMR experiments, the studies confirm the molecular structure in the mesophase and yield the local orientational order parameters. It is observed that the insertion of 4-methoxy benzoic acid not only enhances the mesophase stability but also induces a smectic phase due to an increase in the core length of the hydrogen bonded mesogen.

Full text of DOI:10.1039/c3nj00271c

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DOI: 10.1039/C3NJ00271C
Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
13C-1H Dipolar couplings for probing rod-like hydrogen bonded
mesogens
5
M. Kesava Reddya, K. Subramanyam Reddya, T. Narasimhaswamyb, Bibhuti B. Dasc, Nitin P. Lobod# and
K.V. Ramanathan*e
Received (in XXX, XXX) XthXXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
10 Hydrogen bonding is the most important nonꢀcovalent interaction utilised in building supramolecular
assemblies and is preferred often as a means of construction of molecular, oligomeric as well as
polymeric materials that show liquid crystalline property. In this work, a pyridine based nematogenic
acceptor has been synthesized and mixed with nonꢀmesogenic 4ꢀmethoxy benzoic acid to get a hydrogen
bonded mesogen. The existence of hydrogen bonding between the pyridyl unit and the carboxylic acid
15 was established using FTꢀIR spectroscopy from the observation of characteristic stretching vibrations of
the unionized type at 2425 and 1927 cmꢀ1. The mesogenic acceptor and the complex have been
investigated using 13C NMR in solution, solid and liquid crystalline states. Together with the 2D
separated local field NMR experiments, the studies confirm the molecular structure in the mesophase and
yield the local orientational order parameters. It is observed that the insertion of 4ꢀmethoxy benzoic acid
20 not only enhances the mesophase stability but also induces a smectic phase due to increase in the core
length for the hydrogen bonded mesogen.
45 hence the mesophase property. Accordingly, molecular,
Introduction
macromolecular and supramolecular hydrogen bonded mesogens
have been built with an excellent control on topology as well as
on mesophase morphologies.13,14Thus there is considerable
interest on the synthesis and the study of mesomorphic properties
50 of hydrogen bonded liquid crystals.
The properties of thermotropic liquid crystals in their mesophases
depend on the nature of their constituent units namely, the central
25 core which is fairly rigid and the flexible terminal chains. The
core is constructed with either a linear or a nonꢀlinear chain of
aromatic moieties connected by suitable linking units while either
alkyl or alkoxy chains form the terminal units.1ꢀ3 Depending upon
the composition of the core, the intermolecular interactions differ
30 resulting in a wide variety of mesophase morphologies.4,5 For the
construction of such mesogens, covalent linkages are usually
preferred in view of their better stability.1ꢀ5 By adopting this
approach, a variety of mesogens with different molecular
topologies have been synthesized and their mesophase properties
35 characterized.6ꢀ8 In recent years however, attention is also being
paid to construction of mesogens involving nonꢀcovalent
interactions. Among them, molecular designs that use hydrogen
bonding have achieved a preeminent position.9ꢀ12In this strategy,
the intermediates are usually constructed with covalent bonds and
40 are classified as either donor or acceptor depending upon the kind
of terminal functionality.9,10 These are then allowed to interact
through hydrogen bonding to produce new molecular systems.
The Hꢀbonded systems have the advantages of ease of
construction and the possibility of modulating the interaction and
The construction of materials with hydrogen bonding begins with
the selection of a suitable donor and an acceptor. The mesogenic
property can be introduced either in acceptor or donor moiety and
literature enlists both kinds of systems.9ꢀ14For the purpose having
55 a better control on the mesophase property, efforts in building
hydrogen bonded mesogens have been directed towards the
development of materials in which only one component is
mesogenic and the other is nonꢀmesogenic. In order to achieve
liquid crystalline property in such mesogenic acceptor and nonꢀ
60 mesogenic donor combinations, proper fine tuning of the
molecular structure of the acceptor such as the length of the core
is essential. In this scenario, the pyridine based acceptors have
been generally favored and most often examined as mesogenic
moieties with benzoic acids as donors.15,16One of the means
65 obtaining detailed atomic level information of mesogenic
molecules is the use of solid state NMR. However, only limited
number of studies that utilize this technique to examine hydrogen
bonded mesogens could be found in literature.17ꢀ19We present
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here synthesis of a novel hydrogen bonded liquid crystalline
4ꢀhexyloxy methyl benzoate (9.45 g, 0.04 mol) was placed in 500
ml single neck round bottom flask equipped with double wall
60 water condenser. Ethanol (150 ml) and potassium hydroxide (6 g,
0.1mol) dissolved in distilled water (150 ml) were added to the
flask. The solution was refluxed for two hours and allowed to
cool to room temperature and neutralized with 10% hydrochloric
acid to get a white precipitate.26The compound was purified by
system and a detailed investigation carried out using 13C NMR
and other techniques.
In recent years, high resolution solid state 13C NMR studies have
been used to obtain extensive atomic level information of several
mesogenic systems. Both 13C chemical shifts and 13Cꢀ1H dipolar
couplings have been utilized for this purpose.20ꢀ22The molecular
5
order and relative orientations of different segments of the 65 recrystallizing from methanol.
mesogens have been extracted employing this approach. In the
4-hexyloxy benzoic acid (2)
°
10 present study, the molecular structure of a mesogenic acceptor
and its hydrogen bonded mesogenic complex in solution, solid
state and liquid crystalline phases have been examined using this
Yield: 84.7%, m.pꢀ112 C, FTꢀIR (KBr, cmꢀ1): 2941, 2867 (Cꢀ
Hstr), 2549 (OꢀHstr.of carboxylic acid), 1683 (C=Ostr of carboxylic
acid), 1604, 1511 (C=Cstr aromatic), 1467, 1426 (CꢀHben) 1297,
approach. The mesogenic acceptor is constructed with three 70 1253, 1166, 1121, (CꢀOꢀC
of ether); 1HꢀNMR ppm
asym&symstr
aromatic rings as the core with pyridine as one of the components
15 while 4ꢀmethoxy benzoic acid is employed as the nonꢀmesogenic
donor. Along with 13C NMR, studies using hot stage optical
microscopy (HOPM), differential scanning calorimetry (DSC)
(CDCl3): δ8.06 (d, 2H), 6.93 (d, 2H), 4.01 (t, 2H), 1.78 (m, 2H),
1.46 (m, 2H), 1.33 (t, 4H), 0.90 (t, 3H); 13CꢀNMR ppm (CDCl3):
δ172.38, 163.79, 132.43, 121.48, 114.26, 68.37, 31.64, 29.15,
25.76, 22.69 and 14.13.
and FTꢀIR spectroscopy have been carried out to investigate both 75 Synthesis of 4-aminophenyl 4-hexyloxybenzoate (3,4)
the acceptor and the complex. Studies of structurally simple
20 hydrogen bonded mesogens like the one presented here could be
expected to provide knowledge that can be used for
understanding more complex systems that have been reported in
literature in recent years.23ꢀ25
Synthesis of 4-nitrophenyl 4-(hexyloxy) benzoate (3)
In a representative experiment, 4ꢀhexyloxy benzoic acid (5 g,
0.02 mol) and 4ꢀnitro phenol (2.78 g, 0.02 mol) were placed in a
conical flask. To this, dichloromethane (100 ml) was added and
80 the solution was stirred at room temperature with magnetic
stirrer. 4ꢀDimethylamino pyridine (0.24 g, 0.002 mol) was added
as a catalyst to the solution. After 10 minutes, DCC (5.15 g,
0.025 mol) dissolved in dichloromethane was added to the flask
27 and the solution was allowed to stir for 12 hrs. The precipitated
85 N,N΄ꢀdicyclohexyl urea was filtered off and washed with excess
of dichloromethane (100 ml). The combined organic solution was
taken into separating funnel and then washed twice with 5%
KOH solution followed by distilled water. The yellow solid
obtained was purified by recrystallization from isopropyl alcohol.
90 4-nitrophenyl 4-(hexyloxy) benzoate (3)
Experimental Section
25 Materials
4ꢀpyridine
carboxaldehyde,
nꢀbromo
hexane,
N,N′ꢀ
dicyclohexylcarbodiimide (DCC), 4ꢀdimethylamino pyridine
(DMAP) and 4ꢀmethoxy benzoic acid were purchased from
Aldrich (USA) and used without further purification. Stannous
30 chloride
(SnCl2),
N,
N′ꢀdimethylformamide
(DMF),
tetrahydrofuran (THF), triethylamine, ethanol and methanol (SD
Fine Chemicals, Mumbai) were used as received. 4ꢀhydroxy
methyl benzoate, dichloromethane, ethyl acetate, diethyl ether, nꢀ
hexane, nꢀheptane, acetone, ethyl methyl ketone (EMK) and
35 isopropanolwere obtained from Merck (India) and used as
received. 4ꢀhexyloxy benzoic acid was synthesized in laboratory.
Preparation of 4-{[(E)-pyridin-4-ylmethylidene] amino}
phenyl 4-hexyloxybenzoate (PMAPH):
Yield: 72.4%, m.pꢀ68 ºC, FTꢀIR (KBr, cmꢀ1): 2934, 2865 (CꢀHstr),
1742 (C=Ostr), 1605 and 1516 (C=Cstr aromatic), 1487, 1319 (Cꢀ
Hben), 1343 (ꢀNO2str), 1255, 1164 (CꢀOꢀC
of ester and
asym&symstr
ether ); 1HꢀNMR ppm (CDCl3): δ8.28 (d, 2H), 8.12 (d, 2H), 7.39
95 (d, 2H), 6.98 (d, 2H), 4.03 (t, 2H), 1.86 (m, 2H), 1.47 (m, 2H),
1.34 (m, 4H), 0.90 (t, 3H); 13CꢀNMR ppm (CDCl3): δ164.14,
164.03, 156.06, 145.26, 132.59, 125.27, 122.76, 120.40, 114.59,
68.53, 31.63, 29.12, 25.74, 22.69 and 14.13.
Synthesis of 4-hexyloxybenzoic acid (1,2)
40 Synthesis of 4-hexyloxy methyl benzoate (1)
Reduction of 4-nitrophenyl 4-(hexyloxy) benzoate (4)
In a representative experiment, 4ꢀhydroxy methyl benzoate (7.6
g, 0.05 mol) was placed in a 250 ml three necked round bottom
flask equipped with stirrer and thermometer. To that, DMF (100
ml) and potassium carbonate (10.36 g, 0.075 mol) were added.
45 The resulting mixture was stirred while maintaining the
temperature at 90°C, then nꢀbromohexane (7.04 ml, 0.05 mol)
was added through a pressure equalizing dropping funnel over a
period of 30 minutes and the stirring was continued for about 4
hours and then the reaction mixture was allowed to cool to room
50 temperature, poured into a two liter beaker.26 The contents were
diluted with water (150 ml) and then transferred to 250 ml
separating funnel and diethyl ether was added. The ether layer
collected was washed twice using 10% potassium hydroxide
solution and followed by distilled water. Then the organic layer
55 was dried over anhydrous sodium sulphate and upon evaporation
of ether resulted in liquid 4ꢀhexyloxy methyl benzoate.
100 In this experiment, 4ꢀnitrophenyl 4ꢀ(hexyloxy) benzoate (3.21 g,
0.00936 mol) and SnCl2 (10.53 g, 0.046 mol) were placed in a
single necked round bottom flask along with ethanol and refluxed
for two hrs.28 Then it was cooled to room temperature,
neutralized with 10% NaHCO3 solution then filtered and solid
105 obtained was dried in vacuum oven. The solid was treated with
100 ml EMK for 1 hr and the solvent was evaporated to get light
brown solid which was recrystallized from heptane.
4-aminophenyl 4-hexyloxybenzoate (4)
Yield: 76.1%, m.pꢀ91 °C, FTꢀIR (KBr, cmꢀ1): 3407, 3329
110 (NH2str), 3212 (aromatic CꢀHstr), 2934, 2861 (CꢀHstr), 1721
(C=Ostr), 1607 , 1512 (C=Cstr aromatic), 1467, 1420, 1395 and
1315 (CꢀHben), 1261, 1196 , 1166 (CꢀOꢀC
of ester and
asym&symstr
ether); 1HꢀNMR ppm (CDCl3): δ8.13 (d, 2H), 6.96 (m, 4H), 6.69
(d, 2H), 4.03 (t, 2H), 3.54 (s, 2H), 1.81 (m, 2H), 1.47 (m, 2H),
115 1.35 (m, 4H), 0.92 (t, 3H); 13CꢀNMR ppm (CDCl3): δ165.68,
Hydrolysis of 4-hexyloxy methyl benzoate (2)
2
|Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C3NJ00271C
163.47, 144.31, 143.21, 132.28, 122.46, 121.88, 115.80, 114.31,
68.38, 31.66, 29.17, 25.77, 22.72 and 14.18.
Synthesis of 4-{[(E)-pyridin-4-ylmethylidene] amino} phenyl
4-hexyloxybenzoate (PMAPH) (5)
Solid state NMR measurements
60 Solidꢀstate NMR experiments were carried out on the samples
using a BrukerAvanceꢀIII 500 WB NMR spectrometer. The
proton and carbon resonance frequencies were 500.17 and 125.79
MHz respectively. The spectra in the liquid crystalline phases of
the samples were obtained using a double resonance probe
5
In
a representative experiment, 4ꢀaminophenyl 4ꢀhexyloxy
benzoate (0.939 g, 0.003 mol) and 4ꢀpyridine carboxaldehyde
(0.32 g, 0.003 mol) in equimolar were taken in a 100 ml conical 65 equipped with a 5 mm solenoid coil for static samples using the
flask. Few drops of ethanol were added and the flask was placed
in a microwave oven (power: 40 W) for ten minutes. Then it was
HartmannꢀHahn crossꢀpolarization pulse sequence with a contact
time of 2ms following the 90° proton pulse of width of 4 ꢁs.
30
10 allowed to cool to room temperature, washed with methanol and
recrystallized twice from heptane.
SPINALꢀ64
decoupling was employed during carbon signal
acquisition using a proton decoupling power of 30 kHz. To avoid
Yield: 71.2 %, m.pꢀ105.0 °C, FTꢀIR (KBr, cmꢀ1): 3063 (aromatic 70 sample heating, the recycle delay between each FID was kept as
CꢀHstr), 2927, 2864 (CꢀHstr), 1726 (C=Ostr), 1603, 1507 (C=Cstr
aromatic), 1466, 1416, 1318 (CꢀHben), 1263, 1198, 1167 (CꢀOꢀC
8s. Each spectrum was obtained typically with 256 scans. For
measuring the 13Cꢀ1H dipolar couplings of the molecules in their
mesophase, the SAMPI4 pulse sequence (given in ESI) was
applied on the oriented sample under static conditions.31 Details
15
of ester and ether); 1HꢀNMR ppm (CDCl3): δ8.76 (d,
asym&symstr
2H), 8.48 (s, 1H), 8.15 (d, 2H), 7.77 (d, 2H), 7.30 (m, 4H), 6.97
(d, 2H), 4.03 (t, 2H), 1.83 (m, 2H), 1.47 (m, 2H), 1.35 (m, 4H), 75 of the application of the pulse sequence to liquid crystalline
0.91 (t, 3H); 13CꢀNMR ppm (CDCl3): δ164.96, 163.69,157.90,
150.62, 150.13, 148.39, 142.78, 132.34, 122.67, 122.29, 122.0,
20 121.39, 114.39, 114.39, 68.39, 31.57, 29.09, 25.68, 22.61 and
14.04.
systems have been described in earlier publications32,33and in
caption of Figure 1 in ESI.The method yields a 2D spectrum with
carbon chemical shifts along the direct dimension and the protonꢀ
carbon dipolar coupling frequencies along the indirect dimension.
Synthesis of hydrogen bonding complex:
80
[4-{[(E)-pyridin-4-ylmethylidene]
hexyloxybenzoate and 4-methoxy benzoic acid] (PMAPHMB)
25 (6)
In an experiment, 4ꢀ{[(E)ꢀpyridinꢀ4ꢀylmethylidene] amino}
amino}
phenyl
4-
Results and Discussion
FT-IR and Mesophase characterization
The molecular structures of the mesogenic acceptor PMAPH and
phenyl 4ꢀhexyloxybenzoate (1.3 g, 0.0032 mol) and 4ꢀmethoxy 85 the hydrogen bonded mesogen PMAPHMB are shown in Figure
benzoic acid (0.33 g, 0.0032 mol) were placed in 100 ml conical
flask kept on magnetic stirrer and THF (15 ml) was added.29 The
30 reaction mixture was stirred for about one hour and after that
poured into evaporating bowl and solvent was allowed to
1A&B. As described in the experimental section, the mesogenic
acceptor was synthesized from 4ꢀhexyloxy benzoyloxy aniline
34by reacting it with 4ꢀpyridine carboxaldehyde in ethanol
(Schemeꢀ1). Lee et al. reported the mesophase properties of some
evaporate slowly at room temperature. The solid obtained was 90 of the homologues of the mesogenic acceptor.16The hydrogen
recrystallized twice from heptane.
bonded complex is prepared by mixing equimolar ratio of
mesogenic pyridine and nonꢀmesogenic acid in THF (Schemeꢀ1).
The structural confirmation of the mesogenic acceptor and its
complex with 4ꢀmethoxy benzoic acid has been accomplished by
°
Yield: 69.3%, m.pꢀ101 C, FTꢀIR (KBr, cmꢀ1): 2948, 2865 (Cꢀ
35 Hstr), 2425 (OHstrH Bond) 1710 (C=Ostr), 1605, 1511 (C=Cstr
aromatic), 1471, 1417(CꢀHben), 1255, 1166, 1197 (CꢀOꢀC
1
of ester and ether); HꢀNMR ppm (CDCl3): δ8.79 (d, 95 FTꢀIR and solution NMR (1H and 13C) techniques. For the
asym&symstr
2H), 8.49 (s, 1H), 8.16 (d, 2H), 8.08 (d, 2H), 7.80 (d, 2H), 7.30
hydrogen bonded complex, in addition to 1D solution NMR, 2D
1
(m, 4H), 6.97 (m, 4H), 4.05 (t, 2H), 3.88 (s, 3H), 1.83 (m, 2H),
40 1.49 (m, 2H), 1.37 (m, 4H), 0.92 (t, 3H); 13CꢀNMR ppm (CDCl3):
δ164.97, 163.88, 163.70, 157.75, 150.30, 150.18, 148.32, 143.05,
experiments such as Hꢀ1H DQF COSY and 13Cꢀ1H HSQC were
also carried out for spectral assignment. The existence of
hydrogen bond in the mixture of acceptor and donor is
132.34, 132.28, 122.68, 122.44, 122.07, 122.02, 121.38, 114.39, 100 established by FTꢀ IR spectroscopy. The FTꢀIR spectra of 4ꢀ
113.73, 68.40, 55.48, 31.57, 29.09, 25.68, 22.61 and 14.03.
Instrumental details
methoxy benzoic acid, PMAPH and their hydrogen bonded
complexare shown inESI. The FTꢀIR spectrum of PMAPH
(ESI) shows the following characteristic absorption frequencies :
CꢀH stretching at 2927 and 2864 cmꢀ1, ester carbonyl stretching at
45 FTꢀIR spectra of all the compounds were recorded on an ABB
1
BOMEM MB3000 spectrometer using KBr pellets. H and 13C
NMR spectra of the compounds in CDCl3were run on a JEOL 105 1726 cmꢀ1, ring skeletal (C=C) vibrations at 1603 and 1507 cmꢀ1
500 MHz instrument at room temperature using tetramethylsilane
and CꢀOꢀC stretching at 1263 and 1167 cmꢀ1. For the case of 4ꢀ
methoxy benzoic acid, the O−H stretching frequencies of
carboxylic acid are seen at 2669 and 2557cmꢀ1 which are
considered to be due to Fermi resonances.35,36 The carbonyl
1
as an internal standard. The resonance frequencies of H and 13C
50 were 500.15 and 125.76 MHz respectively. The nature of the
mesophase and the temperature of occurrence of various phases
were determined with an Olympus BX50 polarizing optical 110 stretching absorption band of carboxyl acid (donor) is noticed at
microscope equipped with a Linkam THMS 600 stage with a
TMS 94 temperature controller. The photographs were taken
55 using an Olympus C7070 digital camera. Differential scanning
calorimetry (DSC) traces were recorded using a DSC Q200
1686 cmꢀ1 due to dimerization induced by intermolecular
hydrogen bonding. In the case of the mesogenic complex,
disappearance of the bands at 2669 and 2557 cmꢀ1 which are
noticed for 4ꢀmethoxy benzoic acid (ESI) and appearance of two
instrument with a heating rate of 10°C/minute in nitrogen 115 new bands at 2425 and 1927 cmꢀ1 (ESI) confirms the hydrogen
atmosphere.
bonding between acceptor and donor.37,38These two new bands
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are strong evidence of the intermolecular hydrogen bonding of
range 114ꢀ151 ppm. The interesting feature of the spectrum is
the unionized type between the pyridyl unit and the carboxylic 60 appearance of one of the phenyl methine carbons at 150.6 ppm
acid.37,38 Further, an increase in carbonyl stretching vibration of
acid component is clearly noticed. Thus, the C=O stretching of
carboxyl carbonyl of 4ꢀmethoxy acid is seen at 1686 cmꢀ1
whereas in the hydrogen bonded complex the value shifts and
due to the presence of nitrogen of pyridine ring adjacent to it. The
characteristic imine carbon is noticed at 157.9 ppm. The
quaternary carbons accounting for the entire core unit are well
resolved and the chemical shifts of all core unit carbons are listed
5
appears at 1704 cmꢀ1. The increase in C=O stretching of acid 65 in Table 1. The hexyloxy chain shows six sharp lines among
group is the result of the intermolecular hydrogen bonds.39
Further evidence for the formation of the complex is available
which OCH2 carbon is seen distinctly at 68.3 ppm. The other
methylene and terminal methyl carbon signals are noticed in the
range 22ꢀ32 ppm and 14.0 ppm respectively.
10 from the NMR studies of the mesophase.
The mesophase transition temperatures of the acceptor and the
Spectrum in the solid state
41
hydrogen bonded mesogen have been determined by HOPM and 70 13C CPꢀTOSS spectrum of the mesogen in the solid phase at
DSC methods. For PMAPH, DSC studies reveal a crystal to
nematic phase (CrꢀN) transition at 105.0°C and a transition from
15 the nematic to isotopic phase (NꢀI) at 152.1 °C with the transition
enthalpy values being 3.56 and 0.08K.cal/mole respectively for
room temperature is shown in Figure 3B. The spectrum shows
relatively broad peaks in contrast to solution spectrum. Also more
peaks compared to solution are observed in the solid state. In
solution, for the core unit carbons, 13 peaks are noticed whereas
the two transitions (ESI). In HOPM studies, the sample upon 75 in the solid state, 21 peaks are observed. The additional peaks are
cooling from isotropic phase showed birefringent droplets which
noticed for both the methine and the quaternary carbons of the
core unit. Similar observations have been made for other
mesogenic molecules in the solid state.20,32The reasons for the
appearance of multiple lines in the solid state are many. The
on further cooling transformed to
a homeotropic texture
20 40confirming the occurrence of the nematic phase. The hydrogen
bonded complex (PMAPHMB), on the other hand, showed
smectic A (SA) phase (Figure 2) in addition to nematic phase. 80 freezing of the flip motion about the paraꢀaxis of the benzene ring
°
Accordingly, in DSC, a CrꢀSA transition is noticed at 101.2 C.
SAꢀN transition occurs at 121.0 °C and the NꢀI transition is
has been observed to result in additional lines for the core unit
carbons.32 In this case, the degeneracy of the methine carbon
resonances in the three ring units of the core is removed and six
more carbon peaks are expected. Other reasons include the
°
25 observed at 200.7 C. The transition enthalpies are found to be
9.77, 0.01 and 0.63 K.cal/mole for the three transitions
respectively (ESI). In HOPM studies, the sampleon cooling the 85 presence of more than one molecule in the unit cell or the
isotropic phase, showed birefringent droplets which coalesced to
give homeotropy. On further cooling, NꢀSA phase transition is
30 noticed. Interestingly, the homeotropy persisted in the smectic A
phase also. However, some areas of the sample showed
presence multiple conformers. In the present case, additional
peaks are observed both in the region of the aromatic CH carbons
and in the region of the quaternary carbons indicating that the
peaks arise from molecules differing in their structure or
birefringent polygonal texture characteristic of smectic A phase.40 90 environment. Additional peaks are not seen in the hexyloxy chain
Thus, the HOPM and DSC data unambiguously support the
existence of mesophase both in the acceptor as well as in the
35 hydrogen bonded complex. Since 4ꢀmethoxy benzoic acid is nonꢀ
mesogenic, it showed a single phase transition corresponding to
region in the present case. This can be attributed to the relatively
smaller span of the chemical shift anisotropy of the aliphatic
carbons and the larger line width of the spectra in the solid state.
Spectrum in the mesophase
the CrꢀI transition at 185 ºC.40It is thus interesting to note that the 95 The mesogen exhibits an enantiotropic nematic phase in the
Hꢀbonded complex has more number of phases as well as wider
mesophasic temperature range, exhibiting better mesophase
40 stability.
range105.0ꢀ152.1 ºC. The 13C NMR spectrum of the mesogen was
acquired at several temperatures while heating the sample in the
magnetic field. Well resolved sharp intense peaks were noticed in
the mesophase. A typical spectrum obtained at 125 ºC is shown in
13C NMR Investigations of PMAPH
13C NMR studies of the mesogenic donor and its hydrogen 100 Figure 3C. As seen in the spectrum, the chemical shift values of
bonded complex were carried out in solution, solid phase and
nematic phase with a view to examine the molecular structure. In
45 nematicphase, in addition to static 1D experiments, 2D SLF
experiments are carried out to find the 13Cꢀ1H dipolar couplings
all the carbons in the mesophase are different compared to those
observed in the solution spectrum. For the assignment of the
spectrum in the mesophase, the following procedure was used.
The spectrum shows five sharp intense peaks in the region 135ꢀ
of the phenyl rings and imine units to determine the order 105 170 ppm. Based on their intensities and also from the dipolar
parameter of the core unit.
In solution state
splitting in the 2DꢀNMR spectrum discussed subsequently, these
peaks are assigned to the six nonꢀequivalent methine carbons in
the three methyl rings of the core unit of the mesogen. Among
them, the peak appearing at 148.6 ppm and showing the highest
50 The proton decoupled 13C NMR spectrum of PMAPH in CDCl3
recorded at room temperature is shown in Figure 3A. The
spectrum shows well resolved lines with varying intensities at 110 intensity corresponds to overlapped resonances of two phenyl
different chemical shift values for core as well as terminal chain
carbons. The individual assignment of signals was carried out by
55 iterating the spectrum by the ACD chemsketch software. For the
core unit carbons, the lines are observed in the range 114ꢀ165
ring methine carbons. For the assignment of chemical shifts of
the carbons of ringI, the similarity of ring I to alkoxy benzoic
acids has been utilized. The alkoxy benzoic acids are important
class of mesogens and are often used ascore components
ppm. The mesogen has two phenyl rings and one pyridine ring 115 forawide range of mesogens. Among them three mesogens which
and as a result six methine carbon signals are expected in the
share the 4ꢀalkoxy benzoic acid as a first ring have been
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considered for comparing the chemical shift trends in their
peaks at 170.6, 163.8, 132.3, 122.4 and 113.7 ppm in the
respective mesophases. For instance, in the case of 10B1M7 and 60 aromatic region and a peak corresponding to the methoxy carbon
HZL7 Dong et al.42 and Domenici et al.43 used decyloxy and
heptyloxy benzoic acids for the construction of respective
mesogens. Similarly, the hockeyꢀstick mesogen reported in
literature44is constructed with decyloxy benzoic acid. In all the
at 55.4 ppm are observed. Table 2 lists the chemical shift values
along with peak assignments. Thus, 18 lines seen in the aromatic
part of the spectrum account for both donor and acceptor in the
hydrogen bonded mesogen.
5
cases, the 13C NMR data is available in their mesophase. Though 65 Spectrum in the solid state
these mesogens exhibit smectic phases at different mesophase
temperatures, the pattern of peak appearance and its trend
10 particularly for the 4ꢀalkoxy benzoic acid unit, facilitated the
assignment of ringI carbons. In the case of the carbon of the
The 13C CPꢀTOSS NMR spectrum of the complex in the solid
state is shown in Figure 6B. As in the case of the spectrum of the
acceptor, the solid state spectrum shows, in comparison to the
spectrum in the solution state, several additional peaks in the
azomethine connecting unit, the peak is easily identified at 195.8 70 aromatic range of 117ꢀ169 ppm as well as in the aliphatic range
ppm from the large dipolar splitting observed in the 2D NMR
spectrum.For the rest of the aromatic carbons, the trend observed
15 in the solution NMR spectrum has been utilized and the peaks
assigned. In addition, the 13C spectra have also been recorded as a
of 10ꢀ34 ppm. The spectral features corresponding to the donor
and acceptor moieties in the complex are easily distinguished.
Thus, the characteristic methoxy carbon at 55.9 ppm and the ester
carbonyl at 169.4 ppm indicate the presence of 4ꢀmethoxy
function of temperature and the chemical shifts are shown as a 75 benzoic acid unit in the hydrogen bonded mesogen. Many of the
fuction of temperature in Figure 4A. The figure shows nearly
uniform variation of the aromatic carbon chemical shifts as a
20 function of temperature. For the terminal hexyloxy chain, similar
to solution spectrum, six peaks are observed. The increase of
peaks in the solid state can be identified from comparison to
spectra in the solution phase. However, such assignments are
tentative in the absence of other experimental evidence and hence
are not listed.
chemical shift values of core unit carbons and marginal decrease 80 Spectrum in the mesophase
for hexyloxy chain carbons in contrast to solution spectrum is
indicative of the parallel alignment of the molecules in the
25 magnetic field.45,46 The assigned chemical shift values of all the
carbons of the core unit of the aligned spectrum are listed in
Table 1.
The hydrogen bonded complex (PMAPHMB) shows mesophase
º
in the range 101.2ꢀ200.7 C. It exhibits both smectic A and
º
nematic phases. The CrꢀSA transition is observed at 101.2 C
º
while SAꢀN transition is noticed at 121 C. The spectrum of the
85 static sample in the nematic phase was obtained over a range of
º
2D-SLF study in the mesophase
temperatures and the spectrum obtained atat 130 C is shown in
The 2DꢀSAMPI4 NMR spectrum of the mesogen recorded at 125
30 ºC is shown in Figure 5A. The well resolved contours in the 2D
spectrum provide both 13C chemical shifts as well as 13Cꢀ1H
Figure 6C. The spectral features correspond to a parallel
alignment of the molecule in the magnetic field. The key feature
of the spectrum is the appearance of eight sharp peaks in the
dipolar couplings. The 2D data is thus useful both for confirming 90 range 140ꢀ170 ppm in comparison to that of PMAPH where six
the 1D assignment and also for determining the orientational
order parameters. The spectrum in the region 132ꢀ200 ppm shows
35 7 intense contours arising from CH vectors of the core unit
among which 6 are assigned to CH carbons of phenyl as well as
peaks accounting for three rings in the core are only noticed. The
additional lines belong to the methoxy benzoic acid unit. These
lines are shifted towards the higher frequency range of the
spectrum in comparison to the spectral lines in the isotropic phase
pyridine rings. In 1D static spectrum, the peak appearing at 149.4 95 and with an alignment induced shift very similar to the carbons of
ppm was assigned to two carbons. In the 2D spectrum the
appearance of a correspondingly intense and broad contour is
40 indicative of two carbons with slightly different dipolar couplings
and confirms the 1D assignment. The azomethine carbon is seen
the phenyl rings of the acceptor. In addition, as discussed in the
subsequent section, the 13Cꢀ1H dipolar couplings for the methine
carbons of both the donor and acceptor moieties in the complex
are very similar. In a similar fashion, the methoxy carbon peak of
distinctly due to its attached proton resulting in a large value for 100 the donor is shifted towards the lower frequency range in the
the 13Cꢀ1H dipolar coupling. The quaternary carbons of the core
unit also showed contours due to long range coupling with
45 neighboring CH carbons. For the terminal hexyloxy chain, the 2D
spectrum showed five contours with varying 13Cꢀ1H dipolar
mesophase in a manner similar to the oxymethylene carbon of the
acceptor part. These observations are conclusive proof of the fact
that the benzoic acid unit is a part of the complex. The detailed
spectral assignment of PMAPHMB mesogen has been carried out
couplings. Table 1 lists the 13Cꢀ1H dipolar couplings of all the 105 in a manner similar to the one adopted for the case of the
resolved core unit carbons of the mesogen.
13C NMR Investigations of PMAPHMB
50 In solution state
spectrum of PMAPH. Among the peaks noticed for PMAPHMB
mesogen, the azomethine carbon is seen at 205.9 ppm while the
characteristic ester carbonyl is noticed at 214.7 ppm. Table 2 lists
the 13C chemical shift assignment of all the carbons of
The molecular structure of the mesogen along with carbon
numbering is shown in Figure 1B. The proton decoupled 13C 110 PMAPHMB along with alignment induced chemical shifts. In
NMR of hydrogen bonded mesogen was recorded at room
temperature in CDCl3. The spectrum (Figure 6A) shows well
55 resolved lines and the assignment of the signals was carried out
using the ACD chemsketch software and further confirmed by
this case also, the spectra were recorded at different temperatures
and the plot shown in Figure 4B, enables following the variation
of the 13C chemical shift as a function of temperature.
2D SLF Spectrum
1Hꢀ1H DQF and 13Cꢀ1H HSQC experiments (vide ESI). With the 115 The SAMPI4 pulse sequence was applied on the sample and the
incorporation of 4ꢀmethoxy benzoic acid, an additional set of five
2DꢀSLF spectrum in the nematic phase was obtained at 130ºC.
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The spectrum (Figure 5B) shows well resolved dipolar crossꢀ
peaks for all the carbons. A visual inspection of the crossꢀsections 60 knowledge of the chemical shift anisotropy (CSA) tensor values.
calculating the order parameters.42,51,52 However, this requires a
of the different carbons provides additional support for the
assignment of the 1D spectrum. For instance, eight well
separated contours for the aromatic CH vectors are distinctly
seen. Similarly, the azomethine CH showed contour at 205.9 ppm
Based on the similarity of ring I of both PMAPH and
PMAPHMBto the phenyl ring of 4ꢀhexyloxy benzoic acid
(HBA), we have utilized the CSA tensor values available for
HBA in literature32 and have determined the order parameters of
5
with high 13Cꢀ1H dipolar coupling value (5.14 kHz). On the other 65 ring I at different temperatures as follows. The alignment induced
hand, the quaternary carbons show very small separation. For the
terminal carbons, the characteristic OCH2 of hexyloxy chain and
chemical shift (AIS) which is the difference between the chemical
shift (δlc) in the liquid crystalline phase to that in the solution
phase (δsoln) is related to the components of the chemical shift
tensor δij and the local order parameters S’zz and (S’xx – S’yy) of
10 OCH3 of the 4ꢀmethoxybenzoic acid fragment showed distinctly
different dipolar coupling values making their identification
simpler. Table 2 lists the 13Cꢀ1H dipolar coupling values of all the 70 the phenyl ring according to,21b
core unit carbons of acceptor and donor units. The 2D data in
addition to providing confirmation of the spectral assignment can
15 also be used to obtain the local order parameters of the different
structural units. This is presented in the next section for the
aromatic core units of both the acceptor and the complex.
Orientational order parameters
δlc – δsoln = (2/3 ) S’zz [ P2 (cos
β
) (δ11 – δ22) + (1/2) (δ22 – δ33)] +
(1/3) (S’xx – S’yy) [ δ33 – (cos2
β
) δ22 – (sin2
β
) δ11 (2)
]
Here again, the zꢀaxis is the paraꢀaxis of the ring, the xꢀaxis is in
the plane of the ring and the yꢀaxis is perpendicular to the plane.
75 The angle
β is the angle between the component δ11 of the
chemical shift tensor and the zꢀaxis and is taken as 0° and 60° for
carbons in the para the ortho positions respectively. The obtained
order parameters for ring I are shown as a function of temperature
in Figure 7 for both PMAPH and PMAPHMB. It is noticed that
The 13C NMR data of PMAPH and PMAPHMB mesogens can
20 be used for calculating the orientational order parameters in their
nematic phases. The 2DꢀSAMPI4 experiment is an improved
version of the PISEMA experiment47 and from the dipolar 80 the values of order parameters obtained from using the dipolar
oscillation frequencies obtained from the experiment, the local
order parameters S’zz and S’xxꢀ S’yy of the phenyl rings can
25 bederived as follows.32,48In the case of the methine carbons of the
phenyl ring, the experimental dipolar frequency has two
couplings given in Table 3 compare well with the values obtained
using the chemical shifts. In addition, in Figure 7, it is observed
that at a temperature of 165 C, the order parameter for ring I of
°
PMAPHMB is 0.55. This compares favourably with the order
contributions namely a coupling to the attached proton DCꢀHi and 85 parameter of 0.59 obtained for PMAPH at the corresponding
also a coupling to the remote proton at the ortho position DCꢀHo
.
reduced temperature of 125 °C. It is also observed in the Figure 7
that the range of order parameter values of both the mesogens in
their nematic ranges are similar and typical of values observed
generally for nematogenic mesogens. In the smectic A phase,
The measured frequency is given by49 [(DCꢀHi)2+ (DCꢀHo)2]1/2. The
30 quaternary carbons do not have an attached proton, but have
couplings to two equivalent protons at the ortho positions. In this
case the dipolar frequency is √2*(DCꢀHo). The dipolar couplings 90 PMAPHMB exhibits larger values of the order parameters. The
are related to the local order parameters and are given by the
equation21b, 45
CꢀH =K {½ (3 cos2
(1)
slope of the plot in this phase is also marginally steeper than that
in the nematic phase.
,
35
D
θ
z-1) S’zz+ ½ (cos2θx - cos2θy )(S’xx – S’yy)}
The dipolar coupling measurements at the specified temperatures
provided the local order parameters of individual phenyl rings as
where K = ꢀ hγCγH/4π2r3CH in which γC and γH are gyromagnetic 95 listed in Table 3. The differences in the order parameters between
ratios of carbon and hydrogen nucleirespectively, rCꢀH is the
different phenyl rings in the same molecule can be attributed to
the rings being oriented at a small angle between them. The angle
between the paraꢀaxes of the rings can be calculated from the
values of S’zz for the different rings by making the uniaxial
distance between the nuclei C and H and θx y and z the angles
,
θ
θ
40 the CꢀH vector makes with the coordinate axes. Here, the z, x and
yꢀaxes are defined to be respectively the para axis of the phenyl
ring, an axis perpendicular to the z axis but in the plane of the 100 approximation and by assuming that the long molecular axis is
ring and the axis perpendicular to the plane of the ring.Using the
above expression, the local order parameters S’zz and S’xx – S’yy
45 can be calculated from the experimentally determined dipolar
frequencies for each of the phenyl rings. For calculating K in
nearly parallel to paraꢀaxis of the phenyl ring with the largest
order parameter. Thus values of angles between the para axes of
rings III and II and rings II and I for the case of PMAPH have
been obtained as 14.4° and 15.5° respectively. On the other hand,
eq.1, standard bond distances, namely 1.09 Å for the CꢀH bond 105 for PMAPHMB, the angle between the paraꢀaxes of adjacent
and 1.4 Å for the CꢀC bond have been assumed. For the CCH
bond angle, in view of the uncertainty in the position of the H
50 atom as determined by Xꢀrays, it has been found necessary to
vary the two CꢀCꢀH bond angles, which are found to deviate
rings starting from ring IV are 9.4°, 0° and 5.8°. The reduction in
the angle between the different rings in this case compared to
PMAPH can be interpreted as favoring a rod like molecular
structure with the increase in the length of the core unit.
slightly from the value of 120°expected for an ideal hexagonal 110 It is interesting to note that in the hydrogen bonded mesogen, the
geometry.48,50 Accordingly, in the fitting procedure, the two CꢀCꢀ
H bond angles have also been slightly varied around 120°. Table
55 3 lists the local order parameters thus obtained for each phenyl
ring ofboth the mesogens in their nematic phases. The variation
order parameter of the ring IV belonging to the benzoic acid
moiety attached to the rest of the molecule through hydrogen
bond and those of other phenyl rings connected between
themselves via covalent bonds exhibit very similar local order
of the chemical shifts shown as a function of temperature in 115 parameters. This is akin to the behavior observed for systems
Figure 4 for PMAPH and PMAPHMB can also be utilized for
with phenyl rings, all of which are connected by covalent
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bonds.20 This implies that in favorable cases, the hydrogen
bonded molecules can replace covalent bonded systems, the
Acknowledgements
MKR and TNS sincerely thank Dr. A. B. Mandal, Director,
advantages being ease of synthesis and the possibility of 60 CSIRꢀCLRI for his keen interest and constant support in this
modulating the hydrogen bond and hence the property of the
system.
In the present case, it is observed that the addition of the nonꢀ
mesogenic acid to the mesogenic acceptor enhanced the phase
work. The partial financial support from NWPꢀ23 is duly
acknowledged. The use of the AVꢀIIIꢀ500 Solid State NMR
spectrometer funded by the Department of Science and
Technology, New Delhi, at the NMR Research Centre, Indian
5
and thermal stabilities of the latter and also induced an 65 Institute of Science, Bangalore, is gratefully acknowledged.
enantiotropic smectic A phase. The increase in the mesophase
aDepartment of Chemistry, S.V.University, Tirupati 517 502, India. E-
mails:kesavareddy2000@gmail.com and ksreddy01@rediffmail.com
10 stability as well as the induction of smectic mesophase is
expected as a consequence of the increase in molecular length for
the hydrogen bonded mesogens and the presence of flexible
terminal chains at both ends enhancing the aspect ratio. The
interesting features observed in the present case with the
15 incorporation of nonꢀmesogenic donor indicate that the core
length of mesogenic acceptor and the anisotropic polarizability
have important role to play and that the presence of three phenyl
rings in the core unit of acceptor meets the optimum requirements
for the formation of the hydrogen bonded mesogenic material.
70 bPolymer Laboratory, CSIR-Central Leather Research Institute, Adyar,
Chennai 600 020, India. E-mail:tnswamy99@hotmail.com
cDepartment of Physics, Indian Institute of Science, Bangalore 560012,
India. E-mail:bibidasctc@gmail.com
75
d# Department of Physics, Indian Institute of Science, Bangalore 560012,
India. Current address – Scientist Fellow, Chemical Physics Laboratory,
CSIR-Central Leather Research Institute, Adyar, Chenai 600 020, India;
E-mail: nitinlobo@gmail.com
80 eNMR Research Centre, Indian Institute of Science, Bangalore 560012,
India.E-mail: kvr@sif.iisc.ernet.in
20 Conclusions
A three ring based pyridine containing mesogenic acceptor was
synthesized and mixed with non mesogenic 4ꢀmethoxy benzoic
acid to get a hydrogen bonded complex. The formation of
hydrogen bonding between acceptor and donor was confirmed by
25 FTꢀIR spectroscopy where characteristic bands typical of
hydrogen bonding of unionized type between pyridine and
carboxylic acid was established. The presence of mesophases in
the acceptor and the hydrogen bonded complex were evaluated
by HOPM and DSC. Accordingly, the mesogenic acceptor
30 showed a nematic phase whereas the hydrogen bonded complex
exhibited both smectic A and nematic phases. 13C NMR
investigations in solution, solid state and nematic phase were in
conformity with the expected molecular structure in different
phases. In the nematic phase, the alignment of the mesogens in
35 the magnetic field resulted in well resolved spectra. The spectral
features in solution, solid state and liquid crystalline state
indicated the presence of both acceptor and donor in the hydrogen
bonded complex. The 2D SLF experiment provided 13Cꢀ1H
dipolar couplings which were used for computing the
40 orientational order parameter. These values supported the rodꢀlike
nature for both acceptor and hydrogen bonded complex. The
appearance of the smectic A phase and enhancement in the
mesophase stability for the hydrogen bonded complex was on
expected lines and can be attributed to an increase in the
45 molecular length of the complex. The nearly identical order
parameters obtained for all the phenyl rings in the complex is
very similar to behavior observed for mesogens constructed only
with covalent bonds and indicates that either of the approaches
can be used. It may be emphasized that the Hꢀbonded mesogens
50 are relatively easier to synthesize and are also amenable to
modifications giving rise to possibilities of creating a variety of
different mesogenic systems. It may however be necessary that
the realization of such complexes by incorporating nonꢀ
mesogenic donor will require that the acceptor′s core length is
55 optimally designed.
† Electronic Supplementary Information (ESI) available: [FTꢀIR spectra,
solution 2D NMR data of PMAPHMB, SAMPI4 pulse sequence and DSC
85 data]. See DOI: 10.1039/b000000x/
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42 R.Y. Dong, J. Xu, J. Zhang and C. A.Veracini, Phys. Rev. E, 2005,72,
061701.
43 V. Domenici, C.A. Veracini, V. Novotna and R.Y. Dong,
ChemPhysChem., 2008, 9, 556.
44 B. Das, S. Grande, W. Weissflog, A. Eremin, M.W. Schroder, G.Pelzl,
S.Diele and H. Kresse, Liq. Cryst., 2003, 30, 529.
45 B.M. Fung, Prog. Nucl. Magn. Reson. Spectrosc., 2002, 41, 171.
46 K.V. Ramanathan and N. Sinha, in Current Developments in Solid
State NMR Spectroscopy.Vol VIII, Eds. N. Mueller; P.K., Madhu,
Springer, Wienheim 2003, 55, 45.
47 C.H. Wu, A. Ramamoorthy and S.J. Opella, J. Magn. Reson.A, 1994,
109, 270.
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1998.
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Z. Gan, J. Magn. Res., 2000, 143, 136.
50 B.M. Fung, J. Afzal, T.L. Foss and MꢀH Chau,J. Chem. Phys.,1986,
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70 51 A. Marini and R.Y. Dong, Phys. Rev. E, 2011, 83, 041712.
8
|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

New Journal of Chemistry
Page 10 of 21
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DOI: 10.1039/C3NJ00271C
55
60
5
Table 1 13C NMR data of PMAPH in solution and liquid crystalline
states.The chemical shifts have been referenced to tetramethylsilane at 0
ppm.
65
70
75
80
10
C.No
Solution
(ppm)
Liquid
Crystalline
state (ppm) at
125 ºC
219.8
AIS
(ppm)
13Cꢀ1H Dipolar
Oscillation
Frequencies*
(kHz) at 125 ºC
1
2
3
4
5
6
7
8
9
10
11
12
13
163.6
114.3
132.3
121.3
164.9
148.3
122.0
122.6
150.1
157.9
142.7
122.2
150.6
56.2
23.9
28.4
57.9
40.6
61.9
22.3
26.8
65.4
39.2
67.5
27.2
16.0
1.55
2.33
2.28
1.51
ꢀ
1.59
2.49
2.56
1.50
4.03
1.79
2.56
1.81
138.2
160.7
179.2
205.5
210.2
144.3
149.4
215.5
197.1
210.2
149.4
166.6
15
20
*For the dipolar oscillation frequencies, the errors are ±0.06 kHz for
protonated carbons and ±0.08 kHz for quaternary carbons
85
90
25
95
30
35
40
45
100
105
110
115
120
50
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 |9

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DOI: 10.1039/C3NJ00271C
50
5
Table 2 13C NMR data of PMAPHMB in solution and liquid crystalline
states.The chemical shifts have been referenced to tetramethylsilane at 0
ppm.
55
10
60
65
70
75
80
85
90
95
C. No
Solution
(ppm)
Liquid
AIS
(ppm)
13Cꢀ1H
Dipolar
Oscillation
Frequencies*
(kHz) at
Crystalline
state (ppm)
at 130°C
130°C
1
2
3
4
5
163.7
114.4
132.3
121.4
164.9
227.1
137.8
167.7
184.2
210.9
63.4
23.4
35.4
62.8
46.0
1.58
2.51
2.10
1.45
1.17
15
6
7
148.3
122.0
122.7
150.1
157.7
221.0
151.9
154.7
221.0
202.4
72.7
29.9
32.0
70.9
44.7
1.46
2.84
3.48
1.46
5.14
8
9
10
20
11
12
13
143.1
122.2
150.6
222.5
146.7
161.7
79.4
24.5
11.1
2.02
3.08
2.39
14
15
170.6
122.0
215.0
186.5
44.4
64.5
1.17
1.59
16
17
132.2
113.7
163.3
140.4
31.1
26.7
2.75
2.81
18
163.8
225.1
61.3
1.59
25
*For the dipolar oscillation frequencies, the errors are ±0.06 kHz for
protonated carbons and ±0.08 kHz for quaternary carbons.
100
105
110
115
30
35
40
45
10|Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

New Journal of Chemistry
Page 12 of 21
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DOI: 10.1039/C3NJ00271C
5
Table 3 Orientational Order Parameter values of PMAPH at 125 ˚C and
PMAPHMB at 130 ˚C
c
b
10
15
20
25
a
d
T
(°C)
Angle
Calculated dipolar oscillation
frequencies (kHz)
Ring
S’ZZ
S’xxꢀS’yy
Mesogen
PMAPH
b
c
a*
d
θb
θc
120.6
120.9
119.2
120.6
119.7
118.9
120.5
120.8
120.7
122.0
122.0
117.6
121.2
120.3
0.59
0.65
0.58
0.64
0.65
0.65
0.68
0.058
0.066
0.043
0.058
0.064
0.062
0.069
2.34
2.50
2.56
2.5
2.28
2.56
1.85
2.11
3.48
2.39
2.81
1.37
1.52
1.78
1.48
1.49
2.01
1.58
1.38
1.52
ꢀ
I
125
130
II
III
I
1.51
1.43
ꢀ
2.85
3.07
2.74
II
PMAPHMB
III
IV
1.58
* For carbon a in ring III, the contribution of the azomethine proton has
30 also been taken into account
35
40
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Journal Name, [year], [vol], 00–00 |11

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DOI: 10.1039/C3NJ00271C
Figure Captions
Scheme 1 Synthetic strategy for the target mesogens
60
5
Fig.1 Molecular Structures of (A) PMAPH and (B) PMAPHMB.
65
70
Fig.2 HOPM photographs of mesogens while cooling from isotropic
phase: (A) PMAPH nematic phase at 154 °C and (B) PMAPHMB –
smectic A phase at 116 °C.
10
Fig.313C NMR spectra of PMAPH. Traces from bottom to top correspond
respectively to spectra recorded in (A) solution, (B) solid state and (C)
liquid crystalline phase at 125 °C.
75
15 Fig.4 Plots of 13C chemical shifts for the aromatic carbons of (A) PMAPH
and (B) PMAPHMB in their mesophases as a function of temperature.
The corresponding carbon labels are indicated in the Figure.
80
Fig.5 2D SLF NMR spectra of (A) PMAPH at125°C in the nematic phase
20 and (B) PMAPHMB at 130 °C in the nematic phase.
85
Fig.613C NMR spectra of PMAPHMB. Traces from bottom to top
correspond respectively to spectra recorded in (A) solution, (B) solid
state and (C) liquid crystalline phase at 130 °C.
90
25
Fig. 7 Order parameters, S′zz and S′xx − S′yy for ringꢀI of (A) PMAPH
and (B) PMAPHMB in their mesophases as a function of temperature
obtained by using the CSA values of 4ꢀHexyloxybenzoic acid (HBA)32.
95
30
35
40
45
50
55
100
105
110
115
120
125

New Journal of Chemistry
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5
10
15
20
25
30
35
Scheme 1 Synthetic strategy for the target mesogens

Page 15 of 21
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DOI: 10.1039/C3NJ00271C
5
10
15
20
25
55
60
65
70
75
Fig.1 Molecular Structures of (A) PMAPH and (B) PMAPHMB.
30
35
40
45
50
80
85
90
95
100

New Journal of Chemistry
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55
5
10
15
20
60
65
(B)
(A)
25
Fig.2 HOPM photographs of mesogens while cooling from isotropic
°
phase: (A) PMAPH nematic phase at 154 C and (B) PMAPHMB ꢀ
30
smectic A phase at 116 °C.
35
40
45
50

Page 17 of 21
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DOI: 10.1039/C3NJ00271C
15
.
5
20
25
30
35
40
45
Fig.313C NMR spectra of PMAPH. Traces from bottom to top correspond
respectively to spectra recorded in (A) solution, (B) solid state and (C)
liquid crystalline phase at 125 °C.
10

New Journal of Chemistry
Page 18 of 21
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DOI: 10.1039/C3NJ00271C
5
10
15
20
25
30
35
55
60
65
70
75
80
85
Fig.4 Plots of 13C chemical shifts for the aromatic carbons of (A) PMAPH
40 and (B) PMAPHMB in their mesophases as a function of temperature.
The corresponding carbon labels are indicated in the Figure.
90
95
45
100
50

Page 19 of 21
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DOI: 10.1039/C3NJ00271C
30
35
40
45
50
55
5
°
Fig.5 2D SLF NMR spectra of (A) PMAPH at 125 C in the nematic
phase and (B) PMAPHMB at 130 °C in the nematic phase.
60
65
70
75
10
15
20
25

New Journal of Chemistry
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DOI: 10.1039/C3NJ00271C
30
5
35
40
45
50
55
Fig.6 13C NMR spectra of PMAPHMB. Traces from bottom to top
60
65
70
75
10 correspond respectively to spectra recorded in (A) solution, (B) solid state
and (C) liquid crystalline phase at 130 °C.
15
20
25

Page 21 of 21
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5
25
Fig. 7 Order parameters, S′zz and S′xx − S′yy for ringꢀI of (A) PMAPH
and (B) PMAPHMB in their mesophases as a function of temperature
10 obtained by using the CSA values of 4ꢀHexyloxybenzoic acid (HBA)32.
30
15
35
20