Monodispersive linear supermolecules stabilizing unusual fluid layered phases

Article abstract of DOI:10.1021/ol070808p

Equation Presented The first examples of monodisperse liquid crystalline pentamers, synthesized by covalently linking five mesogenic segments through four flexible spacers, exhibiting mono-/partially bilayered phases with unusual textures, as established

Full text of DOI:10.1021/ol070808p

ORGANIC
LETTERS
2
007
Vol. 9, No. 14
641-2644
Monodispersive Linear Supermolecules
Stabilizing Unusual Fluid Layered
Phases
2
Channabasaveshwar V. Yelamaggad,* Ammathnadu S. Achalkumar,
Doddamane S. Shankar Rao, and S. Krishna Prasad
Centre for Liquid Crystal Research, Jalahalli, Bangalore, 560 013, India
yelamaggad@gmail.com
Received April 4, 2007
ABSTRACT
The first examples of monodisperse liquid crystalline pentamers, synthesized by covalently linking five mesogenic segments through four
flexible spacers, exhibiting mono-/partially bilayered phases with unusual textures, as established by optical, calorimetric, and X-ray diffraction
2
studies, are reported. Importantly, the two types of multifunctional pentamers, namely, unsymmetric and C symmetric are soluble in organic
solvents and exhibit analogous electrochemical features.
The creation of structurally diverse self-organizing functional
single molecules, significant from both fundamental research
and applications points of view, is a contemporary challenge
development of LCs with improved and tunable features,
several giant molecular systems, called supermolecules,
comprising covalently linked functional entities have been
1
2c,5
for chemists. This task, seems to be fulfilled, in part, by
realized.
Such LCs, unlike conventional (rodlike or
design and synthesis of multifunctional molecules stabilizing
liquid-crystal (LC) phases, wherein they self-assemble into
a variety of ordered fluid structures originating either from
their parallel alignment or the nanosegregation of the
immiscible molecular fragments,2 which furnish, respec-
tively, nematic (N) or layered (smectic: Sm) /columnar (Col)
phases. For instance, some of these unique structures have
the potential to serve as a soft medium for electron, ion, or
molecular transporting, and in the fabrication of sensory,
catalytic, and optical materials.3 Of late, toward the further
disklike) systems, provide a variety of fluid structures with
fascinating physical properties and functions.
One of the most important supermolecular examples are
the linear oligomeric LCs (LOLCs) wherein two to several
anisometric segments are covalently linked in end-end
(axial) fashion to one another via flexible spacer(s).6
Importantly, being monodisperse, their fluidity (viscosity)
can be compared to those of low molar mass LCs but still
,3
6
a-c
displays transitional properties of polymers. So far, dimers,
trimers, and tetramers
-5
6d,e
6f-h
comprising two, three, and four
(
1) (a) Lehn, J.-M. In Supramolecular Chemistry: Concepts and
(6) (a) Imrie, C. T.; Henderson, P. A. Curr. Opin. Colloid. Interface
Sci. 2002, 7, 298. (b) Tamaoki, N.; Moriyama, M.; Matsuda, H. Angew.
Chem. 2000, 39, 509. (c) Mallia, A. V.; Tamaoki, N. J. Mater. Chem. 2003,
13, 219. (d) Imrie, C. T.; Henderson, P. A.; Seddon, S. M. J. Mater. Chem.
2004, 14, 2486. (e) Yelamaggad, C. V.; Hiremath, U. S.; Shankar Rao, D.
S.; Krishna Prasad, S. Chem. Commun. 2000, 57. (f) Yelamaggad, C. V.;
Nagamani, S. A.; Hiremath, U. S.; Shankar Rao, D. S.; Krishna Prasad, S.
Liq. Cryst. 2002, 29, 231. (g) Yelamaggad, C. V.; Nagamani, S. A.;
Hiremath, U. S.; Shankar Rao, D. S.; Krishna Prasad, S. Mol. Cryst. Liq.
Cryst. 2003, 397, 207. (h) Imrie, C. T.; Stewart, D.; Remy, C.; Christie, D.
W.; Hamley, I. W.; Harding, R. J. M. J. Mater. Chem. 1999, 9, 2321.
PerspectiVes; Wiley-VCH: Weinheim, Germany, 2001. (b) Elemans, J. A.
A. W.; Rowan, A. E.; Nolte, R. J. M. J. Mater. Chem. 2003, 13, 2661. (c)
Kato, T. Science 2002, 295, 2414.
(
2) (a) Demus, D.; Goodby, J. W.; Gray, G. W.; Spiess, H. W.; Vill, V.
In Handbook of Liquid Crystals; Wiley-VCH: Weinheim, Germany, 1998.
b) Tschierske, C. J. Mater. Chem. 2001, 11, 2647. (c) Tschierske, C. Annu.
Rep. Prog. Chem. Sect. C 2001, 97, 191.
(
(
(
(
3) Kato, T.; Mizoshita, N.; Kishimoto, K. Angew. Chem. 2006, 45, 38.
4) Hamley, I. W. Angew. Chem. 2003, 42, 1692.
5) Saez, I. M.; Goodby, J. W. J. Mater. Chem. 2005, 15, 26.
1
0.1021/ol070808p CCC: $37.00
© 2007 American Chemical Society
Published on Web 06/08/2007

mesogens (separated by one, two, and three spacers),
respectively, have been reported. These studies have dem-
onstrated to an extent that by varying the chemical nature
Scheme 1. Synthesis of Key Intermediates 3a,b
(functionality) of the constituent mesogens, as well as the
length and parity of the spacer, a range of invaluable LOLCs
6
b-e
exhibiting remarkable phase behavior, can be obtained.
In fact, truly multifunctional LCs are the higher oligomers
as they offer an elegant and innovative way of adding various
requisite components. In addition, the organization of these
giant molecules in the mesophase, appear to be rather
intriguing.6 Despite such attractive features, the synthetic
study has been limited to trimers and tetramers of sym-
metrical and nonsymmetrical types. In particular, investiga-
tions on the nonsymmetrical materials, in which the chemical
nature of the constituent anisometric segments is different,
have been scarcely reported. Surprisingly, the synthetic
approaches to higher LOLCs, namely, pentamers, hexamers,
etc. of any kind have not been, to our knowledge, reported
hitherto. This can be ascribed to their immiscibility in organic
d-f
7
solvents, making these LOLCs hard to purify. Owing to their
intrinsic structural diversity, the cumbersome synthetic
pathways are necessary, which perhaps also impede their
growth. Another important factor is that such designs may
not support LC behavior.
Here we report the first successful endeavor of design and
synthesis of mesomorphic, as well as soluble, giant pentamers
composed of five mesogenic segments interlinked through
four flexible spacers varying in their length and parity. The
design is considered to provide functional diversity and
maximize the probability of mesophase formation. In par-
bromoalkanes led to bromides 9a,b, which reacted with 4,4′-
dihydroxybiphenyl in excess to obtain 8a,b. These phenols
were converted to 7a,b. Finally, pentamers 1a,b and 2a,b
were obtained in almost excellent yieds by reacting 7a,b with
3a,b and 7a,b with 4,4′-dihydroxyazobenzene, respectively.
All the intermediates were thoroughly purified by column
chromatography, while the pentamers were purified by
repeated recrystallization in hot DMF. The pentamers were
found to be readily soluble in some of the common organic
solvents. The UV-visible spectrum, of 1a,b showed absorp-
tion maxima at 318 and 355-358 nm, while 2a,b showed
ticular, two sets of both nonsymmetric, 1a,b, and C
symmetric, 2a,b, pentamers have been synthesized. The
2
6f,g
1
13
pentamers 2a,b comprise tolane (half-disc), biphenyl (pro-
an absorption maximum at 360 nm. The H and C NMR
mesogenic,),2 azobenzene (photoactive) mesogenic cores;
a
8
whereas pentamers 1a,b contain cholesterol (chiral, pro-
9
mesogenic, thermochromic,) and naphthalene (mesogenic,
Scheme 2. Synthesis of Precursors 7a,b and Target Pentamers
kinked)10 entities, additionally. The synthesis of pentamers
1a,b and 2a,b required the preparation of key precursors 3a,b
and 7a,b. Schemes 1 and 2 illustrate the synthesis of these
as well as target molecules. Williamson ether synthesis
11
protocol was used for all the transformations. Cholesteryl
6g
ω-bromoalkanoates (6a,b), were reacted with 3-fold excess
of 2,6-dihydroxynaphthalene to obtain monofunctionalized
naphthols 5a,b, which, on refluxing with ∝ ,ω-dibromoal-
kanes, yielded monobromides 4a,b. Treatment of 4,4′-
dihydroxyazobenzene6g in large excess with 4a,b furnished
3
a,b. Scheme 1 depicts the above-mentioned reaction
sequences. As shown in Scheme 2, the condensation of
diphenylacetylene (tolane) 10,6g with 3-fold excess of di-
(
7) For example, several linear pentamers prepared earlier in our
laboratory were found to be insoluble solids with very high melting points.
8) (a) Ikeda, T.; Sasaki, T.; Iehimura. K. Nature 1993, 361, 428. (b)
(
Shishido, A.; Tsutsumi, O.; Kanazawa, A.; Shino, T.; Ikeda, T.; Tamai, N.
J. Am. Chem. Soc. 1997, 117, 7791. (c) Mallia, A.V.; Tamaoki, N. Chem.
Commun. 2004, 2538. (d) Nair, G. G.; Krishna Prasad, S.; Yelamaggad, C.
V. J. Appl. Phys. 2000, 87, 2084.
(
9) (a) Tamaoki, N. AdV. Mater. 2001, 13, 1135. (b) Mallia, A. V.;
Tamaoki, N. Chem. Soc. ReV. 2004, 33, 76.
10) (a) Lauk, U. H.; Skrabal, P.; Zollinger, H. HelV. Chim. Acta 1985,
8, 1406. (b) Duffy, W. L.; Jones, J. L.; Kelly, S. M.; Minter, V.; Tuffin,
R. Mol. Cryst. Liq. Cryst. 1999, 332, 91.
11) See Supporting Information for details.
(
6
(
2642
Org. Lett., Vol. 9, No. 14, 2007

spectra obtained were found to be in complete consonance
with the proposed structure; FAB mass and microanalytical
data further confirmed the molecular structure (SI) of
pentamers. The LC behavior of these compounds was probed
by polarized optical microscopy (POM), differential scanning
calorimetry (DSC), and X-ray diffraction (XRD) studies; the
phase transition temperatures and associated enthalpy values
obtained are shown in Table 1.
texture may suggest the phase to be the smectic Q (SmQ)
1
2
phase. However, as we shall see later, the XRD studies do
not support this conjecture. On cooling the sample at a rate
of 3-5 °C/min from the isotropic phase, a nonspecific pattern
(Figure 1b) was formed. Notably, the usage of slides treated
for planar or homeotropic orientations yielded identical
textures indicating that any aligning influence of the substrate
is ineffective for the M
studies revealed that the M
The C symmetric pentamers 2a,b consisting of different
1
phase. The electrical switching
1
phase is apolar.
2
sets of four spacers exhibit enantiotropic single mesophases
differing slightly in their textures. However, the miscibility
as well as XRD studies revealed that the structures of these
phases are indistinguishable. Thus, it may be sensible to
consider that both pentamers stabilize an identical mesophase,
a
Table 1. Phase Sequence, Transition Temperatures (°C), and
Enthalpies (J/g) of Pentamers 1a,b and 2a,b (Heating/Cooling)
1
1
2
2
a: Cr 131.7 [39.4] M
b: Cr 132.3 [14.9] M
a: Cr 158.1 [17] M 174.7 [11.1] I 173.7 [11.8] M
b: Cr 99.7 [39.4] M 154.2 [11.2] I 153.4 [10] M
1 1
164.6 [12.7] I 163.3 [12.4] M 92.5 [23.6] Cr
1
198.3 [14.1] I 193 [15.9] M
1
88.5 [1.5] Cr
147.8 [23.9] Cr
96.3 [4.9] Cr
2
2
b
2
2
which we abbreviate as M
placed between either treated or untreated glass slides and
cooled from isotropic liquid, exhibits the M phase appearing
initially as tiny batonnets but coalescing to a pattern as
shown in Figure 1c. The pentamer 2b exhibited the M phase
2
phase. Compound 2a when
a
Peak temperatures in the DSC thermograms obtained during the first
heating and cooling cycles at a rate of 5 °C/min. b Cr to Cr transitions was
observed at 78.3 [4.8]. Cr ) Crystal, M1) A monolayer smectic phase;
M2 ) An intercalated smectic phase; I ) Isotropic phase
2
1
1
2
for which the mosaic texture having coffee bean like objects
(Figure 1d) were noted. Here again, the substrate effects are
Despite the fact that the unsymmetrical pentamers 1a,b
possess a different set of spacers, they displayed an identical
negligible. Unlike for the M
nonspecific patterns are obtained upon shearing the M
of both the compounds. Thus, optical textural features seem
to rule out the likelihood of M and M phase being
conventional smectic phases, rather they point to highly
ordered unusual structures.
1
phase, highly birefringent
enantiotropic mesophase (M
alternation can be seen for the isotropic (I) to M
1
) phase. However, a pronounced
phase-
2
phase
1
transition temperatures; pentamer 1b exhibits a higher value
than 1a indicating that the overall molecular shape of the
latter system is bent. Compound 1a or 1b placed between a
1
2
glass slide and cover slip, upon heating, melt into the M
1
XRD experiments were carried out in the M
phases using unaligned samples. The XRD pattern obtained
in the M phase of both pentamers (1a,b), were very similar
and showed two sharp reflections in the low-angle region
with the spacing (d) value for the first one being an exact
multiple of the second (Table 2) suggesting a layered
1 2
and M
phase with a nonspecific texture, which remains unaltered
till the clearing point. The melting and clearing temperatures
seen by POM are in good agreement with the DSC peaks;
the large enthalpy values associated with these transitions
1
are indicative of the M
mesophase. Slow cooling (0.5-1 °C/min) of the liquid led
to the formation of M exhibiting a low birefringent mosaic
1
phase being a highly ordered
1
texture (Figure 1a), which on shearing transforms to a
pseudoisotropic texture indicating that the molecules 1a,b
prefer homeotropic alignment. The low birefringence mosaic
Table 2. The Layer Spacings (d/Å) Obtained in the
Mesophases, the Estimated All-Trans Molecular Length (L/Å) of
Four Pentamers, and the Ratio of d to L
entry (temp/°C)
mesophase
d1, d2 (Å)a
L (Å)b
d/L
1a (150 °C)
1b (160 °C)
2a (160 °C)
2b (140 °C)
M1
M1
M2
M2
88.5, 43.9
91
94
120
115
0.97
1
0.69
0.70
94.22, 47.16
83.04, 39.93
80.35, 38.56
a
The spacings determined by X-ray diffraction. b Molecular lengths
estimated by CS Chem Draw 3D Ultra 9 software.
structure. In addition, a diffuse peak seen in the wide angle
region with d values of about 4.7 Å for both the compounds
proves the fluid nature of the phase. The layer spacing of
the first reflection obtained is close to the molecular length
of 1a,b estimated from the all-trans conformation. This
implies that the molecules form a monolayered orthogonal
1
smectic structure. Thus, assigning the M phase to the
Figure 1. Microphotographs of the textures obtained for the
mesophases of pentamers: (a) M
1
phase at 158 °C of 1a obtained
(
12) (a) Nguyen, H. T.; Ismaili, M.; Isaert, N.; Achard, M. F. J. Mater.
during slow cooling; (b) M phase at 134 °C of 1a formed during
1
Chem. 2004, 14, 1560. (b) Pansu, B. Pramana 2003, 61, 285. (c) Levelut,
A.-M.; Hallouin, E.; Bennemann, D.; Heppke, G.; Lotzsch, D. J. Phys. II
Fr. 1997, 7, 981.
fast cooling; (c) 2a at 170 °C; (d) 2b at 147 °C.
Org. Lett., Vol. 9, No. 14, 2007
2643

aforementioned SmQ phase, reported to have a highly tilted
structure,12 can be undoubtedly ruled out. Figure 2a depicts
a
Table 3. Electrochemical Properties of Pentamers 1a,b and
a,b
2
b
b
b
E HOMO Ec,dLUMO ∆E CV ∆Ec,eUV
b
c
entry E 1ox E 2ox
E red
1a
1b
2a
2b
1.17 1.76 -1.21
1.19 1.83 -1.35
1.26 1.72 -1.25
1.26 1.86 -1.23
5.87
5.89
5.96
5.96
3.49
3.35
3.45
3.47
2.38
2.54
2.5
2.49
2.49
2.49
2.48
2.49
a
Experimental conditions: Pt-disc working electrode, Pt-rod counter
electrode, Ag/AgCl in saturated LiCl solution as reference electrode,
b
(
(
Bu)4NPF6 (0.1 M), room temperature. In volts (V). c In electron volts
d
eV). Estimated from the onset oxidation and reduction potential (vs Ag/
Ag ) plus 4.7 (see ref 11). Band gap determined from the red edge of the
+
e
longest wavelegnth in the UV-vis absorption of pentamers.
between the frontier orbitals was determined from the CV
by taking the difference between the onset of oxidation and
13,14
reduction potential.
The ∆E was also estimated from the
Figure 2. Schematic representation of the self-assembly of
pentamers 1b in a monolayered phase (a) and 2b in an intercalated
layered phase (b).
red edge of the longest wavelength absorption, λred edge
)
11,14
500 nm in the UV-vis spectra.
Apparently, ∆E deduced
from the methods are comparable implying that these
compounds genuinely possess electroactivity.
In conclusion, the syntheses, characterization, and meso-
morphism of the first examples of multifunctional unsym-
metric and C symmetric pentamers comprising five me-
2
sogenic segments interlinked through four flexible spacers
varying in their length and parity, are reported. Apart from
devising synthetic feasibility, their physical properties have
been improved by rendering them soluble in organic solvents
the schematic representation of the local molecular organiza-
tion within the monolayer smectic (M ) phase. It can be
1
imagined that within a layer, these long molecules organize
in an antiparallel fashion wherein the bulky cholesteryl
segment of a molecule moves closer to the alkyl tail regime
of the adjacent molecule to reduce the excluded volume.
Thus, the M phase appears to be a new monolayered smectic
1
modification given the fact it possesses unique textural
pattern.
and by self-organizing to exhibit mesomorphism. The C
2
symmetric pentamers possess tolane, biphenyl, and azoben-
zene cores, while unsymmetrical ones contain cholesterol
and naphthalene entities, additionally. These monodispersive
supermolecules have been evidenced to self-assemble into
monolayered or intercalated phases, which appear to be rather
new smectic modifications given the fact they possess unique
textural pattern. Besides this, they possess electrochemical
activity. Indubitably, this approach offers a unique way to
design a multifunctional material which can bridge the gap
between low molar mass and polymeric LCs, with enhanced
functional capability and ease of processability.
2
The M phase of pentamers 2a,b displayed similar XRD
patterns. In addition to a wide angle peak with d values of
about 4.5 Å, they contained two sharp reflections in the
small-angle region. The d values of these low-angle reflec-
tions (Table 2) suggest a lamellar structure for the M
The layer spacing, as shown in Table 2, is much lower than
the estimated molecular length indicating that the M phase
2
phase.
2
is an intercalated smectic phase. Such an arrangement, as
shown in Figure 2b, perhaps originates from the electrostatic
quadrupolar interaction between the dissimilar anisometric
segments as well as from the mechanism that excludes
volume or space filling constraints. Apparently, the M phase
2
also seems to be a distinct lamellar phase as it displays
unusual textures.
Acknowledgment. We wish to sincerely thank Dr. M.
Pandurangappa and Mr. T. Ramakrishnappa, Central College,
Bangalore, for their help in carrying out cyclic voltammetry
experiments. We also thank Ms. Sandhya D. Hombal
The solution electrochemistry of these four pentamers was
investigated. The oxidation and reduction potentials were
measured by cyclic voltametry (CV) at a scanning rate of
(CLCR) for her technical assistance.
Supporting Information Available: Synthetic procedures
-
1
0
.05 mVs for the millimolar solutions of pentamers in
and characterization data of all intermediates and target
molecules. This material is available free of charge via the
Internet at http://pubs.acs.org.
dichloromethane. All the four giant molecules displayed an
1
1
identical and reproducible cyclic voltammograms. These
voltammograms are characterized by two oxidation waves
centered on E1ox ) 1.22 and E2ox ) 1.78 V and a reduction
wave centered on Ered ) -1.26 V (Table 3). In these
molecular architectures, it is rather complicated to specify
which of the molecular fragments causes the electrochemical
behavior. Nonetheless, owing to their identical redox be-
havior it is reasonable to deduce that the tolane and/or
azobenzene fragments are responding. The band gap (∆E)
OL070808P
(13) Leeuv, D. M.; Simenon, M. M. J.; Brown, A. R.; Einerhand, R. E.
F. Synth. Met. 1997, 87, 53. (b) Cui, Y.; Zhang, X.; Jenekhe, S. A.
Macromolecules 1999, 32, 3284. (c) Wang, H.-Y.; Feng, J.-C.; Wen, G.-
A.; Jiang, H.-J.; Wan, J.-H.; Zhu, R.; Wang, C.-M.; Wei, W.; Huang, W.
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Org. Lett., Vol. 9, No. 14, 2007