Synthesis and properties of copper (II), oxovanadium (IV) and gadolinium (III) complexes derived from polar Schiff's bases

Article abstract of DOI:10.1016/j.molstruc.2011.07.010

The synthesis, liquid crystalline and optical properties of copper (II), oxovanadium (IV) and gadolinium (III) complexes derived from polar Schiff's bases viz., N-(4-n-hexadecyloxysalicylidene)-4′-substituted anilines with methoxy, fluoro and chloro substituents in the 4-position of N-aryl moiety are presented. All the ligands exhibited smectic A phase except methoxy homolog which exhibited nematic phase. Majority of the complexes exhibited smectic A phase with some of them exhibiting smectic E phases. The UV-Visible and photo-luminescent properties of the ligands and complexes are also discussed.

Full text of DOI:10.1016/j.molstruc.2011.07.010

Journal of Molecular Structure 1002 (2011) 135–144
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and properties of copper (II), oxovanadium (IV) and gadolinium (III)
complexes derived from polar Schiff’s bases
Lopamudra Chakraborty, Nirmalangshu Chakraborty, Atiqur Rahman Laskar, Manoj Kumar Paul,
⇑
Nandiraju V.S. Rao
Chemistry Department, Assam University, Silchar 788 011, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2011
Received in revised form 5 July 2011
Accepted 5 July 2011
Available online 10 August 2011
The synthesis, liquid crystalline and optical properties of copper (II), oxovanadium (IV) and gadolinium
(III) complexes derived from polar Schiff’s bases viz., N-(4-n-hexadecyloxysalicylidene)-40-substituted
anilines with methoxy, fluoro and chloro substituents in the 4-position of N-aryl moiety are presented.
All the ligands exhibited smectic A phase except methoxy homolog which exhibited nematic phase.
Majority of the complexes exhibited smectic A phase with some of them exhibiting smectic E phases.
The UV–Visible and photo-luminescent properties of the ligands and complexes are also discussed.
Ó 2011 Elsevier B.V. All rights reserved.
Keywords:
Metallomesogens
Liquid crystals
Smectic phase
Polar compounds
Schiff’s bases
1. Introduction
N-(4-n-alkoxysalicylidene)-n-alkylamines, L1-L4 coordinated to
lanthanide ions, as shown in Fig. 1 [14–17] apart from few examples
Metal containing liquid crystals called as metallomesogens
have received a lot of attention in the last three decades due to
its importance in the basic liquid crystals research as well as its
potential utility in industrial, chemical, medical, and sensor
applications [1–13]. The nature of metal plays an important
role to influence the mesomorphic properties of the organic
ligand to which it coordinates. The importance of Schiff bases
viz., N-(4-n-alkoxysalicylidene)-40-n-alkyl(oxy)anilines lies in
their rich mesomorphism at ambient temperatures and as well
as their suitability for extension with many functional groups
[14–16]. Extensive studies are carried out on metallomesogens of
d-block elements coordinated to N-(4-n-alkoxysalicylidene)-40-n-
alkyl(oxy)anilines because of the ease of synthesis, stability of
the complexes and rich variety of mesophases [10–13]. The incor-
poration of metal ion with unpaired electrons in these Schiff base
ligands can lead to the formation of paramagnetic liquid crystalline
(LC) materials. The alignment of paramagnetic LCs possessing
large magnetic anisotropy can be realized by small magnetic field
strengths. Hence the coordination of paramagnetic lanthanide
ions with N-(4-n-alkoxysalicylidene)-40-n-alkyl(oxy)anilines can
generate low molar mass magnetic liquid crystals which possess
large magnetic anisotropy. However related studies of the
f-block lanthanide ion complexes are relatively limited to
of rare-earth containing metallomesogens with other ligands. The
first calamitic bidentate salicylideneimine nO(OH)mali, possessing
N-aliphatic moiety based lanthanide complexes was reported in
1991 [14] and a new nomenclature of lanthanidomesogens had
been proposed [18–21]. However the efforts in different laborato-
ries [17] to coordinate f-block metals with the salicylideneimine
based ligands with an aryl moiety [14–17] on nitrogen atom of
the central bridging imine group have been unsuccessful. The
difference in basicity of the nitrogens of N-alkyl and N-aryl imines
of the ligand to form a zwitter ion led to the coordination of the
zwitter ionic N-alkyl imines only to form the lanthanide complexes,
while it is not realized with aromatic imines.
N, N0-aromatic donors shown in Fig. 2 [22] viz., 4,7-disubsti-
tuted phenanthroline (L5 and L6), or 4,40-dimethoxy-2,20-bipyri-
dine (L7) coordination with the lanthanide ions was successful,
while diethyl-2,20-bipyridine-4,40-dicarboxylate (L8) did not par-
ticipate in coordination. Hence it has been accounted for an elec-
tronic destabilization of the ester groups on the aromatic
bipyridine core, which is associated with a more flexible conforma-
tion with respect to the phenanthroline to make this ligand not
compatible for chelation with a lanthanide ion of any size. In
fact the influence of the substituents on the basicity and the
coordination ability of the N, N0-aromatic ligands, viz., 4,40-dime-
thoxy-2,20-bipyridine, (L3) the bipyridine bearing a methoxy group
in 4,40-position, produced high yields of the complexes reflect-
ing the dramatically enhanced activation towards coordination.
⇑
Corresponding author. Tel.: +91 9435522541; fax: +91 3842270806.
E-mail address: drnvsrao@gmail.com (N.V.S. Rao).
0022-2860/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2011.07.010

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L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
moiety, characterization of mesomorphism of the ligands as well
as complexes and their spectroscopic characteristics. A comparison
with complexes of d-block elements is also presented.
H2n+1CnO
CH
N CmH2m+1
O H
L1: nOPh(OH).mali
O
H2n+1CnO
2. Results and discussion
O
CH
N CmH2m+1
O H
The synthesis of ligands viz., N-(4-n-decyloxysalicylidene)-40-
substituted-(methyloxy, chloro and fluoro)-anilines (hereafter
abbreviated as 16O(OH)X where X = H, OMe, F, Cl) and their Cu
(II) and VO (IV) (d-block) complexes and Gd (III) (f-block) com-
plexes as detailed in Scheme 1, was carried out following the pro-
cedure described in experimental section. Sm(III) and Pr(III)
complexes were also synthesized and characterized by IR, UV–Vis
and fluorescence studies.
L2: nOPhCOOPh(OH).mali
O
H2n+1CnO
O
O
O
CH
N CmH2m+1
O H
L3: nO(PhCOO)2PhCOO(OH).mali
H2n+1CnO
O
Elemental analysis of the ligands and copper (II) and oxovana-
dium (IV) complexes were consistent with their proposed molecu-
lar formulas (Table 1). Elemental analysis of lanthanide complexes
revealed that the stoichiometry is {[Ln(LH)2L](NO3)2}, where
Ln = Gd, and LH = 2, 3 and 4. We investigated the ionic conductivity
of solutions of the ligand 3 [16O(OH)F], c = 1.0 ꢀ 10ꢁ4 M and 3-Gd
(Gadolinium complex, c = 1.0 ꢀ 10ꢁ3 M) in double distilled and
dried N,N0-dimethylformamide (DMF). The conductivity values
(Table 2) inferred that the ligand is a non-electrolyte. However
the 3-Gd, (Gd complex) is a 2:1 electrolyte with the two nitrate
ions participation from outer coordination sphere [27] confirming
the elemental analysis data. The room temperature infrared spec-
tra of the ligands 1, 2, 3 and 4 exhibited characteristic bands
H2n+1CnO
O
CH
N
H
CmH2m+1
L4: (nO)2PhCOOPh(OH).mali,
n=m=12
O
Fig. 1. Molecular structure of Schiff’s base ligands studied extensively for
complexation [14–17].
R
R
R
R
N
N
N
N
(m
(C@N)) at 1622–1627 cmꢁ1. The C@N stretching vibration is
L7 = R = OCH3
L5 = R = COOC2H5
L6 = R = OC8H17
shifted to lower frequencies for the copper (II) (1608 cmꢁ1) and
oxovanadium (IV) (1608–1610 cmꢁ1) complexes compared to that
of free ligands. This reflects the azomethine N atom is involved in
metal-nitrogen bond formation. The oxovanadium (IV) complexes
exhibit a stretching band at around 966–987 cmꢁ1 assigned to
L8 = R = COOC2H5
R
HN
N
N
m(VAO) suggesting that these complexes have a monomeric struc-
ture [28–31].
N
However the infrared spectra of lanthanide (III) complexes
exhibited characteristic C@N stretching vibration (m(C@N)) at
O(CH2)11
O
CN
ꢂ1640 5 cmꢁ1 and 1616–1622 cmꢁ1 (Table 1) for the complexes
(Representative IR spectra of the complex 2-Gd and 3-Gd are
shown in Fig. 3.), apart from the characteristic bands of aromatic
ring (C@C), Ar(CAH), CH3, and CH2 vibrations. These two bands
confirm two types of coordination one through the oxygen atom
and another through the nitrogen atom. The band at ꢂ1616–
1622 cmꢁ1 reflects the participation of azomethine N atom in me-
tal-nitrogen bond formation in the case of toluidine (2-Gd) and
chloro complexes (4-Gd). However the absence of C@N stretching
vibration for fluoro complex 3-Gd at ꢂ1620 cmꢁ1 reflects the ab-
sence of metal-nitrogen coordination bond formation. The weak
OAH stretching vibration of the ligand at 2870–2850 cmꢁ1, is over-
lapped by the CH modes in the lanthanide complexes reflecting the
NAH vibration of the protonated nitrogen atom in C@N+H as well
as the participation of hydrogen atom in the intramolecular hydro-
gen bonding with the phenolic oxygen atom as reported earlier
[32]. This abnormal behavior may be due to coordination of the
metal through O-atom of the ligand only, the proton of the O-atom
migrate to the N-atom of the imine moiety which may -lead to for-
mation of zwitter ion (C@N+AH) of ligand. The observed vibrations
of two bands at 1635–1645 cmꢁ1 and ꢂ1620 cmꢁ1 for the complex
Gd[(L4H)2(L)]ꢃ(NO3)2, are due to two types of CH@N stretching and
compression vibrations. One of these is the elongated stretching
viz., exactly opposite with an increase in the imine C@N stretching
vibration of the complex (at 1635–1645 cmꢁ1) in the infrared spec-
tra when compared to the stretching vibration in the free Schiff
base ligand, while the other one is a compressed imine bond
appearing at a lower value of 1620 cmꢁ1. The vibration at higher
L9 R =
O(CH2)11
O
CN
H2n+1CnO
N
CmH2m+1
OH
L10: nO(OH).m
Fig. 2. Molecular structure of N, N0-aromatic donors for complexation.
However modification of the phenanthroline ligand (L9) in 5,6-
positions fused with a substituted imidazo ring led the realization
of nematic phase [23].
Hence the design and synthesis of lanthanide containing liquid
crystals viz., salicylideneimine based promesogenic ligands, with a
polar group substituted in an aryl moiety on nitrogen atom of the
central bridging imine group, to coordinate with f-block metals
presents a special experimental challenge. Our previous attempts
were successful in reporting the complexation of different lantha-
nide ions with N-(4-n-alkoxysalicylidene)-40-n-alkylanilines, L10
[24,25]. The lanthanide complexes in general exhibited SmA phase
but some of them exhibited columnar phases [25,26] depending on
the aliphatic chains component in the ligand. In continuation of
earlier work we report here the complexation of different lantha-
nide (III) ions with N-(4-n-hexadecyloxysalicylidene)-40-substi-
tuted-anilines, the substituent being a polar group in N-aryl

L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
137
X
OC16H33
OC16H33
OC16H33
O
O–
N+
O
OH
OC16H33
NO3
O
H
O
O
i
v
NH
Gd
ii
N
X
HN
X
OH
O
N
O
O
O–
N+
CHO
1
HC
H
O3N
O
1 : X =H
2 : X =OMe
3 : X = F
4 : X =Cl
O
2
4
iv
OC16H33
1-Gd : X =H
2-Gd : X =OMe
3-Gd : X = F
4-Gd : X =Cl
X
X
iii
X
N
N
Cu
N
O
O
OC16H33
O
OC16H33
V
N
C16H33
O
O
C16H33
O
O
1-Cu : X =H
2-Cu : X =OMe
3-Cu : X = F
4-Cu : X =Cl
1-VO : X =H
2-VO : X =OMe
3-VO : X = F
4-VO : X =Cl
X
X
Scheme 1. C16H33Br, KHCO3, KI, dry acetone, reflux, 40 h ii. 1 = aniline, 2 = 4-Anisidine, 3 = 4-fluoroaniline, 4 = 4-chloroaniline glacial AcOH, absolute EtOH, Reflux, 4 h iii.
Cu(OAc)2.H2O, EtOH; iv. VOSO4.5H2O, MeOH, TEA; v. Gd(NO3)3; Acetonitrile, Reflux, 4 h. For the sake of clarity the coordination of oxygen atoms of nitrate ion is shown
through nitrate ions.
Table 1
Elemental analysis data (calculated values in parentheses) and yields of the complexes.
Compound
Molecular formula
C (%)
H (%)
N (%)
Yield (%)
IR data (cmꢁ1
)
2
C
C
C
C
C
C
C
C
C
C
C
C
30H45NO3
60H88CuN2O6
60H88N2O7V
90H134GdN5O15
29H42FNO2
58H82F2CuN2O4
58H82F2N2O5V
90H125F3GdN5O12
29H42ClNO2
77.25 (77.04)
72.55 (72.29)
72.14 (72.04)
65.21 (64.22)
76.29 (76.44)
71.15 (71.61)
71.35 (71.36)
62.42 (63.44)
73.98 (73.78)
69.65 (69.26)
69.43 (69.03)
62.55 (61.59)
9.59 (9.70)
8.76 (8.90)
8.65 (8.87)
7.88 (8.02)
9.19 (9.29)
8.39 (8.50)
8.58 (8.47)
7.21 (7.65)
8.91 (8.97)
8.29 (8.22)
8.25 (8.19)
7.65 (7.43)
3.09 (2.99)
2.67 (2.81)
2.65 (2.80)
4.39 (4.16)
2.99 (3.07)
3.01 (2.88)
2.65 (2.87)
4.01 (4.25)
2.88 (2.97)
2.55 (2.79)
2.59 (2.78)
4.31 (4.13)
71
65
58
45
74
71
67
42
65
61
55
42
1627
1608
1608, 987
1641,1622
1625
1608
1608, 970
1645
1627
1608
1610, 966
1635, 1620
2-Cu
2-VO
2-Gd
3
3-Cu
3-VO
3-Gd
4
4-Cu
4-VO
4-Gd
58H82Cl2CuN2O4
58H82Cl2N2O5V
90H125Cl3GdN5O12
Table 2
Electrical conductivity of the compound 3, its Gd(III) complex and Gd(III)NO3 ꢃ 6H2O. Concentration is 1 ꢀ 10ꢁ3 M.
Molar conductivity (ohmꢁ1 cm2 molꢁ1
)
Compound
N,N0-Dimethylformamide
Ethanol
Acetonitrile
3: 16O(OH)F :L3H
3-Gd: [Gd(L3H)2L3](NO3)2
Gd(NO)3.6H2O
15
185
205
15
92
83
9
165
30
frequency (1635–1645 cmꢁ1) is due to the complexation of oxygen
atom with the rare earth ion. This is exactly opposite to the obser-
vation of imine C@N stretching vibration of the copper (II) and VO
complexes of 2 and 4 [Cu(L4)2 at 1608–1610 cmꢁ1] in the infrared
spectra which is lower than the imine C@N stretching vibration of
the free ligand L4H (1627 cmꢁ1), indicating a decrease in strength
of the double bond character of C@N bond. Hence the lower value
of 1620 cmꢁ1 clearly indicated similar type of coordination of the
complexation as in copper complexes which take place through
the deprotonated phenolic oxygen and azomethine nitrogen
atoms. The phenolic CAO stretching vibration, which is observed
in the Schiff base ligands around 1290 cmꢁ1, overlaps in the metal
complexes with a vibration due to the nitrate groups. The four
bands in the IR spectra of all the lanthanide complexes (only Gd(III)
complexes are reported here while Pr(III) and Sm(III) complexes
also gave identical spectra) recorded at 1475, 1286, 1122 and
827 cmꢁ1 can be assigned to the vibrational modes of the coordi-
nated (C2V) nitrate group reflecting the vibrations of m4
and 6 are respectively. Even though it may be assumed that mono
dentate nitrate group participates in coordination, the magnitude
of splitting in the peak positions of the bands of m4 and m1
(186 cmꢁ1) characterize the coordination of nitrate group as biden-
tate rather than mono dentate. The additional bands observed
ꢂ1384 cmꢁ1 suggests the presence of non-coordinated nitrate
groups present in the complex.
,
m1, m2,
m
3. Thermal behavior: DSC and thermal microscopy
3.1. Mesomorphic behavior of ligands
The polarized optical microscopy was performed to confirm the
transition temperatures obtained by DSC studies and to find out

138
L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
influence of the substituent inductive and resonance contributions
(a)
[Gd(L3H)2L3](NO3)2
1477
and the associated change in the geometric anisotropy of the mole-
cule. The contribution of substituent polarizability to the total polar-
izability and polarity of the molecule and the changes induced by the
substituent X in the p-conjugation of ACH@NAC6H4X fragment as
well as the conformation of the molecule determine the thermal sta-
100
1122
1246
1193
825
1286
bility of the compounds. The methoxy group in N-phenyl moiety
with stronger donor properties reduces the effect of p-conjugation
1510
1575
75
1384
of ACH@NAC6H4X which is further augmented by the intramolecu-
lar H-bonding manifested between the proton of ortho-hydroxyl
group in aldehyde moiety and the lone pair of the electrons on the
nitrogen atom in azomethine unit. Both these effects contribute to
increased planar conformation of the molecule in these compounds
in comparison to the noncoplanarity of the phenyl rings on both
sides of azomethine linkage in Schiff bases. Hence the apparent
stronger dispersion attraction between the planar molecules is re-
flected in the increase of crystal-mesophase transition temperature
when H is replaced by methoxy group.
1645
50
800
1000
1200
1400
1600
Wavenumber cm-1
(b)
[Gd(L2H)2L2](NO3)2
100
3.2. Mesomorphic behavior of complexes
1122
1143
1029
827
All the complexes exhibit smectic A phase while an additional
smectic E phase is exhibited by the Cu(II) complexes of fluoro
and chloro compounds and all the Gd(III) complexes. The charac-
teristic textures exhibited by the complexes of compounds 2, 3
and 4 are displayed in Figs. 6–8 respectively. The optical texture
of SmE phase with striations across the fans can be seen in for
Gd(III) complexes (Figs. 6d and 7d) and for Cu(II) complexes
(Fig. 8b). In an earlier report Galyametdinov et al. [35] observed
an increase in mesomorphic range of fluoro homolog [N-(4-n-hep-
tyloxysalicylidene)-40-fluoroaniline] on complexation while a sub-
stantial decrease in mesomorphic range of chloro and methoxy
homologs. However, in the present work with the increase in car-
bon chain length from C7 to C16, enhanced width of mesomorphic
range (Table 4) in all the complexes of Cu(II), VO(IV) and Gd(III)
than that of the ligands is observed. Hence we rationalize that
competing influence between the alkyl chain length and electronic
nature of the substituent plays an important role in the mesomor-
phism of these complexes.
The differential scanning spectra of the fluoro complexes viz., 3-
Cu, 3-VO and 3-Gd is displayed in Fig. 9a–c respectively. The meso-
morphic thermal range increased from 30.1 °C for copper complex
to 103.3 °C and 138.4 °C for oxovanadium and gadolinium com-
plexes, respectively. The predominant influence of metal ion com-
plexation in the manifestation of mesomorphism of a polar ligand
is augmented by the suitable end alkyl chain length to enhance the
mesomorphic range. The larger width of mesomorphic range (92–
136 °C in cooling cycle) particularly in oxovanadium complexes
than the copper complexes is surprising and it may be due to elec-
tronic interactions of VO moiety as well as functional groups rather
than the steric hindrance of the VO group. In general all the Cu(II),
VO(IV) and Gd(III) complexes exhibited focal conic fan texture
characterizing the phase as SmA, while the Cu(II) and gadolinium
(III) complexes of fluoro and chloro compounds exhibited addi-
tional phase texture resembling SmE phase. In contrast to the li-
gands the complexes exhibit wide mesomorphic ranges. The
electronic nature and size of the substituent can also influence
the molecular structure of the ligands and complexes and thereby
affects mesomorphism. It is apparent from the DSC and POM inves-
tigations that the width of the mesomorphic range is substantially
influenced by the nature of substituent as well as the metal ion in
corresponding complexes. These results are in good agreement
with the earlier reported results on similar compounds where
the alkyloxy chain is heptyloxy homolog [33]. The only difference
is the enantiotropic mesophases are observed with the increase
in chain length of salicylidene moiety.
1197
75
50
25
1265
1286
1473
1517
1620
1637
1384
800
1000
1200
1400
1600
Wavenumber cm-1
Fig. 3. FTIR spectra of gadolinium (III) complexes of (a) fluoro and (b) methoxy
homologs.
the type of liquid crystalline phases present in the studied materi-
als. The liquid crystalline phases, phase transition temperatures,
enthalpy and entropy associated with the phase transitions of
the ligands and complexes are summarized in Table 3. All the com-
pounds exhibited strong birefringence and fluidity in the tempera-
ture range characteristic of mesomorphic behavior between the
endothermic or exothermic peaks recorded in DSC thermograms
in the heating or cooling cycle respectively. All the ligands exhibit
mesomorphism. The compound 1 without any substituent on N-
aryl moiety exhibited focal conic fan texture of SmA phase of
0.1 °C. The compound 16O(OH)OMe (2) with methoxy group exhi-
bit schlieren texture characteristic of nematic phase (Fig. 4a) while
the other two compounds 3 and 4 with a polar substituent (F or Cl
respectively) displayed focal conic fan texture characteristic of
SmA phase (Fig. 4b,c). The observed mesophase thermal range
(DT) is 15 °C for 16O(OH)OMe (2), 2.3 °C and 34 °C in case of fluoro
(3) and chloro (4) homologs respectively. A representative DSC
spectrum of fluoro homolog 3 is displayed in Fig. 5. In a previous
report [33–35] on lower homologs of similar compounds viz.,
7O(OH)X, where X = H, OMe, F and Cl, 7O(OH)H do not exhibit LC
behavior while OMe (
T ꢂ 0.5 °C) and chloro
D
T ꢂ 16.9 °C) exhibits nematic phase, fluoro
(D
(
D
T ꢂ 50 °C) homologs exhibit SmA
phases. A comparison of transition temperatures between the
two homologs (Table 4) revealed the increase in melting/crystalli-
zation temperatures. The increase in thermal ranges can be ex-
plained as follows.
The thermal stability of the mesophase in these compounds is
determined by the anisotropy of the contribution of the polarizabil-
ity of X, the electronic structure of the N-phenyl moiety under the

L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
139
Table 3
Phase transition temperatures, associated enthalpy and entropy changes data for the ligands and complexes.
Compound
Transition
T (°C)
D
H (kJ molꢁ1
)
D
S (J molꢁ1 Kꢁ1
)
DT (°C)
1
16O(OH)H = (L1H)
K-I
79.4
67.3
67.2
111.6
77.8
106.6
75.8
100.5
102.2
101.3
86.3
118.5
122.8
141.8
141.0
80.8
66.1
22.8
34.4
60.1
80.2
3.63
2.47
62.77
1.16
1.51
58.9
18.9
47.3
7.01
7.42
54.9
79.3
5.64
3.54
13.3
9.9
187.6
67
I-SmA
SmA-K
K-I
I-K
K-I
I-K
K-N
N-I
I-N
0.1
–
101.1
157.1
228.6
9.56
18.5
185.3
3.09
1-Cu
1-VO
2
Cu(L1)2
VO(L1)2
–
16O(OH)O1 = (L2H)
1.7
15.0
4.03
N-K
K1-K
164.0
48.5
119.5
16.8
17.9
155.3
213.1
12.0
7.66
40.8
26.1
31.1
66.9
2-Cu
Cu(L2)2
K-SmA
SmA-I
I-SmA
SmA-K
K-SmA
SmA-I
I-SmA
SmA-K
K-SmE
SmE-SmA
SmA-I
I-SmA
SmA-SmE
SmE-K
K1-K
19.0
60.2
2-VO
2-Gd
VO(L2)2
99.3
195.0
189.1
53.02
107.7
127.5
194.3
192.1*
126.1*
95.1*
78.9
83.6
74.9
72.6
119.1
120.8
125.7
124.3
97.8
95.7
136.0
[Gd(L2H)2L2](NO3)2
12.4
31.2
–
–
–
86.6
97.1
3
16O(OH)F = (L3H)
Cu(L3)2
0.69
56.1
3.78
55.0
49.5
2.99
5.05
5.10
3.32
60.5
76.4
11.1
10.8
21.0
12.3
–
1.96
157.4
10.8
159.2
126.4
7.60
12.6
12.8
8.90
164.7
193.4
25.6
25.1
63.8
36.2
–
K-I
–
2.3
I-SmA
SmA-K
K-SmE
SmE-SmA
SmA-I
I-SmA
SmA-SmE
SmE-K
K-SmA
SmA-I
I-SmA
SmA-K
K-SmE
SmE-SmA
SmA-I
I-SmA
SmA-SmE
SmE-K
K-SmA
SmA-I
I-SmA
SmA-K
K-SmE
SmE-SmA
SmA-I
I-SmA
SmA-SmE
SmE-K
K-SmA
SmA-I
I-SmA
SmA-K
K-SmE
SmE-SmA
SmA-I
I-SmA
SmA-SmE
SmE-K
3-Cu
6.6
30.1
94.2
3-VO
3-Gd
VO(L3)2
122.2
161.9
160.4
57.1
39.7
103.3
[Gd(L3H)2L3](NO3)2
67.9
139.1*
188.8
183.5*
135.1*
45.1*
87.9
10.0
–
–
–
66.9
5.8
21.7
–
–
120.9
138.4
–
4
16O(OH)Cl = (L4H)
Cu(L4)2
185.4
15.2
14.3
196.5
94.6
42.9
16.6
11.6
55.0
60.7
83.4
15.3
12.7
36.5
132.0
7.8
106.4
105.9
71.9
19.5
34.0
5.43
67.7
36.9
19.1
7.52
5.22
22.9
23.1
32.3
6.40
5.28
11.8
47.6
2.86
6.3
4-Cu
117.4
172.1
179.6
175.6
144.4
108.0
115.5
144.7
143.4
50.8
87.5
92.6
163.0
161.1*
82.5*
45.2*
62.2
67.6
4-VO
4-Gd
VO(L4)2
29.4
92.6
[Gd(L4H)2L4](NO3)2
14.5
–
–
75.5
–
–
–
–
*
Indicates the POM observed values.
solution with concentration (c = 1 ꢀ 10ꢁ5), Fig. 10 (with chloroform
as the solvent being chosen to maximize the molecular solubility)
were studied to obtain the information regarding absorption and
emission maxima, and the Stokes shift of fluorescence. The green
4. Optical absorption and emission studies
The UV–Visible absorption (Table 5) and fluorescence spectro-
scopic properties of compound 4 and its complexes in chloroform

140
L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
(c) Focal conic texture of
(a) Schlieren texture of
(b) Focal conic texture of
nematic phase of 2 at 100.6 0C
SmA phase of 4 at 105.4 0C
SmA phase of 3 at 74 0C
Fig. 4. (a) Schlieren texture of nematic phase of 2 at 100.6 °C, (b) focal conic texture of SmA phase of 3 at 79 °C, and (c) focal conic texture of SmA phase of 4 at 105.4 °C.
in chloroform solution at different concentrations and in thin solid
60
16O(OH)F
film are the large Stokes shifted emission of ꢂ174 nm for 4-Gd com-
Heating
plex and ꢂ122–129 nm (Fig. 10) and broad emissionband with max-
40
ima in the vicinity of ꢂ520 nm and ꢂ471 nm for 4-Gd and 4-Pr
complexes respectively. The Stokes shift, which reflects the struc-
20
tural relaxation of the excited molecule, is in the range reported
for other metallomesogens [37–39] but significantly larger than
Cooling
0
-20
-40
the reported push–pull systems [40–44] exhibiting liquid crystal
behavior, confirmed the molecular conformational changes upon
excitation. Hence it can be assigned to the ligand–ligand charge
transfer transition due to the presence of phenyl rings in the ligands
and also indicative of phosphorescence. Further studies are in pro-
gress to change the design of the ligand to promote metal centered
absorption and emission.
70
75
80
85
90
Temperature (0C)
5. Conclusions
Fig. 5. DSC spectrum of the compound 3.
The synthesis of d- and f-block metal complexes of N-(4-n-hex-
adecyloxysalicylidene)-40-substituted anilines, with the polar sub-
stituents (Cl and F), exhibiting mesomorphism had been
successfully achieved. Proper choice of end alkyl chain length can
influence the manifestation of mesomorphism in ligands as well
as their complexes. Hence competing influence between the alkyl
chain length and electronic nature of the substituent plays an
important role in the mesomorphism of these complexes. In partic-
ular the special experimental challenge of synthesis of lanthanide
containing liquid crystals viz., salicylideneimine based promeso-
genic ligands, with a polar group substituted in an aryl moiety on
nitrogen atom of the central bridging imine group is fulfilled. These
complexes possess large thermotropic mesomorphic range exhibit-
ing SmA and SmE phases. The major advantage of Lanthanide con-
taining metallomesogens is intense luminescence characteristics
as well as large mesomorphic range close to ambient temperature.
The studies on magnetic anisotropy of these complexes is still to be
explored and further work is in progress to synthesize stable room
temperature lanthanidomesogens exhibiting nematic and smectic
phases by suitable changes in the end alkyl chains, substituents
in the aromatic core and counter-ions.
fluorescence of 4-Gd when exposed to UV light is shown in Fig. 11.
The absorption maxima of the lowest optical transition were
observed approximately at 342–355 nm for compound 4 (and other
ligands as well) (ꢂ3.6 eV, ꢂ
two other maxima in at 245 nm and 300 nm. The extended conju-
gated -system is reflected in the electronic absorption spectrum.
e
= 80,000–146,000 L molꢁ1 cmꢁ1) with
p
The electronic properties of these systems are dominated by the do-
nor–acceptor-substituted organic chromophore ligand bands with a
noticeable shift for the d-block complexes. However no noticeable
shift to lower or higher energy levels for the lanthanide complexes
is observed indicating the absence of crystal field effect upon the in-
ter-electronic repulsion between the 4f electrons of the metal ion.
These absorption bands with large molar absorption coefficients re-
flects the p– p-conjugated system of the
pꢄ transition of the highly
ligand surrounding the metal ion core. Luminescence studies of met-
allomesogens are scanty [36] and luminescent studies on the ligand
and complexes were carried out in solution and thin film of the solid
phases. The most important features of the photoluminescence
emission spectra of the lanthanide metallomesogens and ligands
Table 4
Comparison of transition temperatures and mesomorphic range in °C of ligands and complexes.
H
OMe
Me
F
Cl
72.0
87.9
106.4
34.0
Melting point of 7O(OH)X
Melting point/crystallization of 16O(OH)X
Heating
Heating
Cooling
50.9
79.4
–
0.1
–
90.0
100.5
102.2
15.0
64.9
84.6
84.6
1.5
73.0
83.6
83.6
2.3
D
T of 16O(OH)X
Cu(II) complexes of 16O(OH)X
VO(IV) complexes of 16O(OH)X
60.2
3.7
30.1
67.6
–
136.0
33.1
103.3
92.6

L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
141
(a) SmA phase texture of 2-Cu, (b) SmA phase texture of 2-VO
,
[L2]2Cu, at 137.2 0C
[L2]2VO at 127.2 0C
(c) SmA phase texture of 2-Gd
(d) SmE phase texture of 2-Gd
at 192 0C
at 125 0C
Fig. 6. Focal conic fan textures exhibited by complexes of compound 2: 2-Cu, 2-VO and 2-Gd.
(a) SmA phase texture of 3-Cu
,
,
(b) SmA phase texture of 3-VO
,
[L3]2Cu 119 0C
[L3]2VO 151.9 0C
(c) SmA phase texture of 3-Gd
(d) SmE phase texture of 3-Gd
,
[Gd(L3H)2L3](NO3)2 130 0C
[Gd(L3H)2L3](NO3)2 182.6 0C
Fig. 7. Focal conic fan textures exhibited by complexes of compound 3: 3-Cu, 3-VO and 3-Gd.
Chemicals. The solvents and reagents are of AR grade and were
distilled and dried before use. Microanalysis of C, H and N elements
were determined on Carlo-Erba 1106 elemental analyzer. IR spectra
were recorded on a Shimadju FT-IR-prestige-21spectrometer (mmax
6. Experimental
All the chemicals were procured from M/s Alfa Aesar, Tokyo
Kasei Kogyo Co. Ltd. or Lancaster chemicals, Sigma–Aldrich

142
L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
(a) SmA phase t0exture of 4-Cu
,
(b) SmE phase texture of 4-Cu,
[L4]2Cu 172 C
[L4]2Cu 141 0C
(c) SmA phase texture of 4-VO,
(d) SmA phase texture of 4-Gd
[L4]2VO 126.4 0C
[Gd(L4H)2L4](NO3)2 132 0C
Fig. 8. Focal conic fan textures exhibited by complexes of compound 4: 4-Cu, 4-VO and 4-Gd.
in cmꢁ1) over the 400–4000 cmꢁ1 spectral range in KBr pellets.
The 1H NMR spectra of all the organic ligands were recorded in
CDCl3 solution on a JEOL AL300 FTNMR spectrometer. Conductiv-
ity measurements were performed in DMF solution 10ꢁ3 M on a
Systronics CM304 conductivity meter. Chemical shifts are
reported in parts per million (d) relative to tetramethylsilane
(TMS) as internal standard. The phase transition temperatures
and associated enthalpies were recorded with a heating and cool-
ing rate of 5 °C/min using differential scanning calorimetry (DSC)
(Perkin-Elmer Pyris-1 system). The DSC apparatus was calibrated
with indium (156.6 °C, 28.4 J/g) and tin (232.1 °C, 60.5 J/g). The
liquid crystalline properties of the different phases exhibited by
the ligands and complexes were observed and characterized using
polarizing microscope (Nikon optiphot-2-pol attached with hot
and cold stage HCS302, with STC200 temperature controller con-
figured for HCS302 from INSTEC Inc. USA). UV visible absorption
spectra of the compounds in CHCl3 at different concentrations
were recorded on a Shimadzu UV-1601PC spectrophotometer
(kmax in nm). Fluorescence spectra were recorded with a Shimadju
RF-5301PC spectroflouorometer with 150 W xenon lamp as the
excitation source.
son’s method by refluxing 1 mmol of 1-bromohexadecane and
1 mmol of 4-hydroxysalicylaldehyde in the presence of KHCO3
(1 mmol) as base in dry acetone. The product was purified by col-
umn chromatography using hexane as eluent.
Preparation of the metal complexes: The synthesis of copper (II)
complexes was carried out by refluxing the mixture consisting of
slowly added ethanolic copper (II) acetate [Cu(OAc)2 ꢃ H2O]
(1 mmol) solution (20 ml) to a hot solution of the appropriate eth-
anolic imine (2 mmol) solution (ꢂ50 ml) for 2 h. The solution was
cooled to room temperature and filtered to yield the brown copper
(II) complexes which were subsequently recrystallized from a mix-
ture of ethyl acetate and ethanol.
Oxovanadium (IV) complexes were prepared by refluxing the
mixture consisting of slowly added 20 ml of methanolic vanadium
(IV) oxide sulfate (VOS04ꢃ5H2O) (1 mmol) solution to a hot solution
of the appropriate imine (2 mmol) in methanol (50 ml) in the pres-
ence of triethylamine for 4 h. The solution was cooled to room tem-
perature and filtered to yield the green oxovanadium (IV)
complexes which were subsequently recrystallized from a mixture
of ethyl acetate and ethanol.
Gadolinium (III) complexes were synthesized by refluxing by
refluxing the mixture consisting of slowly added gadolinium (III)
nitrate [Gd(NO3)3] (1 mmol) solution in acetonitrile (20 ml) to a
hot solution of the appropriate imine (3 mmol) solution in acetoni-
trile (ꢂ50 ml) for 4 h. The solution was filtered, when it is hot to
avoid the impurity of unreacted ligand and to yield the pale yellow
precipitate of gadolinium (III) complexes which were washed
repeatedly with acetonitrile. The precipitate was dried for further
analysis.
7. Preparation of ligands and metal complexes
Synthesis of the Schiff bases: The free ligands were synthe-
sized following a well known procedure [45–48] by refluxing
a mixture of ethanolic solution of 1 mmol of the 4-n-hexadecyl-
oxysalicylaldehyde with 1 mmol of appropriate amine and few
drops of glacial acetic acid as catalyst for 3 h. The precipitated
product was purified by repeated recrystallization from absolute
ethanol till constant clearing temperatures are achieved (yield
65–75%).
Acknowledgement
Synthesis of aldehyde: 4-n-hexadecyloxy salicylaldehyde was
synthesized as described earlier [46–48] using modified William-
The authors acknowledge the financial assistance from DAE,
DST and UGC.

L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
143
1.4
(a)
(a)
Ligand 4
[L3]2Cu
60
50
40
30
1.2
1.0
0.8
0.6
0.4
0.2
0.0
344
Gd(III)
292
Cu(II)
VO (IV)
Sm(III)
Pr(III)
250
300
350
400
450
90
100
110
120
130
Temperature (0C)
Wavelength nm
15
10
5
(b)
λexc = 340nm
[L3]2VO
(b)
ligand 4 c = 1e-5
80
60
40
20
Gd(III) complex c = 1e-5
Pr(III) complex c = 1e-4
Sm(III) complex c = 1e-4
0
400
450
500
550
600
60
80
100
120
140
160
Wavelength nm
Temperature (0C)
Fig. 10. (a) UV–Visible spectra of compound 4 and its Cu(II), VO(IV) and lanthanide
complexes. (b) Fluorescence spectra of compound 4 and lanthanide complexes.
Conc = 1 ꢀ 10ꢁ5 M.
70
60
50
40
30
20
(c)
[Gd(L3H)2L3](NO3)2
60
80
100
120
140
160
180
200
Temperature (0C)
Fig. 9. (a) DSC spectrum of the compound 3-Cu, (b) DSC spectrum of the compound
3-VO, (c) DSC spectrum of the compound 3-Gd.
Fig. 11. Fluorescent image of 4-Gd in solid state.
Table 5
UV–Visible data for the compound 4 and its complexes. (First row indicates the absorption wavelength in nm and second row indicates the molar extinction coefficient
L molꢁ1 cmꢁ1.)
4
4-Cu
4-VO
4-Gd
4-Sm
4-Pr
245
234
237
241
82,000
300
34,300
284
68,600
292
32,600
282
302
283
81,300
59,900
25,500
312
58,700
18,800
17,500
32,900
346
345
355
344
344
343
133,800
39,100
39,900
107,300
33,400
32,400

144
L. Chakraborty et al. / Journal of Molecular Structure 1002 (2011) 135–144
[22] A. Bellusci, G. Barberio, A. Crispini, M. Ghedini, M. La Deda, D. Pucci, Inorg.
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