Photoluminescent discotic liquid crystals derived from tris(N-salicylideneaniline) and stilbene conjugates: Structure-property correlations

Article abstract of DOI:10.1016/j.dyepig.2016.05.010

Luminescent discotic liquid crystals based on tris(N-salicylideneaniline)s [TSANs] bearing trans-stilbene fluorophores have been synthesized. Their mesomorphic and photophysical properties were studied. These TSANs existing in the form of C_3h and C_s geometrical isomers of the keto-enamine tautomer self-assemble to form the columnar phase and exhibit fluorescence both in solution and mesophase states. These columnar mesophases also exhibit frozen (glassy) columnar phase, which ensures preserved fluorescence intensity, defect free alignment and simultaneous restriction on the motion of ionic impurities. These features make them promising candidates for the use in organic light emitting diodes, particularly as emissive layers. This study also led to an understanding about the dependence of their mesomorphism (mesophase type, stability and thermal range) and photophysical features on the number and pattern of their peripheral substitution, in comparison to the analogous columnar liquid crystalline TSANs reported earlier.

Full text of DOI:10.1016/j.dyepig.2016.05.010

Dyes and Pigments 132 (2016) 291e305
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Photoluminescent discotic liquid crystals derived from
tris(N-salicylideneaniline) and stilbene conjugates:
Structureeproperty correlations
,
Ammathnadu S. Achalkumar b *, B.N. Veerabhadraswamy a, Uma S. Hiremath a,
Doddamane S. Shankar Rao a, Subbarao Krishna Prasad a,
,
**
Channabasaveshwar V. Yelamaggad a
a Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore 560 013, India
b Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Luminescent discotic liquid crystals based on tris(N-salicylideneaniline)s [TSANs] bearing trans-stilbene
fluorophores have been synthesized. Their mesomorphic and photophysical properties were studied.
These TSANs existing in the form of C3h and Cs geometrical isomers of the keto-enamine tautomer self-
assemble to form the columnar phase and exhibit fluorescence both in solution and mesophase states.
These columnar mesophases also exhibit frozen (glassy) columnar phase, which ensures preserved
fluorescence intensity, defect free alignment and simultaneous restriction on the motion of ionic im-
purities. These features make them promising candidates for the use in organic light emitting diodes,
particularly as emissive layers. This study also led to an understanding about the dependence of their
mesomorphism (mesophase type, stability and thermal range) and photophysical features on the
number and pattern of their peripheral substitution, in comparison to the analogous columnar liquid
crystalline TSANs reported earlier.
Received 16 February 2016
Received in revised form
19 April 2016
Accepted 9 May 2016
Available online 10 May 2016
Keywords:
Stilbene
OLEDs
Liquid crystals
Columnar phase
Glassy state
© 2016 Elsevier Ltd. All rights reserved.
1. Introduction
power consumption make OLEDs even more attractive and hopeful
media in display screens. However, all such important features of
The discovery of electroluminescence over five decades ago in
organic semiconductors [1] gave rise to a thriving and promising
research area of organic light emitting diodes (OLEDs). OLEDs,
essentially comprising the organic emitter layer, have been regar-
ded as the forerunner of modern advanced display technology.
Over the years it is well demonstrated that OLED devices have
distinct advantages [2,3] over their nearest competitors like liquid
crystal displays (LCDs) or cathode ray tube (CRT) devices. They can
be effectively used in fabricating micro- to large-size flat (stiff)
panel displays, flexible plastic displays, screens fixed into clothes/
wall hangings, etc. The brighter and higher quality of image, better
contrast, tunability of the color, reduced viewing angle problem,
non-requirement of backlighting, and more importantly the lower
OLED panels vitally depend on the various physical properties of
organic material(s) used. Such molecules comprising chromo-
phores basically absorb incident energy and emit light in the visible
regime. The color of the emitted light (red, green, and blue) largely
depends on the composition of the material. Consequently, the
field, involving molecular design and synthesis, has well-
broadened to include every class of materials viz., small mole-
cules, oligomers, organometallics, polymers etc. [4].
Discotic liquid crystals (discotics), generally formed by cova-
lently linking several paraffinic chains to a disk-like (flat) aromatic
central ring, fall under the class of small molecules. Generally,
owing to strong noncovalent interactions among aromatic cores
(pep stacking), they self-assemble to form fluid columnar (Col)
structures characterized by indefinitely long molecular columns
aggregating into two-dimensional (2D) lattices of varying sym-
metries. The innermost aromatic ring of a disc-like molecule acts as
a conducting unit, while peripheral paraffinic tails act as an insu-
lator. Consequently, the assembly of such molecules into 1D Col
* Corresponding author.
** Corresponding author.
E-mail addresses: achalkumar@iitg.ernet.in (A.S. Achalkumar), yelamaggad@
cnsms.com, yelamaggad@gmail.com (C.V. Yelamaggad).
http://dx.doi.org/10.1016/j.dyepig.2016.05.010
0143-7208/© 2016 Elsevier Ltd. All rights reserved.

292
A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
stacks assists smooth charge-transport along columns than across
the columns; thus, they act as molecular wires. Columnar phases
are the latest entrants in the field of OLEDs. In fact, the 1D columnar
arrays are the ideal alternatives for organic single crystals or
amorphous conjugated polymers which are finding applications in
OLEDs [5e8], solar cells [9e11], and field effect transistors [12e14].
This increasing acceptance of Col phases in such devices, arises
from their inherent properties such as its anisotropic conductivity,
control over the molecular order, solution processability and
structural self healing. Most importantly all these properties can be
tailored by careful molecular engineering which is difficult in the
case of single crystals or conjugated polymers. In the case of OLEDs,
Col phases are mainly used as hole carrying or electron carrying or
emissive layers. Majority of disc-like mesogens like triphenylene,
dibenzopyrene, and hexabenzocoronene etc. comprise electron-
rich (donor) aromatic cores [15e19] and are known as good hole
transporters, whereas DLCs derived from electron-deficient sys-
tems are limited in number. There are few examples like coronene
derivatives substituted with electron withdrawing groups or sys-
tems based on heterocyclic cores such as triazines, quinoxalines
[15e19], hexaazatriphenylene [20] and hexaazatrinaphthylene
[21,22] etc. Unfortunately, most self-assembling aromatic systems
because of their poor emissive nature cannot be used as emissive
layers in OLEDs. Thus many research groups are working on the
design of columnar luminescent LCs which guarantee the orienta-
tionally ordered luminophores [23e29]. Material chemists are in
the hot pursuit of molecules which combine the hole carrying,
electron carrying and emissive features in a single molecule to
avoid the complexities and associated problems in the fabrication
of OLEDs with enhanced efficiencies [30].
Tris(N-salicylideneaniline)s (TSANs) [31e42], relatively a new
class of discotics are encouraging as they can be easily tailored to
introduce features like luminescence, self-organizing ability, mo-
lecular recognition etc. [31e42]. McLachalan et al., reported that
this new class of molecules are existing as an inseparable mixture of
C3h and Cs symmetric regioisomers of the keto-enamine tautomer
[31]. With the introduction of peripheral alkyl tails, the same
compounds stabilized Col phase/s [33]. Further studies revealed the
promising inherent luminescence behavior of these compounds
[34e39]. Lee et al., have shown that intelligently modified the
structure of TSANs, which led to the self-assembly of C3h isomers in
such a way that it enhanced fluorescence efficiency in solutions
[40e42]. It is also postulated that the columnar organization of
discotic molecules with fluorophores is beneficial for the energy
transfer because of the effective Forster mechanism [43]. As part of
our ongoing research in luminescent LCs and our previous work on
TSANs we wanted to incorporate various fluorophores. We suc-
ceeded in obtaining blue light emitting DLCs, by incorporating an
oxadiazole moiety in the molecular structure of TSANs [37]. Thus
we thought of introducing other fluorophores into the molecular
structure of TSANs. It seemed that trans-stilbene chromophore was
an ideal option to enhance the fluorescence property of TSANs [38],
because of their interesting properties [44,45]. Apart from their
commercial use as optical brighteners and laser dyes, their incli-
nation for charge carrier mobility and electroluminescence has
found applications in OLEDs [46e48], field effect transistors and
photovoltaic cells [49]. Initial study in this direction is reported in
our earlier communication [38], while in this paper we are pre-
senting yet another three set of stilbene-TSANs to account for an in-
depth study. Precisely, in this work, we have designed and syn-
thesized three types of luminescent TSANs, labeled as Iaeb, IIaeb
and IIIaed (Fig. 1), bearing trans-stilbene unit where the length,
number and pattern of substitution of exterior alkyl chains have
been varied. These new TSANs were targeted and prepared pri-
marily to compare their mesomorphism and photophysical
behavior with those of the lower homologues of TSANs Iaeb and
IIaeb (Fig. 1) reported previously in the form of a communication
[38]. It may be ideal to mention here that the design and synthesis
of materials herein dealt with are inspired by the potential appli-
cations of disk-like LCs; however, detailed experiments dealing
with technological aspects do not fall within the scope of our work
described herein.
2. Results and discussion
2.1. Synthesis and molecular structural characterization
The target stilbene-based TSANs were synthesized as outlined in
Scheme 1. 1,3,5-Triformyl phloroglucinol (1) was synthesized by
Duff’s formylation as reported earlier [31,33]. As can be seen in
Scheme 1, 1,4-nitrotoluene was subjected to benzylic bromination
by refluxing a solution of N-bromosuccinimide (NBS) in anhydrous
carbon tetrachloride with azobisisobutyronitrile (AIBN) as a radical
initiator to obtain 4-nitrobenzyl bromide (2a). This compound was
reacted with triethyl phosphite following MichaeliseArbuzov
phosphonate synthesis procedure, to obtain diethyl 4-
nitrobenzylphosphonate
(2b).
The
requisite
3,4-
dialkoxybenzaldehydes (3aeb) were prepared in quantitative
yields by O-alkylation of 3,4-dihydroxybenzaldehyde with various
alkyl bromides following Williamson’s ether synthesis protocol.
The WittigeHorner reaction of these benzaldehydes 3aeb with the
diethyl ester 2b in the presence of lithium diisopropyl amide (LDA)
as a base in tetrahydrofuran (THF) at low temperature (ꢀ78 ꢁC)
furnished nitro compounds 4aeb in good yields. These nitro
products were converted to corresponding amines 5aeb by selec-
tive reduction of nitro group using a mixture of indium powder and
hydrochloric acid in THF. On the other hand, the trialkoxy stilbene
amines 10aeb and 13aed were prepared starting from 3,4,5-
trialkoxybenzaldehydes (8aeb) or 2,3,4-trialkoxybenzaldehydes
(11aed) respectively, by following the synthetic steps as
described for the amines 5aeb. 3,4,5-Trialkoxybenzaldehydes
(8aeb) were synthesized, by the oxidation of (3,4,5-tris(alkoxy)
phenyl)methanol (7aeb) using pyridinium chlorochromate (PCC),
while compounds 7aeb were obtained by the lithium aluminium
hydride
(LAH)
mediated
reduction
of
ethyl
3,4,5-
trialkoxybenzoates (6aeb). Finally, the aminostyryl compounds
5aeb or 10aeb or 13aed were reacted with 1,3,5-triformyl phlor-
oglucinol in refluxing ethanol to obtain the respective target mol-
ecules Iaeb, IIaeb and IIIaed in almost quantitative yields.
All the target molecules, Iaeb, IIaeb and IIIaed, existed in the
form of an inseparable mixture of C3h and Cs geometrical isomers of
keto-enamine tautomer as evidenced by 1H and 1He1H COSY NMR
spectra. In fact, as expected, the NMR spectra of TSANs Iaeb and
IIaeb were found to be identical to the spectra of their lower ho-
mologues substituted with n-octyloxy, n-decyloxy and n-dodecy-
loxy terminal chains [38], which are hereafter, respectively, refer to
as I-C8, I-C10 & I-C12 and II-C8, II-C10 & II-C12 for the sake of
convenience and discussion. In particular, the proton spectra
showed multiple peaks between
the resonance of enamine and 2ꢁ-amine protons respectively, and
their coupling. As can be seen (SI), the region viz., 13.1e13.5
d 8.7e8.9 and d 13.1e13.5 due to
d
consisted of four doublets due to the resonance of an amine proton
(He) of C3h isomer and three analogous protons (Hf, Hg and Hh) of Cs
isomer. The enamine region viz.,
d 8.7e8.9 also consisted of four
doublets, which are due to the resonance of a proton (Ha) of C3h
isomer and three protons (Hb, Hc and Hd) of Cs isomer. In principle,
the measure of the integration of He against Hf, Hg and Hh of amine
protons or Ha vs Hb, Hc and Hd of enamine protons can be consid-
ered to calculate the ratio of two isomers. In this discussion, we
considered amine protons for estimating the ratio of the two

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
293
Fig. 1. Molecular structures of new and analogous luminescent discotic TSANs.
isomers, because of the ease in estimation (Scheme 1). 1He1H COSY
NMR spectra confirmed the coupling between the enamine and the
NH protons. Furthermore, the strong coupling (3JHH ~13 Hz) [50]
explains the localization of the NH proton. The proton NMR
spectra for the materials did not show any signals corresponding to
cis isomeric protons, demonstrating that the stilbene units are in
the desired trans form, which is the expected outcome of the
WittigeHornereEmmons method. These trans olefinic protons
were distinguished as doublets in the downfield region with a large
coupling constant (JHH ~16 Hz). In the case of Iaeb and IIaeb series,
two doublets corresponding to these olefinic protons resonate at
sample, Ia with a melting temperature of 81 ꢁC changes into Col
mesophase which stays up to ~224 ꢁC, before transforming into
isotropic state. On cooling the isotropic liquid, a texture, which is a
combination of small focal conics, interspersed with linear bire-
fringent defects, is observed. This pattern grows from an initially
homeotropic dark field of view, which is commonly attributed for
the Colh phase [56]. Further cooling leads to large pseudo focal
conics covering the whole field of view (Fig. 3a). Finally the mes-
ophase crystallized at 29 ꢁC. XRD studies confirm the D6h (hexag-
onal) symmetry of the mesophase. As can be seen in Fig. 3b, the
intensity vs 2q profile extracted from the 2D XRD pattern displays
around
d
7 and 7.05, while in the case of IIIaed series they appear at
two sharp Bragg peaks, an intense one (100) at a spacing 42.21 Å
and a relatively weak one (110) with a d spacing of 24.42 Å. Addi-
tionally a diffused peak in the wide angle was observed with a
spacing of 4.7 Å. The first two peaks in the small angle were in the
ratio of 1:0.58, which proves that the mesophase under investiga-
tion is Colh, with the cell parameter a ¼ 43.74 Å. The XRD pattern
obtained at low temperatures (100 ꢁC) was qualitatively similar but
an additional diffuse peak corresponding to an intra columnar
coreecore stacking distance of 3.46 Å was also observed.
The next compound Ib comprising branched peripheral chains,
showed an enantiotropic columnar phase with much reduced
transition temperatures when compared to the analogous values
obtained for compound Ia or other members of the series. Needless
to say, in this compound there is more fluidic disorder as it pos-
sesses branched alkyl chains at the peripheral region [57e59]. The
thermal behavior of this compound also approves with the fact that
the presence of branched alkyl chains widen the mesophase width,
without altering the mesophase type [5e14,57e59].
d
7 and 7.4. Molecular structures were further confirmed by IR,
UVevis spectroscopy, FABþ mass spectrometry and elemental
analysis.
2.2. Mesomorphic behavior
The phase behavior of the synthesized TSANs were established
with the aid of polarizing optical microscopy (POM), differential
scanning calorimetry (DSC) and X-ray diffraction (XRD) studies; in
fact, XRD data have also been obtained, as representative cases, for
TSANs I-C8 and II-C8. The detailed investigations of the meso-
morphic behavior of these three series of compounds are presented
in the sections to follow. The data obtained from these studies are
summarized in Tables 1 and 2 and Fig. 2.
2.2.1. Series-I
The thermal behavior of the two newly synthesized compounds
were examined and compared with that of previously reported
lower homologues I-C8, I-C10 and I-C12 (Table 1 and Fig. 2). All the
homologues of this series were liquid crystalline; exhibiting
enantiotropic Col phase as per the POM observation. The crystalline
The observed optical pattern for the mesophase was identical to
the one seen for Ia and it is reasonable to adjudge that compound Ib
also stabilizes the Colh phase (Fig. 4a). Notably, the Col phase
formed upon melting the crystalline sample, does not crystallize as

294
A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
aReagents and conditions(i) Hexamine, CF3COOH, 100 oC, 5 h, 3N HCl, 100 oC ; (ii) NBS, AIBN, CCl4, 100 oC, 6 h; (iii)
Triethyl phosphite, 130 oC, 6 h; (iv) 3,7-Dimethyloctylbromide / n-hexadecylbromide, anhydrous K2CO3, DMF, 80 oC, 17 h;
(v) 2b, LDA, THF, -78 oC to rt, 12 h; (vi) Indium powder, Conc. HCl, THF (1:9), rt, 2 h; (vii) 1, ethanol, 80 oC, 6 h. (viii)
LAH, THF, 0 oC to rt, 12 h; (ix) PCC, DCM, rt, 1 h; (x)1-Bromooctane/1-bromodecane/1-bromohexadecane /1-
bromooctadecane, anhydrous K2CO3, DMF, 80 oC, 17 h.bThe ratio of C3h and Cs isomers was determined by comparing the
integral areas of amine protons He (C3h isomer) vs Hf , Hg and Hh(Cs isomer) in 1H NMR.
Scheme 1. aSynthesis of stilbene-based TSANs.
evidenced by the observation that the texture remains unchanged
for a long time. Further, no exothermic or endothermic peaks were
seen in the first cooling or second heating DSC profiles respectively.
The XRD studies carried out at higher temperature (Fig. 4b) and
room temperature (RT) suggested that the compound stabilized
Colh phase. An identical behavior was noticed for lower homo-
logues reported earlier stabilizing Colh mesophase as evidenced by
the POM, DSC and XRD investigations. In this series, on increasing
the chain length, the mesophase range seems to increase margin-
ally in heating cycle (Fig. 2a), while the compound with branched
chain showed a reduced mesophase range (Fig. 2d). Compound I-C8
also exhibits Colh phase as evidenced by the XRD studies. The
absence of multiple reflections in the low angle region of the XRD
pattern could be assigned to a minimum in the form factor. Though,
the existence of a single (100) peak does not allow the unambig-
uous assignment of the Colh phase, the situation is similar to several
previous reports, wherein such an observation coincides with a
hexagonal columnar arrangement [43,51e55].
2.2.2. Series-II
When compared to TSANs Ia and Ib, discotics IIa and IIb showed
a drastic reduction in the melting temperature. Increase in the
number of flexible tails from six to nine enhanced the mesophase
range. Increase in the chain length of the peripheral tails showed a
gradual decrease in the melting and clearing temperatures.
Branching in the peripheral chains (compound IIb) does not show
the pronounced effect as it is in the case of compound Ib. It may
also be noted that, although not related directly, the short armed
TSANs with six peripheral tails exhibit lower melting temperatures
when compared to their analogous compounds comprising nine
alkoxy tails [33,36]. This behavior is contrary to the thermal
behavior of stilbene based TSANs (Series I and II). In this series, on
increasing the chain length, both the clearing temperature and the
mesophase range seems to decrease, as illustrated in Fig. 2b and e.

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
295
Table 1
actual molecular diameter calculated. This may be due to the
interdigitation of peripheral alkyl chains. Since the coreecore dis-
tance is known, i.e. c ¼ 3.61 Å we could calculate lattice area
S ¼ 2466 Å2, lattice volume V ¼ 8889.3 Å3 and number of molecules
within the columnar slice Z ¼ 1.84. The value of Z which is
approximately nearer to two, which means it’s almost two mole-
cules are forming a disc of the individual column. Similar values
have been observed for compounds (Table 2) Ia, Ib, IIa and IIb,
which exhibit Colh phase with prominent coreecore stacking dis-
tance. Even for compound IIIa, which exhibit columnar oblique
(Colob) phase (which we discuss in later sections) the value of Z was
found to be slightly more than two. This observation is common in
the case of polycatenars [63] but not common in discotics and star-
shaped molecules. Lehmann et al. showed that wedge shaped
stilbenoid dendrons stabilized Colh phase by the proper control of
dipole or pushepull character of the central unit [45]. They pro-
posed the packing of four dendron units to form a columnar slice
with their dipoles arranged in antiparallel orientation.
Phase transition temperatures (ꢁC)a and corresponding enthalpies (J/g) of Iaee
series.
Entry
Phase sequence
Heating
Cooling
Series-I
I-C8
I-C10
I-C12
Ia
Cr 125.6 [20.2] Colh 218.4 [1] I
Cr 111.2 [8.1] Colh 229.2 [1.2] I
Cr 107 [35.3] Colh 239.5 [1.7] I
Cr 80.7 [13.5] Colh 223.5 [1.3] I
Cr 65.7 [1.6] Colh 192 [1.1] I
I 214.9 [0.9] Colh 1.3 [0.6] Cr
I 226.6 [1]b Colh
I 237 [1.5]b Colh
I 220.3 [1.2] Colh 29.4 [13.2] Cr
Ib
I 188.2 [1.3]b Colh
Series-II
II-C8
II-C10
II-C12
IIa
IIb
Series-III
IIIa
Cr 72.4 [1.4] Colh 240.4 [1.1] I
Cr 70.1 [1.4] Colh 226.2 [1.4] I
Cr 66.5 [1.5] Colh 199.5 [1.6] I
Cr 41.5 [24.9] Colh 140.2 [0.6] I
Cr 65.7 [0.9] Colh 193.9 [1.3] I
I 236.3 [1.1]b Colh
I 220 [1.2]b Colh
I 196.5 [1.4]b Colh
I 135.5 [0.6]b Colh
I 190 [1.2]b Colh
Cr 117.3 [6.7] I
I 97.2 [1.7] Colob 71.5 [0.5] Cr
I 106.6 [0.5] Colh 60 [0.3] Cr
I 78.7 [1] Colr 72.9 [1.9] Cr
I 56.2 [37] Cr
IIIb
IIIc
IIId
Cr 95.7 [7.2] Colh 114.3 [1.1] I
Cr 77.2 [0.4]c Colr 83.5 [1.9] I
Cr 76.3 [10.6] I
We are proposing a preliminary model to explain the value of Z
in the present work, as in the following. In the case of TSANs the
central core with three ketone groups have three dipoles, where the
negative end is pointed towards the oxygen atom. It should be
noted that these three ketone groups are in conjugation with three
enamine groups. Thus the next neighboring molecule may stack in
such a way so as to avoid repulsion from the partial negative charge
located on oxygen atom. In other words, the next molecule has to
be rotated 60ꢁ with respect to the first molecule. Altogether this
will help in the close packing of these molecules in a unit cell. This
arrangement does not alter the molecular diameter as they are
stacking one above the other. The planarity of the central core is
well established due to the intramolecular H-bonding between the
amine and ketone groups. As a result almost two molecules are
packed in a unit cell. These individual discs are arranged in a hex-
agonal array as shown in Fig. 6. The observed decrease in the value
of lattice parameter ‘a’ with respect to molecular diameter is
explained by the interdigitation of peripheral alkyl tails as shown in
Fig. 6. It is interesting to note that lower homologues II-C8, II-C10,
II-C12 and branched chain derivative IIb also exhibited Colh phase
and an ability to freeze into glassy structure. The branched chain
analogue IIb showed a higher melting and clearing point when
compared to compound Ib. The compound IIb, when compared to
II-C8 which is the straight chain analogue; did not show a huge
decrease in the melting temperature as in the case of Ib with
respect to I-C8 (60ꢁ). In other words, the contrasts in the melting/
isotropization temperatures are apparent for compounds Ib & IIb
and Ia & IIa.
a
Peak temperatures in the DSC thermograms obtained during the first heating
and cooling cycles at 5 ꢁC/min.
b
The mesophase is not crystallizing up to ꢀ60 ꢁC. Cr ¼ Crystal; Colh ¼ Hexagonal
columnar phase; I ¼ Isotropic liquid.
c
CreCr transition is observed at 107.1 [0.4], 69.2 [13.8] and 57.2 [12.7] for IIIa,
IIIc and IIId respectively in the first heating cycle.
Firstly, let us discuss the phase behavior of compound IIa. This
TSAN sandwiched between a glass slide and coverslip, on heating,
melts into a mesophase at about 42 ꢁC having a non-specific
textural pattern; on further heating it transforms into isotropic
liquid at ~140 ꢁC. On cooling from the isotropic phase, a columnar
(Col) phase appears with the texture consisting of a combination of
mosaics with linear birefringent defects and homeotropic regions
(Fig. 5a). The presence of some homeotropic domains strongly
suggests that the phase is hexagonal columnar (Colh). On further
cooling, the compound did not crystallize but appeared to form a
glass nearing about 60 ꢁC. The signatures in DSC thermograms due
to all of the above phase transitions, except for Col-glass, were
obtained. This Col phase was studied further by XRD at two
different temperatures viz. 120 ꢁC and 60 ꢁC, in order to ascertain its
structure. As expected, both the diffractograms were found to be
identical and as a representative case we explain the diffraction
pattern obtained at 60 ꢁC. 1D intensity vs 2
q profile obtained at
60 ꢁC has been shown in Fig. 5b. A diffuse peak in the wide-angle
region with d z 4.5 Å is seen, which can be attributed to the
liquid like (short-range) order of the peripheral alkoxy tails. A
prominent peak in the wide-angle region corresponding to cor-
eecore stacking was observed in the low temperature diffraction
pattern with a d spacing of 3.6 Å. In addition, a sharp reflection was
observed in the low angle regions with the spacing being 46.2 Å.
Additionally the low angle region had peaks corresponding to the
d spacings of 26.8 Å, 17.47 Å and 11.62 Å with Miller indices (100),
(110) and (210). These d spacings were in a ratio of 1:0.58:0.38,
which corresponds to Colh phase. Apparently, this was similar to
the high temperature diffraction pattern obtained at 120 ꢁC (Table 2
and SI), apart from the increase in the lattice parameter a from
49.8 Å to 53.4 Å, which can be attributed to the stretching of the
peripheral alkoxy chains. The mesophase gets freezed on further
cooling which was also confirmed by the DSC scans. Since the
coreecore stacking distance was obtained from the second diffused
peak in all the diffractograms of compound IIa, we could calculate
the lattice parameters (Table 2). For example, if we consider the X-
ray diffractogram for compound IIa at 60 ꢁC (Table 2 and SI), the
calculated lattice parameter a ¼ 49.8 Å, which is 26% less than the
2.2.3. Series-III
The general trend in thermal behavior of these three TSANs
seems to be different from their positional isomeric analogues, the
series II of compounds. In general, these TSANs show lower tran-
sition temperatures but seem to disfavor the mesomorphism. For
example, the first member of the series viz. IIIa comprising three n-
octyloxy tails at 2, 3 and 4 positions of the outermost peripheral
benzene ring displays a monotropic mesophase, as was evidenced
by the observation of textural pattern (Fig. 7a).
In comparison, its positional isomer II-C8 displays an enantio-
tropic Colh phase that exists for a wide thermal range (Table 1 and
Fig. 2). Whereas, the compound IIIb having nine n-dodecyloxy
peripheral tails display an enantiotropic mesophase; however,
unlike in II-C12, the mesophase exists for short thermal range.
Compound IIIc with nine hexadecyloxy chains also exhibits an
enantiotropic Col phase but for a short range. Further increase in
the chain length to octadecyloxy turns the compound IIId crystal-
line. It is interesting to note that first three liquid crystalline

296
A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
Table 2
The results of (hkl) indexation of XRD profiles of the compounds at a given temperature (T) of mesophases.a
Compounds (D/ Phase (T/
Å)
dobs (Å) dcal (Å) Miller indices
Lattice parameters (Å), lattice area S (Å2), molecular volume V (Å3), number of molecules in a unit cell Zc
ꢁC)
hkl
I-C8 (45.85)
Colh (160) 35.87
35.87 100
a: 41.41
4.6 (ha)
Colh (210) 42.21
S ¼ 1485.4
Ia (62.62)
42.21 100
24.37 110
a: 48.74
24.42
4.72
(ha)
S ¼ 2057.2
Colh (100) 45.62
26.43
45.61 100
26.33 110
a: 52.67
c ¼ 3.46
11.88b
S ¼ 2402.26
V ¼ 8312.3
Z ¼ 2.29
4.57
(ha)
3.46 (hc)
Colh (180) 36.23
21.23
Ib (46.85)
36.23 100
20.92 110
18.12 200
13.70 210
a: 41.84
c ¼ 3.71
18.44
S ¼ 1516.04
V ¼ 5620.86
Z ¼ 1.99
13.73
12.67b
5.04
(ha)
3.71 (hc)
Colh (24)
36.77
18.64
13.78
12.62
4.74
36.77 100
18.39 200
a: 42.46
c ¼ 3.66
13.9
210
S ¼ 1561.53
V ¼ 5714.84
Z ¼ 2.02
12.26 300
(ha)
3.66 (hc)
II-C8 (50.19)
IIa (67.62)
Colh (200) 36.41
36.41 100
36.97 110
a: 42.05
S ¼ 1531.0
4.69
(ha)
Colh (25)
36.97
4.39
(ha)
a: 42.69
S ¼ 1578.6
Colh (120) 43.10
43.10 100
24.88 110
16.29 210
a: 49.77
24.94
16.32
4.64
c ¼ 3.76
S ¼ 2145.15
V ¼ 8057.05
Z ¼ 1.67
(ha)
3.76 (hc)
46.21
Colh (60)
46.21 100
26.68 110
17.47 210
10.60 320
a: 53.36
26.80
17.47
11.62
4.52
c ¼ 3.61
S ¼ 2466.03
V ¼ 8889.30
Z ¼ 1.84
(ha)
3.61 (hc)
IIb (49.20)
Colh (180) 36.67
36.67 100
a: 42.35
25.85b
e
210
c ¼ 3.75
13.89
5.0 (ha)
3.75 (hc)
13.86
S ¼ 1552.90
V ¼ 5829.64
Z ¼ 1.61
Colh (24)
37.80
18.96
14.21
4.77
37.8
100
a: 43.65
18.90 200
14.29 210
c ¼ 3.68
S ¼ 1649.99
V ¼ 6074.37
Z ¼ 1.67
(ha)
3.68 (hc)
IIIa (45.9)
Colob (87) 34.32
33.17
34.32 010
33.17 100
18.94 110
17.16 020
16.58 200
14.16 ꢀ230
13.04 ꢀ310
10.64 ꢀ240
a: 41.34
b: 42.77
g
18.87
17.19
16.56
14.31
12.99
10.69
9.69
¼ 36.65ꢁ
c ¼ 3.67
S ¼ 1768.09
V ¼ 6485.87
Z ¼ 2.06
9.85
ꢀ140
4.5 (ha)
3.67 (hc)
Colob (24) 37.28
33.94
37.28 010
33.94 110
29.77 200
19.85 300
18.71 310
a: 60.2
b: 37.7
29.77
19.76
18.75
g
¼ 8.54ꢁ
c ¼ 3.57
S ¼ 2269.46

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
297
Table 2 (continued )
Compounds (D/ Phase (T/
Å)
dobs (Å) dcal (Å) Miller indices
Lattice parameters (Å), lattice area S (Å2), molecular volume V (Å3), number of molecules in a unit cell Zc
ꢁC)
hkl
14.57
17.01
16.57
12.91
11.64
10.9
14.59 410
16.97 220
16.53 ꢀ310
12.68 ꢀ320
11.82 ꢀ130
10.91 ꢀ230
V ¼ 8112.63
Z ¼ 2.58
9.75
9.35
9.89
9.32
ꢀ330
040
4.40
(ha)
3.57 (hc)
IIIb (53.58)
IIIc (70.94)
Colh (100) 37.66
37.66 100
21.75 110
a: 43.49
S ¼ 1638.04
21.68
4.64
(ha)
Colr (75)
43.10
39.97
4.61
43.10 100
39.97 010
a: 43.10
b: 39.97
S ¼ 1722.69
(ha)
a
The diameter (D) of the polycatenar (estimated from Chem 3D Pro 8.0 molecular model software from Cambridge Soft). dobs: spacing observed; dcal: spacing calculated
is the column tilt angle). The spacings marked ha and hc correspond to diffuse
(deduced from the lattice parameters: a for the Colh phase; a and b for the Colr and Colob phase;
g
halos in the wide-angle region arising from correlations between the alkyl chains and core regions, respectively.
b
Diffused peak.
c
Z was calculated only when a relatively strong reflection is observed corresponding to the coreecore stacking.
Fig. 2. Bargraph summarizing the thermal behavior (heating cycle) of stilbene based TSANs with lower homologues (a), (b) and (c); the dependence of transition temperatures on
number and pattern of substitution of peripheral alkoxy tails for stilbene based TSANs (d), (e) and (f).
compounds of this series show Col phases of different symmetry as
explained below.
two sharp reflections with spacings of 37.7 Å and 21.7 Å were seen
with the ratio of their reciprocal spacings being 1:0.58. These re-
flections could be indexed to (100) and (110) planes of a quasi-2D
hexagonal lattice with an intercolumnar distance of 43.5 Å; Thus,
the textural observation coupled with X-ray data suggests it to be a
Colh phase. There was no peak corresponding to coreecore corre-
lation. Enantiotropic Col phase exhibited by compound IIIc showed
focal conic texture (Fig. 7c). Apparently, the identification of the LC
phase on the basis of the optical texture was not clear, and thus, was
investigated with the help of XRD study. XRD studies carried out at
75 ꢁC showed two sharp peaks at low angle with d spacings of 43.1
and 39.97 Å with Miller indices (100) and (010). In the wide angle a
diffused peak with a d spacing of 4.61 Å corresponds to the stacking
of flexible tails. This can be indexed to a Colr phase with lattice
parameters a ¼ 43.10 Å and b ¼ 39.97 Å (Fig. 8c). In order to check
the effect of increasing length of peripheral tails on the mesomor-
phic behavior, compound IIId was prepared. No mesomorphic
Monotropic Col phase of compound IIIa under polarizing optical
microscope showed a mosaic texture (Fig. 7a). The mesophase on
XRD investigation showed many sharp peaks in the low angle re-
gion with two diffuse peaks in the wide-angle region, where the
first one (d ¼ 4.5 Å) corresponds to the stacking of peripheral
flexible tails and the second one (d ¼ 3.67 Å) corresponds to the
stacking of disks within the column (Fig. 8a). The sharp peaks in the
low angle region fits into the lattice of Colob phase with the lattice
parameters a ¼ 41.34 Å and b ¼ 42.77 Å and column tilt angle
g
¼ 36.6ꢁ. Enantiotropic Col phase of compound IIIb shows a
mosaic texture in POM observations (Fig. 7b). The intensity vs 2
q
profile extracted from the 2D X-ray diffraction pattern obtained for
the Col phase of IIIb at 100 ꢁC, as illustrated in Fig. 8b, shows a
diffuse peak in the wide-angle region with spacing (d) of 4.6 Å
indicating the liquid-like in-plane order. In the small angle region

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A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
Fig. 3. Photomicrograph of the optical texture seen for the Colh phase of Ia at 187 ꢁC (a); X-ray intensity profile of the Colh phase of TSAN Ia at 210 ꢁC wherein two reflections at low
angles and a diffuse peak at the wide angle region (b) can be seen.
Fig. 4. Photomicrograph of the optical texture observed for Colh phase of Ib at 147 ꢁC (a); X-ray intensity vs 2 profile obtained for the Colh phase of compound Ib at 24 ꢁC (b).
q
Fig. 5. Photomicrograph of optical texture obtained for Colh phase of IIa at 109 ꢁC (a); X-ray intensity profile obtained for the Colh phase of IIa at 60 ꢁC (b).
behavior was observed for this compound as evidenced by both
POM and DSC studies. Thus, in this series, unlike in Series-I and II,
on increasing the lengths of peripheral alkoxy tails, the mesophase
range gets reduced drastically, as shown in Fig. 2c and f.
symmetry of the Col phase (as in the case of IIIaec) or by desta-
bilizing the Col self-assembly (as evident from the crystalline na-
ture of IIId). In the case of series I and II, the pattern of peripheral
alkoxy tail substitution support the Col packing and it is evidenced
by the enhanced mesophase width and tendency of the meso-
phases to freeze in the glassy state.
In essence, this series shows the importance of the symmetry of
substitution in stabilizing Col phase. Space-filling energy mini-
mized molecular models of representative TSANs I-C8, II-C8 and
IIIa shed some light on this aspect (Fig. 9). When compared to I-C8
and II-C8, the three arms of TSAN IIIa are more twisted to minimize
the repulsion from the alkoxy tail located in the bay region. The
twist of the arms reduces the molecular planarity and as a conse-
quence packing of discs into Col phase becomes very subtle. Thus
increasing the chain length drastically affects the mesomorphic
behavior in this series; either by leading to a change in the
2.3. Photophysical properties
As mentioned in the introduction, we expected the compounds
to be emissive due to the presence of stilbene chromophores and
the large delocalization of electron cloud between the peripheral
region (electron donating) and the central core (electron with-
drawing). We have investigated the UVevis absorption and

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
299
Fig. 6. Schematic showing the self-organization of compound IIa into hexagonal columnar (Colh) lattice with inter-digitation of alkyl tails. Space filling energy minimized (all-trans)
molecular model of IIa derived from molecular mechanics (MM2) method.
Fig. 7. Photomicrographs of optical texture obtained for Colob phase of IIIa at 95 ꢁC (a); Colh phase of IIIb at 83 ꢁC (b) and Colr phase of IIIc at 76 ꢁC (c).
fluorescence behavior of one compound from each series viz., Ia,
IIa and IIIc (all with hexadecyloxy chains) in solution, solid as well
as in the glassy film state. The data obtained from absorption and
emission spectra of these three compounds have been summa-
rized in Table 3. Micromolar solutions in THF, drop-cast solid and
glassy films were used for the study. To begin with, the solution,
solid and thin film spectra were recorded at room temperature.
Solid films were prepared by drop casting the solution of the
compounds in chloroform on glass coverslips and allowing the
solvent to evaporate. Thin glassy films of the compounds were
prepared by, sandwiching the sample between two glass cover-
slips and heating to the isotropic state. From the isotropic state
these compounds were cooled slowly to the glassy state. As shown
in the LHS portion of Fig. 10a,c and d, the absorption spectra (solid
line) of all the three compounds in the solution, solid and frozen
mesophase states, showed two absorption bands corresponding to
emission with an emission maximum at around 510 nm with a
negligible shoulder (Fig. 10a). There was not much difference in
the absorption and emission spectra of compounds Ia, IIa and IIIc.
The Stokes shift was found to be in the range of 65e68 nm as
given in Table 3. In fact, as shown in Fig. 10b, the green emission
was observed on irradiating with a light of 365 nm. The glassy
films of compounds Ia, IIa and IIIc on exciting at the absorption
maxima show a broad emission maxima, with a large Stokes shift
of around 116e137 nm (see right hand side portion of Fig. 10d).
This large red shift can be attributed to the aggregation of mole-
cules leading to strong intermolecular interactions. It should be
noted that, there is a less red shifted emission in the case of glassy
films of compound IIIc, when compared to the glassy films of Ia
and IIa. This may be due to the less efficient packing of molecules
because of the unsymmetrical peripheral substitution in series III
(Fig. 8). The fluorescence intensity decreased with the increase in
the temperature, which is due to the breaking-up of these ag-
gregates and thermally activated radiationless processes (See SI)
[60,61]. Drop-cast films also exhibited emission spectra similar to
those of glassy films (Fig. 10c). The emissive frozen Col phase of
TSANs is technologically important as it possess 2D order and
short intra columnar distance, which supports the charge carrier
mobility with a simultaneous restriction on the mobility of ionic
impurities [62]. Relative quantum yields of these compounds in
solution were measured with respect to tetrakis(octyl)-1H-
pep* and nep* transitions. As expected, the absorption maxima
in the solid as well as glassy state shift slightly to longer wave-
length that can be ascribed to the close proximity of molecules in
frozen 2D structure. Further, in comparison with short-armed
TSANs [35,36], a bathochromic shift of the absorption maxima is
apparent in both the states of these compounds; obviously this
can be attributed to the fact that these long armed compounds
with fluorophores possess an extended
p-conjugation. Irradiation
of these solutions (wavelength of 440 nm) yielded an intense

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A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
Fig. 8. The intensity vs 2
q
profiles obtained for Colob phase of TSAN IIIa at 87 ꢁC (a); Colh phase of IIIb at 100 ꢁC (b); Colr phase of IIIc at 75 ꢁC (c).
Fig. 9. Space-filling energy minimized (all-trans) molecular models of TSANs (a) I-C8; (b) II-C8; (c) IIIa derived from the molecular mechanics (MM2) method.
Table 3
Photophysical properties of luminescent TSANs.
Compound
THF solutiona
Absorptionb
Drop cast solid film
Glassy film
Emissionb,c
Stokes shiftb
Absorptionb
Emissionb,c
Stokes shiftb
Absorptionb
Emissionb,c
Stokes shiftb
Ia
IIa
IIIc
353, 442
353, 440
351, 441
510
505
508
68
65
67
384, 443
379, 440
387, 441
579
563
553
136
121
112
374, 441
370, 441
372, 441
578
567
557
137
126
116
a
Micromolar solutions in THF.
Wavelengths (nm).
The excitation wavelength lex ¼ 440 nm.
b
c
phenanthro[1,10,9,8]carbazole-3,4,9,10-tetracarboxylate [64], and
found to be in the range of 0.29e0.31 (See SI) irrespective of the
substitution pattern. However, detailed investigations addressing
the development of well-aligned films, annealing, film
morphology, luminescence efficiency, electroluminescence, sol-
vent effects etc. are required to go for a device fabrication.
2.4. Electrochemical properties
Cyclic voltammetry (CV) gives valuable information and allows
the estimation of HOMO and LUMO levels of the organic materials.
A 0.1 M solution of tetrabutylammonium hexafluorophosphate, in
acetonitrile was used as a supporting electrolyte (buffer). A

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
301
Fig. 10. Absorption (solid line) and emission spectra (dotted line) in micromolar THF solution of compounds Ia, IIa and IIIc (a); pictures of micromolar solutions of the same
compounds on illuminating with the UV light of 365 nm (b); absorption (solid line) and emission spectra (dotted line) of drop casted films of compounds Ia, IIa and IIIc (c) and
glassy films of the same compounds (d).
standard CV experiment by using a three-electrode cell was carried
out. Specifically, a single compartment cell comprising a 1 mm
diameter platinum (Pt) disc as a working electrode, a Pt wire as an
auxiliary or counter electrode and saturated silver/silver chloride
(Ag/AgCl) as a reference electrode, was used for the study. The
reference electrode was calibrated with the ferrocene/ferrocenium
(Fc/Fcþ) redox couple (absolute energy level of ꢀ4.80 eV to vac-
uum). The redox potentials of these thin-films were measured at a
scanning rate of 50 mV sꢀ1. Compounds Ia, IIa and IIIc were chosen
as representative TSANs from each series and investigated for their
electrochemical behavior. The energy levels and band gaps calcu-
lated from these studies are tabulated in Table 4. The cyclic vol-
tammograms of these compounds are given in Fig. 11. These
compounds were deposited by dropping of the solutions in
dichloromethane on platinum disc surface and then heated gently
with hot air-gun. All the compounds exhibited well-defined irre-
versible oxidation and reduction waves. The optical band gap
(Eg,opt) was estimated from the red edge of the absorption spectra.
ꢀ4.12 eV. The electrochemical band gaps were found to be in the
range of 2.27e2.62 eV, which was comparable to the values of the
optical band gaps observed (2.32e2.35 eV). The observed HOMO,
LUMO levels and band gaps were lower than 1,3,5-tristyryl benzene
derivative which was reported for device fabrication (HOMO:
ꢀ5.71 eV; LUMO: ꢀ2.91 eV; Band gap: 2.8 eV) [65].
2.5. Atomic force microscopy studies
Atomic force microscopy (AFM) is useful tool to study the sur-
face structures at nanometer scales and thus aids in knowing sur-
face morphology to a large extent. We have spin coated the
dichloromethane solutions of compounds Ib, IIa, IIb on quartz
plate. These compounds were chosen because of the reason that
they exhibit frozen glassy columnar phase. The topographic and
phase contrast AFM images recorded for these compounds are
shown in Fig. 12.
As can be seen, the topography shows a homogenous surface
studded with granular structures. Further close examination of the
phase contrast image (Fig. 12b,e,h) suggests the presence of long
fibrils where dissimilar components show different contrasts. The
root mean square (rms) roughness and thickness values of the film
were found to be in the range of ~5e10 nm and 90e140 nm
( 10 nm), respectively. The absorption and emission spectra were
also obtained for these thin-films were found to be identical to the
ones obtained for the drop casted and glassy films. Fig.13 shows the
image of these films in daylight as well as UV light of long
The HOMOeLUMO energy gap (DECV) value was measured from the
difference in potential between oxidation and reduction and was
found to be slightly different from the optical band gap, i.e. the
value deduced from the longest wavelength absorption (lred edge
)
)
(Table 4) using the expression E ¼ h
n
(where ). The IP (EHOMO
n
¼ c/
l
and EA (ELUMO) were determined by adding 4.7 eV (vs Ag/AgCl) to
the oxidation and reduction potentials respectively. The values of
HOMO were found to be in the range of ꢀ6.21 to ꢀ6.39 eV, whereas
the values of LUMO were found to be in the range of ꢀ3.8 to
Table 4
Electrochemical characteristics of the representative compounds.
Compound
E1ox (V)
EHOMO (eV)
E1red (V)
ELUMO (eV)
DEg,CV (eV)
Eg,opt (eV)
Ia
IIa
IIIc
1.51
1.72
1.69
ꢀ6.21
ꢀ6.42
ꢀ6.39
ꢀ0.79
ꢀ0.57
ꢀ0.58
ꢀ3.91
ꢀ3.80
ꢀ4.12
2.30
2.62
2.27
2.32
2.35
2.33

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A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
wavelength (365 nm). From these images it is evident that emission
is red shifted due to the formation of aggregates.
3. Summary
Tris(N-salicylideneaniline)s appended with stilbene fluo-
rophores, which can be regarded as pseudo-triphenylene discotics,
have been designed and synthesized. Thereby, through the incor-
poration of fluorophores we have examined the effects on the
mesomorphic behavior and photophysical properties of discotic
TSANs. Specifically, three series of stilbene-based TSANs were
prepared. Of these, the first series of stilbene-based TSANs con-
sisting of six peripheral alkoxy tails exhibit columnar behavior, and
in some cases this 2D structure freezes. In this series, on increasing
the chain length, the mesophase range seems to increase margin-
ally. The second series of stilbene-based TSANs also exhibit
columnar mesomorphism. In general, on increasing the chain
length, both the clearing temperature and the mesophase range
seem to decrease. All the members of this series freeze the
columnar structure. The general trend in thermal behavior of the
third series of compounds, which are the positional isomeric ana-
logues of second series of TSANs, appear to be different; they show
lower transition temperatures but seem to disfavor the mesomor-
phic behavior as can be seen that only the middle members with
nine n-dodecyloxy or n-hexadecyloxy tails, unlike the n-octyloxy
and n-octadecyloxy analogues, show enantiotropic columnar
mesophase. None of these, unlike other two series of compounds,
freeze the 2D structure. Symmetry of the mesophase is also very
much dependent on length of the alkyl chain. The photophysical
studies carried out for three representative TSANs evidenced their
fluorescence characteristics in solution, mesophase, solid and
glassy states. The bright green light emission was observed in the
solution state. As expected, the emissive maxima in the solid/glassy
state shift slightly to longer wavelength that can be ascribed to the
close proximity of molecules in frozen 2D structure. Absorption and
emission spectra of these three different series does not vary much
depending on the pattern of substitution or on the length of the
peripheral chain. Electrochemical studies revealed that these
compounds exhibit lower HOMO, LUMO levels and band gaps in
comparison to reported tristyryl benzene derivatives. Atomic force
microscopy of the spin coated films revealed a homogeneous sur-
face studded with granular structures, which can be further opti-
mized. In general, it is shown that by varying the nature (polarity)
of fluorophore and peripheral alkoxy chain substitution patterns
affects the co-facial and edge-on interactions among the TSAN
cores, and thus mesophase behavior can be altered. Specifically, the
study clearly indicates that the number, length and positions of
peripheral alkoxy tails strongly affect mesomorphism with almost
no effect on the photophysical property. The observed light emis-
sive feature of the Col phase and the ability to freeze in the glassy
state appear to be promising from the point of OLEDs.
t / e I n r r u C
I
e n r r t u / C
4. Experimental section
4.1. General
Chemicals were obtained from Fluka, Aldrich, Lancaster and
local chemical companies, and were used without any purification;
solvents were dried following the standard procedures. Chroma-
tography was performed using either silica gel (60e120, 100e200
and 230e400 mesh) or neutral aluminium oxide. For thin layer
chromatography, aluminium sheets pre-coated with silica gel
(Merck, Kieselgel 60, F254) were employed. IR spectra were recor-
ded on a Perkin-Elmer Spectrum 1000 FT-IR spectrometer. The
spectral positions are given in wave number (cmꢀ1) unit. NMR
I
/ t n e r r C u

A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
303
Fig. 12. Topographic AFM image, 4
compound IIa (e); and compound IIb (h); topography AFM images of compound Ib (c); compound IIa (f); and compound IIb (i).
m
m ꢂ 4
mm in a 3D perspectives of compound Ib (a); compound IIa (d); and compound IIb (g); phase contrast images of compound Ib (b);
mode using 3-nitrobenzyl alcohol as a liquid matrix. Molecular
length is calculated from the energy minimized structure deduced
from CS Chem 3D version 9 programme. X-Ray diffraction studies
were carried out on powder samples in Lindemann capillaries with
CuKa radiation using high-resolution powder X-ray diffractometer
equipped with a focusing elliptical mirror and a high-resolution fast
detector. The mesogenic compounds were investigated for their
liquid crystalline behavior employing an optical polarizing micro-
scope (Leica DMLP) equipped with a programmable hot stage
(Mettler FP90) and differential scanning calorimeter (Perkin Elmer
DSC7). Above mentioned optical polarizing microscope equipped
with a programmable hot stage and differential scanning calorim-
eter were used to determine the melting points of non-
mesomorphic compounds. Fluorescence emission spectra in solu-
tion state were recorded with luminescence spectrometer (Perkin
Elmer LS 50B), while emission spectra as a function of temperature
in solid state were obtained by using fluorolog spectrofluorimeter
(SPEX) in conjunction with a programmable hot stage (Linkam).
Cyclic voltammograms were recorded using a CH instruments
electrochemical work station equipped with an electrochemical
analyzer (CHI660E). AFM imaging was performed using an Agilent
5500 microscope; intermittent noncontact mode AFM was used for
topographical characterization.
Fig. 13. Photographs of the quartz plates containing spin coated samples of com-
pounds Ib, IIa and IIb under daylight (first row) and under UV light of long wavelength
(365 nm) (second row).
spectra were recorded using either a Bruker DRX-500 (500 MHz) or
Bruker AMX-400 (400 MHz) spectrometer or Bruker Avance series
DPX-200 (200 MHz) spectrometer. For 1H NMR spectra, the
chemical shifts are reported in ppm relative to TMS as an internal
standard. Coupling constants are in Hz. Microanalyses were per-
formed using either a Euro EA3000 model elemental analyzer. Mass
spectra were determined on JEOL JMS-600H spectrometer in FABþ
4.2. Experimental procedures and characterization data
The detailed synthetic procedures and the molecular structural

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A.S. Achalkumar et al. / Dyes and Pigments 132 (2016) 291e305
Org Lett 2005;7:4075e8.
characterization data for the intermediate and target compounds
are provided in the Supporting information section.
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meric materials. J Appl Phys 2008;103:124510/1e124510/6.
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[24] Marguet S, Markovitski D, Goldmann D, Janietz D, Praefcke K, Singer D.
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Acknowledgement
ASA sincerely thanks Science and Engineering Board (SERB),
DST, Govt. of India and BRNS-DAE for funding this work through the
project No. SB/S1/PC-37/2012 and No. 2012/34/31/BRNS/1039
respectively.
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