Star-shaped fluorescent liquid crystals derived from s-triazine and 1,3,4-oxadiazole moieties

Article abstract of DOI:10.1039/c6tc01260d

Star-shaped molecules with a central triazine core appended with three 1,3,4-oxadiazole arms have been designed with the variation in the number, length and pattern of peripheral chain substitution. These compounds were investigated for their thermal, electrochemical and photophysical behavior. These nonconventional molecules stabilized wide range columnar phases and demonstrated how one can tune the liquid crystal self-assembly through simple structural modification. The photophysical properties of these star shaped molecules are extremely dependent on the number and pattern of peripheral chain substitution. These compounds exhibit blue and green luminescence in the solid/liquid crystal state. The ability to overcome aggregation induced quenching is due to the favorable packing of these molecules in the solid state. These solid-state emissive materials with good thermal stability and lower band gap may find applications in the construction of emissive displays and organic lasers.

Full text of DOI:10.1039/c6tc01260d

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Pathak, R. K. Gupta, M. Gupta, S. K. PAL and A. Ammathnadu Sudhakar, J. Mater. Chem. C, 2016, DOI:
10.1039/C6TC01260D.
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DOI: 10.1039/C6TC01260D
Star-shaped Fluorescent Liquid Crystals derived from s-triazine and
1,3,4-oxadiazole moieties
Balaram Pradhana, Suraj Kumar Pathaka, Ravindra Kumar Guptaa, Monika Guptab, Santanu Kumar
Palb and Ammathnadu S.Achalkumara*
aDepartment of Chemistry,
Indian Institute of Technology Guwahati, Guwahati, 781039, Assam, India.
bIndian Institute of Science Education and Research, Mohali,Sectorꢀ81,
Knowledge City, Manauli 140306, Punjab, India.
Abstract: Starꢀshaped molecules with a central triazine core appended with three 1,3,4ꢀ
oxadiazole arms have been designed with the variation in the number, length and pattern of
peripheral chain substitution. These compounds were investigated for their thermal,
electrochemical and photophysical behavior. These nonconventional molecules stabilized wide
range columnar phases and demonstrated how one can tune the liquid crystal selfꢀassembly
through simple structural modification. The photophysical properties of these star shaped
molecules are extremely dependent on the number and pattern of peripheral chain substitution.
These compounds exhibit blue and green luminescence in solid/liquid crystal state. The ability to
overcome aggregation induced quenching is due to the favorable packing of these molecules in
solid state. These solidꢀstate emissive materials with good thermal stability, lower band gap may
find application in the construction of emissive displays and organic lasers.
Introduction
The last four decades have witnessed tremendous research activities with respect to design and
synthesis of various discotic liquid crystals.1Discotic liquid crystals are designed with the general
structural template where a central discꢀlike core is substituted with three or more flexible
chains. Discotic Liquid crystals mainly stabilize two mesophases viz. columnar (Col) phase and
nematic (N) phase. Columnar phase formed by the oneꢀdimensional stacking of discs to form
columns. Based on the arrangement of these columns into different lattices they are classified
into different columnar phases. Nematic phase is formed by the organization of discs with longꢀ
range orientational order but with no positional order. Over the years several innovative
nonconventional molecular designs were shown to stabilize Col phases, which were initially
observed for disc shaped molecules. These classes are polycatenars, dendrimers,
metallomesogens, bent shaped molecules, oligomers, polymers and star shaped molecules.2Star
shaped mesogens or ‘hekates’ are relatively a new class of nonconventional molecules that
stabilize columnar liquid crystalline phases. These molecules are formed by the covalent linking
1

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of three arms symmetrically to a central core.3These arms may be linked to the central core
through flexible or semi flexible or rigid linkers. Shape persistent star shaped mesogens are
obtained, when the linkers connecting central core and three arms are rigid. These molecules lack
the shape anisotropy of discotics required to exhibit mesophases, but their ability to form
mesophases is solely driven by the nanophase segregation of chemically or physically different
molecular subunits and their tendency to fill the space efficiently in bulk. Advantage of this
molecular design with respect to discotics is, the synthetic flexibility provided to incorporate
various functional units into the molecular structure. Hekates or star shaped mesogens can
exhibit a variety of phases like nematic, columnar, cubic or soft crystal phases, because of the
peculiar structure and the ease of tunability associated with this molecular design. The presence
of the voids between the arms of the molecular structure helps the mesophases to freeze in glassy
state. Several 1,3,5ꢀsubstituted benzene based or heterocyclic cores have been utilized to prepare
these star shaped mesogens. sꢀ1,3,5ꢀTriazine is a readily available C3ꢀsymmetrical, trifunctional
core to prepare star shaped mesogens.4Being a C3ꢀsymmetric planar central core which helps in
the selfꢀassembly, it also acts as a nꢀtype semiconductor owing to its electron poor nature.
Triazine core can also be employed as a Hꢀbonding acceptor, which stabilize supramolecular
liquid crystalline selfꢀassembly.5Triazine based central core is also used in the synthesis of
oligomers6 and dendrimers.7This core has also been utilized in the construction of octupolar
materials8 which show nonlinear optical activity with large first hyperpolarizability. 1,3,5ꢀ
Triazine based star shaped mesogens have been prepared by one of the following methods, (i) the
reaction of nucleophilic group functionalized aromatics with cyanuril chloride9; (ii) nucleophilic9
or palladium catalyzed substitutions10 with trihaloꢀ1,3,5ꢀtriazines and (iii) cyclotrimerization of
12b,
nitriles.11,
17Many fluorophores like styrene,9,12 tolane,13 carbazole14 and triazole15 and
thiophenes16 are connected to this central 1,3,5ꢀtriazine core to obtain luminescent mesogens.
Marder et.al. prepared triazine derivatives, where three trialkoxy phenyl substituted 1,3,4ꢀ
oxadiazole moieties are directly connected to the central triazine cores.18Incorporation of 1,3,4ꢀ
oxadiazoles in to the molecular structure has some advantages, as 1,3,4ꢀoxadiazoles are wellꢀ
known for their stability towards thermal, hydrolytic and oxidative degradation, intense
fluorescence and good electron transporting properties. These compounds are widely used as
electron carrier/hole blocking materials and also as electroluminescent layers in organic light
emitting devices. Many other research groups have prepared star shaped molecules containing
oxadiazole moieties connected to central benzene core at 1,3 and 5ꢀpositions.18, 19 In comparison
2

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to these benzene based hekates, reports on the triazine based hekates with 1,3,4ꢀoxadiazole
moieties are scarce. Probably the associated increase in the melting points with the incorporation
of heterocycles discouraged the researchers. In this communication we present hitherto
unreported fluorescent triazine based hekates with three unsymmetrically substituted 1,3,4ꢀ
oxadiazole moieties, which stabilize wide range columnar hexagonal phase. The synthesized
compounds vary from each other with respect to the number and length of the flexible chains.
The number and length of the peripheral tails have a significant effect on the type and range of
the Col phase. Lower chain length usually enhances the coreꢀcore interaction, which leads to the
stabilization of columnar rectangular (Colr) phase, while the higher chain length usually promote
columnar hexagonal (Colh) phase.1d We are attempting to shed light on the structure property
relations in this class of star shaped molecules with respect to their thermal behavior and
photophysical properties.
Results and discussion
Synthesis and Characterization. The synthetic strategy to obtain the target molecules and their
precursors is illustrated in scheme 1. General procedures for the syntheses of ethyl gallate, 3,4ꢀ
hexadecyloxy or 3,4ꢀdodecyloxy ethyl benzoate, 3,5ꢀhexadecyloxy ethyl benzoate, 3,5ꢀdodecyloxy
ethyl benzoate, 3,4,5ꢀhexadecyloxy ethyl gallate,20 3,4,5ꢀdodecyloxy ethyl gallate,20 4ꢀ
hexadecyloxy ethyl benzoate21 and 4ꢀdodecyloxy ethyl benzoate21 are reported earlier. Alkoxy
esters (4a-h) were transformed to their respective hydrazides (3a-h) by treating with hydrazine
hydrate in ethanol or nꢀbutanol as solvent.19,22 Hydrazides 3a-h on refluxing with 4,4',4''ꢀ(1,3,5ꢀ
triazineꢀ2,4,6ꢀtriyl)tribenzoic acid chloride in THF in presence of triethylamine yielded triꢀNꢀ
benzoylbenzohydrazides.19 These compounds were heated with POCl3 to obtain the target
molecules 1a-d and 2a-d.19,22 The key intermediate 4,4',4''ꢀ(1,3,5ꢀtriazineꢀ2,4,6ꢀtriyl)tribenzoic
acid (5) was prepared by chromic oxide mediated oxidation of 2,4,6ꢀtriꢀpꢀtolylꢀ1,3,5ꢀtriazine (6) in
good yield.23 Compound 6 was in turn obtained by the triflic acid catalyzed trimerization of pꢀ
1
tolunitrile.24The structures of all the intermediates and target molecules were confirmed using H
NMR, 13C NMR, IR spectroscopy and MALDIꢀTOF mass spectra. (See the supporting information
(SI) for the details).
Thermal behavior. All the compounds were investigated with the help of Polarizing Optical
Microscopy (POM) and Differential Scanning Calorimetry (DSC). Mesomorphic behavior, phase
3

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transition temperatures and associated enthalpy changes are presented in table 1. Figure 1
graphically represents the thermal behavior of compounds 1a-d and 2a-d in first cooling cycle.
Scheme 1. Synthesis of star shaped LCs.a
a(i) CF3SO3H, dry DCM, 0 OCꢀrt, 12 h, 90%; (ii) CH3COOH, H2SO4, CrO3, Ac2O, 0 OCꢀrt, 24 h, 85%; (iii)
NH2NH2.H2O, ethanol, reflux, 48 h (65–75%); (iv) 4,4',4''ꢀsꢀtriazineꢀ2,4,6ꢀtriylꢀtribenzoic acid chloride, THF,
triethylamine, 6 h, reflux; (v) POCl3, reflux, 17 h (45ꢀ50%).
4

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Cr
Colr1
Colr2
1a
1b
1c
1d
2a
2b
2c
2d
Colh1
Colh2
Colh3
0
54
108
162
216
270
Temperature (oC)
Figure 1. Bargraph summarizing the thermal behavior of compounds 1a-d and 2a-d (first cooling cycle)
Table 1. Phase transition temperaturesa (oC) and corresponding enthalpies (kJ/mol) of hekates.
Entry
Phase sequence
2nd Heating
1st Cooling
I 172.4 (2.4) Cr
Cr 175.4 (3) I
1a
1b
Colh1 53.4 (106.3) Colh2 152.5 (80.6) Colh3
238.5 (18.4) I
I 237.1 (17.2) Colh3 149 (80.2) Colh2
50.37 (207.1) Colh1
Cr 5.4 (127.4) Colh 156.8 (9.9) I
Cr 47.4 (163.8) Colh 240 (34.1) I
Cr1 53.6 (17.2) Cr2 157.7 (126.1) I
Cr 48.8 (225.03) Colh1 161.1 (69.1) Colh2
251.2 (14.7) I
I 154.6 (11.1) Colh 3 (173.7) Cr
I 238.1 (31.7) Colh 39.9 (526.8) Cr
I 154.1 (9.5) Cr2 41.4 (20.8) Cr1
I 255.3 (14.0) Colh2 148.1 (59.9) Colh1
1c
1d
2a
2b
b
c
Cr 48.9 (382.04) Colr1c 90 Colr2 170 Colh 190.1 I 188.1 (20.9) Colh 165 Colr2 84.9 (25.3)
2c
b
(22.3) I
Colr1
Colr 86.6 (135.2) Colh 276.38 (24.8) I
I 267.9 (429.0) Colh 69.7 (26.32) Colrb
2d
aPeak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at 5
b
c
oC/min.Colh = Columnar hexagonal phase; Mesophase freezing in the glassy state; Transition detected
in polarizing optical microscope but not observed in DSC.
Compound 1a with three hexadecyloxy tails did not show any mesophase and showed a crystal
o
to isotropic transition at 173 C. It did not show any mesophase on cooling cycle, instead an
o
isotropic to crystal transition at 172 C. This shows that three flexible tails are not enough to
induce the nanophase segregation of molecular subunits in this class of star shaped molecules.25
POM images of this compound at different temperatures indicated the crystalline nature of the
sample (Fig. S53e and f). This was surprising considering a small enthalpy change (ꢁH = 2ꢀ3
kJ/mol) observed in DSC thermograms (in heating and cooling cycles).
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Compound 1b with six hexadecyloxy tails exhibited enantiotropic mesomorphic
behavior. In the heating cycle the compound showed a crystal to mesophase transition (ꢁH =
104.2 kJ/mol) at a temperature of ≈ 52 oC. This birefringent liquid undergoes further transition at
o
≈ 153oC as observed in DSC scans (See SI), before going to an isotropic liquid state at 239 C.
On cooling the isotropic liquid a dendritic growth was appeared which soon coalesces in to a
mosaic pattern interspersed with homeotropic regions (Fig.2a). At 149 oC a transition was
(b)
(d)
(a)
(c)
Figure 2. Photomicrographs of textures as seen by POM for the Colh3 phase of compound 1b (a)
at 230 °C; (b) Colh3 phase at 155 oC; (c) Colh2 phase at 134 oC; (d) Colh1 phase at 28 oC.
observed in the mesophase, where the texture remains same, but the color of the mosaic pattern
gets changed (Fig.2bꢀc). On further cooling a broad exothermic peak (ꢁH = 207.1 kJ/mol)
corresponding to crystallization was observed in the DSC scan at ≈ 50oC, while the texture
remains unchanged with a concurrent loss of fluidity (Fig. 3a and 2d). In the next heating scan a
transition appeared at a temperature of ≈ 53oC (ꢁH = 106.3kJ/mol) showing that this transition is
not due to glass forming.
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In order to assign the symmetry of the columnar phase powder Xꢀray diffraction of the
sample was carried out at different temperature intervals. Xꢀray diffraction of the sample carried
o
out at 200 C showed a sharp peak centered at 43.2 Å at the lowꢀangle region, followed by two
diffused peaks at 5.18 Å and 3.76 Å respectively (Fig. 3b, Table 2).
Table 2. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature
(T) of mesophase
Compounds
Phase
(T/oC)
dobs(Å)
dcal(Å)
Miller
indices hkl
Lattice
parameter
(D/Å)
(Å)
43.2
100
43.20
45.37
45.33
39.51
a = 49.89
Colh3
(200)
5.18 (ha)
3.76 (hc)
45.37
4.96 (ha)
3.74 (hc)
45.34
4.68 (ha)
3.63 (hc)
39.54
001
100
1b
a = 52.36
Colh2
(100)
(65.78)
001
100
Colh1
(25)
a = 52.35
a = 45.63
001
100
Colh
(140)
5.11 (ha)
3.77 (hc)
001
100
1c
39.56
5.07 (ha)
3.76 (hc)
39.53
39.56
Colh
(100)
(64.31)
a = 45.65
a= 45.69
001
100
39.59
Colh
(25)
4.88 (ha)
3.78 (hc)
41.33
23.04
16.06
5.20 (ha)
3.82 (hc)
41.32
23.51
15.89
001
100
110
210
41.32
23.86
15.62
Colh
(200)
a = 47.72
a = 47.68
1d
001
100
110
210
(66.46)
41.29
23.84
15.60
Colh
(150)
5.12 (ha)
3.76 (hc)
001
Though the observation of a single peak at low angle does not unambiguously confirm the Colh
phase, textural evidence strongly suggests that it is Colh phase. The absence of other peaks in low
angle is generally ascribed to a minimum in the form factor and reported earlier.22,26The sharp
peak at the low angle specifies the distance between the adjacent(100) planes, i.e. d10, from
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which lattice parameter ‘a’ can be calculated. Lattice parameter ‘a’ gives the intercolumnar
distance, which was found to be 49.9 Å.
(a)
(b)
Colh1
Colh2
Colh3
I
Colh3 at 200 oC
Colh1
Colh2
Colh3
I
(c)
(d)
Colh2 at 100 oC
Colh1 at 25 oC
Colh2 at 100 oC
Figure 3. DSC traces obtained for first cooling (blue trace) and second heating (red trace) cycles
o
of compound 1b at a rate of 5 C/min (a). XRD profiles depicting the intensity against the 2θ
obtained for the Colh3 phase of compound 1b at 200 °C (b); Colh2 phase of compound 1b at
100 °C (c) and Colh1 phase of compound 1b at 25 °C; inset shows the image pattern obtained (d).
This is significantly lesser (25 %) than the molecular diameter, i.e. 65.78 Å, which may be due to
the intercalation of peripheral flexible chains. This is understandable considering the large voids
present between the arms of the star shaped molecule, where there is a possibility of alkyl chains
filling up the voids (Fig. 4). The first diffused peak at 5.18 Å in wideꢀangle region corresponds to
the packing of peripheral flexible tails, while the second diffused peak at 3.76Å corresponds to
the stacking of molecules in the individual columns (Fig. 3b). Similar intracolumnar distances
are found in triindoleꢀbased discotics (3.9ꢀ4.4 Å), where the authors ascribed this due to the
presence of phenyl linkers between the central core and peripheral flexible chains.27It is possible
8

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that with the increase in the length of the arms in hekates, the arms are twisted in conformation
with respect to the central planar core and this leads to an increase in the coreꢀcore stacking
distance. Additionally these three different arms may have different diastereomeric
32.89 Å
Figure 4. Schematic showing the selfꢀassembly of compound 1b into Colh phase
conformations too. The small textural change observed at 149 oC along with the exothermic peak
(ꢁH = 80.2 kJ/mol) in DSC prompted us to carry out XRD studies well below this transition
temperature, i.e. at 100 oC. The XRD pattern showed a single peak at low angle centered at 45.37
Å, along with the two diffused peaks at 4.96 Å and 3.76 Å. This diffraction pattern is similar to
the high temperature Colh phase observed for compound 1b. The intercolumnar distance was
found to be 52.4Å. The intercolumnar distance ‘a’ is more than the observed intercolumnar
distance at 200 oC (20 % lesser than molecular diameter). Thus the transition at 153 oC points to
a transition between two Colh phases. Such transition between two Colh phases is not very
common, but has been reported in literature.28 As the intracolumnar distance remains constant at
both temperatures, we observe only an increase in the intercolumnar distance by 5 Å. This may
o
be due to the stretching of peripheral alkyl tails. Considering the transition at 50 C with the
unchanged texture we measured the XRD well below this temperature, i.e. at room temperature.
o
The XRD pattern was almost similar to the measurement carried out at 100 C, with the
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comparable intercolumnar distance. The only difference was the decrease in alkyl chain stacking
and coreꢀcore stacking distance. Thus compound 1b exhibited a transition between three
columnar hexagonal phases (designated as Colh1, Colh2 ad Colh3), as evidenced from POM, DSC
and XRD studies. The difference between these three Colh phases is very subtle as seen from
XRD studies. As you see in the table 2, for other compounds (1c-d), there was no significant
change in intercolumnar distance with respect to temperature, which is very hard to explain.
However we can infer from the data obtained that, compound 1b with peripheral chains
substituted at 3 and 4 positions of the benzene ring selfꢀassemble with more intercalation at
higher temperature Colh3 phase than in low temperature Colh2 and Colh1 phase. There is also
possibility that it can achieve various conformations due to its unsymmetrical peripheral
substitution, which may be a reason for this observation. The decrease in the dꢀspacing values at
wideꢀangle region with the decrease in temperature follows the usual trend. This difference
between compound 1b and 1c-d is again due to the peripheral substitution.
Compound 1c, which is a regioisomer of compound 1b with six hexadecyloxy tails,
exhibited wide range enantiotropic columnar mesophase over 150 degrees including room
temperature. This is quite interesting to see how a small change in the substitution pattern, i.e.
transition from peripheral 3,4ꢀ to 3,5ꢀalkoxyl group substitution improves the mesophase
stability of the star shaped molecules. The texture was a combination of mosaics, small focal
conics and homeotropic regions and it remained unchanged at room temperature (Fig. 5a).
(a)
(b)
Figure 5. Photomicrographs of textures as seen by POM for the Colh phase of (a) compound 1c
at 30 °C and (b) compound 1d at 237 oC.
o
Compound 1c, in the cooling cycle of DSC thermogram showed a peak centered at 3 C (ꢁH =
173.7 kJ/mol) (see SI). Powder XRD studies conducted at different temperature intervals
suggested that the phase under investigation is Colh as explained below. The powder XRD
patterns at different temperatures showed a single reflection corresponding to Miller indices 100
10

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(Fig.6aꢀc). The intercolumnar distance with the decrease in temperature showed little variation in
contrast to the Colh phase exhibited by compound 1b, but higher interdigitation of peripheral
alkyl tails.
(a)
Colh at
(b)
140 oC
(c)
Colh at
100 oC
Colh at
25 oC
Figure 6. XRD profiles depicting the intensity against the 2θ obtained for the Colh phase of
compound 1c at 140 °C (a); Colh phase of compound 1c at 100 °C (b) and Colh phase of
compound 1c at 25 °C; inset shows the image pattern obtained.
d100
200 oC
150 o
C
(deg)
Figure 7. XRD profiles depicting the intensity against the 2θ obtained for the Colh phase of
compound 1d at 200 °C (red trace) and at 150 °C (black trace) (inset shows the image pattern
obtained).
Compound 1d, with nine peripheral hexadecyloxy tails also exhibited wide range columnar
hexagonal mesophase (Fig. 5b) of almost 200 degrees, with higher melting and isotropic
temperatures in comparison to compound 1c, which is confirmed by POM, DSC and XRD
investigations. The powder XRD patterns recorded at 200 oC and 150 oC (Table 2, Fig.7) showed
reflections at low angle corresponding to Miller indices 100, 110 and 210 with d spacing ratio of
1:0.57:0.38, which is fitting well with hexagonal lattice. Thus all the molecules except
compound 1a stabilized Colh phase irrespective of the peripheral substitution. The lattice
parameter of the Colh phase ‘a’ is seemingly a constant with respect to temperature, which
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implies to a small molecular rearrangement within the columns that is occurring due to the
rearrangement of the alkyl chains.
We were curious to know the impact of shortening the length of peripheral alkyl chain on
the thermal behavior of the star shaped molecules as in 2a-d in comparison to compounds 1a-d.
Compounds 2a-d are the lower homologues of compounds 1a-d with nꢀdodecyloxy chains at the
periphery, prepared by maintaining the same substitution pattern. Compound 2a with three nꢀ
dodecyloxy chains was crystalline, but with a lower isotropic temperature than its higher
homologue 1a. Compound 2b with six nꢀdodecyloxy chains exhibited an enantiotropic behavior
with a higher isotropic temperature than compound 1b and hence showing a larger mesophase
width. The compound 2b on cooling from the isotropic liquid state showed a birefringent pattern
of mosaic texture (Fig. 8a), which is frequently observed for Colh phases. DSC scans in cooling
cycle showed a transition at ≈ 148 oC (ꢁH = 60 kJ/mol) (Fig. S57, Table 1), while texture did not
show much variation except the change in color of the mosaic pattern, which remained
unchanged till room temperature (Fig. 8b). The texture is not shearable, thus confirming the
freezing of Colh phase in glassy state. But the second heating showed a crystallization peak at ≈
o
49 C (ꢁH = 225 kJ/mol), before converting to form a birefringent fluidic pattern. XRD studies
have been carried out at 220 oC, 130 oC and 25 oC. The XRD patterns were characteristic of Colh
phase (Table 3, Fig. S58). The peaks at low angle can be indexed into Miller indices 100 and
200. These values are in the ratio of 1:1/√4 conforming the hexagonal lattice (except the missing
1/√3 reflection), which is similar to the XRD pattern observed for compound 1b.
(a)
(b)
Figure 8.POM images obtained for the Colh phase of compound 2b at 238 °C (a) and at room
temperature (b).
The compound 2c exhibited higher melting and isotropic temperatures than compound 1c, which
may be due to the increased coreꢀcore interaction and decreased fluidity of the molecular
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structure. On cooling from the isotropic liquid the compound exhibited a mosaic texture
interspersed with homeotropic domains. This optical texture showed a small change in the color
o
and brightness at a temperature of 165 C (Fig.9a and b), while there was no corresponding
o
signature in DSC scans (Fig.9e). Further cooling showed a transition at ≈ 85 C (ꢁH = 25.3
o
kJ/mol), but the change in texture was minimal. Powder XRD studies carried out at 175 C, 100
oC and at room temperature to investigate these observations (Fig.9f and Table 3). The XRD
o
pattern obtained at 175 C was reminiscent of Colh phase. The pattern consisted of three
reflections at low angle corresponding to the d spacings of 41.48 Å, 23.29 Å and 15.79 Å, which
can be indexed to 100, 110 and 210 reflections with a reciprocal spacing ratio of 1:1/√3:1/√7.
This was accompanied with the two diffused peaks at the wideꢀangle region at d spacings of 5.25
Å and 3.76 Å, which correspond, to the packing of flexible tails and the rigid cores in the Colh
o
phase. The diffraction pattern observed at 100 C showed many peaks at low angle region. The
five d spacings in the low angle region can be assigned to (110), (200), (220), (030) and (400)
reflections from a rectangular lattice. The lattice constants were found to be a = 61.62 Å and b =
Colh
(e)
(a)
(b)
Colr1
I
Colr2
I
Colr2
Colr1
Cr
Colh
Colr2
(f)
Colr1
Colh
(c)
(d)
o
Figure 9.POM images of compound 2c in Colh phase at 175 C (a); Colh to Colr1 transition at
o
o
o
165 C (b); Colr1 phase at 100 C (c); Colr2 phase at 25 C (d); DSC scans for the first cooling
(blue trace) and second heating (red trace) cycles (e); XRD profiles depicting the intensity
o
against the 2θ obtained for the Colh phase of compound 2c at 175 °C, Colr1 phase at 100 C and
Colr2 phase at 25 oC (f).
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Table 3. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature
(T) of mesophase
Compounds
Phase
(T/oC)
dobs(Å)
dcal(Å)
Miller
indices hkl
Lattice parameter
(Å)
(D/Å)
Colh2 (220)
42.97
5.22(ha)
3.75(hc)
45.67
42.94
100
a = 49.59
a = 52.70
001
100
200
Colh2 (130)
45.64
22.82
22.53
5.01(ha)
3.76(hc)
46.78
23.27
16.64
2b
001
100
200
210
Colh1 (25)
46.75
23.38
17.67
a = 53.98
a = 47.87
9.11
4.65(ha)
3.72(hc)
41.48
23.29
15.79
001
100
110
210
Colh (175)
Colr 2(100)
41.46
23.94
15.67
5.25(ha)
3.76(hc)
41.47
30.81
23.67
001
110
200
220
030
400
41.46
30.81
20.73
18.68
15.40
a = 61.62
b = 56.06
18.88
15.47
2c
5.10(ha)
3.73(hc)
39.73
30.75
23.06
18.13
15.21
12.26
001
110
200
210
030
400
500
Colr 1(25)
39.72
30.75
26.47
17.34
15.37
12.3
a = 61.5
b = 52.04
4.50(ha)
3.71(hc)
41.50
22.81
15.61
5.31(ha)
3.83(hc)
41.48
23.56
15.77
5.16(ha)
3.75(hc)
41.47
31.77
23.14
18.33
15.64
12.45
001
100
110
210
Colh (230)
Colh (150)
Colr (25)
41.47
23.94
15.68
a = 47.89
a = 47.87
001
100
110
210
41.46
23.94
15.67
2d
001
110
200
220
030
400
500
41.46
31.77
20.73
18.24
15.88
12.70
a = 63.54
b = 54.73
4.54(ha)
3.71(hc)
001
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56.06 Å. Formation of Colr phase requires higher coreꢀcore interaction as the first column should
know how the neighboring column will organize. It is often found that in a series of compounds
the compounds with shorter chains exhibit Colr phase.29, 1b, 1d In case of a columnar hexagonal to
rectangular transition the (100) splits into two peaks mainly (110) and (200). This is because as
soon as the lattice deviates from the hexagonal symmetry the d110 and d200 spacing no longer
remains degenerate and two separate reflections are observed in the small angle region.1b The
diffused peaks at wideꢀangle region with the d spacings 5.1 Å and 3.73 Å correspond to the
packing of alkyl tails and the stacking of the aromatic cores. The room temperature XRD pattern
also showed the similar diffraction pattern that can be assigned to Colr phase. Since there was an
enthalpy change detected in DSC along with textural variation we have denoted these phases as
Colr1 and Colr2.
(a)
(b)
o
Figure 10.POM images of compound 2d in Colh phase at 130 C (a); and in Colr phase at room
temperature (b).
Compound 2d exhibited an improved thermal behavior in comparison to compound 1d, but
exhibited a transition between wide range Colh phase and Colr phase. Again the optical texture
did not show much variation during this transition (Fig.10), but DSC scans showed the enthalpy
change corresponding to this phase transition, which was further corroborated by XRD studies
carried out at three different temperatures (Table 3, Fig. S58).
Thus the effect of lowering of chain length in these star shaped compounds from nꢀ
hexadecyloxy to nꢀdodecyloxy can be summarized as below. The thermal behavior of the
compounds with three alkyl tails (1a and 2a) was similar as they turned out to be crystalline
irrespective of the chain length. The compounds with six alkyl tails (1b and 2b), where the
alkoxy groups are in adjacent position exhibited Colh phase. The lower chain analogue 2b
exhibited slightly enhanced thermal range. These two compounds show transitions between three
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Colh phases, where the decrease in temperature led to Colh phase with shorter coreꢀcore distance
and longer intercolumnar distance. The transition between different Colh phases may be due to
the possibilities of attaining various conformations with respect to temperature, due to the
unsymmetric peripheral substitution. This observation is limited to the type of peripheral
substitution, which is not seen in the other compounds of the respective series. A marked
difference was visible in the case of compounds 1c and 2c, where the alkyl tails are in alternate
position. Compound 1c exclusively stabilized Colh phase, while 2c was bimesomorphic
exhibiting high temperature Colh and low temperature Colr phases with enhanced thermal range.
Similar behavior was observed in the case of compounds with nine alkoxy tails, i.e. compound
1d and 2d. Overall, the lowering of chain length has increased the coreꢀcore interaction, leading
to stabilization of the Colr phase along with Colh phase and enhanced the thermal range. This
demonstrated how a subtle change in the molecular structure through peripheral substitution
alters the liquid crystal selfꢀassembly.
Overall, these star shaped compounds showed enhanced mesophase stability in
comparison to the earlier reported triazine based polyether derivatives, which showed Col phase
only on complexation with trinitroflurenone (TNF).29 When compared to the similar star shaped
mesogens containing tolane moieties which exhibit short range disordered Colh phases, present
series of compounds exhibit ordered Colh phase over a wide thermal range, which may be due to
the enhanced coreꢀcore interaction caused by the polar oxadiazole units.13
Photophysical properties
We expected an influence on photophysical properties based on the substitution pattern but not
on the length of the peripheral chain.26h The photophysical properties of the star shaped
molecules 1a-d and 2a-d were studied in detail, i.e. in solution and thin film (Table 4).
Absorption and fluorescence spectra of the compounds 1a-d and 2a-d were taken in micromolar
THF solution (Fig. 11, Fig. S62). As can be seen, the absorption spectra for the solutions of
compounds 1a-d showed two absorption maxima in a range of 275ꢀ343 nm, while that of
compound 2a-d showed two absorption maxima in a range of 279ꢀ349 nm. These compounds
showed large values of molar extinction coefficients (ε = 16,610ꢀ28,490 Mꢀ1 cmꢀ1). The
absorption bands are corresponding to πꢀπ* and nꢀπ* transition of this molecular system. The
first band at lower wavelength comes from 1,3,5ꢀtriazine system according to the previous
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reports.13 Optical bandgaps of these systems were calculated from the long edge of the
absorption spectrum and found to fall in the range of 3.12ꢀ3.36eV.
(a)
(b)
(d)
(c)
1a
1c 1d
1b
(e)
Figure 11. Absorption (a) and normalized emission spectra (b) in THF solution (20 ꢁM)
obtained for 1a-d; absorption (c) and normalized emission spectra (d) in thin film state; Pictures
of solutions of compounds 1aꢀd in THF and in thin film state (drop casted from 2 mM solutions
in toluene) as seen with the illumination of 365 nm UV light (e).
Emission spectra of compounds 1a-d and 2a-d exhibited a single emission maximum centered
around 453ꢀ540 nm with large Stoke’s shift of 123ꢀ197 nm. Emission spectra changed with
respect to substitution pattern at the periphery but not on the length of the peripheral tails (Fig.
S62). Compounds 1a and 1c showed emission maxima centered at 458 and 453 nm with a blue
emission in solution (at 365 nm UV light, Fig. 11e). Compounds 1b and 1d showed emission
maxima centered at 506 and 540 nm with a green and yellowꢀgreen emission (at 365 nm UV
light, Fig. 5e). Similar emission behavior was observed for compounds 2a-d (Fig. S62). Relative
quantum yields were measured with respect to quinine sulphate solution (0.1 M H2SO4 with a
quantum yield of 0.54) and found to be in the range of 0.28ꢀ0.33 (Table 4, Fig. S59). Steady state
anisotropy values were in the range of 0.035ꢀ0.086. Thin films of the samples were produced by
drop casting the 2 milimolar solutions of compounds in toluene on glass slides. The absorption
spectra of these films were found to be broad, comprising single or two bands with a small red
shift (Table 4, Fig. 11c, Fig. S62), while the emission spectra did not vary much from the
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Table 4.Photophysical properties of compounds 1a-d in solutiona and film state
Solution State
Thin film State
Absorption Emissionb Stokes λonset ꢂEc,d
Steady
state
anisotropy
Relative Absorption Emissionb
Stokes
shift
(nm)
Entry
g, opt
(nm)
(nm)
shift
(nm)
(nm)
Quantum
Yield
(nm)
(nm)
275,335
286,343
458
506
123
163
386
397
3.21
3.12
0.067
0.083
0.28
0.31
0.29
0.31
0.25
0.33
0.27
356
276, 352
442
495
86
143
1a
1b
285,331
271,343
279, 340
279, 349
270, 329
283, 344
453
540
456
501
451
537
122
197
116
152
122
193
369
395
387
394
366
385
3.36
3.14
3.21
3.15
3.39
3.23
0.070
0.086
0.054
0.035
0.045
0.053
262, 345
352
349
263, 354
267, 346
357
435
495
455
487
449
504
90
1c
1d
2a
2b
2c
2d
143
107
133
105
147
0.31
amicromolar solutions in THF; excited at the respective absorption maxima; Band gap determined from the red edge of the
longest wave length (λonset) in the UVꢀvis absorption spectra; d In volts (eV).
b
c
solution spectra, but they showed a blue shift. In the case of compounds 1a and 1b the observed
blue shift was 16 nm and 11 nm respectively, while in the case of compounds 1c and 1d the blue
shift was found to be 18 nm and 45 nm respectively. Similar behavior was observed in the case
of compounds 2a-d (Fig. S62). Further if we look closely at the absorption spectra of these
compounds in thin film state, we see that there is a band at lower wavelength observed only for
compounds 1b, 1c, 2b and 2c. This is observed specifically to these compounds, which contain
six alkyl tails, confirming that this is arising because of the solidꢀstate packing that is different
from compounds 1a, 1d, 2a and 2d. The emission colors observed under the long wavelength
UV light was almost similar to that obtained in solutions (Fig.11e). Red shifted absorption bands
in the solid state, is usually observed with the formation of Jꢀaggregates; where the molecules are
stacked in a head to tail fashion,30 but we can not rule out this observation due to the
organization of the star shaped molecules in different diastereomeric conformations within the
column.
In order to get a clear view of the aggregation phenomena we have measured the UV and
fluorescence spectra of these compounds as a function of increasing concentration starting from
micromolar solution in THF. At a particular concentration (6.25 × 10ꢀ5 M for compounds 1a-c,
3.12 × 10ꢀ5 M for compound 1d) the luminescence intensity was found to be high (Figure 12a).
Further increase in concentration reduced the luminescence intensity, which may be due to the
aggregation induced quenching. We have measured the fluorescence lifetime at three different
concentrations to understand this observation. Fluorescence lifetime meausrements at
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micromolar concentrations revealed the existence of one species, which is solvated monomer. At
a concentration where the luminescence intensity was high, we observed two species, one with
lower lifetime and the other with higher lifetime. This biexponential decay is due to the presence
of aggregated species and solvated monomers. The presence of two species is may be due to the
high solubility of these compounds in THF. Some of the molecules may prefer to remain as
solvated monomers even at highest concentration (Fig. S63 and Table 1 in SI). The absorption
spectra showed two maxima where the first one at lower wavelength was found with reduced
intensity in comparison to the absorption spectra obtained for micromolar solution (Fig.12b). All
the compounds except compound 1c showed a small red shift in their absorption maximum,
while the absorption maximum of 1c did not change much (Table 4 and 5). Similarly the
(a)
(b)
Figure 12. (a) Emission intensity vs concentration plots of compounds 1a-d in THF solution (a);
Absorption and emission spectra at the concentration where highest emission intensity was
observed (b).
Table 5. Photophysical properties of compounds 1a-c (6.25 × 10ꢀ5M) and 1d (3.12 × 10ꢀ5M) in THF solution and in
thin film drop casted from 2 mM solutions in toluene
Entry Absorpti Emissio
Fraction of
molecules
65%, 35%
53%, 47%
53%, 47%
49%, 51%
Life
Absorptio
n (nm)
356
276, 352
262, 345
352
Emission
(nm)
442
495
435
Fraction of
molecules
86%, 14%
50%, 50%
51%, 49%
74%, 26%
Life
time (ns)
3.64, 0.72
10.19, 2.55
10.83, 2.11
8.65, 2.81
on (nm)
275, 338
293, 346
274, 331
286, 344
n (nm)
452
510
435
535
time (ns)
3.50, 1.67
13.40, 3.01
6.24, 0.82
3.79, 0.93
1a
1b
1c
1d
495
luminescence spectra showed a blue shifted emission maximum for all the compounds except
compound 1b, which showed a small red shift. Thus as we see from Tables 4 and 5, absorption
and emission spectra follow a similar trend (either gradual decrease or increase in the
wavelength) as they move from low to high concentration or to thin film state. The fluorescence
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lifetime measurements carried out for the thin films showed two life times, one is of higher
lifetime corresponding to aggregates, while the lower one is corresponding to monomer species.
The data is comparable to the properties of these compounds in solution state (Table 5).
We were particularly interested to examine the photophysical properties of
compound 1c and 2c, which exhibit room temperature Colh and Colr phase respectively. A thin
film of compound 1c was prepared by heating the sample sandwiched between two glass slides
to isotropic state and annealing it to room temperature. Similarly a thin film of compound 2c was
prepared by annealing it in Colr phase. The liquid crystal film of compound 1c showed an
absorption maximum of 347 nm, which is red shifted by 16 nm in comparison to solution
spectra. Emission spectrum was blue shifted with an emission maximum of 435 nm (Stoke’s
shift ≈ 90 nm) (Fig. 13a). This observation was in line with the data obtained for the drop casted
samples.
(a)
(b)
Figure 13. (a) Normalized absorption (black trace) and emission spectra (red trace) of LC thin film of
compound 1c (inset shows the thin film under UV light (a) and for compound 2c (b).
)
In literature, observation of redꢀshifted absorption maximum in comparison to that in solution
31
state is attributed to the formation of Jꢀtype aggregates. But considering the possibilities of
various diastereomeric conformations that can be attained by these molecules, we cannot specify
this observation to the formation of Jꢀtype aggregates. The absorption spectra of compound 2c
were also redꢀshifted in comparison to the solution spectra by 16 nm (absorption maximum: 349
nm), while there was no much difference with respect to the emission spectra (emission
maximum: 452 nm). This behavior is similar to that of compound 1c irrespective of the chain
length and type of columnar packing. Similar star shaped compounds reported by others, where a
central triazine core connected with transꢀstilbene chromophores showed a blue shifted
emission,12b while the molecules with tolane moieties showed red shifted
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emission.13aPreservation of solidꢀstate/liquid crystal state emission along with good thermal
stability make these compounds promising for organic dye lasers and OLEDs.
IV. Electrochemical properties
Energy levels of frontier molecular orbitals (HOMO and LUMO) and the energy gap of
these levels in star shaped molecules 1a-d were obtained by cyclic voltammetry (CV) and the
data are tabulated in Table 6. All the compounds exhibited irreversible oxidation and reduction
waves (See SI). The optical band gap Eg,opt was estimated from the red edge of the absorption
spectra. Energy levels of LUMO and HOMO were determined by using the formulae ELUMO = ꢀ
(4.8–E1/2, Fc,Fc++ Ered, onset) eV, EHOMO = ꢀ(4.8–E1/2, Fc,Fc++ Eox, onset) eV. There was no much impact
on the energy levels of frontier molecular orbitals with respect to substitution, and the bandgaps
were found to be in the range of 2.53ꢀ2.62 eV. These values are slightly smaller than their
corresponding optical band gap values (Table 4).
Table 6. Electrochemical properties of compounds 1a-d in micromolar dichloromethane solutions
Entry
1a
E1red
E1oxd
EHOMO
ꢀ5.91eV
ꢀ5.82eV
ꢀ5.82eV
ꢀ5.93eV
ELUMO
ꢀ3.32eV
ꢀ3.29eV
ꢀ3.23eV
ꢀ3.31eV
ꢂECV
ꢂE g, opt
3.21eV
3.12eV
3.36eV
3.14eV
ꢀ1.02 V
ꢀ1.05 V
ꢀ1.11 V
ꢀ1.03 V
1.57 V
1.48 V
1.48 V
1.59 V
2.59eV
2.53eV
2.59eV
2.62eV
1b
1c
1d
These values can be compared with 2,4,6ꢀtriphenylꢀ1,3,5ꢀtriazine, which is one of the
oldest materials used in OLED fabrication as a holeꢀblocking material.32 This compound exhibits
a LUMO and HOMO levels of ꢀ2.8 eV and ꢀ6.5 eV respectively. The optical band gap was found
to be 3.71eV,33 which is higher than the present series where three symmetrically substituted
1,3,4ꢀoxadiazoles are connected to the central ring (3.2ꢀ3.4 eV) (Table 4). The LUMO levels of
the compounds 1a-d were lower than 2,4,6ꢀtriphenylꢀ1,3,5ꢀtriazine (ꢀ3.23 to ꢀ3.32 eV), while the
HOMO levels higher and were in the range of ꢀ5.8 to ꢀ5.9eV. Thus the substitution with
alkoxyphenyl oxadiazole moiety on the 2,4,6ꢀtriphenylꢀ1,3,5ꢀtriazine lowers the LUMO;
increases the HOMO level and thus a resultant lowering of the band gap. The lower LUMO level
and bandgaps of these starꢀshaped molecules may reduce the barriers for electrons, with the
simultaneous blocking of the hole movement. Thus the extensive πꢀconjugation and the
introduction of the electron donating groups to the electron deficient central triazine ring is a
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simple method to modify the frontier molecular orbital energy levels and band gap. Additionally
the 1,3,4ꢀoxadiazole units bring about the higher thermal stability and emissive nature that is
very vital for the solidꢀstate display devices. Similar comparable values were observed when the
central triazine unit was connected to fluorene through different linkers.33,34
V. Experimental Section
Commercially available chemicals were used without any purification; solvents were dried
following the standard procedures. Chromatography was performed using either silica gel (60ꢀ
120 and 100ꢀ200) or neutral aluminium oxide. For thin layer chromatography, aluminium sheets
preꢀcoated with silica gel were employed. IR spectra were recorded on a Perkin Elmer IR
spectrometer at normal temperature by using KBr pellet. The spectral positions are given in wave
number (cmꢀ1) unit. NMR spectra were recorded using Varian Mercury 400 MHz or Bruker 600
1
MHz NMR spectrometer (at 298K). For H NMR spectra, the chemical shifts are reported in
ppm relative to TMS as an internal standard. Coupling constants are given in Hz. Mass spectra
were obtained from MALDIꢀTOF mass spectrometer in Laser Desorption positive mode using αꢀ
cyanoꢀ4ꢀhydroxycinnamic acid matrix or from High Resolution Mass Spectrometer.
The mesogenic compounds were investigated for their liquid crystalline behavior by
employing a polarizing optical microscope (POM) (Nikon Eclipse LV100POL) equipped with a
programmable hot stage (Mettler Toledo FP90). Clean glass slides and coverslips were employed
for the polarizing optical microscopic observations. The transition temperatures and associated
enthalpy changes were determined by differential scanning calorimeter (Mettler Toledo DSC1)
under nitrogen atmosphere. Peak temperatures obtained in DSC, corresponding to the phase
transitions were in agreement with the POM observations.The transition temperatures obtained
from calorimetric measurements of the second heating and first cooling cycles at a rate of 5
°C/min are tabulated. In the cases where the DSC signatures are not observed for the phase
transitions, the transition temperatures have been taken from POM observations. Temperature
dependent Xꢀray diffraction studies were carried on unaligned powder samples in Lindemann
capillaries (1 mm diameter) held in programmable hot stage and irradiated with CuKα radiation
(λ = 1.5418 Å). The samples were filled in the capillary tube in their isotropic state and their both
ends were flame sealed. The apparatus essentially consisted of a highꢀresolution powder Xꢀray
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diffractometer (Xenocs) equipped with a focusing elliptical mirror and a highꢀresolution fast
detector. Thermogravimetric analysis (TGA) was performed using thermogravimetric analyzer
(Mettler Toledo, model TG/SDTA 851e) under constant nitrogen flow at a heating rate of 10
°C/min. UVꢀVis spectra were obtained by using PerkinꢀElmer Lambda 750, UV/VIS/NIR
spectrometer. Fluorescence emission spectra in solution state were recorded with Horiba
Fluoromaxꢀ4 fluorescence spectrophotometer or Perkin Elmer LS 50B spectrometer. Cyclic
Voltammetry studies were carried out using a PAR Model 700D series Electrochemical
workstation. Experiments were carried out in micromolar solutions in dichloromethane at room
temperature and experiments were done at a scanning rate of 0.05 mV sꢀ1. For the CV
experiments Ag/AgNO3 as the reference electrode, glassy carbon working electrode, platinum
rod counter electrodes were used. Tetrabutyl ammonium perchlorate (0.1 M) was used as a
supporting electrolyte. The halfꢀwave potential of the ferrocene/ferrocenium (Fc/Fc+) redox
couple (E1/2,
+) was found to be 0.46 V relative to the Ag/Ag+ reference electrode. HOMO
Fc,Fc
energy levels were estimated from the onset oxidation peak values, by using the formula EHOMO
= ꢀ(4.8 ꢀ E1/2,Fc,Fc+ + Eox,onset) eV, while LUMO energy levels were estimated from the onset
reduction peak values by using ELUMO = ꢀ (4.8 ꢀ E1/2,Fc,Fc+ + Ered,onset) eV. Electrochemical band
gaps were estimated from the formula ꢂECV = (ELUMO – EHOMO) eV, while optical band gaps
were determined from the red edge of the longest wavelength in the UVꢀVis absorption spectra.
Time resolved fluorescence was measured using an Edinburgh Instruments Life Spec II
instrument.Here the fluorescence lifetimes were determined by a timeꢀcorrelated single photon
counting method. A Hamamatsu micro channel plate (MCP) detector that has a response time of
50 ps was used in the abovementioned instrument. A picosecond 375 nm laser diode and 336
LED were used as the light sources. The full width at half maxima for the laser diode and the
LED are 90 ps and 570 ps respectively. The fluorescence emission maxima for the corresponding
species has been chosen as the monitoring wavelength in the time resolved fluorescence
measurements. The data were analyzed using a reconvolution method using the software
provided by Edinburgh instruments.
VI. Summary
In summary, we have synthesized eight new starꢀshaped molecules with a central 2,4,6ꢀtriphenyl
triazine core attached with three symmetrically substituted 1,3,4ꢀoxadiazole arms with the
variation in the number, length and pattern of peripheral chain substitution. These compounds
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were investigated for their thermal, electrochemical and photophysical properties with the help
of various techniques. Compound with three flexible chains at the periphery turned out to be
crystalline irrespective of the chain length, where as compounds with six and nine peripheral
chains stabilized Colh phase over a wide thermal range. One of the compounds with peripheral nꢀ
hexadecyloxy substitution at 3 and 4 positions showed a technologically important room
temperature Colh phase. Lowering of chain length in these star shaped compounds from nꢀ
hexadecyloxy to nꢀdodecyloxy enhanced the mesophase range. The compound with three alkyl
tails was crystalline, while the compound with six alkyl tails, where the alkoxy groups are in
adjacent position exhibited Colh phase with a slight enhancement in the thermal range. A marked
difference was visible in the case of compound, where the alkyl tails were in alternate position.
This compound exhibited high temperature Colh and low temperature Colr phases with enhanced
thermal range. Similar behavior was observed in the case of compounds with nine alkoxy tails.
Overall, the lowering of chain length has increased the coreꢀcore interaction, leading to
stabilization of the Colr phase along with Colh phase and enhanced the thermal range. This
demonstrated how a subtle change in the molecular structure through peripheral substitution
alters the liquid crystal selfꢀassembly.
The photophysical properties of these star shaped molecules were extremely dependent
on the number and pattern of peripheral chain substitution. They exhibited blue and green
fluorescence in solid/liquid crystal state. Preserved fluorescence in the solid state is explained
due to the formation of aggregates, which do not quench the fluorescence. The aggregation
quenching is not occurring here probably due to the columnar packing that includes several
different diastereomeric conformations. Cyclic voltammetry studies revealed that, these
molecules possess lower LUMO levels and bandgaps compared to the 2,4,6ꢀtriphenyl triazine
which is used in OLEDs, thus making them better candidates for electron transporting emissive
layers. The ability to overcome the aggregation induced quenching of the fluorescence, higher
thermal stability and beneficial band gap are the properties which make these molecules
promising from the view point of emissive displays and organic lasers.
VIII. Associated Content
1
13
The synthesis and characterization details, H NMR, C NMR spectra of all new compounds,
absorption and emission spectra, POM photographs, DSC thermograms, XRD profiles of LC
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compounds and Cyclic voltammograms are provided as electronic supporting information. This
material is available free of charge via the Internet.
Acknowledgements
ASA sincerely thanks Science and Engineering Board (SERB), DST, Govt. of India and Board
of Research in Nuclear SciencesꢀDepartment of Atomic Energy (BRNSꢀDAE) for funding this
work through the project No.SB/S1/PCꢀ37/2012 and No. 2012/34/31/BRNS/1039
respectively.We thank Ministry of Human Resource Development for Centre of Excellence in
FAST (F. No. 5ꢀ7/2014ꢀTSꢀVII). ASA acknowledges Central Instrumentation Facility, IIT
Guwahati for analytical facilities. We acknowledge Dr. Chandan Mukherjee, IIT Guwahati for
providing his Electrochemical workstation.
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