Novel banana-discotic hybrid architectures

Article abstract of DOI:10.3762/bjoc.5.52

Here we present the design and synthesis of novel banana-discotic dimers and banana-bridged discotic dimers. The chemical structures have been characterized by spectral techniques and elemental analysis. The thermal behaviors of the compounds have been in

Full text of DOI:10.3762/bjoc.5.52

Novel banana-discotic hybrid architectures
Hari Krishna Bisoyi, H. T. Srinivasa and Sandeep Kumar*
Preliminary Communication
Open Access
Address:
Beilstein Journal of Organic Chemistry 2009, 5, No. 52.
Raman Research Institute, C. V. Raman Avenue, Sadashivanagar,
Bangalore, India. Phone: +91 80 23610122, Fax: +91 80 23610492
doi:10.3762/bjoc.5.52
Received: 15 July 2009
Email:
Accepted: 23 September 2009
Published: 07 October 2009
Sandeep Kumar* - skumar@rri.res.in
* Corresponding author
Guest Editor: S. Laschat
Keywords:
© 2009 Bisoyi et al; licensee Beilstein-Institut.
License and terms: see end of document.
bent-core; dimer; discotic; triphenylene
Abstract
Here we present the design and synthesis of novel banana-discotic dimers and banana-bridged discotic dimers. The chemical struc-
tures have been characterized by spectral techniques and elemental analysis. The thermal behaviors of the compounds have been
investigated by polarizing optical microscopy and differential scanning calorimetry. None of these synthesized compounds exhibit
any liquid crystalline property probably because of the incompatibility of the bent-core with the discotic core.
Introduction
Liquid crystals are unique functional self-organized soft mate- addition to various banana phases, these mesogens also display
rials which possess order and dynamics. Recently, banana liquid classical nematic, smectic and columnar phases.
crystals or bent-shaped mesogens have attracted considerable
research interest in the field of soft condensed matter. The On the other hand, the unique geometry of the columnar meso-
mesomorphic properties of a variety of bent shaped molecules phase formed by discotic liquid crystals is of great importance
have been investigated quite extensively. The polar order of not only as models for the study of one-dimensional charge and
these molecules, due to their bent shape, display interesting energy migration in organized systems but also as functional
properties such as ferroelectric or anti-ferroelectric switching materials for device applications such as one-dimensional
[1-7]. The occurrence of superstructural chirality in the meso- conductors, photoconductors, light emitting diodes, photovol-
phase of bent-core compounds with no inherent chirality is not taic solar cells, gas sensors etc. [10-21]. The functional capabil-
only of fundamental scientific interest but also of industrial ities of these materials are due to their easier processibility,
application as this chirality can be switched in external electric spontaneous alignment between electrodes and self-healing of
fields. Various new applications of these materials include defects owing to their dynamic nature. Furthermore, there has
nonlinear optics, flexoelectricity, photoconductivity, molecular been considerable interest in the field of non-conventional low
electronics and the design of biaxial nematic phases [8,9]. In molar mass liquid crystals, especially in liquid crystal dimers
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Beilstein Journal of Organic Chemistry 2009, 5, No. 52.
because of their interesting mesomorphic properties. Liquid banana phases whose potential applications have been
crystal oligomers serve as ideal models for polymers and mentioned in the introductory paragraph. With this idea in
networks due to the striking similarity in their transitional be- mind, we have designed and synthesized novel banana-discotic
havior and like polymers, some oligomers form glassy meso- dimers. A typical five-ring banana liquid crystal and triphen-
phases [22-26]. Their purification and characterization are easy ylene based discotic [27] liquid crystals were chosen to prepare
unlike polydisperse polymers. Owing to the restricted motion of these novel dimers. Here we report the synthesis, characteriza-
the components, liquid crystal oligomers provide and stabilize a tion and phase behavior of these novel dimers.
variety of fluid phases with fascinating functions and oligo-
meric approach provides a wide flexibility in the Results and Discussion
molecular design.
The bend in the rigid cores of the banana liquid crystal
compounds leads to a reduction of the rotational disorder of the
The hybridization of above mentioned two varieties of liquid molecules around their long axes. If segregation of aromatic
crystals may lead to novel nanostructured materials with inter- cores and aliphatic chains is sufficiently strong, the molecular
esting physical properties important for device applications. If structure facilitates an organization into layers. Since the
such architectures could provide columnar mesophases then the molecules are closely packed within the smectic layers and
charge carrier mobilities are expected to increase owing to the additionally, the rotation about their long axes is strongly
presence of conducting aromatic bent-cores in the otherwise hindered, the bent directions align parallel in each layer. As a
insulating alkyl chain mantle around the discotic cores. Hence result of this directed organization, each layer develops a spon-
these compounds are anticipated to display superior electronic taneous polarization. On the other hand, most of the discotic
and optoelectronic performance in organic semiconducting liquid crystalline compounds form columnar phases because of
devices. Moreover, these hybrids may also display new types of the strong π-π interaction of poly aromatic cores. In the
Scheme 1: Synthesis of banana bridged discotic dimer. Reagents and conditions; (i) Br(CH2)12Br, Cs2CO3, MEK, 85 °C; (ii) HO–C6H4–COOCH2Ph,
Cs2CO3, MEK, 85 °C; (iii) 5% Pd-C, H2, 1,4-Dioxane, r.t.
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Beilstein Journal of Organic Chemistry 2009, 5, No. 52.
Scheme 2: Synthesis of banana-discotic dimers.
columnar mesophases, the discotic molecules stack one on top The symmetric dimer 6 melts at 51.1 °C on heating and crystal-
of the other to form columns and the columns so formed lizes at 38.7 °C on cooling. Under polarizing optical micro-
arrange themselves in various two dimensional lattices.
scopy we do not see any other transition, which was confirmed
by differential scanning calorimetry measurements. The
The synthesis of the target compounds, banana-bridged discotic absence of either B-phase or discotic columnar mesophase in
dimer 6 and banana-discotic dimers 8a and 8b are shown in compound 6 may be explained as follows: since the five-ring
Scheme 1 and Scheme 2, respectively. In compound 6 a five- banana unit, which otherwise exhibits mesophase with two
ring banana unit joins two triphenylene discotic cores via two terminal alkyl chains, is now attached with two bulky triphen-
flexible alkyl spacers whereas in compound 8a and 8b a five- ylene units at the ends, the packing of the bent-cores in layers
ring banana unit joins to a triphenylene core via a flexible will not be favored. In other words, the bent core-bent core
alkyl spacer.
interactions are too weak to produce any B-phase. On the other
hand, the two triphenylene units in compound 6 when attached
The chemical structures of the final compounds have been char- with flexible methylene spacers exhibit columnar meso-
acterized by spectral techniques and elemental analysis. The morphism. However, in compound 6 they are attached to each
analytical data is in good agreement with their chemical struc- other with a intervening rigid bent core unit which is probably
tures. The phase behavior of the novel compounds have been not allowing the two discs to lie in the same plane and hence
observed by polarizing optical microscopy and differential hinders columnar mesomorphism in this architecture. So non-
scanning calorimetry. All the solvent crystallized compounds coplanarity of the discs and the rigid bent-core unit in the spacer
pass from crystalline state to isotropic liquid state on heating, seems to be the reason for absence of liquid crystallinity in
and on cooling they crystallize into the solid state without any compound 6. The banana discotic hybrid 8a melts at 92.6 °C to
intervening mesophase. The transition temperatures and their an isotropic liquid and on cooling it slowly crystallizes at
associated enthalpies of the novel compounds are listed in 77.6 °C. Similarly, compound 8b on heating melts to isotropic
Table 1.
liquid at 75.8 °C and on cooling slowly crystallizes at 54.9 °C.
These polarizing optical microscopy observations are confirmed
by differential scanning calorimetry measurements. The
absence of mesomorphism in this architecture may arise from
two competing factors. First, the bent core units will try to pack
in layers by microsegregation but in this kind of packing
arrangement the attached discs have to lie side by side which is
not a preferable option for discs. Moreover, the cross-sectional
area of the disc is such that it will not allow efficient packing of
the bent cores in layers. Secondly, when the discs try to stack
one above the other to produce columnar phases, the bent core
unit will disrupt it owing to its rigidity and high volume frac-
Table 1: Transition temperatures (°C, peak temperature) and enthal-
pies (kJ/mol in parentheses) of the banana-discotic dimers both on
heating and cooling at the scan rate of 5 °C/min.
Compound
Heating
Cooling
6
Cra 51.1 (38.8) Ib
Cr 92.6 (57.7) I
Cr 75.8 (40.9) I
I 38.7 (15.4) Cr
I 77.6 (43.6) Cr
I 54.9 (35.2) Cr
8a
8b
aCr = Crystalline solid, bI = Isotropic liquid
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Beilstein Journal of Organic Chemistry 2009, 5, No. 52.
tion compared to the other alkyl chains surrounding the triphen- and then extracted into chloroform. The organic layer was
ylene disc which otherwise provide space filling effect for washed with water, dried over anhydrous sodium sulfate
columnar mesomorphism. The difluoro compound 8b, has a followed by removal of solvent. The crude product was puri-
lower melting point than its hydrocarbon parent compound 8a fied by column chromatography over silica gel with eluent 2%
which is very common observation in bent-core compounds.
ethyl acetate in hexane. Finally the product was precipitated as
waxy solid from dichloromethane by adding hexane,
yield 2.44 g.
Conclusion
We have designed and synthesized banana-discotic dimers and
a banana-bridged discotic dimer to investigate possible meso- Preparation of benzyl 4-{12-[3,6,7,10,11-pentakis(hexyloxy)-
morphism in such supermolecular architectures. The chemical triphenylene-2-yloxy]dodecyloxy}benzoate (3): Triphenylene
structures have been characterized by spectral techniques and bromide 2 (2.34 g, 2.3 mmol), benzyl 4-hydroxy benzoate (0.45
elemental analysis which establishes the identity and purity of g, 1.98 mmol) and cesium carbonate (2.0 g, 5.94 mmol) in 50
the new compounds. The thermal behavior of the novel meso- mL of MEK was refluxed for 48 h. The reaction mixture was
genic oligomers has been investigated by differential scanning poured into 50 mL of water and then extracted with chloroform,
calorimetry and polarizing optical microscopy. These novel the chloroform layer was washed with 50 mL of water followed
banana-discotic hybrid oligomers are unable to exhibit meso- by drying over anhydrous sodium sulfate. The solvent was
morphism presumably because of the incompatibility of the removed to yield a viscous oily residue 3 which was taken as
bent-core with the discotic core and/or the volume fraction such for hydrogenolysis.
mismatch of the mesogenic rigid cores. Attempts to realize
mesophase in such architectures with different molecular topo- Preparation of 4-{12-[3,6,7,10,11-pentakis(hexyloxy)tri-
logies are currently under way in our laboratory.
phenylene-2-yloxy]dodecyloxy}benzoic acid (4): A mixture of
compound 3 (2.3 g) dissolved in 1,4-dioxane (20 ml) and 5%
Pd-C catalyst (0.4 g) was stirred in an atmosphere of hydrogen
Experimental
All the reagents and solvents were used as received without any until the required quantity of hydrogen was absorbed. The
further purification except CH2Cl2 which was dried and resulting mixture was filtered in hot and the solvent removed
distilled before the reactions. Column chromatographic separa- under reduced pressure. The solid material obtained was recrys-
tion was performed on silica gel (100–200 mesh). 1H NMR tallised using methanol. Yield 1.0 g. mp 68–70 °C.
spectra were recorded in CDCl3 on a 400 MHz (Bruker AMX
400) spectrometer. All chemical shifts are reported in δ (ppm) Preparation of bent-core bridged discotic dimer 6: A
units down field from tetramethylsilane (TMS) and J values are mixture of compound 4 (0.5 g, 0.48 mmol), compound 5
reported in Hz. FT-IR spectra were recorded as KBr discs on a (0.085 g, 0.24 mmol), DCC (0.22 g, 1.07 mmol) and catalytic
Shimadzu FTIR-8400 spectrophotometer. Elemental analysis quantity of DMAP was stirred in dry dichloromethane at room
was performed on a Carlo-Erba Flash 1112 analyzer. Transition temperature for about 24 hours. The solvent was removed and
temperatures were observed using a Mettler FP82HT hot stage the residue purified by column chromatography on silica gel
and FP90 central processor in conjunction with an Olympus using mixture of petroleum ether and ethyl acetate (2%) as
BX51 polarizing microscope. Transition temperatures and asso- eluent. Removal of solvent afforded a residue which was recrys-
ciated enthalpies were measured by differential scanning calo- tallised from dichloromethane and hexane to afford 6. Yield 0.2
rimetry heating from room temperature to isotropic temperat- g. IR (KBr), νmax (cm−1): 2922, 2852, 1734, 1456, 1377, 1261,
ures at the scan rate of 5 °C per minute (Perkin-Elmer Model 1170, 721; 1H NMR (400 MHz, CDCl3, δ/ppm): 8.28 (d, 4H, J
Pyris 1D).
= 8.7 Hz, Ar-H), 8.16 (d, 4H, J = 8.0 Hz, Ar-H), 7.83 (s, 12H,
Ar-H), 7.51 (t, 1H, J = 8.5 Hz, Ar-H), 7.36 (d, 4H, J = 8.7 Hz,
Ar-H), 7.30 (t, 1H, J = 8.2 Hz, Ar-H), 7.26 (d, 2H, J = 8.1 Hz,
Synthesis
Compound 1, compound 5 and Compound 7a–b were prepared Ar-H), 6.98 (d, 4H, J = 8.9 Hz, Ar-H), 4.24 (t, 24H, J = 6.4 Hz,
by following the literature procedures [28-31].
-OCH2), 4.03 (t, 4H, J = 6.2 Hz, -OCH2), 1.97 (m, 28H, -CH2-),
1.69–1.31 (m, 92H, -CH2-), 0.94 (t, 30H, J = 6.1 Hz, -CH3);
Preparation of 2-(12-bromododecyloxy)-3,6,7,10,11-penta- Elemental analysis: Found: C, 76.34; H, 9.01. C154H210O22
kis(hexyloxy)triphenylene (2): A mixture of monohydroxy requires C, 76.64; H, 8.77.
triphenylene (2.0 g, 2.7 mmol), 1,12-dibromododecane (4.4 g,
13.0 mmol), cesium carbonate (1.12 g, 8.8 mmol) and 50 ml of Compounds 8a and 8b were prepared following the similar
methyl ethyl ketone (MEK) was refluxed for 48 h at 85 °C. The procedure for compound 6. 8a: IR (KBr), νmax (cm−1): 2922,
resulting mixture was cooled and poured into 50 ml of water 2850, 1743, 1733, 1602, 1508, 1437, 1259, 1159, 1068, 721; 1H
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4H, J = 8.8 Hz, Ar-H), 7.83 (s, 6H, Ar-H), 7.63 (t, 1H, J = 3.4
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