Room-temperature discotic nematic liquid crystals

Article abstract of DOI:10.1080/15421400390213573

The use of discotic nematic liquid crystals instead of calamitic nematic liquid crystals to improve the viewing angle of a liquid crystal display device has recently been reported. Compared to the large number of calamitic molecules showing nematic phase at room temperature, the number of disk-shaped molecules showing subambient discotic nematic phase N_D are rare. In this paper we present the design, synthesis, mesomorphic behaviour and structureproperty relationship of discotic nematic liquid crystals based on benzene multiyne core.

Full text of DOI:10.1080/15421400390213573

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Room-Temperature Discotic
Nematic Liquid Crystals
Sandeep Kumar a , Sanjay Kumar Varshney a &
Diwakar Chauhan a
a Centre for Liquid Crystal Research, P.O. Box 1329,
Jalahalli, Bangalore, 560013, India
Published online: 18 Oct 2010.
To cite this article: Sandeep Kumar , Sanjay Kumar Varshney & Diwakar Chauhan
(2003) Room-Temperature Discotic Nematic Liquid Crystals, Molecular Crystals and
Liquid Crystals, 396:1, 241-250, DOI: 10.1080/15421400390213573
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Mol. Cryst. Liq. Cryst., Vol. 396, pp. 241–250, 2003
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ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421400390213573
ROOM-TEMPERATURE DISCOTIC NEMATIC LIQUID
CRYSTALS
Sandeep Kumar, Sanjay Kumar Varshney,
and Diwakar Chauhan
Centre for Liquid Crystal Research, P.O. Box 1329,
Jalahalli, Bangalore-560013, India
The use of discotic nematic liquid crystals instead of calamitic nematic liquid
crystals to improve the viewing angle of a liquid crystal display device has
recently been reported. Compared to the large number of calamitic molecules
showing nematic phase at room temperature, the number of disk-shaped
molecules showing subambient discotic nematic phase ND are rare. In this
paper we present the design, synthesis, mesomorphic behaviour and structure-
property relationship of discotic nematic liquid crystals based on benzene
multiyne core.
Keywords: discotic nematic liquid crystals; hexaalkynylbenzene; pentaalkynylbenzene; multi-
ynes; liquid crystal display
INTRODUCTION
Twenty five years ago, in November 1977 a paper appeared in ‘‘Pramana’’,
in which Chandrasekhar and his colleagues reported that not only rod-like
molecules, but also compounds with disc-like molecular shape are able to
form mesophases [1]. This discovery has opened up a whole new field of
liquid crystal research. Mesophases formed by disc-shaped molecules, now
commonly referred to as discotic liquid crystals, are primarily of two
types – nematic and columnar [2–7]. A few discotic molecules are also
reported to form discotic lamellar mesophases [8–12]. Recently, these
materials are attracting much attention due to their remarkable electro-
optical properties. Conductivity along the columns in columnar meso-
phases has been reported to be several orders of magnitude greater than in
the perpendicular direction [13–16]. Thus the columns may be described as
We are very grateful to Professor S. Chandrasekhar for helpful discussions. A research
grant from CSIR (No. 03(0937)/02/EMR-II) is gratefully acknowledged.
241

242
S. Kumar et al.
molecular wires. Discotic liquid crystals based on various aromatic cores
such as triphenylene, phthalocyanine, tricycloquinazoline, etc., have
recently been emerged as a new class of fast photoconducting materials.
Photoinduced charge carrier mobility (hole mobility) in the range of
10ꢀ3 cm2vꢀ1 ꢀ1to 0.38 cm2Vꢀ1sꢀ1 has been reported in various columnar
s
mesophases [17–20]. Electroluminescence and photovoltaic devices based
on discotic liquid crystals have been prepared recently [21–26]. Mechanism
of charge transport and one-dimensional energy migration has also been
studied extensively in these materials [27–33].
In addition to the above mentioned non-display applications, discotic
nematic liquid crystals have recently been reported to be useful for display
applications also. We know that most of the liquid crystal displays (LCDs)
are twisted nematic (TN) and supertwisted nematic (STN) type. The liquid
crystal materials used in these devices are exclusively the calamitic liquid
crystals (composed of rod-shaped molecules). In order to operate the
device at ambient temperature, a number of calamitic nematic liquid
crystals having room temperature mesophases have been synthesised.
Although twisted nematic display devices are working well, the viewing
angle is a critical problem for these displays. Several methods have been
developed to widen the viewing angle of TN and STN devices [34–37] but
some additional complex processes are required.
Unlike calamitic nematic liquid crystals, discotic nematic liquid crys-
tals exhibit negative birefringence ðDn < 0Þ. The negative birefringence
film formed by polymerised nematic discotic LCs can be used as com-
pensation foils to enlarge the viewing angle of twisted nematic LCDs
[38,39]. In fact, this represents the first commercial application of dis-
cotic liquid crystals.
Chandrasekhar and his co-workers recently disclosed
a novel
approach to improve the viewing angle of a display device by utilising
discotic nematic liquid crystals instead of calamitic nematic liquid crys-
tals [40]. The device prepared using hexaalkynylbenzene based discotic
nematic liquid crystal, exhibit wide and symmetrical viewing angle.
However, the material used in the device has high temperature discotic
nematic mesophase and, therefore, the device could not be tested at
room temperature.
Compared to the large number of calamitic molecules showing nematic
phase, the number of disk-shaped molecule showing discotic nematic phase
ND are rare and to our knowledge, only one compound having the discotic
nematic phase at room-temperature is known [41]. In this hexaalk-
ynylbenzene derivative a branched alkyl chain was directly connected to the
phenyl ring to reduce melting and clearing temperatures. Here we present
the synthesis, characterisation, thermal behaviour and structure-property
relationship of several new hexa- and pentaalkynylbenzene based molecules.

Room-temperature Discotic Nematic LCs
243
DESIGN AND SYNTHESIS
In order to understand the structure-property relationship and to prepare
room-temperature discotic nematic liquid crystals, we chose to work on
discotic multiynes primarily because a number of compounds of this class
are already known to show stable discotic nematic phases [42] and,
therefore, some minor structural modifications may lead to materials hav-
ing room-temperature discotic nematic phases. Studying the thermal
behaviour of known compounds carefully, it can be deduced that increasing
the size of peripheral alkyl chains in hexaalkynylbenzene derivatives gra-
dually decreases the melting and clearing temperatures (Fig. 1) [42].
Hexaalkynylbenzene derivatives having only up to nine carbon atoms in the
peripheral chains are known in the literature. Therefore, it would be
interesting to know the thermal behaviour of higher homologues such as
compound number 12a and 12b (Scheme 1).
The use of branched chains in liquid crystals often reduces melting and
isotropic temperatures. The decrease in the transition temperature could
be due to the disorder caused by branched chains and stereoheterogeneity.
This methodology has been successfully applied in the case of several
discotic liquid crystals forming columnar mesophases [43–48]. To check
whether this technique is also effective in the case of alkoxymultiyns based
discotic nematic liquid crystals, we designed compound 17a and 17b
(Scheme 2).
In the case of triphenylene based discotic nematic liquid crystals, it has
been observed that a lateral methyl group substitution reduces the transition
FIGURE 1 Thermal behavior of some of the hexaalkynyl benzene.

244
S. Kumar et al.
SCHEME 1
temperatures significantly [49,50]. Compound 17c, 17d and 17ewere
designed to find the effects of lateral methyl group substitution in alkoxy-
multiyns (Scheme 2).

Room-temperature Discotic Nematic LCs
245
SCHEME 2
From the thermal data of compound 1 and 5 (Fig. 1) [42], it is clear
that when the peripheral alkyl chains are attached to the phenyl ring in
the hexaalkynylbenzene via oxygen atom, the melting and clearing

246
S. Kumar et al.
temperatures are higher compared to when the alkyl chains are directly
attached to the ring. Again, as mentioned above, it is also known that
when the peripheral aliphatic side chains of various cores are branched,
the mesophase is widened but the type of the mesophase formed is not
affected by the introduction of branching in many cases [46]. Therefore,
this may be anticipated that by replacing the normal alkyl chains by
branched alkyl chains connected directly to the phenyl ring, the melting
points of alkynylbenzene derivatives may reduce and thus, stabilize the
mesophase. We have utilised this technology to prepare the first room-
temperature discotic nematic liquid crystal [41]. Now we designed
several more derivatives in which branched chains of various carbon
atoms are attached directly to the core (compound 12c, 12d and 12e,
Scheme 1).
Finally, pentaalkynylbenzene derivatives (compound 19a, 19b and 19c,
Scheme 3) having a combination of branched and normal alkoxy chains
were also designed to achieve room-temperature discotic nematic liquid
crystals.
SCHEME 3

Room-temperature Discotic Nematic LCs
247
SYNTHESIS OF ALKYL-SUBSTITUTED HEXAYNES
All the hexaalkynylbenzene derivatives were prepared following the lit-
erature methods [41,42] as shown in Scheme 1. Commercially available
acids were converted to acid chlorides using oxalyl chloride at room tem-
perature. Friedel-Crafts acylation of the bromobenzene with the above acid
chlorides gives the 1-Bromo-4-alkanoylbenzene 8. Wolff-Kishner reduction
of these ketones furnished 1-Bromo-4-alkylbenzene 9. Pd-catalysed alky-
nylation of different alkyl-substituted bromobenzenes 9. with 2-methyl-
3-butyn-2-ol afforded the protected phenylacetylenes 10. which were
deprotected using KOH in refluxing toluene to yield the phenylacetylenes
11. A palladium-catalysed coupling of these alkyl-chain substituted phe-
nylacetylenes 11 with hexabromobenzene yielded the desired alkyl-sub-
stituted hexaynes 12. All the final compounds were purified by repeated
column chromatography. They were homogenous on TLC plate and their
spectral data were in accordance with the structure.
SYNTHESIS OF ALKOXY-SUBSTITUTED HEXAYNES
Alkoxy-substituted hexaynes can be prepared easily as shown in Scheme 2
[42]. Alkylation of p-bromophenol 13 under classical reaction conditions
furnished 4-alkoxybromobenzenes 14. These bromobenzenes can be con-
verted to free accetylenes 16 as described above (Scheme 2). Final mul-
tiynes can be prepared by a palladium-catalysed coupling of these alkoxy
chain substituted phenylacetylenes 16 with hexabromobenzene. Similarly,
lateral methyl-substituted multiynes can also be prepared starting from
4-bromo-o-cresol.
SYNTHESIS OF ALKOXY-SUBSTITUTED PENTAYNES
Commercially available pentabromophenol can be alkylated to get mono-
alkoxy-pentabromobenzene 18 (Scheme 3). Condensation of acetylenes 16
with 18 as described above yielded desired pentaynes 19.
THERMAL BEHAVIOUR
The thermal behavior of all the compounds was investigated by optical
polarising microscopy. Transition temperatures were measured using a
Mettler FP82HT hot stage and central processor in conjunction with a Leitz
DMRXP polarising microscope.
The crystalline compound 12a melts at 71ꢁC to an isotropic liquid. On
cooling from the isotropic phase, the schlieren texture of nematic phase

248
S. Kumar et al.
appeared at 54ꢁC and crystallises at 34ꢁC. The thermal behaviour is
reproducible and, therefore, the ND phase of 12a is monotropic in nature.
The higher homologue 12b is non-liquid crystalline. It melts at 57ꢁC and
crystals appear from the isotropic melt at about 40ꢁC on cooling. Branched-
chain derivatives 12c and 12d were also found to be non-liquid crystalline.
The 3-methylpentane derivative 12c melts at about 250ꢁC with decom-
position and the 4-methylpentane derivative 12d melts at 195ꢁC with
partial decomposition. Compound 12e was not pure enough to be analysed.
In the alkoxy series (Scheme 2) compound 17a with 3,7-dimethyloctane
periphery melts at 80ꢁC to a nematic mesophase which clears at 124ꢁC. On
cooling the nematic phase appears at 123ꢁC and crystallises at 64ꢁC. The
higher homologue 17b having 3,7,11-trimethyldodecane periphery was in
liquid state (non-liquid crystalline) at room temperature. Compound 17c
having a lateral methyl group substitution and normal octyloxy chains
shows discotic nematic phase at 95ꢁC. It changes to isotropic liquid at
176ꢁC on heating with partial decomposition. Peripheral branched alkoxy
and lateral methyl group substituted compound 17d also gives ND phase at
71ꢁC on heating. Its colour changes from light yellow to brownish on
heating to isotropic temperature (broad 147–160ꢁC) indicating partial
decomposition of the material at higher temperature. Like 17b, the higher
homologue 17e having 3,7,11-trimethyldodecane periphery and a lateral
methyl group was also in liquid state at room temperature.
In the pentayne series, compound 19b was non-liquid crystalline but
other two derivatives 19a and 19c were found to be liquid crystalline at
room temperature. While the discotic nematic mesophase of compound
19a clears at 40ꢁC, it changes to isotropic liquid at 73ꢁC in the case of
19c. On cooling, the ND phase appears with very limited supercooling and
remains stable down to room temperature. Both the compounds show a
glass transition at about ꢀ35ꢁC in DSC heating and cooling cycles. It is
interesting to note that in the case of triphenylene based nematics, a
lateral methyl group substitution usually decreases the isotropic tem-
perature [49,50] but in the case of benzene multiynes the effect is
reverse. All the benzene multiynes having lateral methyl group substitu-
tion show higher clearing temperature compared to materials without
lateral methyl substitution.
CONCLUSION
Several new benzene hexa- and pentaalkynes were prepared and char-
acterised. Two of them show very broad discotic nematic mesophase stable
well below and above the room temperature. These materials may be useful
for wide viewing LCD devices.

Room-temperature Discotic Nematic LCs
249
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