Synthesis, characterization and photophysical studies of rare earth metal complexes with a mesogenic Schiff-base

Article abstract of DOI:10.1016/j.molliq.2016.01.052

A mesogenic (SmC) Schiff-base ligand, N,N′-di-(4′-octyloxybenzoate)salicylidene-1,12-diaminododecane (H₂L), has been synthesized and its phase transition behaviour and thermal stability were investigated by differential scanning calorimetry, th

Full text of DOI:10.1016/j.molliq.2016.01.052

Journal of Molecular Liquids 216 (2016) 510–515
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Synthesis, characterization and photophysical studies of rare earth metal
complexes with a mesogenic Schiff-base
Rajasekhar Yerrasani a, M. Karunakar a, Ragini Dubey a, Angad Kumar Singh a, Rajib Nandi b,
Ranjan K. Singh b, T.R. Rao a,
⁎
a
Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
Department of Physics, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi 221005, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A mesogenic (SmC) Schiff-base ligand, N,N′-di-(4′-octyloxybenzoate)salicylidene-1,12-diaminododecane (H2L),
has been synthesized and its phase transition behaviour and thermal stability were investigated by differential
scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and temperature-dependent
Raman study. Ln(III) complexes (Ln = La, Pr, Nd, Sm, Eu, Tb and Dy) of the H2L were synthesized and the bonding
behaviour/emission property was studied by electronic, IR and NMR and fluorescence spectral data.
© 2016 Elsevier B.V. All rights reserved.
Received 16 September 2015
Received in revised form 19 October 2015
Accepted 15 January 2016
Available online xxxx
Keywords:
Photo-luminescence & Raman spectra
Mesogenic Schiff-base
LnIII complexes
Thermal stability
1. Introduction
2. Experimental section
Metallomesogens (metal containing liquid crystals) have attracted
broad interest because of their unique combination of the characteristic
properties of liquid crystals and metal complexes [1–3]. The lanthanide
containing mesogens (lanthanidomesogens) are the best known class of
luminescent metallomesogens [4–6]. An advantage of luminescence
by trivalent lanthanide ions is that the luminescence spectra consist of
narrow emission lines of intense colour [7–9]. The design of
lanthanide-containing liquid crystals is marred with complication
because of their high coordination numbers that often seem to be
incompatible with the structural anisotropy required to exhibit liquid-
crystalline behaviour [10–11]. Nitrate is often chosen to be the counter-
ion because it can coordinate in a bi-dentate fashion, allowing the lantha-
nide ion to easily obtain a high coordination number. Such materials not
only possess a large magnetic anisotropy but also display interesting
photophysical properties.
2.1. Materials
All the required reagents of analytical grade (AR) were obtained
from commercial sources and used without further purification;
1-bromooctane, 2,4-dihydroxybenzaldehyde, 4-hydroxybenzoic acid
and 1,12-diaminododecane are from Sigma-Aldrich, USA; all the
Ln(NO3)3·xH2O salts are from Alfa Aesar and KHCO3 is from Merck.
The organic solvents obtained from commercial vendors were dried
using standard methods [17] when required.
2.2. Synthesis and analysis
N,N′-di-(4′-octyloxybenzoate)salicylidene-1,12-diaminododecane
(H2L), 3 (Scheme 1), was synthesized from the precursor materials 1 and
2 as reported [18–19]. The LnIII complexes [Ln2(LH2)3(NO3)4](NO3)2
(Ln = La, Pr, Nd, Sm, Eu, Tb and Dy), were prepared by refluxing together
appropriate metal nitrate Ln(NO3)3·xH2O in methanol and the ligand
(H2L) in dichloromethane for ~6 h. The solid complex, yielded by evapo-
rating the solution, was further purified by washing repeatedly with
methanol.
In continuation of the earlier work [12–16] carried out in our labora-
tory on systematic structural and spectroscopic studies of 4f metal com-
plexes of a series of mesogenic organic Schiff-bases, we now report here
the synthesis and spectral studies of a mesogenic Schiff-base, N,N′-di-
(4′-octyloxybenzoate)salicylidene-1,12-diaminododecane (H2L), and
its non-mesogenic rare earth complexes.
2.2.1. Synthesis of N, N′-di-(4′-octyloxybenzoate)salicylidene-1,12-
diaminododecane,(H2L), 3
The ligand (H2L) was synthesized by refluxing together solutions of
4-octyloxy (4′-formyl-3′-hydroxyphenyl)benzoate (7.4 g, 20 mmol)
and 1,12-diaminododecane (2.0 g, 10 mmol) in ethanol for ~3 h in the
⁎
Corresponding author.
E-mail address: drtrrao@gmail.com (T.R. Rao).
http://dx.doi.org/10.1016/j.molliq.2016.01.052
0167-7322/© 2016 Elsevier B.V. All rights reserved.

R. Yerrasani et al. / Journal of Molecular Liquids 216 (2016) 510–515
511
Scheme 1. Reagents and Conditions i) 1-bromooctane, KOH, ethanol, ~12 h reflux ii) 2,4-dihydroxybenzaldehyde, DCC, DMAP, ~24 h stirring at room temp., iii)1 or 2 drops acetic acid,
ethanol, ~3 h reflux iv) Ln(NO3)3·xH2O, Methanol/DCM, ~6 h reflux.
presence of few drops glacial acetic acid; the yellow precipitate thus ob-
tained was washed thrice with hot ethanol and dried at room tempera-
ture: yield: 70%, as yellow-colour solid; mesogenic (SmC); Anal. calcd
for C56H76N2O8 (905.21) (%): C, 74.30; H; 8.46; N, 3.09. Found: C,
74.34; H, 8.44; N, 3.08. 1H NMR (300.40 MHz; CDCl3; Me4Si at 25 °C,
J (Hz), ppm) δ 0.89 (t, J = 6.6, 3H, –CH3), 1.28–1.83 (m, 32H, (–CH2)16),
3.57(t, J = 6.9, 2H, –NCH2), 4.03 (t, J = 6.3, 2H, –OCH2), 6.72 (d, J =
8.4, 1H, Ar–H), 6.78 (s, 1H, Ar–H), 6.96 (d, J = 8.7, 2H, Ar–H), 7.25
(d, J = 6.0, 1H, Ar–H), 8.12 (d, J = 8.7, 2H, Ar–H), 8.30 (s, 1H,CH_N–),
14.09 (s, br, ph–OH); 13C{1H} NMR (75.45 MHz; CDCl3; Me4Si at 25 °C,
ppm) δ: 164.40 (–COO), 163.71(–C4′), 163.65 (–NCH), 163.56 (–C2),
154.15 (–C4), 132.26 (–C6), 131.90 (–C2′), 121.30 (–C1′), 116.47 (–C1),
114.25 (–C5),112.06 (–C3′), 110.60 (–C3), 68.28 (–OCH2) and 58.89
(–NCH2); MS (MALDI-TOF): m/z 905.78 (M + 1), calcd 905.21; IR
(cm−1, KBr): 3441 (ν-OH),1728 (νC_O) 1631 (νC_N), 1142(νCph–O).
ligand, H2L (2.71 g, 3 mmol) at room temperature. The reaction mixture
was refluxed for ~6 h and the pale yellow colour solid that formed was
filtered off and washed thrice with methanol and dried over fused CaCl2
in a desiccator. Yield: 60%, as a pale yellow-colour solid; m.p. 210 °C
(decompose); Anal. calcd for La2C168H228N12O42 (3365) (%): C, 59.96;
H, 6.83; N, 4.99 and La, 8.25. Found: C, 59.88; H, 6.79; N, 4.89 and La,
8.29. 1H NMR (300.40 MHz; CDCl3; Me4Si at 25 °C, J (Hz), ppm)
δ 0.92(t, J = 6.6, 3H, –CH3), 1.78–1.17 (m, 22H, –(CH2)11), 3.49 (t, J =
6.9, 2H, –NCH2), 3.96 (t, J = 6.3, 2H, –OCH2), 6.28–7.25 (m, 7H, Ar–H),
7.93 (s,1H,CH_N), 12.62 (s, br, 1H, –N+H); 13C{1H} NMR (75.45 MHz;
CDCl3; Me4Si at 25 °C, ppm) δ = 176.68 (–NCH), 165.05 (–COO),
164.25 (–C4′), 163.52 (–C2), 158.27 (–C4), 136.95 (–C6), 132.38 (–C2′),
121.07 (–C1′), 114.69 (–C1), 114.17(–C5), 112.71 (–C3′), 110.89 (–C3),
68.30(–OCH2) and 55.39 (–NCH2); IR (cm−1, KBr); 3189 (νN+H), 1724
(νC_O), 1651 (νC_N), 1116 (νph–O).
All the other complexes (LnIII = Pr, Nd, Sm, Eu, Tb and Dy) were syn-
thesized in an analogous way by using the appropriate hydrated salt of
LnIII nitrate; the physical properties and the analytical data of all the
complexes are given in Table 1.
2.2.2. Synthesis of LaIII complex, [La2(LH2)3(NO3)4](NO3)2
A solution of La(NO3)3·6H2O (0.87 g, 2 mmol) in a minimum volume
of methanol was added dropwise to a dichloromethane solution of the
Table 1
Analytical data and general data of lanthanide complexes of H2L.
H2L/complex formula wt. (emp. formula)
Colour yield
m.p.(°C)
Found (calcd)%
C
H
N
M
H2L
Yellow 76%
130
74.34 (74.30)
8.44 (8.46)
3.08 (3.09)
–
905.21(C56H76N2O8)
[La2(LH2)3(NO3)4](NO3)2
3365(La2C168H228N12O42
[Pr2(LH2)3(NO3)4](NO3)2
3369(Pr2C168H228N12O42
[Nd2(LH2)3(NO3)4](NO3)2
3376(Nd2C168H228N12O42
[Sm2(LH2)3(NO3)4](NO3)2
3388(Sm2C168H228N12O42
[Eu2(LH2)3(NO3)4](NO3)2
3391(Eu2C168H228N12O42
[Tb2(LH2)3(NO3)4](NO3)2
3405(Tb2C168H228N12O42
[Dy2(LH2)3(NO3)4](NO3)2
3412(Dy2C168H228N12O42
Light yellow, 65%
Light yellow, 60%
Light yellow, 65%
Light yellow, 60%
Light yellow, 60%
Light yellow, 60%
Light yellow, 65%
210d
230d
210d
220d
200d
210d
210d
59.98 (59.96)
59.79 (59.88)
59.79 (59.77)
59.47 (59.55)
59.41 (59.49)
59.30 (59.25)
59.20 (59.13)
6.79 (6.83)
6.78 (6.82)
6.77 (6.81)
6.71 (6.78)
6.72 (6.78)
6.78 (6.75)
6.81 (6.73)
4.95 (4.99)
4.97 (4.99)
4.96 (4.98)
4.98 (4.96)
4.95 (4.96)
4.89 (4.94)
4.97 (4.93)
8.29 (8.25)
8.41 (8.36)
8.51 (8.54)
8.92 (8.88)
8.99 (8.96)
9.45 (9.33)
9.55 (9.52)
)
)
)
)
)
)
)
d: decomposes.

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R. Yerrasani et al. / Journal of Molecular Liquids 216 (2016) 510–515
2.3. Physical measurements
ligand at 1631 cm−1 undergoes a hypsochromic shift to 1651 cm−1 in
all the complexes on account of zwitterionic formation; further, the ap-
pearance of a weak, broad band at 3189 cm−1 reflects the zwitterionic
(–C_N+H) nature of the ligand [22]. The vibrational modes of the coor-
dinating nitrate groups (C2v) appear at characteristic frequencies
around 1483–1467, 1290–1280 and 853–840 cm−1 in the LnIII com-
plexes [23]; the magnitude of splitting, 194–181 cm−1, at higher ener-
gies may be indicative of a bi-dentate ligation of the coordinated
nitrate groups [23,24]. The additional band observed at 1384 cm−1
may be attributed to the non-coordinated nitrate groups of the
complexes.
The 1H and 13C NMR spectra were recorded on a JEOL AL-300 MHz
FT-NMR multinuclear spectrometer; C, H, N contents were micro
analysed on Vario EL III Carlo Erba 1108 analyser. Infrared spectra
were recorded on JASCO FTIR (model-5300) spectrophotometer in
4000–400 cm−1 region. Mass spectrum was recorded on AB Sciex
TOF/TOF 5800 MALDI mass spectrometer. UV–VIS spectra were record-
ed on Shimadzu spectrophotometer, model, Pharmaspec UV-1700.
Emission spectra were recorded on JY HORIBA Fluorescence spectro-
photometer. The mesophases were identified by using polarized hot-
stage microscope (LOMO, USA) equipped with digital camera (Nikon
Coolpix 4500). Differential Scanning Calorimetry studies were made
on METTLER DSC-25 unit. Molar conductance of the complexes was de-
termined in 0.001 M solutions at room temperature on a CON 510 bench
conductivity metre (cell constant, K = 1.0). Thermogravimetric analysis
(TGA) (RT to 1000 °C) was performed on Perkin-Elmer-STA 6000 under
high purity nitrogen; The Raman spectrum of the ligand was recorded
on Raman spectrometer (Renishaw, UK) equipped with 785 nm lasers;
laser-induced heating in the sample was nullified by using 0.5% of the
diode laser (λ, 785 nm with maximum power of 300 mW) and the inci-
dent laser beam was focussed on the sample by a 50× long distance ob-
jective attached to a Leica DM 2500 M microscope; the Raman scattered
light was collected in back-scattering geometry by the same objective (a
grating of 1200 grooves/mm was used as the dispersion element); var-
iable temperature spectra ( 0.1 K) were recorded by using THM 600
hot stage and a dedicated computer with Wire 4.0 software.
3.2. 1H and 13C NMR spectral studies
The 1H and 13C{1H} NMR spectral data show that the phenolic–OH
signal at 14.09δ of the ligand disappears in the LaIII complex; during
coordination the phenolic protons of the ligand are shifted to the
imine nitrogens, which then get intra-molecularly hydrogen bonded
to the metal-bound phenolate oxygens giving rise to the zwitterionic
structure (_N+–H–O–) and the macrocycle under this condition is
designated as LH2 [25]. We found that the signal at 8.30δ of the imine
hydrogen (–CH_N) shifted to 7.93δ in the LaIII complex with the simul-
taneous formation of a new signal at 12.62δ, characteristic of –N+H res-
onance, in the latter; these observations are in good agreement with
systems reported previously [10,13,26]. The 13C NMR spectra show
a significant shift of the –N_CH– signal of the ligand (163.65δ) to
176.69δ in the LaIII complex. Shifts of similar magnitude were observed
in the case phenolate carbons (owing to direct attachment to the bond-
ing atom) while those observed for the other carbons were of lesser
magnitude. Thus, the NMR spectral data imply bonding through two
phenolate oxygens of the ligand in the zwitterionic form to the LaIII
metal ion.
3. Results and discussion
The structures of the ligand and the corresponding metal com-
plexes were studied by IR & NMR spectral techniques and elemental
analysis. The analytical data (Table 1) of the complexes corresponds
to 2:3 metal to ligand stoichiometry with the general formula
[Ln2(LH2)3(NO3)4](NO3)2. The IR and NMR spectral data imply
bidentate coordination through non-deprotonated zwitterionic form
of the ligand in the LnIII complexes. Further, molar conductance data
(160–165 Ω−1 cm2/mol) measured in 10−3 M DMF solutions corre-
sponds to 2:1 electrolytic behaviour [20] indicating the presence of
two of the six nitrate groups outside the coordination sphere.
3.3. Photophysical properties
The electronic spectra (Fig.1) of the ligand and the complexes were
recorded in chloroform and 3:1(v/v ratio) mixed solution of chloroform
and DMSO. Of the two intra-ligand bands, the one centred at 310 nm
may be due to π–π* transition localized on the aromatic ring while the
other at 405 nm is due to n–π* transition of the imine chromophore.
The spectra of the PrIII, SmIII, NdIII and DyIII complexes show consider-
able red shift in the λmax values in comparison with those of their corre-
sponding aqua ions [27]; these red shifts are presumably due to the
Nephelauxetic effect [28] and thus imply the extent of covalency of
the metal–ligand bond. Various bonding parameters (Table 3), viz.,
Nephelauxetic ratio (β), bonding parameter (b1/2), Sinha's parameter
(%δ) and covalency angular overlap parameter (η), calculated by the
procedures as reported in literature [29] suggest weak covalent nature
of the metal–ligand bonds.
3.1. IR spectral studies
The infrared spectral data assignments regarding bonding through
specific functional groups have been made by a careful comparison of
specific vibrational bands of H2L with the corresponding bands of the
LnIII complexes (Table 2). The broad band at 3441 cm−1 in the IR spec-
trum of the ligand, characteristic of phenolic–OH [21], may be under-
stood to involve considerable amount of intra molecular hydrogen
bonding with –C_N group. This band disappears in the spectra of the
complexes due to shifting of the phenolic proton to the azomethine
nitrogen resulting in the formation of zwitter ion. The νCN band of the
A photoluminescence spectrum of the ligand in CHCl3 solution,
when excited with monochromatic radiation of 390 nm, shows (Fig.1)
a λmax at 435 nm due to intra-ligand transition. The SmIII complex,
Table 2
IR spectral data of the lanthanide complexes of H2L.
H2L/Complex
ν(O–H) Phenolic
ν(N+H)
ν(C_O)
ν(C_N)
ν(C–O) phenolic
ν(NO3)
ionic
ν5
ν1
ν2
ν5–ν1
H2L
3441b
–
1728
1724
1728
1726
1731
1725
1724
1737
1631
1651
1650
1658
1651
1650
1651
1656
1142
1116
1118
1116
1117
1118
1118
1115
–
–
–
–
–
[La2(LH2)3(NO3)4](NO3)2
[Pr2(LH2)3(NO3)4](NO3)2
[Nd2(LH2)3(NO3)4](NO3)2
[Sm2(LH2)3(NO3)4](NO3)2
[Eu2(LH2)3(NO3)4](NO3)2
[Tb2(LH2)3(NO3)4](NO3)2
[Dy2(LH2)3(NO3)4](NO3)2
–
–
–
–
3189w
3189w
3193w
3193w
3193w
3179w
3196w
1384
1383
1383
1384
1383
1384
1384
1469
1467
1479
1479
1483
1469
1493
1280
1283
1287
1290
1289
1280
1312
846
846
845
853
841
846
846
189
184
192
189
194
189
181
–
b: broad, w: weak.

R. Yerrasani et al. / Journal of Molecular Liquids 216 (2016) 510–515
513
Fig. 1. UV–Vis spectrum of ligand, H2L (dotted line) and Photoluminescence spectrum of
ligand, H2L (continuous line) λexc. = 390 nm in CHCl3 (1 × 10−5 M solution).
when excited with 410 nm radiation, shows three major luminescence
bands (Fig.2) at 563, 599 and 645 nm which may be assigned from the
lowest emitting state of 4G5/2 to the 6HJ (J = 5/2, 7/2 and 9/2) states [28,
30,31]. The bright luminescence observed for the SmIII complex is reflec-
tive of a good match between the ligand-centred triplet state and the
three SmIII emissive states; intense emission, however, is only observed
from the 4G5/2 level. It was reported that the close proximity of these
three excited states to each other causes electrons from the higher
states to rapidly relax to the 4G5/2 level, from which radiative transitions
occur [32].
Fig. 2. Photoluminescence spectrum of SmIII complex at λexc., 410 nm.
3.5. Temperature dependent Raman spectral studies
In order to understand changes in intermolecular interactions
and molecular rearrangement upon phase transition, temperature-
dependent Raman spectra (RT − 133 °C) of the ligand (H2L) were re-
corded in 1000–2000 cm−1 region in both heating and cooling cycles.
At the phase transition, Cr → SmC, prominent Raman signatures were
observed that are consistent with DSC and POM studies. A systematic
study of peak position and line width variation of some Raman marker
bands with temperature may give information of structural transforma-
tion and also change in interaction environment during phase transi-
tions. In next sections, we focused on temperature dependence of
Raman bands in two regions, 1500–1850 cm−1 and 1120–1380 cm−1
in cooling cycle.
3.4. Thermal property and liquid-crystallinity behaviour
The phase transitions of the ligand and its complexes were studied
by using polarizing optical microscopy (POM), thermogravimetric anal-
ysis (TGA) and differential scanning calorimetry (DSC). The POM studies
imply smectic–C phase (Fig. 4.) of the ligand (H2L). The corresponding
transition temperatures (Fig. 3.) and enthalpy changes (Table 4) from
DSC and its TGA measurements proved good thermal stability up to
250 °C (Fig. 5). All the LnIII complexes under present study are found
to be non-mesogenic.
3.5.1. The region 1500–1850 cm−1
The Raman spectra (1500–1800 cm−1 region) are presented as a
function of temperature in Fig. 6. At room temperature the bands
at 1580 and 1606 cm−1 are due to C_C stretching modes of the rings.
At Cr → SmC transition the 1580 cm−1 band undergoes a hypsochromic
shift while the 1606 cm−1 band remains unshifted. In SmC phase, the
molecules are loosely packed with one dimensional positional order
Table 3
Electronic spectral data of various metal complexes of the ligand (H2L).
PrIII
NdIII
SmIII
Transitions
DyIII
Transitions
λ
max(cm−1
)
Transitions
λ
max(cm−1
)
λ
max(cm−1
)
Transitions
λ )
max(cm−1
aq. ion
complex
aq. ion
complex
aq. ion
complex
aq. ion
complex
1G4
←
3H4
9900
16,850
20,800
9784
16,863
–
4F3/2
4F5/2
←
←
←
4I9/2
11,450
12,500
13,500
11,507
12,531
13,440
6F9/2
←
←
←
6H5/2
9200
10,500
17,900
9216
9950
–
6H7/2,6F9/2
←
6H15/2
9100
10,200
11,000
–
1
6F11/2
6H5/2
←
9784
11,098
⁎
D2←
3P0←
4F7/2,
4S3/2
4G5/2
6F7/2
←
4F9/2
←
←
←
←
←
14,800
15,900
17,400
19,100
19,500
–
–
P7/2
←
←
26,750
29,100
–
6F5/2
6F3/2
←
←
12,400
13,200
12,406
–
6
⁎
2H11/2
4D7/2
28,735
2
⁎
G7/2
17,182
19,083
–
4G7/2
4G9/2
Spectroscopic bonding parameters
β
0.996
0.044
0.401
0.002
0.997
0.038
0.300
0.002
0.981
0.990
b1/2
%δ
η
0.097
1.936
0.01
0.070
1.010
0.006
*Hypersensitive transition.

514
R. Yerrasani et al. / Journal of Molecular Liquids 216 (2016) 510–515
Fig. 5. TGA thermogram of ligand (H2L).
Fig. 3. DSC thermogram of ligand (H2L).
Table 4
DSC data for H2L and its complexes.
Compound
H2L
Transition
T/°C
ΔH/kJ mol−1
ΔS/J mol−1 K−1
Cr–SmC
SmC–I
I–SmC
122.33
129.86
128.57
70.95
57.11
6.01
8.05
144.41
14.92
20.04
89.89
SmC–Cr
30.93
and orientational order compared to crystal phase, that is characterized
by larger interaction and close molecular packing due to three dimen-
sional positional orders as well as orientational order. This implies
temperature-induced intramolecular changes leading to a small charge
density shift from one region to the other. The band at 1632 cm−1 is due
to C_N stretching vibration of linking group. At room temperature the
C_N bond is involved in intramolecular hydrogen bonding with O–H of
phenyl ring; at Cr → SmC transition this band undergoes hypsochromic
shift with simultaneous increase in intensity and line width. This may be
due to the weakening of intramolecular hydrogen bonding between
C_N and phenolic O–H groups (as in SmC phase) as the molecules
are loosely packed compared to crystal phase. Due to this ortho oxygens
have greater possibility to form lateral interaction with other molecules
to form SmC phase. The ν(C_O)ester band appears at 1732 cm−1 at
room temperature; it is well documented that this type of C_O bonds
are likely to form inter-molecular hydrogen bonds in LCs [33,34]. At
Cr → SmC transition this band shifts to 1737 cm−1 (with broadening)
Fig. 6. Raman spectra of ligand (H2L) in the region 1500–1850 cm−1 at variable
temperatures in cooling cycle.
while at SmC → Iso transition it shifts to 1740 cm−1; these hypso-
chromic shifts may be on account of the change in the micro-
environment around C_O bonds. It is most plausible to assume that
the H− bonds associated with C_O group weaken at SmC phase com-
pared to crystal phase, but in isotropic phase most of the hydrogen
bonds associated with C_O bond are broken. The hydrogen bonds asso-
ciated with C_O group (which are weaker compared to crystal) are
understood to mainly contribute to lamellar SmC phase. It is also interest-
ing to see that all the bands associated with C_C stretching of rings, C_N
Fig. 7. Raman spectra of ligand (H2L) in the region 1120–1380 cm−1 at variable
Fig. 4. Optical Texture of ligand (H2L) at 112 °C.
temperatures in cooling cycle.

R. Yerrasani et al. / Journal of Molecular Liquids 216 (2016) 510–515
515
bond and C_O bond are broadened at Cr
→
SmC transition.
References
The broadening of these bands is due to the changes in intermolecular in-
teractions at Cr → SmC transition and temperature-induced re-
orientational freedom of the bonds in SmC phase.
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3.5.2. The region 1120–1380 cm−1
The temperature-dependent Raman spectra in the region 1120–
1380 cm−1 (Fig. 7) contain prominent bands at 1145, 1168, 1198,
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4. Conclusion
The mesogenic (SmC) Schiff-base, N,N′-di-(4′-octyloxybenzoate)
salicylidene-1,12-diaminododecane (H2L), coordinates to LnIII as a neu-
tral bi-dentate species through two phenolate oxygens in zwitterionic
manner to yield non-mesogenic seven coordinate complexes of the
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Acknowledgements
RY gratefully acknowledges the UGC, New Delhi, for Research
fellowship. RKS is grateful to the DST, New Delhi, for providing Raman
Spectrometer under DST-FIST programme.