Liquid crystalline properties of 3-[4-(4′- alkoxybenzoyloxybenzylidene) amino]-1,2,4-triazines: Synthesis and characterization

Article abstract of DOI:10.1080/15421400903483957

A new series of 3-[4-(4′-alkoxybenzoyloxybenzylidene)amino]-1,2,4- triazines, C₃H₂N₃N=CHC₆H ₄OCOC₆H₄OC_mH_2m+1 where m=6, 7, 8, 9, 10, 11, 12, 14, and 16 have been synthesized. These compounds have been characterized by elemental analyses, FT-IR, UV-visible, ¹H, and ¹³C NMR spectroscopy. Mesomorphic properties of these compounds were studied by differential scanning calorimetry and polarizing microscopy. The mesomorphic nature of these compounds is dependent on the alkoxy chain length. The compounds (m=6, 7) do not exhibit mesomorphism while the compounds (m=8, 9, 11, 12, 14) show a monotropic nematic mesophase in cooling cycle. The 3-[4-(4′-decyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine displays an enantiotropic nematic mesophase and the hexadecyloxy compound exhibits the smectic A mesophase. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/15421400903483957

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Liquid Crystalline Properties of 3-[4-
(4′-Alkoxybenzoyloxybenzylidene)
amino]-1,2,4-triazines: Synthesis and
Characterization
Bachcha Singh a , Ashwini Pandey a & Sachin Kumar Singh a
a Department of Chemistry, Faculty of Science, Banaras Hindu
University, Varanasi, India
Published online: 01 Mar 2010.
To cite this article: Bachcha Singh , Ashwini Pandey & Sachin Kumar Singh (2010) Liquid
Crystalline Properties of 3-[4-(4′-Alkoxybenzoyloxybenzylidene) amino]-1,2,4-triazines:
Synthesis and Characterization, Molecular Crystals and Liquid Crystals, 517:1, 127-137, DOI:
10.1080/15421400903483957
To link to this article: http://dx.doi.org/10.1080/15421400903483957
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Mol. Cryst. Liq. Cryst., Vol. 517: pp. 127–137, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903483957
Liquid Crystalline Properties of
3-[4-(40-Alkoxybenzoyloxybenzylidene)
amino]-1,2,4-triazines: Synthesis and
Characterization
BACHCHA SINGH, ASHWINI PANDEY, AND
SACHIN KUMAR SINGH
Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, India
A new series of 3-[4-(40-alkoxybenzoyloxybenzylidene)amino]-1,2,4-triazines,
=
C3H2N3N CHC6H4OCOC6H4OCmH2m þ 1 where m ¼ 6, 7, 8, 9, 10, 11, 12, 14,
and 16 have been synthesized. These compounds have been characterized by elemen-
tal analyses, FT-IR, UV-visible, 1H, and 13C NMR spectroscopy. Mesomorphic
properties of these compounds were studied by differential scanning calorimetry
and polarizing microscopy. The mesomorphic nature of these compounds is depen-
dent on the alkoxy chain length. The compounds (m ¼ 6, 7) do not exhibit meso-
morphism while the compounds (m ¼ 8, 9, 11, 12, 14) show a monotropic nematic
mesophase in cooling cycle. The 3-[4-(40-decyloxybenzoyloxybenzylidene)amino]-
1,2,4-triazine displays an enantiotropic nematic mesophase and the hexadecyloxy
compound exhibits the smectic A mesophase.
Keywords Differential scanning calorimetry; enantiotropic nematic mesophase;
monotropic nematic mesophase; polarizing microscopy; smectic A mesophase
1. Introduction
1,3,5-Triazine and its derivatives are an interesting class of compounds which show
various biological properties such as antimicrobial and antiviral [1,2], and antiseptic
[3–5] properties. There have been considerable reports on the synthesis and charac-
terization of compounds containing 1,3,5-triazine unit [6] owing to their liquid crys-
talline and non linear optical properties as well as their applications in other fields
[7–19]. The triazine system has been used as a core of liquid crystalline materials
[14,15] and for the development of organic light emitting diodes OLEDS [20–23].
Lee and Yamamoto [24] reported a series of 2,4,6-triphenyl-1,3,5-triazine deriva-
tives having long alkoxy side chains. These compounds behave as liquid crystalline
materials and show quantum yields higher than 50% in photoluminescence. Salisu
[25] reported that 2,4,6-tris-[4-(4-acetylphenyl azo)phenoxyalkoxyethyleneamino]-1,
Address correspondence to Bachcha Singh, Department of Chemistry, Faculty of Science,
BHU, Varanasi 221005, India. E-mail: bsinghbhu@rediffmail.com
127

128
B. Singh et al.
3,5-triazines showing enantiotropic liquid crystal property exhibited smectic
mesophase in both heating and cooling cycles.
Maly et al. [26] reported a new series of supramolecular columnar liquid crystal-
line materials based on self-association of diaminotriazines. These compounds
appear to form disc-like hydrogen-bonded rosettes that are able to form columnar
mesophases by stacking of an extended aromatic core.
Goldman and Nordsieck [27] reported columnar liquid crystalline phase of
2,4,6-triarylamino-1,3,5-triazines substituted with six long peripheral alkoxy chains.
Cowling et al. [28] reported synthesis and properties of new liquid crystals containing
1,2,4-triazine unit which serves as intermediates for the synthesis of new meso-
morphic pyridines. Frolova and coworkers [29] reported new mesogenic discotic tria-
zine based esters which yield a low temperature monotropic columnar mesophase.
The literature survey reveals that relatively less work has been reported on the
synthesis, characterization, and mesomorphic properties of ester based asymmetric
triazines compared to symmetrical ones. The reports on 1,2,4-triazine containing
liquid crystals are, however, relatively rare, which might be due to the direct result
of their high reactivity and, therefore, low stability. The high reactivity makes them
interesting as building blocks for further modification. In view of the above observa-
tions the synthesis, characterization, and the mesomorphic properties of a new series
of ester-based 3-[4-(40-alkoxybenzoyloxybenzylideneamino]-1,2,4-triazines are repor-
ted in this article.
2. Experimental
Materials
3-Amino-1,2,4-triazine, 1-bromoalkanes, 4-hydroxybenzoic acid, 4-hydroxy benzal-
dehyde (all Aldrich), thionyl chloride, and pyridine (Qualigens) were used as
received. All other solvents and reagents were purchased commercially and used
after further purification.
Measurements
Elemental analyses were performed on CE-440 Exeter Analytical CHN analyzer. IR
spectra (4000–100 cmꢀ1) were recorded on Varian 3100 FT-IR Excalibur spectro-
1
photometer. H and 13C NMR spectra were obtained by using a JEOL FT NMR
AL 300 MHz spectrometer using tetramethylsilane as internal standard. Electronic
spectra were recorded on UV-1700 Pharma Spec Shimadzu UV-Visible spectrophot-
ometer. DSC thermograms were recorded by Mettler Toledo TC 15 TA differential
scanning calorimeter at the rate of 10.0 K minꢀ1 under nitrogen atmosphere using
spec pure grade indium as standard by taking samples in close lid aluminum pans.
The transition temperatures from DSC have been determined with accuracy of
ꢁ0.1 K. The mesophase type was assigned by visual comparison with known phase
standards using HT-30.01 NTT 268 LOMO polarizing optical microscope fitted with
a hot stage with temperature accuracy of 0.1 K.
Synthesis of Compounds
Preparation of 4-octyloxybenzoic Acid 2. To a solution containing 4-hydroxybenzoic
acid (1) (2.76 g, 20 mmol), in dry ethanol (50 ml) and KOH (3.36 g, 60 mmol) in dry

Liquid Crystalline 1,2,4-triazines
129
ethanol (20 ml), 1-bromooctane (3.47 ml, 20 mmol) was added in drops with
continuous stirring. The reaction mixture was refluxed for ꢂ14 h under dry
atmosphere and allowed to cool to room temperature. The solid alkoxy potassium
salt was separated and treated with dilute HCl until the pH of the reaction
mixture became ꢂ2. The white solid product was filtered off, washed with water
and recrystallized successively from glacial acetic acid and toluene. Yield: 74%. IR
(KBr, cmꢀ1): 3420 (OH), 2926, 2851 (aliphatic CꢀH), 1688 (C O), 1606, 1513
=
(Ph), 1317, 1296, 1254 (OPh). 1 H NMR (CDCl3, TMS) dH (ppm): 11.90 (s, 1 H,
ꢀOH), 8.06–8.03 (d, 2 H, ArH), 6.94–6.91 (d, 2 H, ArH), 4.04–4.00 (t, 2 H,
ꢀOCH2), 1.85–1.29 (m, 12 H, ꢀ[CH2]n), 0.89–0.86 (t, 3 H, ꢀCH3). 13C NMR dC
(CDCl3): 171.9, 163.6, 132.3, 121.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 68.2 (ꢀOCH2),
31.7, 29.3, 29.2, 29.0, 25.9, 22.6, 14.0 (CH3).
Preparation of 4-(40-Octyloxybenzoyloxy)benzaldehyde 3. 4-Octyloxybenzoic acid
(1.25 g, 5 mmol) dissolved in dry chloroform (50 ml) was heated under reflux with
thionyl chloride (0.57 ml, 8 mmol) for 8 h. The solvent was removed under reduced
pressure to give a yellow oil. It was further treated with a solution of (0.61 g,
5 mmol) of 4-hydroxy benzaldehyde in dry chloroform (50 ml) in presence of 2–3
drops of pyridine. The mixture was then refluxed for 9 h. A light yellow solution
was obtained. It was washed with water (55 ml), dried over anhydrous MgSO4,
and then concentrated to form white crystals of 4-(40-octyloxybenzoyloxy)
benzaldehyde. Yield: 60%, IR (KBr, cmꢀ1): 2923, 2853 (aliphatic CꢀH), 1733
1
=
=
(ester, C O), 1698 (aldehyde C O), 1605, 1511 (Ph), 1308, 1260 (OPh). H NMR
(CDCl3, TMS) dH (ppm): 10.02 (s, 1 H, CHO), 8.15–6.88 (m, 8 H, ArH),
4.07–3.97 (m, 2H, ꢀOCH2), 1.81–1.27 (m, 12 H, ꢀ[CH2]n), 0.90–0.86 (t, 3 H, ꢀCH3).
All the other members of the series were prepared following the above
procedure.
Synthesis of 3-[4-(40-Octyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine 4c. A
mixture of 3-amino-1,2,4-triazine (0.48 g, 5 mmol) and 4-(40-octyloxy benzoyloxy)
benzaldehyde (1.77 g, 5 mmol) in 50 ml of methanol containing 3–4 drops of
glacial acetic acid was heated under reflux for 7 h. A yellow solution thus
obtained was evaporated under reduced pressure to yield reddish brown crystals.
This product was washed with water and recrystallized from chloroform.
All the other homologous members of the series were prepared in a similar
manner. Their IR, analytical, and NMR data are summarized as follows.
3-[4-(40-Hexyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C23H24N4O3 (4a). Yield
67%. IR (KBr, cmꢀ1): 2927, 2857 (aliphatic CꢀH), 1732 (ester C O), 1699 (CH N),
=
=
=
1602 (Ph), 1306 (OPh), 1263 (triazine ring C N conjugated), 1062 (triazine ring CꢀN
non-conjugated), 846 (triazine ring out of plane CꢀH aromatic bend). 1H NMR
=
(CDCl3, TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.14–8.01 (q, 2 H, heterocyclic
ArH), 7.97–6.90 (m, 8 H, ArH), 4.00–3.99 (t, 2 H, ꢀOCH2), 1.85–1.25 (m, 8 H,
ꢀ[CH2]n), 0.91 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3): 191.0, 171.2, 164.2, 163.8
=
(ꢀCH N), 163.4, 155.8, 150.3, 141.1, 133.8, 132.4, 132.1, 131.2, 127.8, 122.5, 121.6,
120.7, 114.3, 114.1, 77.4, 77.0, 76.5 (CDCl3), 68.3 (OCH2), 68.2, 31.5, 29.6, 29.0, 25.6,
22.5, 14.0 (CH3). UV-visible (CHCl3): kmax ¼ 272 nm. Elemental analyses: calculated
(for C23H24N4O3), C, 68.30; H, 5.98; N, 13.85%, found, C, 68.21; H, 5.88; N, 13.67%.

130
B. Singh et al.
3-[4-(40-Heptyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C24H26N4O3 (4b). Yield
62%. IR (KBr, cmꢀ1): 2928, 2855 (aliphatic CꢀH), 1733 (ester C O), 1696 (CH N),
=
=
=
1604 (Ph), 1306 (OPh), 1260 (triazine ring C N conjugated), 1064 (triazine ring C–N
non-conjugated), 844 (triazine ring out of plane CꢀH aromatic bend). 1H NMR
=
(CDCl3, TMS) dH (ppm): 10.01 (s, 1 H, CH N), 8.14–8.01 (q, 2 H, heterocyclic ArH),
7.98–6.88 (m, 8 H, ArH), 4.07–3.97 (t, 2 H, OCH2), 1.82–1.31 (m, 10 H,
ꢀ[CH2]n), 0.88 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3): 166.8, 162.9, 131.5, 122.2,
114.0, 77.4, 77.0, 76.5 (CDCl3), 68.1, 31.7, 29.0, 25.9, 22.5, 14.0 (CH3). UV-visible
(CHC13): kmax ¼ 269 nm. Elemental analyses: calculated (for C24H26N4O3), C, 68.88; H,
6.26; N, 13.38%, found, C, 68.79; H, 5.96; N, 13.31%.
3-[4-(40-Odyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C25H28N4O3 (4c). Yield
64%. IR (KBr, cmꢀ1): 2926, 2854 (aliphatic CꢀH), 1735 (ester C O), 1685 (CH N),
=
=
=
1605 (Ph), 1306 (OPh), 1258 (triazine ring C N conjugated), 1063 (triazine ring
1
CꢀN non-conjugated), 845 (triazine out of plane CꢀH aromatic bend). H NMR
=
(CDCl3, TMS) dH (ppm): 10.01 (s, 1H, CH N), 8.14–8.02 (q, 2 H, heterocyclic
ArH), 7.97–6.90 (m, 8 H, ArH), 4.04–4.00 (t, 2 H, ꢀOCH2), 1.80–1.29 (m, 12 H,
ꢀ[CH2]n), 0.89 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3): 191.0, 171.5, 164.2, 163.8
=
(ꢀCH N), 163.6, 155.9, 133.8, 132.4, 132.3, 131.2, 122.6, 121.3, 120.7, 114.4, 114.1,
77.4, 77.0, 76.5, 68.4 (ꢀOCH2), 68.2, 31.7, 29.6, 29.3, 29.2, 29.0, 25.9, 22.6, 14.0
(CH3). UV-visible (CHC13): kmax ¼ 274 nm. Elemental analyses: calculated (for
C25H28N4O3), C, 69.42; H, 6.52; N, 12.95%, found, C, 69.25; H, 6.39; N, 12.78%.
3-[4-(40-Nonyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C26H30N4O3 (4d). Yield
60%. IR (KBr, cmꢀ1): 2925, 2853 (aliphatic CꢀH), 1733 (ester C O), 1684 (CH N), 1605
=
=
=
(Ph), 1307 (OPh), 1255 (triazine ring C N conjugated), 1059 (triazine ring CꢀN
1
non-conjugated), 844 (triazine out of plane CꢀH aromatic bend). H NMR (CDCl3,
=
TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.14–8.02 (q, 2 H, heterocyclic ArH),
7.99–6.87 (m, 8 H, ArH), 4.04–3.97 (t, 2 H, ꢀOCH2), 1.81–1.28 (m, 14 H,
ꢀ[CH2]n), 0.88 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3): 190.9, 171.5, 166.4, 163.6
=
(ꢀCH N), 162.8, 132.4, 132.2, 131.4, 131.2, 122.6, 122.5, 121.3, 114.4, 114.1, 113.9,
77.4, 77.0, 76.5 (CDCl3), 68.3 (ꢀOCH2), 68.2, 68.1, 60.5, 31.8, 29.6, 29.4, 29.3, 29.2,
29.1, 29.0, 25.9, 22.6, 14.0 (CH3). UV-visible (CHC13): kmax ¼ 285, 259 nm. Elemental
analyses: calculated (for C26H30N4O3), C, 69.93; H, 6.77; N, 12.54%, found, C, 69.44;
H, 5.23; N, 12.48%.
3-[4-(40-Decyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C27H32N4O3 (4e). Yield
65%. IR (KBr, cmꢀ1): 2925, 2854 (aliphatic CꢀH), 1732 (ester C ¼ 0), 1681 (CH N),
=
=
1605 (Ph), 1307 (OPh), 1256 (triazine ring C N conjugated), 1060 (triazine ring CꢀN
1
non-conjugated), 844 (triazine out of plane CꢀH aromatic bend). H NMR (CDCl3,
=
TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.14–8.04 (q, 2 H, heterocyclic ArH),
7.99–6.91 (m, 8 H, ꢀArH), 4.04–3.99 (t, 2 H, ꢀOCH2), 1.80–1.27 (m, 16 H,
ꢀ[CH2]n), 0.88–0.86 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3): 163.6, 132.3, 121.3,
114.1, 77.4, 77.0, 76.5 (CDCl3), 68.2, 31.8, 29.7, 29.5, 29.3, 29.0, 25.9, 22.6, 14.1
(CH3). UV-visible (CHCl3): kmax ¼ 328, 266 nm. Elemental analyses: calculated (for
C27H32N4O3), C, 70.41; H, 7.00; N, 12.16%, found, C, 70.28; H, 6.83; N, 11.90%.
3-[4-(40-Undecyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C28H34N4O3
(4f). Yield 59%. IR (KBr, cmꢀ1): 2924, 2853 (aliphatic CꢀH), 1733 (ester C ¼ 0),
=
1680 (CH N), 1605 (Ph), 1307 (OPh), 1255 (triazine ring C N conjugated), 1060
=

Liquid Crystalline 1,2,4-triazines
131
(triazine ring CꢀN non-conjugated), 845 (triazine out of plane CꢀH aromatic bend).
1H NMR (CDCl3, TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.14–8.01 (q, 2 H,
=
heterocyclic ArH), 7.99–6.87 (m, 8 H, ArH), 4.04–3.97 (t, 2 H, ꢀOCH2),
1.81–1.26 (m, 18 H, ꢀ[CH2]n), 0.88–0.85 (t, 3 H, ꢀCH3). UV-visible (CHC13):
kmax ¼ 267 nm. Elemental analyses: calculated (for C28H34N4O3), C, 70.86; H, 7.22;
N, 11.80%, found, C, 70.81; H, 7.04; N, 11.64%.
3-[4-(40-Dodecyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C29H36N4O3
(4g). Yield 68%. IR (KBr, cmꢀ1): 2921, 2853 (aliphatic CꢀH), 1733 (ester C O),
=
=
=
1651 (CH N), 1604 (Ph), 1306 (OPh), 1263 (triazine ring C N conjugated), 1056
(triazine ring CꢀN non-conjugated), 855 (triazine out of plane CꢀH aromatic
1
=
bend). H NMR (CDCl3, TMS) dH (ppm): 10.02 (s, 1 H, ꢀCH N), 8.14–8.05 (q,
2 H, heterocyclic ArH), 7.97–6.94 (m, 8 H, ArH), 4.07–4.03 (t, 2 H, ꢀOCH2),
1.82–1.26 (m, 20 H, ꢀ[CH2]n), 0.90–0.86 (t, 3 H, ꢀCH3). UV-visible (CHCl3):
kmax ¼ 337, 272 nm. Elemental analyses: calculated (for C29H36N4O3), C, 71.28; H,
7.42; N, 11.46%, found, C, 71.34; H, 7.32; N, 11.34%.
3-[4-(40-Tetradecyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C31H40N4O3
(4h). Yield 70%. IR (KBr, cmꢀ1): 2920, 2851 (aliphatic CꢀH), 1732 (ester C O),
=
=
1680 (CH N), 1606 (Ph), 1307 (OPh), 1257 (triazine ring C N conjugated), 1062
=
(triazine CꢀN non-conjugated), 846 (triazine out of plane CꢀH aromatic bend).
1H NMR (CDCl3, TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.15–8.00 (q, 2 H,
=
heterocyclic ArH), 7.98–6.88 (m, 8 H, ArH), 4.04–3.97 (t, 2 H, ꢀOCH2), 1.81–1.26
(m, 24 H, ꢀ[CH2]n), 0.90–0.85 (t, 3 H, ꢀCH3). Elemental analyses: calculated (for
C31H40N4O3), C, 72.06; H, 7.80; N, 10.84%, found, C, 71.84; H, 7.42; N, 10.62%.
3-[4-(40-Hexadecyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine, C33H44N4O3
(4i). Yield 74%. IR (KBr, cmꢀ1): 2918, 2851 (aliphatic CꢀH), 1736 (ester C O),
=
=
1691 (CH N), 1606 (Ph), 1306 (OPh), 1264 (triazine ring C N conjugated), 1067
=
(triazine ring CꢀN non-conjugated), 845 (triazine out of plane CꢀH aromatic
1
=
bend). H NMR (CDCl3, TMS) dH (ppm): 10.01 (s, 1 H, ꢀCH N), 8.14–8.01 (q,
2 H, heterocyclic ArH), 7.97–6.90 (m, 8 H, ArH), 4.06–4.00 (t, 2 H, ꢀOCH2),
1.84–1.26 (m, 28 H, ꢀ[CH2]n), 0.90–0.85 (t, 3 H, ꢀCH3). 13C NMR dC (CDCl3):
190.9, 163.8, 163.6, 133.8, 132.4, 132.3, 131.2, 122.5, 120.7, 114.4, 114.1, 77.4,
77.0, 76.5 (CDCl3), 68.3, 68.2, (ꢀOCH2), 31.9, 29.6, 29.5, 29.3, 29.0, 25.9, 22.6,
14.1 (CH3). UV-visible (CHCl3): kmax ¼ 339, 270 nm. Elemental analyses:
calculated (for C33H44N4O3), C, 72.76; H, 8.14; N, 10.28%, found, C, 72.38; H,
8.11; N, 10.12%.
3. Results and Discussion
The compounds 3-[4-(40-alkoxybenzoyloxybenzylidene)amino]-1,2,4-triazines 4a–i
have been prepared as shown in Scheme 1. The elemental data are in agreement with
the formulae of the compounds. The IR and NMR spectral data are fully consistent
with the structure. In the IR spectrum of 3-amino-1,2,4-triazine the bands observed
are designated as 3312 vas(NꢀH), 3113 vs(NꢀH), 1650 (NH2 bend), 1341 and
=
1266 (C N conjugated aromatic ring), 1044 (CꢀN non-conjugated aromatic ring)
and 860 cmꢀ1 (CꢀH out of plane bend of aromatic ring) respectively. The IR
spectrum of 4-(40-octyloxybenzoyloxy)benzaldehyde 3 shows absorption bands
=
which are designated as 2923 and 2853 (aliphatic CꢀH), 1733 (C O ester), 1698

132
B. Singh et al.
Scheme 1. Synthetic route for 3-[4-(40-alkoxybenzoyloxybenzylidene)amino]-1,2,4-triazines.
ꢀ1
=
(C O aldehyde), 1605, 1511 (Ph), 1308, and 1260 cm for (OPh), respectively. The
IR spectrum of 3-[4-(40-octyloxybenzoyloxybenzylidene)amino]-1,2,4-triazine 4c
shows absorption bands assigned as 2926, 2854 (aliphatic CꢀH), 1735 (ester
=
=
=
C O), 1685 (CH N), 1605 (Ph), 1306 (OPh), 1258 (triazine ring C N conjugated),
1063 (triazine ring CꢀN non-conjugated), 845 cmꢀ1 (triazine out of plane aromatic
CꢀH bend), respectively. The disappearance of thꢀe1NH2 bending mode of triazine
b(NH2) at 1650 cmꢀ1, and v(C O) band (1698 cm ) of 4-(40-octyloxybenzoyloxy)-
=
benzaldehyde along with the appearance of a new band in the compound 3-[4-(40-
octyloxybenzoloxybenzylidene)amino]-1,2,4-triazine centered at 1685 cmꢀ1 due to
=
n(C N) indicates condensation of amine with aldehyde forming the Schiff base.
The proton NMR spectrum of 3-amino-1,2,4-triazine depicts peaks which are
assigned as d 8.54 (s, 1 H, triazine ring), 8.20 (s, 1 H, triazine ring) and 7.17 (s,
2H, NH2), respectively. The proton NMR spectrum of 4-(40-octyloxybenzoyloxy)-
benzaldehyde exhibits peaks at d 10.02 (s, 1 H, ꢀCHO), 8.15–6.88 (m, 8 H, ArH),
4.07–3.97 (m, 2 H, ꢀOCH2), 1.81–1.27 (m, 12 H, ꢀ[CH2]n), 0.90–0.86 (t, 3 H,
ꢀCH3). The proton NMR spectrum of 3-[4-(40-octyloxybenzoyloxybenzylidene)-
=
amino]-1,2,4-triazine 4c shows peaks at d 10.01 (s, 1 H, CH N), 8.14–8.02 (q, 2
H, heterocyclic ArH), 7.97–6.90 (m, 8 H, ArH), 4.04–4.00 (t, 2 H, ꢀOCH2),
1.80–1.29 (m, 12 H, ꢀ[CH2]n), and 0.89 (t, 3 H, ꢀCH3), respectively. The disappear-
ance of the NMR signals concerning NH2 (d 7.17) of 3-amino-1,2,4-triazine and
CHO (d 10.02) of 4-(40-octyloxybenzoyloxy)benzaldehyde along with the appearance
of a new signal in the compound 4c at d 10.01 confirms the formation of Schiff
base. The signals of two aromatic rings across the ester linkage in compound 4c over-
lap and are observed at d 7.97–6.90 whereas that of heterocyclic aromatic triazine
ring is observed at d 8.14–8.02 ppm. The length of alkyl chain has no significant
=
effect on the position of the signals due to ꢀCH N, heterocyclic aromatic triazine

Liquid Crystalline 1,2,4-triazines
133
ring, phenyl ring, ꢀOCH2, ꢀ[CH2]n and ꢀCH3 protons. The UV-visible spectrum of
3-[4-(40-octyloxybenzoyloxybenzylideneamino)]-1,2,4-triazine 4c shows bands at
274 nm (n ! pꢃ transition, R band) in which an unshaired electron pair on a hetero-
atom is excited to pꢃ antibonding orbital. This involves least amount of energy than
all the transitions, and therefore, this transition gives rise to an absorption band at
longer wavelength.
4. Optical Properties
The liquid crystalline character of 3-[4-(40-alkoxybenzoyloxybenzylidine) amino]-1-
,2,4-triazine 4a–i was investigated by polarizing optical microscope (POM) using a
hot stage. The phase transition temperatures and enthalpies were obtained by carry-
ing out DSC thermal analysis. The phase transition temperatures along with the
enthalpy values for the compounds are summarized in Table 1. The symbol K, N,
SmA, and I are used to denote crystalline, nematic, smectic A, and isotropic phases,
respectively. The compound 3-[4-(40-decyloxybenzoyloxybenzylidene)amino]-1,2,4-
triazine 4e exhibits endothermic peaks at 64.3ꢄC (DH ¼ 0.35 kJmolꢀ1), 82.5ꢄC
(DH ¼ 22.24 kJmolꢀ1), and 91.7ꢄC (DH ¼ 11.44 kJmolꢀ1); the first two peaks are
indicative of a crystal-to-crystal transition, while the third indicates a crystal-to-
mesophase transition, and the one observed at 141.6ꢄC defines a mesophase-to-
isotropic phase transition. The exothermic peaks are observed at 103.8ꢄC
(DH ¼ 0.64 kJmolꢀ1), 86.8ꢄC (DH ¼ 12.07 kJmolꢀ1), 70.9ꢄC (DH ¼ 8.07 kJmolꢀ1),
and 60.5ꢄC (DH ¼ 0.57 kJmolꢀ1), which are attributed to isotropic-to-mesophase
and mesophase-to-crystal phase transitions followed by subsequent crystal phase.
This compound (4e) under polarizing optical microscope reveals an enantiotropic
nematic phase both in heating and cooling cycles at 91.7ꢄC and 103.8ꢄC, respect-
ively. The optical micrograph at 91.7ꢄC is characteristic of a nematic phase with
black brushes 2–4 in numbers are seen emanating from the central nuclei formed
by disclination lines (line defects) giving rise to a schlieren texture. In cooling cycle
nematic droplets appear at 103.8ꢄC which displays the black stripes 2–4 fold with a
schlieren texture. The droplets increase in size and collapse with each other to form a
film showing prominent black stripes. The joining of radiating arms of one nucleus
(molecule) with the other nucleus is clearly seen within the droplets in this phase
of the compound. The compound 4g exhibits endothermic peaks at 46.7ꢄC
(DH ¼ 49.17 kJmolꢀ1) and 65.2ꢄC (DH ¼ 0.42 kJmolꢀ1) due to crystal-to-crystal
phase transitions followed by an isotropic phase transition at 191.8ꢄC. The exother-
mic peaks are observed at 150.1ꢄC (DH ¼ 0.22 kJmolꢀ1) and 109.1ꢄC (DH ¼
0.56 kJmolꢀ1), characteristics of isotropic to mesophase and mesophase-to-crystal
phase transitions, respectively. The optical micrograph reveals monotropic nematic
behavior of this compound which is observed only in cooling cycle at 150.1ꢄC.
The compound 4c exhiꢀb1its endothermic peaks at 71.1ꢄC (DH ¼ 18.66 kJmolꢀ1),
104.6ꢄC (DH ¼ 0.40 kJmol ), 145.4ꢄC (DH ¼ 0.06 kJmolꢀ1), which are due to
successive polymorphic phase involving crystal to crystal transitions and a
peak observed at 186.7ꢄC is attributed to isotropic phase, transition. The exother-
mic peaks for this compound are observed at 150.7ꢄꢀC1 (DH ¼ 0.12 kJmolꢀ1),
109.7ꢄC (DH ¼ 0.25 kJmolꢀ1), 103.0ꢄC (DH ¼ 2.47 kJmol ), and 65.0ꢄC (DH ¼
3.64 kJmolꢀ1), which are attributed to the transition from isotropic-to-mesophase
and mesophase-to-crystal phase followed by the subsequent crystal phases. The
optical micrograph reveals the monotropic nematic phase for this compound with

134
B. Singh et al.
Table 1. Thermal transition temperatures, associated enthalpy (DH), and entropy
(DS), changes of 3-[4-(40-alkoxybenzoyloxybenzylidene) amino]-1,2,4-triazines
Compound
4a
Transitions
T=ꢄC
DH=kJmolꢀ1
DS=JKꢀ1molꢀ1
K1–K2
K2–K3
K3–K4
K4–I
46.0
60.7
73.0
168.6
150.0
108.8
83.1
65.9
57.1
14.13
1.27
0.54
12.73
0.11
0.18
22.28
5.71
14.90
18.66
0.40
0.06
0.15
0.12
0.25
2.47
3.64
0.12
9.93
1.24
12.81
0.35
22.24
11.44
0.14
0.64
12.07
8.07
0.57
0.30
2.78
1.09
1.23
49.17
0.42
0.19
0.22
0.56
0.14
16.93
0.09
20.52
34.37
22.60
44.29
3.80
1.56
28.82
0.26
0.47
62.56
16.84
45.13
54.22
1.05
0.14
0.32
0.28
0.65
6.56
10.76
0.35
28.30
3.87
I–K4
K4–K
K1–I
4b
4c
I–K1
K1–K
K1–K2
K2–K3
K3–K4
K4–I
71.1
104.6
145.4
186.7
150.7
109.7
103.0
65.0
64.2
77.8
46.8
38.2
64.3
82.5
91.7
141.6
103.8
86.8
70.9
60.5
64.2
70.5
66.1
60.2
46.7
65.2
191.8
150.1
109.1
64.4
120.0
109.5
43.6
70.8
89.1
I–N
N–K4
K4–K3
K3–K
K1–K2
K2–I
4d
4e
I–N
N–K
K1–K2
K2–K3
K3–N
N–I
41.16
1.03
62.55
31.36
0.33
I–N
1.69
N–K3
K3–K2
K2–K1
K1–K2
K2–I
33.54
23.46
1.70
0.88
8.09
3.21
3.69
153.80
1.24
0.40
0.52
1.46
0.41
43.07
0.23
64.81
99.97
62.41
4f
I–N
N–K
K1–K2
K2–K3
K3–I
I–N
N–K
K1–K2
K2–I
I–N
N–K
K1–K2
K2–K3
4g
4h
4i
(Continued)

Liquid Crystalline 1,2,4-triazines
135
Table 1. Continued
Compound
Transitions
T=ꢄC
DH=kJmolꢀ1
DS=JKꢀ1molꢀ1
K3–K4
K4–K5
K5–I
104.7
112.1
192.0
113.9
106.3
58.9
0.11
2.69
0.08
3.36
0.80
13.72
0.29
6.98
0.17
8.68
2.10
41.33
I–SmA
SmA–K5
K5–K
the appearance of schlieren texture with black brushes or black stripes emanating
from the central nuclei formed as a result of disclination or line defects in the
compound.
The compound 4d exhibits sharp endothermic peaks at 64.2ꢄC (DH ¼
0.12 kJmolꢀ1) and 77.8ꢄC (DH ¼ 9.93 kJmolꢀ1), which are indicative of crystal-to-
crystal and crystal-to-isotropic phase transitions, respectively. The exothermic peaks
in DSC thermogram of this compound are observed at 46.8ꢄC (DH ¼ 1.24 kJmolꢀ1
)
and 38.2ꢄC (DH ¼ 12.81 kJmolꢀ1) due to isotropic to mesophase and mesophase-to-
crystal phase transition, respectively. The optical micrograph at 46.8ꢄC in cooling
cycle displays numerous nematic droplets (bubbles) with radial outline confirming
nematic phase. The similar phase transitions and texture patterns are observed for
the compounds 4f and 4h.
The compound 4i exhꢀib1its endothermic peaks at 70.8ꢀꢄC1 (DH ¼ 34.37 kJmolꢀ1),
89.1ꢄC (DH ¼ 22.60 kJmol ), 104.7ꢄC (DH ¼ 0.11kJmol ), and 112.1ꢄC (DH ¼
2.69kJmolꢀ1) attributed to subsequent crystal-to-crystal phase transitions (polymor-
phism) and that observed at 192ꢄC defines a crystal-to-isotropic phase transition.
The exothermic peaks for this comꢀp1ound are observed at 113.9ꢄC (DH ¼
3.36kJmolꢀ1), 106.3ꢄC (DH ¼ 0.80kJmol ), and 58.9ꢄC (DH ¼ 13.72 kJmolꢀ1), which
are characteristic of isotropic-to-mesophase and mesophase-to-crystal followed by
another crystal phase transition. This compound exhibits polymorphic changes
(i.e., crystal–crystal transitions) in heating cycle mainly. The optical microscopy dis-
plays monotropic smectic A phase for this compound appearing only in cooling cycle
at 113.9ꢄC. The compound 4a exhibits endothermic peaks at 46.0ꢄC (DH ¼
14.13 kJmolꢀ1), 60.7ꢄC (DH ¼ 1.27kJmolꢀ1), and 73.0ꢄC (DH ¼ 0.54kJmolꢀ1) which
correspond to subsequent crystal-to-crystal phase transitions, and the one observed
at 168.6ꢄC is assigned to a crystal-to-isotropic liquid transition. The exothermic
peaks were observed at 150ꢄC (DH ¼ 0.11kJmolꢀ1
)
and 108.8ꢄC (DH ¼
0.18kJmolꢀ1) which define isotropic-to-crystal phase transition followed by the next
crystal phase. The compound 4b exhibits an endothermic peak at 83.1ꢄC
(DH ¼ 22.28 kJmolꢀ1) which defines a sharp crystal to isotropic phase transition
whereas the appearance of an exothermic peak at 65.9ꢄC (DH ¼ 5.71kJmolꢀ1) and
57.1ꢄC (DH ¼ 14.90 kJmolꢀ1) indicates an isotropic to crystal transition followed
by the next crystal phase. The optical analysis revealed no texture pattern for this
compound both in heating and cooling cycles.
Thus, the interactions between aromatic rings are expected to play an important
role in molecular ordering. The alkyl chain length provides the mobility necessary for
molecular arrangement into an ordered structure. The lower homologs of the series
with m ¼ 6 and 7 exhibit no mesomorphism, the homologs with m ¼ 8, 9, 11, 12, and

136
B. Singh et al.
14 exhibit monotropic nematic mesophase in cooling cycle whereas the member with
m ¼ 10 exhibits enantiotropic nematic mesophases both in heating and cooling cycle.
The higher member of the series with m ¼ 16 displays a monotropic smectic A meso-
phase exclusively. Thus at the shorter alkyl chain length, a monotropic nematic
phase is seen but as the alkyl chain length increases the smectic A phase appears.
Apparently, the lone pairs of electrons on the nitrogen atoms act to broaden the
molecule and also introduce attractive forces which aid smectic formation.
The shape anisotropy in the triazines is responsible for the calamitic mesophase
exhibited by the 3-[4-(40-alkoxybenzoyloxybenzylidene) amino]-1,2,4-triazines. The
mesophase behavior is further affected by the chain length. The intermolecular
forces are strong for C6 and C7, resulting in the crystalline nature. However, for
C8, C9, C11, C12, and C14 the molecular forces are weak yielding thermodynamically
less stable monotropic nematic mesophase. While in the C10 homologue, the thermo-
dynamically stable enantiotropic nematic mesophase is stabilized; the larger chain
length C16 results in the further more stable smectic A mesophase.
5. Conclusions
A new series of 3-[4-(40-alkoxybenzoyloxybenzylidene)amino]-1,2,4-triazines have
been synthesized having terminal alkoxy chains (m ¼ 6ꢂ16). The chemical structures
1
of the final products were investigated by FT-IR, NMR, UV-visible, H, and 13C
NMR spectra. The mesomorphic properties and optical textures of the products
were characterized by DSC and POM. The existence of nematic (schlieren) and
smectic A (focal-conic) texture was confirmed by POM. It was found that the meso-
genic nature of these compounds is dependent on the alkoxy chain length. The lower
homologues with (m ¼ 6, 7) were found to be non-mesogenic, whereas the higher
homologues displayed nematic and smectic A pattern.
Acknowledgment
The authors thank the Head, Department of Chemistry, BHU, Varanasi for provid-
ing laboratory facilities. One of the authors (AP) thanks UGC for financial support
as Project Fellow and BHU for research scholarship. Thanks are also due to
Mr. Satish Tiwari for CHN and DSC analyses, Mr. R. C. P. Bipin for NMR and
Mr. V. N. Pandey for IR and electronic spectral data.
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