Synthesis and mesomorphic properties of uncoordinated liquid crystal dimers bis[4-[(4′-decyloxyphenyl)carboxylate]salicylideneimino] alkanes and their copper(ii) complexes

Article abstract of DOI:10.1080/15421406.2013.822282

Series of symmetrical liquid crystal dimers wherein the spacer connected to two mesogenic units of salicylideneimines consists of the aliphatic chain (CH₂)_n where n adopts even parity numbers from 2 to 10 have been synthesized and ch

Full text of DOI:10.1080/15421406.2013.822282

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Synthesis and Mesomorphic
Properties of Uncoordinated
Liquid Crystal Dimers Bis[4-[(4′-
Decyloxyphenyl)Carboxylate]Salicylideneimi
Alkanes and Their Copper(II) Complexes
Subramanian Balamurugan a , Guan-Yeow Yeap a , Wan Ahmad Kamil
Mahmood a b & Sengodan Senthil b
a Liquid Crystal Research Laboratory, School of Chemical Sciences ,
Universiti Sains Malaysia , Minden , Penang , Malaysia
b Department of Chemistry , Government Arts College , Selam ,
Tamilnadu , India
Published online: 16 Dec 2013.
To cite this article: Subramanian Balamurugan , Guan-Yeow Yeap , Wan Ahmad Kamil Mahmood &
Sengodan Senthil (2013) Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal Dimers
Bis[4-[(4′-Decyloxyphenyl)Carboxylate]Salicylideneimino] Alkanes and Their Copper(II) Complexes,
Molecular Crystals and Liquid Crystals, 587:1, 92-102, DOI: 10.1080/15421406.2013.822282
To link to this article: http://dx.doi.org/10.1080/15421406.2013.822282
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Mol. Cryst. Liq. Cryst., Vol. 587: pp. 92–102, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.822282
Synthesis and Mesomorphic Properties
of Uncoordinated Liquid Crystal Dimers Bis[4-[(4ꢀ-
Decyloxyphenyl)Carboxylate]Salicylideneimino]
Alkanes and Their Copper(II) Complexes
SUBRAMANIAN BALAMURUGAN,1 GUAN-YEOW YEAP,1,∗
WAN AHMAD KAMIL MAHMOOD,1,2 AND SENGODAN
SENTHIL2
1Liquid Crystal Research Laboratory, School of Chemical Sciences, Universiti
Sains Malaysia, Minden, Penang, Malaysia
2Department of Chemistry, Government Arts College, Selam, Tamilnadu, India
Series of symmetrical liquid crystal dimers wherein the spacer connected to two meso-
genic units of salicylideneimines consists of the aliphatic chain (CH2)n where n
adopts even parity numbers from 2 to 10 have been synthesized and characterized.
These dimers function as tetradentate ligands which possess strong affinity toward
the Cu(II) ion leading to the formation of four-coordinated complexes with Cu
N
and Cu O modes. The uncoordinated dimers exhibit enantiotropic nematic as well as
smectic C phases. However, the Cu(II) complexes behave predominantly as nematogens.
Correlation study between the effect of variable spacer length and the mesomophic be-
haviors of these dimers along with their related Cu(II) complexes are reported. The
Cu(II) complexes in which the ligands possess shorter spacers were seemed to exhibit
higher thermal stability.
Keywords Cu(II) complexes; nematic; smectic C; symmetrical dimers; tetradentate
ligands
1. Introduction
It has well been documented in the last few decades that the symmetrical dimers or
"twins" refer to a wide array of liquid crystalline compounds in which a flexible chain
formed a bridge between the two mesogenic units [1–3]. The dimers such as α,ω-bis(4-
alkylanilinebenzylidine-4ꢀ-oxy)alkanes showed a rich smectic polymorphism and the for-
mation of various smectic phases could be influenced by different flexible chain [4]. Earlier
study showed that if the flexible chain is shorter, the dimer will be more rigid owing to
the coupling of the two mesogenic units. However, both mesogenic units become nearly
independent with the increase of the spacer length.
∗Address correspondence to Guan-Yeow Yeap, Liquid Crystal Research Laboratory, School
of Chemical Sciences, Universiti Sains Malaysia, 11800, Minden, Penang, Malaysia. E-mail:
gyyeap@usm.my, or gyyeap liqcryst usm@yahoo.com
92

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal
93
It has also been established that the parity of carbon numbers in the flexible chain
induced the odd–even effect [5,6]. This effect could lead to alternations in clearing tem-
perature and associated enthalpies for the ligand as well as the metal-ligand complexes or
metallomesogens. This phenomenon is interesting because the combination of the proper-
ties of metals (polarizability, color and magnetism) will result in enhanced mesomorphic
behaviors which are essential for the design of new electro-optical devises [7,8].
On the other hand, various studies have established a relationship between these metal
ions and their applications in different fields including the nuclear chemistry, metal ion
separation, pollution control, industrial processes, hydrometallurgy, and polymer drug graft
(antitumor and antibiotic agents) [9].
The Schiff bases or imines possessing ortho-hydroxyl (OH) group are essential for
the formation of metallomesogens. The chelating abilities and analytical applications are
the key factors attracting the interests of researchers in recent years [9]. Previous work has
shown that the complexes containing the tetradentate ligands exhibited prominent magnetic
and photochemical properties [10–13].
In the present work, a series of dimers bis[4-[(4ꢀ-decyloxyphenyl)-carboxylate]
salicylideneimino]alkanes (where the alkanes are (CH2)n– with n = 2, 4, 6, 8, and
10) was designed and synthesized. A comparative study among these imines and the effect
of Cu(II) ion upon the mesomorphic properties of Cu(II) complexes containing these imines
has also been carried out.
The synthetic routes toward the formation of the dimers and their Cu(II) complexes
are shown in Schemes 1 and 2.
2. Experimental
2.1 Characterization
Elemental analysis was carried out with a Perkin–Elmer 240 microanalyzer. Infrared spectra
were obtained on Thermo Electron Corporation Nicolet 380 FT-IR spectrometer. 1H-NMR
(300 MHz) and 13C-NMR (75 MHz) spectra were recorded on a Bruker AM-400 spectrom-
eter with tetramethylsilane as internal standard. Differential scanning calorimetry (DSC)
was conducted on a Perkin–Elmer model DSC Pyris 1 system calibrated with indium and
zinc standards. Polarizing microscopy was performed with a Euromex polarizing micro-
scope equipped with a Linkam HFS 91 heating stage and a TP-93 temperature programmer.
Small quantity of sample was sandwiched between two thin glass covers. The heating and
cooling rates of 5 ◦C min−1 were applied on free imines and Cu(II) complexes.
2.2 Synthesis
All solvents were purified and dried by standard procedures [14]. The reagents 1-decanol,
α, ω-diamine, 2, 4-dihydroxybenzaldehyde, DCC, and DMAP were used as purchased
from Aldrich. Other reagents (1-bromodecane, KOH and p-hydroxybenzoic acid) were
purchased from SRL and used without purification. 1-Bromodecane was reacted with
p-hydroxybenzoic acid to form 4, 4ꢀ-decyloxybenzoic acid by Williamson method. The
acids thus obtained were reacted with 2, 4-dihydroxybenzaldehyde in the presence of DCC
and DMAP to afford the desired aldehyde. The corresponding aldehyde was subsequently
reacted with α, ω-diamine (NH2CnH2nNH2; n = 2, 4, 6, 8, and 10) leading to the formation
of dimers.

94
S. Balamurugan et al.
Scheme 1. Synthetic route for dimers 1–5.
2.2.1 Synthesis of 4-decyloxybenzoic acid. 4-Hydroxybenzoic acid (6.4 g, 46 mmol) and
anhydrous K2CO3 (19 g, 138 mmol) were dissolved in dry acetone (70 mL) and stirred for
1 h at room temperature. 1-Bromodecane (10.1 g, 46 mmol) was added dropwise to this
solution followed by the addition of potassium iodide. The solution was heated to 100◦C
for 24 h with constant stirring. The resulting mixture was filtered and washed with acetone
(3 × 20 mL). Filtrate and washing were collected and concentrated by vacuum distillation.

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal
95
Scheme 2. Synthetic route for complexes 6–10.
The mixture was poured into water (500 mL) and neutralized with 10% HCl solution.
The precipitate thus obtained was filtered and recrystallized from ethanol to form desired
product.
Yield 65%. M.p. 138◦C. C17H26O3: Calc. C, 73.34; H, 9.41; found C, 73.62; H,
9.29. FT-IR (KBr pellet) υmax/cm−1: 2918 (C H), 2546 ( OH of COOH), 1685 (C O),
1256(C O). 1H-NMR (CDCl3), δ/ppm: 11.02 (s, 1H, COOH), 8.12(d, 2H, Ar H),
6.96(d, 2H, Ar H), 4.01(t, 2H, OCH2), 1.81(q, 1H, CH2), 1.24–1.39 (m, 14h, CH2),
0.93(t, 3H, CH3). 13C-NMR (CDCl3) δ/ppm: 172.2, 163.7, 132.3, 121.4, 114.18, 68.2,
31.6, 29.0, 25.8 and 22.5.
2.2.2 Synthesis of 4-formyl-3-hydroxyphenyl-4-decyloxybenzoate. A mixture of 4-
decyloxybenzoic acid (5 g, 18 mmol), 2, 4-dihydroxybenzaldehyde (2.5 g, 18 mmol),
DCC (2.4 g, 20 mmol) and DMAP (0.1 g, 1 mmol) were dissolved in 100 mL of dry
CH2Cl2 and stirred at room temperature for overnight. The precipitated urea was removed
by filtration and the solution was washed with 5% acetic acid followed by saturated brine
solution and water. Organic layer was dried over anhydrous sodium sulphate and the solvent
was removed under vacuum distillation. The residue thus obtained was purified by column
chromatography using 5% methanol in chloroform as eluent.
Yield 70%. C24H30O5: Calc. C, 72.34; H, 7.59; found C, 72.72; H, 7.44. FTIR (KBr
pellet) υmax/cm−1: 3198 (aromatic OH), 2938–2818 (aliphatic CH ), 1696 ( CHO),
1727 cm−1 (ester CO) and 886 (aromatic CH). 1H-NMR (CDCl3), δ/ppm: 0.87 (t, 3H,
CH3); 1.24 (m, 16H, CH2); 4.03 (t, 2H, CH2O); 6.78(s, 1H, Ar); 6.91 (d, 2H, Ar),

96
S. Balamurugan et al.
6.95 (d, 2H, Ar) 7.59(d, 1H, Ar); 8.11 (d, 2H, Ar); 9.87 (s,1H, CHO); 11.53 (s, 1H, OH)
ppm. 13C-NMR (CDCl3) δ/ppm: 194,2, 165.6, 164.5, 163.4, 158.0, 131.9, 130.6, 121.5,
118.2, 114.2, 114.0, 112.6, 68.6, 31.1, 29.4,22.6, 14.1.
2.2.3 Synthesis of N,Nꢀ-bis{4ꢀ-(4ꢀꢀ-decyloxybenzoyloxy)-2ꢀ-hydroxybenzylidine}ethane-
1,2-diamine (1). To a hot solution of the 4-formyl-3-hydroxyphenyl-4-decyloxybenzoate
(0.5 g, 1.3 mmol) in 15 mL absolute ethanol was added dropwise with a solution of 1,2-
diaminoethane (0.03 g, 0.51 mmol) in 2 mL absolute ethanol. The reaction mixture was
refluxed under nitrogen atmosphere for 1 h. Upon cooling, the crystalline product was
separated by filtration and purified several times by recrystallization using acetone/THF
(10:1) mixture. The product crystallized in yellow solids. The above synthetic procedure
was applied to the other homologues and all the analytical data are summarized as follows:
1: Yields 85%. C50H64N2O8: Calc. C, 73.14; H, 7.86; N, 3.41; found C, 73.73; H,
7.32; N, 3.52. FTIR (KBr pellet) υmax/cm−1: 2918 cm−1 (aromatic OH), 2921, 2820
(aliphatic CH), 1718 (ester C O), 1634 (imine C N), and 881 (aromatic CH). 1H-
NMR (CDCl3), δ/ppm: 11.24 (s, 2H, OH), 8.35 (s, 2H, CH N), 8.09 (d, 4H, ArH),
7.61(d, 2H, ArH), 7.21 (d, 2H, ArH), 6.95 (d, 4H, ArH), 6.78(d, 2H, ArH), 4.03 (t, 4H,
CH2O), 3.54 (t, 4H, NCH2), 1.24–1.80 (m, 32H, CH2), 0.85(t, 6H, CH3). 13C-NMR
(CDCl3) δ/ppm: 165.3, 164.0, 160.9, 157.1, 154.8, 129.6, 121.7, 114.1, 110.5, 68.8, 60.4,
55.9, 31.0, 29.7, 26.0, 22.7, 14.2.
2: Yield 79%; C52H68N2O8: Calc. C, 73.55; H, 8.07; N, 3.30; found C, 73.58; H,
8.01; N, 3.29. FTIR (KBr pellet) υmax/cm−1: 2918 (aromatic OH), 2921, 2820 (aliphatic
1
CH), 1724 (ester C O), 1634 (imine C N), and 881 (aromatic CH). H-NMR
(CDCl3), δ/ppm: 11.24 (s, 2H, OH), 8.35 (s, 2H, CH N), 8.07 (d, 4H, ArH), 7.61 (d,
2H, ArH), 7.22 (d, 2H, ArH), 6.94 (d, 4H, ArH), 6.79(d, 2H, ArH), 4.03 (t, 4H, CH2O),
3.54 (t, 4H, NCH2), 1.24–1.99 (m, 36H, CH2), 0.85 (t, 6H, CH3). 13C-NMR (CDCl3)
δ/ppm: 165.3, 164.1, 160.8, 157.0, 154.8, 129.4, 121.7, 114.1, 110.8, 68.8, 60.4, 55.9, 31.0,
29.7, 26.0, 22.7, 14.2.
3: Yield 76%; C54H72N2O8: Calc. C, 73.94; H, 8.27; N, 3.19; found C, 73.67; H, 8.54;
N, 3.11. 1H-NMR (CDCl3), δ/ppm: 2919 (aromatic ;OH), 2920, 2818 (aliphatic CH),
1719 (ester C O), 1632 (imine C N), and 881 (aromatic CH). 1H NMR (300 MHz,
CDCl3, δ): 11.24 (s, 2H, OH), 8.33 (s, 2H, CH N), 8.04 (d, 4H, ArH), 7.61 (d, 2H,
ArH), 7.22 (d, 2H, ArH), 6.96 (d, 4H, ArH), 6.77(d, 2H, ArH), 4.03 (t, 4H, CH2O),
3.54 (t, 4H, NCH2), 1.24–1.99(m, 40H, CH2), 0.85 (t, 6H, CH3). 13C-NMR (CDCl3)
δ/ppm: 165.2, 164.4, 160.6, 157.0, 154.5, 129.6, 121.7, 114.1, 110.8, 68.8, 60.4, 55.9, 31.0,
29.7, 26.0, 22.7, 14.2.
4: Yield 71%; C56H76N2O8: Calc. C, 74.30; H, 8.46; N, 3.09; found C, 74.56; H, 8.65; N,
3.21. FTIR (KBr pellet) υmax/cm−1: 2918 (aromatic OH), 2922, 2818 (aliphatic CH),
1
1726 (ester C O), 1638 (imine C N) and 882 (aromatic CH). H-NMR (CDCl3),
δ/ppm: 11.24 (s, 2H, OH), 8.35 (s, 2H, CH N), 8.08 (d, 4H, ArH), 7.62(d, 2H, ArH),
7.26 (d, 2H, ArH), 6.96 (d, 4H, ArH), 6.77(d, 2H, ArH), 4.03 (t, 4H, CH2O), 3.54 (t,
4H, NCH2), 1.24–1.99 (m, 44H, CH2), 0.85 (t, 6H, CH3). 13C-NMR (CDCl3) δ/ppm:
165.5, 164.2, 160.6, 157.0, 154.0, 129.4, 121.6, 114.1, 110.8, 68.8, 60.4, 55.9, 31.0, 29.7,
26.0, 22.7, 14.2.
5: Yield 79; C58H80N2O8: Calc. C, 74.64; H, 8.64; N, 3.00; found C, 74.62; H, 8.55; N,
3.81. FTIR (KBr pellet) υmax/cm−1: 2918 (aromatic OH), 2921, 2819 (aliphatic CH),
1728 (ester C O), 1636 (imine C N), and 881 (aromatic CH). 1H-NMR (CDCl3),
δ/ppm: 11.24 (s, 2H, OH), 8.34 (s, 2H, CH N), 8.09 (d, 4H, ArH), 7.61 (d, 2H, ArH),
7.22 (d, 2H, ArH), 6.95 (d, 4H, ArH), 6.76 (d, 2H, ArH), 4.07 (t, 4H, CH2O), 3.54 (t,

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal
97
Table 1. Phase transition temperatures (◦C) and associated enthalpies [kJ.mol–1] for dimers
upon cooling
Dimer Cr1
Cr2
SmC
N
Iso
1
2
3
4
5
•
•
•
•
•
98.6 [16.8]
92.4 [20.5]
149.2 [34.5]
65.3 [20.4]
73.2 [25.8]
•
•
128.3 [9.2]
118.1 [13.8]
•
•
•
•
148.0 [2.7]
163.0 [5.8]
177.8 [14.4]
149.0 [1.4]
•
•
•
•
•
•
116.7 [13.0]
•
•
154.5 [5.2]
142.2 [17.3]
Dimers 3 and 4 were compared with those reported by Maja Sepelj et al (2006).
Cr, Crystal; SmC, smectic C; N, nematic; Iso, isotropic.
4H, =NCH2), 1.24–1.80 (m, 48H, CH2), 0.85 (t, 6H, CH3). 13C-NMR (CDCl3) δ/ppm:
165.1, 164.5, 160.7, 157.5, 154.8, 129.6, 121.5, 114.1, 110.5, 68.8, 60.4, 55.9, 31.0, 29.7,
26.0, 22.7, 14.2.
2.2.4 Synthesis of N,Nꢀ-bis{4ꢀ-(4-decyloxybenzoyloxy)-2ꢀ-hydroxybenzylidine}ethane-1,2-
diamine) copper(II) (6). The Cu(II) complexes were synthesized through the reported
procedure [15]. Copper acetate monohydrate (0.04 g, 0.2 mmol) was added to a hot solution
of compound 1 (0.30 g, 0.2 mmol) in 30 mL ethanol. The reaction mixture was refluxed
for 5 h and then cooled to room temperature. The solid, collected by filtration, was washed
with water, ethanol, and recrystallized from CH2Cl2/MeOH to give a green microcrystalline
solid. The same synthetic procedure was adopted for all homologues and their analytical
data are summarized below.
6: Yield 69%. C50H62CuN2O8: Calc. C, 68.04; H, 7.08; N, 3.17; found C, 68.17; H,
7.13; N, 3.24. FTIR (KBr pellet) υmax/cm−1: 2851 (aliphatic CH), 1734 (ester C O),
1615 (imine C N), and 547 (CuO)
Figure 1. Representative DSC traces for dimer 5.

98
S. Balamurugan et al.
Figure 2. Correlation between transition temperature and methylene chain length for dimers 1–5.
7: Yield 78%. C52H66CuN2O8: Calc. C, 68.58; H, 7.31; N, 3.08; found C, 68.46; H,
7.44; N, 3.25. FTIR (KBr pellet) υmax/cm−1: 2851 (aliphatic CH), 1732 (ester C O),
1616 (imine C N), and 540 (CuO)
8: Yield 77%. C54H70CuN2O8: Calc. C, 69.09; H, 7.52; N, 2.98; found C, 69.12; H,
7.76; N, 2.45. FTIR (KBr pellet) υmax/cm−1: 2852 (aliphatic CH), 1734 (ester C O),
1616 (imine C N), and 525 (CuO)
9: Yield 81%; C56H74CuN2O8; C, 69.57; H, 7.72; N, 2.90; found C, 69.88; H, 7.49;
N, 2.75. FTIR (KBr pellet) υmax/cm−1: 2830 (aliphatic CH), 1727 (ester C O), 1616
(imine C N), and 550 (CuO)
10: Yield 74%; C58H78CuN2O8: Calc. C, 70.03; H, 7.90; N, 2.82; found C, 70.15; H,
7.67; N, 2.99. FTIR (KBr pellet) υmax/cm−1: 2838 (aliphatic CH), 1726 (ester C O),
1615 (imine C N), and 540 (CuO).
Table 2. Phase transition temperatures (◦C) and associated enthalpies (kJ.mol–1) for com-
plexes upon heating
Complex
Cr
N
Iso
6
7
8
9
•
•
•
•
•
106.4 [30.0]
152.0 [30.5]
145.2 [33.6]
139.8 [29.7]
123.2 [28.2]
•
•
•
•
•
197.4 [16.8]
254.1 [20.7]
190.2 [23.2]
179.1 [20.8]
166.4 [11.4]
•
•
•
•
•
10
Cr, Crystal; N, nematic; Iso, isotropic.

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal
99
Figure 3. Photomicrograph of complex 6 at 154◦C on heating.
3. Results and Discussion
3.1 Mesomorphic Behaviors
3.1.1 Uncoordinated liquid crystal dimers bis[4-[(4ꢀ-decyloxyphenyl)-carboxylate]sali
cylideneimino]alkanes (where the alkanes are (CH2)n in which n = 2, 4, 6, 8, and
10), 1–5. The synthesis of symmetrical dimers 1–5 have been carried out by conventional
method (Scheme 1). It is well illustrated in Scheme 1 that these dimers consist of four
electron donating atoms and hence function as tetradentate ligands.
All of these dimers show various phases when temperature is varied. The measured
phase transition temperature and associated enthalpy values for compounds 1–5 upon
cooling are shown in Table 1. All of the compounds except compound 5 exhibit SmC
Figure 4. Photomicrograph of complex 8 at 160◦C on heating.

100
S. Balamurugan et al.
Figure 5. Representative DSC thermogram of complex 6.
phase. It is apparent that cooling upon the homologues 1–3 from isotropic liquid led to Iso-
SmC transitions at 148.0◦C, 163.0◦C, and 177.8◦C, respectively. However, when the spacer
length within this series increases from C6 to C8 as that in compound 4, the Iso-N transition
was initially observed at 154.5◦C prior to the N-SmC transition at 149.5◦C. The formation
of the nematic phase in compound 4 can be substantiated on the basis of high mobility of
threads inside the droplet with the flashes when subjected to mechanical stress. From the
same cooling process, ligand 5 experiences the Iso-N transition only before crystallization
at 73.2◦C. A representative DSC thermogram related to compound 5 is shown in Fig. 1.
The correlation between the transition temperatures and methylene chain lengths in dimers
1–5 is shown in Fig. 2. It can also be seen in Fig. 2 that the smectogenic properties vanished
when the spacer length increased from C8 to C10 in respective compounds 4 and 5. The
Cr1-SmC transition temperature reaches the maximum at 149.0◦C in compound 3 leading
to the highest temperature of Iso-SmC transition in comparison to analogues 1 and 2.
Previous reports have established that the presence of smectic phase particularly in
symmetrical dimers could be attributed to the terminal chain length which was greater than
half of the spacer length. This generalization is applicable to current compounds especially
for compounds 1–4 and this observation concurs with that suggested by Date et al. [4].
3.1.2 Cu(II) complexes, 6–10. Knowing from the earlier section that all the dimers 1–5
can be categorized as tetradentate ligands possessing O and N donor atoms, it gives the
dimers an advantage to form the new bonds with electron deficient metal atom or ion [16].
It had been reported that the copper complexes possessing the ligand with longer ter-
minal chain favored the formation of nematic phase but the analogues with shorter terminal
chain were inclined to smectogenic behavior [17]. However, all the present complexes
exhibit exclusively the phase sequence of Cr N Iso (Table 2). The measured phase tran-
sition temperature and associated enthalpy values for Cu(II) complexes 6–10 upon heating
are shown in Table 2. The photomicrographs for complexes 6 and 8 at 154◦C and 160◦C
are illustrated in respective Figs. 3 and 4.
A representative DSC thermogram of complex 6 is shown in Fig. 5. Comparison be-
tween the phase transition temperatures of the ligands (1–5) and their respective complexes

Synthesis and Mesomorphic Properties of Uncoordinated Liquid Crystal
101
Figure 6. Correlation of transition temperature with respect to the chain length of complexes 6–10.
(6–10) shows that transition temperatures of the earlier increase drastically upon com-
plexation. However, the homologues 6–10 on heating led to Cr N transitions at 106.4◦C,
152.0◦C, 145.2◦C, 139.8◦C, and 123.2◦C, respectively.
The correlation between the transition temperatures and respective spacer lengths are
summarized in Fig. 6. One of the notable features is that when the spacer length increased
from C2 to C4, the mesophase range (ꢀN) for complex 6 increased from 91.0◦C to 102.1◦C
in complex 7. However, further increase of the spacer length from C4 to C6 and C8 would
lead to a decline in the thermal stability (45.0◦C and 39.3◦C for complexes 8 and 9,
respectively). This value was seemed to ascend when it moved from complexes 9 to 10
wherein the ꢀN value increased to 43.2◦C.
4. Conclusions
All the target dimers 1–5 exhibited enantiotropic mesophase. The homologous dimers 1-3
exhibit exclusively the SmC phase. While dimer 4 exhibits both N and SmC on cooling
from the isotropic, the member with longest spacer 5 is predominantly nematogen. These
dimers behave as tetradentate ligands coordinating to Cu ion through Cu N and Cu O
modes. All the Cu(II) complexes containing 1–5 were nematogens. The Cu(II) complexes
containing the ligands with shorter spacers (C2 and C4) exhibit higher thermal stability at
the mesophase region in comparison to those with C6, C8, and C10 spacers.
Acknowledgment
The main author (G-Y.Yeap) would like to thank Malaysian Ministry of Higher Education or
MOHE for the Fundamental Research Grant Scheme (FRGS) No. 203/PKIMIA/6711192.
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