Tuning the self-assembly and photophysical properties of bi-1,3,4-thiadiazole derivatives through electron donor-acceptor interactions and their application in OLEDs

Article abstract of DOI:10.1039/c7tc01420a

We report several shape anisotropic molecules that contain two centrally placed 1,3,4-thiadiazole units, which vary from each other with respect to the number and length of the flexible chains at the termini. The number, position and length of the periphe

Full text of DOI:10.1039/c7tc01420a

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DOI: 10.1039/C7TC01420A.
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Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C7TC01420A
Tuning the self-assembly and photophysical properties of bi-1,3,4-
Thiadiazole derivatives through electron donor-acceptor interactions
and their application in OLEDs
Abhay Kumar Yadav,a# Balaram Pradhan,a# Hidayath Ulla,a# Subrata Nath,b Joydip
De,b Santanu Kumar Pal,b M. N. Satyanarayanc and Achalkumar Ammathnadu
Sudhakar*a
aDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
781039, Assam, India. bDepartment of Chemical Sciences, Indian Institute of Science
Education and Research Mohali, Sector-81, Knowledge City, Manauli 140306,
Punjab, India. cDepartment of Physics, National Institute of Technology Karnataka-
Surathkal, Srinivasnagar, Mangalore, 575025, Karnataka, India.
# These authors contributed equally to this work.
*Email - achalkumar78@gmail.com, achalkumar@iitg.ernet.in
Abstract: We report several shape anisotropic molecules containing two centrally
placed 1,3,4-thiadiazole units, which vary from each other with respect to the
number and length of the flexible chains at the termini. The number, position and
length of the peripheral chains connected to the termini showed an impact on the
thermal behavior of these compounds. The compounds with two terminal tails
exhibited enantiotropic smectic C phase, while the compounds with four terminal
tails turned to be crystalline. Surprisingly, among the compounds with six terminal
tails, only the compound with longer terminal chain exhibited columnar phase with
oblique symmetry. It is also to be noted that only compounds with six terminal
chains exhibited gelation in long chain hydrocarbons. The xerogel of the hexacatenar
with six n-decyloxy chains showed the entangled network of nanofibers of several
micrometer lengths. The aggregation behavior of the hexacatenar in hydrocarbon
solvent, is mainly supported by the attractive π-π interactions of the aromatic cores
and van der Waals interactions offered by the peripheral flexible tails. The emission
behavior is dependent on the number of peripheral tails and not on the length.
Further one of the hexacatenar exhibited solvatochromic emissive behavior. This
molecular design helps in the development of long molecular nanowires with a
central conducting core and insulating peripheral sheath, which will be helpful for
the application in organic electronic devices. The application potential of the
columnar liquid crystal material was tested by the fabrication of organic light
emitting diodes (OLEDs) either as single emissive material or as a guest material in a
host polymer. Higher efficiency and brightness was noticed in the host guest OLED,
which exhibited a technologically important bright blue emission.
Introduction
Liquid crystalline semiconductors seem to be a bright option for the application in
organic electronics considering their good solubility, processability and excellent
1,2
reproducibility along with the ease of purification. Additionally, the addressability of
their molecular alignment and charge trapping by defects in the thin films by thermal
1

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DOI: 10.1039/C7TC01420A
annealing provides a major advantage over amorphous polymers and organic single
crystals. Liquid crystalline self-assembly, especially the thermotropic liquid crystals
(LCs) demand an anisotropic molecular structure with chemically or physically
incompatible molecular subunits.3 Conventionally the liquid crystals are designed with a
rod (calamitic) or disc shaped (discotic) rigid core connected with one or several flexible
aliphatic chains. Over the year many nonconventional designs have been explored out of
scientific curiosity and with a quest to obtain novel mesophases. Polycatenars or
‘phasmids’ are one such class of LCs, which form the hybrid structures between the
calamitic and discotic molecular structures.4–6 Obviously they exhibit LC phases that are
common to the both classes of LCs, like nematic phase, smectic or lamellar phase, cubic
phase and columnar phases.3 This depends on the number of flexible terminal tails
connected to the rigid cores. Considering the organic electronics, smectic (Sm) and
columnar (Col) phases are ideal candidates, as they are blessed with the strong π-π
overlap, due to their layered or columnar packing, which is useful for the charge
transport. Design of π-conjugated molecules containing strong donor (D) and acceptor
(A) systems enhance the intramolecular charge transfer and also reduce the band gap,
making them attractive for the applications.7 They are considered to be interesting due to
their improved technological application in the area of organic solar cell, organic field
effect transistors,8,9 organic light emitting diodes (OLEDs),10 chemo sensors11 and crystal
engineering.12–15 This molecular design also enhances the intermolecular interactions
required for the self-assembly process. Incorporation of heterocyclic units in the
molecular structure alters the molecular geometry, provides strong lateral and/or
longitudinal dipoles, enhances the donor-acceptor interaction and alters the electronic
properties of the molecules.16,17 Such weak forces help in the one-dimensional self-
assembly of D-A systems in presence of solvents leading to the formation of stable
organogels, which have the potential in the area of organic photovoltaics, templates for
the growth of inorganic nanostructures, controlled release systems, energy transfer and
fluorescent sensors.18 Combination of good luminescence along with the self-assembled
ordered structures are important in the construction of organic light emitting diodes
(OLEDs), where they can be used as conductive and emissive layers of OLEDs.19
Further, it will be advantageous if the luminescence is not quenched in the aggregated
state, as it will affect the device efficiency.20 As part of our continued efforts to combine
luminescence with liquid crystalline properties, we have synthesized several luminescent
polycatenar mesogens stabilizing Col phases.21–24 Among them 1,3,4-thiadiazole
2

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DOI: 10.1039/C7TC01420A
containing LCs have shown good luminescence and wide range mesophases in
comparison to their oxadiazole counterparts.21,23,24 Some of these derivatives exhibited
gelation, which was mainly driven by the attractive π-π interactions, leading to the
formation of highly ordered structures over a long range.23,24 Recently Li et.al. reported a
series of bi-1,3,4-oxadiazole derivatives with two alkyloxy tails, which stabilized tilted
smectic phases due to the intermolecular donor-acceptor interaction within the smectic
layer.25 This is because in the tilted arrangement, there is more scope for the interaction
of donor moiety (trialkoxy phenyl) of one molecule with the acceptor moiety (1,3,4-
oxadiazole unit) of another molecule. It is interesting to note that the hexacatenars based
on this motif stabilized Col phases of rectangular symmetry and lamellar type.26 They
have also shown that, one of the hexacatenar can self-assemble to form nanoparticles and
further to form helical nanofibers showing bright blue emission in polar solvent like
DMSO. The same molecule exhibited gelation in ethanol with an enhanced emission.27 A
bi-1,3,4-thiadiazole analogue with two n-tetradecyloxy tails was reported to exhibit a
stable smectic C (SmC) phase over a wide thermal range.28 Tetracatenars and
hexacatenars based on this molecular motif have not been published hitherto. In this
paper we report the synthesis and characterization of shape anisotropic molecules
comprising two centrally placed 1,3,4-thiadiazole units (electron deficient, A), which are
covalently linked with peripherally substituted phenyl units (electron rich, D). This forms
a covalently linked D-A-A-D system. The variation in the number and the length of the
flexible chains at the terminal was expected to affect the self-assembly and photophysical
behavior. Their thermal, photophysical and gelation behaviors are thoroughly
investigated. One of the molecules stabilizing columnar mesophase has been tested for its
application in the fabrication of OLEDs.
Results and discussion
Synthesis and Characterization. The synthetic pathway for the target molecules (1T/6-
8, 2T/6-8 and 3T/6-8) and their precursors is represented in Scheme 1. O-alkylation of
ethyl-4-hydroxy benzoate, ethyl-3,4-dihydroxy benzoate and ethyl gallate with
appropriate n-alkyl bromides yielded corresponding alkoxy derivatives (4a-c, 7a-c and
10a-c). These esters were converted to the corresponding hydrazides 5a-c, 8a-c and 11a-
c on treatment with hydrazine hydrate in ethanol. These hydrazides were then treated
with oxalyl chloride29 in THF to obtain the bis-benzohydrazide derivatives 6a-c, 9a-c and
3

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12a-c. After isolation these bis-benzohydrazides were directly refluxed with Lawesson’s
reagent in toluene, yielding target bi-1,3,4-thiadiazole derivatives 1T/6-10, 2T/6-10 and
3T/6-10 respectively. All final compounds as well as all the intermediates were
characterized by various analytical techniques such as 1H NMR, 13C NMR, IR
HO
HO
COOEt
HO
COOEt
(i)
COOEt
COOEt
HO
R
HO
HO
(i)
(i)
R
COOEt
R
COOEt
4a: R = OC6H13
4b: R = OC8H17
4c: R = OC10H21
R
R
10a: R = OC6H13
10b: R = OC8H17
10c: R = OC10H21
R
(ii)
7a: R = OC6H13
7b: R = OC8H17
7c: R = OC10H21
(ii)
R
R
CONHNH2
(ii)
R
CONHNH2
5a: R = OC6H13
5b: R = OC8H17
5c: R = OC10H21
R
CONHNH2
R
11a: R = OC6H13
11b: R = OC8H17
11c: R = OC10H21
(iii)
R
8a: R = OC6H13
8b: R = OC8H17
8c: R = OC10H21
O
O
O
(iii)
O
R
R
R
R
R
HN
NH
HN
NH
O
O
O
O
(iii)
R
R
6a: R = OC6H13
6b: R = OC8H17
6c: R = OC10H21
HN
NH
HN
NH
R
R
O
12a: R = OC6H13
12b: R = OC8H17
12c: R = OC10H21
O
O
O
R
HN
NH
HN
NH
(iv)
R
R
R
R
9a: R = OC6H13
9b: R = OC8H17
9c: R = OC10H21
(iv)
R
R
S
N
S
N
R
R
N
N
R
S
N
R
S
N
(iv)
1T/6: R = OC6H13
1T/8: R = OC8H17
1T/10:R = OC10H21
N
R
N
R
R
S
N
R
S
N
3T/6: R = OC6H13
3T/8: R = OC8H17
3T/10: R = OC10H21
N
N
2T/6: R = OC6H13
2T/8: R = OC8H17
2T/10: R = OC10H21
Scheme 1. Synthesis of bis-1,3,4-thiadiazole based polycatenars. Reagents and
o
conditions (yield) (i) n-Bromoalkanes, anhydrous K2CO3, DMF, 80 C, 24 h (68-85%);
4

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(ii) NH2NH2.H2O, Ethanol, reflux, 48 h (65-80%); (iii) Oxalyl chloride, dry TDHOFI:,101.12039h/,C7TC01420A
RT (iv) Lawesson's reagent, toluene, reflux, 17 h (45-60%).
spectroscopy, and ESI-HRMS or MALDI-TOF analysis (see ESI for details).
Thermal behavior. Polarizing optical microscopy (POM), differential scanning
calorimetry (DSC) and X-ray diffraction (XRD) were employed to analyze the thermal
behavior of new bi-1,3,4-thiadiazole derivatives. The preliminary evidence for the liquid
crystallinity was the conversion of the crystalline sample to a shearable fluidic
birefringent material on heating. The phase transition temperatures and associated
enthalpy changes were observed in the DSC traces were in accordance with the POM
observations. All the mesophases observed were finally confirmed by XRD studies
carried out at different temperatures. The thermal behavior of all the target molecules is
summarized in Table 1 and schematically represented in Fig.1.
a
Table 1. Phase transition temperatures (oC) and corresponding enthalpies (kJ mol-1) of
Entry
Phase sequence
2nd Heating
1st Cooling
I 292.5 (43.1) SmC 211.8 (57.9) Cr
Cr 223.6 (54.9) SmC 301.5 (43.5) I
1T/6
1T/8
Cr1 100.1 (152.3) Cr2 125.9 (91.8) Cr3 I 296.5 (6.2) N 285.2 (58.7) SmC 179.5
190.3 (49.5) SmC 297.2 (70.4) N 305.4 (64.9) Cr3 103.2 (34.6) Cr2 50.4 (54.5) Cr1
(12.4) I
Cr 136.1 (48.3) SmC 220.6 (17) I
Cr1 141.7 (27.5) Cr2 170.6 (7.3) I
I 212.7 (14.5) SmC 127 (48.3) Cr
I 148.9 (27) Cr3 126.3 (11.9) Cr2 69.9 (4.4)
Cr1
1T/10
2T/6
Cr 176.4 (69.9) I
Cr1 132.2 (66.6) Cr2 155.9 (19.4) I
Cr 96.7 (21.6) I
I 164 (59.1) Cr
I 135.3 (46) Cr2 124.99 (7) Cr1
I 86.25 (20) Cr
2T/8
2T/10
3T/6
Cr 86.2 (20.8) I
Cr 73.07 (7.5) Colob 82.3 (7.9) I
I 77.61 (18.6) Cr
I 74.95 (11.4) Colob
3T/8
3T/10
b
the bi-1,3,4-thiadiazole derivatives.
a
Peak temperatures in the DSC thermograms obtained during the first cooling and
second heating cycles at 5 C min-1; The mesophase is freezing in glassy state and not
crystallizing up to room temperature. (Cr = crystal; Colob = columnar oblique; SmC =
Smectic C; N = Nematic; I = isotropic liquid.)
o
b
Thiadiazole derivatives 1T/6-10 with two alkyloxy terminal tails connected to the
terminal benzene rings at 4-position exhibited enantiotropic mesomorphism exhibiting
calamitic mesophases. The crystalline sample of compound 1T/6 on heating showed a
o
transition at a temperature of ≈ 223 C (∆H = 55 kJ/mol) as noticed from DSC. At this
temperature it become shearable to a birefringent fluid.
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1T/6
1T/8
1T/10
2T/6
Crystal
Smectic C Phase
Nematic Phase
Columnar Oblique Phase
2T/8
2T/10
3T/6
3T/8
3T/10
245
310
115
180
50
1O/6
1O/8
1O/10
Crystal
Smectic C Phase
Nematic Phase
Columnar Lamellar Phase
3O/6
3O/8
3O/10
310
245
180
115
50
Temperature (oC)
Figure 1. Bar graph summarizing the thermal behavior of compounds 1T/6-8, 2T/6-8
and 3T/6-8 (first heating scan, upper part) and reported oxadiazole derivatives25,26 (first
heating scan, lower part).
T = 291 oC
T = 220 oC
I
SmC
Cr
Cr
SmC
I
(b)
(c)
(a)
(e)
(d)
L = 32 Å
40 o
(b)
Figure 2. DSC thermograms of compound 1T/6 showing first cooling (blue trace) and
o
second heating (red trace) (a); POM image of compound 1T/6 at 291 C in SmC phase
(b); POM image of compound 1T/6 at 220 oC in SmC phase (after shearing) (c); Intensity
o
vs 2θ profile obtained for compound 1T/6 at 250 C in SmC phase (inset shows the
image pattern) (c); Schematic showing the self-assembly of compound 1T/6 in the SmC
phase (d).
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The high enthalpy changes corresponding to the phase transitions point to tDhOeI: 1s0t.r1o03n9g/C7TC01420A
intermolecular association due to the presence of donor and acceptor moieties in the
molecular structure. Further heating converted the birefringent fluid into isotropic liquid
o
at around ≈ 302 C (Fig.2a and Table 1). Slow cooling of the isotropic melt leads to the
formation of spherulitic droplets which soon elongate to form the batonnet structures and
finally coalesce to form the broken focal conic domains (Fig.2b). The width of the
o
mesophase was found to be 80 degrees in cooling cycle and crystallized at 212 C.
Mechanical shear of the textural pattern leads to a schliren texture, which is the
characteristic of Smectic C (SmC) phase (Fig.2c). However, to confirm the phase
structure, powder XRD studies were carried out in the mesophase. The XRD pattern
o
obtained at 250 C, showed a sharp reflection at small angle with a d-spacing of 24.5 Å
along with a diffused halo at wide angle with a d-spacing of 4.68 Å (Fig.2d). The first d-
spacing corresponds to the layer spacing of SmA phase, while the diffused halo at the
wide angle corresponds to the packing of alkyl tails. The reduced d-spacing with respect
to the molecular length (L) of 32 Å corresponds to the tilting of the molecules with
respect to the layer normal (d/L = 0.8, tilt angle is 40o) (Table 2 and Fig.2e).
Figure 3. DSC thermograms of compound 1T/8 showing first cooling (blue trace) and
o
second heating (red trace) (a); POM image of compound 1T/8 at 302 C in N phase (b);
o
at 296 C in SmC phase (c); Intensity vs 2θ profile obtained for compound 1T/8 at 250
oC in SmC phase (d).
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Compound 1T/8 was bimesomorphic with small width of nematic (N) phase
along with SmC phase. Nematic phase was evidenced by the presence of two fold and
four-fold brush defects in the same sample (Fig.3b). Smectic C phase was visualized by
the broken focal conic texture (Fig.3c), which on shearing transforms to a schlieren
texture.30 The thermal range of the mesophase was greater than that of the lower
homologue 1T/6. The phase was further confirmed by the XRD studies (Fig.3d). Similar
to the case of 1T/6, the d/L ratio observed was 0.74, which confirms the mesophase to be
a tilted SmC phase.28 The higher homologue of this series 1T/10 was monomesomorphic
and exhibited a shorter-range SmC phase as evidenced from POM, DSC and XRD
studies (See Fig.S55, Table 2). It has to be noted that the XRD pattern shows the
presence of two d-spacings at low angle in 1:2 ratio, and they are originating from the
molecular subunits. The d/L ratio was found to be 0.75, which suggested tilting of the
molecules with respect to the layer normal. Tetracatenars 2T/6-10, with four hexyloxy,
octyloxy and decyloxy tails connected to the terminal benzene rings at 3 and 4-positions
turned out to be crystalline in nature. Surprisingly, the hexacatenars 3T/6 and 3T/8 were
also crystalline (Fig.S56-S60).
Table 2. Results of indexation of XRD profiles of the compounds 1T/6-8 at a given
temperature (T) of the mesophase.
Compounds
Phase
(T/oC)
dobs (Å)
Miller
indices
(hkl)
d/L
ratio
Tilt
angle
(L/Å)
SmC
(250)
SmC
(250)
SmC
(200)
24.50
4.68
27.05
4.62
30.74
15.39
5.04
100
0.77
0.74
0.75
40.04
42.35
41.74
1T/6
(32.0)
1T/8
(36.6)
1T/10
(41.2)
100
100
200
Compound 3T/10 with six n-decyloxy chains on cooling from isotropic liquid
showed a growth of leaf like texture, which on further cooling transformed into a mosaic
texture. This texture remained as such till room temperature, and no crystallization was
noticed. The sample was not shearable at room temperature, confirming the frozen nature
of the mesophase in glassy state. From the temperature dependent XRD studies it was
evidenced that Colob phase was stabilized throughout the thermal range. For example, the
XRD pattern at 70 oC exhibited several peaks in low angle to middle angle region with d-
spacing 31.8 Å, 18.6 Å, 16.9 Å, 15.6 Å, 13.5 Å, 11.5 Å, and 11.1 Å corresponding to
8

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Figure 4. DSC thermogram of compound 3T/10 in the second heating (red trace) and
o
first cooling (blue phase) at the scan rate of 5 C/min (a); POM images obtained for
compound 3T/10 at 70 oC (b); at room temperature (c); Intensity vs 2θ XRD pattern
obtained for compound 3T/10 at 70 oC (d) and at 25 oC (e) for Colob phase.
Miller indices (02), (10), (11), (-13), (-14), (-15) and (14) of a Col phase with oblique (p1
symmetry) lattice. Along with this, two peaks at the wide-angle region with the d-
spacings of 4.97 Å and 3.86 Å were observed. They correspond to the packing of flexible
tails and the cores respectively. The peak corresponding to average stacking distance is
quite strong, which signifies the enhanced intermolecular interactions within the
columns. The lattice parameters derived from the reflections at the low angle were found
to be a = 19 Å and b = 65.1 Å, with the angle between them being γ = 77.6o. The value of
γ, which is not equal to 90o, points that the phase under investigation is Colob with a 2D
parallelogram lattice. The lattice area S and volume of the unit cell V were found to be
1210.2 Å2 and 4671.4 Å3 respectively. The number of molecules present in the unit cell,
denoted by Z was found to be 2. The packing of two hexacatenars to form disc-like unit
and their self-assembly into Col phase can be represented as follows. From the XRD data
it is clear that on an average two hexacatenars could be packed inside the unit cell, by
arranging side by side to ensure the space filling (Fig.5).31 These sets of two molecules,
forming a columnar slice, later stack up to form columns, and further these columns self-
organize in a 2D parallelogram lattice with p1 symmetry (Fig.5).32 XRD studies carried
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out at lower temperature, i.e. at 50 C and 25 C (Fig. S61, Fig.4e and Table 3) also
showed the similar diffraction patterns and confirmed the existence of Colob phase
throughout the mesophase range.
Table 3. Results of indexation of XRD profiles of the compounds 3T/10 at a given
temperature (T) of the mesophase.a
Compounds
Phase
Symmetry
(T/oC)
Colob
dobs (Å)
dcal(Å)
Miller
indices
hk
02
10
Lattice parameters (Å),
Lattice area S (Å2),
Molecular volume V (Å3)
a = 19.04
(D/Å)
31.78
18.60
16.90
15.55
13.53
11.52
11.11
4.97 (ha)
3.86 (hc)
31.78
18.42
16.90
15.50
13.50
12.56
11.52
11.13
9.77
31.78
18.60
16.90
15.76
13.61
11.73
10.97
p1
(70)
b = 65.08
3T/10
(43.0)
11
γ = 77.6o
-13
-14
-15
14
S = 1210.2
V = 4671.4
Z = 2.2
Colob
p1
(50)
31.78
18.42
16.90
15.33
13.27
12.71
11.47
11.09
9.68
02
10
11
-13
-14
05
-15
14
a = 18.72
b = 64.61
γ = 79.66o
S = 1189.86
V = 4557.2
Z = 2.2
15
9.28
9.30
-22
4.95 (ha)
3.83 (hc)
31.79
18.22
16.95
15.56
13.52
12.30
11.51
9.74
Colob
p1
(25)
31.79
18.22
16.95
15.45
13.82
12.15
11.96
9.83
9.13
8.66
7.16
6.49
12
22
10
13
35
33
21
14
32
03
15
04
a = 45.11
b = 69.14
γ = 22.07o
S = 1171.9
V = 4464.9
Z = 2.1
9.29
8.63
7.20
6.75
4.88 (ha)
3.81 (hc)
aThe diameter (D) of the disk (as estimated from the Chem 3D Pro 8.0 molecular model software from
Cambridge Soft). dobs: spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a
and b for Colob phase, γ is the tilt angle). The spacing marked ha and hc corresponds to the diffuse reflection
in the wide-angle region arising from correlations between the alkyl chains and cores respectively. Z
indicates the number of molecules per columnar slice of thickness hc (core–core stacking), estimated from
the lattice area S and the volume V.
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Thus on looking at the thermal behavior of these three series, we can conclude that the
length of the peripheral chain and pattern of substitution determines the type of
mesophases. Bis-thiadiazole derivatives 1T/6-10 stabilized calamitic phases over a wide
thermal range. In comparison to the oxadiazole analogues reported earlier,25 these
compounds exhibited higher melting and clearing temperature but with enhanced
mesophase width (Fig.1). The D-A-A-D arrays formed by the donor moities (alkoxy
phenyl) and acceptor moieties (1,3,4-thiadiazole units), forces the molecules within the
same layer to have intermolecular D-A interactions. This is possible only when the
molecules are tilted with respect to the layer normal. In such condition the donor moiety
of one molecule comes closer to the acceptor moiety of the other molecule as shown in
Fig.6. Li et. al. also sugested the similar explanation for the analogous oxadiazole
derivatives.25 The tilt angle of the SmC phase for 1T/6-10 was almost 10o less than that
of corresponding oxadiazole derivatives. This may be due to the wide bending angle of
1,3,4-thiadazoles (164o) in comparison to their 1,3,4-oxadiazole (134o) derivatives.24
Tetracatenars 2T/6-10 turned out to be crystalline. Similarly, the hexacatenars 3T/6 and
3T/8 were crystalline, while compound 3T/10 stabilized Colob phase. This means that
tetractenar design is not favorable to stabilze any calamitic or Col phases.
a = 45.11 Å
γ = 22.1o
b = 69.14 Å
c = 3.81 Å
Figure 5. Schematic showing the self-assembly of compound 3T/10 into Colob phase of
p1 symmetry (for the XRD pattern obtained at 25 oC).
In the case of hexacatenars, the length of the chain also matters, i.e. to provide sufficient
space filling and nanoseggregation. This again confirms the delicate requirements of
molecular design needed for the liquid crystalline self-assembly. Thus hexacatenars 3T/6
and 3T/8 are crystalline, while hexacatenar 3T/10 stabilizes Colob phase, which freezes to
form a glassy state at room temperature. In comaprison to oxadiazole based
hexacatenars26 where all the homologues showed Colr or ColL phase, here only n-
decyloxy derivative (3T/10) stabilized Col phase with oblique symmetry.
11

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DOI: 10.1039/C7TC01420A
D
D
A
D
A
A
D
D
A
D
=
=
A
D
A
D
A
A
A
A
D
D
Figure 6. Schematic diagram showing the self-assembly of compound 1T/6 into SmC
phase, driven by the D-A interactions between the molecules stacked in the layers
resulting in the tilt.
Photophysical properties. Photophysical properties of compounds 1T/6-10, 2T/6-10
and 3T/6-10 in solution was examined and the details are summarized in Table 4.
Absorption and emission spectra of all the compounds were recorded in micromolar THF
solution (Fig. 7). It is clearly visible that, the absorption spectra for the solutions of all
compounds showed a single absorption maximum in the range of 361-380 nm,
irrespective of the substitution. The single absorption band for these compounds are
corresponding to spin allowed π-π* transition of the aromatic system. In comparison to
similar 1,3,4-oxadiazole derivatives, these compounds exhibited a red shifted absorption
and emission behavior.25,26 These bis-1,3,4-thiadiazole derivatives showed large value of
molar absorption coefficients indicating that the systems are highly conjugated. The
values of molar extinction coefficients (ε) of bi-1,3,4-thiadiazole derivatives were in the
range of 25,000-29,000 M-1cm-1 for 1T/6-10, 31,000-32,000 M-1cm-1 for 2T/6-10 and
36,000-39,000 M-1cm-1 for 3T/6-10. The optical band gaps (Eg,opt) estimated from the red
edge of the absorption spectra were found to be 3.02 eV, 2.89 eV and 2.94 eV for the
three different series respectively. Emission spectra of all compounds exhibited a single
emission maximum (for 1T/6-10: 438-443 nm; for 2T/6-10: 467 nm; for 3T/6-10: 494-
496 nm). The emission spectra were much red shifted in the case of 3T/6-10 and they
also showed larger Stoke’s shift in comparison to other two series (Table 4). From the
Fig.7d it is clear that progressive substitution of the cores with alkyl tails leads to red
shifted emissions, as seen from the deep blue emission of compounds 1T/6-10, cyan
colored emission of compounds 2T/6-10 and the bright green emission of compounds
3T/6-10. Steady state anisotropy values of the compounds did not show any general
trend, but compounds 1T/6-10 exhibited larger values compared to compounds 2T/6-10
and 3T/6-10. The relative quantum yields calculated with respect to quinine sulphate
12

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Journal of Materials Chemistry C
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solution (0.1M H2SO4) was found to be promising and were in the range of D0O.3I: 710-.100.3597/C7TC01420A
(Fig.S62).
Table 4. Photophysical properties of compounds bi-1,3,4-thiadiazole derivatives in
solution state a and film state.f
Entry
Solution State
c,
Absorptio Emissio
n (nm)
Stokes
ΔEg,opt
Steady
State
anisotropy
0.073
0.130
0.139
0.078
0.086
0.077
0.053
Quantum
Yielde
d
nb (nm)
shift
(cm-1)
5128
4922
4870
5042
4903
5042
6495
6577
6495
361
361
361
378
380
378
374
374
374
443
439
438
467
467
467
494
496
494
3.02
3.02
3.02
2.89
2.89
2.89
2.94
2.94
2.94
0.37
0.40
0.41
0.45
0.41
0.37
0.48
0.54
0.57
1T/6
1T/8
1T/10
2T/6
2T/8
2T/10
3T/6
3T/8
3T/10
0.068
0.080
amicromolar solutions in THF; bexcited at the respective absorption maxima;
cBand gap determined from the red edge of the longest wave length (λonset) in the
d
e
UV-vis absorption spectra; In electron volts (V). Relative quantum yield
measured with quinine sulfate in 0.1 M H2SO4 solution as a standard.
Table 5. Photophysical properties of compounds bi-1,3,4-thiadiazole derivatives in thin
film state.a
Entry
Absorption
(nm)
Emissionb
(nm)
473
Stokes shift
(cm-1)
5383
Thickness of
the filmsc
88 nm
Absorption
coefficients
7.5 × 104 cm-1
15.8 × 104 cm-1
6.7 × 104 cm-1
377
366
372
3T/6
3T/8
3T/10
475
493
6269
6598
72 nm
70 nm
aSpin coated films on quartz plates using micromolar solutions in chloroform.
bexcited at the respective absorption maxima. cmeasured from ellipsometry.
(b)
(a)
(d)
(c)
1T/6
1T/8
2T/6 2T/8 2T/10 3T/6 3T/8 3T/10
1T/10
Figure 7. Normalized absorption (solid line) and emission (dotted line) spectra of
compounds 1T/6-10 (a); 2T/6-10 (b); 3T6/10 (c) in micromolar THF solutions;
photographs of the same solutions taken under the UV light (λex = 365 nm) (d).
13

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DOI: 10.1039/C7TC01420A
We have tried to prepare the thin films of the compounds by spin coating the solutions of
compounds in chloroform on quartz slides. Compounds 1T/6-10 and 2T/6-10 did not
show good solubility and film forming property. This may be due to the reduced number
of solubilizing chains. Compounds 3T/6-10 were soluble nicely and formed good films
on quartz plates. The solutions of these compounds (8-10 mM) in chloroform were spin
coated at 500 rotations per minute (rpm) speed for initial 30 seconds and at 800 rpm for
the next 30 seconds. They are allowed to dry at ambient temperature. The absorption
bands and emission bands of these thin films were found to be broad in comparison to
their solution spectra (Fig. 8, Table 5). The absorption and emission spectra of compound
3T/10 was almost matching with the solution spectra (Table 4 and 5). The thickness of
the films was measured by ellipsometry technique. From these values, the absorption
coefficients of these thin films were calculated which were found to be in the range of
6.7-7.5 × 104 cm-1 (Table 5). On irradiation of long wavelength UV light, the thin films
of these compounds in solution as well as in thin films showed visually perceivable
bright blue to green emission (Fig. 7c and 8c).
3T/6
(a)
(b)
(c)
3T/8
3T/10
Figure 8. Absorption spectra of compounds 3T/6-10 (a); Emission spectra of compounds
3T6/10 (b) obtained in the thin film state prepared by the spin coating on quartz plates;
(c) the photographs of the thin films taken under the UV light (λex = 365 nm).
In order to understand the behavior of compound with respect to solvent polarity,
photophysical studies of compound 3T/10 as a representative case was carried out in
different solvents (Fig.9, Table 6). The absorption spectra did not vary much with respect
to solvent polarity, pointed to the nonpolar nature of the compound in the ground state. In
contrary the emission spectra of compound 3T/10 showed a red shift on moving from
nonpolar to polar solvents, which indicated the polar nature of the compounds in the
excited state. The red shifted emission in the polar solvent is due to the reduced energy
gap between the ground state and excited state of the compound. This is in turn due to the
14

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presence of donor and acceptor moieties in conjugation, which leads to the deloDcOaIl:i1z0a.1t0i3o9n/C7TC01420A
of electron cloud towards acceptor side from the donor side. This leads to an internal
charge-transfer state in addition to the locally excited state.24 The internal charge-transfer
state is favored by the increased solvent polarity. Thus the energy gap between the
ground and excited states is reduced in polar solvents, resulting in a red shifted emission
(Fig.9).
Figure 9. Normalized Absorption spectra (a); emission spectra of compound 3T/10 in
different solvents (b); Photographs taken under the UV light (λex = 365 nm) in decane
(nonpolar) and chloroform (polar) solvents in micromolar concentrations (c).
Table 6. Photophysical properties of compound 3T/10 in different solventsa
Solvent
Decane
Hexane
CCl4
Toluene
DCM
THF
Chloroform
EtOH
Absorption (nm)
Emissionb (nm)
Stokes shift (nm)
374
373
377
378
375
375
377
373
374
456
452
469
475
510
494
494
540
540
82
79
92
97
135
119
117
167
166
DMF
amicromolar solutions in different solvents; bexcited at the respective
absorption maxima.
Electrochemical properties. Energy levels of HOMO and LUMO levels of the
representative bi-1,3,4-thiadiazole derivatives were obtained by cyclic voltammetry (CV)
and the data are tabulated in table 6. CV scans of the compounds displayed irreversible
oxidation and reduction waves (See Figure S64). The optical band gap Eg,opt was
estimated from the absorption onset obtained from the absorption spectra in solution.
Energy levels of LUMO and HOMO were determined by using the formulae ΔECV
=
ELUMO – EHOMO eV, EHOMO = -(4.8 - E1/2,Fc,Fc+ +Eox, onset) eV and ELUMO = - (4.8 - E1/2,Fc,Fc+
15

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+ Ered,onset) eV. Compounds 1T/10, 2T/10 and 3T/10 exhibited LUMO levels oDfO-I3: 1.01.10e3V9/C, 7TC01420A
-3.27 eV and -3.13 eV, and HOMO levels of -5.99 eV, -6.17 eV and -6.03 eV. The
LUMO and HOMO levels of compound 2T/10 found to be slightly lower than those of
compounds 1T/10 and 3T/10.
Table 7. Electrochemical properties of representative compoundsa,b
d
d
c,e
c,f
c,g
c, h
Entry
E
E
E
E
ΔE
ΔE
g, opt
1red
1oxd
HOMO
LUMO
CV
1.65
1.83
1.69
-1.24
-1.07
-1.21
-5.99
-6.17
-3.10
-3.27
-3.13
2.89
2.90
2.90
3.02
2.89
2.94
1T/10
2T/10
3T/10
-6.03
a Micromolar solutions in THF. Experimental conditions: Ag/AgNO3 as
the reference electrode, glassy carbon working electrode, platinum rod
counter electrode, TBAP (0.1 M) as a supporting electrolyte, room
b
temperature, scanning rate of 0.05 mV s-1. In electron volts (eV). In
c
d
e
volts (V). Estimated from the onset oxidation peak values by using the
f
formula EHOMO = -(4.8 - E1/2,Fc,Fc+ + Eox,onset) eV. Estimated from the
onset reduction peak values by using ELUMO = - (4.8 - E1/2,Fc,Fc+ + Ered,onset
)
g
eV. Estimated from the formula ΔECV = ELUMO - EHOMO eV. h Band gap
determined from the red edge of the longest wave length in the UV-Vis
absorption spectra.
Gelation studies. Considering the ability of the polycatenar 3T/10 to self-assemble into
Colob phase (in bulk), we were curious to study its ability to form self-assembled
superstructures in presence of different solvents. Thus the ability of the compound to
form aggregates was examined by mixing the compound 3T/10 separately in different
polar aprotic solvents like chloroform, dichloromethane, tetrahydrofuran,
dimethylsulfoxide (DMSO), dimethylformamide (DMF), polar protic solvents like
ethanol, n-butanol, nonpolar aromatic solvents like benzene, toluene, m-xylene, nonpolar
aliphatic solvents like hexane, decane, dodecane and hexadecane (Table 1 in SI). The
process of gelation involved three sequential steps. The first step was mixing the
compound with solvent in sealed glass vial at room temperature. In the second step
attempt was done to get a clear solution by heating the mixture. The last step was to
allow the solution to cool to ambient temperature. The formation of the gel was
confirmed by “stable to inversion of the glass vial” method, i.e. after inverting the glass
vial if the gel was unable to flow, then it confirms the formation of stable gel. The critical
gelation concentration (CGC) (wt. %), i.e. is the minimum concentration necessary for
gelation, and Tgel (oC) i.e. is the thermal stability of the gels determined by ‘dropping
ball’ method.33 The compound 3T/10 was found to have a CGC of 2.0 wt. % and Tgel of
16

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Journal of Materials Chemistry C
o
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DOI: 10.1039/C7TC01420A
42 C. Further we checked the gelation behavior of other compounds 1T/6-10 and 2T/6-
10 in decane (Table 8), as we were interested to know their self-assembling behavior
towards the solvent.
Table 8. Gelation behavior and CGCs of compound 1T/6-10, 2T/6-10 and 3T/6-10 in
decane.
Entry
1T/6
1T/8
1T/10
2T/6
2T/8
2T/10
3T/6
3T/8
3T/10
Properties
CGC (wt. %)
-----
Tgel (oC)
-----
-----
-----
-----
-----
-----
40
P
P
P
P
P
-----
-----
-----
-----
-----
2.7
2.2
2.0
P
G(O)
G(O)
G(O)
40
42
G = stable gel; P = precipitate; O = opaque. The critical gelation
concentration (wt. %) is the minimum concentration necessary
for gelation. Tgel (oC) is the thermal stability of the gels.
(a)
(b)
(c)
(d)
Cooling
Cooling
Figure 10. Emission spectra showing a decrease in the emission intensity with the
decrease in temperature from solution to gel state for compound 3T/10 in decane (15
mM, λex = 368 nm) (a); Normalized emission spectra showing a red shift on gelation of
compound 3T/10 (in decane 15 mM) (b); Plot showing a red shift in the emission
wavelength with respect to temperature (c); Plot showing the decrease in the emission
intensity at 474 nm with respect to temperature (d).
These compounds got precipitated in decane after the heat assisted dissolving. Other
hexacatenars 3T/6 and 3T/8 also formed organogels in decane. The gelation of these
17

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compounds is mainly supported by the aromatic π-π interactions, dipole-dipole
interactions and hydrophobic alkyl chain interactions with the long chain aliphatic
solvents. Further we investigated the organogel of hexacatenar 3T/10 by extensive
photophysical studies, microscopy and XRD studies.
The UV-vis spectrum of compound 3T/10 in 15 mM decane solution showed an
absorption maximum centered at 374 nm, while the emission spectrum showed an
emission maximum centered at 474 nm (Table 6 and Fig.11b). The process of the
organogelation was followed by fluorescence spectroscopy with respect to temperature.
The emission intensity of compound 3T/10 decreased on cooling down the temperature
o
o
from 95 C to 5 C, with a simultaneous red shift from 474 nm to 492 nm (Fig. 10a-c).
There was a visually perceivable change in the color of the solution under the UV light of
long wavelength. The color change is from cyan blue in solution to bright green in the
gel state with a reduction in the emission intensity (15 mM in decane, Fig.10a-d and
11c). Thus, this phenomenon is aggregation caused quenching of emission (ACQ). The
ACQ is due to the strong attractive intermolecular π−π stacking interactions, due to
which the excited states of the aggregates come back to the ground state via non-radiative
Solution Gel Solution Gel
(a)
(b)
UV light
(λ = 365nm)
Daylight
(c)
3T/10
(e)
(d)
Figure 11. Images of the solution and gel at CGC in daylight and UV light (λ = 365 nm)
(a); Absorption, emission and excitation spectra of compound 3T/10 in 15 mM decane
solution (b); The decrease in the emission intensity on moving from solution state to
gelation at CGC (Concentration: 15 mM in decane), and to the thin film state (c);
Reversible change in emission intensity at 474 nm by repeated sol-gel transition (d); The
fluorescence decay of compound 3T/10 in decane at 15 mM (red trace) and 20 µM (black
trace) concentrations (blue trace is instrument response function: IRF; λexc = 375 nm) (e).
18

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channels, leading to the emission quenching.34-36 Similar aggregation quenching was
View Article Online
DOI: 10.1039/C7TC01420A
observed in the case of thin film prepared by spin coating (Fig.11c).
The solution to gel interconversion with respect to change in temperature was
seen for numerous cycles of heating and cooling as demonstrated by the change in its
emission intensity with respect to its emission maximum (Fig.11d). Excited state
fluorescence lifetimes of compound 3T/10 in the very dilute condition (20 μM) and at
CGC (15 mM) in n-decane were measured by monitoring their emission decay at 456 nm
for dilute solution and 492 nm for the concentrated solution (ex = 475 nm). The
emission decay profile in the diluted condition can be fitted with a bi-exponential decay
lifetime values of 1 = 0.80 ns (1 = 6%) and 2 = 2.22 ns (2 = 94%) indicating the
presence of two-emitting species. The solution with higher concentration also showed the
presence of two excited state species, however, the relative amplitude () of short and
long lifetime components were found to be changed significantly, such as 1 = 0.66 ns
(1 =54%), 2 = 4.25 ns (2 = 46%)
(Fig.11e). This observation confirmed the
formation of higher-order aggregates in the solution at higher concentration.
Figure 12. AFM image obtained for the xerogel of compound 3T/10 obtained from 15
mM decane solution (The thickness of the fiber marked is ≈ 200 nm) (a); SEM image
obtained for the xerogel of compound 3T/10 obtained from 15 mM decane solution
(scale bar is 1 μm)
The morphology of organogel formed from the hexacatenar 3T/10 in decane was
probed by different microscopic techniques like field emission scanning microscopy
(FESEM) and atomic force microscopy (AFM). The xerogel under investigation was
prepared by evaporating the decane solution of compound 3T/10 in air followed by
19

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DOI: 10.1039/C7TC01420A
vacuum evaporation for 2-3 h. The AFM images of the dry gel of 3T/10 exhibited dense
network of intertwined fibers of several micrometers long and thickness of ≈ 200 nm.
(Fig.12a). These results were again supported through SEM micrographs of 3T/10, which
also confirmed the entangled network of nanofibers (Fig. 12b). Further the structure and
arrangement of molecule in the xerogel was extensively studied with the help of powder
X-ray diffraction studies. The XRD pattern for xerogel 3T/10 at room temperature
showed several peaks at low angle and wide angle showing the crystalline arrangement
of the molecules in the gel state. However, the indexation of the peaks at low angle
suggested the arrangement of the molecules in a columnar structure with oblique
symmetry.32 (Table 2 in SI and Fig. S64).
Electroluminescence properties. Ability of the hexacatenar mesogen 3T/10 to exhibit
enhanced emission in the Col phase, motivated us to study its application as an
electroluminescent material in the fabrication of OLEDs. To evaluate the performance of
this material in OLEDs, we have made three types of device architectures, which are as
follows. Device 1: ITO/PEDOT: PSS (50 nm)/ emissive layer (80 nm)/ LiF (1 nm)/Al
(120 nm). Here, 3T/10 was used as a single emission layer. Device 2: ITO/PEDOT: PSS
(50 nm)/ emissive layer (80 nm)/ LiF (1 nm)/Al (120 nm). Here, the emissive layer was
composed of 5 wt.% 3T/10 as dopant with PVK (Poly(9-vinylcarbazole) as host material.
Device 3-4 had the configuration ITO/PEDOT: PSS (50 nm)/ emissive layer (80 nm)/
BCP (6 nm)/ Alq3 (35 nm)/ LiF (1 nm)/Al (120 nm). Here, 5 wt.% 3T/10 doped in PVK
formed the emissive layer. Figure 13 shows the energy level diagram of the materials
used and the schematic of the device structure. Here, ITO (indium tin oxide) served as a
transparent anode. PEDOT: PSS (poly(3,4-ethylenedioxy-thiophene)/poly(styrene-
sulfonate)) was used as HIL (hole injection layer) which can smoothen the ITO surface,
decrease the turn-on voltage, reduce the probability of electrical shorts, and extend the
lifetime of the device. PVK was used as host material due to its excellent film forming
property and good miscibility with different emissive material.37,38 BCP (Bathocuproine)
was used as HBL (hole-blocking layer) to confine the reductant holes in the emitting
layer which didn’t recombine with the electrons in the emitting zone.39-41 Alq3 (tris-(8-
hydroxyquinoline)aluminum) is widely used as a green emitting ETL (electron-
transporting layer) material and in the present device architecture it was used to facilitate
electron injection into the emitting layer. To improve electron injection, a bilayer cathode
consisting of LiF (lithium fluoride) and Al (aluminum) was used, where LiF works as
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EIL (electron-injecting layer). The key device performance data is summarized in Table
8. Figure 14a depicts the current density-voltage-luminance (J-V-L) characteristics of the
fabricated OLEDs. The OLED composed of 3T/10 as the sole emission layer (device 1)
exhibits high drive voltages (Vonset, corresponding to 1 cdm-2) of 8.66 V indicating high
charge injection barriers, low luminance (Lmax) of 38 cdm-2, current efficiency (ηc) of
0.11 cdA-1, power efficiency (ηp) of 0.033 lmW-1, and external quantum efficiency (ηEQE
)
of 0.07 %. There was a slight increase in the device performance when PVK: 3T/10 (5
wt.%) emitting layer (device 2) was used as an emissive layer. However, a drastic
increase in the device performance was observed with PVK: 3T/10 (5 wt.%) as emitting
layer (device 3) together with BCP and Alq3 as HBL and ETL. The efficiency of device
3 was almost 9 times higher than that of device 1/2 with Lmax of 1118 cdm-2, ηc of 0.96
cdA-1, ηp of 0.29 lmW-1 and ηEQE of 0.62%. Also, the device showed a low Vonset of 5.73
V. The improvement in the device performance can be ascribed to efficient transfer of
exciton energy from the host material to emissive dopant and/or a balanced injection of
hole and electron carriers into the emission layer. Furthermore, the performance of the
device was improved with an annealed emission layer (device 4). In this case, Vonset was
further reduced to 4.89 V, with better efficiencies of ηc: 1.26 cdA-1, ηp: 0.43 lmW-1, ηEQE
:
0.75% and Lmax of 1717 cdm-2 compared to device 3. All the fabricated devices emitted
in the blue region when ITO was positively biased. The steady-state EL spectra (Figure
15b) of the devices at the driving voltage of 15 V showed emission peaks at ~490 nm.
The emission peaks of EL spectra are almost same as compared to the thin film
fluorescence spectra of 3T/10 (Figure 14b), which imply that both are originating from
the same excited state. The devices exhibited excellent spectral stability over a wide
range of operating voltages (Figure 14c). The CIE chromaticity coordinates are
determined using the EL spectra of the devices at 15 V. The chromaticity diagram is
shown in Figure 14d, and the CIE coordinates are tabulated in Table 8 corresponds to a
bright blue emission. It is to be noted that the devices are unoptimized and further
optimizations can lead to much better performance including the brightness and
efficiency. Though there is an improvement in the EL in comparison to a bithiadiazole
derivative reported earlier,42 there is a further scope to improve, by minor alterations in
the molecular design and device architecture.
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Figure 13. Energy-level diagram of the materials used in the OLEDs and schematics of
the device architectures.
Figure 14. (a) J-V-L characteristics, (b) PL spectra of PVK: 3T/10 thin film (inset) and
EL spectra, (c) EL spectra of device 4 at different bias voltages, (inset shows an image of
the device) and (d) chromaticity diagram of the OLEDs.
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Table 9. Electroluminescent performance data of OLEDs
i
Device Vonset
Lmax
(cd m-2)f
38
79
1118
1717
ηc
(cd A-1) g
0.11
0.18
0.96
ηp
(lm W-1)g
0.03
ηEQE
(%)g
0.07
0.11
0.62
0.75
λem
(nm)h
491
491
489
CIE (x,y)
(V)e
1a
2b
3c
4d
8.66
7.73
5.73
4.89
0.142, 0.379
0.144, 0.371
0.147, 0.302
0.149, 0.363
0.07
0.29
0.43
1.26
489
a Device Configuration: ITO (120 nm)/ PEDOT: PSS (50 nm)/ 3T/10 (80 nm)/ LiF (1 nm)/ Al (120 nm).
b Device Configuration: ITO (120 nm)/ PEDOT: PSS (50 nm)/ PVK: 5wt.% 3T/10 (80 nm)/ LiF (1 nm)/ Al (120 nm).
c Device Configuration: ITO (120 nm)/ PEDOT: PSS (50 nm)/ PVK: 5wt.% 3T/10 (80 nm)/ BCP (6 nm)/ Alq3 (35
nm)/ LiF (1 nm)/ Al (120 nm).
d Device Configuration: ITO (120 nm)/ PEDOT: PSS (50 nm)/ annealed - PVK: 5wt.% 3T/10 (80 nm)/ BCP (6 nm)/
Alq3 (35 nm)/ LiF (1 nm)/ Al (120 nm).
e Vonset: turn-on voltage at luminance of 1 cd m-2;
f Lmax : Luminance at 20 V;
g current efficiency (ηc), power efficiency (ηp) and external quantum efficiency (ηEQE
h λem: Emission wavelength maximum at 15 V;
i CIE color coordinate
)
Conclusion. Several bi-1,3,4-thiadiazole based shape anisotropic molecules have been
synthesized, with the variation in the number, length and position of the flexible chains at
the terminal phenyl groups. The rod-like molecules, which had the two terminal tails,
stabilized tilted layered or smectic C phase with a high tilt angle. This organization is
driven by the donor acceptor interactions of the adjacent molecules within the layer. The
tetracatenars were crystalline, while among the hexacatenars, the compound with six n-
decyloxy chains stabilized columnar oblique phase. It is noteworthy that, only the
compounds with six peripheral tails exhibited gelation in long chain hydrocarbon solvent
decane. This shows that the gelation is facilitated by the donor-acceptor interactions,
attractive π-π interactions of the aromatic cores and van der Waals interactions offered by
the peripheral flexible tails. These molecules exhibited a red shift in the emission with
the increase in the number of peripheral tails, while the length of the peripheral tails did
not have any impact. One of the molecules exhibited solvatochromism, where a red
shifted emission with the increase in the solvent polarity was observed. In comparison to
the solution state, the organogel and thin film showed aggregation caused quenching of
emission (ACQ). Thus we have established a structure-property relationship on the self-
assembly behavior and the photophysical properties of these novel series of donor-
acceptor molecules. This molecular design is able to produce molecular nanowires of
several micrometers, with a central conducting core and insulating peripheral sheath,
which will be helpful for the application in organic electronics. The ability of the
hexacatenar columnar LC, to exhibit technologically important bright blue
23

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electroluminescence may increase the interest among the scientific community working
in the area of liquid crystals and OLEDs.
Supporting Information. Materials and methods, Molecular structural characterization
1
data, H and 13C NMR spectra and the data related to the thermal, photophysical,
electrochemical and gelation behavior are supplied as Supporting Information.
Author Contributions. ABY and BP synthesized the compounds. BP and SN carried
out the photophysical and gelation studies. H.U. designed, executed and analyzed the
OLEDs. M.N.S. supervised the OLED investigations. JD carried out the XRD and
ellipsometry studies. SKP supervised these studies. ASA designed the overall project,
analyzed the results and wrote the manuscript.
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 BRNS-DAE for funding this work through project No.
SB/S1/PC-37/2012 and No.2012/34/31/BRNS/1039 respectively. H.U. acknowledge
DST-SERB, India (NPDF Award No.PDF/2016/000711) for the financial support. CIF
IIT Guwahati is acknowledged for the analytical facilities. We thank the Ministry of
Human Resource Development for Centre of Excellence in FAST (F. No. 5-7/2014-TS-
VII).
References
1. Q. Li, Liquid Crystals Beyond Displays, John Wiley & Sons, Inc., 2012.
2. Q. Li, Self-Organized Organic Semiconductors, John Wiley & Sons, Inc., 2011.
3. C. Tschierske, Handbook of Liquid Crystals: Fundamentals, Wiley- VCH, 2014.
4. J. Malthête, A. M. Levelut and N. H. Tinh, J. Phys. Lett., 1985, 46, 875–880.
5. J. Malthěte, H. T. Nguyen and C. Destrade, Liq. Cryst., 1993, 13, 171–187.
6. H.-T. Nguyen, C. Destrade and J. Malthécte, Adv. Mater., 1997, 9, 375–388.
7. K. B. Woody, B. J. Leever, M. F. Durstock and D. M. Collard, Macromolecules,
2011, 44, 4690–4698.
24

Page 25 of 27
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C7TC01420A
8. S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hägele, G. Scalia, R. Judele,
E. Kapatsina, S. Sauer, A. Schreivogel and M. Tosoni, Angew. Chem. Int. Ed.,
2007, 46, 4832–4887.
9. I. Cho, S. K. Park, B. Kang, J. W. Chung, J. H. Kim, W. S. Yoon, K. Cho and S.
Y. Park, J. Mater. Chem. C, 2016, 4, 9460–9468.
10. X. Chen, L. Chen, K. Yao and Y. Chen, ACS Appl. Mater. Interfaces, 2013, 5,
8321–8328.
11. V. Bhalla, A. Gupta, M. Kumar, D. S. S. Rao and S. K. Prasad, ACS Appl. Mater.
Interfaces, 2013, 5, 672–679.
12. H. Bock, W. Seitz, M. Sievert, M. Kleine and J. W. Bats, Angew. Chem. Int. Ed.
Engl., 1996, 35, 2244–2246.
13. M. L. Greer, J. R. Duncan, J. L. Duff and S. C. Blackstock, Tetrahedron Lett.,
1997, 38, 7665–7668.
14. E. Bosch, S. M. Hubig, S. V. Lindeman and J. K. Kochi, J. Org. Chem., 1998, 63,
592–601.
15. G. W. Coates, A. R. Dunn, L. M. Henling, J. W. Ziller, E. B. Lobkovsky and R.
H. Grubbs, J. Am. Chem. Soc., 1998, 120, 3641–3649.
16. A. Seed, Chem. Soc. Rev., 2007, 36, 2046–2069.
17. B. Roy, N. De and K. C. Majumdar, Chem. – Eur. J., 2012, 18, 14560–14588.
18. S. S. Babu, V. K. Praveen and A. Ajayaghosh, Chem. Rev., 2014, 114, 1973–
2129.
19. A. C. Grimsdale, K. Leok Chan, R. E. Martin, P. G. Jokisz and A. B. Holmes,
Chem. Rev., 2009, 109, 897–1091.
20. L. Xiao, Z. Chen, B. Qu, J. Luo, S. Kong, Q. Gong and J. Kido, Adv. Mater.,
2011, 23, 926–952.
21. S. K. Pathak, S. Nath, R. K. Gupta, D. S. S. Rao, S. K. Prasad and A. S.
Achalkumar, J. Mater. Chem. C, 2015, 3, 8166–8182.
22. S. K. Pathak, M. Gupta, S. K. Pal and A. S. Achalkumar, ChemistrySelect, 2016,
1, 5107–5120.
23. B. Pradhan, M. Gupta, S. K. Pal and A. S. Achalkumar, J. Mater. Chem. C, 2016,
4, 9669–9673.
24. B. Pradhan, V. M. Vaisakh, G. G. Nair, D. S. S. Rao, S. K. Prasad and A. A.
Sudhakar, Chem. - Eur. J., 2016, 22, 17843–17856.
25. S. Qu, X. Chen, X. Shao, F. Li, H. Zhang, H. Wang, P. Zhang, Z. Yu, K. Wu, Y.
Wang and M. Li, J. Mater. Chem., 2008, 18, 3954–3964.
26. S. Qu and M. Li, Tetrahedron, 2007, 63, 12429–12436.
27. S. Qu, L. Zhao, Z. Yu, Z. Xiu, C. Zhao, P. Zhang, B. Long and M. Li, Langmuir,
2009, 25, 1713–1717.
28. H. Wang, F.-Q. Bai, H. Liu, B. Bai, X. Ran, S. Qu, J. Shi, D. Xie, H.-Y. Li, M. Li
and H.-X. Zhang, Phys. Chem. Chem. Phys., 2011, 13, 9697–9705.
25

Journal of Materials Chemistry C
Page 26 of 27
View Article Online
29. F. Chen, T. Tian, C. Zhao, B. Bai, M. Li and H. Wang, J. Mol. StruDcOt.I:,102.1001396/,C7TC01420A
1109, 239–246.
30. I. Dierking, in Textures of Liquid Crystals, Wiley-VCH Verlag GmbH & Co.
KGaA, 2003, pp. i–xi.
31. K. Raïs, M. Daoud, M. Gharbia, A. Gharbi and H. T. Nguyen, ChemPhysChem,
2001, 2, 45–49.
32. N. Godbert, A. Crispini, M. Ghedini, M. Carini, F. Chiaravalloti and A. Ferrise, J.
Appl. Crystallogr., 2014, 47, 668–679.
33. A. Takahashi, M. Sakai and T. Kato, Polym. J., 1980, 12, 335–341.
34. G. J. B. Birks: Photophysics of Aromatic Molecules. Wiley-Interscience, 1970.
35. Z. Zhelev, H. Ohba and R. Bakalova, J. Am. Chem. Soc., 2006, 128, 6324–6325.
36. F. Würthner, C. Thalacker, S. Diele and C. Tschierske, Chem. – Eur. J., 2001, 7,
2245–2253.
37. M. Cai, T. Xiao, E. Hellerich, Y. Chen, R. Shinar and J. Shinar, Adv. Mater.,
2011, 23, 3590–3596.
38. R. K. Gupta, D. Das, M. Gupta, S. K. Pal, P. K. Iyer and A. S. Achalkumar, J.
Mater. Chem. C, 2017, 5, 1767–1781.
39. S. Chidirala, H. Ulla, A. Valaboju, M. R. Kiran, M. E. Mohanty, M. N.
Satyanarayan, G. Umesh, K. Bhanuprakash and V. J. Rao, J. Org. Chem., 2015,
81, 603–614.
40. H. Ulla, M. Raveendra Kiran, B. Garudachari, M. N. Satyanarayan, G. Umesh
and A. M. Isloor, Opt. Mater., 2014, 37, 311–321.
41. H. Ulla, M. Raveendra Kiran, B. Garudachari, T.N. Ahipa, Kartick Tarafder, A.
V. Adhikari, G. Umesh and M.N. Satyanarayan, J Mol. Struc. 2017, 1143, 344-
354.
42. M. Sato, R. Ishii, S. Nakashima, K. Yonetake and J. Kido, Liq. Cryst., 2001, 28,
1211–1214.
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176x89mm (300 x 300 DPI)