The effect of regioisomerism on the mesomorphic and photophysical behavior of oxadiazole-based tris(: N -salicylideneaniline)s: Synthesis and characterization

Article abstract of DOI:10.1039/c7nj01766a

Two new regioisomeric star-shaped tris(N-salicylideneaniline)s are synthesized and characterized. The arms of these star-shaped mesogens differ from each other with respect to the substitution on the 3,5-positions of the central 1,2,4-oxadiazole moieties. The unsymmetrical nature of substitution leads to a change in the distribution of electron density, which will have an effect on the type of columnar self-assembly. One of these molecules stabilizes the columnar hexagonal phase, while the other one stabilizes the columnar rectangular phase. The columnar rectangular phase, which requires enhanced intermolecular interactions, is observed in the case of the star-shaped molecule, where the trialkoxy phenyl group is connected at the 5-position of the heterocycle, whereas the columnar hexagonal phase is observed in the case of the star-shaped molecule, where the trialkoxy phenyl group is connected at the 3-position of the heterocycle. These compounds showed reduced melting points, clearing points and an enhanced mesophase range with respect to their symmetric counterpart (1,3,4-oxadiazole derivative) reported earlier. All the molecules exhibited green light emission in solution, with good quantum yield. The emission of 1,2,4-oxadiazole derivatives was considerably red-shifted in comparison to those of 1,3,4-oxadiazole derivatives. This study emphasizes how a minor change in the molecular structure brings about a beneficial change in the self-assembly characteristics of star-shaped molecules.

Full text of DOI:10.1039/c7nj01766a

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Effect of regioisomerism on the mesomorphic and photophysical behavior
of oxadiazole-based tris(N-salicylideneaniline)s: Synthesis and
characterization
aΨ
aΨ
b
b
Suraj Kumar Pathak, Subrata Nath, Joydip De, Santanu Kumar Pal and
a
Ammathnadu S. Achalkumar *
a
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
b
7
81039, Assam, India. Department of Chemical Sciences, Indian Institute of Science
Education and Research Mohali, Sectorꢀ81, Knowledge City, Manauli 140306, Punjab,
India.
Ψ
Equal contribution
Abstract: Two new regioisomeric starꢀshaped tris(Nꢀsalicylideneaniline)s are synthesized and
characterized. The arms of these starꢀshaped mesogens differ from each other with respect to
the substitution on the 3,5ꢀpositions of the central 1,2,4ꢀoxadiazole moieties. The
unsymmetrical nature of substitution leads to a change in the distribution of electron density,
which will have an effect on the type of the columnar selfꢀassembly. One of these molecules
stabilize columnar hexagonal phase, while the other one stabilize columnar rectangular phase.
Columnar rectangular phase, which require enhanced intermolecular interactions are observed
in the case of starꢀshaped molecule, where the trialkoxy phenyl group is connected at the 5ꢀ
position of the heterocycle, whereas columnar hexagonal phase was observed in the case of
starꢀshaped molecule, where the trialkoxy phenyl group is connected at the 3ꢀposition of the
heterocycle. These compounds showed reduced melting point, clearing point and enhanced
mesophase range with respect to the symmetric counterpart (1,3,4ꢀoxadiazole derivative)
reported earlier. All the molecules exhibited green light emission in solution, with good
quantum yield. The emission of 1,2,4ꢀoxadiazole derivatives was much redꢀshifted in
comparison to 1,3,4ꢀoxadiazole derivatives. This study emphasizes how a minor change in the
molecular structure brings about a beneficial change in the selfꢀassembly characteristics of
starꢀshaped molecules.
Introduction
Over the years several molecular motifs have been tried to stabilize selfꢀassemblies of πꢀ
conjugated molecules, which will enhance the one dimensional (1D) charge migration. This
capability garners increased attention nowadays, due to the potential advantages in organic
1
optoelectronics. In this context; high quality single crystals, amorphous polymers and
columnar liquid crystals are the major contenders to take over the mantle of next generation
semiconductor industries. Each one of them has their own advantages and disadvantages,
where one has to make a clever trade off to make a successful choice and tailor them
according to the demand. For example, the single crystals vouch for the highest order and
consequently the conductivity but obtaining a defect free single crystal (defects act as charge
2
traps) and to handle them in device fabrication is not an easy task. Amorphous polymers
have the problem of batchꢀtoꢀbatch reproducibility, difficulty of purification and low
1

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conductivity. On the other hand columnar liquid crystals (Col LCs) formed by the discꢀlike
organic molecules are known to have superior order, processability and good anisotropic
conductivity. Col LCs are often compared to molecular wires, where the one dimensionally
stacked aromatic cores form a central conducting unit, while the peripheral tails function as an
insulating sheath, thus leading to a large difference in the charge carrier mobility along the
3
column and across the column. Additionally they have a selfꢀhealing ability to overcome the
defects, which act as charge traps. Further benefits are the ease of synthesis, tunability of the
structure as per the requirement, reproducibility and ease of processability (by solution or by
4
vapor deposition). Though the stabilization of Col LCs was first reported with disc shaped
5
molecules, consequently several novel molecular designs like polycatenars, star shaped
molecules, dendrimers and metallomesogens have shown the ability to stabilize this selfꢀ
6
assembly. Considering the potential applicability of Col LCs as a bright option in the
7
8
fabrication of organic photovoltaics (OPVs), organic light emitting diodes (OLEDs) and
9
organic field effect transistors (OFETs) , one need to have a flexible platform, where the
desired properties can be incorporated by an easy synthetic route, while maintaining Col
assembly intact. Starꢀshaped molecules are relatively a new class of materials that stabilize
1
0
Col LC phases, with a synthetic flexibility to include various functional properties. These
molecules have three covalently linked arms to the central core through flexible or rigid
linkers. Though these molecules lack shape anisotropy as in the case of discotics, the
nanophase segregation of incompatible molecular subunits promote the stabilization of Col
LC phases. Recently tris(Nꢀsalicylideneanilines) (TSANs) are receiving great attention from
researchers as a useful molecular design to stabilize different selfꢀassemblies. With a careful
design this core can provide smart materials, due to its synthetic flexibility, which will help in
tuning their material properties. Several research groups utilized this core in the development
1
2
13
14
15
of molecular switches, sensors, covalent organic frameworks, chiral catalysts. We were
able to synthesize several luminescent Col LCs based on TSANs and shown that their selfꢀ
1
6
assembly and luminescence can be tuned by appropriate design of the arms.
Introduction of heterocycles in the liquid crystal design is of immense interest in
recent days, because of the sheer number of structural varieties possible and hence the control
17
on their properties. The heterocyclic molecular structure provides a reduced symmetry,
strong lateral and/or longitudinal polar induction. Further their incorporation leads to donor–
acceptor interactions within the molecules due to unsymmetrical distribution of electron
17d
density. All these properties in turn affect the photophysical and selfꢀassembly behavior.
2

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Additionally, it provides an uneven electron density distribution in the molecule and hence a
wide range of emission colors are possible. Thus, heterocyclic mesogens are finding a strong
1
footing in the area of OLEDs. Such oriented mesogens exhibit polarized emission. 1,3,4ꢀ
Oxadiazoles are widely studied heterocycles in liquid crystals and OLEDs, due to their high
stability towards hydrolytic, thermal and oxidative deterioration. These materials are known
for high luminescence quantum yield and superior electron transporting properties. Thus they
are finding application as electron transporting/hole blocking or electroluminescent layers in
17,18
OLEDs.
In the recent past we have reported TSANs containing unsymmetrically
substituted 1,3,4ꢀoxadiazole arms and they exhibited greenish blue emission in solution state
16e
and stabilized columnar hexagonal phase. However, they showed high melting and clearing
temperatures. The regioisomeric 1,2,4ꢀoxadiazole is also known for its good emissive nature
and high thermal stability, but less studied unlike its 1,3,4ꢀisomer. The unsymmetric ring
structure of 1,2,4ꢀoxadiazole moiety provides a strong lateral dipole leading to the
stabilization of different mesophases in comparison to the LCs derived from 1,3,4ꢀ
1
8–24
oxadiazoles.
Unsymmetrical substitution of 1,2,4ꢀoxadiazole is another effective handle
to tune the resultant material properties. 1,2,4ꢀoxadiazole derivatives are known to possess
high band gap and high S1 and T1 energies. These properties are favorable for the design of
high triplet energy host materials which are required for the fabrication of efficient
2
6
phosphorescent OLEDs. Phosphorescent OLEDs are superior in comparison to OLEDs,
because they trap both the singlet and triplet excitons, to increase the efficiency. Literature
reports suggest that 1,2,4ꢀoxadiazole derivatives have acceptable thermal stability, lower
18
melting points and wider mesophase range in comparison to their 1,3,4ꢀoxadiazole isomers.
o
Further the bending angle of the 1,2,4ꢀoxadiazole core is 140 which is slightly more than the
o
19
bending angle of the 1,3,4ꢀoxadiazole core (134 ). Only few reports are there till now on
20ꢀ25
1,2,4ꢀoxadiazole containing mesogens exhibiting Col phase.
Hence we were interested in
the introduction of 1,2,4ꢀoxadiazole moieties in TSANs core and study the thermal and
photophysical behavior with respect to the earlier reported 1,3,4ꢀoxadiazole based TSANs.
Synthesis and Characterization. The synthetic approach for the preparation of the target
molecules is presented in scheme 1 and 2. The 1,3,5ꢀtrimethoxybenzaldehyde was converted
to corresponding nitrile (1) up on treating with hydroxylamine hydrochloride in
o
dimethylsulfoxide (DMSO) at 100 C for 3 h. Further reflux of nitrile (1) with hydroxylamine
hydrochloride in ethanol led to benzamidoxime intermediate (2). Compound 2 on refluxing
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Cr
Columnar rectangular phase
Columnar hexagonal phase
134
o
1a
40o
1b
1
1c
25
80 135 190 245 300
1
40o
o
Temperature ( C)
Figure 1. Molecular structures of the regioisomeric TSANs 1a and 1b-c and their thermal
behavior (based on the heating scans from DSC)
with 4ꢀnitrobenzoyl chloride in pyridine afforded 5ꢀ(4ꢀnitrophenyl)ꢀ3ꢀ(3,4,5ꢀ
trimethoxyphenyl)ꢀ1,2,4ꢀoxadiazole (3) which on demethylation furnished the key precursor,
namely, 5ꢀ(5ꢀ(4ꢀnitrophenyl)ꢀ1,2,4ꢀoxadiazolꢀ3ꢀyl) benzeneꢀ1,2,3ꢀtriol (4), in 87% yield.
Compound 4 on Oꢀalkylation with 1ꢀbromohexadecane yielded the nitro compound 5, which
on catalytic reduction provided the required aniline 6. Finally, the condensation of aniline 6
with 1,3,5ꢀtriformylphloroglucinol (7) afforded TSANs 1b (Scheme 1). Similarly, the 4ꢀ
nitrobenzaldehyde was converted to its respective nitrile 9 up on heating with hydroxylamine
o
hydrochloride in DMSO at 100 C for 3 h. Further the nitrile (9) was converted to
benzamidoxime intermediate (10) as mentioned earlier, which on further refluxing with 3,4,5ꢀ
trimethoxybenzoyl
trimethoxyphenyl)ꢀ1,2,4ꢀoxadiazole (11). This trimethoxy derivative on demethylation with
BBr furnished the key precursor, 5ꢀ(5ꢀ(4ꢀnitrophenyl)ꢀ1,2,4ꢀoxadiazolꢀ3ꢀyl)benzeneꢀ1,2,3ꢀ
chloride
in
pyridine
afforded
3ꢀ(4ꢀnitrophenyl)ꢀ5ꢀ(3,4,5ꢀ
3
triol (12) in 57 % yield. The triol 12 on Oꢀalkylation with 1ꢀbromohexadecane yielded the
nitro compound 13, which on catalytic reduction provided the corresponding aniline 14.
Finally, the reaction of aniline 14 with 1,3,5ꢀtriformylphloroglucinol afforded TSANs 1c
(Scheme 2). These compounds existed as an inseparable mixture of isomers of C3h and C
s
symmetry, which was quantified by proton NMR by integrating the ratio of the protons
corresponding to each isomer (Scheme 1 and 2, Table 1). This can be explained as follows.
1
The H NMR spectra showed multiple peaks between 8.6ꢀ8.9 and 13ꢀ13.4 due to coupling
b d
between enamine (vinylic: H , H , H
f
, and H
h
) and secondary amine (NH: H
a
, H
c e
, H , and H
g
)
protons. The integrated area under the peaks of H
a
vs. H , H , and H of amine protons or H
c
g
e
b
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d f h
vs. H , H , and H can be considered to calculate the ratio of two forms. However, owing to
the close proximity of chemical shifts of enamine protons, we have considered the secondary
Scheme 1.
Reagents and Conditions (i) NH OH.HCl, DMSO, reflux, 3 h (85%); (ii) NH OH.HCl,
2
2
triethylamine, CH
80%); (iv) BBr , Dry DCM, 0 CꢀRT, 12 h (87%); (v) 1ꢀBromohexadecane, anhyd.K
DMF, 80 C, 12 h (64%); (vi) Pd/C, H , RT, 6 h (70%); (vii) Ethanol, reflux, 6 h (65%).
3
CN, reflux, 24 h (90%); (iii) 4ꢀnitrobenzoyl chloride, pyridiine, reflux, 6 h
o
(
3
2 3
CO ,
o
2
amine protons for estimating the ratio of the two isomers of different rotational symmetry
shown in Table 1). The structures of all the intermediates and target molecules were
confirmed using routine spectroscopic analysis and ESIꢀHRMS or MALDIꢀTOF analysis.
See the supporting information (SI) for the details).
(
(
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Scheme 2.
Reagents and Conditions (i) NH OH.HCl, DMSO, reflux, 3 h (95%); (ii) NH OH.HCl,
2
2
triethylamine, CH CN, reflux, 24 h (90%); (iii) trimethoxy benzoyl chloride, pyridine, reflux,
3
o
6
3
h (61%); (iv) BBr , Dry DCM, 0 CꢀRT, 12 h (57%); (v) 1ꢀBromohexadecane,
o
anhyd.K
2 3 2
CO , DMF, 80 C, 12 h (80%); (vi) Pd/C, H , RT, 6 h (41%); (vii) Ethanol, reflux, 6
h (67%).
Thermal behavior
The mesogenic behavior of these star shaped molecules was studied with the help of
polarizing optical microscopy (POM), differential scanning calorimetry (DSC) and powder Xꢀ
ray diffraction (XRD) studies. Thermal gravimetric analyses (TGA) of these compounds
o
showed that they are stable at least upto 250 C (Fig. S35). The phase transitions and the
accompanying enthalpy changes are summarized in Table 1 and graphically represented in
Figure 1.The data obtained from XRD studies are tabulated in Table 2. The thermal behavior
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1
6e
of compound 1a was reported earlier and for the sake of comparison with the regioisomeric
,2,4ꢀoxadiazole derivatives, it is provided in Table 1.
1
a
o
Table 1. Phase transition temperatures ( C) and corresponding enthalpies (kJ/mol) of regioisomeric TSANs
Entry
Heating
Cooling
b
(
1
1
1
s
C3h: C )
c
a (1:2.6)
(2:1)
(1.8:1)
Cr 63.4 (131.6) Cr 157.3 (29.4) Col 269.2 (5.6) I
I 265.7 (4.9) Col 24.6 (51.5) Cr
1
2
h
h
b
Cr 32.4 (8.9) Col
Cr 33.4 (7.5) Col
h
213.6 (0.1) I
214.6 (0.2) I
I 203.1 (0.2) Col
h
26 (9.9) Cr
c
r
I 204 (0.2) Col 25.7 (8.5) Cr
r
a
Peak temperatures in the DSC thermograms obtained during the second heating and first cooling cycles at 5
o
b
1
C/min.; Col = Columnar hexagonal phase, Col = Columnar rectangular phase; Ratio obtained from H NMR,
h
r
c
See Fig.S13 and Fig.S26; Reported in reference 16e.
Table 2. Results of the (hkl) indexation of the XRD profiles of the compounds at a given temperature (T) of the
a
mesophases .
Compounds
D/Å)
Phase
(T/ C)
Symmetry
d
obs (Å)
d
(Å)
cal
Miller
indices
(hk)
Lattice parameters (Å)
o
2
(
Lattice area S (Å ),
3
Molecular volume V (Å )
a = 52.9; S = 2423.42;
V = 12117.12.
1b
Col
h
45.48
5.00 (h
45.48
10
(65)
(180)
P6mm
a
)
)
)
Col
100)
P6mm
Col
28)
P6mm
Col
h
47.83
4.98 (h
a
47.83
47.85
10
10
a = 55.23; S = 2641.61;
V = 13155.2.
(
h
47.85
4.75 (h
a
a = 55.3; S = 2648.31;
V = 12579.45.
(
r
45.52
24.27
12.70
45.52
24.27
12.87
01
10
13
a = 24.27, b = 45.52;
S = 1104.8;
V = 4109.74;
Z = 0.8.
1c
(140)
P2mm
(60)
5
3
.09 (h
.72 (h
a
)
c
)
Col
60)
P2mm
r
45.54
24.91
12.69
45.54
24.91
12.46
01
10
13
a = 24.91, b = 45.54;
S = 1134.4;
V = 4174.6;
(
4.96(h
a
)
Z = 0.9.
3.68(h
c
)
a
The diameter (D) of the disk (estimated from Chem 3D Pro 8.0 molecular model software from Cambridge Soft,
calculated for the C3h isomer). dobs: spacing observed; dcal: spacing calculated (deduced from the lattice
parameters; a for Col phase, a, b for Col phase). The spacings marked h and h correspond to diffuse
reflections in the wideꢀangle region arising from correlations between the alkyl chains and core regions,
h
r
a
c
c
respectively. Z indicates the number of molecules per columnar slice of thickness h , estimated from the lattice
area S and the volume V.
From the Table 1 and Figure 1, it is evident that the compound 1a exhibited columnar
hexagonal (Col ) phase with higher melting and clearing temperatures in comparison to
h
o
compound 1b and 1c. Compound 1b melted at 32 C with an enthalpy change of 8.9 kJ/mol to
o
form a birefringent fluid. This on further heating cleared to form an isotropic liquid at 214 C.
o
The isotropic liquid on cooling at a rate of 5 C/min exhibited the formation of rectilinear
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birefringent defects in the pseudoisotropic texture, which is a characteristic pattern observed
for Col phases (Fig. 2a). The texture remained as such till room temperature (RT), even
though with a concurrent decrease in the fluidity one lowering the temperature. No signs of
o
crystallization were observed at RT, but the DSC scans showed crystallization 26 C with an
enthalpy change of 9.9 kJ/mol (Fig. 2b).
Figure 2. POM image of compound 1b in Col
compound 1b obtained for second heating (heating: red trace) and first cooling (cooling:
black trace) (b); XRD profiles depicting the intensity against the 2θ obtained for the Col
h
phase at room temperature (a); DSC scans of
h
o
o
o
phase of compound 1b at 180 C, 100 C and at 28 C (c).
The XRD studies were carried out for the sample taken in capillary at different temperature
o
o
o
intervals. The XRD diffraction patterns obtained at 180 C, 100 C and 28 C, exhibited a
single peak at low angle region along with a diffused peak in the wide angle (Fig. 2c, Table
2
). The single peak at low angle can be assigned to the 100 reflection of a hexagonal lattice
P6mm), while the diffused peak at wide angle originates from the packing of flexible alkyl
tails. Though the presence of a single peak at the low angle is not sufficient to assign the
phase to be Col , there are several reports in the literature where the single peak at the low
(
h
27
angle is attributed to a minimum in the form factor. The XRD pattern of compound 1b at 28
o
C showed peak with a dꢀspacing of 47.9 Å, from which the hexagonal cell parameter ‘a’ was
calculated and found to be 55.3 Å (Fig.3). In comparison to the molecular diameter of 65 Å
(for C3h isomer, Fig.6), the observed hexagonal lattice constant is 15% less, which indicates
the intercalation or folding of the peripheral alkyl tails. Considering the large voids present in
the star shaped architecture, it is possible that these flexible tails may fill this vacant space
1
also. Even though the C
s
isomer exists in the Col phase (50% according to the H NMR,
Table 1, Fig.7) formed by compound 1b the molecular diameter does not vary much in
comparison to C3h isomer and it will be averaged out. Basically the C3h and C
s
isomers
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resemble the extensions of shape persistent pseudotriphenylene and benz[d]ꢀanthracenylium
structures, due to the strong intramolecular hydrogen bonding. However owing to the
presence of peripheral flexible alkyl tails the shape is almost discꢀlike even for the C
s
isomer,
16d
which is not a circular core. Further the unit cell area, ‘S’ and unit cell volume ‘V’ were
2
3
found to be 2423.4 Å and 12117.12 Å respectively. A relatively sharp peak at wide angle
corresponding to the stacking of the cores is not observed for 1b even at RT in comparison to
16e
compound 1a, which exhibited a coreꢀcore stacking distance of 3.44 Å. This means that the
coreꢀcore interaction is low in compound 1b in comparison to its 1,3,4ꢀoxadiazole analogue.
The selfꢀassembly of compound 1b is schematically represented in Fig.3.
a = 55.3 Å
3
2.5 Å
Figure 3. Schematic showing the selfꢀassembly of compound 1b in Col phase of p6mm
h
o
symmetry (Derived from the XRD pattern obtained at 28 C, only C isomer shown for the
3h
sake of clarity)
o
o
Compound 1c, melted at a temperature of ≈ 33 C, and had a clearing point of 215 C,
which is similar to that of compound 1b (Fig.4b). It is understandable that the melting point is
related to the melting of alkyl chains, while the clearing point is due to the unstacking of the
cores from the columns. Thus the melting and clearing points remain almost same for both the
compounds. This shows that the change in the orientation of heterocycle i.e. the 3,5ꢀ
disubstitutedꢀ1,2,4ꢀoxadiazole do not have much influence on the melting and clearing points
(Fig.1, Table 1). On cooling the isotropic liquid a mosaic texture was appeared with
birefringent defects in a homeotropic background (Fig.4a). Often the textural pattern of Col
phase is not specific to the symmetry of the Col phase and it has to be crossꢀchecked with the
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XRD studies. XRD studies of the fluidic phase revealed that, it is a Col phase with a
o
o
rectangular symmetry. Powder XRD studies were carried out at 140 C and 60 C. Both the
diffraction patterns were similar and showed a strong reflection at the low angle along with
two weak reflections. In the wide angle two diffused peaks were observed, where the first one
was corresponding to the packing of alkyl tails, while the second one corresponding to the
o
packing of cores. For example the diffraction pattern obtained at 60 C showed three
reflections at low angle corresponding to 45.54 Å, 24.91 Å and 12.69 Å, which can be
indexed to the Miller indices 01, 10 and 13 of a rectangular lattice (P2mm) (Fig.4c).
o
Figure 4. POM image of compound 1c in Col
r
phase at 30 C (a); DSC scans of compound 1c
obtained for second heating (heating: red trace) and first cooling (cooling: black trace) (b);
XRD profiles depicting the intensity against the 2θ obtained for the Col
r
phase of compound
o
1c at 60 C (c).
b = 45.5 Å
a = 24.3 Å
3
0 Å
r
Figure 5. Schematic showing the selfꢀassembly of compound 1c in Col phase of P2mm
o
symmetry (Derived from the XRD pattern obtained at 60 C, only C3h isomer shown for the
sake of clarity)
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The rectangular unit cell has lattice constants a = 24.9 Å and b = 45.5 Å with an area and
2
3
volume of 1134.4 Å and 4174.6 Å respectively (Fig.5, Table 1). The diffused peaks at wide
angles centered at 4.96 Å and 3.68 Å, correspond to the packing of alkyl tails and the stacking
of cores. The presence of the coreꢀcore peak at 3.68 Å signifies the enhanced intermolecular
interaction of these star shaped molecules within the columns. Thus the number of molecules
Figure 6. Molecular models obtained for star shaped compounds 1a, 1b and 1c; Molecular
diameters (D) are given in the inset (estimated from Chem 3D Pro 8.0 molecular model
software from Cambridge Soft for C isomer).
3h
Figure 7. Molecular models obtained for star shaped compounds 1a, 1b and 1c; Molecular
diameters (D) are given in the inset (estimated from Chem 3D Pro 8.0 molecular model
software from Cambridge Soft for C isomer).
s
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(Z) present in the unit cell was calculated and found to be approximately equal to 1. This is
not observed in the case of compound 1b. It is to be noted that Col phases will be stabilized,
r
only when there are enhanced intermolecular interactions between the molecules in the
column.
Further to understand the different behavior of these regioisomeric compounds we have
carried out the molecular modeling studies. The molecular diameters of the star shaped
molecules (estimated from Chem 3D Pro 8.0 molecular model software from Cambridge Soft,
for C3h isomer) were found to be 62 Å for compound 1a, 65 Å for compound 1b and 60 Å for
compound 1c. This means that compound 1a and 1b are more stretched (more planar) in
comparison to compound 1c (Fig.6). Compound 1c attains more of a propeller shape and
because of this the molecules are forced to pack in certain order. This result in the increased
intermolecular interactions within the column, that leads to the stabilization of Col
well as the appearance of coreꢀcore stacking peak in the XRD pattern. However, earlier
reported 1,3,4ꢀoxadiazole based TSANs have shown Col phase, and the presence of coreꢀ
r
phase as
h
core peak. But in the case of 1,2,4ꢀoxadiazole derivatives, the direction of the net dipole
moment is changed in comparison to 1,3,4ꢀoxadiazole derivatives. This difference in the
direction and the extent of dipole moment is the result of bending angle provided by the type
of heterocycle in the arms of the star shaped molecule. It should be noted that the amount of
C isomers in both compounds 1b and 1c were found to be almost 50% (Table 1 and Fig.7),
s
16e
while in the case of compound 1a it is 2.6 times more than the C3h isomer. But it has to be
noted that, irrespective of the difference in the shape of the isomers, these molecules selfꢀ
assembled into columnar structure having a 2D lattice of different symmetries. This shows
that the difference in the shape of the molecules did not have a major impact on the
mesophase stabilization. It is interesting to note that the columnar selfꢀassembly is flexible
enough to accommodate this difference in the molecular shape (Fig.6 and 7).
Photophysical behavior
Photophysical properties of these compounds 1a, 1b and 1c were studied in solution as
well as in thin films. The absorption spectra of compounds in micromolar THF solutions were
taken. Compound 1a and 1b exhibited similar absorption spectra with two absorption bands
centered at 343 and ≈ 423 nm irrespective of the structural difference. Compound 1c showed
a blue shifted absorption maximum centered at 411 nm. The optical band gap calculated from
the red edge of the absorption onset was found to be in the range of 2.62ꢀ2.7 eV. The relative
quantum yields measured were in the range of 0.27ꢀ0.31 (see the SI, Fig. S32). Thin films of
12

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these LCs were prepared by annealing the samples in glass slides by annealing them from
their isotropic state. Absorption spectra of all the three compounds in thin film showed a blue
shift. Emission spectra of compounds 1a and 1b in solution exhibited an emission maximum
centered at 473ꢀ475 nm with shoulders at 496ꢀ499 nm (Stoke’s shift of 51 nm). Emission
spectrum of compound 1c was slightly blue shifted with an emission maximum centered at
463 nm with a shoulder band at 491 nm. All the solutions exhibited a bright green
fluorescence under the UV light of long wavelength (λex = 365 nm). All the thin films showed
a broad red shifted emission spectra in the range of 557ꢀ620 nm with shoulder bands (Fig.8
and Table 3).
Table 3. Photophysical properties of TSANs 1a-c.
Solution
Emission
(nm)
Thin film
Absorption Emission
a
a, b
c, d
e
Entry
Absorption
nm)
Stoke’s
shift
λ
onset
ꢁE g,opt
ε
ꢀ1 ꢀ1
(
(nm)
(nm)
(nm)
(M cm )
ꢀ
1
(cm )
1
a
343, 424
343, 422
411
475, 499
473, 496
51
474
470
460
2.62
2.64
2.70
366, 420
361, 409
351, 396
557, 602
614
73049
64713
72808
1b
51
1
c
463, 491
52
545, 620
a
b
c
d
Micromolar solutions in THF. Excited at the respective absorption maxima. In electron volts (eV). Band gap
was determined from the red edge of the longest wave length in the UVꢀVis absorption spectra. Emission
obtained by exciting the thin films at their respective absorption maxima.
e
1a
(c)
(b)
(a)
!
"#
! $#
! %#
1b
1c
Figure 8. Normalized absorption (solid line) and emission spectra (dotted line) in THF
solution obtained for 1a-c (a); Pictures of micromolar solutions of compounds 1a-c in THF as
seen with the UV illumination (λ = 365 nm)(b); Normalized emission spectra obtained for
compounds 1a, 1b and 1c in the thin film state (c), right side shows the images of thin films
with the UV illumination (λ = 365 nm).
Conclusions
We have reported two new regioisomeric starꢀshaped tris(Nꢀsalicylideneaniline)s, whose arms
differ from each other with respect to the substitution at the 3,5ꢀpositions of the central 1,2,4ꢀ
oxadiazole moieties. The effect of this unsymmetrical substitution led to a change in the
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electron density distribution, which exhibited an effect on the type of the columnar selfꢀ
assembly. One of these molecules stabilized columnar hexagonal phase, while the other one
stabilized columnar rectangular phase. Columnar rectangular phase, which require enhanced
intermolecular interactions was observed in the case of starꢀshaped molecule, where the
trialkoxy phenyl group is connected at the 5ꢀposition of the heterocycle, whereas columnar
hexagonal phase was observed in the case of starꢀshaped molecule, where the trialkoxy
phenyl group is connected at the 3ꢀposition of the heterocycle. These compounds showed
reduced melting point, clearing point and enhanced mesophase range with respect to the
symmetric counterpart (1,3,4ꢀoxadiazole derivative) reported earlier. All the molecules
exhibited green light emission in solution, with a comparable quantum yield. 1,2,4ꢀoxadiazole
derivatives exhibited much redꢀshifted emission in comparison to 1,3,4ꢀoxadiazole derivatives
in annealed thin film state. This study provides an interesting view on how a simple structural
variation, tunes the selfꢀassembly behavior of liquid crystalline molecules. Considering the
ability of these molecules to stabilize luminescent Col phases, these molecules have a bright
potential for the application in OLEDs.
Conflicts of interest. There are no conflicts of interest to declare
Acknowledgements. ASA sincerely thanks Science and Engineering Board (SERB), DST,
Govt. of India and BRNSꢀDAE for funding this work through project No. SB/S1/PCꢀ37/2012
and No.2012/34/31/BRNS/1039 respectively. We thank the Ministry of Human Resource
Development for Centre of Excellence in FAST (F. No. 5ꢀ7/2014ꢀTSꢀVII).
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