Homo dinuclear lanthanide(III) complexes of a mesogenic Schiff-base, N,N′-di-(4-decyloxysalicylidene)-1′,6′-diaminohexane: Synthesis and characterization

Article abstract of DOI:10.1016/j.saa.2011.05.030

A mesogenic Schiff-base, N,N′-di-(4-decyloxysalicylidene)-1′, 6′-diaminohexane, H₂ddsdh (abbreviated as H₂L ²) that exhibits smectic-B (SmB) mesophase, was synthesized and its structure studied by elemental analysis, mass spectrometry, NMR & IR spectral techniques. The Schiff-base, H₂L², upon condensation with hydrated lanthanide(III) nitrates, yields Ln^III complexes of the general composition [Ln₂(L²H ₂)₃(NO₃)₄](NO₃) ₂, where Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho. IR and NMR spectral data imply a bi-dentate bonding of the Schiff-base in its zwitterionic form (as L²H₂) to the Ln^III ions through two phenolate oxygens. The POM and DSC studies reveal that none of the Ln ^III complexes exhibits mesomorphism. Fluorescence studies show that the Tb^III complex displays characteristic metal-centered fluorescence (solution state).

Full text of DOI:10.1016/j.saa.2011.05.030

Spectrochimica Acta Part A 79 (2011) 1654–1659
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Homo dinuclear lanthanide(III) complexes of a mesogenic Schiff-base,
N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-diaminohexane: Synthesis and
characterization
Pawan Raj Shakya, Angad Kumar Singh, T.R. Rao∗
Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
a r t i c l e i n f o
a b s t r a c t
A mesogenic Schiff-base, N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-diaminohexane, H2ddsdh (abbreviated
as H2L2) that exhibits smectic-B (SmB) mesophase, was synthesized and its structure studied by
elemental analysis, mass spectrometry, NMR & IR spectral techniques. The Schiff-base, H2L2, upon
condensation with hydrated lanthanide(III) nitrates, yields LnIII complexes of the general composition
[Ln2(L2H2)3(NO3)4](NO3)2, where Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho. IR and NMR spectral data imply
a bi-dentate bonding of the Schiff-base in its zwitterionic form (as L2H2) to the LnIII ions through two phe-
nolate oxygens. The POM and DSC studies reveal that none of the LnIII complexes exhibits mesomorphism.
Fluorescence studies show that the TbIII complex displays characteristic metal-centered fluorescence
(solution state).
Article history:
Received 22 October 2010
Received in revised form 7 May 2011
Accepted 17 May 2011
Keywords:
Mesogenic Schiff-base
Zwitterionic-coordination
LnIII complexes
Fluorescence
Mono-capped octahedron
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
purification: 1-bromodecane, 2,4-dihydroxybenzaldehyde and 1,6-
diaminohexane are from Sigma–Aldrich, USA; all the Ln(NO)3·xH2O
Lanthanide(III) ions have been widely studied for a number of
applications such as luminescent metallomesogens [1–6], lumines-
cent materials [2,7], chemosensors [1,8], fluorescence labels [3],
and as photoluminescence devices [9,10] for their photophysical
properties arising from f–f transitions. By incorporating lanthanide
ions into liquid crystals, one can obtain luminescent liquid crystals,
which are useful for the design of emissive LCDs. In continuation of
the earlier work [11–14] carried out in our laboratory on systematic
structural and spectroscopic studies of 3d and 4f metal complexes
of mesogenic organic Schiff bases, we now report here synthesis
and spectroscopic characterization of some LnIII complexes with a
Schiff-base, N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-diaminohexane
(H2L2). Besides the mesogenic properties of the synthesized lig-
and, the fluorescence properties of some representative complexes
(SmIII, EuIII, TbIII and DyIII) are also studied.
salts are from Indian Rare Earths Ltd.; KI and KHCO3 are from Merck.
The organic solvents received from commercial sources were dried
using standard methods [15] when required.
The 1H and 13C NMR spectra were recorded on a JEOL AL-
300 MHz FT-NMR multinuclear spectrometer. Infrared spectra
were recorded on a JASCO FT/IR (model-5300) spectrophotometer
in the 4000–400 cm−1 region using KBr pellets. FAB-mass spectrum
was recorded on JEOL SX-102 FAB mass spectrometer. The metal
contents of the complexes were determined by complexometric
titrations against EDTA using xylenol orange as an indicator. C, H,
and N contents were micro analyzed on an Exeter Analyzer, Model
CE-440 CHN. Magnetic susceptibility measurements were made at
room temperature on a Cahn–Faraday balance using Hg[Co(NCS)4]
as the calibrant. UV–vis spectra were recorded on Shimadzu spec-
trophotometer, model Pharmaspec-UV 1700. Molar conductance
of the complexes in 0.001 M solutions were determined on a CON
510 bench conductivity meter, using a 2-ring stainless steel ultem-
body conductivity electrode (cell constant, K = 1.0) with built-in
temperature sensor. Mesophases were identified by the defect
textures observed using polarized hot-stage binocular microscope
(LOMO, USA) equipped with digital camera (Nikon Coolpix 4500).
Differential scanning calorimetry studies were made on METTLER
DSC-25, Mettler STARe SW 9.00 unit. Fluorescence measurements
were made in solution state (mixed solvent of CHCl3/DMSO; 3:1,
v/v) on a Perkin Elmer LS-45 luminescence spectrometer with a
10 nm slit width on both excitation and emission.
2. Experimental
2.1. Materials and physical measurements
All the required reagents of analytical grade (AR) were
obtained from commercial sources and used without further
∗
Corresponding author. Tel.: +91 542 6702459; fax: +91 542 2368127.
E-mail address: drtrrao@gmail.com (T.R. Rao).
1386-1425/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2011.05.030

P.R. Shakya et al. / Spectrochimica Acta Part A 79 (2011) 1654–1659
1655
OH
OH
Dry acetone, KI
Reflux ~ 30 h
H2O
C10H21Br
CHO
+ CO2
+
+ KHCO3
KBr
+
+
HO
CHO
C10H21O
CHO
1
OH
Ethanol / Acetic acid
Reflux , ~ 1.5 h
2
H2N
O
NH2
C10H21
3
4
O
OH
2
1
H
C10H21
6
3
2'
4'
6'
1
2
N
5
N
5
2H2O
6
1'
3'
5'
C10H21
H
HO
O
4
2
THF
Stir, ~3 h.
3 H2L2
2 Ln(NO3)3. xH2O
[Ln2(L2H2)3(NO3)4](NO3)2
3
Scheme 1. Reaction steps involved in the synthesis of 4-decyloxysalicylaldehyde, 1, N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-diaminohexane, H2ddsdh, 2 (H2L2), and LnIII
complexes, 3.
2.2. Synthesis
The synthesis
thus obtained was filtered off under suction, thoroughly washed
with cold ethanol and dried at room temperature.
of
N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-
diaminohexane (H2L2), 2, was achieved by proceeding through
two major steps, viz., alkylation of 2,4-dihydroxybenzaldehyde
with 1-bromodecane, followed by Schiff-base formation. All the
experimental details are given in Scheme 1. The LnIII complexes,
[Ln2(L2H2)3(NO3)4](NO3)2 (Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and
Ho), 3, were prepared by reacting together the appropriate metal
nitrate [Ln(NO3)3·xH2O] and the ligand, H2L2 at room temperature.
2.2.3. Synthesis of LaIII complex, [La2(L2H2)3(NO3)4](NO3)2, 3
The LaIII complex was prepared by drop-wise addition of a solu-
tion of La(NO3)3·6H2O (0.87 g, 2.0 mmol in 20 mL THF) to a solution
of the ligand, H2L2 (1.91 g, 3.0 mmol in 30 mL THF) under magnetic
stirring. The resultant solution turned cloudy after ∼15 min; stir-
ring was continued for ∼3 h at room temperature; the solid product
separated out was filtered, washed repeatedly with cold methanol
and dried over fused CaCl2.
2.2.1. Preparation of 4-decyloxysalicylaldehyde, 1
Equimolar amounts of 2,4-dihydroxy benzaldehyde (50 mmol,
6.91 g), 1-bromodecane (50 mmol, 10.4 mL) and potassium bicar-
bonate (∼55 mmol, 5.51 g) were added to a solution of dry acetone
(100 mL). The mixture was refluxed for ∼30 h in the presence of
KI (0.1–0.2 g) as a catalyst and filtered while hot to remove the
insoluble solids. Dilute hydrochloric acid (6 N) was added to the fil-
trate until the point of neutralization and the product was then
extracted twice with CHCl3 (100 mL portions). The chloroform
extract was concentrated to obtain a straw-yellow solid which was
purified by column chromatography over SiO2 eluting first with
n-hexane and then with a mixture of n-hexane and chloroform
(v/v, 1/1); evaporation of this purified extract finally yielded 4-
decyloxysalicylaldehyde in the form of a white solid; yield: 62%
(8.63 g).
All the other rare-earth complexes (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy
and Ho) were synthesized in an analogous way using the appropri-
ate hydrated salt of Ln(III) nitrate; the physical properties and the
analytical data of all the complexes are given in Table 1 while the
NMR data of the ligand (H2L2) and the LaIII complex are presented
as supplementary data.
3. Results and discussion
3.1. Properties of the complexes
The shining yellow-colored minute crystalline Schiff-base lig-
and (H2L2) was found to react with Ln(NO3)3·xH2O to yield LnIII
complexes. All the complexes are insoluble in common organic
solvents but soluble in hot DMSO. The analytical data (Table 1) of
the complexes indicate 2:3 metal to ligand stoichiometry with the
general formula [Ln2(L2H2)3(NO3)4](NO3)2. The nitrate groups are
present both within and outside the coordination sphere; the molar
conductance data (106–119 ꢀ−1 cm2 mol−1) measured in 10−3 M
mixed solvent (CHCl3 and DMSO; 3:1, v/v) imply 2:1 electrolytic
nature [16] of the complexes.
2.2.2. Synthesis of
N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-diaminohexane, 2
The
ligand,
N,Nꢀ-di-(4-decyloxysalicylidene)-1ꢀ,6ꢀ-
diaminohexane, was prepared by refluxing absolute ethanolic
solutions of 4-decyloxysalicylaldehyde (30 mmol, 8.34 g in 50 mL)
and 1,6-diaminohexane (15 mmol, 1.74 g in 15 mL) for ∼1.5 h in the
presence of few drops of glacial acetic acid. The resulting mixture
was left over night; minute yellow-colored crystalline product, 2,

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3.2. FAB-mass spectral study of H2L2
The structure of the Schiff-base ligand was confirmed by
FAB mass spectrum in addition to the spectral evidences of
IR and NMR spectroscopy to be discussed later. The mass
spectral features of H2L2 were characterized by the molecu-
lar ion peak as well as base peak corresponding to the m/e
value of 636, which matches with the molecular weight of
636.95 of H2L2 of the molecular formula, C40H64N2O4. The 100%
intensity of the molecular ion peak as the base peak is as
expected for the molecule on the basis of its predominant aro-
matic character; the major fragment peaks (m/e = 509, 304) are
+
due to C10H21OC6H3(OH)CH N(CH2)6N CH(OH)C6H3OCH2 and
+
C10
H
21OC6H3(OH)CH NCH2CH2 species respectively.
3.3. IR spectral studies
The prominent infrared spectral data of the Schiff-base (H2L2)
and its LnIII complexes are presented in Table 2.
Assignments regarding bonding through specific functional
groups have been made by a careful comparison of specific vibra-
tional bands of H2L2 with the corresponding bands of the metal
complexes. The broad absorption centered on 3453 cm−1 in the IR
spectrum of the ligand, characteristic of ꢂ(O–H)phenolic [17], may
be understood to involve considerable amount of H-bonding (to
the ortho
C N group presumably of intramolecular type) under
the experimental conditions; this band totally disappears in the
spectra of the complexes due to shifting of the phenolic proton to
the azomethine nitrogen atom resulting in the formation of zwit-
ter ion. The weak/medium intensity bands centered on 1146 cm−1
are assignable to ꢂ(C–O)phenolic. The strong intensity band occurring
at 1624 cm−1, assignable [18] to ꢂ(C N) absorption of the azome-
thine moiety, undergoes a hypsochromic shift in all the complexes
on account of zwitter-ion formation. Thus, the complexation to the
LnIII ion results in migration of the phenolic protons onto the two
uncoordinated imino nitrogens, which then get intramolecularly
hydrogen-bonded to the metal-bound phenolate oxygens to give
rise to the zwitterionic structure, N+–H· · ·O−. Similar zwitter-ionic
behaviour has been reported by others for acyclic Schiff’s base lan-
thanide complexes [19]. The formation of a zwitterionic form can
be rationalized by the tendency of the lanthanide ions to coordi-
nate to negatively charged ligands with a preference for O-donor
ligands. By transfer of the phenolic proton to the imine nitrogen,
the phenolic oxygen becomes negatively charged thereby facili-
tating coordination to the lanthanide ion. The initial evidence for
zwitter-ionic formation was also obtained from 1H NMR spectra
while further evidence was given by infrared spectroscopy and,
more particularly, by the band frequencies of the ꢂ(C N) vibration.
The shift to higher wave numbers upon complexation implies the
presence of C–N+ group [20] and the non-involvement of nitrogen
atom in complex formation. Further, all the present complexes are
characterized by a strong band due to ꢂ(C N) at 1653(30) cm−1 and
a weak broad band at about 3198–3179 cm−1 due to the hydrogen-
bonded N+–H· · ·O− vibration of the protonated imine [21]. Thus,
the ligand is found to coordinate to the metal ion via the nega-
tively charged phenolic oxygen while no binding occurs between
the lanthanide ion and the imine nitrogen.
The LnIII complexes also exhibit three additional bands around
1470–1467, 1293–1287 and 846–844 cm−1, which can be assigned
to the vibrational modes of the coordinated (C2v) nitrate groups
[22]. The profile and magnitude of separation of the modes asso-
ciated with asymmetric nitrate vibrations have been used as a
criterion to distinguish between mono- and bidentate chelating
nitrates. Accordingly, the magnitude of splitting, 180–177 cm−1, at
higher energies may be indicative of a bidentate interaction of the

P.R. Shakya et al. / Spectrochimica Acta Part A 79 (2011) 1654–1659
1657
Table 2
The important infrared frequencies (in cm−1) of H2L2 and its LnIII complexes.
H2L2/complex
ꢂ(O–H)phenolic
ꢂ(N+H)
ꢂ(C N)
ꢂ(C–O)phenolic
ꢂ(NO3)
ꢂ5
Ionic
ꢂ1
ꢂ2
ꢂ5–ꢂ1
H2L2
3453b
–
3194w
3197w
3197w
3195w
3188w
3179w
3193w
3193w
3198w
1624
1653
1653
1653
1653
1653
1653
1653
1654
1654
1146
1121
1121
1121
1122
1121
1121
1122
1122
1121
–
1467
1467
1468
1467
1468
1469
1470
1469
1470
–
1375
1378
1376
1381
1378
1378
1380
1378
1378
–
–
–
180
178
179
177
177
177
178
178
177
[La2(L2H2)3(NO3)4](NO3)2
[Pr2(L2H2)3(NO3)4](NO3)2
[Nd2(L2H2)3(NO3)4](NO3)2
[Sm2(L2H2)3(NO3)4](NO3)2
[Eu2(L2H2)3(NO3)4](NO3)2
[Gd2(L2H2)3(NO3)4](NO3)2
[Tb2(L2H2)3(NO3)4](NO3)2
[Dy2(L2H2)3(NO3)4](NO3)2
[Ho2(L2H2)3(NO3)4](NO3)2
–
–
–
–
–
–
–
–
–
1287
1289
1289
1290
1291
1291
1292
1291
1293
844
844
845
845
845
846
846
846
846
b: broad; w: weak.
coordinated nitrate anions with the lanthanide ions [22,23]. The
additional bands observed at 1381–1375 cm−1 may be attributed
to the non-coordinated nitrate groups of the complexes.
complex in a zwitter-ionic form, with the deprotonated phenolic
oxygen and the protonated imine nitrogen as shown in Scheme 2.
1
The 13C{ H} NMR spectra show a shift of the –NCH signal from
ı, 165.39 (in the case of H2L2) to ı, 166.07 (in the case of the LaIII
complex). Similar shifts were observed in the case of the carbon
atoms directly attached to the bonding atoms (phenolate carbons)
while those for the other carbons were of lesser magnitude. Thus,
the NMR spectral data imply bonding through the two phenolate
oxygens of the ligand in the zwitter-ionic form to the LaIII metal
ion.
3.4. 1H and 13C NMR spectral studies
1
A comparison of the NMR spectral data [1H and 13C{ H}] of
the ligand with that of the LaIII complex shows the absence of
the phenolic –OH signal in the latter. It is important to note that
the composition of the LaIII complex, [La2(L2H2)3(NO3)4](NO3)2,
implies coordination of the ligand (H2L2) as a neutral species;
however, it may be inferred from the 1H NMR spectrum of
the compound that the phenolic protons are shifted to the two
uncoordinated imino nitrogens, which then get intramolecularly
hydrogen-bonded to the metal-bound phenolate oxygens to give
rise to the zwitterionic structure ( N+–H· · ·O−). The macrocycle
under this condition is designated as L2H2 [21]. We found that the
signal corresponding to the imine hydrogen,–CH–N, was broad-
ened in the LaIII complex (ı, 8.36) when compared with that of the
ligand (␦, 8.32); further, a new signal, characteristic of –N+H reso-
nance, appears in the spectrum of the LaIII complex at 9.27ı while
the parent ligand does not show any such signal. These observa-
tions are in accordance with those made by Binnemans et al. [24]
who reported similar results while working on rare earth contain-
ing magnetic liquid crystals of the formula, [Ln(LH)3(NO3)3], where
LH = 4-alkoxy-N-alkyl-2-hydroxy benzaldimine. Thus, it may be
confirmed that the present Schiff-base (H2L2) exists in the metal
3.5. Magnetic and electronic spectral studies
The ꢁeff values (at RT; Table 1) of all the present LnIII complexes
(5.25, 4.89, 2.06, 4.52, 12.38, 14.91, 15.29 and 16.41 BM for Ln = Pr,
Nd, Sm, Eu, Gd, Tb, Dy, and Ho respectively), have been found to be
higher than the reported van Vleck values presumably on account
of metal–metal interactions [25–27] that may be present in all such
complexes with abnormal ꢁeff values.
The electronic spectra (qualitative solution-state spectra in a
mixed solvent (CHCl3 and DMSO; 3:1, v/v) in the 200–1100 nm
region) recorded for only the PrIII, NdIII, SmIII and DyIII complexes
show considerable red shift in the ꢃmax values in comparison with
those of their corresponding aquo ions [28]. These red shifts are pre-
sumably due to the Nephelauxetic effect [29] and are regarded as a
measure of covalency of the bonding between the metal ion and the
ligands. Various bonding parameters (Table 3), viz., Nephe1auxetic
ratio (ˇ), bonding parameter (b1/2), Sinha’s parameter (%ı) and
O
C H
10 21
H
H
OH
N
N
C H
10 21
HO
O
O
C H
10 21
H
H
O
HN
N
H
C H
10 21
O
O
Scheme 2. Depiction of migration of phenolic protons to imine nitrogens of the ligand, H2L2, during the formation of zwitter ion.

1658
P.R. Shakya et al. / Spectrochimica Acta Part A 79 (2011) 1654–1659
Table 3
Electronic spectral data of various metal complexes of H2L2.
PrIII
NdIII
Transitions/bonding
parameters
ꢃmax (cm−1) aq. ion
ꢃmax(cm−1) complex
Transitions/bonding
parameters
ꢃmax (cm−1) aq. ion
ꢃmax (cm−1) complex
1G4
←
3H4
9900
16,850
20,800
9775
16,835
20,661
4F3/2
4F5/2
4F7/2
4F9/2
←
←
4I9/2
11,450
12,500
13,500
14,800
15,900
17,400
19,100
19,500
11,481
12,484
12,423
14,684
15,949
17,094
19,011
19,531
0.996
1D2*←
3P0←
,
4S3/2
←
←
2H11/2
←
2G7/2*←
4G7/2
4G9/2
←
←
ˇ
0.993
0.059
0.705
0.003
b1/2
%ı
ꢄ
0.045
0.402
0.002
SmIII
DyIII
Transitions/bonding
parameters
ꢃmax (cm−1) aq. ion
ꢃmax(cm−1) complex
Transitions/bonding
parameters
ꢃmax (cm−1) aq. ion
ꢃmax (cm−1) complex
6F9/2
←
6H5/2
9200
9174
6H7/2
6F9/2
6H5/2
,
9100
–
←
6H15/2
6F11/2
4G5/2
←
10,500
17,900
20,800
26,750
9852
–
←
10,200
11,000
12,400
13,200
9756
←
6F7/2
6F5/2
6F3/2
←
←
←
11,001
12,392
13,193
0.989
0.074
1.112
0.005
4M15/2
←
20,000
25,907
0.967
0.129
3.413
0.017
6P7/2*←
ˇ
b1/2
%ı
ꢄ
* Hypersensitive band.
a mixed solvent of CHCl3/DMSO (3:1, v/v; 1.0 × 10−4 mol L−1) were
recorded at room temperature. Among the complexes studied, the
TbIII complex emits characteristic metal-centered luminescence
[Fig. 2(a)] when excited with monochromatic radiation of 380 nm;
besides the broad emission peak at 451 nm, the spectrum consists of
covalency angular overlap parameter (ꢄ), calculated by the pro-
cedures as reported in the literature [30], suggest weak covalent
nature of the metal–ligand bonds.
On the basis of the IR, NMR and electronic spectral data
it may be proposed that the mesogenic Schiff-base, N,Nꢀ-di-(4-
decyloxysalicylidene)-lꢀ,6ꢀ-diaminohexane, (H2L2) coordinates to
LnIII as a neutral bi-dentate species to yield seven-coordinate com-
plexes of the general formula, [Ln2(L2H2)3(NO3)4](NO3)2, where
Ln = La, Pr, Nd, Sm, Eu, Gd, Tb, Dy and Ho, the metal-centered poly-
hedron being possibly distorted mono-capped octahedron.
7
four typical emission bands of the TbIII ion at 490 nm (5D4 → F6),
547 nm (5D4 → F5), 587 nm (5D4 → F4) and 621 nm (5D4 → F3)
respectively [31]; further, the spectrum shows that the first two
emission bands are about 15 times more intense than the bands at
the lower energy region. Thus, it may be inferred that the ligand,
H2L2 is likely to be a suitable organic chelator to absorb energy and
transfer the same to TbIII ion. The spectrum of the SmIII complex
[Fig. 2(b)] is devoid of any bands characteristic of metal-centered
emission, but shows two emission bands at 326 and 353 nm (exci-
tation energy of 270 nm) which may be attributed to the free ligand
emission [32]. The fluorescent intensities observed in the EuIII and
7
7
7
3.6. Mesogenic studies
The mesophases were identified by polarized hot-stage
microscopy and DSC. The POM studies imply smectic-B phase of
H2L2; the corresponding transition temperatures and enthalpy
changes are given in Table 4 while the optical texture is shown
in Fig. 1. None of the LnIII complexes under present study is found
to be mesogenic in behaviour.
3.7. Fluorescence studies
Fluorescence emission spectra (with the excitation and emission
slit widths of 10.0 nm) of the SmIII, EuIII, TbIII and DyIII complexes in
Table 4
Transition temperatures and enthalpy changes.
Compound
Transitiona
Tb (◦C)
ıHb (kJ mol−1
)
Cr–Cr
80.80
91.23
107.24
104.40
72.51
3.85
66.49
1.69
1.89
62.23
Cr–SmB
SmB–I
I–SmB
SmB–Cr
H2L2
a
Cr: crystal; SmB: smectic B; I: isotropic liquid.
Data as obtained from the first DSC cycle.
Fig. 1. The optical texture of H2L2 (SmB) at 97 ◦C.
b

P.R. Shakya et al. / Spectrochimica Acta Part A 79 (2011) 1654–1659
1659
80
70
60
50
40
30
20
10
0
(a)
(b)
8
6
4
2
0
400
500
600
300
400
500
Wavelength (nm)
Wavelength (nm)
Fig. 2. The fluorescence spectra of (a) TbIII and (b) SmIII complexes.
DyIII complexes were very weak (spectra not shown) under the
same experimental conditions. Energy transfer between the Schiff-
base ligand and the TbIII ion appears to follow the well-known
intramolecular energy transfer mechanism exhibited by lanthanide
Schiff-base complexes [33].
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The mesogenic Schiff-base, N,Nꢀ-di-(4-decyloxysalicylidene)-
lꢀ,6ꢀ-diaminohexane (H2L2), coordinates to LnIII as
a neutral
bi-dentate species to yield seven-coordinate complexes of the gen-
eral formula, [Ln2(L2H2)3(NO3)4](NO3)2, where Ln = La, Pr, Nd, Sm,
Eu, Gd, Tb, Dy and Ho, the polyhedron being possibly distorted
mono-capped octahedron. The zwitterionic-species of the neutral
bi-dentate ligand, H2L2, coordinates to the LnIII metal ion through
two phenolate oxygens. The POM and DSC studies reveal that only
the ligand (H2L2) is mesogenic (smectic-B phase) but not the LnIII
complexes. The fluorescence studies indicate that the ligand, H2L2,
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We wish to acknowledge recording of FAB Mass spectra at the
Central Drug Research Institute, Lucknow, India. The financial assis-
tance received from the CSIR, New Delhi (vide Sanction letter No.
01(2321)/09/EMR-II) and the fellowship awarded to Mr. Pawan R.
Shakya by the University Grants Commission, Kathmandu, Nepal,
are gratefully acknowledged.
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Supplementary data associated with this article can be found, in
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