.
Angewandte
Communications
DOI: 10.1002/anie.201209453
Liquid Crystals
A Modulated Helical Nanofilament Phase**
Ethan Tsai, Jacqueline M. Richardson, Eva Korblova, Michi Nakata, Dong Chen,
Yongqiang Shen, Renfan Shao, Noel A. Clark, and David M. Walba*
The modern study of the liquid crystal (LC) phases formed by
bent-core molecules[1] has led to many interesting and
unexpected phenomena, including the first example of
spontaneous reflection symmetry breaking and conglomerate
formation in a bulk fluid phase.[2] Indeed, exploration of the
unique behavior of bent-core materials is currently one of the
most productive frontier research areas in soft matter
science.[3] Perhaps the most complex of the known bent-core
phases, the helical nanofilament (HNF) phase (also known as
the B4 banana phase) has been under serious investigation
since 1997,[1c,d] and continues to be a focus of interest in the
bent-core materials constellation. Here, we report full char-
acterization of the first example of a new phase in this family,
HNF(mod), composed of simple alkoxybiphenylcarboxylate
units and lacking the Schiff base groups found in previously
known HNF mesogens.
The classic HNF phase possesses a unique hierarchical
nanostructure: An assembly of twisted layers stacked to form
well-defined chiral nanorods (individual HNFs ca. 40 nm
diameter), with a structure driven by intra-layer frustration
leading to spontaneous saddle splay, and formation of layers
with negative curvature.[4–6] Solid state NMR data suggest that
within individual HNF layers the structure is crystalline,[7]
though electron diffraction shows that no interlayer positional
correlation exists.[4] The HNFs in turn form a kind of hexatic
LC phase, which freezes into a glassy state at ca. 1108C. When
the mesogens are achiral or racemic, an LC conglomerate of
large heterochiral domains is easily seen in LC cells by
polarized optical microscopy. The bulk HNF phase is
porous,[4,6] and when grown in the presence of other materials,
can produce nanostructured composites.[8] Potential applica-
tions of the HNF phase and composites include nonlinear
optics, organic electronics, photovoltaics, and chiral separa-
tions.
ble benzylideneaniline (Schiff base) moiety. This is problem-
atic for some potential applications. Also, the rarity of the
known HNF phases in the bent-core “structure space”
motivates discovery of new HNF phases in order to better
understand the molecular structural factors leading to their
formation, and to allow future design of functional HNF
materials.
Here we report characterization of seven homologues of
biphenyl-3,4’-diyl bis-(4’-alkoxybiphenyl-4-carboxylate),[10]
(diesters 1(n) (n = 9–15), Figure 1a). All of these exhibit
a new HNF phase possessing in-layer modulation in addition
to the characteristic negative curvature of the layers.
Bent-core phases with in-layer modulation are known,
including for example a tilted polar smectic (SmCP) phase
possessing undulated smectic layers (also known as the B7
banana phase).[11] Here, we designate the new modulated
HNF phase HNF(mod), with the proposed hierarchical struc-
ture for individual HNFs illustrated in Figure 1. As for the
classic HNF phase, we propose that the aromatic cores in the
new phase are tilted, while the tails are extended almost
normal to the local layer plane.[4]
However, for the HNF(mod), in-layer modulation with
a spacing of about 40 ꢀ, not seen in the HNF phase, is
suggested by experimental data. We propose this secondary
modulation results from a structural periodicity with wave
vector parallel to the layers and normal to the polar axis (P),
as indicated in Figure 1c. The precise nature of this periodic
defect structure is not known, though the lack of two-fold
symmetry for rotation about b in diesters 1(n) could
reasonably produce such a periodic structural change by
simple 1808 rotation about b, mediating in-layer “stripes”
about eight molecules wide, as indicated (Figure 1c). Finally,
as for the HNF phase, the twisted layers stack to form helical
nanofilaments (Figure 1d).
Interestingly, of the great many bent-core structures
reported to date, with the exception of a single reported
outlier[9] all HNF mesogens include the hydrolytically unsta-
Evidence for this structural picture of the HNF(mod) phase
formed by diesters 1(n) derives from: 1) polarized optical
microscopy (POM); 2) differential scanning calorimetry
(DSC); 3) X-ray diffraction (XRD) at small angle; and 4)
transmission electron microscopy (TEM). The phase assign-
ments, transition temperatures and enthalpies on cooling, and
HNF(mod) layer spacing (d1) and in-layer modulation dimen-
sions (d2) for diesters 1(n), are given in Table 1. A brief
discussion of the key observations and interpretations leading
to the model illustrated in Figure 1 follows.
As indicated in Table 1, all of the homologues are very
similar in their basic behavior. All show a transition from
isotropic to a more conventional bent-core phase, tentatively
assigned as either a B1 (columnar) phase, or a B2 (tilted
smectic) phase. For the lower homologues, (n = 9–12) the high
[*] Dr. E. Tsai, Dr. J. M. Richardson, Dr. E. Korblova,
Prof. Dr. D. M. Walba
Department of Chemistry and Biochemistry
University of Colorado, 215 UCB
Boulder, CO 80309-0215 (USA)
E-mail: walba@colorado.edu
Dr. M. Nakata, Dr. D. Chen, Y. Shen, R. Shao, Prof. Dr. N. A. Clark
Department of Physics, University of Colorado, 390 UCB
Boulder, CO 80309-0390 (USA)
[**] We thank the Liquid Crystal Materials Research Center (NSF MRSEC
award no. DMR-0820579) for financial support of this work.
Supporting information for this article is available on the WWW
temperature phases are monotropic with respect to HNF(mod)
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 5254 –5257