A low-molar-mass, monodispersive, bent-rod dimer exhibiting biaxial nematic and smectic A phases

Article abstract of DOI:10.1002/anie.200453908

A "peelable banana" is formed when a bent-core molecule is linked to a rodlike mesogen through a flexible aliphatic spacer. This is an appropriate description of this novel low-molar-mass organic system, which displays a transition from a biaxial nematic

Full text of DOI:10.1002/anie.200453908

Angewandte
Chemie
uniaxial nematic (Nu) phase, in which the constituent rodlike
molecules, on average, orient about a common axis called the
director n (Figure 1a). In contrast, one of the most sought
after phases in thermotropic LCs is the biaxial nematic (Nb)
Figure 1. Structural representation of a) the Nu phase formed by con-
ventional rodlike mesogens; b) the Nb phase in which the constituent
molecules are boardlike; c) the SmA phase with D8¥ symmetry formed
by rodlike molecules; d) the SmAb phase with the local D2h symmetry
built up by boardlike mesogens; e) the SmAPF phase (C2v symmetry)
formed by bent-core molecules having a common bent direction;
f) the SmAPA phase in which the bent direction of molecules in the
neighboring layer alternates; g) the SmAAP phase formed by bent-core
molecules with a random orientation of molecules; h) a quartet struc-
ture for the SmAb phase with D2h symmetry, in which a pair of nonsym-
metric dimeric molecules in the plane of the paper have their m direc-
tor antiparallel to the pair below that plane; and i) the dimeric mole-
cules in the Nb phase, which is based on ideas of the frustrated spin
gas model.
Liquid Crystals
A Low-Molar-Mass, Monodispersive, Bent-Rod
Dimer Exhibiting Biaxial Nematic and Smectic A
Phases
Channabasaveshwar V. Yelamaggad,
Subbarao Krishna Prasad,* Geetha G. Nair, Indudhara
Swamy Shashikala, Doddamane S. Shankar Rao,
Chetan V. Lobo, and Sivaramakrishna Chandrasekhar[+]
phase, in which there is, additionally, a correlation of the
molecules in a direction m perpendicular to the prime
director n (Figure 1b). Chandrasekhar et al.[3] were the first
to observe this phase, which is yet to be universally
accepted.[4] Designing a molecular structure for realizing a
Nb phase is not a trivial task, necessitating careful molecular
engineering. Nonetheless, several different classes of LCs
have been realized[4–8] in which the shape biaxiality[9] of the
rodlike or disclike entity is optimized by breaking the shape
symmetry. Another phase, the smectic A (SmA) phase,
generally has a uniaxial character (Figure 1c), although the
possibility of a biaxial smectic A (SmAb) phase in which the
molecules are along the layer normal, but have an additional
director m in the plane of the layers, was pointed out.[10] Brand
et al.[11] developed this idea and proposed a SmAb phase
whose symmetry depends on the type of constituent mole-
cules: D2h if the molecules are boardlike (Figure 1d) and C2v if
they are bent-core or banana-shaped molecules (Figure 1e,f).
The phase will be SmAAP if the bent-core molecules are
randomly oriented within the smectic layer (Figure 1 g) The
SmAb phase was first observed in a polymeric system[12] and
In memory of V. N. Raja
Liquid crystal (LC) phases are a unique state of matter. Apart
from their importance in life sciences, they have been used as
novel media in many practical applications.[1,2] Of the variety
of thermotropic LC phases known, the simplest but most
exploited phase in display device technology is the optically
[*] Dr. C. V. Yelamaggad, Dr. S. Krishna Prasad, Dr. G. G. Nair,
I. S. Shashikala, Dr. D. S. Shankar Rao, C. V. Lobo,
Prof. S. Chandrasekhar+
Centre for Liquid Crystal Research
Jalahalli, Bangalore-560 013 (India)
Fax: (+91)80-2838-2044
E-mail: skprasad1@yahoo.com
[+] Deceased.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453908
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3429

Communications
subsequently in a mixture of rodlike and bent-core mole-
cule that generally supports the formation of smectic phases
through a central flexible spacer containing an odd or even
number of methylene units.
cules,[13] pure bent-core molecules,[14] and a metallomesogen
mixed with 2,4,7-trinitrofluorenone (TNF).[15] The nematic–
smectic A transition (N–SmA) is perhaps the most exhaus-
tively studied, but is still the least understood with quite a few
open questions.[16] Investigations on the Nb–SmAb transition
that could answer some of these questions have thus far been
limited to a report on a side chain LC polymer[12] in which the
authors mentioned the difficulties of working with polymeric
material. Thus the need arises to observe such a transition in a
low-molecular-weight system. Here we report the first
example of a low-molar-mass system displaying a Nb–SmAb
transition, as evidenced by textural pattern, in-plane birefrin-
gence, conoscopic, calorimetric, electro-optical switching, and
X-ray studies.
The synthesis of the unique dimers 1a and 1b, which
contain a pentamethylene and a hexamethylene spacer,
respectively, is depicted in Scheme 1 (see the Supporting
Information for data on 1a and 1b). Owing to the presence of
the terminal polar group and the asymmetric structure, the
molecule can be regarded as a “peelable banana” in which the
top (with the stalk) can be differentiated from the bottom.
This new structural feature is the important factor in
stabilizing the mesomorphic sequence observed for 1b (see
below).
Contrary to our expectations, dimer 1a is non-liquid
crystalline, perhaps due to the stiffer conformation of the
spacer. On the other hand, three mesophases could be
detected for 1b by polarizing microscopy, calorimetry, and
X-ray investigations. The lowest temperature mesophase (M)
was found to be uniaxial with a layered structure but with
some unusual textures. Detailed characterization is underway
to establish the nature of this phase. From preliminary
microscopic observations the other two phases could easily be
identified as nematic (N) and smectic (Sm) phases, respec-
tively. First, we will discuss the characterization of the Sm
phase. The X-ray measurements were carried out by placing
1b in a Lindemann capillary tube and aligning it in the
In view of the fact that bent-core molecules have strong
shape biaxiality,[17,18] we introduced a new molecular engi-
neering concept in which the shape biaxiality of a rodlike
mesogen is enhanced by covalently linking the mesogen
linearly to a bent-core molecule through a flexible poly-
methylene spacer to form nonsymmetrical dimers.[19,20] Addi-
tional support for this idea came from the results of the
frustrated spin-gas model.[21] Thus we produced linear dimers
consisting of a cyanobiphenyl unit—that is, a rodlike molecule
known to exhibit the nematic phase—covalently attached to a
salicylaldimine-based, unsymmetrical, banana-shaped mole-
Scheme 1. Synthesis of the nonsymmetrical, bent-rod dimers 1a and 1b. a) K2CO3, DMF, 808C, 12 h; b) 5a or 5b, Jones’s reagent (CrO3, H2SO4,
H2O); c) 2,4-dihydroxybenzaldehyde, DCC, DMAP, CH2Cl2, RT, 2 h; d) 3-nitrophenol, DCC, DMAP, CH2Cl2, RT, 4 h; e) H2/Pd–C (10%), EtOH,
16 h; f) abs. EtOH, AcOH (one drop), reflux, 2 h. DCC=N,N’-dicyclohexylcarbodiimide, DMAP=4-dimethylaminopyridine. Phase transition tem-
peratures and transition enthalpies: 1a: Cr 141.78C (35.8 JgÀ1) I. 1b: I 162.18C (0.4 JgÀ1) Nb 135.68C (0.1 JgÀ1) SmAb 119.28C M 105.58C
(49.6 JgÀ1) Cr. (Cr crystalline, I isotropic, M uniaxial smectic, N nematic, Sm smectic).
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presence of a magnetic field (ꢀ 2 T) by cooling it from the
isotropic (I) phase through the N phase. The X-ray diffraction
pattern collected using a two-dimensional imaging plate
showed a pair of small-angle reflections and a pair of wide-
angle diffuse arcs orthogonal to the small-angle ones (Fig-
ure 2a). The extracted spacing of the low-angle reflection (see
the Supporting Information) is comparable to the length of
the molecule in its most extended configuration, showing that
the phase is SmA.
isogyres (Figure 2e), a characteristic feature of biaxial phases.
Two other aspects that need to be mentioned are the strong
intensity fluctuations and the formation of a striped texture
(see the Supporting Information), both features being
reported for the SmAb/SmAp phase.[14,15] All these points
unambiguously prove that this phase is the SmAb. Quantita-
tive measurements of the in-plane birefringence (Dm) using a
tilting plate compensator yielded a value of 0.013 at ꢀ 108C
away from the transition to the N phase.
When the sample was taken between either untreated or
polyimide-coated, unrubbed plates, the N phase showed the
typical schlieren texture, except that the pattern consisted
entirely of two-brush disclinations (Figure 2d), which is an
indication of biaxiality.[6] This is supported by the topological
considerations of Mermin[22] and recent computer simula-
tions.[23] To ensure that the biaxiality was not introduced by
the glass surfaces, we examined the textures formed in both
the N and SmAb phases in a free-standing film. Again only S =
Æ 1/2 spontaneous defects were observed (see the Supporting
Information). The conoscopic observations carried out in the
N phase as described above showed clear separation of the
isogyres (Figure 2 f), confirming the biaxiality of the phase,
although Dm is only 0.005. The X-ray pattern (Figure 2b)
obtained in this phase was identical to that in the SmAb phase,
except that the width of the reflections was more indicative of
a lack of positional order along any direction. Furthermore,
samples taken between kapton sheets that were coated with a
polyimide solution and unidirectionally rubbed also yielded
essentially similar patterns in both the SmAb and Nb phases.
As discussed earlier, two apparent forms of molecular
packing for SmAb having ferroelectric and antiferroelectric
arrangements of the dimers are similar to the structures
shown in Figures 1e and 1 f. However, the observation of the
S = Æ 1/2 defects and the absence of any antiferroelectric-like
electric switching signal rules out these structures. (Even with
moderately high fields we did not observe any switching.
However, the presence of a cyano group adds a new
dimension to the problem, namely, that higher fields result
in Freedericksz transition. Therefore, measurements at higher
fields were not possible.) Although a packing of the molecules
such that the m director of the neighboring molecules lies in
the molecular plane, but points randomly to the left or to the
right (Figure 1 g), can explain the observed features, it is not
efficient from the point of view of excluded volume. The
quartet structure[14] (Figure 1 h) in which four dimers form a
group is a possibility since this structure, as an entity, has D2h
symmetry and accounts for all the features observed for the
SmAb phase. The quartet model can also be used to explain a
nonpolar Nb phase, in which the quartets are arranged in such
a fashion as to loose the one-dimensional positional order of
the smectic phase, leading to the appearance of the Nb phase.
For the Nb phase, the problem of excluded volume mentioned
above does not place any serious restrictions on the packing
of the molecules. Hence, the structure shown in Figure 1i is
more probable than the quartet structure. Furthermore, from
a molecular point of view, the appearance of the Nb phase in a
system of bent-core molecule of the type we have synthesized
can be explained on the basis of the quite successful frustrated
spin gas model developed by Berker and Walker.[21]
Figure 2. The X-ray (a, b), textural (c, d), and conoscopic (e, f) pat-
terns obtained for the SmAb (a, c, e) and Nb phases (b, d, f). The tex-
ture consists of only 1/2 strength disclinations and isogyres in the
conoscopic pattern are well separated.
In the optical studies, with the substrates treated for
planar configuration of the molecules, a focal conic texture
was observed. However, even glass plates treated with
octadecyltriethoxysilane failed to produce a perfectly home-
otropically aligned sample. Instead
a low-birefringence
schlieren texture consisting of exclusively two-brush disclina-
tions (strength S = Æ 1/2, Figure 2c) was seen, which has some
important implications. Firstly, it rules out the phase being a
SmC phase. The Æ 1/2 defects have been predicted[11] as well
as observed for the SmAb/SmAp phase.[12–15] However, the
ordering of the bent-core molecules in a layer such that their
“arrow” directions are uniformly oriented would result in a
polar layer (Figure 1e) with an electric polarization parallel
to the in-plane director m. Since m is polar, such a structure
will not support the formation of Æ 1/2 defects. But if the
direction of m alternates between adjacent layers (Figure 1 f),
it can exhibit a two-brush schlieren pattern and the structure
would be antiferroelectric in character. However, the absence
of any switching upon the application of square-wave as well
as triangular-wave, low-frequency (1–10 Hz) fields rules out
the antiferroelectric structure. The optical biaxiality of the
phase was evidenced by conoscopic studies on a homeotropi-
cally aligned sample in which the molecules are uniformly
oriented with the director n normal to the substrate.
Application of a 1-kHz sine wave, in-plane electric field
(70 V magnitude) to this sample aligned the director m. The
conoscopic pattern obtained shows clear separation of the
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[19] Several monodispersive cyclic compounds formed by combining
a banana-shaped unit and a rodlike moiety have been reported
to form uniaxial nematic and smectic mesophases: A. Godt, S.
Duda, O. Unsal, J. Thiel, A. Harter, M. Roos, C. Tschierske, S.
Diele, Chem. Eur. J. 2002, 8, 5094.
[20] “Liquid Crystal Dimers and Oligomers”: C. T. Imrie, G. R.
Luckhurst in Handbook of Liquid Crystals, Vol. 2B (Eds.: D.
Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, V. Vill), Wiley-
VCH, Weinheim, 1998, chap. X.
[21] A. N. Berker, J. S. Walker, Phys. Rev. Lett. 1981, 47, 1469.
[22] N. D. Mermin, Rev. Mod. Phys. 1979, 51, 591.
[23] C. Chiccoli, I. Feruli, O. D. Lavrentovich, P. Pasini, S. V.
Shiyanovskii, C. Zannoni, Phys. Rev. E 2002, 66, 030701.
In summary, we report the first occurrence of the Nb–
SmAb phase transition in a low-molecular-weight LC system.
This was achieved by a careful design of the constituent
molecules that incorporates covalently linking a bent-core
molecule to a rodlike mesogen through a flexible spacer.
Realization of a biaxial nematic phase in a low-molar-mass
system is significant since it opens up the possibility of
fabricating new types of liquid crystal display devices. A
device employing such a material is attractive as the switching
of the in-plane m director is expected to be much faster than
the switching of the n director.
Received: February 3, 2004 [Z53908]
Keywords: biaxial phases · liquid crystals · mesogens ·
.
phase transitions
[1] S. Chandrasekhar, Liquid Crystals, Cambridge University Press,
Cambridge, 1992.
[2] P. G. de Gennes, J. Prost, The Physics of Liquid Crystals,
Clarendon, Oxford, 1993.
[3] S. Chandrasekhar, B. K. Sadashiva, B. R. Ratna, V. N. Raja,
Pramana 1988, 30, L-491.
[4] G. R. Luckhurst, Thin Solid Films 2001, 393, 40.
[5] K. Praefcke, B. Kohne, B. Gundogan, D. Singer, D. Demus, S.
Diele, G. Pelzl, U. Bakowsky, Mol. Cryst. Liq. Cryst. 1991, 198,
393.
[6] S. Chandrasekhar, G. G. Nair, D. S. Shankar Rao, S. Krishna
Prasad, K. Praefcke, D. Blunk, Liq. Cryst. 1998, 24, 67; S.
Chandrasekhar, G. G. Nair, D. S. Shankar Rao, S. Krishna Pra-
sad, K. Praefcke, D. Blunk, Curr. Sci. 1998, 75, 1042.
[7] J. Li, V. Percec, C. Rosenblatt, O. D. Lavrentovich, Europhys.
Lett. 1994, 25, 199.
[8] a) B. R. Acharya, A. Primak, T. J. Dingemans, E. T. Samulski, S.
Kumar, Pramana 2003, 61, 231; b) L. Omnes, B. A. Timimi, T.
Gelbrich, M. B. Hursthouse, G. R. Luckhurst, D. W. Bruce,
Chem. Commun. 2001, 2248; c) R. W. Date, D. W. Bruce, J.
Am. Chem. Soc. 2003, 125, 9012.
[9] Based on considerations of symmetry, a feature required to
realize biaxial phases is molecular biaxiality. This could be due to
the molecules having biaxial correlations or to their shape
biaxiality. Here we consider only the latter.
[10] W. L. McMillan, Phys. Rev. A 1973, 8, 1921.
[11] H. R. Brand, P. E. Cladis, H. Pleiner, Macromolecules 1992, 25,
7223.
[12] H. F. Leube, H. Finkelmann, Makromol. Chem. 1991, 192, 1317.
[13] R. Pratibha , N. V. Madhusudana, B. K. Sadashiva, Science 2000,
288, 2184.
[14] K. J. K. Semmler, T. J. Dingemans, E. T. Samulski, Liq. Cryst.
1998, 24, 799; A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W.
Weissflog, J. Salfetnikova, H. Kresse, Phys. Rev. E 2001, 64,
051701; B. K. Sadashiva, R. A. Reddy, R. Pratibha, N. V.
Madhusudana, J. Mater. Chem. 2002, 12, 943.
[15] T. Hegmann, J. Kain, S. Diele, G. Pelzl, C. Tschierske, Angew.
Chem. 2001, 113, 911; Angew. Chem. Int. Ed. 2001, 40, 887.
[16] P. Barois in Physical Properties of Liquid Crystals, (Eds.: G. W.
Gray, V. Vill, H.-W. Spiess, D. Demus, J. W. Goodby), Wiley-
VCH, Weinheim, 1999.
[17] T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, J.
Mater. Chem. 1996, 6, 1231.
[18] a) D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan, D. R.
Link, M. A. Glaser, N. A. Clark, Science 2000, 288, 2181; b) T. C.
Lubensky, Science 2000, 288, 2146.
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