Phasmidic indigoid liquid crystals

Article abstract of DOI:10.1039/b925049m

Two new phasmidic mesogens with an integrated indigo core have been synthesized and their thermotropic properties were investigated by polarization microscopy and differential scanning calorimetry. Their surprising textural features are reminiscent of tho

Full text of DOI:10.1039/b925049m

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www.rsc.org/materials | Journal of Materials Chemistry
Phasmidic indigoid liquid crystals†
*
Jan H. Porada and Dirk Blunk
Received 27th November 2009, Accepted 2nd February 2010
First published as an Advance Article on the web 17th February 2010
DOI: 10.1039/b925049m
combined with its rather small spatial demand, its stability and its C2h
symmetry, which eases a symmetric bis-substitution.
Two new phasmidic mesogens with an integrated indigo core have
been synthesized and their thermotropic properties were investigated
by polarization microscopy and differential scanning calorimetry.
Their surprising textural features are reminiscent of those known
from some bent core mesogens. Common properties and differences
are discussed.
These phasmidic indigo derivatives 1 and 2 were synthesized by
reductive condensation6 of the corresponding isatin derivatives 7
and 9, which were obtained by Suzuki-coupling between 2,3,4-tri-
dodecyloxyphenylboronic acid (5) and the 5- and 6-bromoisatin
(6 and 8), respectively, cf. Scheme 1. Detailed descriptions of the
synthetic procedures, the analytical data of all new compounds and
further photographs of the textures is given in the ESI.† In contrast to
many other indigo derivatives, compounds 1 and 2 are well soluble in
solvents like chloroform, indicating a sterical demand which reduces
the intermolecular hydrogen bond interactions.
Discotic molecules capable of forming columnar liquid crystalline
phases are of increasing interest due to their possible applications in
molecular electronics or organic photovoltaics.1,2 Besides the classical
discotic mesogens (most often an aromatic core substituted with six
alkyl or alkoxy chains) the class of phasmidic or hexacatenar meso-
gens also exhibits columnar organized phase types and therefore
offers an alternative approach, to which comparatively little attention
has been given up to now.
Examination of the thermotropic liquid crystalline properties with
polarization microscopy (PM) and differential scanning calorimetry
(DSC) reveal an unusual behaviour for 1 and 2.
The indigo derivative 1 is crystalline at room temperature and
shows the phase transitions listed in Table 1. The high transition
enthalpy from the Col2 to the isotropic phase indicates a high degree
of molecular order in the Col2 phase. By slowly cooling from the
isotropic phase the microscopic texture presented in Fig. 2 (crossed
polarizers, more examples are shown in the ESI†). In this texture
small domains pervaded with numerous defects are characteristic.
The appearance is dominated by the growth of myelin like filaments
that develop to focal conic domains when adsorbed to the cover
plate, and to helical structures of varying pitch length of both
orientations when suspended in the bulk isotropic phase.
Phasmidic mesogens consist of an elongated, rod like central part
and two peripheral wing groups, each of the latter substituted with
three aliphatic chains.3–5 In contrast to conventional discotic meso-
gens this molecular setup allows an easy incorporation of functional
molecular moieties as e.g. chromophores in the core and to simply
attach the whole wing groups as substituents at only two positions.
In this context we decided to design the phasmidic mesogens 1 and 2
with an integrated indigo motif as shown in Fig. 1. The choice for
indigo was based on its remarkable chromophoric properties
Elongated domains of uniform birefringence arise in a very rapid
growth process. A change of the texture when cooling from Col2 to
Col1 could not be observed.
To the best of our knowledge, up to now likewise textures with
spontaneous formation of helical filaments are only known for
lyotropic mesogens7,8 or bent-core mesogens with a so called B7
phase.9–12 The optical similarity of the texture in Fig. 2 to a B7
phase is deceptive, since the B7 phase is exclusively formed by
bent-core mesogens and of a smectic nature. The driving force
for the appearance of the B7 phase is a polarization modulation
by periodic splay defects.12 Polarity and a bent molecular shape is
thus an essential requirement for the B7 phase. Both are not
present in 1 and the application of an electric field showed – as
expected – no result.
The phase transition data of compound 2, crystalline at room
temperature, are compiled in Table 1. Upon slowly cooling from the
isotropic liquid, the Col3 phase grows with occasional big focal conic
domains of differing visual appearance as presented in Fig. 3. Most
frequently observed are blue domains, showing irregular concentric
stripes and an extinction cross orientation of 45ꢀ to the crossed
polarizers (Fig. 3a). Rather seldom compared to these blue domains
are red domains which show an extinction cross orientation
congruent to the crossed polarizer and analyser (Fig. 3c). A third
Fig. 1 Phasmidic indigo derivatives carrying 2,3,4-tridodecyloxyphenyl
substituents at the 5,50- (1) or 6,60-positions (2).
€
Universitat zu Koln, Institute of Organic Chemistry, Greinstr. 4, 50939
Ko€ln, Germany
€
† Electronic supplementary information (ESI) available: Experimental
details, NMR spectra and further texture photographs (polarization
microscopy). See DOI: 10.1039/b925049m
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Scheme 1 Syntheses of the phasmidic indigo mesogens 1 and 2.
Table 1 Thermal dataa of the compounds 1 and 2
Cr
M
M
I
1
2
C
C
–b/61.7 (50.8) Col1 –b/77.6 (14.1) Col2 218/219.6 (32.8)
–/–
–b/126.2 (66.1) Col3 143/143.1 (18.4)
C
C
—
a
PM/DSC [ꢀC] (DH [kJ molꢁ1]), Cr: crystalline, M: mesophase, Colx:
b
distinct columnar mesophases, I: isotropic liquid. This transition
could not be observed by PM.
Fig. 3 Texture of the Col3 phase of 2 at 144 ꢀC between crossed
polarizers (a and c, polarizers vertical and horizontal) and with only one
horizontal polarizer (b and d). Pictures a and b show the same sector of
the preparation, as do c and d.
above these formations radially oriented, elongated small red
segments become visible, as can be seen in the top right quarter of
Fig. 3a or in Fig. 3c at the very top, center to left. When the field of
vision is shifted slightly, these red sections, appearing to lie distinc-
tively above the other parts of the sample, move with respect to the
other sections of the preparation. At the current state of investigation
the origin of this parallactic effect is unclear, though we assume
a surface induced nucleation process at the glass slides as important
contribution.
Fig. 2 Texture of the Col2 phase of 1 at 211 ꢀC between crossed
polarizers showing focal conical domains, helical filaments and elongated
domains of uniform birefringence.
The blue domains can be categorized in two groups of inverse
behaviour concerning the sequence of the colour shade at the arms of
the extinction cross and its rotational direction by rotating the
sample. The domains I and II in Fig. 3a belong to the same group,
while domain III behaves in the opposite way.
textural feature are areas which appear brownish and could be
identified as a superposition of blue and red domains. With the
brownish areas and occasionally also with the blue domains a strong
parallactic effect can be observed: seemingly with a clear distance
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The spatially oriented chromophoric indigo core unit serves as
a molecular probe. The difference of the blue domain types becomes
obvious by examination with only one polarizer due to their direc-
tional opposed dichroism (Fig. 3b). As shown in Fig. 3d (arrow)
a blue spherulite can be composed of blue domains of either orien-
tation. The dichroism of the red domains is more pronounced and
always shows the highest absorption parallel to the polarizer
(Fig. 3d).
and thus to textures as in the case of 1. Further studies, especially
X-ray investigations, to clarify the molecular architecture of these
interesting mesophases, displaying those unusual textures, are in
progress.
Notes and references
€
1 S. Laschat, A. Baro, N. Steinke, F. Giesselmann, C. Hagele, G. Scalia,
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2 G. P. Crawford and R. H. Hurt, in Encyclopedia of Nanoscience and
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The situation with three distinct domain types in which two are
complementary is reminiscent again of a texture so far exclusively
observed for bent-core mesogens, here the B2 phase. In the B2 phase
the complementary domains are synclinic enantiomeric arrange-
ments, while the third type of domains is formed by an anticlinic
racemic assembly. The bent molecular shape together with the tilt of
the molecules within the layers of the B2 phase generates a chiral
situation, which can be switched upon application of an electric
field.9,13 Since the indigo derivative 2 is centrosymmetric and therefore
non-polar a likewise phase architecture is not necessarily chiral. By
application of an electric field therefore a switching event was not be
observed.
^
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In conclusion we presented the two new phasmidic indigo meso-
gens 1 and 2 representing the first examples of liquid crystalline indigo
derivatives. Their unusual optical features remind of the textures of
the B7- or B2-phase, respectively, found for some bent-core meso-
gens. Differences can be observed upon the reaction on an applied
electric field. Besides some rare examples in which a biaxial nematic
phase is claimed14–16 phasmidic mesogens only form columnar pha-
ses.4,5,17 Therefore it is reasonable to assume a columnar type of phase
here as well, most likely of oblique geometry. The strong core-core
interactions demanded for Colob-phases1 might originate from the
central H-bond donator acceptor system or from the intramolecular
push-pull situation of the chromophore leading to strong interactions
between adjacent molecules, e.g. in terms of Coulomb or charge-
transfer interactions. Since both 1 and 2 are centrosymmetric but
non-polar we suggest that the steric interactions of the wing groups
may cause a manipulating force for the formation of the columnar
phases leading to some kind of sterically induced splay deformations
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2958 | J. Mater. Chem., 2010, 20, 2956–2958
This journal is ª The Royal Society of Chemistry 2010