A new strategy towards the synthesis of a room-temperature discotic nematic liquid crystal employing triphenylene and pentaalkynylbenzene units

Article abstract of DOI:10.1039/c6cc09509g

A new approach is reported for the design of a room-temperature discotic nematic (N_D) liquid crystal (LC) dimer consisting of a triphenylene and a pentaalkynylbenzene unit linked via flexible alkyl spacers. The formation of the N_D phase is realized most likely through folding of the dimeric molecule that prevent stacking between the triphenylene units, as suggested by modelling in the mesophase derived from X-ray scattering results and high-level DFT calculations.

Full text of DOI:10.1039/c6cc09509g

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A new strategy towards the synthesis of a room-temperature
discotic nematic liquid crystal by employing triphenylene and
pentaalkynylbenzene units
Received 00th January 20xx,
Accepted 00th January 20xx
Monika Gupta,a Santosh Prasad Gupta,a M. V. Rasna,b Debashis Adhikari,a Surajit Dharab and
DOI: 10.1039/x0xx00000x
Santanu Kumar Pal*a
www.rsc.org/
A new approach is reported for the design of a room-temperature strategy to achieve room-temperature nematic discogens by
discotic nematic (ND) liquid crystal (LC) dimer consisting of a perturbing lateral sidearm of pentaalkynylbenzene with
a
triphenylene and a pentaalkynylbenzene unit linked via flexible substitution ortho to the ethynyl group on peripheral phenyl ring.9
alkyl spacers. The formation of ND phase is realized most likely They also established further that room-temperature ND phase can
through folding of the dimeric molecule that prevent stacking be obtained by introducing an attraction-enhancing unit in the
between the triphenylene units, as suggested by modelling the lateral side arm of hexa(phenylethynyl)benzene.10
mesophase derived from X-ray scattering results and high-level
DFT calculations.
In this communication, we report a new design for the realization
of room-temperature discotic ND dyad consisting of a triphenylene
& pentaalkynylbenzene unit linked via flexible alkyl spacers. The
Disc shaped molecules showing nematic phase (ND) are rare but
of utmost importance in many display device applications. They
have garnered particular interest owing to their commercialization
as optical compensation films to enhance the viewing angle of
commonly used LC display based on polymerized nematic
discogens.1 Unfortunately, most of the discotic nematogens
reported so far exhibit ND behaviour at high temperature and over a
narrow temperature range.2 In contrary, implementation of
nematic discogens in devices necessitates ND phase at room
temperature. A thorough literature survey reveals that hexa- and
pentaalkynylbenzene derivatives have been the most investigated
system among various discotic nematogens.2 ND mesophase has
also been observed for star-shaped 1,3,5-trisalkynylbenzene and
triphenylene units linked through a short rigid π-conjugated
diacetylene spacer as well as thiophene units.3-5 Hydrogen bonding
between phenol and pyridine moieties as well as between
phloroglucinol and alkoxystillbazole moieties has also led to
fabrication of ND phase.6 However, till date, only a few approaches
have been reported for the formation of room-temperature ND
phase. For instance, Kumar et al. reported first example of room-
temperature ND mesogen by using pentaalkynyl benzene derivative
with branched alkyl spacers.7 The room-temperature ND phase has
also been observed for the discotic triphenylene core possessing
poly(ethylene oxide) side chains.8 Lee & co-workers developed a
earlier
examples
of
symmetric
dimers
based
on
pentaalkynylbenzene show ND phase at higher temperature.11 We
believe that incompatibility of the two discs in the folded mesogen
leads to improper packing resulting into the formation of a room-
temperature ND phase which persists over a wide range. Earlier we
demonstrated that linking a pentaalkynylbenzene unit with a
triphenylene core through flexible alkyl spacer containing a short
rigid ester group in centre leads to columnar mesophase at ambient
temperature.12 In contrast, further increasing the flexibility of
spacer, i.e., by introducing only alkyl chains leads to the desired
mesophase.
The target compounds (7a-d) were synthesized by the route as
depicted in scheme 1. The synthesis of intermediate compounds 2,
3, 5 and 6 has been discussed elsewhere.13 Compounds 7a-d were
prepared by reacting the pentayne
triphenylene 6 in presence of cesium carbonate and potassium
3 and monohydroxy
iodide using butanone as solvent under reflux condition (see ESI).
All compounds were characterised by H & 13C NMR, IR, UV-Vis and
1
mass (MALDI) spectrometry (ESI, Fig. S1-S9). The thermal behaviour
of synthesized compounds was investigated by differential scanning
calorimetry (DSC) and the mesophase behaviour was analysed by
polarized optical microscopy (POM) and X-ray scattering studies.
All compounds exhibited light blue fluorescence in solution. The
representative absorption spectrum obtained for 7b showed the
maxima centred at 264, 280, 338 and 416 nm with two shoulder
peaks at 237 & 380 nm (ESI, Fig. S10). Emission spectra of all
compounds were recorded by exciting their solutions at their
absorption maxima. The spectra showed blue light emission with
the maxima centred at 452 nm for all fluorophores. The
a.Department of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Mohali, Sector-81, Knowledge City, Manauli-140306, India. E-
mail: skpal@iisermohali.ac.in
b.School of Physics, University of Hyderabad, Hyderabad-500046, India
†
Electronic Supplementary Information (ESI) available: Detailed synthetic
procedure, characterization details, 2D diffraction patterns and
photoluminescence spectra. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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representative emission spectrum for compound 7b is also shown
in Fig. S10, ESI.
13
12
11
10
9
View Article Online
DOI: 10.1039/C6CC09509G
a
b
Compounds 7a and 7d were found to be non-liquid crystalline.
Compound 7b was solid at room temperature and displayed a
melting transition (centred at 39.93 °C) to the mesophase with a
heat of transition (ΔH) of 1.57 kJ mol-1. On further heating,
mesophase was cleared at 57.48 °C (ΔH = 0.48 kJ mol-1). On the
other hand, cooling scan showed the appearance of a well-defined
schlieren texture (ESI, Fig. S11) at 55 °C that remained stable down
to room temperature (even upto -19.4 °C). Compound 7c was liquid
crystalline at room temperature and exhibited isotropization at 50
°C (ΔH = 0.30 kJ mol-1). On further cooling, mesophase emerged at
46.83 °C (ΔH = 0.29 kJ mol-1) which was stable upto -22.36 °C. The
DSC traces obtained from heating and cooling runs with 7c are
shown in Fig. 1a. Gratifyingly, the photomicrograph of compound 7c
at 40 °C (Fig. 1b) obtained during cooling from the isotropic liquid
clearly displays a signature of ND phase.
8
7
20 μm
6
-20
0
20
40
60
Temperature (C)
Fig. 1 (a) DSC trace of compound 7c on heating and cooling (scan rate 5 °C
min-1). (b) Polarizing optical photomicrograph of compounds 7c at 40 °C (on
cooling, crossed polarizers).
(side by side separation) along the disc plane and approximates the
diameter of the composite disc unit,
wide angle peak is calculated to be 4.68 Å which mainly originates
̴19 Å. The spacing from the
To investigate the detailed structure of the ND phase, X-ray from the liquid-like correlation of the molten chains. The reflection
in the mid-angle region corresponds to a distance of 8.78 Å which is
attributable to the weak correlation of dyad along the normal to the
disc plane (face to face correlation) and corresponds to the
diffraction experiments were performed. Diffractograms of
mesophases of both 7b and 7c (Fig. 2 & Fig. S12) displayed a sharp
small angle peak corresponding to the presence of local columnar-
like orientational alterations within the nematic phase, and a broad
peak at wide angle which is ascribed to liquid like order of alkyl
chains. This pattern conclusively affirms the presence of a ND
phase.14 In addition, one broad peak in the mid-angle range was
also observed which was further confirmed by full q range fitting of
diffraction pattern with Lorentzian profiles.
effective thickness of the folded dimer. This thickness, ̴8 Å is a
result of folding of the dyad which was further supported by DFT
calculations (vide infra). The diffraction pattern for the nematic
phases of compound 7c is very similar to 7b and provides very
similar conclusions (Fig. 2b).
The correlation length, the degree of order within the
mesophases, was calculated using the formula ξ = [k2π]/[(Δq)]
which is equivalent to Scherrer’s equation, ξ = [k λ]/[(Δ2θ) Cosθ]. In
the above formula, k is the shape factor whose typical value is 0.89,
λ is the wavelength of the incident X-ray, Δ2θ is the broadening in
2θ at half of the maximum intensity (FWHM) in radian unit, θ is the
maximum of the reflection, q is the scattering vector (q= 4πSinθ/ λ)
and the Δq is broadening in q at half of the maximum intensity. The
Δq is obtained by Lorentzian fitting of the diffraction pattern. For
compound 7b, the correlation lengths for the reflections 19.81, 8.78
and 4.68 Å are calculated to be 43.05, 9.48 and 8.66 Å respectively
(Table 1). For compound 7c the respective correlation lengths have
been increased and this is more prominent for the reflection in the
small angle region (Table 1). For direct comparison, the correlation
length was divided by d-spacing, which results in a measure for the
spatial order in terms of dimensions of the molecular length scale.
Fig. 2c displays the azimuthal plot for small and wide angle peaks of
compounds 7b and 7c. Azimuthal intensity variations in small and
and wide angles are in opposite phase which reflects that the
growth of the side by side correlation of the composite disc and
chain-chain correlation are in orthogonal plane. Since the intensity
variation is broad in nature which further resonates that the
structure is locally planar.
The calculated d- spacing of 19.81 Å for the signal in the small
angle for 7b (Fig. 2a) represents the average inter-disc distance
C5H11
C5H11
C5H11
Br
Br
O
Br
Br
Br
Br
Br
Br
Br
Br
(i)
(ii)
OH
O
(CH2)n
(CH2)n
C5H11
C5H11
Br
2
Br
3
1
C5H11
C5H11
C6H13
O
OC6H13
(v)
+
C5H11
O
(CH2)n
O
OC6H13
OC6H13
C6H13
O
7a: n = 6
7b: n = 8
7c: n = 10
7d: n = 12
7
C5H11
C5H11
To investigate the folding behavior of two discs in 7b, we carried
out a high-level DFT analysis. The computational analysis on a
slightly truncated model (see ESI for coordinates) reveals that the
folded form of the molecule is electronically stable by 22.6 kcal mol-
OC6H13
OH
OC6H13
OC6H13
C6H13
C6H13
O
O
C6H13
C6H13
O
OC6H13
OC6H13
(iii)
(iv)
O
1
(B3LYP-D3/6-311G**) over its unfolded, linear congener.15
OC6H13
OC6H13
OC6H13
Considering the entropic penalty associated with the folding event,
free energy difference (ΔG) was calculated to be 14.7 kcal mol-1 in
favor of the folded state. As can be observed from Fig. 3, one of the
phenyl group of the phenyl acetylenes is nicely engaged in π-π
interaction with the triphenylene unit, keeping the interacting C…C
distances within 3.5-3.7 Å. To ensure further that the stabilization
for folded state is not much biased by the choice of the level of
OC6H13
4
6
5
Scheme 1 Synthesis of the target compounds 7. Reagents and Conditions: (i)
Dibromoalkane, K2CO3, KI, butanone, 80 °C, 18 h, 89%; (ii) Pd(Ph3)2Cl2, 4-
pentylphenylacetylene, PPh3, CuI, Et3N,100 °C, 15h, 82%; (iii)FeCl3, CH2Cl2,
H2SO4, RT, 1 h, 70%; (iv) Cat-B-Br, CH2Cl2, RT, 48 h, 40%; (v) Cs2CO3, KI,
butanone, 80 °C, 18 h, 85%.
2 | J. Name., 2012, 00, 1-3
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b
a
c
View Article Online
DOI: 10.1039/C6CC09509G
1200
750
1400
900
600
300
0
675
600
525
450
1050
700
350
0
0.0
0.5
1.0
q (Å-1)
1.5
2.0
0.0
0.5
1.0
q (Å-1)
1.5
2.0
0
50
100
150
200
Azimuthal angle [Degree]
Fig. 2 X-ray diffraction patterns of compound (a) 7b and (b) 7c (half filled diamond in dark cyan colour) along with the Lorentzian fittings). Black,
red and blue colour is the Lorentzian profiles corresponding to small, mid and wide angle peak, respectively. Purple colour is the sum of all these
Lorentzian profiles and fits to the full q range profile of the data. (c) Azimuthal plot for small and wide angle peak of the compound 7b (blue &
dark yellow) and 7c (red & purple) respectively.
theory, we recalculated the electronic energy difference in M06-
D3/6-311G**.16 The difference came out to be 29.4 kcal mol-1 in
favor of folded State, unambiguously confirming that the
optimized folded state is significantly stable over the linear
geometries and such a large difference further indicates that the
molecule may remain preferentially folded even in neat
condition. Most notably, this folded behavior of 7b is also
consistent with our X-ray analysis.
To investigate the physical properties of the room
temperature ND LCs, dielectric constant and birefringence were
measured as a function of voltage and temperature (details of
the cell preparation for these measurements are given in the
ESI). The dielectric constant of samples was measured as a
function of voltage at a fixed frequency of 1 KHz (Fig. 4a). The
dielectric constant increases beyond the Freedericksz’ threshold
voltage and tend to saturate at higher voltages. In particular, for
sample 7b and 7c the threshold voltages are 9.5 V and 11.8 V,
respectively. The director is homeotropic below the Freedericksz
threshold voltage and becomes planar at much higher voltage.
This voltage-scan enables us to measure the effective dielectric
anisotropy at a particular temperature. The dielectric anisotropy
of the samples was found to be negative and very small (≈ -
0.01). Although the value of the dielectric constant is
comparable to other room-temperature ND LCs, the anisotropy
is much smaller than most of the other ND compounds known in
the literature.17
As shown in Fig. 4c, the birefringence increases gradually with
voltage along the isotropic to ND transition instead of a
discontinuous jump, often seen for first order phase transitions.
This is due to the large isotropic-ND coexisting region (≈ 4 to 5
°C), which was observed indeed under POM. The maximum
birefringence obtained at room temperature for the compounds
7b and 7c are ≈ 0.082 and ≈ 0.071, respectively. The very small
dielectric anisotropy and the optical birefringence indicate that
the orientational order in such systems is relatively lower than
many other discotic compounds at room temperature.6,7,17 Most
possibly, this stems from the folding of the two disc units and
the presence of
a flexible linking chain. Such linking is
responsible for poor stacking of the two disc-like units that
prevent short-range columnar order in the nematic phase.
Notably,the present study did not show any evidence of phase
biaxiality in these systems, which may originate from improper
stacking of the two neighbouring disc-like units.
To measure the change in birefringence (Δn = ne-no) of
samples, its voltage dependence was tested in the ND phase at a
fixed temperature which is shown in Figure 4b. Above the
Freedericksz threshold voltage, Δn increases gradually and
saturates at ≈20 V. The saturation of Δn indicates the planar
state of the director as discussed earlier. Furthermore, a voltage
of 23 V was applied to measure the temperature dependent
birefringence.
Table 1 X-ray reflections and corresponding correlation lengths
obtained after fitting the full q range profile with Lorentzian
Fig. 3 Panel A: Computionally optimized structures of (i) linear and (ii)
folded dimer. The folded form is electronically stable by 22.6 kcal mol-1
(B3LYP-D3/6-311G**) over unfolded form. Panel B: Schematic of (i) the
structure of compounds 7. (ii) Modeled as two discs connected with a
flexible chain (T- triphenylene, P- pentaalkynylbenzene) in which disc P is
truncated. (iii) A composite disc results after folding of these two discs.
(iv) A possible schematic representation of ND phase of the composite
discs. Alkyl chains are omitted for better clarity. Arrow in black color
show the direction of the composite disc normal.
profiles in the nematic phases of compound 7b
& 7c.
Sample
Name
Properties
Small
peak
angle
Mid
peak
angle
Wide angle
peak
d spacing d(Å)
Correlation
19.81±0.01
43.05±0.03
8.78±0.02
9.48±0.12
4.68±0.01
8.66±0.02
7b
Length (ξ) (Å)
ξ/d
2.17
1.08
1.85
d spacing d(Å)
Correlation
19.67±0.01
51.21±0.03
8.54±0.02
9.62±0.12
4.68±0.01
8.69±0.02
7c
Length (ξ) (Å)
ξ/d
2.60
1.13
1.86
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a
b
c
View Article Online
DOI: 10.1039/C6CC09509G
Δn 7b
Δn 7c
3.09
3.08
3.07
n-1b
0.08
0.06
0.04
0.02
0.00
Δn 7b
n-1b
0.08
n-1c
n-1c
Δn 7c
ε 7bb


ε 7cc
0.06
0.04
0.02
0.00
3.03
3.02
3.01
5
0
5
10
15
20
25
10
15
20
-15
-10
-5
T-Tni
0
5
V
V
Fig. 4 (a) The voltage dependent dielectric constant for 7b and 7c. (b) Voltage dependent birefringence of compounds 7b (37 °C) and 7c (43 °C) and (c) the
temperature dependent birefringence of compounds 7b and 7c.
2
3
4
H. K. Bisoyi and Sandeep Kumar, Chem. Soc. Rev., 2010,
39, 264-285.
B. G. Kim, S. Kim and S. Y. Park, Tetrahedron Lett., 2001,
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(a) S. Kumar and S. K. Varshney, Org. Lett., 2002, 4, 157-
Earlier reports disclosed that the mesophase behaviour of a
discotic dimer generally relies on the position of equilibrium
existing between the folded and unfolded conformers which in
turn depends on the subtle interactions between the two discs,
linking as well as the peripheral alkyl chains.18 In general,
discotic dimers having shorter spacers tend to fold and arrange
themselves in intra-1,3 or intra-1,4 fashion to give columnar
arrangement. In our studies, we indicate a possibility of folding
of the molecule due to π-π interactions between the
triphenylene unit and phenyl rings of pentaalkynylbenzene.
However, due to poor tendency of pentaalkynylbenzene units to
pack into columns, a ND mesophase is observed. In addition, we
propose the possibility of folding in compound 7 from three
different experimental results; i) the spacing of the mid angle
peak in X-ray diffraction corresponding to effective thickness of
the folded dimer ii) DFT calculations disclose that the folded
state is significantly more stable than the unfolded state iii)
smaller values of dielectric anisotropy and optical birefringence.
On the basis of these results we propose a schematic of ND
phase shown in Fig. 3, where, mesogenic dyad (7b or 7c) is
modelled as two discs linked via flexible chains, in which one of
the disc is truncated. These discs of the dyad can fold to form a
composite disc which further assembled into a ND phase. The
slight non-planarity of these composite discs avoids the
formation of columnar phase as it prevents the π-π stacking
among these units. Also, the incompatibility of the two discs
present in the composite disc leads to improper packing
resulting into mesophase at room temperature.
159; (b) S. Kumar and S. K. Varshney, Liq. Cryst., 2001, 28
161-163.
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(a) L. Zhang, D. L. Hughes and A. N. Cammidge, J. Org.
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S.-Y. Park, Tetrahedron Lett., 2005, 46, 7143-7146.
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8
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10 H.-H. Chen, H.-A. Lin, S.-C. Chien, T.-H. Wang, H.-F. Hsu,
T.-L. Shih and C. Wu, J. Mater. Chem., 2012, 22, 12718-
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13 (a) M. Gupta and S. K. Pal, Liq. Cryst., 2015, 42, 1250-
In conclusion, we have synthesised two new discotic dyads
showing ND phase at room temperature possibly arising due to
folding of two discs through π-π interactions. XRD analysis, DFT
calculations as well as the lower values of the birefringence and
dielectric anisotropy are consistent with the prescribed model of
the compounds.
We are grateful to NMR, HRMS and SAXS/WAXS facility at IISER
Mohali. Dr. SK Pal and Dr. S Dhara are grateful for financial
support from INSA (bearing sanction No. SP/YSP/124/2015/433)
and DST, FIST-II, respectively. M. Gupta acknowledges the
receipt of a graduate fellowship from IISER Mohali.
1256; (b) M. Gupta and S. K. Pal, Langmuir, 2016, 32
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