A novel calamitic liquid crystalline oligomer composed of three non- identical mesogenic entities: Synthesis and characterization

Article abstract of DOI:10.1039/a907974b

The synthesis and evaluation of the mesomorphic properties of the first trimesogen consisting of three non-identical calamitic mesogenic entities have been described.

Full text of DOI:10.1039/a907974b

A novel calamitic liquid crystalline oligomer composed of three non-identical
mesogenic entities: synthesis and characterization
C. V. Yelamaggad,* U. S. Hiremath, D. S. Shankar Rao S. Krishna Prasad
Centre for Liquid Crystal Research P.B. No. 1329, Jalahalli, Bangalore-560013, India.
E-mail: uclcr@giasbg01.vsnl.net.in
Received (in Cambridge, UK) 4th October 1999, Accepted 23rd November 1999
The synthesis and evaluation of the mesomorphic properties
of the first trimesogen consisting of three non-identical
calamitic mesogenic entities have been described.
The liquid crystalline properties of CII-105 were studied
using an optical polarizing microscope (Leitz DMRXP) and a
differential scanning calorimeter (Perkin Elmer DSC7). For
polarizing microscopic observations, glass plates coated with a
polyimide solution and unidirectionally rubbed were used. On
cooling from the isotropic phase (I) at 247.7 °C the N* phase
with a characteristic planar texture is seen in which the nematic
director lies in the plane of the substrates. This ensures that the
helical axis is perpendicular to the glass plates, i.e. along the
viewing direction. At a second transition below 215 °C, the
texture changes to a square grid pattern, which persists down to
119 °C, when it transforms into a third mesophase. If the sample
is observed in a wedge type cell with similar surface treatment
to the parallel cell, Grandjean Cano dislocation lines are
observed in both the cholesteric and second mesophases
demonstrating that there is a helical twist normal to the plates in
both cases. Thus the low temperature phase shows the Cano
lines superposed on the square grid pattern (see Fig. 1). If the
plates are treated to give a homeotropic alignment, a filament
texture is observed with an undulatory structure whose
Liquid crystalline dimers1 (dimesogens) consisting of two
identical (symmetrical) or non-identical (non-symmetrical)
mesogenic units have gained attention as these are regarded as
model compounds for main chain2 and side chain liquid
crystalline polymers.3 The non-symmetrical dimers are mark-
edly different from those of symmetrical ones as they exhibit
interesting polymorphic sequence4 and stabilize wide range
chiral nematic (N*)5a and smectic A (SmA) mesophases.5b The
addition of one more mesogenic moiety to a dimer via a flexible
spacer results in the next higher oligomer, which has been called
a trimesogen, trimer or triplet liquid crystal.6 We shall adopt the
term ‘trimesogen’ in this article. Based on the molecular
structure of the individual calamitic entities there can be three
possible combinations: (i) all of them are structurally identical,
(ii) two of them are identical while the third entity is different,
and (iii) all three are different. These trimesogens are of interest
because of the recent demonstration that a flexible backbone
based virtual trimer model successfully accounts for the
transitional properties of side chain liquid crystalline poly-
mers.3b Trimesogens of type (i) and (ii) were reported for the
first time in 19867 and since then a few other examples of these
types have been reported.6 To the best of our knowledge
trimesogens of type (iii) have not been reported to date. In
continuation of our work on oligomeric liquid crystals, here we
present the synthesis of the first type (iii) trimesogen composed
of three different calamitic mesogenic entities, namely, chol-
esteryl ester, diphenylacetylene (tolan) and azobezene moie-
ties.
In our recent investigations we had demonstrated that the
combination of a cholesteryl ester unit and a tolan unit through
an n-pentyl (odd) spacer stabilizes a wide-range N* phase.5a
Owing to a change in the helical pitch the N* phase shows a
temperature dependent wavelength of selective reflectivity
(thermochromism). Another field of materials research that has
been of considerable current interest is the phenomenon of
photochromism, which holds lot of promise for applications
such as optical information storage technology.8 Therefore we
aimed to synthesize a trimesogen which is multifunctional,
satisfying two criteria: (i) it exhibits thermochromism over a
wide range of temperature, and (ii) it is photochromic at the
same time. A major advantage of single component multi-
functional liquid crystal is that the problem of phase separation,
which occurs in mixtures of mesogenic monomers and
functional monomers, is avoided.9 Here we report the synthesis
of such a multifunctional liquid crystal and a preliminary
investigation of the mesomorphic properties of this novel
trimesogen.
The synthetic approach employed for the trimesogen,
cholesteryl 6-{4-[4-[(4-butylphenylazophenoxy)butoxy]phenyl-
ethynyl]phenoxy}hexanoate (CII-105), is shown in Scheme 1.
It was conceived that CII-105 could be prepared by combining
two key fragments, the non-symmetrical dimeric phenol CII-76
and butylbromo azobenzene 7. Accordingly, both the fragments
were synthesized and coupled under mild basic conditions to
give CII-105 as a yellow crystalline compound.†
Scheme 1 Reagents and conditions: i, anhydrous K2CO3, cat. KI, butan-
2-one, 70 °C, 24 h, 84%; ii, dihydropyran, PPTS, CH2Cl2, room temp., 12
h, quant.; iii, 2-methylbut-3-yn-2-ol, (PPh3)2PdCl2, CuI, PPh3, Et3N, 65 °C,
16 h, 80%; iv, KOH, toluene, 110 °C, 2 h, 64%; v, (PPh3)2PdCl2, CuI, PPh3,
Et3N, 75 °C, 16 h, 75%; vi, TsOH, MeOH–THF, room temp., 30 min, 68%;
vii, NaNO2, HCl, phenol, 0–5 °C, 2 h, 78%; viii, 1,4-dibromobutane,
anhydrous K2CO3, acetone, 60 °C, 90%; ix, anhydrous K2CO3, DMF,
80 °C, 24 h, 52%.
Chem. Commun., 2000, 57–58
This journal is © The Royal Society of Chemistry 2000
57

Scheme 2 The number in parenthesis indicates the transition enthalpy; X is
a highly ordered mesophase and yet to be fully characterized; K
crystal.
=
entities connected by flexible spacers. The compound is
multifunctional, containing themochromic and photochromic
mesogenic units. Such substances have potential as materials
for information storage technology and replace mixtures of
mesogenic compound in different applications, thereby solving
phase separation problems associated with such mixtures. These
may perhaps also serve as model compounds (which are yet to
be realized) for co-polymeric liquid crystals. Interestingly, this
oligomer stabilizes a recently discovered UTGCC* phase—
supposed to be a highly frustrated phase—over a wide
temperature range of 90 °C and thus invites systematic
investigations so as to understand this behavior.
We are grateful to Professor S. Chandrasekhar for many
useful discussions and thankfully acknowledge Dr Geeta G.Nair
for her help in the experiments.
Notes and references
† The trimesogen CII-105 and other intermediates exhibited spectral data
consistent with their molecular structure. Selected data for CII-105:
nmax(KBr)/cm21 2950, 2863, 1729, 1607 and 1578; dH(500 MHz, CDCl3)
7.89 (d, J 8.9, 2H, Ar), 7.8 (d, J 8.15, 2H, Ar), 7.44 (d, J 5.9, 2H, Ar), 7.42
(d, J 5.8, 2H, Ar), 7.3 (d, J 8.15, 2H, Ar), 7.0 (d, J 8.95, 2H, Ar), 6.87 (d,
J 8.75, 2H, Ar), 6.84 (d, J 8.85, 2H, Ar), 5.36 (br d, J 4.85, 1H, olefinic), 4.6
(m, 1H, CHOCO), 4.12 (br t, 2H, OCH2), 4.06 (br t, 2H, OCH2), 3.96 (t, J
6.4, 2H, OCH2), 2.68 (t, J 7.65, 2H, ArCH2), 2.31 (m, 4H, 2 3 CNCH2),
2.01–0.9 (m, 40H, 17 3 CH2, 6 3 CH), 1.02 (s, 3H, CH3), 0.94 (t, J 7.25,
3H, CH3), 0.91 (d, J 6.5, 3H, CH3), 0.87 (d, J 2.1, 3H, CH3), 0.86 (d, J 2.1,
3H, CH3) and 0.67 (s, 3H, CH3); dC(100 MHz, CDCl3) 172.93, 161.22,
158.87, 158.75, 151.05, 147.1, 145.73, 139.67, 132.83, 128.99, 124.54,
122.59, 122.51, 115.8, 115.61, 114.67, 114.5, 88.04, 87.91, 73.82, 67.77,
67.69, 67.48, 56.69, 56.16. 50.05, 42.31, 39.74, 39.51, 38.16, 36.99, 36.59,
36.19, 35.77, 35.52, 34.54, 33.41, 31.88, 28.87, 28.2, 27.82, 25.95, 25.57,
24.76, 24.26, 23.83, 22.77, 22.53, 22.3, 21.03, 19.28, 18.71, 13.88 and
11.835; m/z (FAB) 1001 [MH]+ (calc. for C67H88N2O5).
Fig. 1 Optical microscopic texture observed (magnification 4003) for the
UTGBC* phase at 212.3 °C while cooling from the N* phase. (a) The
Grandjean Cano lines (the two striations running from the top to the bottom
of the picture) indicate the presence of a helix whose axis is normal to the
photograph.The square grid patterns [shown on an enlarged scale in (b)]
arise from a two-dimensional modulation perpendicular to the helix. The
simultaneous existence of both these features is supposed to be proof of the
existence of the UTGCC* phase.
periodicity is approximately the same as that of the spacing in
the square grid pattern mentioned above. Again, the filament
texture remains unchanged down to 119 °C. The features
described here for the low temperature mesophase are identical
to the textural observations for the very recently reported10
undulated twist grain boundary (UTGBC*) phase in a binary
system as well as in a single compound. While in the binary
mixture the UTGBC* phase is reported to occur over a narrow
range of temperature ( ~ 4 °C),10a the range is not mentioned in
the case of the single compound.10c In view of these
observations it is remarkable that the second mesophase in CII-
105, which shows the essential features of the UTGBC* phase,
viz. simultaneous existence of a square grid texture and a helical
structure perpendicular to it, exists over a large temperature
range of 90 °C. According to the proposed model,10a the
UTGBC* phase is a highly frustrated phase characterized by
modulations in all the three dimensions, with the two-
dimensional undulation of the smectic C*-like blocks being
orthogonal to the helix of the TGB structure. Hence, it is all the
more interesting to see that a phase with such a highly-frustrated
structure exists over a wide temperature range. The mesophase
below 119 °C seems to be a highly ordered one, and its detailed
characterisation is underway.
1 For a brief but recent review on dimers and oligomers see: C. T. Imrie
and G. R. Luckhurst, HandBook of Liquid Crystals, ed. D. Demus, J.
Goodby, G. W. Gray, H.-W. Spiess and V. Vill, Wiley-VCH,
Weinheim, 1998, vol. 2B, p. 801.
2 A. C. Griffin and T. R. Britt, J. Am. Chem. Soc., 1981, 103, 4957;
J. C. W. Chien, R. Zhou and C. P. Lillya, Macromolecules, 1987, 20,
2340.
3 (a) C. T. Imrie, F. E. Karasz and G. S. Attard, Macromolecules, 1993,
26, 545; (b) C. T. Imrie, F. E. Karasz and G. S. Attard, Macromolecules,
1993, 26, 3803.
4 J.-I. Jin, H.-S. Kim, J.-W. Shin, B. Y. Chung and B.-W. Jo, Bull. Korean
Chem. Soc., 1990, 11, 209; F. Hardouin, M. F. Achard, J.-I. Jin, J.-W.
Shin and Y.-K. Yun, J. Phys. II, 1994, 4, 627; C. V. Yelamaggad, A.
Srikrishna, D. S. Shankar Rao and S. Krishna Prasad, Liq. Cryst., 1999,
26, 1545.
5 (a) C. V. Yelamaggad, Mol. Cryst. Liq. Cryst., 1998, 326, 149; (b) C. V.
Yelamaggad, D. S. Shankar Rao and S. Krishna Prasad, unpublished
work.
6 G. S. Attard and C. T. Imrie, Liq. Cryst., 1989, 6, 387; R. Centore, A.
Roviello and A. Sirigu, Mol. Cryst. Liq. Cryst., 1990, 182B, 233; T.
Ikeda, T. Miyamoto, S. Kurihara, M. Tsukada and S. Tazuke, Mol.
Cryst. Liq. Cryst., 1990, 182B, 357; A. T. M. Marcelis, A. Koudijs and
E. J. R. Sudholter, Liq. Cryst., 1995, 18, 851; A. T. M. Marcelis, A.
Koudijs and E. J. R. Sudholter, Liq. Cryst., 1996, 21, 87; J. Andersch, S.
Diele, D. Lose and C. Tschierske, Liq. Cryst., 1996, 21, 103.
7 H. Furuya, K. Asahi and A. Abe, Polym. J., 1986, 18, 779.
8 S. H. Chen, D. Katsis, A. W. Schmid, J. C. Mastrangelo, T. Tsutsui and
T. N. Blanton, Nature, 1999, 397, 506; D. J. Broer, J. Lub and G. N.
Mol, Nature, 1995, 378, 467; E. Sackmann, J. Am. Chem. Soc., 1971,
93, 7088.
The transition temperatures obtained from the cooling mode
DSC scan along with the enthalpies are given in Scheme 2 (the
transition from the cholesteric to the second mesophase was too
weak for any reliable calculation of the enthalpy). It is worth
mentioning here that the trimesogen CII-105 is thermally stable
during repeated heating and cooling cycles through the
1
mesophases–isotropic transition, as confirmed by a H NMR
9 G. Attard and G. Williams, Nature, 1987, 326, 544.
10 (a) P. A. Pramod, R. Pratibha and N. V. Madhusudhana, Curr. Sci.,
1997, 73, 761; (b) G. G. Nair, Curr. Sci., 1998, 74, 98; (c) B. K.
Sadashiva, Pramana, 1999, 53, 213.
scan of the same sample which had been subjected to the DSC
experiments. The profile obtained was identical, within experi-
mental error, to that for the pristine sample.
In conclusion we have achieved the synthesis of the first
trimesogen containing three non-identical calamitic mesogenic
Communication a907974b
58
Chem. Commun., 2000, 57–58