Synthesis and characterization of 1,4-phenylenediamine derivatives containing hydroxyl and cyclotriphosphazene as terminal group

Article abstract of DOI:10.1016/j.molstruc.2019.02.013

A series of compounds with two Schiff base linking units and four different substituents (heptyl, dodecyl, methoxy and chloro) have been successfully synthesized. Further reactions form new monosubstituted cyclotriphosphazene based molecules with different substituents. These compounds were characterized using FT-IR (Fourier Transform Infrared), NMR (Nuclear Magnetic Resonance) and CHN elemental analysis. The transition mesophase(s) of these compounds were determined using POM (Polarized Optical Microscope) and DSC (Differential Scanning Calorimetry). Two compounds with heptyl and dodecyl substituents were found to be mesogenic with smectic C phases while monosubstituted cyclotriphosphazene compounds of the same substituents (heptyl and dodecyl chains) were also found to be mesogenic. Cyclotriphosphazene compounds with heptyl chain shows smectic C and nematic phases while compound with dodecyl chain shows only the nematic phase. However, compounds with methoxy and chloro substituents were found to be non-mesogenic.

Full text of DOI:10.1016/j.molstruc.2019.02.013

Accepted Manuscript
Synthesis and characterization of 1,4-phenylenediamine derivatives containing
hydroxyl and cyclotriphosphazene as terminal group
Zuhair Jamain, Melati Khairuddean, Siti Amira Saidin
PII:
S0022-2860(19)30137-1
DOI:
https://doi.org/10.1016/j.molstruc.2019.02.013
MOLSTR 26170
Reference:
To appear in: Journal of Molecular Structure
Received Date: 5 July 2018
Revised Date: 1 February 2019
Accepted Date: 4 February 2019
Please cite this article as: Z. Jamain, M. Khairuddean, S.A. Saidin, Synthesis and characterization of
1,4-phenylenediamine derivatives containing hydroxyl and cyclotriphosphazene as terminal group,
Journal of Molecular Structure (2019), doi: https://doi.org/10.1016/j.molstruc.2019.02.013.
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ACCEPTED MANUSCRIPT
Synthesis and characterization of 1,4-phenylenediamine derivatives containing
hydroxyl and cyclotriphosphazene as terminal group
Zuhair Jamaina, Melati Khairuddeana, Siti Amira Saidina.
aSchool of Chemical Sciences, Universiti Sains Malaysia (USM), 11800 Penang, Malaysia
ABSTRACT
A series of compounds with two Schiff base linking units and four different substituents
(heptyl, dodecyl, methoxy and chloro) have been successfully synthesized. Further
reactions form new monosubstituted cyclotriphosphazene based molecules with different
substituents. These compounds were characterized using FT-IR (Fourier Transform
Infrared), NMR (Nuclear Magnetic Resonance) and CHN elemental analysis. The transition
mesophase(s) of these compounds were determined using POM (Polarized Optical
Microscope) and DSC (Differential Scanning Calorimetry). Two compounds with heptyl
and dodecyl substituents were found to be mesogenic with smectic C phases while
monosubstituted cyclotriphosphazene compounds of the same substituents (heptyl and
dodecyl chains) were also found to be mesogenic. Cyclotriphosphazene compounds with
heptyl chain shows smectic C and nematic phases while compound with dodecyl chain
shows only the nematic phase. However, compounds with methoxy and chloro substituents
were found to be non-mesogenic.
KEYWORDS: Cyclotriphosphazene, Schiff base, mesogenic, nematic, smectic C

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INTRODUCTION
The study of liquid crystal materials has expanded greatly over the years. Even
though a large number of liquid crystal compounds have been synthesized and
characterized, not many cyclotriphosphazene liquid crystal molecules have been reported.
Liquid crystal is a state between solid (crystal) and liquid (isotropic) states. Liquid crystal
may flow like a liquid but its molecule has some solid properties and is oriented in a
crystal-like way. The study of LC properties began in 1888 when an Austrian botanist,
Friedrich Reinitzer discovered a material known as cholesteryl benzoate with two distinct
melting points. Upon heating, the solid changes into a hazy liquid and further heating
transformed it into a clear liquid. This finding started the discovery of a new phase of
matter called the liquid crystal phase [1].
The phosphorus-nitrogen chemistry, in particular the study of cyclotriphosphazenes
started since the nineteenth century but has been intensively investigated only in the mid
1950’s [2]. Cyclotriphosphazene is a ring compound consisting of alternating phosphorus
and nitrogen atoms with two substituents attached to the phosphorus atoms (Figure 1).
Cl
Cl
Cl
R
R
N
N
P
P Cl
R
P
P
R
N
N
N
N
P
P
Cl Cl
R R
R = can be any nucleophiles
Figure 1. Structure of cyclotriphosphazene.
Much attention has been focused on these interesting compounds because they
consist of inorganic backbones as well as organic side-chains. They have remarkable

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chemical stability and synthetic versatility which enables the introduction of any side
group, R on phosphorus [3]. Previous studies reported that substituents such as –NH2, -OR,
-OC6H5 or fluorinated derivatives are able to enhance the oxidative and thermal stabilities
[4]. Allcock and Klingenberg (1995) reported the aromatic azo phosphazene liquid crystals,
investigating the effect of the substituents on the aromatic unit, the function of the chain
length and different phosphazene skeletal structures [5]. Schiff base unit when attached to
the cyclotriphosphazene core were reported to have variety of applications as flame
resistance, biomedicine, semiconductors, dyes and catalysis [6-9].
This research work involved the synthesis and characterization of new compounds
having two Schiff base linking units with different terminal substituents. These
intermediates were further attached to the cyclotriphosphazene core through mono
substitution reaction. The aim is to determine the liquid crystal properties of these
cyclotriphosphazene compounds. In order to avoid complications from varieties of terminal
group associations, the main variables that will be studied are the effect of the chain length
and the type of terminal substituents towards the liquid crystal properties.
MATERIALS AND METHODS
CHEMICALS
The chemicals used in this study are listed as follows: 1-bromoheptane (MERCK
Schuchardt OHG), 1-bromododecane (MERCK Schuchardt OHG), 4-hydroxybenzaldehyde
(MERCK Schuchardt OHG), 1,4-phenylenediamine (MERCK Schuchardt OHG), 4-
chlorobenzaldehyde (MERCK Schuchardt OHG), 4-methoxybenzaldehyde (MERCK
Schuchardt OHG), methanol (QREC (ASIA) Sdn Bhd), N,N-dimethylformamide (QREC

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(ASIA) Sdn Bhd), potassium carbonate (QREC (ASIA) Sdn Bhd), potassium iodide
(MERCK Schuchardt OHG), anhydrous sodium sulphate (MERCK Schuchardt OHG), n-
hexane (QREC (ASIA) Sdn Bhd), ethyl acetate (BDH Laboratory Supplies Poole),
dichloromethane (QREC (ASIA) Sdn Bhd), tetrahydrofuran (MERCK Schuchardt OHG)
and phosphonitrilic chloride trimer (SIGMA-ALDRICH Co.).
EXPERIMENTAL
Compounds 1a-b were synthesized using the alkylation reaction of p-
hydroxybenzaldehyde (Scheme 1). Further reaction of phenyldiamine with some para
substituted benzaldehyde afforded compounds 2a-b, 3 and 4 (Scheme 2). Reaction of
cyclotriphosphazene ring with 4-hydroxybenzaldehyde gave compound 5 (Scheme 3)
which was used for further reaction to yield the final compounds 6a, 6b, 7 and 8 (Scheme
4)
O
O
K2CO3, KI, DMF
reflux, 12 h
OH
O R
+ R-Br
R= C7H15, 1a; C12H25, 1b
Scheme 1. Alkylation reaction of intermediates 1a and 1b [10].

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O
O
H
H2N
NH2
X
HO
+
+
H
MeOH
rt. 2 h
N
N
HO
X
X= OC7H15, 2a; OC12H25, 2b; OCH3, 3; Cl, 4
Scheme 2. Condensation reaction of compounds 2a, 2b, 3 and 4 [11].
Cl Cl
Cl Cl
P
P
O
H
Et N, THF
N
P
N
P
3
O
H
N
P
N
P
Cl
Cl
HO
+
O
Cl
Cl
rt, 4 days
N
Cl
Cl
N
Cl
Scheme 3. Formation of monosubstituted cyclotriphosphazene, 5 [12].
Cl
Cl
P
N
O
H
N
P
N
P
O
H
O
+
X
H2N
NH2
+
Cl
Cl
Cl
MeOH
rt, stir
2h
Cl Cl
P
N
N
N
P
N
P
O
X
Cl
Cl
N
Cl
X= OC7H15, 6a; OC12H25, 6b; OCH3, 7; Cl, 8
Scheme 4. Condensation reaction of compound 6a, 6b, 7 and 8 [13]
.

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(1a) Synthesis of 4-heptyloxybenzaldehyde
4-hydroxybenzaldehyde (6.11 g, 0.05 mol) and 1-bromoheptane (8.95 g, 0.05 mol) were
dissolved in N,N-dimethylformamide, DMF (10 mL), separately. Both solutions were
mixed in a 100 mL round bottom flask. Potassium carbonate (10.37 g, 0.06 mol) and
potassium iodide (0.83 g, 0.005 mol) were added into the mixture and was refluxed for 12
hours. The reaction was monitored using TLC. Upon completion, the mixture was poured
into 250 mL cold water and was extracted using ethyl acetate. The organic layers were
collected and dried in anhydrous sodium sulphate. The product was filtered, evaporated and
dried in vacuum oven overnight. The same method was used to synthesis 1b. Yield: 7.89 g
(71.73 %), light-yellow oil. FT-IR (cm-1): 2928 and 2858 (asymmetrical and symmetrical
Csp3-H stretching), 2769 (H-CO, aldehydic stretching), 1691 (C=O stretching), 1600 (C=C
1
stretching), 1252 (C-O stretching). H-NMR (500 MHz, DMSO-d6) δ, ppm: 9.86 (s, 1H),
7.81 (d, J=10.0 Hz, 2H), 7.00 (d, J=10.0 Hz, 2H), 3.94 (t, J=7.5 Hz, 2H), 1.66-1.69 (m,
13
2H), 1.32-1.36 (m, 2H), 1.20-1.24 (m, 6H), 0.82 (t, J=7.5 Hz, 3H). C-NMR (125 MHz,
DMSO-d6) δ, ppm: 190.20, 163.60, 131.44, 129.55, 114.44, 67.86, 31.28, 28.59, 28.57,
25.44, 22.09, 13.59.
(1b) 4-dodecyloxybenzaldehyde
Yield: 10.79 g (74.38 %), dark-yellow oil. FT-IR (cm-1): 2922 and 2855 (asymmetrical and
symmetrical Csp3-H stretching), 2733 (H-CO, aldehydic stretching), 1688 (C=O
stretching), 1600 (C=C stretching), 1255 (C-O stretching). 1H-NMR (500 MHz, DMSO-d6)
δ, ppm: 9.83 (s, 1H), 7.80 (d, J=10.0 Hz, 2H), 7.02 (d, J=10.0 Hz, 2H), 3.98 (t, J=7.5 Hz,
2H), 1.66-1.69 (m, 2H), 1.33-1.35 (m, 2H), 1.12-1.28 (m, 16H), 0.80 (t, J=7.5 Hz, 3H). 13C-

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NMR (125 MHz, DMSO-d6) δ, ppm: 190.76, 163.58, 131.59, 129.50, 114.60, 67.88, 31.34,
29.12, 29.09, 29.07, 29.05, 28.82, 28.79, 28.52, 25.43, 22.10, 13.74.
(2a) Synthesis of 4-({4-[(4-heptyloxybenzyliden)amino]phenylimino}methyl) phenol
A mixture of 4-hydroxybenzaldehyde (0.50 g, 4.10 mmol) and 1,4-phenylenediamine (0.44
g, 4.10 mmol) in methanol (15 mL) were placed in a round bottom flask. 4-
heptyloxybenzaldehyde (0.90 g, 4.10 mmol) was added into the mixture and the reaction
mixture was stirred at room temperature for two hours. The reaction progress was
monitored by TLC. Upon completion, the mixture was cooled in ice water and the
precipitate formed was filtered and dried. The crude was recrystallized using methanol. The
same method was used to synthesis 2b, 3 and 4. Yield = 1.29 g (76.09%), mp: 115.2-117.7
°C, yellow powder. FT-IR (cm-1): 3188 (O-H stretching), 3069 (Csp2-H stretching), 2925
and 2861 (asymmetrical and symmetrical Csp3-H stretching), 1600 (C=N stretching), 1511
1
(C=C stretching), 1249 (C-O stretching), 1163 (C-N stretching). H-NMR (500 MHz,
DMSO-d6) δ, ppm: 8.57 (s, 1H), 8.50 (s, 1H), 7.87 (d, J=10.0 Hz, 2H), 7.78 (d, J=5.0 Hz,
2H), 7.27 (d, J=10.0 Hz, 2H), 7.25 (d, J=10.0 Hz, 2H), 7.05 (d, J=10.0 Hz, 2H), 6.90 (d,
J=5.0 Hz, 2H), 4.08 (t, J=5.0 Hz, 2H), 1.75-1.78 (m, 2H), 1.43-1.48 (m, 2H), 1.31-1.39 (m,
6H), 0.89 (t, J=7.5 Hz, 3H). 13C-NMR (125 MHz, DMSO-d6) δ, ppm: 161.56, 160.64,
158.83, 158.73, 149.43, 149.40, 133.32, 132.46, 130.43, 130.29, 121.66, 121.57, 115.70,
114.91, 68.02, 31.09, 28.61, 28.25, 25.36, 21.84, 13.66. CHN elemental analysis:
Calculated for C27H30N2O2: C: 78.23 %, H: 7.29 %, N: 6.76 %; Found: C: 78.01 %, H: 7.27
%, N: 6.75 %.

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(2b) 4-({4-[(4-dodecyloxybenzylidene)amino]phenylimino}methyl) phenol
Yield = 1.33 g (67.02%), mp: 120.1-122.8°C, yellow powder. FT-IR (cm-1): 3277 (O-H
stretching), 3066 (Csp2-H stretching), 2917 and 2852 (asymmetrical and symmetrical Csp3-
H stretching), 1598 and 1576 (C=N stretching), 1511 (C=C stretching), 1250 (C-O
1
stretching), 1170 (C-N stretching). H-NMR (500 MHz, DMSO-d6) δ, ppm: 8.56 (s, 1H),
8.50 (s, 1H), 7.87 (d, J=10.0 Hz, 2H), 7.78 (d, J=5.0 Hz, 2H), 7.27 (d, J=10.0 Hz, 2H),
7.25 (d, J=10.0 Hz, 2H), 7.05 (d, J=10.0 Hz, 2H), 6.90 (d, J=5.0 Hz, 2H), 4.08 (t, J=7.5
Hz, 2H), 1.77-1.74 (m, 2H), 1.43-1.45 (m, 2H), 1.28-1.36 (m, 16H), 0.87 (t, J=7.5 Hz, 3H).
13C-NMR (125 MHz, DMSO-d6) δ, ppm: 161.46, 160.56, 158.83, 158.62, 149.43, 149.40,
134.95, 133.37, 130.43, 130.27, 121.64, 121.57, 115.70, 114.90, 68.01, 31.16, 28.88, 28.86,
28.82, 28.73, 28.70, 28.60, 28.52, 25.38, 21.90, 13.67. CHN elemental analysis: Calculated
for C32H40N2O2: C: 79.30 %, H: 8.32 %, N: 5.78 %. Found: C: 79.29 %, H: 8.32 %, N: 5.77
%.
(3) 4-({4-[(4-methoxybenzyliden)amino]phenylimino}methyl)phenol
Yield: 1.02 g (75.38%), mp: 175.1-178.4 °C, yellow powder. FT-IR (cm-1): 3163 (O-H
stretching), 3023 (Csp2-H stretching), 2879 (Csp3-H stretching), 1594 (C=N stretching),
1511 (C=C stretching), 1252 (C-O stretching), 1160 (C-N stretching). 1H NMR (500 MHz,
DMSO-d6) ˬ, ppm: 8.59 (s, 1H), 8.51 (s, 1H), 7.90 (d, J=5.0 Hz, 2H), 7.78 (d, J=10.0 Hz,
2H), 7.30 (d, J=5.0 Hz, 2H), 7.27 (d, J=10.0 Hz, 2H), 7.08 (d, J=10.0 Hz, 2H), 6.88 (d, J=
10.0 Hz, 2H), 3.84 (s, 3H). 13C NMR (125 MHz, DMSO-d6) ˬ, ppm: 161.84, 160.51,
159.13, 159.09, 149.26, 149.23, 134.63, 133.32, 131.58, 130.39, 121.85, 121.75, 115.62,

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114.25, 55.38. CHN elemental analysis: Calculated for C21H18N2O2: C: 76.34%, H: 5.49%,
N: 8.48%. Found: C: 76.11%, H: 5.45%, N: 8.46%.
(4) 4-({4-[(chlorobenzyliden)amino]phenylimino}methyl) phenol
Yield: 0.95 g (75.61%), mp: 178.2-180.7 °C, yellow powder. FT-IR (cm-1): 3185 (O-H
stretching), 3026 (Csp2-H stretching), 2882 (Csp3-H stretching), 1600 (C=N stretching),
1493 (C=C stretching), 1276 (C-O stretching), 1163 (C-N stretching), 824 (C-Cl bending).
1H-NMR (500 MHz, DMSO-d6) δ, ppm; 8.71 (s, 1H), 8.51 (s, 1H), 7.97 (d, J=10.0 Hz, 2H),
7.78 (d, J=10.0 Hz, 2H), 7.60 (d, J=10.0 Hz, 2H), 7.37 (d, J=10.0 Hz, 2H), 7.27 (d, J=10.0
Hz, 2H), 6.88 (d, J= 10.0 Hz, 2H). 13C NMR (125 MHz, DMSO-d6) δ, ppm: 160.52,
159.14, 158.92, 149.27, 149.12, 135.98, 134.92, 133.43, 130.58, 130.25, 128.97, 122.12,
121.75, 115.62. CHN elemental analysis: Calculated for C20H15N2OCl: C: 71.75 %, H: 4.52
%, N: 8.37 %. Found: C: 71.60 %, H: 4.50 %, N: 8.35 %.
(5) Synthesis of monosubstituted cyclotriphosphazene benzaldehyde
4-hydroxybenzaldehyde (3.54 g, 0.029 mol) and triethylamine (2.93 g, 0.029 mol) were
dissolved in tetrahydrofuran, THF (10 mL). The mixture was cooled to 0 °C in an ice bath
and was stirred vigorously for an hour. Then, phosphonitrilic chloride trimer (10.0 g, 0.029
mol) which was dissolved in THF (10 mL) was added dropwise into the mixture. A white
salt began to precipitate within few minutes. After three hours at 0 °C, the reaction was
allowed to attain at room temperature and was continued to stir for an additional 93 hours.
The reaction was monitored using TLC. Upon completion, the mixture was filtered to
remove the triethylamine hydrochlroride salt. THF solution was removed by rotary

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evaporation. The residue was then dissolved in dichlromethane and washed several times
with distilled water. The organic layer was collected and the dichlromethane solution was
removed by rotary evaporation. The yellow oil residue was purified using column
chromatography. Yield: 8.56 g (68.15%), light yellow oil. FT-IR (cm-1): 1706 (C=O
stretching), 1500 (C=C stretching), 1238 (C-O stretching), 1176 (P=N stretching), 977 (P-
1
O-C stretching). H-NMR (500 MHz, CDCl3) δ, ppm: 10.00 (s, 1H) , 7.94 (d, J=10.0 Hz,
2H), 7.43 (d, J=5.0 Hz, 2H). 13C-NMR (125 MHz, CDCl3) δ, ppm: 190.53, 153.57, 134.65,
31
131.69, 131.67, 122.16, 122.12. P-NMR (500 MHz, CDCl3) δ, ppm: 22.48 (d, J=155.0
Hz, 2P), 11.71 (t, J=155.0 Hz, 1P).
(6a) Synthesis of N-(4-heptyloxybenzylidene)-N’-[4-(2,4,4,6,6-pentachloro-2λ5,4λ5,6λ5-
[1,3,5,2,4,6]-triazatriphosphinin-2-yloxy)benzaldehydene] benzene-1,4-diamine
A mixture of monosubstituted cyclotriphosphazene compound, 5 (1.00 g, 2.31 mmol) and
1,4-phenylenediamine (0.25 g, 2.31 mmol) in methanol (15 mL) was placed in a round
bottom flask. Then, 4-heptyloxybenzaldehyde (0.51 g, 2.31 mmol) was added dropwise into
the mixture. The reaction was stirred at room temperature for two hours. The reaction was
monitored by TLC. Upon completion, the mixture was cooled in ice water and the
precipitate formed was filtered and dried. The crude was recrystallized using methanol. The
same method was used to synthesis 6b, 7 and 8. Yield: 1.15 g (69.09%), mp: 118.1-120.8
ºC, yellow powder. FT-IR (cm-1): 2922 and 2857 (asymmetrical and symmetrical Csp3-H
stretching), 1605 (C=N stretching), 1510 (C=C stretching), 1249 (C-O stretching), 1187
1
(P=N stretching), 1169 (C-N stretching), 1013 (P-O-C stretching). H-NMR (500 MHz,
DMSO-d6) δ, ppm: 8.68 (s, 1H), 8.58 (s, 1H), 7.96 (d, J=5.0 Hz, 2H), 7.88 (d, J=10.0 Hz,

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2H), 7.53 (d, J=5.0 Hz, 2H), 7.34 (d, J=10.0 Hz, 2H), 7.30 (d, J=15.0 Hz, 2H), 7.06 (d,
J=10.0 Hz, 2H), 4.07 (t, J=7.5 Hz, 2H), 1.74-1.77 (m, 2H), 1.43-1.45 (m, 2H), 1.31-1.36
13
(m, 6H), 0.89 (t, J=5.0 Hz, 3H). C-NMR (125 MHz, DMSO-d6) δ, ppm: 161.97, 160.90,
160.05, 159.55, 150.34, 149.46, 136.80, 135.53, 131.72, 130.86, 129.21, 122.36, 122.24,
115.51, 68.43, 31.62, 29.10, 28.79, 25.88, 22.40, 14.24. 31P-NMR (500 MHz, DMSO-d6) δ,
ppm: 23.24 (d, J=150.0 Hz, 2P), 12.45 (t, J=155.0 Hz, 1P). CHN elemental analysis:
Calculated for C27H29N5O2Cl5P3: C: 44.68 %, H: 4.03 %, N: 9.65 %. Found: C: 44.62 %, H:
4.03 %, N: 9.66 %.
(6b) N-(4-dodecyloxybenzylidene)-N’-[4-(2,4,4,6,6-pentachloro-2λ5,4λ5,6λ5-[1,3,5,2,4,6]-
triazatriphosphinin-2-yloxy)benzaldehydene] benzene-1,4-diamine
Yield: 1.23 g (67.43%), mp: 121.7-124.1 ºC, yellow powder. FT-IR (cm-1): 2918 and 2853
(asymmetrical and symmetrical Csp3-H stretching), 1608 (C=N stretching), 1510 (C=C
stretching), 1252 (C-O stretching), 1194 (P=N stretching), 1169 (C-N stretching), 1024 (P-
1
O-C stretching). H-NMR (500 MHz, DMSO-d6) δ, ppm: 8.68 (s, 1H), 8.58 (s, 1H), 7.95
(d, J=10.0 Hz, 2H), 7.88 (d, J=10.0 Hz, 2H), 7.53 (d, J=5.0 Hz, 2H), 7.34 (d, J=10.0 Hz,
2H), 7.30 (d, J=10.0 Hz, 2H), 7.06 (d, J=5.0 Hz, 2H), 4.07 (t, J=5.0 Hz, 2H), 1.74-1.76 (m,
13
2H), 1.43-1.45 (m, 2H), 1.27-1.35 (m, 16H), 0.87 (t, J=5.0 Hz, 3H). C-NMR (125 MHz,
DMSO-d6) δ, ppm: 162.80, 161.98, 160.05, 159.54, 150.31, 149.43, 136.85, 135.47,
131.72, 130.86, 129.05, 122.35, 122.24, 115.34, 68.41, 31.69, 29.41, 29.38, 29.35, 29.25,
29.12, 29.11, 29.06, 25.88, 22.44, 14.24. 31P-NMR (500 MHz, DMSO-d6) δ, ppm: 23.55 (d,
J=155.0 Hz, 2P), 12.75 (t, J=155.0 Hz, 1P). CHN elemental analysis: Calculated for

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C32H39N5O2Cl5P3: C: 48.29 %, H: 4.94 %, N: 8.80 %. Found: C: 48.22 %, H: 4.93 %, N: 8.81
%.
(7) N-(4-methoxybenzylidene)-N’-[4-(2,4,4,6,6-pentachloro-2λ5,4λ5,6λ5-[1,3,5,2,4,6]-triaza
triphosphinin-2-yloxy)benzaldehydene] benzene-1,4-diamine
Yield: 1.07 g (72.51%), mp: 207.3-210.5 ºC, yellow powder. FT-IR (cm-1): 2839 (Csp3-H
stretching), 1601 (C=N stretching), 1503 (C=C stretching), 1296 (C-O stretching), 1198
1
(P=N stretching), 1169 (C-N stretching), 980 (P-O-C stretching). H-NMR (500 MHz,
DMSO-d6) δ, ppm: 8.90 (s, 1H), 8.69 (s, 1H), 7.96 (d, J=5.0 Hz, 2H), 7.54 (d, J=5.0 Hz,
2H), 7.53 (d, J=5.0 Hz, 2H), 7.36 (d, J=15.0 Hz, 2H), 7.30 (d, J=5.0 Hz, 2H), 7.08 (d, J=
15.0 Hz, 2H), 3.90 (s, 3H). 13C-NMR (125 MHz, DMSO-d6) δ, ppm: 160.38, 160.06,
159.29, 159.24, 149.82, 149.28, 136.06, 133.13, 131.42, 130.30, 128.81, 122.01, 121.91,
31
112.01, 55.78. P-NMR (500 MHz, DMSO-d6) δ, ppm: 22.50 (d, J=150.0 Hz, 2P), 12.11
(t, J=170.0 Hz, 1P). CHN elemental analysis: Calculated for C21H17N5O2Cl5P3: C: 39.31 %,
H: 2.67 %, N: 10.92 %. Found: C: 39.09 %, H: 2.64 %, N: 10.87 %.
(8) N-(4-chlorobenzylidene)-N’-[4-(2,4,4,6,6-pentachloro-2λ5,4λ5,6λ5-[1,3,5,2,4,6]-triazatri
phosphinin-2-yloxy)benzaldehydene] benzene-1,4-diamine
Yield: 0.96 g (64.61%), mp: 220.5-223.0 ºC, orange powder. FT-IR (cm-1): 1612 (C=N
stretching), 1492 (C=C stretching), 1274 (C-O stretching), 1205 (P=N stretching), 1180 (C-
1
N stretching), 1075 (P-O-C stretching), 842 (C-Cl bending). H-NMR (500 MHz, DMSO-
d6) δ, ppm: 8.71 (s, 1H), 8.69 (s, 1H), 7.97 (d, J=10.0 Hz, 2H), 7.95 (d, J=10.0 Hz, 2H),
7.60 (d, J=10.0 Hz, 2H), 7.54 (d, J=5.0 Hz, 2H), 7.38 (d, J=5.0 Hz, 2H), 7.37 (d, J= 5.0 Hz,

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13
2H, ). C-NMR (125 MHz, DMSO-d6) δ, ppm: 160.05, 158.88, 158.77, 154.48, 149.49,
148.90, 136.04, 134.94, 131.44, 130.23, 128.95, 128.64, 122.11, 122.02. 31P-NMR (500
MHz, DMSO-d6) δ, ppm: 22.54 (d, J=155.0 Hz, 2P), 11.74 (t, J=155.0 Hz, 1P). CHN
elemental analysis: Calculated for C20H14N5O2Cl6P3: C: 37.18 %, H: 2.18 %, N: 10.84 %.
Found: C: 37.12 %, H: 2.20 %, N: 10.83 %.
RESULTS AND DISCUSSION
FT-IR spectral discussion
The research works involved the alkylation reaction of p-hydroxybenzaldehyde with
alkylbromide in the presence of potassium carbonate and potassium iodide to produce
intermediates 1a and 1b (Scheme 1).The IR data for intermediates 1a and 1b showed the
absorption bands at 2890 and 2920 cm-1 for the symmetrical and asymmetrical C-H (sp3)
stretching. The band at 2730 cm-1 was assigned for the aldehydic C-H stretching while
bands at 1689, 1602 and 1253 cm-1 were attributed to the C=O, C=C and C-O stretching,
respectively. No broad absorption band for the O-H stretching (3300 cm-1) was observed
which confirmed that the insertion of the alkyl group into the benzaldehyde was successful.
Further reaction of p-phenylenediamine with intermediates 1a (heptyl chain), 1b
(dodecyl chain) and two other benzaldehydes (methoxy and chloro) produced four Shiff
base compounds (2a, 2b, 3 and 4) with different substituents (Scheme 2). All compounds
showed a broad absorption of the O-H stretching in the region of 3220-3560 cm-1. The
absorption bands for C=N, C=C and C-N stretching were observed at 1600, 1500 and 1160
cm-1 respectively. Compounds 2a and 2b showed strong absorption bands at 2850 and 2980
cm-1 due to the symmetrical and asymmetrical Csp3-H stretching of the heptyl and dodecyl

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chains [14]. Compound 3 showed only the Csp3-H stretching at 2879 cm-1. Absorptions
were observed at 1250 cm-1 for C-O stretching of compounds 2a, 2b and 3. A band at 824
cm-1 was assigned for C-Cl stretching in compound 4. No absorption bands of the aldehydic
C-H stretching in all the intermediates but instead all intermediates showed the C=N
stretching at 1612 cm-1 which confirmed the formation of the Schiff base moiety.
The monosubstituted cyclotriphosphazene compound, 5 has been synthesized by
substitution reaction of p-hydroxybenzaldehyde with cyclotriphosphazene as shown in
Scheme 3. The IR spectrum of monosubstituted cyclotriphosphazene, 5 showed the
absorptions 1706, 1500 and 1238 cm-1 for the C=O, C=C and C-O stretching, respectively.
A strong absorption at 1176 cm-1 was attributed for P=N stretching of the
cyclotriphosphazene ring. The P-O-C bending can be observed at 977 cm-1. No broad
absorption band for the O-H stretching, which confirmed that the insertion of 4-
hydroxybenzaldehyde into cyclotriphosphazene core was successful.
The synthesized monosubstituted compound, 5 was used to produce new
monosubstituted cyclotriphosphazene based molecules with different substituents (Scheme
4). The IR spectra of compounds 6a, 6b, 7 and 8 showed the same absorption pattern
(Figure 2). All compounds showed the absorption bands for C=N, C=C, C-O and C-N
stretching at 1600, 1510, 1250 and 1160 cm-1. Absorption band at 1180 cm-1 was assigned
to P=N stretching while the absorption in the region of 980 to 1070 cm-1 was assigned to P-
O-C bending. Compounds 6a and 6b showed absorptions at 2839 and 2900 cm-1which were
assigned for the symmetrical and asymmetrical C-H (sp3) stretching of the aliphatic chains
in the molecules. Compound 7 showed similar absorption band but with weaker intensity,
due to the methoxy substituent in the molecule. The absorption band for C-Cl bending of

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compound 8 was observed at 842 cm-1. The absence of the aldehydic C-H stretching of the
starting material indicated that the condensation reaction was completed.
Figure 2. The FT-IR overlay spectra for compounds 6a, 6b, 7 and 8
1H and 13C NMR spectral discussion
The 1H NMR of intermediates 1a and 1b revealed the same pattern of signals where
the aldehydic proton was observed at δ 9.83 and 9.87 ppm, respectively. Two doublets of
the aromatic protons resonated at δ 7.00, 7.81 ppm and 7.02, 7.80 ppm, respectively while
15 aliphatic protons resonated in the upfield region from δ 0.83 to 3.95 ppm for 1a and 25

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protons in the region of δ 0.80 to 3.98 ppm for 1b. The 13C NMR for intermediates 1a and
1b showed 12 and 17 carbon signals, respectively.
1
The H NMR of all compounds 2a, 2b, 3 and 4 showed two singlets of the
azomethine protons in the most deshielded region. Six doublets for the aromatic protons
were observed in the range of δ 6.90 to 7.87 ppm. Besides, compounds 2a and 2b showed
15 and 25 aliphatic protons for the heptyl and dodecyl chains in the upfield region,
respectively. The detailed 13C NMR data of all compounds were shown in the experimental
section. All compounds showed similar patterns with a methyl carbon at δ 55.38 ppm in
compound 3. Compounds 2a and 2b showed the presence of 27 and 32 carbon signals,
respectively.
1H-NMR spectrum of monosubstituted cyclotriphosphazene benzalehyde, 5 showed
the presence of a singlet at δ 9.97 ppm of the carbonyl proton and two doublets of the
13
aromatic protons at δ 7.92 and 7.41 ppm. C NMR spectrum (Appendix 7b-ii) showed a
signal at the most downfield region (δ 190.49), assigned for the carbonyl carbon. Two
quaternary carbons were assigned for P-O-C (δ 153.56) and C=O (δ 134.66) while other
aromatic carbons resonated at δ 122.11, 122.15, 131.67 and 131.69 ppm. 31P NMR
spectrum (Figure 3) showed two different phosphorus atoms. A triplet at δ 11.71 ppm was
assigned to the phosphorus atom attached to 4-hydroxybenzaldehyde, which coupled to
other phosphorus in the cyclotriphosphazene ring. Another two phophorus atoms coupled to
the substituted phosphorus to give a doublet at δ 22.48 ppm.

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Figure 3. 31P spectrum of monosubstitited cyclotriphosphazene, 5
1
13
The detailed H and C NMR data were listed in the experimental section. All the
compounds showed the similar pattern with the presence of the methoxy proton and carbon
for compound 7. Besides, compounds 6a and 6b showed aliphatic protons and carbons for
1
13
the heptyl and dodecyl chains in the upfield region, respectively. The H and C NMR of
compound 6a is shown in Figure 4 as an example. The 31P NMR spectra of all compounds
showed the same pattern as 5 with two signals since there were two different electron
environment of phosphorus in the molecule.

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Figure 4. The 1H (500 MHz, DMSO-d6) and 13C (125 MHz, DMSO-d6) spectra of
compound 6a
Determination of liquid crystal properties using POM
Determinations of mesophase(s) behaviour for all compounds were done using
polarized optical microscope (POM). The heating and cooling rates used for all compounds

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were 5oC/min. Compounds 2a, 2b, 6a and 6b were found to exhibit liquid crystal
mesophase. These compounds can be termed as mesogenic compounds. However, no liquid
crystal mesophase was observed for compounds 3, 4, 7 and 8. These compounds are non-
mesogenic. In the heating cycle, compound 2a exhibited SmC phase of a broken focal conic
fan texture at 118.0ºC before it became the isotropic phase at 145.9ºC. Upon cooling, the
SmC phase appeared at 140.3ºC and became the crystal phase at 101.7ºC. The texture of
this mesophase is illustrated in Figure 5.
(a)
(b)
Figure 5. The POM photographs of broken fan-shaped texture of compound 2a, observed
upon cooling at 139.7ºC. (a) magnification: 20 0.40 and (b) magnification: 50 0.50
×
×
In the heating cycle, compound 2b showed a SmC phase of a broken focal conic fan
texture at 121.5ºC and became isotropic at 128.2ºC. Upon cooling, the isotropic became
SmC phase at 122.5ºC before it was transformed into the crystal phase at 109.3ºC. This
SmC texture is illustrated in Figure 6.

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(a)
(b)
Figure 6. The POM photographs of broken fan-shaped texture of compound 2b, observed
upon heating with magnification of 20 0.50 at (a) 123.4ºC and (b) 126.3ºC
×
Compound 6a exhibited two types of liquid crystal phases. In the heating cycle, the
crystal phase became the SmC phase of a broken focal conic fan texture at 120.6ºC before it
was transformed into the nematic phase of a thread-like texture at 148.5ºC and finally
became the isotropic phase at 162.5ºC. Upon cooling, the nematic phase appeared at
172.8ºC, followed by the SmC at 155.6ºC and became the crystal phase at 140.4ºC. The
SmC and the nematic textures are illustrated in Figure 7.
(a)
(b)

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Figure 7. The POM photographs of compound 6a, observed upon heating with
magnification of 20
×
0.50. (a) broken fan-shaped texture of SmC at 121.5ºC and (b)
thread-like nematic phase at 149.9ºC
Compound 6b exhibited the nematic phase of a thread-like texture at 123.9ºC,
which then became the isotropic phase at 202.8ºC in the heating cycle. Upon cooling, the
nematic phase appeared at 193.1ºC and became the crystal phase at 110.8ºC. The nematic
texture is illustrated in Figure 8.
(a)
(b)
Figure 8. The POM photographs of a thread-like nematic phase texture of compound 6b,
observed with magnification of 20 0.25 at (a) 197.5ºC (heating) and (b) 191.7ºC (cooling)
×
Determination of thermal transitions using DSC
Compounds 2a, 2b, 6a and 6b with liquid crystal mesophases were further
confirmed using differential scanning calorimetry (DSC) to determine their thermal
transitions in the heating and cooling cycles of the compounds. Thermal enthalpy, ∆H
(kJ/mol) of each transition was calculated and summarized in Table 1 and 2.

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Table 1: DSC thermal transitions of compounds 2a, 2b, 6a and 6b (Heating Cycle)
Compound
Transition temperature (ºC)
Enthalpy, ∆H (kJ/mol)
Cr
•
SmC
N
-
I
•
116.05
5.19
•
145.25
4.91
2a
2b
6a
6b
-
-
•
•
•
121.16
11.93
•
•
-
127.51
2.41
•
•
•
-
•
•
119.04
13.90
146.58
11.53
161.39
2.44
122.30
201.73
-
59.89
12.13
Note: Cr = Crystal, SmC = Smectic C, N= Nematic, I = Isotropic
Table 2: DSC thermal transitions of compounds 2a, 2b, 6a and 6b (Cooling Cycle)
Compound
Transition temperature (ºC)
Enthalpy, ∆H (kJ/mol)
I
N
SmC
Cr
•
-
-
-
140.95
-4.50
•
100.27
-1.54
•
2a
2b
6a
6b
•
•
•
123.07
-2.16
•
•
-
109.68
-11.46
•
•
•
-
•
•
174.81
2.41
156.90
2.20
141.96
7.15
194.70
111.53
-
-8.66
-46.04
Note: Cr = Crystal, SmC = Smectic C, N= Nematic, I = Isotropic

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The DSC data of compounds 2a, 2b and 6b showed two endotherms in the heating
and cooling cycles. The thermogram showed the transitions of crystal-SmC-isotropic phase
for compounds 2a and 2b while compounds 6b showed the transitions of the crystal-N-
isotropic phase were observed. The DSC thermogram of compounds 6a showed three
endotherms in both cycles which attributed to the phase transitions of SmC and nematic.
The DSC thermogram of compound 6a is shown in Figure 9.
SmC → N
N → I
I → N
Cr → SmC
SmC → Cr
N → SmC
Figure 9. DSC thermogram of compound 6a.
Structure property relationship
Molecular shape is important in self-assembly of a molecule which gives the impact
on the ordering abilities of the mesogenic molecules. In general, a rod-like molecule is
composed of flat, rigid cores, linking units and terminal groups of flexible chains [15]. In
liquid crystal molecules, linking unit connects one core to another, linking the terminal
chain to the core. Schiff base is an example of a linking unit which is very useful in liquid
crystal research due to low temperature of phase transitions [16]. Schiff base offers the

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possibility of controlling the alignment and orientation of a molecule to generate liquid
crystal materials [17]. The linking unit of two Schiff base moieties between the aromatic
rings is to increase the length of the molecules, as well as to alter the polarizability and
flexibility of the molecules, in which it restricts the freedom of rotation. The extension of
the linking unit greatly enhances the nematic-isotropic transition temperature and increases
the thermal stability of the nematic order.
The linking unit at the para position of the rings is to preserve the linearity of the
molecule [18-19]. The interconnected cyclic rings (in this case is benzene) causes the
resulting compound to have a linear planar conformation. The nature of the terminal
substituents has profound influence on the liquid crystal properties of a compound. It was
reported that compounds with shorter alkoxy chains (n=1,3,4) usually able to exhibit
nematic phases, while compounds with longer alkoxy chains (n=6,7,8,10,12,14) prone to
exhibit SmC phases [20].
The most common core units in liquid crystals are aromatic moieties which are
connected through linking unit and they are very useful in providing rigidity to the
molecule. It affects the liquid crystal stability and other physical properties, allowing linear
configuration. However, decreased ring flexibility has the effect of increasing transition
temperature [21-24]. It was reported that conjugation has dramatic effect on the ability of
the molecules to self-assemble into liquid crystal mesophases [25].
In this study, compounds (2a, 2b, 6a and 6b) with terminal alkyl chains of heptyl
and dodecyl showed liquid crystal properties since they possessed stronger attractive forces.
The long alkyl chains add flexibility to the rigid core and stabilized the molecular
interactions which are needed for the formation of liquid crystal mesophase. This in turn,

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incereased the melting temperature, Tm and widen the range of liquid crystal mesophases
[26-27]. The increasing alkoxy chain length in a molecule increases the molecular ordering,
resulting in more stable mesophases with higher clearing temperatures, Tc, as shown in
compounds 6a and 6b [20]. There are intermolecular interactions due to the coordinated
oxygen atom [28]. The final compounds with flexible alkyl chains attached to a rigid
cyclotriphosphazene core showed increased in rigidity. The cyclotriphosphazene ring
provides the chemical stability to the structures in the compounds. Thus, it forms more
stable mesophases with higher temperatures.
In this work, compounds 4 and 8 with chloro substituent did not exhibit any liquid
crystal phases while compounds 3 and 7 with a methoxy substituent also did not exhibit any
liquid crystal property. They have low thermal stability to induce liquid crystal mesophase.
CONCLUSION
Intermediates, 1a and 1b were synthesized through the alkylation reaction. Further
condensation reaction produced a series of Schiff base compounds 2a, 2b, 3 and 4. These
compounds were characterized using FT-IR, 1D and 2D NMR spectroscopy and CHN
elemental analysis. Reaction of cyclotriphosphazene ring with 4-hydroxybenzaldehyde
gave compound 5 which was used for further reaction to yield the final compounds 6a, 6b,
7 and 8. These final compounds were also characterized. Compounds having two Schiff
base linking units (2a, 2b, 3 and 4) were compared with the same compounds attached to
cyclotriphosphazene ring (6a, 6b, 7 and 8). Determination of their liquid crystal mesophase
using POM showed that compounds 2a and 2b with heptyl and dodecyl chains,
respectively, exhibited SmC phases with broken-fan textures. Compound 6a which is

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attached to the cyclotriphosphazene core showed a SmC and nematic phases while
compound 6b showed only a nematic phase. However, compounds 3, 4, 7 and 8 were found
to be non-mesogenic without liquid crystal mesophase. The thermal transitions for the
mesophase transitions were calculated using the DSC thermogram.
Acknowledgement: The authors would like to thank the University Sains Malaysia (USM)
to provide the lab facilities and the RUI (USM) grant (1001/PKIMIA/811332).
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HIGHLIGHTS
•
•
•
Compounds with two Schiff base linking units and four different substituents was
synthesized and characterized.
The liquid crystal properties of this compounds was determined by polarized optical
microscope (POM) and differential scanning calorimeter (DSC).
Compounds with heptyl and dodecyl substituents showed the liquid crystal properties
while compounds with methoxy and chloro substituents were found to be non-
mesogenic.