Enantiotropic nematics from cross-like 1,2,4,5-tetrakis(4'-alkyl-4- ethynylbiphenyl)benzenes and their biaxiality studies

Article abstract of DOI:10.1002/chem.201103453

The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross-shaped compound 3, which showed enantiomeric nematic phase behavior. This cross-like core structure could alternatively be viewed as two fused V-shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface-induced biaxiality was evidenced by optical studies. Cluster-size analysis within the nematic phase of compound 3 revealed the formation of meta-cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small-angle 2D X-ray diffraction patterns of magnetic-field-aligned samples indicated that the nematic phase was composed of small smectic C-like clusters with the tilting of molecules within the clusters. The wide-temperature-range enantiomeric nematic phase of cross-like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent-rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher-order liquid-crystalline phases and crystallization, for future biaxial investigations. Take up your cross: An enantiotropic nematic phase with a wide temperature range has been achieved by using cross-like mesogens. Biaxiality investigations by using conoscopic and 2D XRD studies imply the formation of small smectic C-like meta-cybotactic clusters in the nematic arrangement. Copyright

Full text of DOI:10.1002/chem.201103453

FULL PAPER
DOI: 10.1002/chem.201103453
Enantiotropic Nematics From Cross-Like 1,2,4,5-Tetrakis(4’-Alkyl-4-
Ethynylbiphenyl)Benzenes And Their Biaxiality Studies
Hsiu-Hui Chen,[a] Hsing-An Lin,[a] Yin-Hui Lai,[a] Shu-Yu Lin,[a] Chien-Hung Chiang,[a]
Hsiu-Fu Hsu,*[a] Tzenge-Lien Shih,*[a] Jey-Jau Lee,[b] Chien-Chen Lai,[c] and
Ting-Shen Kuo[d]
In memory of Lian-Jye Yu
Abstract: The theoretically predicted
optimum length/breadth/width ratio for
maximizing shape biaxiality was inves-
tigated experimentally by the facile
and successful synthesis of cross-
shaped compound 3, which showed
enantiomeric nematic phase behavior.
This cross-like core structure could al-
ternatively be viewed as two fused V-
shaped mesogens, which have recently
immerged as a new direction in biaxial
nematic research, at the bending tips
that can act as a new structure for biax-
ial investigations. Whilst the thermal
analysis data of compound 3 did not
meet the expected theoretical values
for biaxial nematics, surface-induced
biaxiality was evidenced by optical
studies. Cluster-size analysis within the
nematic phase of compound 3 revealed
the formation of meta-cybotactic nem-
atics, which approached the cluster
sizes of cybotactic nematics. The split
small-angle 2D X-ray diffraction pat-
terns of magnetic-field-aligned samples
indicated that the nematic phase was
composed of small smectic C-like clus-
ters with the tilting of molecules within
the clusters. The wide-temperature-
range enantiomeric nematic phase of
cross-like compound 3 enabled the mo-
lecular skeleton to serve as an alterna-
tive skeleton to bent-rod mesogens,
which exhibited nematic phases with
the potential competition of transitions
to
higher-order
liquid-crystalline
phases and crystallization, for future
biaxial investigations.
Keywords: arenes
·
mesogens
·
nematic phase · liquid crystals ·
X-ray diffraction
Introduction
proposed to afford the Nb phase in thermotropic systems:
1) by mixing rod/disc mesogens physically or chemically[4] or
2) by achieving the shape biaxiality of a single compo-
nent.[5–7] In terms of shape biaxiality, board-like nematogens
were originally proposed to lead to the Nb phase; however,
all board-like or cross-like mesogens reported to date have
shown only monotropic nematic behavior or columnar
phases. For example, Goodby and co-workers reported the
synthesis of board-like, or, more precisely, cross-like (Fig-
The theoretical prediction[1] of biaxial nematics (Nb) was
first proposed more than four decades ago and, since then,
much effort has been devoted to the realization of this
exotic phase from low-molecular-weight thermotropic liquid
crystals, which have promising applications for improving
the response times and viewing-angle characteristics of
liquid-crystal displays.[2,3] Two main approaches have been
ure 1a),
1,2,4,5-tetrakis(4-tridecylphenylethynyl)benzene
(1), which showed a monotropic nematic phase within
a small temperature range and its poor liquid crystallinity,
which was ascribed to its structural deviation from being
rod-like and disc-like, hindered further investigation of the
nematic phase.[5] Increasing the inter-core stacking interac-
tions of board-like mesogens led to the disappearance of the
nematic structure.[6] The incorporation of molecular biaxiali-
ty into board-like Pd complexes generated a monotropic
nematic phase with a weakly first-order nematic-to-isotropic
(N-to-Iso) transition but a Nb phase could not be con-
firmed.[7] The generation of biaxiality in thermotropic nem-
atic LC systems by using the most-obvious model of board-
like molecules has yet to be successfully performed.
[a] Dr. H.-H. Chen, H.-A. Lin, Y.-H. Lai, S.-Y. Lin, C.-H. Chiang,
Prof. H.-F. Hsu, Prof. T.-L. Shih
Department of Chemistry, Tamkang University
251 Taipei (Taiwan)
Fax : (+886)02-2620-9924
E-mail: hhsu@mail.tku.edu.tw
[b] Dr. J.-J. Lee
National Synchrotron Radiation Research Center of Taiwan
300 Hsinchu (Taiwan)
[c] Prof. C.-C. Lai
Department of Chemistry, National Chung-Hsing University
402 Taichung (Taiwan)
[d] T.-S. Kuo
Department of Chemistry, National Taiwan Normal University
106 Taipei (Taiwan)
Recently, bent-core mesogens have immerged as a new di-
rection in biaxial nematic research.[8,9] Of the reported bent-
core nematogens, Lehmann and co-workers reported
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201103453.
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These results suggested that judicious molecular design for
fine-tuning inter-cross-interactions for the generation of
nematic supra-structures seems to be feasible to allow the
exploration of their potential optical biaxiality. Herein, com-
pounds 2 and 3 (Figure 1c) were designed by extending the
peripheral phenyl groups in cross-like compound 1 to bi-
phenyl groups to retain the aspect ratio of 1.67 and to en-
hance inter-cross-p–p interactions between the side-arms.
Enantiotropic nematic behavior over wide temperature
ranges was achieved by the successful synthesis of cross-like
compounds 3a–3d with biphenyl peripheries. A new plat-
form for biaxial investigations is reported and investigations
into the potential biaxiality of the nematic phase of com-
pound 3d were conducted by using various methods, includ-
ing conoscopy and 2D X-ray diffraction.
Results and Discussion
First, cross-like alkoxy derivative 2 was prepared because of
its facile synthetic preparation (Scheme 1). However, diffi-
culties in purification were encountered owing to the poor
solubility of compound 2 in organic solvents and satisfactory
elemental analysis was never achieved. Nevertheless, from
preliminary differential scanning calorimetry (DSC) and po-
larizing optical microscopy (POM) studies, compound 2
showed an enantiotropic nematic mesophase. Encouraged
by these results, the synthesis of alkyl analogues (3) was car-
ried out in the hope of solving the problem of low solubility
for further investigations (Scheme 1). After various at-
tempts, optimized yields of cross-like 1,2,4,5-tetrakis(4’-
alkyl-biphenyl-4-ethynyl)benzenes (3a–3d) of 73–95% were
achieved by reacting 10 equivalents of the biphenyl-contain-
ing side-arm with 1,2,4,5-tetrabromobenzene under the cou-
pling conditions described in the Experimental Section.
Starting from tetraiodobenzene instead of its bromo ana-
logue did not give higher yields. Compounds 3 are soluble in
Figure 1. a) Chemical structure of compound 1; b) schematic representa-
tion of the cross-like geometry, which can also be viewed as two bent-
cores that are fused at the bending tips; c) chemical structures of com-
pounds 2 and 3.
a system with an ideal tetrahedral bending structure that fit
the requirements to form a biaxial nematic phase.[8] Howev-
er, the nematic phases of bent-core mesogens typically have
potential competition in the form of transitions into higher-
order LC phases and crystallization.[10] Nevertheless, the suc-
cess in biaxial bent-core mesogens makes cross-like materi-
als a good model for producing biaxiality because a cross
can alternatively be viewed as two V-shaped mesogens that
are fused at the bending tips (Figure 1b) to give a more-
symmetrical molecular structure for wider LC ranges. How-
ever, as mentioned above, obtaining a reasonable nematic
temperature range for cross-like molecules has been a major
obstacle to further investigations of biaxiality. Better molec-
ular design of lozenge-shaped mesogens is needed for the
generation of wide-temperature-range nematic arrange-
ments. Recently, with the aim of solving this problem, rigid
Y-shaped nematogens that were derived from compound
1 were investigated to give stable nematic phases by the ma-
nipulation of intermolecular interaction through employing
various substitution patterns on the central aromatic ring.[11]
Scheme 1. Synthesis of compounds 2 and 3.
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Enantiotropic Nematics From Cross-Like Structures
FULL PAPER
Table 1. Phase behavior of compounds 3a–3d.[a]
most polar organic solvents, such as CH2Cl2 and THF. The
facile and efficient synthesis of compounds 3 facilitated fur-
ther investigations into the optical biaxiality of the meso-
phases.
Compound
Phase behavior
T [8C] (DH [kJmolꢀ1])
DS/R
[c]
3a
3b
3c
3d
Cryst!N!Isop.d.
Cryst!N!Isop.d.
Cryst!N!Isop.d.
Cryst!N!Iso
236.0 (50.74), 250.6 (0.74[b]
192.3 (45.50), 232.3 (0.73)
162.1 (36.54), 197.9 (0.88)
149.9 (45.24), 190.9 (1.20)
)
–
–
[c]
0.22
0.31
The molecular structure of compound 3a was determined
by single-crystal X-ray diffraction (Figure 2a). The molecu-
lar core of compound 3a is a rigid rectangular scaffold with
an average length and width of 23.4 and 8.5 ꢁ, respectively.
The alkyl chains are approximately oriented along the long
side of the core to furnish a prolonged molecular rectangle.
Along the long molecular axis, chain interdigitations from
adjacent molecules lead to the formation of an infinite 1D-
chain structure (Figure 2b). Along the short axis, intermo-
lecular p–p interactions are responsible for the sheet assem-
bly. The p–p interactions are likely to be quadrupolar and,
thus, should lead to a slipped-parallel arrangement of the
molecules, as in the case of compound 3a, or an arrange-
ment in which the minor axes are orthogonal. These sheets
pile to render the overall molecular packing with parallel
molecular planes, long axes, and short axes, respectively
(Figure 2c).
[a] Cryst, N, Iso, and Isop.d. denote crystalline, nematic, and isotropic
phases, and an isotropic liquid with partial decomposition, respectively.
For compounds 3a–3c, owing to thermal decomposition upon clearing,
the data were obtained from the first DSC heating run at 108Cminꢀ1. For
compound 3d, the data were obtained from the second DSC heating run
at 108Cminꢀ1. [b] The transition was accompanied by a severe thermal
decomposition. [c] The N!Iso transition was accompanied by severe de-
composition and the DS/R value could not be unambiguously deter-
mined.
followed by dark homeotropic textures prior to clearing,
were observed for all compounds. When approaching iso-
tropic temperatures, compounds 3c and 3d showed flicker-
ing dark/bright textures. The Schlieren textures of the nem-
atic phase of compounds 3a–3d that were sandwiched be-
tween glass slides showed solely two-brush disclinations
(Figure 3a,b) upon slow cooling or slow heating (ꢁ
It has been proposed by using molecular field theory that
Figure 3. Optical micrographs of a) compound 3c (heating 1838C) and
b) compound 3d (cooling 1608C), sandwiched between glass slides be-
tween cross-polarizers. Scale bar: 50 mm.
18Cminꢀ1). With increasing chain length, both the melting
and clearing temperatures decreased whilst a general trend
in increasing the mesophase range was observed, although
the mesophase ranges of compounds 3a and 3b could not
be unambiguously determined owing to severe thermal de-
composition prior to clearing. The nematic temperature
range of compound 3a is 158 and those for compounds 3b,
3c, and 3d are 408 or more. Compound 3d is the only com-
pound that did not suffer from thermal decomposition prior
to clearing and an enantiotropic nematic phase could be ob-
tained. The enantiotropic behavior of the nematic phase of
compound 3, contrary to the monotropic nematic behavior
of compound 1, can be attributed to the incorporation of bi-
phenyl groups for increasing intermolecular p–p interac-
tions, presumably electrostatic quadrupolar interactions to
lead to a slipped-parallel arrangement, similar to the pack-
ing pattern that is observed in the crystal structure. More-
over, the larger L/W and B/W ratios of compounds 3 than
those of compound 1 give rise to more-restricted rotations
along the long and short molecular axes to give enhanced
intermolecular p–p interactions.
Figure 2. Single-crystal X-ray diffraction studies: a) the molecular struc-
ture of compound 3a and packing diagrams normal to (b) and parallel to
the molecular plane (c).
the maximum shape biaxiality can be reached when the
biaxiality parameter (l) is equal to 6ꢀ1/2, which implies a spe-
cific length/breath/width (LBW) ratio of effectively
15:5:3.[12] From the single-crystal structure, the biaxiality pa-
rameter and LBW ratio of the core of compound 3a, assum-
ing that the thickness of the molecule is 4.5 ꢁ,[10] were deter-
mined to be 0.50273 and about 15:5.4:2.9, respectively,
which were close to their respective optimum values.
The mesogenic properties of compounds 3a–3d, which
were studied by a combination of polarizing optical micros-
copy (POM) and differential scanning calorimetry (DSC),
are summarized in Table 1. Upon heating, Schlieren or mar-
bled textures, which are indicative of a nematic mesophase,
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It has been predicted by
theory that the optimal bending
angle of the bent rods for maxi-
mum biaxiality is 109.58.[13]
When viewing compound 3 as
two bent rods that are fused at
the bending tips, the two types
of interarm bending angles are
approximately 608 and 1208,
which are significantly off from
the optimal value. The theory
predicts that such
a system
should show a uniaxial nematic
(Nu) phase with a biaxial nem-
atic phase, which will not be de-
tected by DSC, hidden below
the freezing point. According to
this theory, the biaxial nematic
phases of compounds 3c and
3d would not be observable.
For compounds 3c and 3d, no
transition was detected between
the Cryst!N and N!Iso tran-
sitions by POM and DSC. It
was also predicted by theory
that the Nb!Iso transition
should be second order[3,14] and
that the entropy change (DS/R)
should approach zero (when
Figure 4. Conoscopic interference figures with a) a l-plate inserted and b) a l/4 plate inserted for the nematic
phases of compounds 4 (668C), 3d (1768C), and 5 (808C) for the determination of the optical sign and optical
biaxiality of the nematic phase of compound 3d, respectively. The arrow denotes the rubbing direction.
l is the ideal value of 6ꢀ1/2) or be significantly smaller (when
l is away from the ideal value) than those for the Nu!Iso
transitions (ꢂ0.4). For compounds 3c and 3d, which have
minimal thermal decomposition, the DS/R values were 0.22
and 0.31, respectively; these values are not much smaller
than 0.4 and, thus, suggest that the nematic phases are not
biaxial. These values may have reflected the l value of com-
pounds 3 from the crystal structure, which was off the ideal
value.
The molecular biaxiality of cross-like compounds 3 and
the calamitic nature of the resulting nematic structure were
confirmed by single-crystal structure determination, where-
by the biaxial nematic molecular packing would rely on the
restricted rotation along the long and short molecular axes,
which may be understood by determination of the optical
sign. By conoscopic studies on the homeotropically aligned
sample with the insertion of a lambda retardation plate, the
optical sign of the nematic phases of cross-like compound
3d, calamitic compound 4,[15] and discotic compound 5[16]
were investigated and compared. Discotic compound 5 was
chosen for the conoscopic studies because it was the only
discotic compound that showed homeotropic alignment by
the alignment method in this study. As shown in Figure 4a,
for the optically positive nematic phase of compound 4, an
interference pattern of yellow vertices of the second/fourth
quadrants (quadrants of addition) and blue vertices of the
first/third quadrants (quadrants of subtraction) was ob-
served. For the optically negative uniaxial nematic phase of
compound 5c, oppositely placed colors of the vertices of the
quadrants were observed. The homeotropic alignment of the
nematic phase of compound 3d, which was achieved by
sandwiching the sample between rubbed poly(vinylalcohol)
(PVA)-coated glass slides that were separated by a spacer of
5 mm thickness, was confirmed by its indistinguishable dark
texture with the dark image of a blank cell under the same
POM conditions (see the Supporting Information, Fig-
ure S1). No birefringence was observed for the homeotropic
texture of compound 3d, which may imply that its nematic
phase is non-biaxial or that it consists of small biaxial nem-
atic domains that are isotropically distributed. For cross-like
compound 3d, the colors of the vertices resemble those of
optically negative discotic compound 5, although not as dis-
tinct (Figure 4b), thus suggesting that the large index of re-
fraction is parallel to the plane of the substrate.
For the nematic phase of compound 3d, its calamitic
nature implies that the chains are orientated along the long
axis of the molecular core and the negative optical sign may
imply that the rotations of the core along the long and short
molecular axes are limited. These results imply that the
nematic phase is a biaxial nematic phase.
Conoscopy was further utilized for the realization of opti-
cal axiality of the nematic phase of compound 3d. Prior to
the conoscopic studies, the homeotropic alignment of the
nematic phase of compound 3d was again confirmed. As
shown in Figure 4b, the interference pattern of the hyper-
bolic figures, that is, the open isogyres, of the homeotropi-
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Enantiotropic Nematics From Cross-Like Structures
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cally aligned nematic phase of compound 3d corroborates
its optical biaxiality, as opposed to those of cross-isogyres
for calamitic compound 4[15] and discotic compound 5,[16]
both of which are uniaxial.
It has been reported that the small-angle-signal splitting
of magnetically aligned nematics could originate from
a tilted organization in meta-cybotactic nematic phases,
which are composed of locally ordered smectic C (SmC)
The mesophases of compounds 3 were further elaborated
by X-ray diffraction (XRD) analysis (Table 2). As an exam-
ple, the 2D XRD pattern of the crystalline state of com-
pound 3d revealed multiple rings with sharp intensities (Fig-
ure 5a). When entering the mesophase, only two rings were
observed. The broader halo in the wide-angle regime (d-
spacingꢃ4.8 ꢁ) corresponds to the correlation of the
molten alkyl chains, thereby confirming the formation of the
mesophase. The wide-angle
Table 2. Powder X-ray diffraction data of compounds 3a–3d.
Compound
T [8C]
d-Spacing [ꢁ]
3a
3b
3c
3d
245
225
190
185
24.3, 4.9
26.2, 4.9
29.1, 4.8
33.6, 4.8
signal may also include the
inter-disc correlation along the
disc-normal but this fact could
not be verified. The sharper
signal in the small-angle region
corresponds to inter-core corre-
lations along the long molecular
axis. With increasing chain
length, the d-spacing of the
small-angle signals increases.
Under an applied magnetic
field (B) of 0.3 T normal to the
X-ray beam and capillary, the
two rings of the nematic phase
of compound 3d were no
longer circular and were cen-
tered orthogonally to each
other. In the wide-angle regime,
a pair of alkyl halos appeared
(Figure 5b), the intensities of
which peaked on the equator
(perpendicular to B). In the
small-angle region, two pairs of
sharp arcs were detected along
the meridian (Figure 5c). As
shown in the azimuthal scan
over 2q=2–48 (Figure 5d), the
splitting angles (Dc/2) were
nearly
identical
(43–448,
Table 3) within the nematic
temperatures and the splitting
becomes less distinct as the
temperature approaches the
clearing point. On further heat-
ing to the isotropic phase, the
pair of split arcs collapses into
two arcs with the maximum in-
tensities along meridian. Com-
pound 3c also exhibited split
small-angle scattering in the
nematic temperature range with
splitting angles (Dc/2) of 448
(Table 3; also see the Support-
ing Information, Figure S4).
Figure 5. Variable-temperature 2D XRD patterns of compound 3d: a) unaligned; b) aligned by a 0.3 T mag-
netic field; c) zoomed-in plots of the small-angle regions in (b); d) azimuthal scans of 2q=2–48 of (c) at 165,
175, 185, and 1908C.
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H.-F. Hsu, T.-L. Shih et al.
Table 3. Variable-temperature cluster size (Lj j and L⊥) and splitting
(Dc/2) of the small-angle XRD scattering in the nematic phase of mag-
netically aligned samples of compounds 3c and 3d.
Compound
B
[T]
T[a]
[8C]
d[b]
[ꢁ]
Dc
[8]
Dc/2
[8]
Lj j
[ꢁ]
L?
[ꢁ]
3d
0.3
165
175
185
175
185
170
180
190
180
185
35.3
34.7
34.0
33.8
33.6
31.7
31.6
29.9
29.2
29.1
85
85
87
–
43
43
44
–
62
62
61
64
64
40
44
35
42
42
8
7
7
10
10
7
8
8
10
10
0
–
–
3c
0.3
87
88
87
–
44
44
44
–
0
–
–
[a] Temperature on heating. [b] d-Spacing of the small-angle scattering.
Figure 6. Variable-temperature 1D X-ray diffractograms of magnetic-
field-aligned compound 3d (normalized to the same intensity of the
wide-angle signal); Inset: a zoomed-in plot of the small-angle signals.
clusters, and/or the biaxial cybotactic nematic phase.[10,17–20]
Herein, the term “meta-cybotactic phase” represents the
nematic phase of clusters that, in some literature reports,
are called cybotactic nematics and the term “cybotactic
phase” represents the normal nematic phase with very
short-range, non-specific, nearest-neighbor molecular pack-
ing, which, in some literature reports, is called classical cybo-
tactic or ordinary nematics.[10,17–20] Simulations have shown
that the form factor, that is, molecular structure, dominates
in biaxial cybotactic nematics, to exhibit temperature-inde-
pendent splitting, whilst the structure factor, that is, the cor-
relation of molecular packing, dictates in meta-cybotactic
nematics, to give temperature-dependent splitting.[21] More-
over, the splitting that is caused by the form factor will be
considerably larger than that caused by the structure
factor.[21] The temperature-independent splitting by the form
factor can be easily understood because rigid core structures
do not significantly distort as the temperature changes. On
the other hand, tilting of the smectic-like clusters in meta-cy-
botactic nematics may change upon variation in the temper-
ature. Experimentally, for bent-rod mesogens, Dc/2 in-
creased with increasing temperature within meta-cybotactic
nematics, whereas Dc/2 decreased within cybotactic nemat-
ics.[19] Moreover, for bent-rods nematics, the Dc/2 values in-
crease with shortening chain length in the meta-cybotactic
arrangement and stay roughly constant with changing the
chain length in cybotactic nematics.[10,19] For both com-
pounds 3c and 3d, the differences in the Dc/2 values in the
their nematic phase at different temperatures are within 18
(Table 3). However, the temperature dependence was only
investigated only over 20 K, whereas the trends in previous
reports were observed over much-wider temperature
ranges.[10,19] Moreover, the difference in chain length be-
tween compounds 3c and 3d is only two carbon atoms.
Therefore, the chain-length- and temperature dependence of
Dc/2 of compounds 3c and 3d may have not been truly re-
vealed by the data within the 20 K range that was investigat-
ed.
with respect to that of the wide-angle signal of compound 3
(Figure 6) raises the question of a field-induced meta-cybo-
tactic nematic arrangement.[18] It has been reported that, for
nematic phases, a much-stronger small-angle-scattering in-
tensity than the wide-angle-scattering intensity would sug-
gest a meta-cybotactic nematic phase.[20] In the XRD pattern
of the nematic phase of compound 3d at various tempera-
tures (Figure 6), the small-angle-scattering intensity is much
stronger than the wide-angle-scattering intensity. Quantita-
tively, meta-cybotactics can be distinguished by their signifi-
cantly larger cluster sizes, that is, considerably larger corre-
lation lengths with Lj j/dj j ꢂ2 and L?/d? ꢃ10 than those of
cybotactics, which only have nearest-neighbor correlation
with Lj j/dj j ꢃ2 and L?/d? ꢃ4.[20] For compound 3d, without
the applied magnetic field, the longitudinal and transverse
cluster sizes (Lj j and L?, respectively) by using Scherrer’s
equation[22] within the nematic temperature range at 165 and
1758C are about 55 (d=34.5 ꢁ) and 9 ꢁ (d=4.8 ꢁ), respec-
tively, without the applied magnetic field and about 62 ꢁ
(d=35.0 ꢁ) and 8 ꢁ (d=4.8 ꢁ), respectively, under the ap-
plied field (Table 3). For compound 3d, the applied magnet-
ic field results in a small change in both Lj j and L?: a small
increase in Lj j, and a small decrease in L?. The longitudinal
and transversal L/d values are both approximately 2, thus
suggesting that the nematic phase of compound 3d is (classi-
cal) cybotactic. Notably, cluster sizes that are determined by
using the Scherrer equation are only estimations and that,
when cluster sizes are translated into their corresponding
numbers of molecules (L/d+1), the difference will be less
significant. The respective cluster sizes of Lj j and L? in the
nematic phase of compound 3d correspond to an aggregate
of about three molecules in both the longitudinal and trans-
versal directions, that is, a short-range small cluster of 3ꢂ3ꢂ
3 molecules. The almost-field-independent cluster sizes indi-
cate that the magnetic field did not induce further clustering
of compound 3d. In summary, the cluster-size calculation
suggests that the clusters in the nematic phase of compound
3d are small, which suggests a classical cybotactic nematic.
The strong applied field that was used herein may induce
the formation of the meta-cybotactic nematic arrangement
and the relatively higher-intensity of the small-angle signal
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Similar results were found for compound 3c, but with small-
er cluster sizes (Table 3).
For cross-like molecules of compounds 3, three distinct
scatterings, owing to the length, width, and breadth correla-
tions, could most probably be indicative of a biaxial arrange-
ment. For the biaxial packing of cross-like compounds 3, the
scattering that is due to the length correlation is apparent
and those that are due to the other two dimensions are not
apparent. For compounds 3, the minimum achievable aver-
age thickness, that is, breadth, of compounds 3, by allowing
the rotational disorder of the biphenyl groups along the re-
spective sidearm axis, is about 4.5 ꢁ,[10] which is very close
to the d-spacing of the alkyl halo. Because the wide-angle
scattering is very diffuse in the nematic phases, the differen-
tiation of signals owing to molecular width and breadth
from the alkyl halo is not apparent for the nematic phase of
compound 3. However, when comparing the 1D diffracto-
grams of compound 3d in the nematic phase with that in the
isotropic phase (Figure 6), two very broad signals at 10.2 (2q
ꢃ10.28) and 3.2 ꢁ (2qꢃ23.98) can be observed that may be
attributed to the molecular width and breadth correlations,
respectively. The cross-like molecules in the nematic phase
are likely to be weakly associated to render the observed
weak and broad correlation along the width dimension.
More likely, the molecules can pack in a slipped-parallel
manner in the clusters, similar to the pattern that is ob-
served in the crystal structure, rather than in a side-by-side
manner, to show split small-angle scattering in the aligned
samples. Under a magnetic field, the nematic phase of com-
pounds 3c and 3d are composed of aligned small clusters,
that is, short-range molecular organization of about three
molecules, both along the long/short molecular axis and per-
pendicular to the molecular plane. Within each small cluster,
the molecules pack in an in-plane, slipped-parallel organiza-
tion along the molecular plane, thereby resembling the
SmC-like organization. Such organization will lead to split
small-angle scattering, as observed in Figure 5c.
Figure 7 shows the XRD setup and the proposed molecu-
lar packing of short-range SmC-like clusters in the nematic
phase of compounds 3 based on the 2D XRD results. Two
possible SmC-like cluster orientations are proposed (for de-
tailed analysis of 2D XRD and molecular packing, see the
Supporting Information, Figures S7, S8, and S9). Figure 7b
demonstrates the SmC-like cluster packing when the long
molecular axis is aligned along the magnetic field. Two alter-
native views of the structure of compound 3, as two tilted
rods that are fused at the center of the rods (Figure 7b, left
panel) or as two V-shaped mesogens that are fused at the
bending tips (Figure 7b: right panel), both render the same
2D XRD patterns (Figure 5c). In Figure 7c, the SmC-like
clusters are orientated so that one of the long rods in the
cross that is formed by fusing two tilted rods at the centers
is aligned along the magnetic field. Herein, the two types of
clusters with opposite tilting directions with respect to the
magnetic field are both present to lead to the obtained
XRD pattern.
Figure 7. a) XRD experimental setup, in which the sample is shown as
a three-cluster aggregate and b,c) the proposed molecular packing in
a SmC-like cluster. (b) A SmC-like cluster, which shows two alternative
views of the structure of compound 3 as two tilted rods that are fused at
the centers of the rods (left) or as two V-shaped mesogens that are fused
at the bending tips (right). Herein, the long molecular axis is aligned
along the magnetic field (B). (c) SmC-like clusters with compound 3; the
samples are viewed as two fused cross-rods and one of the long rods is
aligned along the magnetic field. In (b) and (c), short dashed lines repre-
sent the layer planes.
Conclusion
The theoretically predicted optimum length/breadth/width
ratio for maximizing shape biaxiality was investigated exper-
imentally by the successful synthesis of compound 3. The
wide temperature range of the nematic phase that was ex-
hibited by compound 3 enabled further biaxiality studies.
The thermal analysis data, DS/R, did not meet the expected
theoretical values for biaxial nematics. The conoscopy stud-
ies indicated the presence of restricted rotations along the
long- and short molecular axes within the molecular pack-
ing. Cluster-size analysis within the nematic phase of com-
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H.-F. Hsu, T.-L. Shih et al.
pounds 3c and 3d indicated that their nematic phase was
NcybC and the split small-angle 2D XRD patterns of magnet-
ic-field-aligned samples suggested an approximate 448 tilting
of the molecules within the clusters. Moreover, the enantio-
tropic nematic behavior of cross-like compound 3 was ob-
served without the detection of other higher-order meso-
phases; this result is in contrast to the nematic phases of
bent-core mesogens, which face potential competition from
transitions into higher-order LC phases and crystallization.
Contrary to numerous previously reported attempts to gen-
erate enantiotropic nematics from board-like- and cross-like
molecules, the results reported herein should evoke a renais-
sance in interest in the investigation of biaxiality on board-
like or cross-like nematogens. The wide-temperature-range
nematic phases of compound 3 enable the molecular skele-
ton to act as an alternative skeleton to bent-rod nematogens
for future biaxial investigations. Further structural modifica-
tion to afford lower melting points, expanding the nematic
temperature range, and to induce biaxiality, for example by
the incorporation of molecular biaxiality,[7] will be pursued
in the near future.
Acknowledgements
We thank the National Science Council of Taiwan for supporting this
work.
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Experimental Section
All reactions were conducted under an atmosphere of nitrogen in oven-
dried glassware by using standard Schlenk techniques. THF was distilled
over sodium/benzophenone. All other solvents and chemicals were re-
agent grade (Aldrich Chemical Co.) and used without further purification
unless otherwise described. 1H and 13C NMR spectra were recorded on
a Bruker Avance 600 spectrometer. Chemical shifts are reported in ppm
relative to residual CHCl3 (1H: d=7.26 ppm; 13C: d=77.0 ppm). Absorp-
tion spectra were recorded on a Jasco V-550 spectrometer. Photolumines-
cence spectra were recorded on a Hitachi F-2500 fluorescence spectro-
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a PerkinElmer Pyris1 with heating and cooling rates of 5 and 108C
minꢀ1, respectively. Polarized optical microscopy (POM) was carried out
on a Nikon Eclipse E600 POL with a Mettler FP90/FP82HT hot stage
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tween two rubbed glass slides that were separated by a 5 mm spacer.
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l=(3/2)1/2
L
(BꢀW)/[L
E
(1)
(2)
l=6ꢀ1/2 ꢃ0.40825 when Bꢀ1 =(Wꢀ1Lꢀ1)/2
However, the LBW ratio of 5:3:1 in Equation (1) gives l=0.87482.
The best LBW ratio (15:5:3) is calculated herein.
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programmable by a PC with a PID feedback system. The 2D XRD pat-
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a Heraeus CHN-O-Rapid Analyzer at the NSC Regional Instrumental
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Enantiotropic Nematics From Cross-Like Structures
FULL PAPER
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Received: August 19, 2011
Revised: April 14, 2012
Published online: && &&, 0000
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H.-F. Hsu, T.-L. Shih et al.
Take up your cross: An enantiotropic
nematic phase with a wide tempera-
ture range has been achieved by using
cross-like mesogens. Biaxiality investi-
gations by using conoscopic and 2D
XRD studies imply the formation of
small smectic C-like meta-cybotactic
clusters in the nematic arrangement.
Arenes
H.-H. Chen, H.-A. Lin, Y.-H. Lai,
S.-Y. Lin, C.-H. Chiang, H.-F. Hsu,*
T.-L. Shih,* J.-J. Lee, C.-C. Lai,
T.-S. Kuo ...................... &&&& — &&&&
Enantiotropic Nematics From Cross-
Like 1,2,4,5-Tetrakis(4’-Alkyl-4-Ethy-
nylbiphenyl)Benzenes And Their Biax-
iality Studies
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