Light emitting, star-shaped tris(N-salicylideneaniline) discotic liquid crystals bearing trans-stilbene fluorophores: Synthesis and characterization

Article abstract of DOI:10.1016/j.tetlet.2012.10.090

Star-shaped tris(N-salicylideneaniline) discotics possessing trans-stilbene fluorophores have been synthesized by condensing 1,3,5-triformylphloroglucinol with 4-(alkoxystyryl)benzen-amines, and characterized. ¹H and 2D ¹H-¹

Full text of DOI:10.1016/j.tetlet.2012.10.090

Tetrahedron Letters 53 (2012) 7108–7112
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Light emitting, star-shaped tris(N-salicylideneaniline) discotic liquid
crystals bearing trans-stilbene fluorophores: synthesis and characterization
⇑
A. S. Achalkumar , C. V. Yelamaggad
Centre for Soft Matter Research, Prof. U. R. Rao Road, Jalahalli, Bangalore 560013, Karnataka, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Star-shaped tris(N-salicylideneaniline) discotics possessing trans-stilbene fluorophores have been synthe-
sized by condensing 1,3,5-triformylphloroglucinol with 4-(alkoxystyryl)benzen-amines, and character-
ized. 1H and 2D 1H–1H COSY NMR experiments reveal their occurrence in two inseparable keto-enamine
tautomeric forms featuring C3h and Cs rotational symmetries. Their columnar liquid crystal behavior has
been ascertained unequivocally with the aid of a polarizing optical microscope and a differential scanning
calorimeter. Photoluminescence property has been examined both in solution and columnar states. Green
light emission observed in the solution state is especially noteworthy. Thus, intrinsic self-assembly into a
fluid columnar state exhibiting light emission property clearly illustrates their significance in technological
applications.
Received 14 August 2012
Revised 15 October 2012
Accepted 18 October 2012
Available online 26 October 2012
Keywords:
Tris(N-salicylideneaniline)
trans-Stilbene
Discotics
Columnar phase
Light emission
Ó 2012 Elsevier Ltd. All rights reserved.
In the past few decades,
p
-conjugated organic compounds, such
tronic and optoelectronic devices6,8 but also for academic
reasons.6 In general, they are formed by the self-assembly of
disc-like molecules, discotics, into indefinitely long columnar
stacks that in turn aggregate into two-dimensional (2D) lattices
of different symmetries.6,7 Notably, individual columns of the
phase serve as molecular wires as the central aromatic core of
the constituent discotics act as the conducting unit, while periph-
eral flexible tails behave as an insulator. In other words, the exten-
as small molecules (monomers), oligomers and polymers, have
been extensively studied1 because of their ability to serve as an
essential chemical component in the fabrication of devices viz., or-
ganic light emitting diodes (OLEDs),2 photovoltaic cells,3 field ef-
fect transistors (FETs)4 etc. Molecular crystals derived from such
substances are especially promising as they exhibit light absorp-
tion and emission in the visible region, photoconductivity, and
electrical conductivity, and thus behave like semiconductors. Gi-
ven the fact that electronic transport in semiconductors is often
perturbed by residual impurities and/or chemical or morphological
defects, accomplishing organic crystals free from such errors is
rather essential. 1a As is well known, realizing ultrapure and defect
free organic crystals is extremely difficult as their synthesis in-
volves multiple steps.1 However, molecular self-organization phe-
nomenon has been regarded as one of the important and
alternative strategies for the development of supramolecular struc-
tures facilitating directional charge transport and photophysical
properties.5–7
sive p-orbital overlap of the discotics within the column leads to a
high degree of uniaxial charge-carrier mobilities. In fact, Col phases
possess a remarkable ability to transport electrons9 and holes.10
Another important aspect is that they can be easily processed from
the isotropic liquid phase of discotics to lessen/eliminate the struc-
tural/electronic defects. 6 Thus, Col phases can be used as the med-
ia in OLEDs,11 solar cells12 and FETs.13 Discotics have been also
used as the electron and hole transport layers in device fabrica-
tion.8,11 Most importantly, for the purpose of using them as emis-
sive layer as well, there have been constant efforts in realizing
discotics exhibiting high fluorescence.6,8,14,15
In this work, we report the synthesis and characterization of no-
vel fluorescent discotics derived from tris(N-salicylideneaniline)
[TSANs] where an electron accepting central core, cyclohexane-
1,3,5-trione, is substituted, for the first time, with trans-stilbene
fluorophores. For some time now, we have been working on discotic
TSANs to reveal their structure-property correlations.16–18 Such
compounds are rather special as they can be readilyobtained in good
quantities through simple, high yielding synthetic steps, and most
appreciably, they exhibit fluid/frozen columnar supramolecular
structures capable of displaying tunable photoluminescence.16–20
Columnar (Col) liquid crystal (LC) phases,6 discovered by Chan-
drasekhar and co-workers in 1977,7 are one of the best-known and
most widely studied examples of such supramolecular structures.
They, being highly anisotropic and ordered structures, have at-
tracted much attention not only for their use in designing elec-
⇑
Corresponding author. Tel.: +91 80 28381119; fax: +91 80 2838 2044.
E-mail
addresses:
yelamaggad@csmr.res.in,
yelamaggad@gmail.com
(C.V. Yelamaggad).
Present Address: Department of Chemistry, Indian Institute of Technology
Guwahati, Guwahati, 781039 Assam, India.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.090

A. S. Achalkumar, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 7108–7112
7109
Thus, the design of present work originated from the fact that TSANs
provide unique ways to infuse and tune both luminescence and
molecular material features. For example, we have demonstrated
that a TSAN appended with three fluorophore 1,3,4-oxadiazole arms
displays emissive characteristics both in solution and Col phase.17
Notably, solution emitted intensive blue fluorescence, while
glassy/fluid Col phase displayed light emission behavior with inher-
ent 2D order. In continuation of this work we chose to introduce a
stilbene core as it is one of the most widely explored conjugated or-
ganic systems in devising luminescent materials.21 Precisely, we in-
tended to append three stilbene cores with dialkoxy and trialkoxy
tails, as fluorophore arms, to a central cyclohexane-1,3,5-trione core
so as to realize scarcely reported fluorescent discotic LCs. The molec-
ular structures of the accomplished discotic TSANs, Ia–c and IIa–c,
existing in two inseparable keto-enamine tautomeric forms, are
shown in Figure 1.
The synthetic steps adopted to realize the target TSANs and
their precursors are shown in Scheme 1. The benzylic bromination
of 4-nitrotoluene with N-bromosuccinimide (NBS) in the presence
of catalytic amount of azobisisobutyronitrile (AIBN) afforded
4-nitrobenzyl bromide (1) in 80% yield; the Michaelis–Arbuzov
reaction of this bromide with triethyl phosphite yielded
diethyl(4-nitrobenzyl)phosphonate (2). Horner–Wadsworth-Em-
mons (HWE) reaction between 3,4-dialkoxybenzaldehydes (3a–c)
and 2 in the presence of lithium diisopropyl amide (LDA in THF
at À78 °C furnished 1,2-bis(alkoxy)-4-(4-nitrostyryl)-benzenes
(4a–c) which on reduction with indium powder in hydrochloric
acid at room temperature yielded 4-(3,4-bis(alkoxy)styryl)benzen-
amines (5a–c). Ethyl 3,4,5-trialkoxy-benzoates (6a–c) were treated
with lithium aluminium hydride (LAH) in THF at 0 °C to obtain
(3,4,5-tris(alkoxy)phenyl)-methanols (7a–c) which upon oxidation
with pyridinium chlorochromate (PCC) in dichloromethane gave
3,4,5-trialkoxy-benzaldehydes (8a–c). Subsequent reaction of
these aldehydes with phosphonate 2 under HWE reaction condi-
tions resulted in the formation of 4-(3,4,5-tris(alkoxy)styryl)nitro-
benzenes (9a–c) which upon subjecting to reduction yielded
4-(3,4,5-tris(alkoxy)-styryl)benzeneamines (10a–c). Finally, three
folds of amines 5a–c and 10a–c were respectively, reacted with
1,3,5-triformylphloroglucinol in refluxing ethanol to accomplish
the target TSANs Ia–c and IIa–c as dull yellow crystalline samples,
which were repeatedly recrystallized from ethanol until constant
melting and clearing temperatures were observed. Pure, bright yel-
lowish, solid samples thus obtained were fully characterized on the
basis of microanalyses and UV–Vis, FTIR and NMR spectral data.
The IR spectra of the TSANs showed two prominent, sharp
bands in the region of 1618–1620 and 1513–1574 cmÀ1, perhaps
assignable, respectively, to the C@O and C@C groups of the central,
highly conjugated core. The 1H NMR spectra recorded in CDCl3
were well resolved and found to possess the expected spectral pat-
tern. Notably, as shown in Figure 2, as a representative case, multi-
ple peaks were observed in the region of d 8.7–9.0 and 13–13.5 due
to enamine and 2o-amine protons and their coupling. The former
region comprised of two distinct doublets as well as two merged
doublets (with varying intensities), all arising due to the resonance
of a proton (Ha) of C3h isomer and three protons (Hb, Hc and Hd) of
Cs form. While the latter region consisted of three distinct doublets
differing in their relative intensities; each of these originate due to
the resonance of an amine proton (He) of C3h isomer and three
analogous protons (Hf, Hg and Hh) of Cs isomer. 1H–1H COSY
NMR spectrum (see inset of Fig. 2) further evidenced the coupling
among the enamine and the NH protons.16–19 The identified NMR
peaks and patterns are in good agreement with those reported
for discotic TSANs existing in an inseparable mixture of two
keto-enamine tautomeric forms having C3h and Cs symmetries.16–
R2
R1
R3
19
It may be noted here that the integrations of the amine peaks
have been considered for estimating the ratio of the two geometri-
cal isomers and is shown against each discotic TSAN in Scheme 1.
The occurrence of these discotics in keto-enamine (NH) form, in-
stead of enol-imine (OH) form, can be interpreted in terms of elec-
tronic factors such as localization/delocalization of electrons and
the extent/strength of H-bonding leading to the generation of an
additional, thermally and hydrolytically stable, pseudo-aromatic
ring. Precisely, the strong electron-pulling ability of the central ring
and electron-pushing nature of substituents over the imine func-
tionality leads to a substantial withdrawal of the negative charge
from the oxygen atom in the OH group facilitating the proton
transfer to the nitrogen atom of the imine group leading to the for-
mation of NH isomers.
Liquid crystal behavior of the synthesized discotics was investi-
gated by two complementary experimental techniques: (i) polariz-
ing optical microscopy (POM) and (ii) differential scanning
calorimetry (DSC). The results emanating from these studies are
accumulated in Table 1. From these results it can be seen that all
the TSANs exhibit enantiotropic LC behavior, in particular, Col
phase that was ascertained by the observation of characteristic
optical textures. The crystalline sample Ia, held between two un-
treated glass substrates, upon heating melts into Col mesophase
at about 125 °C that transforms into isotropic state at ꢀ218 °C.
On cooling from the isotropic liquid, the transition to Col phase oc-
curs at ꢀ215 °C, which grows as a fern leaf (Fig. 3a); such a textural
pattern is commonly seen for the columnar phase.6 On further
cooling, it develops into large pseudo focal conic fans covering
the field of view and supercools till it reaches room temperature
(RT) where it becomes highly viscous. The DSC traces of the first
heating-cooling cycles corroborate the optical observation
R3
a
H
N
N
O
H
O
O
H
R2
H
N
e
R1
R2
H
R3
H
R1
C
3h Isomer
+
R1
H
N
b
O
O
N
O
H
R2
R1
R3
H
c
H
f
Cs Isomer
N
H
R2
g
d
H
h
R3
Ia
: R1 = R2 = OC8H17, R3 = H
Ib : R1 = R2 = OC10H21, R3 = H
Ic : R1 = R2 = OC12H25, R3 = H
R3
IIa : R1 = R2 = R3 = OC8H17
R1
R2
IIb
: R1 = R2 = R3 = OC10H21
IIc : R1 = R2 = R3 = OC12 25
H
Figure 1. The molecular structure of discotic TSANs existing in two inseparable
keto-enamine tautomeric (C3h and Cs) forms.

7110
A. S. Achalkumar, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 7108–7112
R
R
R
O
H
O
(v)
O2N
O2N
CH3
R
R
R
OH
OEt
R
R
(i)
3a: R= n-OC8H17 (83 %)
3b: R= n-OC10H21(84 %)
6a: R= n-OC8H17 (74 %)
7a: R= n-OC8H17 (70 %)
6b:
6c:
7b:
7c:
R= n-OC10H21 (70 %)
R= n-OC12 (82 %)
R= n-OC10H21 (73 %)
CH2Br
3c: R= n-OC12 25
H (82 %)
H
25
R= n-OC12
H
(72 %)
25
(iii)
(vi)
1 (80 %)
R
R
R
(ii)
R
R
(iii)
NO2
OEt
OEt
CH2P
NO2
R
R
O2N
O
4a:
4b:
R= n-OC8H17 (60 %)
R= n-OC10H21(60 %)
4c: R= n-OC12 25
O
9a:
R= n-OC8H17 (60 %)
R= n-OC10H21 (61 %)
R
2
(70 %)
9b:
9c:
H
(62 %)
8a: R= n-OC8H17 (80 %)
8b: R= n-OC10H21 (68 %)
R= n-OC12
H
(63 %)
25
(iv)
(iv)
8c: R= n-OC
H
(72 %)
12 25
R
R
R
R
NH2
NH2
R
5a:
5b:
R= n-OC8H17 (70 %)
R= n-OC10H21 (73 %)
10a: R= n-OC8H17 (60 %)
10b: R= n-OC10H21 (62 %)
10c: R= n-OC12H25 (64 %)
5c: R= n-OC
H (74 %)
12 25
Ia
(60 %) (1:2.08)
OHC
HO
OHC
OH
IIa (70 %) (1:2.40)
IIb
(vii)
(vii)
Ib (60 %) (1:2.32)
Ic (62 %) (1:2.25)
(71 %) (1:2.45)
IIc (65 %) (1:2.41)
CHO
OH
Scheme 1. Reagents and conditions: (i) NBS, AIBN, CCl4, reflux, 6 h; (ii) triethyl phosphite, 130 °C, 6 h; (iii) 2, LDA, THF, À78 °C to rt, 12 h; (iv) indium powder, concd HCl, THF
(1:9), rt, 2 h; (v) LAH, THF, 0 °C to rt, 12 h; (vi) PCC, DCM, rt, 1 h; (vii) ethanol, 80 °C, 6 h.
(Fig. 3b); importantly, the cooling trace indicated that the Col
phase transforms in to crystal state at ꢀ1 °C. The higher members
Ib and Ic showed an identical behavior. That is, the observed opti-
cal textural pattern for the mesophase of these two discotics was
indistinguishable to the one seen for Ia. In fact the mesophases
of these three compounds could be well mixed and thus, it is log-
ical to adjudge that compounds Ia, Ib and Ic stabilize a Col phase of
the same nature. However, it may be remarked here that the Col
phase of Ib and Ic, unlike in the case of Ia, does not crystallize
and instead it supercools well below the RT (À50 °C). Next three
discotic TSANs, IIa, IIb and IIc derived from 3,4,5-tri-alkoxystilbene
cores, display mosaic textural pattern upon cooling from their iso-
tropic liquid state. Figure 4a depicts, as a representative case, such
a pattern observed for discotic IIa. No textural change was ob-
served even when the samples were cooled down to RT. However,
the samples lose their fluidity at about 60 °C suggesting that Col
phase freezes into the glassy state at about this temperature. The
frozen Col state remains unchanged till À50 °C as evidenced by
DSC traces, for example see Fig. 4b. Thus, compounds IIa–c behave
identically with the exception that their melting temperature (Tm)
and clearing temperature (Tc) decrease with the increase in the
length of terminal tails. Overall, it is immediately apparent that
TSANs Ia–c show higher Tm values when compared to that of
IIa–c. This disparity originates because Ia–c comprise six periphe-
ral chains while IIa–c possess nine.
Figure 2. 1H NMR spectrum of discotic Ia in CDCl3; inset shows the 1H–1H COSY
NMR spectrum of the same compound where the coupling among enamine and
amine protons can be seen.
These star-shaped discotics are anticipated to exhibit photo-
physical properties as they are made by tethering three fluoro-
phore arms with an electron accepting central core. Thus, the
optical absorption and fluorescence behavior of Ia and IIa, as rep-
resentative cases, were studied in solution and mesophase. The top
panels of Figure 5a,b show the UV–Vis and photoluminescence
spectra of Ia and IIa in solution (THF). The absorption spectra (blue
traces) of the two compounds show two bands at about 350 nm
Table 1
Phase transition temperatures (°C)a and associated enthalpies (J/g), of star-shaped
discotics Ia–c and IIa–c. Cr = Crystal; Col = Columnar phase; Iso = Isotropic phase
TSANs
Phase sequence
Heating; cooling
Ia
Ib
Ic
IIa
IIb
IIc
Cr 125.6 [20.2] Col 218.4 [1] I; I 214.9 [0.9] Col 1.3 [0.6] Cr
Cr 111.2 [8.1] Col 229.2 [1.2] I; I 226.6 [1]b Col
Cr 107 [35.3] Col 239.5 [1.7] I; I 237 [1.5]b Col
Cr 72.4 [1.4] Col 240.4 [1.1] I; I 236.3 [1.1]b Col
Cr 70.1 [1.4] Col 226.2 [1.4] I; I 220 [1.2]b Col
Cr 66.5 [1.5] Col 199.5 [1.6] I; I 196.5 [1.4]b Col
and 440 nm corresponding to
p–
p⁄ and n–p⁄ transitions. Upon
irradiation with 440 nm light, emission maxima occur at around
510 nm and 505 nm (top panel, orange traces) for Ia and IIa,
respectively. Solutions of the compounds, placed in quartz cuv-
ettes, before, and after light irradiation are shown in the inset of
a
Peak temperatures in the DSC traces obtained during the first heating—cooling
cycles at 5 °C/min.
b
The Col phase did not crystallize up to À50 °C.

A. S. Achalkumar, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 7108–7112
7111
(a)
24
(b)
Colh
(a)
I
I
Cr
22
20
18
Cr
Colh
-50
0
50
100
150
200
Temperature (oC)
Figure 3. (a) Microphotograph of the optical texture observed for the Col phase of Ia at 211 °C. (b) DSC traces of the first heating-cooling cycles obtained for Ia.
(b)
(a)
(a)
20.5
I
Cr
Colh
Colh
20.0
19.5
I
-50
0
50
100
150
200
Temperature (oC)
Figure 4. (a) Photomicrograph of the mosaic texture for the Col phase of IIa at 78 °C. (b) DSC traces of the first heating-cooling cycles recorded for IIa.
12
8
6
4
2
0.6
0.4
0.2
0.0
0.6
0.6
0.4
0.2
(b)
(a)
8
4
0.0
0
0
12
RT
RT
100oC
150oC
200oC
230oC
15
150 o
175 o
200 o
215 o
C
C
C
C
0.08
12
9
9
6
3
0.04
0.00
0.3
6
0.0
400
500
600
700
300
400
500
λ
600
(nm)
700
λ
(nm)
Figure 5. UV–Vis (left peaks) and emission (right peaks) spectra of Ia and IIa in their solution (top panels) and Col phase (lower panels). Insets show the pictures of solution
as seen before and after illumination with 365 nm light.
Table 2
Photophysical properties of discotics Ia and IIa
THF solutiona
Emissionb,c
Glassy film
Emissionb,c
TSANs
Absorptionb
Stokes shiftb
Absorptionb
Stokes shiftb
Ia
IIa
353, 442
353, 440
510
505
68
65
366, 442
370, 444
629
629
187
185
a
b
c
micromolar solutions in THF.
Wavelengths (nm).
The excitation wavelength kex = 440 nm.

7112
A. S. Achalkumar, C. V. Yelamaggad / Tetrahedron Letters 53 (2012) 7108–7112
3. (a) Shirota, Y.; Kageyama, H. Chem. Rev. 2007, 107, 953–1010; (b) Ma, C. Q.;
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top panels of Figure 5a,b where the visually perceivable green light
in the emissive state can be seen. As shown in Table 2, the Stokes
shifts, the difference between positions of the band maxima of the
absorption and emission spectra, were found to be nearly over
60 nm. The lower panels of Figure 5a,b illustrate the absorption
spectra of the frozen Col phase (blue traces) and the photolumines-
cence spectra recorded at different temperatures while cooling the
Col phase from isotropic phase till RT. Analogous to the solution
spectra, the UV–Vis spectra of the frozen Col state at RT display
two absorption bands. Upon excitation with 440 nm light, both
the films, at RT, show a broad emission band at 629 nm with a neg-
ligible shoulder (see red traces of lower panels of Figure 5a,b). The
emission spectra of the fluid Col phase, recorded at four different
temperatures, exhibit, as shown in the right hand side of lower
panels of Fig. 5a,b, a broad emission band at around 630 nm. It
can be seen that the intensity of the emission peak increases pro-
gressively with the decrease in the temperature; this can be attrib-
uted to breaking of larger columnar stacks into smaller ones and
thermally activated radiationless processes.22
Apparently, the emission maxima of the neat/frozen Col phase,
when compared to the solution, exhibit a strong bathochromic
(red) shift that can be attributed to the strong co-facial proximity
of C3h and Cs isomers within the Col structure and perhaps the
presence of excimers/aggregates. Generally, the excimers are re-
garded as molecular dimers or a stoichiometric complex with asso-
ciated excited electronic states, dissociative ground states, and
structure-less emission spectra.23 In essence, the intimate overlap
of the discotic cores in the fluid columnar/glassy state brings the
energy levels closer and thus, a large red shift in the emission max-
ima occurs. Besides, to some extent, the thickness of the sample
also contributes to the shift in emission wavelength. Therefore,
for very thin (spin coated or vacuum deposited) samples there is
the possibility that emission might shift toward the blue region.
These results clearly illustrate the photoluminescence property of
the present discotics both in solution and mesomorphic state and
thus, further substantiate our view that discotic TSANs hold prom-
ise for diverse technological applications.
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In conclusion, the synthesis and characterization of new discot-
ic LCs derived from tris(N-salicylideneaniline)s are reported. The
novelty of these discotics stems from the fact that three stilbene
fluorophores, varying in the number and length of terminal chains,
are joined to an electron accepting central core. All six compounds,
occurring in two inseparable keto-enamine tautomeric forms fea-
turing C3h and Cs rotational symmetries, show Col LC behavior.
Their solutions and fluid/glassy Col phases exhibit photolumines-
cence. In view of the fact that these TSANs stabilize light emissive
Col phase, aided by the self-assembly of two tautomeric forms,
they can be regarded as technologically important organic
substances.
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Acknowledgement
We gratefully acknowledge the financial support from the
Department of Science Technology, Govt. of India, provided under
the SERC scheme No. SR/S1//OC-06/2007.
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