Showcasing research from the group of Dr Santanu Kumar Pal
at Department of Chemical Sciences, Indian Institute of
Science Education and Research Mohali, India

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A new strategy towards the synthesis of a room-temperature
discotic nematic liquid crystal employing triphenylene andpentaalkynylbenzene units
Two new discotic dyads showing ND phase at room temperature
were prepared. Modelling the mesophase as derived from X-ray
scattering results, high-level DFT calculations as well as dielectricand birefringence studies suggested that the formation of ND
phase was realized most likely through folding of the dimericmolecules through π–π interactions.
See Santanu Kumar Pal et al.,Chem. Commun., 2017, 53, 3014.

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Cite this: Chem. Commun., 2017,53, 3014

Published on 03 February 2017. Downloaded on 27/03/2017 04:48:03.

Received 29th November 2016,
Accepted 3rd February 2017
DOI: 10.1039/c6cc09509g

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A new strategy towards the synthesis of aroom-temperature discotic nematic liquid
crystal employing triphenylene andpentaalkynylbenzene units†
Monika Gupta,a Santosh Prasad Gupta,a M. V. Rasna,b Debashis Adhikari,aSurajit Dharab and Santanu Kumar Pal*arsc.li/chemcomm

A new approach is reported for the design of a room-temperature
discotic nematic (ND) liquid crystal (LC) dimer consisting of a triphenylene
and a pentaalkynylbenzene unit linked via flexible alkyl spacers. The
formation of the ND 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.

Disc shaped molecules showing a nematic phase (ND) are rare but
are 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 ofcommonly used LC displays based on polymerized nematic
discogens.1 Unfortunately, most of the discotic nematogens reported
so far exhibit ND behaviour at high temperature and over a narrowtemperature range.2 In contrast, implementation of nematic discogens
in devices necessitates the ND phase at room temperature. A thoroughliterature survey reveals that hexa- and pentaalkynylbenzene
derivatives have been the most investigated systems among
various discotic nematogens.2 The ND mesophase has also been
observed for star-shaped 1,3,5-trisalkynylbenzene and triphenylene
units linked through a short rigid p-conjugated diacetylene spacer
as well as thiophene units.3–5 Hydrogen bonding between phenol
and pyridine moieties as well as between phloroglucinol andalkoxystilbazole moieties has also led to fabrication of the ND
phase.6 However, to date, only a few approaches have been reported
for the formation of the room-temperature ND phase. For instance,
Kumar et al. reported the first example of a room-temperature ND
mesogen by using a pentaalkynyl benzene derivative with branchedalkyl spacers.7 The room-temperature ND phase has also been
observed for the discotic triphenylene core possessing poly(ethyleneaDepartment of Chemical Sciences, Indian Institute of Science Education andResearch (IISER) Mohali, Sector-81, Knowledge City, Manauli-140306, India.
E-mail: skpal@iisermohali.ac.inbSchool 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/c6cc09509g

3014 | Chem. Commun., 2017, 53, 3014--3017

oxide) side chains.8 Lee & co-workers developed a strategy to achieveroom-temperature nematic discogens by perturbing the lateral sidearm of pentaalkynylbenzene with a substitution ortho to the ethynyl
group on the peripheral phenyl ring.9 They also establishedfurther that the room-temperature ND phase can be obtained byintroducing an attraction-enhancing unit in the lateral side armof hexa(phenylethynyl)benzene.10
In this communication, we report a new design for therealization of room-temperature discotic ND dyads consistingof triphenylene & pentaalkynylbenzene units linked via flexible
alkyl spacers. The earlier examples of symmetric dimers based on
pentaalkynylbenzene show the ND phase at higher temperature.11
We believe that the incompatibility of the two discs in the folded
mesogen leads to improper packing resulting in the formation of aroom-temperature ND phase which persists over a wide range.
Earlier we demonstrated that linking a pentaalkynylbenzene unit
with a triphenylene core through flexible alkyl spacers containing ashort rigid ester group in the centre leads to a columnar mesophase at ambient temperature.12 In contrast, further increasing theflexibility of the spacers, i.e., by introducing only alkyl chains, leads
to the desired mesophase.
The target compounds (7a–d) were synthesized by the route
depicted in Scheme 1. The synthesis of intermediate compounds 2, 3,
5 and 6 has been discussed elsewhere.13 Compounds 7a–d wereprepared by reacting the pentayne 3 with monohydroxy triphenylene 6
in the presence of cesium carbonate and potassium iodide using
butanone as a solvent under reflux conditions (see the ESI†). Allcompounds were characterised by 1H & 13C NMR, IR, UV-Vis andmass (MALDI) spectrometry (ESI,† Fig. S1–S9). The thermal behaviourof the synthesized compounds was investigated by differential scanning calorimetry (DSC) and the mesophase behaviour was analysed bypolarized 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 allcompounds were recorded by exciting their solutions at theirabsorption maxima. The spectra showed blue light emission withThis journal is © The Royal Society of Chemistry 2017

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Scheme 1 Synthesis of the target compounds 7. Reagents and conditions:(i) dibromoalkane, K2CO3, KI, butanone, 80 1C, 18 h, 89%; (ii) Pd(Ph3)2Cl2,
4-pentylphenylacetylene, PPh3, CuI, Et3N, 100 1C, 15 h, 82%; (iii) FeCl3, CH2Cl2,
H2SO4, RT, 1 h, 70%; (iv) Cat-B–Br, CH2Cl2, RT, 48 h, 40%; and (v) Cs2CO3, KI,butanone, 80 1C, 18 h, 85%.

Fig. 1 (a) DSC traces of compound 7c upon heating and cooling (scanrate 5 1C min 1). (b) Polarizing optical photomicrograph of compound 7c
at 40 1C (upon cooling, crossed polarizers).

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the maxima centred at 452 nm for all fluorophores. The representativeemission spectrum for compound 7b is also shown in Fig. S10, ESI.†
Compounds 7a and 7d were found to be non-liquid crystalline.
Compound 7b was solid at room temperature and displayed amelting transition (centred at 39.93 1C) to the mesophase with aheat of transition (DH) of 1.57 kJ mol 1. Upon further heating, themesophase was cleared at 57.48 1C (DH = 0.48 kJ mol 1). On theother hand, the cooling scan showed the appearance of a welldefined schlieren texture (ESI,† Fig. S11) at 55 1C that remained
stable down to room temperature (even up to 19.4 1C). Compound
7c was liquid crystalline at room temperature and exhibited isotropization at 50 1C (DH = 0.30 kJ mol 1). Upon further cooling, themesophase emerged at 46.83 1C (DH = 0.29 kJ mol 1), which wasstable up to 22.36 1C. The DSC traces obtained from heating andcooling runs with 7c are shown in Fig. 1a. Gratifyingly, thephotomicrograph of compound 7c at 40 1C (Fig. 1b) obtained
during cooling from the isotropic liquid clearly displays asignature of the ND phase.
To investigate the detailed structure of the ND phase, X-ray
diﬀraction experiments were performed. Diﬀractograms of mesophases of both 7b and 7c (Fig. 2 and Fig. S12, ESI†) displayed asharp 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 the liquid likeorder of the alkyl chains. This pattern conclusively aﬃrms thepresence of a ND phase.14 In addition, one broad peak in the midangle range was also observed, which was further confirmed by fullq range fitting of diﬀraction patterns with Lorentzian profiles.
The calculated d-spacing (19.81 Å) for the signal in the small
angle for 7b (Fig. 2a) represents the average inter-disc distance(side by side separation) along the disc plane and approximates
the diameter of the composite disc unit, B19 Å. The spacing fromthe wide angle peak is calculated to be 4.68 Å, which mainly
originates from the liquid-like correlation of the molten chains.
The reflection in the mid-angle region corresponds to a distanceof 8.78 Å which is attributable to the weak correlation of the dyad
along the normal to the disc plane (face to face correlation) andcorresponds to the eﬀective thickness of the folded dimer. This
thickness (B8 Å) is a result of folding of the dyad which was
further supported by DFT calculations (vide infra). The diﬀraction

Fig. 2 X-ray diﬀraction patterns of compounds (a) 7b and (b) 7c (half filled diamonds in dark cyan colour) along with the Lorentzian fittings. Black, redand blue curves are the Lorentzian profiles corresponding to small, mid and wide angle peaks, respectively. The purple curve is the sum of all these
Lorentzian profiles and fits to the full q range profile of the data. (c) Azimuthal plots for small and wide angle peaks of compounds 7b (blue & dark yellow)
and 7c (red & purple) respectively.

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Table 1 X-ray reflections and the corresponding correlation lengths
obtained after fitting the full q range profile with Lorentzian profiles in
the nematic phases of compounds 7b & 7c

Published on 03 February 2017. Downloaded on 27/03/2017 04:48:03.

Samplename Properties

Small angle Mid angle Wide angle
peak
peak
peak

7b

d spacing, d (Å)
19.81  0.01 8.78  0.02 4.68  0.01
Correlation length, x (Å) 43.05  0.03 9.48  0.12 8.66  0.02
x/d
2.17
1.08
1.85

7c

d spacing, d (Å)
19.67  0.01 8.54  0.02 4.68  0.01
Correlation length, x (Å) 51.21  0.03 9.62  0.12 8.69  0.02
x/d
2.60
1.13
1.86

pattern for the nematic phase of compound 7c is very similar to
7b and provides very similar conclusions (Fig. 2b).
The correlation length, the degree of order within themesophases, was calculated using the formula x = [k2p]/[(Dq)]
which is equivalent to Scherrer’s equation, x = [kl]/[(D2y)cos y].
In the above formula, k is the shape factor whose typical value is 0.89,
l is the wavelength of the incident X-rays, D2y is the broadening in 2yat half of the maximum intensity (FWHM) in radians, y is the
maximum of the reflection, q is the scattering vector (q = 4p sin y/l)
and the Dq is broadening in q at half of the maximum intensity. The
Dq is obtained by Lorentzian fitting of the diﬀraction pattern. For
compound 7b, the correlation lengths for the reflections at 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 thereflection in the small angle region (Table 1). For direct comparison,
the correlation length was divided by the d-spacing, which results ina measure for the spatial order in terms of dimensions of themolecular 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 wide angles are in opposite phases, whichreflects that the growth of the side by side correlation of thecomposite disc and chain–chain correlation are in the orthogonalplane. The intensity variation is also broad in nature, which further
supports that the structure is locally planar.
To investigate the folding behavior of the two discs in 7b, wecarried out a high-level DFT analysis. The computational analysis on aslightly truncated model (see the ESI† for coordinates) reveals that thefolded form of the molecule is electronically stable by 22.6 kcal mol 1(B3LYP-D3/6-311G**) over its unfolded, linear congener.15 Considering
the entropic penalty associated with the folding event, the free energy
diﬀerence (DG) was calculated to be 14.7 kcal mol 1 in favor of thefolded state. As can be observed from Fig. 3A, one of the phenylgroups of the phenyl acetylenes is nicely engaged in p–p interaction
with the triphenylene unit, keeping the interacting C  C distances
within 3.5–3.7 Å. To ensure further that the stabilization for the folded
state is not much biased by the choice of the level of theory, werecalculated the electronic energy difference at M06-D3/6-311G**.16
The difference came out to be 29.4 kcal mol 1 in favor of the folded
state, unambiguously confirming that the optimized folded state issignificantly stable over the linear geometries and such a large
difference further indicates that the molecule may remain preferentially folded even under neat conditions. Most notably, thisfolded behavior of 7b is also consistent with our X-ray analysis.

3016 | Chem. Commun., 2017, 53, 3014--3017

Fig. 3 Panel A: Computationally optimized structures of (i) linear and (ii)folded dimers. The folded form is electronically stable by 22.6 kcal mol 1(B3LYP-D3/6-311G**) over the unfolded form. Panel B: Schematic of (i) the
structure of compounds 7. (ii) Modeled as two discs connected with aflexible chain (T, triphenylene; P, pentaalkynylbenzene) in which disc P istruncated. (iii) A composite disc that results after folding of these two discs.
(iv) A possible schematic representation of the ND phase of the composite
discs. Alkyl chains are omitted for better clarity. Arrows in black color show
the direction of the composite disc normal.

To investigate the physical properties of the room temperature
ND LCs, the 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 the samples was measured as a function ofvoltage at a fixed frequency of 1 kHz (Fig. 4a). The dielectric
constant increases beyond the Fréedericksz threshold voltage andtends to saturate at higher voltages. In particular, for samples 7band 7c the threshold voltages are 9.5 V and 11.8 V, respectively.
The director is homeotropic below the Fréedericksz thresholdvoltage and becomes planar at a much higher voltage. This
voltage-scan enables us to measure the eﬀective dielectric anisotropy at a particular temperature. The dielectric anisotropy of thesamples was found to be negative and very small (E 0.01).
Although the value of the dielectric constant is comparable toother room-temperature ND LCs, the anisotropy is much smaller
than most of the other ND compounds known in the literature.17
To measure the change in the birefringence (Dn = ne no) of
the samples, its voltage dependence was tested in the ND phaseat a fixed temperature, which is shown in Fig. 4b. Above theFréedericksz threshold voltage, Dn increases gradually and saturates
at E20 V. The saturation of Dn indicates the planar state of thedirector as discussed earlier. Furthermore, a voltage of 23 V wasapplied to measure the temperature dependent birefringence.
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 tothe large isotropic–ND coexisting region (E4 to 5 1C), which wasobserved indeed under POM. The maximum birefringence values
obtained at room temperature for compounds 7b and 7c are E0.082
and E0.071, respectively. The very small dielectric anisotropy andThis journal is © The Royal Society of Chemistry 2017

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Fig. 4 (a) The voltage dependent dielectric constant for 7b and 7c. (b) Voltage dependent birefringence of compounds 7b (37 1C) and 7c (43 1C) and(c) the temperature dependent birefringence of compounds 7b and 7c.

the optical birefringence indicate that the orientational order in such
systems is relatively lower than many other discotic compounds atroom temperature.6,7,17 Most possibly, this stems from the folding of
the two disc units and the presence of a flexible linking chain. Suchlinking is responsible for poor stacking of the two disc-like units thatprevent short-range columnar order in the nematic phase. Notably,
the present study did not show any evidence of phase biaxiality inthese systems, which may originate from improper stacking of thetwo neighbouring disc-like units.
Earlier reports disclosed that the mesophase behaviour of adiscotic dimer generally relies on the position of the equilibrium
existing between the folded and unfolded conformers, which in turndepends on the subtle interactions between the two discs, linking aswell as the peripheral alkyl chains.18 In general, discotic dimers
having shorter spacers tend to fold and arrange themselves in an
intra-1,3 or intra-1,4 fashion to give a columnar arrangement. In ourstudies, we indicate a possibility of folding of the molecule due top–p interactions between the triphenylene unit and the phenyl ringsof pentaalkynylbenzene. However, due to the poor tendency of thepentaalkynylbenzene units to pack into columns, a ND mesophase isobserved. In addition, we propose the possibility of folding incompound 7 from three different experimental results: (i) the
spacing of the mid angle peak in X-ray diffraction corresponding
to the effective thickness of the folded dimer; (ii) DFT calculations
that disclose that the folded state is significantly more stable thanthe unfolded state; and (iii) smaller values of dielectric anisotropy
and optical birefringence. On the basis of these results, we propose
a schematic of the ND phase shown in Fig. 3, where the mesogenic
dyad (7b or 7c) is modelled as two discs linked via flexible chains, inwhich one of the discs is truncated. These discs of the dyad can foldto form a composite disc, which can further assemble into a ND
phase. The slight non-planarity of these composite discs avoids theformation of a columnar phase as it prevents the p–p stacking
among these units. Also, the incompatibility of the two discs present
in the composite disc leads to improper packing resulting in amesophase at room temperature.
In conclusion, we have synthesised two new discotic dyads
showing the ND phase at room temperature possibly arisingfrom the folding of two discs through p–p interactions. XRD
analysis, DFT calculations as well as the lower values of theThis journal is © The Royal Society of Chemistry 2017birefringence and dielectric anisotropy are consistent with theprescribed model of the compounds.
We are grateful to the NMR, HRMS and SAXS/WAXS facilities
at IISER Mohali. Dr SK Pal and Dr S Dhara are grateful for thefinancial 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.

Notes and references
1 (a) M. Lu and K. H. Yang, Jpn. J. Appl. Phys., 2000, 36, L412–L415;(b) H. Mori, Y. Itoh, Y. Nishuira, T. Nakamura and Y. Shinagawa, Jpn.
J. Appl. Phys., 1997, 36, 143–147.
2 H. K. Bisoyi and S. Kumar, Chem. Soc. Rev., 2010, 39, 264–285.
3 B. G. Kim, S. Kim and S. Y. Park, Tetrahedron Lett., 2001, 42, 2697–2699.
4 (a) S. Kumar and S. K. Varshney, Org. Lett., 2002, 4, 157–159;(b) S. Kumar and S. K. Varshney, Liq. Cryst., 2001, 28, 161–163.
5 (a) L. Zhang, D. L. Hughes and A. N. Cammidge, J. Org. Chem., 2012,
77, 4288–4297; (b) L. Zhang, H. Gopee, D. L. Hughes andA. N. Cammidge, Chem. Commun., 2010, 46, 4255–4257.
6 (a) J. H. Lee, I. Jang, S. H. Hwang, S. J. Lee, S. H. Yoo and J. Y. Jho, Liq. Cryst.,
2012, 39, 973–981; (b) J. H. Lee, M.-J. Han, S. H. Hwang, I. Jang, S. J. Lee,S. H. Yoo, J. Y. Jho and S.-Y. Park, Tetrahedron Lett., 2005, 46, 7143–7146.
7 (a) S. Kumar and S. K. Varshney, Angew. Chem., Int. Ed., 2000, 112,
3270–3272; (b) S. Kumar, S. K. Varshney and D. Chauhan, Mol. Cryst.
Liq. Cryst., 2003, 396, 241–250; (c) S. K. Varshney, V. Prasad and
H. Takezoe, Liq. Cryst., 2011, 38, 53–60.
8 S. Kohmoto, E. Mori and K. Kishikawa, J. Am. Chem. Soc., 2007, 129,
13364–13365.
9 S. C. Chien, H. H. Chen, H. C. Chen, Y. L. Yang, H. F. Hsu, T. L. Shihand J. J. Lee, Adv. Funct. Mater., 2007, 17, 1896–1902.
10 H.-H. Chen, H.-A. Lin, S.-C. Chien, T.-H. Wang, H.-F. Hsu, T.-L. Shihand C. Wu, J. Mater. Chem., 2012, 22, 12718–12722.
11 K. Praefcke, B. Kohne, D. Singer, D. Demus, G. Pelzl and S. Diele,
Liq. Cryst., 1990, 7, 589–594.
12 M. Gupta, I. Bala and S. K. Pal, Tetrahedron Lett., 2014, 55, 5836–5840.
13 (a) M. Gupta and S. K. Pal, Liq. Cryst., 2015, 42, 1250–1256;(b) M. Gupta and S. K. Pal, Langmuir, 2016, 32, 1120–1126.
14 C. T. Imrie, Z. Lu, S. J. Picken and Z. Yildirim, Chem. Commun., 2007,
1245–1247.
15 (a) A. D. Becke, J. Chem. Phys., 1993, 98, 5648–5652; (b) S. Grimme,
J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104.
16 Y. Zhao and D. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241.
17 D. VenkataSai, G. Mirri, P. Kouwer, R. Sahoo, I. Musevic and
S. Dhara, Soft Matter, 2016, 12, 2960–2964.
18 (a) K. J. A. Bozek and V. E. Williams, Soft Matter, 2014, 10, 5749–5754;(b) N. Boden, R. J. Bushby, A. N. Cammidge, A. El-Mansoury, P. S. Martin
and Z. Lu, J. Mater. Chem., 1999, 9, 1391–1402; (c) M. C. Tzeng, S. C. Liao,
T. H. Chang, S. C. Yang, M. W. Weng, H. C. Yang, M. Y. Chiang, Z. Kai,
J. Wu and C. W. Ong, J. Mater. Chem., 2011, 21, 1704–1712.

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