A new room temperature dark conglomerate mesophase formed by bent-core molecules combining 4-iodoresorcinol with azobenzene units

Article abstract of DOI:10.1039/c3cc45938a

The first bent-core molecules comprising 4-iodoresorcinol as the central core unit and incorporating azobenzene units have been synthesized. A new type of dark conglomerate phase (DC phase) is observed, which remains over a wide temperature range down to room temperature without crystallization.

Full text of DOI:10.1039/c3cc45938a

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A new room temperature dark conglomerate
mesophase formed by bent-core molecules combining
4-iodoresorcinol with azobenzene units†
Mohamed Alaasar,*ab Marko Prehma and Carsten Tschierske*a
Cite this: Chem. Commun., 2013,
49, 11062
Received 3rd August 2013,
Accepted 4th October 2013
DOI: 10.1039/c3cc45938a
www.rsc.org/chemcomm
The first bent-core molecules comprising 4-iodoresorcinol as the dilution with a large excess of an achiral nematic phase.8 In contrast
central core unit and incorporating azobenzene units have been to the soft crystalline B4 phase the formation of the completely fluid
synthesized. A new type of dark conglomerate phase (DC phase) is DC phases without crystalline nano-filaments is more difficult to
observed, which remains over a wide temperature range down to understand.5 A common way to distinguish both types of DC phases
room temperature without crystallization.
is by XRD where the soft crystalline B4 phases are characterized by
the presence of sharp wide angle reflections, whereas only one
Chirality has been one of the most attractive themes in chemistry diffuse halo around d = 0.45 nm is found for the fluid DC phases.
since Pasteur’s famous resolution experiment showing the handed- Herein we report a new subtype of DC phases which appears
ness of tartaric acid.1 Since that time spontaneous formation of to be intermediate between these two types. Moreover, the
chiral phases by the self-assembly of achiral molecules or self- reported compounds forming this phase are the first examples
resolution of racemic conglomerates has been a well known of bent-core liquid crystals (BCLCs) containing a 4-iodoresorci-
phenomenon, often observed in the crystalline solid state.2 How- nol unit as the central core structure. These materials also
ever, with bent-core molecules this phenomenon was also found in contain photoisomerizable azobenzene groups which can be
soft crystals and even in liquid crystals.3 Spontaneous macroscopic exploited for optical and optoelectronic devices.9 To the best of
optical activity has been observed for these achiral molecules in the our knowledge, the compounds reported herein are the first
soft crystalline helical nano-filament phases (assigned as HNF examples of azo functionalized BCLCs showing DC phases.‡
phases or B4 phases)4 and in fluid polar smectic phases (B2 phases).5
The synthetic pathway leading to the bent core compounds
These optically nearly isotropic mesophases are composed of con- A12 and Bn is given in Scheme 1. The detailed synthetic proce-
glomerates of macroscopic chiral domains, which can easily be dures, purification and analytical data are reported in the ESI.†
distinguished under a polarizing microscope (dark conglomerate
Compound A12 without any lateral substituent on the outer
phases, DC phases). Conglomerates of chiral domains were also benzene rings (X = H) was found to be non-mesomorphic, having a
found in the recently reported birefringent SmCsPR phase6 and in melting point at T = 112 1C and crystallizing at T = 77 1C. Replacing the
nematic phases formed by some bent-core mesogens,7 however in hydrogen atoms next to the alkoxy chain by fluorine led to compounds
these cases their formation is dependent on surface interactions Bn, (X = F, n = 8–14), which all give optically isotropic mesophases
which are required to stabilize the chiral domains. The B4 phase is composed of a conglomerate of chiral domains (DC phases, see Fig. 2).
the best investigated of the DC phases. In these mesophases the
chirality arises from a crystalline packing of the aromatic cores in
left- or right-handed helical nano-filaments. The packing of the
terminal alkyl chains is frustrated and therefore these chains
remain in a disordered liquid state.4 Another characteristic feature
of these soft crystalline phases is that the chirality is retained upon
a Institute of Chemistry, Martin-Luther University Halle-Wittenberg,
Kurt-Mothes Str.2, D-06120 Halle/Saale, Germany.
E-mail: carsten.tschierske@chemie.uni-halle; Fax: +49 (0)3455527346;
Tel: +49 (0)34555256664
b Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt.
E-mail: M_Alaasar@yahoo.com; Fax: +20 235727556; Tel: +20 235676595
† Electronic supplementary information (ESI) available: Synthesis, analytical
Scheme 1 Synthetic route to the bent core molecules An and Bn.10,11
data, and additional data. See DOI: 10.1039/c3cc45938a
c
11062 Chem. Commun., 2013, 49, 11062--11064
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Table 1 Phase transition temperatures (T/1C), mesophase types, and transition
enthalpies [DH/kJ molꢀ1] of compounds A12 and Bna
by ‘‘crystallization’’ from solvents appear to be in the mesomorphic
state. Uncrossing the polarizers by a small angle leads to the
appearance of dark and bright domains with a maximum contrast
at an angle of ca. 71. Uncrossing the polarizer in the other direction
reverses the dark and bright domains (Fig. 2a and c). Rotating the
Compd
n
1st heating
1st cooling
2nd heating
A12
B8
B10
B12
B14
12
8
10
12
14
Cr 112 [35.1] I
DC 116 [31.5] I
DC 116 [25.0] I
DC 114 [25.8] I
DC 116 [30.2] I
I 77 [33.7] Cr
Cr 112 [34.9] I
DC 116 [30.6] I
I 107 [27.9] DC
I 102 [29.3] DC
I 103 [23.7] DC
I 109 [32.0] DC
DC 115 [23.9] I sample between crossed polarizers does not lead to any change and
DC 111 [25.3] I
DC 116 [30.1] I
this indicates that the distinct regions represent chiral domains
with opposite handedness.
a
Peak temperatures from DSC at a rate of 10 K minꢀ1; abbreviations: Cr =
No current peak could be observed for compound B14 in
electrooptical investigations of the mesophase and no birefrin-
gent mesophase is induced under an applied triangular wave
voltage of up to 200 Vpp in a 6 mm ITO cell. These features are
also very similar to those of the soft crystalline B4 type DC
phases (helical nanofilament phases).
crystalline solid; DC = dark conglomerate phase; I = isotropic liquid.
However the XRD pattern (see Fig. 3 and Fig. S2, ESI†) is different
from those of the B4 phases. For compound B14 an intense small
angle reflection is observed, corresponding to a distance of
d = 4.63 nm (90 1C). From this d-value and the molecular length
L = 6.1 nm (in a 1201 V-shaped conformation and all-trans alkyl
chains) a d/L ratio of 0.76 results, which allows a tilt angle of the
molecules up to b = 411 (cos b = d/L). Whether there is a tilt of the
molecules or whether the difference between d and L is only due to
chain folding and chain interdigitation cannot be decided at
Fig. 1 DSC heating and cooling curves (10 K minꢀ1) observed for B14.
The transition temperatures of the synthesized compounds are present. The reasons are that aligned samples cannot be obtained
summarized in Table 1. All compounds Bn exhibit only one for DC phases due to their locally distorted structure (see Fig. S2a,
mesophase (the DC phase), and the phase transition temperatures ESI†) and that optical methods cannot be used for tilt angle
are almost the same irrespective of the terminal chain length. determination of these isotropic mesophases.
Compound B14, as a representative example, was investigated in
With the exception of the intense small angle scattering all other
more detail. Upon cooling B14 under the polarizing microscope the scatterings have only very low intensity and are more or less diffuse.
isotropic liquid becomes highly viscous with glass-like appearance The low intensity could, at least partly, be due to the absorption by
at 109 1C but no birefringence is observed between crossed polar- the heavy atom iodine incorporated in the molecular structure. A
izers (Fig. 2b). Fig. 1 shows the heating and the cooling differential broad scattering is found at about 2y = 9.31, corresponding to a
scanning calorimetry (DSC) curves obtained for compound B14. The mean distance of 1.0 nm and in the wide angle region there are two
formation of this phase is associated with a relatively high transition additional even weaker diffuse scattering maxima corresponding to
enthalpy of DH B 30 kJ molꢀ1, which is a value similar to typical d-values of 0.52 and 0.43 nm. So the majority of intensity of the
transition enthalpies of the B4 to isotropic liquid transition.4 The diffuse scattering is found in the medium angle region. This kind of
onset temperatures of the transition peaks upon heating and cool- XRD pattern is distinct from the B4 phases, which show three or
ing are approximately the same (B111 1C) indicating the absence of more sharp wide angle reflections at larger angles in the 2y range
a hysteresis. Also the transition peaks upon heating and cooling of 181–281. However, it is also distinct from the fluid DC phases
have approximately the same enthalpy values in the first and all showing only one diffuse halo in the wide angle region at 2y B 201
following heating–cooling cycles.
besides the small angle layer reflection.5a
Upon further cooling no crystallization occurs down to room
Though the precise origin of this diffraction pattern is still
temperature (Fig. 1). Even after storage of the sample at room unclear, in a tentative model it can be assumed that the individual
temperature for 6 weeks the same transition temperatures and bent-core molecules form densely packed crystalline nano-clusters
enthalpies were obtained and there was no indication of any
crystallization (see Fig. S1 in the ESI†), even the samples obtained
Fig. 2 Textures of the DC phase of compound B14 at T = 80 1C: (b) between
crossed polarizers and (a) after rotating one polarizer by 71 from the crossed position
Fig. 3 XRD pattern of the DC phase of compound B14 at T = 90 1C; the inset
shows the magnified (B40 fold) wide angle region after baseline correction.
in the clock-wise direction and (c) in the anticlockwise direction, showing dark and
bright domains, indicating the presence of areas with opposite chirality sense.
c
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incorporating around 4–6 molecules. The mean distance between photoisomerizable azobenzene units could lead to interesting
these nano-crystallites is assumed to give rise to the weak diffuse perspectives for chirality switching and phase modulation by
scattering with a maximum around 1.0 nm. The further growth of interaction with circularly polarized light.9,14
these clusters is obviously inhibited. This might be caused by the
M.A. acknowledges the support from the Alexander von
packing of helical conformers, which leads to a twist and inhibits Humboldt Foundation for the research fellowship at Martin-
the further growth to larger nano-filaments or crystalline layers. Luther University Halle-Wittenberg, Germany.
Nano-crystallites with identical twist sense arrange in layers with
only short range order in the layers. As packing of nano-crystallites
with identical chirality is favorable macroscopic chirality develops.
Notes and references
‡ B4-type DC phases have been reported for bent shaped mesogenic
The chirality induces twist and bend of the layers which, together
dimers combining two rod-like azobenzene units.15 A B4-like DC phase was
with the effects of steric and packing frustrations,5 gives rise to a
found recently for one homologue in a series based on a 4-bromoresorcinol
strong non-regular deformation or fragmentation of these layers.
The correlation length of uniformly oriented domains is in the
range of 35 nm (as estimated from the full width at half-maximum
of the layer reflection peak) which is below the wavelength of light,
thus leading to an optically isotropic appearance of the phase. Some
parts of the alkyl chains seem to remain in a disordered state and
contribute to the diffuse wide angle scatterings (see Fig. 3).
core with rod-like azobenzene units upon cooling from a nematic phase,
but with a very short range before crystallization.11
§ Besides the B4-type helical filament phases a soft crystalline dark
conglomerate phase with a 2D lattice17 and for the fluid DC phases,
which usually have a single layer structure, an intercalated B6-like
version was also reported recently,18 moreover an achiral cubic phase
composed of vesicular layer aggregates has been reported.19
¶ Dark conglomerate phases were also observed for some 4-bromoresorcinol
esters with Schiff base wings.16
As it is known that B4 phases can be diluted by nematic LC hosts
to a high degree (>95%) without loss of the chirality8 we investigated
a 1:1 mixture and a 1:9 mixture of B14 with 40-pentyl-4-cyano-
biphenyl (5-CB). In the 1 : 1 mixture the DC-Iso transition is reduced
to 60 1C, but the chiral domains are still clearly visible (see Fig. S3a–c,
ESI†). This DC phase rapidly crystallizes at 46 1C (Fig. S3d, ESI†) and
upon heating the crystalline material melts at 86 1C to an isotropic
liquid. In the 1 : 9 mixture no DC phase is formed, again compound
B14 crystallizes from the nematic phase at T = 35 1C and these
crystals melt at 52 1C. So, in contrast to B4 phases there is a clear
destabilizing effect of 5-CB on the DC phase. Simultaneously,
formation of a crystalline phase is strongly enhanced. It seems that
the 5-CB molecules can reduce the frustration of the packing of the
molecules and allow the growth of the small nano-crystallites to a
macroscopic crystalline phase. It is however not clear if pure B14
crystallizes or if these crystals involve additional 5-CB molecules.
It appears that the presence of the relatively large and highly
polarizability of iodine in the 4-position of the resorcinol unit
provides bent-core mesogens with new interesting phase struc-
tures. The observed kind of DC phase is not reported for related
molecules with chlorine at the same position.11 Bromine seems
1 L. Pasteur, C. R. Acad. Sci., 1848, 26, 535.
2 Chirality at the Nanoscale, ed. D. B. Amabiliino, Wiley-VCH,
Weinheim, 2009.
3 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe,
J. Mater. Chem., 1996, 6, 1231; D. R. Link, G. Natale, R. Shao,
¨
J. E. Maclennan, N. A. Clark, E. Korblova and D. M. Walba, Science,
1997, 278, 1924; R. A. Reddy and C. Tschierske, J. Mater. Chem., 2006,
16, 907; H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006, 45, 597.
4 T. Sekine, T. Niori, J. Watanabe, S. W. Choi, Y. Takanishi and
H. Takezoe, J. Mater. Chem., 1997, 7, 1307; D. M. Walba,
¨
L. Eshdat, E. Korblova and R. K. Shoemaker, Cryst. Growth Des.,
2005, 5, 2091; E. Bialecka-Florjanczyk, I. Sledzinska, E. Gorecka and
J. Przedmojski, Liq. Cryst., 2008, 35, 401; L. E. Hough, H. T. Jung,
D. Kruerke, M. S. Heberling, M. Nakata, C. D. Jones, D. Chen,
D. R. Link, J. Zasadzinski, G. Heppke, J. P. Rabe, W. Stocker,
E. Korblova, D. M. Walba, M. A. Glaser and N. A. Clark, Science,
2009, 325, 456; D. Chen, J. E. Maclennan, R. Shao, D. K. Yoon,
¨
H. Wang, E. Korblova, D. M. Walba, M. A. Glaser and N. A. Clark,
J. Am. Chem. Soc., 2011, 133, 12656.
5 (a) G. Dantlgraber, A. Eremin, S. Diele, A. Hauser, H. Kresse, G. Pelzl and
C. Tschierske, Angew. Chem., Int. Ed., 2002, 41, 2408; (b) L. E. Hough,
M. Spannuth, M. Nakata, D. A. Coleman, C. D. Jones, G. Dantlgraber,
¨
C. Tschierske, J. Watanabe, E. Korblova, D. M. Walba, J. E. Maclennan,
M. A. Glaser and N. A. Clark, Science, 2009, 325, 452.
6 M. Alaasar, M. Prehm, M. Nagaraj, J. K. Vij and C. Tschierske, Adv.
Mater., 2013, 25, 2186.
7 G. Pelzl, A. Eremin, S. Diele, H. Kresse and W. Weissflog, J. Mater.
to be a bit more favorable for the formation of DC phases,11
§
¨
Chem., 2002, 12, 2591; V. Gortz, Liq. Cryst. Today, 2010, 19, 37.
8 T. Otani, F. Araoka, K. Ishikawa and H. Takezoe, J. Am. Chem. Soc.,
2009, 131, 12368.
9 H. M. Dhammika Bandarab and S. C. Burdette, Chem. Soc. Rev.,
2012, 41, 1809.
10 I. Thomsen and K. B. G. Torssell, Acta Chem. Scand., 1991, 45, 539.
11 M. Alaasar, M. Prehm and C. Tschierske, Liq. Cryst., 2013, 40, 656.
whereas iodine seems to have a much stronger DC-promoting
effect. The bulky iodine might lead to strongly twisted helical
molecular conformations by twisting the adjacent COO group
out of the planes of the adjacent benzene rings,12 thus favour-
ing layer distortion and formation of chiral superstructures. 12 S. Ananda Rama Krishnan, W. Weissflog and R. Friedemann, Liq.
Cryst., 2005, 32, 847.
13 D. W. Bruce, Struct. Bonding, 2008, 126, 161.
14 H. Takezoe, Top. Curr. Chem., 2012, 318, 303.
Iodine–iodine interactions or weak halogen bonding inter-
actions13 could also contribute to this effect.
In summary, we report herein the first bent-core liquid crystal- 15 S.-W. Choi, T. Izumi, Y. Hoshino, Y. Takanishi, K. Ishikawa,
J. Watanabe and H. Takezoe, Angew. Chem., Int. Ed., 2006, 45, 1382;
G. Lee, R. J. Carlton, F. Araoka, N. L. Abbott and H. Takezoe, Adv. Mater.,
2013, 25, 245.
line materials containing 4-iodine substitution in the central core
unit. These compounds exhibit a new type of DC phase¶ occurring
in broad temperature ranges including room temperature. More- 16 S. Kang, J. Thisayukta, H. Takezoe, J. Watanabe, K. Ogino, T. Doi
and T. Takahashi, Liq. Cryst., 2004, 31, 1323.
17 E. Tsai, J. M. Richardson, E. Korblova, M. Nakata, D. Chen, Y. Shen,
over, these are the first examples of azobenzene-based BCLC
showing DC phases.‡ The DC phase formed by this kind of
R. Shao, N. A. Clark and D. M. Walba, Angew. Chem., Int. Ed., 2013,
molecules is different from the previously reported types and could
contribute to an improved understanding of these phases and the
occurrence of spontaneous achiral symmetry breaking in soft
matter in general. Moreover, the possibilities provided by the
52, 5254.
18 H. Ocak, B. Bilgin-Eran, M. Prehm and C. Tschierske, Soft Matter,
2013, 9, 4590.
19 S. Kang, M. Harada, X. Li, M. Tokita and J. Watanabe, Soft Matter,
2012, 8, 1916.
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11064 Chem. Commun., 2013, 49, 11062--11064
This journal is The Royal Society of Chemistry 2013