Mesomorphic, micro-Raman and DFT studies of new calamitic liquid crystals; Methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates

Article abstract of DOI:10.1016/j.saa.2014.02.112

The mesomorphic properties of newly synthesized homologous series of calamitic liquid crystals; methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino] benzoates, H_2n+1C_nOC ₆H₄COOC₆H₄C(H)N C₆H ₄COOCH₃; n = 6, 8, 10, 12, 14, 16 (MABBAB-n) containing ester and Schiff base groups as linker have been studied by temperature dependent micro-Raman study, differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). All members of this series exhibit enantiotropic smectic A (SmA) mesophase with oily streak and focal conic textures. Analyses of Raman marker bands of phenyl rings, Schiff base and ester groups of MABBAB-10 confirm the phase transitions. The Raman study also gives an evidence of breaking of weak intermolecular hydrogen bonds associated with ester groups and formation of new hydrogen bonds through CN bond at Cr → SmA phase transition. The monomer and dimer were optimized and vibrational assignment of MABBAB-10 was also done with density functional theoretical (DFT) technique to understand the experimental results.

Full text of DOI:10.1016/j.saa.2014.02.112

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Mesomorphic, micro-Raman and DFT studies of
new calamitic liquid crystals; methyl
4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates
a,
Rajib Nandi a,1, Hemant Kumar Singh b,1, Sachin Kumar Singh b, Bachcha Singh b, , Ranjan K. Singh
⇑
⇑
a Department of Physics, Banaras Hindu University, Varanasi 221005, India
b Department of Chemistry, Banaras Hindu University, Varanasi 221005, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
A new liquid crystalline material is
synthesized.
Temperature dependent Raman, DFT,
DSC and POM studies have been done.
Peak shift and linewidth changes of
marker bands give clear signatures of
transitions.
ꢀ
Evidence of breaking and formation of
H-bonds at crystal ? smectic A
phase.
a r t i c l e i n f o
a b s t r a c t
Article history:
The mesomorphic properties of newly synthesized homologous series of calamitic liquid crystals; methyl
4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates, H2n+1CnOC6H4COOC6H4C(H)@N C6H4COOCH3;
n = 6, 8, 10, 12, 14, 16 (MABBAB-n) containing ester and Schiff base groups as linker have been studied
by temperature dependent micro-Raman study, differential scanning calorimetry (DSC) and polarizing
optical microscopy (POM). All members of this series exhibit enantiotropic smectic A (SmA) mesophase
with oily streak and focal conic textures. Analyses of Raman marker bands of phenyl rings, Schiff base and
ester groups of MABBAB-10 confirm the phase transitions. The Raman study also gives an evidence of
breaking of weak intermolecular hydrogen bonds associated with ester groups and formation of new
hydrogen bonds through C@N bond at Cr ? SmA phase transition. The monomer and dimer were opti-
mized and vibrational assignment of MABBAB-10 was also done with density functional theoretical
(DFT) technique to understand the experimental results.
Received 18 November 2013
Received in revised form 13 February 2014
Accepted 16 February 2014
Available online 6 March 2014
Keywords:
Liquid crystals
Smectic A mesophase
Polarizing optical microscopy
Differential scanning calorimetry
Raman spectroscopy
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
various fields such as optics, electro-optics, thermo-conducting
materials and fast switching devices [1–5] makes it an extraordinary
Liquid crystals (LCs) have potential for technological applications
due to its unique mesogenic behaviour. The applications of LC in
material in science. The LCs continue to have a revolutionary
technological impact and also consistently pose new challenges of
fundamental understanding. For more advanced applications of LC
phases, it is necessary to understand more deeply the structure–
property correlation as well as molecular origin of mesophases.
The chemical properties and phase behaviour of LC systems de-
pend on two factors; the properties of the molecules themselves
⇑
Corresponding authors. Tel.: +91 542 2369881; fax: +91 542 2368174.
E-mail addresses: bsinghbhu@rediffmail.com (B. Singh), ranjanksingh65@rediff
mail.com (R.K. Singh).
1
Main workers with equal contribution.
http://dx.doi.org/10.1016/j.saa.2014.02.112
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
249
i.e. shape and size of the molecules, and the interactions through
which a molecular property influences the macroscopic behaviour.
The average molecular field produced by LC molecules is different
in different mesophases and that is influenced by inter- and intra-
molecular interaction as well as symmetry of the molecules. The
internal vibrations of an individual constituent molecule are there-
fore modified during LC transition. And therefore vibrational spec-
troscopy in general and Raman spectroscopy in particular has great
potential to monitor the molecular dynamics during LC phase tran-
sitions [6–10]. The precise study of linewidth changes, peak shifts
and variations of integrated intensities of Raman marker bands
help to understand the dynamics and molecular realignment in-
duced during transition [11–20]. The whole analysis in Raman
study is based on behaviour of molecular vibration and therefore
the precise knowledge of the vibration under study through den-
sity functional theoretical (DFT) technique is of additional help.
For vibrational assignment single molecule DFT calculation is a
good approximation [21–23].
obtained by the condensation reaction of methyl-4-aminobenzoate
with appropriate 4-(40-alkoxybenzoyloxy)benzaldehydes in reflux-
ing ethanol (details of synthesis and characterization are given in
supplementary data).
Characterization of mesophase
Differential scanning calometric (DSC) and polarizing optical
microscopic (POM) studies
Phase transitions and thermodynamic parameters of MABBAB
series were determined from DSC thermograms in heating and
cooling cycle scans by a Mettler Toledo TC 15 TA differential scan-
ning calorimeter at the rate of 5.0 °C minꢁ1. The transition temper-
atures from DSC thermograms have been determined with an
accuracy of 0.1 °C. The mesophase type was identified by visual
comparison with known phase standards using an HT 30.01 NTT
268 Lomo polarizing optical microscope fitted with a hot stage
with temperature controlling accuracy of 0.1 °C.
The synthesis of new liquid crystals in a systematic manner by
changing the linking group and aromatic ring in the core part and
studying the effect on transition temperatures as well as under-
standing the changes at molecular level during mesomorphic tran-
sitions may lead to quantitative as well as qualitative knowledge of
LCs. Vikram et al. investigated the nature of phase transition of
4DBA by Raman spectroscopy and found that the strong intermo-
lecular hydrogen bonding due to carboxylic acid terminal group fa-
vours dimer structure that is responsible for smectic and nematic
phases [16]. Further we investigated low temperature behaviour
of 4DBA by Raman spectroscopy and observed the evidence of slow
structural transformation due to more strong hydrogen bonds [21].
In another work, we synthesized a novel LC system; ethyl-[4-(4’-
decyloxy)benzoyloxy]-benzoate (4-EDBB) containing diester
groups by protecting the carboxylic acid terminal group of 4DBA
with the curiosity to see the effect on mesomorphic behaviour
when hydrogen bonds are weak. In 4-EDBB conformational
changes through breaking of lateral hydrogen bonds were ob-
served at crystal (Cr) ? smectic A (SmA) phase transition [22]. A
novel liquid crystalline homologous series methyl 4-(4’-n-alkoxyb-
enzylideneamino)benzoate (MABAB) containing Schiff base as link-
ing group was synthesized and the Raman study revealed an
evidence of induced co-planarity of the rings of MABAB-10 at
Cr ? SmA phase transition [23].
Temperature-dependent Raman spectral measurement
Raman spectra of MABBAB-10 were recorded on a micro-Raman
setup from Renishaw, UK equipped with a grating of 2400 lines/
mm and a Peltier cooled charge coupled device (CCD). The excita-
tion source was 514.5 nm line of Ar+ laser. The spectral resolution
of the spectrometer with 50 l .
m slit opening was less than 1 cmꢁ1
The reported accuracy in the measurement of temperature
is 0.1 °C. Other details of spectrometer were given elsewhere
[23]. No preferential direction exists in our cells for the phase
ordering and, therefore, the crystal and LC phases are multi-
domain.
Theoretical approach
All quantum chemical calculations were done using DFT with
Gaussian-03 program package [24]. The method adopted for this
calculation was hybrid density functionals, that mixes the Becke
three hybrid functionals for exchange part and the Lee, Yang and
In the present study we have synthesized a novel LC system;
methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates
(MABBAB) containing esterandSchiff base groupsas linkerwiththree
aromaticrings. Thedifferentphaseswereidentifiedbythedifferential
scanning calorimetry (DSC), polarizing optical microscopic (POM)
and temperature dependent Raman microscopy. Few possible dimers
as well as monomer are optimized using DFT. Temperature depen-
dent analysis of Raman bands belonging to CAH in-plane bending
and C@C stretching of rings, Schiff base and ester linking groups gives
anevidence of breakingofweakintermolecularhydrogen bondsasso-
ciated with ester groups and formation of new hydrogen bonds asso-
ciated with C@N bond at Cr ? SmA phase transition.
Methodology
Synthesis
The synthetic route for preparation of Schiff base ligands-methyl
4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates is out-
lined in Scheme 1. Alkylation of 4-hydroxybenzoic acid with appro-
priate alkyl bromides in the presence of KOH in EtOH solution leads
to formation of 4-alkoxybenzoic acids, which on further esterifica-
tion with 4-hydroxy benzaldehyde gave 4-(40-alkoxybenzoyl-
oxy)benzaldehydes. The Schiff bases of MABBAB 6-16 were
Scheme 1. Reactions and reagents: (a) RBr (1.0 equiv.), KOH (3.0 equiv.), refluxing
in EtOH, 7 h, 74%, (b) SOCl2 (3.0 equiv.), refluxing in dry chloroform, 7 h, (c) pyridine
(catalyst), refluxing in dry chloroform, 66%. (d) Acetic acid (3 drops), refluxing in
EtOH, 4 h, 90–94%.

250
R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
Parr hybrid functional for correlation part (B3LYP) [25,26]. The
optimization of monomer and frequency calculation were done
with basis set 6-31G(d,p). We have also evaluated dimers with ba-
sis set 6-31G(d). The input structure was drawn and the output
geometry as well as vibrational animations were observed in Gauss
View 4.1 [27]. Potential energy distribution (PED) for accurate
vibrational assignment was done using GAR2PED software [28].
The Gaussian-03 output gives the Raman activity which comes
from the second derivative of the polarizability with respect to
the normal coordinate [29,30] and can be converted into Raman
intensity. Raman intensity is proportional to the Raman scattering
The lower and higher homologues of MABBAB with hexyloxy and
octyloxy chain lengths (MABBAB-6 and MABBAB-8) and dodecyloxy
to hexadecyloxy chain lengths (MABBAB-12, 14 and 16) show
similar phase behaviour and transition behaviour as obtained
through POM and DSC studies for MABBAB-10. The observation of
SmA phase for all the compounds of the series could be expected. In-
deed, lamellar mesophases are most commonly observed for cala-
mitic (rod like) compounds with polar end group. The strong
dipole moment in the series of compound due to polar methyl ben-
zoate group at one end and anisotropic tail at opposite end promotes
intermolecular interactions between neighbouring layers by
cross section; d
r
/d
x
, which is given by the expression:
dipole–dipole interactions and hence stabilizes
arrangement.
Table 1 summarizes the thermodynamic parameters associated
with the mesomorphic transitions of the MABBAB. The large en-
a lamellar
2
3
4
d
r
24p4
45
ð
m0
ꢁ
mj
Þ
h
4
5
ꢀ
ꢁ
¼
Sj
8p
2cmj
ꢁhcmj
dx
1 ꢁ exp
kT
thalpy (DH) and entropy changes (DS) are observed for these com-
pounds during the transitions from crystal to liquid crystal and
liquid crystal to isotropic states for lower to higher homologues.
In the series of MABBAB 6 ? 16, liquid crystal transition tem-
perature decreases from hexyloxy (MABBAB-6) to decyloxy deriv-
ative (MABBAB-10) and again increases for dodecyloxy
(MABBAB-12) to hexadecyloxy (MABBAB-16) in heating and cool-
ing cycle both as shown in Fig. 3. Thus, in the series of MABBAB
6 ? 16, a random order of melting (Tm) and isotropization temper-
atures (Ti) is found. Although, Tm and Ti generally decrease with in-
crease in the spacer length due to the plasticization effect of the
long alkyl spacers in mesogenic compounds [34]. Therefore, it
can be concluded that along with packing/disassembling some
molecular phenomenon is also playing role which affects the Tm
and Ti of the mesogens. It will be further discussed in DFT section.
where m0 is the excitation frequency, mj is the harmonic vibrational
wavenumber of the jth normal mode and Sj is the Raman activity of
the jth normal mode. The Raman scattering cross section for the
individual modes is calculated using the above relationship, which
gives the relative intensity.
Result and discussions
Mesomorphic property
Differential scanning calorimetry (DSC) and polarized optical
microscopic (POM) studies
The mesomorphic properties of MABBAB were characterized and
investigated by DSC and POM. The phase transition temperatures
and thermodynamic data of the compounds are listed in Table 1.
All the compounds exhibit enantiotropic SmA mesophase.
The DSC thermograms of all compounds of MABBAB series are
shown in Fig. 1. In the first heating scan of MABBAB-10, two sharp
peaks appear at 95.9 and 143.3 °C (Fig. 1a). The DSC thermogram re-
corded in the cooling scan exhibits two separate peaks at 133.3 and
83.5 °C (Fig. 1b). POM observations reveal that upon heating from
crystal state to 95.9 °C, oily streak textures with homeotropic areas
emerges as shown in Fig. 2. It is to be noted that oily streak texture is
commonly observed for chiral compounds as cholesteric mesophase
while fan shaped texture is characteristic of smectic phase [31]. Our
compounds are simple rod like in nature and are not chiral at all. Oily
streak texture for smectic phase is very rare, however, few such re-
ports of oily streak texture for smectic phase are available in the lit-
erature [32,33]. Furthermore, taking thin layer of MABBAB-10
exhibits typical focal conic textures of the SmA mesophase with
some oily streak textures under cross polarizer in heating cycle
(Fig. 2b). In cooling cycle, POM observation reveals typical focal
conic textures of SmA mesophase with hexagonal patterns that
emerges from 133.3 °C and solidifies at 83.5 °C (Fig. 2c). Reheating
regenerates the same mesophase with similar pattern; that is, the
mesomorphism is enatiotropic.
DFT study
The various inter-molecular and induced intra-molecular inter-
actions decide the symmetry and packing of the molecules that
play a crucial role in formation of LC phases and their physical
properties. Therefore optimization of monomer and dimers, calcu-
lation of harmonic wavenumber and vibrational assignment with
PED have been done to investigate the molecular properties using
the DFT method. We optimized all the members of MABBAB. The
optimized structure of MABBAB-10 molecule is shown in Fig. 4.
The absence of any imaginary frequency in the calculated vibra-
tional frequencies ensures that the optimized geometry corre-
sponds to a true energy minimum and most of the molecules are
favoured to present at this structure. The dihedral angles of linking
groups and dipole moment of all compounds of series are almost
same for all members.
The room temperature experimental and theoretical spectra of
MABBAB-10 are shown in Fig. 5. In the DFT calculation, the
B3LYP function tends to overestimate the wavenumber of the fun-
damental modes compared to the experimentally observed values
due to the combination of electron correlation effects and basis set
deficiencies. In order to obtain a considerably better agreement
Table 1
Thermal transitions and corresponding thermodynamic parameters of MABBAB 6-16.
No.
Compounds
T (°C) [
DH, kJ/mol; DS, J/(mol K)]
Heating
Cooling
1
2
3
4
5
6
MABBAB-6
MABBAB-8
MABBAB-10
MABBAB-12
MABBAB-14
MABBAB-16
k 118.9 (33.2; 84.7) SmA 158.6 (6.0; 14.1) i
k 117.5 (24.5; 62.8) SmA 155.0 (9.0; 21.1) i
k 95.9 (21.0; 56.9) SmA 143.3 (22.1; 52.9) i
k 102.7 (28.5; 75.7) SmA 137.5 (15.9; 39.3) i
k 106.8 (74.5; 196.1) SmA 136.5 (12.2; 29.8) i
k 97.8 (66.4; 178.9) SmA 130.9 (12.9; 31.8) i
i 135.7 (ꢁ10.8; ꢁ26.4) SmA 102.6 (ꢁ32.0; ꢁ85.2) k
i 141.8 (ꢁ2.6; ꢁ6.2) SmA 111.7 (ꢁ22.1; ꢁ57.4) k
i 133.3 (ꢁ16.7; ꢁ41.2) SmA 83.5 (ꢁ21.8; ꢁ61.1) k
i 127.5 (ꢁ13.7; ꢁ33.6) SmA 88.3 (ꢁ31.9; ꢁ88.4) k
i 125.6 (ꢁ12.2; ꢁ30.6) SmA 85.7 (ꢁ29.9; ꢁ83.5) k
i 126.6 (ꢁ8.3; ꢁ20.7) SmA 65.3 (ꢁ29.6; ꢁ87.4) k
Abbreviations: k = crystalline state, SmA = smectic A mesophase, i = isotropic liquid.

R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
251
Fig. 1. DSC thermograms of methyl 4-[4-(4-alkoxy benzoyloxy)benzylideneamino]benzoates (MABBAB) in (a) heating cycle and (b) cooling cycle.
Fig. 2. Microphotographs of the compounds MABBAB in SmA phase (a) oily streak texture of MABBAB-10 with homeotropic domains at 100.2 °C in heating cycle, (b) broken
fan texture of MABBAB-10 at 105.3 °C in heating cycle, (c) broken fan texture with hexagonal patterns of MABBAB-10 at 129.8 °C in cooling cycle, (d) broken fan texture of
MABBAB-6 at 123.4 °C in heating cycle, (e) broken fan texture of MABBAB-14 at 118.7 °C in heating cycle and (f) SmA to crystal transition of MABBAB-10 at 85.1 °C in cooling
cycle.
with the experimental data, scaling factor 0.9614 [35] in the region
600–1800 cmꢁ1 has to be used. The mismatch between the exper-
imental and theoretical spectra is due to large intermolecular
interaction in the crystalline phase whereas in single molecular le-
vel calculation no such interaction is considered. The vibrational
assignment with PED of experimentally observed Raman bands
of MABBAB-10 as shown in Table 2 has been done for further
meaningful discussion in temperature dependent Raman study in
next section.
interactions and charge transfer interactions. The typical strength
of such hydrogen bonds is 2–10 kJ/mol and that for Van der Waals
forces is 1–3 kJ/mol but these relatively small forces have great
contribution to determine the mesophases in liquid crystalline sys-
tems [1,2]. We have optimized the different type of possible dimer
structures of MABBAB-10 which are shown in Fig. 6 in which weak
H-bonds (Oꢂ ꢂ ꢂHAC) and dipole–dipole interactions are responsible
for dimer formation.
Due to the presence of di-ester group, nitrogen atom in the
Schiff base linking group and polar nature of the methyl benzoate
terminal group dimer formation is possible through the supra-
molecular interactions such as hydrogen bonding (Oꢂ ꢂ ꢂHAC,
Nꢂ ꢂ ꢂHAC), Van der Waals interactions, dipolar and quadrupolar
Temperature dependent micro-Raman study
To understand the molecular rearrangement and changes in
intra- and inter-molecular interactions, we have recorded
temperature dependent Raman spectra of MABBAB-10 from room

252
R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
Fig. 3. Phase diagram of the mesogen MABBAB-n (n = 6–16) in (a) heating and (b) cooling cycles.
Fig. 4. The optimized structure of MABBAB-10 molecule using DFT method employing B3LYP/6-31G(d,p) functional.
Temperature dependent Raman spectra in region 1135–1225 cmꢁ1
This region contains CAH in-plane bending of phenyl rings, CAO
stretching and CAN stretching vibrations. This region is highly sen-
sitive to the phase transitions [14–18,22,23] and suitable analysis of
these marker bands gives very clear information about the molecu-
lar structural transformation at phase transitions. At room temper-
ature the CAH in-plane bending mode has two components which
are at 1162 and 1173 cmꢁ1 as shown in Fig. 7. They correspond to
calculated bands at 1144 cmꢁ1 and 1154 cmꢁ1 and their assign-
ments are given in Table 2. The band at 1162 and 1173 cmꢁ1 are
mainly due to the CAH in-plane bending vibration of ring II and ring
III respectively (Table 2). The origin of two components of CAH in-
plane bending mode lies in the fact that the phenyl rings experience
different neighbouring interactions. The difference in interaction
might be due to different micro-environment owing to non-
coplanerity of rings as shown in Fig. 4. The dihedral angle between
ring I and ring II is 46.62° and that for ring II and ring III is 36.30°.
The CAH in-plane bending mode has two components for whole
temperature range; room temperature – isotropic phase. In order
to make correct interpretation, precise analysis by non-linear fitting
considering the band shape as mixture of Gaussian and Lorentzian
has been done using Spectral Calc software. The peak positions and
linewidth of these bands are presented as a function of temperature
in Fig. 8(a) and (b). In the present study, we have observed a different
temperature behaviour of the CAH in-plane bending vibration than
previously studied LC systems; 4-EDBB [22] and MABAB [23]. Both
components of CAH in-plane bending vibration (1162 and
1173 cmꢁ1) suddenly shift to the lower wavenumber at Cr ? SmA
transition. A sudden change in peak position during Cr ? SmA
transition clearly indicates a first order transition. In the SmA phase
the molecules are loosely packed compared to the crystal phase but
have one-dimensional positional and orientational order. The crys-
tal phase is characterized by the larger interactions and close molec-
ular packing due to three-dimensional positional order as well as
orientational order. In the high temperature phase (SmA) the
Fig. 5. The experimental and DFT /B3LYP 6-31(d,p) derived Raman spectra at room
temperature of MABBAB-10 molecule.
temperature to 145 °C at different temperature in the spectral
region 1000–2000 cmꢁ1. The DSC and POM studies of MABBAB-
10 have revealed Cr ? SmA and SmA ? isotropic phase transitions
at 95.9 °C and 143.3 °C in heating cycle. The Raman signatures in
heating cycle for Cr ? SmA transition is observed between 94
and 97 °C. In order to understand the changes in intra/inter-
molecular interactions and the resulting dynamics of the core as
a function of temperature, we have concentrated on temperature
dependence of Raman bands associated with the core part of the
molecule in two regions, 1135–1225 cmꢁ1 and 1540–1760 cmꢁ1
.
A systematic study of the peak position, the integrated intensity
and the linewidth variation of these bands give a brief idea about
the structural transformation at the transition point which will
be discussed in the next individual sections.

R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
253
Table 2
Calculated harmonic wavenumbers, observed Raman wavenumbers and their assignments along with PED obtained from DFT calculations of MABBAB-10.
Harmonic
Experimental
Assignmenta and PED
wavenumber (cmꢁ1
)
wavenumber (cmꢁ1
)
1740
1728
1640
1600
1585
1566
1408
1724
1714
1630
1608
1598
1584
1417
m
m
m
m
m
m
m
(C11@O12) (82)ꢁ
m
(C3AC11) (7)
(C40@O41) (80)ꢁ
m(C37AC40) (6) + bIN(C37C40) (5)ꢁ
m(C40AO45) (5)
(C23@N25) (51)ꢁbIN(C23H24) (10) + bIN(C21C23) (7)
(C16@C19) (10) +
(C31@C33) (12) +
m
m
(C15@C17) (10)ꢁ
m
m
m
(C31@C33) (6) +
(C23@N25) (7) +
(C14@C16) (13) +
m
m
m
(C4@C5) (6)ꢁ
m
(C32@C35) (6) +
m
(C1@C2) (5)
(C15@C17) (5)
(C32@C35) (12)ꢁ
(C16@C19) (6)ꢁ
m
(C17@C21) (5) + m
(C14@C15) (19)ꢁ
m
m
(C17@C21) (13)ꢁ
(C19@C21) (13) + dASYM RII (7)ꢁbIN (C14O13) (6)
(C15@C17) (18)ꢁ
(C16@C19) (15) + bIN(C21C23) (10)ꢁbIN(C23H24) (8)ꢁbIN(C15H18) (7) + bIN(C17H22
)
(7)ꢁbIN(C14O13) (6)ꢁbIN(C19H75) (6)ꢁbIN(C16H20) (5)
1260
1239
1233
1198
1181
1154
1144
1292
1267
1260
1208
1198
1173
1162
m
m
m
m
m
(C37AC40) (30)ꢁ
(C3AC11) (31)ꢁ
(C30AN25) (16)ꢁ
(C14AO13) (42)ꢁdTRI RII (8)ꢁ
(C30AN25) (16)ꢁd(C21C23N25) (12) +
m
m
(C40AO45) (23) + bIN(C40O41) (13) + dSYM RIII (5)
(C11AO13) (7) + dASYM(C14O13C11C3) (7) + bIN(C40O41) (7)ꢁ
(C21AC23) (13) + bIN(C23H24) (9)ꢁ (C3@C11) (7)
(C11AO13) (7) + bIN(C19H75) (6) + bIN(C17H22) (6)
(C21AC23) (12) + dTRI RII (6)ꢁbIN(C33H38) (5)ꢁ
(C32@C35) (5)ꢁ
(C14@O13) (8)ꢁ (C17@C21) (5)
m(C1@C2) (7)ꢁ
m(C3@C4) (5)
m
m
m
m
m
(C32@C35) (5)
(C30@N25) (5)
bIN(C32H36) (19) + bIN(C35H39) (14) + bIN(C31H34) (13)ꢁbIN(C33H38) (6)ꢁ
m
m
bIN(C15H18) (12)ꢁbIN(C17H22) (10)ꢁbIN(C19H75) (9)ꢁbIN(C16H20) (9) +
m
m
a
m: Stretching,
l
: ring puckering, b: bending,
c
: twisting, d: deformation, : rocking, : torsion, S: scissoring, W: wagging, IN: in plane, OUT: out of plane, sym: symmetric,
q
s
asym: asymmetric, LIN: linear bending, RI: ring(1 2 3 4 5 6), RII: ring(14 15 17 21 19 16), RIII: ring(30 31 33 37 25 32).
Fig. 6. Different type of optimized dimer structures calculated at DFT /B3LYP 6-31(d) level; (a) anti-parallel dimer considering four weak hydrogen bonds having binding
energy 27.96 kJ/mol, (b) anti-parallel dimer through two weak hydrogen bonds having binding energy 14.16 kJ/mol and (c) dimer through the alkoxy–alkoxy chain
interactions having binding energy 4.25 kJ/mol.
intermolecular interactions through the phenyl rings decrease and
the individual CAH in-plane bending oscillator experiences less
interaction with the surrounding, resulting into decrease in wave-
number. The most probable reason for decrease in interaction
through the rings may be due to breaking of hydrogen bonds associ-
ated with hydrogen atoms of rings. The hydrogen bonds dimers
(optimized structures shown in Fig. 6(a) and (b)) are breaking at
Cr ? SmA transition. Presence of two components of the CAH in-
plane bending band confirms the non planer molecular geometry
of MABBAB-10 at SmA phase. The linewidth of these bands is shown
in Fig. 8(b). The broadness of CAH in-plane bending mode at
Cr ? SmA phase transition is due to the fact that the individual
CAH oscillator has different intermolecular interaction environ-
ment in SmA phase due to loose packing and they appear at slightly
shifted wavenumbers causing inhomogeneous broadening.
There is another band which has two components at 1198 and
1208 cmꢁ1 as shown in deconvoluted spectrum (Fig. 7). They ap-
peared in calculated spectrum at 1181 and 1198 cmꢁ1, respectively
Fig. 7. Temperature dependent Raman spectra of MABBAB-10 at different temper-
ature in the range 1135–1225 cmꢁ1
.
(Table 2). The band at 1198 cmꢁ1 is mainly associated with CAN

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R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
Fig. 8. Variation of (a) peak position and (b) linewidth of ꢃ1162, 1173 and 1198 cmꢁ1 bands with temperature.
stretching vibration in the linking group which is mixed with CAC
stretching and CAH in-plane bending vibration of rings (Table 2).
The weak band at 1208 cmꢁ1 is mainly due to the CAO stretching
vibration. The band at ꢃ1198 cmꢁ1 shifted to lower wavenumber
at Cr ? SmA phase transition as shown in Fig. 8(a). The CAN band
is characteristic of single bond character. In LC phase transition,
breaking and formation of hydrogen bonds simultaneously take
place at phase transitions, which affect the charge distribution of
different single bonds of the molecule. Weakening of this band
indicates that the some charge shift takes place from CAN bond.
Temperature dependent Raman spectra in region 1540–1760 cmꢁ1
This region contains C@C stretching of phenyl rings, C@N
stretching of linking group AC(H)@NA and C@O stretching of car-
bonyl groups shown in Fig. 9. The bands at 1584, 1598 and
1608 cmꢁ1 are mainly due to C@C stretching of rings respectively
(Fig. 9). They are also known as quadrant stretching vibration of
aromatic rings [36,37]. The assignments of these bands are given
in Table 2. The band at 1629 cmꢁ1 consists of mainly C@N stretch-
ing and CAH bending motions of linking group AC(H)@NA
(Table 2). Other two bands at 1714 and 1724 cmꢁ1 are due to the
C@O stretching vibration (Fig. 9). The band at 1714 cmꢁ1 is due
to terminal carbonyl group and the band at 1724 cmꢁ1 is attributed
to linking carbonyl group (Table 2).
Fig. 9. Temperature dependent Raman spectra of MABBAB-10 at different temper-
ature in the range 1540–1760 cmꢁ1
.
higher at Cr ? SmA transition (Fig. 10). The reason for higher shift
of both carbonyl groups is the breaking of hydrogen bonds [22].
The hydrogen bonds associated with C@O bonds in dimers as
shown in Fig. 6(a) and (b) are broken at Cr ? SmA transition. These
types of dimers are present in crystal phase but in SmA phase a
small residue of it may exist.
From the another point of view it is very interesting to see that
the CAH in plane bending and C@C stretching vibrations of rings,
and CAN and C@N of vibrations of AC(H)@NAC linking group shift
lower where the bands due to C@O stretching vibration shift high-
er at Cr ? SmA transition (Fig. 10). We cannot avoid the role of
charge transfer phenomena that may take place at Cr ? SmA tran-
sition. In LC systems, which consists generally large number of
atoms, the intra-molecular strain on different bonds of the mole-
cule changes resulting in charge density shift from one part to
the other [38]. For the MABBAB-10 molecule, charge transfer takes
place from rings and AC(H)@NAC linking group to both C@O
bonds.
The linewidth of the bands at 1584, 1598, 1629, 1714 and
1724 cmꢁ1 increases suddenly at Cr ? SmA transition shown in
Fig. 10(b). The experimentally observed Raman linewidth has con-
tributions from both the intrinsic linewidth and the temperature-
dependent reorientation part. Due to increase in temperature,
there is a consistent increase in linewidth that is the reorientation
contribution to the linewidth, leading to the band broadening,
which may be assumed to be more or less same for all bands.
All bands in this region show significant spectral changes at
Cr ? SmA phase transition. The peak position and linewidth varia-
tion of these bands with temperature are shown in Fig. 10(a) and
(b). The bands corresponding to C@C stretching of rings shifted
lower and broadened. The most plausible reason for lower shift
of C@C stretching of rings at transition may be due to the
p–p
stacking between the rings of neighbouring molecules being larger
in LC phase than that in crystal phase. The phenyl rings of different
molecules may come closer in layered like structure at SmA phase,
resulting in
molecules. The
the stacking reduce the force constant of the C@C bond. The
a
p–p
stacking between neighbouring mesogen
p-electrons of the phenyl rings participating in
p–p
C@N stretching band shifted lower at Cr ? SmA transition and
on further increasing temperature it shifts higher gradually as
shown in Fig. 10(a). This may be due to fact that in SmA phase
the C@N bond suffers some kind of interaction which is absent in
crystal phase. This indicates that new kind of hydrogen bond pat-
tern (C@Nꢂ ꢂ ꢂH) may exist in SmA phase. In isotropic phase, the
hydrogen bonds associated with C@N bond in SmA phase are bro-
ken. From this we conclude that C@N bonds are free from hydro-
gen bonds in crystal phase but involved in hydrogen bonds in
SmA phase. Both bands due to C@O stretching suddenly shifted

R. Nandi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 128 (2014) 248–256
255
Fig. 10. Variation of (a) peak position and (b) linewidth of ꢃ1584, 1598, 1608, 1629, 1714 and 1724 cmꢁ1 bands with temperature.
The sudden increase in the linewidth is due to the large inter-
molecular interaction as well as structural changes at the
Cr ? SmA transition. Another reason for broadening of bands
may be presence of more species in SmA phase. The sudden in-
crease in linewidth of C@O bands is due to breaking of hydrogen
bonds involved in dimer formation through the oxygen atom
(Fig. 6) at Cr ? SmA transition.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2014.02.112.
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Acknowledgements
Rajib Nandi is grateful to the University Grants Commission, In-
dia for providing fellowship under Research Fellowship Scheme for
Meritorious Students. HKS, SKS and RKS are grateful to Council of
Scientific and Industrial Research (CSIR), New Delhi, India, for
financial support. SKS and RKS are grateful to DFG, Germany. BS
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