Synthesis and phase transition behaviours of a homologous series of pyridine symmetrical dimers

Article abstract of DOI:10.14233/ajchem.2013.14295

A series of symmetrical dimers bearing heterocyclic ring, a,w-bis-(4-pyridine-4-methylideneaminoxy) alkanes, were prepared and all the members are differed by the alkyl spacer length (CnH2n, where n = 6, 8, 10, 12). Differential scanning calorimetry and p

Full text of DOI:10.14233/ajchem.2013.14295

Asian Journal of Chemistry; Vol. 25, No. 11 (2013), 6151-6155
http://dx.doi.org/10.14233/ajchem.2013.14295
Synthesis and Phase Transition Behaviours of a
Homologous Series of Pyridine Symmetrical Dimers
1
2,3,*
4
LAY-KHOON ONG , SIE-TIONG HA
and GUAN-YEOW YEAP
1
2
3
4
Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jln Genting Klang, Setapak, 53300 Kuala Lumpur, Malaysia
Faculty of Science, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
Centre for Biodiversity Research, Universiti Tunku Abdul Rahman, Jln Universiti, Bandar Barat, 31900 Kampar, Perak, Malaysia
Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti Sains Malaysia, 11800 Minden, Penang, Malaysia
*Corresponding author: E-mail: hast_utar@yahoo.com
(
Received: 12 July 2012;
Accepted: 30 April 2013)
AJC-13423
A series of symmetrical dimers bearing heterocyclic ring, α,ω-bis-(4-pyridine-4-methylideneaminoxy) alkanes, were prepared and all the
members are differed by the alkyl spacer length (C 2n, where n = 6, 8, 10, 12). Differential scanning calorimetry and polarizing optical
n
H
microscopy techniques were employed to study their transition temperatures and mesophase characteristics. Differential scanning calorimetry
thermograms displayed direct isotropization and recrystallization during heating and cooling processes, respectively. The crystal phase
texture changed to dark region (isotropic stage) without exhibiting any mesophase. Influence of molecular structure on the mesomorphic
properties of symmetrical dimers was also reported.
Key Words: Dimer, Schiff base, Pyridine, Phase behaviour, Synthesis.
study of dimers has developed steadily in the recent years and
this could be attributed to the enormous use of dimers as model
compounds to understand the importance of polymeric structure
INTRODUCTION
Development of liquid crystal science and technology has
led to the study of numerous new mesogens especially, thermo-
10-12
containing both semi-flexible main chain and side group
.
1,2
tropic liquid crystals . Most thermotropic liquid crystals are
calamitic-shaped molecules having rigid core consisted of two
or more aromatic rings and one or more flexible terminal chains.
Schiff base is a well-known linking group used in connecting
core groups. Though it provides a stepped core structure, yet it
maintains molecular linearity, therefore providing better stability
In addition, liquid crystal dimers are also of important interest
because these materials exhibit quite different behaviour as
13
compared to conventional low molar mass mesogens . These
mesogens can be generally categorized into two categories:
(
a) symmetrical dimers in which the two mesogenic cores are
identical and (b) unsymmetrical dimers which contain two non-
identical mesogenic units.
In continuation of our research on heterocyclic liquid
3,4
and enabling mesophase formation . Comprehensive research
on Schiff base system had been carried out ever since the
discovery of 4-methoxybenzylidene-4'-butylaniline which
14-20
crystals , we are prompted to introduce heterocyclic pyridine
5
exhibited nematic phase at room temperature .
into dimer system through production of α,ω-bis-(4-pyridine-4-
methyleneaminoxy) alkanes. The phase transition and the effect
of changing the spacer length of the dimers were identified,
wherein these dimers differ from one another in spacer length
Over the past few decades, heterocyclic liquid crystals
have received much attention due to their unique properties
including a reduced packing ability (basically giving rise to
lower melting points than their phenyl counterparts), a medium
to strong lateral dipole, high anisotropy of the polarizability,
2 n
(CH ) in which n = 6, 8, 10 and 12. Owing to the presence of
pyridine as donor group, these compounds may also serve as
mesogenic charge-transfer donors in our future studies by inter-
21
acting with acceptor molecules to form binary liquid crystals .
6
-8
low viscosity etc. . Early results have found that the incorpo-
ration of heterocyclic ring influenced the mesomorphic prop-
erties of the calamitic molecules due to their unsaturation and/
9
EXPERIMENTAL
or polarizability . The system involving heterocyclic liquid
crystals has extended from low-molar mass molecules to
dimeric or oligomeric structures. The importance towards the
4-Aminophenol, pyridine-4-carboxyaldehyde, 1,6-
dibromohexane, 1,8-dibromooctane, 1,10-dibromodecane,

6
152 Ong et al.
Asian J. Chem.
TABLE-1
PERCENTAGE YIELDS AND IR FREQUENCIES OF SYMMETRICAL DIMERS
-
1
IR ν (cm )
o
m.p. ( C)
Compound
Yield (%)
ν(CH
3
)
ν(C=N)
1622
ν(C=C)
ν(C-O)
1253
1256
1258
1251
1,4-disubstituted aromatic ring
6
8
BP3A
BP3A
40
46
24
25
160
152
148
80
2955, 2822
2932, 2859
2939, 2853
2924, 2822
1599
1584
1582
1587
835
835
835
834
1623
1
1
0BP3A
2BP3A
1622
1621
1 13
The IR, NMR ( H and C) and mass spectral data of the
1
,10-bromododecane and potassium carbonate anhydrous
were obtained commercially. The synthetic route for the title
compounds are illustrated in Fig. 1.
representative compound, 8BP4A are summarized as below.
+
8BP4A: EI-MS m/z (rel. int. %): 506 (100) [M] , 198 (65); IR
-1
(
KBr, νmax, cm ): 2932, 2859 (C-H aliphatic), 1623 (C=N),
1
584 (C=C), 1256 (C-O); H NMR (400 MHz, CDCl
O
1
1.41-1.43 {m, 8H, 2 x -O(CH ) (CH ) }, 1.78-1.85 {p, 4H, 2
3
, δ ppm):
C
N
HO
NH2
2
2
2 2
+
H
2 2 2 2 2
x -O(CH )(CH )(CH ) }, 4.0 (t, 4H, 2 x -OCH ), 6.95 (d, 4H,
Ar-H), 7.27 (d, 4H,Ar-H), 7.74 (d, 4H, pyridine protons), 8.48
1
3
C H OH
(s, 2H, CH=N), 8.74 (d, 4H, pyridine protons); C NMR (100
MHz, CDCl δ ppm): 26.00, 29.30, 30.95 for {-(CH (CH )O},
8.24 for {-(CH )O}, 115.05, 122.09, 122.60, 143.15, 143.38,
150.49, 155.21 for aromatic and pyridine carbons, 158.78
CH=N).
2
5
3
)
2 3
2
6
2
H
N
(
C
N
HO
RESULTS AND DISCUSSION
The percentage yields and selected IR data are tabulated
K CO , BrC H2nBr,
2
3
n
in Table-1. The purity of the products, nBP4A (where n = 12,
4, 16, 18) was analyzed by TLC on silica gel plates and was
chromatographically pure, as indicated by a single spot. R
1
(
CH ) CO
3 2
f
values for two effluents system for all compounds were pro-
vided in Table-2. The phase transition temperatures and their
associated enthalpy changes obtained from differential scanning
calorimeter analysis over heating and cooling cycles are tabulated
in Table-3.
O
O
H
C
C H
H
C
n
2n
N
N
N
N
Fig. 1. Synthetic route of symmetrical dimers, nBP4A
TABLE-2
RETENTION FACTOR (R ) VALUES OF
SYMMETRICAL DIMERS
f
Electron ionization mass spectrum was recorded by a
Finnigan MAT95XL-T mass spectrometer operating at 70 eV
ionizing energy. FT-IR spectra were acquired on Perkin Elmer
R value**
f
Compound*
EAC37
0.61
ACC46
0.77
6
BP4A
BP4A
2
000-FTIR spectrophotometer in the frequency range of 4000-
8
0.65
0.85
-
1
4
were recorded in CDCl
00 cm with samples prepared as KBr pellets. NMR spectra
by utilizing JEOL 400MHz NMR
1
0BP4A
2BP4A
0.76
0.85
3
1
0.73
0.77
Spectrometer with TMS as internal standard. The phase transi-
tion temperatures were measured by Mettler Toledo DSC823
differential scanning calorimeter at a scanning rate of 10 ºC/
min. Liquid crystalline properties were investigated using a
Carl Zeiss polarizing optical microscope attached to a Linkam
Hotstage.
Single spot detected in TLC analysis; **EAC37 = Ethyl acetate:
Chloroform (3:7); ACC46 = Acetone: Chloroform (4:6)
TABLE-3
TRANSITION TEMPERATURES, ENTHALPY CHANGES AND
PHASE TRANSITIONS OF SYMMETRICAL DIMERS
Synthesis of
noxy) alkanes, nBP4A: Schiff base intermediate, 4-[(pyridin-
-ylmethylene)amino]phenol, P4OH was obtained through
α,
ω-bis-(4-pyridine-4-methylideneami-
-1
Transition temperatures, ºC (∆H, kJ mol )
Compound
Heating
Cooling
4
6BP4A
8BP4A
Cr 159.2 (65.3) I
Cr 152.0 (70.9) I
Cr 148.1 (70.9) I
Cr 79.2 (50.2) I
Cr 156.9 (65.5) I
Cr 138.2 (69.2) I
Cr 136.5 (69.4) I
Cr 60.2 (51.4) I
17
step 1 (Fig. 1) according to previously reported method . Then,
P4OH (2 mmol) dissolved in a minimum amount of DMF was
transferred into acetone solution (30 mL) of dibromoalkane
10BP4A
12BP4A
(
1 mmol) and anhydrous potassium carbonate (1 mmol). The
EI mass spectrum (Fig. 2) showed the molecular ion peak
at m/z = 506 which was corres-ponding to molecular mass of
suggested that 8BP4A was successfully synthe-
resulted mixture was stirred and heated under reflux for 24 h.
The acetone was removed via evaporation at room temperature.
The final products, nBP4A were recrystallized by ethanol until
constant transition temperature is obtained.
32 34 4 4
C H N O
sized.

Vol. 25, No. 11 (2013)
Synthesis and Phase Transition Behaviours of Pyridine Symmetrical Dimers 6153
Fig. 2. EI mass spectrum of symmetrical dimer, 8BP4A
30.4
30
29
28
27
26
25
24
23
22
21
20
19
18
17.6
4000.0
3600
3200
2800
2400
2000
1800
1600
1
1400
1200
1000
800
600
400.0
–
cm
cm-1
Fig. 3. FTIR spectrum of symmetrical dimer, 8BP4A
-1
Strong absorption bands emerged at 2932 and 2859 cm
in FTIR spectrum of 8BP4A (Fig. 3) confirmed the presence
to methylene protons of long alkyl chain. Two distinct doublets
detected at δ = 6.95 and 7.27 ppm are belonged to the reso-
nance of four aromatic protons. A singlet peak observed at δ =
-1
of aliphatic C-H in alkyl chain. A sharp band at 1256 cm can
be ascribed to C-O bonds of aromatic ether linking group.
2
2-24
8.48 ppm is due to proton of imine linking group . Signals
(doublets) attributed to the pyridine carbons were observed at
δ = 7.74 and 8.74 ppm. The molecular structure of 8BP4A
-1
Absorption band appeared at 1623 cm designates for C=N
linking group. This value falls within the frequency range
2
2-24
13
reported for Schiff base linkage
1
.
In H NMR spectrum of 8BP3A, a triplet signal observed
was further verified by using C NMR spectroscopy. A peak
at δ = 68.24 ppm is attributed to the methylene carbons attached
at δ = 4.0 ppm, is atributed to the methylene protons attached
2
to oxygen atom (-CH O-). The remaining methylene carbons
to oxygen atom (-CH O-). The multiplet and pentet signals
2
were resonated at δ = 26.00, 29.30 and 30.95 ppm. Peaks at δ
= 115.05-155.21 ppm is assigned to aromatic and pyridine
appeared at δ = 1.41-1.43 and 1.75-1.85 ppm can be assigned

6
154 Ong et al.
Asian J. Chem.
carbons. The most downfield peak at δ = 158.78 ppm is due to
existence of azomethine carbon.
the melting temperatures illustrate a gradual descending trend
with the increase in the length of alkyl spacer from n = 6 to n
= 12. This phenomenon can be explained in terms of dilution
of the core units which was influenced by the increase in the
Phase transition behaviours and optical texture studies:
All homologous members were non-mesogenic compounds.
In the representative differential scanning calorimeter thermo-
gram of 8BP3A (Fig. 4), it showed single endotherm and
exotherm, respectively during both heating and cooling cycles.
This observation indicates direct melting of the crystal phase
to the isotropic liquid phase and vice versa. Under polarizing
optical microscope observation, crystal changed to dark region
isotropic during heating run. No liquid crystal texture was
observed during cooling process. Other members, nBP4A
2
5,26
flexibility of spacer . It is also important to note that upon
increasing the number of carbon in C 2n units, a significant
n
H
decrease of recrystallization point is observed from n = 6 to n
= 12.Again, this phenomenon can be ascribed to the flexibility
of the spacer. It is also interesting to point out that there is a
severe drop in the melting temperature when the alkyl space
2 2
is changed -(CH )10- to -(CH )12-.
170
170
(where n = 6, 10, 12) showed the similar characteristics as
those discussed for 8BP4A.
Melting point
Cr-I
I-Cr
150
150
Heating
I
Recrystallization point
Cr
130
130
110
110
90
90
Cr
Cooling
I
70
70
50
50
6
00
80
100
Temperature (ºC)
Fig. 4. DSC thermogram of symmetrical dimer, 8BP4A
120
140
160
66
.
88
1
1
0
0
11 22
Number of carbons (n) in the alkyloxy chain
Number of carbons (n) in the alkyloxy chain
Fig. 5. Plot of transition temperatures versus number of carbons (n) in the
alkyloxy chain of symmetrical dimers
Effect of alkyl chain length on transition temperatures:
A further inspection on differential scanning calorimeter data
also illustrates the evolution of the Cr-to-I transition with the
lengthening of spacers. Fig. 5 illustrates the correlation between
the transitional temperatures (Cr-to-I, I-to-Cr) with the variation
Structure-mesomorphic property relationships: The
mesomorphic behaviour of organic compounds is sensitive to
its molecular architecture; a slight modification of molecular
geometry brings about substantial change in its mesomorphic
n
in the length of alkyl spacer C H2n. This study revealed that
TABLE-4
COMPARISON OF MESOMORPHIC BEHAVIOUR OF NBP4A WITH STRUCTURALLY RELATED DIMERS
o
Phase Transition ( C)
Compound
Structure
O
O
H
C
H
C
(
C12H24)
12BP4A
Current work]
Cr 79.2 I
N
N
[
N
N
1
2BA6: Cr 129.1 N
32.2 I
12BA7: Cr 127.0 N
33.0 I
1
2
8,29
nBAm
1
1
2BA10: Cr 118.2
SmG 131.3 I
2
9
Cr 117.6 SmA 125.8
N 154.7 I
1
0BABP10
2
6
Cr 144.2 SmA 160.7
N 173.2 I
1
0BABZ
3
0
10BATP: Cr 149.6 N
2
nBATP
07.6 I

Vol. 25, No. 11 (2013)
Synthesis and Phase Transition Behaviours of Pyridine Symmetrical Dimers 6155
27
behaviour . Investigation of these factors by experimental or
theoretical means should hopefully lead to a better understanding
of structure-property relationships on the synthesized compounds
in liquid crystals.
REFERENCES
1
2
.
.
F. Yuksel, D. Atilla and V. Ahsen, Polyhedron, 26, 4551 (2007).
B.Y. Zhang, F.B. Meng, M. Tian and W.Q. Xiao, React. Funct. Polym.,
6
6, 551 (2005).
In order to relate the transitional properties with the chemical
constitution of molecules, the present dimers nBP4A are
compared with the related liquid crystal dimers. The general
3. P.J. Collings and M. Hird, Introduction to Liquid Crystals Chemistry
and Physics, Taylor and Francis Ltd., London, pp. 43-176 (1998).
4
.
S. Singh and D.A. Dunmur, Liquid Crystals: Fundamentals. World
Scientific Publishing Co. Pvt. Ltd., London, pp. 92-277 (2002).
H. Kelker and B. Scheurle, Angew. Chem. Int. Ed., 8, 884 (1969).
2
7,28
molecular structure for these dimers labeled as nBAm
2
,
0BABP10 , 10BABZ and nBABTP and their phase
5
.
8
26
30
1
6. A. Seed, Chem. Soc. Rev., 36, 2046 (2007).
.
7
M.L. Parra, C.G. Saavedra, P.I. Hildago and E.Y. Elgueta, Liq. Cryst.,
5, 55 (2008).
M.L. Parra, J.Alderete, C. Zuniga, H. Gallardo, P. Hindalgo, J. Veragara
sequence and transition temperature are shown in Table-4.
Terminal groups showed strong influence on the meso-
morphic properties of a molecule (Table-4). It can be seen
that the dimers with terminal alkyl chain, nBAm exhibited
mesomorphic properties and dimers without terminal group,
nBP4A are non-mesogens. Terminal chains bring flexibility
3
8
.
and S. Hernandez, Liq. Cryst., 28, 1659 (2001).
9. L.L. Lai, C.H. Wang, W.P. Hsien and H.C. Lin, Mol. Cryst. Liq. Cryst.,
87, 177 (1996).
2
0. C.T. Imrie, Struct. Bonding, 95, 149 (1999).
1. C.T. Imrie, F.E. Karasz and G.S. Attard, Macromolecules, 26, 3803
1
1
31
into the molecule which is required to stabilize mesophases .
Dimers with short terminal chain, 12BA6 and 12BA7 are
nematogens and longer terminal chain promotes smectic phase
in 12BA10. As for 10BABP10, one side terminal chain is
replaced with a polar cyano substituent but it still able to display
mesophase. High rigidity of biphenyl ring in 10BABP10 also
enhances its phase stability wherein its clearing temperature
(
12. G.R. Luckhurst, Macromol. Symp., 96, 1 (1995).
1993).
1
1
3. C.T. Imrie and P.A. Henderson, Curr. Opin. Colloid Interf. Sci., 7, 298
2002).
(
4. S.T. Ha, T.M. Koh, S.L. Lee, G.Y. Yeap, H.C. Lin and S.T. Ong, Liq.
Cryst., 37, 547 (2010).
15. S.T. Ha, T.M. Koh, G.Y. Yeap, H.C. Lin, S.L. Lee, Y.F. Win and S.T.
Ong, Phase Trans., 83, 195 (2010).
6. S.T. Ha, T.M. Koh, G.Y. Yeap, H.C. Lin, S.L. Lee, Y.F. Win and S.T.
Ong, Mol. Cryst. Liq. Cryst., 582, 10 (2010).
1
1
1
1
(
(
154.7 ºC) is much higher than 12BP4A (79.2 ºC) and nBAm
131-133 ºC).
Presence of pyridine ring in nBP4A is not the main reasons
7. S.T. Ha, L.K. Ong, Y.F. Win, G.Y. Yeap, N.L. Bonde and P.L. Boey,
Am. J. Appl. Sci., 7, 656 (2010).
8. S.T. Ha, L.K. Ong, Y.F. Win, Y. Sivasothy, G.Y. Yeap and P.L. Boey,
Aust. J. Basic Appl. Sci., 4, 1146 (2010).
that prevent current dimers from exhibiting mesophases. This
can be evident from the case of 10BABZ and 10BATP wherein
these compounds having heterocyclic benzothiazole and
thiophene, respectively, are mesogens. However, it needs to
complement with terminal polar chloro substituent like in the
case of 10BABZ or having a longer molecular core through
additional aromatic ring in 10BATP. Increasing the core length
by inserting short unsaturated linkages (COO, ester) between
phenyl and thiophene rings enhances the clearing temperature
of 10BATP.
9. S.T. Ha, T.M. Koh, G.Y. Yeap, H.C. Lin, P.L. Boey, Y.F. Win, S.T. Ong
and L.K. Ong, Mol. Cryst. Liq. Cryst., 506, 56 (2009).
20. S.T. Ha, T.M. Koh, H.C. Lin, G.Y.Yeap,Y.F. Win, S.T. Ong,Y. Sivasothy
and L.K. Ong, Liq. Cryst., 36, 917 (2009).
2
2
2
2
2
1. S.H. Han, H. Yoshida, Y. Nobe, M. Fujiwara, J. Kamizori, A. Kikuchi,
F. Iwahori and J. Abe, J. Mol. Struct., 735-736, 375 (2005).
2. G.Y. Yeap, S.T. Ha, P.L. Boey, W.A.K. Mahmood, M.M. Ito and Y.
Youhei, Mol. Cryst. Liq. Cryst., 452, 73 (2006).
3. G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, M.M. Ito, S. Sanehisa and V.
Vill, Mol. Cryst. Liq. Cryst., 452, 63 (2006).
4. G.Y. Yeap, S.T. Ha, P.L. Lim, P.L. Boey, M.M. Ito, S. Sanehisa and Y.
Youhei, Liq. Cryst., 33, 205 (2006).
Conclusion
5. P.J. Collings and M. Hird, Liquid Crystal: Nature's Delicate Phase of
Matter, IOP Publishing Ltd., Bristol, England (1990).
26. G.Y. Yeap, E.A. Al-Taifi, C.H. Ong, W.A.K. Mahmood, D. Takeuchi
In this paper, we report the synthesis and characterization
of new symmetrical dimers, α,ω-bis-(4-pyridine-4-methylene-
aminoxy) alkanes. The influence of dimer spacer length,
terminal (polar and alkyl chain) groups and core rings on
mesomorphic properties were discussed and compared with
reported dimer liquid crystals. The presence of the pyridine
group renders this system to be suitable components for building
charge transfer-based liquid crystals with an appropriate choice
of acceptor molecules.
and M.M. Ito, Phase Trans., 85, 483 (2012).
2
2
7. A.K. Prajapati, N.L. Bonde, H.M. Patel, Phase Trans., 78, 507 (2005).
8. D. Bhuyan, P. Pardhasaradhi, K.N. Singh, D. Madhavi Latha, P.V. Datta
Prasad, P.R. Alapati and V.G.K.M. Pisipati, World J. Condensed Matter
Phys., 1, 167 (2011).
2
3
3
9. B. Gogoi, P.R. Alapati and A.L. Verma, Cryst. Res. Technol., 37, 1331
(
2002).
0. G.Y. Yeap, T.C. Hng, D. Takeuchi, K. Osakada, W.A.K. Mahmood and
M.M. Ito, Mol. Cryst. Liq. Cryst., 506, 134 (2009).
1. V. Vill, Encyclopedia of Materials: Science and Technology, Elsevier
Science Ltd., p. 4545 (2001).
ACKNOWLEDGEMENTS
The authors thank Universiti Tunku Abdul Rahman and
Ministry of Higher Education Malaysia for the financial support
and research facilities.