Benzimidazole based mesogenic Schiff-bases: Synthesis and characterization

Article abstract of DOI:10.1016/j.molliq.2017.05.061

Two homologous series of mesogenic Schiff-bases, N-4-((alkoxy)-(phenyl-3-hydroxy-4-(4-(5-methylbenzimidazol))-2-alkoxysalicylaldimine)benzoate (7a–d) and N-4′-(5-methyl-benzimidazole)-phenyl-4-alkoxysalicylaldimine (8a–d) incorporating benzimidazole moiety have been prepared and the molecular structures studied by FT-IR, NMR and ESI-MS spectrometry. Mesogenic behaviour was investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and variable temperature powder X-ray diffraction (PXRD) techniques. Changing the spacer (ester-linked to non-ester linked) of the Schiff-base results in enhancement of thermal stability and phase transition temperature. The members of series-I show monotropic SmA while those of series-II reflect enantiotropic SmA mesomorphism. An electrochemical study of a representative Schiff base in each series (7d and 8c) showed an electrical band gap 1.26?eV and 1.22?eV respectively.

Full text of DOI:10.1016/j.molliq.2017.05.061

Accepted Manuscript
Benzimidazole based mesogenic Schiff-bases: Synthesis and
characterization
Ragini Dubey, Rajasekhar Yerrasani, M. Karunakar, Angad
Kumar Singh, Rupali Gupta, Vellaichamy Ganesan, T.R. Rao
PII:
DOI:
Reference:
S0167-7322(17)30545-7
doi: 10.1016/j.molliq.2017.05.061
MOLLIQ 7353
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
7 February 2017
11 May 2017
14 May 2017
Please cite this article as: Ragini Dubey, Rajasekhar Yerrasani, M. Karunakar, Angad
Kumar Singh, Rupali Gupta, Vellaichamy Ganesan, T.R. Rao , Benzimidazole based
mesogenic Schiff-bases: Synthesis and characterization, Journal of Molecular Liquids
(2017), doi: 10.1016/j.molliq.2017.05.061
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ACCEPTED MANUSCRIPT
[Revised Manuscript MOLLIQ_2017_503_R1]
Benzimidazole based Mesogenic Schiff-Bases: Synthesis and characterization
Ragini Dubey, Rajasekhar Yerrasani, Karunakar M., Angad Kumar Singh,
Rupali Gupta, Vellaichamy Ganesan and T. R. Rao*
Department of Chemistry, Banaras Hindu University, Varanasi-221 005, India
Email: drtrrao@gmail.com
Abstract
Two homologous series of mesogenic Schiff-bases, N-4-((alkoxy)-(phenyl-3-hydroxy-
4-(4-(5-methylbenzimidazol))-2-alkoxysalisylaldimine)benzoate (7a-d) and N-4’-(5-methyl-
benzimidazole)-phenyl-4-alkoxysalisylaldimine (8a-d) incorporating benzimidazole moiety
have been prepared and the molecular structures studied by FT-IR, NMR and ESI-MS
spectrometry. Mesogenic behaviour was investigated by polarizing optical microscopy
(POM), differential scanning calorimetry (DSC) and variable temperature powder X-ray
diffraction (PXRD) techniques. Changing the spacer (ester-linked to non-ester linked) of the
Schiff-base results in enhancement of thermal stability and phase transition temperature. The
members of series-I show monotropic SmA while those of series-II reflect enantiotropic SmA
mesomorphism. An electrochemical study of a representative Schiff base in each series (7d
and 8c) showed an electrical band gap 1.26 eV and 1.22 eV respectively.
Keywords: Liquid crystal, Benzimidazole, Schiff-base, SmA
1. Introduction
Liquid crystals are self assembled dynamic functional soft materials having both order
and mobility at molecular, supramolecular and macroscopic levels; thus, they are fascinating
materials in many areas of applied science1. It is believed that mesomorphic behaviour of an
organic compound can be varied by modifying its molecular structure2-4. Recently, there has
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been a renewed interest in the incorporation of heterocycles in liquid crystals due to their
optical and electronic behaviour5-7. These functional properties arise due to the inclusion of
hetero atoms (S, O and N) which considerably change the polarity, polarizability and to a
certain extent the geometry of the molecule, thereby influencing the type of mesophase, the
phase transition temperatures, dielectric constants and other properties of the mesogens.
Liquid crystal containing heterocycles such as 1,3,4-oxadiazole, 1,2,3-triazole, pyrazoles,
pyridine, pyrimidine, etc., have recently been explored8. On the other hand, heterocyclic rings
fused with benzene rings are also becoming popular mesogenic cores to be incorporated into
the molecular structure; thus, liquid crystals containing fused heterocyclic ring systems such
as coumarin, benzothiazole and benzothiadiazole are reported in literature9. However, only
few reports on liquid crystal-containing terminal benzimidazole groups are available in
literature10. The type of linking group between the spacer and mesogenic units (e.g., ether,
ester, etc) is believed to be responsible for stabilising the molecular orientations necessary for
liquid crystal phase generation11.
Hence, with a view of expanding the existing literature on mesomorphic properties of
heterocyclic-based liquid crystals comprising a benzimidazole unit, we have undertaken a
systematic study on synthesis, mesomorphic and photo-physical properties of two
homologous series with varying alkyl chains (series-I, with ester link and series-II without
ester link) and the results are reported in this paper.
2. Experimental
2.1 Materials and Methods
All reagents are purchased from commercial sources; 2,4-dihydroxybenzaldehyde, 1-
bromoalkanes and potassium bicarbonate are from Sigma Aldrich, USA, while 4-
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nitrobenzaldehyde, 4-methyl-o-phenylenediamine, and SnCl2.2H2O are from Alfa-aesar and
KOH is from Merck. All the required solvents were dried using standard methods12.
Elemental analyses for C, H & N were performed on Vario EL III Carlo Erba 1108
1
analyser. H and 13C NMR spectra were recorded on JEOL AL-300/500 MHz FT-NMR
spectrometer. Mass spectrometric analysis was carried out on WATERS-Q-TOF Premier-
HAB213. FT-IR spectra were recorded on JASCO FTIR (model-5300) spectrophotometer in
4000–400 cm-1 region. The UV-visible absorption (Shimadzu spectrophotometer model,
Pharmaspec UV-1700) and fluorescence emission spectra (JY HORIBA Fluorescence
spectrophotometer) were recorded at room temperature in 10−5 M THF solutions. DSC
thermograms (at the heating rate of 5.0 °C min−1, accuracy of ±0.1 °C) were recorded on a
METTLER DSC-25 under nitrogen atmosphere. Mesophases were identified by using
polarized binocular microscope (LOMO, USA) equipped with hot-stage and digital camera
(Nikon Coolpix 4500); TG Analysis was carried out on PerkinElmer-STA 6000 apparatus
under nitrogen atmosphere. Powder XRD studies (unoriented samples filled in Lindemann
capillary) were made on Panalytical (Empyrean) X-ray diffractometer and electrochemical
measurements on a potentiostat/galvanostat (Model CHI 660C, CH Instruments, USA).
Quantum chemical calculations were made using DFT as implemented in the GAUSSIAN-09
package.
2.2 Synthesis
The synthetic route for preparing the homologous series and intermediates is depicted
in Scheme 1. The general procedure for alkylation of 2,4-dihydroxybenzaldehyde and its
esterification was as earlier reported method13-15
.
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2.2.1 Synthesis of 5-methyl-2-nitro-benzimidazole, 1.
The precursor material, 5-methyl-2-nitro-benzimidazole, 1, was prepared following
literature procedure16; final crude product was purified by column chromatography on silica
gel using hexane/ethyl acetate (10:1,v/v) as eluent to get a brown solid after evaporating the
solvent; yield: 5.0 g, 69%; Found: C, 66.39; H, 4.37; N, 16.57 %; Calc. for C14H11N3O2: C,
1
66.40; H, 4.38; N, 16.59. H NMR (300.40 MHz, DMSO-d6), δ (ppm): 13.08 (b, 1H, -NH),
8.37-8.27 (m, 4H, Ar-H),7.49-7.37(m, 2H, Ar-H), 7.02 (d, 1H, J = 8.4 Hz, Ar-H), 3.50 (s, 3H,
-CH3); 13C NMR (75.45 MHz, DMSO-d6), δ (ppm): 152.64, 148.67, 147.61, 136.14, 135.80,
132.94, 127.18, 124.48, 124.17, 118.86, 21.37.
2.2.2 Synthesis of 5-methyl-benzimidazol-2-amine, 2.
The compound, 5-methyl-benzimidazol-2-amine, 2, was obtained by reducing 1;
yield, 2.7 g, 45%; Found: C, 75.30; H, 5.86; N, 18.80%; Calc. for C14H13N3: C, 75.31; H,
5.87; N, 18.82. 1H NMR (300.40 MHz, DMSO-d6), δ (ppm): 12.24 (b, 1H, -NH), 8.32 (d, 2H,
J = 6.9 Hz Ar-H),7.81 (d, 2H, J = 8.1 Hz Ar-H) 6.92 (d, 1H, Ar-H, J = 8.1 Hz), 6.65 (m, 2H,
Ar-H) 5.53 (s, 2H, -NH2), 3.34(s, 3H, -CH3) and 13C NMR (75.45 MHz, DMSO-d6), δ (ppm):
164.65, 158.06, 155.86, 150.32, 127.54, 127.15, 124.20, 122.49, 117.59, 113.45, 21.24.
2.2.3 Synthesis of 3-hydroxy-4-(((5-methyl-benzimidazol-2-yl)imino)methyl)phenyl 4-
(octyloxy)benzoate,7a;
It was prepared by drop-wise addition of methanolic solution of
methylbenzoimidazol-2-amine to a methanolic solution of 4-(octyloxy)phenyl-4-formyl-3-
hydroxybenzoate followed by addition of a few drops of glacial acetic acid. The reaction
mixture was refluxed for ~4h and then cooled to room temperature. The microcrystalline
yellow coloured product was filtered off by suction, thoroughly washed with cold ethanol,
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recrystallised from absolute ethanol and dried at room temperature; yield 3.91 g (68%).
Elemental analysis for C36H37N3O4, found (calculated) %: C, 75.14 (75.11); H, 6.49 (6.48);
1
N, 7.30 (7.32); H NMR (300.40 MHz, CDCl3), δ (ppm): 13.46 (b, 1H, OH), 8.64 (s, 1H,
CH=N), 8.17 (d, 2H, J = 8.7 Hz), 8.08 (d, 2H, J = 8.7 Hz), 7.41-7.26 (m, 6H, Ar-H, NH),
7.11 (d, 1H, J = 8.1 Hz, Ar-H), 7.00-6.89(m, 3H, Ar-H), 6.82(d, 1H, J = 8.7 Hz, Ar-H), 4.07
(t, 2H, J = 6.6 Hz, -OCH2), 2.48 (s, 3H, ph-CH3), 1.82-1.30 (m, 12H, (-CH2)6), 0.89 (t, 3H, J
13
= 6.6 Hz, -CH3), C NMR (75.45 MHz, CDCl3, ppm) δ :166.77, 164.88, 163.09, 161.00,
156.41, 150.46, 149.82, 149.58, 136.39, 133.37, 132.41, 131.11, 129.83, 127.51, 127.13,
124.62, 123.33, 121.85, 117.65, 115.55, 114.59, 114.41, 111.33, 68.49, 21.71. IR (KBr) in
cm-1: 1169 (C–Oether), 1315 (C–Ophenolic), 1623 (C=N), 1721 (C=Oester), 2919–2851 (C–Halkyl).
ESI-MS m/z 576.89 [M+H]+ calculated 575.27.
2.2.4 Synthesis of 4-(decyloxy)-phenyl-3-hydroxy-4-(((4-(5-methyl-benzimidazol-2-yl)
phenyl)imino)benzoate, 7b;
Yield 4.80 g (75%); Elemental analysis for C38H41N3O4, found (calculated) %:
C,75.62 (75.60), H, 6.85 (6.84), N, 6.96 (6.64); 1H NMR (500.15 MHz, CDCl3), δ (ppm):
13.46 (b, 1H, OH), 8.62 (s, 1H, CH=N), 8.15 (d, 2H, J = 9.5 Hz, Ar-H,), 8.05 (d, 2H, J = 8.5
Hz, Ar-H), 7.33-7.08 (m, 6H, Ar-H, NH), 6.99 (d, 1H, J = 9.5 Hz, Ar-H), 6.89-6.75(m, 3H,
Ar-H), 6.92 (d, 1H, J = 8.5 Hz, Ar-H) 4.05 (t, 2H, J = 7.0 Hz, -OCH2), 2.47 (s, 3H, ph-CH3),
13
1.83-1.28 (m, 16H, (-CH2)8 ), 0.99 (t, 3H, J = 6.0 Hz, -CH3) and C NMR (125.76 MHz,
CDCl3, ppm) 165.20, 164.74, 163.91, 162.80, 162.07, 155.20, 149.26, 133.49, 132.53, 132.8,
128.59, 127.59, 121.95, 121.09, 117.16, 114.51, 113.38, 110.82, 68.48, 21.83. IR (KBr) in
cm-1: 1169 (C–Oether), 1315 (C–Ophenolic), 1623 (C=N), 1722 (C=Oester), 2920–2852 (C–Halkyl).
ESI-MS m/z 604.75 [M+H]+ calculated, 603.75.
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2.2.5 Synthesis of 4-(dodecyloxy)-phenyl-3-hydroxy-4-(((4-(5-methyl-benzimidazol-2-yl)
phenyl)imino) benzoate, 7c;
Yield 4.52 g (72 %): Elemental analysis for C40H45N3O4, found (calculated) %: C,
1
76.05 (76.07); H, 7.18 (7.19); N, 6.66 (6.67) ; H NMR (300.40 MHz, CDCl3), δ (ppm):
13.42 (b, 1H, OH), 8.65 (s, 1H, CH=N), 8.16 (d, 2H, J = 7.8 Hz, Ar-H), 8.08 (d, 2H, J = 6.9
Hz, Ar-H), 7.41-7.19 (m, 6H, Ar-H, NH),7.11(d, 1H, J = 7.5 Hz Ar-H), 6.99-6.90(m, 4H, Ar-
H), 4.05(t, 2H, J = 7.0 Hz, -OCH2), 2.47(s, 3H, ph-CH3), 1.82-1.27(m, 20H, (-CH2)10), 0.86
13
(t, 3H, J = 6.0 Hz, -CH3) and C NMR (75.45 MHz, CDCl3, ppm) 165.82, 162.21, 160.10,
157.25, 152.79, 152.38, 151.29, 140.94, 135.94, 133.63, 132.03, 131.34, 129.66, 127.76,
127.16, 125.71, 125.21, 123.04, 122.82, 121.79, 120.43, 116.06, 115.80, 115.51, 114.48,
68.70, 21.27. IR (KBr) in cm-1: 1169 (C–Oether), 1315 (C–Ophenolic), 1623 (C=N), 1722
(C=Oester), 2920–2852 (C–Halkyl). ESI-MS m/z 632.56 [M+H]+ calculated 631.34.
2.2.6 Synthesis of 4-(tetradecyloxy)phenyl-3-hydroxy-4-(((4-(5-methyl-benzimidazol-2-
yl)phenyl)imino)methyl)benzoate, 7d;
Yield 4.50 g (66 %): Elemental analysis for C34H41N3O4, found (calculated) %: C,
73.49 (73.50); H, 7.49 (7.48); 1H NMR (300.40 MHz, CDCl3), δ (ppm): 13.63 (b, 1H, OH),
8.63 (s, 1H, CH=N), 8.14-8.06 (m, 4H, Ar-H), 7.72-7.13(m, 6H, Ar-H, NH), 7.13(d, 1H, J =
9.9 Hz, Ar-H), 6.97-6.86(m, 4H, Ar-H), 4.05 (t, 2H, J = 7.0 Hz, -OCH2), 2.48 (s, 3H, ph-
CH3), 1.82-0.98 (m, 24H, (-CH2)12), 0.88 (t, 3H, J = 6.0 Hz, -CH3) and 13C NMR (75.45
MHz, CDCl3, ppm) 165.10, 161.70, 160.98, 156.14, 152.61, 152.15, 140.92, 135.03, 133.38,
132.74, 131.24, 127.81, 127.47, 127.21, 125.35, 124.98, 121.92, 115.04, 114.50, 68.67,
21.97. IR (KBr) in cm-1: 1169 (C–Oether), 1315 (C–Ophenolic), 1623 (C=N), 1722 (C=Oester),
2920–2852 (C–Halkyl); ESI-MS m/z 660.56 [M+H]+ calculated, 659.34.
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2.2.7 Synthesis of (4’-(5”-methyl-1H-benzimidazole)-phenyl)-4-octyloxysalisylaldimine,
8a:
It was prepared by drop-wise addition of methanolic solution of methylbenzimidazole
amine to a methanolic solution of 4-octyloxy-2-hydroxybenzaldehyde and refluxing the
reaction mixture in presence of a few drops of glacial acetic acid for ~4h followed by cooling
to room temperature. The microcrystalline yellow coloured product was filtered off by
suction, washed with cold ethanol; the product was recrystallised from absolute ethanol and
dried at room temperature; Yield 3.10 g, 68%; Found C, 76.47; H, 7.29; N, 9.20%; Calc for
C29H33N3O2: C, 76.45; H, 7.30; N, 9.22%; 1H NMR (300.40 MHz,CDCl3), δ (ppm): 13.69 (b,
1H, OH), 8.42 (s, 1H, -CH=N), 8.08(d, 2H, J =7.8 Hz, Ar-H),7.53(d, 1H, J = 7.5 Hz, Ar-H),
7.25-7.16 (m, 4H, Ar-H, NH), 7.07(d, 1H, J = 7.8 Hz, Ar-H), 6.52-6.42(m, 3H, Ar-H), 3.93
(t, 2H, J = 7.5 Hz, -OCH2,), 2.43(s, 3H, ph-CH3), 1.79-1.31 (m, 12H, (-CH2)6), 0.86 (t, 3H, J
= 6.3 Hz -CH3,) and 13C NMR (75.45 MHz, CDCl3), δ (ppm): 164.24, 163.86, 161.26,
150.93, 148.96, 135.20, 133.54, 132.65, 127.68, 127.49, 124.27, 121.30, 112.93, 112.54,
107.69, 101.69, 68.07, 21.46 ;IR (KBr pellet, cm-1) 3415, υ(OH/NH); 1629, υ(CH=N); ESI-
MS m/z 456.20 [M+H]+ calculated, 455.53.
All the other members of the series were prepared in a similar manner. The yield, IR,
NMR and elemental data for the compounds are summarised below:
2.2.8 Synthesis of 4’-(5”-methyl-benzimidazole)-phenyl)-4-decyloxysalisylaldimine, 8b:
Yield, 3.20 g, 66%; Found: C, 76.92; H, 7.70; N, 8.71%; Calc. for C31H37N3O2: C,
76.98; H, 7.71; N, 8.69;1H NMR (300.40 MHz, CDCl3), δ (ppm): 13.68 (s, 1H, OH), 8.43 (s,
1H, -CH=N), 8.09(d, 2H, J = 6.3 Hz, Ar-H), 7.53 (d, 1H, J = 6.0 Hz), 7.37-7.05(m, 6H, Ar-H,
NH), 6.44(m, 2H, Ar-H) 3.95 (t, 2H, -OCH2, J = 6 Hz), 2.43 (s, 3H, ph-CH3), 1.77-0.88 (m,
13
16H (-CH2-)8), 0.88 (t, 3H, -CH3, J = 6.3 Hz) and C NMR (75.45 MHz, CDCl3), δ (ppm):
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164.41, 164.04, 161.47, 151.1, 149.16, 140.79, 133.74, 130.72, 130.10, 127.92, 127.52,
124.44, 121.50, 112.75, 107.82, 101.52, 68.17, 21.68; IR (KBr pellet, cm-1) 3415, υ(-
OH/NH); 1627, υ(CH=N); ESI-MS m/z 484.20, [M+H]+ calc: 483.28.
2.2.9. Synthesis of 4’-(5”-methyl-benzimidazole)-phenyl)-4-dodecyloxysalisylaldimine,
8c:
Yield, 3.4 g, 65%; mp, 287 °C; Found C,77.45; H, 8.05; N, 8.20%; Calc for
C33H41N3O2: C, 77.46; H, 8.08; N, 8.21%; 1H NMR (300.40 MHz,CDCl3), δ (ppm): 13.61 (b,
1H, OH), 8.49 (s, 1H, CH=N), 8.07(d, 2H, J = 8.4 Hz, Ar-H), 7.54(d, 2H, J = 9.0 Hz, Ar-H),
7.39(s, 1H, Ar-H), 7.28-7.21(m, 3H, Ar-H, NH), 7.09(d, 1H, J = 8.1Hz), 6.47-6.45 (m, 2H,
Ar-H) 3.99 (t, 2H, J = 6.6 Hz ), 2.46 (s, 3H, ph-CH3), 1.76-1.26 (m, 20H, (-CH2-)10), 0.85 (t,
3H, -CH3, J = 6.9 Hz) and 13C NMR (75.45 MHz, CDCl3), δ (ppm): 164.29, 164.06, 161.72,
150.56, 149.56, 133.88, 133.01, 127.77, 125.14, 124.58, 122.20, 121.63, 115.31, 112.87,
107.97, 101.58, 68.22; ESI-MS : m/z 512.20, [M+H]+; calc. 511.28; IR (KBr pellet, cm-1):
3415, υ(-OH/NH); 1627, υ(CH=N).
2.2.10. Synthesis of 4-(5’-methyl-benzimidazole)-phenyl)-4-tetradecyloxysalisylaldimine,
8d:
Yield 3.90 g, 70%; Found C, 77.95; H, 8.45; N, 7.82%; Calc. for C35H45N3O2: C,
77.88; H, 8.40; N, 7.79%; 1H NMR (300 MHz, CDCl3), δ (ppm): 13.67 (b, 1H, OH), 8.43 (s,
1H, -CH=N), 8.08(d, 2H, J = 8.4 Hz, Ar-H), 7.53(d, 1H, J = 7.5 Hz, Ar-H),7.38(s, 1H, Ar-H),
7.25-7.05(m, 5H, Ar-H, NH),6.44-6.42 (m, 2H, Ar-H), 3.92 (t, 2H, -CH2, J = 6.3 Hz), 2.43 (s,
3H, ph-CH3), 1.79-0.85 (m, 24H, (-CH2-)12), 0.85(t, 3H, -CH3, J = 6.0 Hz) and 13C NMR
(75.45 MHz, CDCl3): δ(ppm): 164.45, 164.09, 161.49, 151.31, 149.22, 142.56, 138.24,
133.76, 132.88, 127.92, 127.70, 124.56, 121.40, 112.50, 107.92, 101.07, 68.18, 21.68; IR
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(KBr pellet, cm-1): 3416, υ(-OH/NH); 1628, υ(CH=N); ESI-MS m/z 540.35, [M+H]+ calc.
539.75.
3. Results and Discussion
3.1 Synthesis and Chemical structure characterisation
Synthetic routes of the compounds are outlined in the scheme 1. The Schiff bases
were obtained by condensation reaction of 5-methyl-benzimidazol-2-amine with appropriate
aldehyde in refluxing methanol. All the ligands were characterized by elemental analyses and
standard spectroscopic techniques.
The FT-IR spectra of compounds in both the series showed considerable similarities;
the diagnostic absorption band of the azomethine (C=N) group appeared in the 1620-1623
cm-1 range. The spectral bands appearing within the frequency ranges of 2921-2923 cm-1 and
2851-2854 cm-1 may be respectively attributed to v(C–H)asy and v(C–H)sym of –CH3. The
broad band centred at 3351-3354 cm−1 is characteristic of ν(O–H/N-H) frequency. The
members of series-I are characterized by (C=O)ester band appearing at 1721-1723 cm-1.
Further, resemblance of both the series was also reflected in their 1H NMR spectra; phenolic-
OH appeared as a singlet at 13.43-13.69 ppm, the aromatic protons gave rise to multiplets
over the range of 8.62-6.23 ppm and –CH=N of the Schiff-base moiety appeared as a singlet
at 8.42-8.62 ppm. 13C NMR spectral data (given below in the running text under ‘synthesis’)
is used to further substantiate the molecular structures of the compounds.
3.2 Mesomorphic study of the benzimidazole based Schiff bases
Thermotropic behavior of the benzimidazole based Schiff-base ligands was studied on
the basis of data obtained from thermogravimetric analyses (TGA), differential scanning
calorimetry (DSC), polarized optical microscopy (POM) and X-ray scattering on powder
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samples XRD studies (in selected cases). Table 1 presents the phase sequence, transition
temperatures and enthalpies of the transitions; the phase transition temperatures are deduced
from calorimetric measurements of the second heatingcooling cycles at a rate of 5 °C min-
1. In the absence of transition peaks in DSC traces, wherever applicable, the phase transition
temperatures (noted during the POM study) are taken as such.
Thermal stability of these compounds (series-I) up to ~250 °C (Fig. 1(a)) has been
inferred from TGA measurements and monotropic behaviour implied17 by DSC studies:
endotherms observed during heating [corresponding to the crystal phase  the isotropic
liquid at 228 °C (n = 8) and at 214 °C (n = 14)] and exotherms in cooling cycle [characteristic
of isotropic  mesophase transition (213 °C (n = 8), 208 °C (n = 14) and mesophase 
crystal transitions at 149 °C (n = 8) and at 68° C (n = 14), Fig. 2]. It was observed that the
clearing temperatures of the compounds and the phase transition temperatures decreased
slightly with the increase of chain length (Table 1). A broken fan-shaped SmA mesophase
(from 208 to 68 °C; Fig. 3) was implied by POM studies of 7d of series-I; similar optical
textures were observed for the rest of the members of Series-I. The wide range of the
mesophase of 7d was further corroborated by XRD diffraction patterns at 204 °C (Fig. 4);
these patterns consist of (i) a sharp intense low angle peak (2θ = 2.15 Å), and (ii) a weak
broad diffuse peak at wide angles (2θ = 17 Å) characteristic of liquid-like arrangement of the
molecules within the layers. These features, in agreement with the POM observations,
confirm the SmA mesophase. A comparison of the experimental d-spacing (d, 40.57 Å) with
the DFT-calculated [B3LYP/6-311G (d,p)] molecular length (l, 41 Å, Fig. 5) implies d/l ratio
of ~1, which suggests a monolayer arrangement18.
TGA measurements performed on the members of Series-II (having no ester linkage)
imply thermal stability up to ~300 °C (Fig. 1(b)); POM studies show that the initial batonnets
got transformed into typical fan-shaped focal conic smectic textures, characteristic of SmA
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mesophase (the representative textures of 8a and 8c are shown in Figs. 6 & 7); in case of 8c
DSC thermogram showed three sharp endothermic peaks (at 81 °C, 122 °C and 294 °C) and
two sharp exothermic peaks (at 284 °C and 67 °C) (Fig. 8) while POM studies reveal that
heating beyond 225 °C leads to formation of fully grown focal conic SmA mesophase having
fluid property with higher viscosity. The DSC data thus, implies crystal transformation (Cr–
Cr1) characteristic of polymorphism2,17,19-20. Powder XRD patterns (Fig. 9) of 8c at 225 °C
show a sharp reflection in the low-angle region (2θ = 2.48 Å) presumably due to the layered
structure of the smectic A phase, and a broad peak in the wide-angle region (2θ = 4.45 Å)
characteristic of liquid-like arrangement of the molecules within the layers. On the basis of
the d/l ratio (d/l ~1) of 8c (d, the interlayer distance calculated from XRD, 35.6 Å, and l, the
calculated length of the optimized structure, 32.7 Å, Fig.10) the SmA phase was suggested to
have a monolayer arrangement.
3.3 Structure–mesomorphic property relationship
The observed differences in the mesomorphic properties and thermal stability of the
members of series-I (having ester linkage) and series-II (similar to series-I devoid of ester
linkage) may be attributed to their structural differences. Although both the series exhibited
SmA mesophase, the mesogenic behaviour of series-I, with thermal stability upto ~250 °C, is
limited to monotropic type while that of the series-II is expanded to enantiotropic type with
thermal stability upto ~300 °C. Further, the transition temperatures of series-II are spanned
relatively over a wider range. These differences may be understood in terms of restricted
rotation/flexibility in series-I due to ester linkage which alters the polarity and intermolecular
interactions between the layers21; on the contrary, members of series-II (with no such ester
linkage) tend to be more linear with a higher degree of self-assembly, thus resulting in a
wider of SmA phase.
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3.4 Photo physical properties
The photo physical properties of these mesogens were recorded in micromolar
solutions of THF (Table 2); both the series showed similar absorption patterns with the
lowest energy band arising at 363–365 nm (Fig. 11). The high molar absorption coefficients
(ε, ~28,000 dm3 mol−1 cm−1), attributed to π–π* transitions in the hetero-aromatic and
aromatic rigid portions of the molecules, are in accordance with the conjugated nature of the
systems. Emission spectral bands of the compounds (ex, 330 nm, Fig. 12) span over the
range of 380-410 nm with small stokes shift (Table 2). No significant difference in the
absorption and emission spectra has been observed across the homologous series (n = 8 to n =
14).
The theoretical HOMO-LUMO plots (TD-DFT/B3LYP/6-311G (d,p)) of 7d and 8c
(Fig. 13) imply that the electron density in the HOMO is confined mainly onto the
benzimidazole and benzene moieties, while that of the LUMO is distributed onto the benzene
moiety (imine system).
3.5 Electrochemical properties
Cyclic voltammetry (CV) studies of one representative compound from each series
(7d, 8c) (Fig. 14) have been carried out in micromolar dichloromethane solution (potential
range of 2 V to +2 V at a scan rate of 50 mVs1 under nitrogen atmosphere) for investigating
electrochemical behaviour. For these studies 0.1M solution of tetrabutylammonium
hexafluorophosphate was used as supporting electrolyte; a single compartment cell fitted with
a Ag/AgNO3 reference electrode, a platinum wire counter electrode, and a glassy carbon
working electrode was used for the experiment; the reference electrode was calibrated with
ferrocene/ferrocenium (Fc/Fc+) redox couple (with an absolute energy level of 4.80 eV with
respect to vacuum)22. On the basis of the cyclic voltammograms for 7d & 8c the HOMO
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energies have been calculated as −5.06 eV and −4.92 eV, and the LUMO energy values as
−3.80 eV and −3.70 eV respectively. The electrical band gaps (as per the CV data; Table 3)
were calculated as 1.26 eV and 1.22 eV for of 7d and 8c which are much smaller than optical
band gaps23.
4. Conclusion
In summary, two homologous series of benzimidazole based mesogenic Schiff-bases
were synthesised and characterised. Series-I (having ester linkage) showed monotropic SmA
mesophase while series-II (with no ester linkage) corresponded to enantiotropic SmA
mesophase. Enhanced thermal stability of the members of series-II has been discussed on the
basis of structure-mesomorphism relationship. Electrochemical studies of a representative
compound from each series imply low HOMO-LUMO band gap (1.22 & 1.26 eV).
Acknowledgements
Ragini Dubey thanks the University Grants Commission, New Delhi, India, for
Senior Research Fellowship [R/Dev./Sch.- (UGC-SRF)/S-01]. The help received from Prof.
Sandeep Kumar, Raman Research Institute, Bengaluru for powder XRD measurements is
gratefully acknowledged.
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References
[1] (a) T. Geelhaar, K. Griesar, B. Reckmann, Angew. Chemie - Int. Ed. 52 (2013)
8798-8809; (b) J.P.F. Lagerwall, G. Scalia, Curr. Appl. Phys. 12 (2012) 1387–1412;
(c) H. K. Bisoyi, S. Kumar, Chem. Soc. Rev. 40 (2011) 306–319; (d) D. Wang, S. Y.
Park, I. K. Kang, J. Mater. Chem. C, 3 (2015) 9038.
[2] (a) C. Tschierske, Angew. Chem., Int. Ed., 52 (2013) 8828–8878; (b) Handbook of
Liquid Crystals: Fundamentals, ed. J. W. Goodby, P. J. Collings, T. Kato, C.
Tschierske, H. Gleeson and P. Raynes, Wiley-VCH, Weinheim, Germany, 2014,
vol. 1
[3] (a) G. Y. Yeap, W. S. Ooi, Y. Nakamura, Z. Cheng, Mol. Cryst. Liq. Cryst. 381
(2002) 169–178; (b) G. Y. Yeap, S. T. Ha, P. L. Lim, P.L. Boey, M. M. Ito, S.
Sanehisa, Y. Youhei, Liq. Cryst. 33 (2006) 205–211; (c) B. K. So, W. J. Kim, S. M.
Lee, M. C. Jang, H. H. Song, J. H. Park, Dyes and Pigments 75 (2007) 619–623; (d)
S. T. Ha, L. K. Ong, S. T. Ong, G. Y. Yeap, J. P. W. Wong, T. M. Koh, H. C. Lin,
Chin. Chem. Lett. 20 (2009) 767–770; (e) S. T. Ha, L. K. Ong, J. P. W. Wong, G. Y.
Yeap, H. C. Lin, S. T. Ong, T. M. Koh, Phase Transitions 82 (2009) 387–397. (f) B.
T. Thaker, B. S. Patel , D. B. Solanki , Y. T. Dhimmer, J. S. Dave, Mol. Cryst. Liq.
Cryst. 517 (2010) 63-80
[4] (a) A. N. Cammidge, R. J. Bushby, Handbook of Liquid Crystals; Wiley-VCH:
Weinheim, 1998; (b) M. Funahashi, J. Hanna, Adv. Mater. 17 (2005) 594-598; (c) H.
Maeda, M. Funahashi, J. I. Hanna, Mol. Cryst. Liq. Cryst. 366 (2001) 369-376; (d) S.
Mery, D. Nicoud, J.-F. Haristoy, D. Guillon, S. Diele, H. Monobe, Y. Shimizu, J.
Mater. Chem. 12 (2002) 37-41.
14

ACCEPTED MANUSCRIPT
[5] (a) J. Han, J. Mater. Chem. C 1 (2013) 7779–7797; (b) A. Seed, Chem. Soc. Rev. 36
(2007) 2046–2069.
[6] V. V. Titov, A. I. Pavlyuchenko, Chem. Heterocycl. Compd. 16 (1980) 1–13.
[7] M. P. Aldred, P. Vlachos, D. Dong, S. P. Kitney, W. C. Tsoi, M. O’Neill, S. M.
Kelly, Liq. Cryst. 32 (2005) 951–965.
[8] (a) B. Roy, N. De, K. C. Majumdar, Chem.–Eur. J. 18 (2012) 14560–14588; (b) Han,
J. Mater. Chem. C., 1 (2013) 7779–7797; (c) P. Vlachos, B. Mansoor, M. P. Aldred,
M. O’Neill, S. M. Kelly, Chem. Commun. 23 (2005) 2921–2923; (d) S. Yamane, Y.
Sagara, T. Mutai, K. Araki, T. Kato, J. Mater. Chem. C. 1 (2013) 2648–2656; (e) R.
Giménez, M. Piñol, J. L. Serrano, Chem. Mater. 16 (2004) 1377–1383.
[9] (a) Sie-Tiong Ha, Teck-Ming Koh, Guan-Yeow Yeap, Hong-Cheu Lin, Siew-Ling
Lee, Yip-Foo Win, Siew-Teng Ong, Phase Transitions 83 (2010) 95-204; (b) N. K.
Chudgar, M. K. Parekh, S. S. Madhavrao, H. C. Sharma, Liq. Cryst. 19 (1995) 807-
810; (c) A. K. Prajapati, N. L. Bonde, Mol. Cryst. Liq. Cryst. 501 (2009) 72-85; (d) C-
Shu. Wang, I-W. Wang, K.-L. Cheng, C. K. Laia, Tetrahedron 62 (2006) 9383–9392;
(e) H-M. Kuo, W-P. Ko, Y-T. Hsu, G-H. Lee, C. K. Lai, Tetrahedron 72 (2016) 6321-
6323.
[10] (a) S. Roddecha, M. Anthamatten, Liq. Cryst., 37 (2010) 389–397; (b) O. N. Kadkin,
J. Tae, S. Y. Kim, E. H. Kim, E. Lee, M. G. Choi, Liq Cryst. 36 (2009) 1337–1347; (c)
S. H Seo, G. N. Tew, J. Y. Chang, L. Zhang et al. Soft Matt. 2 (2006) 886–891.(d) L.
Zhang, X. Chen, F. Zhao, X. Fan, P. Chen, Z. An, Liq. Cryst. 40 (2013) 396-410 (e)
F. F. Jian, K. F. Wang, Liq. Cryst. 35 (2008) 1415–1421; (f) K. Goossens, P.
Nockemann, K. Driesen, B. Goderis, C. G. Walrand, K. V. Hecke, L. V. Meervelt, E.
Pouzet, K. Binnemans, T. Cardinaels, Chem Mater. 20 (2008) 157–168.
15

ACCEPTED MANUSCRIPT
[11](a) J. Belmar, Liq. Cryst. 26 (1999) 9-15; (b) P. A. Henderson, J. M. Seddon, C. T.
Imrie, Liq. Cryst. 32 (2005) 1499-1513.
[12] A. Weisberger, F. S. Praskver, Organic Solvents, International Publishers Inc.: New
York, 1956, 1263.
[13] R. Yerrasani, M. Karunakar, R. Dubey, A. K. Singh, R. Nandi, R. K. Singh, T. R.
Rao, J. Mol. Liq. 216(2016)510–515.
[14] A. K. Singh, S. Kumari, T. R. Rao, Mater. Sci. Eng., C. 31 (2011) 1144–1147.
[15] A. K. Singh, S. Kumari, K. Ravi Kumar, B. Sridhar, T. R. Rao, Polyhedron 27 (2008)
1937-1941.
[16] Y. Caoa, M. Yanga, Y. Wanga, H. P. Zhoua, J. Zhengc, X. Zhanga, J. Wua, Y. Tiana,
Z. Wud, J. Mater. Chem. C 2 (2014) 3686-3694.
[17] D. Demus, L. Richter, Textures of Liquid Crystals, Verlag Chemie: New York, 1978.
[18] C. C. Liao, C. S. Wang, H. S. Sheu and C. K. Lai, Tetrahedron 64(2008)7977–7985.
[19] I. Dierking, Textures of Liquid Crystals, Wiley-VCH: Weinheim, 2003.
[20] Z. Galewaki, Mol. Cryst. Liq. Cryst. 249(1994)43-49.
[21] D. H. Croll, P. K. Sripada, J. A. Hamilton, J. Lipid Research 28(1987)1444-1454.
[22] (a) M. Gupta, S. K. Pal, Langmuir 32(2016)1120−1126, (b) F. A. Olate, J. A. Ulloa, J.
M. Vergara, S. A. Sanchez, J. Barbera, M. L. Parra, Liq. Cryst. 41(2016)811–82
[23] B. Y. Zhang, Y. G. Jia, D. S. Yao, X. W. Dong, Liq. Cryst. 31(2004)339–345.
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Scheme 1. Reagents and conditions: (i) RBr, K2CO3, DMF, ~5h reflux; (ii) KOH, ethanol, ~2h
reflux; (iii) 2,4-dihydroxybenzaldehyde, DCC, DMAP, DCM, stirring about ~24h; (iv) RBr,
K2CO3, DMF, ~5h reflux; (v) KI, DMF, 24h reflux; (vi) SnCl22H2O, dry ethanol, ~4h reflux; (vii)
& (viii) few drops of acetic acid, ethanol, ~4h reflux
(a)
(b)
Fig. 1. TGA thermograms of (a) Series-I and (b) Series-II
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Fig. 2. DSC thermograms of (a) 7c and (b) 7d
Fig. 3. Optical textures (broken-fan like, SmA) of (a) 7c at 210 °C and (b) 7d at 206 °C
Fig. 4. XRD pattern for Smectic A phase of 7d at 204 °C
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l = 41 Å
Fig. 5. DFT optimized structure of 7d and its molecular length
Fig. 6. Optical textures (SmA) of 8a: (a) Batonnets at 274 °C (b) at 220 °C (c) at 187 °C
and (d) transition from SmA to Crystalline state at 79 °C
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(a)
(b)
Fig. 7. Optical photomicrographs of 8c: (a) Batonnets at 284 °C and (b) SmA at 205°C.
Fig. 8. DSC thermograms of (a) 8a and (b) 8c
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Fig. 9. XRD pattern (SmA) of 8c at 225 °C
Fig. 10. DFT optimized structure of 8c and its molecular length
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Fig. 11. Absorption spectra of 7c and 8c
Fig. 12. Emission spectra of 7c & 8c
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Compound HOMO
7d
8c
LUMO
HOMO
LUMO
Fig. 13. HOMO & LUMO orbitals of 7d & 8c
7d
8c
Fig. 14. Cyclic voltammograms of 7d and 8c
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Table 1: Thermal Transitions and thermodynamic parameters
T, °C [ΔHb, kJ/ mol]
Compounds
Heating
Cooling
Series-I
Cr 184.9 (6.0) Cr1 228c I
Cr 184.4 (5.1) Cr1 223c I
Cr 165 (9.3) Cr1 220c I
Cr 157.7 (8.5) Cr1 214c I
I 226c SmA 149.1 (5.1) Cr
I 219 c SmA 138.6 (4.3) Cr
I 212 (0.3) SmA 130.9 (6.1) Cr
I 208 (1.7) SmA 68 (1.2) Cr
7a
7b
7c
7d
Series-II
8a
Cr 73 (17.9) Cr1 146 (13.6) SmA 283c I
Cr 79 (34) Cr1 120 (14.5) SmA 287c I
I 276c SmA 78 (1.2) Cr
I 275 (0.5 ) SmA 71.4 (4.5) Cr
8b
Cr 81 (17) Cr1 122 (15) SmA 294 (7.11) I I 284 (1.6 ) SmA 67 (2.7) Cr
8c
Cr 76(3.8), Cr1 113 (3.2) SmA 279c I
I 276 (0.4) SmA 60 (1.2) Cr
8d
Cr = crystalline state; SmA = Smectic A mesophase; i = isotropic liquid; b = Data as obtained from DSC;
c = Observed by POM
Table 2: Optical absorption and emission data (nm)
Compound
Band maximum
absorption
Emission
385
363
365
364
365
364
365
364
363
7a
7b
7c
7d
8a
8b
8c
8d
385
386
388
409
409
410
411
Table 3: Electrochemical data
HOMOa(eV) LUMOa(eV) ∆Eg (eV)b ∆Eg(eV)c
Compounds
−5.06
−4.92
−3.80
−3.70
1.26
1.22
2.57
2.67
7d
8c
a= Calculated from ferrocene standard.
b= Calculated as the difference between HOMO & LUMO.
c= Optical band gap determined from the onset of the lower energy of the
absorption maxima.
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Graphical abstract
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highlights:
1. Two homologous series of benzimidazole-based Schiff-bases have been synthesized.
2. Structural, thermal and optical studies made by FTIR, NMR, ESI-MS, POM, DSC
and PXRD techniques.
3. Monotropic and enantiotropic mesomorphism identified in series-I and series-II
respectively.
4. Differences in mesomorphism and thermal stability correlated to structural variations.
5. Low electrochemical band gaps found for representative members.
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