Liquid-crystalline heterodimesogens and ABA-heterotrimesogens comprising a bent 3,5-diphenyl-1,2,4-oxadiazole central unit

Article abstract of DOI:10.3762/bjoc.8.54

Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4- oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4- thiadiazole core, connected by flexib

Full text of DOI:10.3762/bjoc.8.54

Liquid-crystalline heterodimesogens and
ABA-heterotrimesogens comprising a bent
3,5-diphenyl-1,2,4-oxadiazole central unit
Govindaswamy Shanker, Marko Prehm and Carsten Tschierske*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 472–485.
Institute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle/Saale, Germany,
Tel: ++49 (0) 345 55 25664, Fax: ++49 (0) 345 55 27346
doi:10.3762/bjoc.8.54
Received: 09 December 2011
Accepted: 07 March 2012
Published: 30 March 2012
Email:
Carsten Tschierske* - carsten.tschierske@chemie.uni-halle.de
This article is part of the Thematic Series "Progress in liquid crystal
chemistry II".
* Corresponding author
Keywords:
Guest Editor: S. Laschat
bent-core mesogens; cybotactic nematic phases; dimesogen; liquid
crystals; 1,2,4-oxadiazoles; trimesogen
© 2012 Shanker et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Three new types of terminally connected ABA-heterotrimesogens and heterodimesogens, composed of a bent 3,5-diphenyl-1,2,4-
oxadiazole central unit and one or two rod-shaped 4-cyanobiphenyl cores or one 2-phenyl-1,3,4-thiadiazole core, connected by
flexible spacers, have been synthesized, and their mesomorphic behavior was studied by optical polarizing microscopy (PM),
differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All dimesogens exhibit broad ranges of cybotactic nematic
phases (NcybA and NcybC), in some cases accompanied by additional mesophases (CybA or SmC) at lower temperature. The
combination of the 3,5-diphenyl-1,2,4-oxadiazole unit with one cyanobiphenyl core leads to the removal of tilted smectic and
cybotactic nematic phases (SmC, NcybC), which are replaced by the nontilted CybA phases and nematic phases composed of
SmA-type clusters (NcybA). The orthogonal cybotactic nematic phases of bent-core mesogens are of special interest for achieving
biaxial nematic phases of the orthorhombic type. The orthogonal (NcybA) and skewed (NcybC) cybotactic nematic phases were
distinguished by XRD and optical observations.
Introduction
Liquid-crystalline (LC) dimers of low-molar-mass compounds mesogenic dimers (homodimesogens), composed of identical
formed by the coupling of two mesogenic segments are of mesogenic units, and heterodimesogens combining different
contemporary interest [1-4]. These compounds exhibit fasci- types of units, such as two distinct rod-like units [4], or
nating properties, often different from the single mesogens, and combining rod-like with discotic [1,9,10], phasmidic or bent-
they can also be considered as model compounds for main- core units, linked together through flexible chains [1]. These
chain LC polymers [5-8]. Dimesogens can be categorized into flexible chains, which in most cases represent linear alkyl
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Beilstein J. Org. Chem. 2012, 8, 472–485.
chains, connect the distinct mesogenic units, but depending on The 3,5-diphenyl-1,2,4-oxadiazole segment has recently
their length and degree of flexibility they can also decouple the attracted significant attention as a central building block for
segmental motions of the two interconnected mesogenic units, bent-core LC molecules (angle ~140°) [59-63], due to the ferro-
and hence these chains are also assigned as spacers. The prop- electric-like polar switching observed in the nematic phases of
erties of these dimesogens strongly depend on the topology of some of these compounds under applied electric fields [64,65].
connection [11], the spacer length and spacer parity, resulting in However, these nematic phases were only observed at high
linear and bent [1,3], T-shaped [12-14], Y-shaped [15] or temperature, which makes their investigation difficult.
H-shaped molecules [12,13,16-19]. Moreover, combinations of
mesogenic units differing not only in shape [20-22], but also in Herein, three types of compounds combining the bent
polarity or compatibility of the mesogenic units [1-4,23], and 3,5-diphenyl-1,2,4-oxadiazole unit terminally with one or two
dimesogens incorporating chiral [24-26] segments often result rod-like mesogenic units through flexible spacers are reported
in unique interesting properties, which are fundamentally (Scheme 1). The trimesogens CB-Ox-CB/n combine the
different from the individual mesogens.
3,5-diphenyl-1,2,4-oxadiazole unit with two nematogenic
cyanobiphenyl (CB) units (Scheme 2), in the dimesogens
Recent interest in dimesogens has been focused on combina- CB-Ox/n only one CB unit is incorporated, and in compounds
tions of two rod-like molecules through relatively short odd- Thia-Ox/n a 3-heptyl-5-phenyl-1,3,4-thiadiazole unit is incor-
numbered spacer units to obtain dimesogens with an overall porated (Scheme 3). All compounds form nematic phases over
bent shape [27-31] exhibiting properties that are characteristic wide temperature ranges, in some cases accompanied by add-
for bent-core mesogens, such as polar ferroelectric or antiferro- itional nontilted (CybA) or tilted (SmC) mesophases at lower
electric switching LC phases [32-35] and spontaneous achiral temperatures.
symmetry breaking [34-36]. Also dimesogens combining two
Results and Discussion
bent-core units terminally [37-43] or laterally [44], or
combining a bent-core segment with a rod-shaped mesogen Synthesis and characterization
[40,45-54], and related end-to-end connected ABA-heterotrime- The dimesogens and trimesogens were obtained by following
sogens combining a bent-core segment with two rod-like units the synthetic protocols described in Scheme 2 and Scheme 3 by
[55], or a rod-like core with two bent-core units [56], were DCC coupling of the ω-mesogen-functionalized alkanoic acids
synthesized and studied. These compounds are of interest with 2 [66] and 4 [66] with 4-[(4-hydroxyphenyl)-1,2,4-oxadiazol-5-
respect to the potential biaxiality of the nematic (N) [45,50,57] yl]phenyl 4-hexylbenzoate (3) [67] and 3,5-bis(4-hydroxy-
and nontilted smectic phases of these compounds [45]. Bent- phenyl)-1,2,4-oxadiazole (1) [65], respectively. The syntheses
core units have also been combined with disc-like units, but in of the acids 2 and 4, and the 1,2,4-oxadiazole substituted
this case the incompatibility and steric mismatch of the distinct phenols 1 and 3 were reported previously in the cited refer-
units led to a loss of LC properties [58].
ences. The final compounds were purified by crystallization
Scheme 1: Structures of the investigated ABA-heterotrimesogens CB-Ox-CB/n and heterodimesogens CB-Ox/n, Thia-Ox/n.
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Beilstein J. Org. Chem. 2012, 8, 472–485.
Scheme 2: Synthesis of the ABA-heterotrimesogens CB-Ox-CB/n.
Scheme 3: Synthesis of the dimesogens CB-Ox/n and Thia-Ox/n.
from ethyl acetate/ethanol mixtures. All compounds represent synthesized di- and trimesogens exhibit liquid-crystalline
the first examples in their classes and their molecular structure behavior.
was probed by 1H-, 13C NMR and microanalytic techniques
(see Experimental).
Trimesogens CB-Ox-CB/n
The trimesogens CB-Ox-CB/n comprising rod-shaped CB
segments on either side of the 2,5-diphenyl-1,2,4-oxadiazole
unit exhibit exclusively nematic phases (N). Upon cooling of
Investigation of the liquid-crystalline behavior
Methods
The obtained di- and trimesogens were investigated by PM these compounds from the isotropic liquid phase, Schlieren
(Optiphot 2, Nikon) in conjunction with a heating stage textures were observed, which are evidence of nematic phases
(FP82HT, Mettler) and differential scanning calorimetry (DSC, [68,69]. These trimesogens have very high clearing tempera-
DSC-7, Perkin-Elmer). The assignment of the mesophases is tures at around T = 320 °C for compound CB-Ox-CB/3 and at
based on the combined results of optical textures and X-ray T = 245 °C for CB-Ox-CB/4. The major reason for the much
diffraction (XRD) studies. XRD investigations on aligned higher nematic phase stability of CB-Ox-CB/3 compared to the
samples were performed by using a 2D wire detector (HI-Star, higher homologue may be the result of decreased chain decou-
Siemens AG). Alignment was achieved either in thin capillaries pling of the mesogenic units by the shorter spacers and the fact
under a magnetic field (B = 1 T) or by slow cooling of a small that the spacers are even numbered (considering the ether
drop of the sample on a glass substrate; the incident X-ray beam oxygens and the COO groups as parts of the spacers), which
was in this case nearly parallel to the glass plate. The transition gives a more linear coupling of the mesogenic units. XRD
temperatures, transition enthalpies and observed phase types are measurement of CB-Ox-CB/4 confirms the N phase by the
collated in Table 1. It is apparent from this table that all of the presence of two diffuse scattering peaks with maxima at
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Beilstein J. Org. Chem. 2012, 8, 472–485.
Table 1: Phase-transition temperature (T/°C) and associated enthalpy values (in square brackets, ΔH/kJ mol−1) of the synthesized di- and trimeso-
gens.a
Compound
Phase transition on heating
Phase transition on cooling
—b
CB-Ox-CB/3 Cr 187
[29.2]
CB-Ox-CB/4 Cr 164
[38.1]
Cr 162
[40.1]
Cr 130
[36.2]
Cr 144
[41.6]
Thia-Ox/10 Cr1 132
[84.7]
N
320
Iso
(dec)
N
246
Iso
Iso
—b
Iso
Iso
Iso
242
[−3.5]
N
123
[−25]
Cr
[4.2]
CB-Ox/3
CB-Ox/4
Thia-Ox/5
CybA
CybA
SmC
Cr2
210
[<0.01]
NcybA
NcybA
NcybC
NcybC
302
(dec)
Iso
Iso
Iso
Iso
148
[<0.01]
255
[1.7]
251
[−1.9]
NcybA
NcybC
NcybC
146
[<0.01]
CybA
SmC
Cr
84
[−15.1]
Cr
Cr
173
[0.4]
224
[2.9]
223
[−2.1]
172
[−0.7]
114
[−37.2]
137
178
[0.9]
174
123
[33.6]
[−1.0]
[53.4]
aPeak temperatures in the DSC thermograms obtained during the first heating and cooling cycles at 10 K/min; abbreviations: Cr = crystalline solid; Iso
= isotropic liquid, N = nematic LC phase; NcybA = cybotactic nematic phase formed by small SmA-type (nontilted) cybotactic clusters; NcybC = cybo-
tactic nematic phase formed by small SmC-type (tilted) cybotactic clusters; CybA = LC phase formed by extended SmA-type clusters; SmC = smectic
C phase; dec = decomposition. bDue to decomposition at the N–Iso transition no cooling curve could be recorded.
d-values of 1.5 nm (very weak) and 0.46 nm, assigned to the Dimesogens CB-Ox/n
average longitudinal distances and the average lateral distances The dimesogens CB-Ox/3 and CB-Ox/4, having a polar group
between the molecules, respectively (Figure 1). The longitu- at one end and an n-hexyl tail at the other, exhibit enantiotropic
dinal distance is only about one third of the total molecular CybA–N phase sequences. Similar to the trimesogens CB-Ox-
length in the most extended conformation, Lmax = 4.6 nm, and CB/n, a strong influence of spacer length and spacer parity on
hence it is most probably determined by the average length of the phase-transition temperatures for these dimesogens is also
the three individual aromatic cores. The absence of sufficiently observed. The shorter compound CB-Ox/3 with even-numbered
long aliphatic segments and also the electrostatic interaction spacer (O(CH2)3COO) has much higher temperatures of the
between the terminal CN groups and the aromatics seem to be CybA–N and N–Iso transitions. Upon cooling a sample of
responsible for the absence of any smectic phase and the CB-Ox/4 from the isotropic liquid, the nematic phase forms
complete mixing of the aromatics in the nematic phase. The low with a typical highly birefringent Schlieren texture (Figure 2b)
intensity of the diffuse small-angle scattering, due to the low [68]. This indicates a predominately homogeneous alignment
electron-density contrast of these molecules with relatively (director n on average parallel to the substrate surface) of the
short spacers does not, in this case, allow further conclusions sample. On reducing the temperature below 146 °C a change of
about the details of the structure of these nematic phases to be the texture is observed upon which the Schlieren texture
drawn.
changes into a multidomain texture composed of small fan-like
Figure 1: XRD pattern of a (partially) surface-aligned sample of the N phase of compound CB-Ox-CB/4: (a) diffraction pattern at 170 °C; (b) θ-scan at
170 °C.
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Beilstein J. Org. Chem. 2012, 8, 472–485.
XRD patterns of the nematic phase of compound CB-Ox/4,
obtained with a magnetically aligned sample (B = 1 T), show
diffuse scattering in the wide-angle region with two crescent-
like maxima at d = 0.46 nm centered on the equator (Figure 3a).
In the small-angle region a second diffuse scattering peak with a
maximum at d = 4.23 nm (T = 150 °C) has clear maxima on the
meridian of the diffraction pattern. This indicates an arrange-
ment of the molecules with their long axes parallel to the
magnetic field direction. The intensity of this small-angle
scattering is higher than that of the wide-angle scattering
(Figure 3c), which confirms the presence of cybotactic clusters
with short-range smectic order [69-74]. Because the maxima of
the diffuse peaks in the wide- and small-angle regions are
perpendicular to each other the cybotactic clusters are of the
nontilted SmA type (NcybA) [57,75,76]. The transversal period-
icity in these cybotactic clusters (d = 4.23 nm) is comparable to
the molecular length Lmol = 4.5–4.6 nm in the most stretched
conformation (determined with CPK models). This indicates a
kind of monolayer organization of the molecules in the cybo-
tactic clusters. In the SmA-like clusters the alkyl chains are
segregated from the aromatic cores, and the aromatics are orga-
nized on average antiparallel to each other with complete inter-
calation of the aromatic units, i.e., the cyanobiphenyls and
diphenyl-1,2,4-oxadiazole units appear not to be segregated
(Figure 4a). In the aliphatic regions the alkyl chains are inter-
digitated and conformationally disordered. The size of the SmA
type clusters can be estimated from the width of the small-angle
scattering at the peak half maximum. The correlation length in
the nematic phase at T = 150 °C, estimated according to
ξ
= 2/Δq from the full width at half maximum (Δq) [77], in
the longitudinal direction is ξ = 5.3 nm and in the transversal
direction is ξ = 1.8 nm. Hence, the dimensions of the cybo-
tactic clusters (L ) can be approximated to L
= 3ξ
[78],
leading to the values L = 16 nm and L = 5 nm. Accordingly,
the clusters are relatively large, composed on average of about
3–4 layers, and about 11 molecules are arranged in the cross
section.
Figure 2: Dimesogen CB-Ox/4: (a) DSC traces obtained during initial
heating and cooling cycles scanned at a rate of 10 K min−1; (b,c)
textures as seen between crossed polarizers for a homogenously
aligned sample; (b) Schlieren texture of the NcybA phase at T = 245 °C;
(c) polydomain texture of the CybA phase at T = 135 °C.
On reduction of the temperature further, there is a significant
change in the XRD pattern, indicating a phase transition. The
domains (Figure 2c). This texture remains unaltered down to a diffuse scattering on the meridian becomes sharper and strongly
temperature of 84 °C, below which crystallization takes place. rises in intensity (Figure 3c). In addition, a weak second-order
Shearing the sample gives rise to reorganization with reflection becomes visible (Figure 3b). The maximum of the
homeotropic alignment, which appears completely dark small-angle scattering is slightly shifted from d = 4.23 nm in the
between crossed polarizers, and no birefringence occurs in this NcybA phase at T = 150 °C to d = 4.31 nm in the low-tempera-
homeotropically aligned sample until crystallization sets in. ture phase at T = 130 °C (Figure 3c). The diffuse outer scat-
This indicates the formation of a uniaxial mesophase, but tering remains completely diffuse, confirming the absence of
surprisingly, no enthalpy change is associated with this tran- any in-plane order. As the maxima of the scattering peaks in the
sition as observed in the DSC traces (Figure 2a). Similar wide- and small-angle regions are perpendicular to each other,
features can be observed for the shorter homologue CB-Ox/3 the molecules are on average nontilted. Because the position of
(see Table 1).
the small-angle scattering is not significantly shifted, the orga-
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Beilstein J. Org. Chem. 2012, 8, 472–485.
Figure 3: XRD data of the dimesogen CB-Ox/4: (a,b) diffraction patterns of a magnetic-field-aligned sample (the direction of the magnetic field is
shown as a white arrow), the insets show the scattering in the small-angle region: (a) NcybA at 150 °C; (b) CybA at 130 °C; (c) θ-scans at 150 °C and
130 °C in the small-angle regions of both the NcybA and the CybA phase.
nization of the molecules should be identical to the model seems not to be truly long-range. It appears that for this reason
proposed for the SmA-type clusters forming the NcybA phase there is no measurable transition enthalpy for this phase tran-
(Figure 4a). This means that at this phase transition the molec- sition (see DSC traces in Figure 2a) as would be expected for a
ular organization does not change fundamentally, but instead transition to a SmA phase, and hence this phase is tentatively
the short-range SmA-like clusters simply become fused to much assigned as CybA. Related tilted [69] and nontilted phases [79]
larger clusters. The correlation length in this phase (T = 130 °C) with structures intermediate between the Ncyb phases and undis-
was estimated to 27 nm in the longitudinal direction and 23 nm torted smectic phases have been observed previously and seem
in the transversal direction. This is somewhat smaller than to represent a typical phenomenon for molecules at the border-
usually observed for typical SmA phases. Hence, although the line between a bent-core and rod-like shape.
correlation length is significantly increased, the smectic order
Dimesogens Thia-Ox/n
The dimesogens Thia-Ox/5 and Thia-Ox/10 have a molecular
structure that is quite distinct from that of compounds
CB-Ox/n. Firstly, they have alkyl tails at both ends, and
secondly, they incorporate a 1,3,4-thiadiazole unit, which is not
strictly linear (bending angle 162°) [80] and within which the
direction of the major dipole moment is perpendicular to the
molecular long axis [80-82]. Finally, the spacer units of these
thiadiazoles are also significantly longer (n = 5, 10) than those
used for the related cyanobiphenyl-containing dimesogens
(n = 3, 4). As a result, LC phases with a tilted organization of
the molecules become dominant. The dimesogen Thia-Ox/5,
with the shorter spacer, displays a SmC–NcybC dimorphism and,
remarkably, the SmC phase is removed for the longer homo-
logue Thia-Ox/10.
A N-to-SmC transition is observed at T = 172–173 °C for the
dimesogen Thia-Ox/5 on reduction of the temperature
(Figure 5) [68]. In contrast to the NcybA–CybA-transition, for
which no transition enthalpy could be detected (see Figure 2a),
Figure 4: Models showing the suggested organizations of dimeso-
gens in the smectic phases and in the preferred local structure in the
cybotactic clusters of the related nematic phases: (a) monolayer struc-
a small but clearly visible enthalpy change is observed for the
NcybC–SmC transition of Thia-Ox/5 (Figure 5a). Under
ture (d ~ L); (b) intercalated structure (d ~ ½ L). The molecular tilt and
the bent-shape of the mesogenic units are not considered.
homeotropic boundary conditions (Figure 5c) the texture of the
low-temperature phase is a Schlieren texture, indicating a
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Beilstein J. Org. Chem. 2012, 8, 472–485.
Figure 5: Dimesogen Thia-Ox/5: (a) DSC traces obtained during first heating and cooling cycles scanned at a rate of 10 K min−1; (b,c) textures as
observed between crossed polarizers, (b) N phase having a Schlieren texture at T = 220 °C and (c) SmC phase at T = 160 °C under homeotropic
boundary condition; (d) N phase and (e) SmC phase under homogeneous boundary conditions; the same temperatures as (b,c), observed in poly-
imide-coated cells, 6 μm.
biaxial smectic phase. The smectic phase was investigated by phase. It is split into maxima located beside the meridian, indi-
XRD of a surface-aligned sample. The 2D XRD profile cating a tilted organization of the molecules in the layers
(Figure 6a) has a sharp Bragg peak (d = 4.4 nm) with its (Figure 6b). From the positions of the diffuse wide-angle scat-
second-order reflection located on the meridian, confirming tering peaks a tilt angle of approximately 25° can be estimated.
a well-defined layer-like organization. In the wide-angle However, it is not possible to decide from the scattering pattern
region there is a diffuse scattering peak with a maximum at whether there is a synclinic or anticlinic arrangement of mole-
d = 0.46 nm providing evidence for a liquid-like ordered LC cules in adjacent layers, but the relatively high birefringence of
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Beilstein J. Org. Chem. 2012, 8, 472–485.
the texture (Figure 5c) is more in line with a synclinic organiza- disorder. Hence, it is in line with a monolayer structure of the
tion. Moreover, as is typical for synclinic tilted SmC phases, SmC phase in which the aromatic cores and the relatively short
domains with different director orientation can be observed in spacer units form one type of layer, which is separated by the
homogeneously aligned samples (Figure 5d and Figure 5e), layers formed by the aliphatic end-chains (Figure 4a). In the
indicating an optical tilt corresponding to the XRD tilt.
layers of the core units, the 3,5-diphenyl-1,2,4-oxadiazole-,
2-phenyl-1,3,4-thiadiazole and the relatively short C5 spacer
units are mixed, leading to an antiparallel organization of the
molecules on average. In the NcybC phases the fundamental
structure should be retained but becomes a short-range local
structure in the cybotactic clusters.
Compound Thia-Ox/10, with a much longer spacer unit than
Thia-Ox/5, has exclusively a nematic phase and no transition to
a SmC phase could be observed optically on cooling the sample
down to 123 °C, at which point crystallization starts. The XRD
pattern of a magnetically aligned sample (B = 1 T) of this
nematic phase shows a crescent-like diffuse wide-angle scat-
tering peak centered on the equator (Figure 7a). In the small-
angle region a diffuse scattering peak with a maximum at
d = 2.5 nm (T = 150 °C) is extended to a line parallel to the
equator. The χ-scan over this scattering is split into two maxima
located beside the meridian (Figure 7b). This indicates the pres-
ence of SmC-type cybotactic clusters with a tilt (β) of the mole-
cules of about β = Δχ/2 = 37–39°. The estimated cluster size is
L = 5 nm and L = 1.4 nm, corresponding to about two layers
of aromatic cores and about 3 × 3 molecules in the transversal
directions. Hence the size of these clusters is significantly
smaller than for the NcybA phase of compound CB-Ox/4. The
d-value of the small-angle scattering maximum (d = 2.5 nm)
is much smaller than the molecular length (Lmol = 5.9 nm)
in the most stretched conformation. Considering the significant
tilt the effective molecular length, calculated according to
Leff = d/cos β = 3.2 nm, corresponds to 0.54 Lmol. This small
value, close to half the molecular length Lmol, suggests that in
the cybotyctic clusters there is a mixed side-by-side packing of
the individual aromatic units, separated by the aliphatic
domains composed of the mixed terminal chains and C10 spacer
units (“intercalated” structure, see Figure 4b) [3,4].
Figure 6: XRD data of the SmC phase of the dimesogen Thia-Ox/5:
(a) diffraction pattern at T = 160 °C; (b) χ-scans over the diffuse scat-
tering in the wide-angle region (black line: 2θ = 15–25°) and the 01
reflection (blue line: 2θ = 1–3°) at 160 °C; the lower part of the diffrac-
tion pattern is shaded by the heating stage, therefore, the intensity
below the equator is diminished.
Hence, for the dimesogens Thia-Ox/n, elongation of the
spacers leads to a change of the molecular organization as indi-
The nearly equal distribution of the diffuse scattering intensity cated by a strong change of the d-spacing. Whereas in the case
on the left and the right of the XRD pattern (Figure 6a) can be of Thia-Ox/5 the rigid cores together with the C5 spacers are
explained with a multidomain structure of the sample with organized side-by-side and separated from the alkyl chains in
nearly equal contributions of the distinct tilt directions. The the monolayer smectic phases and in the cybotactic clusters of
layer spacing of d = 4.4 nm is smaller than the length of the the nematic phase (Figure 4a), in the case of compound
molecule (Lmol = 5.3 nm in the most stretched conformation). Thia-Ox/10 the longer C10 spacers cannot be accommodated
Considering the tilt, a calculated d-value of dcal = 4.8 nm would between the aromatic cores. These long spacers preferably mix
be expected according to dcal = Lmol cos β for the molecule in with the terminal alkyl chains, and hence only the aromatics are
the most stretched conformation and a reduction to the experi- organized side-by-side and the two distinct cores are mixed
mental value of d = 4.4 nm is easily possible by conformational randomly (Figure 4b). Due to the significant mismatch of the
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Beilstein J. Org. Chem. 2012, 8, 472–485.
ordered organization in the SmC-type cybotactic clusters
forming the NcybC phase. Hence, for the thiadiazoles
Thia-Ox/n, elongation of the alkylene spacer unit connecting
the two mesogenic cores removes the SmC phase, which is
contrary to the usually observed effect that elongation of alkyl
chains stabilizes smectic phases due to improved segregation.
The promotion of nematic phases by elongation of an aliphatic
spacer unit was previously observed for dimesogens combining
two rod-like segments [3,4,83]. As a general rule, for this type
of dimesogen the terminal chains must be longer than half the
spacer length to form smectic phases [3,4]. According to this
rule, the formation of smectic phases should in principle be
possible for both compounds, Thia-Ox/5 and Thia-Ox/10. It
appears that the very different lengths of the two mesogenic
units and probably also the bent shape of these units contribute
additionally to destabilization of the smectic phase.
Although these two thiadiazole-containing dimesogens only
form tilted LC phases (SmC, NcybC), for Thia-Ox/5 with a rela-
tively short spacer the tilt is significantly reduced (25°)
compared to the related 2,5-diphenyl-1,2,4-oxadiazoles without
an attached rod-like unit (40–50°) [64,65]. In the dimesogen
Thia-Ox/10, in which the two mesogenic units are more decou-
pled, the reduction of the tilt is much less (37–39° tilt). Hence,
not only the type of rod-like mesogen, but also the degree of
coupling of the rod-like units to the 3,5-diphenyl-1,2,4-oxadia-
zole core, seems to be important for the degree of tilt.
Distinguishing skewed and orthogonal cybotactic
nematic phases
Figure 7: XRD pattern of a magnetic-field-aligned sample of the NcybC
phase of the dimesogen Thia-Ox/10: (a) diffraction pattern at
T = 150 °C (inset = small-angle scattering); (b) χ-scan over the small-
angle scattering (2θ = 2.5–5.0°). Besides the scattering at d = 2.5 nm
there is a second very weak scattering with a maximum at a smaller
θ-angle, corresponding to approximately twice the d-value. This would
indicate a monolayer structure in the cybotactic clusters, similar to that
described for Thia-Ox/5. It is unlikely that the much more intense scat-
tering at a higher d-value represents the second order of the weak
reflection. A possible explanation could therefore be a heterogeneous
structure of the sample, probably provided by the interaction of the
cybotactic clusters with the surface of the capillary, in which the
majority represents the intercalated structure described in the text and
the minority is formed by a monolayer structure as found for
Thia-Ox/5. The proposed formation of a SmC surface layer is basi-
cally in line with texture observations made for compound Thia-Ox/5
(see Figure 8c,d) in which no significant change of the texture is found
at the NcybC-to-SmC transition. It could be expected that the energetic
difference between these two distinct types of layer structures is small,
and hence variation of the cluster size under the influence of surface
interactions could have an effect on the structure.
Usually the two distinct types of cybotactic nematic phases,
NcybC and NcybA, can be distinguished by analyzing the shape
of the diffuse small-angle X-ray scattering of magnetically
aligned samples, which is split into two maxima beside the
meridian for NcybC phases (Figure 8a and Figure 8b) and is
nonsplit and centered on the meridian for NcybA phases
(Figure 3a). During investigation of compounds Thia-Ox/5 and
CB-Ox/4 we found that there are also some differences in the
optical textures of NcybC and NcybA phases. On cooling from
the isotropic liquid state, the textures occur as highly birefrin-
gent Schlieren textures (Figure 8a and Figure 8e) in both cases.
On further cooling, the texture changes to a much less birefrin-
gent appearance for both compounds, attributed to an anchoring
transition in which the direction of the molecules changes from
parallel to the surface (homogeneous) to perpendicular or tilted
with respect to the surface (homeotropic) (Figure 8b and
core length of the two mesogenic units, there is some unfavor- Figure 8f) [52]. This anchoring transition, which depends on the
able overlapping of alkyl chains and aromatics and therefore the LC material as well as on the properties of the surfaces, could
layers are not well developed (see Figure 4b). Hence, a long- be influenced by temperature-dependent changes of the dielec-
range smectic ordering cannot develop upon decreasing the tric anisotropies and steric surface–molecule interactions
temperature, and the molecules only retain an orientationally [52,84], but, as proposed previously, it is most probably also
480

Beilstein J. Org. Chem. 2012, 8, 472–485.
related to the size of the cybotactic clusters forming the nematic birefringent (colorful) textures, as shown in Figure 8a and
phase [69]. At high temperature the clusters are small, and in a Figure 8e. As the cluster size grows with decreasing tempera-
homogeneous cell the alignment is determined by the organiza- ture the preferred orientation becomes that with the “layer
tion of the mesogenic cores parallel to the surface, giving highly planes” of the cybotactic smectic clusters being parallel to the
Figure 8: Comparison of the optical textures of distinct types of 1,2,4-oxadiazole based dimesogens as observed between nontreated glass plates
(Menzel cover glasses, Menzel GmbH, Braunschweig) between crossed polarizers: (a–d) compound Thia-Ox/5 at (a) T = 206 °C; (b) T = 197 °C;
(c) T = 178 °C; (d) T = 173 °C NcybC-to-SmC transition, the dotted line indicates the phase boundary; (e–h) compound CN-Ox/4 at (e) T = 254 °C;
(f) T = 232 °C; (g) T = 178 °C; (h) T = 150 °C (at T = 146 °C in the CybA phase all birefringence has disappeared).
481

Beilstein J. Org. Chem. 2012, 8, 472–485.
cell-surface. In this homeotropic alignment the birefringence is could be expected for cybotactic nematic phases composed of
significantly reduced and the textures appear either completely tilted SmC clusters in which the biaxiality is additionally influ-
dark (molecules on average perpendicular to the surface, see enced by the tilt of the molecules. Therefore, the rarely occur-
Figure 8g and Figure 8h) or retain a low birefringence and then ring NcybA phases [75,76] are of significant interest in the
appear with gray Schlieren texture (molecules are tilted with search for biaxial nematic phases [57]. It is important to note
respect to the surface, Figure 8c and Figure 8d). Due to surface here that the nematic phases of some 3,5-bis(4-hydroxyphenyl)-
interactions, the cybotactic clusters can in some cases become 1,2,4-oxadiazole bis(4-alkyloxybenzoates) have been identified
aligned at the surfaces even in the nematic phase by forming as ferroelectric-like switching cybotactic nematic phases
smectic surface layers, and these layers align the clusters of the (NcybCP) [64,65], but in these NcybCP phases the molecules are
bulk sample. In this way the surface anchoring determines the strongly tilted (40–50° tilt), such that the field-induced polar
optical properties of the investigated samples. If the surface and biaxial nematic phases of these compounds represent
layer is nontilted (SmA-like) then the sample adopts a subtypes of the monoclinic Nbm phases. It has been shown here
homeotropic alignment, which appears completely dark. Indeed that the combination of the 2,5-diphenyl-1,2,4-oxadiazole core
for compounds CB-Ox/n with a NcybA–CybA transition the with one CB unit in dimesogens leads to the removal of the tilt
birefringent Schlieren texture disappears upon approaching the and promotes an orthogonal organization of the molecules
transition to the CybA phase (Figure 8g and Figure 8h). In (CybA, NcybA). However, replacing the CB group by a
contrast, for compound Thia-Ox/5, with a NcybC–SmC tran- 2-phenyl-1,3,4-thiadiazole core retains the tilted SmC-like
sition (and also for Thia-Ox/10 without SmC phase), the bire- organization of the simple 1,2,4-oxadiazole-based bent-core
fringent texture, as typical for tilted smectic phases, is retained mesogens [81,82].
on cooling down to the transition to the SmC phase (Figure 8c
Experimental
and Figure 8d). In fact the NcybC-to-SmC transition is hardly
detected by polarizing microscopy (see Figure 8d), but it is Preparation of the trimesogens CB-Ox-CB/n
clearly confirmed by the DSC peak (Figure 5a) and the XRD A mixture of 3,5-bis(4-hydroxyphenyl)-1,2,4-oxadiazole (1)
pattern (Figure 6a). However, the absence of birefringence in [65] (0.39 mmol, 1 equiv), ω-(4’-cyanobiphenyl-4-yloxy)alka-
the homeotropically aligned sample does not prove the pres- noic acids (2a,b) [66] (0.78 mmol, 2 equiv) and a catalytic
ence of a NcybA phase, as in a NcybC phase randomization of the amount of DMAP were dissolved in dry CH2Cl2 and stirred for
tilt direction of the SmC clusters can also result in a completely 5 min. To the above clear solution, N,N’-dicyclohexylcarbodi-
dark appearance of the homeotropic samples, and in this case imide (DCC, 1.2 mmol, 3 equiv) was added and stirring was
birefringence occurs only upon approaching the transition to the continued for 12 h at rt, followed by solvent evaporation and
SmC phase. Hence, optical investigation can only be used as a column chromatography with using EtOAc/n-hexane 2:8, to
first indication of NcybA and NcybC phases if the transition to a yield a solid, which was further purified by crystallization from
homeotropically aligned smectic phase is observed, but it EtOAc/EtOH 2:8.
usually requires additional confirmation by XRD of magneti-
cally aligned samples.
CB-Ox-CB/3: 3,5-bis{4-[4-(4’-Cyanobiphenyl-4-
yloxy)butanoyloxy]phenyl}-1,2,4-oxa-diazole; colorless solid;
yield 71%; 1H NMR (400 MHz, CDCl3) δ 8.25 (d, J = 8.4 Hz,
Conclusion
The first examples of end-to-end connected bent-core–rod 2H, Ar), 8.16 (d, J = 8.0 Hz, 2H, Ar), 7.70 (dd, 1J = 8.2 Hz, 2J
couples and rod–bent-core–rod trimesogens incorporating a = 8.4 Hz, 8H, Ar), 7.56 (d, J = 8.4 Hz, 4H, Ar), 7.32 (d, J = 8.8
bent 3,5-diphenyl-1,2,4-oxadiazole core have been synthesized Hz, 2H, Ar), 7.27 (d, J = 8.0 Hz, 2H, Ar), 7.22 (d, J = 8.8 Hz,
and their LC phase behaviors were studied. All molecules show 4H, Ar), 4.17 (t, J = 5.8 Hz, 4H, 2 × OCH2), 2.85 (m, 4H, 2×
broad regions of nematic phases. The heterotrimesogens with CH2COO), 2.32 (m, 4H, 2 × CH2); Analysis calcd for
two CB units (CB-Ox-CB/n) form exclusively nematic phases, C48H36N4O7: C, 73.83; H, 4.65; N, 7.18; found: C, 73.76; H,
whereas related dimesogens with only one CB unit (Ox-CB/n) 4.45; N, 6.91.
show an additional CybA phase. The nematic phases of these
dimesogens represent cybotactic nematic phases [68,71-73] CB-Ox-CB/4: 3,5-bis{4-[5-(4’-Cyanobiphenyl-4-
composed of small SmA-like clusters. These cybotactic nematic yloxy)pentanoyloxy]phenyl}-1,2,4-oxa-diazole; colorless solid;
phases composed of clusters with (on average) nontilted mole- yield 75%; 1H NMR (400 MHz, CDCl3) δ 8.22 (d, J = 8.0 Hz,
cules are of significant interest, as restricted rotation around the 2H, Ar), 8.17 (d, J = 8.0 Hz, 2H, Ar), 7.68 (dd, 1J = 8.4 Hz, 2J
long axes of the molecules would in this case lead to biaxial = 8.4 Hz, 8H, Ar), 7.52 (d, J = 8.8 Hz, 4H, Ar), 7.28 (d, J = 8.8
nematic phases of the orthorhombic type (Nbo) [57,73]. In Hz, 2H, Ar), 7.21 (d, J = 8.0 Hz, 2H, Ar), 7.00 (d, J = 8.8 Hz,
contrast, biaxial nematic phases of the monoclinic type (Nbm) 4H, Ar), 4.09 (t, J = 5.2 Hz, 4H, 2 × OCH2), 2.71 (m, 4H, 2 ×
482

Beilstein J. Org. Chem. 2012, 8, 472–485.
CH2COO), 1.97 (m, 8H, 4 × CH2); 13C NMR (125 MHz, Thia-Ox/5: 4-[3-(4-{6-[4-(5-Heptyl-1,3,4-thiadiazol-2-
CDCl3) δ 175.01, 171.44, 171.21, 168.30, 159.50, 159.48, yl)phenoxy]hexanoyloxy}phenyl)-1,2,4-oxadiazol-5-yl]phenyl
154.13, 152.94, 145.19, 145.16, 132.55, 131.57, 131.54, 129.63, 4-hexylbenzoate; colorless solid; yield 69%; 1H NMR (400
128.84, 128.36, 127.08, 124.48, 122.43, 122.08, 121.76, 119.03, MHz, CDCl3) δ 8.24 (d, J = 8.8 Hz, 2H, Ar), 8.15 (d, J = 8.8
115.08, 115.06, 110.15, 110.12, 67.46, 67.43, 33.96, 28.52, Hz, 2H, Ar), 8.07 (d, J = 8.4 Hz, 2H, Ar), 7.80 (d, J = 8.8 Hz,
21.60, 21.57; Anal. calcd for C50H40N4O7: C, 74.24; H, 4.98; 2H, Ar), 7.38 (d, J = 8.8 Hz, 2H, Ar), 7.28 (d, J = 8.8 Hz, 2H,
N, 6.93; found: C, 73.94; H, 4.74; N, 6.62.
Ar), 7.19 (d, J = 8.4 Hz, 2H, Ar), 6.91 (d, J = 8.8 Hz, 2H, Ar),
4.01 (t, J = 6.4 Hz, 2H, 1 × OCH2), 3.05 (t, J = 7.6 Hz, 2H, 1 ×
CH2), 2.66 (t, J = 7.2 Hz, 2H, 1 × CH2), 2.60 (t, J = 7.2 Hz, 2H,
CH2COO), 1.80–0.82 (m, 30H, 12 × CH2, 2 × CH3); Anal.
Preparation of the dimesogens CB-Ox/n and
Thia-Ox/n
A mixture of 4-[3-(4-hydroxyphenyl)-1,2,4-oxadiazol-5-yl]phe- calcd for C48H54N4O6S: C, 70.74; H, 6.87; N, 5.87; found: C,
nyl 4-hexylbenzoate (3) [67] (0.34 mmol, 1 equiv), ω-(4’- 70.57; H, 6.54; N, 6.67.
cyanobiphenyl-4’-yloxy)alkanoic acid (2a,b) [66] (0.34 mmol,
1 equiv) or ω-[4-(5-heptyl-1,2,4-thiadiazol-2yl)phenoxy]alka- Thia-Ox/10: 4-[3-(4-{11-[4-(5-Heptyl-1,3,4-thiadiazol-2-
noic acids (4a,b) [66] (0.34 mmol, 1 equiv) and a catalytic yl)phenoxy]undecanoyloxy}-phenyl)-1,2,4-oxadiazol-5-yl]phe-
amount of DMAP were dissolved in dry CH2Cl2. To the above nyl 4-hexylbenzoate; colorless solid; yield 72%; 1H NMR (400
clear solution, N,N’-dicyclohexylcarbodiimide (DCC, 0.51 MHz, CDCl3) δ 8.23 (d, J = 8.8 Hz, 2H, Ar), 8.14 (d, J = 8.8
mmol, 1.1 equiv) was added and stirred for 12 h at rt, followed Hz, 2H, Ar), 8.07 (d, J = 8.4 Hz, 2H, Ar), 7.79 (d, J = 8.8 Hz,
by solvent evaporation and column chromatography with 2H, Ar), 7.37 (d, J = 8.8 Hz, 2H, Ar), 7.27 (d, J = 8.8 Hz, 2H,
EtOAc/n-hexane 2:8 to yield a solid, which was further puri- Ar), 7.18 (d, J = 8.4 Hz, 2H, Ar), 6.89 (d, J = 8.8 Hz, 2H, Ar),
fied by crystallization from EtOAc/EtOH 2:8.
3.96 (t, J = 6.4 Hz, 2H, 1 × OCH2), 3.05 (t, J = 7.6 Hz, 2H, 1 ×
CH2), 2.66 (t, J = 7.2 Hz, 2H, 1 × CH2), 2.54 (t, J = 7.2 Hz, 2H,
CB-Ox/3: 4-(3-{4-[4-(4’-Cyanobiphenyl-4-yl)oxybutanoyl- CH2COO), 1.77–0.79 (m, 40H, 17 × CH2, 2 × CH3); 13C NMR
oxy]phenyl}-1,2,4-oxadiazol-5-yl)phenyl 4-hexylbenzoate; (125 MHz, CDCl3) δ 175.09, 171.85, 169.47, 168.33, 168.11,
colorless solid; yield 70%; 1H NMR (400 MHz, CDCl3) δ 8.24 164.60, 161.27, 154.57, 153.03, 149.87, 130.34, 129.66, 129.27,
(d, J = 8.8 Hz, 2H, Ar), 8.16 (d, J = 8.8 Hz, 2H, Ar), 8.07 (d, J 128.83, 128.76, 126.35, 124.43, 122.86, 122.67, 122.11, 121.73,
= 8.4 Hz, 2H, Ar), 7.64 (dd, 1J = 8.8 Hz, 2J = 8.8 Hz, 4H, Ar), 114.91, 68.18, 36.09, 34.38, 31.62, 31.61, 31.03, 30.17, 30.05,
7.49 (d, J = 8.4 Hz, 2H, Ar), 7.38 (d, J = 8.8 Hz, 2H, Ar), 7.28 29.43, 29.31, 29.30, 29.18, 29.12, 29.04, 28.94, 28.87, 28.86,
(d, J = 8.4 Hz, 2H, Ar), 7.21 (d, J = 8.8 Hz, 2H, Ar), 6.96 (d, J 25.96, 24.84, 22.55, 22.54, 14.03, 14.02; Anal. calcd for
= 8.8 Hz, 2H, Ar), 4.11 (t, J = 6.0 Hz, 2H, 1 × OCH2), 2.81 (t, J C53H64N4O6S: C, 71.92; H, 7.29; N, 6.33; found: C, 71.65; H,
= 7.2 Hz, 2H, 1 × CH2), 2.67 (t, J = 7.6 Hz, 2H, CH2COO), 7.25; N, 6.29.
2.23 (m, 2H, CH2), 1.63–1.12 (m, 8H, 4× CH2), 0.84 (t, J = 6.8
Hz, 3H, 1 × CH3); 13C NMR (125 MHz, CDCl3) δ 175.13, Acknowledgements
171.24, 168.29, 164.61, 159.36, 154.60, 152.88, 149.89, 145.15, This work was supported by the EU within the FP7 funded
132.55, 131.74, 130.34, 129.67, 128.87, 128.77, 128.40, 127.11, Collaborative Project BIND (Grant No 216025) and the Cluster
126.33, 124.60, 122.69, 122.06, 121.71, 119.03, 115.10, 110.18, of Excellence “Nanostructured Materials”.
66.71, 36.09, 31.62, 31.04, 30.97, 28.88, 24.52, 22.54, 14.03;
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