Novel tetrahedratic smectic C and nematic mesophases in unsymmetrically 1,1-bis-substituted ferrocenomesogens

Article abstract of DOI:10.1002/chem.200901349

Non-conventional mesophases have been observed in novel ferrocene-containing liquid crystals. The formation of helical and garland-like superstructures, and spontaneous resolution into macroscopic chiral domains occur in the lamellar mesophase. The nemati

Full text of DOI:10.1002/chem.200901349

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DOI: 10.1002/chem.200901349
Novel Tetrahedratic Smectic C and Nematic Mesophases in Unsymmetrically
1,1’-Bis-substituted Ferrocenomesogens
Oleg N. Kadkin,* Eun Ho Kim, Young Joon Rha, So Yeon Kim, Jigeon Tae, and
Moon-Gun Choi*[a]
Liquid crystals are anisotropic fluids that exhibit proper-
ties between those of conventional liquids and solid crys-
tals.[1] During the last decades liquid crystals based on ferro-
cene derivatives attracted significant interest of researchers
engaged in the area of metal-containing liquid crystals or
metallomesogens.[2] The conformation of 1,1’-bis-substituted
ferrocenes is not always known with certainty. The rotation-
al barrier of cyclopentadienyl rings in ferrocene was estimat-
ed as ~0.5 kJmolÀ1 on the basis of DFT calculations,[3] and
statistically as ~4.6 kJmolÀ1 from the data of X-ray structure
analysis.[4] Flexibility in the center of the rigid rod in ferro-
cenomesogens may lead to non-conventional molecular or-
ganization in the liquid crystal state. We have prepared un-
symmetrically 1,1’-bis-substituted ferrocenomesogen 1 (see
below), and observed rather unusual thermooptical behavior
munity in recent years because of their extraordinary prop-
erties. Many of the banana mesophases exhibited antiferro-
electric and ferroelectric properties, and also spontaneously
generated chirality from achiral molecules.[8] However, the
observed properties of 1 are not comparable with the
known types of banana mesophases, especially regarding
their X-ray diffraction patterns. Taking into account uncom-
mon optical textures and X-ray scattering data we suggest a
tetrahedral symmetry for the observed lamellar and nematic
mesophases of 1, and the notations SmCT* and NT*, where
T stands for tetrahedratic order and * for chirality. Interest-
ingly, a theoretical possibility of the tetrahedral symmetry in
nematic liquid crystals was introduced earlier by Fel,[9] and
similar theoretical concept was extended later for banana-
shaped liquid crystals.[10]
Thermal phase transitions of 1 were studied by means of
polarized optical microscopy (POM), microcalorimetric
(DSC) and X-ray diffraction (XRD) methods (see Table 1
and SI). Schlieren textures were observed under a polarizing
microscope, which are typical for smectic mesophases with
tilted layers (see Supporting Information, Figure S2). Be-
sides, the formation of garland-like and helical superstruc-
tures (see Figure 1a,b) and large separate domains of oppo-
site chirality were observed (Figure 1c,d). Turning polarizer
or analyzer consecutively clockwise and counter clockwise
by the angle of 158 from the crossed position leads to
switching in brightness of chiral domains of the opposite
handedness.[11] Beside the above mentioned banana liquid
crystals spontaneously generated chirality from achiral mol-
ecules has been observed earlier in some nematics formed
from hydrogen-bonded carboxylic acids[12] and bow-shaped
molecules,[13] and also in double layered anticlinic SmCA
mesophases.[14]
and XRD patterns. In many respects the properties of 1 are
reminiscent of the banana-shaped mesogens, though such
behavior was detected in none of the earlier reported sym-
metrically[5] and unsymmetrically[6] 1,1’-bis-substituted ferro-
cenomesogens. In general, banana liquid crystals are charac-
terized with the bent molecular shapes and a great variety
of distinct mesophases labeled from B1 to B8.[7] They have
attracted the enormous attention of the liquid-crystal com-
[a] Dr. O. N. Kadkin, E. H. Kim, Y. J. Rha, S. Y. Kim, J. Tae,
Prof. M.-G. Choi
Department of Chemistry, Yonsei University
262 Seongsanno, Seodaemun-gu, Seoul 120-749 (Korea)
Fax : (+82)2-364-7050
In our opinion, the origin of macroscopic chiral domains
and helical superstructures in 1 is adequately explainable
considering pseudotetrahedratic associates. One can see that
there are several kinds of mirror relationships involving the
E-mail: onk@yonsei.ac.kr
choim@yonsei.ac.kr
central ferrocene fragment can be established in
(Figure 2). Firstly, conformational chirality may be caused
1
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901349.
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Table 1. Phase transition parameters and XRD characteristics of 1 in the heating course.
The
pseudotetrahedratic
Phase[a]
T
DH
XRD features
dimers seem to be more capa-
ble of preserving their shapes
in comparison with the single
flexible conformers. Perhaps,
plain-to-plain p–p electronic
interactions of the benzene
rings and n–p interactions be-
tween the free electron pair of
the nitrogen atom and aromat-
ic rings are the main driving
forces for the dimer formation.
Coplanarity of the conjugated
aromatic system with the cy-
clopentadienyl rings of the fer-
rocene moiety could be a very
important factor as well. The
majority of the earlier reported
symmetrically and unsymmetri-
cally 1,1-bis-substituted ferro-
cenomesogens are derivatives
of the carboxylated or alkyl
[8C] [kJmolÀ1
]
Cr1
Cr2
–
–
numerous SAXS peaks which are grouped around 3 main (4.60, 3.92, 3.41 nm) in-
terlayer distances, numerous small and 1 sharp (0.44 nm) WAXS peak
layered structure with small multiplicity in a SAXS peak (4.80), a number of
small and 3 main (0.54, 0.49, 0.46 nm) WAXS peaks
the same as the preceding crystal phase, a SAXS peak is gradually decreasing
with increase in temperature, WAXS peaks are not changing
two diffuse halos (4.28, 0.55 nm)
91.9 2.71
SmCT* 203.4 23.88
NT*
I
215.5 0.42
272.4 1.61
–
[a] Cr1, Cr2: crystal, SmCT*: tetrahedratic smectic C, NT*: tetrahedratic nematic, I: isotropic liquid phases.
substituted
ferrocenes,[5–6]
where such a rigidly conjugat-
ed aromatic system is missing.
In the reported cases of 1,1’-bis
arylated ferrocenomesogens[5f,i]
the formation of dimeric mo-
lecular associates seems to be
prevented by repulsion be-
tween the free electronic pairs
of the azomethine nitrogens.
Computer-aided calculations of
the pseudotetrahedratic dimers
gave the energy drop of
Figure 1. POM textures of the SmCT* mesophase of 1: a) a magnified area with garland-like and helical forma-
tions at 2108C; b) a magnified area with a helical string; c-d) spontaneous chiral resolution in the SmCT* mes-
ophase at 2058C.
by different orientation of the fluoro-substituents in relation
to a three-dimensional structure of ferrocene. Obviously,
such conformers are characterized by relatively small degree
of asymmetry, and it would be rather difficult to elucidate
the observed macroscopic chiral domains basing solely on
this. Secondly, chiral relationship occurs between the
bended conformers. Spontaneous organization into chiral
smectic layers in this case is also questionable, as it should
be considered that the arm substituents have different
lengths and random orientation, and the chiral conforma-
tions are easily interconvertible because of flexibility in the
central part of 1. And finally, the bended conformers can be
coupled in four different kinds of dimers, or two kinds of en-
antiomeric pairs. The internal order of the mesophases com-
prised of such kind of coupled conformers will be similar to
some of the specifically classified mesophases with a tetra-
hedral symmetry theoretically described by Lubensky and
Radzihovsky.[10b]
Figure 2. Possible chiral relationships in 1.
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Ferrocenomesogen
COMMUNICATION
41.2 kJmolÀ1 for the untilted enantiomers and 47.3 kJmolÀ1
for the tilted one. Accordingly, the tilted dimers are more
stable thermodynamically in comparison with their untilted
counterparts. In fact, the
the NT* mesophase of 1. First of all, brightness of the
sample alternates when the cell is turned in relation to the
crossed polarizers (Figure 4). Secondly, conoscopic observa-
bended enantiomers of differ-
ent handedness can not couple
into the tetrahedratic dimers,
and, hence, this provides an in-
itial mechanism for stereospe-
cific supramolecular assem-
bling. Notably, the untilted
dimers are not well adapted
for packing into the lamellar
smectic C layers together with
the tilted enantiomers, and,
possibly, they segregate into
separate helical and garland-
like superstructures.
The broad nematic meso-
phase with non-conventional
textural patterns follows the
Figure 4. Possible optical biaxiality of the NT* mesophase of 1 detected by POM experiments at 2408C: a)
switching brightness of the homeotropically aligned sample of ~10 m thickness by turning the crossed nicols
from the angle 0 to 45 and 908 (top pictures); b) conoscopic pictures of the homeotropically aligned sample at
the angles of 08 and 458 to the crossed nicols (lower pictures).
SmCT* mesophase (Figure 3).
The NT* mesophase has a dark
appearance, and only some
momentary birefringent flash
can be observed on transition
to the isotropic liquid state. Chiral nematic droplets and bi-
refringent walls are observed in perimeter areas of the
sample. A boundary of the pseudoisotropic domains has a
complex texture pointing out non-conventional supramolec-
ular organization of the mesophase (see Figure 3d). Addi-
tionally, certain signs of optical biaxiality were detected in
tions revealed a large symmetry disturbance of the “Maltese
cross”. Indeed, the above optical observations might be con-
nected with the surface alignment and not with the bulk
biaxial mesophase. But at any rate these optical phenomena
are indicative of biaxial geometry of the molecules. In this
connection, the distorted tetrahedratic dimers are very simi-
lar to rectangular board-
shaped mesogens with molecu-
lar biaxiality except for two
vertices are slightly twisted in
relation to two others in the
first case. In summary, all of
the observed optical properties
conform well to the suggested
tetrahedratic order.
X-ray diffraction patterns of
1 at the different phase states
are represented in Figure 5. At
room temperature
a SAXS
pattern consist of a number of
scattering peaks, which is sug-
gestive of the complex internal
arrangement of the molecules.
In a WAXS pattern one sharp
intensive peak corresponding
to the interlayer spacing dis-
tance of 0.44 nm is explicitly
exposed among numerous
small reflections. Most likely,
the latter points out strong
one-dimensional plain-to-plain
Figure 3. POM textures of the NT* mesophase of 1: a) dark droplets on cooling from the isotropic state at
2728C; b) an equilibrated transition from the isotropic liquid (left) to NT* (right) mesophases at 2728C; c)
chiral nematic droplets at 2658C; d) the intricate structure of the NT* pseudoisotropic domain boundary,
2458C.
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O. N. Kadkin, M.-G. Choi et al.
not straightforward in this
stage to give assignments for
all the reflection peaks in the
SmCT* state. However, in a
case of cross-penetrated pack-
ing of the pseudotetrahedrons
X-ray scattering reflections
might appear due to the elec-
tronic density periodicities
other than imaginable from the
simple 2D or 3D order of the
molecules. For instance, some
sensible interpretation of this
would be a superposition of
two or several 2D lattices
within the smectic layers,
Figure 5. Selected SAXS (left) and WAXS (right) patterns of 1 in the different phase states.
stacking of the ferrocene moieties in the room temperature
crystal form. After a transition to the Cr2 state the SAXS di-
agrams reveal a layered structure with interlayer spacing of
~4.8 nm (001 reflection). At the same time in the WAXS di-
agrams three relatively intensive peaks appear among nu-
merous smaller reflections. The same aspect of the SAXS
and WAXS patterns are sustained in the SmCT* mesophase.
The interlayer spacing distance of ~4.8 nm is very persistent
and the remnant weakly layered structure remains even in
the nematic state up to 2208C. Molecular modelling explains
sufficiently well the appearance of the periodicity of
~4.8 nm from the layered packing of the tilted pseudotetra-
hedratic dimers (Figure 6). As for the WAXS signals it is
which certainly will give rise to a series of peaks in the wide
angle region. A schematic illustration of possible periodici-
ties of the electronic density in a case of tetrahedratic smec-
tic layers is given in Figure 6. The difference between the
Cr2 and SmCT* is that conformational melting of the termi-
nal alkyl chains gives rise to fluidity and liquid crystal prop-
erties in the latter case, while the central parts of the meso-
genic molecules will be arranged in a flexible interpenetrat-
ed network.
Briefly, very unusual mesophases were observed in 1,
which are distinguished by spontaneous mesoscopic and
macroscopic chiral degeneracy, non-conventional XRD pat-
terns and explicit signs of molecular biaxiality. The fairly
plausible tetrahedratic order
can be deduced from these ex-
perimental observations. Ac-
cordingly, dark textures of the
NT* mesophase seem to be a
natural
consequence
of
random orientation of the ter-
minal alkyl chains from the
centres of elongated tetrahe-
drons. Intricate boundaries of
the NT* pseudoisotropic do-
mains is another strong indica-
tion of complex internal organ-
ization of the observed meso-
phase. It should be noted that
a justification of the observed
properties of 1 without engag-
ing the suggested pseudotetra-
hedratic dimers would be quite
ambiguous.
Experimental Section
Figure 6. Molecular models illustrating tetrahedratic dimers (in the upper left corner), the SmCT* layers with
interlayer spacing distance of ~4.8 nm (in the lower left corner) and schematic illustration of possible different
periodicities of the electronic density depending on the point of view in a case of interpenetrated packing of
the pseudotetrahedratic dimers within the smectic layers (in the right).
The synthetic procedures for prepara-
tion of compound 1 and other experi-
mental details are given in Supporting
Information to this article.
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Ferrocenomesogen
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Acknowledgements
This work has been supported by a grant from the Korea Science and En-
gineering Foundation (KOSEF) through the Centre for Bioactive Molec-
ular Hybrids (CBMH), and the program Brain Korea 21 (BK-21). The
Pohang Accelerator Laboratory is gratefully acknowledged for providing
X-ray beam from their synchrotron facilities. We thank Prof. Piotr Kas-
zynski for the original proposition about hinge-like mesogenic molecules
from unsymmetrically 1,1’-bis-substituted ferrocenes. We also thank Prof.
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Keywords: chirality · ferrocenes · helical structures · liquid
crystals · metallomesogen
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Received: May 20, 2009
Published online: September 8, 2009
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