Intramolecular charge transfer interactions and molecular order of rod like mesogens

Article abstract of DOI:10.1039/c5ra22261c

A mesogenic 4-((4-(alkoxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)benzoate series with terminal chains varying from C₂ to C₁₂ carbons (even number carbons only) are synthesised and their mesophase transitions are examined by hot-stage optical polarising microscopy as well as differential scanning calorimetry. Accordingly, enantiotropic nematic mesophase for all the homologs and an additional smectic A phase for the C₁₂ homolog is observed. Powder X-ray diffraction studies confirm the interdigitated bilayer organization in the smectic A phase for the C₁₂ homolog. It is remarkable that the mesogens under investigation only differ in the linking unit i.e. ester versus imine in contrast to recently reported mesogens, yet show a large difference in certain properties. Accordingly, the crystal structure of the C₄ homolog reveals a triclinic lattice with P1 space group in which the molecules are packed in a slipped co-facial configuration. Additionally, a detailed investigation of the C₁₂ mesogen by UV-visible and fluorescence spectroscopy as well as computational methods unveils interesting features. The fluorescence spectrum of the C₁₂ mesogen is observed at 366 nm with a shoulder at 433 nm and a large solvent polarity induced red-shift is noticed in contrast to a structurally similar homolog examined recently. Further, the C₁₂ mesogen in solvents such as ethyl acetate, dichloromethane, chloroform, tetrahydrofuran, acetonitrile and dimethyl sulfoxide exhibited dual emission. Therefore, density functional theory and time dependent density functional theory calculations are utilized to obtain insight. Besides variation in the dihedral angle between rings B and C for the C₁₂ mesogen, it is found that the highest occupied molecular orbital (HOMO) is localized on the N,N-dimethylaminobenzene moiety while the lowest unoccupied molecular orbital (LUMO) is mostly concentrated on the phenyl benzoate unit. Time dependent-density functional theory (TD-DFT) calculations disclose the orbitals involved in the dominant excited state electronic transitions and their corresponding energies together with oscillator strength. The high resolution 1D and 2D separated local field (SLF) solid state ¹³C NMR investigation of the C₁₂ mesogen lead to the orientational order parameters of the phenyl rings of the core in the SmA_d phase. The temperature versus alignment induced chemical shifts reveals an increase in chemical shifts with a decrease in temperature in the smectic A_d phase in concurrence with order parameter values. Thus, understanding the photophysical properties of mesogens with dimethylamino moieties would facilitate better design of molecules for application in organic light emitting diodes for polarized emission.

Full text of DOI:10.1039/c5ra22261c

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PAPER
Intramolecular charge transfer interactions and
molecular order of rod like mesogens†
Cite this: RSC Adv., 2015, 5, 105066
M. Guruprasad Reddy,a Nitin P. Lobo,b E. Varathan,c S. Easwaramoorthic
a
*
and T. Narasimhaswamy
A mesogenic 4-((4-(alkoxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)benzoate series with terminal
chains varying from C2 to C12 carbons (even number carbons only) are synthesised and their mesophase
transitions are examined by hot-stage optical polarising microscopy as well as differential scanning
calorimetry. Accordingly, enantiotropic nematic mesophase for all the homologs and an additional
smectic A phase for the C12 homolog is observed. Powder X-ray diffraction studies confirm the
interdigitated bilayer organization in the smectic A phase for the C12 homolog. It is remarkable that the
mesogens under investigation only differ in the linking unit i.e. ester versus imine in contrast to recently
reported mesogens, yet show a large difference in certain properties. Accordingly, the crystal structure
of the C4 homolog reveals a triclinic lattice with P1 space group in which the molecules are packed in
a slipped co-facial configuration. Additionally, a detailed investigation of the C12 mesogen by UV-visible
and fluorescence spectroscopy as well as computational methods unveils interesting features. The
fluorescence spectrum of the C12 mesogen is observed at 366 nm with a shoulder at 433 nm and a large
solvent polarity induced red-shift is noticed in contrast to a structurally similar homolog examined
recently. Further, the C12 mesogen in solvents such as ethyl acetate, dichloromethane, chloroform,
tetrahydrofuran, acetonitrile and dimethyl sulfoxide exhibited dual emission. Therefore, density functional
theory and time dependent density functional theory calculations are utilized to obtain insight. Besides
variation in the dihedral angle between rings B and C for the C12 mesogen, it is found that the highest
occupied molecular orbital (HOMO) is localized on the N,N-dimethylaminobenzene moiety while the
lowest unoccupied molecular orbital (LUMO) is mostly concentrated on the phenyl benzoate unit. Time
dependent-density functional theory (TD-DFT) calculations disclose the orbitals involved in the dominant
excited state electronic transitions and their corresponding energies together with oscillator strength.
The high resolution 1D and 2D separated local field (SLF) solid state 13C NMR investigation of the C12
mesogen lead to the orientational order parameters of the phenyl rings of the core in the SmAd phase.
The temperature versus alignment induced chemical shifts reveals an increase in chemical shifts with
a decrease in temperature in the smectic Ad phase in concurrence with order parameter values. Thus,
understanding the photophysical properties of mesogens with dimethylamino moieties would facilitate
better design of molecules for application in organic light emitting diodes for polarized emission.
Received 23rd October 2015
Accepted 4th December 2015
DOI: 10.1039/c5ra22261c
www.rsc.org/advances
1. Introduction
Thermotropic liquid crystals are an important class of
molecular materials and their growing prominence is attrib-
uted to a spontaneous response to external stimuli.1 The
supramolecular organization at the molecular level and
macroscopic ow of these materials lead to exciting and
interesting mesophases with intricate morphologies.2 Innu-
merous molecular designs for complex mesogens have
emerged in recent literature with a radical change in synthetic
strategy.3 Despite these exciting developments, the funda-
mental interest on the structure-mesophase correlation of
rod-like (calamitic) mesogens is enduring since subtle
changes in the molecular structure, and dramatic changes in
aPolymer Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai,
India-600020. E-mail: tnswamy99@hotmail.com
bChemical Physics Laboratory, CSIR-Central Leather Research Institute, Adyar,
Chennai, India-600020
cChemical Laboratory, CSIR-Central Leather Research Institute, Adyar, Chennai,
India-600020
† Electronic supplementary information (ESI) available: It contains the synthetic
details, experimental details, single crystal data and check le for BPCPDB, 1H,
13C NMR spectra of synthesized mesogens and powder X-ray diffraction prole
for DdPCPDB in nematic phase at 180 ꢀC. CCDC 1415623. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra22261c
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properties are regularly seen.4 Therefore, the rod-like mole-
cules serve as standard mesogens for evaluating the role of
different moieties such as the core, linking units as well as
terminal chains. In this class of rod-like mesogens, for several
reasons, core unit built with three phenyl rings are distinct.
For example, the presence of three phenyl rings in the core
ensures liquid crystallinity and by judicious selection of
linking units as well as terminal groups, polymesomorphism
can be induced.5 It is well-known that attachment of lengthy
alkoxy chains to the core in rod-like molecules favor smectic
mesophases while shorter chains produce nematic phase.6
Additionally, the insertion of terminal polar groups like NO2,
CN not only favours smectic polymesomorphism but also
ensure the re-entrant phenomena.7,8 Also, thermal stability of
three ring core mesogens is reasonably high in contrast to two
ring molecules due to conjugation of phenyl rings mediated
by linking units which enhance the molecular anisotropic
polarizability.5
2. Experimental section
The synthesised mesogens are listed below with expanded
names and codes:
(a) 4-((4-(Ethoxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)-
benzoate {5(a)}: EPCPDB.
(b) 4-((4-(Butoxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)-
benzoate {5(b)}: BPCPDB.
(c) 4-((4-(Hexyloxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)-
benzoate {5(c)}: HPCPDB.
(d) 4-((4-(Octyloxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)-
benzoate {5(d)}: OPCPDB.
(e) 4-((4-(Decyloxy)phenoxy)carbonyl)phenyl-4-(dimethylamino)-
benzoate {5(e)}: DPCPDB.
(f) 4-((4-(Dodecyloxy)phenoxy)carbonyl)phenyl-4-(dimethyl-
amino)benzoate{5(f)}: DdPCPDB.
(g) 4-(Dodecyloxy)phenyl-4-(benzoyloxy)benzoate (DdPBB).
(h)
4-(((4-Dodecyloxyphenyl)imino)methyl)phenyl-4-(dim-
ethylamino)benzoate (DdIMPDB).16
Molecular materials with dimethylamino group have
attracted considerable attention due to their fundamental
importance in photo-induced intramolecular charge transfer
and their applications in sunscreens.9 Of particular interest is
on 4-dimethylaminobenzoates in which dimethylamino acts
as a donor while the benzoate moiety as acceptor.10 As a result
these molecular systems have been used as important model
molecules in probing many aspects of excited intramolecular
charge transfer state.11 Further, the 4-(N,N-dimethylamino)
benzoate based molecules are found to exhibit twisted intra
molecular charge transfer interactions with dual uores-
cence.12 Despite many studies on dimethylamino based
molecular systems, detailed investigations on mesogens with
dimethylamino group are limited.13 Furthermore, the current
emphasis on donor–acceptor based p-conjugated molecules
for their applications in organic light emitting diodes as well
as organic photo voltaics also augmented the curiosity on
molecules with dimethylamino moiety.14,15 Recently, we re-
ported mesogens consisting of three phenyl ring core with
a terminal N,N-dimethylamino group which exhibited inter-
esting photophysical properties in addition to enantiotropic
mesophases.16 In this work, we report yet another series of
mesogens built with ester linking groups since the imine
linkage has less conformational exibility and less hydrolytic
stability than ester unit. The main motivation of the investi-
gation is the observation of contrasting photo physical prop-
erties as well as the molecular packing of the mesogens
explored in this work as against the earlier series16 though the
change in the molecular structure is subtle. Thus the work
reports besides the crystal structure of C4 mesogen, the liquid
crystalline properties of homologs by hot-stage polarizing
microscopy (HOPM) and differential scanning calorimetry
(i) 4-(((4-(Hexyloxy)phenyl)imino)methyl)phenyl-4-(dimethyl-
amino) benzoate (HIMPDB).16
The experimental protocols and the spectral data of inter-
mediates and the target mesogens are provided in the ESI.†
Instrumental details
FT-IR spectra of the compounds were recorded on ABB
1
BOMEM MB3000 spectrometer using KBr pellet. H and 13C
solution NMR spectra of the mesogen in CDCl3 were
acquired on a Bruker 400 MHz Avance III HD Nano Bay NMR
spectrometer at room temperature using tetramethylsilane
(TMS) as an internal standard. The resonance frequencies of
1H and 13C were 400.23 and 100.64 MHz respectively. Optical
polarizing microphotographs were taken using Carl Zeiss
Axiocam MRC5 polarizing microscope equipped with
a Linkam THMS 600 stage with a TMS 94 temperature
programmer. The samples were placed between 12 mm glass
cover slips and transferred to heating stage and were heated
with a programmed heating rate. The photographs were
taken using imager A2M digital camera. Differential scan-
ning calorimetry traces were recorded using a DSC Q200
instrument with a heating rate of 10 ꢀC minꢁ1 in nitrogen
atmosphere. The data obtained from second heating is
considered for discussion. UV-visible absorption spectra
were measured using Shimadzu UV-1800 spectrophotom-
eter. The steady state uorescence measurements
were carried out by Varian Cary Eclipse uorescence
spectrophotometer.
Computational details
(DSC) as well as photo physical properties. Further, the vari- The ground (S0) geometries of model systems DdPCPDB and
able temperature powder X-ray diffraction (VT-XRD) of one of DdIMPDB were optimized by density functional theory (DFT)
C12 homolog revealed interdigitated bilayer smectic A phase based method with Becke's three-parameter functional and
(SmAd) while the solid state 13C NMR studies offered orienta- the Lee-Yang–Parr functional (B3LYP)17,18 with 6-31G* basis
tional order parameters of the phenyl rings of the core unit in set. Based on the gas phase optimized geometry of
liquid crystalline phase.
DdPCPDB, DdIMPDB and DdPBB spectral properties in
chloroform were calculated by time dependant density
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Scheme 1 Synthetic route adopted for realizing mesogens.
functional theory (TD-DFT) method with Polarizable
Continuum Model (PCM) at B3LYP/6-31G* level. All the
calculations were carried out using Gaussian 09 program
package.19
Powder X-ray measurements
Powder X-ray diffraction (XRD) studies of the un-oriented
samples (Lindemann capillary, diameter of 1 mm, Hampton
Research, Aliso Viejo, CA, USA) were carried out using PAN-
alytical instrument (DY 1042-Empyrean) operating with a line
Fig. 1 HOPM photographs of DdPCPDB on cooling the isotropic
phase (a) birefringent nematic droplets at 185.1 ꢀC, (b) nematic threads
at 178.0 ꢀC, (c) nematic to SmAd phase transition at 176.0 ꢀC and (d) fan
texture of SmAd at 138.3 ꢀC.
Fig. 2 DSC second heating and cooling scans of DdPCPDB.
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Table 1 Transition temperatures and enthalpy values of synthesized mesogens 5(a–f)a
Code
TC–N/C–SmAd (ꢀC)
DH (kcal molꢁ1
)
TSmAd–N (ꢀC)
DH (kcal molꢁ1
)
DT, TSmAd–N
TN–I (ꢀC)
DH (kcal molꢁ1
)
DT, TN–I
EPCPDB
BPCPDB
HPCPDB
OPCPDB
DPCPDB
DdPCPDB
192.2
161.7
121.2
114.7
112.8
109.3
8.76
6.80
4.04
9.22
8.55
6.47
—
—
—
—
—
176.5
—
—
—
—
—
0.0196
—
—
—
—
—
67.2
262.4
233.7
205.8
194.1
186.4
185.2
0.41
0.33
0.31
0.28
0.26
0.24
70.2
72.0
84.6
79.4
73.6
8.7
a
C-crystal; SmAd-interdigitated bilayer smectic A; N-nematic; I-isotropic.
˚
Table 2 Single crystal data for BPCPDB
focused Ni-ltered Cu-Ka (l ¼ 1.54 A) beam and a linear
detector (PIXcel 3D). The sample temperature was controlled
with a precision of 0.1 ꢀC using a heater and a temperature
controller (Linkam).20
Identication code
Empirical formula
Formula weight
Temperature
BPCPDB
26H27NO5
433.49
293(2) K
0.71073 A
C
Wavelength
ꢀ
Solid state NMR measurements
Crystal system, space group Triclinic, P1
Unit cell dimensions
a ¼ 9.8058(2) A, alpha ¼ 64.9330(10) deg.
The solid-state NMR experiments were carried out on a Bruker
b ¼ 15.4400(3) A, beta ¼ 83.4090(10) deg.
1
Avance III HD 400 WB NMR spectrometer (9.4 T). H and 13C
c ¼ 16.9339(3) A, gamma ¼ 88.3180(10)
resonance frequencies were 400.07 and 100.61 MHz, respec-
tively. The spectra in the liquid crystalline phases of the sample
were recorded using a double resonance 5 mm static probe with
a horizontal solenoid coil. For measurement, the sample
alignment was achieved by rst heating to the isotropic phase
deg.
Volume
2306.46(8) A3
4, 1.248 Mg mꢁ3
0.086 mmꢁ1
920
Z, calculated density
Absorption coefficient
F(000)
Crystal size
0.20 ꢂ 0.15 ꢂ 0.10 mm
and then slowly cooling to the respective mesophases. The Theta range for data
1.34 to 26.45 deg.
proton 90ꢀ pulse width was 5 ms. The 1D 13C NMR spectra in
collection
Limiting indices
ꢁ12 # h # 12, ꢁ18 # k # 19, ꢁ21 # l #21
99.6%
SmAd phase was obtained by cross-polarization (CP) scheme
Reections collected/unique 35 532/9485 [R(int) ¼ 0.0644]
with a 50 kHz radio frequency (RF) eld strength on both the 1H
Completeness to theta ¼
and 13C channels during the contact time of 3 ms, number of
26.45
scans 128 and a recycle delay of 8 s. High resolution 2D SLF Absorption correction
Semi-empirical from equivalents
Full-matrix least-squares on F2
Max. and min. transmission 0.9914 and 0.9829
spectra were obtained under static condition using the SAMPI-4
pulse sequence that correlates the 13C chemical shi with the
associated 13C–1H dipolar coupling. Experimental conditions
for SAMPI-4 pulse scheme for liquid crystalline samples were
Renement method
Data/restraints/parameters 9485/0/583
Goodness-of-t on F2
0.927
Final R indices [I > 2sigma(I)] R1 ¼ 0.0510, wR2 ¼ 0.1038
described in our earlier work.21 The CP contact time s was 3 ms, R indices (all data)
R1 ¼ 0.1539, wR2 ¼ 0.1445
Largest diff. peak and hole 0.206 and ꢁ0.174 e Aꢁ3
number of t1 increments was 128 and the signal was acquired
with 24 transients for each t1 increments. The recycle delay of 15
s was used to minimize RF heating effects. The data were zero
lled in both the t2 and t1 dimensions, yielding a 4096 ꢂ 256
real matrix. A shied sine bell window function was applied to
the time domain data and the spectrum was processed in the
CCDC no.
1415623
phase sensitive mode. SPINAL-64 (ref. 22) heteronuclear
decoupling scheme with a 30 kHz RF eld strength was used in
all the experiments. The sample temperature was regulated by
a Bruker BVTB-3500 variable temperature unit and measured
from 207Pb NMR chemical shi of Pb(NO3)2.23 The adamantane
methine carbon signal at 29.5 ppm was used as an external
13
reference for C chemical shis calibration.24
3. Results and discussion
Scheme 1 shows the synthetic route implemented for realising
the mesogens. Thus, 4-alkoxy phenols are condensed with 4-
dimethylamino benzoyloxy benzoic acid which is prepared from
4-dimethylamino benzoic acid and 4-hydroxy benzaldehyde
Fig. 3 ORTEP diagram of BPCPDB.
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compare with those reported recently.16 Hence, sufficient
structural characterization is attempted using the HOPM, DSC,
single crystal as well as powder XRD, UV-visible absorption and
uorescence spectroscopy. Further, highest occupied molecular
orbital-lowest unoccupied molecular orbital (HOMO–LUMO) by
DFT as well as TD-DFT calculations are carried out to get the
molecular insight on the photo physical properties. The high
resolution solid state 13C NMR of representative mesogen
(DdPCPDB) is accomplished to get the molecular order.
HOPM, DSC and XRD studies
The synthesised mesogens are investigated using HOPM and
DSC to identify and establish the liquid crystalline properties.
In HOPM, the samples are taken to isotropic phase and on
cooling, the phase formed is identied by noticing the charac-
teristic textures. The existence of nematic phase for all the
mesogens is established by seeing the birefringent droplets.25
On further cooling, the droplets coalesced to give threaded
texture. For the case of DdPCPDB, the C12 homolog, in addition
to nematic phase, SmAd is also noticed (Fig. 1). The sample on
cooling the isotropic phase showed threaded nematic texture
and at 176.5 ꢀC, the sample changes in to fan texture signifying
the phase change.26 The HOPM observations are supported by
DSC studies and accordingly two transitions i.e. crystal–nematic
and nematic–isotropic are noted. However, for DdPCPDB, an
additional transition at 109.3 ꢀC is observed which is attributed
to crystal-SmAd phase (Fig. 2). Table 1 lists the transition
temperatures and the enthalpy values of all the synthesized
mesogens. The interesting feature of the DSC data is low tran-
sition enthalpy for SmAd to nematic phase indicating pseudo
rst order nature of the transition. The general trends observed
from the HOPM and DSC studies include the progressive
decrease in the nematic–isotropic transition temperatures (TN–I
)
with increase in terminal chain length. Further, the appearance
of SmAd phase only in DdPCPDB is owing to the presence of
lengthy dodecyloxy chain which enhances the aspect ratio and
favour the segregation of core and terminal chains leading to
lamellar order.27 The mesophase range ꢃ70–85 ꢀC for the
homologs suggests that the molecular anisotropic polarizability
is good. This can be appreciated since the core unit has three
phenyl rings that are connected by ester groups which mediate
the conjugative interactions. Further, the terminal alkoxy and
dimethylamino also enhance the anisotropic interactions in the
molecules due to the hetero atoms. A comparison of mesophase
transitions of earlier mesogens with the current homologs
indicate marginally higher mesophase range for earlier series
which could be attributed to the presence of rigid imine link in
Fig. 4 Powder X-ray diffraction profile for DdPCPDB SmAd phase (a) at
110 ꢀC, (b) at 120 ꢀC, and (c) at 165 ꢀC.
followed by oxidation. The terminal alkoxy chain is varied from the core. The SmAd mesophase range, on the other hand, found
C2 to C12 with even number carbons only. The structural to be high for DdPCPDB than DdIMPDB indicating the inu-
assignment of all the synthesized mesogens in solution is ence of two ester linking units.
established by 1H and 13C NMR spectral techniques (Fig. S1 and
The important and consist feature of the DdIMPDB as well
S2 of ESI†). As stated earlier, the present homologs are struc- as DdPCPDB is the observation of SmAd phase for respective
turally similar to recently reported dimethylamino based C12 homologs. Usually, the interdigitated partial bilayer
mesogen series and only differ in the second linking unit.16 The organization in smectic A phase is noticed for rod-like mole-
importance of the present work is to explore the photophysical cules possessing terminal polar cyano or nitro groups.28
as well as mesophase properties of the current mesogens and However, the similar organization in dimethylamino based
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Fig. 5 (a) Molecular structure and (b) energy optimized space filled model, magnitude and direction of dipole moment of DdPCPDB.
Fig. 7 UV-visible absorption (solid line) and fluorescence (dashed line)
spectra of compounds DdPCPDB and DdIMPDB in chloroform. The
fluorescence spectra recorded by exciting the samples at their
respective absorption maximum wavelength.
Fig. 6 Solution absorption spectra of DdPCPDB in different solvents
with varying polarity.
rod-like mesogens is not known in the literature. In order to
understand the SmAd formation in dimethylamino based
mesogens, the existing model for the polar mesogens is criti-
cally examined. Due to the presence of nitro or cyano group at
one end of the core unit, the terminal dipoles contribute for
the overall polarity of the rod-like mesogens. In such meso-
gens, in order to minimize the dipolar energy, the molecules
adopt antiparallel packing leading to smectic Ad formation.
Such an arrangement is favored as it provides better way to
escape from macroscopic polarity. Similar arguments can be
considered for explaining the SmAd organization in dimethy-
lamino based mesogens. In these molecules owing to the
presence of excellent electron donating dimethylamino group,
the rest of the core unit acquires negative charge resulting in
polarization of the molecules. In order to overcome the dipolar
energy, these molecules also adopt antiparallel packing
leading to smectic Ad phase. Thus, the main difference
between the mesogens with polar cyano or nitro versus dime-
thylamino is the difference in the location of the charge. In
other words, in terminal polar mesogens, the negative charge
rests on polar group while in mesogens with dimethylamino
group, the negative charge locates on the core unit. Hence, the
antiparallel packing of neighboring molecules in these
mesogens leads to SmAd organization.
The single crystal structure of BPCPDB is show in Fig. 3 and
the data is listed in Table 2 (Table S1–S7 of ESI†). The molecular
packing in the crystal leads to triclinic lattice with P1 space
group. Interestingly, the HIMPDB, reported recently, also
showed triclinic structure. However, the BPCPDB mesogen
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ꢀ
˚
165 C is found to be 16.42 A which is equal to the length of
dodecyloxy chain. This suggests the antiparallel arrangement of
neighboring molecules with overlapping of rigid cores leading
to SmAd organization.
Effect of linking units on twisted intramolecular charge
transfer interactions
Intramolecular charge transfer interactions of DdPCPDB and
DdIMPDB that originates from N,N-dimethylaminobenzoate
moiety and the inuence of remotely substituted carboxylate
and azomethine groups are studied using computational and
experimental methods. N,N-Dimethylamino benzoate, a well-
known molecular probe oen discussed for twisted intra-
molecular charge transfer interactions (TICT) shows character-
istic dual emission corresponding to the radiative transition
from local excited (LE) state and TICT state.32,33 UV-visible
absorption spectrum of the compounds DdPCPDB in different
solvents with varying polarity is shown in Fig. 6. For instance,
the measurement in chloroform solvent shows absorption band
at 328 nm as depicted in Fig. 6. It is clear from the Fig. 7 that
DdIMPDB exhibit slightly broader absorption band (FWHM ¼
4430 cmꢁ1) as against to DdPCPDB (FWHM ¼ 3330 cmꢁ1). The
uorescence spectrum (Fig. 8) of DdPCPDB is observed at 366
nm with a shoulder at 433 which is tailing over 500 nm. On the
other hand, the uorescence spectrum of DdIMPDB, is
comparatively broad with a maximum at 485 nm. Despite
similar chromophoric skeleton, a large red-shi in uorescence
ca. 119 nm for DdIMPDB in contrast to DdPCPDB is really
intriguing. This could be due to replacement of carboxylate
group of DdPCPDB with azomethine unit in DdIMPDB. The
calculated Stokes shis for DdPCPDB and DdIMPDB respec-
tively are 3540 and 9960 cmꢁ1 which suggests that the structural
change between the ground and excited state is more predom-
inant for DdIMPDB. Further, the larger Stokes shi for
DdIMPDB can also be due to the intramolecular charge transfer
interactions between the electron rich N,N-dimethylamino-
benzene (donor) and electron decient ester group (acceptor).
Moreover, to understand the intramolecular charge transfer
interactions, solvent polarity dependent uorescence spectral
studies are carried out. In our recent report, it is observed that
DdIMPDB with N,N-dimethylaminobenzoate moiety does not
show any dual emission, a characteristic feature of TICT inter-
actions.16 Indeed DdIMPDB exhibit negative solvatochromism
in uorescence, a rare phenomenon, suggesting a large dipole
moment in the ground state than in the excited state. On the
other hand, DdPCPDB shows progressive red-shi in uores-
cence while going from nonpolar toluene to polar acetonitrile
solvent. Interestingly, only one emission peak is observed in
hexane, toluene and methanol respectively at 344, 358 and 367
nm, while in ethyl acetate, dichloromethane, chloroform,
tetrahydrofuran, acetonitrile and dimethyl sulfoxide, two peaks
are found characteristic to the nature of the solvent. The higher
energy emission originates from the local excited (LE) state or
Franck Condon state and the anomalous, second emission peak
observed at longer wavelength region would have been origi-
nated from the TICT state. The ratio of the relative intensity
Fig. 8 Solution fluorescence spectra of DdPCPDB in different polarity
solvents.
shows parallel packing with slipped co-facial arrangement
whereas HIMPDB exhibited antiparallel packing.16 Thus subtle
alteration in the molecular structure, the spatial arrangement of
molecules in crystalline solid state is changed even though the
triclinic lattice is common for both the mesogens. Despite the
similar crystal lattice, the variation in molecular packing i.e.
antiparallel versus parallel packing for HIMPDB and BPCPDB
could be argued based on the dipole moment values. The DFT
calculations of quantum chemical methods reveal the dipole
moments for BPCPDB and HIMPDB as 7.19 D and 4.77 D
respectively. Thus the very high dipole moment for BPCPDB
induced by the second ester linking unit could be responsible
for slipped co-facial arrangement of the molecules in crystal
lattice as against to HIMPDB where anti parallel packing is
preferred.
For DdPCPDB which exhibit both nematic and SmAd phases,
the powder X-ray diffraction is carried out in both the phases to
further conrm the phase assignment. Fig. 4 shows the XRD
prole recorded at three different temperatures in SmAd phase
(110–165 ꢀC) while the XRD scan in nematic phase is shown in
ESI (Fig. S3†). In the nematic phase, in the small angle region,
no sharp reection is noticed while a broad hump is clearly
observed at wide angle. On the other hand, the XRD prole of
SmAd phase (Fig. 4) shows a sharp and intense reection in
small angle region and broad diffuse peak at the wide angle
region. The broad hump in the wide angle region indicates the
absence of in plane order and liquid like nature of molecules in
the layer.29 These features support the layer ordering typical of
smectic mesophase.30 The layer spacing (d) determined from
small angle reection at 110 ꢀC, 120 ꢀC and 165 ꢀC is found to be
˚
54.46, 53.47 and 51.90 A respectively. Further, d/L ratios in
which L represents the length of the DdPCPDB mesogen (35.48
˚
A) calculated from energy optimized structure by DFT method
of quantum chemical calculations (Fig. 5) are in the range of
˚
1.53–1.46 A. Thus the d/L ratio which is higher than 1.1 and
lower than 2 suggest that the phase is partially bilayer smectic A
phase (SmAd).31 Further, d–L value for the measurement at
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Fig. 9 Optimized geometries of DdPCPDB, DdIMPDB and DdPBB at B3LYP/6-31G* level of theory.
between the TICT and LE state is also found to depend upon the charge transfer interactions. To get more insight in to the
solvent. Particularly, in ethyl acetate, dichloromethane, chlo- experimental ndings, DFT and TD-DFT calculations are con-
roform, tetrahydrofuran, acetonitrile and dimethylsulfoxide ducted with the Gaussian 09 program using the B3LYP method
higher ratio is noticed in contrast to other non-polar solvents. and 6-31G* basis set. Fig. 9 displays the optimized molecular
The subtle structural differences between DdPCPDB and structures wherein, the phenyl rings are designated as A, B, and
DdIMPDB, though not directly linked to the N,N-dimethyla- C for clarity. As can be seen from the dihedral angles summa-
mino benzoate has drastic changes in the intramolecular rized in Table 3, almost similar value is observed between the
Table 3 Calculated dihedral angles using B3LYP/6-31G* optimized geometries
Dihedral angle
Compounds
C1–C2–C3–O4
C3–O4–C5–C6
C7–C8–C9–O10
C9–O10–C11–C12
DdPCPDB
DdIMPDB
DdPBB
179
179
178
141
142
140
179
178
179
132
150
132
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Fig. 10 FMO distribution of DdPCPDB and DdIMPDB (isosurface value ¼ 0.03 au) at B3LYP/6-31G* level of theory.
ring A and B. This suggests that neither ester nor azomethine Table 5 Calculated HOMO, LUMO and HOMO–LUMO gap (DH–L) at
B3LYP/6-31G* level
between B and C exerts geometrical impact on dimethylamino
benzoate group. However, the dihedral angle between rings B
Compounds
HOMO
LUMO
DH–L
and C (C9–O10–C11–C12) is found to vary signicantly from 150ꢀ
to 132ꢀ. It is clear that, ester substituted mesogens (DdPCPDB
and DdPBB) loses co-planarity at ring B which in turn reduces
the extended p-conjugation in contrast to imine based mesogen
(DdIMPDB). As a result of loss of co-planarity in DdPCPDB, the
ring B and C with delocalized p-conjugation acts as a chromo-
phoric unit, whereas for DdPCPDB and DdPBB, the ring A and B
are responsible for electronic properties. These ndings are
akin with frontier orbital contour plots as shown in Fig. 10. For
mesogens DdPCPDB, the HOMO is localized on N,N-dimethy-
laminobenzene moiety, and LUMO is predominantly concen-
trated on the phenyl benzoate unit. In the case of DdIMPDB and
DdPBB, the HOMO is concentrated on the benzylidene-phenyl
unit, whereas LUMO is delocalized on the phenyl benzoate
moiety. For all the cases, the HOMO / LUMO transition
induces signicant redistribution of electron density due to
intramolecular charge transfer interactions with characteristic
to the nature of the substituent.
DdPCPDB
DdIMPDB
DdPBB
ꢁ5.55
ꢁ5.22
ꢁ5.75
ꢁ1.26
ꢁ1.34
ꢁ1.65
4.28
3.89
4.09
charge transfer peak is observed at 340.3, 320.5 and 320.2 nm
respectively, which constitutes the HOMO / LUMO transition
(Table 5).
High resolution solid state 13C NMR spectroscopy
The solution proton decoupled 13C NMR spectrum of DdPCPDB
(CDCl3) is shown in Fig. 11(a) where well resolved lines are seen
from 14–165 ppm. In the range 110–165 ppm, 14 lines with
varying intensities arising from the core unit carbons are
observed. Among them, six lines with comparable intensity are
assigned to three phenyl ring CH carbons. The remaining low
intense lines are due to quaternary carbons of phenyl ring as
well as ester carbonyl carbons. The individual assignment of all
the carbons are carried out by making use of 13C chemical shis
computed from DFT calculations. Table 6 list the experimental
13C chemical shis as well as those calculated from DFT
method of core unit carbons. For the terminal units i.e. dode-
cyloxy chain and dimethylamino carbons, the lines have
TD-DFT calculations are also performed to nd out the
orbitals involved in the dominant excited state electronic tran-
sitions and their corresponding energies and oscillator strength
and the results are listed in Table 4. A close analysis of these
values shows that the calculated spectral properties of both the
model systems are in close agreement with experimental values.
In all the systems (DdPCPDB, DdIMPDB and DdPBB), the
Table 4 Summary of the excited state electronic transitions obtained from the TD-DFT calculations at B3LYP/6-31G* level
Compounds
DdPCPDB
Solvent
CHCl3
States
Absorption (nm)
Energy (eV)
Oscillator strength (f)
Dominant contributiona (%)
Exp. (nm)
328
S1
S2
S3
S1
S2
S1
S2
S3
340.3
390.4
291.9
320.5
266.3
330.2
270.0
241.3
3.64
3.88
4.25
3.87
4.66
3.75
4.59
5.14
0.4324
0.1255
0.6476
0.9480
0.6233
0.1335
0. 5681
0.2942
H / L (98%)
Hꢁ1 / L (94%)
H / L+1 (94%)
DdIMPDB
DdPBB
CHCl3
CHCl3
H / L (67%), H / L+1 (18%)
Hꢁ1 / L (38%), Hꢁ6 / L (28%)
H / L (90%)
H / L (94%)
Hꢁ5 / L (55%), Hꢁ1 / L+1 (34%)
328
338
a
H denotes HOMO and L denotes LUMO.
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Fig. 11 13C NMR spectra of DdPCPDB in (a) solution, SmAd phase (b) at
115 ꢀC and (c) at 155 ꢀC.
Fig. 12 2D SAMPI-4 spectra of DdPCPDB in SmAd phase (a) at 115 ꢀC
and (b) at 155 ꢀC.
appeared in the range 14–69 ppm. Of them, the OCH2 carbon of
dodecyloxy chain is easily identied at 68.4 ppm whereas the
methyl carbons of dimethylamino unit are seen at 40.0 ppm.
The terminal methyl carbon of the dodecyloxy chain is noted at
14.1 ppm and rest of the methylene carbons are noticed in the
range 22–32 ppm.
than SmAd phase (ꢃ67 C), the 13C NMR experiments are per-
ꢀ
formed only in SmAd phase. The spectrum shows 9 peaks in the
range 132–234 ppm contributed by core unit carbons arising
from phenyl rings as well as ester linking units. The peaks
appeared in the range 132–165 ppm are relatively more intense
than the remaining (173–234 ppm) peaks. These high intense
The static ꢀ13C NMR spectra of DdPCPDB recorded in SmAd
ꢀ
phase at 115 C and 155 C are shown in Fig. 11(b) and (c). As
the mesogen exhibits narrow nematic mesophase range (ꢃ8 ꢀC)
Table 6 13C NMR data of DdPCPDB in solution and liquid crystalline statea
115 ꢀC
155 ꢀ
C
SmAd
phase
(ppm)
13C–1H
dipolar oscillation
frequencies (kHz)
SmAd
phase
(ppm)
13C–1H
dipolar oscillation
frequencies (kHz)
C.
Solution
DFT
(ppm)
AIS
(ppm)
AIS
(ppm)
No (ppm)
1
2
3
4
5
6
7
8
153.9
110.8
132.2
115.3
164.9
155.7
122.4
131.7
126.7
165.0
144.2
122.2
115.1
156.9
150.4
108.3
132.6
116.5
161.6
156.3
121.1
132.0
125.3
162.6
144.7
123.9
116.1
156.1
219.1
132.4
164.3
178.1
209.0
214.7
152.6
163.9
198.0
204.6
214.7
152.3
142.5
233.4
65.2
21.6
32.1
62.8
44.1
59.0
30.2
32.2
71.3
39.6
70.5
30.1
27.4
76.5
1.64
3.19
2.71
1.59
0.60
1.63
2.58
2.71
1.65
0.56
1.63
2.58
2.80
1.64
211.1
130.2
159.8
170.9
203.0
205.9
148.0
159.8
188.7
200.2
205.9
148.0
139.0
223.9
57.2
19.4
27.6
55.6
38.1
50.2
25.6
28.1
62.0
35.2
61.7
25.8
23.9
67.0
1.49
2.81
2.44
1.44
0.64
1.47
2.36
2.44
1.52
0.99
1.47
2.36
2.50
1.48
9
10
11
12
13
14
a
DFT-density functional theory; CS-chemical shis; AIS-alignment induced chemical shis.
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Table 7 Orientational order parameter values of DdPCPDB
Calculated dipolar oscillation
frequencies (kHz)
Angles
T (ꢀC)
Ring
qb
qc
Szz
(Sxx ꢁ Syy)
b
c
a
d
RMSD (kHz)
115
I
119.3
121.2
121.2
119.5
121.1
121.0
120.7
120.8
120.5
120.8
120.9
120.5
0.70
0.71
0.70
0.63
0.64
0.63
0.068
0.066
0.070
0.062
0.058
0.060
3.18
2.59
2.59
2.81
2.36
2.37
2.74
2.72
2.81
2.44
2.41
2.51
1.59
1.66
1.65
1.44
1.49
1.47
1.63
1.65
1.63
1.47
1.49
1.46
0.03
0.02
0.01
0.03
0.03
0.01
II
III
I
II
III
155
the chemical shis of core unit carbons and a marginal
decrease for the terminal chain carbons indicate that the
alignment of the long axis is parallel to the magnetic eld.34
Similar trends are noticed for the static spectrum measured at
155 ꢀC in SmAd and the corresponding chemical shi values are
listed in the Table 6.
The 2D solid state 13C NMR of DdPCPDB is carried out in
SmAd phase with a view to nd the molecular order using the
13C–1H dipolar couplings. Typically the 2D spectrum provides
the information about the carbon chemical shi and 13C–1H
dipolar couplings which can be utilized to nd the order
parameter of phenyl rings of the core unit. The 2D spectra
measured at 115 ꢀC and 155 ꢀC in SmAd phase are shown in
Fig. 12(a) and (b). Similar to the 1D static spectrum, the 2D also
shows 9 contours in the range 132–234 ppm which are mainly
arising from core unit carbons. The important features of the
2D spectra is observation of four contours for phenyl ring
methine carbons in the range 132–165 ppm among which the
contours appeared at 164.3 and 152.6 ppm are more intense and
is due to the overlapping of C3 and C8 as well as C7 and C12
carbons respectively. A close examination of 2D spectrum in the
range 204–210 ppm, however, shows low intense contours from
ester linking units. The chemical shi assignment of ring I
carbons is carried out using the 13C NMR data of recently re-
ported mesogen which also shows SmAd phase and has struc-
turally similar ring I.16 For ring III also 13C NMR data in liquid
crystalline phase is available from the literature.35 Table 6 lists
the 13C–1H dipolar couplings of core unit carbons measured in
SmAd phase at two different temperatures.
Fig. 13 Plot of temperature versus alignment induced chemical shifts
(AIS) in SmAd of DdPCPDB.
signals are assigned to phenyl ring methine carbons of core
units. In solution 13C NMR spectrum, six lines are clearly
observed for the methine carbons of three phenyl rings of the
core unit. The less number of peaks observed in the static
spectrum indicates that some of the carbons are overlapping
due to the similar chemical environment. The complete
assignment of the spectrum is carried out by making use of 13
C
NMR data of recently published mesogen as well as with the use
of 2D SLF spectrum.16 For the terminal chain carbons, the peaks
are observed in the range 11–63 ppm. Among them, the
distinguishable peaks are from OCH2 carbon (62.2 ppm),
methyl carbons of dimethyl group (40.5 ppm) and terminal
methyl carbon (11.5 ppm). The methylene carbons of dodecy-
loxy chain, on the other hand, are noticed in the range 20–28
ppm. It is clear from the comparison of chemical shi values of
solution and static 13C NMR spectrum that in SmAd phase, the
molecules are aligned in the magnetic eld. Further, increase in
By making use of 13C–1H dipolar couplings and employing
the eqn (1) mentioned below, the order parameter of the phenyl
rings of the core unit is determined. The method adopted for
extracting the dipolar couplings from experimentally deter-
mined dipolar oscillation frequency and their use for deter-
mining the order parameter is briey mentioned. The dipolar
oscillation frequency corresponds directly to the dipolar
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coupling only for an isolated C–H pair whereas for a carbon interdigitated bilayer organization in smectic A phase. In these
coupled to more than one proton, the dipolar couplings have to molecules owing to the presence of excellent electron donating
be extracted from the oscillation frequency. For instance, in the dimethylamino group, the rest of the core unit acquires nega-
case of phenyl ring methine carbons, the experimental dipolar tive charge resulting in polarization of the molecules. In order
frequency has two contributions namely, a coupling to the to overcome the dipolar energy, these molecules adopt anti-
attached proton DC–Hi and also a coupling to the remote proton parallel packing leading to the formation of partial bilayer
at the ortho position DC–Ho. Then the measured frequency is smectic A organization. Further, the slipped co-facial arrange-
2
+ (DC–Ho)2]1/2
.
For the case of quaternary ment of molecules in crystal lattice for BPCPDB is attributed to
36
given by [(DC–Hi
)
carbons, couplings to two equivalent protons at the ortho high dipole moment (7.19 D) in contrast to C6 mesogen of
positions are possible and hence the dipolar frequency will be earlier series. Interestingly, UV-visible and uorescence spec-
O2 ꢂ (DC–Ho). Further, these dipolar couplings are related to the troscopy accompanied by computational methods disclose the
local order parameters, namely, Szz and (Sxx ꢁ Syy) by the existence of twisted intramolecular charge transfer state.
following equation,37
Further the computational methods provided the information
about the dihedral angles between the rings which were corre-
lated to the delocalized p-conjugation of chromophoric unit.
The TD-DFT calculations disclosed the orbitals involved in the
dominant excited state electronic transitions and their corre-
sponding energies together with oscillator strength. A close
scrutiny of these values shows that the calculated spectral
properties of both the systems were found to be in close
agreement with experimental values. Thus the charge transfer
peak observed at 340.3, 320.5 and 320.2 nm respectively were
originated from HOMO / LUMO. The orientational order
parameters of the core unit in SmAd phase determined from the
high resolution solid state 13C NMR are found to be 0.71 and
DCH ¼ K[1/2(3 cos2 qz ꢁ 1)Szz + 1/2(cos2 qx
ꢁ cos2 qy)(Sxx ꢁ Syy)]
(1)
where K ¼ ꢁhgHgC/4p2rCH3, with gH and gC are the gyromagnetic
1
ratios of H and 13C nuclei respectively, rCH is the inter-nuclear
vector, qx, qy and qz are angles between rCH and the correspond-
ing molecular axes. For the phenyl rings, the molecular frame is
dened by taking para-axis (C2 axis) as the z-axis, the x-axis is in
the plane of the ring while the y-axis is perpendicular to the
˚
plane. Further, the standard bond distances of rCH ¼ 1.1 A for the
˚
C–H bond and rCC ¼ 1.4 A for the C–C bond are considered. In
order to arrive at the best t, the two C–C–H bond angles are also
varied slightly ꢃ120ꢀ. By employing the eqn (1), the local order
parameters Szz and (Sxx ꢁ Syy) are calculated from the experi-
mentally determined dipolar frequencies for each of the phenyl
rings at different temperatures in SmAd phase and are listed in
Table 7. It is clear from the Table 7 that the ring II order
parameter is slightly higher than ring I as well as ring III, a trend
normally encountered in three ring mesogens.38,39 The Szz values
are in consistent with the literature data of those rod-like
mesogens that exhibit SmAd phase.5,39
ꢀ
0.64 at 115 ꢀC and 155 C respectively.
Acknowledgements
The authors thank Dr A. B. Mandal, Outstanding Scientist, CLRI
and Prof. K. V. Ramanathan, NMR Research Centre, Indian
Institute of Science, Bangalore, India for their support and keen
interest in this work. The authors are thankful to Dr V. Sub-
ramanian, CLRI, for his help and support. We are also indebted
to Prof. V. A. Raghunathan and Ms K. N. Vasudha, Raman
Research Institute, Bangalore, India for the powder X-ray
measurements. The partial nancial support from NWP-55 is
duly acknowledged. MGR thanks Council of Scientic and
Industrial Research (CSIR), New Delhi for Senior Research
Fellowship.
Since the 1D static 13C experiments are carried out at different
temperatures covering SmAd phase for DdPCPDB, AIS vs.
temperature data is plotted as shown in Fig. 13. It is clear from
the plot that with increase in temperature a decrease in AIS
values is noticed. The trend observed in the plot is in consistent
with the order parameters calculated from 13C–1H dipolar
couplings. It is pertinent to mention that in a recent work,
Domenici et al. have investigated the 13C NMR of three ring
mesogens which shows smectic de Vries phase. The study
revealed that at the phase transition temperature, conforma-
tional change of the molecules is clearly noticed. However, in the
present work such conformation change is not observed with in
the SmAd phase for which the chemical shi data is available.
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